Platinum-Group Element Distribution in Base- Metal Sulfides of the Merensky Reef and UG2 from the Eastern and Western Bushveld Complex, South Africa

PhD thesis Inga Osbahr

Platinum-group element distribution in base- metal sulfides of the Merensky Reef and UG2 from the eastern and western Bushveld Complex, South Africa

(Platingruppenelement - Verteilung in Sulfiden des Merensky Reef und UG2 im Bushveld Komplex, Südafrika)

Der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr. rer. nat.

Vorgelegt von Inga Osbahr

aus Berlin

PhD thesis Inga Osbahr

Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 09.08.2012 Vorsitzender der Promotionskommission: Prof. Dr. Rainer Fink Erstberichterstatter: Prof. Dr. Reiner Klemd Zweitberichterstatter: Prof. Dr. Karsten Haase

PhD thesis Inga Osbahr

ZUSAMMENFASSUNG Buntmetall-Sulfide (BMS) sind wichtige Träger von Platingruppenelementen (PGE) in magmatischen Ni-Cu-PGE Lagerstätten. Die Verteilung und Konzentration von PGE in Pentlanditen, Pyrrhotinen, Chalkopyriten und Pyriten wurde an Proben des Merensky Reef und des UG2 aus dem westlichen und östlichen Bushveld Komplex in Südafrika untersucht. An insgesamt vier Bohrkernen aus dem Merensky Reef und zwei Bohrkernen aus dem UG2 wurden ausgewählte Buntmetall-Sulfide analysiert. Diese Sulfide stammen aus dem mineralisierten Bereich des Merensky Reef und des UG2. Zur Bestimmung der Hauptelemente wurden Elektronenstrahlmikrosonden-Analysen durchgeführt und zur Bestimmung der Spurenelemente (PGE, Ag und Au) Laser Ablation-Inductively Coupled Plasma- Mass Spektrometer (LA-ICP-MS)-Analysen. Zusätzlich wurden Gesamt- gesteinsanalysen an repräsentativen Proben durchgeführt.

Die Verteilung von Pt und Pd im Gesamtgestein des Merensky Reef des westlichen Bushveld Komplexes zeigt eine sogenannte „top-loaded“ Mineralisation, was bedeutet, dass die höchsten PGE-Konzentrationen befinden sich im Bereich des oberen Chromititbands und seiner direkten Umgebung. Im östlichen Bushveld Komplex hingegen ist die Verteilung komplexer, da die maximalen Pt- und Pd-Konzentrationen im Merensky Reef auf das untere und obere Chromititband verteilt sind. Im UG2 sind die höchsten Konzentrationen von Pt und Pd auf den unteren und oberen Bereich des UG2 Chromitit beschränkt.

Die Ergebnisse der LA-ICP-MS Analysen zeigen, dass die PGE am stärksten im Pentlandit angereichert sind, wohingegen Pyrrhotin deutlich geringere PGE-Gehalte aufweist und im Chalkopyrit kaum nachweisbare PGE-Konzentrationen enthalten sind. Pentlandit, sowohl aus dem Merensky Reef, als auch aus dem UG2, enthält wesentliche Pd- und Rh- Konzentrationen. Pentlandite im Merensky Reef des östlichen Bushveld Komplex zeigen Konzentrationen von bis zu 700 ppm Pd und bis zu 130 ppm Rh. Im westlichen Bushveld wurden im Pentlandit sogar bis zu 1750 ppm Pd und 1000 ppm Rh nachgewiesen. Pyrrhotin kann deutliche Konzentrationen von IPGE (bis zu je 33 ppm Ru, Os und Ir) aufweisen. Chalkopyrit enthält geringe PGE-Konzentrationen, kann aber hohe Silbergehalte (bis zu 50 ppm) aufweisen.

Im UG2 des westlichen Bushveld Komplex können im Pentlandit bis zu 400 ppm Pd und 200 ppm Rh nachgewiesen werden, im östlichen Bushveld können es sogar bis zu 1000 ppm Pd und 140 ppm Rh sein. Auch deutliche Gehalte von Ir (150 ppm) und Ru (175 ppm) wurden gemessen. Auch im UG2 enthält der Chalkopyrit kaum nachweisbare PGE-Konzentrationen, eine Ausnahme bildet das Pt, welches in wenigen Proben bis zu 20 ppm erreichen kann.

PhD thesis Inga Osbahr

Aus einer Massenbilanzierung wird ersichtlich, dass Pentlandit aus dem feldspatführenden Pyroxenit und dem pegmatoidalen feldspatführenden Pyroxenit des Merensky Reef bis zu 100 % des Pd und Rh einbaut, ebenso 10-40 % des Os, Ir und Ru. Pyrrhotin und Chalcopyrit enthalten weniger als 10 % der im Gesamtgestein enthaltenen PGE. Die übrigen PGE (darunter bis zu 100 % des Platins) sitzen in Platingruppenmineralen, wie Cooperit/Braggit, Moncheit, Sperrylit und Isoferroplatinum. Im UG2 des westlichen Bushveld Komplex sind nur ca. 5 % des gesamten Pd und 15 % des gesamten Rh im Pentlandit eingebaut. Im östlichen Bushveld sind es bis zu 40 % des Pd und 30 % des Rh und insgesamt weniger als 20 % des Os, Ir und Ru. Ähnlich wie im Merensky Reef liegen, auch im UG2, fast 100 % des Platins in Form von diskreten Platingruppenmineralen vor.

Im Merensky Reef des Bushveld Komplex zeigt die Verteilung von Cu, Ni und S gegenüber Pd und Pt sogenannte „offsets patterns“. Das Verteilungsmuster dieser „offsets“ entspricht vom Liegendem zum Hangendem: Pd in pentlandite → Pd/Pt im Gesamtgestein → (Cu, Ni, S) im Gesamtgestein. Diese Abfolge konnte für einen Großteil der Sequenzen beobachtet werden. Die höchste Pd Konzentration im Pentlandit scheint an die früheste, volumenmäßig aber eher geringe sulfidische Schmelze, gebunden zu sein, welche sich im Liegenden des Merensky Reef befindet. Eine mögliche Erklärung für das Auftreten von „offset patterns“ bietet die Rayleigh Fraktionierung.

PhD thesis Inga Osbahr

ABSTRACT Base-metal sulfides (BMS) in magmatic Ni-Cu-PGE deposits are important carriers of platinum-group elements (PGE). The distribution and concentrations of PGE in pentlandite, pyrrhotite, chalcopyrite and pyrite were determined in samples from the mineralized proportion of four Merensky Reef and two UG2 intersections from the eastern and western Bushveld Complex. Electron microprobe analysis was used for major elements, and in situ LA-ICP-MS for trace elements (PGE, Ag and Au). Whole-rock trace element analyses were performed on representative samples to obtain mineralogical balances.

Both Pt and Pd in Merensky Reef samples from the western Bushveld show a “top loaded” mineralization, mainly concentrated in the upper chromitite stringer and its immediate vicinity. Samples from the eastern Bushveld reveal more complex distribution patterns, since the concentration maxima are located in the lower and upper chromitite stringer area. In UG2 samples of the eastern and western Bushveld, the highest Pd and Pt concentrations are located in the lower and upper portion of the UG2 chromitite.

In situ LA-ICP-MS analyses of PGE in sulfides reveal that pentlandite carries the highest and most distinctly elevated PGE contents, whereas pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite is the principal host of Pd and Rh in the ores of the Merensky Reef and UG2, being incorporated in the crystal lattice. Palladium and Rh concentrations in pentlandite reach up to 700 ppm and 130 ppm, respectively, in the Merensky Reef samples from the eastern Bushveld, and up to 1750 ppm Pd and 1000 ppm Rh in those from the western Bushveld. Only traces of Pt are found in the BMS. Pyrrhotite contains significant, though generally lower amounts of Ru, Os and Ir, but hardly any Pd or Rh. Chalcopyrite contains most of the Ag but carries extremely low PGE concentrations. In the UG2 samples, Pd and Rh in pentlandite reach up to 400 ppm and 200 ppm, respectively, in the western Bushveld and up to 1000 ppm Pd and 140 ppm Rh in the eastern Bushveld Complex. Significant amounts of Ir (150 ppm) and Ru (175 ppm) were found in the UG2 samples. Pyrrhotite contains significant amounts of Rh (100 ppm), Os (70 ppm) and Ru (200 ppm). Chalcopyrite contains very low PGE concentrations, although maximum Pt concentrations of 20 ppm were detected.

Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal pyroxenite hosts up to 100 % of the Pd and Rh and smaller amounts (10-40 %) of the Os, Ir and Ru. Pyrrhotite and chalcopyrite usually contain less than 10 % of the whole-rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100 %), are present in the form of

PhD thesis Inga Osbahr discrete platinum-group minerals (PGM) such as cooperite/braggite, sperrylite, moncheite and isoferroplatinum. In the UG2 of the western Bushveld pentlandite hosts 5 % of the whole-rock Pd and 15 % of the whole-rock Rh while in the eastern Bushveld it is 40 % of the Pd and 30 % of the Rh. Less than 20 % of whole-rock Os, Ir and Ru is incorporated in the pentlandite of the UG2. Similar to the Merensky Reef, in the UG2 the remaining PGE and almost 100 % of the Pt are present in the form of discrete platinum-group minerals.

Distribution patterns of whole-rock Cu, Ni and S versus whole-rock Pd and Pt commonly show distinct offsets in the Merensky Reef. The general sequence of “offset patterns” of PGE and BMS maxima, in order from bottom to top, is Pd in pentlandite → Pd/Pt in whole-rock → (Cu, Ni, S) in whole-rock, and is quite obvious in most of our samples. Occasionally, only partially similar or more complex trends are also present.

PhD thesis Inga Osbahr

Table of Contents

1. Aim of this Study 10

2. Introduction 11 2.1 The Bushveld Complex 11 2.2 Stratigraphy 15 2.2.1 Merensky Reef 15 2.2.2 UG2 17 2.3 Platinum-group elements 18 2.4 Previous studies on sulfide liquid fractionation 19

3. Sample Description and Petrography 21 3.1 Merensky Reef 21 3.2 UG2 25

4. Analytical Methods 28 4.1 Transmitted and reflected light microscopy 28 4.2 Electron microprobe analyses (EMPA) 28 4.3 Laser ablation-ICP-MS 29 4.4 Electric pulse disaggregation (EPD) and Hydroseparation (HS) 33 4.5 Ni-fire assay 34

5. Mineral Chemistry 35 5.1 Merensky Reef 35 5.1.1 Orthopyroxene 35 5.1.2 Clinopyroxene 35 5.1.3 Plagioclase 37 5.1.4 Phlogopite 38 5.1.5 Olivine 38 5.1.6 Chromite 39 5.1.7 Pentlandite 41 5.1.8 Chalcopyrite 46 5.1.9 Pyrrhotite and troilite 46 5.1.10 Pyrite 47 5.1.11 Cubanite 47 5.1.12 Platinum-group minerals 50 5.2 UG2 60 5.2.1 Orthopyroxene 60 5.2.2 Clinopyroxene 61

PhD thesis Inga Osbahr

5.2.3 Plagioclase 61 5.2.4 Phlogopite 62 5.2.5 Olivine 62 5.2.6 Chromite 63 5.2.7 Pentlandite 66 5.2.8 Chalcopyrite 69 5.2.9 Cubanite 69 5.2.10 Troilite 70 5.2.11 Platinum-group minerals 70

6. Analytical Results-Merensky Reef 73 6.1 Whole-rock chemistry 73 6.1.1 Western Bushveld 73 6.1.2 Eastern Bushveld 75 6.2 PGE in base-metal sulfides 84 6.2.1 Stratigraphic variation of PGE 84 6.2.2 PGE concentration in base-metal sulfides (western Bushveld) 86 6.2.2.1 SD124 86 6.2.2.2 SD134 90 6.2.3 PGE concentration in base-metal sulfides (eastern Bushveld) 95 6.2.3.1 US200 95 6.2.3.2 US186 99 6.2.4 Silver and gold concentration in BMS (eastern and western Bushveld) 105 6.3 Summary 106

7. Analytical Results-UG2 108 7.1 Whole-rock chemistry 108 7.1.1 SD124 (western Bushveld) 108 7.1.2 DT46 (eastern Bushveld) 109 7.2 PGE in base-metal sulfides 114 7.2.1 SD124 (western Bushveld) 114 7.2.2 DT46 (eastern Bushveld) 116 7.3 Summary 120

