Genesis and magmatic-hydrothermal evolution of the Silurian Pingtan W deposit, Nanling Range, South China: Constraints from scheelite and apatite geochemistry and geochronology

A cluster of Silurian W mineralization events have been reported in South China but the ore-forming processes remain not well-constrained, impeding a comprehensive understanding of deposit genesis and hindering the deployment of further exploration programs. In this study, we examine the newly discovered Silurian Pingtan W deposit as a case study. We undertook a comprehensive study of microtextures and geochemistry on scheelite and apatite, and U–Pb geochronology on apatite to determine the timing of W mineralization as well as the origin and evolution of the ore-forming fluids. Based on BSE and CL images, we identify two types of scheelite (Sch) and apatite (Ap): Sch A (A1 and A2) and Ap I in the altered granite-scheelite stage, and Sch B (B1 and B2) and Ap II in the quartz-scheelite stage. The Mo contents and Eu/Eu* ratios of multi-stage scheelite suggest a decrease in fluid fO₂ from the early to late stages within each mineralization phase. The Eu/Eu* ratios of multi-stage apatite indicate an increase in fluid fO₂ from the altered granite-scheelite stage to the quartz-scheelite stage. The Y/Ho ratios of these phases of apatite and scheelite growth imply that the initial fluids originated from the host porphyritic biotite monzogranite, and early-stage apatite crystallization led to the depletion of Y and Ho in scheelite. A support vector machine biplot shows that Ap I is magmatic in origin, while Ap II is hydrothermal in origin. The hydrothermal Ap II yields a U–Pb age of 433.7 ± 9.7 Ma (2σ, MSWD = 1.9), aligning with the emplacement ages (431–430 Ma) of the porphyritic biotite monzogranite. These data imply that the Pingtan W deposit represents a typical altered-granite type W deposit genetically linked to its host Silurian porphyritic biotite monzogranite. In addition, the estimated F and Cl contents in melt and fluid based on their contents in apatite indicate that the volatile-rich altered porphyritic biotite monzogranite has great W mineralization potential. Considering previous studies on the Silurian W mineralization, we propose that the Silurian may represent another significant period for W mineralization in South China, with contemporaneous highly evolved and volatile-rich granites serving as a new exploration target for W deposits.