Mineralogy and paragenesis of red-bed copper mineralization in the Lower Permian of South central Kansas

Subeconomic concentrations of copper sulfides within greenish gray shales, argillaceous dolomites, and limestones of the Lower Permian red-bed sequence of south central Kansas have been investigated. The host rocks are similar to those hosting red-bed mineralization in Oklahoma and Texas. Copper sulfides occur principally as replacements of earlier formed diagenetic pyrite. Two distinct, spatially unrelated sulfide assemblages have been identified in drill cores. In the northern portion of the area pyrite is replaced by chalcopyrite and then bornite at shallower depths. In the southern section pyrite is replaced by "chalcocite" nearer the surface. The depth zonation strongly suggests that descending fluids produced the Cu mineralization. Electron microprobe studies of the sulfide minerals indicate that bornite, as in other red-bed deposits, is sulfur rich. In addition, true chalcocite (Cu 2 S) was not found in the studied area. Instead, two phases having compositions of Cu (sub 1.78 + or - 0.04) S (similar to anilite) and Cu (sub 1.91 + or - 0.03) S (similar to djurleite) were identified. Thermochemical calculations suggest that continuous depletion of a Cu-rich ore fluid and variations in pH and f (sub O 2 ) are responsible for producing the pyrite-chalcopyrite-bornite and pyrite-"chalcocite" assemblages.