CRC LEME
Open File Report 74
ABSTRACT
Regolith-landform relationships and the petrological, mineralogical
and geochemical characteristics of lags, Lawlers District, Western
Australia
Anand, R.R., Churchward, H.M. and Smith, R.E.
This study of the regolith in the Lawlers district is focussed
upon the Meatoa, Brilliant, and Agnew-McCaffery areas. The regolith
units were mapped using 1:25 000 colour air photographs and three
times enlargements (1:8333) of selected air photographs as the base.
The regolith stratigraphy has been established for each of these
areas and some selected surface units characterised petrologically,
mineralogically and geochemically. The regolith patterns observed
in these areas are explained in terms of the distribution of (a)
regimes of erosion of the laterite profile to the level of saprolite
and bedrock resulting in terrain characterised by low hills, (b)
regimes where the essentially complete laterite profile is preserved,
commonly forming gentle ridge crests and backslopes, and (c) regimes
characterised by depositional accumulations of detritus derived
by the erosion of the laterite profile, burying the partly truncated,
and in places complete laterite profile in the lower slopes of colluvial-alluvial
outwash plains. Idealized regolith-landform models for these areas
have been established for the purpose of predicting regolith relationships
in comparable terrain elsewhere.
The soils occurring within those truncational regimes which have
mafic or ultramafic bedrock lithologies are predominantly red-coloured
light clays and red sandy clay loams. They are often acidic and
commonly are underlain by a red-brown hardpan. The red clays often
contain pseudomorphic grains after amphiboles, further evidence
of their mafic origin. The occurrence of pedogenic calcrete at shallow
depths in the erosional regimes generally relates to a mafic lithology.
Soils on felsic lithologies are acidic, yellowish brown, sandy loams.
Residual regimes are dominated by acidic, brown gravelly sandy loams
and sandy clay loams and generally red-brown hardpan is not developed.
The soils within the depositional regimes are developed in colluvium-alluvium
and are acidic, gravelly sandy clay loams and light clays.
The distribution and characteristics of lag gravels have been placed
within the regolith-landform framework established during this study.
At Meatoa, four classes of lag have been recognized: black, coarse
cobbles after ferruginized saprolite, yellow to reddish-brown lateritic
lithic fragments, lateritic lag, and fine lag of mixed origin. The
fine lag of mixed origin, which is widespread in the Meatoa area,
comprises a variety of clasts derived from the breakdown of large
lateritic lithic fragments and lateritic duricrust, both of which
occur in the local uplands. These four lag types differ in their
morphological, chemical, and mineralogical characteristics and include
systematic variations in the Al substitution within the Fe-oxides.
Some lateritic lithic fragments preserve the original rock fabric.
Lateritic lithic lag is relatively richer in kaolinite while lags
of ferruginized saprolite and lateritic lag are poorer in kaolinite.
Petrographic examination of the lag gravels has shown that kaolinite
is progressively replaced by hematite and goethite. Goethites of
ferruginized saprolite contain lowest Al substitution (<5 mole%),
whereas those of lateritic lag contain highest Al substitution (19-26
mole%). The low Al substitution in goethites of ferruginized saprolite
suggests that these have formed by absolute accumulation of Fe in
an environment almost free of available Al. The data on Al substitution
of various regolith materials from this study have shown that the
degree of Al substitution in goethite can be used to predict the
weathering status of regolith materials and the environments within
which they have formed. The lags of lateritic lithic fragments and
lateritic lag show relatively high levels of As, Pb, and Ga, Sn,
W, and Bi occur in low concentrations and did not show any variations
between the lag types. The lags of ferruginized saprolite have significantly
higher levels of Mn, Zn, and Co than the other lag types. The differences
in the geochemistry of various types of lags are related to the
nature of bedrock, degree of weathering, and mechanism of accumulation
of weathering products. The concentrations of Cr and Ni were found
to be useful in discriminating the origin (mafic vs ultramafic)
of lag gravels. The levels of Au are low in the range of 0-0.034
ppm. Investigation suggests that the Fe-rich duricrusts are probably
formed by absolute accumulation of Fe. One possible explanation
is that Fe originally impregnated the soils and sediments in local
valleys which now occur as ridge crests in the present landscape
because of inversion of relief.
Gold in lateritic nodules from the North Pit location occurs in
two forms (i) grains up to 15 µm in diameter, occurring in
cracks, and (ii) large, dendritic Au grains which reach 70 µm
in diameter and are attached to the surface of goethite. Both occurrences
of Au appear to be secondary and are almost free from Ag (<1%
Ag). In the lateritic nodules, As and Mn are strongly associated
with Fe oxides, while Cu is associated with kaolinite.
Last updated: Thursday, January 06, 2000 11:49 AM
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