GEOCHEMICAL FOOTPRINTS OF IOCG DEPOSITS BENEATH THICK COVER: INSIGHTS FROM THE OLYMPIC CU-AU PROVINCE, SOUTH AUSTRALIA

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Footprints of IOCG deposits
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GEOCHEMICAL FOOTPRINTS OF IOCG DEPOSITS
BENEATH THICK COVER: INSIGHTS FROM THE OLYMPIC
CU-AU PROVINCE, SOUTH AUSTRALIA
Adrian Fabris12, Simon van der Wielen23, Tim Keeping12, Georgina Gordon12
1 Geological Survey of South Australia - Department of State Development, 4/101
Grenfell St Adelaide, South Australia, AUS
2 Deep Exploration Technology Cooperative Research Centre, 26 Butler Boulevard
Burbridge Business Park, Adelaide Airport, 5950, South Australia, AUS
3 University of Adelaide, Adelaide, South Australia, AUS
Introduction
South Australia hosts one of the world’s great iron oxide copper gold (IOCG)
terranes. Termed the Olympic Cu-Au Province (Skirrow et al. 2007), this belt is
renowned as the host to Olympic Dam, the type example of breccia-hosted,
hematite-rich IOCG deposits (Groves et al. 2003). Other significant hematite-
dominated deposits include Prominent Hill and Carrapateena (Belperio et al. 2007;
Porter 2010). Post-mineralisation cover, including Proterozoic to Recent rocks and
sediments commonly exceed 400m and poses a significant risk and cost to mineral
exploration. Exploration typically consists of single drill holes into geophysical
targets.
The South Australian Geological Survey in collaboration with the Deep
Technology Cooperative Research Centre (DET CRC) have undertaken a program
of reassaying historic drill cores in order to identify characteristic geochemical
‘footprints’ of IOCG deposits under cover. The specific aim is to identify distal
geochemical footprints of IOCG deposits, both within and above basement,.
Methodology
Most data were obtained on samples from drill holes held in storage by the
Department of State Development, with the remaining samples from company drill
holes. One-metre basement intersections were sampled every 10 m, with flexibility to
vary the sample spacing to gain the most representative samples. Where feasible, a
sample was also taken above and below the sediment cover-basement
unconformity, specifically targeting any basal conglomerate units containing bedrock
fragments. Over 100 drill holes totalling >2500 samples were analysed (Figure 1).

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Figure 1. Location of drill holes sampled within the Olympic Cu-Au Province
(red outline), Gawler Craton, South Australia.
Whole rock geochemistry
Chemical analyses on drill core samples were done by Intertek-Genalysis in
Adelaide. Each sample was crushed, pulverised and analysed using the following
methods;
• Lead collection fire assay on 25g samples (ICP-MS) – Au, Pt, Pd
• 4 acid (ICP-OES) – Cu, Li, Ni, Pb, S, Zn
• 4 acid (ICP-MS) – Ag, As, Bi, Cd, Co, Cs, Ge, In, Mo, Nb, Re, Sb, Se, Te, Tl
• Carbonate fusion/SIE – F
• Lithium borate fusion (ICP-OES) – Al2O3, CaO, Cr, Fe2O3, K2O, MgO, MnO,
Na2O, P, SiO2, TiO2, V
• Lithium borate fusion (ICP-MS) – Ba, Be, Ce, Dy, Er, Eu, Ga, Gd, Hf, Ho, La,
Lu, Nd, Pr, Rb, Sc, Sm, Sn, Sr, Ta, Tb, Th, Tm, U, W, Y, Yb, Zr

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Spectral mineralogy
Spectral scans of drill core were made using the semi-automated
hyperspectral logging tool, HyLoggerTM. Reflectance spectra were measured for all
drill holes in visible-near infrared (400-1100 nm) through to short wave infrared
(1100–2500 nm) wavelengths. Spectral data were used to interpret mineralogy using
The Spectral Geologist™ (TSG) software package.
Petrophysical measurement
Drill hole sampling was accompanied by measurements of magnetic
susceptibility and specific gravity. Magnetic susceptibility measurements (Terraplus
KT9) were obtained every two metres and specific gravity measurements
approximately every three metres. Magnetic susceptibility values of more than 0.05
SI units were used to indicate the presence of magnetic minerals.
Classification of alteration
Rock alteration in each sample was classified using a combination of spectral
(HyLoggerTM), petrophysical and geochemical data (e.g., Figure 2; Fabris et al.
2013). Once classified, relationships between each alteration assemblage and the
key economic elements were evaluated. Alteration assemblages related to Cu-Au
mineralisation were established by identifying assemblages with a strong relationship
to Cu and Au values of many times the average crustal abundance (Figure 3).
Additional elements associations with these alteration assemblages were then
determined.

