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

Abstract

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 ~79Ma (78.1±0.9Ma; 79.0±1.2Ma; 79.3±0.8Ma), consistent with the mineralization age of the Xishan Sn–W deposit constrained by molybdenite Re–Os isochron age (79.4±4.5Ma) and LA-ICP-MS cassiterite U–Pb ages (78.1±0.9Ma and 79.0±1.2Ma) 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=7.88–10.07wt.%), high Ga/Al ratios (10000*Ga/Al>2.6) and high Zr+Nb+Ce+Y concentrations (>350ppm). 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.