Platinum-Group Element Abundances in Pyrite from the Main Sulfide Zone, Great Dyke, Zimbabwe

Table 2. Representative compositional microprobe analyses of PGM inclusions in chromite from the Central Vardar Zone, Kosovo territory. 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 al. 2014; Holwell et al. 2014; Vukmanovic et al. 2014). exploration in Kosovo. However, further research is Dyke of Zimbabwe hosts the world’s second largest Recently, several studies indicate that pyrite from Ni- needed to characterize chromitites and their associated 6 Summary and conclusions reserve of platinum-group elements (PGE). It comprises Cu-(PGE) magmatic sulfide deposits can host significant PGE mineralization within the Central Vardar Unit. a sulfide assemblage made up of pyrrhotite, pentlandite, amounts of PGE (e.g., Oberthür et al. 1997; Dare et al. Investigated chromitites show semi-massive (68-85 chalcopyrite and minor pyrite. Several studies have 2011; Djon and Barnes 2012; Piña et al. 2013; Smith et vol% chromite) cataclastic texture. They mainly consist highlighted that pyrite can be an important carrier of PGE Acknowledgements al. 2014). These studies show that pyrite contains similar of unaltered chromite with primary and secondary and therefore, we have measured PGE and other trace amounts of Os, Ir, Ru and Rh to coexisting pyrrhotite silicates that fill interstices and fissures. Moreover, small The authors are very grateful to Gabriela Kozub-Budzyń, element abundances in pyrite of the MSZ from the and pentlandite, and is the only base metal sulfide amounts of magnetite and pyrophanite were described in for her great help during the electron microprobe Hartley, Ngezi, Unki and Mimosa mines by LA-ICP-MS. Pyrite occurs as individual euhedral or subhedral grains hosting significant amounts of Pt. Detection of trace samples. Numerous fractures and microcracks are analyses. The investigations were partly sponsored by or clusters of crystals mostly within chalcopyrite and amounts of Pt in pyrite is especially relevant because Pt evidence of late-stage dynamic metamorphism. AGH grant 11.140.320. and the ReAlloys Company. pentlandite (occasionally in form of symplectitic is typically not present in the other base metal sulfides Alteration of chromite grains was limited to some intergrowths) and is generally absent within pyrrhotite. At but occurs as discrete platinum-group minerals (PGM, thin rims bordering the chromite crystals or the cracks References Hartley and Ngezi, pyrite contains higher Os, Ir, Ru, Rh e.g., sperrylite PtAs2). within them. Recrystallised chromites in altered area are and Pt contents than co-existing pyrrhotite, pentlandite In this contribution, we report new PGE, Au, Ag, Co, richer in iron and/or aluminium and poorer in chromium. Beak Consultants GmbH (2006) Geological map of Kosovo 1:200 and chalcopyrite from the same sulfide aggregate. In Se, As, Te, Bi and Sb abundances in pyrite from the Hence, it is deduced that primary crystallization 000. Independent Commission for Mines and Minerals. Online contrast, at Mimosa and Unki, PGE values in pyrite are Version, available at: http://www.arsi-group.com/wp- Main Sulfide Zone in four mines of the Great Dyke of conditions and ores origin were preserved within the low and similar to coexisting sulfides. Pentlandite is content/uploads/2013/04/Geological_Map_Kosovo.pdf Last Zimbabwe. In addition to assessing the role of pyrite as Kosovo chromitites. always the main sulfide carrying Pd. Although the origin accessed: 27 Feb 2015 of pyrite in this type of mineralization is commonly carrier of PGE in this mineralization, we use trace PGE mineralization associated with chromitites is Elezaj Z (2009) Geodynamic evolution of Kosovo during Triassic attributed to the activity of hydrothermal fluids, we element contents and textures of pyrite to discuss the represented by PGE-bearing sulphides (laurite) and and Jurassic. Yerbilimleri 30(2):113-126 suggest that pyrite may have formed by late, low possible mechanisms involving in its formation. alloys (isoferroplatinum, Os-Ir alloy). The euhedral Elezaj Z, Korda A (2008) Gjeologjia e Kosoves. Printing Press, temperature (< 300ºC) decomposition of residual Ni-rich shape of PGM suggests primary formation of inclusions Universitetit të Prishtinës, Prishtinë Elezaj Z, Kodra A (2012) Geology of Kosova. Printing Press, mss. during chromite crystallization. 