Quartz solubility in the H2O-NaCl system: a framework for understanding vein formation in porphyry copper deposits.

Porphyry copper deposits consist of low-grade stockwork and disseminated sulphide zones that contain characteristic vein generations formed during the evolution of the magmatic-hydrothermal systems. The present contribution proposes an interpretive framework for the formation of porphyry veins that is based on quartz solubility calculations in the H2O-NaCl system at temperatures of 100 to 1 000 degrees C and pressures of 1 to 2 000 bar. Based on the derived quartz solubility constraints, a model was developed linking the formation of different crosscutting vein types to the P-T evolution of porphyry-related magmatic-hydrothermal systems. Petrographic data and fluid inclusion studies performed previously were compiled to show that the model correctively predicts the sequence of vein formation in porphyry deposits and the crosscutting relationships among A to E veins. This suggests that the developed model has real genetic implications providing a universal explanation of mineralisation and alteration patterns in this deposit type. Quartz solubility in the H2O-NaCl-CO2 system cannot presently be fully modelled as thermodynamic data required to evaluate the influence of CO2 on quartz solubility are not yet available for the entire range of upper-crustal pressure and temperature conditions.
Porphyry copper deposits consist of low-grade stockwork and disseminated sulphide zones that contain characteristic vein generations formed during the evolution of the magmatic-hydrothermal systems. The present contribution proposes an interpretive framework for the formation of porphyry veins that is based on quartz solubility calculations in the H2O-NaCl system at temperatures of 100 to 1 000 degrees C and pressures of 1 to 2 000 bar. Based on the derived quartz solubility constraints, a model was developed linking the formation of different crosscutting vein types to the P-T evolution of porphyry-related magmatic-hydrothermal systems. Petrographic data and fluid inclusion studies performed previously were compiled to show that the model correctively predicts the sequence of vein formation in porphyry deposits and the crosscutting relationships among A to E veins. This suggests that the developed model has real genetic implications providing a universal explanation of mineralisation and alteration patterns in this deposit type. Quartz solubility in the H2O-NaCl-CO2 system cannot presently be fully modelled as thermodynamic data required to evaluate the influence of CO2 on quartz solubility are not yet available for the entire range of upper-crustal pressure and temperature conditions.