An experimental study to elucidate magnetite solubility and metal partitioning in a magnetite-silicate melt-vapor-brine-gold metal assemblage

Experiments were performed to quantitatively determine magnetite solubility and the partitioning ore metals in a vapor-brine-rhyolite melt-magnetite assemblage at 800°C and fO2 = NNO. The primary goal was to determine the effect of reduced pressure and concomitant changes in salinity on magnetite solubility. Experiments were performed at 100, 110, 130, 140 and 145 MPa with vapor salinities of 1.8, 2.3, 5.1, 9.2 and 19 wt. % NaCl eq., respectively, and brine salinities of 64, 57, 51, 42 and 35 wt. % NaCl eq., respectively.; The data indicate that the pressure at which a melt undergoes volatile exsolution plays a determinant role in the mass transfer of iron from the melt phase to an exsolved volatile phase. The concentration of iron in magmatic vapor increases by an order of magnitude whereas the concentration of iron in magmatic brine remains relatively constant as pressure increases from 100 to 145 MPa. Calculated partition coefficients for iron between vapor and brine, Dn/bFe decrease significantly with decreasing pressure, indicating that the ability of magmatic vapor to transport iron is enhanced significantly if volatile exsolution occurs at higher values of reduced pressure.; Determined partition coefficients for gold between rhyolite melt and magnetite crystals grown from the rhyolite melt indicate that the magmatic crystallization of magnetite can play a mitigating role in the ability of a calc-alkaline magmatic system to yield a gold-rich ore fluid. The partition coefficient for gold between vapor and brine, Dn/bAu , converges upon unity as the critical pressure is approached. These data indicate that the efficacy with which magmatic vapor scavenges and transports gold is equivalent to that of brine as the pressure of the magmatic system approaches the critical pressure.; The results also indicate that the magnetite crystallization affects significantly the cobalt and nickel budgets of an evolving rhyolite melt whereas copper partitions equally between a rhyolite melt and magnetite. The ability of magnetite crystallization to reduce the efficiency with which magmatic vapor or brine scavenge copper from melt is enhanced at high mass fractions of magnetite and in melts with a low Co,meltH2O/Cs,melt H2O .