PGE mineralization at the Alard stock: Implications for the porphyry to epithermal transition, La Plata Mountains, Colorado. by Erin Sloane Summerlin A thesis submited to the Graduate Faculty of Auburn University in partial fulfilment of the requirements for the Degree of Master of Science Auburn, Alabama May 4 th , 2014 Keywords: Porphyry, PGE-Copper, Alkaline, Colorado, Telurides Copyright 2014 by Erin S. Summerlin Approved by James A. Saunders, Chair, Profesor of Geology and Geography Wilis E. Hames, Profesor of Geology and Geography Haibo Zou, Asociate Profesor of Geology and Geography ! ii Abstract Geochemical and petrographic studies on the Alard stock, which hosts a porphyry Cu-Ag-Au-Te-PGE deposit in the La Plata Mountains of southwestern Colorado, suggest mantle sources for both metals and sulfur. Microprobe analysis of ores verifies the PGE host mineral phase(s) as Pd?Pt?Bi-telurides and Ag?Pd telurides, and a possible previously unidentified mineral with a suggested formula of PdTe 2 . The La Plata mining district, named after the lacolithic La Plata mountains, lies ~20 km northwest of Durango, Colorado, at the southwestern-most part of the Colorado Mineral Belt. Intrusion of Cretaceous lacolithic diorite-monzonite porphyries from igneous activity that propagated inward from the flat-slab subuction of the Faralon plate during the Laramide Orogeny (70-80 Ma) caused domal uplift of strata and contact metamorphosed the surrounding country rocks. Younger alkaline stocks appear to have initiated the ore- forming proceses in the district by providing fluids and reopening existing faults. One of these alkaline porphyry stocks, The Alard stock (~67 Ma), hosts both masive, stockwork, and diseminated chalcopyrite and pyrite, Ag-Au telurides, and PGEs. Sulfur isotope values for the Alard stock range from -7.8 to +1? ? 34 S (VCDT) , indicating a magmatic source with a minor 32 S-enriched component of contamination apparently derived from the country rocks. Copper isotope values range from +0.965 to +2.700? ? 65 Cu, within the range for typical porphyry coppers. Lead isotope data suggests a mantle source for lead from sulfides in the stock mineralization. Due to the ~1 iii ! km of topographic relief, the transition betwen porphyry and epithermal styles of mineralization is well exposed and amenable to sampling in the La Plata district. Mineralization in the district includes porphyry mineralization (the Alard stock) as wel as contact-metamorphic Au-bearing sulfides, limestone replacement (skarn) with sulfides and telurides, Au-bearing pyrite, mixed sulfides with free Au, chalcocite veins, ruby silver veins, and Au-Ag-teluride deposits. Because of their close spatial asociation, hydrothermal fluids that mineralized the Alard stock porphyry are likely coeval or analogous to fluids that mineralized the Au-Ag-telurides in the epithermal deposits and perhaps even the skarn deposits. Geochemicaly, the porphyry and epithermal ores in the La Plata district are extremely similar (Au, Ag, Te). Therefore the migration of fluids from the mantle source, to the porphyry regime, and finaly to the epithermal environment sems a likely model for ore deposition in the La Plata district. ! iv Acknowledgments I am greatly appreciative to the wonderful family, friends, and colleagues that have loved and supported me throughout the completion of my MS degree. Michael Mason, my office mate and felow student of Dr. Saunders, carried many heavy rocks, double checked my math, and was an Excel formula master, to him I am very grateful. My advisor, Dr. Saunders, was unbelievably patient and encouraging during the entirety of my MS project, and even before I chose to come to Auburn. Without his wealth of knowledge and guidance, this thesis would not have been possible. I would also like to thank the National Science Foundation who funded Dr. Saunder?s grant, NSF grant #1004381370012000, which in turn provided me with graduate research asistant funding for my second year at Auburn and helped to fund some of my fieldwork and travel expenses for sulfur and microprobe analyses. I am also very thankful to the Society of Economic Geologists, the Southeastern Section of the Geological Society of America, and the Colorado Scientific Society for finding my grant proposals worthy enough to fund. Thank you also to Dr. Wilis E. Hames, Dr. Haibo Zou, and Dr. Robert B. Cook for providing editing and advice. ! v Table of Contents Abstract ............................................................................................................................... ii Acknowledgments .............................................................................................................. iv List of Tables .................................................................................................................... vii List of Figures .................................................................................................................. viii List of Abbreviations ..................................................................................................... xvii Chapter 1: Introduction ......................................................................................................1 Chapter 2 Previous Works .................................................................................................7 History of the La Plata Mining District .................................................................7 Geology of the La Plata District ............................................................................8 Ore Deposits of the La Plata District ...................................................................13 Bedrock Gulch: A Detailed Report ......................................................................17 Recent Study ........................................................................................................18 Laramide Orogeny and Flat Slab Subduction Influence on COMB Ore Deposits .34 Alkaline hosted PGE Porphyry Deposits ...........................................................31 PGE-Copper Asociation in Other Deposit Types ...............................................35 Porphyry to Epithermal Transition .....................................................................36 Chapter 3: Methodology ....................................................................................................41 Field Methods ......................................................................................................41 Ore Petrography ...................................................................................................43 vi ! Geochemistry .......................................................................................................44 Isotopic Analyses .................................................................................................45 Chapter 4 Results .............................................................................................................48 Geochemistry .......................................................................................................48 Statistical Analysis ...............................................................................................52 Isotope Geochemistry ..........................................................................................55 Petrology and Petrography of Ores and Asociated Rocks .................................60 Chapter 4 Discussion ........................................................................................................94 Chapter 5 Conclusions ....................................................................................................127 References .......................................................................................................................130 ! vii List of Tables Table 1 Historical Production in the La Plata District from Eckel (1949) ..........................9 Table 2 Stratigraphy in the La Plata District .....................................................................12 Table 3 Possible Exploration Model from Schiowitz et al. (2008) ....................................26 Table 4 La Plata District Geochemical Data ......................................................................50 Table 5 Log Normal Correlation Matrix ............................................................................53 Table 6 Raw Correlation Matrix ........................................................................................54 Table 7 Sulfur Isotope Values for AT and CH ..................................................................55 Table 8 Lead Isotope Analysis Results .............................................................................59 Table 9 Alard Stock Ore Minerals in Order of Abundance for At and CH ......................68 Table 10 Microprobe Analyses for Copper Hil ................................................................75 ! vii List of Figures Fig. 1. Location map of the La Plata Mountains in Southwestern Colorado, USA in asociation with the southern extent of the Colorado Mineral Belt, (Wegert and Parker,2011)???????????????????????????.2 Fig. 2. Photography of a portion of the hinge fold creating the domal structure of the La Plata mountains. From Eckel (1949)?????????????????.11 Fig. 3. Detailed geologic map of the Alard stock, modified from Werle et al. (1983) and adapted from Ft. Lewis College student Goncalves? senior thesis (2010)???19 Fig. 4. Gold geochemical basins map with concentrations in ppb (Schiowitz et al., 2008) ????????????????????????????????25 Fig. 5. Figure based on concepts in Saunders and Bruseke (2011) showing Laramide flat-slab subduction and lithospheric mantle devolitization. The Alard stock fals within the Colorado Plateau?s Te-rich zone enriched with the metals Te-Au-Ag sourced from the subducted slab??????????????????????????????.28 Fig. 6. Grade tonnage plot for alkaline-related gold deposits, from Jensen and Barton (2000). Dashed contours indicate total gold in deposit. Cripple Creek and the La Plata district have each been plotted as two points. The point labeled LP Mtns represents historic production from high-grade epithermal veins. The point labeled LP ? Alard Stock represents the porphyry-style Cu?Au?PGE deposit. Cripple Creek is plotted as CC I and CC II, high grade vein systems and low- grade ``diseminated'' deposits, respectively most districts include al gold produced, including that which is in vein, brecia, porphyry, skarn, or placer deposits. Because most of the deposits are incompletely explored, the data shown on this plot are not suitable for use as a predictive model. (Taken from Kely and Ludington,2002)????????????????????????.... 31 ! ix Fig. 7. Location map of alkalic porphyry Cu ? Mo ? Au ? Pt ? Pd ore deposits. Modified from Economou-Eliopolous (2005)?????????????????????????????.32 Fig. 8. Plot of Pt+Pd (ppb) grade and ore-tonnage (milion tons) from John and Taylor (2014). The Alard stock plots the highest grade for the post-collisonal porphyry copper deposits on the plot. A tonnage of 200 Mt is shown for the Alard stock, as reported by Neubert et al. (1992?????????????????????????????..34 Fig. 9. Illustration of the ?porphyry-epithermal? transition and possible asociated deposit types, modified from Hedenquist and Lowenstern (1994)?????????37 Fig. 10. Schematic cross section of a magmatic-hydrothermal system showing source magma reservoir, and the two possible ore-fluid evolutions in the transition betwen porphyry and epithermal environments (Heinrich, 2005)?????...38 Fig 11. A geologic map showing a portion of the La Plata district and the field area for this study. Mine locations are indicated by a colored circle and the appropriate abbreviation. Modified from Schiowitz (2008) who digitized this version, and from Eckel (1949) who published the original map???????????..42 Fig. 12. Distribution of sulfur isotope values for La Plata analyses, n=17 samples. Values reported as ? 34 S (VCDT) . Includes two data points from commercial analyses CH-1 (? 34 S = -5.8) and AT-1 (? 34 S = -6.0)?????????????????..56 Fig. 13. Distribution of copper isotope values for La Plata analyses, n=4 samples. Values reported as ? 65 Cu. Samples collected from the Alard Stock by Dr. Gonzales at Ft. Lewis College yielded ? 65 Cu of 0.965 ? and 1.380 ?. Samples collected by the author were AT-9 (2.700 ?) and CH-6 (2.570 ?)???????????...57 ! x Fig 14. Plots of lead isotope results showing Alard Tunnel as closed diamands and Copper Hil as open diamonds . Present day values are presented on the left and age corrected values are on the right?????????????????..58 ! ! Fig. 15. Photomicrographs of mineralization from Alard Tunnel. From top left to bottom right: An example of hydrothermal breciation, stockwork pyrite and chalcopyrite veins within pink syenite, breciation cemented by chalcopyrite and chalcopyrite, propylitic alteration and offset veining, and pyrite, chalcopyrite, hematite, and goethite as evidence of oxidation??????????????????...62 Fig. 16. Photomicrograhs of Alard tunnel samples in reflected light. A) pyrite surrounded by chalcopyrite in veinlets. B) masive chalcopyrite with pyrite, C) veinlets and blebs of chalcopyrite and pyrite, note finer grain size than previous photomicrograph, D) pyrite grain surrounded by chalcopyrite and sphalerite with minor magnetite?????????????????????????.63! ! ! Fig. 17.Photomicrographs of hand samples from the Copper Hil glory hole show extensive chalcopyrite mineralization within a contact-metamoprhic seting betwen the Alard stock syenite and the surrounding sedimentary rocks (likely the Pony Expres limestone). Mineralization at copper hil contains much les pyrite than mineralization at the Alard tunnel and far more chalcopyrite. Copper oxidation minerals at Copper Hil consist of malachite and azurite, however some chalcocite and chrysocolla are also present in outcrop at the glory hole???...64! ! ! Fig 18. Photomicrographs in reflected lightfrom Copper Hil. A) Common relationship betwen chalcopyrite and magnetite, B) magnetite wrapping around chalcopyrite within a veinlet, C) masive chalcopyrite around of apatite and minor quartz, D) Example of diseminated magnetite and chalcopyrite??????????...65 ! ! Fig. 19. Outcrop photographs from the Copper Hil glory hole and the NE-trending vein mined there. The first photo looks down on the vein from the entrance to the glory hole, and the second photo looks up the vein from the bottom of the glory hole. This main vein, however, is not the only source of chalcopyrite in the vicinity of the glory hole. Chalcopyrite is masive in and around the glory hole????..66 ! xi Fig. 20. Reflected light photomicrograph of the paragenetic sequence at the Alard stock (minus pyrite). Minerals include chalcopyrite (cpy), bornite (bn), covelite (cov), and sphalerite (sph)????????????????????????69 Fig. 21. Reflected light photomicrograph of AT-4 showing the relationship betwen chalcopyrite (cpy), bornite (bn), and covelite (cov) with a posible bornite exsloution texture (bn to cpy????????????????????..70 Fig. 22. Reflected light Photmicrograph of a calcite vein crosscutting potasium feldspar in sample AT-1. Note the asociation of opaque minerals (pyrite) within the hydrothermal calcite vein. Ore minerals chalcopyrite and pyrite are abundant in calcite veins in the Alard stock???????????????????.71 Fig. 23. Photomicrograph using both transmited and reflected light to show the ore minerals (pyrite and chalcopyrite) as wel as the calcite vein. Framboidal pyrite also exists in the Alard stock (grain directly under the chalcopyrite arrow??.72 Fig. 24. Photomicrograph of pyrargryite asociated with masive chalcopyrite at Copper Hill??????????????????????????????.72 Fig. 25. Photomicrograph of acanthite asociated with masive chalcopyrite at Copper Hill??????????????????????????????.73 Fig. 26. Photos and photomicrographs of La Plata district epithermal and limestone replacement-type deposits. A) Native gold on Au-Ag telurides, Cumberland. B) Au-Ag telurides in hand sample with quartz and barite. C) Cumberland hand sample showing azurite and malachite (supergene?) in asociation with telurides (coloradoite, hesite, etc) and pink barite with a typical epithermal open-space- filing vein texture with banding. D) Photomicrograph of a Besie G sample in reflected light showng native Au asociated with hesite. E) Reflected light photomicrograph of a Matyday sample showing sphalerite, galena, and pyrite. E) Handsample from Mayday showing vein sulfites likely in asociation with telurides????????????????????????????.74 ! xii Fig. 27. CH-3-3 backscater electron (BSE) image showing PGE-mineral surrounded by quartz (qtz) and rutile (rut), and in close asociation with apatite (ap), chalcopyrite (cpy), and titatinite,/sphene (tit/sphn)?????????????????76 Fig. 28. EDS spectrum for CH-3-3. Note the peaks for Pt, Bi, Pd, and Te. Cu, and S peaks are background from the surrounding chalcopyrite, and the Si peak resulted from the background feldspar??????????????????????.77 Fig. 29. EDS element maps of CH-3-3. From left to right, images are as follows: A) Calcium map showing sphene, B) Backscater electron image, C) Copper map showing chalcopyrite, D) Composite element map, note platinum map of PGE- mineral grain in the center of the image, E) Titanium element map showing rutile and titanite/sphene, F) Platinum element map. Note concentration of Pt as bright red, however other areas of the image may show some indistinct noise, G) Bismuth and platinum element map at a larger scale, showing bismuth in chalcopyrite, but not within the PGE-mineral grain???????????..78 Fig. 30. CH-3-4 EDS element maps showing a PGE-mineral in direct asociation with chalcopyrite. Other phases in the vicinity of the grain (not shown in image) include magnetite and apatite. A) is a backscater electron image with the PGE- mineral circled in red. B) shows a composite element map showing copper in chalcopyrite and the PGE-mineral. C) a WDS element map showing platinum. D) an element map showing copper in chalcopyrite????????????...79 Fig. 31. EDS spectrum for CH-3-4. Note the peaks for Pt, Pd, Bi, and Pd. Cu and Fe peaks are from background chalcopyrite and the Si peak is due to background silicate (feldspar)?????????????????????????80 Fig. 32. Backscater electron image and a composite element map for CH-2-2. PGE- mineral is surrounded by potasium feldspar (kspr), plagioclase (plg), and a barium-strontium sulfate. The element map shows paladium in red, potasium in blue (to signify potasium feldspar), sodium in green (to signify plagioclase, specificaly albite), and sulfur in yelow (to signify Ba-Sr-sulfate)?????...81 ! xii Fig. 33. EDS spectra for CH-2-2. The Pd peak is larger relative to the Pt peak for this particular sample. Te appears to be a greater component in the mineral than Bi..82 Fig. 34. Element maps of sample CH-2-2. From left to right, A) Backscater electron image, B) Element map for paladium (Pd), C) element map for sodium (Na), signifying albite, D) element map of copper (Cu) signifying chalcopyrite, E) element map of sulfur (S) showing chalcopyrite and or pyrite, F) element map showing potasium (K) as evidence for potasium feldspar????????..83 Fig. 35. Backscater electron image of CH-4-1. The PGE grain is circled in red with surrounding minerals biotite (bt), chalcopyrite (cpy) potasium feldspar (kspr), an alkali-rich pyroxene (pyx), calcite (cal), clinopyroxene (cpx), and plagioclase/albite (plg)??????????????????????...84 Fig. 36. EDS spectra for CH-4-1. Note the absence of the Pt peak and the abundance of Ag, relative to Pd and Te. Cu, Fe, and S are background spectra from the nearby chalcopyrite???????????????????????????85 Fig. 37. Element maps for sample CH-4-1. A) BSE image, B) silicon element map, signifying feldspar and biotite, C) Pd element map, D) composite element map showing phase relationships, E) magnesium element map signifying biotite, and F) copper element map signifying chalcopyrite. ????????????...86 Fig. 38. Backscater electron image of CH-Z-2 showing a PGE-mineral grain hosted inside a Ca-Mg rich pyroxene phase (likely augite), that is near to but not hosted in chalcopyrite (cpy)???????????????????????..87 Fig. 39. EDS spectra for CH-Z-2. Pt and Te peaks are apparent, as it?s the absence of Pd and Bi peaks. Cu, Mg, and Fe are background spectra from the surrounding mineral phases?????????????????????????....88 ! ! xiv Fig. 40. Backscater electron images for sample CH-2-1. The PGE-mineral grain (circled in red) is surrounded by chalcopyrite (cpy), an alkali-rich aphibole (aegerine?) (amph), and an alkali-rich pyroxene (augite?) (pyx)???????????.89 Fig. 41. EDS spectra for CH-2-1. Pt, Pd, and Te al show prominent peaks. Cu, Fe, and S are background peaks from the nearby chalcopyrite???????????.90 Fig. 42. Backscater electron image of sample CH-Z-1. Three PGE mineral grains (circled in red) are surrounded by epidote (ep) and plagioclase (plg). Biotite (bt), magnetite (mgt), and chalcopyrite (cpy) are also in the vicinity of the PGE- mineral grains. ?????????????????????????...91 Fig. 43. EDS spectra for CH-Z-1, the central circled grain in Fig. 42. Peaks for a Pt-Pd- Bi-Te mineral are evident. Background Fe, Al, and Na are background spectra from the surrounding plagioclase??????????????????...92 Fig. 44. EDS element maps of sample CH-Z-1. From left to right, A) BSE image, B) sodium eleent map, C) iron element map??????????????...93 ! ! Fig. 45. Covariant plots of geochemical (Pt, Pd, and Cu) data in the La Plata district. Samples from the Alard tunnel, Copper Hil, Cumberland, Besie G., and May Day deposits are included?????????????????????..95 Fig. 46. Pd, Pd, Se, and Te covariant plots for La Plata district samples. Note the discrimination of locationand porphyry, epithermal and skarn deposit types?...96 Fig. 47. Total alkali versus Silica (TAS) diagram for the La Plata district. Gray shaded region based on data range for the Alard stock from Werle et al. (1984). Abbreviations are as follows: AT = Alard tunnel, FSY = fresh syenite, AVGSY = USGS syenite STM-1 standard ???????????????????97 ! xv Fig. 48. TAS plot from Wegert and Parker (2001) superimposed with data from this study. Red stars are Alard tunnel, white starts are fresh syenite or diorite, and blue is the USGS STM-1 syenite standard, following the same symbology as the previous TAS graph???????????????????????...99 Fig. 49. Alard stock data plotted on an MgO versus SiO 2 graph (modified from Wegert and Parker (2011) with data from previous studies on the Alard stock, the Mcdermott Formation, and the lower crustal xenolith average. The USGS STM-1 syenite standard is also included for reference?????????????100 Fig. 50. Range of sulfur isotope values for sulfides from meteorites, mantle xenoliths, diamonds, igneous rocks, and modern sediments. Figure from Marini et al. (2001)????????????????????????????104 Fig. 51. Sulfur isotope range for various hydrothermal ore deposits, showing ? 