THE LASER ABLATION MICROPROBE (LAM)-ICP-MS: APPLICATIONS TO ORE SYSTEMS AND MINERAL EXPLORATION

Simon E. Jackson, Norman J. Pearson and William L. Griffin,

GEMOC, Macquarie

Laser ablation microprobe (LAM)-ICP-MS is firmly established as a fast, sensitive and reliable technique for in situ determination of trace elements in minerals and their inclusions. The technique couples the high resolution sampling capabilities of a pulsed UV laser with the extraordinary detection capabilities of the ICP-MS. Current instrumentation is capable of simultaneously determining 30-40 elements in spots of ca. 30 µm diameter at detection limits down to low ppb level for many elements. Minimal sample preparation is required and a typical analysis takes less than 2 minutes. Using the depth profiling capability of the laser, it is also possible to assess the homogeneity of the ablation volume and so determine whether elements are concentrated homogeneously in the lattice structure, or whether they are chemically zoned or occur in inclusions. The technique already has very useful isotopic applications (e.g., U/Pb dating) and, with the recent advent of multi-collector ICP-MS instrumentation, an age of rapid, in situ, high precision isotope ratio analyses (e.g., U/Pb, Re/Os, Sm/Nd, Lu/Hf, Rb/Sr) is dawning.

Applications of LAM-ICP-MS in mineral exploration are numerous and developing rapidly. This paper will briefly describe the technique and detail several case studies of applications of LAM-ICP-MS in the study of mineral deposits.

Indicator minerals: LAM-ICP-MS has tremendous potential for the analysis of indicator minerals for assessing the mineral potential of their source rocks. For example, trace element data from chrome-pyrope garnets in heavy mineral concentrate provide information that can be used to quantify the diamond grade of a kimberlite or lamproitic host rock. The relationships between elements such as Zr, Y and Ti have been used to identify the chemical signatures of different types of mantle processes. Plots involving these elements show that garnet inclusions in diamond and a relatively high proportion of garnets from high-grade diamondiferous kimberlites have depleted trace element concentrations with Zr < 20 ppm, Y < 8 ppm and Ti from 10-2000 ppm. The depleted nature of garnets associated with diamond is also borne out in REE patterns, with many garnets having strongly depleted HREE and enriched MREE, indicative of metasomatic processes which appear to be strongly correlated with the presence of diamonds. Plots involving element ratios indicative of the shape of the REE patterns (e.g., Nd/Y, Sc/Y) provide simple discrimination tests to estimate the diamond potential of a pipe. Case studies of pipes from S. Africa and Siberia will be presented. Other resistate indicator minerals (e.g., tourmaline, chromite, ilmenite, zircon) have tremendous and largely unexplored potential for prospecting for other classes of deposit.

Gangue minerals: Studies of trace element chemistry of hydrothermal gangue minerals (e.g., carbonates, fluorite) in gold and base metal deposits have revealed that complex and extreme zonation of elements, such as REE, is typical and may be indicative of processes that have lead to the economic concentration of the ore minerals. In prospecting for ore deposits formed in submarine environments, systematic variations in REE patterns of gangue carbonate, which are sensitive indicators of hydrothermal fluid chemistry (e.g., sea water vs. magmatic) can be used in tracing the mineralised feeder zone(s). A case study of the Castellanos deposit, Cuba, will be presented.

Ore minerals: The trace element signatures of many ore minerals and their associated sulphides reflect the setting of the mineralisation, allowing quick identification of the geological environment of a showing. The application of LAM-ICP-MS to characterisation of Australian molybdenite occurrences will be given by Blevin et al. (this volume).

Mapping: In zones of very intense weathering, where field mapping can be very difficult, the trace element signatures of certain resistant minerals has been used for correlating volcanic and intrusive units. An example of the application of LAM-ICP-MS to mapping and correlating potentially diamondiferous units using the chemistry of resistant mineral phases will be presented.

Fluid inclusions: LAM-ICP-MS has the capability of sampling and analysing individual fluid inclusions in the ore and associated minerals. This has critical implications for refining our models of ore forming systems.

Age dating: In addition to its many trace element application, LAM-ICP-MS can produce accurate in situ U-Pb age determinations of zircons and other minerals. The speed, precision and accuracy of the technique make it ideally suited for exploration purposes.

The application of LAM-ICP-MS to the study of mineral deposits is in its infancy. However, the wide ranging capabilities, speed and economy of the technique offers a powerful new tool in exploration geology and can also provide data that will give new insights into the genesis of mineral deposits.

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