Guelph Scanning Proton Microprobe
SULFIDE MINERAL ANALYSIS
Evaluation of ore bodies and subsequent exploitation to recover silver, gold, platinum-group, and other elements, demand knowledge of the distribution of minor and trace elements in the various sulfide mineral species of the ore. The need to analyze well-characterized individual grains demands an in-situ microprobe technique. Analysis of co-existing minerals e.g. silicates may also be necessary in arriving at a complete understanding of elemental distribution.
Micro-PIXE provides this analysis, with detection limits of a few ppm. Micro-PIXE has already proved its value in measurements of Ag concentration in minor carriers such as chalcopyrite and sphalerite. It has had extensive use in the study of PGE elements in pentlandites from various massive sulfide deposits.
| Proton Energy: | 3 MeV |
| Beam Current: | 14 nA |
| Spot Size: | 5 x 10 µm |
| Measuring Time: | 10 minutes |
Detection Limits in ppm. (wt.):
| Pyrrhotite | Chalcopyrite | |
| Se | 4 | 6 |
| Mo | 4 | 5 |
| Rh | 4 | 5 |
| Pd | 5 | 6 |
| Ag | 6 | 7 |
| Sn | 9 | 10 |
GARNETS, CHROMITES, ETC. AND DIAMOND EXPLORATION
Considerable experience has been built-up at Guelph in the micro-PIXE analysis of garnets, chromites and ilmenites, whose trace element content may help to distinguish diamondiferous from barren rock types. The trace nickel content of garnets is of special interest, in view of the well-defined inverse correlation between nickel content and formation temperature reported in the geochemical literature. The nickel range of interest, 10-100 ppm, which is below electron probe detection limits, is easily handled by micro-PIXE.
Results from a typical garnet analysis:
| Proton energy: | 3 MeV |
| Beam current: | 7.5 nA |
| Spot size: | 10 x 10 µm |
| Measuring time: | 4.5 minutes |
Detection limits in ppm (wt.)
| Element | Ni | Cu | Zn | Ga | As | Zr | Mo |
| LOD ppm | 5 | 4 | 3 | 3 | 2.5 | 4 | 5 |
Micro-PIXE measurements for major elements (Cr, Fe) agree well with electron probe results; measurements for trace nickel agree with those for reference standards. A variety of reference materials have been investigated, e.g. BHVO-1 (basalt).
SR ZONING IN FISH OTOLITHS
Fish
otoliths are composed of calcium carbonate. In the case of anadromous fish,
which migrate between fresh and salt-water, there are significant differences
in the concentration of strontium that is incorporated in the calcite,
reflecting the low Sr content of inland lakes and the high Sr content of
ocean water. The growth bands are visible in optical microscopy, and their
Sr content can be measured by micro-PIXE, whose limit of detection for
Sr is about 1-2 ppm. Linescans conducted across a series of zones reveal
an oscillatory behaviour in the Sr concentration for fish that migrate
annually to the ocean. Comparison of the Sr banding to the optical banding
can reveal in which particular years since birth a fish has migrated, and
in which particular years it remained in spawning grounds. Absolute levels
of Sr in the primordium assist in the identification of fish with particular
lakes.
Micro-PIXE
imaging and analysis of Sr in otoliths thus provides a new quantitative
tool for studies of fish migration and for population discrimination. It
may prove useful in the context of aboriginal land claims in Canada. The
method is also relevant to long-term studies of aquatic pollution by mining
operations, since the otoliths incorporate other trace elements.
Sample preparation methodology has been developed by a collaboration involving fisheries scientists from DFO, earth scientists from the University of Manitoba and physicists from the University of Guelph. To provide statistically useful data, many otoliths should be examined in any particular study. A large number of specially sectioned otoliths are "potted" in particular orientations in a standard mineral mount, and optically imaged to provide photographs of the band structure. A complete micro-PIXE study of these otoliths is then pre-programmed at the GSPM and the analysis is conducted with minimal operator intervention.
Three posters dealing with specific otolith projects are available for download in PDF format that can be viewed with Acrobat. They are listed below:
