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Open File Report 22

Laterite geochemistry in the CSIRO-AGE Database for the Central Yilgarn Region (Barlee, Bencubbin, Corrigin, Hyden, Jackson, Kalgoorlie, Kellerberrin, Southern Cross sheets)

Grunsky, E.C.

A multi-element geochemical study has been carried out based upon laterite samples that cover parts of the main greenstone belts and portions of the granitoid-gneiss terrain of the BARLEE, JACKSON, KALGOORLIE, BENCUBBIN, KELLERBERRIN, SOUTHERN CROSS, CORRIGIN, and HYDEN 1:250 000 map sheets. This report presents a summary of the data and a provisional interpretation of selected parts of the data. The data used in the study are contained in the accompanying diskette (in the back pocket).

The sampling arose as part of a combined research programme between CSIRO and an experimental exploration programme (the AGE Joint Venture Programme) during the period 1983 to 1986. The study provides geochemical knowledge of the element abundance levels and shows variations in the geochemistry of lateritic and associated ferruginous materials, geochemistry that complement more specific information arising from orientation studies about mineral deposits.

The database which was used for the study is dominated by two major lithological groups: greenstone and granitoid rocks. The two lithologies have different geochemical characteristics, and thus have been, for the most part, treated separately. The database consists of both regional and follow-up samples. Most of the investigation for this report has emphasized the results from the regional samples as this will provide information for additional regional sampling programmes for the sponsors.

Laterite is by far the dominant material sampled in the area. The ferricretes, which are second priority in sampling, are represented by 184 samples, while the laterites are represented by 1815 samples.

In the region covered by the present report, a total of 2102 samples were analyzed for 30 elements. Summary statistics, histograms, and maps of the percentile classes are presented for selected elements in laterites. Several numerically-based procedures were applied for the purposes of outlining regional trends and detecting areas of relatively-high abundances of selected elements (anomalies). Numerical techniques included the use of ranking of individual elements, principal components analysis, CHI-6*X, PEG-4, and NUMCHI indices, and multivariate ranking of the chalcophile elements chi-squared plots).

The resulting scores of these techniques have been ranked and plotted on maps and scatter plots. The most anomalous samples tend to occur as outliers when these methods are applied. The results of these applications confirm the presence of some broad regional geochemical trends that are related to bedrock lithologies and possible regional alteration processes.

The dominant geochemical features are:

  • Several multi-element associations occur with Au, primarily in the greenstone belts, and a few in the granitoid-gneiss terrain. The characteristic elements are, Sb, Se, Ag, W, As, and Sn.
  • Several Au anomalies occur in the Diemals, Marda, and Southern Cross greenstone belts.
  • Regional, geochemical features that are characteristic of the greenstone belts are defined by higher relative abundances of Cr, Mn, V, Zn, Ni, Co, and Fe2O3.
  • A chalcophile trend may be present striking parallel to the Southern Cross belt, parallel to the Johnston Range in the Diemals area, and parallel to the stratigraphy in the Marda complex.
  • Additional elevated abundances of Sn, W, Mo and P occur throughout the felsic and greenstone terrains and may be the result of mineralization, alteration, or fractionation. These anomalous areas should be followed up with several models in mind.
  • Broad, regional, geochemical features, characteristic of the granitoid-gneiss terrain, are defined by higher relative abundances of Nb, Ga, Mo, and Pb.
  • Elevated Au and chalcophile abundances occur within the gneiss terrain in the Kellerberrin area. These areas may be previously unrecognized greenstone enclaves and may warrant further investigation.

The use of ranked data, empirical indices, principal components, and chi-square plots is discussed as a means of determining anomalies as target sites for mineral exploration.

Last updated: Tuesday, January 04, 2000 03:07 PM


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