Cu, Fe and S concentrations in magmatic/hydrothermal fluids: Evidence from natural and synthetic fluid inclusions

Fluid inclusions containing chalcopyrite (Cpy) daughter minerals have been recognized in many magmatic/hydrothermal ore deposits and have been used by many workers to estimate the copper concentrations of ore-forming fluids. The authors have developed a graphical technique that facilitates estimation of the concentrations of Cu, Fe and S in individual fluid inclusions. Application of this technique to natural fluid inclusions indicates a wide range in copper concentrations in ore-forming fluids, from a few hundred ppm to a few wt. % copper. This same technique has been applied to synthetic fluid inclusions to estimate the solubilities of metals and sulfur at magmatic/hydrothermal conditions. For example, at 600 C and 3 kbar, this technique gives solubilities of 1,910, 1,680, and 1,890 ppm Cu, Fe and S, respectively, in a 10 wt.% NaCl solution in equilibrium with Cpy. A diagram summarizes the relationship between the size of the Cpy daughter mineral as seen in two dimensions, the volume percent Cpy in the inclusion, and the concentration of copper required to produce a Cpy daughter mineral of that size.