Magnetite composition in Ni-Cu-PGE deposits worldwide: application to mineral exploration

    Magnetite composition in Ni-Cu-PGE deposits worldwide and its application to mineral exploration Emilie Boutroy, Sarah A.S. Dare, Georges Beaudoin, Sarah-Jane Barnes, Peter C. Lightfoot PII: DOI: Reference:

S0375-6742(14)00176-9 doi: 10.1016/j.gexplo.2014.05.010 GEXPLO 5386

To appear in:

Journal of Geochemical Exploration

Received date: Revised date: Accepted date:

7 October 2013 16 April 2014 13 May 2014

Please cite this article as: Boutroy, Emilie, Dare, Sarah A.S., Beaudoin, Georges, Barnes, Sarah-Jane, Lightfoot, Peter C., Magnetite composition in Ni-Cu-PGE deposits worldwide and its application to mineral exploration, Journal of Geochemical Exploration (2014), doi: 10.1016/j.gexplo.2014.05.010

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Title : Magnetite composition in Ni-Cu-PGE deposits worldwide and its application to mineral exploration

Emilie Boutroy1, Sarah A. S. Dare 2, Georges Beaudoin1*, Sarah-Jane Barnes2 and Peter C. Lightfoot3 1

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Département de géologie et de génie géologique, Université Laval, Québec (Qc) G1V 0A6. Canada ([email protected], [email protected]) 2 Sciences de la Terre, Université du Québec à Chicoutimi, Saguenay (Qc) G7H 2B1. Canada ([email protected]; [email protected] ) 3 Vale, Exploration, Highway 17 West, Sudbury, ON, P0M 1N0. Canada ([email protected] )

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Keywords : Magnetite; Ni; Cu; sulfides; Mineral chemistry; Discriminant diagram.

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ACCEPTED MANUSCRIPT Abstract The concentration of trace elements in magnetite is determined primarily by the geological

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environment at the time of its formation, which makes magnetite a useful indicator mineral in the

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exploration of ore deposits. Magnetite is a common accessory mineral in magmatic Ni-Cu-Platinum-

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Group Element (PGE) sulfide deposits forming primary grains in massive sulfide ore and/or secondary grains during alteration of sulfide ore and host rock. We report magnetite composition from massive and

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disseminated sulfide samples (n = 94), representative of 13 major Ni-Cu-PGE deposits, worldwide, and from a range of geological environments and formation ages (Archean to Permo-Triassic). The samples

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are divided into 6 different types according to the composition of the parental host magma: (1) komatiite, (2) ferropicrite, (3) picrite-tholeiite, (4) anorthosite-troctolite, (5) flood basalt and (6) impact melt. The

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minor and trace element composition of magnetite was measured by electron probe micro-analysis and a

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subset of samples (n = 61) by laser ablation-inductively coupled plasma-mass spectrometry.

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The composition of all primary magnetite, crystallized from an immiscible sulfide liquid, plots on a trend in Ti vs. Cr and V vs. Cr. However, primary magnetite trace element content is controlled by element partitioning during sulfide liquid differentiation, from early-forming Fe-rich monosulfide solid solution (MSS) to Cu-rich intermediate solid solution (ISS). The concentration of most lithophile elements (Cr, Ti, V, Al, Sc, Nb, Ga, Ta, Hf and Zr) is highest in the early-forming magnetite, which crystallized from Fe-rich MSS. The fractionated Cu-rich liquid was depleted in lithophile elements so that lateforming magnetite, which crystallized from the Cu-rich ISS, has a low concentration of these elements. Compatible chalcophile elements partitioning into magnetite depends largely on the co-crystallizing sulfide mineral. Elements such as Ni partition preferentially in Fe-rich sulfide minerals (pyrrhotite or pentlandite) such that coprecipitated magnetite is depleted in chalcophile elements compatible with the sulfides. In contrast, magnetite co-crystallized with Cu-rich ISS is enriched in Ni because Ni is not compatible with the co-crystallizing Cu sulfides. Magnetite composition is also dependent on parental magma composition. Magnetite in massive sulfides from ultramafic parental magmas is most similar in

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ACCEPTED MANUSCRIPT compositions to primitive magnetite from Fe-rich sulfides in fractionated intermediate to mafic hosted orebodies.

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Two types of secondary magnetite are distinguished from the komatiite-hosted Thompson Nideposit (Manitoba, Canada): (1) magnetite formed by replacement of pyrrhotite and (2) magnetite formed

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during the serpentinization of the ultramafic host rocks. Secondary magnetite is depleted in most trace

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elements (Ni, Mn, V, Ti, V, Al, Cr), with the exception of Si and Mg, which are enriched. To be useful as a tool for mineral exploration, primary and secondary magnetite should not have

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the same minor and trace element composition. Primary magnetite from all 13 major Ni-Cu deposits plot

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in the field for Ni-Cu sulfide deposits in the Ni+Cr vs. Si+Mg discrimination diagram. Secondary magnetite plot outside of the field for Ni-Cu deposits. Therefore using this diagram, we can discriminate between primary and secondary magnetite using the low Ni+Cr content of secondary magnetite. Magnetite

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is resistant to surficial weathering and destruction during mechanical transport, such that it is a useful

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indicator mineral in exploration that can be used to detect eroded Ni-Cu-PGE deposits in surficial

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sediments.

1. Introduction

Iron oxides are major to accessory minerals in most magmatic, sedimentary or hydrothermal mineral deposit types. The metallogenic environment at the time of formation of a mineral deposit determines the minor and trace element compositions of magnetite. The chemical characteristics can be used as an indicator mineral in the exploration for ore deposits (Carew, 2004; Gosselin et al., 2006; Singoyi et al., 2006; Rusk et al., 2010; Dupuis and Beaudoin, 2011; Nadoll et al., 2012) and as an indicator of ore-forming processes (Reguir et al., 2008; Dare et al., 2012; Angerer et al., 2012, 2013; Nadoll et al. 2014). Indicator minerals have characteristics which are specific to a type of mineral deposit or that are useful to assess the fertility for mineralization in a geological setting. Useful indicator minerals

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ACCEPTED MANUSCRIPT in exploration must resist mechanical abrasion and chemical weathering during transport and have physical properties, such as density, color, and magnetic susceptibility, to easily identify them

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(McClenaghan, 2005; McClenaghan et al., 2013). Dupuis and Beaudoin (2011) proposed a set of discriminant diagrams that allow the distinction of

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a range of mineral deposit types based on the chemical composition of magnetite and hematite analyzed

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by electron probe micro-analysis (EPMA). The Ni+Cr vs. Si+Mg diagram was shown to be useful to identify samples containing magnetite from magmatic Ni-Cu-PGE and Cr deposits versus those from all

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other deposit types. Similarly, the Al/(Zn+Ca) vs. Cu/(Si+Ca) diagram was proposed to discriminate

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samples from Cu-Zn-Pb volcanogenic massive sulfides (VMS) deposits versus magnetite in samples from other deposit types. Iron oxide compositions that plot outside the Ni-Cu-PGE and the VMS fields belong to groups of samples from Iron-Oxide-Copper-Gold (IOCG), Kiruna-type, porphyry-Cu, Banded Iron

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Formation (BIF), skarn, Fe-Ti, and V, deposits can be discriminated using the Ni/(Cr+Mn) vs. Ti+V or the

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Ca+Al+Mn vs. Ti+ V diagrams. In order to further develop the use of magnetite as a robust indicator

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mineral in exploration it is critical to: 1) understand the processes that control the composition of magnetite in each deposit-type and, 2) test these discriminant criteria on a set of representative samples from a number of world-class ore deposits. Magmatic sulfide deposits are an important resource of Ni, Cu and platinum group elements (PGE). This class of deposits is associated with ultramafic to mafic rocks from a range of tectonic settings and ages (Naldrett, 2004). Experiments have shown that up to 30 % magnetite may crystallize at high temperatures (~ 1000°C) directly from the sulfide liquid (Naldrett 1969; Fonseca et al., 2008). Dare et al. (2012) showed that magnetite chemistry from massive sulfide ores from Sudbury (Canada), one of the world’s largest NiCu-PGE mining camp, records the change in composition of the sulfide liquid during sulfide fractionation. Magnetite starts to crystallize with early forming Fe-rich monosulfide solid solution (MSS, Fig.1A) at high temperatures (1180 – 940°C; Naldrett, 1969), and continues to crystallize from the residual sulfide liquid together with Cu-rich intermediate solid solution (ISS, Fig.1B) at lower temperatures (940 – 800°C;

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ACCEPTED MANUSCRIPT Craig and Kullerud, 1969; Fleet and Pan, 1994). Pyrrhotite and a minor amount of pentlandite exsolve from the MSS during cooling, and chalcopyrite and a minor amount of cubanite and pentlandite crystallize

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from the ISS (Barnes et al., 2001; Dare et al. 2012). In sulfide melt, the lithophile elements partition into magnetite whereas the availability of the chalcophile elements to partition into magnetite depends on the

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co-crystallizing sulfide mineral (Dare et al., 2012). Magnetite in Ni-Cu deposits is compositionally

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different from magnetite in layered intrusions (such as the Bushveld Complex) because sulfide and silicate liquids evolve differently during fractional crystallization (Dare et al., 2013).

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In this study we expand on the previous work on magnetite in massive sulfide ore from 3 Ni-Cu-

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PGE deposits from Sudbury (Dare et al. 2012), which is related to a one-of-a-kind impact melt, to a global scale in order to investigate (1) the behavior of trace elements in magnetite during sulfide fractionation in other types of Ni-Cu-PGE deposits, and (2) the trace element composition of magnetite relative to that of

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the parental magma. Magnetite was analyzed, using both EPMA and LA-ICP-MS, from 13 major Ni-Cu-

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PGE deposits formed in a range of geological environments, parental host magma compositions and ages

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that are considered representative for this class of ore deposit (Table 1). We report detailed results from two Ni-Cu-PGE deposits: 1) Voisey's Bay (Labrador, Canada) Ni-Cu-Co deposit, where a zoned sulfide orebody (Ovoid) formed by closed system crystallization (Naldrett et al., 2000) in contrast to the open system, where the MSS and ISS are physically separated, inferred in some of the deposits at Sudbury (Li et al., 1992; Farrow and Lightfoot, 2002; Dare et al., 2012); 2) Thompson Nickel Belt Ni-Cu deposits (Manitoba, Canada; TNB) where some of the magmatic sulfides have been remobilized in metasedimentary rocks of the Ospwagan Formation during low pressure, high temperature upper amphibolite grade regional metamorphism (e.g. Lightfoot et al., 2012). Secondary magnetite from Thompson, formed by replacement of sulfide and silicate minerals during serpentinization of the ultramafic host rocks (Layton-Matthews et al., 2007; Couëslan and Pattison, 2012), allows us to compare the composition of secondary magnetite with that of primary magnetite.

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ACCEPTED MANUSCRIPT 2. Compositional diversity in the selected Ni-Cu-PGE

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deposits Representative samples (n = 173) of sulfide ore and host rocks from 13 Ni-Cu-PGE deposits

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formed in a range of geological environments, primary magma composition (ultramafic, mafic and

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intermediate) and age of formation from Archean to Permo-Triassic were selected to represent the class of Ni-Cu-PGE sulfide deposits (Table 1). We have extended the initial work of Dupuis and Beaudoin (2011;

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Cape Smith, Voisey’s Bay, Sudbury and Blue Lake) and Dare et al. (2012; Sudbury) with data from 9

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additional Ni-Cu-PGE deposits. We divide the deposits based on the parental magma compositions into: (1) komatiite and basaltic-komatiite (e.g., Thompson Nickel Belt, Canada, number of samples, n = 29), (2)

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ferropicrite (e.g., Pechenga, Russia, n =17), (3) picrite-tholeiite (e.g., Jinchuan, China, n = 29), (4) flood

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basalt (e.g., Noril’sk-Talnakh, Russia, n = 37), (5) anorthosite-troctolite (e.g., Voisey’s Bay, Canada, n = 22) and (6) intermediate composition impact melt (Sudbury, Canada, n = 39). The tectonic setting of the

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selected deposits (Table 1) ranges from continental rift, volcanic arcs, rifted continental margin to meteoritic impact. They include five of the largest Ni deposits in the world: Noril’sk-Talnakh, Sudbury, Jinchuan, Nebo-Babel and Pechenga (Barnes and Lightfoot, 2005). Three of the selected deposits are differentiated (Sudbury, Talnakh and Voisey’s Bay), from Fe-rich ore (pyrrhotite-rich with minor pentlandite and chalcopyrite) to Cu-rich ore (chalcopyrite-rich with minor cubanite, pentlandite and in some cases pyrrhotite), as a result of sulfide fractionation and segregation of the residual Cu-rich liquid (Naldrett, 2004; Barnes and Lightfoot, 2005).

