L. C. Hsu
Effects of oxygen and sulfur fugacities on the scheelite-tungstenite and powellite-molybdenite stability relations
Economic Geology and the Bulletin of the Society of Economic Geologists (July 1977), 72(4):664-670

Abstract:
The stability relations between scheelite and tungstenite and between powellite and molybdenite were experimentally determined on the basis of the following reactions:CaWO ₄ + SiO ₂ + S ₂ = CaSiO ₃ + WS ₂ + 3/2 O ₂ CaMoO ₄ + SiO ₂ + S ₂ = CaSi ₃ + MoS ₂ 3/2 O ₂ Under hydrothermal conditions, the oxygen and sulfur fugacities for the above reactions were controlled simultaneously by a single or combination of solid buffer assemblages. At P _f = 1,000 bars and T = 577 degrees C, the equilibrium curve for reaction (1) passes through these points on the log f (sub S ₂ ) - log f (sub O ₂ ) diagram: (-3.0, -21.7), (--7.1, -24.4), and (-12.9, --28.3), whereas the equilibrium curve for reaction (2) passes through these points on the same diagram: (-3.0, -16.3), (-7.1, -19.0), and (-12.9, -22.9).With a proper supply of Ca, Mo, and W, the formation of powellite requires over five orders of magnitude of f (sub O ₂ ) higher than the formation of scheelite unler the same f (sub S ₂ ) conditions. The restriction of f (sub S ₂ ) imposed upon the formation of tungstenite as compared with molybdenite is even more rigorous under the same f (sub O ₂ ) conditions, i.e., over eight orders of magnitude difference. The common natural association of scheelite with molybdenite is consistent with their wide overlap of log f (sub S ₂ ) -- log- f (sub O ₂ ) stability fields. It is also concluded that powellite cannot occur with tungstenite and the formation of either of the two minerals alone requires an unusual geological environment, i.e., very high f (sub O ₂ ) /low f (sub S ₂ ) for the former and very low f (sub O ₂ ) /high f (sub S ₂ ) for the latter.