Abstract

Copper sulfide mineralization occurs within reduced sedimentary rocks of the Permianage Wellington Formation and Ninnescah Shale in south-central Kansas. There are two distinct sulfide assemblages. Pyrite-digenite is found in gray-green shale units within red beds of the Ninnescah Shale. Chalcopyrite, bornite, and chalcocite occur in carbonate units within the upper Wellington Formation to lower Ninnescah Shale transition. Digenite occurs as a replacement of pyrite whereas copper sulfides in carbonate units occur in vugs and small fractures. Chalcocite and barite fill vugs in carbonates occurring within red bed-dominated stratigraphy, whereas the assemblage calcite-chalcopyrite-bornite is prevalent in veinlets within carbonate units situated near a major stratigraphic color change from underlying gray units to red beds.Sulfide sulfur and organic carbon concentrations are generally low (<0.2 wt % for both) in Cu sulfide-bearing rocks. It is possible that initial organic carbon and sulfide sulfur contents were low prior to copper mineralization or that there was interaction between an oxidized, Cu-bearing fluid and reduced, organic carbon-bearing sediments. The delta 34 S values for pyrite range from -40 to -26 per mil. Digenite replacing pyrite is characterized by a similar range in delta 34 S values, whereas chalcopyrite, bornite, and chalcocite in carbonate hosts are significantly heavier, with a range in delta 34 S values of -19 to -7 per mil. Textures of mineralized rocks, S/C relationships, and delta 34 S values probably reflect digenite-pyrite mineralization involving sulfur-conserving replacement reactions, whereas sulfide deposition in carbonates followed partial reduction of sulfate (delta 34 S value near 11ppm) contained in the Cu-bearing fluid. Sulfide mineral distribution and copper contents of reduced and oxidized sedimentary rocks suggest that large-scale fluid infiltration was minimal and that mineralization resulted from small-scale fluid migration within the sedimentary column.

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