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Silcrete — a potential new
exploration sample medium:
a case study from the Challenger gold deposit
Melvyn J. Lintern (Cooperative Research Centre for Landscape Evolution and Mineral Exploration)
Malcolm J. Sheard (Principal Geologist, Mineral Resources Group, PIRSA)
Introduction
Recent geochemical research undertaken at the
Challenger gold deposit in the Gawler Craton has indicated
that silcrete has potential use as a sample medium to aid gold
exploration in specific regolith–landform environments.
Silcrete, like calcrete, is:
• a common material found in various arid to semi-arid
•
•
parts of Australia
easily identified and occurs at or near the surface
readily analysed using standard laboratory procedures.
The discovery of silcrete as a potential sampling medium
follows on from that of calcrete which has been shown to be
of immense importance in opening large areas of South
Australia and Australia to mineral exploration (Lintern,
1997). As with any geochemical survey technique, anomaly
threshold values for silcrete should be established for each
area being explored. A knowledge of the regolith
stratigraphy is important since thresholds may vary
considerably and silcrete sampling may not be applicable in
areas of transported overburden.
Silcrete lag amongst ‘grey billy’ silcrete outcrop.
(Photo 46356)
The composition and origins of silcrete
Silcrete is a secondary cementation feature imposed on
existing sediments or weathered rocks. Of the two broad
classes of silcrete — pedogenic silcrete formed in the soil
and groundwater silcrete formed at or near the watertable —
only pedogenic silcrete has been investigated in this study.
Silcrete is silicified material that retains the properties and
fabric of its precursor, but diluted by added silica and
modified by the processes that accompanied precipitation.
Silcrete is usually very hard but extremely brittle, and breaks
with a conchoidal fracture such that the fracture propagates
across the cemented grains rather than around them. Freshly
broken silcrete shards commonly have a pungent odour, a
vitreous sheen and are very sharp. Silcrete has been widely
mapped in South Australia since the 1960s and appears with
a lithotype designation or as a host-rock overprint symbol on
many State geological maps. Many pedogenic silcretes have
high concentrations of Ti (1–15%, to a maximum of >50%),
Zr (500–>2000 ppm), Nb (to >25 ppm) and Y (to >5 ppm).
In contrast, Hutton et al. (1978) and Thiry and Milnes (1991)
found that groundwater silcretes tend to have lower
concentrations of Ti (<1%), Zr (<100 ppm) and Nb
(<20 ppm).
Pedogenic silcrete forms within the soil profile, at or near
the land surface, by chemical leaching, illuviation and
mineral modification. In general, pedogenic silcrete may
display diagnostic features in the upper portions that are
16
Vertical view of silcrete lag showing occurrence on land surface.
Field of view is ~0.8 x 0.5 m. (Photo 46355)
consistent with slope and soil leaching processes. These
include nodules (30–300 mm), pisoliths (5–30 mm),
concretions or concretionary laminated overgrowths, forms
similar to dripping candle wax, columnar bodies with
internal structures (draped banding), silica-cemented
rounded silcrete clasts, silcrete breccias, hyaline silica
and/or opal overgrowths, cutans, veins and patches. The
silcrete may also contain strongly segregated silica–titania
(anatase or leucoxene) accumulations and resistate, and
minerals such as zircon and monazite. A complex silica
deposition scenario was advanced by Thiry and Milnes
(1991), whose model is driven by cyclic seasonal wet and
dry periods, leaching silica in low concentrations (~6 ppm
SiO2) with deposition by evaporative concentration.
MESA Journal 11
October 1998
Methods
180
Zone 1
3
Zone 2
Zone 3
• It crosses several zones of mineralisation (Zones 1–3).
The in situ regolith consists of Archaean Christie Gneiss,
highly weathered in its upper part, and the transported unit of
fluvial deposits of presumed Tertiary age.
Regolith stratigraphy
October 1998
366000mE
362000mE
1000
METRES
Gold in calcrete
5–10 ppb
Challenger ore shoots
projected to surface
10–100 ppb
Regolith line
>100 ppb
Sample point
Gold mineralisation
in RAB >100 ppb
Gold (ppb) from original
sampling program
7
98-1264
Fig. 1 Plan of the Challenger gold deposit showing gold in
calcrete anomaly, mineralisation (as defined by RAB), ore shoots
(as defined by RC) and the regolith line referred to in this study
(after Bonwick, 1977).
situ regolith is usually characterised by the presence of small
(a few millimetres), rounded quartz grains.
