• W-Sn mineralization processes in the Shanhu W-Sn deposit can be divided into three stages. • The magmatic hydrothermal fluids were progressive diluted by meteoric water. • Cassiterite precipitation resulted from fluid cooling and redox reactions. • Wolframite deposition was triggered by fluid mixing. The Shanhu W-Sn deposit, a super-large quartz-vein type deposit within the Nanling Range of South China, exhibits a distinct mineralization age (∼100 Ma) compared to other deposits in the region. Despite its significance, the ore-forming mechanism remains poorly constrained. To elucidate the origin and evolution of ore-forming fluids and the precipitation mechanisms of W and Sn, we conducted a comprehensive study of fluid inclusions and oxygen isotopic compositions in cassiterite and wolframite. Based on vein types and mineral assemblages, W-Sn mineralization in the deposit can be divided into three stages: the muscovite-cassiterite stage (Stage I), the quartz-wolframite-sulfide stage (Stage II), and the calcite-scheelite stage (Stage III). Stage I is characterized by fluid inclusions in cassiterite (216-324°C, 6.2–7.8 wt% NaCl eq ) and quartz (175-291°C, 3.1–12.4 wt% NaCl eq ), with associated fluorite showing lower temperature-salinity ranges (177-215°C, 2.9–7.9 wt% NaCl eq ). Stage II exhibits fluid inclusions in wolframite (183-355°C, 3.2–7.2 wt% NaCl eq ) and quartz (136-267°C, 2.4–6.6 wt% NaCl eq ), with δ 18 O fluid values of + 3.1-+5.1‰, indicating significant meteoric water influx relative to Stage I (δ 18 O fluid =+11.2-+12.0‰). Stage III contains the lowest-temperature-salinity fluid inclusions in fluorite (144-192°C, 0.9–1.6 wt% NaCl eq ). Raman spectroscopy reveals a fluid evolution from CO 2 –CH 4 -bearing (Stage I) to CO 2 -dominated (Stage II). The progressive decrease in temperature and salinity, coupled with increasing oxygen fugacity from Stage I to III, reflects the dilution of magmatic fluids by meteoric water. In-situ oxygen isotope variations in single cassiterite and wolframite crystals provide critical constraints on the precipitation mechanisms of ore minerals. Cassiterite precipitation in Stage I resulted from fluid cooling and redox reactions between Sn (II)-Cl complexes and As (III). In contrast, wolframite deposition in Stage II was triggered by a significant influx of meteoric water that reduced temperature and salinity.
Cheng et al. (2026) studied this question.