2.1. Sample selection For each deposit, up to twenty polished thin sections, representative of massive (> 60% sulfides), semi-massive (30-60%) and disseminated (< 30%) sulfide zones, and, where available, the maficultramafic host rocks, were examined (Appendix 1). The sample suite comprises 75 massive sulfide

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ACCEPTED MANUSCRIPT samples, from every deposit except Jinchuan, Nebo-Babel and Shaw Dome, 12 semi-massive samples from Thompson, Pechenga, Eagle, Jinchuan and Montcalm, and 15 disseminated sulfide samples. For the

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two differentiated sulfide deposits studied here (the Talnakh deposit from Noril’sk-Talnakh and the Ovoid deposit at Voisey’s Bay), representative samples were selected from both Fe-rich and Cu-rich sulfide

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zones. In komatiite- and picrite-hosted deposits, pyrrhotite and chalcopyrite are in similar proportion in

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our samples, forming undifferentiated Ni-rich, Cu-poor sulfides (Barnes and Lightfoot, 2005). In general, 3 to 4 magnetite grains per thin section were selected for analysis by EPMA and LA-ICP-MS, based on

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grain size (> 200μm), lack of inclusions, and distribution in thin section. Two of the deposits were studied in more detail to investigate the compositional diversity of

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magnetite from 1) the differentiated orebody of the Ovoid at Voisey’s Bay, Canada (Naldrett et al., 2000; Lightfoot et al. 2012) to investigate the change of minor and trace element concentration of magnetite

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during sulfide liquid fractionation and 2) the amphibolite grade Thompson Deposits, Canada (Layton-

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Matthews et al., 2007) to compare magnetite composition of primary magnetite, formed by fractional

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crystallization, with that of secondary magnetite, formed during metamorphism.

2.2. Voisey’s Bay Ni-Cu-Co deposit (Labrador, Canada) The 1333 Ma Voisey’s Bay deposit (Fig. 2) is located close to the tectonic contact between the Archean Nain and the Paleoproterozoic Churchill provinces (Amelin et al., 1999). Resources have been historically estimated at 142 Mt grading 1.6 wt% Ni, 0.85 wt% Cu, 0.09 wt% Co and less than 0.5 g/t PGE (Inco, 2005; Voisey’s Bay Nickel Co. website, 27th October 2006). The host intrusions of the Voisey's Bay deposit comprise ferrodiorite, ferrogabbro, troctolite and olivine gabbro that are associated with an anorthosite complex (Ryan, 2000). The parental mafic melt contained ~ 8 wt% MgO (Scoates and Mitchell, 2000) and was emplaced at a depth of 12 to 15 km (Naldrett, 2004). The magma was contaminated by crustal rocks, in particular sulfur-rich gneisses and organic carbon-rich sediments (Ripley et al., 1999). The R factor (ratio silicate magma /sulfide liquid) is estimated at 200 (Naldrett, 2004). The Ovoid orebody is a lens of massive sulfides, 110 m thick, hosted in the upper part of the feeder dyke (Fig.

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ACCEPTED MANUSCRIPT 2C). It comprises a dominant Fe-rich margin of po-pn-cpy-rich ore and a small core of cubanite-rich ore. Naldrett et al. (2000) proposed that the Ovoid orebody formed from the solidification of a sulfide melt in a

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closed system, with crystallization of sulfides from the margin to the core. Twenty samples of massive sulfides were collected at regular depths along a cross section through the Ovoid orebody (Fig. 2) with 16

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samples being representative of Fe-rich sulfide and 4 samples representative of Cu-rich sulfide. The Fe-

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rich ore (Fig. 1A) contains pyrrhotite with lesser pentlandite and chalcopyrite, and represents MSS. The Cu-rich ore (Fig. 1B) contains dominantly cubanite with lesser chalcopyrite, pyrrhotite, and pentlandite.

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The Fe-rich and Cu-rich samples represent a transition from MSS to ISS (Naldrett et al., 2000; Lightfoot

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et al., 2012).

2.3. Thompson Nickel Belt (Manitoba, Canada)

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The Thompson Nickel Belt (TNB) is located in the Circum-Superior Boundary Zone of the

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Canadian Shield (Fig. 3). The Thompson deposits are concentrated in a narrow, northeast-trending

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tectonic belt that represents the collisional remnants of the Archean Superior Province, autochthonous Proterozoic supracrustal sequences (Ospwagan Group), and allochthonous Proterozoic rocks related to the Trans-Hudson Orogen (Layton-Matthews et al., 2007; Lightfoot et al., 2012). The Ni-Cu mineralization of the TNB is associated with variably serpentinized komatiitic intrusions that range in composition from dunite to pyroxenite, which are hosted by clastic and chemical sediments of the Ospwagan Group (Bleeker, 1990). Past, proven and probable reserves for the district were estimated at 150 Mt grading 2.32 wt% Ni, 0.16 wt% Cu and 0.046 wt% Co, 0.83 g/t PGE by Naldrett (2004). The primary characteristics of the Thompson ore deposits and associated ultramafic bodies have been modified by deformation and metamorphism under upper amphibolite facies, and serpentine±carbonate alteration that resulted from the infiltration of post-peak metamorphism, volatile-rich, fluids from metasedimentary country rocks (LaytonMatthews et al., 2007; Couëslan et al., 2011; Lightfoot et al., 2012). Massive sulfide mineralization occurs in two settings at Thompson (Layton-Matthews et al., 2007): 1) within ultramafic bodies (e.g., Pipe deposits; Fig 3A) and 2) within metasedimentary rocks of the Ospwagan Formation (e.g. the T1 and T3

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ACCEPTED MANUSCRIPT deposits of the Thompson Dome; Fig 3B), where it is believed that the magmatic sulfides were remobilized during low pressure, high temperature regional metamorphism (Layton-Matthews et al.,

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2007). Twenty-eight samples including both sulfide-rich and serpentinized host rocks were available from

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the Thompson Ni-Cu deposits (Fig. 3; Pipe: n=11; T3: n=7; and T1: n=1).

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3. Petrography

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3.1. Magnetite from massive sulfides

Magnetite (defined as < 2 wt.% trace elements; Dupuis and Beaudoin, 2011) was found in the

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majority of massive sulfide samples (75 of 82) except for the massive sulfides from Thompson (Appendix 1). The massive sulfides contain 3 textural types of magnetite (Fig. 4):

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(1) Disseminated (1 to 20 vol. %) primary, coarse-grained magnetite are subhedral to anhedral with a

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grain size of 0.1 to 2 mm. Magnetite occurs in textural equilibrium with pyrrhotite, chalcopyrite,

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pentlandite or cubanite and it typically does not contain inclusions or exsolutions (Figs. 4 A, B and C). (2) Thin magnetite veins (10 to 200 µm in width) that cross-cut both primary magnetite and sulfides and which is interpreted to be secondary in origin (Fig. 5A). (3) Anhedral secondary magnetite partially replacing sulfide in massive ore occurs in the altered komatiite hosting the Alexo deposit (Fig. 5C).

In the case of differentiated deposits (Voisey’s Bay and Talnakh) primary magnetite is present in both the Fe-rich and Cu-rich samples (Appendix 1; Figs. 1 and 4A-C). Ti-rich magnetite (defined as > 2 wt.% Ti) was only found in the Fe-rich samples from the Talnakh deposit (Fig. 4D) where it is subhedral to anhedral with a grain size of 0.05 to 0.5 mm but contains no visible ilmenite exsolutions. The distribution of Fe-oxides at Talnakh and Voisey’s bay is similar to that described for the variably differentiated sulfides from the deposits of Sudbury (Dare et al., 2012) where ilmenite exsolution is common in Ti-rich magnetite in Fe-rich samples.

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ACCEPTED MANUSCRIPT 3.2. Magnetite from semi-massive and disseminated sulfides Magnetite is less common than chromite in ultramafic-hosted semi-massive and disseminated

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sulfides and less common than ilmenite and Ti-rich magnetite in mafic-hosted sulfides. Chromite aggregates (Fig. 5B) and ilmenite are associated with silicates or at the sulfide-silicate grain boundary

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(Appendix 1). Primary magnetite is absent in semi-massive and disseminated sulfides, however sulfides

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from serpentinized deposits contain irregular aggregates of anhedral secondary magnetite (100-500 µm in diameter) that partially replace pyrrhotite or chalcopyrite, which can form up to 5 % volume of a section

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(Fig. 5C). Thin secondary magnetite veins cut sulfides from Jinchuan.

3.3. Magnetite from the host rocks

With the exception of the serpentinized deposits, igneous host rocks from other Ni-Cu deposits

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contain no magnetite. Primary ilmenite and Ti-rich magnetite are the dominant Fe-oxide in mafic host

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rocks, whereas chromite predominates in ultramafic rocks (Table 2). Most magnetite grains from

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serpentinized and metamorphosed ultramafic host rocks at Thompson are subhedral to anhedral, with a diameter ranging from 0.005 to 2 mm, with an average grain size of 0.01 mm (Fig. 5). Secondary magnetite from the serpentinized host rocks are divided texturally into: (1) Disseminated magnetite in serpentine (e.g. Pechenga) (2) Magnetite overgrowth (5-10 µm-thick) around magmatic chromite (Fig. 5B, the Pipe deposit). (3) Narrow (<200 µm in width) magnetite-pyrrhotite veins, where anhedral magnetite constitutes 70 to 90 vol.% of the vein (Fig. 5D, the Pipe deposit). (4) Magnetite-dolomite veins (0.5 to 5 mm in width). The proportion of magnetite ranges from 90 vol.% to less than 5 vol.% in veins (Figs. 5E and F, the Pipe deposit).

4. Analytical Methods 4.1 Electron Probe Micro-Analysis

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ACCEPTED MANUSCRIPT Magnetite was analyzed at Université Laval, Quebec, with a CAMECA SX-100 Electron Probe Micro-Analyzer using a 10-μm diameter beam with a voltage of 15 kV, and a current of 100 nA. The wide

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beam diameter is necessary to prevent heating of magnetite under the high current. Minor and trace elements K, Ca, Al, Si, Ti, Mg, Mn, Cr, V, Sn, Cu, Zn, and Ni were measured. Analytical conditions are

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similar to those described by Dupuis and Beaudoin (2011) except for the addition of Sn to the element list

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and the use of large, high sensitivity crystals to improve the detection limit of Zn and Cu (Table 2). Calibration was achieved using a range of natural and synthetic standards, comprising simple oxides

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(GEO Standard Block of P and H Developments) and natural minerals (Mineral Standard Mount MINM

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25–53, Astimex Scientific; Jarosewich et al. 1980). The background was measured on both sides of the peak for 15-20 s at a position free of interfering element X-ray and the concentration was counted over the peak for 20 to 60 s depending on the element. Relative standard deviation ranges from 3% (Cr) and 30%

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(Ti).

