UBC Theses and Dissertations
Beyond the confines of the ore deposit : mapping low temperature hydrothermal alteration above, within, and beneath Carlin-type Gold deposits Ahmed, Ayesha Doris
Multiple analytical techniques were employed to investigate distal patterns in low temperature hydrothermal fluid flow into and out of Carlin-type gold deposits in two study areas: the Leeville deposit and the Shoshone range including the Pipeline, Gold Acres and Elder Creek deposits. Previous studies indicate that gold is hosted in lower Paleozoic carbonate rocks overlain by thick sequences of similarly aged siliciclastic rocks. Patterns in δ¹⁸O depletion (<20‰VSMOW), and Au, As, Sb, Hg, Tl, and Te concentrations in lower Paleozoic carbonate rock identified three disconnected lateral fluid pathways into the Pipeline deposit: a main conduit providing gold-bearing fluid to the main ore body, the Abyss fault located ~300m below the main ore zone, and the RMT located underneath the Abyss fault. Following gold precipitation in the Pipeline deposit, gold-depleted fluids were likely exhausted laterally, at least initially, along the same structures as those that allowed fluid to enter the deposit. Upon intersecting the RMT fault, fluid either exploited the fault to reach surface, or transgressed overlying siliciclastic rocks via small scale faults and fractures. δ¹⁸O and δD values of H₂O in equilibrium clay minerals, and the concentration and crystallinity of illite outlined multiple zones of hydrothermal alteration in surface rocks from both the Shoshone Range and Leeville study areas, however no genetic link was established to Carlin-type gold mineralization at depth. Similarities in trace element geochemistry, ore assemblage, and alteration assemblages however, suggest that the Elder Creek deposit may represent low temperature (200°C) gold mineralization resulting from the exhaust of Carlin-type ore forming fluid. The region above the surface projection of the Leeville deposit exhibits multiple zones of hydrothermal fluid upflow resulting in pervasive illitization of surface siliciclastic rocks. The Pipeline/ Gold Acres also contain abundant crystalline illite. The presence of highly crystalline illite highlights zones of focused fluid upflow, typically along faults and other secondary permeability structures such as breccias.
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