Gold in iron oxide copper–gold deposits

Gold in iron oxide copper–gold deposits

    Gold in Iron Oxide Copper-Gold Deposits Zhimin Zhu PII: DOI: Reference: S0169-1368(15)00178-X doi: 10.1016/j.oregeorev.2015.07.001 O...

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    Gold in Iron Oxide Copper-Gold Deposits Zhimin Zhu PII: DOI: Reference:

S0169-1368(15)00178-X doi: 10.1016/j.oregeorev.2015.07.001 OREGEO 1552

To appear in:

Ore Geology Reviews

Received date: Revised date: Accepted date:

13 May 2015 2 July 2015 6 July 2015

Please cite this article as: Zhu, Zhimin, Gold in Iron Oxide Copper-Gold Deposits, Ore Geology Reviews (2015), doi: 10.1016/j.oregeorev.2015.07.001

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ACCEPTED MANUSCRIPT Gold in Iron Oxide Copper-Gold Deposits Zhimin Zhu

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Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Chengdu 610041, China

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Abstract Iron oxide copper-gold (IOCG) deposits contain economic or anomalous gold. Little is known

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about the gold distribution in IOCG deposits and its controlling factors. The gold grades and tonnages of selective deposits in some well-defined IOCG provinces are compiled in this paper. The gold grades in these IOCG deposits range from 0.01 to 1.41 g/t (averaging 0.41g/t), and ~90% of them are less than 1g/t. Their

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gold tonnages range from 0.6 to 483 t (averaging 64 t), but with the exception of the giant Olympic Dam containing gold of 2968 t. The Cu/Au ratios of IOCG deposits are significantly variable, ranging from 0.7 to The

gold

is

present

as

three

forms:

(1)

native

gold,

(2)

electrum,

and

(3)

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64.

gold-bismuth-antimony-tellurium alloy. The gold distribution is controlled by gold contents in host rocks and the efficiency of precipitation mechanism (e.g., cooling, fluid-rock interaction and fluid mixing).

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Keywords Gold distribution; Iron oxide copper-gold deposits; Host rocks

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Introduction

Since the discovery of the giant Olympic Dam deposit in Australia in 1975, Iron Oxide Copper-Gold

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(IOCG) deposits have become the largest producer of uranium (Hitzman and Valenta, 2005; Skirrow, 2011), the third producer of copper (Sillitoe, 2012) and significant producer of gold (Kerrich et al., 2000; Frimmel, 2008; Porter, 2010). These deposits may also contain high contents of light rare earth elements (LREEs),

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silver, molybdenum, nickel, cobalt, barium, fluorine and phosphorus (Williams et al., 2005; Barton, 2014). Consequently, IOCG deposits are considered as profitable exploration targets during the past decades. IOCG deposits are a relatively newly defined clan of ore deposits (Meyer, 1988; Hitzman et al., 1992; Barton and Johnson, 1996; Williams et al., 2005; Porter, 2010; Chen, 2013). They are a class of Cu ± Au deposits containing abundant low-Ti iron oxide (magnetite and/or hematite) and extensive hydrothermal alkali (Na/Ca/K) alteration. Such deposits also have strongly structural controls, and temporal but not a close spatial association with igneous rocks. They formed in rift, subduction zone and basin collapse (Hitzman, 2000) from Late Archean to Pliocene (Groves et al., 2010). According to above definition, the carbonatite-related deposits (e.g., Palabora in South Africa and Bayan Obo in China, Groves and Vielreicher, 2001; Smith and Wu, 2000), Kiruna-type or Chilean coastal belt Fe



Corresponding author: [email protected]

ACCEPTED MANUSCRIPT deposits (Billström et al., 2010; Chen, 2013), skarn deposits (eg., the Lower Yangtze Fe±Cu deposits in China, Angara-Ilim Fe deposits in Siberia and Turgai Fe deposits in Kazakhstan, Meinert et al., 2005; Groves et al., 2010; Yang et al., 2011), hybrid porphyry-IOCG Aitik Cu-Au deposits in Sweden

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(Wanhainen and Martinsson, 2010) and Cu-Au deposits in Tennant Creek (Groves et al., 2010) are not IOCG deposits sensu stricto. Therefore, only eleven IOCG provinces sensu stricto (Fig.1) have been

