Magmatic (Ni-Cu-Cr-PGE) mineral systems: ore forming processes and geodynamic setting 971

Table 2. Representative compositional microprobe analyses of PGM inclusions in chromite from the Central Vardar Zone, Kosovo territory.

6 Summary and conclusions Investigated chromitites show semi-massive (68-85 vol% chromite) cataclastic texture. They mainly consist of unaltered chromite with primary and secondary silicates that fill interstices and fissures. Moreover, small amounts of magnetite and pyrophanite were described in samples. Numerous fractures and microcracks are evidence of late-stage dynamic metamorphism.

Alteration of chromite grains was limited to some thin rims bordering the chromite crystals or the cracks within them. Recrystallised chromites in altered area are richer in iron and/or aluminium and poorer in chromium. Hence, it is deduced that primary crystallization conditions and ores origin were preserved within the Kosovo chromitites.

PGE mineralization associated with chromitites is represented by PGE-bearing sulphides (laurite) and alloys (isoferroplatinum, Os-Ir alloy). The euhedral shape of PGM suggests primary formation of inclusions during chromite crystallization.

The present data reveal new PGM occurrences within ophiolitic fragments related to the Vardar and Mirdita-Gjakove zones. This may open up new horizon for PGE

exploration in Kosovo. However, further research is needed to characterize chromitites and their associated PGE mineralization within the Central Vardar Unit. Acknowledgements The authors are very grateful to Gabriela Kozub-Budzyń, for her great help during the electron microprobe analyses. The investigations were partly sponsored by AGH grant 11.140.320. and the ReAlloys Company. References Beak Consultants GmbH (2006) Geological map of Kosovo 1:200

000. Independent Commission for Mines and Minerals. Online Version, available at: http://www.arsi-group.com/wp-content/uploads/2013/04/Geological_Map_Kosovo.pdf Last accessed: 27 Feb 2015

Elezaj Z (2009) Geodynamic evolution of Kosovo during Triassic and Jurassic. Yerbilimleri 30(2):113-126

Elezaj Z, Korda A (2008) Gjeologjia e Kosoves. Printing Press, Universitetit të Prishtinës, Prishtinë

Elezaj Z, Kodra A (2012) Geology of Kosova. Printing Press, Ministria e Zhvillimit Ekonomik, Prishtinë

Kodra A, Gjata K, Xhomo A (2000) Tectonic history of Mirdita oceanic basin (Albania). Buletini Shkencave Gjeologjike 1: 5-26

Platinum-Group Element Abundances in Pyrite from the Main Sulfide Zone, Great Dyke, Zimbabwe Rubén Piña Dpto. Cristalografía y Mineralogía, Fac. Ciencias Geológicas, Universidad Complutense de Madrid, Spain Fernando Gervilla Dpto. Mineralogía y Petrología and Instituto Andaluz de Ciencias de la Tierra, Fac. Ciencias, Universidad de Granada-CSIC, Granada, Spain Sarah-Jane Barnes Sciences de la Terre, Université du Québec à Chicoutimi, Canada Thomas Oberthür Bundesanstalt für Geowissenschaften und Rohstoffe, Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany Rosario Lunar Dpto. Cristalografía y Mineralogía, Fac. Ciencias Geológicas, Universidad Complutense de Madrid, and Instituto de Geociencias IGEO (UCM-CSIC), Madrid, Spain Abstract. The Main Sulfide Zone (MSZ) of the Great Dyke of Zimbabwe hosts the world’s second largest reserve of platinum-group elements (PGE). It comprises a sulfide assemblage made up of pyrrhotite, pentlandite, chalcopyrite and minor pyrite. Several studies have highlighted that pyrite can be an important carrier of PGE and therefore, we have measured PGE and other trace element abundances in pyrite of the MSZ from the Hartley, Ngezi, Unki and Mimosa mines by LA-ICP-MS. Pyrite occurs as individual euhedral or subhedral grains or clusters of crystals mostly within chalcopyrite and pentlandite (occasionally in form of symplectitic intergrowths) and is generally absent within pyrrhotite. At Hartley and Ngezi, pyrite contains higher Os, Ir, Ru, Rh and Pt contents than co-existing pyrrhotite, pentlandite and chalcopyrite from the same sulfide aggregate. In contrast, at Mimosa and Unki, PGE values in pyrite are low and similar to coexisting sulfides. Pentlandite is always the main sulfide carrying Pd. Although the origin of pyrite in this type of mineralization is commonly attributed to the activity of hydrothermal fluids, we suggest that pyrite may have formed by late, low temperature (< 300ºC) decomposition of residual Ni-rich mss. Keywords. Pyrite, sulfides, platinum-group elements, LA-ICP-MS, Great Dyke, Zimbabwe 1 Introduction Pyrite is a relatively common minor sulfide in Ni-Cu-PGE magmatic sulfide deposits, however its origin is poorly understood. Experimental studies show that pyrite may form by decomposition of S-rich monosulfide solid solution (mss) at temperatures below 700ºC (Naldrett et al. 1967; Craig 1973). However, this process does not seem to be common because most natural sulfide melts do not contain the required amount of sulfur. Alternatively, it has been proposed that pyrite forms as result of partial to total replacement of pre-existing sulfides (mainly, pyrrhotite) due to the activity of late magmatic, hydrothermal and/or metamorphic fluids (e.g., Djon and Barnes 2012; Piña et al. 2013; Smith et

