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.
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
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