Mass Transfer during Hematitization and Implications for Uranium Mineralization in the Zoujiashan Deposit, Xiangshan Volcanic Basin

The Zoujiashan uranium deposit in the Xiangshan ore field is the largest volcanic-related uranium deposit in China. Hematite- and fluorite-type ores are the predominant mineralization styles. Hematitization in the Xiangshan ore field is closely associated with uranium mineralization, mainly occurring as hematitized rocks enclosing fluorite-type vein ores developed in pre-ore illitized porphyritic lava. Detailed petrographic and mass balance calculation studies were conducted to evaluate the mechanisms for uranium precipitation and mass transfer during hematitization. Petrographic observations suggest that in the hematitized rocks, orthoclase is more altered than plagioclase, and quartz dissolution is common, whereas in the illitized rocks, pyrite commonly occurs within the altered biotite grains, and chlorite grains are locally found. Mass balance calculations indicate that Na2O and U were gained, K2O, CaO and SiO2 were lost, whereas Fe2O3-t remained more or less constant during hematitization. These observations suggest that the hydrothermal fluids were Na- and U-rich and Ca-K-poor, and the Fe2+ used for hematitization was locally derived, most likely from biotite, pyrite and chlorite in the host rocks. The Fe2+ is inferred to have played the role of reductant to precipitate uranium, and calculation indicates that oxidation of Fe2+ provided by host rocks is sufficient to form ores of economic significance. Consequently, the hematite-type ore is interpreted to be generated by the reaction between oxidized ore fluids and reduced components in host rocks. The development of calcite and pyrite in the fluorite ores suggests that perhaps mixing between the U-rich fluid and another fluid carrying reduced sulfur and carbon may have also contributed to uranium mineralization, in addition to temperature and pressure drop associated with the veining.