Whole-rock and biotite geochemistry of granites from the Miao’ershan batholith, South China: Implications for the sources of granite-hosted uranium ores

Abstract

Granite-hosted uranium deposits are the most important types of uranium deposits in South China, but the metal sources of these deposits are still poorly constrained. The Miao’ershan batholith (South China) is a natural laboratory to address this issue, as well as to highlight the use of biotite composition as an indicator for the petrogenesis and uranium fertility of granites. Here, we present whole-rock and biotite geochemical data for U-bearing (Zhangjia and Douzhashan) and barren (Xiangcaoping and Paleozoic) granites of the Miao’ershan batholith. Contents of Li, Be, Mn, Sn, In, Nb, Ta, Ga, W, and U in the studied biotites increase from barren to U-bearing granites, while Mg, Ni, Co, V, and Rb display the opposite trend, which may indicate higher degrees of fractionation of the U-bearing granites. An Fe/(Fe + Mg) ratio of 0.65 in biotite appears to be an efficient threshold to discriminate between U-bearing and barren granites. New ternary discrimination diagrams are introduced, based on Li–V × 20–Mn and Sn–Co × 5–Nb, respectively, to aid the recognition of the U-bearing and barren granites. The relatively high whole-rock U concentrations (8.0–38.6 ppm) and low whole-rock ratios of Th/U (0.5–3.2) and REE/U (2.1–13.5) of the Zhangjia and Douzhashan granites favored the crystallization of uraninite. The samples experiencing fluid-wallrock interaction from the Zhangjia granite are characterized by low whole-rock U concentrations of 3.0 to 4.2 ppm and high Th/U ratios of 8.2 to 10.1, suggesting that a significant part of the U has been leached out by oxidizing meteoric waters. This confirms that the granite represents the primary source of U at the Zhangjia uranium deposit.