Northwest
0
129
Southeast
RC holes
125
120
115
110
105
100
2
4
In situ
saprolite
6
e
MESA Journal 11
0
edg
The transported regolith occurs in the eastern part of the
section and consists, from the surface down to ~3 m, of
calcrete and silcrete; drill cuttings indicate that white and pink
coloured, variably silicified clay and silt (porcelanite) are
dominant from ~3 to ~16 m. Underlying some of the lower
portions of the silicified unit is brightly coloured yellow and
red clay up to 3 m thick. Two palaeochannels, one extending
down to ~25 m and the other to ~21 m, incise the base of the
saprolite. The position of some silcrete at the base of the
(deeper) palaeochannel suggests it to be of groundwater
origin. The eastern (shallower) palaeochannel is mostly filled
with clay. The unconformity between the transported and in
6692000 mN
Palaeochannel
The in situ regolith occurs principally in the central and
western part of the regolith line. In the upper in situ regolith
(0–6 m) at Challenger, deep weathering has yielded a
generally pale coloured (mainly yellow and red), clay-rich
saprolite (Fig. 2). The clays are predominantly kaolinite,
illite and smectite, with residual quartz fragments and clasts.
The saprolite has been overprinted and coloured by iron and
manganese oxides and, within the upper few metres, is
variably cemented by silica, carbonate and gypsum. Some
of the saprolite displays features consistent with surficial
physical disruption and dislodgment, to form locally derived
slope talus breccias. These can contain resistate mineral
grains that have low to moderate rounding due to transport;
some of these have been incorporated deeper into the profile
via surficial shrinkage cracks. The lower regolith consists of
either (i) mottled clay, overlying variably coloured clay (and
partly ferruginous weathered rocks) and abundant quartz
grading to less ferruginous weathered rock and abundant
quartz with depth or (ii) as above but with mottled clay
absent. The boundary between highly weathered, mostly
clay-rich saprolite and moderately weathered saprolite
containing appreciable quantities of partly weathered rock is
variable, but lies between 20 and 40 m.
5
0
Depth (metres)
•
•
Zone 1 is by far the richest and almost crops out. Zone
2 is weakly mineralised and has 1–2 m of transported
material. Zone 3 is weakly mineralised and is buried
beneath >20 m of transported overburden.
It traverses in situ and transported regolith units.
Surface drilling contamination was minimal, permitting detailed geochemical research work.
7
6694000 mN
364000mE
The study area is located near the Challenger gold
deposit of Resolute Ltd and Dominion Mining Ltd (the
Gawler Joint Venture), 750 km northwest of Adelaide.
Geochemical sampling of a variety of materials was
undertaken at ~50 m intervals along a 1.45 km ‘regolith line’
orientated northwest–southeast (Fig. 1). Detailed
stratigraphy and sample descriptions were undertaken at
several pits and from thirty 6 m deep holes drilled especially
for the study. The regolith line is located close to existing
RAB and RC drilling done by the Gawler Joint Venture and
was chosen by the authors as it fulfilled several requirements:
0
500
Transported fluvial
material
METRES
Lithological boundary
Base of carbonate
impregnation
Gypsum–gypcrete
zone
Ferruginous zone
Boundary to iron staining
Silcrete
Base of silcrete
Boundary between trans–
ported and in situ regolith
Alluvium
Calcrete
Saprolite
Mineralisation
at depth
98-1265
Fig. 2 Section along the regolith line (0–6 m) at Challenger. The
pink colours represent in situ regolith and blues represent
transported regolith, separated by a red unconformity line.
17
Both the in situ and transported regolith materials have
been affected by silcrete formation. Silcrete occurs on and
below much of the surface as lag, outcrop or subcrop and is
mostly of pedogenic origin. The base of silcrete averages
~2.5 m within the saprolite but increases to >6 m within the
palaeochannel. This feature may reflect primary porosity–
permeability differences, with the sandier palaeochannel
sediments being more amenable to silicification than silt and
clay of the saprolite. Pit exposures revealed a high degree of
complexity of siliceous cementation, brecciation, re-cementation and textural variability. In addition, the first 2–3 m of
the upper regolith is strongly overprinted by calcrete.
Carbonate coatings mask much of the complexity in hand
specimens and pit-face exposures. Porcelanite dominates
the profile below 1 m in palaeochannel sediments (under a
massive ‘grey billy’ silcrete). Second generation and later
silica cementation is mostly as isotropic hyaline forms and
occurs as cutans, layered overgrowths, veinlets and,
commonly, as a clast cement in breccias.
