CRC LEME
Open File Report 3
ABSTRACT
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.
Mineralogy
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 %.
Geochemistry
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.
Keywords:
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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