Abstract

Published interpretations on the timing of copper and copper-iron sulfide deposition in stratiform copper deposits hosted by low-energy sediments range from those postulating deposition during sedimentation to those postulating deposition later in the diagenetic history of the host sediments, with most favoring deposition during diagenesis. Interpretations of sulfide precipitation during diagenesis of the host sediments commonly favor a sulfide replacement of biogenic pyrite, and in some instances, replacement of anhydrite as well.A qualitative reexamination was made of published data on (1) sulfur isotopes, sulfide textures, and sulfide-nonsulfide mineral relationships in stratiform copper deposits hosted by low-energy sediments and on (2) timing of bacterial sulfate reduction, timing of pyrite formation, and timing of authigenic mineral formation in modern analogues of host sediments to ore. The reexamination is permissive of the hypothesis that chalcocite, but not necessarily copper-iron sulfides, in stratiform copper deposits was precipitated within 50 cm of the sediment surface and that most precipitated within a few centimeters of the surface, as a result of bacterial sulfate reduction. The reexamination reveals that evidence for a pyrite replacement origin of chalcocite is based upon equivocal interpretations of sulfide textures. Textures displayed by most bornite, chalcopyrite, and digenite can be interpreted to indicate an origin by replacement of preexisting pyrite-chalcocite composite grains.Limited porosity and permeability data on modern analogues of the host sediments and on clays are compatible with the hypothesis. The porosity and permeability data and published models of compacting shale and sandstone sequences indicate that flow of copper-transporting water was probably along, rather than across, the host-rock bedding.

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