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Open File Report 148

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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