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recognized including: (1) Gawler Craton, South Australia; (2) Carajás mineral province, Brazil; (3) Mount Isa Inlier, Northern Australia; (4) Central Andean Coastal Belt, Southern Peru and Northern Chile; (5) Kangdian Copper Belt, Southwest China and Northern Vietnam; (6) Khetri Copper Belt, India; (7) Lufilian

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Arc, Zambia; (8) Great Bear Magmatic Zone, Canada; (9) Southeast Missouri Iron Province, America; (10) Fennoscandian Shield, North Finland; (11) West African Craton, Mauritania. In above provinces, most

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deposits have economic or anomalous gold (Hitzman et al., 1992; Foster et al., 2007). However, we know little about the gold distribution in IOCG deposits and its potential controlling factors, partly owing to a

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lack of detailed compilation and studies on well-established IOCG deposits.

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In this contribution, the gold grades and tonnages of selective deposits in above eleven IOCG provinces are summarized. The main objective of this contribution is to discuss the gold distribution in IOCG

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deposits and its possible controls, as well as implication for exploration and ore processing.

Gold in IOCG deposits

The data used in this contribution are mainly from a global compilation of well-established IOCG

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deposits in Williams et al. (2005), Cox and Singer (2007), Porter (2010) and other updated references shown in table 1. Deposits without available gold grades (e.g., Gameleira in Carajás mineral province, Boss-Bixby in Southeast Missouri Iron Province, Vi Kem in North Vietnam and some deposits in Khetri Copper belt) are excluded. Therefore, only thirty-six deposits from ten IOCG provinces worldwide are complied in the dataset. The gold grades of these IOCG deposits are between 0.01 g/t and 1.41 g/t (averaging 0.41g/t), but cluster at low values with ~90% of the data below 1g/t (Fig. 2-A). There is a positive correlation between Cu content and its gold grade for a specific deposit (Fig. 2-B). The deposits have a range of gold tonnage from 0.6 to 483 t (averaging 64 t), but with the exception of the Olympic Dam containing gold of 2968 t (Fig. 2-C). The Cu/Au ratios of IOCG deposits are significantly variable. They range from 0.7 to 64, but most values fall between 0.7 and 6.0 and only six of them fall between 6.0 and 64 (Fig. 2-D). The gold in IOCG deposits is present as three forms: (1) native gold, (2) electrum, and (3)

ACCEPTED MANUSCRIPT gold-bismuth-antimony-tellurium alloy. Native gold and electrum commonly occurred as intergranular particles and tiny inclusions in sulfide, hematite and gangue minerals (quartz, calcite and siderite) in many deposits, e.g., Ernest Henry, Igarape Bahia, Boss-Bixby, Starra/Selwyn, Sin Quyen, Guelb Moghrein

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(Foster et al., 2007; McLean , 2001; Seeger, 2000; Sleigh, 2002; Strickland and Martyn, 2002; Tazava and de Oliveira, 2000). Native gold and electrum also occur as fine particles in fractures in sulfide and quartz

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(e.g., Ernest Henry, Lala, Starra/Selwyn, Foster et al., 2007; Rotherham, 1997; Sun et al., 1994). However, Gold-bismuth-antimony-tellurium alloy is only reported in NICO deposit in Great Bear Magmatic Zone, Canada (Goad et al., 2000).

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Discussion and Conclusions

Several important factors govern the grade and tonnage of gold and copper in hydrothermal ore deposit

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are: (1) the availability of source of gold; (2) the solubility of gold in solution; (3) the flux of hydrothermal fluid through the rock volume; and (4) the efficiency of the precipitation mechanism (Skinner, 1997; Wood,

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2002). Lines of evidence indicate that IOCG deposits formed from variable but generally high temperature,

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high salinity, oxidized and S-poor hydrothermal fluids (Hitzman et al., 1992; Barton and Johnson, 1996; Kerrich et al., 2000; Barton, 2014). Under these conditions, gold and copper would be transported

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predominantly as chloride complexes (Williams-Jones et al., 2009; Pokrovski et al., 2013; Pokrovski et al., 2014). Thus, for a given IOCG deposit where the flux of fluid through the rock volume is constant, the main controls on the gold distribution are the source, solubility and depositional mechanism of gold.