8. Geochemistry 121 8.1 Merensky Reef 121 8.1.1 Whole-rock trace element geochemistry 121 8.1.2 Trace element correlation in pentlandite 124 8.1.3 Summary 127 8.2 UG2 129 8.2.1 Whole-rock trace element geochemistry 129

PhD thesis Inga Osbahr

8.2.2 Trace element correlation in pentlandite 130 8.2.3 Summary 133

9. Mass Balance Calculation 135 9.1 Merensky Reef 137 9.1.1 Eastern Bushveld (US200) 137 9.1.2 Western Bushveld (SD124, SD134) 140 9.2 UG2 145 9.2.1 Western Bushveld (SD124) 145 9.2.2 Eastern Bushveld (DT46) 146

10. Summary and Discussion 150 10.1 Whole-rock “Offset”-feature in the Merensky Reef 152 10.2 Pd-enrichment in pentlandite 153

11. Conclusions 156

12. References 158

PhD thesis Inga Osbahr 1. Aim of this Study

1. Aim of this Study The aim of this present study is to obtain a detailed overview of the occurrence and small- scale distribution of the platinum-group elements (PGE) in base-metal sulfides (BMS) in the Merensky Reef and the UG2 of the eastern and western Bushveld Complex in order to improve our understanding of the genetic processes which formed the mineralization of the Merensky Reef and the UG2. Furthermore, the aim is to point out differences or similarities in the mineralization between the eastern and western Bushveld Complex and also between the Merensky Reef and the UG2.

In order to accomplish this aim, in situ major and trace element analyses on BMS, silicates and chromites were undertaken by electron-microprobe and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), respectively, in order to determine the composition of the mineral phases. In addition whole-rock major and trace element (including PGE) analyses were conducted for selected samples. For this study, six drill cores from the Critical Zone of the Rustenburg Layered Suite (RLS) were geochemically and petrologically investigated. Two of the drill cores display the entire section of the mineralized part of the Merensky Reef in the eastern (US200, US186) and two drill cores of the western Bushveld Complex (SD124 and SD134). Two additional drill cores of the UG2 (DT46 from the eastern and SD124 from the western Bushveld) were supplied by Anglo American Research laboratories.

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PhD thesis Inga Osbahr 2. Introduction

2. Introduction 2.1. The Bushveld Complex The Bushveld Complex is part of the Palaeoproterozoic Bushveld Igneous Province. Large Igneous Provinces (LIP) are defined as “massive crustal emplacements of predominantly mafic Mg- and Fe-rich extrusive and intrusive rocks which originate via processes other than normal seafloor spreading and include continental flood basalts, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups and ocean basin flood basalts” (Coffin and Eldholm 1994, p. 1).

The Bushveld Complex in South Africa, which constitutes the world largest mafic layered intrusion also contains the world’s most important PGE reserves (e.g. Willemse et al. 1969; von Gruenewaldt 1977, 1979; Hulbert and von Gruenewaldt 1982; Vermaak 1995; Cawthorn 1999; Misra 2000). It was emplaced at 2.05 Ga in an essentially intraplate setting and today covers an area of ca. 65.000 km2, with a thickness of 5-10 km (Walraven et al. 1990; Eales and Cawthorn 1996; Cawthorn et al. 2006b). The Bushveld Complex outcrops in four major limbs, which form a huge elliptical structure (Fig. 2.1).

Fig. 2.1: a) Simplified geological map of the Bushveld Complex, South Africa, with the locations of drill cores SD124 and SD134 (western limb, Styldrift farm; SD) and US200, US186, DT46 (eastern limb, Lebowa Mine, farm Umkoanesstad and Diamond; US, DT), modified after Cawthorn et al. (2002b). The map also shows the localities of previous whole-rock PGE studies of the Merensky Reef at Rustenburg (Ballhaus and Sylvester 2000; Godel et al. 2007), Styldrift (Naldrett et al. 2009) and Winnaarshoek (Mitchell and Scoon 2007) and of the UG2 at Dwars River (Voordouw 2009) as well as LA-ICP-MS studies on PGE in base-metal sulfides at Rustenburg (Ballhaus and Sylvester 2000; Godel et al. 2007) b) Simplified stratigraphy of the Bushveld Complex (after Eales and Cawthorn 1996).

