Abstract

Oil field brines commonly contain ppm quantities of Zn, Pb, and H 2 S but negligible quantities of Cu. Theoretical analysis of the water-rock interactions expected during migration through evaporitic and red-bed sequences suggests that these reducing oil field brines can become transformed into far more oxidized fluids capable of transporting Cu, Pb, Zn, and sulfate and of forming sediment-hosted Cu-rich deposits. Calculation of the chemical mass transfer accompanying reaction of a typical reducing brine with anhydrite, hematite, and quartz, followed by the assemblage quartz, K-feldspar, muscovite, hematite, chlorite, and cuprite, reveals that the aqueous phase becomes greatly undersaturated with respect to sphalerite, galena, chalcopyrite, bornite, pyrite, and pyrrhotite as it achieves log f (sub O 2 ) values well above the magnetite-hematite buffer consistent with saturation with respect to anhydrite, hematite, and chalcocite and ore-forming concentrations of copper and sulfate. Consequently, evaporites are not only possible sources of salinity and (oxidized) sulfur for evolving basinal brines but may be crucial to the development of the f (sub O 2 ) conditions required to transport Cu in ore-forming concentrations. Reaction of the oxidized brine with organic matter (approximated by graphite) and diagenetic pyrite results in precipitation of the zonation sequence chalcocite, bornite, chalcopyrite, and pyrite with variable amounts of hematite, covellite, sphalerite, and galena, depending on the relative rates of reaction of the graphite and pyrite.

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