Mineral Ridge: A Late Cretaceous Orogenic Gold System in the Miocene Silver Peak-Lone Mountain Metamorphic Core Complex
AuthorClaypoole, Micah Noah
AdvisorRessel, Michael W
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Gold deposits in rocks of the Silver Peak metamorphic core complex in west-central Nevada are contained in highly deformed, sheeted milky quartz veins that cut Neoproterozoic metasedimentary rocks and Cretaceous peraluminous granite and migmatite. Gold production at Mineral Ridge since discovery in 1864 has been approximately 1 million oz. Gold-bearing veins are concentrated in a major low-angle ductile shear system active during the Late Cretaceous. However, a broadly coincident brittle detachment cuts the ductile shear, separating metasedimentary and foliated granitic rocks from overlying non-metamorphosed Cambrian carbonates and Cenozoic volcanics. The Mineral Ridge biotite-muscovite granite pluton bounds the ductile shear along its roof, defining a wedge of prospective metamorphic rocks generally <50-m-thick. Zones of intense shearing generally correspond to highest vein density and therefore, gold grades, and such zones are overprinted with strong but narrowly defined quartz-sericite-pyrite-carbonate alteration. Vein boudins are subparallel to the strong planar to anastomosing shear fabric best developed over 1-10 meter thicknesses at contacts between muscovite schist and/or marble, and foliated granite. Gold-bearing veins contain Au:Ag of ≥4, zoned base metals, low As, Sb, and Hg, but high Bi, the latter correlating with gold.Vein-bearing granites at Mineral Ridge yield U/Pb zircon ages of 91.7+/-0.3 Ma and 89.9+/-0.5 Ma and likely derive from crustal melts during peak metamorphism. Hydrothermal monazite in gold-bearing quartz veins yielded a U/Pb age of 76.0+/-1.8 Ma, indicating veins are unrelated to granites and formed during retrograde greenschist conditions. Previous cooling ages on mica in granites of ~42 and 53 Ma further define the cooling path to ~300°C. Mineral Ridge is interpreted as an orogenic gold system associated with a brittle-ductile shear that formed at depths as much as 11-20 km during the waning stages of Late Cretaceous Sevier contraction. Major exhumation occurred in the late Miocene in association with detachment and transtension in the Walker Lane deformation belt.