PIXE Analysis of Hydrothermal Fluids PIXE analysis of fluid inclusion composition using the CSIRO-GEMOC Nuclear Microprobe enables direct analysis of ore elements, and elements that provide clues to fluid sources and evolution, in preserved ore fluids trapped as inclusions in minerals such as quartz, and makes experiments possible that shed light on ore-metal mobility. See Ryan et al. (Ryan, 2001) and the review paper (Ryan, 1999) for more details and examples.

Granite-Hosted Copper-Gold Deposits Brine and vapour inclusions in quartz from the Kidston granite-related breccia gold-copper deposit in North Queensland, Australia, show strong partitioning of Cu into the vapour phase and the presence of S in the vapour (Heinrich et al., 1993a). The PIXE spectra below show Cu as the strongest cation in the vapour, while most other cations partition into the brine phase. This effect was first observed and quantified using PIXE at the CSIRO for brine and vapour inclusions from the Yankee Lode Sn deposit within the Mole Granite of New England, Australia, (Heinrich et al., 1993b). In the Yankee Lode case, the partitioning was even more pronounced with the ratio of vapour/brine Cu of 10:1. Clearly brine-vapour segregation of trace metals, and transport in the vapour phase, is likely to play an important role in the formation of Cu deposits in these systems. If a S complex is responsible for preferential partitioning of Cu into the vapour, then Au is predicted to show a similar, but probably more extreme behaviour. This helps to understand the formation of deposits such as Kidston in terms of vapour transport of Cu and Au. Imaging Copper Partitioning A vivid example of strong Cu partitioning is seen in images of brine and vapour fluid inclusions.

Modelling Fluid Evolution Detailed PIXE analyses of fluids enables modelling of the fluid evolution which can help to explain the detailed zonation of ore rich provinces.

VMS Deposits PIXE microanalysis of fluid inclusions provides clues to the origin of ore forming fluids. Trace elements such as As and K in fluid inclusions from the Mt. Chalmers VMS gold- copper deposit (Hunns et al., 1994) provides evidence for magmatic fluid input into this system. Ca/K and Br/Cl ratios indicate mixing between magmatic fluids and sea-water. Similar clues of fluid origin and mixing have been obtained from PIXE analyses of fluid inclusions in quartz from the Hellyer, Tasmania, (Zaw et al., 1996) and Tennant Creek, Northern Territory, (Zaw et al., 1994) ore deposits.

Experimental partitioning between Sulfide, Metal and Fluid PIXE analysis of fluid inclusions trapped in quartz provides a tool for the experimental study of element partitioning between sulfide, metal and fluid phases needed for better understanding of metal transport in fluids in ore-forming systems.

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

• Heinrich, C.A., C.G. Ryan and T.P. Mernagh (1993a), "Ore Metals in Magmatic Brine and Vapour: New Evidence from PIXE Micronalysis of Fluid Inclusions", Extended Abstract, 'Specialist Group in Economic Geology' meeting, Armidale, Australia, 2-6 February 1993.
• Heinrich, C.A., C.G. Ryan, T.P. Mernaugh and P.J. Eadington (1993b), "Segregation of Ore Metals betweeen Magmatic Brine and Vapor: a Fluid Inclusion Study using PIXE Microanalysis", Economic Geology 87, 1566-1583.
• Hunns, S.R., Khin Zaw, R.R. Large, J. A. Dean, C.G. Ryan and J. McPhie (1994), "Preliminary Geochemical Results Constraining the Formation of the Mount Chalmers Volcanic-hosted Massive Sulfide Deposit", proceedings of Conf. on "New Developments in Geology and Metallogeny: Northern Tasman Orogenic Zones", Ed. R.A. Henderson, Extended Abstracts Volume, Contributions of the Economic Geology Research Unit, No. 50, 117-124.
• Ryan, C.G., B.M. McInnes, P.J. Williams, G. Dong, T.T. Win and C.J. Yeats, "Imaging Fluid Inclusion Content using the New CSIRO-GEMOC Nuclear Microprobe", Nucl. Instr. Meth. B181 (2001) 570-577.
• Ryan, C.G., (1999), "Advances in fluid inclusion analysis using the Nuclear Microprobe", Nucl. Instr. Meth. B158 (1999) 523-532.
• Zaw, K., J.B. Gemmel, R.R. Large, T. Mernagh and C.G. Ryan (1996), "Evolution and source of ore fluids in the stringer system, Hellyer VHMS deposit, Tasmania, Australia: Evidence from fluid inclusion microthermometry and geochemistry", Ore Geology Reviews 10, 251-278.
• Zaw, K., D.L. Huston, R.R. Large, T. Mernagh and C.G. Ryan (1994), "Geothermometry and compositional variation of fluid inclusions from the Tennant Creek gold-copper deposits, Northern Territory: Implications for exploration of auriferous ironstones", proceedings of AusIMM annual conference, Darwin, Australia, August 1994.