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CSIRO-GEMOC Nuclear Microprobe
The CSIRO-GEMOC Nuclear Microprobe (NMP) provides a powerful mix of features for non-destructive, simultaneous multielement quantitative analysis and imaging at high resolution and high sensitivity.

A powerful technique called Dynamic Analysis has been developed at the CSIRO for un-mixing complex spectral data in real-time. When combined with beam scanning, it provides a tool for producing quantitative images of major and trace element distribution that portray accurate spatial information.

The deep penetration and straight-line paths of MeV ions also permit the analysis of fluid inclusions and solid phases locked within host minerals and the detection of buried precious metals.



Sensitivity


Low level image of Br in melt inclusion


Image of Fe in melt inclusion

Sensitivity

Strong PIXE X-ray production and the large solid-angle, close detection geometry of the new NMP enable detection limits as low as 0.2 ppm to be achieved in sub-regions of images. Minimum detection limits in sulfides range between 1-2 ppm and light matrices, such as diamond, give detection limits below 50 ppb.

The images on the left show major element Fe and trace Br distributions in a quenched melt inclusion in clinopyroxene. Note the clear distribution of Br that is discernible at an average concentration of 2.6 ppm over the area of the inclusion. The detection limit (99% confidence) is 0.3 ppm.

Resolution

High sensitivity analysis demands beam-currents of between 0.5 and 1 nA for fluid inclusions and 5-15 nA for sulfide and silicate imaging. An important design feature of the new NMP is a slow rate of growth of beam-spot with increasing current. So from a spot-size of 1.3 µm at 0.5 nA the spot grows only to 1.8-2 µm at 10 nA, ideal for imaging applications.

In order to benefit from high resolution, a microprobe must also provide ample sensitivity to obtain sufficient statistics to render fine spatial detail. The large detector solid-angle and high beam-currents of the new NMP provide the sensitivity to image major and minor elements at full beam resolution and to discern subtle trace element images. The image on the right shows the fine-detail observed in some minor elements in sulfides.



Resolution


High resolution image of As in ore sample


Accuracy


Accurate spatial distribution of As in epithermal ore


Accurate spatial distribution of trace Au in epithermal ore

Spatial Accuracy

PIXE and electron probe images can suffer from artefacts due to element overlaps, detector artefacts and pile-up. Images produced by the new NMP use a powerful technique (Dynamic Analysis) developed at the CSIRO to un-mix complex spectral signatures.
The resulting images are quantitative, stored in ppm-charge units, and strongly reject artefacts due to element overlaps and detector response effects. These operations are now available using the new GeoPIXE II software package.

This is particularly important for imaging trace elements such as Au in sulfide assemblages. The image of Au to the left shows the spatial variation of Au from 100 ppm (blue) to 5000 ppm (red) in pyrite from the Emperor Mine, Fiji, and is free of artefacts that can potentially arise from Zn, As, W and background. Image counting time: 45 minutes.

Quantitative Analysis

The Dynamic Analysis transform for imaging is built on a standardless PIXE analysis method developed at the CSIRO for the quantitative analysis of PIXE spectra. The resulting images are stored in ppm-charge units, and can be interrogated directly to determine the concentrations of all detected elements in portions of the images, using the new GeoPIXE II software package.

The example on the right illustrates this function for 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%).



Quantitative


seafloor sulfide: Quantitative Cu image


Penetration


Cu image in sulfide ore sample


Buried Au particles in sulfide ore image

Depth of Penetration

MeV energy protons penetrate several tens of microns into minerals, and provide sensitivity to buried structures. Two important examples of the application of this characteristic are in situ fluid inclusion analysis, and the detection of rare precious metal inclusions and phases in sulfides.
The images show an example that illustrates rare Au detection. The small gold-electrum phases in this sulfide assemblage are contained within Cu-rich veins locked within pyrite enclosed within magnetite. The observation of this trapped gold texture, which used Au detected at any depth throughout the thin-section, may help explain low recoveries in the associated ore deposit.

Non-destructive Analysis

MeV protons deposit very little energy over the first ~30 microns of their path in minerals, and cause negligible damage. This enables the in situ analysis of fluid inclusions, preserved within minerals such as quartz. The protons excite X-rays and gamma-rays from elements within the inclusion enabling the imaging of fluid inclusons and the quantitative analysis of their contents.

The figure on the right shows Fe and Cu images (images for 14 elements obtained) from two coexisting brine and vapour inclusions in quartz from a porphyry-Cu-Au deposit. The brine Fe image shows Fe in solution and a hematite daughter around a large vapour bubble and daughters of halite and sylvite (inferred from Cl and K images). The vapour inclusion Cu image shows the high concentration of Cu in solution around a large central vapour bubble and the presence of a chalcopyrite daughter.

Note that solid phases outside the inclusion can be recognized and avoided using this NMP PIXE imaging approach. That is, they are not included in the integration performed for quantitative analysis.



Damage


Intact fluid inclusion after NMP imaging
For further information contact: Dr Jamie Laird or Dr Chris Ryan
Phone +61-3-8344 8375
Fax orders +61-8-6436 8586
CSIRO Earth Science and Resource Engineering
c/o School of Physics, University of Melbourne, VIC 3010, Australia
CSIRO Australia

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