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Figure 2. Plot of K/Al versus Na/Al using molar ratios. Sample points have
been classified by a combination of spectral, petrophysical and geochemical
data. Samples with minimal alteration have similar sodic to potasic feldspar
content and therefore plot in the centre of the diagram (labelled regionally-
altered samples). These can subsequently be classified as background
samples. Alteration in most samples are characterised by Na depletion. The
variation in K/Al commonly relate to K feldspar alteration (high values), sericite
alteration (moderate values) and chlorite and/or Fe oxide alteration (low
values).
Increasing degree of
sericite alteration
Regionally-altered samples
Relatively unaltered Gawler
Range Volcanics
Relatively unaltered
Donington Suite

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Figure 3. Probability plots for Cu, Au, U, Ag, Ce and La with
respect to alteration mineral assemblages. Significant trends
are those that diverge from the x-axis and other mineral
assemblages. The dashed line on each graph signifies 10 times
the crustal abundance for that element.
Results
Associations between elements and alteration types
Element associations with each of the alteration assemblages were examined
using probability plots. In probability plots, values are plotted against the N score for
each sample where N = (X-mean)/standard deviation. Elements typically associated
with IOCG mineralisation (viz. Cu, Au, U, Ag, Ce, La) were enriched in the sericite-
Fe oxide and chlorite-Fe oxide alteration assemblages (Figure 3). Relative to
average crustal abundance (Rudnick and Gao 2003), many of the samples from the
central Olympic IOCG Province are strongly anomalous in these elements.
Samples in this dataset showed strongly anomalous values for pathfinder
elements (Figure 4). High W and Mo values were most commonly associated with
the Fe-oxide dominated mineral assemblages (Fe-oxide, sericite-Fe oxide and
chlorite-Fe oxide) and often with samples containing magnetite. Highly anomalous
As, Co and Te values were most commonly associated with the chlorite-Fe oxide
alteration assemblage. Values above 10 times crustal abundance for Sb, Se and Bi
were typical in all alteration assemblages and were therefore not as biased towards

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mineral assemblages containing Fe oxide as with many other pathfinder elements in
the dataset. Other elements with high values across all alteration assemblages
include Ba, Cu, Ag ± W, S (Figure 4).
In addition to establishing a relationship between alteration styles and trace
element anomalism, correlation coefficients and element concentrations that were
many times the average crustal abundance (Rudnick & Gao 2003) were used to
define the following list of elements with a relationship to IOCG mineralisation:
Au, Ag, As, Ba, Bi, Cd, Co, Cs, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Re, S, Sb,
Se, Sn Te, Ti, Tl, Tm U, W, Zn, LREE (Ce, Eu, Gd, La, Pr, Nd, Sc, Sm), HREE (Dy,
Er, Ho, Lu, Tb, Y, Yb)
Figure 4. Probability plots for the pathfinder elements As, Bi, Co, Mo, Sb, Se,
Te and W. The dashed line on each graph signifies 10 times the crustal
abundance for that element. Note: some extreme values plot off the graphs.

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Regional geochemical trends
The spatial distribution of trace elements associated with IOCG mineral
systems within the eastern Gawler Craton were mapped in 3D (Figure 5). Around
hematite-dominated IOCG systems, key associated elements were subdivided
based on their distance from significant Cu mineralisation
• Local – Ce, La, Te ± Co±Cd±Mn
• Moderate – Au, Ba, Mo, S
• Broad – Bi, Ag, As, Cu, Fe, Sb, Se, W.
Figure 5. 3D perspective view of Cu and Te values from down hole samples,
central eastern Gawler Craton, South Australia. Significant mineral
occurrences are labelled. High Te values are only associated with ore grade
intersections.
IOCG prospectivity Index
The recognition of size variation of geochemical halos for certain groups of
trace elements that relate to Cu mineralisation in IOCG systems makes it possible to
derive an index that measures how many key elements have values above a certain
threshold (Fabris et al. 2013). This IOCG prospectivity index, now incorporated into
IoGas v5.2, provides a method to vector using multi-element geochemical data.
Basal unconformity sampling

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High values in similar elements to that evident from basement mineralisation
were also found in gravel samples from just above the sedimentary cover-basement
unconformity. Although geochemical trends were not as consistent as those from
basement samples, the use of element combinations identified samples that were
proximal to known mineralisation (Figure 6).
Figure 6. Ce values from base of cover sampling in the central eastern Gawler
Craton, South Australia on a solid geology base map. High Ce values in
combination with high Cu and La values can be used to identify proximal
mineralisation within basement.
Discussion & Conclusions
Over 80% of South Australia is covered by transported regolith and these
materials commonly hinder the detection of ore deposits at the surface. In addition to
regolith materials, most of the highly prospective Olympic Cu-Au Province of the
eastern Gawler Craton also contains several hundreds of metres of sedimentary rock
High Cu-Ce-La
(Ag, As, Bi, U, W)
?

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which has resulted in many under explored regions, in spite of the fact that it hosts
one of the world’s richest orebodies, the Olympic Dam deposit. Exploration in the
region has historically relied on geophysical methods. Thorough analysis of
prospective basement and cover rocks by the Geological Survey of South Australia
has shown that there is an important role for geochemistry in the exploration
workflow and that there are coherent and very broad trace element patterns around
IOCG deposits, and these can be used to recognise ‘halos’ within mineral systems.
Samples collected from the base of the sedimentary cover-basement unconformity
were found to typically reflect underlying or proximal mineralisation and provide an
important sample media in areas of thick cover.
The concepts derived from this project are being tested within a ~$2M, State
government funded drilling program in partnership with mineral explorers and DET
CRC that is planned for mid-2015.
References
BELPERIO, A., FLINT, R. & FREEMAN, H. 2007. Prominent Hill: A Hematite-
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FABRIS, A.J., HALLEY, S., VAN DER WIELEN, S., KEEPING, T. & GORDON, G.
2013. IOCG-style mineralisation in the central eastern Gawler Craton, SA;
characterisation of alteration, geochemical associations and exploration vectors,
Report Book 2013/00014. Department of Innovation, Manufacturing, Trade,
Resources and Energy, South Australia, Adelaide.
GROVES, D.I., BIERLEIN, F.P., MEINERT, L.D., & HITZMAN, M.W. 2010. Iron oxide
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