34 S for sulfides and sulfates. Sources of data are detailed in the source publication for this figure, Rye and Ohmoto, 1974?????????????????????????????...105 Fig. 52. Histogram diferentiating Alard Tunnel samples from Copper Hil samples with respect to their sulfur isotope values (relative to VCDT, per mil)?????...106 Fig. 53. Schematic plot of some typical copper porphyry sulfur isotope data. Modified from Barnes (1997)???????????????????????..107 Fig. 54. Plot of orphyry copper (-gold) asociated sulfide and sulfate ranges (? 34 S per mil). Some granitic rocks also included. Figure from Wilson et al. (2007)??108 Fig. 55. Histogram of ? 34 S CDT values for the Cenozoic ore deposits in metalogenic belts in Kamchatka, Russia (Takahaski et al., 2005)?????????????110 ! xvi Fig. 56. Plot of ? 34 S sulfide vs ? 34 S sulfate values for epithermal hydrothermal systems from Seal, 2006. Included are the 0? line, indicated by the red arrow, the typical porphyry field, shown by the crosshatched area, and the dashed field for epithermal ores. Values in per mil (VCDT). Note the clustering around 0? and the overlap betwen porphyry and epithermal fields??????????...111 Fig. 57. Histograms of epithermal Ag-Au sulfur data from Shikazono (1995)???...112 Fig. 58. Histograms showing Mule Canyon sulfur isotope data, primarily on pyrite, marcasite, and arsenopyrite. White = open-space-filing sulfides, gray = replacement sulfides, patern ? mixtures, and black = igneous pyrrhotite; cpy-tet = chalcopyrite/tetrahedrite mixture, Ign = igneous, and stib = stibnite???......113 Fig. 59. Plot of ulfur isotope fractionations, considering sulfur species species and hydrothermal minerals, al plotted with respect to pyrite. Solid lines indicate minerals and dashed lines indicate species in solution (Rye and Ohmoto, 1974)?????????????????????????????114 Fig. 60. Compilation of ? 65 Cu isotope data from a variety of sources (Li et al. 2010). Original data sources listed in Li et al. (2010). Unfiled bars represent oxidized ores in porphyry deposits reported by Mathur et al. (2009), however some extreme values from this study are not plotted due to scale restrictions???.116 Fig. 61. Generalized profile of the typical porphyry copper stratigraphy, mineralogy, and ? 65 Cu of leached, hypogene, and supergene reservoirs in a typical porphyry deposit. Figure from Mirnejad et al. (2010) with data from Mathur et al. (2009)????????????????????????????...117 Fig. 62. Histogram of porphyry copper minerals ? 65 Cu composition. Histogram from Mathur et al. (2012). Data taken from Mathur et al. (2005, 2010) and Zhu et al. (2000)????????????????????????????...118 ! xvii Fig. 63. Plots of ? 65 Cu versus ? 34 S plot from two ore bodies from Northparkes, Australia (Li et al. 2010). No systematic relationship betwen Cu and S was determined from their study. ????????????????????????..119 Fig. 64. Plot of ? 34 S of sulfide minerals versus copper grade (milion metric tonnes) of porphyry copper deposits (Hatori and Keith, 2001). Deposit abbreviations are as follows: Bg=Bingham, Utah, Bt=Butte, Montana, El-Sv = El Salvador, Chile, Ch=Chino, New Mexico, Yr=Yerrington, Nevada, Bis=Bisbee, Arizona, Lp=Le Panto Far Southeast, Philipines, Aj=Ajo, Arizona, Sg=Sungun, Iran, Fr=Frieda River in Papua New Guinea, VC=Valey Copper, British Columbia, Pang=Pangua, Papua New Guinea, Tn=Tintic, Utah, GC=Galore Creek, British Columbia, GM=Globe-Miami, Arizona, Cm=Craigmont, British Columbia, GSs, Gasp? Copper, Qu?bec, CV= Cerro Verde-Santa Rosa, Peru, MP=Mineral Park, Arizona, Sk=Skouries, Greece, Hb=Hilsboro, New Mexico, GS=Golden Sunlight, Montana. Solid squares are averages and bars represent sample ranges. Individual deposit data sources listed in Hatori and Keith, (2001)?????120 Fig. 65 Plot of ? 34 S and ? 65 Cu values for samples from the Alard stock porphyry along with epithermal samples from the Northern Great Basin (NGB). Samples were analyzed for sulfur isotopes at either a commercial lab (NGB-1) or at UGA?s Stable Isotope Lab (NGB-2), and splits of the same samples were analyzed for copper isotopes. NGB ores include Buckskin National, NV; Midas, NV, Dewey, Trade Dollar, Idaho Tunnel, ID. Gray field indicates the range for a completely mantle sourced Cu and S magmatic signature. NGB epitheram samples are from MS thesis research by Michael Mason, Auburn University, unpublished??????????????????????????..121 Fig. 66. Plots of lead isotope compositions ( 206 Pb/ 204 Pb versus 208 Pb/ 204 Pb, and 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb). Alard stock data indicated by the bold triangles. Plot modified from Keley and Ludington (2002). Closed circles from the COMB are intrusions spatialy asociated with Au-Te mineralization in the Central City (Eldora Stock) and Boulder County district (Jamestown Porphyry). The SK line corresponds to the Stacey and Kramers (1975) growth curve, where tick marks show time in 100-milion year intervals???????????????..124 Fig. 67. Lead isotope composition graphs with Alard stock data superimposed on a graph from Bouchet et al (2014). Data plot along the 1.7 Ga reference isochron and show a relatively unradiogenic signature, likely that of mantle derived lead?????????????????????????????...125 ! xvii Fig. 68. Plot of lead isotope data for the La Plata district. Alard stock samples circled in red and epithermal samples circled in blue. Note slope diference and trend line??????????????????????????????129 ! xix List of Abbreviations AT Alard Tunnel BSE Backscater Electron CH Copper Hil Glory Hole COMB Colorado Mineral Belt EDS Energy-dispersive X-ray Spectroscopy ICP-MS Inductively Coupled Plasma Mas Spectrometry LMI Layered Mafic Intrusion PGE Platinum Group Element ! ! ! ! ! 1 1. INTRODUCTION ! ! Two important and distinct clases of hydrothermal metalic ores are often found in close proximity to one another other: (1) Typical porphyry ore deposits are adjacent to or hosted by an igneous intrusion, form betwen <600 ? 300? C at betwen 2-5 km below the surface, and contain metals such as copper (Cu), molybdenum (Mo), gold (Au), and tungsten (W) or tin (Sn). Certain clases of porphyries may also anomalously include silver (Ag), telurium (Te), and platinum group elements (PGEs), and (2) Epithermal ore deposits, which fal into two main clases: high sulfidation and low sulfidation systems. The epithermal environment is above or more distal to intrusive bodies, is near surface (>1.5 km) and cooler (<300? C), and typicaly contains Au-Cu- lead (Pb), and zinc (Zn). Epithermal deposit types are clasified by proximity to the intrusion (high sulfidation is more proximal) and by mineral asemblage (Hedenquist and Lowenstern, 1994). Both porphyry and epithermal ore deposit types occur in the La Plata Mountains of southwestern Colorado (Fig. 1). Understanding the probable genetic connection betwen ore-forming proceses in both seting types is an area of current research caled the ?porphyry to epithermal transition.? Discerning the source(s) of fluids, metals, ligands, and other constituents that interplay to create these ore deposits is crucial for constructing genetic models for ore- forming proceses. PGEs include the six transition metals ruthenium (Ru), rhodium (Rh), 2 ! paladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt). These economicaly valuable elements are primarily used for their catalytical properties, mainly in catalytic converters in the transportation industry. Due to their economic importance and their anomalous occurrence, understanding PGE mineralization in the Alard Stock is one focus of this study. Fig. 1. Location map of the La Plata Mountains in Southwestern Colorado, USA in association with the southern extent of the Colorado Mineral Belt, (Wegert and Parker, 201). Discerning where ore-forming species originate, how they travel, and how they precipitate to make ores involves determination of their sources, source material, and the geochemical proceses controlling transportation and ore phase formation. Porphyry deposits are high temperature (>700?C to <250?C), low-grade stockwork ores within ! 3 asociated sub-volcanic porphyritic intrusive bodies and surrounding country rock. In contrast, epithermal deposits are shalow (<1 km deep) diseminated, stockwork, or vein deposits that were hydrothermaly formed. Delineating the connection and transition betwen the two regimes is a necesity for fully characterizing ore deposits and formation on the ?district? scale. Further, PGE occurrence in the Alard stock porphyry mineralization is both anomalous and perhaps economicaly significant because PGEs are typicaly asociated with mafic igneous rocks, rather than alkaline (felsic) rocks. PGE and Te deposits deemed economicaly viable typicaly occur in layered mafic intrusions (LMIs) and therefore originate via magmatism. Interestingly, along with telurium, PGEs may also occur in a distinctly alkaline subclas of copper porphyry deposits. Famous LMI's include South Africa's Bushveld complex, USA's Stilwater complex and the Duluth Gabbro, Canada?s Sudbury LMI, and Russia's Noril'sk LMI (Naldret, 1999; Cabri, 2002; Arndt et al., 2005; Barnes and Lightfoot, 2005; and Cawthorn et al., 2005). In the U.S., mining for PGEs is planned for in the Duluth Gabbro (as byproducts of Cu-Ni ores) and are produced at the Stilwater Complex. Utah's Bingham copper mine may also begin production of PGEs as byproducts in the near future. Exploration for more ?nontraditional? PGE sources wil become more important as the traditional (LMI) sources are progresively mined out. PGEs have a high value because of their uses in industry and due to their low crustal abundance, which is a function of their siderophile nature. Most chondritic meteorites contain ~100-600 ppb of PGEs, indicating that PGEs are likely enriched in the outer core rather than in Earth?s crust (Anders and Grevese, 1989). Although the US produces some PGEs currently, it imports roughly 75% of the PGE's used in domestic ! 4 industry. In light of this statistic, exploring alternative sources for PGE deposits has become of interest. Hydrothermal systems as conduits and sources for PGE deposits could provide alternative exploration targets for PGE-Te production. Te is used in the creation of Cd-Te films for photovoltaic solar cels and as an additive to stel, copper, and lead aloys. These hydrothermal systems are most commonly asociated with hypabyssal alkaline porphyry intrusions, and asociated ores are within a subclas of porphyry copper deposits and may be transitional to ?subepithermal? (LeFort et al., 2011) and also epithermal ores (Saunders and May, 1986; Mutschler et al., 1997). Alkaline intrusions hosting these ores may have been enriched in PGE's via deeper and earlier magmatic proceses afecting the area; however, PGEs are integrated into epigenetic hydrothermal sulfide minerals that originated at the same time as their emplacement and crystalization (Werle, 1983; Werle et al., 1984; Eliopoulos and Economou-Eliopoulos, 1991; Mutschler et al., 1997). Porphyry deposits supply about 60% of the world?s copper as wel as significant amounts of silver and gold. In addition, Te, Se, Re are concentrated localy in some Cu deposits and these deposits wil likely provide most of the world?s Te, Se, and Re in the future. Recent research on porphyry Cu deposits has been extensive (Silitoe, 2000, 2005, 2010; Richards 2003, 2009, 2011; John et al. 2010). Porphyry Cu deposits and systems consist of Cu-rich stocks or intrusions that are surrounded by large volumes of hydrothermaly altered rock. Porphyry Cu deposits may include spatialy asociated deposits such as epithermal, skarn, carbonate replacement, and sediment hosted base and precious metal deposits (Silitoe, 2010). The formation of porphyry Cu systems usualy occurs in subduction-related environments and may be the result of the intrusions of calc- ! 5 alkaline or even alkaline magmas, as in the case of the Alard stock. Te, Se, Re, PGMs, Ag, As, W, U, and Zn are byproducts recovered from porphyry Cu deposits (Silitoe, 1983) and both Te and Pd?Pt are enriched in the Alard stock. In the La Plata district, bismuth minerals are quite abundant and occur within mineralization in porphyry, epithermal, and skarn-type deposits as Bi-telurides. John and Taylor (2014, in pres) comment on the Te connection betwen porphyry and epithermal environments by stating that ?Many Au-rich epithermal deposits with significant Te concentrations may be post- porphyry ores, thus reflecting the late-stage events in an evolving porphyry-epithermal magmatic-hydrothermal system most consistently of an alkaline nature.? The La Plata district hosts Au-Te epithermal mineralization in close spatial asociation with Cu-Ag- Au-Te-PGE porphyry mineralization and is therefore a prime location for investigating the porphyry to epithermal transition resulting from a single or coeval magmatic- hydrothermal system(s). Geochemicaly, PGE's can be both calcophile and siderophile, and they can make relatively strong aqueous hydrosulfide and chloride complexes (Gamons and Bloom, 1993). Te should also have a strong afinity for disolved H 2 S (Saunders and May, 1986; Zhang and Spry, 1994) and combines with noble metals such as Au, Ag, and PGEs to form ore minerals of economic value. Investigating the porphyry-related hydrothermal occurrences of PGE and Te mineralization, as wel as characterizing the porphyry to epithermal transition for ore deposits, could provide the data to develop models for new mineral resource occurrence and characterization in the US and worldwide. For example, the USGS is currently developing a model for ?alkalic? noble metal deposits, like those in the proposed research area. Both porphyry and epithermal ore deposits are exposed in ! 6 the La Plata district due to the high (~ 1 km) topographic relief of the La Plata Mountains. ! 7 2. PREVIOUS WORK History of the La Plata Mining District Background information discussed below on the La Plata District (also previously named the California District) history, geology and ore deposits comes largely from the definitive published research on the area, USGS Profesional Paper 219 by Edwin B. Eckel (1949). The La Plata Mountains lie in southwestern Colorado betwen the San Juan Mountains and the Colorado Plateau, roughly 12-32 km southwest of Durango, Colorado (Eckel, 1949). Mining in the 310 km 2 La Plata District began sometime in the 18 th century when Spanish explorers visited the La Plata Mountains. However, no documented record of mining in the area exists before 1861 when placer gold was found near Durango, Colorado along the Animus River. Mining in the district officialy began in 1873 when placer gold was discovered along the La Plata River, which runs through the center of the district. Despite unrest with the Ute Indian tribes in the area, prospecting and mining continued to thrive wel into the 1930s, and by 1900, more than 200-patented mining claims existed in the La Plata district, along with nine miling sites. Production in the district from 1878-1937 is shown in Table 1. Production in the district prior to 1900 was comparatively smal due to unsuccesful miling atempts and intermitent production eforts. Despite miling site dificulties, the remote location and rugged terrain, and character of deposits, the district took an upturn in both new discoveries and production after the early 1900s. Mining activity persisted until 1937, 8 ! due to the discovery and exploitations of a few smal but high grade deposits (Neglected, Red Arrow, Gold King, Incas, and May Day/Idaho), which yielded more than two-thirds ($1,000,000) of the district?s total production. Exploratory work continues in the district today, with the only known recently smal-scale producing mines are Red Arrow and May Day/Idaho. Geology of the La Plata District The La Plata mountains lie at the southwestern end of the Colorado Mineral Belt (COMB) and have >1 km of topographic relief. The mountains, characterized by a domal structure, were created by the uplift of sedimentary rocks via the intrusion of numerous igneous stocks, dikes, and sils. The areal stratigraphy comprises Devonian through Upper Cretaceous strata, and beds dip away from central high areas. The central part of the mountains is nearly encircled by a steply dipping horseshoe-shaped hinge fold (Fig. 2), however dips on both sides of the fold are relatively horizontal. Large displacement faults cut the outer part of the dome, and there are numerous faults and fractures within the main dome. Both porphyritic and nonporphyritic igneous rocks occur in the district, however they vary widely in both composition and form. Diorite-monzonite porphyry, the most common porphyry in the district, occurs as contemporaneous sils, stocks, and dikes that intruded into the overlying strata. Generaly younger than the porphyry, the nonporphyritic rocks (syenite, monzonite, and diorite) occur as irregular stocks asociated with many dikes. 9 ! T a bl e 1: H i s t ori c a l P roduc t i on i n t he L a P l a t a D i s t ri c t f rom E c ke l , (1949.) !! ! 10 T a bl e 1 Cont i nue d : H i s t ori c a l P roduc t i on i n t he L a P l a t a D i s t ri c t f rom E c ke l , (1949.) ! 11 ! These nonporphyritic stocks pervasively metamorphosed the sedimentary strata within the center of the district. Sedimentary strata within the district, including sequence and description, are described in Table 2. Fig. 2. Photograph (top) of a portion of the hinge fold creating the domal structure of the La Plata Mountains. From Eckel (1949) and a structure map (botom) of the La Plata district, showing the hinge fold outlined in black and the Alard stock outlined in red, from Wiser (1960). ! 