The relative solubilities of wolframite and scheelite in hydrothermal fluids: Insights from thermodynamic modeling

Abstract

The occurrence of either scheelite (CaWO4) or wolframite ([Fe,Mn]WO4) in tungsten deposits is generally explained by the Ca content of the host rocks, but scheelite/wolframite ratios in granitoids-hosted tungsten deposits are highly variable, ranging from scheelite-dominated to wolframite-dominated. Wolframite is also commonly replaced by scheelite or vice versa- among many granitoids-hosted tungsten deposits. To fill this gap in understanding the stabilities of wolframite and scheelite under hydrothermal conditions, the relative solubilities of scheelite and wolframite in hydrothermal fluids were constrained using two modeled chemical systems: W-Ca-Mn-Cl-Na-O-H and W-Ca-Fe-Cl-Na-O-H. The two modeled chemical systems can well reproduce the available experimental data for scheelite, ferberite, and hübnerite solubilities under hydrothermal conditions. The modeling results indicate that scheelite is more soluble than hübnerite and ferberite under most W-mineralizing conditions (300–400 °C, 500–1500 bars, and 1–3 mol/kg NaCl). Scheelite can replace ferberite (hübnerite) over a wide range of temperatures and pressures; therefore, the replacement of ferberite (hübnerite) by scheelite does not necessarily represent late stages of mineralization. Two mechanisms can cause scheelite to replace ferberite (hübnerite). The first mechanism is cooling with an increase in the ∑Ca∑Fe (∑Ca∑Mn) molality ratio in fluids, and the second mechanism is a decrease in fluid pressure with constant ∑Ca∑Fe (∑Ca∑Mn). Neither leaching Ca from Ca-rich wallrocks nor removing Fe (Mn) from hydrothermal fluids is required for the second mechanism. The second mechanism may account for vein-type or disseminated scheelite mineralization in host rocks whose Ca is low compared to carbonates.