Redox-controlled hydrothermal remobilization of stratabound Zn and Pb mineralization: A case study of the Jiashengpan deposit, northern China

The Jiashengpan deposit in the Langshan–Zhaertai belt, northern China, is a large Zn–Pb deposit in which the present high-grade orebodies have been interpreted as products of late hydrothermal remobilization of earlier stratabound Zn–Pb enrichment. However, the physicochemical process responsible for this remobilization remains unclear. In this study, ore petrography, Raman analysis of fluid inclusions, and thermodynamic modeling were combined to constrain fluid composition and evolution and the mechanism of Zn–Pb remobilization during the late hydrothermal stage. Petrographic observations have identified an earlier pyrite–dolomite–quartz assemblage and a later pyrrhotite–sphalerite–galena–quartz assemblage within the same hydrothermal vein. The replacement of pyrite by pyrrhotite and sphalerite, together with the absence of cross-cutting relationships between these assemblages, indicates progressive mineralogical evolution rather than two separate hydrothermal events. Raman analyses of quartz-hosted fluid inclusions, together with previous fluid inclusion studies, indicate that the late hydrothermal fluid involved in Zn–Pb remobilization was a low-salinity, carbon-bearing hydrothermal fluid that evolved from CO 2 -rich to reduced CH 4 -rich conditions. CH 4 -rich inclusions occur in quartz coexisting with pyrrhotite, sphalerite, and galena, suggesting that Zn–Pb sulfide precipitation was closely related to the reduced fluid. Thermodynamic modeling indicates that the calculated Zn and Pb solubilities in the low-salinity, CO 2 -rich hydrothermal fluid are about 6–20 ppm and about 2 ppm, respectively. These values are at the lower end of the concentration range reported for natural ore-forming hydrothermal fluids, indicating that the low-salinity, CO 2 -rich hydrothermal fluid can remobilize Zn and Pb but has a limited metal-transport capacity. When the fluid evolved to CH 4 -rich conditions, total Zn solubility decreased by about one to three orders of magnitude, to approximately 0.0026–0.65 ppm, leading to the precipitation of sphalerite. In contrast, the calculated Pb solubility is lower in the CH 4 -rich fluid than that in the CO 2 -rich fluid below 300 °C, but increases rapidly at higher temperatures. This indicates that galena precipitation is favored by fluid reduction and further promoted by cooling of the reduced fluid. Flow-through modeling further indicates that interaction with organic matter in the carbonaceous host rocks can drive the evolution of the fluid from CO 2 -rich to CH 4 -rich conditions and promote substantial Zn–Pb sulfide precipitation. In the flow-through model without organic matter, this redox evolution was limited, and the modeled amounts of precipitated sphalerite and galena were one to two orders of magnitude lower than those in the model with organic matter. These results indicate that late Zn–Pb remobilization and mineralization at Jiashengpan were mainly redox-controlled. Due to its limited capacity to transport metal, the late hydrothermal fluid is unlikely to transport large amounts of Zn and Pb over long distances. The present high-grade orebodies at Jiashengpan are therefore best interpreted as products of local remobilization and redistribution of pre-existing stratabound Zn–Pb enrichment during interaction between a carbon-bearing hydrothermal fluid and carbonaceous host rocks.