An experimental investigation of ore metals in silicate melt-volatile phase systems

Vapor, brine, or vapor + brine magmatic volatile phase(s), MVP, were equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa. The goals of this project were two fold: (1) study the relationship between MVP composition and melt composition and structure, and (2) examine the concentration, speciation and partitioning of copper, gold, and iron in silicate melt-MVP-metallic gold systems. The starting composition of the NaCl-KCl-HCl-H ₂O MVP varied from 3 to 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. Starting KCl/HCl was varied from 3.2 to 104 to evaluate element distribution as a function of the concentration of HCl in the MVP $CMVPHCl.$; The data indicate that the equilibrium constant for the exchange of potassium and sodium between the melt and brine, $K/meas$ (K, Na), decreased as the concentration of HCl in the brine $CbHCl$ increased. The results suggest: (1) $CbHCl$ reacts with the NaOH component in the melt to produce NaCl and H ₂O; and (2) either the proportion of the NaOH component is greater than the KOH component in the melt, or NaOH is more reactive than KOH relative to HCl.; Analyses of fluids trapped at 800°C and 100 MPa indicate that high concentrations of gold, copper and iron can be transported by a MVP. Metal speciation was found to vary according to the H₂S/HCl of the MVP. Copper and gold in a MVP with high H₂S/HCl will be dominated by metal-sulfo species, CuHS and AuHS, respectively. Decreasing the H₂S/HCl of a MVP to low values will change the speciation from metal-sulfo to metal-chloro complexes (e.g. CuCl and AuCl₂H).; The results of this study indicate that gold and copper can be scavenged from a silicate melt by a high-HCl brine, although, a low-HCl brine is capable of transporting a considerable amount of gold and copper into an associated magmatic-hydrothermal system as well. Examination of the pertainment equilibria suggest that a significant amount of gold and copper can be transported by brines regardless of the fugacities of sulfur-bearing species, but that low-HCl brines require a high fugacity of H₂S in order to transport large quantities of both gold and copper.