CRC LEME
Open File Report 62
ABSTRACT
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.
Soils
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.
Lags
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.
Hardpan
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
|
