Geology, petrology, and chemistry of the Leadville Dolomite: host for uranium at the Pitch Mine, Saguache County, Colorado

Abstract

Newly documented uranium ore in the Pitch Mine occurs chiefly in brecciated Mississippian Leadville Dolomite along the Chester reverse fault 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-mineralized zones are generally thicker, more consistent, and of higher grade in dolomite than in other hosts, and roughly 50 percent of the new reserves are in dolomite. Strong physical control by dolomite is evident, as this is the only lithology that is pervasively brecciated within the fault slices that make up the footwall of the reverse fault zone. Other lithologies tend to either remain unbroken or undergo ductile deformation. Chemical controls are subtle and appear to involve chiefly formation of FeS2 as pyrite and marcasite, which accompany uranium. Leadville Dolomite in the area is about 130 m thick and is predominantly nonfossiliferous dolomicrite. In the Pitch Mine, Leadville Dolomite is bound by faults and maximum known thickness is about 17 m. Mud texture, paucity of fossils and other allochems, thin laminations, and probable algal mat structures suggest sedimentation in a tidal-flat (possibly supratidal) environment. Preservation of mud texture and lack of replacement features indicate that dolomitization was an early, prelithification process, as in modern tidal flats, and produced a chemically and texturally uniform rock over tens of meters with relatively few limestone beds surviving. The sedimentary and diagenetic environment of the tidal-flat dolomite, apparently most favorable for uranium deposits, probably obtained over a large area and should consistute an exploration target over a broad area of central Colorado. Carbonate rocks of the Belden Formation, in contrast to those of the Leadville, contain calcite in great excess of dolomite, more than 5 percent silt-size quartz and clay, and abundant fossils and oolites. Belden limestones (sandy micrite and sandy wackestone) probably were deposited in an intertidal or subtidal environment. Very little uranium ore occurs in these rocks. Chemical aspects, such as the iron, sulfur, and organic carbon contents, are very similar to those of Leadville dolomites, and hence seem favorable, but Belden limestones generally are only mildly fractured. The minor-element content of ore-bearing dolomites is generally normal judging from the relatively scarce data yet published for comparable rocks. Elements enriched in ore include iron, sulfur, molybdenum, and lead. One surface expression of ore in dolomite is ocher-colored, leached, porous gossan that is characterized by residual silica and limonite and by high radioactivity but low chemical uranium content.