Much of the massive silcrete is coloured grey to greenish
grey to pale yellow-brown, but ferruginous inclusions and
staining can yield strong yellows, reds and browns. The
porcelanite can be white, cream, pale to strong yellow and
brown. Manganese oxides occur as black, threedimensional dendrite inclusions in some of the hyaline
silica. There are no readily observable stratigraphic
separating sequences between the various silicification
events. However, there is macroscopic evidence that a first
generation silcrete has been partly disrupted by surface
processes, mostly mechanical, but also including some
minor fluvial–colluvial activity in its upper portions. A
second generation silcreting event then re-cemented the
silcrete scree talus and colluvium derived from the earlier
forms into complex breccias.
Ferruginous silcrete
sample GC 038
Grey silcrete
sample GC 037
600 ppb Au
180 ppb Au
400
120
200
60
0
0
1
Bulk
2
3
4
Sub-samples
5
The preliminary survey involved the collection of five
bulk samples (several kilograms) of coarse (6 mm) silcrete
lag (all types) from near Zone 1. The bulk samples were split
in half and analysed for gold by atomic absorption
spectrometry. Five random sub-samples of individual lag
specimens were analysed by instrumental neutron activation
analysis. Some lag specimens were broken into two or three
pieces and analysed separately. The results indicated that
gold concentrations are highly variable, with one subsample containing >10 ppm (Fig. 3), suggesting a ‘nugget
effect’ caused by coarse grains and that at least some of the
silcrete lag (containing high gold concentrations) must have
18
1
Bulk
Ferruginous silcrete
sample GC 123LVC
2
3
4
Sub-samples
5
Ferruginous silcrete
sample GC 041
200 ppb Au
400 ppb Au
100
200
0
0
1
Bulk
2
3
4
Sub-samples
5
Bulk
1
2
3
4
Sub-samples
5
Coarse-grained silcrete
sample GC 039
1 000 ppb Au
5470 ppb
GC 037-39,41
GC 123LVC
Zone 1
500
Zone 2
Geochemical results
Only the results for the surficial silcrete lag are presented
here, since this material is easier to collect than the
underlying silcrete from which it is derived. Two
geochemical orientation surveys of lag were undertaken. A
preliminary survey of siliceous lag from close to the main
mineralised zone (Zone 1) showed a number of different
types, including:
• uncommon rounded clasts with sedimentary features
• grey ‘normal’ silcrete
• ferruginous silcrete
• silcrete containing coarse-grained quartz.
The ferruginous silcrete has a red to dark red appearance,
but analyses indicate an iron content of <3%.
12 600 ppb
Sample locations relative
to regolith line
0
Zone 3
Bulk
1
2
3
4
Sub-samples
5
98-1266
Fig. 3 Variation in gold content (ppb) of silcrete (three types, five
samples) from Challenger. Individual silcrete clasts (1–5) were
randomly sub-sampled and analysed separately from the bulk
sample. In some cases, large individual clasts were broken into
two or three pieces, represented by multiple bars for the one
sub-sample number.
Captions to photos (opposite):
(a) Silicified sandy clay containing ferruginous material exhibiting
a replacement texture from in situ regolith unit. Cut slab (section)
and outside view. (Photos 45675, 45673)
(b) Ferruginous silcrete lag. Cut slab (section) and outside view.
(Photos 45679, 45678)
(c) A tray of ferruginous silcrete showing variety of form and
colour. (Photo 45687)
(d) A laminar mega-breccia of calcrete-impregnated silcrete from
transported regolith unit at 1.8 m depth. Yellow label is 50 mm
long. (Photo 45444)
(e) Grey silcrete lag. Cut slab (section) and outside view.
(Photos 45537, 45536)
(f) A tray of grey silcrete lag showing variety of form and colour.
(Photo 45676)
(g) A tray of siliceous lag showing sedimentary structures.
(Photo 45681)
(h) Grey silcrete showing pedogenic drape features. (Photo 45519)
MESA Journal 11
October 1998
(a)
(e)
(b)
(f)
(c)
(g)
(d)
(h)
MESA Journal 11
October 1998
19
been locally derived. Inclusions of mineralised quartz
and/or the silicified, clay-rich saprolite (derived from
mineralised Christie Gneiss) combined with the lag’s
location close to mineralisation suggest it to be a ‘true’
deflation (let-down) lag that has not been transported any
appreciable distance. A bulk sample of the uncommon
rounded lag clasts yielded a gold concentration of 3 ppb.
The follow-up survey involved collecting 30 bulk
samples along the entire regolith line followed by sieving in
the laboratory to retain the coarse (>6 mm) silcrete (all
types). Gold concentrations were found to be highly
anomalous over Zone 1 mineralisation (105 ppb maximum),
close to background (3 ppb) at Zone 2, and background (<1
ppb) at Zone 3 (Fig. 4) over transported overburden. Silcrete
lag was absent near the middle of the regolith line due to the
presence of Pleistocene aeolian sand spreads.
As a comparison, calcrete was collected from the
regolith line and analysed for gold. Concentrations were
much higher and contrasts greater than from silcrete (Fig. 5).
Gold concentrations peaked at 2370, 50 and 13 ppb over
Zones 1, 2 and 3 respectively. Zones 1 and 2 are particularly
100 ppb Au
50
well defined by calcrete sampling while Zone 3 samples
appear to be broadly anomalous in gold without actually
defining the mineralisation.
Conclusions
The results of this orientation survey indicate that
silcrete can be used as an exploration sample medium for
gold. It fulfils the criteria for a good geochemical sample
medium in that it is easy to collect, widespread, easily
identified and can be analysed by standard procedures.
However, even though the Challenger area is liberally
covered by silcrete lag, it is not as effective a sample
medium as calcrete, since contrasts and absolute concentrations are lower and often close to detection limits, particularly over the weak Zone 2 mineralisation. Silcrete
appears to work well at Challenger over Zone 1 because
there the regolith is in situ and not transported. Unlike
calcrete, silcrete does not appear to preferentially concentrate gold but merely retains some or all of the gold already
present in the precursor. The results suggest that where
calcrete is absent, silcrete lag may be used, with caution, as
an alternative sample medium, although the sampling
density needs to be further tested. Examination of samples
from a series of soil pits and drill cuttings from over the
Challenger gold deposit shows a probable genetic
relationship between quartz-rich silcrete lag, saprolite and
quartz veining associated with mineralisation. Although
much of the silcrete lag in this study has a subrounded to
rounded form, the high gold concentrations over
mineralisation clearly indicate that the morphology is more
likely due to weathering and erosion rather than
transportation.
Acknowledgments
0
1500
1000
Zone 2
500
0
Zone 3
Easting (m)
Zone 1
Mineralised zones
98-1267
Fig. 4 Gold concentrations in coarse (6 mm) silcrete lag from the
Challenger regolith line.
260 ppb
100 ppb Au
2370 ppb
145 ppb
This research was funded by the Cooperative Research
Centre for Landscape Evolution and Mineral Exploration
(CRC LEME), PIRSA Mineral Resources Group and the
Gawler Joint Venture. The research was part of a much
larger 12-month project involving regolith characterisation,
multi-element geochemistry and regolith-landform
mapping. David Gray and Charles Butt (CRC LEME),
Stuart Robertson (PIRSA) and Jerome Gillman (Gawler
Joint Venture) are thanked for comments on the manuscript.
CRC LEME is supported by the Australian Cooperative
Research Centres Program.
For further information contact Mel Lintern (ph. 08
8338 0071) or Malcolm Sheard (ph. 08 8338 0073).
References
Bonwick, C.M., 1997. Discovery of the Challenger gold deposit:
implications for future exploration on the Gawler Craton. In:
New Generation Gold Mines ’97. Case histories of discovery.
Conference proceedings, AMF, Adelaide, pp.7-1–7-15.
50
no calcrete
sampled here
0
1500
1000
Zone 1
Zone 2
500
Zone 3
0
Easting (m)
Mineralised zones
98-1268
Fig. 5 Gold concentrations in calcrete from the Challenger
regolith line. Data have been truncated at 100 ppb.
20
Hutton, J.T., Twidale, C.R. and Milnes, A.R., 1978. Characteristics
and origin of some Australian silcretes. In: Langford-Smith, T.
(Ed.), Silcretes in Australia. University of New England,
Department of Geography, pp.19-39.
Lintern, M.J., 1997. Calcrete sampling for gold. MESA Journal,
5:5-8.
Thiry, M. and Milnes, A.R., 1991. Pedogenic and groundwater
silcretes at Stuart Creek Opal Field, South Australia. Journal of
Sedimentary Petrology, 61:111-127. +
MESA Journal 11
October 1998