Heavy minerals as guides to uranium-vanadium ore deposits in the Slick Rock district, Colorado

Abstract

Heavy-mineral studies were made of sandstone from the Morrison formation of Late Jurassic age in the Disappointment Valley area, Slick Rock district, San Miguel County, Colo. The studies were made in conjunction with exploratory drilling for the U.S. Geological Survey for uranium-vanadium deposits in the Salt Wash member. Important production has come from the Salt Wash in the district. The Disappointment Valley area lies in a syncline between two salt anticlines and is bordered on either side by fault zones. One of the fault zones seems to be spatially related to a zone of alteration and to an area that includes almost all the known uranium-vanadium deposits in the district. Heavy-mineral fractions from about 230 samples of sandstone from the Brushy Basin and Salt Wash members of the Morrisoii taken from 61 drill holes were studied to determine identity and relative amounts of minerals. Among the heavy minerals recognized are black opaque minerals (which are now principally hematite), zircon, tourmaline, apatite, rutile, garnet, leucoxene, barite, anatase, pyrite, and galena. Three distinct lithologic types of sandstone were recognized, all of which contain virtually the same heavy minerals, but in different proportions. Light reddish-brown sandstone, which contains no ore deposits and in general is outside the zone of alteration associated with faults, is characterized by the dominance of black opaque minerals. Light-gray carbonaceous sandstone that is not closely associated with ore deposits and is most abundant in the ore-bearing sandstone unit of the Salt Wash member is characterized by abundant black opaque minerals (less than in light reddish-brown sandstone), and a small amount of pyrite. Within the third lithologic type one variant is light-gray sandstone (light buff at the surface where weathered) in the zone of alteration associated with faults, which is characterized by very sparse black opaque minerals, and some pyrite. A second variant is light-gray sandstone near mineralized layers, which also is characterized by very sparse black opaque minerals, and contains more anatase and pyrite than the other types, and minor galena. Probably shortly after deposition of the Morrison formation and before deep burial, black opaque minerals became oxidized in place, largely to hematite but 169 170 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM in part to leucoxene. Some iron moved locally and was redepo-sited as hematite films on other detrital grains; although altered, the black opaque minerals were not destroyed. This diagenetic change produced the red beds as such. In places in the Morrison, especially in the ore-bearing sandstone, carbonaceous material inhibited oxidation of black opaque minerals. No hematite films formed on other detrital grains, and the sandstone remained light gray. Nevertheless some black opaques were probably leached and destroyed and iron was redeposited as sparse pyrite, but most of the black opaque minerals remained in the sandstone. This change also is considered to be diagenetic. At a later time, after consolidation of sediments, and after the rocks were faulted, altering solutions moved along the fault zones and converted the light reddish-brown sandstone near the faults to light-gray sandstone by leaching black opaque minerals and the hematite films. This change is considered to be epigenetic. Where the solutions moved through light-gray carbonaceous sandstone in the ore-bearing unit, black opaque minerals were similarly leached. In addition, ore minerals, as well as some anatase, pyrite, and galena, were precipitated because of the chemical interaction of the leaching solutions and the stagnant, reducing solutions surrounding areas containing carbonaceous material. The almost complete absence of black opaque minerals in light-gray carbonaceous sandstone in the ore-bearing sandstone of the Salt Wash member of the Morrison formation in the Disappointment Valley area is considered to be a positive indication of proximity to mineralized rock.