The formation of the Late Cretaceous Xishan Sn-W deposit, South China: Geochronological and geochemical perspectives

The Xishan Sn-W deposit is spatially related to K-feldspar granites in the Yangchun basin, western Guangdong Province, South China. LA-ICP-MS zircon U-Pb dating for the Xishan pluton defines an emplacement age of ~ 79 Ma (78.1 ± 0.9 Ma; 79.0 ± 1.2 Ma; 79.3 ± 0.8 Ma), consistent with the mineralization age of the Xishan Sn-W deposit constrained by molybdenite Re-Os isochron age (79.4 ± 4.5 Ma) and LA-ICP-MS cassiterite U-Pb ages (78.1 ± 0.9 Ma and 79.0 ± 1.2 Ma) for the cassiterite-quartz vein. These indicate a close genetic relationship between the granite and Sn-W mineralization. The Xishan K-feldspar granites have geochemical characteristics of A-type granites, e.g., high total alkali (Na2O + K2O = 7.88-10.07 wt.%), high Ga/Al ratios (10000*Ga/Al > 2.6) and high Zr + Nb + Ce + Y concentrations (> 350 ppm). They are further classified as A2-type granites. The whole-rock isotopic compositions of K-feldspar granites (initial 87Sr/86Sr = 0.705256-0.706181; εNd(t) = − 5.4 to − 4.8) and zircon εHf(t) values (− 7.8 to 2.0) suggest a mixed magma source. The low zircon Ce4 +/Ce3 + ratios (12-88) of K-feldspar granites suggest low oxygen fugacities, which is key for enrichment of tin in primary magmas. The K-feldspar granites have experienced strong differentiation as indicated by their high Rb/Sr and K/Rb ratios, and low Nb/Ta and Zr/Hf ratios, which play an important role in ore-forming element transportation and concentration. A-type granite characteristics of the Xishan pluton show that it formed in an extensional environment. The high F and low Cl characteristics of the K-feldspar granite are most probably attributed to slab rollback. In the Late Cretaceous, the Xishan Sn-W deposit was located near the interaction of the circum-Pacific and the Tethys tectonic realms. Late Cretaceous Sn-W deposits, including the Xishan deposit, form an EW-trending belt from Guangdong to Yunnan Province in South China. This belt is in accordance with the direction of the Neo-Tethys slab rollback in the Late Cretaceous. In addition, the NS-trending extension has been recognized in the Late Cretaceous in South China. We propose that the Xishan Sn-W deposit should be attributed to the Neo-Tethys slab rollback in the Late Cretaceous.