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4.2. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry

The list of minor and trace elements analyzed was established according to recent studies of laser ablation performed on magnetite (Carew, 2004; Singoyi et al., 2006; Rabayrol and Barnes, 2009; Dare et al., 2012): 24Mg, 27Al, 45Sc, 47Ti, 51V, 52Cr, 55Mn, 60Ni, 66Zn, 75As, 59Co, 69,71 Ga, 74Ge, 89Y, 90,92 Zr, 95Mo, 101

Ru, 105Pd, 111Cd, 118Sn, 121Sb, 93Nb, 107Ag, 115In, 178Hf, 181Ta, 182W, 187Re, 193Ir, 195Pt, 197Au, 208Pb and

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Bi. Sulfur, Si, Ca and Cu, as well as the elements listed above, were monitored to detect inclusions. The

content of Si and Ca of magnetite were taken from the EPMA results. Multiple isotopes of Zr and Ga were measured to resolve any isobaric interference as discussed in Dare et al. (2012). LA-ICP-MS analysis was carried at Université du Québec à Chicoutimi (UQAC) using two instrumental procedures. The first LA-ICP-MS system consisted of a New Wave Research 213 nm Nd:YAG UV laser coupled to a Thermo X7 ICP-MS with a high-performance interface. LA-ICP-MS analytical method follows Dare et al. (2012) using a beam size of 80 μm, a speed stage of 5μm per second, a laser frequency of 10 Hz, and a power of 0.3 mJ/pulse. Calibration was based on a range of Fe-bearing

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ACCEPTED MANUSCRIPT international reference materials (BCR2-g, NIST361 and MASS-1) due to the lack of a single reference material that contained all the required elements at that time. We used the semi-quantitative option of the

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Plasmalab software to estimate Ru, Pd and Re but these elements were near or below detection limit. We used GOR-128g and a natural magnetite (BC-28) from the Bushveld Complex to monitor data quality

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(Table 3).

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The more recent LA-ICP-MS system installed at UQAC is a RESOlution M-50 Excimer 193nm laser coupled to an Agilent 7700x ICP-MS. A beam size of 25 to 80 µm, a speed stage of 3 to 15 µm/s, a

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laser frequency of 10 Hz, and a power of 5 mJ per pulse were used. A single Fe-rich reference material,

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GSE-1G containing all the required elements, was used for calibration (Savard et al., 2012). To monitor the quality of the analyses, reference materials GSD-1G and BC-28 were routinely analyzed (Table 3). Data reduction was carried out using the software Iolite.

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Using both instruments, lines were ablated across the width of a magnetite grain (Fig. 1) for a

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period of 20 to 60 seconds depending on the grain size, after monitoring a gas blank for 20–30 seconds.

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An ablation line across the grain allows measuring the initial composition of a grain by including exsolutions formed by subsolidus exsolution-oxidation processes. Iron was used as the internal standard assuming a stoichiometric value. Both instrumental procedures yield high precision and accurate results based on repeated analysis of our internal standard BC28 (Table 3) and of other reference materials (Savard et al., 2012), such that data from the two LA-ICP-MS instruments can be combined. Detection limits are 0.01 to 0.02 ppm for 24Mg, 59Co, 89Y, 90.92 Zr, 93Nb, 101Ru, 105Pd, 107Ag, 115In, 181Ta, 182W, 187Re, 197

Au, 208Pb, 209Bi; 0.025 to 0.05 ppm for 45Sc, 51V, 95Mo, 178Hf; 0.055 to 0.1 ppm for 65Cu, 71Ga, 111Cd,

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Sb, 193Ir, 195Pt; 0.1 to 0.5 ppm for 27Al, 47Ti, 60Ni, 66Zn, 74Ge, 75As, 118Sn; 0.55 to 1 ppm for 52Cr and

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Mn. Dare et al. (2012) showed a strong correlation between EPMA and LA-ICP-MS methods except

near the limit of detection and demonstrated that results from the two analytical methods can be combined.

4.3. Estimation of average composition We used nonparametric distribution modeling to estimate the average composition of magnetite

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ACCEPTED MANUSCRIPT for EPMA analysis because the data are typically censored as they contain non-detect values that are below the minimum detection limits (Helsel, 2005). The standard nonparametric Kaplan–Meier method

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was chosen to estimate the average composition of our censored data because this method does not require the assumption of a population distribution type (Lee and Helsel, 2007). Analyses with more than 2 wt.%

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of the elements listed in Table 2 and 3 were not used to compute deposit average composition to plot on

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the discrimination diagrams, following the method of Dupuis and Beaudoin (2011).

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5. Results

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5.1. Trace element composition of primary magnetite

We report EPMA analyses of 230 primary magnetite grains from 40 massive sulfide samples and

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LA-ICP-MS analyses of the 160 larger grains studied by EPMA (Appendix 1). The trace element

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concentration of the primary magnetite is variably rich in Zn, Ni, Mn, Cr, V, Ti, Al, Si, Mg, Co, Cu, Sn,

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W and Ga (concentration > 100 ppm). Potassium, Ca, Sc, Ge, Zr, Nb, Sb and Mo are present in trace amounts. Yttrium, Ta, Ir, Ag, Cd, In, Hf, Au, Pb, Bi are typically below or close to the lower limits of detection.

5.1.1. Differentiated ore deposits

Magnetite from Fe-rich and Cu-rich samples from the Talnakh and Voisey’s Bay deposits were normalized to the composition of the continental crust (Rudnick and Gao, 2003), following the method of Dare et al. (2012), and plotted on multi-element variation diagrams to compare them to magnetite from the differentiated ore deposits of Sudbury (Fig. 6). The lithophile elements are plotted separately from the chalcophile elements as they behave differently during sulfide fractionation: lithophile elements are controlled by the crystallization of magnetite whereas the chalcophile elements are controlled by the crystallization of sulfides (Dare et al. 2012). Magnetite shows similar lithophile element patterns for the three differentiated sulfide deposits (Figs. 6A, C and E). Ti-rich magnetite from primitive Fe-rich sulfide has high concentrations for several

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ACCEPTED MANUSCRIPT lithophile elements, in particular Cr and V, in comparison to magnetite (Ti < 2 wt.%) from Fe-rich sulfide. Magnetite from Fe-rich sulfide has high concentrations for all lithophile elements compared to magnetite

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in Cu-rich samples, which is depleted to these elements. Chrome, V, Ti, and in the case of the Talnakh deposit, Mg and Al, show the largest depletion between Fe-rich and Cu-rich samples. Mn varies the least.

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The concentration of chalcophile elements in magnetite shows variable behavior during sulfide

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fractionation in the three deposits (Figs. 6 B, D and F). There are greater similarities between Talnakh and Voisey’s Bay deposits than with Sudbury. The concentrations of Ni and Co are similar in magnetite from

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Fe- and Cu-rich sulfide at Talnakh and Voisey’s Bay (Figs. 6B and D). This is in contrast to the McCreedy deposit (Sudbury) in which Ni and Co are highest in magnetite from the Cu-rich sulfide (Fig. 6F).

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Magnetite in Fe-rich sulfide from Voisey’s Bay and Talnakh are slightly enriched in Mo compared to the Cu-rich sulfide, whereas the inverse relation is found for samples from the McCreedy East Deposit at

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Sudbury (Fig. 6F). Tin is enriched in magnetite associated with Cu-rich sulfide at Talnakh and McCreedy

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(Figs. 6B and F) but there is little difference between Fe- and Cu-rich sulfide magnetite at Voisey’s Bay

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(Fig. 6D). Zinc is enriched in magnetite from Fe-rich sulfide in all three deposits (Figs. 6B, D and F). 5.1.2. Multi-element patterns

In order to compare the composition of magnetite in massive sulfides that formed from different parental magmas, it is necessary to compare samples at similar levels of sulfide fractionation. Thus the multi-element diagrams shown in Figure 6 have been extended to combine both lithophile and chalcophile elements in one diagram to facilitate comparison of the patterns for all elements (Fig. 7). The elements are ordered from left to right with increasing compatibility into magnetite, following Dare et al. (2013). Magnetite compositions are plotted in Figure 7 according to their sulfide assemblage (Fe-rich, Cu-rich and undifferentiated). The sulfide deposits that are differentiated, with both Fe-rich (Fig. 7A) and Cu-rich (Fig. 7B) orebodies, are associated with mafic (Talnakh and Voisey’s Bay) and intermediate (Sudbury) magmas. In this study, deposits containing undifferentiated sulfides, that contain pyrrhotite and chalcopyrite in equal proportions (Fig. 7C), are predominantly associated with ultramafic parental magmas (e.g., komatiites and picrites), with the exception of the Eagle deposit (mafic).

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ACCEPTED MANUSCRIPT Magnetite and Ti-rich magnetite from Fe-rich ore show a regular increase of normalized minor and trace element compositions, from left to right, with relative enrichments in Pb and Sn, and depletions

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in Sc, Mg and Co (Fig. 7A). Magnetite from Cu-rich ore has a sawtooth pattern characterized by enrichments in chalcophile elements (Pb, Sn, Mo, Zn and Ni) and Mn and depletion in several lithophile

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elements (Al, Sc, Ga, Mg, Ti, V and Cr). Compared to magnetite from Fe-rich sulfide, Cu-rich sulfide

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magnetite is depleted in most lithophile elements and enriched in several chalcophile elements (Pb, Sn and Ni). Magnetite from undifferentiated sulfides from the mafic-hosted Eagle deposit has a pattern similar to

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magnetite from Cu-rich sulfide in mafic and intermediate rock-hosted deposits (Fig. 7B). However,

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magnetite from undifferentiated sulfides in ultramafic rocks have a pattern more similar to that of magnetite in Fe-rich sulfide from mafic and intermediate rock-hosted deposits, with enrichments in Ge, W, Sn, Mo, Cr and depletions in Al, Sc and Mg (Fig. 7C). Compared to other magnetite from mafic–

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hosted Fe-rich sulfide, magnetite from ultramafic-hosted sulfides is depleted in Zr, Al, Ta, Nb, Zn, and in

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some cases Mg, and enriched in Ge, W and Cr. These differences indicate that the composition of the

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parental magma has some control on the minor and trace element composition of magnetite. However, the compositional differences caused by magma composition are smaller than the difference caused by sulfide fractionation between magnetite from Fe-rich and Cu-rich sulfide from the same magma type (Fig. 7B).

5.2. Trace element composition of secondary magnetite We report analyzes of 147 grains of secondary magnetite from 11 semi-massive sulfide zones, 22 disseminated sulfides, and 17 host rock samples by EPMA, and LA-ICP-MS analysis of the 32 larger magnetite grains studied by EPMA. Secondary magnetite grains from the Thompson Ni Belt (TNB) were large enough to be analyzed by LA-ICP-MS. However, most of the secondary magnetite grains in other deposits were too small (< 100 µm) for LA-ICP-MS analysis, or contained abundant inclusions, and thus only EPMA data are available to document the chemical signature of secondary magnetite. The different types of secondary magnetite from the Pipe Deposit (TNB) are plotted on extended multi-element diagrams (Fig. 8) according to their chemical patterns. In each diagram, the average

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ACCEPTED MANUSCRIPT composition of primary magnetite, found in one massive sulfide sample from the Pipe Deposit (TNB), is plotted for comparison with secondary magnetite (Fig. 8). Primary magnetite is enriched in nearly all of

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the elements compared to secondary magnetite, except for Si, Ca, W, Sc, Mg and in a few cases Co and Ni. In Figure 8A, secondary magnetite replacing sulfides, disseminated magnetite in dolomite vein,

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magnetite-pyrrhotite vein in serpentinite, and magnetite rimming chromite show a similar pattern.

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However, disseminated magnetite from a dolomite vein is slightly enriched in Ca and W and magnetite rimming chromite is enriched in Ni, Cr and Co (Fig. 8A). Magnetite veins containing disseminated

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dolomite (Fig. 8B) have magnetite with low concentrations of Al, Ge, Sn, Mo, Ti, V, Cr, and in some

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cases Zn, Co and Ni, but are enriched in Ca (250 ppm) similar to disseminated magnetite in dolomite veins. Thin magnetite veins in sulfides form another distinct group that are characterized by magnetite with high W and Mn (± Ge, Sn, Ga) and low Al, Ti, Co and Cr contents (Fig. 8C) compared to the pattern

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of secondary magnetites plotted in Figure 8A.