12 Table 2: Stratigraphy in the La Plata District Table 2: Modified stratigraphy of the La Plata district, from Eckel (1949). Age Formation Name in local use Thickness (m) Description Upper Cretaceous Mancos Shale Mancos Shale 366 Soft dark-gray to black carbonaceous shale with thin lenses and concretions of impure limestone. Only the lower part is exposed within the La Plata district. Dakota Sandstone Dakota Sandstone 30-46 Gray or brown sandstone with variable conglomerate at or near base. Carbonaceous shale partings and coal at several horizons. Forms cliffs. Should be favorable to ore, but little of it occurs in the mineralized area. Upper Jurassic Morrison Formation McElmo Formation 122-190 Alternating friable fine-grained yellowish-brown to gray sandstones and variegated shales with one or more lenses of conglomerate near top. Largely altered to dense light- colored quartzite and hornfels in central part of district. Generally unfavorable to ore deposits. Junction Creek Sandstone Upper La Plata Sandstone 49-152 Massive friable white sandstone, distinctly crossbedded. Altered to hard white to brown quartzite in central part on the district. Contains much ore in several places. Wanakah Formation Middle La Plata Shale 8-38 Alternating pink to red sandy marls with lenses of friable white or light-colored sandstone. Similar to Morrison formation where metamorphosed. Generally unfavorable to ore deposits. La Plata Limestone 0-8 Medium-gray to black massive un-fossiliferous limestone. Locally replaced by pyrite or by telluride minerals. Largely altered to contact-metamorphic minerals in central part of district. Contains much ore in several places. Entrada Sandstone Lower La Plata Sandstone 30-50 Similar to Junction Creek sandstone. Jurassic and Upper Triassic Dolores Formation Dolores Formation ?Red Beds? 152-229 Salmon-pink to bright-red mudstones and fine-grained mudstones. Several beds and lenses of limestone-shingle conglomerate and of light-gray slabby mudstone at or near base. Where metamorphosed it has same character as shale from Morrison formation. Limestone conglomerate beds altered to contact-metamorphic minerals in places. Generally unfavorable to ore deposits. Permian Cutler Formation Cutler Formation ?Red Beds? 457-671 Alternating dull-red arkosic sandstones, conglomerates, limy shales, and mudstones. Similar to Morrison where metamorphosed. Nodules of limestone, unaltered in places but elsewhere represented by garnet, epidote, etc. Favorable to ore deposits where rocks are silicified. 30-91 Dull-red shale, sandstone, and thin beds of sandy fossiliferous limestones. Similar to Morrison where metamorphosed. Contains ore deposits in places. Rico Formation Rico Formation Pennsylvanian Hermosa Formation Hermosa Formation 854? Alternating green to gray and occasionally dull-red arkosic sandstone, shale, fossiliferous limestone, and gypsum. Only the upper 500 ft is exposed within the La Plata district. Favorability to ore deposits not known. Molas Formation 0-23 (?) Red limy shale. May not be present beneath La Plata district. Mississippian Leadville Limestone 18 (?) Fossiliferous massive to laminated limestone. Upper Devonian Ouray Limestone 23 (?) Fossiliferous limestone, sandy limestone, and quartzite. Elbert Formation 0-40 (?) Shale, Limestone, and sandstone. Upper Cambrian Ignacio Quartzite 15-30 (?) Massive to thin-bedded quartzite, with some conglomerate at base. Precambrian (?) (?) Thought to be similar to basement rocks along the Animus River and in the Needle Mountains. Evidence points to granite and hornblendite, but volcanics, schists, and gneisses could also be present. Unconformity Unconformity Unconformity Unconformity ! 13 Precambrian basement rock is not exposed in the district, but its existence and character are inferred from outcrops in the nearby Nedle Mountains. The intrusion of porphyritic bodies into the district is the main cause for the uplift of the La Plata dome, but the nonporphyritic stocks crosscut the stratigraphy and preexisting sils without disturbing them. Contacts betwen the stocks and enclosing sedimentary strata include are sharply defined, breciated, or exhibit gradational contacts. The La Plata dome is the most noticeable structure in the district and is roughly 24 km in diameter and it grades into the flank of the more regional San Juan uplift. The La Plata dome is asymmetrical due to its superimposition upon the 5? regional dip of the San Juan Mountains. The horseshoe-shaped hinge fold encircles the central part of the mountains and there are several large faults on the south, northern, and northwestern margins of the dome, initiated by the intrusion of the porphyries prior to metamorphism. Short, smal displacement discontinuous faults, many of which trend east or radiate from the dome center, occurred during the doming event. With the emplacement of the nonporphyritic stocks, these older smal faults were reopened and new similar faults were formed. These later faults are the location of many of the district?s mineral deposits, as they provided convenient conduits for mineralizing fluids. The ore-bearing fractures tend to be concentrated along the hinge fold, where beds dip 25? to 60?, and where shearing and breciation is most common. Ore Deposits of the La Plata District As mentioned earlier, most of the district?s deposits are asociated with faults. District faults are either ?barren? or ?ore-bearing.? Generaly, the barren faults are much ! 14 older and exhibit larger displacement, and are situated in the northwestern and southern margins of the dome. Ore-bearing faults tend to concentrate towards the center of the dome, although in general these fractures are widely distributed. The relationship of the minor ore-bearing or mineralized faults to the larger barren faults suggests that most of the smaler faults formed prior to the intrusion of the nonporphyritic rocks, and after the porphyry emplacement and faulting asociated with the domal uplift. When renewed faulting occurred after the intrusion of the nonporphyritc stocks, due to the crustal readjustment at the end of igneous activity, reopened and new fractures became prime sites for ore-bearing solutions to precipitate ore. Gold and silver constituted more than 98% of the district?s metal production and the district is best known for its veins and replacement deposits of Au?Ag-bearing telurides. The district also contains diseminated PGE-bearing chalcopyrite, gold with sulfides in contact-metamorphic deposits and Au-bearing pyrite in veins, replacement bodies, and brecia bodies, mixed sulfides with silver and free gold, chalcocite veins, ruby silver veins, and gold placers. The following section describes the ore deposits and mines focused on for this study. Alard Tunnel (AT) The Alard tunnel, located near an upper tributary to Bedrock Creek, at approximately 3124 m elevation, intercepts a deposit within a mineralized section of the syenite porphyry. Alard Mining Company drove the tunnel on an unpatented claim in 1921 in order to explore the diseminated copper deposits in the area, however no ore was produced (Eckel, 1949). Acording to Eckel (1949) mineralization in the vicinity of ! 15 the Alard tunnel extends at least 457 m westward and 122-152 m verticaly from the tunnel. Diseminated chalcopyrite, cupiferous pyrite (Cu, Au), and ruby silver veins occur in the vicinity of the Alard tunnel. Copper Hil Glory Hole (CH) Located on the ridge separating Bedrock and Boren Creeks, at relatively the same elevation as the Alard tunnel is the Copper Hil glory hole. The Copper Hil glory hole exploited the only known PGE-bearing ore body within the district. The most obvious feature of this deposit is the glory hole that is 15-23 m at its widest and is 9-15 m deep. A ~183 m tunnel is also present within the workings. Copper Hil is situated along the border of the syenite porphyry (the Alard stock) and the metamorphosed sediments. Mineralization occurs as veins containing chalcopyrite, magnetite, and hematite in a metamorphosed augite-orthoclase-rich rock. Cumberland (CB) This Au-Ag-Te low sulfidation epithermal deposit lies in the eastern part of the Cumberland Basin, along the western slope of Cumberland Mountain, in the basin below the San Juan Mountains lookout and the notch leading to the Besie G mine. The workings range from 3511-3764 m and the mine exists predominantly within the Cutler and Delores Formation red beds. Discovered in 1878, Cumberland was worked intermitently until 1937, with periods of activity that lead to the existence of a 30-stamp mil, a power plant, and machine shop in the Cumberland Basin. The vein, striking N 60? to 80? E, has been partialy worked through a vertical range of 302 m, with four levels and at least two known portals. ! 16 Besie G (BG) Originaly named the Egyptian Queen when opened around 1880, the Besie G mine is a high grade low sulfidation epithermal gold teluride deposit located near the divide betwen Columbus Basin and Hefernan Gulch, at an elevation of 3065 m. This patented claim has been worked several times and production records indicate that roughly 500 kg of gold, and of silver, have been produced from Besie G since 1887 (Saunders and May, 1986). With two portals and an upper and lower level, Besie G follows an Au-rich quartz vein within the silicified redbeds of the Dolores formation that is cut by a complex irregular sils and dikes of diorite porphyry. Both a shear zone and brecia zone exist in places along the vein, where roscoelite (V-rich mustovite), and native gold are localy visible. May Day/ Idaho (MD) The May Day and Idaho Ag-Au-Te limestone replacement (skarn) and vein deposit mines, located at the mouth of the La Plata river canyon, are responsible for more than half of the reported total production in the La Plata District, through 1949. Production numbers at that time for Idaho and May Day are 1359.7 kg Au and 10857.9 kg Ag, and 2123.8 kg Au and 21516.8 kg of Ag respectively. These mines are located at comparatively low elevations and are more easily acesible than other mines in the district. Due to abundant faulting (the May Day/Idaho fault system), veining is common at this location, and porphyry dikes contact sedimentary units (calcareous) to form the limestone replacement bodies and veins. The major eastward-trending reverse faults were important in localizing ore here, however a series of northward-trending, steply dipping ! 17 normal faults with adequate displacement contain the principal ore bodies mined at May Day/Idaho. Bedrock Gulch: A Detailed Report Lonsdale (1921) completed a graduate thesis on the La Plata District, specificaly on the Bedrock Gulch area, where the Alard Stock is located. Lonsdale?s main focus was to bridge gaps in existing data on the district as a whole, but ended up concentrating on only the geology and ore deposits of Bedrock Gulch. The smal mining town of La Plata was located at the mouth of Bedrock Gulch, three miles from May Day and the Rio Grande Southern Railroad spur servicing May Day (Lonsdale, 1921). The area experienced periods of intermitent exploration and production, but once the deposit at May Day was discovered, renewed exploration occurred, and the town of La Plata grew (Lonsdale 1921). During this period of resurgence, the deposits on Bedrock Gulch were prospected and developed, with the result that roughly 3,000 tons of copper ore were shipped to Durango, CO to be smelted (presumably from Copper Hil). Lonsdale (1921) reported no significant ore production (no copper bodies were realy developed, and no significant Au or Ag was economicaly produced) by the Alard Mining Company at Alard tunnel. Lonsdale (1921) describes the contact-metamorphism and ?baking? of the Dolores Formation in Bedrock Gulch, which is silicified, grey, and more competent and dense, as compared to the un-metamorphosed characteristic ?redbeds? sen in other parts of the district. The author discusses the ?briliantly white? quartzite of the metamorphosed La Plata sandstone in the vicinity of Bedrock Gulch, also due to intrusion of igneous bodies ! 18 (the Alard stock). Lonsdale (1921) reports three diferent types of igneous rocks in the vicinity of the gulch, however the geologic map from Eckel (1949) does not go as deeply into detail. Lonsdale (1921) cited petrographic and geochemical data for a diorite- monzonite porphyry, an augite-syenite, and a diorite in Bedrock Gulch. Two types of deposits, a diseminated low-grade copper ore, and lead-silver- bismuth enriched veins (within the Black Rock Claim), are reported to occur within Bedrock Gulch (Lonsdale, 1921). The Pb-Ag-Bi veins are apparently not found elsewhere within the district and Eckel (1949) does not mention them. Lonsdale (1921) also states that throughout Bedrock Gulch, there are copper-bearing ores similar to those at Copper Hil, specificaly within the exposures of syenite and diorite-monzonite porphyry in the gulch. A smal occurrence of chalcocite and several veins of cuprite are also noted to be in Bedrock Gulch. Lonsdale (1921) suggests that the diorite may be the most likely source of ore forming fluids, but offers no evidence to corroborate that idea. He does afirm that ?geologic conditions are favorable for continuity of depth and size of ore bodies? in Bedrock Gulch (Lonsdale, 1921). Recent Study After a ~30-year gap in publications on the La Plata District, several studies conducted research that built upon the extensive USGS Profesional Paper by Eckel (1949). Werle et al. (1984) updated geochemical data on the Alard Stock and detailed the evolution of the stock and its mineralization in seven major stages. Werle et al. (1984) discussed how the Alard stock's mineralization is unlike the typical calc-alkaline copper porphyry mineralization characteristic of the western United States and Canada. They ! 19 compared it to the alkaline suite porphyry copper deposits of British Columbia (Barr et al. 1976), as opposed to the calc-alkaline porphyry coppers in western North America. Werle et al. (1984) also studied the trace element geochemistry of the Alard Stock, noted the extensive K-metasomatism of the syenites, and formulated a genetic model of the Alard Stock mineralization. Werle et al. (1983) also mapped the Alard stock in greater detail (Fig. 3) than previously published Eckel (1949). Fig. 3. Detailed geologic map of the Allard stock, modified from Werle et al. (1983) and adapted from Ft. Lewis Colege student Goncalves? senior thesis (2010) ! 20 The following intrusive event descriptions are quoted directly from Werle et al. (1983). Stage 1 The bulk of the Alard stock is made up of its earliest intrusive units, a composite group of hydrothermaly altered grey syenite plutons. Localy wel-developed phenocryst lineations and occasional occulsions of Precambrian basement rocks demonstrate the intrusive nature of these plutons. Wal rocks show litle evidence of lateral displacement adjacent to these plutons, suggesting that the intrusives made room for theselves by lifting roof rocks verticaly . . . rather than by shouldering aside wal rocks. The grey syenites are seriate porphyritic to equigranular rocks composed of alkali feldspar and plagioclase (oligoclase) with minor biotite, hornblende, quartz, and acesory apatite, sphene, zircon, alanite, garnet, and opaques. Two magmatic feldspars are usualy present: (1) an earlier, larger grained, microperthitic, often Carlsbad-twinned variety with an orthoclase structure as indicated by the X-ray method of Wright (1968); and (2) a later interstitial, les pertthitic variety with an orthoclase or anomalous structure. Stage 2 After their emplacement the gray syenites were hydrothermaly argilized. Plagioclase, and to a leser extent, orthoclase were replaced by fine-grained aggregates of kaolinite and sericite, and mafic minerals were altered to chlorite. Argilic alteration extends over 1000 m beyond the limits of the stock. Stage 3 Intrusive plutons of mafic syenite were emplaced along two northeast-treanding shear zones cutting the altered grey syenites. Mafic syenites constitute about 15% of the Alard stock, at its present level of exposure, and are typicaly hypidiomorphic-heterogranular to porphyritic rocks composed of varying amounts of orthoclase, plagioclase, augite, hornblende, and biotite, with trace amounts of apatite, sphene, and opaques. A smal body of chemicaly distinct layered mafic syenite is exposed in the glory hole of the Copper Hil mine. These cumulate rocks exhibit fine-scale rhymithic layering consisting of (1) layers composed of 0.1 m cumulus aegirine-augite, magnetite, apatite, zircon, and sphene together with intercumulus alkali feldspar; some aegerine-augite ezhibits adcumulate overgrowths that have produced crescumulate crystals up to 2 m long; and (2) layers of cumulus plagioclase (An 25-35 ), aegerine-augite, apatite, zircon, and sphene in a matrix of intercumulus alkali feldspar. Sulfides, predominantly chalcopyrite, are found interstitial to the cumulus crystals in both types of layers. Localy the layered rocks are cut by veinlets, or mottled by patches of deuteric(?) garnet, calcite, biotite, albite, hematite, and sulfides. With the exception of the Copper Hil rocks, most of the stage 3 syenites are the only rocks from the Alard stock that have Ab/Or/Q ratios plotting close to the thermal ! 21 minimum on the Ab-Or liquidus surface. In the vicinity of the Alard tunnel, inclusions of stage 3 mafic syenite, some of which show distinct layering, occur in stage 5 pink syenite. Stage 4 Violent escape of volatiles localy formed shatered cylindrical or eliptical prisms of rock within the stock, and fragments of the shatered rock were rotated and carried upward, producing intrusive brecia pipes. The matrix of these brecias tanges from fine-grained high sanidine to finely comminuted rock fragments. The compositional and textural similarities betwen sanidine in the fine-grained brecia matrix, the widespread replacement sanidine elsewhere in the stock, and sanidine in mineralized veins suggest a close temporal relationship betwen brecia pipe formation (stage 4), K-metasomatis (stage 5), and mineralization (stage 7). Stage 5 Intense K-metasomatism profoundly altered the composition of the older stock units and the wal rocks adjacent to the stock. Introduced sanidine replaced argilized feldspars in the gray syenite and formed a network of replacement veins in both stage 1 and stage 3 syenites. Replacement sanidine is not as abundant in stage 3 mafic syenites as it is in argilized stage 1 gray syenites, suggesting that the agrilized feldspars were more succeptible to replacement. The extensive K- metasomatism at Alard is comparable to the potasic fenite halos asociated with some carbonatite-alkaline complexes, but is more extensive than the K- metasomatic zones in most calc-alkaline porphyry copper deposits. Dikes of pink syenite, consisting largely of equigranular alkali feldspar, and dikes of pink porphyritic syenite were emplaced during stage 5. These dikes contain up to 15% interstitial calcite, which appears to have crystalized simultaneously with alkali feldspar. Stage 6 Emplacement of trachyte and pegmatite dikes marks the transition from magmatic to pneumatolitic activity. Trachyte dikes, best exposed in the south-central part of the stock, consist of plagioclase phenocrysts in a groundmas of high sanidine laths with a trachytoid texture. Pegmatite dikes are common in the vicinity of the Alard tunnel where they fil tension fractures resulting from right lateral movement on a northeast-trending shear zone. Pegmatite dikes range in thicknes from a few centimeters to 2 m and have exposed strike lengths of up to 50 m. Two types of pegmatites have been recognized: older syenite pegmatites and younger mafic pegmatites. The syenite pegmatites consist predominantly of orthoclase, with subordinate plagioclase, augite, calcite, and quartz. The mafic pegmatites consist of intergrown dark green augite, orthoclase, calcite, and quartz. Some of the mafic pegmatites are symmetricaly zoned with borders of augite, followed inward by orthoclase zones surrounding a core of quartz, chalcedony, ! 22 tremolite(?), and calcite. Sulfides, dominantly chalcopyrite with subordinate pyrite and bornite, occur as coarse interstitial mases and fracture filings in al of the pegmatite dikes. The mafic pegmatites are the only acmite-normative (peralkaline) rocks of the Alard stock. Stage 7 Hydrothermal solutions deposited metalic minerals, sanidine, quartz, calcite, and fluorite as veinlets and diseminated replacement mases throughout the stock, localy producing significant volumes of stockwork mineralization containing more than 0.1% copper. A generalized paragenetic sequence for the hypogene opaque minerals is (1) iron oxides (magnetite and hematite); followed by (2) pyrite (continuing through al the later stages) and arsenopyrite; followed by (3) chalcopyrite, enargite, sphalerite, bornite, and chalcocite; followed by (4) marcasite and galena. At the copper Hil mine, trace amounts of domeykite and tennanite formed before chalcopyrite. The gangue minerals in order of formation is (1) sanidine, (2) minor quartz, and (3) calcite and fluorite. The following sequence of vein types is also apparent: (1) iron oxide ? pyrite ? copper sulfarsenite ? copper sulfide ? calcite ? quartz veins; (2) calcite ? quartz ? sphalerite ? galena ? chalcopyrite ? pyrite veins; (3) fluorite ? quartz ? pyrite ? chalcopyrite ? marcasite veins. Smal base metal sulfide veins and mantos and quartz ? pyrite ? fluorite ? teluride veins that occur peripheral to the stock may have formed at the same time as vein types (2) and (3) within the stock. The amount of replacement and vein calcite introduced during stage 7 ranges from 2 to 12 vol. %. That over 6 km 2 of rock was afected by carbonatization indicates a significant source of CO 2 at depth. Trace element chemistry shows volatile enrichment and anomalous concentrations of barium, rubidium, and strontium, as wel as high amounts of arsenic, telurium, and bismuth (Werle et al., 1983). The stock is enriched with up to 10% CO 2 , which occurs as extensive calcitic alteration of both feldspars and mafic mineral species. Fluid inclusion studies indicate a CO 2 -saturated fluid phase was present during crystalization of rocks in stages 4, 5, and 6, as wel as during early vein formation. The most important finding from Werle et al. (1983), other than their genetic model and geochemical analyses, are the sulfur isotope values cited. The authors do not go into detail about the sulfur isotopes but do report, ?The Alard sulfides have an average ? 