The petrogenesis and metallogenesis of the ore-forming granites in the Tongmukeng Sn deposit, Jiangnan Orogenic Belt, South China

The Tongmukeng tin deposit is situated in the Yunshan area of the Jiangnan orogenic belt in South China. The ore body is primarily distributed along the joints and fractures within the granites, with some occurrences found in the contact zones between granite and surrounding rocks. Two types of granites, fine- and coarse-grained granites related to the ore body, were comprehensively investigated using whole-rock geochemistry, zircon and cassiterite U-Pb dating, zircon Lu-Hf isotopes, as well as whole-rock and cassiterite oxygen isotopes. The zircon U-Pb ages of the two types of granites are constrained to ∼ 127 Ma, which is consistent with the cassiterite U-Pb age of the altered granite at ∼ 133 Ma. This consistency suggests a genetic relationship between the Tongmukeng granite and tin mineralization during the Early Cretaceous. The two types of granites show elevated levels of K2O + Na2O and high Al2O3/(MgO + FeOT) and A/CNK ratios, indicative of an S-type granite affinity. The high Rb/Sr ratios and low Nb/Ta and Zr/Hf ratios further suggest highly fractionation in the two types of granites. Zircon εHf(t) values of both granites ranging from −12.0 to −0.47 indicate that the primary sources of these granites predominantly derived from the mature materials of the surrounding Shuangqiaoshan Group. The major element compositions reveal that biotite in the granites underwent re-equilibration at low temperatures (<500 °C) and low oxygen fugacity. However, the low Ce4+/Ce3+ values of < 5 in Cretaceous zircons from the two granites indicate that the granitic magma exhibited a reductive nature, with log(fO2) values below −18. Cassiterite with a low-δ18O value of ∼ 4.68 ‰ is notably lower than the normal mantle δ18O values of 5.3 ± 0.3 ‰ and those of altered and fresh granites (7.85–10.37 ‰). This suggests the involvement of meteoric water during the formation of cassiterite. Therefore, the high degree of fractionation and low oxygen fugacity play a key role in the enrichment of tin, while the rise in oxygen fugacity and fall in temperature caused by the injection of meteoric water along faults and fractures are crucial for the migration and precipitation of tin. Nevertheless, the decoupling distribution of W and Sn deposits in northern Jiangxi Province (NJP) remains unclear and requires further investigation to elucidate the genesis of W and Sn deposits.