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The source of gold

The average gold abundance of the upper continental crust is 1.3 ppb (Rudnick and Gao, 2013). Even as co-products or by-products from the production of copper or other metals, the ores must contain amounts of gold no less than 0.1 ppm, 100-fold enrichment over upper continental crust. Therefore, to form Au-rich IOCG deposits, the source rocks must be pre-enriched in gold. There are two possible sources for gold in hydrothermal deposits: (1) volcano-sedimentary sequences including basalts, carbonaceous sedimentary rocks (Large et al., 2011; Gaboury, 2013; Pitcairn et al., 2015); and (2) felsic-intermediate magmas, which release magmatic-hydrothermal fluids during magma crystallization (Pollard, 2006; Groves et al., 2010; Richards and Mumin, 2013a, 2013b). However, the IOCG deposits have temporal but not close spatial relationships with magmatic rocks. Moreover, lines of evidences have shown that IOCG deposits formed from one or more of fluids (eg. Barton, 2014; Williams et al., 2010; Porter, 2010), including magmatic-hydrothermal fluids, basinal or surficial brines and metamorphic-hydrothermal fluids. As a

ACCEPTED MANUSCRIPT consequence, leaching gold from host rocks by the above fluids is a possible mechanism. Such a model is evident by the similarity of element association between the ore and host rock in the Carajás mineral province (Xavier et al., 2012), the high initial Os isotope ratios of ores indicating crustal origin of ore

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metals in the Lala deposit and some deposits of the Central Andean Coastal Belt (Ruiz et al., 1997; Mathur et al., 2002; Zhu and Sun, 2013), and high gold concentration in host rocks in the Mantoverde district of

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Chile, Candelaria district of Chile and the Lala district of China (Maschik et al., 2000; Rieger et al., 2010; Zhu, 2011).

The gold grades of IOCG deposits vary significantly, but at a province scale, the ranges of gold grades

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for most deposits are narrow. For example, at Gawler Craton, the gold grades range from 0.11 to 0.81 g/t, At Carajás mineral province, the gold grades range from 0.3 to 0.86 g/t. At Mount Isa Inlier, the gold

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grades range from 0.26 to 1.4 g/t. At Kangdian Copper Belt and Lao Cai district, the gold grades range from 0.18 to 0.44 g/t. At Central Andean Coastal Belt (except Mina Justa), the gold grades range from 0.11

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to 0.93 g/t. At Lufilian Arc, the gold grades range from 0.01 to 0.05 g/t. The narrow range in gold grades

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deposits at the province scale suggests that source rock might control the gold contents in IOCG deposits. The precipitation mechanism of gold

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In the IOCG deposits, AuCl2- and CuCl2- is likely the main complex that transports the gold and copper in high temperature hydrothermal fluids (Williams-Jones et al., 2009; Pokrovski et al., 2013; Pokrovski et

al., 2009):

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al., 2014). So the solubility of metal (gold or copper) will be governed by the reaction (Williams-Jones et

MeCl 2  1 / 2H 2O ⇌ Me  2Cl   H   1 / 4O2 (where Me is gold or copper) Under this condition, the deposition of gold or copper will be promoted by a decrease in the activity of Cl- and fO2 and an increase of pH. As a consequence, cooling, decompression, phase separation, fluid-rock interaction and fluid mixing are all possible mechanisms leading to gold and copper deposition. In the Andean Coastal Belt, the deposits were formed by cooling, fluid-rock interaction and fluid mixing (Chen et al., 2010). In the Mount Isa Inlier (Rotherham, 1997; Williams and Skirrow, 2000; Williams et al., 2001), the Fennoscandian Shield (Billström et al., 2010) and the Khetri Copper Belt (Knight et al., 2002), the ore-forming fluids interaction with ironstone or carbonaceous rocks lead to mineralization. In the Gawler craton (Haynes et al., 1995), the Raul-Condestable (de Haller and Fontboté, 2009), and the Carajás province (Xavier et al., 2012), the deposits were formed as a result of mixing of at least two fluids. In

ACCEPTED MANUSCRIPT conclusion, cooling, fluid-rock interaction and fluid mixing are three main controls on gold and copper precipitation in IOCG deposits. The efficiency of copper and gold deposition controls the Cu/Au ratios of IOCG deposits (Rotherham,

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1997; Davidson, 2002). Grainger et al. (2002) noticed that in Carajás mineral province the high-temperature deposits have lower gold grades and higher Cu/Au ratios (e.g., Salobo with gold grade of