al. 2014; Holwell et al. 2014; Vukmanovic et al. 2014). Recently, several studies indicate that pyrite from Ni-

Cu-(PGE) magmatic sulfide deposits can host significant amounts of PGE (e.g., Oberthür et al. 1997; Dare et al. 2011; Djon and Barnes 2012; Piña et al. 2013; Smith et al. 2014). These studies show that pyrite contains similar amounts of Os, Ir, Ru and Rh to coexisting pyrrhotite and pentlandite, and is the only base metal sulfide hosting significant amounts of Pt. Detection of trace amounts of Pt in pyrite is especially relevant because Pt is typically not present in the other base metal sulfides but occurs as discrete platinum-group minerals (PGM, e.g., sperrylite PtAs2).

In this contribution, we report new PGE, Au, Ag, Co, Se, As, Te, Bi and Sb abundances in pyrite from the Main Sulfide Zone in four mines of the Great Dyke of Zimbabwe. In addition to assessing the role of pyrite as carrier of PGE in this mineralization, we use trace element contents and textures of pyrite to discuss the possible mechanisms involving in its formation.

2 Pyrite from the Great Dyke of Zimbabwe The Great Dyke of Zimbabwe (2575.4 ± 0.7 Ma, Oberthür et al. 2002) represents the world’s second largest reserve of PGE after Bushveld Complex in South Africa (Oberthür 2011). It is a 550 km long and 4 to 11 km wide, linear, mafic and ultramafic layered intrusion emplaced into Archean granites and greenstone belts of the Zimbabwe craton. The Great Dyke shows a well-defined igneous stratigraphy divided into a lower Ultramafic Sequence (dunite, harzburgite and pyroxenite) and an upper Mafic Sequence (gabbro and norite) (Wilson and Prendergast 1989). The economic PGE mineralization is restricted to disseminations of intercumulus sulfides (from 0.1 to 10 vol. %) in the several meters-thick Main Sulfide Zone (MSZ) situated in pyroxenites some meters below the transition from the Ultramafic to the Mafic Sequence. Based on the degree of sulfide mineralization and PGE ratios, the MSZ is

MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 3972

divided in a basal PGE-rich subzone that slightly overlaps with an overlying base metal sulfide (BMS)-rich subzone.

Pyrite mainly occurs in the upper part of the PGE-rich subzone and in the BMS-rich subzone of the MSZ. Sulfides occur as disseminations of polymineralic aggregates interstitial to silicates. These aggregates are made up of pyrrhotite, equal amounts of pentlandite and chalcopyrite and minor pyrite. Pentlandite mostly forms coarse grains and minor flame-shaped exsolution lamellae in pyrrhotite, and chalcopyrite typically occurs along the peripheries of sulfide aggregates or as isolated monomineralic grains. According to Oberthür (2011), the presence of pyrite indicates increasing fS2 up sequence in the MSZ. 2.1 Pyrite textures The studied samples come from the Hartley, Ngezi, Unki and Mimosa mines (located from north to south along the Great Dyke). Samples are located just above the Pt-peak of the MSZ. At Unki and Mimosa, pyrite forms clusters of small individual euhedral to subhedral crystals within chalcopyrite, pentlandite and, to a lesser extent, pyrrhotite (Fig. 1a). At Hartley and Ngezi, pyrite forms small individual grains within pentlandite and chalcopyrite (Fig. 1b). Locally, these grains appear to have coalesced during growth, resulting in poikiloblastic aggregates that enclose chalcopyrite and pentlandite grains. Some pyrite grains seem to form symplectitic intergrowths with chalcopyrite and pentlandite (Fig. 1c). 2.2 PGE and other trace element abundances Trace elements (PGE, Au, Ag, Re, Co, Se, As, Te, Bi and Sb) were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at LabMaTer, Université du Quebec à Chicoutimi (UQAC), Canada, using a Resonetics Resolution 193 nm Excimer laser with a M-50 ablation cell and an Agilent 7,700x quadrupole mass spectrometer, and following the analytical protocol of Piña et al. (2013).

