Fine-grained tracing of mineralization process in the Shimensi giant tungsten deposit, Jiangnan Orogenic Belt: Constraints from in situ analysis of quartz and scheelite

The Shimensi super-large W-polymetallic deposit in northern Jiangxi represents a significant discovery within the Jiangnan Orogen in recent years, and its metallogenic mechanism holds substantial research importance. To elucidate the origin and evolution of ore-forming fluids, this study employed in situ analytical techniques, including SEM-CL, LA-ICP-MS and SIMS, to systematically investigate the compositional and oxygen isotopic characteristics of quartz from mineralization stages (pre-ore stage and three ore-forming stages) and scheelite from three generations. The results indicate that the quartz in early-stage Qtz1 (pre-ore) and Qtz2 [wolframite (Qtz2-1)-scheelite (Qtz2-2) assemblage stage] exhibit oscillatory zoning, while that in the late-stage Qtz3 (molybdenite stage) and Qtz4 (chalcopyrite stage) lack zoning. The Ti content in quartz decreases from 10.9×10-6 in pre-ore stages to 5.77×10-6 in the wolframite stage, 5.57×10-6 in the scheelite stage, 3.45×10-6 in the molybdenite stage and 0.63×10-6 in the late chalcopyrite stage, suggesting a progressive decline in mineralization temperature. Meanwhile, the Al content decreases from 174×10-6 to 85.8×10-6, reflecting increasing fluid alkalinity (pH elevation). Quartz-derived δ18OFluid values range from 0.69‰ to 10.76‰, revealing magmatic-dominated fluids (δ18OFluid=9.55) during early W-Mo mineralization and significant meteoric water input (δ18OFluid=1.40) in the late Cu stage. Scheelite textures and REE patterns demonstrate the following distinct characteristics: (1) Sch1 (wolframite stage) displays subhedral crystals with oscillatory zoning, LREE enrichment, and Eu negative anomalies, where REEs substitute Ca2+ via a vacancy-coupled mechanism (3Ca2+↔2REE3++□Ca); (2) Sch2 (scheelite stage) exhibits euhedral morphology with Eu positive anomalies; (3) Sch3 (molybdenite-chalcopyrite stage) shows euhedral-subhedral crystals, HREE enrichment and Eu positive anomalies; (4) Sch2 and Sch3 following the substitution mechanism 2Ca2+= REE3++Na+. The rising Mo content in scheelite further supports the rising trend of the oxygen fugacity. This study demonstrates that the mineralization process of the Shimensi deposit experienced a progressive process of the oxygen fugacity increase, stepwise alkalinity increases in ore-forming fluids, and temperature decrease of the ore-forming fluids. In the early stage, the low-oxygen fugacity environment was conducive to the precipitation of wolframite; while the late-stage high oxygen fugacity caused by fluid alkalinity increase and redox front migration was crucial for massive scheelite deposition. Sulfide mineralization (molybdenite, chalcopyrite) was synergistically controlled by cooling, alkalinity increase, and meteoric water mixing. The geochemical characteristics of quartz and scheelite provide crucial evidence for revealing the fluid source and evolution of super-large tungsten deposits.