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Open File Report 62

Regolith-landform development and consequences on the mineralogical and geochemical characteristics of regolith units, Lawlers District, Western Australia

Anand, R.R., Churchward, H.M., Smith, R.E. and Grunsky, E.C.

Regolith-Landform Relationships

The regolith patterns observed in the Lawlers district are explained in terms of the distribution of (a) regimes of erosion of the laterite profile to the level of saprolite, saprock, bedrock resulting in terrain characterized by low hills, (b) regimes where the essentially-complete laterite profile is preserved. commonly forming gentle ridge crests and backslopes, and (c) regimes characterized by depositional accumulations of detritus derived by erosion of the laterite profile, burying the partly-truncated, and in places complete, laterite profile in the lower slopes of colluvial-alluvial outwash plains. In the depositional areas, the sediments reach up to 30 m in thickness. It is now well established that buried residual laterite profiles are widespread beneath the colluvium and alluvium.

Studies in three type areas (the Agnew-McCaffery, Meatoa, and Brilliant areas) provide an understanding of regolith relationships, regolith stratigraphy, and the origin of regolith units. Criteria are established for distinguishing residual regolith from transported regolith applicable to drill hole logging. A regolith-landform model for the Lawlers district presents relationships in terms of erosion and burial of complete and partly truncated lateritic profiles.


The soils occurring within those truncated 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. Black, ferruginous cobbles of iron segregations, fragments of ferruginous saprolite, and vein quartz occur largely on erosional areas (Units 2a, 2b). Lag of lateritic pisoliths and nodules occurs on residual areas (Units 1a, 1b) overlying complete or nearly complete laterite profiles. The lag of mixed origin, comprising lithic fragments, quartz, lateritic pisoliths and nodules, and fragments of ferruginous saprolite, is abundant on colluvial-alluvial outwash plains.

Lateritic residuum

The top of the residual laterite profile is composed of a layer of lateritic residuum averaging some 3-8 m in thickness comprising a sub-unit of loose pisoliths and nodules which may be underlain by a sub-unit of nodular duricrust. A zone of ferruginous saprolite characterized by bodies of iron segregations generally underlies the lateritic residuum. It is established that ferruginous saprolite forms a blanket deposit up to several metres thick in many areas in the Lawlers district and is preferentially developed over mafic and ultramafic lithologies. In turn, ferruginous saprolite grades into a thick saprolite zone, which extends to vertical depths of 50 to 70 m.

Development of many nodules and pisoliths in lateritic residuum is associated with fragmentation of ferruginous saprolite. Fragmentation of bodies of iron segregations can also yield nodules and pisoliths which become incorporated within the lateritic residuum. 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.


At Lawlers, hardpan has developed within in situ regolith and detritus resulting from the erosional modification of the old surface. Cementation of these materials by Si and Fe to form the hardpan is a relatively recent process.

Discrimination between sample types

The 181 samples collected from the McCaffery-North Pit area were separated into four broad groups based mainly upon their morphological characteristics and regolith-landform framework. These include materials from both surface and subsurface units of the weathering profiles. The four groups recognized are: colluvium, lateritic residuum, ferruginous saprolite, and iron segregations. These four groups are shown to have different morphological, mineralogical, and geochemical characteristics. Iron segregations can be recognized by their irregular, black, non-magnetic pitted surfaces. Internal surfaces of iron segregations may show goethite and hematite pseudomorphs after sulphides. Lateritic pisoliths and nodules of lateritic residuum typically have 1 to 2 mm thick yellowish-brown to greenish cutans around black to red nuclei. The presence of cutans may be used to recognize nodules and pisoliths derived from the breakdown of lateritic residuum.

Mineralogy has been shown to give valuable information concerning which part of the weathering profile is exposed at the surface. Iron segregations differ from lateritic residuum by having abundant goethite and less hematite and kaolinite. Maghemite is typically absent in iron segregations. Lateritic residuum can be distinguished from ferruginous saprolite by having abundant hematite and less kaolinite. Colluvium differs from the other groups in having abundant quartz, kaolinite, and some heavy minerals.

The four sample media also show differences in the degree of Al substitution in goethite which appears to be related to the maturity of the regolith, level of truncation, and may also reflect the environments in which the particular regolith unit has formed. Evaluation and identification of various sample media by the degree of Al substitution in goethite looks to be very promising.

Iron segregations are dominated by Fe2O3, Mn, Zn, Co, Ba, and goethite and these elements can be used to discriminate iron segregations from lateritic residuum, ferruginous saprolite, and colluvium. Many of the chalcophile elements and Au exhibit lower levels of abundances to those in lateritic residuum and ferruginous saprolite. However, the prominent regional distribution of iron segregations, often as scree on pediment surfaces in partly-stripped profiles, offers potential for use as a geochemical sampling medium.

Whilst the Fe2O3 contents of the ferruginous saprolite are comparable with those of the lateritic residuum, there are strong geochemical distinctions between the two types. Lateritic residuum has relatively higher levels of Cr, V, Ni, As, and Pb. Conversely, ferruginous saprolite carries significantly higher levels of Cu, Sb, Bi, and Au. The concentrations of SiO2, MgO, TiO2, Zr, and Nb are higher in colluvium than in lateritic residuum and ferruginous saprolite. These differences may be due to the degree of weathering, mineralization, mechanism of accumulation of the secondary weathering products, and origin.

The group separation using canonical variate analysis and all possible subset calculations has indicated that effective separation of the four sampling media exists. A combination of 14 elements (Fe, Mn, Cr, V, Pb, Zn, Ni, Co, As, Sb, Bi, W, Zr, Nb) would seem to be the most useful for separation of the groups.

Siting and bonding of elements

Gold in lateritic nodules from the North Pit location occurs as (i) grains up to 15 µm in diameter, occurring in cracks, and (ii) relatively large dendritic Au grains, which reach 70 µm in diameter, 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 11:09 AM


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