Calculation of magmatic fluid contributions to porphyry-type ore systems: Predicting fluid inclusion chemistries

Abstract

The composition of published fluid inclusion analyses from a porphyry-type deposit are compared with the composition of a calculated magmatic aqueous phase. The calculations are performed by modeling the partitioning of chlorine between an arc magma and an aqueous phase using a Nernstian law, and modeling the partitioning of the elements sodium, potassium, hydrogen, calcium, magnesium, copper, manganese and barium into the chloride-bearing fluid according to experimentally determined exchange constants. The instantaneous concentration of the above elements in successive aliquots of magmatic aqueous phase evolved during second boiling can be calculated as a function of the vapor evolution progress variable. Further, an average concentration of an element in the magmatic aqueous phase (averaged over the course of vapor evolution progress) can also be calculated. The composition of the fluid inclusions can be reproduced using both methods. Magmatic fluid compositions with 2000-3000 ppm copper, 10 wt% chlorine and K/Na ratios=0.5 result from the calculations. Whereas potassium concentrations are slightly depressed, and calcium and magnesium concentrations are elevated in the analyzed fluid inclusions relative to the calculated fluids, the sodium, chlorine, copper, manganese and barium concentrations in fluids extracted from fluid inclusions can be reproduced with these calculations. This study suggests that fluid inclusions of the potassic zone of some porphyry-type deposits represent cooled magmatic fluids which have undergone some exchange with wall rocks but which contains magmatic concentrations of copper, manganese and barium at temperatures below 450°C.