Weakly fractionated I-type granitoids and their relationship to tungsten mineralization: A case study from the early Paleozoic Shangmushui deposit, Dayaoshan area, South China

Tungsten deposits related to weakly fractionated granitoids are scarce worldwide. The geological significance of early Paleozoic magmatism and associated mineralization in South China remains poorly understood compared with well-documented Mesozoic magmatism and related mineralization. Early Paleozoic magmatic rocks, previously thought to be only rarely associated with mineralization, crop out widely in the Dayaoshan area, South China, and have significant prospecting potential for tungsten polymetallic deposits. Shangmushui is a newly discovered W-bearing skarn (with minor Cu) deposit in the Dayaoshan area. The zircon U–Pb age of the granodiorite that hosts the tungsten deposit is 465.4 ± 1.5 Ma, which is consistent with scheelite Sm–Nd age (466.6 ± 5.5 Ma) within the error. The present data, combined with previous findings, indicate a protracted period of magmatism and related W–Mo polymetallic mineralization during 470–420 Ma in the Dayaoshan area. Elevated zircon δ¹⁸O_V-SMOW values (5.5‰ to 6.6‰) and mostly negative ε_Hf(t) values (–9.5 to +0.7), together with T_DM2 of 1.6 Ga indicate that these plutons formed via partial melting of Mesoproterozoic crust with some contributions from mantle materials. Estimates of the oxygen fugacity of the magmas show that the Shangmushui granodiorite is relatively reduced (FMQ–0.11). Shangmushui granodiorite is a weakly fractionated I-type granitoid with low differentiation index values (DI = 59.6–66.9), B contents (8.6–30.1 ppm), and Rb/Sr ratios (0.34–0.73), which are different from most highly fractionated tungsten-bearing S- and I-type granitoids globally. The low oxygen fugacity and high W contents of the granodiorites, and the close spatio-temporal relationship between the granodiorites and the tungsten ore bodies, indicate that early Paleozoic weakly fractionated I-type granitoids in the Shangmushui region have a close genetic relationship with tungsten mineralization.