U-Pb Geochronology and Geochemistry of U-Rich Garnet from the Giant Beiya Gold-Polymetallic Deposit in SW China: Constraints on Skarn Mineralization Process

Yu Fu,Xiaoming Sun,Dengfeng Li,Hai Lin

doi:10.3390/min8040128

Yu Fu, Xiaoming Sun + Show 2 more

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https://doi.org/10.3390/min8040128

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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.

Highlights

Garnet is a common mineral in metamorphic rocks and hydrothermally-altered rocks such as skarn
Garnet from different rock/ore-forming origins exhibits distinct major and trace element features, trace element geochemistry of garnet can serve as an indicator of its formation environment [5,16,17,18,19]
Based on the mineral assemblage and texture, the U-rich garnet from W297 and W281 may represent for two distinct types of garnet that are well developed in the Beiya skarn deposit

Summary

Introduction

Garnet is a common mineral in metamorphic rocks and hydrothermally-altered rocks such as skarn. Geochemical characteristics of garnet have been widely used to investigate complex magmatic, metamorphic and hydrothermal processes [1,2,3,4,5,6,7]. Dating of these processes could be achieved by garnet Sm-Nd or Lu-Hf geochronology [8,9,10]. Minerals 2018, 8, 128 in-situ and rapid analyses, and has been widely applied to date various magmatic and hydrothermal processes [14,15]. Garnet from different rock/ore-forming origins exhibits distinct major and trace element features, trace element geochemistry of garnet can serve as an indicator of its formation environment [5,16,17,18,19]

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Discussion

Conclusion