Ore-forming processes of the Dafulou tin deposit in the Dachang orefield: Evidence from geological, petrographic, fluid inclusion, and sulfur isotope analyses

The Dafulou tin deposit, a representative deposit in the eastern ore zone of the Dachang tin orefield (located in the Youjiang Basin of South China), exhibits distinctive geological features that provide important insights into the ore-forming processes and genetic model of tin-polymetallic mineralization. Based on petrographic observations, sulfur isotope analysis, and fluid inclusion studies—integrated with the geological characteristics of the deposit—the following findings were obtained: The spatial distribution of ore bodies in the Dafulou deposit is jointly controlled by NW-trending deep faults and an interstratified fracture zone within the first segment of the Devonian Tangding Formation. The ore-forming process can be divided into three stages: (1) the sulfide stage (Stage 1: pyrrhotite + pyrite + sphalerite + stannite + minor cassiterite and arsenopyrite); (2) the cassiterite-sulfide stage (Stage 2: cassiterite + arsenopyrite + pyrrhotite + pyrite + stannite); and (3) the calcite stage (Stage3: minor pyrrhotite + pyrite with calcite). High-grade cassiterite-sulfide and quartz stockwork-type ore bodies formed during Stage 2 are superimposed upon or crosscut the lower-grade sulfide-type ore bodies formed during Stage 1. Although fluid inclusions in quartz from Stages 1 and 2 show broadly similar homogenization temperatures and salinities (344–380 °C and 16.62–20.27 wt%, respectively), a higher abundance of hydrocarbon-bearing inclusions (10–15 %) is observed in Stage 1. In addition, the δ³⁴S values of sulfides from Stage 1 (−14.9 ‰ to −9.0 ‰) differ significantly from those of Stage 2 (centered around −3 ‰), indicating the involvement of organic matter in the sulfur cycle, including bacterial sulfate reduction, thermochemical sulfate reduction, and pyrolysis of organic matter. These data suggest that the Stage 1 ore-forming fluid was generated by the mixing of stratum-derived, organic sulfur-rich fluids with magmatic fluids, while the Stage 2 fluid was derived solely from a magmatic source. Mineralogical evidence indicates a paragenetic sequence from stannite to cassiterite, reflecting a transition from a strongly reducing environment in Stage 1—characterized by high sulfur fugacity (ƒS₂) and low oxygen fugacity (ƒO₂)—to a more oxidizing and alkaline environment with decreasing ƒS₂ and increasing ƒO₂ and pH. Moreover, the assemblage of cassiterite with arsenopyrite and calcite suggests that fluid boiling (CO₂ escape) and redox reactions involving As(III) complexes were the primary mechanisms responsible for large-scale cassiterite precipitation. In summary, the Dafulou deposit is classified as a distal magmatic-hydrothermal system, whose formation likely involved pulsating ascent of tin-rich fluids driven by a concealed deep pluton, followed by differential unloading during fluid mixing with organic sulfur-rich fluids from the surrounding strata (Stage 1) and subsequent fluid boiling (Stage 2) within the ore-hosting space. This process ultimately resulted in the formation of the orebody framework and superimpositional mineralization features. This study provides a theoretical basis for future deep exploration in the region.