CRC LEME
Open File Report 18
ABSTRACT
Report on laterite geochemistry in the CSIRO-AGE Database for
the Northern Murchison Region (Cue, Belele, Glengarry, Sandstone
sheets)
Grunsky, E.C., Smith, R.E. and Perdrix, J.L.
A multi-element geochemical study has been carried out upon laterite
and ferricrete samples that cover parts of the main greenstone belts
of the CUE, BELELE, GLENGARRY, and SANDSTONE 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 at
CSIRO and an experimental exploration programme (the AGE Joint Venture
Programme) during the period 1983 to 1986.
In the region covered by the present report a total of 1065 samples
were analyzed for 30 elements. Summary statistics, histograms, and
maps of the percentile classes, are presented for selected elements
in laterites and some very Fe-rich materials from partly eroded
profiles which have been referred to as ferricretes.
The database that was used for the study is dominated by two major
sample types, laterites and ferricretes. The two sample media have
different geochemical characteristics, and thus have been treated
separately. Lateritic sample media are abundant in the southwestern
parts of the area and ferricrete sample media are more abundant
in the northern and eastern parts of the area. The study also provides
knowledge of the element abundance levels and variation of laterite
and ferricrete geochemistry that complement information arising
from orientation studies about mineral deposits.
The geochemical characteristics of the area have been studied using
four techniques:
- Exploratory data analysis techniques were employed to rank the
data for each element. Samples that rank in the upper percentiles
for selected elements (chalcophile) have been considered as significant
for further exploration follow-up.
- Principal components analysis of the multi-element data was
used for the purpose of isolating significant linear combinations
of elements that are associated with dispersion haloes within
potentially mineralized zones. The results of the analysis also
confirm the presence of some broad regional geochemical trends
that are related to bedrock lithologies and regional alteration
processes.
- Chalcophile and pegmatophile indices were computed for the purposes
of isolating areas that are enriched in elements that are commonly
associated with several types of mineral deposits. These indices
are based on the cumulative concentrations of selected elements
and outline regions where further exploration follow-up may be
warranted.
- Chi-square plots provide a means of isolating multi-element
outliers from the background population of samples. These outliers
may be related to multi-element dispersion haloes that surround
many types of mineral deposits and may warrant further exploration
follow-up.
Application of the above methods of data analysis have outlined
the following dominant geochemical features:
- district-scale (10 to 30 km in length) patterns in the distribution
of Cu + Zn + Ni + Co ± Cr; V + Sn; and Zr + Nb; each pattern
appearing to relate to the dominant characteristics of varying
bedrock associations;
- an As + Sb + Mo + Sn + Be + W + Au association in the Weld Range
- Meekatharra -Gnaweeda greenstone belts trending northeasterly
which infers the presence of a chalcophile corridor;
- an association of Au and W within selected laterite samples
in the Dalgaranga greenstone area;
- an association of Au and Sb within selected ferricrete sample
in the Weld Range - Meekatharra -Gnaweeda greenstone areas; and,
- several anomalies at the 1 km scale that require follow-up sampling
in order to assess their continuity and significance. These anomalies
have been determined by a variety of methods. The most anomalous
samples tend to occur as outliers when these methods are applied.
Sporadic Au anomalies also occur. However, the general sample spacing
of 3 km with fill-in sampling at 1 km is too wide for reliable interpretation
of Au patterns in laterites. Laterite and ferricrete geochemistry
at these low sample densities generally requires the use of intermediate
zonal targets, such as anomalous chalcophile envelopes, which can
occur about individual deposits.
Last updated: Tuesday, January 04, 2000 12:56 PM
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