Geology and uranium-vanadium deposits of the slick rock district, San Miguel and Dolores counties, Colorado

Sedimentary rocks known in the Slick Rock district in southwestern Colorado range in age from Devonian (?) to Cretaceous, and aggregate about 13,000 feet in maximum thickness. Important uranium-vanadium production has come from deposits in the Salt Wash member of the Morrison formation of Late Jurassic age.

The sedimentary rocks are gently folded in the Dolores and Glade anticlines and the Disappointment syncline, and are cut by the Dolores fault zone in the north part of the district and by the Glade fault zone in the south part of the district. Principal fracture sets are oriented approximately parallel to the major faults.

Detrital hematite, magnetite, and ilmenite in rocks of the Morrison formation not affected by epigenetic alteration contain appreciable amounts of several of the elements found in the ore deposits. Epigenetic alteration processes have bleached large volumes of rock and largely destroyed these minerals. Such alteration is spatially associated with the Dolores fault zone.

Most of the known ore deposits are in the north part of the Slick Rock district in a belt called the Dolores ore zone. The zone lies along the Dolores fault zone but is wider than the fault zone. All known deposits are associated with abundant carbonaceous plant material. Uranium-vanadium deposits in the district are chiefly tabular to lenticular and are roughly parallel to the sedimentary bedding. Some ore bodies, however, are narrow, elongate, and curve sharply across bedding; these bodies have been called "rolls" by the miners. Mineral zoning is evident in some roll bodies; carbonates, goethite (altered from pyrite), selenides, and sulfides are commonly found in concentric layers at the concave edge of rolls. This zoning, and the relationship of roll ore bodies to sedimentary structures and lithology, suggest that ore was deposited at an interface between two solutions, possibly cool connate water and a warmer ore solution.

On a district scale, copper and lead are distinctly most abundant in the ore deposits within and immediately adjacent to the Dolores fault zone, and less abundant in deposits toward the edge of the zone. Uranium-vanadium deposits in the district occur only in sandstone that is considered to be epigenetically altered, and the most extensive epigenetic changes have occurred close to ore bodies.

It is concluded that ground water, heated and set into circulation near the end of Cretaceous time by igneous intrusions in the La Sal and other centers on the Colorado Plateau, picked up elements from sedimentary rocks where they had been faulted and fractured, and deposited the elements at solution interfaces where accumulations of carbonaceous material provided favorable chemical conditions for precipitation.