CRITICAL MINERAL (PGE, NI, CO) PRODUCTION AND CONCEALED RESOURCES IN THE WESTERN US

Critical mineral (CM) production in California, Oregon, and Nevada includes ~33,340 oz. platinum group elements (PGE), ~0.3 Mt Ni, and minor Co and V that were recovered from placer, laterite, carbonate replacement (CRD), and vein deposits, and from sulfides in ultramafic rocks mined primarily for Au and Cu. These CM occur in alloys, pentlandite, tellurides, selenides, and silicate-arsenate-carbonate minerals. Most PGE and Ni deposits are spatially associated with or in ultramafic rocks and serpentinite. Placer PGE, possibly authigenic, and Ni were derived from disseminated and podiform sulfide aggregates like those exposed in accreted and exhumed oceanic mantle peridotite, and from weathering of peridotite.

CRD and vein deposits near and in intrusions in Mojave Desert districts contain variable proportions of PGE, Ni, Co, and V, in addition to Cu and Au that enabled mining. At Copper King, Nevada PGE alloys and minerals are disseminated in altered peridotite dikes in 1.7 Ga felsic to mafic metamorphic rocks or occur in networks of Cu-Fe-Ni-Co-S-Te minerals and electrum in dikes. Mineral assemblages, metal proportions, and isotope values (S, Pb) consistent with a mantle source suggest that deposits represent hydrothermally modified sulfide-silicate melts derived from a concealed mafic/ultramafic intrusion. The Cu-Ni-Te-PGE mineral associations largely conform to experimentally determined partitioning of metals between silicate and sulfide liquids, followed by fractionation of Fe-Ni-Cu-S liquid to a Cu-rich sulfide liquid with elevated concentrations of PGE, Te, Au, and Ag.

At Goodsprings, Nevada stratigraphy, metal proportions, and temporal relationships among diverse Zn-Pb-Ag-Au-PGE deposits reflect different ages, metal sources, and metal concentration mechanisms. Co-spatial Zn-V-Ni in Mississippi Valley-type CRD mined primarily for Zn and Ag supports metal derivation from Paleozoic sedimentary strata during basin-scale fluid flow, whereas confinement of most Co to hydrothermal Cu-PGE-Au-Ag vein and CRD associated with Triassic intrusions is consistent with hydrothermal extraction of metals from sulfide cumulate in 1.7 Ga crystalline rocks, as at Copper King. Characteristics of these exposed deposits imply concealed PGE-Ni-Co resources in the Mojave Desert.