Abstract The Stibnite-Yellow Pine district, Valley County, Idaho, includes diverse deposits from which nearly 31 tonnes (t) Au (~1 Moz), ~65 t Ag (~2.1 Moz), and significant quantities of W, Sb, and Hg have been recovered, and large Au, Ag, and Sb resources recently defined. Gold-Ag-W-Sb deposits and resources occur in brecciated, veined, and replaced wall rock fragments and septa in the Meadow Creek fault zone, a near-vertical, N- to NE-trending, meter- to tens-of-meters-wide structure in Late Cretaceous felsic plutons of the Idaho batholith. Other Au-Ag deposits and resources and all productive Hg deposits occur in Neoproterozoic-early Paleozoic metasedimentary strata of the Stibnite roof pendant in and east-southeast of the West End fault zone, which parallels the Meadow Creek fault zone. The paragenesis and zoning of the deposits presented herein are based on field relationships, core logs and analyses, petrographic observations, geochronology, and chemical and isotopic analyses of minerals and rocks by conventional and microbeam methods. Of the five hydrothermal events that deposited epigenetic minerals in granitic and metasedimentary rocks, events 2 through 5 provided nearly all Au-Ag-W-Sb-Hg production and make up the resources. Small-volume 86 to 82 Ma quartz ± sulfide veins of event 1A are dispersed in granitic and metasedimentary rocks and are economically insignificant, whereas calc-silicate minerals on contacts of granitic and pendant strata contain minor to mined amounts of Au, Ag, and Sb deposited by later events. Aggregates of quartz, muscovite, pyrite, and pyrrhotite on other contacts, event 1B, are coeval with 78 to 75 Ma granitic intrusions exposed ~14 km west of the district but undated in the district. Event 2 mineral assemblages include sector- and growth-zoned As ± Au pyrite, arsenopyrite, K-feldspar, muscovite, and rutile. These and other minerals replaced igneous biotite, plagioclase, and Fe-Ti oxides of granitic rocks in and adjacent to the Meadow Creek fault zone. Zoned As ± Au pyrite in the Meadow Creek fault zone and disseminated in pendant strata near the West End fault zone is the largest source of produced and resource Au in the district. The event 3 minerals—quartz, K-feldspar, galena, sphalerite, Pb-Sb-Ag sulfides, dolomite, and scheelite—occur in breccia matrices and veins, partly replace granitic rock fragments and septa of the Meadow Creek fault zone, and are the source of recovered W. Event 2 and some event 3 epigenetic minerals, including K-feldspar, muscovite/illite, pyrite, and arsenopyrite, extend tens to hundreds of meters into granitic rocks from mine and resource boundaries defined by concentrations of zoned As ± Au pyrite. This pervasive potassic alteration accompanied deposition of events 2 and 3 minerals in the Meadow Creek fault zone between ca. 70 and 58 Ma and between ca. 58 and 56 Ma, respectively, based on Ar and U-Pb geochronology. Quartz-adularia-pyrite-electrum veins of event 4, unobserved in Meadow Creek fault zone exposures and drill core, are more frequently encountered in pendant strata, where they apparently make up significant proportions of some Au-Ag mines and resources in and adjacent to the West End fault zone. These veins may be represented in the Meadow Creek fault zone by μm-sized Au particles in heavy mineral concentrates of drill hole intervals composited for metallurgical tests. Event 5 minerals include quartz and K-feldspar in addition to stibnite, miargyrite, cinnabar, and minor Hg-Zn-Te-S minerals, which are the sources of most produced and resource Sb and Ag in the Meadow Creek fault zone and Hg in pendant strata. Spatial zoning of epigenetic minerals including As ± Au pyrite, scheelite, stibnite, and cinnabar and elements including Au, Ag, W, Sb, and Hg is evident from their distribution and concentration, respectively, in the Meadow Creek, West End fault zones, and metasedimentary strata east and southeast of those zones. Proposed lateral fluid-flow vectors, based on mineral and element spatial relationships, event succession, calculated fluid δ34S values, and fluid inclusion microthermometry, show that event 2 fluids (est. 200°C) primarily used the Meadow Creek fault zone and West End fault zone, with lesser flow through pendant strata to the southeast. The main conduit of events 3 and 5 fluids was the Meadow Creek fault zone, and those fluids permeated pendant rocks to the southeast for several kilometers, where minor stibnite and most cinnabar were deposited at relatively low temperatures (est. 175°–100°C). Other mining districts and geothermal fields display Sb and Hg zoning similar to that of the Stibnite-Yellow Pine district, but none replicates the variety and ~24-m.y. span of Au-Ag-W-Sb-Hg events at Stibnite-Yellow Pine. Pervasive fault zones in other plutons of the Idaho batholith and adjacent pendants may contain deposits similar to those of the Stibnite-Yellow Pine district.
Vikre et al. (Mon,) studied this question.