CRC LEME
Open File Report 92
ABSTRACT
Regolith geology and geochemistry Mt Magnet District - Geochemical
orientation studies, Stellar and Quasar deposits
Robertson, I.D.M., King, J.D., Anand, R.R. and Butt, C.R.M.
Recent exploration under the lag-strewn colluvium-alluvium covered
plain, mantling the Boogardie Synform, has located concealed Au
mineralisation at Stellar and Quasar. The area is difficult to explore
due to structural complexity in the basement, high geochemical backgrounds
from numerous mineralised settings, a variably stripped, residual
regolith beneath the transported overburden and scattered, high
Au contents in the overburden. Pit exposures at Stellar and drilling
at Quasar also revealed palaeochannel sediments hidden beneath the
colluvium.
Regolith-landform relationships in the district were established
by mapping the regolith of an area of 25 km2 around the mines and
inspecting drill cuttings and pit exposures. A palaeochannel, filled
with mega-mottled, grey clay and sandy clay, with some sepiolite
towards the top and detrital, ferruginous, lateritic nodules, pisoliths
and authigenic black granules towards the base, is exposed at Stellar.
At Quasar, a similar palaeochannel occurs south-west of the pit
and was detected by drilling. These fluvial channels appear to have
been incised in already weathered basement and the sediments themselves
have undergone post-depositional weathering. At Stellar, a clay-rich
lateritic duricrust has formed by the weathering and later cementation
of a partly transported unit overlying both the felsic and mafic-ultramafic
bedrocks. Contemporary weathering of clays in the palaeochannel
was probably responsible for the development of mega-mottles and
some ferruginous granules. The colluvial-alluvial overburden was
derived from dismantling the lateritic regolith. It contains lateritic
nodules and pisoliths, probably from a proximal source, but is dominated
by polymictic fragments, including BIF, ferruginous saprolite and
saprolite, set in a silty-clay matrix. Its upper 2-3 m is silicified
to red-brown hardpan.
Analytical data for a suite of 24 elements was provided by Hill
50 NL; the Ti and Zr data were unreliable and not used. Although
geochemical backgrounds and thresholds have been established for
the major regolith units, these must be regarded as of local application
only because no data from distant background areas was available.
Elevated concentrations of Cr, Ni, Cu, V and, possibly, Zn indicate
mafic-ultramafic rocks. The saprolite and mottled zone have a significantly
greater Cr/Fe ratio than the transported materials. Although the
laterite has a high Cr/Fe ratio, Cr and Ni were ineffective for
discriminating between laterites developed over felsic and mafic-ultramafic
rocks. Regolith differentiation can be improved by canonical analysis;
the most useful elements appear to be Al, Fe, Ni, Cr, Ga, Y, Zn,
Th and Cu.
The Au mineralisation is poor in pathfinder elements such as As
and Sb but there are weak anomalies in Bi and Pb. There is no correlation
between the composition of the basement and the overlying colluvium,
nor between nearly adjacent layers within the colluvium and its
overlying lag. Unless partial extraction geochemistry can indicate
basement-related anomalies in the colluvium-alluvium, this blanket
must be penetrated to reach the weathered Archaean beneath. Comparison
of sampling the top of the basement with that of the unconformity,
or interface, between basement and colluvium, indicates lower order
but broader and more consistent anomalies in the interface; this
can be improved using additive indices of Bi, Pb and Zn. The interface
sample is, therefore, the preferred sample medium, in this area
of buried, stripped regolith, except where there are palaeochannels.
Where there has been less regolith stripping and a buried laterite
occurs, as at Stellar, this is the preferred sample medium.
Last updated: Friday, July 21, 2000 04:24 PM
|