2 Pyrite from the Great Dyke of Zimbabwe Ministria e Zhvillimit Ekonomik, Prishtinë The present data reveal new PGM occurrences within Keywords. Pyrite, sulfides, platinum-group elements, LA- Kodra A, Gjata K, Xhomo A (2000) Tectonic history of Mirdita The Great Dyke of Zimbabwe (2575.4 ± 0.7 Ma, ophiolitic fragments related to the Vardar and Mirdita- oceanic basin (Albania). Buletini Shkencave Gjeologjike 1: 5- ICP-MS, Great Dyke, Zimbabwe Oberthür et al. 2002) represents the world’s second Gjakove zones. This may open up new horizon for PGE 26 largest reserve of PGE after Bushveld Complex in South 1 Introduction Africa (Oberthür 2011). It is a 550 km long and 4 to 11 Pyrite is a relatively common minor sulfide in Ni-Cu- km wide, linear, mafic and ultramafic layered intrusion PGE magmatic sulfide deposits, however its origin is emplaced into Archean granites and greenstone belts of poorly understood. Experimental studies show that pyrite the Zimbabwe craton. The Great Dyke shows a well- may form by decomposition of S-rich monosulfide solid defined igneous stratigraphy divided into a lower solution (mss) at temperatures below 700ºC (Naldrett et Ultramafic Sequence (dunite, harzburgite and al. 1967; Craig 1973). However, this process does not pyroxenite) and an upper Mafic Sequence (gabbro and seem to be common because most natural sulfide melts norite) (Wilson and Prendergast 1989). The economic do not contain the required amount of sulfur. PGE mineralization is restricted to disseminations of Alternatively, it has been proposed that pyrite forms as intercumulus sulfides (from 0.1 to 10 vol. %) in the result of partial to total replacement of pre-existing several meters-thick Main Sulfide Zone (MSZ) situated sulfides (mainly, pyrrhotite) due to the activity of late in pyroxenites some meters below the transition from the magmatic, hydrothermal and/or metamorphic fluids Ultramafic to the Mafic Sequence. Based on the degree (e.g., Djon and Barnes 2012; Piña et al. 2013; Smith et of sulfide mineralization and PGE ratios, the MSZ is

Magmatic (Ni-Cu-Cr-PGE) mineral systems: ore forming processes and geodynamic setting 971 divided in a basal PGE-rich subzone that slightly Pd. Osmium, Ir, Ru, Rh and Pt are positively correlated hydrothermal alteration must be considered to explain 100000 overlaps with an overlying base metal sulfide (BMS)- each other, whereas Pd poorly correlates with all other a) Hartley and Ngezi the pyrite formation. rich subzone. PGE. In contrast to the PGE, Au, Te, Ag, Sb and Bi 10000 Pyrite mainly occurs in the upper part of the PGE- contents are higher in pyrite from Mimosa and Unki than 1000 Pn Py rich subzone and in the BMS-rich subzone of the MSZ. in pyrite from Hartley and Ngezi (Fig. 2). Gold is 100 Sulfides occur as disseminations of polymineralic positively correlated with Te and Bi. In the pyrite 10 aggregates interstitial to silicates. These aggregates are samples from Hartley and Ngezi, Pt correlates with Au. Po ppm 1 made up of pyrrhotite, equal amounts of pentlandite and 50 chalcopyrite and minor pyrite. Pentlandite mostly forms a 0,1 Pn Cpy 0.2 mm coarse grains and minor flame-shaped exsolution 0,01 Ir Po 0 lamellae in pyrrhotite, and chalcopyrite typically occurs Se Re Co Ni Os Ir Ru Rh Pt Pd Te Au Cu Ag Pb Cd Sb Bi As Zn

