GEOCHRONOLOGY AND MINERALOGY OF THE BAIYANGHE VOLCANIC-HOSTED BE-U DEPOSIT, XINJIANG PROVINCE, NW CHINA

Previous studies of the BaiYangHe deposit have reported that it is hosted by a granite porphyry, with U-Pb ages for the host rock ranging from 309 Ma to 317 Ma, and a Rb-Sr age of 293±15 Ma. Supposed pitchblende was dated by the U-Pb method, which gave an age of 224±3 and 237±3 Ma. However, the genesis of the deposit, and the relationship between U and Be mineralization is poorly understood. Therefore, the objectives of this study are to (a) characterize the Be and U mineralization, (b) re-evaluate the age of U mineralization using secondary ion mass spectrometry (SIMS), and (c) develop a genetic model.

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.