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EPMA data for secondary magnetite from other Ni-Cu sulfide deposits, which are all hosted in

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ultramafic rocks, are plotted on multi-element diagrams (Fig. 9) and compared to primary magnetite from undifferentiated massive sulfides in ultramafic-hosted deposits (Fig. 9A). Secondary magnetite in veins contains only a few elements above EPMA detection limit: Si, Ca, Al, Mg, Zn and Ni, and in the case of magnetite vein in sulfides, Mn (Fig 9B). The composition of secondary magnetite in veins from the Pipe Deposit (TNB) is distinct from primary magnetite from all ultramafic-hosted deposits (Fig. 9B). Secondary magnetite in veins has a similar range in concentration of Si, Mg and Zn as primary magnetite (Fig. 9B) but is consistently depleted in Ti, V, Ni, Cr and Mn (except for magnetite veins in sulfides from Pipe Deposit and Jinchuan) and in a few cases slightly enriched in Ca. Disseminated magnetite in serpentinite (Fig. 9C) have a multi-element pattern similar to magnetite veins but typically contains higher, but variable, concentrations of Si, Mg, Ti, V and Ni but the Cr content of disseminated secondary magnetite is lower than that in the majority of primary magnetite. Secondary magnetite replacing sulfides show multi-element patterns not dissimilar to those of primary magnetite (Fig. 9D). Compared to secondary magnetite replacing sulfides from Pipe Deposit

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ACCEPTED MANUSCRIPT (TNB), the composition of magnetite replacing sulfides from all the other deposits has higher

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concentrations of Mn, Ti and V. Most secondary magnetites have lower Ni contents (Fig. 9D).

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6. Discussion

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The trace element composition of primary and secondary magnetite from a range of Ni-Cu-PGE deposits allows us to 1) document the evolution of the sulfide liquid from early-forming MSS to later-

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forming magnetite hosted in the Cu-rich ISS at Voisey’s Bay and Talnakh, and compared the evolution trend with magnetite recorded at Sudbury deposit (Dare et al., 2010); 2) determine if the composition of

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the parental magma has an influence on trace element composition of magnetite; and 3) determine if primary and secondary magnetite have the same trace elements compositions in order to develop an

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exploration tool for Ni-Cu-PGE deposits.

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6.1. Evolution of magnetite composition during sulfide fractionation The behavior of chalcophile elements during sulfide fractionation is well constrained from studies of whole rock geochemistry and experiment of studies: Fe, Co, IPGE (Os, Ir, Ru, Rh) and Re partition into MSS such that the residual liquid, from which the ISS crystallizes, is enriched in Cu, Pt, Pd, Au, Ag, As, Bi, Cd, Pb, Sb, Sn, Te and Zn (Distler et al., 1977; Zientek et al., 1994; Li et al., 1996; Barnes et al., 1997, 2001; Prichard et al., 2004; Mungall et al., 2005; Sinyakova and Kosyakov, 2007; Dare et al., 2012). Nickel remains more or less constant in abundance throughout sulfide liquid fractional crystallization because its partition coefficient into MSS is close to 1 (Li et al, 1996). In contrast, the behavior of lithophile elements is less well constrained because they are not concentrated in the sulfide liquid and the whole rock analyses of massive sulfide samples are commonly contaminated by minor inclusions of silicates. However, the presence of up to 2-5 wt.% Ti, V and Cr in magnetite in massive sulfide from Sudbury indicates that a small amount of these lithophile elements does partition into the sulfide liquid or is associated with the sulfide liquid, and become concentrated in accessory magnetite. Dare et al., (2012)

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ACCEPTED MANUSCRIPT proposed that the concentration of lithophile elements in magnetite is useful in tracking sulfide fractionation, as magnetite continuously crystallizes from the sulfide melt an early stage (with Fe-rich

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MSS) to the end of fractionation (with Cu-rich ISS). The changing composition of magnetite with sulfide fractionation can be monitored using the (Pt+Pd)/(Os+Ir+Ru+Rh) ratio of whole rock, which increases

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during fractionation of the sulfide liquid. Lithophile element concentrations in magnetite negatively

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correlate with the (Pt+Pd)/(Os+Ir+Ru+Rh) ratio indicating that lithophile element concentrations in Feoxide systematically decrease as the sulfide liquid fractionates. This indicates that lithophile elements

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concentration is controlled by the crystallization of magnetite from the sulfide liquid. Dare et al. (2012)

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defined 3 magnetite compositional fields according to the degree of fractionation of the sulfide liquid at Sudbury: (1) primitive Fe-rich MSS from Creighton, (2) evolved Fe-rich MSS from McCreedy East Main orebody and (3) residual Cu-rich ISS from McCreedy East 153 orebody (Fig. 10). At Sudbury, Cu-rich

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liquid was separated at the early stage from the MSS by draining into fractures in the footwall, and MSS

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rarely crystallized with ISS (Farrow and Lightfoot, 2002).

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At Voisey’s Bay, Naldrett et al. (2000) showed that the Ovoid massive sulfide orebody crystallized from the margin inwards, with a small amount of fractionated, residual, Cu-rich liquid becoming concentrated toward the center of the Ovoid. In Talnakh sulfide, the mineralization took place as dozens of sulfide shoots in the near-bottom part of the intrusion and in hornfelses beneath it (Spiridonov, 2010). The lower and peripheral parts of the sulfide shoots are usually composed of pyrrhotite-group minerals, which are gradually changed by pyrrhotite and chalcopyrite, and/or cubanite, then by cubanite and chalcopyrite, and, finally, by chalcopyrite and/or sulfur-undersaturated chalcopyrite group minerals (Spiridonov, 2010). In samples from the Ovoid and Talnakh deposits, pyrrhotite (i.e., MSS) is present throughout the fractional crystallization of sulfide liquid. Magnetite composition from Fe-rich MSS and Cu-rich ISS from Voisey’s Bay and Talnakh are compared to those from Sudbury in Figure 10. At Voisey’s Bay, magnetite from the Ovoid orebody displays a continual decrease in Cr concentration with increasing (Pt+Pd)/IPGE ratio in the whole rock. MSS magnetite plots in the Sudbury field for evolved Fe-rich MSS whereas Cu-rich ISS magnetite plots

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ACCEPTED MANUSCRIPT towards the Sudbury ISS field (Fig. 10A). Because early-formed magnetite is enriched in lithophile elements such as Cr, Ti and V, later-formed magnetite in Cu-rich ISS becomes depleted (Figs. 10B, C).

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Dalley (2012) analyzed magnetite from the Ovoid and other orebodies at Voisey’s Bay. His Ovoid data exhibit features similar to our results whereas magnetite from Fe-rich sulfides in other orebodies (e.g.,

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Reid Brook) are richer in Cr, Ti and V and plot in the field for primitive Fe-rich MSS. Ti-rich magnetite

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from Talnakh plots in the Sudbury‘s Primitive Fe-rich magnetite field, but most magnetite from Fe-rich MSS and Cu-rich ISS plot at the intersection of the Evolved Fe-rich and Cu-rich ISS fields from Sudbury

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(Figs. 10B, C). This confirms that the behavior of lithophile elements is the same for all of these Ni-Cu-

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PGE deposits because lithophile elements partition dominantly into magnetite during its crystallization from the sulfide liquid.

For chalcophile elements, Dare et al. (2012) showed that Ni, Mo and Sn increase in concentration

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in magnetite as the whole rock (Pt + Pd)/IPGE ratio increases during sulfide fractionation whereas Co

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remains more or less constant and Zn decreases in magnetite from Sudbury massive sulfides. The

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concentration of Ni into magnetite does not decrease during fractionation of the sulfide liquid but increases in concentration in the Cu-rich samples probably because ISS crystallized without MSS from the residual liquid in the Cu-rich veins of Sudbury (Fig.10D). This indicates that the bulk partition coefficient is not controlled simply by the crystallization of magnetite but most likely by the sulfide assemblage. For the Voisey’s Bay and Talnakh deposits, sulfide fractionation occurred in a closed system so that ISS (now ccp-cub) crystallized from the residual sulfide liquid together with minor MSS (now po-pn). In this case the crystallizing MSS should compete with magnetite for Ni. The binary plots of Ni vs. Cr and Ni vs. (Pt + Pd)/(Os+Ir+Ru+Rh) ratio show that Ni does not increase in magnetite during differentiation of the Ovoid and Talnakh deposits (Figs. 10D-F), because unlike at Sudbury MSS was co-crystallizing with the Cu-rich ISS. However the Ni content in magnetite is higher at Talnakh than magnetite from the Ovoid, probably reflecting the Ni tenor of the sulfide liquid. The settings at Talnakh and Ovoid are different from that of Sudbury in which the two sulfide solid-solutions were physically separated. These results show that magnetite trace element composition is mainly controlled by sulfide liquid fractionation in Ni-Cu-PGE

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ACCEPTED MANUSCRIPT deposits. Detailed sampling of the differentiated Ovoid orebody shows that the Ti content of magnetite

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tracks sulfide fractionation from rim to core (Fig. 11). Using the (Pt+Pd)/(Os+Ir+Ru+Rh) ratio in whole rock as a proxy for sulfide fractionation (Fig. 11A), the concentration of Ti in magnetite decreases from

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the Fe-rich rim to the Cu-rich core (Fig. 11B).

6.2. Parental magma

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During fractional crystallization of a sulfide liquid, lithophile elements have the same behavior for

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all of these Ni-Cu-PGE deposits whereas the chalcophile element partitioning into magnetite depends on physical separation of MSS and ISS. However, the relative concentrations of lithophile elements differ somewhat in the different parental magmas.

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The compositional differences of primary magnetite among the 3 differentiated deposits (Sudbury,

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Ovoid and Talnakh), could be attributed to differences in magma composition. Sudbury magma has an

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intermediate composition approximating bulk continental crust (Lightfoot et al., 1997; Ames et al., 2002). The most primitive magnetite (Ti-rich) from Talnakh massive sulfide is richer in Mg (Fig. 7) and Ni (Fig. 10F) than the most primitive magnetite from Voisey’s Bay (Reid Brook) and Sudbury (Creighton). This could be interpreted to indicate a more primitive mafic magma at Talnakh, as suggested by cumulus chromite in the host mafic intrusion (Barnes and Kunilov, 2000). Chromite is absent in host intrusions at Voisey’s Bay and Sudbury, where instead, intercumulus ilmenite and rare Ti-rich magnetite occur (Li et al., 2000). Although the massive sulfide sample from the mafic-hosted Eagle deposit is undifferentiated, with equal proportions of MSS and ISS, its magnetite is similar in composition to that from Cu-rich ISS sulfide assemblages (Fig. 7). This implies that the sulfide liquid that formed the sample at Eagle was most likely from an evolved sulfide liquid unlike the other undifferentiated orebodies hosted in ultramafic rocks, which plot in the primitive MSS field (Figs. 12). In ultramafic-hosted deposits (komatiites, basaltic komatiites and ferropicrites), the first forming oxide from the sulfide liquid is chromite, because of high Cr content of parental magma (Barnes and

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ACCEPTED MANUSCRIPT Roeder 2001), later followed by Cr-bearing magnetite (Ewers et al. 1976). Magnetite is likely to crystallize from an ‘evolved’ sulfide liquid but because there is not much Cu in these magmas (i.e.

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Ni/Cu=10-20 for TNB and Cape Smith, Ni/Cu =3-10 for Pechenga; Barnes and Lightfoot, 2005) this evolved sulfide liquid may not be significantly Cu-rich. That may explain why there is no magnetite from

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ultramafic hosted deposit plotting further down the fractionation trend (Fig. 12). The composition of

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evolved magnetite in massive sulfides from ultramafic parental magmas is most similar to that of primitive magnetite in differentiated mafic-hosted orebodies, except that the former have higher Cr and Ni, and

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lower Al contents, reflecting the more primitive nature of the parental magma (Figs. 7C and 12; Barnes

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and Lightfoot, 2005).

