Fluid evolution and mineralization mechanisms in Helong quartz vein-type W-Cu deposit: insights from fluid inclusion microthermometry and in situ geochemical analysis of minerals

The Helong W-Cu deposit, located within the Nanling region, contains quartz vein type W and Cu mineralization. The Helong deposit is a prime example of a polymetallic mineralization characterized by significant and economically valuable resources of both W and Cu. Specifically, it hosts substantial reserves of 40,000 tonnes of WO₃ and 2000 tonnes of Cu, highlighting its dual-metal endowment. Three mineralization stages were identified, characterized by wolframite-quartz veins, scheelite-quartz veins and chalcopyrite-quartz veins, respectively. We employed techniques such as fluid inclusion microthermometry and in situ trace element analysis of minerals to explore the nature of the fluids responsible for W and Cu mineralization. The analytical results reveal that liquid-dominated aqueous fluid inclusions are recorded in both wolframite, scheelite and quartz associated with chalcopyrite. These liquid-dominated aqueous fluid inclusions contain a vapor bubble, with the remainder occupied by liquid NaCl-H₂O. Microthermometric analyses indicate that the fluid inclusions in the early-stage wolframite have homogenization temperatures and salinities of 294–338 °C and 6.4–8.9 wt% NaCl equivalent, respectively, suggesting that the early-stage ore-forming fluid was characterized by high temperature and medium salinity. The broad range of homogenization temperatures suggests that natural cooling of the fluid was the primary factor triggering the precipitation of wolframite. The transition from the wolframite stage to the scheelite stage is characterized by a consistent decrease in the homogenization temperatures and salinities of fluid inclusions, suggesting that the precipitation of scheelite may be attributed to fluid mixing, which may be accompanied by fluid-rock interaction. Fluid inclusions in quartz associated with chalcopyrite have homogenization temperatures of 194–233 °C and salinities of 4.4–6.1 wt% NaCl equivalent, indicating that the latest stage of fluid evolution was characterized by low temperature and low salinity. The absence of boiling fluid inclusion assemblages (which would indicate phase separation) and the relatively broad range of homogenization temperatures suggest that simple cooling—rather than fluid boiling or mixing—was the primary mechanism for chalcopyrite precipitation. Wolframite is significantly enriched in heavy rare earth elements (HREE) and high field strength elements such as Nb and Ta, exhibiting a typical left-leaning pattern. This pattern, coupled with the crystal-chemical mechanism of REE partitioning in wolframite (i.e., preferential incorporation of HREE into its crystal lattice relative to LREE), suggests that the ore-forming fluid for wolframite was derived from a highly fractionated granitic magma—consistent with the geochemical signatures of HREE-enriched fluids exsolved from fractionated granitoids. Most scheelite samples display a typical positive Eu anomaly and have low Mo contents, suggesting a reducing fluid environment. However, a small number of scheelite samples exhibit negative Eu anomalies, indicating that scheelite crystallization occurred under dynamic redox conditions.