title: Age and Genesis of W-Mo-Cu Mineralization, Gold Hill, Utah
Creators: name: Carey, Nathan
Publisher: name: University of Nevada, Las Vegas
Year: 2022

description: Utah's Gold Hill mining district hosts a Jurassic felsic pluton (~80 km^2) that was emplaced into a Mississippian to Pennsylvanian carbonate-dominated sedimentary sequence and is spatially associated with numerous polymetallic mineral deposits. This study focuses on W-Mo-Cu mineralization at the Lucy L., Doctor, Yellow Hammer, Reaper, and Rustler deposits within the district. The new, detailed petrographic observations, mineral chemistry (on scheelite, molybdenite, chalcopyrite, magnetite, garnet, and pyroxene), and Re-Os molybdenite geochronological data presented here allows for the determination of key mineral associations and paragenesis, the characterization of deposit types and their evolution, and the constraining of the relative and absolute timing of mineralization and genetic relationships with the Jurassic pluton. Altogether, this research applies modern analytical techniques to a historic mining district and reveals several key insights into the diverse metal potential of Jurassic plutons in the Great Basin.Mineral chemistry and paragenetic relationships indicate that the deposits at Lucy L., Doctor, Yellow Hammer, and Rustler formed in a skarn environment containing both endoskarn (Doctor, Yellow Hammer, and Rustler) and exoskarn (Lucy L. and Doctor) mineralization that is genetically related to the emplacement of the Jurassic pluton. Mo-rich scheelite formed during the prograde skarn stage and Mo-poor scheelite, chalcopyrite, and molybdenite formed during the retrograde skarn stage of mineralization. Five of six Re-Os molybdenite ages (ranging from 156.8 ± 2.2 to 154.4 ± 2.2 Ma) are within uncertainty of the U-Pb zircon ages for the Jurassic pluton, supporting this genetic relationship and constraining the timing of W-Mo-Cu mineralization. Grandite garnet and scheelite chemistries are indicative of highly fluctuating fluid compositions during prograde skarn formation and suggest that the prograde stage formed in oxidizing conditions and the retrograde stage formed in more reducing conditions. Following skarn formation, a stage of late supergene Cu alteration and oxidation formed malachite and iron oxide mineralization, the exact timing of which remains uncertain. One anomalous deposit within the district is the Reaper deposit, which has a somewhat unique paragenesis compared to the other deposits within the system. The new data presented here suggests that the Reaper deposit formed initially as a Mo-rich scheelite-bearing pegmatite pipe that was overprinted by W-Mo-Cu skarn mineralization and was later overprinted by epithermal-type mineralization and supergene Cu enrichment, with W-Mo-Cu skarn and supergene Cu enrichment being temporally consistent with mineralization in the rest of the study area. Potential epithermal-type mineralization at Reaper is evidenced by the presence of crustiform-colloform banding and quartz-tourmaline breccia, scheelite cathodoluminescence zoning that differs from that present elsewhere within the district, and one Reaper molybdenite sample that yielded an apparently older age (165.6 ± 2.4) most likely due to interaction with later epithermal fluids that removed Re from the molybdenite after skarn mineralization. This study also reveals key insights into the deportment of metals present as minor inclusions or mineral phases, many of which are important base (Pb and Zn) and precious (Au and Ag) metals or are on the 2022 US Federal Government and USGS List of Critical Minerals (As, Ba, Bi, Ce, La, Sb, and Te), indicating the potential importance of this district for the future domestic exploration targeting and production of these metals.