6.3. Magnetite discriminant diagrams: application to exploration

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The Ni+Cr vs. Si+Mg diagram (Fig. 13) was proposed to discriminate the composition of

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magnetite (< 2 wt.% Ti, Cr, etc) in Ni-Cu-PGE deposits from that of other deposit types (Dupuis and

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Beaudoin, 2011). This was based on EMPA data of primary magnetite in massive sulfide ores from deposits hosted in ultramafic (Cape Smith, Blue Lake), mafic (Voisey’s Bay) and intermediate (Sudbury) composition rocks. As shown above, primary magnetite mainly forms in large massive sulfides bodies and is rarely associated with rocks containing smaller volumes of sulfides (i.e., semi-massive and disseminated). To determine whether primary magnetite may serve as an indicator mineral for exploration, it is necessary 1) to demonstrate that primary magnetite from all Ni-Cu-PGE deposits have a common and distinctive composition and 2) to be able to distinguish primary massive sulfide magnetite from that from host rocks and secondary magnetite. Chromite is the common primary oxide in ultramafic host rocks whereas ilmenite and Ti-rich magnetite typically form in mafic and intermediate host rocks. These Cr- and Ti-rich oxides typically form at the sulfide/silicate interface in semi-massive and disseminated ores instead of magnetite (Fonseca et al. 2008). 6.3.1. Primary Magnetite The average composition of massive sulfide ore magnetite, analyzed by EPMA, from 9 Ni-Cu-

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ACCEPTED MANUSCRIPT PGE deposits, including samples from the differentiated orebodies of Talnakh and Ovoid at Voisey’s Bay is reported (Fig. 13). Following the method of Dupuis and Beaudoin (2011) only magnetite containing < 2

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wt.% of other elements was used to calculate the average composition for the deposit. Although magnetite from ultramafic-hosted deposits, and those in primitive Fe-rich sulfides in differentiated mafic- and

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intermediate-hosted deposits (e.g., Creighton), are typically rich in Ti, V and Cr (Figs. 10 and 12), only a

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small proportion contain > 2 wt.% total of these elements and are thus excluded from the discriminant diagram. For differentiated deposits, the composition of magnetite is the average of that in Fe-rich ore and

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that in Cu-rich ore.

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The new data, reported here, is combined with that of Dupuis and Beaudoin (2011) to provide a more comprehensive record of the compositional diversity of magnetite from a total of 13 Ni-Cu-PGE deposits worldwide. The average composition of magnetite plots in the field proposed for Ni-Cu-PGE

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deposits on the Ni+Cr vs. Si+Mg diagram, confirming that this discriminant diagram effectively identifies

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magnetite from Ni-Cu-PGE deposits from other deposit types (Fig. 13A). This can be applied to all Ni-

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Cu-PGE deposits that contain massive sulfides for all different parental magma compositions except those associated with komatiites because primary magnetite is rare and chromite is more common. In this case, the composition of chromite could be used instead to discriminate sulfide ore from host rock using the work of Barnes and Roeder (2001) and McClenaghan et al. (2012). Currently, uneconomic Ni-Cu massive sulfide deposits, such as Blue Lake, have magnetite that is compositionally similar to magnetite in massive sulfide from economic Ni-Cu deposits because the magnetite chemistry is controlled by the sulfide liquid fractionation and not the amount of metals accumulated. The variation in Ni+Cr content (0.1 – 1 wt.%) of magnetite is a consequence of magmatic sulfide fractionation. Although the Cr content of magnetite may decrease (3 wt.% to 1 ppm: Figs. 10 - 12) during sulfide fractionation of mafic/intermediate hosted orebodies, the relatively high content of Ni (> 400 ppm) in massive sulfide ore magnetite, compared to all other magnetite from hydrothermal ore deposits, ensures that even magnetite from the more fractionated sulfides (i.e., evolved Fe-rich sulfide and residual Cu-rich sulfide from Ovoid and Talnakh, Fig. 13B) plot within or near the lower limit of the Ni-Cu field. The Ni

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ACCEPTED MANUSCRIPT content of magnetite in most of the samples remains more or less constant, around 500 ppm for Voisey’s Bay (mafic) and around 1000 ppm for ultramafic-hosted deposits (and Talnakh), reflecting the Ni content

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of the sulfide liquid (Fig. 10 – 12). The Ni content of magnetite has low variance in these samples because they contain pyrrhotite (after MSS), even in the more evolved Cu-rich samples. As a result, Ovoid and

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Talnakh samples plot at low Ni+Cr values in the field for Ni-Cu-PGE deposits (Fig. 13A, B). It is only in

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Cu-rich samples that contain no MSS/pyrrhotite, such as those formed by open system fractionation at McCreedy East (Sudbury), that contain magnetite richer in Ni (2000 – 5000 ppm) and plot well within the

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middle of the Ni-Cu field (Dare et al. 2012).

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However, there is a wide range in Ni+Cr values for Fe-rich samples because of the wide variation of Cr during fractionation from primitive to evolved MSS (e.g., Fig. 10). Magnetite from the Eastern Deeps sulfide at Voisey’s Bay has higher Ni+Cr content than that of the Ovoid (evolved MSS) because

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the former is hosted in primitive Fe-rich MSS, similar to those found in the Reid Brook or Creighton

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deposits. For other Fe-rich samples from Sudbury, such as from Victor and Craig deposits, their magnetite

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has a low Ni+Cr content similar to those from evolved MSS of the Ovoid and McCreedy East deposit.

6.3.2. Secondary Magnetite

In general, secondary magnetites associated with serpentinized hosted rocks are depleted in Al, Mn, Ti, V and Cr, and enriched Si, Ca and Mg compared to primary magnetite (Fig. 10). When plotted on the Ni+Cr vs. Si+Mg diagram (individual analyses), nearly all secondary magnetite plot outside the field for Ni-Cu-PGE deposits with the exception of three magnetite grains that replace sulfides, which have high Cr content (Fig. 14A). The compositions of individual analyses of secondary magnetite are plotted on the Al/(Zn+Ca) vs. Cu/(Si+Ca) diagram (Dupuis and Beaudoin, 2011). All but five secondary magnetite compositions plot within the field for volcanogenic massive sulfides (VMS) deposits (Figs. 14 B and D). Magnetite in VMS is commonly a product of the replacement of sulfides at low temperature (> 300ºC; Galley, 2000). The detailed petrographic study of the altered Pipe Deposit (TNB) identified 5 different types of

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ACCEPTED MANUSCRIPT secondary magnetite all of which have a composition different to primary magnetite (Fig. 5). All the secondary magnetite from Pipe Deposit have similar low Ni+Cr concentrations and plot below the Ni-Cu

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field in Figure 14B. The minor and trace element composition of three types of secondary magnetite associated with 1) sulfide, 2) chromite and 3) dolomite veins are similar (Fig. 8A) despite their different

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textural associations. From this we infer that they formed from a similar fluid under similar physical-

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chemical conditions during the low temperature serpentine ± carbonate alteration documented by LaytonMatthews et al. (2007). Thus the replacement of sulfide and chromite by magnetite was most likely

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contemporaneous with the replacement of olivine by serpentine, and formation of disseminated magnetite

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in magnetite-carbonate veins (e.g., Ripley et al., 2005). Small deviations in magnetite composition could be explained by the nature of the mineral they are replacing (e.g., high Cr when replacing chromite) or by the nature of the fluid (e.g., high Ca when associated with dolomite-forming fluids), which could vary

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with temperature and/or source. The temperature of formation of magnetite by serpentinization (150-550

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ºC; Boschi et al., 2008; Murata et al., 2009) is lower than that of primary magnetite crystallizing from a

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sulfide liquid (800-1000ºC; Naldrett, 1969; Craig and Kullerud, 1969). Under high temperature magmatic conditions, many elements are available to partition into magnetite from the melt as shown by the relatively enriched primary magnetite from the magmatic sulfide ore. In contrast, when fluids alter a rock at lower temperature, fewer elements are available to enter into the magnetite structure (Nadoll et al., 2012, 2014), most likely because of the lower solubility of trace elements in a low-temperature hydrothermal fluid (Agranier et al., 2007).

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ACCEPTED MANUSCRIPT 7. Conclusion The study of primary magnetite from 13 world-class Ni-Cu-PGE deposits from a range of setting

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and age of formation demonstrates that magnetite trace element composition is mainly controlled by

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sulfide liquid fractionation. The composition of primary magnetite records the evolution of the sulfide

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liquid from early-forming Fe-rich monosulfide solid solution and later-forming magnetite hosted in the Cu-rich intermediate solid solution.

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The availability of lithophile and chalcophile elements depends on the sulfide mineral crystallizing with magnetite. The composition of the parental silicate magma exert only a minor control of

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the composition of minor and trace elements of primary magnetite. Primary magnetite in sulfides hosted by ultramafic rocks shows enrichment in Cr, whereas magnetite in Fe-rich sulfide hosted in mafic rocks

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shows enrichment in Al.

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There is a compositional difference between primary magnetite formed during sulfide

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crystallization and later, secondary magnetites which are depleted in most trace elements (Ni, Mn, V, Ti, V, Al, Cr), with the exception of Si and Mg which are enriched in some samples. This study demonstrates the efficiency of the (Ni+Cr) vs (Si+Mg) diagram to discriminate between Ni-Cu-PGE deposit primary magnetite from magnetite from other types of deposits and host rock. Magnetite chemistry is considered a good indicator mineral for Ni-Cu-PGE deposits that have been eroded in glacial, fluviatile or aeolian environments. A detailed study demonstrated that a subsample of 100 grains of the ferromagnetic fraction of a till sample, from the 0.25-1.0 mm grain size fraction, is statistically acceptable for determining the mineralogical and compositional range of iron oxides (Sappin et al., in press).

Acknowledgments We thank Marc Choquette (Laval) and Dany Savard (UQAC) for their assistance with EPMA and LAICP-MS analyses, respectively. This project is funded by the DIVEX Network, Vale Exploration Canada, the Geological Survey of Canada, and the Natural Science and Engineering Research Council of

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ACCEPTED MANUSCRIPT Canada. Vale Brownfields Exploration provided for representative samples. Special thanks to Rob Stewart and Dawn-Evans Lamswood for facilitating this. Thank to P.V. Sunder Raju for some of the Voisey’s Bay

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LA-ICP-MS data collection. Thomas Angerer and Michael Zientek are thanked for their insightful reviews

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that improved this paper significantly.

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Bibliography

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Abzalov, M.Z. (1998) Chrome-spinels in gabbrowehrlite intrusions of the Pechenga area, Kola Peninsula,

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Russia: emphasis on alteration features. Lithos 43 (3), 109-134.

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Agranier A., Lee C.-T.A., Li Z.-X.A., and Leeman W.P., 2007. Fluid-mobile element budgets in serpentinized oceanic lithospheric mantle: Insights from B, As, Li, Pb, PGEs and Os isotopes in the

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Feather River Ophiolite, California. Chemical Geology 245 (3-4), 230-241.

Amelin, Y., Li, C., and Naldrett. A.J., 1999. Geochronology of the Voisey's Bay intrusion. Labrador. Canada, by precise U-Pb dating of coexisting baddeleyite. zircon and apatite. Lithos 47, 33-51.

Ames, D. E., Golightly, J. P., Lightfoot, P. C., and Gibson, H. L., 2002. Vitric compositions in the onaping formation and their relationship to the Sudbury Igneous Complex, Sudbury Structure. Economic Geology 97, 1541-1562.

Angerer, T., Hagemann, S.G., Danyushevsky, L.V., 2012. Geochemical Evolution of the Banded Iron Formation-Hosted High-Grade Iron Ore System in the Koolyanobbing Greenstone Belt, Western Australia. Economic Geology 107, 599-644.