PhD thesis Inga Osbahr 2. Introduction

The Western limb extends from near Thabazimbi to north of Pretoria. The Eastern limb outcrops from Stoffberg to Chuniespoort and is ca. 200 km long (Eales and Cawthorn 1996). The eroded Far Western limb, which extends to the Botswana border and the Northern or Potgietersrus limb, is partially hidden beneath younger rocks (Eales and Cawthorn 1996). The Northern and Western limbs are not well exposed, whereas the Eastern limb has much better exposures. The Southeastern or Bethal limb was only identified on the basis of a high gravity, and is known only from bore-core information (Eales and Cawthorn 1996). Every limb of the Bushveld is subdivided into the mafic rocks of the Rustenburg Layered Suite (RLS) the Lebowa Granite Suite, the Rashoop Granophyre Suite and the siliceous rocks of the Rooiberg Group (SACS 1980; Hatton and Schweitzer 1995).

The Rustenburg Layered Suite has been divided into the Marginal, Lower (LZ), Critical (CZ), Main (MZ) and Upper Zones (UZ) (Kruger 1990) and has been dated at 2054.4 ± 2.8 Ma (U/Pb zircon age, Harmer and Armstrong 2000). The Marginal Zone consists of medium-grained, unlayered rocks, which are mainly norite with variable proportions of accessory clinopyroxene, quartz, biotite and hornblende. Quartz and biotite reflect assimilation of shale (Cawthorn et al. 2002b). The rocks of the Marginal Zone may reach 800 m in thickness and probably represent multiple magmatic intrusions of magma (Cawthorn et al. 2006b). The Lower Zone is divided into the Lower Pyroxenite, Harzburgite and Upper Pyroxenite Subzones, all with less than 1 % chromite (Cawthorn et al. 2006b). The thickness of the Lower Zone reaches 1300 m (Cawthorn et al. 2002b). The Critical Zone is divided into a Lower and Upper Critical zone and hosts huge deposits of chromite within the Lower (LG), Middle (MG) and Upper (UG) Group chromitite layers and the world’s largest PGE-bearing ore bodies, namely the UG2, Merensky Reef and Platreef (Hatton and Schweitzer 1995; Schürmann et al. 1998). Each layering of chromitite, pyroxenite, norite and anorthosite displays a cyclic unit. The Lower Critical Zone contains up to seven chromitite layers, namely the LG1-LG7. The Middle Group shows four chromitite layers (MG1-MG4) that are located at the top of the Lower Critical Zone and at the base of the Upper Critical Zone. At the top of the Upper Critical Zone two thick chromitite layers occur, namely the UG1 and the UG2 (the UG3 is restricted to the eastern Bushveld), while the top two cyclic units are the Merensky Reef and the Bastard Reef (Cawthorn et al. 2002b). The term “reef” is a mining term rather than a lithological term and designates a layer, which is sufficiently enriched in PGE to be exploited (Lee 1996). The Merensky Reef dips to the centre of the Complex with an angle of 9-27 ° (Du Plessis and Kleywecht 1987; von Gruenewaldt et al. 1990). The PGE grade of the Merensky Reef of the western and eastern Bushveld is 3.5-9.5 g/t and 4.6-6.8 g/t, respectively, (Cawthorn et al.

PhD thesis Inga Osbahr 2. Introduction

2002b), however, the highest PGE grades tend to be found within the two thin chromitite layers of the Merensky Reef, with lower grade between them (Cawthorn 1999). The UG2 is considered as the largest PGE resource on earth (Vermaak 1985 and 1995). The UG2 represents only one lithological unit and dips to the centre of the Complex with an angle of between 10-26 ° (Cawthorn et al. 2002b). The UG2 has a PGE grade between 4.8-6.5 g/t and 4.5-8.0 g/t PGE on average in the western Bushveld and eastern Bushveld, respectively (Cawthorn et al. 2002a).

The Platreef is located in the northern limb of the Bushveld Complex and comprises pyroxenite with PGE and BMS mineralization (Van der Merwe 1976). The Platreef is associated with the Critical Zone (Wagner 1929; White 1994), however, van der Merwe (1978) regarded the Platreef as the base of the Main Zone. The Platreef also hosts significant PGE contents but differs from the UG2 and Merensky Reef concerning the PGE grade and the Pt/Pd ratio. The Platreef typically contains 1-2 g/t over many meters while the Pt/Pd ratio is approximately 1 (Kinnaird et al. 2005).

