COPPER AND GOLD IN PORPHYRY ORE FLUIDS

Laser ablation ICPMS analysis of fluid inclusions from porphyry Cu, Cu-Mo and Cu-Au ore deposits is providing new insights into how these metals are transported and deposited in these systems. In particular, the importance of low salinity fluids (both liquid-like and vapors) and fractionation of copper and gold from other metals by partitioning between brines and vapors in space and time have been increasingly reported. A review of the current data on copper and gold concentrations shows that brines typically display an upper threshold of Cu concentrations of around 1 wt% (higher at Bingham: 2.6 wt%) and a relatively narrow range (~2 orders of magnitude). Low salinity fluids and vapors display similar upper Cu concentration limits (higher at Bajo de la Alumbrera: 3.3 wt% Cu) but are much more variable, over four orders of magnitude. This is likely to reflect, at least in part, greater degrees of precipitation from the low salinity fluids and vapors during evolution and trapping. Other metals typically vary much less, consistent with the lower abundance of their sulfides or oxides in the main Cu orebodies. Au has only been measured in a few cases and the accuracy of the data is still uncertain. Concentrations range up to 1.3 ppm in brines and up to a few ppm in vapors. From these data it is apparent that the brines, vapors and intermediate density liquids that characterize most porphyry systems can all transport similar concentrations of Cu and Au, thus it is the relative mass proportions of these fluid types and their distributions through space and time that will control Cu and Au dispersion. Vapor separation is a likely mechanism for fractionation of Cu and Au from other metals, but only where vapors physically escape from the zone of phase separation.