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Open File Report 3

Regolith-landform evolution and geochemical dispersion from the Boddington Gold Deposit, Western Australia

Anand, R.R.

Regolith-Landform Relationships

A map of regolith-landform patterns was produced over an 11 km x 11 km area at Boddington. The regolith stratigraphy was established, the regolith units characterized, and a regolith-landform model established for the Boddington district. The model presents relationships in terms of preservation and erosion of the lateritic weathering profile and of local deposition.

The regolith of Boddington, as in other parts of the Yilgarn Craton, has developed over a long period, which is inferred to have taken place under a seasonally-humid climate during the Tertiary. The laterite profile is now undergoing degradation through physical and chemical processes.

A deep lateritic mantle occurs extensively over, much of the Boddington district. The profile comprises a gravelly, sandy soil containing loose lateritic pisoliths over a lateritic duricrust (Lateritic residuum), thence a bauxite zone, clay zone, saprolite, saprock, and bedrock. The thickness of the horizons varies considerably, depending upon the nature of the parent bedrock. Lateritic residuum forms a continuous blanket over some 30% of the landscape; however, the thickness and facies of lateritic residuum vary within the landscape. Lateritic residuum is either more developed or more preserved on mid-slopes than on crests or lower slopes. The mid-slope positions are dominated by pisolitic duricrust which is typically underlain by fragmental duricrust. By contrast, pisolitic duricrust is generally absent on crests and up-slope positions where fragmental duricrust is dominant. Lower slope positions are occupied by loosely-packed pisolitic gravels which can reach a thickness of 4 m.

The lateritic duricrust and associated loose lateritic pisoliths and nodules at Boddington are largely residual, but transported nodules and pisoliths are on lower slopes. Several types of nodules and pisoliths occur in the lateritic units and have been classified as lithic, non-lithic, and of mixed origin. Relict textures after andesite are visible through some of the profiles to the level of fragmental duricrust and these correlate with the bedrock relationships which have been established through drilling. Feldspars, in fabrics similar to those of bedrock have been pseudomorphed by gibbsite, showing that at least some of the lateritic duricrust is residual. Relict textures, derived from dolerite, are also present as residual ilmenite in the bauxite zone, as well as in the fragmental and pisolitic duricrusts. The dolerite results in a redder fragmental duricrust and bauxite zone than in those horizons derived from the felsic andesite.


The saprock consists of smectite, kaolinite, and mixed-layer minerals with relict primary minerals from the bedrock. Kaolinite is the dominant mineral in the clay zone and saprolite. The bauxite zone is characterized by replacement of kaolinite by gibbsite, and by the presence of hematite. In the overlying duricrusts and the lateritic pisoliths and nodules, hematite becomes predominant over goethite. Maghemite and amorphous Al-oxide appear in the pisolitic duricrust and become major constituents of the loose lateritic pisoliths and nodules. Hematite and goethite are relatively more abundant in the weathering profiles formed from dolerite than in those derived from andesite. Anatase is an important secondary mineral over dolerite.

Aluminium substitution increases up the profile. In the bauxite and duricrust horizons, Al substitution in goethite ranges from 20-33 mole % whereas in saprolite it is only from 8-18 mole %.


Chemical analyses of 284 samples of various regolith units, collected systematically from surface and from pit walls, document the multi-element characteristics of the lateritic Au deposits including dispersion during lateritic weathering of the protore and hosting lithologies. Significantly anomalous concentrations of Au occur close to the surface within the lateritic residuum although the supergene Au ore occurs at greater depth (10 m to 30 m). The concentrations of Au generally decrease into the overlying laterite units, i.e. from the bauxite zone through the lateritic duricrust to loose pisoliths. Gold in pisolitic and nodular lag is variable and is generally much weaker than the underlying pisolitic duricrust.

The protore mineralization is depicted by a multi-element (W, Mo, As, Sn, Cu, Bi) geochemical halo in the lateritic residuum, both in the duricrust and in the lateritic pisoliths and nodules. The element association is As, Bi, Mo, Sn and W, with more erratic Cu and Au. Tungsten, Mo, As and, to some degree, Sn, show a more widespread and homogenous distribution than Cu and Bi. Variations in the contents of As, W, Sn, Mo, and Au were observed between profiles that reflect variations in the parent rock and bedrock mineralization.

The trends in element behaviour in profiles formed from andesite and dolerite are very similar, with minor differences due to the contrasting chemistry and mineralogy of the host rock. The bauxite zone and duricrusts formed from dolerite are richer in Fe203, TiO2, Mn, V, and Zn, than the same horizons from felsic andesite. The Ti and Zr contents of the laterite have been used to interpret the origin of the lateritic residuum following the method proposed by Hallberg (1984) for saprolite. Most of the samples appear to have been derived from felsic andesite rocks, the remainder from dolerite.

The Mn, Cu, Zn, Ni, and Co are relatively depleted in the upper horizons of the profile, while Fe, Al, Ti, V, Cr, As, Bi, Sn, Ga, W, Zr, Nb, Mo, and Pb are retained or enriched throughout the whole profile.

These elements have accumulated in the lateritic residuum and are either associated with Fe-oxides and gibbsite, or occur as resistant primary minerals such as zircon, cassiterite, and scheelite.

Gold, Cu, and Al are enriched in the non-magnetic pisoliths contrasting with Fe, and As that are relatively more abundant in magnetic pisoliths.

The Boddington Au deposit highlights some of the problems of Au exploration in lateritic terrains which are exemplified by the leaching of Au from surface and near-surface lateritic pisoliths and nodules. A large strong, consistent multi-element anomaly at surface, with or without Au, seems to be the best and most reliable indicator of Au deposits. For Au exploration in the Boddington area, samples of fragmental duricrust instead of loose lateritic pisoliths or pisolitic duricrust, should be collected.


Geomorphology, residual regime, erosional regime, depositional regime, regolith stratigraphy, andesite, bauxite zone, lateritic residuum, soil, pisolitic lag, nodular lag, dolerite, geochemistry, mineralogy, geochemical dispersion, magnetic nodule.

Last updated: Friday, December 24, 1999 10:47 AM


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