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0.49 g/t and Cu/Au ratio of 1.68), whereas the low-temperature deposits have the higher gold grades and lower Cu/Au ratios (e.g., Igarape Bahia-Alemáo with gold grade of 0.86 g/t and Cu/Au ratio of 1.63). In Mount Isa Inlier, the low-temperature deposits also have higher gold grades and lower Cu/Au ratios (e.g.,

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Osborne with gold grade of 0.80 g/t and Cu/Au ratio of 1.75) than high-temperature deposits (e.g., Ernest Herry deposit with gold grade of 0.51 g/t and Cu/Au ratio of 2.16). Skirrow (2010) found that the shallow

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low-temperature hematite-group IOCG deposits have lower Cu/Au ratio than the deep high-temperature magnetite-group IOCG deposits. The gold and copper solubility as chloride complexes declines with

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decreasing temperature, but the CuCl2- focused to a narrow temperature window whereas the AuCl2- not

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(CuCl2- and AuCl2- stabilize as low as 300–250°C. Below these temperatures, most Cu precipitates as sulfide but AuCl2- stabilize over a wide temperature range; Pokrovski et al., 2014). Therefore,

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low-temperature deposits likely have higher gold grade but lower Cu/Au ratio than high-temperature deposits. In addition, Cu/Au ratio can be significantly affected by supergene processes (Williams et al., 2005) and this mechanism may expound the high Cu/Au ratios (18.8-64) of IOCG deposits in Lufilian Arc

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(Lobo-Guerrero, 2010; Wilhelm, 2008). Implications for Ore Processing and Exploration Gold is considered as a by-product from the production of copper and in some cases as primary commodity (e.g., NICO deposit). However, most of native gold grains are too small to be liberated and will be lost during crushing and grinding processes into the tailing (Foster et al., 2007). To utilize those fine particle gold, specific ore processing technology must be designed. In this paper, pre-enriched host rocks and efficient depositional mechanism are suggested to control gold grade and tonnage in IOCG deposits. As a consequence, in order to find gold-rich IOCG deposits, a province scale the most important things are to focus on areas both with gold-rich host rocks and efficient depositional mechanism (e.g., cooling, fluid-rock interaction and fluid mixing) operating. Moreover, since the lower-temperature deposits generally have higher gold grades than those of high-temperature deposits, at a deposit scale the exploration of gold-rich deposits should also focus on low-temperature deposits.

ACCEPTED MANUSCRIPT Acknowledgements This study was supported by National Natural Science Foundation of China (41102044) and Sichuan Youth Science&Technology Foundation of China (2012JQ0026). I would like to thank Dr. Zhang

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Rongqing and Dr. Zhu Kongyang for paper preparation.

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Freitas, e Silva, F.H., 2012. The Iron Oxide Copper-Gold Systems of the Carajás Mineral Province, Brazil. Society of Economic Geologists Special Publication 16, 433-454.

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metallogenic province. Mineralium Deposita 46, 731-747. Zhu, Z.M., 2011. Lala iron oxide copper gold deposit: metallogenic epoch and metal sources. (Unpublished thesis) Chengdu University of Technology.

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Zhu, Z.M., Sun, Y.L., 2013. Direct Re-Os dating of chalcopyrite from the Lala IOCG deposit in the Kangdian Copper Belt, China. Economic Geology 108, 871-882.

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Fig. 1 Geographical distribution of IOCG provinces with selected deposits worldwide (modified after Porter, 2010)

Fig. 2. A. Frequency histogram of gold grades in IOCG deposits, B. Cu contents versus Au grades for IOCG deposits, C. Frequency histogram of gold grades in IOCG deposits, D. Frequency histogram of Cu/Au ratios for IOCG deposits.

ACCEPTED MANUSCRIPT Table 1 Tonnage and grades for selected IOCG deposits worldwide Deposit

Tonnage (Mt)

Cu (%)

Au (g/t)

Host Rock

Age

Sources of data

1 Gawler Craton, South Australia Olympic Dam

9576

0.82

0.31

Granite breccia

Carrapateena

203

1.31

0.56

Brecciated granitoids

Prominent Hill

283

0.89

0.81

Volcanic rocks and sedimentary rocks

Hillside

170

0.70

0.20

Moonta/Wallaroo

10.1

3.70

0.42

Cairn Hill

11.4

0.37

0.11

Volcanic rocks and sedimentary rocks

Salobo

986

0.82

0.49

Metagraywacke and amphibolite

Igarape Bahia/Alemáo

219

1.40

0.86

Siderite-chlorite breccia

Cristalino

500

1.00

0.30

Mafic to felsic volcanic rocks

Sossego/Sequeirinho

245

1.10

0.28

Brecciated metavolcanic rocks

Alvo 118

170

1.00

0.30

Metavolcanic rocks, granite and gabbro

226

1.10

0.51

Intermediate to felsic volcanic rocks

Osborne

27

1.40

0.80

Eloise

3.1

5.50

1.40

Mount Elliott

570

0.44

0.26

Starra/Selwyn

253

0.34

0.48

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Hayward and Skirrow,

Mesoproterozoic

Meta-sediments intrude by igneous rocks

2010; Ehrig et al., 2012.

