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Uranium ore in the Pitch Mine occurs chiefly in brecciated Mississippian Leadville Dolomite along the Chester upthrust zone and, to a lesser extent, in sandstone, siltstone, and carbonaceous shale of the Pennsylvanian Belden Formation and in Precambrian granitic rocks and schist. Uranium mineralization is generally thicker, more consistent, and of higher grade in dolomite than in other hosts; roughly 50% of new reserves are found in dolomite. Most ore distribution is controlled by dolomite and probably by brittle behavior (pervasive faulting and brecciation) in a forced fold environment during Laramide basement uplift. Leadville Dolomite in the ore zone is bounded by faults and its maximum known thickness in the area is about 17 m. The Leadville is predominantly dolomicrite End_Page 755------------------------------ with minor textural variations, but transgressive biomicrite facies occur locally. Mud texture, scarcity of fossils and other allochems, thin lamination, and probable algal-mat structures suggest sedimentation in a tidal-flat environment; dolomitization was pervasive and probably before lithification. Fracture- and breccia-controlled pitchblende-coffinite ores are associated with epigenetic pyrite and marcasite; magnesium, iron, sulfur, molybdenum, and lead are enriched in the ore and uranium is independent of organic carbon. One surface expression of ore is ocher-colored, leached, porous gossan, characterized by residual silica and limonite and by high radioactivity but low uranium. Guides to this type of deposit appear to include upthrust faulting, a thick section of brittle rocks attached to the basement, nearby radioactive plutonic or volcanic rocks, and presence of anomalous iron, magnesium(?), sulfur, and molybdenum. Sulfides, carbon, or hydrocarbons are possible reductants. End_of_Article - Last_Page 756------------
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