Can economic porphyry copper mineralization be generated by a typical calc‐alkaline melt?

Abstract

Numerical simulation of chlorine and copper partitioning between a crystallizing melt and exsolving aqueous fluids indicates that “typical” calc‐alkaline magmas contain sufficient copper, chlorine, and water to produce economic porphyry copper mineralization. Neither an elevated copper content in the magma nor an additional large volume of magma are required to provide metals or volatiles. The most important variables that determine the volume of melt necessary to produce sufficient copper are the degree of compatible behavior displayed by copper, the ratio of initial water in the melt to the water saturation level, and the initial chlorine/water ratio of the melt. The absolute values for initial water in the melt and water content at saturation are relatively unimportant in determining the required melt volume. The bulk salinity of the exsolved fluid may vary from < 2.0 wt.% NaCl to saturation levels (84 wt.% NaCl at 700°C) indicating that boiling is not necessary to produce high salinity brines. At appropriate P‐T‐XNaCl conditions the magmatic aqueous fluid separates into a saline liquid, which transports most of the copper, and a low‐salinity vapor. The salinities of the two immiscible phases are governed by the P‐T conditions, while the bulk fluid salinity determines the mass fractions of liquid and vapor formed. Pressure quenching causes rapid crystallization of the aplitic groundmass in porphyritic rocks associated with copper mineralization and significantly reduces the amount of chlorine and copper partitioning to the aqueous fluid. This results in abrupt and possibly large reductions in fluid salinity and causes copper to become concentrated in the melt. As copper is transported from the melt by the earliest exsolving fluids in deep (2.0 kbar) systems and by late exsolving fluids in shallow (0.5 kbar) systems, the relative timing of pressure quenching/aplite formation and fluid transport of copper from the melt can vary significantly in systems produced under different confining pressures. Model results incorporating petrologic constraints determined for Yerington, Nevada, are in good agreement with observed mineralization.