There are notable differences between trace element abundances of pyrite from Hartley and Ngezi mines (located in the North Chamber of the Great Dyke) and pyrite from Mimosa and Unki mines (located in the South Chamber) (Fig. 2). At Hartley and Ngezi, pyrite is relatively rich in PGE with contents ranging from 0.3 to 99 ppm Pt, 0.7 to 61.1 ppm Rh (both Pt and Rh contents are typically > 10 ppm), 1.2 to 47.1 ppm Ru, 0.1 to 7.8 ppm Os and 1.0 to 20.2 ppm Ir. These values are higher than those in co-existing pyrrhotite, pentlandite and chalcopyrite (Fig. 2a). PGE abundances in pyrite from Hartley and Ngezi mines are in agreement with previous contents reported by Oberthür et al. (1997) from the Hartley mine using micro-pixe (40 ppm Ru, 10 ppm Rh, 9 ppm Pd and 233 ppm Pt on average). At Mimosa and Unki mines, PGE concentrations in pyrite are low (< 0.11 ppm Pt, < 0.34 ppm Rh, < 2.5 ppm Ru, < 0.37 ppm Ir and < 0.40 ppm Os) and similar to those in pyrrhotite and pentlandite (Fig. 2b). Palladium in pyrite is commonly below 2 ppm in all studied mines except in one sample from Ngezi which contained up to 60.4 ppm

Pd. Osmium, Ir, Ru, Rh and Pt are positively correlated each other, whereas Pd poorly correlates with all other PGE. In contrast to the PGE, Au, Te, Ag, Sb and Bi contents are higher in pyrite from Mimosa and Unki than in pyrite from Hartley and Ngezi (Fig. 2). Gold is positively correlated with Te and Bi. In the pyrite samples from Hartley and Ngezi, Pt correlates with Au.

Figure 1. Representative photomicrographs in reflected-light, showing pyrite from the Mimosa (a), Ngezi (b) and Hartley (c) mines. Pyrite (Py) occurs mostly within pentlandite (Pn) and chalcopyrite (Cpy) and not within pyrrhotite (Po).

Trace element distribution between co-existing pyrrhotite, pentlandite, chalcopyrite and pyrite in a single polymineralic aggregate from Ngezi mine is shown in Figure 3. Platinum, Os, Ir, Ru, Rh, Co, Se and Bi are preferentially concentrated in pyrite (as also indicated by the primitive mantle-normalized profiles of Figure 2), whereas Pd prefers pentlandite.

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Figure 2. Primitive mantle-normalized trace element average patterns as determined by LA-ICP-MS for pyrite (black), pyrrhotite (red), pentlandite (green) and chalcopyrite (violet) from: a) Hartley (solid lines) and Ngezi (dashed lines), and b) Unki (solid lines) and Mimosa (dashed lines). Elements are ordered from left to right according to decreasing mss compatibility. 3 Discussion 3.1 Mechanisms of pyrite formation The formation of pyrite in Ni-Cu-(PGE) magmatic sulfide deposits is commonly attributed to the activity of late magmatic or hydrothermal fluids that trigger the replacement of pyrrhotite by pyrite due to addition of S (i.e., increasing fS2) or removal of Fe (Oberthür 2011; Djon and Barnes 2012; Piña et al. 2013; Smith et al. 2014; Holwell et al. 2014; Duran et al. 2014). Although previous studies described that pyrite in the MSZ of the Great Dyke predominantly occurs within pyrrhotite (Oberthür et al. 2003) as observed in other magmatic sulfide ore deposits (e.g., McCreedy in Sudbury, Canada, Dare et al. 2011; Aguablanca, Spain, Piña et al. 2013), in our study, pyrite is typically closely associated with pentlandite and chalcopyrite and generally absent in pyrrhotite. This feature is relevant for the origin of pyrite. If hydrothermal fluids had played some role in the formation of pyrite, the alteration mechanism should successfully explain the selective replacement of pentlandite and chalcopyrite (not pyrrhotite) by pyrite, since secondary pyrite typically forms at the expense of pyrrhotite. In addition, some studies have suggested that secondary pyrite inherits PGE contents from the replaced sulfides (Dare et al. 2011; Piña et al. 2013; Smith et al. 2014). Pyrite within chalcopyrite has relatively high Os, Ir, Ru and Rh contents (Fig. 4). However, host chalcopyrite does not contain Os, Ir or Ru (typically below the detection limit) and thus these elements could not be inherited directly from chalcopyrite. Therefore, we suggest that some other mechanism different to

