Abstract

Four generations of pitchblende and pitchblende alteration products have been identified in the ore-mineral assemblage of the Midwest deposit. Each generation is texturally and compositionally distinctive. The formation of each generation appears to have been preceded by an episode of fracturing and brecciation, indicating extensive reworking of the deposit with time. Microprobe analysis of each generation shows decreasing Pb and increasing Si, Ca, Al, and As with successive reworking events.The earliest pitchblende noted is botryoidal and contains more Pb than subsequent generations. The botryoids were fractured and altered in an episode of brecciation that was accompanied or followed by the introduction of fluids. Second-generation massive pitchblende and pitchblende alteration products (including coffinite and uranium arsenate) enclose altered fragments of first-generation pitchblende. Of the second-generation materials, this massive pitchblende contains slightly lower amounts of Pb than first-generation botryoidal pitchblende, and the alteration products of pitchblende contain substantially less Pb. Coffinite and some As-rich pitchblende contain water. A second episode of fracturing and partial dissolution of the earlier assemblages preceded precipitation of third-generation colloform pitchblende, which contains significantly less Pb than do previous generations. The fourth and youngest episode of uranium deposition is represented by disseminated pitchblende and coffinite and by pitchblende crust and veinlets. These last phases contain the least Pb and are enriched in many minor elements. All generations are accompanied by a variety of Co-bearing Ni minerals and by illite or sericite. Other important gangue minerals are quartz, hematite, siderite, calcite, and goethite.

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