CRC LEME
Open File Report 105
ABSTRACT
Geochemical dispersion in transported and residual regolith,
Fender Gold Deposit, Cue, Western Australia
Butt, C.R.M.
Fender is a small Au deposit (248 000t @ 2.4 g/t Au) approximately
2 km south of Big Bell, WNW of Cue, on the margin of a colluvial-alluvial
plain. The deposit itself is entirely overlain by a thin (2-5 m)
cover of transported overburden and does not outcrop. The overburden
consists of two units, fine- to coarse-grained sandy clay, sand
and gravel, overlying silty clays. Both the sands and the silty
clays locally contain detrital lateritic gravels. The sands are
weakly cemented in the top metre to form hardpan and some deeper
sediments are mottled; there is no pedogenic carbonate. The sediments
contain feldspar grains (0.5-1.0 cm) and are probably derived from
the granites to the west. There are two principal regolith situations
beneath the overburden. In the south, the lateritic profile appears
largely complete and a small Au resource is hosted by lateritic
residuum and ferruginous saprolite. In the north, the lateritic
profile is truncated and the sediments are deposited on saprolite
which, in some places, is depleted in Au but, in others, has Au
at ore-grade concentrations immediately beneath the unconformity.
Similar situations are present 200 m to the west, where lateritic
residuum and saprolite outcrop. There is no surface geochemical
expression of the deposit in soils (15-30 cm depth), determined
by conventional total analysis or by bulk cyanide leach (BLEG) analysis,
nor in composite samples (4 m) of the sediments, except where drilling
has penetrated into the concealed lateritic residuum. Possible geochemical
dispersion into the sediments has been investigated by careful sampling
of the sediments and uppermost residuum by RAB drilling, with care
taken to avoid cross-hole contamination. The primary mineralization
(Au >1000 ppb) is characterized by enrichment in Ag (mean 1.4
ppm), As (145 ppm), Sb (450 ppm), W (130 ppm), Cd (1.2 ppm) Mo (37
ppm), Tl (7 ppm), Zn (475 ppm) and Hg (100 ppm). However, of these,
only As, Sb and Ware detectable in the near-surface samples. In
the residuum, the W distribution indicates the weathered primary
mineralization, even where Au has been depleted in saprolite or
enriched and dispersed in lateritic residuum. In comparison, As
and Sb are both enriched and widely dispersed in the nodular ferruginous
clays and ferruginous saprolite to give broad anomalies. Concentrations
are homogeneous and remain anomalous (>50 ppm As, 70 ppm Sb)
in shallow ferruginous saprolite and outcropping lateritic gravels
for at least 200 W of the subcropping mineralization. Gold abundances
are generally <5 ppb in the sediments over saprolite, except
for some spot concentrations (80-245 ppb) immediately above subcropping
mineralization, and an associated weak enrichment (5-16 ppb) extending
50 m down slope. However, they are significantly anomalous (60 ppb
Au) for over 100 m east of the subcropping lateritic residuum. In
comparison, As (40-120 ppm), Sb (12-50 ppm) and, over saprolite,
W (5-17 ppm) are anomalous in the clays for at least 200 m down
slope to the east. Essentially all of the As and Sb in the silty
clay unit is hosted by mechanically transported ferruginous nodules
(80 to over 100 ppm Sb, 300-450 ppm As). Neither Au nor W are concentrated
in the nodules. The sands and soils contain background concentrations
of Au, As, Sb and W except where directly overlying lateritic residuum.
It is concluded that a sampling strategy that targets lateritic
residuum, whether outcropping or buried, would locate this deposit.
Where the regolith is truncated, restricted dispersion in residual
and transported units implies that analysis for Au alone is unsatisfactory
in the top 20 m. Multi-element analysis would reveal broad, low-order
As+Sb+/-W anomalies in the silty clays, which can be markedly enhanced
by selective sampling of ferruginous nodules. Such selective sampling
is preferable to the common practice of compositing samples over
intervals of 2-6 m. However, composites of ferruginous nodules from
the lowermost 2-4 m of the sediments may be suitable, especially
if they are scarce. The preferential concentration of As and Sb
in detrital ferruginous nodules, rather than in the matrix, of the
silty clays, implies that there has been little or no post-depositional
chemical dispersion.
Last updated: Sunday, August 05, 2001 13:59:33
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