along the peripheries of sulfide aggregates or as isolated 100000 monomineralic grains. According to Oberthür (2011), the b) Unki and Mimosa 10000 presence of pyrite indicates increasing fS2 up sequence in Py the MSZ. 1000 120 250 100 2.1 Pyrite textures 10 Rh Pd 0 0 The studied samples come from the Hartley, Ngezi, Unki 1 and Mimosa mines (located from north to south along 0.2 mm 0,1 the Great Dyke). Samples are located just above the Pt- 0,01 peak of the MSZ. At Unki and Mimosa, pyrite forms Se Re Co Ni Os Ir Ru Rh Pt Pd Te Au Cu Ag Pb Cd Sb Bi As Zn b Cpy clusters of small individual euhedral to subhedral 150 100 crystals within chalcopyrite, pentlandite and, to a lesser Figure 2. Primitive mantle-normalized trace element average extent, pyrrhotite (Fig. 1a). At Hartley and Ngezi, pyrite patterns as determined by LA-ICP-MS for pyrite (black), Pt Se 0 forms small individual grains within pentlandite and pyrrhotite (red), pentlandite (green) and chalcopyrite (violet) 0 from: a) Hartley (solid lines) and Ngezi (dashed lines), and b) chalcopyrite (Fig. 1b). Locally, these grains appear to Py Unki (solid lines) and Mimosa (dashed lines). Elements are have coalesced during growth, resulting in poikiloblastic Po ordered from left to right according to decreasing mss aggregates that enclose chalcopyrite and pentlandite Pn compatibility. grains. Some pyrite grains seem to form symplectitic 100 intergrowths with chalcopyrite and pentlandite (Fig. 1c). 3 Discussion 14000

Bi Co 0

2.2 PGE and other trace element abundances 0 0.2 mm 3.1 Mechanisms of pyrite formation

analytical protocol of Piña et al. (2013). Pn previous studies described that pyrite in the MSZ of the The pyrite textures observed in the samples from There are notable differences between trace element Great Dyke predominantly occurs within pyrrhotite Hartley and Ngezi mines are quite similar to those abundances of pyrite from Hartley and Ngezi mines (Oberthür et al. 2003) as observed in other magmatic described for pyrites from the Keivitsansarvi Ni-Cu-PGE (located in the North Chamber of the Great Dyke) and sulfide ore deposits (e.g., McCreedy in Sudbury, Canada, sulfide deposit in northern Finland (Gervilla and pyrite from Mimosa and Unki mines (located in the Dare et al. 2011; Aguablanca, Spain, Piña et al. 2013), in Kojonen 2002), from orogenic peridotites of the French South Chamber) (Fig. 2). At Hartley and Ngezi, pyrite is Cpy our study, pyrite is typically closely associated with 0.2 mm Pyrenees (Lorand and Alard 2011) and from the UG-2 relatively rich in PGE with contents ranging from 0.3 to pentlandite and chalcopyrite and generally absent in reef in the Bushveld Complex, South Africa (Naldrett et 99 ppm Pt, 0.7 to 61.1 ppm Rh (both Pt and Rh contents pyrrhotite. This feature is relevant for the origin of al. 2009). In all these cases, pyrite mainly occurs are typically > 10 ppm), 1.2 to 47.1 ppm Ru, 0.1 to 7.8 pyrite. If hydrothermal fluids had played some role in the associated with pentlandite and/or chalcopyrite forming ppm Os and 1.0 to 20.2 ppm Ir. These values are higher formation of pyrite, the alteration mechanism should Figure 1. Representative photomicrographs in reflected-light, locally fine symplectitic intergrowths. Experimental than those in co-existing pyrrhotite, pentlandite and showing pyrite from the Mimosa (a), Ngezi (b) and Hartley (c) successfully explain the selective replacement of studies in the system Fe-Ni-S (Craig 1973; Misra and chalcopyrite (Fig. 2a). PGE abundances in pyrite from mines. Pyrite (Py) occurs mostly within pentlandite (Pn) and pentlandite and chalcopyrite (not pyrrhotite) by pyrite, Fleet 1973) demonstrate pyrite only coexists with Ni- Hartley and Ngezi mines are in agreement with previous chalcopyrite (Cpy) and not within pyrrhotite (Po). since secondary pyrite typically forms at the expense of rich pentlandite at temperatures below 300-250ºC. In the contents reported by Oberthür et al. (1997) from the pyrrhotite. In addition, some studies have suggested that UG-2 reef, Naldrett et al. (2009) indicate that loss of Fe Hartley mine using micro-pixe (40 ppm Ru, 10 ppm Rh, Trace element distribution between co-existing secondary pyrite inherits PGE contents from the replaced to the chromite from sulfides on cooling, accompanied 9 ppm Pd and 233 ppm Pt on average). At Mimosa and pyrrhotite, pentlandite, chalcopyrite and pyrite in a sulfides (Dare et al. 2011; Piña et al. 2013; Smith et al. by significant rise in fS2, caused the formation of pyrite- Unki mines, PGE concentrations in pyrite are low (< single polymineralic aggregate from Ngezi mine is 2014). Pyrite within chalcopyrite has relatively high Os, pentlandite assemblages by decomposition of Ni-rich 0.11 ppm Pt, < 0.34 ppm Rh, < 2.5 ppm Ru, < 0.37 ppm shown in Figure 3. Platinum, Os, Ir, Ru, Rh, Co, Se and Ir, Ru and Rh contents (Fig. 4). However, host mss at temperatures below 250ºC. At Keivitsansarvi, the Ir and < 0.40 ppm Os) and similar to those in pyrrhotite Bi are preferentially concentrated in pyrite (as also chalcopyrite does not contain Os, Ir or Ru (typically reaction of Fe-rich vaesite (NiS2) with the coexisting mss and pentlandite (Fig. 2b). Palladium in pyrite is indicated by the primitive mantle-normalized profiles of below the detection limit) and thus these elements could on cooling gave rise to a mss richer in Ni from which commonly below 2 ppm in all studied mines except in Figure 2), whereas Pd prefers pentlandite. not be inherited directly from chalcopyrite. Therefore, pyrite and pentlandite ultimately formed. In contrast, one sample from Ngezi which contained up to 60.4 ppm we suggest that some other mechanism different to

972 MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 3 divided in a basal PGE-rich subzone that slightly Pd. Osmium, Ir, Ru, Rh and Pt are positively correlated hydrothermal alteration must be considered to explain 100000 overlaps with an overlying base metal sulfide (BMS)- each other, whereas Pd poorly correlates with all other a) Hartley and Ngezi the pyrite formation. rich subzone. PGE. In contrast to the PGE, Au, Te, Ag, Sb and Bi 10000 Pyrite mainly occurs in the upper part of the PGE- contents are higher in pyrite from Mimosa and Unki than 1000 Pn Py rich subzone and in the BMS-rich subzone of the MSZ. in pyrite from Hartley and Ngezi (Fig. 2). Gold is 100 Sulfides occur as disseminations of polymineralic positively correlated with Te and Bi. In the pyrite 10 aggregates interstitial to silicates. These aggregates are samples from Hartley and Ngezi, Pt correlates with Au. Po ppm 1 made up of pyrrhotite, equal amounts of pentlandite and 50 chalcopyrite and minor pyrite. Pentlandite mostly forms a 0,1 Pn Cpy 0.2 mm coarse grains and minor flame-shaped exsolution 0,01 Ir Po 0 lamellae in pyrrhotite, and chalcopyrite typically occurs Se Re Co Ni Os Ir Ru Rh Pt Pd Te Au Cu Ag Pb Cd Sb Bi As Zn along the peripheries of sulfide aggregates or as isolated 100000 monomineralic grains. According to Oberthür (2011), the b) Unki and Mimosa 10000 presence of pyrite indicates increasing fS2 up sequence in Py the MSZ. 1000 120 250 100 2.1 Pyrite textures 10 Rh Pd 0 0 The studied samples come from the Hartley, Ngezi, Unki 1 and Mimosa mines (located from north to south along 0.2 mm 0,1 the Great Dyke). Samples are located just above the Pt- 0,01 peak of the MSZ. At Unki and Mimosa, pyrite forms Se Re Co Ni Os Ir Ru Rh Pt Pd Te Au Cu Ag Pb Cd Sb Bi As Zn b Cpy clusters of small individual euhedral to subhedral 150 100 crystals within chalcopyrite, pentlandite and, to a lesser Figure 2. Primitive mantle-normalized trace element average extent, pyrrhotite (Fig. 1a). At Hartley and Ngezi, pyrite patterns as determined by LA-ICP-MS for pyrite (black), Pt Se 0 forms small individual grains within pentlandite and pyrrhotite (red), pentlandite (green) and chalcopyrite (violet) 0 from: a) Hartley (solid lines) and Ngezi (dashed lines), and b) chalcopyrite (Fig. 1b). Locally, these grains appear to Py Unki (solid lines) and Mimosa (dashed lines). Elements are have coalesced during growth, resulting in poikiloblastic Po ordered from left to right according to decreasing mss aggregates that enclose chalcopyrite and pentlandite Pn compatibility. grains. Some pyrite grains seem to form symplectitic 100 intergrowths with chalcopyrite and pentlandite (Fig. 1c). 3 Discussion 14000

Bi Co 0

2.2 PGE and other trace element abundances 0 0.2 mm 3.1 Mechanisms of pyrite formation

