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Open File Report 74

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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