THE REPLACEMENT CHARACTER OF COPPER-SILVER MINERALIZATION IN THE PERMIAN FLOWERPOT SHALE AND WELLINGTON SANDSTONE FORMATIONS OF OKLAHOMA

Disseminated copper-silver mineralization occurs within Permian dark gray shales and sandstones at many localities in Kansas, Oklahoma, and Texas. Ore microscopic studies of two localities, Creta and Paoli, Oklahoma, provide clear evidence that the mineralization has formed by selective replacement of favorable minerals and that those minerals differed significantly between the two deposits.

Shale-hosted, stratiform, copper mineralization was mined at Creta, Oklahoma for about 10 years beginning in 1965. Mining was largely confined to a dark greenish gray shale that is about 8 inches thick. The copper-bearing dark greenish gray shale lies beneath a thin gypsum bed within a sequence of red shales and gypsum beds. The copper grade averaged about 2% and ranged up to 4.5%.

Sandstone-hosted copper mineralization has been drilled extensively but not mined at Paoli, Oklahoma. Intervals of copper mineralization 8-13 feet thick are localized along the reduced side of roll fronts within sandstone channels. Copper grades measured about 1-4% and the silver content is about 0.5-10 oz/ton, but locally ranges up to more than 200 oz/ton.

Ore microscopy shows that three types of copper sulfide grains were deposited at Creta by replacement of: 1) spores, 2) pyrite, and 3) pyrrhotite. Replacement of megaspores began in their interiors and gradually progressed to their spore cases forming obate grains about 120 µm across and with smooth outer margins. The spore replacements diminished downward beneath the ore zone. Colloform pyrite grains, about 40 µm in diameter and with lobate outer margins, are thoroughly replaced within the ore zone, and the extend of their replacement diminishes upward. The smallest copper sulfide grains are 10 µm long, prismatic in shape, and they replaced pyrite pseudomorphic after pyrrhotite. The copper sulfide minerals are chalcocite, digenite, anilite, and djurleite. Silver occurs as stromeyerite.

In contrast, hematite grains were selectively replaced at Paoli, although some cubes and pyritohedrons of pyrite as well as pyrite cement around quartz sand grains also were replaced. The paragenetic sequence of replacement was: native silver, chalcopyrite (early), bornite, digenite, and chalcocite.