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Skarn and porphyry hosted copper mineralization at the Thompson Knolls discovery, Utah
Mann, Chad
Mann, Chad
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2025
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This is a first description of the recently discovered Cu mineralization at Thompson Knolls, UT including petrographic observations, geochemical whole rock data, fluid inclusions, and LA-ICP-MS U-Pb ages of skarn-related garnet. The bulk of the yet discovered Cu mineralization is hosted by discontinuous, relative flat dipping skarn zones that were intercepted in five drillholes. The Thompson Knolls skarn mineralization is hosted in Devonian dolomite and dolomitic limestone and associated with granodioritic dykes. Exoskarn is invariably magnesian composition with mostly pyroxene, tremolite, talc, calcite, and serpentine. Endoskarn altered granodioritic dykes comprise mainly pyroxene and lesser garnet overprinted by actinolite and epidote. Disseminated chalcopyrite is the main Cu mineral, which is intimately intergrown with retrograde tremolite, talc, serpentine, and lesser epidote in exo- and endoskarn units. Skarn mineralogy, geometry and geochemistry imply a relatively shallow formational environment in an intermediate to distal position relative to a causative intrusion. The recognition of systematic geochemical and mineralogical zoning across the intersected skarn zones may provide useful criteria for exploration targeting. The zoning includes a decrease in the skarn Cu/(Zn+Pb) bulk rock ratios approximately from TK8 to TK14 which is consistent with a decrease in fluid inclusion homogenization temperatures from 468.2° C to 277.9° C, implying a W, or SW to NE fluid migration. A total of six garnet samples from drillholes TK6, TK8, and TK14 and were dated successfully by in-situ U-Pb LA-ICP-MS. The ages range from 155 ± 2 Ma to 164 ± 3 Ma. The overlapping garnet ages with previous U-Pb zircon dates of 152.7 ± 1.3 Ma, 156 ± 1.8 Ma, and Re-Os molybdenite date of 158 ± 0.8 Ma suggests one magmatic event causing mineralization at Thompson Knolls. Placed in the Cordilleran magmatic back arc of the Great Basin region, the mid to late-Jurassic age of the Thompson Knolls proposes a connection of back arc magmatism (~164 to 145 Ma) and the Nevadan Orogeny (~165 to 155 Ma) to mineralization. The subduction-related magmatism and compressional orogeny presents locally ideal conditions for a calc-alkalic hydrous and oxidized magma to mobilize copper into a thickened upper crust (~35 to 45 km), adding potential for concealed Cu-mineralization in the Great Basin.
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