LAM-ICPMS analysis of resistate minerals: applications to mineral exploration

W.L. Griffin, N.J. Pearson and S.Y. O'Reilly, GEMOC, Macquarie

Resistate Indicator Minerals (RIMs) are minor components of ores or ore-related rocks, that are resistant to chemical weathering and survive mechanical transport in surface environments. They are physically recognisable, can be recovered from stream or soil samples, and form a dispersion halo that is larger than the ore body, and thus provides a larger exploration target. This technique, originally developed by the diamond exploration industry, can be applied to exploration for a range of other ore deposit types. It is especially useful in terrains such as Australia, where the ore bodies themselves may be heavily weathered so that only RIMs have survived at the surface.

The main problem in applying the RIMs technique is that similar minerals may be shed into drainages both by ore bodies and by unmineralised rocks. Cr-pyrope garnets are commonly used as indicator minerals to find potentially diamondiferous kimberlites and lamproites, but 99% of the bodies found are uneconomic. Similarly, Cr-rich spinels may be associated both with kimberlites and lamproites, but also with magnesian volcanic rocks (komatiites) that do not contain diamond -- but some of which may contain economic Ni deposits. Considerable effort has been expended to define geochemical discriminants to separate ore-related RIMs from misleading ones; because the technique must be applied on a grain-by-grain basis, most of these attempts have been based on major-element analysis by electron microprobe. While major-element analysis may give a useful first cut, it commonly leaves a large degree of ambiguity. The application of trace-element data has been more successful, for several reasons (1) trace elements show larger concentration ranges than major elements; (2) they have larger interphase partition coefficients and thus are more process-sensitive; (3) they provide a larger number of variables to be used in multi-element discriminants.

LAM-ICPMS is an ideal tool for RIMs analysis: its high sensitivity provides simultaneous quantitative analysis of a wide range of elements above MDL (25-30 elements in a typical array); a spatial resolution of 30-50 µm allows multiple analyses of individual grains (typically 200-500 µm diameter); it requires little sample preparation and analysis is rapid. Using a UV laser microprobe built by Memorial University, coupled to a PE Elan 5100 ICPMS, GEMOC routinely analyses a range of RIMs for research purposes and as a contract service to the mineral exploration industry. EMP data are used to provide internal standarization, and most analyses are carried out with the NIST 610 glass as the external standard; the techniques are described by Norman et al. (1996).

Examples of applications:

(1) Cr-pyrope garnets in diamond exploration. The major/trace-element combination provides early discrimination between diamondiferous and barren deposits, primarily using Ni, REE and HFSE elements. The data also allow characterization of the lithospheric mantle beneath an area; this information is useful in area selection for diamond exploration.

(2) Cr-spinels in diamond and Ni exploration. A variety of HFSE elements can be used to recognise chromites derived directly from the mantle and carried up in kimberlites, lamproites and other potentially diamondiferous rocks (Yao et al., this meeting). Komatiitic chromites can be clearly discriminated from kimberlitic/lamproitic ones, which is important for exploration in areas with both rock types (such as West Australia).

(3) Tourmaline (a complex borosilicate) is associated with many Pb-Cu-Zn ore deposits, but also with many barren rocks. These two occurrences can be recognised on the basis of trace-element patterns and Fe/Zn ratios; Cu-Pb-Zn ratios in tourmalines mirror those in associated sulfide ores.

(4) Zircons from kimberlites can be discriminated from crustal zircons on the basis of REE, Th and U contents. Work in progress (E. Belousova) is aimed at recognition of zircons from a wide variety of rocks and ore deposit types.

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