34 S!of -6.5? (range -4.7 to -7.9).? They comment that other alkaline suite porphyry copper systems have similar values (i.e. ! 23 Shasket Creek, WA, Goose Lake, MT, Copper Mountain, BC, Galore Creek BC). A comparison of the geochemical data obtained from Werle et al. (1983) with the data gathered by this study is discussed further in the Geochemistry section of this thesis. Saunders and May (1986) published data on the Besie G mine, described in the above ore deposits section. Saunders (1986) proposed that the Cu-Au-Te-PGE enriched Alard stock was actualy geneticaly related to the epithermal mineralization, although the authors mainly focused on the Besie G vein for his PhD research. The authors state, ?The close spatial and temporal asociation betwen the gold teluride mineralization at the Besie G deposit with Tertiary alkalic stocks suggests that they are geneticaly related.? Saunders and May (1986) go on to propose the idea that Besie G is underlain by a syenite or monzonite stock at depth, and this stock provided precious metals and telurium to the epithermal system, which produced the high grade bonanza gold deposit. The Alard stock is enriched in Au, Ag, and Te, and Saunders and May (1986) postulate that a similarly enriched stock could perhaps underlie Besie G. Gold teluride mineralization is considered the latest hydrothermal event in the district, due to crosscutting relationships betwen the ore-bearing veins and the district?s youngest intrusive bodies. Alkalic rocks are asociated with other gold teluride deposits in Colorado, such as the Cripple Creek district. The Alard stock is enriched in Au, Ag, Te, Bi, Ba, Pt, and Pd (Werle et al. 1984) and Besie G has a very similar elemental suite asociated with its mineralization. Saunders and May (1986) cite that Besie G contains a maximum of 80 ppb Pt+Pd. Comparing mineralization at Besie G to the Alard stock thus leads to a comparison of the La Plata district?s epithermal and porphyry mineralization, and to the porphyry to epithermal transition in general. Saunders (1986, ! 24 1991) and Saunders and Bookstrom (1998) also proposed a genetic link betwen deeper porphyry-style ores and shalower Au-Ag-Te epithermal mineralization. In 1992, the US Bureau of Mines conducted a Mineral Appraisal of the San Juan National Forrest, including the La Plata mining district, and published the report in the Mineral Land Asesment Open File Report 2-92 (Neubert et al., 1992). In their report on the Alard stock, the MLA cites that the Alard stock has been driled numerous times by several diferent mining companies. No dril holes apparently contained high enough grade or tonnage to make mining the Alard stock economicaly profitable, however the report cites that the Bedrock Creek copper mineralization offers a inferred exposed 200 milion ton resource grading 0.4% copper, 60 ppb gold, 7 ppm silver, and 5 ppb PGEs. If the dril holes are representative of the subsurface, then the inferred resource would be 300 milion tons grading 0.34% copper, and with similar precious metal contents as sen at the surface. At Copper Hil, a resource of 30,000 tons also exists, with 1.45% copper and 0.42 oz/ton silver and unknown amount of gold and PGEs. This report concludes that large-scale mining of the Alard stock could occur if copper prices rose about $2.50/lb (currently the price of copper fluctuates around $3.00/lb) or if exploration eforts revealed larger tonnages or higher grades of ore (Neubert et al. 1992). Schiowitz et al. (2008) completed stream sediment geochemistry for 41 samples from the La Plata district in their GIS analysis of the district. They created a GIS geodatabase complete with geology, structural features, mine locations, sample locations, and watersheds within the district. Spatial analysis was done in order to help visualize the relationships betwen geology, structure, and geochemistry. An ?exploration model? was ! 25 created (Table 3), detailing the favorable situations for gold occurrence, and they mapped the geochemical concentrations, as shown in figure 4. Fig. 4. Map showing stream sediment geochemistry for drainage basins in the La Platas, highlighting the gold geochemical basins with concentrations in ppb (Schiowitz et al., 2008) ! 26 Table 3: Possible exploration model from Schiowitz et al., (2008). Laramide Orogeny and Flat Slab Subduction Influence on COMB Ore Deposits The Laramide orogeny began in the Late Cretaceous (70-80 Ma) and continued wel into the Eocene (33-55 Ma). During this event, the Faralon/Proto-Pacific plate collided with the North American plate and active subduction and mountain building occurred. In Colorado, the Laramide orogeny began during the Late Campanian (~72 Ma) as the Late Cretaceous sea underwent its final regresion from the Western Interior; orogeny began in Colorado before marine deposition ended, in the southwest and northeast respectively (Tweto, 1980). Uplifts and consequent subsidence (structural basin development) acompanied the Laramide orogeny in Colorado, and major erosion of Laramide features took place during the Oligocene tectonic quiescence in the area. Development of the Rio Grande Rift triggered rejuvenation of these uplifts via block- faulting (Tweto, 1979). Prior to the start of the Laramide orogeny, major shear zone systems formed in the ! 27 Precambrian, acompanied by deformation including deep-seated folding and plastic flow and shalower britle deformation and retrograde metamorphism. This northeast-trending shear system was recognized as a precursor and structural controlling mechanism for the COMB (Tweto and Sims, 1963). As the Laramide orogeny persisted, it provided an opportune tectonomagmatic seting for intrusive events, which sourced the metals and hydrothermal fluids responsible for the COMB ore deposits. Without the structural set up of the COMB in the Precambrian, the Laramide intrusions and asociated mineralization would not have formed what we now know as the northeast-trending COMB (Tweto and Sims 1963; Tweto, 1980). Flat-slab subduction asociated with the Laramide orogeny led to a fertile lithospheric mantle via mantle devolitization reactions (Fig. 5). This fertile mantle became enriched in volatile ore-forming species such as water, CO 2 , sulfur, and metals/metaloids (Saunders and Brueseke, 2011). As the flat-slab subducts, it sinks and experiences increasing temperatures and presures (increasing metamorphism), followed by melting, and then devolitization (Saunders and Bruseke, 2011). Metals/metaloids can be either fluxed into the overlying asthenospheric wedge or into the lithospheric mantle, however the key inference is that they are driven off into whatever type of mantle overlies the subduction zone. In the western USA, the flat-slab subduction of the Laramide orogeny caused volatiles to flux from the Faralon Plate into the North American lithosphere (English et al., 2003; Humphreys et al., 2003, Crossey et al., 2009). ! 28 Fig. 5. Figure based on concepts in Saunders and Bruseke (201) showing Laramide flat-slab subduction and lithospheric mantle devolitization. The Alard stock falls within the Colorado Plateau?s Te-rich zone enriched with the metals Te-Au-Ag sourced from the subducted slab. Richards (2009) discusses ?postsubduction? magmatisim and Cu-Au porphyry and Cu epithermal deposits as products of remelted subduction-modified lithosphere. In this case, the enriched postsubdicition lithospheric mantel can partialy melt. This partial melting can be triggered by a number of tectonomagmatic proceses. Richards (2009) gives evidence that these types of deposits are predominantly asociated with alkaline rocks, as are the ?synsubduction? versions of this proces (i.e. the intrusion of the Alard stock). Furthermore, magmatism becomes progresively alkalic (K-rich) deeper and farther from the trench (Saunders personal communication 2014). In reference to the Laramide events and the COMB, intrusions appear to get younger from the southwest to the northeast (with some of the oldest being the La Plata intrusives). In opposition, once the Rio Grande Rift initiated, new tetonomagmatic events and magmatism became progresively older from the northeast to the southwest (Art Bookstrom, personal communication to Saunders). ! 29 Mutschler et al. (1997a,b) and Keley and Ludington (2002) note the alkaline nature of the Alard stock in the La Plata district in their overview papers concerning alkaline magmatism and Au-Te mineralization throughout Colorado. Several types of productive gold deposits (79 Ma to 26 Ma) in the Rocky Mountains (porphyry Cu-precious metal systems ? PGEs, transitional types, and Au-Ag epithermal systems) exhibit a close spatial, temporal, and genetic asociation with alkaline igneous rocks. These alkalic rocks apparently represent mantle melts that fractionated in crustal level magma chambers. Coeval calc-alkaline rocks, which formed from crustal melting and magma mixing, also occur with these alkaline stocks in many locations (Mutschler et al. 1997a,b). Porphyry Cu>Ag>Au ? PGE deposits (Alard Stock - CO, Goose Lake Stock ? MT, Cerilos district ? NM) occur in or adjacent to shoshonitic syenite stocks and mineralization occurs as stockworks, diseminations, veins, pegmatite dikes and segregations, endoskarns, exoscarns, and local imiscible sulfide concentrations. These deposits usualy contain high sulfur abundances and precious metals are byproducts or coproducts of copper production. In British Columbia, deposits of this nature are being actively mined, although they occur in Mesozoic acreted terranes (Schroeter et al., 1989; Mutschler et al., 1997a,b). Epithermal gold deposits asociated with alkaline rocks, as discussed by Mutschler et al. (1997a,b) occur throughout the Rocky Mountains, and the Besie G mine fit the criteria, as does the Cripple Creek district, and the Boulder County teluride camps (Saunders, 1991). Keley and Ludington (2002) note the formation of alkaline igneous rocks and asociated hydrothermal ore deposits as faling into two distinct time periods ? (1) during the (70-40 Ma) Laramide orogeny (deposits restricted to the COMB) and then later more widespread mineralization ! 30 asociated with alkalic igneous bodies (35-27 Ma) during the transitional period from compresional to extensional regimes. The most important factor in these gold deposits? formation sems to be the opportune near-surface emplacement of volatile-rich, relatively oxidized alkaline magmas, that source from subduction-modified subcontinental lithosphere, as evidenced by Sr and Pb isotope compositions (data for the La Plata Mountains is unavailable). Mostly al alkaline-related gold deposits have five characteristics, that are typical of economic alkaline-related ore deposits around the world: (1) they occur in areas of multiple episodes of intrusive activity (with or without prior calc-alkaline activity); (2) they contain evidence of magmatic-hydrothermal activity such as brecia pipes and stockworks; (3) mineralization and the hydrothermal system came late in the evolution of the igneous body; (4) alteration asemblages are potasic (K-feldspar, sericite, and biotote); (5) consistent paragenesis can be sen in veins (early base-metal sulfides, then gold or Au-bearing telurides) (Mutschler and Mooney, 1993; Richards 1995; Jensen and Barton 2000). Figure 6 shows a grade tonnage plot for alkaline-related gold deposits as discussed in Keley and Ludington (2002). It should be noted that although Au-Cu alkaline related deposits have been isolated as a clas, most of their fundamental characteristics are basicaly the same of calc-alkaline influenced deposits. Only the occurrence of roscoelite (V-bearing muscovite), anomalous high Te and F contents, pervasive potasic alteration, and deficiency of quartz as a gangue mineral are the truly distinctive characteristics of these deposits (Bonham, 1988; Jensen and Barton, 2000; Silitoe, 2002), albeit none exhibit al of these atributes. ! 31 Fig. 6. Grade tonage plot for alkaline-related gold deposits, from Jensen and Barton (200). Dashed contours indicate total gold in deposit. Cripple Crek and the La Plata district have each been plotted as two points. The point labeled LP Mtns represents historic production from high-grade epithermal veins. The point labeled LP ? Alard Stock represents the porphyry-style Cu?Au?PGE deposit. Cripple Crek is plotted as C I and C I, high grade vein systems and low-grade `diseminated'' deposits, respectively most districts include al gold produced, including that which is in vein, brecia, porphyry, skarn, or placer deposits. Because most of the deposits are incompletely explored, the data shown on this plot are not suitable for use as a predictive model. (From Kely and Ludington, 202). Alkaline-hosted PGE Porphyry Deposits Alkaline porphyry deposits (Fig. 7) constitute a significant global resource due to their large size. As previously stated, the Alard stock hosts anomalous PGEs, most notably at the Copper Hil deposit. The Alard stock is part of a distinct Cu?Au?PGE?Te subclas of porphyry deposits that are known worldwide. Most deposits of this type are located in Eastern Europe, such as Bulgaria?s Elatsite and Chelopech and Serbia?s Bor and Madjanpek, which occur in the Srednogorie zone. The Srednogorie zone is an east- west striking belt consisting of a Late Cretaceous subduction/island arc system that hosts both porphyry and epithermal deposits asociated with sub-alkaline igneous rocks. ! 32 Fig. 7. Location map of alkalic porphyry Cu ? Mo ? Au ? Pt ? Pd ore deposits. Modified from Economou- Eliopolous (205). The Skouries alkaline porphyry Cu-Au-PGE-Te deposit in northern Greece exhibits strongly fractured and intensely altered country rocks due to hydrothermal fluids which additionaly caused silicification, potasic, phyllic, and propylitic alteration asociated with a 18 Ma monzonitic-trachytic mineralized stock (Elipolous and Economou-Elipolous, 1991). Porphyry mineralization includes chalcopyrite, pyrite, bornite, and chalcocite in veins, diseminations, and stockworks. Sulfides contain includions of native gold and electrum and the geochemical signature is characterized by PGEs with Cu, Au, Ag, B, Bi, Co, Se, and Te. Barium and Sr contents are also high while Rb is low. Paladium concentrations range from les than 2 to 480 ppb. Ruthenium content ranges from 8 to 41 ppb, Pt from 2 to 10 ppb, and Te up to 2700 ppb (Elipolous and Economou-Elipolous, 1991; Economou-Elipolous, 2005). PGEs occur in merenskyite [(Pd,Pt)Te 2 ] and a (Pt,Pd,Bi)Te phase (michenerite?) within chalcopyrite and bornite, or enclosed by electrum and hesite (Ag 2 Te) inclusions in chalcopyrite (Economou- Elipolous, 2005). Of the seven alkaline-hosted Cu-Au porphyry deposits in British Allard Stock ! 33 Columbia, Galore Creek, Mt. Polley, Afton/Ajax and Mt. Miligan are known to contain significant PGEs (Thompson et al., 2001, Economou-Elipoulos, 2005; LeFort et al., 2011), and localy are also enriched in Te such as Mt. Miligan (LeFort et al., 2011). The youngest (183 Ma) and most wel studied of the British Columbia deposits is Mt. Miligan, which contains PGE telurides, base-metal sulfides, arsenides, antimonides and various Au-Ag-Te-Bi minerals with PGE contents up to 6.4 ppm (Thompson et al., 2001; LeFort et al., 2011). LeFort et al. (2011) hypothesize that the Cu-Au-PGE-Te veinlets are transitional (?subepithermal?) betwen the porphyry and epithermal environments, although no epithermal mineralization is present there, unlike in the La Plata district. The common link betwen these porphyry Cu-Au+PGE+Te deposits aside from their similar mineralization is their alkaline nature. While alkaline magmas can form in virtualy every tectonomagmatic seting, many are asociated with subduction and/or areas that have been afected by past subuduction. In the aforementioned deposits, the magmas were al noted to have sourced from previously metasomatized, subduction- afected mantle (Thompson et al., 2001; Economou-Eliopoulos, 2005; Auge et al., 2005). The Alard stock was emplaced in such a tectonomagmatic seting, which afected southwestern Colorado during the Laramide and throughout the Cenozoic (Jones et al., 2011). The alkaline chemistry of the magmas is a key characteristic that influences PGE enrichment in these deposits, evidenced by the geochemistry of both the Alard stock and the porphyry deposits listed above. John and Taylor (2014, in pres) plotted the grade- tonnage for PGMs in porphyry Cu and Mo deposits (Fig. 8). Note that the Alard stock is estimated to contain a resource of ~200 milion tons with a Pt+Pd content of 72.3 ppb. ! 34 Fig. 8. Plot of Pt+Pd (pb) grade and ore-tonage (million tons) from John and Taylor (2014). The Allard stock plots the highest grade for the post-colisonal porphyry coper deposits on the plot. A tonage of 20 Mt is shown for the Alard stock, as reported by Neubert et al. (1992). ! 35 PGE-Copper Asociation in Other Deposit Types PGEs can be asociated with copper in other types of precious metal deposits. For example, a Cu-Ag-Au mine in the Revais Creek district of Montana produced roughly 300 ounces of platinum metals during its lifetime (Mertie, 1969). This deposit, named the Green Mountain mine, shows a similar mineral asemblage to that of Copper Hil, including the supergene minerals chrysocolla and malachite. The tenor of Pd and Pt at this mine was ~3.0 g/t for each. In Nevada, the Boss mine in the Yelow Pine district also produced Cu, Au, and PGEs. Ore minerals consisted of chalcopyrite, limonite, chrysocolla, bornite, chalcocite, malachite, and cuprite, however the mineralization was hosted in a quartz vein. Bismuth was apparently part of the system as wel, similar to the situation at the Alard stock. The Boss mine reported a tenor of 14.6 oz/ton Pt and 64.2 oz/ton Pd (Mertie, 1969). In addition, supergene PGE deposits occur asociated with Cu-Ni sulfide deposits. The Key West deposit, in the Copper King/Bunkervile mining district in Nevada, contains chalcopyrite, pyrite, pyrrhotite, milerite, pentlandatie, and magnetite. Pt and Pd exists as sulfarsenides [mainly Irarsite (Ir, Pt, Rh, Ru)AsS] and telurides, however secondary PGE-bearing phases consist of Pd-oxides ? Te-Bi, Pd-Cu-rich oxides, and Pd- Fe-rich oxides. Key West averaged roughly 871 ppm Pt, 2612 ppb Pd, and 496 ppm Au (Suarez et al., 2012). Clearly there is an important asociation of Pt and Pd with Cu, Au, Te, and Bi, and they are likely transported together in chloride- or hydroxide- complexes (Wood, 1992). Bi-telurides commonly occur in gold skarns (Meinert, 1992), which gives further evidence of a magmatic hydrothermal source for Bi and Te, and by extension ! 36 PGEs. The PGEs in the Alard stock occur as Pt ? Pd bismuth-telurides with or without Ag. ! ! Porphyry to Epithermal Transition ! ! ! ! Historicaly, two schools of thought concerning precious metal sources in hydrothermal ores exist, where the metals are either (1) leached from the surrounding rocks or (2) are sourced directly from the magmas. The Hedenquist and Lowenstern (1994) model links epithermal precious metal deposits to porphyry systems indicating that magmatic fluids are important contributors to epithermal hydrothermal systems (Fig. 9). Magmas provide the heat source needed to sustain hydrothermal systems throughout their lifespans. Perhaps magmas also provide water, metals, ligands, and other species to these hydrothermal systems. Due to their close spatial asociation, there are chemical and physical connections betwen ore-forming fluid proceses in porphyry and epithermal environments and the transition zone betwen them (Gamons and Wiliams-Jones, 1997). Studies on active hydrothermal systems experimentaly show that magmas do contribute such components to hydrothermal systems, however as distance from the intrusion increases, evidence of a connection becomes weaker. Modeling experiments show that metals and ligands can be entirely mobilized from magmas (Hedenquist and Lowenstern, 1994). Au, Sn, Mo, PGEs and other siderophile elements have low crustal abundances relative to the Earth?s mantle and core, however Fe-Ni sulfides in the mantle contain these metals. During partial melting, these sulfides can contribute metals into the ! 