Trace elements (PGE, Au, Ag, Re, Co, Se, As, Te, Bi The formation of pyrite in Ni-Cu-(PGE) magmatic Figure 3. Trace element maps of coexisting pyrrhotite, and Sb) were determined by laser ablation inductively c Po sulfide deposits is commonly attributed to the activity of pentlandite, chalcopyrite and pyrite from Ngezi mine. Mapping coupled plasma mass spectrometry (LA-ICP-MS) at late magmatic or hydrothermal fluids that trigger the was carried out by line scans ablating the whole aggregate. The LabMaTer, Université du Quebec à Chicoutimi (UQAC), replacement of pyrrhotite by pyrite due to addition of S maps show semi-quantitative values. All elements shown are preferentially concentrated into pyrite with the exception of Pd Canada, using a Resonetics Resolution 193 nm Excimer Py (i.e., increasing fS2) or removal of Fe (Oberthür 2011; laser with a M-50 ablation cell and an Agilent 7,700x Djon and Barnes 2012; Piña et al. 2013; Smith et al. that concentrates in pentlandite. Osmium and Ru (not shown) follow the same distribution as Ir and Rh. quadrupole mass spectrometer, and following the 2014; Holwell et al. 2014; Duran et al. 2014). Although analytical protocol of Piña et al. (2013). Pn previous studies described that pyrite in the MSZ of the The pyrite textures observed in the samples from There are notable differences between trace element Great Dyke predominantly occurs within pyrrhotite Hartley and Ngezi mines are quite similar to those abundances of pyrite from Hartley and Ngezi mines (Oberthür et al. 2003) as observed in other magmatic described for pyrites from the Keivitsansarvi Ni-Cu-PGE (located in the North Chamber of the Great Dyke) and sulfide ore deposits (e.g., McCreedy in Sudbury, Canada, sulfide deposit in northern Finland (Gervilla and pyrite from Mimosa and Unki mines (located in the Dare et al. 2011; Aguablanca, Spain, Piña et al. 2013), in Kojonen 2002), from orogenic peridotites of the French South Chamber) (Fig. 2). At Hartley and Ngezi, pyrite is Cpy our study, pyrite is typically closely associated with 0.2 mm Pyrenees (Lorand and Alard 2011) and from the UG-2 relatively rich in PGE with contents ranging from 0.3 to pentlandite and chalcopyrite and generally absent in reef in the Bushveld Complex, South Africa (Naldrett et 99 ppm Pt, 0.7 to 61.1 ppm Rh (both Pt and Rh contents pyrrhotite. This feature is relevant for the origin of al. 2009). In all these cases, pyrite mainly occurs are typically > 10 ppm), 1.2 to 47.1 ppm Ru, 0.1 to 7.8 pyrite. If hydrothermal fluids had played some role in the associated with pentlandite and/or chalcopyrite forming ppm Os and 1.0 to 20.2 ppm Ir. These values are higher formation of pyrite, the alteration mechanism should Figure 1. Representative photomicrographs in reflected-light, locally fine symplectitic intergrowths. Experimental than those in co-existing pyrrhotite, pentlandite and showing pyrite from the Mimosa (a), Ngezi (b) and Hartley (c) successfully explain the selective replacement of studies in the system Fe-Ni-S (Craig 1973; Misra and chalcopyrite (Fig. 2a). PGE abundances in pyrite from mines. Pyrite (Py) occurs mostly within pentlandite (Pn) and pentlandite and chalcopyrite (not pyrrhotite) by pyrite, Fleet 1973) demonstrate pyrite only coexists with Ni- Hartley and Ngezi mines are in agreement with previous chalcopyrite (Cpy) and not within pyrrhotite (Po). since secondary pyrite typically forms at the expense of rich pentlandite at temperatures below 300-250ºC. In the contents reported by Oberthür et al. (1997) from the pyrrhotite. In addition, some studies have suggested that UG-2 reef, Naldrett et al. (2009) indicate that loss of Fe Hartley mine using micro-pixe (40 ppm Ru, 10 ppm Rh, Trace element distribution between co-existing secondary pyrite inherits PGE contents from the replaced to the chromite from sulfides on cooling, accompanied 9 ppm Pd and 233 ppm Pt on average). At Mimosa and pyrrhotite, pentlandite, chalcopyrite and pyrite in a sulfides (Dare et al. 2011; Piña et al. 2013; Smith et al. by significant rise in fS2, caused the formation of pyrite- Unki mines, PGE concentrations in pyrite are low (< single polymineralic aggregate from Ngezi mine is 2014). Pyrite within chalcopyrite has relatively high Os, pentlandite assemblages