hydrothermal alteration must be considered to explain the pyrite formation.

Ir

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Figure 3. Trace element maps of coexisting pyrrhotite, pentlandite, chalcopyrite and pyrite from Ngezi mine. Mapping was carried out by line scans ablating the whole aggregate. The maps show semi-quantitative values. All elements shown are preferentially concentrated into pyrite with the exception of Pd that concentrates in pentlandite. Osmium and Ru (not shown) follow the same distribution as Ir and Rh.

The pyrite textures observed in the samples from Hartley and Ngezi mines are quite similar to those described for pyrites from the Keivitsansarvi Ni-Cu-PGE sulfide deposit in northern Finland (Gervilla and Kojonen 2002), from orogenic peridotites of the French Pyrenees (Lorand and Alard 2011) and from the UG-2 reef in the Bushveld Complex, South Africa (Naldrett et al. 2009). In all these cases, pyrite mainly occurs associated with pentlandite and/or chalcopyrite forming locally fine symplectitic intergrowths. Experimental studies in the system Fe-Ni-S (Craig 1973; Misra and Fleet 1973) demonstrate pyrite only coexists with Ni-rich pentlandite at temperatures below 300-250ºC. In the UG-2 reef, Naldrett et al. (2009) indicate that loss of Fe to the chromite from sulfides on cooling, accompanied by significant rise in fS2, caused the formation of pyrite-pentlandite assemblages by decomposition of Ni-rich mss at temperatures below 250ºC. At Keivitsansarvi, the reaction of Fe-rich vaesite (NiS2) with the coexisting mss on cooling gave rise to a mss richer in Ni from which pyrite and pentlandite ultimately formed. In contrast,

Magmatic (Ni-Cu-Cr-PGE) mineral systems: ore forming processes and geodynamic setting 973

divided in a basal PGE-rich subzone that slightly overlaps with an overlying base metal sulfide (BMS)-rich subzone.

Pyrite mainly occurs in the upper part of the PGE-rich subzone and in the BMS-rich subzone of the MSZ. Sulfides occur as disseminations of polymineralic aggregates interstitial to silicates. These aggregates are made up of pyrrhotite, equal amounts of pentlandite and chalcopyrite and minor pyrite. Pentlandite mostly forms coarse grains and minor flame-shaped exsolution lamellae in pyrrhotite, and chalcopyrite typically occurs along the peripheries of sulfide aggregates or as isolated monomineralic grains. According to Oberthür (2011), the presence of pyrite indicates increasing fS2 up sequence in the MSZ. 2.1 Pyrite textures The studied samples come from the Hartley, Ngezi, Unki and Mimosa mines (located from north to south along the Great Dyke). Samples are located just above the Pt-peak of the MSZ. At Unki and Mimosa, pyrite forms clusters of small individual euhedral to subhedral crystals within chalcopyrite, pentlandite and, to a lesser extent, pyrrhotite (Fig. 1a). At Hartley and Ngezi, pyrite forms small individual grains within pentlandite and chalcopyrite (Fig. 1b). Locally, these grains appear to have coalesced during growth, resulting in poikiloblastic aggregates that enclose chalcopyrite and pentlandite grains. Some pyrite grains seem to form symplectitic intergrowths with chalcopyrite and pentlandite (Fig. 1c). 2.2 PGE and other trace element abundances Trace elements (PGE, Au, Ag, Re, Co, Se, As, Te, Bi and Sb) were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at LabMaTer, Université du Quebec à Chicoutimi (UQAC), Canada, using a Resonetics Resolution 193 nm Excimer laser with a M-50 ablation cell and an Agilent 7,700x quadrupole mass spectrometer, and following the analytical protocol of Piña et al. (2013).