37 melt, which then ascends to the crust. Once extruded, these basaltic magmas (the ocean floor) are altered and then subducted, providing abundant Cl - , H 2 O, alkali-group elements, oxidized S, Zn and Cu, thereby enriching the subducted mantle wedge. Fig. 9. Illustration of the ?porphyry-epithermal? transition and posible associated deposit types, modified from Hedenquist and Lowenstern (194). Ligands that transport ore metals can include chloride, bisulfide, and hydroxyacids, and are dependent on fluid composition. Fluid inclusion studies of minerals in ore deposits that formed at 700-800? C have recently shown that hypersaline brines can contain 40-60% NaCl and 250-5000 ppm Cu. Evidence supporting the transportation of magma-sourced metals and fluids from the porphyry to the high- sulfidation epithermal environment is beter supported due to the proximity of high- ! 38 sulfidation systems to intrusive porphyries. One goal of this study is to test the hypothesis that epithermal and porphyry deposits can be geneticaly connected. Understanding the evolution of a magmatic fluid and its origin is central to understanding the transition betwen porphyry and epithermal environments. A fluid can be a single magmatic volatile phase that separates from a magma chamber at depth and is then transported to the epithermal environment, or it may separate into two phases at the source chamber or during ascent (Fig. 10). After separation from dense brine in the porphyry environment, the les dense, more buoyant vapor phase may continue to rise until it reaches the epithermal environment (Henley and McNabb, 1978; Silitoe, 1983; Hedenquist et al., 1998; Heinrich et al., 1999; Muntean and Einaudi, 2001; Heinrich et al., 2004). Fig. 10. Schematic cros section of a magmatic-hydrothermal system showing source magma reservoir, and the two posible ore-fluid evolutions in the transition betwen porphyry and epithermal environments (Heinrich, 205). ! 39 Both carbon and sulfur isotopes in epithermal environments can certainly show a magmatic signature, however some input from sedimentary sulfur and organic carbon also exists (Hedenquist and Lowenstern, 1994). Porphyry deposits typicaly show the same distribution, with most yielding evidence of magmatic sources for metals and others with indications of country rock sources. The influence of country rock leaching of metals does not exclude the existence of an original and overarching magmatic source. This model provides implications for metal sources in porphyry and epithermal environments. Later work has shown that magmatic fluids can transport Au from the porphyry environment into shalower epithermal systems (Heinrich et al., 2004; Heinrich, 2005). Henrich et al. (2004) used fluid inclusion studies, thermodynamic modeling, and gold solubility experiments to show that epithermal and porphyry mineralization can both be linked to the cooling of hydrous magma chambers. Phase stability modeling of the NaCl- H 2 O system has shown stability fields where vapor, a single-phase fluid, hypersaline liquid brine, and an acqueous in presure, temperature, and salinity space (Heinrich, 2005). The isotopic composition of lead locked in gold ores can be used to interpret the origin of the precious metal. Pb isotope compositions of mid-Miocene epithermal deposits in the Northern Great Basin suggest a mafic magma Pb source (Kamenov et al. 2007). Trace lead in gold, electrum, and naumanite from the ores as wel as Pb from local mafic rocks in close proximity to these deposits show similar compositions to the middle Miocene bimodal volcanics and to the Columbia River Basalts. The 15.5-16.5 Ma age range for both the ores and for the magmatism asociated with the track of the Yelowstone hotspot/plume coupled with the overlap in lead compositions, indicates a ! 40 magmatic lead source for local mafic rocks and for the Au?Ag metal in the considered ore deposits (Kamenov et al., 2007). The Ladolam epithermal deposit of Lihir Island, Papua New Guinea similarly showed lead isotopic compositions suggestive of a magmatic Pb source (Kamenov et al., 2005). Precious metals in sulfide mineralization at the Taupo Volcanic Zone, New Zealand show isotopic Pb signatures consistent with a magmatic origin as wel (Hedenquist and Gulson, 1992). Exsoluton of magmatic volatiles from magmas at depth produces a low-density brine carrying Pb, Zn, Sn into the ?deep porphyry? environment while a vapor phase likely transports Au, Cu, As, and Hg into shalower epithermal systems (Heinrich, et al., 1994, 2004; Kamenov et al., 2007). Therefore the comparison betwen Pb isotope compositions of metals in epithermal deposits and spatialy asociated porphyry deposits may suggest the presence of a related Pb source. ! 41 3. METHODOLOGY Field Methods Fieldwork was completed during the summer of 2013 in the La Plata Mountains, north of Mancos and west of Durango, Colorado. Samples for this study (Fig. 11) were collected from both porphyry and epithermal deposits in the district, but sampling focused on the mineralization asociated with the Alard stock. Samples collected from historic mine dumps and from outcrops were described in the field and their locations were recorded using a Garmin GPS receiver unit. Sampling focused on finding the least altered samples of syenite and diorite-monzonite porphyry, on obtaining both masively mineralized samples and stockwork samples, and on gathering samples to represent the overal character of the ores as a whole. Fieldwork was conducted by a team of three from Auburn University: the author, Dr. James Saunders, and felow graduate student Michael Mason. Mines visited during fieldwork included the Alard Tunnel, the Copper Hil Glory Hole, the May Day/Idaho mine, and the mine dumps of Cumberland, Columbus, Tomahawk, and Besie G. mines. Samples totaling 250 pounds were shipped back to Auburn University for further study. 42 ! Fi g 1 . A g e o lo g ic m a p s h o w in g a p o r tio n o f th e L a P la ta d is tr ic t a n d th e f ie ld a r e a f o r th is s tu d y . M in e lo cat i o n s ar e i n d i cat ed b y a co l o r ed ci r cl e an d t h e ap p r o p r i at e ab b r ev i at i o n . Mo d i f i e d f r o m S c h i o w i t z ( 2 0 8 ) w h o d i g i t i z e d t h i s v e r s i o n , a n d f r o m E c k e l ( 1 9 4 9 ) w h o p u b l i s h e d t h e o r i g i n a l m a p . N 43 ! Ore Petrography Samples were sorted and thin section blanks were selected and cut with the goal of characterizing the most representative or interesting features of each deposit Quality Thin Sections in Tuscon, Arizona prepared the standard and polished thin sections used in this study. Polished ore mounts were prepared by the author in the thin section lab at Auburn University. Petrography was preformed using a transmited and reflected light microscope. Mineral identification, mineral relationships, and textural relationships were aided by the acompanying geochemical data and with the electron microprobe at the University of Georgia. Using the JEOL 8600 electron microprobe at the University of Georgia Geology Department, minerals from six samples were analyzed using an acelerating voltage of 15 kV and a beam current of 15 nA. Pure element, aloy, or metal standards were used to calibrate the microprobe before analysis. Mineral grains were quantitatively identified using UGA?s Beuker 5010 Silicon Drift Detector (SD) energy dispersive X-ray (EDS) detector controlled by a Bruker Quantax energy dispersive spectrometers (WDS) automated with Advanced Microbeam, Inc. electronics and Probe for EPMA software, using 10 second counting times and natural and synthetic mineral standards. Counting times increased to 30 or 60 seconds to verify measurements as needed. Analyses were calculated using Armstrong?s (1988) Phi-Rho-Z matrix correction model. Backscatered electron (BEI) and X-ray maps were acquired using imaging software of the Quantax analysis system. ! 44 Geochemistry A majority of the samples used for geochemical analysis in the present study were collected in the field. Several samples were collected and provided by David A. Gonzales from Fort Lewis College, in Durango, Colorado. Samples from Besie G. were supplemented from the collection of Dr. James Saunders, which he collected during his PhD research on the Besie G mine. Sample splits were made and al samples were analyzed by ACME Analytical Labs LTD in Vancouver, British Columbia. Each sample was sawed in half, and the remaining split was used to make a thin section, ore mount, or was used for isotope analyses. Geochemical samples were selected to contain a high ratio of ore to gangue and host rock. Therefore, precious metal concentrations were maximized and do not necesarily indicate prospective mining grades for each deposit. At ACME, 6 of the 23 samples (250 g) were crushed, split, and pulverized to 200 mesh; 17 samples were pulverized to 85% pasing 200 mesh. After samples were appropriately prepared, 21 samples (15 g each) underwent 1:1:1 (1 part sample: 1 part HCl: 1 part HNO 3 ) Aqua Regia digestion Ultratrace ICP-MS analysis. Two samples (30 g each) then underwent fire asay fusion for Au-Pt-Pd by ICP-ES. 4 samples (0.2 g each) underwent whole rock analyses (oxide weight %) via LiBO 2 /Li2B 4 O 7 fusion ICP-ES. ! 45 Isotopic Analyses Sulfur, copper, and lead isotope analyses were completed on selected samples from this study. Samples selected generaly consisted of remaining portions of samples used for ore mounts, thin sections, or for geochemical analyses. Sulfur isotope analyses were performed by the author and felow graduate student Michael Mason at the University of Georgia?s Stable Isotope laboratory. Chalcopyrite and pyrite were driled out of samples from Alard Tunnel and Copper Hil under a binocular microscope using a dental dril. Driling out of the minerals continued until sufficient mas to perform analyses was gathered in powder form (chalcopyrite 6 to 8 mg, pyrite 4-6 mg). The mas was multiplied by the theoretical yield of SO 2 for respective minerals (chalcopyrite 10.9, pyrite 16.67). The result was then divided by two, which gave the mas needed of each of the three reagents, vanadium pentoxide (V 2 O 5 ), copper metal powder, and quartz powder. V 2 O 5 provides the necesary oxygen to react with sulfur to make SO 2 gas and the copper metal powder buffers the oxygen fugacity to suppres the formation of SO and SO 3 . Silica powder acts as an abrasive during grinding. The mineral sample and the reagents were combined with an agate mortar and pestle under a fume hood, until combined enough so that the mineral sample began to bond with the reagents (approximately 5-10 minutes). The sample was then scraped out of the mortar and pestle and was then loaded into a 6 mm quartz tube, which was packed with silica fibers on either end. Samples were labeled and loaded into the desicator to await analysis. The sample was then loaded into a 1050? C furnace and combusted for 10 minutes. The resultant SO 2 gas was then cryogenicaly isolated from the other gases ! 46 produced (non-condensable gases, CO 2 , and H 2 O) on the cryogenic purification line. The resultant amount of isolated SO 2 gas was measured using a calibrated mercury manometer and reported as the actual SO 2 yield. Actual yield was compared to theoretical yield and recorded as percent yield. The gas was collected in a Pyrex breakseal tube and was analyzed using a Finnigan MAT 252 mas spectrometer. An error of ? 0.03 ? was calculated on replicate analysis of standards. Results are presented relative to the standard, the Vienna Canyon Diablo Troilite (VCDT). Copper Isotopes Dr. Ryan Mathur of Juniata College, using the laboratory facilities at the University of Arizona, conducted copper isotope analyses on samples of chalcopyrite for this study. Chalcopyrite rich samples were disolved at Auburn University using HF and the residuals were separated under a binocular microscope. The chalcopyrite samples were sequentialy pased through two sets of column chemistry to separate the Cu from the possible Fe. Resulting samples were injected into a Multicollector Inductively- Coupled-Plasma mas spectrometer (MC-ICP-MS), the Micromas Isoprobe at the University of Arizona in low-resolution mode using a microconcentric nebulizer to increase sensitivity. The nebulizer flow was adjusted so that the intensity of the 63 Cu beam remained constant at 2 V. Both on and off peak blank corrections were applied to the data and yielded the same result. The standard used was the NIST 976 Cu standard. Two blocks of 25 ratios are reported as an average for each run. Within each run the ! 47 measurement error was les than ? 65 Cu = 0.01? for al analyses. This protocol was adapted from Mathur et al. (2009). Lead Isotopes Dr. George Kamenov analyzed six mineralized porphyry samples for this study at the Department of Geological Sciences at the University of Florida. Samples were disolved in HF and the residual sulfide or silicate mineral was picked for Pb isotopic analysis. Analyses were conducted using a Nu Plasma multi-collector ICP-MS (Nu Instruments, UK). Fresh Pb-Tl mixtures were prepared following the Tl normalization technique developed by Kamenov et al. (2004). Both sample and standard solution were aspirated into the plasma source using a Micromist nebulizer with a GE spray chamber. Preamplifier gain calibrations were determined before each analysis. Analyses were conducted in static mode simultaneously acquiring 202 Hg on low-1, 203 Tl on low-2, 204 Pb on Axial, 205 Tl on high-1, 206 Pb!on high-2, 207 Pb on high-3 and 208 Pb on high-4 Faraday detectors. Al standard and sample results were normalized with 205 Tl/ 203 Tl = 2.38750 and age corrected using an approximate age for the Alard stock (67 Ma). Results are reported as ratios of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb. ! 48 4. RESULTS Geochemistry The Alard stock contains considerable Cu relative to Ag>Au, and is therefore clasified as a Cu-Ag-Au porphyry. The USGS Porphyry Copper Model database estimates that the Alard stock has a 200 milion metric ton resource at 0.4% Cu and 5 g/t Ag (John et al., 2010). The most intriguing feature of the stock, however, is the occurrence of PGEs at the Copper Hil mine, first noticed by Eckel (1949). For this study, whole rock analyses and trace element concentrations were conducted on samples from the La Plata District (Table 4). Both representative and ?high-graded? samples were sent for geochemical analysis. In addition to samples from the porphyry mines, Alard Tunnel and Copper Hil, samples were also analyzed from the epithermal Cumberland and Besie G deposits along with apparent the skarn-type deposits May Day and Idaho. The goal was to evaluate data that might show a link betwen porphyry and epithermal environments in the La Plata district. Al of the Copper Hil samples analyzed and some of the Alard Tunnel samples contain ? 1% Cu, 2.5-10 g/t Ag, and 0.01-1.39 g/t Au. Pd values range from 0.026-0.96 ppm, however most analyses from Alard Tunnel were below the 10 ppb detection limit for Pd. Pt values range from 0.005-1.67 ppm. Both Pt and Pd values are noticeably higher in samples from Copper Hil than in samples from Alard Tunnel. Fire asay analyses for ! 49 Alard Tunnel yielded 0.096 ppm Au, ?0.003 ppm Pt, and 0.003 ppm Pd, while Copper Hil asays yielded 1.34 ppm Au, 1.09 ppm Pt, and 0.855 ppm Pd. ! 50 T a bl e 4. L a P l a t a D i s t ri c t G e oc he m i c a l D a t a Ta b l e 4 ; : G e o c h e m ic a l d a ta f o r Al l a r d t u n e l ( AT ) , C o p e r Hi l ( C H) , C u m b e r l a n d ( C B ) , B e s i e G ( B G) , M a y d a y ( M D) , a n d co u n t r y r o ck s , ( F S Y , F R DM P ) an al y s es ar e i n p ar t s p er m i l i o n ( p m ) . ! 51 T a bl e 4 Cont d. Ta b l e 4 Co n t d ; G e o c h e m i c a l d a t a f o r Al l a r d t u n e l ( AT ) , C o p e r Hi l ( C H) , C u m b e r l a n d ( C B ) , B e s i e G ( B G) , M a y d a y ( M D) , a n d co u n t r y r o ck s , ( F S Y ) . Al l an al y s es ar e i n p ar t s p er m i l i o n ( p m ) . ! 52 Statistical Analysis A Pearson correlation matrix was calculated in Excel, using the geochemical data on the La Plata district gathered for this study. Linear correlations were determined for every pair of elements calculated from Mo, Cu, Pb, Zn, Ag, Ni, Co, Mn, Fe, U, Au, Cd, Sb, Bi, V, Cr, Hg, Se, Te, Pd and Pt. Although there are far too few data points to acurately characterize the district overal, results indicate several moderate to strong correlations betwen diferent elements. Plots were produced using both log normalized (Table 5) and ?raw? (Table 6) concentration data. There is a strong correlation betwen Pb and Ag, presumably within galena. Co, Mn, Fe, Bi, V, Se, and Pt are strongly correlated with Cu, while Au and Ag show a negative correlation to Cu. Ag is positively correlated to Au, Cd, Sb, Se, and Te. Sb, Hg, and Te are al positively correlated with Au, which likely occur as gold-bearing telurides. Bi and Sb show a strong positive correlation, and the Alard Stock contains anomalous bismuth (Werle et al., 1984). Pt and Pd strongly correlate with Cu, Co, Fe, U, Bi and Se. Pt and Pd show are not correlated with Te. ! 53 T a bl e 5. L og N orm a l Corre l a t i on M a t ri x T a bl e 5: Bol d num be rs s i gni f y s t rong pos i t i ve c orre l a t i ons . Cu Pb Zn Ag Co Mn Fe Au Cd Sb Bi V Cr Hg Se Mo 0. 222682 0. 178916 0. 012894 *0.2889 0. 277419 0. 319431 0. 271456 *0.42179 *0.02594 *0.13251 0. 130395 0. 203933 0. 080462 *0.43227 0. 210572 Cu 0. 073896 0. 21561 0. 153893 0.678549 0.596137 0.526534 *0.52966 0. 268656 *0.31709 0.87783 0.526797 0. 05474 *0.57031 0.795976 Pb 0.877794 0.639489 *0.25891 *0.39533 *0.26626 0. 394835 0.831731 0. 456119 0. 159597 *0.18346 0.543546 0. 23566 0. 34714 Zn 0.569352 *0.27115 *0.33568 *0.30266 0. 309717 0.935136 0. 479321 0. 159391 *0.18533 0.550896 0. 255378 0. 405383 Ag *0.12888 *0.44815 *0.22186 0.675806 0.698374 0.50706 0. 382708 *0.17522 0. 297672 0. 324056 0.564643 Co 0.789521 0.867364 *0.62394 *0.22255 *0.76111 0.669563 0. 439226 *0.1707 *0.86287 0. 439335 Mn 0.765569 *0.87052 *0.3511 *0.73363 0. 382675 0.654993 *0.07135 *0.79477 0. 147747 Fe *0.63008 *0.27257 *0.75087 0.550254 0. 448138 *0.06251 *0.87443 0. 247291 As 0.500756 0. 296067 0.533543 *0.25148 *0.41779 0. 003027 0.667355 *0.09651 Au 0. 411863 0.686862 *0.2618 *0.53568 0. 161938 0.728151 *0.04899 Cd 0.527714 0. 271569 *0.21426 0. 421479 0. 295193 0.551912 Sb *0.28372 *0.35603 0. 316068 0.847213 0. 081765 Bi 0. 333631 0. 01917 *0.50905 0.827546 V 0. 11712 *0.49711 0. 214975 Cr 0. 098059 0. 041439 Hg *0.25456 SeTePt Pd *0.24467 0. 412196 0. 060069 *0.02087 0. 438071 0.5858790. 125761 0.515698 *0.26904 0. 070832 0. 093791 *0.38252 0.6867440. 074609 *0.16406*0.42964 0. 491088 0. 052872 Pt *0.17682 0.555709 *0.00306*0.03961 0. 292228 0.6387820. 372262 0.667752 *0.45663*0.16171 0. 057509 *0.50287 0.7359310. 308924 *0.01942*0.59649 0. 476743 *0.16787 0.898924 ! 54 T a bl e 6. Ra w Corre l a t i on M a t ri x Cu Pb Zn Ag Co Mn Fe Au Cd Sb Bi V Cr Hg Se Mo !0.06475 !0.08854 !0.12497 !0.33694 0. 234188 0. 184053 0. 143474 !0.16456 !0.14907 !0.134 !0.0014 0. 12103 !0.16277 !0.21617 !0.15375 Cu !0.26442 !0.26431 !0.06582 0.653445 0.557894 0.604814 !0.45331 !0.2689 !0.14575 0. 393675 0. 395807 !0.1652 !0.41699 0. 398773 Pb 0.979713 0. 475823 !0.24298 !0.26912 !0.21975 0. 260122 0.967112 0. 040921 0. 041618 !0.2158 0.728679 0. 367122 !0.10695 Zn 0. 466048 !0.31135 !0.28215 !0.26806 0. 198868 0.990284 0. 165201 !0.07617 !0.19655 0.748951 0. 460196 !0.081 Ag !0.11041 !0.51395 !0.1508 0. 482952 0.531885 0. 427189 0. 400692 !0.33563 0. 426782 0.626335 0.601522 Co 0. 348162 0.764589 !0.37385 !0.30897 !0.32858 0.501699 0. 126066 !0.26398 !0.49444 0. 363657 Mn 0.535001 !0.34567 !0.32098 !0.29093 !0.03816 0.74693 !0.07402 !0.43582 !0.19281 Fe !0.33428 !0.28728 !0.31907 0. 389093 0. 364375 !0.09493 !0.4549 0. 263232 As 0. 091152 0. 402165 0.896739 !0.14476 !0.27283 0. 358015 0.726973 0. 454772 Au 0. 220581 0. 030626 !0.14474 !0.20471 0. 202488 0. 372252 !0.1995 Cd 0. 24626 !0.04448 !0.24353 0.750051 0.512247 0. 006134 Sb !0.10082 !0.15785 0. 412082 0.67807 0. 495445 Bi !0.10836 0. 050259 !0.18206 0.57419 V 0. 023937 !0.1719 !0.11195 Cr 1 0. 362885 0. 