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)
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
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
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