Segregation of ore metals between magmatic brine and vapor; a fluid inclusion study using PIXE microanalysis

Abstract

Nondestructive proton-induced X-ray emission (PIXE) analysis is used to measure the concentrations of base metals and other heavy elements in single hypersaline brine inclusions and low-salinity vapor inclusions, which are intimately associated within a granite-hosted quartz-cassiterite vein in the Mole Granite (New South Wales, eastern Australia). Microther-mometric measurements and Raman microspectrometry on the same inclusions complement the data.The brine inclusions have an estimated composition (in wt %): NaCl, 20; KCl, 7; CaCl 2 , 0.9; MnCl 2 , 4; FeCl 2 , 14; ZnCl 2 , 1; plus (in ppm by weight) Co, 300; Cu, 900; As, 500; Br, 400; Rb, 1,700; Sn, approximately 400; Cs, 2,000; Pb, 3,000. Cation ratios in this brine compared with trace element concentrations in the source granite closely match experimental data on equilibrium metal distribution between chloride-bearing aqueous fluid and silicate melts, confirming that the brine inclusions represent near-pristine samples of magmatic fluid.Low-salinity vapor inclusions occurring in the same sample are interpreted on the basis of phase equilibria to have coexisted with the magmatic brine at 500 degrees to 600 degrees C and 500 to 800 bars, prior to some cooling and decompression to trapping conditions of 380 degrees to 450 degrees C and approximately 300 bars. Compared with the brine inclusions, they contain higher concentrations of CO 2 and S (of unknown speciation) and about 1 percent Cu as the most abundant cation detected by PIXE analysis. Concentration ratios of copper to all other first-row transition metals (e.g., Cu/Fe, Cu/Zn, and also Cu/Pb) are more than two orders of magnitude higher in the vapor than in the brine inclusions.The fluid inclusion results indicate that partitioning of base and precious metals between magmatic vapor and hypersaline brine may be substantial and highly element specific, probably as a result of contrasting metal complexing in the two fluid phases. Volatile sulfur complexes of copper could explain preferential partitioning of copper (and by inference, of gold) into the magmatic vapor phase, whereas chloride complexing causes most other heavy metals to partition into the coexisting brine. The analytical data emphasize the importance of magmatic vapor and vapor condensate as ore-forming solutions and suggest that brine-vapor separation may be an important mechanism of base and precious metal segregation in high-temperature hydrothermal systems. This possibility has not received much attention so far, but could be a significant factor in the formation of porphyry-style and other magmatic hydrothermal base and precious metal deposits.