GEMOC ARC National Key Centre

Chris G. Ryan1, Esmé van Achterbergh2, Brent M. McInnes1, Patrick J. Williams3, Guoyi Dong3 and Khin Zaw4
1. CSIRO Exploration and Mining, 2. GEMOC Macquarie, 3. Economic Geology Research Unit, James Cook University, 4. Centre for Ore Deposit Studies, University of Tasmania

The new CSIRO-GEMOC Nuclear Microprobe (NMP)
The instrument was designed specifically for minerals analysis and imaging and to achieve ppm to sub-ppm sensitivity at a spatial resolution of 1-2 _m using X-rays and _-rays induced by MeV energy ion beams.  The key feature of the design is a unique magnetic quadrupole quintuplet ion focussing system that combines high current with high spatial resolution (Ryan et al., 1999).  These design goals have been achieved or exceeded.  On the first day of operation, a spot-size of 1.3 _m was obtained at a beam current of 0.5 nA, suitable for fluid inclusion analysis and imaging.  The spot-size grows to just 1.8 _m at 10 nA (3 MeV protons), ideal for mineralogical samples with detection limits down to 0.2 ppm achieved in quantitative, high resolution, trace element images. Applications of the NMP include: research into ore deposit processes through trace element geochemistry, mineralogy and fluid inclusion analysis of ancient deposits and active sea-floor environments, ore characterization, and fundamental studies of mantle processes and extraterrestrial material.

Quantitative True Elemental Imaging
Dynamic Analysis is a method for projecting quantitative major and trace element images from proton-induced X-ray emission (PIXE) data obtained using the NMP (Ryan et al., 1995). The method un-mixes full elemental spectral signatures to produce quantitative images that can be directly interrogated for the concentrations of all elements in selected areas or line projections, etc.

Fluid Inclusion Analysis and Imaging
The analysis of fluids trapped as fluid inclusions in minerals holds the key to understanding ore metal pathways and ore formation processes. PIXE analysis using the NMP provides a direct non-destructive method to determine the composition of these trapped fluids with detection limits down to 20 ppm.  However, some PIXE results have been controversial, such as the strong partitioning of Cu into the vapour phase (e.g. Yankee Lode, Mole Granite, NSW [Heinrich et al., 1993] and Batu Hijau, Indonesia [McInnes et al., 1999]), and the high concentrations of some elements in many ore-related fluid inclusions [e.g. Pb ~4 wt% at Hellyer, Tasmania (Khin Zaw et al., 1996) and Ba ~9 wt% at Starra, Cloncurry district, Queensland (Williams et al., 2000)].  Now, using the NMP, the internal contents of individual fluid inclusions can be imaged to show clearly that these elements reside within the fluid inclusions, and to discrimination against solid phases outside the inclusion volume.

Melt Inclusion Analysis and Imaging
Samples of melts and fluids, responsible for metasomatic change and evolution of the earth's upper mantle are often preserved as inclusions in xenoliths.  However, their quench textures can often conceal rare minor phases that concentrate important trace elements (e.g. HFSE and REE).  The penetration of MeV protons enables the detection of these contributions to ~40 _m depth, thus providing a tool to determine reliable melt composition, with detection sensitivities down to 0.2 ppm, and to image spatial variation in component elements at 1-2 _m resolution.

References
Heinrich, C.A., Ryan, C.G., Mernaugh, T.P., and Eadington, P.J. (1993), Econ. Geol. 87, 1566-1583.
McInnes, B.M., and Ryan, C.G., (1999), unpublished data.
Ryan, C.G., Jamieson, D.N., Churms, C.L., Pilcher, J.V. (1995), Nucl. Instr. Meth. B104, 157.
Ryan, C.G., Jamieson, D.N., Griffin, W.L., Cripps, G. (1999), Nucl. Instr. Meth. B158, 18-23.
Williams, P.J., Dong, G., Ryan, C.G., Pollard, P.J., Rotherham, J.F., Mernagh, T.P., Chapman, L.H. (2000), submitted to Econ. Geol.
Zaw, K., J.B. Gemmel, R.R. Large, T. Mernagh and C.G. Ryan (1996), Ore Geol. Reviews 10, 251-278.