Paragenesis of Uranium Minerals in the Grants Mineral Belt, New Mexico: Applied Geochemistry and the Development of Oxidized Uranium Mineralization

Abstract

The Grants Mineral Belt of northwestern New Mexico was mined from the 1940s to the late 1980s, with more than 340 million pounds of U3O8 extracted during that time (McLemore et al., 2013). Currently, the Grants Mineral Belt contains over 400 million pounds of U3O8 (McLemore et al., 2013), once again bringing the region under consideration for inexpensive in-situ recovery (ISR) of uranium. This study focuses on providing a preliminary mineral profile for portions of the Ambrosia Lake and Laguna sub-districts in order to aid leaching tests directed at assessing the feasibility of ISR for recovery of Grants-style mineralization. X-ray diffraction (XRD) analysis was employed as the primary means of identifying reduced and oxidized uraniumbearing phases and other minerals associated with sandstone-hosted uranium. Thin section and polished-block petrography, and electron microprobe analyses were employed to evaluate and confirm XRD results. Host-rock constituents identified in thin section and XRD analysis include quartz, microcline, and orthoclase, with albite, kaolinite, and illite as the volumetrically-dominant alteration products of magmatic feldspars (Austin, 1980). Calcite was identified in barren sandstone as cement. Analysis of reduced mineralization from the Jackpile-Paguate and St. Anthony mines identify coffinite [generally U(SiO4)1-x(OH)4x] as the dominant crystalline phase in these mines. Very fine-grained uraninite (UO2) overgrowths on coffinite were identified via polished petrographic analysis in reduced samples containing abundant carbonaceous matter in the Mt. Taylor and Section 31 mines. Fine-grained pyrite is observed with carbonaceous matter from numerous uranium occurrences via polished petrography, including the Mt. Taylor, St. Anthony, and Section 31 mines. Microprobe analysis of black ore from the Mt. Taylor Mine identified the mineraloid ilsemannite [Mo3O8•n(H2O)] in the carbonaceous material, associated with weakly crystalline coffinite. Oxidized uranium species are mineralogically diverse, reflecting availability of oxyanions and other metals in oxidizing groundwaters; this diversity is reflected in the abundance of sulfate, carbonate, and phosphate minerals identified in this study. The St. Anthony mine hosts abundant uranyl-sulfate and -phosphate minerals, with lesser carbonates. Dominant uranyl-sulfate phases occurring in the St. Anthony mine are zippeite [K3(UO2)4(SO4)2O3(OH) • 3H2O] and jachymovite [(UO2)8(SO4)(OH)14 • 13(H2O)], with ubiquitous gypsum (CaSO4 • 2H2O). Several phosphates are identified, with (meta-) autunite [Ca(UO2)2(PO4) 2 • 10-12H2O] the dominant phosphate, with trace meta-ankoleite [K2(UO2)2(PO4)2 • 6(H2O)] and phurcalite [Ca2(UO2)3O2(PO4)2 • 7(H2O)]. The uranylvanadates carnotite [K2(UO2)2(VO4)2•3H2O] and meta-tyuyamunite [Ca(UO2)2 (VO4)2 • (35)H2O] are dominant where vanadium is present, such as at the Piedra Triste mine in the Laguna District (Fig. 3). Samples from the St. Anthony and Section 31 mines contain phases with multiple oxyanions, such as zippeite + autunite (St. Anthony), and andersonite + gypsum. These minerals reflect the composition of post-deposition oxidizing groundwaters and, in some cases, post-mine and meteoric waters. Consideration for uranium recovery at the St. Anthony Mine should focus on employing oxidizing, carbonate-bearing solutions with a weakly-acidic to neutral pH to treat reduced mineralization; this would allow mobilization and transport of uranium as uranyl-carbonate complexes so as prevent uranium precipitation as uranyl-phosphates. Vanadium, molybdenum, and selenium are geochemically scant at the majority of locations for this study, but should be considered as potential products during recovery in the reduced mineralized horizons being explored for uranium potential. Importantly, vanadium greatly restricts uranium mobility when uranium is oxidized and, similar to uranyl-phosphates, is stable under acidic conditions. During in-situ leaching, the use of alkaline, carbonate-bearing solutions increases the solubility of uranyl-phosphates and uranyl-vanadates (see Garrells and Christ, 1990) reducing their ability to precipitate. Although pyrite is present in trace quantities at the St. Anthony Mine, quantification of pyrite in the reduced mineralization horizons should be considered, as pyrite would be expected to react with oxidizing leach solutions, consequently reducing the pH of the leaching environment, possibly decreasing uranium solubility, and allowing for precipitation of uranyl-phosphates. Attention must also be given to the abundant calcite in the barren sandstones of the St. Anthony Mine and in the Grants Mineral Belt, as calcite will prevent oxidizing solutions from reaching reduced uranium mineralization. Although detailed geochemical evaluation of Mt. Taylor and Section 31 ores with respect to ISR requires a larger sample size in order to obtain a more complete and quantitative profile of the reduced and oxidized mineralization, this study suggests that carbonate-bearing leach solutions would oxidize and transport uranium effectively and without development of competing uranium species.