AGE AND GENESIS OF W–MO–CU MINERALIZATION, GOLD HILL, UTAH

Utah's Gold Hill mining district hosts a Jurassic felsic pluton emplaced into a Mississippian to Pennsylvanian carbonate-dominated sedimentary sequence that is spatially associated with numerous polymetallic (potentially skarn-type) mineral deposits. This study focuses on the understudied W–Mo–Cu mineralization associated with the Jurassic pluton and will determine the relationships between geological setting, magma composition, paleo-fluid evolution, and the resulting characteristics of these polymetallic systems. Here, we present the preliminary results of in-depth petrographic analyses from several mines in the district to outline deposit paragenesis, key mineral assemblages, and cross-cutting and overprinting mineral relationships to determine the timeline of alteration, mineralization, and remobilization. The following distinct phases of skarn-like alteration and mineralization have been identified to date: (1) early actinolite, apatite, diopside, and scheelite(?); (2) molybdenite, chalcopyrite, bornite, pyrite, magnetite, and calcite; and (3) later supergene malachite, chalcocite, and limonite.

Further research to be undertaken includes determining hydrothermal fluid compositions and the nature of the mineralization and associated alteration (whole rock geochemistry, thin sections, and fluid inclusions), and the timing of mineralization (molybdenite Re-Os dating). The timing of the mineralization in the Gold Hill district has not previously been constrained, and new Re-Os geochronological data, combined with existing U-Pb zircon dates for the spatially associated Jurassic pluton, will provide the relative and absolute timing of the mineralizing events in this district. These data will elucidate the genesis of W–Mo–Cu mineralization in the Gold Hill district and present insight for future exploration targeting in similar terranes.