There are notable differences between trace element abundances of pyrite from Hartley and Ngezi mines (located in the North Chamber of the Great Dyke) and pyrite from Mimosa and Unki mines (located in the South Chamber) (Fig. 2). At Hartley and Ngezi, pyrite is relatively rich in PGE with contents ranging from 0.3 to 99 ppm Pt, 0.7 to 61.1 ppm Rh (both Pt and Rh contents are typically > 10 ppm), 1.2 to 47.1 ppm Ru, 0.1 to 7.8 ppm Os and 1.0 to 20.2 ppm Ir. These values are higher than those in co-existing pyrrhotite, pentlandite and chalcopyrite (Fig. 2a). PGE abundances in pyrite from Hartley and Ngezi mines are in agreement with previous contents reported by Oberthür et al. (1997) from the Hartley mine using micro-pixe (40 ppm Ru, 10 ppm Rh, 9 ppm Pd and 233 ppm Pt on average). At Mimosa and Unki mines, PGE concentrations in pyrite are low (< 0.11 ppm Pt, < 0.34 ppm Rh, < 2.5 ppm Ru, < 0.37 ppm Ir and < 0.40 ppm Os) and similar to those in pyrrhotite and pentlandite (Fig. 2b). Palladium in pyrite is commonly below 2 ppm in all studied mines except in one sample from Ngezi which contained up to 60.4 ppm

Pd. Osmium, Ir, Ru, Rh and Pt are positively correlated each other, whereas Pd poorly correlates with all other PGE. In contrast to the PGE, Au, Te, Ag, Sb and Bi contents are higher in pyrite from Mimosa and Unki than in pyrite from Hartley and Ngezi (Fig. 2). Gold is positively correlated with Te and Bi. In the pyrite samples from Hartley and Ngezi, Pt correlates with Au.

Figure 1. Representative photomicrographs in reflected-light, showing pyrite from the Mimosa (a), Ngezi (b) and Hartley (c) mines. Pyrite (Py) occurs mostly within pentlandite (Pn) and chalcopyrite (Cpy) and not within pyrrhotite (Po).

Trace element distribution between co-existing pyrrhotite, pentlandite, chalcopyrite and pyrite in a single polymineralic aggregate from Ngezi mine is shown in Figure 3. Platinum, Os, Ir, Ru, Rh, Co, Se and Bi are preferentially concentrated in pyrite (as also indicated by the primitive mantle-normalized profiles of Figure 2), whereas Pd prefers pentlandite.

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Figure 2. Primitive mantle-normalized trace element average patterns as determined by LA-ICP-MS for pyrite (black), pyrrhotite (red), pentlandite (green) and chalcopyrite (violet) from: a) Hartley (solid lines) and Ngezi (dashed lines), and b) Unki (solid lines) and Mimosa (dashed lines). Elements are ordered from left to right according to decreasing mss compatibility. 3 Discussion 3.1 Mechanisms of pyrite formation The formation of pyrite in Ni-Cu-(PGE) magmatic sulfide deposits is commonly attributed to the activity of late magmatic or hydrothermal fluids that trigger the replacement of pyrrhotite by pyrite due to addition of S (i.e., increasing fS2) or removal of Fe (Oberthür 2011; Djon and Barnes 2012; Piña et al. 2013; Smith et al. 2014; Holwell et al. 2014; Duran et al. 2014). Although previous studies described that pyrite in the MSZ of the Great Dyke predominantly occurs within pyrrhotite (Oberthür et al. 2003) as observed in other magmatic sulfide ore deposits (e.g., McCreedy in Sudbury, Canada, Dare et al. 2011; Aguablanca, Spain, Piña et al. 2013), in our study, pyrite is typically closely associated with pentlandite and chalcopyrite and generally absent in pyrrhotite. This feature is relevant for the origin of pyrite. If hydrothermal fluids had played some role in the formation of pyrite, the alteration mechanism should successfully explain the selective replacement of pentlandite and chalcopyrite (not pyrrhotite) by pyrite, since secondary pyrite typically forms at the expense of pyrrhotite. In addition, some studies have suggested that secondary pyrite inherits PGE contents from the replaced sulfides (Dare et al. 2011; Piña et al. 2013; Smith et al. 2014). Pyrite within chalcopyrite has relatively high Os, Ir, Ru and Rh contents (Fig. 4). However, host chalcopyrite does not contain Os, Ir or Ru (typically below the detection limit) and thus these elements could not be inherited directly from chalcopyrite. Therefore, we suggest that some other mechanism different to

hydrothermal alteration must be considered to explain the pyrite formation.

