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Quantitative PIXE Imaging of Sulfides

Nuclear Microprobe investigations of sulfide ores and mineral assemblages focus on questions of the detailed mineralogical residence of precious metals, such as gold, from a mineral processing standpoint and also on trace element zoning that may shed light on mineral growth processes, and the changing physical and chemical environment, recorded in time as zoned compositional gradients and other growth features in sulfide minerals.
CSIRO Australia

Quantitative PIXE images

The Dynamic Analysis Approach to Imaging

The Dynamic Analysis method uses a matrix transform to un-mix characteristic elemental spectral components from PIXE/SXRF event streams to build elemental images that strongly reject overlap interference and detector response artefacts Ryan et al. (2000, 2001, 2005). The resulting images are stored in ppm-charge/flux units, and can be interrogated directly to determine the concentrations of all detected elements in portions of the images using the interactive GeoPIXE II software.

The example on the left illustrates this function for PIXE images of a sulfide growth feature from a sea-floor "black- smoker", Manus Basin, north of Papua New Guinea. The figure shows concentrations extracted directly from the images for a sub-set of detected elements (25 in all) for a small sub-region near the core of this growth feature (values in ppm or wt%).

The Emperor Mine, Fiji

The Dynamic Analysis method was applied to image a gold-bearing vein from the Emperor gold-telluride deposit in Fiji, a hydrothermal vein system with economic gold occurring as native gold and gold-silver-tellurides. Much of the gold is believed to be in solid solution in pyrite and/or arsenopyrite.

The images show an area of 2000 x 2000 µm2 spanning a sulfide vein, collected for 45 minutes using 3 MeV protons from the CSIRO-GEMOC Nuclear Microprobe. Gold concentration, for example, varies between trace levels in pyrite (dark blue 100 ppm; red 5000 ppm) to major element levels (white) in Au-Ag-tellurides Ryan et al. (2001).

The images show an area of pyrite banded with changing concentrations of Cu, Au, As, Mo, Sb, Pb, Ag, Te and Se. These growth band suggest changes in fluid chemistry or the physicochemical conditions. For example, bands of high Mo may indicate redox changes. The band of high Au (red band) correlates with a minimum in Cu concentration. This may provide clues to the speciation of Au in these fluids.

Quantitative Fe image, epithermal ore Quantitative trace Cu image, epithermal ore
Quantitative trace Au image, epithermal ore Quantitative As image, epithermal ore

Quantitative Zn image, epithermal ore Quantitative Te image, epithermal ore
Quantitative Mo image, epithermal ore Quantitative Ag image, epithermal ore
Quantitative Pb image, epithermal ore Quantitative Se image, epithermal ore

Gold incorporation in pyrite

The general zonation of Au in these images argues strongly for solid solution, or at least extremely fine-grained inclusions. Phase equilibria in the Fe-As-S system argue for a maximum concentration of As in pyrite of 0.5 wt% at 500 C (Clark, 1960). Yet in natural systems, As concentrations of 8.9 wt% have been observed (Griffin et al., 1991). Fleet et al. (1989) interpreted this as deposition of a metastable arsenian pyrite phase, observed in TEM as 1 nm lamellae in pyrite with a marcasite structure.

It has been suggested that such a layer may be the site for incorporation of Au in pyrite. Arsenopyrite has a marcasite structure, so the common association of Au and As in hydrothermal sulphides, may be partly a result of disequilibrium deposition of arsenopyrite on pyrite. Cook and Chryssoulis (1990) propose that As substitution in pyrite leads to [AsS]3- anion pairs. Charge can then be balanced by the substitution of Fe2+ by a trivalent cation such as Au3+, As3+ or Sb3+. This mode of incorporation of Au in pyrite is then similar to that in arsenopyrite ([Fe]3+ [AsS]3-) which often shows high concentrations of "invisible gold".

However, while these images and other work suggest a general broad correlation of Au with As, in detail at high resolution these images show no clear Au correlation with As. Indeed, the maximum Au trace zonation (red band) corresponds to a minimum in As.

More high resolution work using the new NMP may provide data to elucidate the mechanisms for gold incorporation, and provide clues to Au speciation in ore fluids.

Ion Microchanneling

The definitive test, to distinguish between Au in microscopic inclusions or in solid solution, is to use ion microchanneling to determine if Au is substitutional in the pyrite lattice. Jamieson and Ryan (1993) succeeded in channeling beams of 3 MeV protons and 2 MeV He+ ions into the <210> oriented pyritohedral face of a 400 µm pyrite grain, from the Emperor Mine ore-zone.

They observed reduced scattering yield from trace Au (yellow curve) when the beam was channeled, relative to the un-channeled (random; pink curve), consistent with a ~50% substitutional Au component. This result confirmed the significant solid-solution Au component suggested in the PIXE true elemental imaging.

Channeling spectrum showing Au in lattice

These examples provide some idea of the information that can be extracted from sulfide minerals using the Nuclear Microprobe methods at the CSIRO. The methods can be applied to basic ore studies or to solve ore processing problems.

See also what can be learnt from the analysis of ore fluids preserved as fluid inclusions in minerals.

For further information contact: Dr. Chris Ryan via email: (Chris.Ryan@csiro.au)
Phone +61-3-9905 9087
Fax orders +61-7-3327 4455
CSIRO Exploration and Mining
Bayview Road, Clayton VIC 3168
Australia
CSIRO Australia

CSIRO Exploration and Mining

References

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