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EVIDENCE OF MESOZOIC HYDROTHERMAL EVENTS PRIOR TO CARLIN-TYPE MINERALIZATION WITHIN AND BELOW THE TWIN CREEKS MEGA PIT GOLD DEPOSIT, TURQUOISE RIDGE DISTRICT, NORTHERN NEVADA USA
An early, long-lived, poly-phased, high-heat event characterized by a scheelite-arsenopyrite-chalcopyrite±rutile±ilmenite mineral assemblage is associated with multiple Mesozoic intrusions which metasomatized complexly deformed allochthonous Paleozoic stratigraphy hosting gold of the TRD. Two distinct fluid temperature ranges mark multiple pulses (315-375°C and 205-265°C). Geochemically this is expressed as anomalous Bi-W-Pb-Cu-Mo-Sb±As in 4-acid data. Petrography indicates that this older event predates a cooler Carlin-Type gold fluid even (195-255°C). Typical Carlin-Type acidic fluids caused corrosion of scheelite and calcite (decalcification) and a strong association of fine-grained arsenian pyrite and illite+kaolinite. Geochemically, the Carlin event is expressed as anomalous Au-As-Sb-Hg. Late calcite veinlets are interpreted to be formed from re-precipitation of calcite after dissolution, barely post-dating the Carlin-Type gold fluid system.
This newly identified and documented paragenesis, combined with detailed lithologic, structural and alteration observations, indicates that Mesozoic magmatism likely prepared the ground and metallogenic conditions that were overprinted by Carlin-Type mineralization in the middle Eocene. Together, with many other geological elements, this data supports the idea that the Twin Creeks Mega Pit gold deposit is poly-metallic, like the Vista gold deposit. These observations and data challenge the conventional Carlin Ore Systems Model and indicate that Mesozoic magmatic fluids were likely influential for TRD Carlin-Type gold mineralization. Similar conditions may prevail in other districts hosting Carlin-type deposits.