Genesis of the Lamasu large skarn Cu deposit, northern Xinjiang: Constraints from garnet U-Pb dating, micro-textural and geochemical analyses

The Lamasu deposit is the first large copper (Cu) deposit with -0.6 Mt Cu reserve in the Chinese Western Tianshan Orogen (NW China). The Cu orebodies are mainly hosted in the exoskarn of the Proterozoic Kusongmuqieke Group. Now, the metallogenic mechanism of newly discovered skarn Cu orebodies (0.5 Mt Cu) remains poorly constrained. In this study, we conducted LA-ICP-MS U-Pb isotope dating, EPMA and LA-ICP-MS geochemical analyses on different generations of garnet from Lamasu to elucidate the magmatic hydrothermal evolution and its timing. Garnets from the Lamasu exoskarn can be divided into three types: reddish-brown coarse-grained Grt-I, light brown fine-grained Grt-II and yellowish-green heterogranular Grt-III, with U-Pb age of 389.1 +/- 2.0 Ma, 387.1 +/- 1.8 Ma and 387.0 +/- 2.3 Ma, respectively. These dates represent the oldest Cu mineralization age in the Chinese Western Tianshan, coeval with the Middle Devonian subduction of the North Tianshan oceanic plate. Grt-I to Grt-III particles are mainly andradite with minor grossularite, and they have different REE compositions. The Grt-I has total REE contents (SREE) of 101.47 to 262.87 ppm, with steeply right-inclining REE distribution patterns (SREE/HREE of 3.81 to 68.50) and positive Eu anomaly. The Grt-II core has SREE of 163.49- 249.52 ppm, LREE/HREE of 2.00-4.71, and negative Eu anomaly. The Grt-II rim has SREE of 46.34-99.99 ppm, with SREE/HREE of 18.06-177.23, and positive Eu anomaly. The SREE of Grt-III range from 31.71 to 219.02 ppm, with flat REE distribution pattern and positive Eu anomaly, and the LREE/HREE ranges from 2.16 to 9.07. These garnets have similar trace element compositions, featured by LILEdepletions (e.g., Rb, Ba and Sr) and HFSE enrichments (e.g., Th, U, Nb and Ce). Micro texture and geochemical composition of garnets indicate that the Lamasu magmatic hydrothermal system could have changed from an open to a closed environment, and from infiltration metasomatism to diffusive metasomatism, which formed Grt-I, Grt-II and Grt-III successively. These garnets generally formed in a relatively oxidized fluid environment, which inhibited early sulphide precipitation and favoured for Cu enrichment during the ore fluid evolution.