The Main Zone is a thick succession of norite and gabbronorite, with minor anorthosite and pyroxenite layers, while olivine and chromitite are absent (Cawthorn et al. 2006b). The Main Zone is > 3000 m thick and does not show such spectacular layering as the Critical Zone, however modally layered rock packages can be identified (Molyneux 1974, Mitchell 1990, Nex et al. 1998 and 2002). The Upper zone is the most laterally extensive zone. It is well layered with a thickness of ca. 2000 m and its most prominent feature is the presence of 24 magnetite layers (Cawthorn et al. 2006b).

The Lebowa Granite Suite comprises, as the name implies, different types of granite. It comprises a series of sheeted intrusion between 1.5 and 3.5 km in thickness (De Beer et al. 1987; Molyneux and Klinkert 1987; Kleemann and Twist 1989). The granites underlie the predominantly felsic volcanics of the Rooiberg Group and mostly consist of alkali feldspar granites with iron-rich ferromagnesian minerals. The Rooiberg Group occurs mainly above the RLS and can be divided into several formations. The bulk of the succession comprises siliceous volcanic rocks with sandstone and shale intercalations. The Rooiberg Group has been dated at 2057.3 ± 3.8 Ma (U/Pb zircon age; Harmer and Armstrong 2000). The Rashoop Granophyre Suite represents the acid phase of the Bushveld Complex (Cawthorn et al. 2006b).

PhD thesis Inga Osbahr 2. Introduction

The rocks of the Bushveld Complex intruded into the sediments of the Transvaal Supergroup and are now buried under a younger sedimentary cover, which was partly removed by erosion (Cheney and Twist 1991).

PhD thesis Inga Osbahr 2. Introduction

2.2. Stratigraphy 2.2.1. Merensky Reef

An average Merensky Reef sequence consists of a pegmatoidal (feldspathic) pyroxenite which is occasionally replaced by pegmatoidal feldspathic harzburgite at its base. Two to four chromitite layers, reaching 1-2 cm in thickness, characterize the upper and lower limits of the main economic mineralization. The Reef undulates into a footwall, which consists of an approximately 5 m thick poikilitic anorthosite layer and occasionally of gabbronorite. The 1.5 to 3.5 m thick hanging wall of the Reef consists of feldspathic pyroxenite (Merensky Pyroxenite) or norite as displayed in Figure 2.2 or described by Wagner (1929) and Vermeulen (2010).

Fig. 2.2: Sketches showing the distribution of rock units and mineralization in “normal” Merensky Reef of the western Bushveld (Rustenburg area) after Vermaak (1976), and in the eastern Bushveld (Lebowa mine) after Schwellnus et al. (1976), modified after Naldrett (1989).

Different classifications on Reef-type facies were undertaken by several authors. For example, Cawthorn (2011) summarized the Reef type facies from the eastern and western Bushveld Complex, namely the contact or thin Reef facies, the pegmatitic pyroxenite Reef facies and the thick Reef facies. The contact or thin Reef facies is characterized by only one chromitite layer and the absence of a pegmatitic pyroxenite (Cawthorn 2011). The pegmatitic pyroxenite Reef facies is characterized by two chromitite stringers that may be separated by only a few cm to several tens of cm. The unit between the two stringers usually is a pegmatitic pyroxenite and the unit above the upper chromite stringer a feldspathic pyroxenite (Cawthorn 2011). To the east in the Lebowa mines the Reef becomes thicker and lithologically more variable. Anorthosite or norite occurs below the lower chromitite stringer and pyroxenite with pegmatitic lenses may occur above the stringer. Another chromite layer 15

PhD thesis Inga Osbahr 2. Introduction

occurs on top of this package. Occasionally an intermediate chromite stringer may occur (Cawthorn 2011).

Furthermore, the Merensky Reef shows many variations in host rock lithologies and distribution of the mineralization, and was for example further divided in a 4-20 cm wide thin reef facies, a 20-40 cm wide medium reef facies and a 1-2 m wide thick reef facies in the Rustenburg area of the western Bushveld e.g. by Wilson et al. 1999.

