Formation of the Sanchachong skarn W deposit and associated weakly evolved granitoids in South China: Insights from whole-rock and mineral geochemistry

Abstract

Globally, many tungsten (W) deposits are associated with highly evolved magma systems, but some W deposits are genetically linked with weakly evolved granitoids. Nevertheless, the key factors controlling the W enrichment in such systems remain obscure. The ∼106 Ma Sanchachong W deposit (50 Kt WO₃ at 0.35 %) from the southern Qinzhou–Hangzhou belt of South China is a typical example, with the causative intrusion being a granodiorite. In this contribution, we unravel the magmatic processes, volatile compositions, and ore-forming metal characteristics at Sanchachong, shedding light on the mechanisms driving W mineralization of weakly evolved granitoids. The geochemical characteristics of bulk rocks and key minerals (zircon, apatite, and biotite) jointly denote that the ore-related intrusion is a weakly evolved I-type granitoid. The magmas have experienced the slight-moderate separations of amphibole and biotite, as evident by the elevated Sr/Y ratios and low Rb/Sr and Rb/Ba ratios. Additionally, the low zircon δ¹⁸O of 6.5 to 8.2 ‰ and apatite (⁸⁷Sr/⁸⁶Sr) of 0.7071 to 0.7077, and high apatite ε_Nd(t) of −3.2 to −7.5 indicate derivation from lower continental crustal sources. Distinct from adjacent barren weakly evolved S-type granites, the fertile Sanchachong granodiorites exhibit: (1) elevated magmatic water content, as demonstrated by high zircon H₂O contents (330–490 ppm), amphibole chemistry calculations (∼7.3 wt% H₂O) and apatite Sr/Th ratios (37.8–123); (2) intensive fluid exsolution signatures, reflected in low apatite X_Cl/X_OH ratios of 0.01–0.04 and highly variable X_F/X_Cl ratios of 100–380; and (3) low F and W concentrations in melt, as evidenced by apatite X_F < 0.89, biotite F < 0.85 wt% and W < 0.6 ppm. These characteristics, resembling equivalent granitoids associated with W deposits from the Jiangnan ore belt, reflect that an enrichment in water is more vital to promote W mineralization for weakly evolved granitoids compared with high magmatic W and F contents.