Abstract

The Pitch Uranium Mine is located on the southwestern flank of the Sawatch Range in the northeastern part of Saguache County, Colo. This area is characterized by a variety of Precambrian rocks with scattered faulted remnants of Paleozoic sedimentary rocks. Crossing the Pitch Mine area with a northerly trend is the reverse fault zone known as the Chester fault zone. The Chester fault zone has helped to localize mineralization at most of the important mines and prospects in the district, including the Pitch Mine. Although some uranium ore at the Pitch Mine occurs in the Belden Formation and in Precambrian rocks, the highest grade ore is concentrated in an organic-rich dolomite breccia that occurs at the contact of the Belden (Pennsylvanian) and Leadville (Mississippian) formations as well as within the Leadville Formation. Petrographic studies of the character of this black breccia, at the Pitch Mine and regionally, indicate that it is karst related and not a fault breccia, as interpreted by previous workers. The presence of fossil karst towers west of the Pitch Mine indicates that this area was a tower karst regime during the Late Mississippian. A study of the thickness variations of the Leadville Formation and of Belden lithologies was helpful in determining that the black breccia fills the intertower areas.

The uranium mineralization, as indicated by sparse trace element data and mineral assemblages, formed at low temperatures and consists almost exclusively of pyrite, pitchblende, and amorphous uranous oxides. It is believed that the volcanics that overlap the southern part of the area are likely source rocks for the uranium and that a residual elevated geothermal gradient was responsible for enhancing ground-water circulation, leaching, and mineralization.