0151 Hg 0. 247922 Se 1 Te Pd !0.18003 0. 427791 !0.08403!0.11399 0. 338548 0. 460057 !0.14403 0. 397169 !0.16471!0.15228!0.11264!0.14647 0.521182 !0.11993!0.13201 !0.2158 0.616724 !0.18583 Pt !0.16446 0. 38332 !0.08385!0.10292 0. 296006 0. 397692 !0.11725 0. 384254!0.1612 !0.14003!0.10343 !0.1307 0. 434604 !0.08505!0.12004!0.19573 0.581186 !0.16834 0.987106 55 Isotope Geochemistry Isotopic analyses conducted as part of this study on the Alard stock include isotopes ratios for sulfur, copper, and lead isotopes. Mineralized samples of chalcopyrite and pyrite from both Alard Tunnel and Copper Hil were analyzed for sulfur isotopes (Table 7). Relative to standards, results had an error of ? 0.03!?. A histogram of ? 34 S (VCDT) values is shown in Figure 12. Sulfur values fal within the normal expected range for porphyry Cu deposits. The range ? 34 S (VCDT) for the Alard Stock is perhaps indicative of a slight sedimentary sulfur input from wal rocks (as evidenced by a depleted ? 34 S), coupled with a mostly magmatic sulfur signature, at 0?. Table 7. Sulfur isotope values for AT and CH. Table 7. Table of sulfur isotope analyses conducted at the UGA Stable Isotope Laboratory, n=15 samples. ! 56 Fig. 12. Distribution of sulfur isotope values for La Plata analyses, n=17 samples. Values reported as ? 34 S (VCDT) . Includes two data points from comercial analyses CH-1 (? 34 S = -5.8) and AT-1 (? 34 S = -6.0). Copper Isotopes Analyses carried out by Dr. Ryan Mathur are shown in Figure 13. Separates from chalcopyrite mineralization at Alard Tunnel and Copper Hil were analyzed for this study, and fal within the typical range for porphyry Cu-deposits (? 3 ? ? 65 Cu) that show a magmatic Cu source. Two samples were collected by Dr. Gonzales from Ft. Lewis College and therefore have no concrete spatial data to acompany them. The other two samples were collected by the author during field work and are splits of samples used for other geochemical analyses and for petrography. Copper isotope ratios are reported in the familiar delta notation: 0! 1! 2! 3! 4! 5! (8! (7! (6! (5! (4! (3! (2! (1! 0! 1! 2! F r e q u e n c y ? 34 S Allard Stock S-Isotopes ! 57 Fig. 13. Distribution of coper isotope values for La Plata analyses, n=4 samples. Values reported as ? 65 Cu. Samples colected from the Alard Stock by Dr. Gonzales at Ft. Lewis Colege yielded ? 65 Cu of 0.965 ? and 1.380 ?. Samples collected by the author were AT-9 (2.700 ?) and CH-6 (2.570 ?). Lead Isotopes Lead isotope analyses were conducted on ten La Plata district samples to determine the source of lead within the sulfide mineralization, with 3 from Alard tunnel, 3 from Copper Hil, 2 from Besie G, and 2 from Cumberland (Table 8 and Fig. 14). Samples include AT-1, AT-9, CH-1, CH-3, and CH-6. 206/204 Pb values range from 17.91 to 19.5, 207/204 Pb values range from 15.59 to 15.67, and 208/204 Pb values range from 37.8 to 38.51. 0! 1! 2! 3! -5 -4 -3 -2 -1 0 1 2 3 4 5 F r e q u e n c y ? 65 Cu Allard Stock Cu-Isotopes ! 58 ! Fi g 1 4 . P lo ts o f le a d is o to p e r e s u lts sh o w i n g A l a r d T u n e l ( cl o s ed di a m onds ) a nd C oppe r H i l ( ope n di a m onds ) . P r e s e nt da y va l ue s a r e pr e s e nt e d on t he l e f t a nd ag e co r ect ed v al u es ar e o n t h e r i g h t . P r e s e n t D ay V al u e s A ge C or r e c t e d V al u e s 59 Table 8. Lead Isotope Analysis Results Pb Isotopes and U, Th, Pb Concentrations Present Day Sample 206 Pb/ 204 Pb 207 Pb/ /204 Pb 208 Pb/ 204 Pb AT-1 HF fraction (silicate) 19.50015.66638.618 AT-1 sulfide 19.39315.66638.575 AT-9 sulfide 19.51315.67438.513 CH-1 sulfide 18.70115.55438.191 CH-3 sulfide 18.51915.52138.032 CH-6 sulfide 18.66115.55138.127 CB-1 silicate 21.53315.86038.996 CB1-naumanite 21.83215.89639.108 BG-1 silicate 22.19715.92639.233 BG-1 naumanite 20.62215.77038.558 Sample Pb (ppm) U (ppm) Th (ppm) AT-1 HF fraction (silicate) 0.2380.4870.781 AT-1 sulfide 21.1192.6780.165 AT-9 sulfide 165.5721.3551.908 CH-1 sulfide 11.7672.06710.852 CH-3 sulfide 5.4820.211 1.091 CH-6 sulfide 46.5551.6039.837 CB-1 silicate 306.4960.2000.041 CB1-naumanite 30.0760.2560.051 BG-1 silicate 6.6050.4482.718 BG-1 naumanite 3.7950.0320.285 Sample 204 Pb 238 U/ 204 Pb 235 U/ 204 Pb 232 Th/ 204 Pb AT-1 HF fraction (silicate) 1.337151.9741.102245.826 AT-1 sulfide 1.3409.3960.0680.582 AT-9 sulfide 1.3390.6070.0040.861 CH-1 sulfide 1.36212.811 0.09367.733 CH-3 sulfide 1.3692.7940.02014.541 CH-6 sulfide 1.3642.5060.01815.496 CB-1 silicate 1.2920.0500.0000.010 CB1-naumanite 1.2850.6570.0050.133 BG-1 silicate 1.2765.2790.03832.242 BG-1 naumanite 1.3170.6430.0055.698 Age Corrected (67 Ma) Sample 206 Pb/ 204 Pb 207 Pb/ /204 Pb 208 Pb/ 204 Pb AT-1 HF fraction (silicate) 17.91315.59137.802 AT-1 sulfide 19.29515.66238.573 AT-9 sulfide 19.50615.67338.510 CH-1 sulfide 18.56815.54737.966 CH-3 sulfide 18.48915.51937.984 CH-6 sulfide 18.63515.55038.075 CB-1 silicate 21.53215.86038.996 CB1-naumanite 21.82515.89539.108 BG-1 silicate 22.14215.92339.126 BG-1 naumanite 20.61515.77038.539 ! 60 Petrology and Petrography of Ores and Asociated Rocks Eckel (1949) provided a generalized but thorough petrographic analysis of ore samples and country rocks in the La Plata district, however only a cursory description of the Alard stock and asociated mineralization was completed. This study further investigates the host rock and ore petrography at the Alard tunnel and Copper Hill deposits within the Alard stock. A brief analysis of the Cumberland, Besie G. and May Day deposits? ore petrography is also included. Throughout the district, gold, telurium, copper and silver are the most abundant elements in mineralization with gold (both native and in Au-Ag telurides) being the largest commodity mined historicaly. Lead, silver, and copper were also productively mined but to a leser extent. What Eckel (1949) refers to as diorite monzonite porphyry, is actualy a range of monzonitic to dioritic to syenitic intrusive porphyries. Eckel (1949) chose the naming scheme of diorite monzonite porphyry and syenite porphyry as simplistic field names to diferentiate betwen two general types of porphyries in the district. Samples collected in this study plot in the monzonite, monzodiorite, and syenite fields on a total alkali vs silica (TAS) diagram, further evidencing their overlap. The diorite monzonite porphyry (DMP) has a porphyritic texture with feldspar ? hornblende ? augite in a gray to brown groundmas. It is much darker in color than the syenite porphyry and has smaler phenocrysts of feldspar. The DMP is not as extensively mineralized as the syenite (Eckel, 1949). The syenite porphyry contains larger feldspar phenocrysts, is more feldspathic, and contains augite rather than hornblende. The syenite porphyry is also more heavily altered than the diorite monzonite porphyry and feldspars are converted to sericite ? ! 61 kaolinite ? zoistite ? carbonate (ankerite?) and pyroxenes have altered to hornblende ? limonite ? chlorite. ?Fresh? syenite contains plagioclase ? orthoclase ? augite ? apatite ? sphene ? chlorite. Even the most unaltered syenite found during field work for this study showed moderate seritizitation and pyritization, evidence of the pervasive potasic alteration, local propylitic (Fig. 15) alteration, the slight and quartz-sericite-pyrite alteration of the Alard stock. Ore minerals in the Alard stock consist of chalcopyrite ? pyrite ? magnetite ? hematite (specular) ? galena ? sphalerite (Fig. 16). Mineralization at Alard tunnel typicaly occurs within brecias, stockwork veins (Fig. 15) and is also moderately diseminated throughout most of the Alard stock. At Copper Hil, the glory hole contains masive chalcopyrite (Fig. 17) with azurite ? malachite with minor covelite ? bornite (Fig. 16). A major northeast trending vein appears to be the conduit for mineralizing fluids at Copper Hil (Fig. 18), as is the trend throughout the district. Alard tunnel samples contain roughly 30% more pyrite relative to chalcopyrite than those from Copper Hill. ! 62 Fi g . 1 5 . Ph o t o m i c r o g r a p h s of m i ne r a l i z a t i on f r om A l a r d T unne l . F r om t op l e f t t o bot t om r i ght : A n e xa m pl e of hydr ot he r m a l br e c i a t i on, s t oc kw or k pyr i t e an d ch al co p y r i te v e in s w ith in p in k s y e n ite , b r e c ia tio n c e m e n te d b y c h a lc o p y r ite a n d c h a lc o p y r ite , p r o p y litic a lte r a tio n a n d o f s e t v e in in g , a n d p y r ite , c h a lc o p y r ite , he m a t i t e , a nd goe t hi t e a s e vi de nc e of oxi da t i on . ! 63 Fi g . 1 6 . P h o to m ic r o g r a p h s o f A lla r d tu n e l s a m p l es i n r ef l ect ed l i g h t . A ) p y r i t e s u r o u n d ed b y ch al co p y r i t e i n v ei n l et s . B ) m as s i v e ch al co p y r i t e w i t h p y r i t e, C ) ve i nl e t s a nd bl e bs of c ha l co p y r i t e an d p y r i t e, n o t e f i n er g r ai n s i ze t h an p r ev i o u s p h o t o m i cr o g r ap h , D ) pyr i t e gr a i n s ur r ounde d by c ha l co p y r i t e an d s p h al er i t e w i t h mi n o r ma g n e t i t e . ! 64 Fi g . 1 7 . Ph o t o gr a phs of h an d s am p l es f r o m t h e C o p er H i l g l o r y h o l e s h o w ex t en s i v e ch al co p y r i t e m i n er al i zat i o n w i t h i n a co n t act - me t a mo p r h i c s e t i n g b e t w e n th e A lla r d s to c k s y e n ite a n d th e s u r o u n d in g s e d im e n ta r y ro c k s (l i k e l y t h e P o n y E x p re s l i m e s t o n e ). M i n e ra l i z a t i o n a t c o p e r h i l c o n t a i n s m u c h l e s p y ri t e t h a n mi n e r a l i z a t i o n a t t h e A l a r d t u n e l a n d f a r mo r e c h a l c o p y r i t e . C o p e r o x i d a t i o n mi n e r a l s a t C o p e r H i l c o n s i s t o f ma l a c h i t e an d azu r i t e, h o w ev er s o m e c ha l c oc i t e a nd c hr ys oc ol l a a r e a l s o pr e s e nt i n out c r op a t t he gl or y hol e . ! 65 Fi g 1 8 . P h o to m ic r o g r a p h s in r e f le c te d lig h t fr o m C o p e r H i l . A ) C o m o n r e l a t i o n s h i p b e t w e n c h a l c o p y r i t e a n d m a g n e t i t e , B ) m a g n e t i t e w r a p i n g a r o u n d ch al co p y r i t e w i t h i n a v ei nl e t , C ) m a s i ve c ha l c opyr i t e a r ound of a pa t i t e a nd m i nor qua r t z , D ) E xa m pl e of di s e m i na t e d m ag n et i t e an d ch al co p y r i t e. . ! 66 Fi g . 1 9 . O u t c r o p p h o t o g r a p h s f r o m t h e C o p e r H i l g l o r y h o l e a n d t h e N E - tr e n d in g v e in m in e d th e r e . T h e f ir s t p h o to lo o k s d o w n o n th e v e in f r o m th e en t r an ce t o t h e g l o r y h o l e, an d t h e s eco n d p h o t o l o k s u p t h e v ei n f r o m t h e b o t o m o f t h e g l o r y h o l e. T h i s m ai n v ei n , h o w ev e r, i s n o t t h e o n l y s o u rc e o f ch al co p y r i t e i n t h e v i ci n i t y o f t h e g l o r y h o l e. C h al co p y r i t e i s m as s i v e i n an d ar o u nd t he gl or y hol e ! 67 Ore mineral paragenesis in the Alard stock is typicaly uniform; however some diferences in mineralogy do exist betwen the samples from Alard tunnel and Copper Hil (Table 9). The generalized paragenetic sequence determined by this study is as follows. (1) Pyrite and bornite appear to be the earliest ore phases, followed by (2) covelite, magnetite/iron oxides, and sphalerite, then ruby silvers and acanthite, and chalcopyrite, galena, and Pt-Pd-Bi-telurides sem to be the latest ore phases. Gangue minerals within hypogene veins include (1) sanidine, (2) quartz, and (3) calcite?fluorite. Vein calcite is abundant and vein quartz is also present. Werle et al. (1984) detailed the vein occurrences within the Alard stock: (1) iron oxide ? pyrite ? copper sulfarsenite ? copper sulfide ? calcite ? quartz veins; (2) calcite ? quartz ? sphalerite ? galena ? chalcopyrite ? pyrite veins; (3) fluorite ? quartz ? pyrite ? chalcopyrite ? marcasite veins. Smal base metal sulfide veins and mantos and quartz ? pyrite ? fluorite ? teluride veins that occur peripheral to the stock may have formed at the same time as vein types (2) and (3) within the stock. Sample AT-4 shows the best evidence for the paragenetic sequence in the Alard tunnel samples (Fig. 20). Bornite with possible chalcopyrite exsolution textures is surrounded by covelite, both are rimed by chalcopyrite, and sphalerite occurs after chalcopyrite. Covelite, bornite, and chalcopyrite are al hydrothermal hypogene minerals. Another example from AT-4 shows an interesting exsolution texture within bornite, which sems to be indicative of chalcopyrite exsolving bornite (Fig. 21). Chalcopyrite occurs much les abundantly within the Alard tunnel samples than within the Copper Hil samples. ! 68 Table 9: Alard Stock Ore Minerals in order of Abundance for AT and CH Alard Tunel Copper Hil Pyrite Chalcopyrite Chalcopyrite Bornite Sphalerite Pyrite Magnetite (minor) Covelite Galena (minor) Magnetite/ Iron Oxides - Sphalerite -- Galena - Ruby silvers (Pyrargyrite) - Acanthite - Pt+Pd+Bi+Telurides ! 69 Fig. 20. Reflected light photomicrograph of the paragenetic sequence at the Allard stock (minus pyrite). Minerals include chalcopyrite (cpy), bornite (bn), covelite (cov), and sphalerite (sph). Field of view is 1 mm. cpy sph cov bn! ! 70 Fig. 21. Reflected light photomicrograph of AT-4 showing the relationship betwen chalcopyrite (cpy), bornite (bn), and covelite (cov) with a possible bornite exsloution texture (bn to cpy). Field of view is 1 mm. Hydrothermal calcite veining is abundant in the Alard stock, as is the asociated propylitic alteration of the porphyry system (Fig. 15). Clearly a CO 2 rich source existed at depth to provide such volatile rich fluid when this deposit formed. Ore minerals chalcopyrite and pyrite are abundant in calcite veins in the Alard stock (Fig. 22), and most commonly occurs within the Alard tunnel samples. Framboidal pyrite also appears in mineralization at the Alard stock (Fig. 23.) Ruby silvers (pyrargyrite?proustite) were found using reflected light microscopy (Fig. 24) as was acanthite (Fig. 25). Using the electron microprobe, Ag-telurides were also identified. co v cp y bn xso ln txt ! 71 Fig. 2. Transmitted light Photomicrograph of a calcite vein croscutting potasium feldspar in sample AT- 1. Note the asociation of opaque minerals (pyrite) within the hydrothermal calcite vein. Ore minerals chalcopyrite and pyrite are abundant in calcite veins in the Alard stock. Field of view is 2 m. cc vein ksp r py ! 72 Fig. 23. Photomicrograph using both transmitted and reflected light to show the ore minerals (pyrite and chalcopyrite) as well as the calcite vein. Framboidal pyrite also exists in the Alard stock (grain directly under the chalcopyrite arow). Fig. 24. Photomicrograph of pyrargryite asociated with masive chalcopyrite at Coper Hill. Field of view is 1 mm. py cpy cc vein cpy Pyrargyrite ! 73 Fig. 25. Photomicrograph of acanthite asociated with masive chalcopyrite at Coper Hill. Field of view is 1 m. Two other epithermal deposits and one limestone replacement/skarn deposit were included in this study (Fig. 26). At Cumberland and Besie G, Au-Ag telurides are common as is native gold. Cumberland samples contain conspicuous azurite and malachite, possibly as remnant evidence of Cu-telurides. Mayday ore occurs as veins within a limestone replacement/skarn type deposit with galena?sphalerite?telurides Acanthite cpy ! 74 Fi g . 2 6 . Ph o t o s a n d p h o t o m i c r o g r a p h s o f L a Pl a t a d i s t r i c t e p i t h e r m a l a n d l i m e s t o n e r e p l a c e m e n t - ty p e d e p o s its . A ) N a tiv e g o ld o n A u - Ag t e l u r i d e s , Cu m b e r l a n d . B) Au - Ag t e l u r i d e s i n h a n d s a m p l e wi t h q u a r t z a n d b a r i t e . C ) C u b e r l a n d h a n d s a m p l e s h o wi n g a z u r i t e a n d m a l a c h i t e ( s u p e r g e n e ? ) i n as s o ci at i o n w i t h te llu r id e s ( c o lo r a d o ite , h e s ite , e tc ) a n d p in k b a r ite w ith a ty p ic a l e p ith e r m a l o p e n - sp a c e - fi l i n g v e i n t e x t u r e w i t h b a n d i n g . D ) P h o t o m i c r o g r a p h o f a B e s i e G sa m p l e i n r e f l e c t e d l i g h t sh o w n g n a t i v e A u a sso c i a t e d w i t h h e ssi t e . E ) R e f l e c t e d l i g h t p h o t o m i c r o g r a p h o f a M a t y d a y sa m p l e sh ow i ng s pha l e r i t e , ga l e na , a nd pyr i t e . E ) H a nds a m pl e f r om M a yda y s how i ng ve i n s ul f i t e s l i ke l y i n a s oc i a t i on w i t h t e l ur i de s .! Au -Ag t e l l u ri d e s t e l l u ri d e cl u st e r w i t h n a t i ve Au Au -Ag t e l l u ri d e s p i n k b a ri t e + ve i n t e xt u re ma l a ch i t e a n d a zu ri t e n a t i ve Au h e ssi t e py sp h gln ve i n su l f i d e s +/ - t e l l u ri d e s AB C DDE ! 75 Electron microprobe analysis of 6 samples from the Copper Hil glory hole resulted in the recognition of PGE-bearing mineral grains with several diferent compositions and mineral asociations. Samples for microprobe analysis were chosen based on Pt and Pt compositions from geochemical analyses, with concentrations >1 ppm selected as the most favorable for analysis. Element peaks, element maps, and compositional analyses were gathered for grains if they were sufficiently large enough (>3 ?m) to permit acurate compositional analysis. Atomic proportions and elemental weight percent of compositional analyses (Table 10) are reported for PGE-mineral grains, and several Ag-Se-Bi minerals. Microprobe-EDS images and spectra are presented below in figures 27-44 and wil be discussed in the following section of this thesis. Table 10. Microprobe Analyses for Copper Hil Atmoic Proportions Fe S As Cu Se Ag Bi Te Au Pt Pd CH-2-10.3717.742.243.961.7972.511.170.070.030.090.03 -3-111.3440.02-0.0226.402.430.450.0419.36-0.040.010.01 CH-3-210.2139.36-0.1026.632.820.40-0.0220.760.00-0.070.02 -3-314.490.260.141.57-0.030.382.7652.771.1019.317.25 CH-3-41.050.35-0.051.610.070.784.6159.740.4814.6716.69 -4-13.1210.63-0.065.060.4629.950.0129.060.07-0.1021.81 CH-2-20.210.040.000.110.011.120.8364.300.403.5929.39 -Z-12.1231.360.0310.680.062.4311.2621.260.429.6310.77 Weight Percent Fe S As Cu Se Ag Bi Te Au Pt Pd CH-2-10.225.931.752.621.4781.572.550.090.060.180.03 -3-19.7119.68-0.0225.732.940.740.1437.89-0.110.030.01 CH-3-28.4018.59-0.1124.923.280.63-0.0539.02-0.01-0.200.03 -3-36.310.060.080.78-0.020.324.4952.451.6929.356.01 CH-3-40.460.09-0.030.800.050.667.5159.440.7422.3113.85 -4-11.563.06-0.042.880.3228.980.0233.260.12-0.1720.82 CH-2-20.100.010.000.060.011.011.4568.670.665.8626.18 -Z-11.2010.250.026.920.052.6723.9727.640.8319.1511.68 Weight Percent Oxide FeO SO 3 As 2 O 3 Cu 2 O SeO 2 Ag 2 O Bi 2 O 3 TeO 2 Au 2 O 3 PtO PdO CH-2-10.2814.812.312.952.0787.622.840.110.070.200.04 -3-112.5049.14-0.0328.974.140.790.1647.39-0.120.030.02 CH-3-210.8146.41-0.1528.064.610.68-0.0548.81-0.02-0.220.03 -3-38.110.160.110.88-0.020.345.0165.601.8931.766.91 CH-3-40.590.22-0.040.900.060.718.3774.340.8324.1415.93 -4-12.017.63-0.053.250.4531.130.0241.600.13-0.1823.95 CH-2-20.120.020.000.070.011.081.6285.890.746.3430.11 -Z-11.5525.580.037.790.072.8726.7234.570.9420.7213.43 ! 76 Fig. 27. A PGE-mineral grain (merenskyite) (red circle) in sample CH-3-3, shown in a backscater electron (BSE) image. The PGE-mineral is surounded by quartz (qtz) and rutile (rut), and in close association with apatite (ap), chalcopyrite (cpy), and titatinite,/sphene (tit/sphn). cpy cpy ap ap qtz rut tit/sphn ap 30 ?m ! 77 Fi g . 2 8 . Th e c o r e s p o n d i ng ED S s p e c t r u m f o r a P G E - mi n e r a l g r a i n (m e re n s k y i t e ) fo u n d i n CH - 3 - 3 (F i g . 2 7 ) . N o te th e p e a k s f o r P t, B i, P d , a n d T e . C u , an d S pe a ks a r e ba c kgr ound f r om t he s ur r oundi ng c ha l c opyr i t e , a n d th e S i p e a k r e s u lte d f r o m th e b a c k g r o u n d f e ld s p a r . . ! 78 F ig . 2 9 . E D S e le m e n t m a p s o f th e P G E - mi n e r a l g r a i n (m e re n s y k i t e ) in s a m p le CH - 3 - 3 (F i g . 2 7 , F i g . 2 8 ) . F r o m le f t to r ig h t, im a g e s a r e a s f o llo w s : A ) C a lc iu m ma p s h o w i n g s p h e n e , B ) B a c k s c a t e r e l e c t r o n i ma g e , C ) C o p e r ma p s h o w i n g c h a l c o p y r i t e , D ) C o mp o s i t e e le m e n t m a p , n o te p la tin u m m a p o f P G E - mi n e r a l gr a i n i n t he c e nt e r of t he i m a ge , E ) T i t a ni um e l e m e nt m a p s how i ng r ut i l e a nd t i t a ni t e / s phe ne , F ) P l a t i num e l e m e nt m a p. N ot e c onc e nt r a t i on of P t a s br i ght r e d, how e ve r ot he r a r e a s of t he i m a ge m a y s how s om e i ndi s tin c t n o is e , G ) B is m u th a n d p la tin u m e le m e n t m a p a t a la r g e r s c a le , s h o w in g b is m u th in c h a lc o p y r ite , b u t not w i t hi n t he P G E - mi n e r a l g r a i n . A BC D EFG ! 