Chemical responses to hydraulic fracturing and wolframite precipitation in the vein-type tungsten deposits of southern China

Abstract

Wolframite is the main ore mineral in the vein-type tungsten deposits of southern China. Much progress has been made on the characteristics of the mineralizing fluids, but the mechanisms of wolframite precipitation remain poorly understood. Hydraulic fracturing driven by high-pressure fluids is a common mechanical process during magmatic-hydrothermal transition, but it is uncertain whether and how this mechanical process may affect chemical equilibrium and cause wolframite precipitation. This paper examines how a hydraulic fracturing process affects solubility of tungsten in CO2-saturated NaCl solutions using a hydro-mechanical numerical model coupled with a multi-component thermodynamic model. The thermodynamic model presented here is in the system of Fe-W-Cl-Na-C-O-H. The modeling results indicate that fluid pressure exerts a significant influence on chemical equilibrium where CO2 solubility in NaCl solutions decreases with decreasing fluid pressure and pH increases with decreasing fluid pressure. An increase in pH reduces the concentrations of the dominant iron-bearing species (FeCl20) and the dominant tungsten-bearing species (HWO4−) in fluids. Tungsten solubility in fluids reaches tens of ppm. Over ten fluctuations of fluid pressure are identified in the numerical experiments of hydraulic fracturing. These pressure fluctuations cause a decrease in solubility of tungsten by over 30% of the maximum solubility. Repeated drops of fluid pressure during hydraulic fracturing processes cause CO2 loss and could be efficient processes for precipitating wolframite from mineralizing fluids. These findings may also offer an insight into the precipitation mechanisms of other metals from CO2-bearing hydrothermal fluids.