A field and geochemical investigation of tourmaline occurrences around the Hog Mountain gold deposit, AL

Abstract

Tourmaline can be a powerful mineral as its properties allow it to preserve information that can be used to uncover its geologic history. Abundant, previously undocumented occurrences of tourmaline have been discovered in the Wedowee Group adjacent to the Hog Mountain orogenic gold deposit in Tallapoosa County, Alabama. Field work revealed four distinct groups: pegmatitic, metamorphic, quartz-hosted, and aggregate, which were compared to tourmaline within a Hog Mountain tonalite (hmt) quartz vein within the gold deposit. Petrography, geochemical analyses, and microscopic imaging were used to determine the mineralogy, textures, elemental concentrations, and compositional zoning patterns of 21 samples. Trace element analyses revealed distinct compositions of each group; the pegmatitic tourmaline is relatively enriched in Pb, metamorphic has the highest Li concentrations, aggregate tourmaline is relatively enriched in Ce, and tourmaline from the hmt quartz vein contains high Sr, Sn, and Pb. Elemental maps show that zoning reflects changes in Fe, Mg, Ti, and occasionally other elements, indicating changes in fluid composition during mineral growth or diffusion during metamorphism. These findings demonstrate that the tourmaline in the Hog Mountain area has great potential as a geochemical indicator and geochronological target. They suggest that further analysis of the tourmaline and their host rocks in and around the orogenic gold deposit could refine the geologic history of Hog Mountain.