Sand, Salt, and Copper: An Introduction to Sediment-hosted Copper Deposits, Paradox Basin Evaporites, and the Cashin Cu-Ag Deposit
Dr. Timothy MacIntyre
MacIntyre Geologic Consulting
Abstract:
As global electrification accelerates, copper will prove to be one of the most essential “critical minerals,” with sediment-hosted Cu deposits supplying a significant portion of global production. The discovery of the giant, high-grade Kamoa-Kakula deposit in Central Africa (the 5th largest Cu mine by contained metal) has renewed attention on this previously overlooked deposit class.
Sediment-hosted Cu (Sed-Cu) deposits form when saline, oxidized, Cu-bearing fluids are reduced in the presence of sulfur leading to the precipitation of Cu-sulfides. When this process is repeated with Cu-rich fluids focused into a S-rich trap, it can form large high-grade deposits that are economic to mine. Critical components to consider are the source of Cu and S, basin architecture and flow paths, quality of the trap, timing of fluid movement, and energy to drive fluid migration. The vast majority of giant Sed-Cu deposits occur in the Katangan Basin (Central African Copperbelt - Zambia/DR Congo) and the Southern Permian Basin (Kupferschiefer - Poland/Germany). However, incredible outcrops in the Paradox Basin provide a remarkable opportunity to study simple Sed-Cu deposits, such as the Cashin Mine, which clearly illustrate core elements of the model.
The historic Cashin Cu-Ag Mine is hosted in the Wingate Sandstone along the southwest flank of the Paradox Valley salt anticline. Cu-Ag minerals are localized in veins and pods along a small normal fault, with lower-grade disseminated mineralization extending 10’s to 100’s of meters into reduced “bleached” sandstone. Originally deposited as eolian red beds, the Wingate was later reduced by hydrocarbons, creating buff to white, pyrite-bearing zones (paleo-hydrocarbon reservoirs) with suitable chemistry to precipitate Cu from mineralizing fluids. Cu sulfides (predominantly chalcocite, bornite, chalcopyrite, and tennantite), replace pyrite, bitumen, carbonate cements, and locally quartz with barite and dolomite gangue in veins. Fluid inclusion data indicate moderate salinity (17-21% NaCl eq.), low-temperature (90-110°C) mineralizing fluids. Metal zoning from Cu-Ag to Pb-Zn supports upward and outward fluid flow. The Cashin deposit provides a clear example of how structure, host-rock chemistry, and the timing of fluid migration must be considered to effectively explore for metals in sedimentary basins.
Bio:
Tim MacIntyre is an independent consultant specializing in sediment-hosted mineral deposits and evaporite related systems. He was first introduced to copper deposits in the Paradox Basin by Dr. Jon Thorson and went on to complete a MS thesis on the Cashin deposit at the Colorado School of Mines under Dr. Murray Hitzman. Tim then joined Ivanhoe Mines during the Kamoa discovery, spending four years exploring across the Central African Copperbelt before returning to CSM for a PhD focused on First Quantum’s Kansanshi Cu-Au deposit. Since completing his PhD, Tim has worked as a consultant, VP Exploration, and project manager on a variety of copper and gold exploration programs worldwide. He is a Fellow in the Society of Economic Geologists.
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