Probing Ore Forming Processes Using the Scanning Proton Microprobe

C.G. Ryan1, Khin Zaw2, C.A. Heinrich3, D.N. Jamieson4 and E. van Achterbergh1
1 GEMOC and CSIRO Exploration and Mining, PO Box 136, North Ryde NSW 2113, Australia, 2 CODES, Geology Department, University of Tasmania, Hobart, TAS 7001, Australia, 3 Department Erdwissenschaften, ETH Zentrum, Z|rich CH-8092, Switzerland, 4 School of Physics, University of Melbourne, Parkville VIC 3152, Australia.

The large penetration depths and predictable nature of MeV proton trajectories has permitted the development of standardless quantitative methods for microanalysis of minerals at ppm levels, trace-element imaging and non-destructive analysis of individual fluid inclusions, all of which offer particular benefits to ore-formation research.

FLUID INCLUSION ANALYSIS

By modelling Proton Induced X-ray Emission (PIXE) yields from the complex 3D geometry of an inclusion in its host mineral, and by using beam-scanning to control the proton dose distribution across an inclusion, the CSIRO method enables quantitative analysis of fluid inclusions of 5-20 mm in di ameter with sensitivities for the ore elements down to 40 ppm in the fluid (Ryan et al., 1995).
Research at the CSIRO is focused on the analysis of ore elements in fluids and experimental studies. Much of the work is centred on the analysis of hydrothermal fluids associated with copper-gold deposits. A good example is the Kidston granite-related breccia gold-copper deposit in North Queensland, Australia. Brine and vapour inclusions show strong partitioning of Cu into the vapour phase and the presence of S in the vapour (Fig. 1). This, and other work, suggests that brine-vapour segregation of trace metals, and transport in the vapour phase, plays an im(Heinrich et al., 1993).

TRACE ELEMENT IMAGING

A new method called Dynamic Analysis (DA) developed at the CSIRO formulates the PIXE analysis problem as a matrix transform that can directly un-mix elemental components to yield accurate major- and trace-element images of spatial distribution in real-time (Ryan et al., 1996). This technique permits the imaging of precious metal distribution in sulfides, for example, for basic studies of ore formation and as a tool for mineral processing. Fig. 2 illustrates the capability with an example of trace Au distribution in pyrite from the Emperor Mine, Fiji. The image shows a record of pyrite growth and reveals a detailed correlation of Au with As incorporation in pyrite. Pb (also Mo and Sb) deposition occurs in distinct episodes, and is mutually exclusive to Cu. Au ranges from 180-2000 ppm in these pyrites.

Besten, J. den, Jamieson, D.N., and Ryan, C.G., "Lattice location of gold in natural pyrite crystals", in prep.

Heinrich, C.A., Ryan, C.G., Mernaugh, T.P., and Eadington, P.J., 1993, Economic Geology 87, 1566-1583.

Ryan, C.G., Heinrich, C.A., Van Achterbergh, E., Ballhaus, C., and Mernagh, T.P., 1995, Nucl. Instr. Meth. B104, 182-190.

Ryan, C.G., Van Achterbergh, E., Jamieson, D.N., and Churms, C.L., 1996, Nucl. Instr. Meth. B109/110, 154-160.

Ryan, C.G., Van Achterbergh, E., Jamieson, D.N., and Churms, C.L., 1996, Nucl. Instr. Meth. B109/110, 154-160.

Figures available from Chris Ryan.

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