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Figure 3. Trace element maps of coexisting pyrrhotite, pentlandite, chalcopyrite and pyrite from Ngezi mine. Mapping was carried out by line scans ablating the whole aggregate. The maps show semi-quantitative values. All elements shown are preferentially concentrated into pyrite with the exception of Pd that concentrates in pentlandite. Osmium and Ru (not shown) follow the same distribution as Ir and Rh.

The pyrite textures observed in the samples from Hartley and Ngezi mines are quite similar to those described for pyrites from the Keivitsansarvi Ni-Cu-PGE sulfide deposit in northern Finland (Gervilla and Kojonen 2002), from orogenic peridotites of the French Pyrenees (Lorand and Alard 2011) and from the UG-2 reef in the Bushveld Complex, South Africa (Naldrett et al. 2009). In all these cases, pyrite mainly occurs associated with pentlandite and/or chalcopyrite forming locally fine symplectitic intergrowths. Experimental studies in the system Fe-Ni-S (Craig 1973; Misra and Fleet 1973) demonstrate pyrite only coexists with Ni-rich pentlandite at temperatures below 300-250ºC. In the UG-2 reef, Naldrett et al. (2009) indicate that loss of Fe to the chromite from sulfides on cooling, accompanied by significant rise in fS2, caused the formation of pyrite-pentlandite assemblages by decomposition of Ni-rich mss at temperatures below 250ºC. At Keivitsansarvi, the reaction of Fe-rich vaesite (NiS2) with the coexisting mss on cooling gave rise to a mss richer in Ni from which pyrite and pentlandite ultimately formed. In contrast,

MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 3974

Lorand and Alard (2011) suggest that the formation of pyrite intergrowth with pentlandite and chalcopyrite was due to subsolidus sulfurization processes that increased the fS2 of the system. They suggest that pentlandite may be first sulfurized into mss at temperatures above 300ºC and then the mss broke down into pentlandite + pyrite symplectites upon cooling below 300ºC.

It is suggested here that in the MSZ, pyrite may form during low-temperature equilibration of Ni-rich mss coexisting with intermediate solid solution (iss). Pentlandite composition is closely related to sulfides with which the pentlandite is associated. The tentative <135ºC Fe-Ni-S phase diagram indicates that pentlandite coexisting with pyrite is relatively rich in Ni (Naldrett et al. 2009). Pentlandite in the pentlandite-pyrite assemblages from the Great Dyke has Fe:(Fe+Ni) wt. % ratios ranging from 0.46 to 0.49. These composition ranges are similar to those observed in pentlandite equilibrated with pyrite and pyrrhotite in the Marbridge deposit, Quebec (Graterol and Naldrett 1971).

Figure 4. Ruthenium versus Ir contents for pyrite from the Great Dyke, Zimbabwe. 3.2 Pyrite as sulfide hosting PGE Previous studies have indicated that most Pd and Rh are hosted in pentlandite, whereas Pt is dominantly present in form of PGM in the MSZ (Oberthür et al. 2003). Our results indicate that among all base metal sulfides, pyrite is individually the sulfide phase hosting the highest contents of PGE (with the exception of Pd that is preferentially concentrated in pentlandite). Mass balance calculation to determine the percentage of each PGE present in each sulfide has not been carried out to date and, thus, the importance of pyrite as carrier of PGE in the mineralization is not quantified. Nevertheless, since most of PGE appear show higher preference for pyrite when present (Fig. 3), we envisage that pyrite may account for significant amounts of PGE (particularly, Rh and Pt) in those zones with the highest modal abundances of pyrite. Acknowledgements We would like to thank Dany Savard and Sadia Mehdi for their assistance during the laser ablation analyses. The research work was financed by the Spanish research project CGL2010-17668, the Canada Research Chair in Magmatic Metallogeny and the German Federal Institute for Geosciences and Natural Resources (BGR).

