科研之友
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摘要
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 (Adr(69.0)Grs(26.8)-Adr(93.5)Grs(1.5)) is slightly richer in Fe than Grt I (Adr(68.5)Grs(20.3)-Adr(86.3)Grs(3.4)), 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.
- 出版日期2018-4
- 单位中山大学
