AU-AG-S-SE DEPOSITS IN THE WESTERN US

Vein, breccia, and stockwork Au-Ag sulfide deposits that contain selenide minerals, primarily naumannite, occur in at least 14 mining districts in WA, NV, and ID. Much of the production and resources (Moz Au; Moz Ag) in the largest districts, Republic, WA (3; 17), Midas, NV (2.5; 11), Ivanhoe, NV (2; 8.4), Delamar, ID (1;38), and Sleeper, NV (1.7; 2), has occurred or been defined in the last two decades. Resources in 1 to >2 m wide veins at Republic, Midas, and Ivanhoe are ~2-5 Mt at 0.7-1 opt Au and 4-8 opt Ag; ~20-55 Mt of stockwork, veins, and breccia at 0.03-0.05 opt Au and °Â 2 opt Ag were surface-mined at Delamar and Sleeper. Smaller Au-Ag sulfide-selenide resources in other NV districts (National, Mountain View, Jarbidge, Mule Canyon, Buckhorn, Seven Troughs, Hog Ranch, Rosebud, and Tuscarora) have been mined or identified by drilling. Elevated Se also occurs in Au-Ag districts in the Walker Lane structural zone of NV and CA (including Goldfield, Tonopah, and Comstock Lode), and in S¨¬ in geothermal systems and Pliocene Au deposits, in Sb deposits, and in colluvium in northern NV.

Delamar and most NV Au-Ag-Se deposits formed at 16-14 Ma contemporaneous with or immediately following rhyolite, andesite, basalt volcanism; some deposits occur in volcaniclastic strata and pre-Tertiary rocks. The 39.3 Ma Tuscarora deposits are coeval with an eruptive center, and ~50 Ma Republic vein systems formed in a graben filled with eruptive and volcaniclastic rocks of the same age. In addition to naumannite, ore minerals are electrum and silver sulfides (acanthite, tetrahedrite, pyrargyrite, miargyrite, stephanite). Aguilarite, clausthalite and HgSeS minerals occur in some deposits. Veins commonly consist of alternating bands of sulfide-selenide minerals, quartz/chalcedony, and adularia-K-mica. They formed at temperatures °Ã 200¨¬ C from low salinity aqueous fluids that surfaced in thermal springs in some deposits, and are enveloped by zoned wall rock selvages containing quartz, illite/K-mica, pyrite, chlorite, and calcite. Spalled vein bands, deformed sinter clasts, draped protuberances in bands, and relict chalcedony attest to colloid aggregation, particulate transport, and silica recrystallization. Paragenetically late calcite pseudomorphs, and alunite and kaolinite replacement of silicate minerals in veins and wall rocks record volatile exsolution, suggesting that water table subsidence terminated some systems.