About 3 % BMS, namely pentlandite, pyrrhotite, chalcopyrite and minor pyrite, occur interstitially with the silicates and the chromite (Lee 1996). The BMS and PGE correlate with each other in the mineralized part of the Reef (Lee 1983). In most of the Merensky Reef facies the economic PGE mineralization is concentrated in the pegmatoidal feldspathic pyroxenite with highest concentrations in the range of the chromitite stringers (Lee 1996). However, in thinner Reef facies the economic mineralization of BMS and PGE is frequently dispersed into the hanging wall and footwall rocks (Kinloch 1982). The Merensky Reef mineralization, depending on the location of the maximum PGE concentration, is called “top” loaded if the maximum PGE concentration is in the range of the upper chromitite layer or “bottom” loaded if it is in the range of the lower chromitite stringer. Otherwise it displays a more “complex” distribution. The Pt/Pd ratio is up to 5 in the Merensky Reef (Cousins and Kinloch 1976; Hey 1999).

The PGM mineralogy shows lateral variations in the Merensky Reef (Kinloch 1982; Mossom 1986; Kinloch and Peyerl 1990), however, Vermaak and Hendriks (1976) suggest an order of abundance of braggite and Pt-Fe alloy over cooperite and laurite.

PhD thesis Inga Osbahr 2. Introduction

2.2.2. UG2

The UG2 chromitite layer appears 15-400 m below the Merensky Reef. The UG2 in general is characterized by a ca. 60 cm to 110 cm thick chromitite seam (Mathez and Mey 2005), however, its thickness varies between 15 to 255 cm across the whole Bushveld (McLaren and DeVilliers 1982). It is considered as a single chromitite layer (Cawthorn 2011). However, Lee (1996) pointed out that a break occurs in the middle of the chromitite layer where the texture becomes distinctly poikilitic and that this textural change is accordance with a change in PGE concentration. The foot and hanging wall comprises pyroxenite, harzburgite, norite or anorthosite. In the immediate hanging wall two to four “leader” chromitite layers occur ranging from 1 mm to 12 cm in thickness (Cawthorn et al. 2002b). The chromitite contains of 60-90 % of compact chromite with lesser amounts of pyroxene and plagioclase and trace amounts of BMS. The amount of sulfides in the UG2 is very low (< 0.05 %) and a mineralization above the UG2 has never been reported up to now. However, the presence of PGE in the pegmatitic footwall has been reported by Hiemstra (1982), McLaren and De Villiers (1982) and Lee (1996).

The PGM mineralogy in the UG2 varies on a regional scale along the strike (Peyerl 1982, McLaren and de Villiers 1982, Kinloch 1982) and is dominated by PtFe, PGE arsenides and laurite (Grimbeek 1996). The grain size of the PGM is always smaller than that of the BMS by a factor of 10 (McLaren and de Villiers 1982). However, the grain size of the PGM of the UG2 is usually smaller than that of the PGM of the Merensky Reef (Cawthorn et al. 2002b).

A difference between the eastern and western Bushveld is the Pt/Pd ratio of the UG2, which is much lower in the eastern Bushveld (1.0-1.6) than in the western Bushveld (1.9-2.3) (Cawthorn et al. 2002b). The distribution of PGE in the UG2 reveals a top and bottom loaded mineralization with grades of up to 20 ppm, while in the middle of the UG2 the grades can decrease to ~3 ppm.

PhD thesis Inga Osbahr 2. Introduction

2.3. Platinum-group elements The platinum group elements include platinum, palladium, rhodium, osmium, iridium and ruthenium (Fig. 2.3). They can be divided, either into light PGE, including ruthenium, rhodium and palladium (highlighted in yellow), and heavy PGE, including osmium, iridium and platinum (highlighted in blue), whereas the light elements have only half of the density of the heavy PGE (Buchanan 1988), or due to their coherent behavior during magmatic processes they can further be divided into the Platinum-group (PPGE) consisting of Pd, Pt, and Rh (black font) and the Iridium-group (IPGE) consisting of Os, Ir, and Ru (red font) (Barnes et al. 1985; Mungall 2005). In the latter case the elements of the two subgroups have similar melting points, ranging between 1552 and 1966 °C for the PPGE group and between 2310 and 3045 °C for the IPGE group. The PGE share the tendency to prefer the formation of metallic bonds with Fe and Ni. In addition, they share the tendency with Cu and Ag to favor the formation of covalent bonds with sulfur. This behavior gives the PGE a siderophile and chalcophile character, respectively (Mungall 2005). Besides high melting points, PGE are known for their chemical inertness and for their ability to catalyze chemical reactions (Brenan 2008). Due to all these unique properties, there is great industrial demand for the PGE from the automobile and chemical industries or from the jewelry industry.