by decomposition of Ni-rich 0.11 ppm Pt, < 0.34 ppm Rh, < 2.5 ppm Ru, < 0.37 ppm shown in Figure 3. Platinum, Os, Ir, Ru, Rh, Co, Se and Ir, Ru and Rh contents (Fig. 4). However, host mss at temperatures below 250ºC. At Keivitsansarvi, the Ir and < 0.40 ppm Os) and similar to those in pyrrhotite Bi are preferentially concentrated in pyrite (as also chalcopyrite does not contain Os, Ir or Ru (typically reaction of Fe-rich vaesite (NiS2) with the coexisting mss and pentlandite (Fig. 2b). Palladium in pyrite is indicated by the primitive mantle-normalized profiles of below the detection limit) and thus these elements could on cooling gave rise to a mss richer in Ni from which commonly below 2 ppm in all studied mines except in Figure 2), whereas Pd prefers pentlandite. not be inherited directly from chalcopyrite. Therefore, pyrite and pentlandite ultimately formed. In contrast, one sample from Ngezi which contained up to 60.4 ppm we suggest that some other mechanism different to

Magmatic (Ni-Cu-Cr-PGE) mineral systems: ore forming processes and geodynamic setting 973 Lorand and Alard (2011) suggest that the formation of References pyrite intergrowth with pentlandite and chalcopyrite was Proterozoic Black-Shales: a Source of Sulfur and Semi- due to subsolidus sulfurization processes that increased Craig JR (1973) Pentlandite-pyrrhotite and other low-temperature relations in the Fe-Ni-S systems. Amer J Sci 273:496-510 the fS2 of the system. They suggest that pentlandite may Metals for the Formation of Ni-Cu-Platinum-Group Dare SAS, Barnes S-J, Prichard HM, Fisher PC (2011) be first sulfurized into mss at temperatures above 300ºC Chalcophile and platinum-group element (PGE) concentrations Element Deposits and then the mss broke down into pentlandite + pyrite in the sulfide minerals from the McCreedy East deposit, symplectites upon cooling below 300ºC. Sudbury, Canada, and the origin of PGE in pyrite. Miner It is suggested here that in the MSZ, pyrite may form Deposita 46:381-407 Nadège Samalens, Sarah-Jane Barnes, Edward William Sawyer during low-temperature equilibration of Ni-rich mss Djon MLN, Barnes S-J (2012) Changes in sulfides and platinum- Unité des Sciences de la Terre, Université du Québec à Chicoutimi, G7H 2B1, QC, Canada coexisting with intermediate solid solution (iss). group minerals with the degree of alteration in the Roby, Twilight, and High Grade Zones of the Lac des Iles Complex,

Pentlandite composition is closely related to sulfides Ontario, Canada. Miner Deposita 47:875-896 Abstract. The basal unit of the Duluth Complex with which the pentlandite is associated. The tentative Duran CJ, Barnes S-J, Corkery JT (2014) Sulfide-rich pods from between xenoliths of the Bedded Pyrrhotite Unit and the (Minnesota, USA) contains Ni-Cu sulfides. The S in these <135ºC Fe-Ni-S phase diagram indicates that pentlandite the Lac des Iles Pd-deposits, western Ontario, Canada. 12th enclosing mafic magma in the basal part of the Partridge is thought to be derived from a sulfide-rich black shale coexisting with pyrite is relatively rich in Ni (Naldrett et International Platinum Symposium, Abstract Vol. pp 245-246 River Intrusion. Gervilla F, Kojonen K (2002) The platinum-group minerals in the unit known as the Bedded Pyrrhotite Unit (BPU) a al. 2009). Pentlandite in the pentlandite-pyrite stratigraphic unit within part of the host rocks. Numerous assemblages from the Great Dyke has Fe:(Fe+Ni) wt. % upper section of the Keivitsansarvi Ni-Cu-PGE deposit, 2 Geological setting northern Finland. Can Miner 40:377-394 boreholes have been drilled through the basal unit of the

