Tungsten mineralization in the Huangjindong gold deposit, southern China: Insights from scheelite texture, in-situ trace elements and Sr isotope compositions

Scheelite and wolframite are two primary tungsten minerals in nature. The occurrence of tungsten minerals varies in different types of deposits, with scheelite rather than wolframite typically forming in orogenic gold deposits. However, the explanation for this phenomenon remains unclear. Huangjindong deposit, located in the center of the Jiangnan Orogenic Belt in southern China, is a typical orogenic gold deposit with scheelite mineralization. Texture, in-situ trace elements and Sr isotope compositions of scheelite from this deposit were investigated to explore the formation process of scheelite and the source of tungsten of similar orogenic gold deposits. Three hydrothermal stages have been identified at this deposit: milky scheelite + quartz (pre-gold Stage I); gold + sericite + sulfide + quartz (syn-gold Stage II); and dolomite + quartz (post-gold Stage III). Scheelite primarily occurs during the initial pre-gold stage. Local color differences in CL image have developed along rims and in domains adjacent to fractures of scheelite due to slight late-stage hydrothermal alteration of scheelite along their fractures. The intense development of sericite and chlorite alteration in the host rocks indicates that the water-rock reaction consumed a large amount of H+. Within the scheelite-quartz veins, the presence of brecciated slate enclaves and localized quartz geodes, indicating that the fluid experienced significant pressure drop. This indicates that the water-rock reaction and the decrease in pressure resulted in the precipitation of scheelite. The dominance of tungsten mineral precipitation due to pressure reduction serves as the main factor driving the development of scheelite over wolframite within the deposit, which also elucidates the reason that tungsten mineralization associated with orogenic gold deposits predominantly features scheelite. The characteristics of scheelite without oscillatory zoning, high Sr (1180-8600 ppm) and low Mo (bdl-0.18 ppm) concentrations, and flat REE pattern are similar to those of scheelite in orogenic gold deposits worldwide. The high 87Sr/86Sr ratio (0.7539-0.7663) indicates that the tungsten-bearing ore-forming fluids derived from deep-seated metamorphic fluids.