Porter, 2010; Neoarchean

Xavier et al., 2012. .

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2 Carajás mineral province, Brazil

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3 Mount Isa Inlier, Northern Australia Ernest Herry

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Porter, 2010;

Siliciclastic rocks and iron formation Meta-arkose and amphibolite

Porter, 2010; Mesoproterozoic Duncan et al., 2014.

Amphibolite and trachyandesite

Biotite schist, amphibolite and iron stone

Mina Justa, Peru

347

0.71

Raúl-Condestable, Peru

>32

Monterrossas, Peru

1.9

La Candelaria, Chile

470

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4 Central Andean Coastal Belt, Southern Peru and Northern Chile

Cerro Negro, Chile Punta del Cobre, Chile El Espino, Chile

Andesite and volcanic sediments

0.30

Andesite lava, tuff

2.00

0.93

Gabbro-diorite

0.95

0.22

Andesite breccia

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Manto Verde, Chile

0.03

1.70

Sidder, 1987; Porter, 2010; Mesozoic

410

0.58

0.11

Andesite lava

Chen, 2010;

249

0.40

0.15

Andesite lava, tuff

Lopez et al., 2014

>120

1.50

0.2-0.6

Andesite-basalt lava

123

0.66

0.24

Volcanic-sedimentary rocks

Dahongshan, China

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5 Kangdian Copper Belt and Lao Cai district, Southwest China and North Vietnam 192

0.90

0.18

Marble and meta-volcanic rocks

106

0.87

0.19

Albitite, mica-schist and quartzite

52.8

0.91

0.44

Gneiss and schist

140

1.1-1.7

0.50

Quartzite and schist

Chimiwungo

761

0.64

0.01

Malundwe

162

0.89

0.03

Mumbwa-Kitumba

87

0.94

0.05

0.80

0.07

Lala, China

Sin Quyen, Vietnam

McLean, 2002; Mesoproterozoic

Zhao and Zhou, 2010; Zhu and Sun, 2013.

6 Khetri Copper Belt, India Khetri

Neoproterozoic

Knight et al., 2002.

7 Lufilian Arc, Zambia Mafic-felsic plutonic and

Porter, 2010; Neoproterozoic

volcanic rocks

Wilhelm, 2008.

8 Great Bear Magmatic Zone, Canada Sue-Dianne

8.4

Rhyodacite ignimbrite

Paleoproterozoic

Corriveau et al., 2010.

9 Fennoscandian Shield, North Finland Hannukainen

170.7

0.20

0.10

Laurinoja

4.56

0.88

1.00

Mafic-felsic

Billström et al., 2010; Paleoproterozoic

volcanic sequence and ironstone Rautuvaara

2.8

0.48

0.20

1.12

1.41

Niiranen et al., 2007.

10 West African Craton, Mauritania Guelb Moghrein

33.4

Meta-carbonate rocks and metabasalt

Neoarchean

Kolb et al., 2010.

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Figure and table captions Fig. 1 Geographical distribution of IOCG provinces with selected deposits worldwide (modified after Porter, 2010) Fig. 2. A. Frequency histogram of gold grades in IOCG deposits, B. Cu contents versus Au grades for IOCG deposits, C. Frequency histogram of gold grades in IOCG deposits, D. Frequency histogram of Cu/Au ratios for IOCG deposits. Table 1 Tonnage and grades for selected IOCG deposits worldwide.

ACCEPTED MANUSCRIPT Conflict of interest

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No conflicts of interest to declare.

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Highlights 1. Iron oxide copper-gold (IOCG) deposits contain economic or anomalous gold. 2. The gold is present as native gold, electrum, and gold-bismuth-antimony-tellurium alloy. 3. The gold distribution is controlled by gold contents in host rocks and the efficiency of precipitation mechanism