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Open File Report 28

Laterite geochemistry in the CSIRO-AGE Database for the Albany-Fraser Region (Collie, Dumbleyung, Mt Barker, Pemberton sheets)

Grunsky, E.C.

A multi-element geochemical study has been carried out on laterite and associated ferruginous samples that cover parts of the granitoid-gneiss terrain of the COLLIE, DUMBLEYUNG, MOUNT BARKER, and PEMBERTON 1:250 000 map sheets. The report presents a summary of the data and a provisional interpretation of selected parts of the data. The data used in this 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 database which was used for the study is composed of laterite and associated ferruginous samples collected over predominantly gneissic and felsic intrusive rocks that span the Archaean Yilgarn Block and the Proterozoic Albany-Fraser Province. The data were split into two groups representing the distinctions between the two geological provinces. Laterite is the most abundant material. The laterites are predominantly composed of loose nodules and pisoliths and number 543 samples in the Yilgarn block, and 456 samples in the Proterozoic province.

A total of 1026 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 principal components analysis, and ranking of individual elements, ranking of CHI-6*X, PEG-4, and NUMCHI indices, and multivariate ranking of selected chalcophile elements (chi-squared plots).

The resulting ranked scores of these techniques have been 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 most probably related to lithological variation within the granitoid-gneiss terrane.

There are significant geochemical distinctions between the laterites developed over the Archaean terrain (Yilgarn) and the laterites developed over the Albany-Fraser Orogenic belt based on the examination of histograms, order statistics, and a discriminant function analysis. Yilgarn laterite samples contain greater mean abundances for Ti, Mn, V, Zn, Sn, W, Ga, Nb, Zr, and Ba. Albany-Fraser laterite samples contain greater mean abundances for Cr, Ni, As, Sb, Bi, Mo, Se and Au.

The essential geochemical features of the area are:

Yilgarn Laterites

  • Gold occurs as individual Au anomalies as well as multi-element associations with Sb, W, Mo, Pb, and As in Whistlers, Darling Hill, Muradup, Boscabel, north of Trollup Hill, Peringillup, and Cranbrook areas.
  • Areas with the greatest Sn, W, Nb, Ta potential occur in the Whistlers, Darling Hill, Darkan, Quindanning, Boscabel, north of Trollup Hill and Peringillup areas.
  • Molybdenum occurs with Sn and As in the Whistlers, Darling Hill, Darkan, Boscabel and north of Trollup Hill areas.
  • Tungsten is associated with As, Mo, Sb, Pb, and Au and as isolated anomalies. Tungsten anomalies occur in the Whistlers, Darling Hill, Darkan, Quindanning, and Boscabel areas.
  • Silver appears to have a very limited multi-element association with the exception of a slight association with Pb and Ga. Elevated Ag occurs in the Whistlers, Darling Hill, Darkan, and Boscabel areas.

Albany-Fraser Laterites

  • Elevated multi-element abundances of Au occur with Sn, Sb, W, and Sn. The areas which contain these multi-element associations include the Carbarup Hill, Mt Barker, Denbarker, Lake Katherine, and Lake Muir areas.
  • Tin and Nb occur as single and multi-element associations southeast and southwest of Denbarker, north of Mt Barker, and the Lake Muir areas. Tin is also associated with Nb, Mn, Zr, Au, Mo, Sb, and Se in the Denbarker to Lake Katherine and Lake Muir areas.
  • Molybdenum occurs as single and multi-element associations with As, Rb, Pb; As, Ni, Zn, and Cr; Ni, Sb, and Co in the Carbarup Hill, Denbarker to Lake Katherine, north of Lake Muir, and Mt Barker areas.
  • Tungsten occurs with little or no multi-element signature. Elevated abundances of W occur in the Denbarker and Lake Muir areas.
  • Silver occurs with virtually no multi-element signature. Elevated abundances of Ag occur south of Denbarker, west of Lake Katherine, and the Lake Muir areas.
  • Other elements are difficult to assess individually. Since most economic commodities being sought have multi-element geochemical signatures, it makes sense to employ methods that make use of these multi-element characteristics. The results of the principal components analysis, the CHI-6*X, PEG-4, and NUMCHI indices, and Mahalanobis distance methods all show zones that have multi-element enrichment and indicate that the areas mentioned above may warrant additional follow-up investigation. Exploration for Au and associated precious metal deposits may be assisted by the use of several of these multi-element methods.

The data and results presented in this report, plus additional geophysical lithological, lithogeochemical, and structural data, may provide sufficient information for a selective and cost efficient exploration programme.

Last updated: Tuesday, January 04, 2000 03:46 PM


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