26

ACCEPTED MANUSCRIPT Angerer, T., Hagemann, S.G., Danyushevsky, L.V., 2013. High-grade Iron ore at Windarling, Yilgarn craton: a product of syn-orogenic deformation, hydrothermal alteration, and supergene modification in an

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Archean BIF–basalt lithostratigraphy. Mineralium Deposita 48, 697-728.

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Barnes, S. J., Coats, C. J. A., and Naldrett, A. J., 1982. Petrogenesis of a Proterozoic Nickel Sulfide-

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Komatiite Association : the Katiniq Sill, Ungava, Quebec. Economic Geology 77, 413-429.

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Barnes, S.-J., Makovicky, E., Karup-Moller, S., Makovicky, M., and Rose-Hanson, J., 1997. Partition

MA

coefficients for Ni, Cu, Pd, Pt, Rh and Ir between monosulfide solid solution and sulfide liquid and the implications for the formation of compositionally zoned Ni–Cu sulfide bodies by fractional crystallization

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of sulfide liquid. Canadian Journal of Earth Sciences 34, 366–374.

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Barnes, S. J. and Kunilov, V.Y., 2000. Spinels and Mg Ilmenites from the Noril’sk 1 and Talnakh

1717.

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Intrusions and other mafic rocks of the Siberian Flood Basalt Province. Economic Geology 95, 1701–

Barnes, S.-J., Acterberg, E., Makovicky, E., and Li, C., 2001. Proton probe results for partitioning of platinum group elements between monosulphide solid solution and sulphide liquid. South Africa Journal of Geology 104, 337–351.

Barnes, S.-J., Melezhik, V. A., and Sokolov, S. V., 2001. The composition and mode of formation of the Pechenga Nickel deposits, Kola Peninsula, Northwestern Russia. Canadian Mineral 39 (2), 447-471.

Barnes S. J., and Roeder P. L., 2001. The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks. Journal of Petrology 42 (12), 2279-2302.

27

ACCEPTED MANUSCRIPT Barnes, S.-J. and Lightfoot, P.C., 2005. Formation of magmatic nickel sulphide ore deposits and processes

PT

affecting their copper and platinum group element contents. Economic Geology 100th, 179-213.

Barnes, S. -J., Cox, R. A., and Zientek, M. L., 2006. Platinum-group element, Gold, Silver and Base Metal

SC

Contributions to Mineralogy and Petrology 152, 187 – 200.

RI

distribution in compositionally zoned sulfide droplets from the Medvezky Creek Mine, Noril'sk, Russia.

NU

Barrie, C.T., Corfu, F., Davis, P., Coutts, A.C., and MacEachern, D., 1999. Geochemistry of the

MA

Dundonald komatiite-basalt suite and genesis of the Dundeal Ni deposit, Abitibi subprovince, Canada. Economic Geology 94, 845–866.

D

Beaudoin, G., Laurent, R., and Ohnenstetter, D., 1990. First report of Platinum-group minerals at Blue

AC CE P

TE

Lake, Labrador Trough, Quebec. Canadian Mineralogist 28, 409-418.

Bleeker, W., 1990. Evolution of the Thompson nickel belt and its nickel deposits, Manitoba, Canada: Ph.D. thesis, University of New Brunswick, Fredericton, New Brunswick, 444 p.

Boschi, C., Dini, A., Früh-Green, G.L., and Kelley, D.S., 2008. Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30°N): Insights from B and Sr isotope data, Geochimica et Cosmochimica Acta 72 (7), 1801-1823.

Carew, M. J., 2004. Controls on Cu-Au mineralisation and Fe oxide metasomatism in the Eastern Fold Belt, N.W. Queensland, Australia: Unpub. Ph.D. Thesis. James Cook University, Australia, 308 p.

Chen, Y., Fleet, M.E., and Pan, Y., 1993. Platinum-Group Minerals and Gold in Arsenic-Rich ore at the Thompson Mine, Thompson Nickel Belt, Manitoba, Canada. Mineralogy and Petrology 49, 127-146.

28

ACCEPTED MANUSCRIPT

Couëslan, C. G., Pattison, D.R.M., and Tinkham D.K., 2011. Regional low-pressure amphibolite-facies

PT

metamorphism at the Pipe II mine, Thompson Nickel Belt, Manitoba, and comparison of metamorphic

RI

isograds in metapelites and meta-iron formations. The Canadian Mineralogist 49, 721-747.

SC

Couëslan, C. G., Pattison, D.R.M., 2012. Low-pressure regional amphibolite-facies to granulite-facies metamorphism of the Paleoproterozoic Thompson Nickel Belt, Manitoba. Canadian Journal of Earth

NU

Sciences 49, 1117-1153.

MA

Craig, J. R. and Kullerud, G., 1969. Phase relations in the Cu-Fe-Ni-S system and their application to

D

magmatic ore deposits. Economic Geology Monographs 4, 344–358.

TE

Dalley, T., 2010. An examination of the petrography and geochemistry of magnetite at the Voisey’s Bay

AC CE P

Ni-Cu-Co deposits. Bachelor thesis, Memorial University of New Foundland, 84 p.

Dare, S. A. S., Barnes, S.-J., Prichard, H. and Fisher, P. C., 2011. Chalcophile and platinum-group (PGE) concentrations in the sulfide minerals from the McCreedy East deposit, Sudbury, Canada, and the origin of PGE in pyrite. Mineralium Deposita 46, 381–407.

Dare, S.A.S., Barnes, S-J, and Beaudoin, G., 2012. Variation in trace element contents of magnetite crystallized from a fractionating sulphide liquid, Sudbury, Canada: Implications for provenance discrimination. Geochimica et Cosmochimica Acta 88, 27–50.

Dare, S.A.S., Barnes, S-J, Beaudoin, G., Méric J. and Boutroy E., 2013. The use of trace elements in magnetite as provenance and petrogenetic indicators in magmatic and hydrothermal environments. SGA 2013 Abstract.

29

ACCEPTED MANUSCRIPT

PT

Ding, X., Li, C., Ripley, E. M., Rossell, D., and Kamo, S., 2010.The Eagle and East Eagle sulfide ore bearing mafic!ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and

SC

RI

petrologic evolution. Geochemistry, Geophysics, Geosystems 11 (3), 22 p.

Distler, V.V., Malevsky, A.Y., and Laputina, I.P., 1977. Distribution of platinoids between pyrrhotite and

MA

NU

pentlandite in crystallization of a sulphide melt. Geochemistry International 14, 30–40.

Dupuis, C., and Beaudoin, G., 2011. Discriminant diagrams for iron oxide trace element fingerprinting of

D

mineral deposit types. Mineralium Deposita 46, 319–335.

TE

Ewers, W. E., Graham, J., Hudson, D. R., and Rolls, J. M., 1976. Crystallization of chromite from nickel–

AC CE P

iron sulfide melts. Contributions to Mineralogy and Petrology 54, 61–64.

Farrow, C.E.G., and Lightfoot, P.C., 2002. Sudbury PGE revisited: toward an integrated model. In The geology, geochemistry, mineralogy and mineral beneficiation of platinumgroup element (ed. L. J. Cabri). Canadian Institute of Mining, Metallurgy and Petroleum, Special volume 54, 13–130.

Fleet, M. E., and Pan, Y., 1994. Fractional crystallization of anhydrous sulfide liquid in the system Fe–Ni– Cu–S, with application to magmatic sulfide deposits. Geochimica et Cosmochimica Acta 58, 3369–3377.

Fonseca, R.O.C., Campbell, A.H., O’Neill, H.S.C., and Fitzgerald, J.D., 2008. Oxygen solubility and speciation in sulphide-rich mattes. Geochimica et Cosmochimica Acta 72, 2619–2635.

30

ACCEPTED MANUSCRIPT Galley, A.G., Jonasson, I.R., and Watkinson, D.H., 2000. Magnetite-rich calc-silicate alteration in relation to synvolcanic intrusion at the Ansil volcanogenic massive sulfide deposit, Rouyn-Noranda, Quebec,

PT

Canada. Mineralium Deposita 35, 619-637.

RI

Gosselin, P., Beaudoin, G., and Jébrak, M., 2006. Application of the geochemical signature of iron oxides

SC

to mineral exploration. GAC-MAC Annual Meeting Program with Abstracts.

MA

NU

Helsel, D., 2005. Nondetects and data analysis. Wiley, New York.

Jarosewich, E., Nelen, J.A., Norberg, J.A., 1980. Reference samples for electron microprobe analysis.

D

Geostand Newsl 4, 43–47.

TE

Krivolutskaya, N. A., Ariskin, A. A. , Sluzhenikin, S. F., and Turovtsev, D. M., 2001. Geochemical

AC CE P

thermometry of rocks of the Talnakh intrusion: Assessment of the melt composition and the crystallinity of the parental magma. Petrology 9 (5), 389–414.

Layton-Matthews, D., Lesher, C.M., Burnham, O.M., Liwanag, J., Halden, N.M., Hulbert, L., Peck, D.C., 2007. Magmatic Ni-Cu-platinum-group element deposits of the Thompson Nickel Belt, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication 5, 409-432.

Lee, L., Helsel, D., 2007. Statistical analysis of water-quality data containing multiple detection limits II: S-language software for nonparametric distribution modelling and hypothesis testing. Computer methods in Geosciences 33, 696–704.

31

ACCEPTED MANUSCRIPT Lesher, C. M., and Sproule, R. A., 2007. Geochemistry of komatiites in the Eastern Goldfields Superterrane, Western Australia and the Abitibi Greenstone Belt, Canada, and implications for the

PT

distribution of associated Ni–Cu–PGE deposits. Applied Earth Science (Trans. Inst. Min. Metall. B) 116

RI

(4), 167-187

SC

Li C., Naldrett A. J., Coats C. J. A. and Johannessen P., 1992. Platinum, palladium, gold, and copper-rich stringers at the Strathcona mine, Sudbury: their enrichment by fractionation of a sulfide liquid. Economic

MA

NU

Geology 87, 1584–1598.

Li, C., Barnes, S.-J., Makovicky, E., Rose-Hansen, J., and Makovicky, M., 1996. Partitioning of Ni, Cu, Ir, Rh, Pt and Pd between monosulfide solid solution and sulfide liquid: effects of composition and

TE

D

temperature. Geochimica et Cosmochimica Acta 60, 1231–1238.

AC CE P

Li, C., Lightfoot, P. C., Amelin, Y., and Naldrett, A. J., 2000. Contrasting Petrological and geochemical Relationships in the Voisey’s bay and Mushua Intrusions, Labrador, Canada: Implications for Ore Genesis. Economic Geology 95, 771-799.

Lightfoot, P.C., Keays, R.R., Morrison, G.G., Bite, A., and Farrell, K., 1997. Geochemical Relationships in the Sudbury Igneous Complex: Origin of the Main Mass and Offset Dikes. Economic Geology 92, 289307.

Lightfoot, P.C., Keays, R.R, Evans-Lamswood, D.E., and Wheeler, R., 2012. Crustal contamination and multiple S-saturation events in Nain Plutonic Suite magmas: evidence form Voisey’s Bay, Labrador, Canada. Mineralium Deposita 47, 23-50.

32

ACCEPTED MANUSCRIPT McClenaghan, B., 2005. Indicator mineral methods in mineral exploration. Geochemistry: Exploration,

PT

Environment, Analysis 5, 233-245.

McClenaghan, M. B., Kjarsgaard, J.M., Averill, S.A, Layton-Matthews, D., Crabtree, D., Matile, G.,

RI

McMartin, I., Pyne, M., 2013, Indicator mineral signatures of magmatic Ni-Cu deposits, Thompson Nickel

SC

Belt, Manitoba: Part 2 - till data: Geological Survey of Canada. Open File 7200.

NU

Méric, J., Barnes S-J, and Dare, S.A.S., 2011. Caractérisation géochimiques des magnétites de la zone critique de l’intrusion magmatique de Sept-Îles (Québec, Canada) et intégration à une base de données

MA

utilisant la signature géochimique des oxydes de fer comme outil d’exploration. Rapport de stage de fin

D

d’étude de Master, 66 p.