79 Fig. 30. A PGE-mineral grain (merenskyite) in sample CH-3-4 EDS with element maps showing the PGE-mineral in direct asociation with chalcopyrite. Other phases in the vicinity of the grain (not shown in image) include magnetite and apatite. A) is a backscater electron image with the PGE- mineral circled in red. B) shows a composite element map showing coper in chalcopyrite and the PGE-mineral. C) a WDS element map showing platinum. D) an element map showing coper in chalcopyrite. cpy cpy cpy A B C D ! 80 Fi g . 3 1 . Co r e s p o n d i n g ED S s p e c t r u m f o r th e P G E - mi n e r a l g r a i n ( me r e n s k y i t e ) in CH - 3 - 4 (F i g . 3 1 ) . N o te th e p e a k s f o r P t, P d , B i, an d P d . C u an d F e pe a ks a r e f r om ba c kgr ound c ha l c opyr i t e a nd t he S i pe a k i s due t o ba c kgr ound s i l i c a t e ( f e l ds pa r ) ! 81 Fig. 32. Backscatter electron image and a composite element map for a PGE-mineral grain (new mineral?) (circled in red) found in sample CH-2-2 with phases labeled (top). The PGE-mineral is surounded by potasium feldspar (kspr), plagioclase (plg), and a barium-strontium sulfate. The element map shows paladium in red, potasium in blue (to signify potasium feldspar), sodium in gren (to signify plagioclase, specificaly albite), and sulfur in yelow (to signify Ba-Sr-sulfate) (botom). ! 82 Fi g . 3 . Co r e s p o n d i n g ED S s p e c t r a f o r th e P G E - mi n e r a l g r a i n (n e w m i n e ra l ? ) in CH - 2 - 2 (F i g . 3 2 ) . T h e P d p e a k is la r g e r r e la tiv e to th e P t p e a k f o r th is p a r tic u la r sa m p l e . T e a p e a r s t o b e a g r e a t e r c o m p o n e n t i n t h e m i n e r a l t h a n B i . ! 83 Fig. 34. Individual element maps of the unnamed/new PGE-mineral grain in sample CH-2-2 (Fig. 32, Fig. 33). From left to right, A) Backscatter electron image (same as Fig. 32, without phases labeled), B) Element ap for paladium (Pd), C) element map for sodiu (Na), signifying albite, D) element map of coper (Cu) signifying chalcopyrite, E) element map of sulfur (S) showing chalcopyrite and or pyrite, F) element map showing potassium (K) as evidence for potassium feldspar. Pd A B C D E F ! 84 Fig. 35. Backscatter electron image of a PGE-mineral grain (sopcheite) found in sample CH-4-1. The PGE grain is circled in red with surrounding minerals biotite (bt), chalcopyrite (cpy) potasium feldspar (kspr), an alkali-rich pyroxene (pyx), calcite (cal), clinopyroxene (cpx), and plagioclase/albite (plg). bt cal cpx plg cpy cpy kspr pyx pyx ! 85 Fi g . 3 6 . Co r e s p o n d i n g ED S s p e c t r a f o r th e P G E - mi n e r a l (s o p c h e ite ) gr a i n i n s a m pl e CH - 4 - 1 (F i g . 3 5 ) . N o te th e a b s e n c e o f t h e P t p eak an d t h e ab u n d an ce o f A g , re l a t i v e t o P d a n d T e . C u , F e , a n d S a re b a c k g ro u n d s p e c t ra f ro m t h e n e a rb y c h a l c o p y ri t e . ! 86 Fig. 37. Individual element maps for the PGE-mineral grain (sopcheite) in sample CH-4-1 (Fig. 35, Fig. 36). A) BSE image, B) silicon element map, signifying feldspar and biotite, C) Pd element map, D) composite element map showing phase relationships, E) magnesium element map signifying biotite, and F) coper element map signifying chalcopyrite. ! 87 Fig. 38. Backscater electron image of a PGE-mineral grain in sample CH-Z-2 showing a PGE-mineral grain (circled in red) hosted inside a Ca-Mg rich pyroxene phase (likely augite), that is near to but not hosted in chalcopyrite (cpy). cpy cpy cpy aug ! 88 Fi g . 3 9 . Co r e s p o n d i n g ED S s p e c t r a f o r th e P G E - mi n e r a l g r a i n i n s a mp l e C H - Z - 2 (F i g . 3 8 ) . P t a n d T e p e a k s a r e a p a r e n t, a s it? s th e a b s e n c e o f P d a n d B i p e a k s . Cu , M g , a n d F e a r e b a c k g r o u n d s p e c t r a f r o m t h e s u r o u n d i n g m i n e r a l p h a s e s . ! 89 Fig. 40. Backscatter electron images for a PGE-mineral (mocheite) in sample CH-Z-1. The PGE-mineral grain (circled in red) is surrounded by chalcopyrite (cpy), an alkali-rich amphibole (aegerine?) (amph), and an alkali-rich pyroxene (augite?) (pyx). cpy cpy amph pyx ! 90 Fi g . 4 1 . C o r e s p o n di ng ED S s p e c t r a f o r th e P G E - mi n e r a l g r a i n (m o n c h e i t e ) in s a m p le CH - Z - 1 (F i g . 4 0 ) . P t, P d , a n d T e a ll s h o w p r o m in e n t p e a k s . C u , F e , a n d S ar e b ack g r o u n d p eak s f r o m t h e n ear b y ch al co p y r i t e. ! 91 Fig. 42. Backscater electron image of thre PGE-mineral grains in sample CH-Z-3. The thre PGE mineral grains (circled in red) are surounded by epidote (ep) and plagioclase (plg). Biotite (bt), magnetite (mgt), and chalcopyrite (cpy) are also in the vicinity of the PGE-mineral grains. ep mgt plg ep bt cpy plg ! 92 Fi g . 4 3 . Co rre s p o n d i n g ED S s p e c t r a f o r one of t he P G E - mi n e r a l g r a i n s ( c e n t e r g r ai n i n F i g . 4 2 ) i n s am p l e C H - Z - 3 . Pe a k s f o r a Pt - Pd - Bi - Te m i n e r a l a r e e v i d e n t . Ba c k g r o u n d F e , A l , a n d N a a r e b a c k g r o u n d s p e c t r a f r o m t h e s u r o u n d i n g p l a g i o c l a s e . 93 Fi g . 4 . In d i v i d u a l ED S el em en t m ap s o f th e P G E - mi n e r a l g r a i n s i n sa m p l e C H - Z - 1. F r om l e f t t o r i ght , A) B S E i m a g e , B ) s o d i u m e l e m e n t m a p , C ) i r o n e l e m e n t ma p ABC D EFG Pt ! 94 5. DISCUSION Discussion of Geochemical and Whole Rock Data Covariant plots for geochemical data in this study display element-element correlations in more detail. A covariant plot of Pt and Pd (Fig. 45) shows a strong positive correlation betwen the two elements, which supports the results from the above correlation matrix. This suggests that the Pt and Pd likely coexist together in a mineral phase. Also, phases with low Pd generaly have two orders of magnitude more Pt. When concentrations of Pt or Pd approach the range of 10-100 ppb, a very strong positive correlation betwen Pt and Pd is evident (Fig. 45). Covariant plots for Pt and Pd versus Cu show no correlation until Cu concentrations reach 10,000 ppm, at which point Pt concentration increase from ~5 ppb to > 1 ppm, whereas Pd concentration increases from ~30 ppb to ~750 ppb. Generaly, more Pt exists within the Alard stock, relative to Pd. Eckel (1949) reported that Pt and Pd likely occurred within or in asociation with chalcopyrite. From this geochemical data, this is only probably true in samples with at least 1% Cu, where Pt and Pd reach economicaly viable concentrations. Diagrams plotting Pt and Pd versus Se and Te concentration within the porphyry and epithermal or skarn deposits (Fig. 46) show a generaly positive correlation trend betwen Pt and Pd and Se and Te for the Alard stock porphyry deposits. The epithermal/skarn deposits do not fit this general trend, but have more Te relative to Se. ! 95 The presence of more Te is due to the occurrence of Au-Ag-teluride minerals at al three of the epithermal or skarn type deposits sampled for this study (Besie G, Cumberland, May Day). Fig. 45. Covariant plots of geochemical (Pt, Pd, and Cu) data in the La Plata district. Samples from the Allard tunel, Coper Hil, Cumberland, Besie G., and May Day deposits are included. 1 10 100 1,000 10,000 1 10 100 1,000 P t (p p b ) Pd (ppb) 1 10 100 1,000 10 100 1,000 10,000 P d (p p b ) Cu (ppm) 1 10 100 1,000 10,000 10 100 1,000 10,000 P t (p p b ) Cu (ppm) 96 Fig. 46. Pd, Pd, Se, and Te covariant plots for La Plata district samples. Note the discrimination of locationand porphyry, epithermal and skarn deposit types. 1 10 100 1,000 10,000 0.01 0.10 1.00 10.00 100.00 1,000.00 P t+ P d (p p b ) Te (ppm) AT CH Epithermal and Skarn 0.01 0.10 1.00 10.00 100.00 1,000.00 0.1 1.0 10.0 100.0 T e (p p m) Se (ppm) AT CH Epithermal and Skarn 1 10 100 1,000 10,000 0.1 1.0 10.0 100.0 P t+ P d (p p m) Se (ppm) AT CH Epithermal and Skarn ! 97 Whole rock analyses of samples of syenite from the La Plata district for this study yielded similar values to those of Werle et al. (1984) and Wegert and Parker (2011). As compared to Werle et al. (1984), the whole rock analyses from this study do plot within the same field (Fig. 47), however Werle et al. (1984) recorded data with higher overal SiO 2 and Na 2 O+K 2 O values. Both studies show the alkaline nature of the Alard stock, as al analyses plot within the alkaline field. An average syenite composition, the USGS Syenite-STM1 standard, is also plotted for reference. Analyses for this study show an elevated K 2 O/Na 2 O (0.65-0.97) ratio, similar to shoshonites, as Werle et al. (1984) also found. Fig. 47. TAS diagram for the La Plata district. Gray shaded region based on data range for the Alard stock from Werle et al. (1984). Abreviations are as folows: AT = Alard tunel, FSY = fresh syenite, AVGSY = USGS syenite STM-1 standard. ! 98 Data from Wegert and Parker (2011) comparing the McDermott formation, the Allad stock, other La Plata district rocks, and the Four Corners lower crustal xenolith average (Fig. 48) follows the same trend as data from Werle et al. (1984) and this study. Note that both Wegert and Parker (2011) and Werle et al. (1984) have analyses that plot farther into the trachyte field. Alard stock TAS values trend betwen basaltic trachyandesite, trachyandesite, to trachyte compositions. Samples from this study fal in the shoshonite field, as Na 2 O ? 2.0 < K 2 O, however data from Werle et al. (1984) and Wegert and Parker (2011) plot as latite compositions. Alard stock samples AT-1, AT-9 and FSY and FSY2 follow the same trend of decrease (Fig. 49) in MgO content relative to increase in SiO 2 as Wegert and Parker (2011) showed. This trend begins at the Four Corners lower crustal xenolith average and SiO 2 content increases as fractional crystalization increases in the system, removing MgO. ! 99 Fig. 48. TAS plot from Wegert and Parker (201) superimposed with data from this study (stars). Red stars are Alard tunel, white stars are fresh syenite, and the blue star is the USGS STM-1 syenite standard, folowing the same symbology as the previous TAS graph (Fig. 47). ! 100 Fig. 49. Allard stock data ploted on an MgO versus SiO 2 graph {modified from Wegert and Parker (201)] with data from previous studies on the Allard stock, the Mcdermot Formation, and the lower crustal xenolith average. The USGS TM-1 syenite standard is also included for reference. ! 101 Microprobe Analyses: Finding PGE-mineral Grains in Copper Hil Samples In samples from Copper Hil that contained ~1 ppm Pt ?Pd, microprobe analyses indicated discrete grains of at least four PGE-minerals, which include merenskyite (Pd,Pt)(Te,Bi) 2 , moncheite (Pt,Pd)(Te,Bi) 2 , sopcheite (Ag 4 Pd 3 Te 4 ), and an unnamed Pd-Te mineral (PdTe 2 ). EDS spectra and compositional results for these minerals can be found in Table 10 of the results section of this thesis. Merenskyite occurs most notably in the Merensky Reef of the Bushveld Complex, it also appears in the Great Dike in Zimbabwe, the Stilwater complex, Montana; the New Rambler Cu?Ni mine, Encampment district, Wyoming,the Key West Mine, Bunkervile district, Nevada, in Kambalda, Western Australia, in the Lac des Iles complex, Ontario, Canada, and in the Noril?sk region of Russia. Several of these deposits or districts were noted for PGE occurrence in the previous works section of this thesis. In the Alard stock, merenskyite was found in CH-3-3 (Fig. 27) occurring with rutile + quartz + apatite + sphene + chalcopyrite, and in CH-3-4 (Fig. 30) with chalcopyrite + magnetite + apatite. Moncheite, similar to merenskyite but usualy more Pt rich, also occurs in the Bushveld complex, at Sudbury, Noril?sk, Stilwater, Kambalda, the Lac des Iles complex, and in the New Rambler and Key West mines. Typicaly moncheite occurs in smal amounts in Pt?Pd-bearing masive Cu?Ni sul?de deposits, however at the Alard stock it occurs in asociation with epidote+plagioclase+biotite in sample CH-Z-1 (Fig. 40). It is obvious that these minerals are often asociated with one another due to their similar chemical formulas and to the fact that Pt and Pd often alow and travel together in systems. Sopcheite typicaly occurs in veinlets cutting chalcopyrite (Monchegorsk deposits, ! 102 Russia), and has been found at the Lac des Iles complex, at Sudbury, and at the Santo Tomas II porphyry copper deposit, Benguet, Philippines. At Copper Hil, sopcheite was found in CH-4-1 (Fig. 35) surrounded by chalcopyrite+biotite+pyroxene. The unnamed, Pd-teluride is perhaps a new mineral, and occurs in CH-2-2 (Fig. 32) enclosed completely in epidote, although plagioclase and biotite are also in close asociation. PGE-grains range betwen 3-35?m in length. Diseminated Pt?Pd may also occur within chalcopyrite at the Alard stock, but other than the geochemical and asay analyses in this study, there are no data to support or negate the presence of diseminated PGEs within the Alard stock not in asociation with Te or within sulfides. ICM-MS analysis has shown that Pd can be homogenously distributed within chalcopyrite and tetrahedrite and could potentialy be bound within the minerals? crystal latices (Pasava et al, 2010; John and Taylor, 2014 in pres). Sulfur Isotope Geochemistry of the Alard Stock Since the late 1940s, sulfur isotope geochemistry has been used to investigate many geologic proceses including ore deposits, the origin and evolution of sulfur species in natural waters (including present-day and ancient marine), fractionation in bacterialy influenced proceses, and acid mine drainage (Marini et al., 2011). Sulfur isotope values are reported as per mil (?) relative to Canyon Diablo Troilite (CDT) or to Vienna Canyon Diablo Troilite (VCDT), a fixed standard relative to CDT. Meteoritic sulfur (generaly regarded as approximate bulk composition of Earth?s primordial sulfur, relative to CDT or VCDT) and seawater are the most commonly used reference ! 103 reservoirs for sulfur isotopes in terrestrial systems. Geochemical conditions influence how sulfur partitions in a system. Relative to the parent material, oxidation produces resultants enriched in 34 S, while reduction produces species depleted in 34 S (Seal, 2006). Sakai (1968) first suggested that sulfur isotope fractionation in minerals could be controlled by the geochemistry of the ore-forming fluids. Marini et al. (2011), compiled extensive data that shows sulfur isotope ranges from a variety of sources (Fig. 50). Mantle sulfur isotope composition, conventionaly considered to be 0 ? 2? (Thode et al., 1961), could be heterogeneous as evidenced by the ranges for mantle xenoliths, mid ocean ridge basalt (MORB) sulfides, and ocean island basalt (OIB) sulfides (Fig. 51). Despite the evidence supporting sulfur isotope heterogeneity, values for magmatic sulfur do cluster around 0? (Seal, 2006). Rye and Ohmoto (1974) state, ?Igneous sulfur is derived from the upper mantle or from the homogenization of large volumes of deeply buried or subducted materials.? Sulfur isotope studies on sulfur-bearing minerals in hydrothermal ore deposits yield a wide range of ? 34 S values (Fig. 51), depending on the degree of fractionation and the isotopic signature of the original source of sulfur. Many porphyry deposits typicaly show a strong ? 34 S signature of around 0? (Fig. 53), however porphyry deposits sulfur isotope values can range betwen ?6 and +4? (Marini et al., 2001). Hydrothermal ore deposits range from ~ ?30? (Fig. 51) but show an overal signature clustering around 0? ? 34 S. Sulfur analyses on samples from Alard Tunnel and Copper Hil (Fig. 52) range from -7.7 to +0.3? ? 34 S VCDT , with an average of -5.2? ? 34 S VCDT and a mode of - 6.0 ? 34 S VCDT . Therefore the Alard stock sulfur isotope range is comparable to that of most typical porphyry copper deposits (Fig. 52). ! 104 Fig. 50. Plot showing the range of sulfur isotope values for sulfides from eteorites, mantle xenoliths, diamonds, igneous rocks, and modern sediments with Alard stock data overlayed in red. Figure from Marini et al. (201). ! 105 Fig. 51. Plot showing the sulfur isotope range for various hydrothermal ore deposits, showing ? 34 S for sulfides and sulfates with Alard stock data overlayed in red. Figure from Rye and Ohmoto, 1974; ! 106 Fig. 52. Histogram showing data from Allard Tunel samples and from Coper Hill samples with respect to their sulfur isotope values (relative to VCDT, per mil). Figure 52 above shows the sulfur isotope data from both Alard Tunnel and Copper Hil. Sulfur analyses for the Alard Stock (n=14) appear to be enriched in the lighter isotope, 32 S relative to the heavier isotope, 34 S. As evidenced by the similarities to many porphyry copper deposits shown above, sulfur at the Alard Stock is apparently predominantly magmatic in origin, but apparently include some contribution of 32 S- enriched sedimentary sulfur from the strata intruded by the porphyry, potentialy from the Cutler formation. No sulfate minerals are known to exist within the mineralization at the Alard Stock and were therefore not analyzed. A more robust study including sulfur analyses is needed to definitively determine the origin of sedimentary sulfur, and to beter constrain the sulfur isotopic signature of this alkalic porphyry. 0 1 2 3 4 5 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 F r e q u e n c y ? 34 S Allard Stock ? 34 S Values CH AT ! 107 Fig. 53. Schematic plot of some typical copper porphyry sulfur isotope data with Allard stock data overlayed in red.. Modified from Barnes (197). Sulfur isotope fractionation arises from temperature diferences, or most importantly, from oxidation and reduction reactions acting on the sulfur species. Other aqueous sulfur species may become important during ore deposition (HSO 4 - , KSO 4 - , NaSO 4 - , and HS - ) but the major factors controlling sulfur isotope composition for hydrothermal minerals are (1) temperature, determining interplay of the sulfur-bearing species? fractionations, (2) total ? 34 S in the source and (3) amount of oxidized vs. reduced species in the ore forming solutions (Rye and Ohmoto, 1974). Oxygen fugacity and pH also induce variations ? 34 S in hydrothermal minerals (Rye and Ohmoto, 1974). Under igneous system conditions, at high temperature, redox reactions tend to occur under equilibrium conditions, whereas disequilibrium conditions are present under low temperature conditions (Seal, 2006). Galore Creek, BC Valley Copper, BC Craigmont, BC Butte, MT Bingham, UT Tintic, UT Marysdale, UT Chino, NM Ajo, AZ Sierrita, AZ Twin Butte, AZ Morocacha, Peru El Salvador, Chile Sulfides Sulfates 0 +20-20 ? 34 S, ? ! 108 Fig. 54. Plot of porphyry copper (-gold) asociated sulfide and sulfate ranges (? 34 S per mil). Some granitic rocks also included. Allard stock data is shown in red. Figure from Wilson et al. (207). Wilson et al. (2004) ilustrates ranges of ? 34 S sulfide values determined for minerals asociated with porphyry copper (-gold) deposits and with granitic rocks (Fig. 54). Based on this histogram, mean ? 34 S sulfide is -0.96?, while the range for sulfides asociated with porphyry copper deposits is ?4.20 to +2.27?, within one standard deviation of the mean. This generalized range does not take into acount the individual ranges of sulfur values for each deposit or the sulfate analyses. Again, sulfide data cluster around 0? for the most of the porphyry deposits in the United States, Canadian ! 109 Cordilera, South American Cordilera, and the Philippines. Deposits in Australia show enrichment in 34 S, as does one Canadian deposit, Galore Creek, which shares similarities with the Alard Stock (Werle et al., 1984; Jensen and Barton, 2000; Wilson et al., 2004). Note that unaltered igneous rocks samples (granites) from Japan and Australia plot betwen ? 10?, supporting the idea that 34 S for unmineralized rocks of mantle origin can range outside of exactly 0? if crustal contamination occurs. The connection betwen porphyry and epithermal environments could perhaps be further strengthened by comparing the stable and radiogenic isotopes of both deposit types. In particular, it is especialy useful where both environments overlap, such as in the La Plata district. Although not measured as part of this study, sulfur isotope studies on epithermal (Au-Ag-Te) deposits can yield somewhat similar ? 34 S values. Takahashi et al. (2005) report the sulfur isotopic range for hydrothermal deposits in Kamchatka, Russia (Fig. 55). Averaged ? 