Neoproterozoic W[sbnd]Sn mineralization in the western Jiangnan Orogen, South China: Insights from the Fanjingshan S-type granites

Abstract

The Jiangnan Orogen boasts abundant Neoproterozoic granitoids alongside locally significant WSn mineralization in South China. However, the factors governing the relationships between granites and the WSn mineralization remain elusive. Notably, Neoproterozoic WSn mineralization locally occurred within the western margin of the Jiangnan Orogen, encompassing important deposits like the Jiumao, Baotan and Heiwanhe deposits. The W-Sn-bearing tourmaline muscovite (Taoshulin) granite intruded into Neoproterozoic Fanjingshan Group, comprising of low-degree metamorphosed sedimentary rocks that locally contain granitic gravel (two-mica granite). These granites in this study have high SiO2 (73.6–77.4 wt%), Al2O3 (12.8–15.1 wt%), Na2O (3.16–5.25 wt%) and high A/CNK values (1.28–1.47), which display co-variation trends. They have extremely low rare earth element (REE) concentrations (< 35 ppm) and exhibit rare earth element (REE) patterns characterized by strong negative Eu anomalies (< 0.28), implying highly fractional crystallization. Zircon and apatite LA-ICP-MS UPb dating reveal that the emplacement of the Taoshulin tourmaline muscovite granite occurred around 830 Ma, similar to the UPb age (835 ± 7 Ma) of cassiterite, establishing the clear temporal-genetic relationship between granites and WSn mineralization. The zircon UPb age of 847 ± 14 Ma and the apatite UPb age of 852 ± 7 Ma obtained from the two-mica granite likely indicate a ca. 850 Ma granitic magmatism event. These two-episode granites exhibit negative εHf(t) values (+0.9 to −8.0), corresponding to two-stage Hf model ages of 1.7 to 2.2 Ga, indicating their derivation from Paleoproterozoic metasedimentary basement rocks. It is noteworthy that the W-Sn-bearing granite has high tetrad effect (TE1,3 = 1.16–1.24) and very low Nb/Ta ratios (mostly bellow four) that may have resulted from magmatic-hydrothermal alteration. Moreover, the ca. 830 Ma W-Sn-bearing granite is characterized by a notable concentration of high fluorine, distinctively negative Eu anomalies, and non-chondritic REE behavior in apatite. Hence, the WSn mineralization in the Fanjingshan region was probably constrained by various factors, including source materials, magmatic processes, physicochemical conditions and magmatic-hydrothermal processes. Our findings highlight the pivotal role played by Neoproterozoic muscovite tourmaline granites as hosts for WSn mineralization in the Fanjingshan region. In summary, this study emphasizes the significance of the ca. 830 Ma highly differentiated S-type granites holding significant potential for the exploration of critical metals such as W, Sn, Nb, and Ta in the western part of the Jiangnan Orogen and in geologically analogous regions across the globe.