In situ LA-ICP-MS U-Pb geochronology and geochemical characteristics of garnet from the Zhuxi skarn W-Cu deposit, South China

Abstract The Zhuxi skarn W-Cu ore deposit (reserves of 3.44 Mt @ 0.5% WO3, 0.11 Mt @ 0.57% Cu) is located in the Jiangnan Orogen, South China, which is the largest tungsten deposit in the world. The W mineralization mainly occurs in skarns related to the Late Jurassic granitoids, and less in greisens and scheelite-quartz-muscovite veins. There is still a lack of direct timing of skarn formation in the Zhuxi deposit, which is crucial to understand the mineralization process. Garnet is typically the earliest characteristic paragenetic mineral crystallized during skarn formation, representing the onset of the hydrothermal event. The Zhuxi garnet yields a compositional range of Adr16.29-40.47Grs55.71-78.36Sps0.83-7.34, and displays chondrite-normalized LREE-enriched and HREE-depleted patterns with positive to negative Eu anomalies. The inapparent correlations between ΣREE and Al, Fe3+, Fe2+, Mg, and Ca altogether imply that the REE3+ incorporation into the Zhuxi garnet is most probably controlled by a coupled substitution mechanism via Ca site vacancies, and can also be intervened by external factors, such as fluid chemistry and physicochemical parameters (e.g., pH, fO2). The strongly positive ΣREE vs. Y correlation suggests that REE3+ and Y3+ have similar geochemical behavior. The garnet LA-ICP-MS U-Pb dating yielded ages of 153.0 ± 4.4 and 148.7 ± 7.9 Ma, indicating that the skarn mineralization at Zhuxi is coeval with the emplacement of the Late Jurassic granitoids. Combined with published dates reported by a variety of methods in the ore district, it is advocated that the ore-related magmatism, skarn, and W mineralization in the Zhuxi deposit all occurred at ca. 153 Ma in an extensional tectonic setting resulted from the tearing of the Izanagi slab subduction. This study highlights the reliability and viability of grossular garnet U-Pb dating for constraining the timing of skarn formation and its relevant mineralization.