SE-RICH PRECIOUS METAL DEPOSITS AND THE SIGNIFICANCE OF SE IN SINTER, NORTHERN GREAT BASIN

Many precious metal vein and stockwork deposits in Miocene volcanic rocks in northern Great Basin-SW Idaho mining districts, including National, Midas, Sleeper, Ivanhoe, Jarbidge, Delamar, Deep Hole (Mountain View), and Argenta (Mule Canyon), contain dominant to subordinate Ag selenide minerals (naumannite, aguilarite), Hg-Pb-Sb-Cu selenide minerals (tiemannite, clausthalite, antimonselite, berzelianite), and Se. Deposits of known age formed c.16-14.5 Ma, coinciding with the onset of bimodal (basalt-rhyolite) volcanism and development of fluvial-lacustrine systems in Miocene volcanic fields. Sinter or near-surface hydrothermal mineral assemblages remain in most districts.

Aqueous selenide species (H₂Se; HS^-) in 300 to 100° C hydrothermal fluid are stable to higher fO₂ than aqueous sulfide species, and they can coexist with aqueous sulfate species, alunite, and hematite. Thus, selenide minerals may occur with sulfate and oxide minerals in relatively oxidized thermal spring deposits in which sulfide minerals other than cinnabar are unstable. On Buckskin Mountain (National district), sinter and volcaniclastic deposits contain Hg-Se-S-Cl phases, hematite, and elevated cospatial abundances of Ag, Au, Sb, Se, S and other elements in sub-microscopic forms, as determined by LA ICP-MS. These element associations reflect compositions of selenide and sulfide minerals in subjacent precious metal veins ~ 250 m beneath sinter. A high thermal gradient and elevated Se/S in conductively cooled hydrothermal fluid, as indicated by fluid inclusion microthermometry and S isotope fractionations, enabled transport of significant amounts of Ag, Au, and other metals to the paleosurface, apparently as metal-selenide complexes. Although subsequent depression of the water table caused fluid boiling, H₂S exsolution and oxidation to H₂SO₄, leaching of sinter and volcaniclastic deposits, and widespread formation of replacement and crustiform alunite, primary Se phases were preserved. Compositions of sinters and near-surface altered rocks in high temperature, Se-rich hydrothermal systems, determined by high resolution-high sensitivity methods, can be used to qualitatively predict precious metal accumulations at depth.