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)
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