PIXE MICROANALYSIS OF FLUID INCLUSIONS AND ITS APPLICATION TO STUDY ORE METAL SEGREGATION BETWEEN MAGMATIC BRINE AND VAPOR

The composition of fluid trapped as inclusions during the growth of mi nerals, or during later healing of fluid-filled cracks, provides infor mation on the processes that form rocks and ore deposits. A method has been developed for the quantitative PIXE microanalysis of intact flui d inclusions. The method addresses the complex 3D geometry of the incl usion, and its orientation, internal structure and relation to X-ray t ake-off angle. The effects of inclusion geometry are significant for l ow energy X-rays due to the increased absorption caused by the increas ed path length through the mineral that is not accounted for in a plan ar yield model. Consideration of these effects yields a more reliable analysis method for lighter elements, such as Cl. The method was appli ed to study low-salinity vapor and high-salinity brine inclusions in q uartz from a granite-hosted quartz-cassiterite vein in the Mole Granit e (New England plateau, Australia). Cation ratios in the brine compare d with the source granite closely match experimental data on equilibri um metal distribution between chloride-bearing aqueous fluids and sili cate melts, confirming an earlier interpretation that these inclusions represent samples of magmatic fluid. The analysis of the vapor inclus ions showed strong partitioning of Cu into the vapor phase. This surpr ising result indicates that liquid-vapor separation and preferential t ransport of Cu (and by inference Au) may be a major mechanism of base and precious metal segregation and ore formation within magmatic hydro thermal systems.