The precipitation mechanisms of scheelite from CO2-rich hydrothermal fluids: Insight from thermodynamic modeling

Abstract

Scheelite and wolframite are two main tungsten-bearing ore minerals in tungsten deposits. Compared to wolframite formed mostly by NaCl–H₂O fluids, scheelite in many but not all tungsten deposits is associated with CO₂–bearing hydrothermal fluids, but its precipitation mechanisms from these fluids are poorly understood. The replacement of wolframite by scheelite is common among tungsten deposits where these two tungstates coexist, but how CO₂, as the pH-buffering agent, affects their replacement remains unclear. To answer these questions, we first tested the pH-buffering effect of CO₂ by comparing available experimental pH values of H₂O–CO₂±NaCl solutions at 200–280 °C and the corresponding thermodynamic modeling results. The mean absolute errors between the calculated and experimental pH values are 0.14 log units in H₂O–CO₂ solutions and 0.13 log units in H₂O–CO₂–NaCl solutions, indicating that the calculated acitivities of H⁺ in CO₂-bearing solutions are reliable. Then, the solubilities of scheelite, scheelite-ferberite, and scheelite-hübnerite in NaCl–H₂O–CO₂ systems were modeled in this study. Our modeling results suggest that scheelite solubility in CO₂-rich fluids is highly dependent on fluid pressure but is insensitive to fluid temperature. A decrease in fluid pressure from lithostatic to hydrostatic levels at a depth of 3–8 km causes CO₂ escape and pH rise and precipitates over 70 % of tungsten contents in fluids on average. Therefore, CO₂ escape is an efficient mechanism for precipitating scheelite from CO₂–rich hydrothermal fluids. The independence of scheelite solubility on temperature at high pressures and the slightly retrograde solubility at low pressures allows CO₂–rich hydrothermal fluids to carry a great amount of tungsten far away from its sources. This may be one of reasons why some scheelite deposits occur at greater distances (>500 m) from the granite or extend along its strike for several kilometers. Similar to the cases in NaCl–H₂O systems, two mechanisms for scheelite replacing wolframite in CO₂–rich fluids are identified, a single decrease in fluid pressure or simple cooling plus an increase in the ratios of total Ca contents to total Fe (or Mn) contents in fluids.