U-Pb geochronology and geochemistry of U-rich garnet from the giant beiya gold-polymetallic deposit in SW China: Constraints on skarn mineralization process

Abstract

The giant Beiya Au skarn deposit (over 300 tonnes Au metal reserve) is located in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt. The deposit is the largest Au skarn deposit and third largest Au deposit in China. In this paper, we present garnet U-Pb ages and trace element geochemical data from two types of skarn-related U-rich garnet from Beiya, and discuss their implications on skarn metallogenesis. Based on optical characteristics and major element compositions, the older Grt I and the younger Grt II (both belong to the grossular-andradite solid solution) are distinguishable: Grt II (Adr69.0Grs26.8–Adr93.5Grs1.5) is slightly richer in Fe than Grt I (Adr68.5Grs20.3–Adr86.3Grs3.4), with the average FeO contents being 25.0% and 24.5%, respectively. LA-ICP-MS garnet U-Pb dating yielded lower intercept ages of 35.8 ± 0.8 Ma (MSWD = 0.9, n = 20) and 34.0 ± 2.1 Ma (MSWD = 1.2, n = 16), respectively. These ages represent the timing of the garnet crystallization, and are consistent with published alteration ages within error, which suggests that the technique presented provides a reliable alternative in dating skarn alteration. Grt I has higher U content and lower Th/U ratio than Grt II, indicating a lower oxygen fugacity for the earlier skarn alteration. Grt II shows weak (or no) Eu anomaly and lower LREE/HREE ratios than those of Grt I, which indicates that Grt I and Grt II may have formed under a mildly acidic and nearly neutral condition, respectively. From the early prograde skarn (Grt I) to late prograde skarn (Grt II) stage, the ore-forming fluids may have changed from a relatively reduced acidic to a relatively oxidized neutral pH condition.