In this chapter we report on the use and limitations of oxygen and halogen fugacities to characterize granitoids associated with tungsten deposits. Tungsten-related granitoids generally occupy an intermediate geochemical position, less evolved than tin granites, but more differentiated than porphyry copper and molybdenum granodiorites. Microprobe analyses of the different populations of biotite in the CanTung system (in granite, aplite, lamprophyre, vein, and skarn) show differences in major-element chemistry. Fluorine-chlorine intercept values, however, indicate that the halogens in the biotites reequilibrated with one fluid. This reequilibration probably was at relatively low temperatures (300° to 350°C), but might have been at higher temperatures (500°C) if the fluid did not change its halogen fugacity during cooling. In both cases, this implies that the initial halogen content of the magmatic biotites is overprinted by the hydrothermal fluid. The observation that an aplite, away from the mineralization, has a lower intercept value (higher fluorine fugacity) may indicate that the mineralizing fluid evolved from the main biotite granite body and not from the leucocratic phase. High ferrous/ferric ratios in biotites, the absence of magnetite and titanite in the granite, and the skarn mineralogy indicate reducing conditions of the magmatic-hydrothermal fluids of the CanTung and some other high-grade deposits. We propose that the oxidation state of the magma, the amount of initial water, and the depth of emplacement play an important role in the formation of large tungsten deposits.