34 S CDT values indicate a mostly positive signature of -0.7 to +3.8? (av. +1.7?) for East Kamchatka and ?1.8 to +2.0? (av. -0.1?) for Central Kamchatka. Negative values for Central Koryak (av. -2.8?) are atributed to slight contamination by isotopicaly light ( 32 S-enriched) sedimentary sulfur from surrounding country rocks. Takahashi et al. (2005) report isotopic data that is consistent with data from neighboring metalogenic belts. It has been suggested that perhaps the addition of an external sulfur source (sedimentary sulfur) is necesary to make economic PGE-Ni- Cu ores (Lesher and Keays, 2002). ! 110 Fig. 5. Histogram of ? 34 S CDT values for the Cenozoic ore deposits in metalogenic belts in Kamchatka, Rusia (Takahaski et al., 205). Allard stock data is shown in red. Data for low sulfidation epithermal systems are somewhat limited, however there are good constraints on high sulfidation epithermal systems (Seal, 2006). Seal (2006) notes that sulfur geochemistry of epithermal deposits can be identified using ? 34 S values of ! 111 sulfide/sulfate mineral pairs of pyrite, pyrrhotite, chalcopyrite, anhydrite, alunite, barite, and sulfate-bearing apatite (Fig. 56). These epithermal deposits show a wider range of ? 34 S compositions compared to typical porphyries, due to the lower temperature conditions needed to form epithermal ores. This is consistent with an inferred genetic relationship to intrusions (and porphyries) and equilibrium conditions are asumed (Seal, 2006). Fig. 56. Plot of ? 34 S sulfide vs ? 34 S sulfate values for epithermal hydrothermal systems from Seal, 206. Included are the 0? line, indicated by the red arrow, the typical porphyry field, shown by the croshatched area, and the dashed field for epithermal ores. Values in per mil (VCDT). Note the clustering around 0? and the overlap between porphyry and epithermal fields. Shikazono (1995) plots the ? 34 S values of Japanese Ag-Au vein-type epithermal deposits. Green tuff-type deposits occur in a submarine volcanic region, while non-green ! 112 tuff-type deposits occur in a subaerial volcanic region (Fig. 57). Both subsets show clustering around 0?, however there is also evidence for the asimilation of both biogenic sulfate ( 32 S enriched) and seawater sulfate ( 34 S enriched) into the epithermal systems. Fig. 57. Histograms of epithermal Ag-Au sulfur data originaly obtained from Ishihara et al. (1986), Shikazono (1987), Hedenquist et al. (1994), Shikazono an Shimizu (1993), Ishihara and Sasaki (1994), Shimizu et al. (195), and Nakata (unpublished). Figure is from Shikazono (195). Allard stock data shown in red. ! 113 The best data available for sulfur isotopes on epithermal ores in the western US come from studies on the Mule Canyon deposit in Nevada. John et al. (2003) discuss the geochemistry, stable isotope, and fluid inclusion results for the Mule Canyon low sulfidation epithermal Au-Ag deposit. Sulfur isotope data for the Mule Canyon deposit (Fig. 58) show a range of ? 34 S with a strong trend betwen 0 and +5?. Fig. 58. Histograms showing Mule Canyon sulfur isotope data, primarily on pyrite, marcasite, and arsenopyrite. Allard stock sulfide data is shown in red. White = open-space-filing sulfides, gray = replacement sulfides, patern ? mixtures, and black = igneous pyrhotite; cpy-tet = chalcopyrite/tetrahedrite mixture, Ign = igneous (melt inclusions), and stib = stibnite. Figure from John et al. (203). Sulfide ? 34 S ranges for epithermal Au-Ag deposits of the Hauraki Goldfield, New Zealend, fal betwen -3.1 to +4.9? (n=399 analyses) and sulfur is suggested to be of magmatic origin. The ? 34 S H 2 S in solution was likely similar to the ? 34 S of the sulfide ! 114 minerals present at Hauraki (Christie and Robinson, 1992; Christie et al., 2007). Averaged ranges for sulfide minerals include chalcopyrite (2.0 ? 0.8 per mil), sphalerite (2.0 ? 1.1 per mil), pyrite (2.1 ? 1.5 per mil) and galena (0.3 ? 1.0 per mil) (Christie et al., 2007). Fig. 59. Plot of ulfur isotope fractionations, considering sulfur species species and hydrothermal minerals, all ploted with respect to pyrite. Solid lines indicate minerals and dashed lines indicate species in solution (Rye and Ohmoto, 1974). ! 115 Ohmoto and Rye (1974) compiled sulfur isotope fractionation curves (Fig. 59) to model how sulfur species fractionate in minerals and waters with change in temperature. Fractionation in the high temperature porphyry environment (<700?C to <250?C) remains relatively negligible for pyrite, chalcopyrite, sphalerite and pyrrhotite. Due to this lack of considerable fractionation in ? 34 S betwen the aqueous species and the mineral, pyrite and chalcopyrite are suitable mineral species for acurate ? 34 S analyses and the result is more or les the same as the source sulfur reservoir. If temperature is known (via fluid inclusion studies) the curves (Fig. 52) can be used to determine the ? 34 S fluid composition from the ? 34 S mineral composition. Both porphyry and epithermal ore deposits may show sulfide signatures indicative of magmatic origins, as evidenced above. Alard Stock Copper Isotopes in Relation to Other Porphyry Copper Deposits Copper isotope geochemistry can be useful in constraining the source(s) of metals in hydrothermal ore deposits, however the interpretation of Cu isotope data in relation to metals in ore deposits is stil being developed. Studies using copper isotopes to beter understand high and low temperature aqueous proceses have recently been published in the literature (Kimbal et al. 2009; Mathur et al. 2005, 2009, 2010; Mathur and Schilt 2010; Asael et al., 2007, 2009; Haest et al., 2009, Larson et al., 2003; Maher and Larson 2007; Rouxel et al., 2004; Li et al., 2010, Palacios et al., 2001). Li et al. (2010) compiled copper isotope data from several types rocks, sediments, and ore deposits including skarns, prophyries, and masive sulfides (Fig. 60). ! 116 Fig. 60. Histograms showing a compilation of ? 65 Cu isotope data from a variety of sources (Li et al. 2010). Original data sources listed in Li et al. (2010). Unfiled bars represent oxidized ores in porphyry deposits reported by Mathur et al. (209), however some extreme values from this study are not plotted due to scale restrictions. Allard stock Cu data is shown in red. Based on the above histogram, ? 65 Cu data for porphyries, skarns, masive sulfides, and other hydrothermal deposits tends to cluster around 0?, with a general range of ? 3 ? (Fig. 60). Oxidized copper in solution appears to be isotopicaly heavier than the mineral it is derived from via low temperature reactions inducing the disolution of copper- bearing sulfides, namely chalcopyrite and chalcocite (Mathur et al., 2005; Seo et al., ! 117 2007; Borrok et al., 2008; Pokrovsky et al., 2008; Kimbal et al., 2009; Mathur et al., 2009). Oxidation of Cu-rich minerals should therefore produce isotopicaly lighter signatures. Supergene zones are isotopicaly heavier, and leached zones are isotopicaly lighter than the hypogene zone in a given porphyry system (Fig. 61). Copper minerals precipitated by high temperature fluids show much lower fractionation than copper minerals precipitated by low temperature fluids (Mathur et al., 2012). The ? 65 Cu patern observed by Mathur et al. (2012) is that the ? 65 Cu values of the leach cap minerals > hypogene minerals > enrichment minerals. Fig. 61. Generalized profile of the typical porphyry coper stratigraphy, mineralogy, and ? 65 Cu of leached, hypogene, and supergene reservoirs in a typical porphyry deposit. Figure from Mirnejad et al. (2010) with data from Mathur et al. (209). This patern most likely reflects the continued weathering proceses due to uplift proceses where shalower portions are exposed to greater degrees of weathering whereas the deeper portions of the porphyry system are not exposed so such a great degree of oxidative weathering. The disolution of Cu-rich minerals leaves depleted ? 65 Cu residues (Fig. 62), and as the shalower part of the system weathers, the isotopic ! 118 composition of the residue wil also become progresively depleted (Mathur et al., 2012). Fig. 62. Histogram of porphyry copper minerals ? 65 Cu composition. Histogram from Mathur et al. (2012). Data taken from Mathur et al. (2005, 2010) and Zhu et al. (2000). Allard stock data is shown in red. Mathur et al. (2012) state, ?Copper migration and sequestration can be controlled by climate, pyrite/chalcopyrite ratio (ability to create sulfuric acid), fracture abundance (secondary permeability), metalic surfaces for copper to precipitate on, uplift, erosion, preservation, solubility/precipitation of minerals, and preservation of enrichment blankets.? Cu isotope fractionation in high temperature hydrothermal systems may be the result of variation in ? 65 Cu compositions in source materials, or Cu fractionation during ! 119 the ore-forming proces (mobilization, transport, and deposition), or both (Li et al., 2010). Copper preferentialy partitions into the vapor phase rather than into brine (Heinrich et al., 1999; Williams-Jones and Heinrich, 2005). Simon et al. (2006) reported that this partitioning is promoted by low sulfur. Li et al. (2010) plot ? 65 Cu against ? 34 S for sample pairs from the Northparkes porphyry deposit (Australia), however no systematic correlation is evident (Fig. 73). They ascribe this to the difering chemical behavior of Cu and S in porphyry systems, where SO 2 and H 2 S exhibit a strong equilibrium isotopic fractionation. Copper fractionates betwen two species only in oxidized, low-temperature hydrothermal setings, and only one main species occurs at high temperatures (Li et al. 2010). Rouxel et al. (2004) found similar results concerning the relationship betwen copper and sulfur isotopes in hydrothermal environments. Fig. 63. Plots of ? 65 Cu versus ? 34 S plot from two ore bodies from Northparkes, Australia (Li et al. 2010). No systematic relationship between Cu and S was determined from their study. In their study of two porphyry copper deposits, Mount Pinatubo (Philipines) and Bingham Canyon (Utah, USA), Hatori and Keith (2001) compared major Cu porphyry ! 120 deposits? ? 34 S values with their respective copper grades (milion tonnes, Mt). The prospective range for the Alard Stock is denoted with a red box (Fig. 63). This placement is based on sulfur isotope values obtained by this study and the Alard Stock?s estimated resource. The USGS porphyry copper model cites that the Alard Stock has 200 Mt at 0.4% copper and geochemical data from this study shows that the Alard Stock has <1% copper. Both tonnages are considered for the figure below in order to place the Alard Stock in perspective relative to other porphyry copper deposits. Fig. 64. Plot of ? 34 S of sulfide minerals versus coper grade (milion metric tones) of porphyry coper deposits (Hatori and Keith, 2001). Deposit abbreviations are as follows: Bg=Bingham, Utah, Bt=Butte, Montana, El-Sv = El Salvador, Chile, Ch=Chino, New Mexico, Yr=Yerington, Nevada, Bis=Bisbe, Arizona, Lp=Le Panto Far Southeast, Philipines, Aj=Ajo, Arizona, Sg=Sungun, Iran, Fr=Frieda River in Papua New Guinea, VC=Valey Coper, British Columbia, Pang=Pangua, Papua New Guinea, Tn=Tintic, Utah, GC=Galore Crek, British Columbia, GM=Globe-Miami, Arizona, Cm=Craigmont, British Columbia, GSs, Gasp? Coper, Qu?bec, CV= Cero Verde-Santa Rosa, Peru, MP=Mineral Park, Arizona, Sk=Skouries, Grece, Hb=Hilsboro, New Mexico, GS=Golden Sunlight, Montana. Solid squares are averages and bars represent sample ranges. Individual deposit data sources listed in Hattori and Keith, (201). ! 121 A ? 34 S versus ? 65 Cu graph, showing Alard stock samples and samples from Northern Great Basin epithermal ores (NGBs), is presented below. The graph (Fig. 65) shows no direct correlation betwen sulfur and copper isotope values, however both copper and sulfur values for porphyry and epithermal samples plot in or very near a ?magmatic source? field. This further adds to supporting evidence that both copper and sulfur show a significant mantle signature, and that both metals and sulfur in hydrothermal fluids are derived from the mantle. Fig. 65 Plot of ? 34 S and ? 65 Cu values for samples from the Alard stock porphyry along with epithermal samples from the Northern Great Basin (NGB). Samples were analyzed for sulfur isotopes at either a comercial lab (NGB-1) or at UGA?s Stable Isotope Lab (NGB-2), and splits of the same samples were analyzed for coper isotopes. NGB ores include Buckskin National, NV; Midas, NV, Dewey, Trade Dolar, Idaho Tunel, ID. Gray field indicates the range for a completely mantle sourced Cu and S magmatic signature. NGB epitheram samples are from MS thesis research by Michael Mason, Auburn University, unpublished. ! "10! "8! "6! "4! "2! 0! 2! 4! 6! 8! 10! "10! "8! "6! "4! "2! 0! 2! 4! 6! 8! 10! ? 43 S (?) ? 65 Cu (?) NGB Epithermal -1 NGB Epithermal - 2 Allard Stock ! 122 Sources of Pb in the La Plata District Ores Lead isotopes can be used as radiogenic tracers for sources of lead in both porphyry and epithermal deposits (and other Pb-bearing deposits). U, Th, and Pb strongly partition into the upper and lower portions of the crust (Zartman and Haines, 1988), and radiogenic lead isotopes 206 Pb, 207 Pb, 208 Pb arise from complex decay chains that begin at 238 U, 235 U, and 232 U, respectively. Half-lives vary but al are isotopes are reported relative to 204 Pb, the only non-radiogenic stable lead isotope. Doe and Zartman (1979) and Zartman and Doe (1981) originaly provided the plumbotectonic model for variations in average lead isotopic compositions within diferent tectonic environments. This model, revised in Zartman and Haines (1988), hinges on the idea that U, Th, and Pb are extremely sensitive to bidirectional transport among reservoirs, due to the abundance of these elements in the crust relative to the mantle. By analyzing the lead isotope values for ore deposits, the source of lead in the system, whether from the crust, mantle, or sediments, or perhaps mixtures, may be determined. Generaly, lead isotope values for the Alard stock show a ?primitive? mantle, les radiogenic signature. Data plot within the general area and trend for Kely and Ludington (2002) show for the Colorado Plateau volcanics, the COMB, and the San Juan Volcanic Field (Fig. 66). Kely and Ludington (2002) published data on alkaline gold related deposits (particularly Cripple Creek). In their study, lead isotope analyses for samples from an asortment of COMB deposits show a les radiogenic, more mantle like Pb signature. Data from the Alard stock plot along the same trend as the other alkaline Au-Te teluride districts (i.e. Boulder County) in the COMB. However several ! 123 samples lie outside of the scale of Kely and Ludington?s (2002) plot, but the trend is maintained. 206 Pb/ 204 Pb and 207 Pb/ 204 Pb compositions of al the alkaline rocks (Fig. 66) are consistent with a mantle Pb source (Zartman and Haines, 1988; Kely and Ludington 2002). Lower crust input into the evolution of rocks in (plotting above the SK reference line) the northern part of the region could have been significant, as evidenced by the 208 Pb/ 204 Pb (Stein, 1985; Keley et al., 1988). Post-subduction generated alkaline magmas could have likely asimilated with lower crustal material (Keley and Ludington, 2002). Alard stock values plot below this line and show les crustal contamination than the rocks in the northern part of the COMB. La Plata district data fal in the general range of the San Juan Volcanics, roughly 56 km NW of the La Plata Mountains (Fig 1). Relative to the 1.7 Ga reference isochron, al Alard stock lead isotope values appear to be rather unradiogenic and and fal along the general trend line, not above, as would be indicative of extensive crustal contamination of lead (Fig. 67). Plotting along the 1.7 Ga isochron indicates that source rock formation likely occurred during the formation of the Proterozoic Yavaipi Orogenic events. If crustal contamination did play a role in the tectonomagmatic and hydrothermal proceses that created the Alard stock, it is minor, relative to magmatic lead input, but could be likely due to the overabundance of lead in crustal rocks. As is the case with the sulfur isotope values of the Alard stock, some crustal or wal rock contamination is evident, but is negligible relative to the overal mantle signature. ! 124 Fig. 66: Plots of lead isotope compositions ( 206 Pb/ 204 Pb versus 208 Pb/ 204 Pb, and 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb). Alard stock data indicated by the bold triangles. Plot modified from Keley and Ludington (202). Closed circles from the COMB are intrusions spatially asociated with Au-Te mineralization in the Central City (Eldora Stock) and Boulder County district (Jamestown Porphyry). The SK line coresponds to the Stacey and Kramers (1975) growth curve, where tick marks show time in 100-milion year intervals. ! 125 Fi g . 6 7 . Le a d i s o t o p e c o m p o s i t i o n g r a p h s w i t h Al l a r d s t o c k d a t a (b l a c k t ri a n g l e s ) su p e r i m p o se d o n a g r a p h f r o m B o u c h e t e t a l ( 2 0 1 4 ) . D a t a p l o t a l o n g t h e 1 . 7 Ga r e f e r e n c e i s o c h r o n a n d s h o w a r e l a t i v e l y u n r a d i o g e n i c s i g n a t ur e , l i ke l y t ha t of m a nt l e de r i ve d l e a d. 126 Four epithermal samples (2 from Besie G and 2 from Cumberland) were analyzed for lead isotopes for comparison to the Alard stock porphyry Pb isotope values. While the porphyry values are les radiogenic, epithermal Pb values are more radiogenic and fal along a trend line relative to the porphyry samples (Fig. 68). Two scenarios are possible to explain this trend line for the La Plata district Pb isotopes. (1) The 1740?170 Ma calculated trend line is a real age for the Pb in the ores, meaning that the hydrothermal fluids that precipitated the mineralization in the La Plata district originaly gathered the Pb from 1.7 By old basement rocks and recorded the basement Pb signature in the ores. (2) The ?age? given is a coincidence and the trend line is simply a mixing line betwen an unradiogenic mantle Pb source (lower Pb ratios) and a more radiogenic crustal Pb source (higher Pb ratios). Both instances are plausible explanations for the lead isotope signature of the La Plata district. Fig. 68. Plot of lead isotope data for the La Plata district. Alard stock samples circled in red and epithermal samples circled in blue. Note slope diference and trend line. 127 5. Conclusions 1). Geochemical data from this study support a genetic link betwen porphyry and epithermal deposits in the La Plata mining district and betwen the hydrothermal fluids that deposited them, including Ag ? Au ? Cu ? Te ? Pt ? Pd. 2). Sulfur isotope data from this study yield a range of ? 34 SVCDT betwen -7.0 to +1.0?, indicating a primarily magmatic source mixed with of 32 S enriched sulfur, likely from sediments the Alard stock intruded. 3). Copper isotope analyses also indicate a magmatic source of copper for the porphyry copper mineralization at the Alard stock. ? 65 Cu values range from 0.956 to 2.7?. 4). Samples of mineralization analyzed by two diferent techniques from the Copper Hil Glory Hole show several high values of > 1 ppm Pt and Pt. Most samples show >1.0% copper, 2.5 to 100 ppm Ag, and 0.1 to 1.4 ppm Au. 5). PGEs have been reported in the Alard stock as early as 1943, however no published analysis of phase occurrence exists. Using microprobe analyses based on selection by geochemical Pt and Pd compositions, Pt and Pd in the Alard stock occur in at least four phases: merenskyite, moncheite, sopcheite, and an unnamed Pd-Te 128 6). The possible new paladium teluride mineral has a formula of PdTe 2 . 7). Lead isotope values indicate a mostly mantle Pb source, and Alard data follow the trend of much of the rest of the Colorado Mineral Belt and have the same lead values as the San Juan Volcanic Field. 8). The La Plata district fits the Saunders and Brueske (2012) model for Se, Te enrichment of the Western U.S. due to flat slab subduction. The La Plata district, abundant in Te, plots in the Te-enriched region on their proposed map. 9). General ore paragenesis from earliest to latest is as follows: pyrite, bornite, covelite, sphalerite, magnetite, acanthite, chalcopyrite, and Pt?Pd?Bi-telurides. 10). High bismuth and telurium content in both porphyry and epithermal deposits in the La Plata district further indicate a magmatic source for precious metals. 11). Further research in the district is needed to beter characterize the geochemical signatures of the porphyry and epithermal ores. More sulfur, copper, and lead isotope analyses would help to verify the results of this study and make the Pb isotope interpretations for the area les ambiguous. More detailed microprobe analysis would ensure that al occurring PGE minerals are identified. 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