U-Pb Geochronology and Geochemistry of U-Rich Garnet from the Giant Beiya Gold-Polymetallic Deposit in SW China: Constraints on Skarn Mineralization Process
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
... r 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. , and has been widely applied to date various magmatic and hydrothermal processes [14, 15] . Furthermore, garnet from different rock/ore-forming origins exhibits distinct major and trace element features, thus trace element geochemistry of garnet can serve as an indicator of its formation environment [5,     . The giant Beiya Au-polymetallic skarn deposit is situated in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt. The deposit is the largest Au skarn deposit in China with significant amounts of Fe, Cu, Pb, Zn and Ag. The Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt is an important Cu-Au province in SW China, and contains a large number of deposits that are space-time associated with the Cenozoic (Himalayan) alkaline magmatism     . Ore deposit geology, ore fluid composition and metallogenic studies of the Beiya Au mineralization all suggest that the Beiya deposit resembles typical porphyry-related Au skarn deposits worldwide       . Garnet is a common mineral in the Beiya skarn ores, and provides a suitable target for geochronology and geochemistry study. In this study, we present new U-Pb and trace element data for two U-rich garnet types from Beiya. Our new data are integrated with published results to discuss the ore-forming environment and constrain the Beiya skarn mineralization processes. Moreover, we examine the precision and effectiveness of the LA-ICP-MS garnet U-Pb dating technique, and its applications on constraining the timing of skarn mineralization. Geological Setting and Ore Geology Regional Geology The Beiya deposit is the largest gold deposit in the region and ranks as the third largest gold deposit in China, with estimated reserves of 130.8 million tonnes (Mt) Au ore at 2.47 g/t, 170 Mt Fe ore at 33.3 wt %, and 125 Mt Cu ore at 0.52 wt %, along with considerable amounts of Pb, Zn and Ag [30, 31] . The Beiya skarn gold-polymetallic deposit is located to the east of the Jinshajiang suture (western margin of the Yangtze Craton) in SW China (Figure 1) , and situated in the middle part of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt  . Among the many porphyry-related deposits in the belt, Beiya is the most representative with regards to scale, and covers an area of 800 km 2 . The deposit is located at the limbs of the NS-trending Beiya syncline, and comprises six ore segments, namely: Weiganpo, Bijiashan, Guogaishan, Wandongshan, Hongnitang and Jingouba. These segments are divided into two zones, with the former three and latter three located on the eastern and western limb of the Beiya syncline, respectively (Figure 2a ). Among these segments, the Wandongshan section (contains mainly the KT52 orebody) is the largest and contains most of the Au resources (99 Mt @2.61 g/t Au  ). for advising on the LA-ICP-MS analysis. Wenchang Li and Zhangrong Liu are thanked for the fieldwork and sampling support. Two anonymous reviewers are thanked for constructive reviews that greatly improved the quality of this paper.