PIXE microanalysis of fluid inclusions and its application to study ore metal segregation between magmatic brine and vapor

Abstract

The composition of fluid trapped as inclusions during the growth of minerals, or during later healing of fluid-filled cracks, provides information on the processes that form rocks and ore deposits. A method has been developed for the quantitative PIXE microanalysis of intact fluid inclusions. The method addresses the complex 3D geometry of the inclusion, and its orientation, internal structure and relation to X-ray take-off angle. The effects of inclusion geometry are significant for low energy X-rays due to the increased absorption caused by the increased path length through the mineral that is not accounted for in a planar yield model. Consideration of these effects yields a more reliable analysis method for lighter elements, such as Cl. The method was applied to study low-salinity vapor and high-salinity brine inclusions in quartz from a granite-hosted quartz-cassiterite vein in the Mole Granite (New England plateau, Australia). Cation ratios in the brine compared with the source granite closely match experimental data on equilibrium metal distribution between chloride-bearing aqueous fluids and silicate melts, confirming an earlier interpretation that these inclusions represent samples of magmatic fluid. The analysis of the vapor inclusions showed strong partitioning of Cu into the vapor phase. This surprising result indicates that liquid-vapor separation and preferential transport of Cu (and by inference Au) may be a major mechanism of base and precious metal segregation and ore formation within magmatic hydrothermal systems.