Mineral Chemistry and Sulfur Isotope Geochemistry from Tonalite-Hosted, Gold-Bearing Quartz Veins at Hog Mountain, Southwestern Appalachians: Implications for Gold Precipitation Mechanism, Sulfur Source, and Genesis

Abstract

AbstractMineralized quartz veins within Phanerozoic orogenic belts provide important insights into fluid source and transport, source(s) of S, and genetic implications for ore deposit formation. Here, investigations on mineralized veins hosted by the Hog Mountain tonalite, southernmost Appalachians, were performed using scanned electron microscopy, microprobe analysis, and secondary ion mass spectrometry.The small, reduced Hog Mountain tonalite hosts three different mineralized vein types dominated by Au-bearing quartz veins with a simple base metal sulfide assemblage and spatially associated Au phases (electrum > native gold > maldonite) with Bi phases (native bismuth > unnamed Bi3Te ≈ hedleyite > bismuthinite). In contrast, barren to low-grade arsenopyrite and sphalerite-pyrite-arsenopyrite-galena-chalcopyrite (Zn-Fe-As-Pb-Cu) veinlets are rare. Base metal sulfides are stoichiometrically homogeneous with the exception of sphalerite and galena that have noticeable enrichments in Fe and Cd, and Bi and Ag, respectively. Trace element concentrations (Au, Ag, Bi, Te, Se, and transition metals) in major and minor base metal sulfides are generally low. In situ sulfur isotope analyses on base metal sulfides from different vein types show two distinct populations: (1) δ34Ssulfide = 12.9 ± 1.4‰ in low-grade arsenopyrite and Zn-Fe-As-Pb-Cu veinlets and (2) δ34Ssulfide = 7.7 ± 0.9‰ in mineralized quartz veins.Based on mineral chemistry, two fluid phases were responsible for metal deposition at Hog Mountain. Fluid phase I with low-soluble Au formed rare, barren to low-grade arsenopyrite and Zn-Fe-As-Pb-Cu veinlets from highly acidic, relatively high temperature fluid with moderate ƒS2 and variable ƒO2. In contrast, a reduced, near-neutral, low ƒS2 fluid phase II had higher soluble Au concentrations and created the dominant Au-bearing quartz veins in which Au was deposited via scavenging by Bi melts syngenetic at lower temperatures. Isotopic modeling shows that both fluid phases sourced their S from the metasedimentary rocks hosting the tonalite and adjacent, coeval gold deposits, with the tonalite contributing some S to fluid phase II, resulting in lower δ34S values. Our results are consistent with mineral assemblage, mineral chemistry, and sulfur source(s) from other intrusion-hosted Phanerozoic orogenic gold deposits and support the syngenetic Au scavenging model by Bi melts as a viable Au deposition process in orogenic gold deposits.