Fluid Inclusion Microanalysis using PIXE Recent advances in technique now permit routine non-destructive analysis and imaging of fluid inclusion in minerals using Proton Induced X-ray Emission (PIXE) and beams of 3 MeV protons from the CSIRO-GEMOC Nuclear Microprobe. Development is continuing to augment this capability with Proton Induced Gamma-ray Emission (PIGE) for light element detection and 3D fluid mapping using elastic recoils. Using PIGE we can now provide images of light elements such as Na and F.

Method Energetic protons pass easily through minerals like quartz to depths of more than 80 µm, and excite X-rays from elements within trapped fluid inclusions. The X-rays are counted using a cooled germanium or lithium-drifted silicon detector. By focussing the proton beam to about 1.3 µm in diameter, and raster scanning the beam over each inclusion, individual fluid inclusions can be imaged and analyzed non-destructively. In order to relate the detected X-rays to concentration of an element in an inclusion, a model has been developed at the CSIRO for calculating X-ray yields from specific 3D inclusion geometries (inclusion size, shape, density, orientation and depth) (Ryan et al., 1993). Now by raster-scanning the beam over a fluid inclusion to provide a uniform dose, and using the 3D model, the method permits the extraction of the concentration of all detected elements in the inclusion (Ryan et al., 1995). If there has been no loss from the inclusion, this provides the composition of the original homogeneous trapped fluid. Imaging The high resolution of the new CSIRO-GEMOC Nuclear Microprobe also provides a tool for imaging the internal contents of individual fluid inclusions.

Sensitivity The sensitivity of the Proton Microprobe enables the analysis of individual fluid inclusions down to ~ 5 µm in diameter. In typical inclusions (10-15 µm) ore-related elements (such as Cu, Zn, Au and Pb) can be detected and analyzed down to concentrations of ~20 ppm. Accuracy Test performed using synthetic fluid inclusions in quartz have shown the method to be accurate to 10-15% for undersaturated solutions (no daughter minerals present). With the presence of daughter minerals, and due to the uncertainties associated with the positions of these minerals, uncertainties typically grow to ~30%. Application The PIXE technique for the non-destructive analysis of fluid inclusions is in routine use at the CSIRO for the analysis of ore-fluids associated with porphyry Cu-Au and VMS deposits worldwide.

For further information contact: Dr. Chris Ryan via email: (Chris.Ryan@csiro.au) +61-2-9490 8673 +61-2-9490 8909 CSIRO Exploration and Mining PO Box 136 North Ryde NSW 2113 Australia

CSIRO-GEMOC Nuclear Microprobe CSIRO Exploration and Mining

References

• Ryan, C.G., C.A. Heinrich and T.P. Mernagh (1993), Nucl. Instr. Meth. B77, 463-471.
• Ryan, C.G., Heinrich, C.A., Van Achterbergh, E., Ballhaus, C., and Mernagh, T.P., (1995), Nucl. Instr. Meth. B104, 182-190.