TE

Mungall, J.E., Andrews, R., Cabri, L.J., Sylvester, P.J., and Tubrett, M., 2005. Partitioning of Cu, Ni, Au,

AC CE P

and platinum-group elements between monosulfide solid solution and sulfide melt under controlled oxygen and sulfur fugacities. Geochimica et Cosmochimica Acta 69, 4349–4360.

Murata, K., Maekawa, H., Yokose, H., Yamamoto, K., Fujioka, K., Chiba, H., Ishii, T., Wada, Y., 2009. Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific. Geosphere 5 (2), 90–104.

Nadoll, P., Mauk, J.L., Hayes, T.S., Koenig, A.E., Box, S.E., 2012. Geochemistry of magnetite from hydrothermal ore deposits and host rocks of the mesoproterozoic Belt Supergroup, United States Economic Geology 107 (6), 1275-1292.

Nadoll, P., Angerer, T., Mauk, J. L., French, D., Walshe, J., 2014. The chemistry of hydrothermal magnetite: A review. Ore Geology Reviews 61, 1–32.

33

ACCEPTED MANUSCRIPT

Naldrett, A. J., 1969. A portion of the Fe-O-S system between 900 and 1080ºC and its application to

PT

sulfide ore magma. Journal of Petrology 10, 171-201.

RI

Naldrett, A. J., Singh, J., Krstic, S., and Li, C., 2000. The Mineralogy of the Voisey’s Bay Ni-Cu-Co

SC

Deposit, Northern Labrador, Canada: Influence of Oxidation State on Textures and Mineral Compositions.

NU

Economic Geology 95, 889–900.

MA

Naldrett, A.J., 2004. Magmatic Sulfide Deposits. Geology, geochemistry and exploration Springler, Berlin, 727 p.

D

Prichard, H.M., Hutchinson, D., and Fisher, P.C., 2004. Petrology and crystallization history of multiphase

AC CE P

Economic Geology 99, 365–376.

TE

sulfide droplets in a mafic dike from Uruguay: implications for the origin of Cu–Ni–PGE sulfide deposits.

Puchtel, I.S., Humayun, M., Campbell, A. J., Sproule, R. A., and Lesher, M. C., 2004. Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada. Geochimica et Cosmochimica Acta 68 (6), 1361–1383.

Rabayrol, F., Barnes, S-J., 2009. Méthode de calibration des éléments mineurs et traces dans les magnétites par LA-ICP-MS. DIVEX Sous-Projet SC29, 37p.

Reguir, E.P., Chakhmouradian, A.R., Halden, N.M., and Yang, P., 2008. Early magmatic and reactioninduced trends in magnetite from the carbonatites of Kerimasi, Tanzania. Canadian Mineralogist 46, 879– 900.

34

ACCEPTED MANUSCRIPT Ripley, E. M., Park, Y-R., Li, C., and Naldrett, A.J., 1999. Sulfur and oxygen isotopic evidence of country

PT

rock contamination in the Voisey's Bay Ni-Cu-Co deposit, Labrador Canada. Lithos 47, 53-68.

Ripley, E. M., Sarkar, A., and Li, C., 2005. Mineralogic and stable isotope studies of hydrothermal

SC

RI

alteration at the Jinchuan Ni-Cu Deposit, China. Economic Geology 100th, 1349–1361.

Rusk, B.G., Oliver, N.H.S., Cleverley, J.S., Blenkinsop, T.G., Zhang, D., Williams, P.J. and Habermann,

NU

P., 2010. Physical and chemical characteristics of the Ernest Henry iron oxide copper gold deposit,

MA

Australia; implications for IOCG genesis; in Porter, T.M., (ed.), Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v. 3 - Advances in the Understanding of IOCG Deposits; PGC

D

Publishing, Adelaide.

TE

Ryan, B., 2000. The Nain Churchill boundary and the Nain plutonic suite: a regional perspective on the

AC CE P

geologic setting of the Voisey's Bay Ni-Cu-Co deposit. Economic Geology 95, 703-724.

Sappin, A.A., Dupuis, C., Beaudoin, G., Pozza, M., McMartin, I., McClenaghan, M.B., in press. Optimal ferromagnetic fraction in till samples along ice-flow paths: Case studies from the Sue-Dianne and Thompson deposits, Canada. Geochemistry: Exploration, Environment, Analysis

Savard, D., Barnes, S.-J., Dare, S., and Beaudoin, G., 2012. Improved calibration technique for magnetite analysis by LA-ICP-MS. Goldschmidt 2012 Abstract

Scoates, J.S., and Mitchell, J.N., 2000. The evolution of troctolitic and high-Al basaltic magmas in Proterozoic anorthosite plutonic suites and implications for the Voisey’s Bay massive Ni-Cu sulfide deposit. Economic Geology 95, 677-702.

35

ACCEPTED MANUSCRIPT Seat, Z., Beresford, S.W., Grguric, B.A, Gee, M.A.M., and Grassineau, N. V., 2009. Reevaluation of the role of external sulfur addition in the genesis of Ni-Cu-PGE deposits: Evidence from the Nebo-Babel Ni-

PT

Cu-PGE deposit, West Musgrave, Western Australia. Economic Geology 104, 521–538.

RI

Singoyi, B., Danyushevsky, L., Davidson, G. J., Large, R. et Zaw, K., 2006. Determination of trace

SC

elements in magnetites from hydrothermal deposits using the LA-ICP-MS technique. In: Abstracts of Oral

NU

and Poster Presentations from the SEG2006 Conference, Keystone, USA, 367-368.

MA

Sinyakova, E. F., and Kosyakov, V. I., 2007. Experimental modeling of zoning in copper–nickel sulfide ores. Doklady Earth Sciences 417A, 1380–1385.

TE

AC CE P

Geophysics 51, 1059–1077.

D

Spiridonov, E. M., 2010. Ore-magmatic systems of the Noril’sk ore field. Russian Geology and

Tucker Barrie, C., and Naldrett, A. J., 1990. Geochemical constraints on the genesis of the Montcalm gabbroic complex and Ni-Cu deposit, Western Abitibi subprovince, Ontario. Canadian Mineralogist 28, 451-474.

Yang , X.-Z., Ishihara, S., and Zhao, D.-H., 2006. Genesis of the Jinchuan PGE deposit, China: evidence from fluid inclusions, mineralogy and geochemistry of precious elements. Mineralogy and Petrology 86, 109–128.

Zientek, M. L., Likhachev, A. P., Kunilov, V. E., Barnes, S. -J., Meier, A.L., Carlson, R.R., Briggs, P.H., Fries, T.L., and Adrian, B.M., 1994. Cumulus processes and the composition of magmatic ore deposits: examples from the Talnakh District, Russia. In Proceedings of the Sudbury-Noril’sk Symposium (eds. P. C. Lightfoot and A. J. Naldrett). Ontario Geological Survey, Special publication Issue 5, pp. 373–392.

36

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Zwanzig, H.V., Macek, J. J., and McGregor, C.R., 2007. Lithostratigraphy and Geochemistry of the

PT

High-Grade Metasedimentary Rocks in the Thompson Nickel Belt and Adjacent Kisseynew Domain,

AC CE P

TE

D

MA

NU

SC

RI

Manitoba: Implications for Nickel Exploration. Economic Geology 102, 1197–1216.

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ACCEPTED MANUSCRIPT Fig. 1. Reflected light photographs of massive sulfides from the Ovoid orebody of Voisey’s Bay deposit, Labrador. (A) Pyrrhotite-rich (Po) ore, with minor pentlandite (Pn) and disseminated coarse-grained

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magnetite (Mgt; 5%), located at the edge of the ore body. This sample represents the Fe-rich MSS cumulate that crystallized early from the sulfide liquid. (B) Cu-rich ore containing chalcopyrite (Ccp) and

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cubanite (Cub), minor pyrrhotite and magnetite (1%). This sample crystallized from the residual Cu-rich

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liquid and is located at the core of the Ovoid ore body. Field of view of polished block is 2.5 cm.

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Fig. 2. (A) Location map (inset) and geological setting of Voisey’s Bay Ni-Cu-Co deposit (in red box),

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Labrador and New Foundland (modified from Naldrett and Li, 2007). (B) Geological map of the Voisey’s Bay intrusion (modified from Li et al., 2000). Red line represents the transversal section of the Ovoid. (C) West facing geologic section through the Ovoid orebody drawn from N-S Section 55837.00E. Sample

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locations of massive sulfides (white circles) taken from drill core.

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Fig. 3. (A) Location map (inset) and geological setting of Thompson Nickel Belt (TNB), Manitoba (modified from Zwanzig et al., 2007). Sample locations from Thompson deposit (in red box) and Pipe mine shown as stars. (B) Geological setting of Thompson mine showing the location of T1 and T3 mines (stars; modified from Chen et al., 1993).

Fig. 4. Photomicrographs of primary magnetite (Mag) in massive sulfides. (A) Magnetite in textural equilibrium with massive pyrrhotite-rich ore (Talnakh deposit); (B) Magnetite in textural equilibrium with chalcopyrite (Ccp) in Cu-rich ore (Talnakh deposit); (C) Magnetite with subhedral to anhedral texture in contact with chalcopyrite (Ccp) and pyrrhotite (Po) in massive sulfide (Mesamax deposit); (D) Ti-rich magnetite in textural equilibrium with massive pyrrhotite-rich ore (Talnakh deposit);

Fig. 5. Photomicrographs of Fe-oxide magnetite grains in semi-massive to disseminated sulfides and serpentinized host rocks of the Pipe Mine deposit (TNB). (A) Primary, euhedral magnetite in pyrrhotite

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ACCEPTED MANUSCRIPT cut by secondary magnetite vein (arrow); (B) Rim of magnetite (Mag) overgrowing primary chromite (Chr); (C) Anhedral aggregates of magnetite replacing pyrrhotite in semi-massive sulfides (Pipe Mine);

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(D) Thin magnetite-pyrrhotite vein in serpentinite; (E) Massive magnetite vein (1.5 mm thick) containing disseminated dolomite in serpentinite (Dol); (F) Narrow dolomite vein with disseminated magnetite in

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serpentinite.

Fig. 6. Multi-element diagrams for lithophile elements (left column) and chalcophile elements (right

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column) in magnetite from zoned sulfide ore deposits by LA-ICP-MS (A, B) Talnakh, Russia; (C, D)

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Voisey’s Bay, Labrador and Newfoundland; (E, F) Sudbury (Creighton and McCreedy), Ontario (Dare et al., 2012). Values of bulk continental crust are taken from Rudnick and Gao (2003). The order of compatibility of each element into magnetite increases to the right (Dare et al., 2013). In C-D, magnetite

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from Voisey’s Bay Ovoid orebody of analyzed in this study is similar in composition to that from the

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literature (Lit; Dalley, 2010). Each symbol represents the average composition for a sulfide association

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normalized to bulk continental crust. DL is the detection limit and n is number of samples.

Fig. 7. Extended multi-element diagrams average composition of primary magnetite per deposit and sulfide association normalized to bulk continental crust. (A) Magnetite in Fe-rich ore associated with mafic and intermediate magmas. Ti-rich magnetite from Talnakh and Creighton are also notably enriched in Cr; (B) Magnetite from Cu-rich ore associated with mafic and intermediate magmas. Magnetite in undifferentiated sulfides from mafic-hosted Eagle deposit is more similar in composition (low Ti, V and Cr) to magnetite from Cu-rich ore than from Fe-rich ore (A); (C) Magnetite (excluding Ti-rich magnetite) from mafic-hosted Fe-rich sulfides. Magnetite from ultramafic-hosted deposits are hosted in undifferentiated sulfides. Grey field in B and C is the range for Fe-rich ore magnetite excluding Ti-rich magnetite from A.

