EXPLORATION FOR OVERPRINTING DEPOSIT STYLES IN THE BASIN AND RANGE; INSIGHTS FROM A DETAILED STUDY OF W-MO-CU MINERALIZATION IN GOLD HILL, UTAH

Utah's Gold Hill mining district hosts a Jurassic felsic pluton (~80 km²) that was emplaced into a Paleozoic carbonate-dominated 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 this district. We characterize the formation and evolution of each of the deposits and, in doing so, reveal key exploration insights for the potential of overprinting deposit types within the Basin and Range. Mineral chemistry and paragenetic relationships indicate that a majority of these W-Mo-Cu deposits formed in a skarn environment and are genetically related to the emplacement of the Jurassic pluton. Five of six new Re-Os molybdenite ages (ranging from 156.8 ± 2.2 to 154.4 ± 2.2 Ma) are within uncertainty of existing U-Pb zircon ages for the Jurassic pluton, supporting a genetic relationship and constraining the timing of W-Mo-Cu mineralization. A stage of late supergene Cu alteration and oxidation followed skarn formation.

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 suggest that the Reaper deposit formed initially as a Mo-rich scheelite-bearing pegmatite pipe that was overprinted by W-Mo-Cu skarn mineralization. Later, the deposit was again overprinted by epithermal-type Au mineralization and experienced supergene Cu alteration. Potential epithermal-type mineralization at Reaper is evidenced by the presence of crustiform-colloform banding, quartz-tourmaline breccia, and scheelite cathodoluminescence zoning that differs from the other deposits studied. In addition, one Reaper molybdenite sample 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. Overall, the complexly related mineralization styles at Gold Hill are well-exposed examples of potentially similar, undiscovered mineral systems in the Basin and Range. The tectonomagmatic history of this region is favorable for overprinting deposit types that may contain both base and precious metals, along with a strong critical mineral potential.