Geology of dolomite-hosted uraniumd deposits at the Pitch Mine, Saguache County, Colorado
J. Thomas Nash
Newly documented 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-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 rock type that is pervasively brecciated within the fault slices that make up the footwall of the reverse-fault zone. Other rocks tended to either remain unbroken or undergo ductile deformation. Chemical controls on uranium deposition are subtle and appear chiefly to involve coprecipitation of FeS2, as pyrite and marcasite, suggesting that sulfide ion may be the reductant.
Leadville Dolomite in the area is about 130 m thick and is pre-dominantly 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 vertically with relatively few limestone beds surviving. 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. Not much uranium ore occurs in these rocks. Chemical aspects, such as the iron, sulfur, and organic carbon contents, are similar to those of Leadville dolomites, and hence seem favorable, but limestones in the Belden generally are only mildly fractured.
The content of most minor elements in ore-bearing dolomites is normal for rocks of this composition, but iron, sulfur, molybdenum, and lead are enriched in ore. One surface expression of ore in dolomite is ocher, leached, porous gossan that is characterized by residual silica and limonite and by high radioactivity but low uranium content.
- Nash, J. Thomas, 1981, Geology of dolomite-hosted uraniumd deposits at the Pitch Mine, Saguache County, Colorado, in: Western slope Colorado--western Colorado and eastern Utah, Epis, Rudy C.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 32nd Field Conference, pp. 191-198.