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Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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
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
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,.
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).
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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
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
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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
(11002500 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
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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
Increasing degree of
sericite alteration
Regionally-altered samples
Relatively unaltered Gawler
Range Volcanics
Relatively unaltered
Donington Suite
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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.
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
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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.
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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
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
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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)
Footprints of IOCG deposits
Authors listed here (Arial 11 pt) April 2014
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.
BELPERIO, A., FLINT, R. & FREEMAN, H. 2007. Prominent Hill: A Hematite-
Dominated, Iron Oxide Copper-Gold System. Economic Geology 102: 14991510
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
copper-gold (IOCG) deposits through Earth history: implications for origin,
lithospheric setting, and distinction from other epigenetic iron oxide deposits:
Economic Geology, v. 105, p. 641-654.
PORTER, T.M., 2010. The Carrapateena Iron Oxide Copper Gold Deposit, Gawler
Craton, South Australia: a Review; in PORTER. T.M., (ed.), 2010 Hydrothermal Iron
Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing,
Adelaide v. 3, pp. 191-200.
RUDNICK, R.L. & GAO, S., 2003. Composition of the Continental Crust. In:
RUDNICK, R.L., HOLLAND, H.D. & TUREKIAN, K.K. (eds). Treatise on
Geochemistry, Vol 3. Elsevier, p.1-64.
We study the relations between several selected elements present in the Sin Quyen IOCG deposit, Lào Cai, North Vietnam, and interpret the obtained correlations, especially with a coefficient higher than 0.7. The correlations with high coefficients are mainly observed for the elements belonging to the chalcophile group (Cu, Ag, Au, Te, and Bi) and for the relation between uranium and Ag, Au, Cu, Pb, and Bi. Although the S-, Fe-, and REE-bearing minerals are predominant in the studied deposit, no strong correlation between them and the other elements was observed, even with Cu. The phenomena are primarily explained based on the geochemical properties of the mentioned elements and the characteristics of IOCG deposits.
Full-text available
The discovery of iron oxide copper-gold (IOCG) mineralization at Prominent Hill in the Mount Woods inlier (South Australia) by Minotaur Resources Ltd. in November 2001 was based on an empirical exploration model that focused specifically on the hematite end member of the IOCG ore deposit model type. Copper and gold-bearing hematite-rich breccias formed by repetitive hydrothermal brecciation, milling, and explosive venting within a volcanic setting. High-pressure spalling, chemical corrosion of breccia clasts, and compositional layering within the breccias are distinctive features. Iron oxide-sericite-silica alteration is pervasive within and marginal to the main breccias and is surrounded by a wider zone of less intense alteration. Iron, copper, gold, barium, fluorine, uranium, cerium, and lanthanum are strongly correlated and associated with a relatively low temperature hydrothermal system. Mineralization was synchronous with volcanism and sedimentation within a narrow east-west–trending graben that developed at ~1585 Ma.
This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.
The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, lias progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxideassociated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, highgrade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supereontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental cmst and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle cmstal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supereontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate worldclass IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated.