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Open File Report 61

Regolith-landform development and siting and bonding of elements in regolith units, Mt Gibson District, Western Australia

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

Regolith-Landform Relationships

The complex regolith of the Mt Gibson district is explained in terms of the distribution of: (a) regimes where the essentially complete laterite profile is preserved, commonly forming broad crests and upper gentle slopes, (b) regimes of erosion of the laterite profile to the level of saprolite, saprock, bedrock resulting in terrain characterized by low to medium hills, and (c) regimes characterized by depositional accumulations of detritus provided by the erosion of the laterite profile, commonly burying the complete and partly-truncated lateritic weathering profile in the lower slopes and valley floors. In the latter, sediments reach 30 m in thickness and residual laterite up to 6 m thick was observed to occur under the sediments.

The regolith units were mapped over a 17 km x 10 km area, the regolith stratigraphy established, and the regolith units were characterized. A regolith-landform model for the Mt Gibson district describes relationships in terms of erosion and burial of complete and partly truncated profiles. Lateritization and post-lateritization processes responsible for the formation of a variety of regolith types are discussed.

Midway North Area

The pattern of regolith at Midway North which characterizes the district relates closely to the erosional and depositional modification of the deeply-weathered mantle. A total of 25 samples representing various regolith units were characterized petrologically, mineralogically, and geochemically. Systematic mineralogical and geochemical differences occur between transported and residual regolith units. For example, the colluvial units (soils and hardpanized colluvium) contain higher amounts of kaolinite and quartz relative to the underlying residual regolith units. Hematite increases upwards in the residual weathering profile. The Al content in goethite also tends to increase towards the top of the residual profile.

Lateritic residuum is enriched in Cu, Pb, Zn, As, W, Ag, and Au which are associated with hematite and goethite. The calcareous clays overlying acid, plastic clays largely consist of dolomite, kaolinite with small amounts of hematite, calcite, halite, and palygorskite. These clays have low contents of chalcophile elements, but contain significant amounts of Au. However, the amounts of Au are very low in clays relative to the lateritic residuum. The upper saprolite is depleted compared to the lower saprolite in Au and chalcophile elements. Acid, red plastic clays, developed from the weathering of underlying saprolite, are also low in Au.


Scanning electron microscopic studies of calcareous soils and nodular calcretes from the Mt Gibson Au deposits reveal a fossilized community of soil micro-organisms dominated by filamentous structures preserved in fine detail by calcite. The calcite forming the filaments has a variety of crystal habits. Calcified filaments observed in samples of nodular calcrete suggest that biological activity could have played a significant role in the formation of the carbonates in the regolith.

Siting and Bonding of Elements and Dispersion Processes

The bulk samples of soils, lateritic residuum, hardpanized colluvium (red-brown hardpan), and calcareous soils were separated into various morphological groups, such as magnetic vs., non magnetic nodules-clasts, matrix vs., clasts, calcareous fragments vs., ferruginous clasts, and cores vs., cutans. The petrological, mineralogical, and geochemical characteristics of these materials were established. Non-magnetic lateritic nodules and clasts are the dominant fraction of both soils and lateritic residuum. The magnetic and non-magnetic nodules and clasts have different internal fabrics. The cores of magnetic nodules and clasts are black and massive, whereas those of nonmagnetic nodules and clasts are yellowish brown and porous. Hematite is a dominant mineral in both magnetic and non-magnetic lateritic nodules and clasts. The non-magnetic lateritic nodules and clasts contain higher amounts of goethite and kaolinite relative to the magnetic lateritic nodules and clasts, while maghemite is present in the magnetic nodules and clasts. The cutans of nodules and pisoliths are dominated by goethite. Goethite in the lateritic residuum is highly Al-substituted (17 mole%).

Iron, Cr, V, Pb, As, W, Sb, Bi, and Zn are enriched in magnetic lateritic nodules and clasts relative to the non-magnetic lateritic nodules and casts. By contrast, Al, Si, Cu, Ag, Au, and Ni are relatively more abundant in.ts of Au than cutans.

The matrix of hardpanized colluvium (red-brown hardpan) consists of kaolinite, quartz, and amorphous silica. The mean values of Au and Ag for the matrix of hardpan are higher than, or very similar to, those for lateritic nodules.

Carbonate fractions separated from nodular calcrete are anomalous in Au. The magnitude of the anomaly is, however, smaller than that of lateritic nodules and pisoliths.

The possible associations of Au, Pb, Zn, As, Cu, Ni, Cr, and V with various secondary mineral species are discussed. Chromium, V, Ga, Pb, As, and Sb are strongly associated with hematite. Goethite and kaolinite appear to have strong affinities for Cu, Ni, Ag, and Au. Hydromorphic dispersion appears to be the major dispersion process that extended the geochemical halo in the upper part of the regolith.

Last updated: Thursday, January 06, 2000 11:11 AM


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