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ACCEPTED MANUSCRIPT Fig. 8. Multi-element diagram of trace element composition of primary and secondary magnetite from the Pipe Mine deposit (TNB), analyzed by LA-ICP-MS. (A) Primary magnetite compared to various texture

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of secondary magnetite; (B) Magnetite vein with disseminated dolomite and disseminated magnetite associated with chromite compared to primary magnetite and secondary magnetite from (A); (C)

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Secondary magnetite vein in sulfides compared to primary magnetite and secondary magnetite from (A);

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Each symbol represents the average composition in a thin section of one textural type of secondary

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magnetite.

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Fig. 9. Multi-element diagram of trace element composition of primary and secondary magnetite analyzed by EPMA. (A) Primary magnetite from massive sulfides hosted by ultramafic rocks (average per thin section); (B) Magnetite veins in massive to disseminated sulfides and in serpentinite compared to primary

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magnetite from (A); (C) Disseminated, secondary magnetite in hosts rocks compared to primary magnetite

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from (A); (D) Magnetite replacing sulfides from semi-massive to disseminated sulfides compared to

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primary magnetite from (A). Each symbol represents the average composition in a thin section of one textural type of secondary magnetite.

Fig. 10. (A) Binary plot of Cr in primary magnetite vs. whole rock Pt+Pd/(IPGE) for massive sulfides from Ovoid orebody (Voisey’s Bay), plotted on trends of sulfide fractionation (shaded fields) defined by magnetite from Sudbury (Dare et al. 2012). During fractionation, Fe-rich monosulfide solid solution is the first to crystallize (grey fields) followed by Cu-rich intermediate solid solution (yellow fields) from the residual liquid. Binary plots of Ti vs. Cr (B), V vs. Cr (C) and Ni vs. Cr (D) for primary magnetite, from the zoned sulfides orebodies from Talnakh and Voisey’s Bay. (E) Binary plot of Ni in primary magnetite vs. whole rock Pt+Pd/(IPGE) for massive sulfides from Ovoid orebody, Voisey’s Bay. (F) Binary plot of Ni vs. Cr for primary magnetite, from the zoned sulfides orebody from Talnakh. Black arrow represents the Voisey’s Bay trend. Each symbol represents the average composition in a thin section of LA-ICP-MS analysis.

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Fig. 11 Spatial zonation of (A) Pt+Pd/ (Os+Ir+Ru+Rh) ratio in whole rocks and (B) Ti content in

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magnetite (ppm) in massive sulfide of the Ovoid orebody (Voisey’s Bay). Pt+Pd/ (Os+Ir+Ru+Rh) ratio

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increases from the outer Fe-rich rim to the Cu-rich core, whereas the concentration of Ti decreases.

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Fig. 12 Binary plots of Ti vs. Cr (A), V vs. Cr (B) and Ni vs. Cr (C) for primary magnetite, from undifferentiated sulfides orebodies from Eagle, Pechenga, Raglan, Mesamax, Expo 2003 and Pipe Mine,

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plotted on trends of sulfide fractionation (shaded fields) defined by magnetite from Sudbury (Dare et al.,

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2012). Each symbol represents the average composition in thin section of LA-ICP-MS analysis.

Fig. 13 (A) Average primary magnetite composition, analyzed by EPMA, for individual Ni-Cu-PGE

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deposits plotted on the Ni+Cr vs. Si+Mg discriminant diagram of Dupuis and Beaudoin (2011). Data for

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other deposits from Dupuis and Beaudoin (2011). Each symbol represents the average composition per

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deposit; (B) Primary magnetite from the zoned orebodies of Talnakh and Voisey’s Bay deposits plotted on on the Ni+Cr vs. Si+Mg discriminant diagram. Magnetite from Fe-rich and Cu-rich sulfides assemblages have similar Ni+Cr. Talnakh has higher Ni+Cr than that of Voisey’s Bay. Each symbol represents the EPMA average composition in a thin section.

Fig. 14 Secondary magnetite from Pipe Mine (A, B) and the ultramafic-hosted deposits (C, D) plotted on the Ni+Cr vs. Si+Mg discriminant diagram (A, C) and the Al/(Zn+Ca) vs. Cu/(Si+Ca) discriminant diagram (B, D) of Dupuis and Beaudoin (2011). Each symbol represents the EPMA average composition in a thin section of one textural type of secondary magnetite.

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Table 1. List of deposits studied Location

Past production and resources

Age

Related Magmatism

Alexo

Ontario, Canada

52 000 t @ 4.5% Ni; 0.37% Cu; 0.23 %Co

Archean

Komatiite

Montcalm

Ontario, Canada

3.56 Mt @ 1.44% Ni; 0.68% Cu; 0.23 %Co

Archean

Shaw Dome

Ontario, Canada

3.4 Mt @ 1.24% Ni

Thompson Nickel Belt

Manitoba, Canada

Cape Smith : Raglan, Expo and Mesamax

Tectonic settings*2

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Deposit

References

Volcanic arc

Peridotitic komatiite *

Puchtel et al. (2004)

Picritetholeiite

Greenstones belts (rift?)

Cumulate gabbro

Barrie et al. (1990)

Archean

Komatiite

Volcanic arc

Peridotitic komatiite*

Barnes et al. (2007)

150.3 Mt @ 2.32% Ni; 0.16 % Cu; 0.046% Co

Paleoproterozoic

Komatiite

Rifted continental margin

Variably serpentinized dunite to pyroxenite*

Layton-Matthews et al. (2007)

Québec, Canada

24.7 Mt @ 2.72% Ni; 0.7% Cu; 0.054% Co

Paleoproterozoic

Basaltickomatiite

Rifted continental margin

Differentiated ultramafic intrusions*

Barnes et al. (1982)

Blue Lake

Québec, Canada

4.03 Mt @ 0.52% Ni; 0.85% Cu

Paleoproterozoic

Basaltickomatiite

Rifted continental margin

Peridotite and gabbro

Beaudoin et al. (1990)

Pechenga

Kola Peninsula, Russia

339 Mt @ 1.18% Ni; 0.63% Cu; 0.045% Co

Mesoproterozoic

Ferropicrite

Rifted continental margin

Eagle

Michigan, USA

3.6 Mt @ 3.8% Ni; 2.9% Cu

Mesoproterozoic

Picritetholeiite

Continental rift

Jinchuan

Gansu Province, China

515 Mt @ 1.06% Ni; 0.75% Cu; 0.019% Co

Mesoproterozoic

Picritetholeiite

Rifted continental margin

Dunite, lherzolite, olivine websterite and websterite*

Ripley et al. (2005)

Sudbury

Ontario, Canada

1648 Mt @ 1.2% Ni; 1.08% Cu; 0.038% Co

Mesoproterozoic

Impact Melt (andesite)

Meteoric impact

Norite, felsic norite, quartz gabbro and granophyre

Ames et al. (2002) Dare et al. (2012)

Nebo-Babel

West Musgrave, Australia

392 Mt @ 0.3% Ni; 0.33% Cu

Neoproterozoic

Flood basalt

Volcanic arc

Gabbronorite

Seat et al. (2009)

Newfoundland, Canada

142 Mt @ 1.59% Ni; 0.85% Cu; 0.09% Co

Neoproterozoic

Anorthositetroctolite

Continental rift

Troctolite

Naldrett et al. (2000)

Voisey’s Bay

AC

CE

PT ED

MA

NU

SC

RI

Host Rocks

42

Layered intrusions of gabbro-wehrlite and peridotite* Melatroctolite, olivine melagabbro and feldspathic peridotite*

Barnes et al. (2001)

Ding et al. (2010)

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Talnakh

Siberia, Russia

1903 Mt @ 1.77% Ni; 3.57% Cu; 0.061% Co

Permo-Triassic

Flood basalt

Rifted cipontinental margin

AC

CE

PT ED

MA

NU

SC

RI

PT

* Partially serpentinized; *2 Naldrett, 2004.

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Gabbro

Barnes et al. (2000) Krivolutskaya et al. (2001)

ACCEPTED MANUSCRIPT Table 2. Analytical conditions for electron probe microanalysis Element

Crystal

Line

Sin1 Peak

Counting time (s) Background

Peak

Range of detection limits (ppm)

Background

AC CE P

TE

D

MA

NU

SC

RI

PT

V LIF 0.62175 0.61575 60 20 49-56 K Cr LIF 0.56869 0.56369 60 20 47-55 K Zn LLIF 0.35633 0.35133 20 15 102-103 K Cu LLIF 0.38247 0.37747 20 20 81-82 K Ni LLIF 0.41167 0.40567 20 20 61-62 K Mn LLIF 0.52210 0.51460 20 20 40-42 K K LPET 0.42723 0.42223 20 20 14-15 K Sn LPET 0.41117 0.40817 20 20 42-43 L Ca LPET 0.38387 0.37787 20 20 15-19 K Ti LPET 0.31447 0.30847 20 20 19-25 K Al TAP 0.32458 0.31858 20 20 18-21 K Si TAP 0.27737 0.27237 40 20 15-19 K Mg TAP 0.38497 0.37897 40 20 22-25 K 1 =2d Sin, where  is the wavelength and d is the interplanar distance of the analytical crystal.

44

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Table 3. Comparison between the two LA-ICP-MS instrumental procedures

LA-ICPMS Resonetics M-50 excimer

Rd4

11618

45

49

51

52

20787

31

87615

9603

1172

2125

241

11561 7139 62 100 10551 1319 13

20349 3821 19 98 19572 2642 13

32 34 107 103 32 5 17

93077 79439 85 106 76984 8787 11

9005 1381 15 94 9234 462 5

1299 224 17 111 1396 77 6

2024 627 31 95 2141 123 6

294 26 9 122 292 26 9

91

94

103

88

96

119

121

Ti

V

Cr

55

Mn

59

Co

101

60

Ni

65

Cu

573

66

Zn

178

Hf

181

1

Ta

33

588

0.58

0.07

572 68 12 100 641 87 14

45 36 81 136 39 24 61

590 137 23 100 491 72 15

1.46 2.21 152 252 0.92 0.18 20

0.43 1.58 370 614 0.18 0.12 65

112

118

84

159

257

PT

Average Std2 Rsd3 Rd4 Average Std2 Rsd3

Sc

Al

RI

LA-ICPMS Thermo X7

27

Mg

SC

Monitor BC-281

24

NU

Elements (ppm)

AC CE P

TE

D

MA

Magnetite from Upper Zone (Magnet Heights) of the Bushveld Complex, South Africa, Barnes et al (2004), working value published in Dare et al. (2012); 2 standard deviation; 3 relative standard deviation (100*std/average; %); 4 relative difference to working value (average/concentration; %). Working values for BC28, a natural massive magnetite from Magnet Heights, Bushveld Complex published in Barnes et al. (2004), taken from Dare et al. (2012): whole rock INAA data recalculated to 100% magnetite by a factor of 1.07

45

AC CE P

TE

D

MA

NU

SC

RI

PT

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46

AC CE P

TE

D

MA

NU

SC

RI

PT

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AC CE P

TE

D

MA

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SC

RI

PT

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AC CE P

TE

D

MA

NU

SC

RI

PT

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AC CE P

TE

D

MA

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SC

RI

PT

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AC CE P

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D

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SC

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PT

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AC CE P

TE

D

MA

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SC

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PT

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AC CE P

TE

D

MA

NU

SC

RI

PT

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AC CE P

TE

D

MA

NU

SC

RI

PT

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AC CE P

TE

D

MA

NU

SC

RI

PT

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55

AC CE P

TE

D

MA

NU

SC

RI

PT

ACCEPTED MANUSCRIPT

56

AC CE P

TE

D

MA

NU

SC

RI

PT

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57

AC CE P

TE

D

MA

NU

SC

RI

PT

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AC CE P

TE

D

MA

NU

SC

RI

PT

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59

ACCEPTED MANUSCRIPT Highlights

PT

RI SC NU MA D TE

-

Representative magnetite samples from 13 major Ni-Cu-PGE deposits Minor and trace element composition of magnetite by EPMA and LA-ICP-MS Magnetite trace element composition is mainly controlled by sulfide liquid differentiation Compositional difference between primary and secondary magnetite Discriminant diagram for primary magnetite for Ni-Cu-PGE deposits

AC CE P

-

60