Elemental Sulfur (S°) Deposits and S° Associated with Precious Metals, Mercury, and Thermal Springs in the Great Basin

ABSTRACT
Minor to minable amounts of elemental sulfur (S¡), sometimes
associated with Au, Ag, Cu, and Hg, occur in the Walker Lane structural
zone, in northwestern Nevada, and in southwestern Utah in
three environments: (1a) vuggy-silica sulfide rocks, (1b) stratiform
deposits, and, (2) thermal springs and fault zones. The first two environments
are found in highly altered Miocene or younger volcanic
and sedimentary rocks; they are parts of, or genetically related to,
precious metal deposits of the Òhigh sulfidationÓ type. Vuggy-silica
sulfide rocks and stratiform S¡ deposits are situated on the western
and eastern margins of the Great Basin; all vuggy silica-sulfide S¡
occurrences are in the Walker Lane structural zone of Nevada and
California. Most thermal springs and fault zone S¡ occurrences odf
the third environment are clustered in an area of northwest Nevada
characterized by active faults, thin crust and elevated heat flow.
S¡ in all three environments is paragenetically late, encrusting
surfaces and filling fractures, breccia matrices, leached phenocryst
sites, and intergranular pores. Physical properties and textures of S¡
partially constrain depositional temperatures to <160¡C. In vuggy silica-sulfide rock and stratiform S¡ deposits processes of sublimation and precipitation occured, as S¡ abruptly polymerizes to high viscosities at 160¡C and melts at <119¡C. S¡ in thermal springs sublimed or precipitated at <119¡C temperatures, in part through biologically mediated processes. Isotope and selected minor element abundances in Great Basin S¡ deposits distinguish between deposit types. S¡ in vuggy silicasulfide rocks and some stratiform deposits often coexists with copper- arsenic sulfosalts, pyrite, and alunite, and contains concentrations of selenium, tellurium, arsenic, antimony and copper that are several to several hundred times assumed magmatic abundances. It is weakly to moderately depleted (relative to Ca–on Diablo Troilite) in 34S (-0.4 to -18.4ä) as are associated enargite-luzonite (-3.5 to - 11.8ä) and pyrite (-2.4 to -18.9ä), suggesting common sulfur sources. Following depression of static groundwater saturation levels, vuggy silica-sulfide rock S¡ sublimed or precipitated from magmatically derived, partially oxidized, isotopically light H2S, with atmospheric oxygen as the oxidant. Less isotopically depleted vuggy silica-sulfide rock S¡, with isotopic compositions near zero permil, was derived from a H2S-rich magmatic fluid in which SO2 reactions were insignificant. Stratiform deposits S¡ both sublimed and precipitated, in the presence of ferrous iron in lake sediments, from partially oxidized, isotopically light H2S. The isotopically light H2S was residual in an isotopically heavy, SO2-rich fluid from which alunite and other sulfates had earlier precipitated as a result of SO2 disproportionation. d34SS¡ in most thermal springs and in fault zones ranges from near zero to moderately positive (-1.4 to 17.1ä) and contains very low minor element abundances. S¡ in low grade gold deposits associated with thermal springs has isotopic compositions (d34S = -13.4 to +10.7ä) and chemical compositions intermediate to those of the other two environments. S¡ in some long-lived systems in rangemargin fault zones could be derived from later fluids unrelated to gold mineralization. Minor element and isotope abundances in Great Basin S¡ occurrences are controlled by magmatic H2S/SO2, as well as by depth and proximity to a thermal source which, for thermal springs S¡, may be a high geothermal gradient and for the other two environments, magma. Depths of S¡ deposition depend on depth to water-saturated rocks of degassing magma, and range from the surface to hundreds of feet in thermal springs, from tens to hundreds of feet in stratiform deposits and from hundreds to more than a thousand feet in vuggy silica-sulfide rock occurrences.