PETROGRAPHY AND GEOCHEMISTRY OF THE BAIYANGHE VOLCANIC-HOSTED BE-U DEPOSIT, XINJIANG AUTONOMOUS REGION, NW CHINA
Samples from the ore zone contain up to 10,000 ppm U. Based on whole rock geochemistry and petrography, the suite of rocks associated with the BaiYangHe deposit is a volcanic pile, ranging in composition from alkali basalt to rhyolite. However, U and Be mineralization is predominantly hosted in the peralkaline to weakly peraluminous, strongly fractionated Nb-rich rhyolites.
Powder XRD, SEM, and EMPA analyses show that the dominant U mineral is uranophane (Ca(UO2)2[HSiO4]2·5H2O), which occurs along fractures, and is associated with Mn and Pb oxides. These results contrast with previous reports, which identified uraninite (UO2) as the dominant uranium mineral. Beryllium mineralization occurs as bertrandite (Be4(Si2O7)(OH)2), and is associated with earlier generations of fluorite (CaF2).
Although the source of both U and Be is the rhyolite, the controlling factors for Be mineralization differ from those of U. Multiple generations of fluorite occur along the same fractures that host pyrite (FeS2), bertrandite, uranophane, and hematite (Fe2O3). The fluorite, bertrandite, and pyrite predate the uranophane and hematite. Mineral textures suggest that fluids (meteoric water) interacted with the fluorite, leaching U, Ca, F, Pb and Mn from host rocks, and oxidized the pyrite to hematite. The oxidation of pyrite produced an acidic fluid (e.g., FeS2 + H2O = Fe2O3 + H + H2SO4) favorable for the remobilization of F- and U6+. Therefore, U was most likely transported as a uranyl-fluoride complex (e.g., UO2F+) until a change in pH caused the precipitation of uranophane. In open fractures, evaporation and supersaturation are the mechanism for precipitation.
Rare earth element patterns for rhyolites in the BaiYangHe deposit show a strongly depleted Eu anomaly, in contrast to the rest of the volcanic suit, therefore REE patterns may be used as a geochemical exploration tool for these types of deposits