Ore microscopy of the Paoli silver-copper deposit, Oklahoma

The Paoli silver-copper deposit is located in south-central Oklahoma, 56 km south-southeast from Norman, Oklahoma. It was mined for high-grade silver-copper near the beginning of this century, and intensive exploratory drilling during the early 1970's delineated unmined portions of the deposit. A collaborative study between the U.S.G.S., the Kansas Geological Survey, and the University of Missouri-Rolla was undertaken to provide new information on the character of red bed copper deposits of the Midcontinent region. The Paoli deposit has been interpreted to occur as a roll-front type of deposit. The silver and copper mineralization occurs within paleochannels in the Permian Wellington Formation. The silver-copper interfaces appear to be controlled by oxidation-reduction interfaces that are marked by grey to red color changes in the host sandstone. Ore microscopic examinations of polished thin sections show that unoxidized ore consists of chalcocite, digenite, chalcopyrite, covellite and pyrite; and oxidized ores are characterized by covellite, bornite, hematite and goethite. In sandstone-hosted ores, chalcocite and digenite replace dolomite and border clastic quartz grains. In siltstone-hosted ores, the copper sulfide grains have varied shapes; most are irregular in shape and 5-25 ??m across, others have euhedral shapes suggestive of pyrite crystal replacements, and some are crudely spherical and are 120-200 ??m across. Chalcopyrite is the predominant copper sulfide at depth. Covellite and malachite replace chalcocite and digenite near the surface. Silver only occurs as native silver; most as irregularly shaped grains 40-80 ??m across, but some as cruciform crystals that are up to 3.5 mm across. The native silver has been deposited after copper sulfides, and locally replaces chalcocite. Surficial nodules of pyrite, malachite and hematite locally are present in outcrops at the oxidation-reduction fronts. Polished sections of the nodules show that malachite forms a cement around quartz sand grains, and brecciated pyrite grains are surrounded by rims of hematite and goethite. Dolomite is the principal sandstone cement. Cathodoluminescence microscopic study of the mineral has shown that it was deposited during seven periods before the copper sulfide mineralization. ?? 1991.