CRC LEME
Open File Report 148
ABSTRACT
Regolith, geomorphology, geochemistry and mineralisation of the
Sussex-Coolabah area in the Cobar-Girilambone region, North-western
Lachlan Fold Belt, NSW
R.A. Chan, R.S.B. Greene, N. de Souza Kovacs, B.E.R. Maly, K.G.
McQueen and K.M. Scott
A joint study by NSW Department of Mineral Resources (DMR) and
the Cooperative Research Centre for Landscape Evolution and Mineral
Exploration (CRC LEME) of the Sussex-Coolabah area was undertaken
to assist geological mapping and mineral exploration in the relatively
unknown Girilambone belt of the Lachlan Fold Belt. This study was
based around a shallow drilling program of 138 air core drill holes,
spaced mostly at 1 km intervals along 3 road traverses. This CRC
LEME report details the regional mineralisation, weathering and
geochemical setting, based on previous studies, and the methodologies
and results of investigations of the regolith geology within the
study area. This report complements the NSW DMR report (GS2001/200).
Regolith landform mapping, incorporating remotely sensed data,
and selected soil analyses enabled identification of the distribution
and landform association of dominant surficial regolith materials
in the study area, and their processes of formation. Much of the
surficial material is sheetwash colluvium with intervening modern
alluvial tracts, and minor residual soils: an aeolian component
appears to be widespread in the top 0.5 to 3 m of soils. Enhanced
airborne magnetics enabled identification of subsurface magnetic
sediments associated with widespread networks of palaeovalleys.
Field and microscopic logging of the drill holes, assisted by Portable
Infrared Mineral Analyser (PIMA) analysis, enabled differentiation
of transported and in situ regolith and the description of material
attributes. PIMA analysis also identified variations in mineralogy
of the highly weathered sedimentary bedrocks. Regolith profiles
were interpreted for all drill holes, and used as the basis for
deriving sections along the 3 traverses. The areal data from the
regolith landform mapping and the magnetics provide a context for
extrapolation from the sections, and thereby an indication of the
3D regolith architecture of the study area. Palaeosediments cover
about 50% of the area. They infill palaeovalleys up to 40 m in depth
and 7 km wide, and overtop palaeohighs in places.
Regolith and bedrock samples were geochemically analysed for a
suite of 22 elements. A number of in situ and displaced geochemical
anomalies worthy of follow up investigation were detected. Ten profiles
with anomalous or interesting geochemistry (involving 7 elements)
are discussed with regard to the relationship between geochemistry,
mineralogy and regolith setting. Mafic dykes (commonly with high
Cr) are indicated by combined X-ray Diffraction and PIMA analyses
of saprolite.
The regolith and landform evolution of the Sussex-Coolabah area
is placed in a regional framework and is taken back to the Mesozoic,
allowing geochemical anomalies to be placed in an evolutionary context.
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