In-Situ Leaching of Crownpoint, New Mexico, Uranium Ore: Part 1 Mineralogical Frame of Reference

Abstract

Vogt, T.C.,** SPE, Mobil Research and Development Corp. Dixon, S.A.; Mobil Research and Development Corp. Strom, E.T.; Mobil Research and Development Corp. Johnson, W.F.; SPE, Mobil Research and Development Corp. Venuto, P.B.; SPE, Mobil Research and Development Corp. Summary Before assessing potential for in-situ leaching, it is advantageous to identify the nature of probable reactants in the ore horizon, including both the uranium species and those in the gangue material. We report mineralogical data on an areally broad suite of samples from the Westwater Canyon member of the Jurassic Morrison formation in the Crownpoint area of the Grants mineral belt. Optical microscope, electron microprobe, X-ray diffraction, and spectroscopic and wet chemical analyses were conducted. In these samples, the uranium occurs predominantly as either coffinite or uraninite. The host predominantly as either coffinite or uraninite. The host rock is arkosic sandstone comprising mainly detrital quartz and feldspar. Kaolinite and chlorite are present in claystone clasts, and calcite occurs as a cement. Major trace elements include molybdenum, vanadium, iron, selenium and sulfur. There is carbonaceous organic material in the ore, and it is intimately associated with the uranium mineral when it is coffinite. Geochemical, pyrolytic and 13C nuclear magnetic resonance (NMR) pyrolytic and 13C nuclear magnetic resonance (NMR) analyses indicate that the organic material is probably a very mature, coaly kerogen derived from plant components. Introduction An understanding of reactions involved in leaching chemistry is a key aspect of successful in-situ uranium leaching. Mineralogic characterization of samples from the ore deposit provides identification of reactive uranium and gangue minerals and shows the association and distribution of labile and relatively inert and nonreactive species. Certain criteria have been established to judge the suitability of an ore deposit for uranium recovery by in-situ leaching. The ore deposit must be beneath the water table and must be restricted by impermeable strata above and below the permeable ore zone. The uranium minerals must be contacted easily by leachate injected into the deposit. Subsurface conditions should allow oxidation of the uranium and formation of a soluble complex, by the oxygenated leachate containing a complexing agent, so that the uranium solution can be pumped to the surface for recovery. pumped to the surface for recovery. We are among the first to test in-situ leaching in New Mexico and have pioneered leaching at depths of 2,000 ft (609.6 m); underground or strip mining are the conventional techniques in use in New Mexico.Our Crownpoint project areas are in McKinley County, NM, and are part of the Grants mineral belt (Fig. 1). The origin and the nature of these uranium deposits have been subjects of continuing research. The leases lie to the west and east of the town of Crownpoint. These leases are on the southern edge of the San Juan basin, about 40 miles (64 km) northeast of Gallup and about as far to the northwest of Grants. The surface of the properties is essentially flat, with an elevation of about 6,800 ft properties is essentially flat, with an elevation of about 6,800 ft (2073 m). The major uranium deposits in this area occur in the Westwater Canyon member of the Jurassic Morrison formation (Fig. 2). The subsurface ore depth is about 2,000 ft (609.6 m) in the project areas where the Westwater member is about 260 ft (79.2 m) thick and consists of interbedded sandstone and claystone. The sandstones are crossbedded and contain pebbles and silicified log fragments. The Westwater member is the major water-bearing member of the Morrison formation. The uranium mineralization typically is confined by impermeable claystone strata.The sandstone of the Westwater Canyon member can be classified compositionally as arkose. Detrital components of the sandstone are quartz, feldspar, rock fragments, and carbonaceous debris. JPT P. 2200