Effects of oxygen and sulfur fugacities on the scheelite-tungstenite and powellite-molybdenite stability relations

The stability relations between scheelite and tungstenite and between powellite and molybdenite were experimentally determined on the basis of the following reactions:CaWO (sub 4) + SiO (sub 2) + S (sub 2) = CaSiO (sub 3) + WS (sub 2) + 3/2 O (sub 2) CaMoO (sub 4) + SiO (sub 2) + S (sub 2) = CaSi (sub 3) + MoS (sub 2) 3/2 O (sub 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 (sub 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 (sub 2) ) - log f (sub O (sub 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 (sub 2) ) higher than the formation of scheelite unler the same f (sub S (sub 2) ) conditions. The restriction of f (sub S (sub 2) ) imposed upon the formation of tungstenite as compared with molybdenite is even more rigorous under the same f (sub O (sub 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 (sub 2) ) -- log- f (sub O (sub 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 (sub 2) ) /low f (sub S (sub 2) ) for the former and very low f (sub O (sub 2) ) /high f (sub S (sub 2) ) for the latter.