Sources of Sulfide Mineral Components in Au-Ag-W-Sb-Hg Deposits, Stibnite-Yellow Pine District, Idaho: Genetic Relationships to Host and Subjacent Rocks, and Regional Context

Abstract Approximately 5.6 million ounces (Moz) of Au (~174 tonnes [t]) and significant Sb, Ag, W, and Hg were produced from deposits in the Stibnite-Yellow Pine district, Valley County, Idaho, or exist in drill-defined resources. Epigenetic minerals of deposits and resources occur in fault zones in late Cretaceous granitic rocks of the Idaho batholith, in late Proterozoic-early Paleozoic metasedimentary rocks of the Stibnite pendant, and in vein and replacement deposits in pendant strata. Deposits and resources formed during at least five paragenetically distinct events that spanned approximately 24 m.y. The district-wide distribution of Au, Sb, W, and Hg are consistent with temperature gradient zoning in, adjacent to, and distal to major fluid conduits represented by the high-angle, north- to northeast-trending Meadow Creek and West End fault zones. Sulfide minerals of four events and host rocks were evaluated for component sources using Pb, Sr, and S isotope values—which are determined by microbeam and bulk analytical techniques—and by comparison to isotope values, stratigraphy, and ages of host rocks of polymetallic Ag-Au-Cu-Co-Pb-Zn deposits, host rocks, and plutonic rocks in Idaho, Montana, Alberta, and the Great Basin. Based on initial 206Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr isotope values (13.17–21.26, 31.93–41.24, and 0.703–0.725, respectively) of sulfide minerals and whole rocks, sulfide mineral paragenesis, and comparisons, event 1A pyrite in vein and contact zone deposits (ca. 86–75 Ma); event 2 As ± Au pyrite, which contains most mined and resource Au (ca. 70–58 Ma); and some event 5 stibnite (ca. 49–46 Ma) were sourced from late Proterozoic-early Paleozoic and older, more radiogenic metasedimentary rocks. Alternatively, they were indirectly from Cretaceous intrusions in the Stibnite-Yellow Pine district that obtained Pb from these metasedimentary rocks. Less radiogenic Pb isotope values of other event 5 stibnite and cinnabar in mined deposits and resources are largely indistinguishable from those of the areally extensive Atlanta lobe of the Idaho batholith, reflecting mixed crust sources including Archean, Mesoproterozoic, and Paleozoic rocks. The broad range of δ34S values of sulfide minerals in deposits and host rocks of the Stibnite-Yellow Pine district (–19 to 9‰) and large masses of metasedimentary rocks adjacent to deposits reflect reduction of pore seawater sulfate following sedimentation and transfer of S in sedimentary pyrite to deposits. The relatively confined range of mostly positive δ34S values of minerals comprising events 1, 2, and 5 in granitic rocks (~1–9‰) implies that sedimentary pyrite was formed by thermochemical sulfate reduction (TSR) with δ34S values homogenized by high-temperature fractionation during Cretaceous magmatism. The broader range of δ34S values of event 5 cinnabar and realgar in metasedimentary rocks (~19–3‰) reflects direct replacement of sedimentary pyrite formed by thermochemical and biogenic sulfate reduction. Sulfur and Pb isotope systematics link many polymetallic deposits in Idaho, Montana, and the Great Basin, including those of the Stibnite-Yellow Pine district, to sedimentary source rocks and Mesozoic magmatism and demark a dominant metallogeny of large parts of the interior western United States.