ratios ranging from 0.46 to 0.49. These composition Graterol M, Naldrett AJ (1971) Mineralogy of the Marbridge No. 3 Duluth Complex into the country rock and these allow ranges are similar to those observed in pentlandite & No. 4 Nickel-Iron Sulfide Deposits. Econ Geol 66:886-900 investigation of the processes whereby S in the country The Duluth Complex is a Keweenawan-age (1100 Ma) equilibrated with pyrite and pyrrhotite in the Marbridge Holwell DA, Keays RR, Firth EA, Findlay J (2014) Geochemistry rock was transferred to the mafic magma. The basal unit mafic complex located in Minnesota, USA. The deposit, Quebec (Graterol and Naldrett 1971). and mineralogy of platinum-group element mineralization in contains partially melted xenoliths of the Bedded Complex is composed of several mafic intrusions related the River Valley intrusion, Ontario, Canada: a model for early Pyrrhotite Unit and the mafic rocks surrounding these to a mantle plume that was located beneath the mid- stage S saturation and multi-stage emplacement and the show a progressive decreases in: S/Se, δ34S, and semi- continental rift system (Ojakangas et al. 2001). The implications for ‘contact-type’ Ni-Cu-PGE mineralization. metal contents away from the xenoliths. The partial melt country rocks to the Duluth Complex consist of Econ Geol 109:689-712 of the xenoliths contained small sulfide droplets. We sediments ranging from banded iron formation through Lorand J-P, Alard O (2011) Pyrite tracks assimilation of crustal suggest that sulfur and semi-metals (Sb, Bi, As and Pb) carbonates, but are predominately greywackes and sulfur in Pyrenean peridotites. Miner Petrol 101:115-128 were transferred to the mafic magma via the droplets. Misra KC, Fleet ME (1973) The chemical composition of synthetic The sulfide droplets then equilibrated with the mafic pelites. Of particular interest is the Bedded Pyrrhotite and natural pentlandite assemblages. Econ Geol 68:518-539 magma and collect Ni, Cu and platinum-group elements Unit, a sulfide-rich black shale believed to have been Naldrett AJ, Craig JR, Kullerud G (1967) The central portion of the deposited in restricted anoxic basins. This unit is Fe-Ni-S system and its bearing on pentlandite exsolution in (PGE). During the final stage of crystallization of the iron-nickel sulfide ores. Econ Geol 62:826-847 sulfide liquid the semi-metals combined with the PGE to approximately 200 m thick, but has a sporadic Naldrett AJ, Kinnair J, Wilson A, Yudovskaya M, McQuade S, form platinum-group minerals. distribution. Chunnett G, Stanley C (2009) Chromite composition and PGE The Duluth Complex can be divided into three mafic content of Bushveld chromitites: Part 1 - the Lower and Keywords. Duluth Complex, Proterozoic black-shales, intrusions: the Partridge River Intrusion, the Bathtub Middle Groups. Applied Earth Science: Transactions of the in-situ assimilation, Ni deposits, magmatic sulfides, LA- Intrusion and the South Kawishiwi Intrusion. The basal Ruthenium versus Ir contents for pyrite from the ICP-MS Figure 4. Institution of Mining and Metallurgy, Section B 118:131-161 unit (Unit I) of the Partridge River Intrusion contains Great Dyke, Zimbabwe. Oberthür T (2011) Platinum-group element mineralization of the abundant xenoliths of the host-rocks and most of the Main Sulfide Zone, Great Dyke, Zimbabwe. Reviews in Econ 1 Introduction sulfide mineralization (Severson and Hauck 2008). Unit 3.2 Pyrite as sulfide hosting PGE Geol 17:329-349 Oberthür T, Cabri LJ, Weiser TW, McMahon G, Müller P (1997) Most of world’s Ni-Cu-PGE deposits were formed after 1 is composed of the following lithologies: norite, Previous studies have indicated that most Pd and Rh are Pt, Pd and other trace elements in sulfides of the main sulfide gabbronorite and troctolite, from the base to the center of zone, Great Dyke, Zimbabwe: a reconnaissance study. Can the addition of sulfur to the magma from an external hosted in pentlandite, whereas Pt is dominantly present source in the country rocks (Ripley and Li 2013). It is the intrusion. Norites are found near to the contact with Miner 35:597-609 the Virginia Formation country rocks and around in form of PGM in the MSZ (Oberthür et al. 2003). Our Oberthür T, Davis DW, Blenkinsop TG, Höhndorf A (2002) Precise generally thought that this S is derived from black results indicate that among all base metal sulfides, pyrite U-Pb mineral ages, Rb-Sr and Sm-Nd systematics for the shales. Black shales are also rich in semi-metals (As, Sb, xenoliths of country rock. Disseminated to massive Ni- is individually the sulfide phase hosting the highest Great Dyke, Zimbabwe – constraints on late Archean events in Te and Bi). Interestingly, these elements are necessary Cu sulfides occur in Unit I. Sulfides are commonly contents of PGE (with the exception of Pd that is the Zimbabwe craton and Limpopo belt. Precambrian Res for the formation of many of the platinum-group found around the xenoliths. 