CRC LEME
Open File Report 143
ABSTRACT
Mineralogical and geochemical aspects of the regolith at the
Brahman Au Prospect, Charters Towers area, N.E. Queensland
K.M. Scott and S.J. Fraser
The regolith at the Brahman Au prospect, is characterised by the
presence of ferruginous pisoliths above mottled transported sandstone
(Campaspe Formation) which unconformably overlies residual clay
and saprolitic granite. Within the profile, Au is laterally dispersed
for up to several hundred metres at the top and bottom of the saprolite
as well as in the surficial pisoliths. Arsenic, Cu and Pb contents
are also elevated in the ferruginous pisoliths. Zones of lateral
dispersion of Au with a zone of depletion beneath the surficial
anomaly are similar to those commonly observed in theYilgarn Craton
of Western Australia. However, at Brahman, the Au-enriched pisolitic
unit and Au-depleted mottled zone are developed in transported material,
i.e. Au has been hydromorphically dispersed into the overlying sediments.
Manganese oxides and associated elements (Ce, Co, La and Zn) are
well developed in the clay zone of the profile, but are also present
in transported material suggesting that they are also dispersed
late in the history of regolith development.
Despite their probable development in transported material, the
ferruginous pisoliths represent a good sampling medium. Orientation
results suggest that low level Au, As, Cu end Pb halos at least
700 x 500 m are present even in five metre composite samples that
include a large component of non--ferruginous material. More intense
anomalies, with better signal-to-noise ratios, would be expected
with purer pisolitic samples which have not been diluted with depleted
material. The development of such extensive hydromorphic dispersion
over minor mineralisation is encouraging for exploration within
the Charters Towers region. Anomalous Th (+U) with the pisoliths
suggest that airborne radiometrics may be a useful method to assist
in the planning a regional pisolith sampling programme.
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