GEOCHRONOLOGY AND MINERALOGY OF THE BAIYANGHE VOLCANIC-HOSTED BE-U DEPOSIT, XINJIANG PROVINCE, NW CHINA
Based on whole rock geochemistry and petrography, the host rock is predominantly a peralkaline to weakly peraluminous, strongly fractionated, Nb-rich A-type rhyolite. Other rocks associated with the BaiYangHe deposit range in composition from alkali basalt to rhyolite. 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 U mineral. Using data obtained by SIMS and the 206Pb/238U isochron method, three groups of ages for uranophane are identified: ~20 Ma, 40-70 Ma, and 100-130Ma, indicating multiple fluid events, which precipitated more than one generation of uranophane. 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. Mineral textures suggest that fluids (meteoric water) interacted with fluorite, leaching U, Ca, F, Pb and Mn from host rocks, and oxidized the pyrite to hematite. This produced an acidic fluid favorable for the remobilization of F- and U6+. Altered and corroded zircons in the vicinity of the uranophane, suggest that a fluorine-rich fluid was associated with U mineral precipitation. Therefore, U was transported as a uranyl-fluoride complex and a change in pH caused the precipitation of uranophane. In open fractures, evaporation and supersaturation are the mechanisms for precipitation.