CRETACEOUS STRUCTURAL CONTROLS AND EVOLUTION OF GOLD- AND REE-BEARING COBALT-COPPER DEPOSITS, BLACKBIRD DISTRICT, EAST-CENTRAL IDAHO

Blackbird Au- and REE-bearing Co-Cu ore deposits, east-central Idaho, previously were classified as Besshi-type VMS, sedex, and IOCG deposits in an intact stratigraphic unit. Our new studies indicate that metals were introduced into metamorphic tectonites by structurally controlled veins. The vein-hosting structures and the mineral-deposit and host-rock tectonic evolution were kinematically linked to Late Cretaceous thrust faulting.

Host rocks are two schist units of undetermined age and three Mesoproterozoic phyllite units, all footwall to a regional Late Cretaceous thrust fault. The metaevaporite-bearing Mesoproterozoic Yellowjacket Formation formed the allochthonous upper plate (above Blackbird-deposit host rocks). Mineralization was localized in a footwall duplex structure at an oblique thrust ramp; the duplex is composed of inverted metamorphic domains (amphibolite to greenschist facies).

This local oblique dilational setting hosted a consistent sequence of vein and veinlet types associated with discrete sulfide mineral assemblages. Both early and late quartz veins contain chalcopyrite and pyrite whereas intermediate-age biotite (+tourmaline) veins and associated replacement zones host cobaltite. REE minerals are in wall-rock lithic fragments in biotite veinlets but mostly are constituents in both biotite and late quartz veins. Early and late albite and late carbonate (generally siderite) veinlets are barren. Mineralized zones occupy dilatant and permeable axial-planar cleavage, shears, and fractures. Early quartz veins and biotite veins and replacements were introduced during folding and underwent differing degrees of syn- to late-tectonic prograde and retrograde metamorphism. These early veins are discordant to bedding in the mildly tectonized domain but are concordant to metamorphic layering in polydeformed, pervasively tectonized domains. Late quartz veinlets are discordant and ⁴⁰Ar/³⁹Ar dating of unoriented muscovite from the selvage of a late quartz vein is about 83 Ma, indicating the minimum age of tectonism and mineralization. We conclude that these ore deposits formed in the Cretaceous-aged duplex zone by regional chemical exchange, brine production, and metal scavenging, facilitated by the thrust-related crustal thickening and associated heat and fluid flow.