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Open File Report 149

Regolith architecture and geochemistry of the Hermidale area of the Girilambone region, North-western Lachlan Fold Belt, NSW

A joint project between CRC LEME and NSW DMR
R.A. Chan, R.S.B. Greene, M. Hicks, B.E.R. Maly,
K.G. McQueen and K.M. Scott

Stage 2 of the Girilambone (Cobar-Bourke) Project has involved collaborative work between CRC LEME and the NSW Department of Mineral Resources in the Hermidale area. This work has provided 2275 m of drilling (49 holes, generally 2-4 km apart, along 5 road traverses) for regolith.

Regolith-landform mapping conducted in association with drill hole logging reveals that colluvial and alluvial sediments cover most of the Hermidale area. Colluvial regolith-landform units are widespread on plains, as well as in areas of higher elevation and relief in the central-south and western parts of the area. Alluvial regolith-landform units are dominant on plains in the centralnorth and eastern parts. There are also numerous palaeochannels, many of which are defined on 1.5 Vertical Derivative magnetic imagery. However, drilling and mapping indicate that palaeosediments are more extensive than the obvious extent of magnetic sediments. The variable thickness of sediments, both in palaeovalleys within the erosional domain in the west and beneath depositional plains in the east, indicates a palaeo-relief significantly greater than the present relief.

Logging of regolith units through palaeovalley sediments indicates a change in composition from brown to grey clays. This could reflect a change in depositional environment in the palaeovalleys or weathering associated with a palaeo-redox front from higher palaeo-water tables. More detailed facies interpretation is required to resolve these possibilities. The present water table typically ranges from 30-67 m depth and intersects the saprolite well below the base of transported regolith and sediments. Palynology indicates that reduced sediments in the palaeochannel deposits are most likely Late Miocene to Early Pliocene in age.

Dominant bedrock lithologies are phyllite/siltstone, sandy phyllite and sandstone, which are mostly highly weathered to at least 80 m depth. Less weathered felsic volcanic lithologies occur in the southwestern portion of the area. Some mafic and ultramafic bedrock units have also been detected. Regolith carbonates are common throughout the area and are associated with the transported/saprolite or bedrock interface or with sediments higher in the profile.

Mineralogical and geochemical studies of the regolith profiles indicate different mineral assemblages within transported and in situ regolith. The transported regolith shows assemblages with kaolinite±illite±smectite, whereas the in situ saprolite contains kaolinite±muscovite/phengite illite±smectite mineral assemblages.

The change in drilling procedure (i.e. purging the vacuum chamber by drilling 2 m of local material and discarding before re-starting drilling at a particular site) has avoided the cross-hole contamination seen during the earlier Sussex phase of drilling. Some caution is advised when interpreting geochemical results for some elements analysed using multi-acid dissolution methods, (e.g., high Cr identifies mafic dykes but the absence of Cr does not preclude material from being mafic). Specifically, Ba, Cr, Ti and Zr values determined by ICP analysis should be regarded as minimum values because of the possibility of incomplete dissolution. Similarly, K and Rb contents may also be low relative to XRF-determined values.

Many near-surface samples show elevated Au, together with Ca, Mg and in some cases Ba-Sr, and there is a high probability that such Au is associated with secondary regolith carbonate. Weak Au-As mineralisation occurs in CBAC 142, along strike from the Muriel Tank mineralisation.

Mafic dykes occur in the rocks to the north and east of the Babinda Volcanics. These dykes can be enriched in chalcophile elements, including Co, Cu and Zn.

Mineralisation-associated elements may be present in Zn-rich profiles in the Babinda Volcanics, possibly analogous to the Mt Windsor Volcanics (in which stratiform mineralisation generally occurs at a particular level in the volcanic sequence). Weak As-Sb-Mo-W-Zn-(Cu)-(Au) mineralisation occurs in CBAC 167 and probably reflects the edge of more Pb-rich mineralisation.


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