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Proterozoic Black-Shales: a Source of Sulfur and Semi-Metals for the Formation of Ni-Cu-Platinum-Group Element Deposits Nadège Samalens, Sarah-Jane Barnes, Edward William Sawyer Unité des Sciences de la Terre, Université du Québec à Chicoutimi, G7H 2B1, QC, Canada Abstract. The basal unit of the Duluth Complex (Minnesota, USA) contains Ni-Cu sulfides. The S in these is thought to be derived from a sulfide-rich black shale unit known as the Bedded Pyrrhotite Unit (BPU) a stratigraphic unit within part of the host rocks. Numerous boreholes have been drilled through the basal unit of the Duluth Complex into the country rock and these allow investigation of the processes whereby S in the country rock was transferred to the mafic magma. The basal unit contains partially melted xenoliths of the Bedded Pyrrhotite Unit and the mafic rocks surrounding these show a progressive decreases in: S/Se, δ34S, and semi-metal contents away from the xenoliths. The partial melt of the xenoliths contained small sulfide droplets. We suggest that sulfur and semi-metals (Sb, Bi, As and Pb) were transferred to the mafic magma via the droplets. The sulfide droplets then equilibrated with the mafic magma and collect Ni, Cu and platinum-group elements (PGE). During the final stage of crystallization of the sulfide liquid the semi-metals combined with the PGE to form platinum-group minerals. Keywords. Duluth Complex, Proterozoic black-shales, in-situ assimilation, Ni deposits, magmatic sulfides, LA-ICP-MS 1 Introduction Most of world’s Ni-Cu-PGE deposits were formed after the addition of sulfur to the magma from an external source in the country rocks (Ripley and Li 2013). It is generally thought that this S is derived from black shales. Black shales are also rich in semi-metals (As, Sb, Te and Bi). Interestingly, these elements are necessary for the formation of many of the platinum-group minerals (PGM). Thus the question arises: are these elements also derived from the black-shales?

The Partridge River Intrusion of the Duluth Complex represents an ideal and well-documented location for studying contamination processes. Mineralization is found in the basal part of the intrusion close to its contact with sedimentary country rocks of the Virginia Formation (Severson and Hauck 2008).

A particular unit in the Proterozoic Virginia Formation, called Bedded Pyrrhotite Unit, is composed of pyrrhotite-rich black-shales. Thériault and Barnes (1998) proposed that sulfur was transferred to the mafic magma after partial melting of entrained xenoliths in the magma. Subsequently, the Bedded Pyrrhotite Unit has been identified as the source of the sulfur that contaminated the mafic magma (Queffurus and Barnes 2014). However, the exact mechanism by which the S is transferred by the partial melt is poorly understood.

Therefore, we have carried a petrographic, mineralogical and geochemical study of the interactions

between xenoliths of the Bedded Pyrrhotite Unit and the enclosing mafic magma in the basal part of the Partridge River Intrusion. 2 Geological setting The Duluth Complex is a Keweenawan-age (1100 Ma) mafic complex located in Minnesota, USA. The Complex is composed of several mafic intrusions related to a mantle plume that was located beneath the mid-continental rift system (Ojakangas et al. 2001). The country rocks to the Duluth Complex consist of sediments ranging from banded iron formation through carbonates, but are predominately greywackes and pelites. Of particular interest is the Bedded Pyrrhotite Unit, a sulfide-rich black shale believed to have been deposited in restricted anoxic basins. This unit is approximately 200 m thick, but has a sporadic distribution.

The Duluth Complex can be divided into three mafic intrusions: the Partridge River Intrusion, the Bathtub Intrusion and the South Kawishiwi Intrusion. The basal unit (Unit I) of the Partridge River Intrusion contains abundant xenoliths of the host-rocks and most of the sulfide mineralization (Severson and Hauck 2008). Unit 1 is composed of the following lithologies: norite, gabbronorite and troctolite, from the base to the center of the intrusion. Norites are found near to the contact with the Virginia Formation country rocks and around xenoliths of country rock. Disseminated to massive Ni-Cu sulfides occur in Unit I. Sulfides are commonly found around the xenoliths.

Samples were chosen on from boreholes that crossed the contact between the Virginia Formation and Partridge River Intrusion at the Dunka Road, Mesaba and Wetlegs deposits.

3 Results 3.1 Petrography Temperatures of greater than 800°C have been reported from diatexite migmatites found close to the contact with the intrusion (Sawyer 2014).

The Bedded Pyrrhotite Unit is divided into unmetamorphosed and metamorphosed depending upon whether inside or outside of the contact aureole that is developed around the intrusion. Unmetamorphosed Bedded Pyrrhotite Unit, outside the contact aureole, contains pyrite and few grains of chalcopyrite.