INFLUENCE OF INTERNAL FLUID GENERATION MECHANISMS ON COPPER MINERALIZATION IN THE LISBON VALLEY, PARADOX BASIN, UTAH

Conceptual models of Paradox Basin Copper and Uranium ore formation call upon an initial fluid flow event in which reducing basinal brines migrate from the Paradox formation via faults bleaching overlying red beds. The presence of hydrocarbons associated with many Cu/U ore deposits within the Paradox Basin suggests that ore formation must have occurred following hydrocarbon generation and perhaps following maximum burial. Here, we developed and applied a one-dimensional hydrologic model to investigate the viability of internal fluid sources driving ore forming fluids up basin bounding faults across the Paradox Basin. The internal fluid generation mechanisms we considered included mechanical compaction, petroleum and natural gas generation, aquathermal expansion of water, and clay dewatering. We ruled out gypsum dehydration as this likely occurred at shallow depths and temperatures of less than 20 ^oC. We found that sediment compaction and clay dewatering had the largest impacts on anomalous pore pressure generation within the Paradox Formation. We calculated the net flux of fluid migration along basin bounding faults. We assumed that the flux to the fault was either 50% or 100% of the fluid generation source terms depending on whether a pore pressures approached the lithostatic load. Peak fluid migration occurred during hydrofracturing events which also increased the permeability 100-fold when the pore pressure exceeded 85% lithostatic pressure. The model was calibrated using pressure, temperature, and vitrinite reflectance data. We estimate that the timing of peak fluid transport to a fault and amount of copper deposited using a conservative concentration of 10 ppm Cu in the fluid. Typically, we saw two peak flow events during basin evolution. The range in timing of peak flow events across the Paradox basin varied between: 7-100 and 250-310 Ma. This was largely due to differences in subsidence rates. Peak fluid migration occurred concurrently with peak oil and gas production. Vertical flow rates within the fault zone within the Lisbon Valley basin during peak fluid expulsion range from 0.02 to 0.05 m/yr and deposit a total of about 10⁷ kg Cu for a 1 km fault long (map view) distance. This represents about 1% of the total copper endowment at the Lisbon Valley Copper Mine (~10⁹ kg).