Add time:08/16/2019         Source:sciencedirect.com

The Baiyanghe Be-U deposit is located in the Xuemisitan volcanic belt of the northwest margin of the Junggar plate, NW China. It is the largest beryllium deposit in Asia, but the physicochemical conditions of ore precipitation and mechanisms of transportation and precipitation of Be and U are still poorly constrained. Here we present detailed petrographic study, whole-rock major and trace elements of the host rocks, and mineral chemistry of this deposit to address these issues. The alteration minerals associated with Be-U mineralization mainly include fluorite, hematite, quartz, chlorite, manganese oxides, and muscovite. Fluorite is generally colorless, green, or light purple to dark purple in this deposit. The colorless and green fluorite and calcite display low concentrations of Be and U. The light purple to purple fluorite is characterized by high Be concentration but low U concentration. The dark purple fluorite shows high concentrations of Be and U. Both primary mica in the Yangzhuang granite porphyry and hydrothermal muscovite in the ore are enriched in F (4.26–7.78 wt%, 4.22–6.70 wt% respectively). These results indicate that the ore-forming fluids were enriched in F, and this deposit might have experienced at least two hydrothermal events that are responsible for the precipitation of Be and U minerals. All investigated chlorites are trioctahedral and belong to Fe-chlorites, and they precipitated at temperatures of 125–200 °C. Thermochemical constraints suggest that the ore-forming fluids had logfO2 about −25 to −30 and pH about 4.5–5.2. Our study indicates that the Yangzhuang granite porphyry may represent the primary sources of Be and U, and that post-magmatic hydrothermal processes accompanied with extensive hydrothermal alteration caused remobilization of Be, U, and other elements from the granite porphyry. Both Be and U were mainly transported as fluoride complex ions, as indicated by the abundant fluorite and other F-bearing gangue minerals. The contact zones between the granite porphyry and Devonian volcanic rocks provide favorable reservoirs for Be and U minerals. The precipitation of Be was mainly resulted from the decreasing temperature and fluorine concentration in fluids, and the decreasing fO2 and increasing pH were responsible for the precipitation of U.

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