Abstract

In this study motivated by detailed field work and laboratory analysis, we numerically simulate the supergene enrichment of porphyry copper deposits. We utilize a new program developed to model simultaneously advective solute transport in variably saturated porous media and chemical fluid-rock interactions incorporating mineral dissolution kinetics. Field and laboratory analytic and mineralogic data from supergene enrichment systems at Butte, Montana, and Ok Tedi, located in western Papua New Guinea, are used to provide geochemical, lithologic, and hydrologic initial and boundary conditions required for geologically relevant modeling.We have simulated the weathering and supergene enrichment of porphyry copper potassium silicate proto-ore or "protore" containing disseminated pyrite and chalcopyrite in a one-dimensional, vertically oriented flow region under steady flow conditions for a total model time of about 12,000 yr. During this time primary chalcopyrite dissolves completely from the leached and blanket zones, pyrite is destroyed in the leached zone, and magnetite dissolves from the entire flow region. We therefore investigate both supergene enrichment processes and the response of the system to complete dissolution of a protore reactant.Computed mineral assemblages are generally in excellent agreement with those observed in natural systems. Although copper is remobilized in the simulation by leaching above the model ground-water table and reprecipitation below it, the resulting differentiated weathering profile is chemically closed with respect to copper. This is because nearly all of the transported copper is fixed below the ground-water table by secondary sulfide precipitation. Therefore, an important conclusion to be drawn from the modeling is that in natural oxidative weathering, vertical copper fluxes out of supergene systems are probably negligible even over extremely long periods of geologic time. These theoretical results confirm earlier calculations of amounts of erosion and erosion rates, and reconstructions of paleotopography assuming copper mass balance in regions of vertical fluid flow (Brimhall et al., 1985; Alpers and Brimhall, 1988, 1989). Sulfur, in contrast, is mobile during the simulation and significant quantities of sulfate leave the flow region with time. Modeling results indicate that the dominant source of sulfur for secondary copper sulfide mineral formation in the enrichment blanket is the preexisting protore sulfides, not electrochemical reduction of the sulfate transported out of the leached zone. The numerical simulation demonstrates that oxygen fugacity is the primary control governing the nature and extent of low-temperature reequilibration of the fossil magmatic-hydrothermal system in the near-surface weathering environment.

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