Platinum-Group Element Abundances in Pyrite from the Main Sulfide Zone, Great Dyke, Zimbabwe
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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 ordered from left to right according to decreasing mss Po 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.