Calculated compositions of fluids released from a crystallizing granitic melt: Importance of pressure on the genesis of ore forming fluid

抄録

Chemical compositions of magmatic fluids released from a water-saturated, crystallizing granitic melt are calculated quantitatively. The calculations were performed using the model originally proposed by Holland (1972) incorporating currently available experimental partition coefficient data. The results indicate that the partitioning of metal cations into the fluid phase is strongly dependent on pressure, their fluid-melt partitioning coefficients and chlorine content in the initial granitic melt. Concentration of chlorine and most cations in the fluid phase will generally increase at pressures below 1 kbar, whereas decrease above it, with the progress of crystallization. While crystallization of a granitic magma over a wide range of pressures will yield a fluid phase with high concentration of sodium and potassium compared to calcium, enrichment of the fluid in lead and zinc will likely occur if the fluid release occurs at pressures less than 1 kbar. The calculation suggests that the initial granitic magma should contain chlorine at least in a range of 200-1000 ppm to transport base metals into the aqueous phase effectively. The calculations reported in this study strongly support the validity of the classical hypothesis that chlorine-rich magmatic fluids evolved at shallow depths (below 2 kbar) have the potential of forming hydrothermal base metal deposits.