CARLIN-TYPE GOLD DEPOSITS RELATED TO THE BINGHAM CANYON, UTAH, PORPHYRY CU DEPOSIT; THERMAL AND ISOTOPIC CONSTRAINTS

The Barneys Canyon and Melco Carlin-type gold deposits are 6 to 8 km north of the Bingham Canyon porphyry Cu-Au-Mo deposit. Earlier studies have shown these gold deposits are near the outer limit of an irregular As-Ag geochemical halo, about 3 km beyond the outer limits of a Pb-Zn halo, and 7 km beyond the outer limits of a pyrite halo, centered on the 37 Ma porphyry. Sedimentary rocks from the porphyry deposit northward across the gold deposits have not been heated beyond about 150°C. An asymmetric paleothermal anomaly (PTA) formed by hydrothermal fluids circulating around the Bingham Canyon stock extends at least 10 km north as measured by the systemmatic increase (12.5 to 15 permil) in d ¹⁸O values of the matrix of quartzites away from the stock. Reset apatite fission-track ages indicate that the gold deposits are located approximately on the 100°C isotherm of the 37 Ma PTA. The maximum temperature at Melco, 6 km north of the Bingham pit was between 100°C and 140°C, and at Barneys Canyon about 100°C, as indicated by conodont color alteration indexes of 1.5 to 2, mean random solid bitumen reflectance of about 1.0 %, lack of annealing of zircon fission tracks, and partial to complete annealing of apatite fission tracks. Recrystallized limestones at Melco and Barneys Canyon have the highest d ¹⁸O values (~30 permil) while limestones in the Cu zone of the porphyry have the lowest values (~10 permil). The high d ¹⁸O values strongly suggest that mineralization was related to low temperature fluids with exceptionally high d ¹⁸O_H2O values such as could be derived from a crater lake or near fumaroles on the surface of an active volcano. The gold deposits appear to be genetically related to the formation of the 37 Ma Bingham ore-forming system. We interpret the gold mineralization to be related to the redistribution of dispersed Au into late, permeable structures during the collapse of the Bingham Canyon hydrothermal system in which isotopically heavy, acid-sulfate, surface waters, mixed with and displaced fluids in the collapsing main-stage hydrothermal system.