Stratigraphy, Ore Mineralogy, Geochemistry, and Genesis of Gold-Bearing Quartz Veins at the Hog Mountain Tonalite, Southwestern Appalachians, USA

Abstract

Hog Mountain of the southern Appalachians is a gold deposit in the eastern Blue Ridge of Alabama. The mineralized quartz veins found there have an average Au grade of 0.14 oz/t (maximum: 2.4 oz/t Au) and occur within a tonalite that intruded into the moderately metamorphosed phyllites of the Wedowee Group during the Neoacadian orogeny. Vein formation and mineralization is epigenetic and took place during the Pennsylvanian-Permian Alleghanian orogeny. This study employed a multi-faceted observational and geochemical approach to understanding the genesis of Hog Mountain; core logging, microscopy (reflected light, scanning electron microscope), and electron microprobe analysis provided important textural and compositional data, whereas sulfur isotopic analysis allowed for source fingerprinting of the ore-forming fluids. The quartz ± plagioclase ± calcite veins at Hog Mountain have an ore assemblage dominated by pyrrhotite with minor to trace pyrite and trace arsenopyrite – chalcopyrite ± sphalerite – Bi-Te-Au±S phases. Gold mineralization is spatially related to Bi±Te±S phases and it occurs dominantly as native gold and electrum and rare maldonite. Electron microprobe analyses show a wide range of Bi (80-509 ppm), Au (97-627 ppm), and Co (33-1130 ppm) within base metal sulfides. Native bismuth and Bi-chalcogenides have relatively elevated concentrations of Ag, Sb, Pb, S, and Se up to several 1000 ppm due to preferred substitution of these elements for Bi and Te. Electrum has an Au/Ag ratio of 10.4. Sulfur isotopic compositions of sulfides emerge as two populations: Group 1, δ34Ssulfide = 5.2 to 9.0 ‰, average: 7.7 ± 0.9 ‰; Group 2, δ34Ssulfide = 11.6 to 15.2 ‰, average: 12.9 ± 1.4 ‰. The sulfides in each group differ in occurrence and texture; Group 1 represents the dominant ore assemblage and main stage of mineralization, whereas Group 2 sulfides were late stage. The ore mineralogy at Hog Mountain is consistent with reduced, nearly neutral, relatively H2S- and CO2-rich, low saline fluids with relatively high ƒTe2 that allowed telluride precipitation. Fluid temperatures ranged between 400-200 ºC. Sulfur and metals originated from the surrounding metasedimentary rocks and variations in δ34S reflect changes in the redox state of the hydrothermal fluid. Interaction with tonalite or mixing with meteoric water is excluded as S and metal sources. Gold precipitation occurred via Au scavenging by Bi(±Te) melts that formed directly from the reduced hydrothermal fluid at temperatures > 271 ºC due to the breakdown of Bi2S2OH-complexes. Molten droplets of Bi±Te±Au±S were remobilized during syngenetic vein and sulfide crystallization. With these new data, this study confidently classifies Hog Mountain as an orogenic gold deposit rather than a reduced intrusion-related gold deposit despite overlapping characteristics (e.g., hosted by a reduced tonalite). Importantly, no metal zoning occurs around the veins, veins are younger than the intrusion, and they display no distinct changes in ore grade. Establishing this genetic model for Hog Mountain impacts on-going and future exploration in the southernmost Appalachians.