113:293-305 preferentially concentrated in pentlandite). Mass balance minerals (PGM). Thus the question arises: are these Samples were chosen on from boreholes that crossed Oberthür T, Weiser TW, Gast L, Kojonen K (2003) Geochemistry the contact between the Virginia Formation and calculation to determine the percentage of each PGE and mineralogy of platinum-group elements at Hartley elements also derived from the black-shales? present in each sulfide has not been carried out to date Platinum Mine, Zimbabwe. Miner Deposita 38:327-343 The Partridge River Intrusion of the Duluth Complex Partridge River Intrusion at the Dunka Road, Mesaba and, thus, the importance of pyrite as carrier of PGE in Piña R, Gervilla F, Barnes S-J, Ortega L, Lunar R (2013) Platinum- represents an ideal and well-documented location for and Wetlegs deposits. the mineralization is not quantified. Nevertheless, since group elements-bearing pyrite from the Aguablanca Ni-Cu studying contamination processes. Mineralization is sulfide deposit (SW Spain): a LA-ICP-MS study. Eur J most of PGE appear show higher preference for pyrite found in the basal part of the intrusion close to its 3 Results Mineral 25:241-252 when present (Fig. 3), we envisage that pyrite may Smith JW, Holwell DA, McDonald I (2014) Precious and base contact with sedimentary country rocks of the Virginia account for significant amounts of PGE (particularly, Rh metal geochemistry and mineralogy of the Grasvally Norite- Formation (Severson and Hauck 2008). 3.1 Petrography and Pt) in those zones with the highest modal Pyroxenite-Anorthosite (GNPA) member, northern Bushveld A particular unit in the Proterozoic Virginia Temperatures of greater than 800°C have been reported abundances of pyrite. Complex, South Africa: implications for a multistage Formation, called Bedded Pyrrhotite Unit, is composed from diatexite migmatites found close to the contact with emplacement. Miner Deposita 49:667-692 of pyrrhotite-rich black-shales. Thériault and Barnes Vukmanovic Z, Reddy SM, Godel B, Barnes SJ, Fiorentini ML, the intrusion (Sawyer 2014). (1998) proposed that sulfur was transferred to the mafic Acknowledgements Barnes S-J, Kilburn MR (2014) Relationship between The Bedded Pyrrhotite Unit is divided into magma after partial melting of entrained xenoliths in the microstructures and grain-scale trace element distribution in unmetamorphosed and metamorphosed depending upon We would like to thank Dany Savard and Sadia Mehdi komatiite-hosted magmatic sulphide ores. Lithos 184-187:42- magma. Subsequently, the Bedded Pyrrhotite Unit has whether inside or outside of the contact aureole that is for their assistance during the laser ablation analyses. 61 been identified as the source of the sulfur that developed around the intrusion. Unmetamorphosed The research work was financed by the Spanish research Wilson AH, Prendergast MD (1989) The Great Dyke of Zimbabwe contaminated the mafic magma (Queffurus and Barnes Bedded Pyrrhotite Unit, outside the contact aureole, project CGL2010-17668, the Canada Research Chair in - I. Tectonic setting, stratigraphy, petrology, structure, 2014). However, the exact mechanism by which the S is emplacement and crystallization. In: Prendergast MD, Jones contains pyrite and few grains of chalcopyrite. Magmatic Metallogeny and the German Federal Institute MJ (eds) Magmatic sulfides - the Zimbabwe Volume. transferred by the partial melt is poorly understood. for Geosciences and Natural Resources (BGR). Institution of Mining and Metallurgy, London, pp 1-2 Therefore, we have carried a petrographic, mineralogical and geochemical study of the interactions

974 MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 3