Fig. 2.3: PGE, Re and Au and their position in the PSE with melting points and density.

PhD thesis Inga Osbahr 2. Introduction

2.4. Previous studies on sulfide liquid fractionation The distribution of the PGE is mainly the result of sulfide liquid fractionation, as revealed by experimental investigation (Barnes and Picard 1993; Barnes et al. 1997, 2001; Li et al. 1996; Naldrett 2004; Peregoedova et al. 2004; Mungall et al. 2005). The experimental studies postulated that at high temperatures (<1000 °C) a mss and a Cu-rich residual liquid were subsequently formed due to the separation of a Fe-Ni-Cu sulfide liquid from a silicate magma. The PGE partition into the (Fe, Ni, Cu)-sulfide liquid rather than into basaltic melt, with a partition coefficient in the range of 102-105 (Bezmen et al. 1994).

At 950-840 °C, an iss phase is thought to crystallize from the Cu-rich liquid (e.g. Kullerud et al. 1969; Dare et al. 2010). At temperatures below 650 °C, mss exsolves into pyrrhotite and below 610 °C pentlandite becomes stable. The latter was formed through a reaction of a nearly pure FeS composition of mss at 1000 °C and the (Ni,Fe)3±xS at 806 °C (Craig and Scott 1976). At 557 °C, chalcopyrite exsolves from iss followed by cubanite at 210 °C (Craig and Scott 1976). During the crystallization of mss and iss the PGE, Au and Ag behave differently. Pt and Pd together with Cu behave incompatibly during mss crystallization and strongly partition into the sulfide liquid, with partition coefficients for Pt and Pd between 0.05-0.16 and 0.08-0.27, respectively, and 0.17-0.27 for Cu. Os, Ir, Ru and Rh partition into mss. Ir and Rh have a partition coefficient ranging from 1.06-13 and from 0.37 to 8.23, respectively (Li et al. 1996), while the partition coefficient for Ru ranges from 1.8 to 14.5 (Mungall et al. 2005) and for Os between 3.36 and 10.61 (Barnes et al. 2006). The partition coefficient of Ag between the mss and the sulfide liquid is 0.38 (Barnes et al. 2006). Au is the most incompatible element, with a partition coefficient of ≤ 0.01 (Mungall et al. 2005). Ni partitions almost equally between the mss and the Cu-rich liquid. However, the partition coefficients strongly depend on the bulk S content in the system (Barnes et al. 2001). With increasing S content the partition coefficient increases in mss and liquid (Li et al. 1996). The effect of temperature is observed for Rh. The partition coefficient of Rh increases with decreasing temperature (Li et al. 1996).

Accordingly, pyrrhotite and pentlandite should be enriched in Os, Ir, Ru and Rh and chalcopyrite in Pd, Pt, Ag and Au, however, this effect was not observed in previous studies of different Ni-Cu-PGE deposits (Barnes et al. 2006; Godel et al. 2007; Dare et al. 2010). Consequently, other processes than sulfide liquid fractionation are thought to be responsible for a redistribution of trace elements and PGEs between BMS during recrystallization (see Dare et al. (2010) for discussion and references). For instance, Barnes et al. (2006) and Dare et al. (2010) suggested that the enrichment of Pd in pentlandite is due to diffusion during its exsolution from the mss. A small quantity of Pd is believed to stem from the Pd that had originally partitioned into the mss, whereas most of the Pd was derived by diffusion from the nearby iss (Dare et al. 2010). 19

PhD thesis Inga Osbahr 2. Introduction

There are several processes that may be responsible for the origin of the PGE mineralization. For example, Kinloch (1982), Campbell et al. (1983) and Naldrett (1986) suggested that the PGE precipitated from the magma and accumulated on the top of the crystal pile thus representing the “downers” theory. On the other hand, Vermaak (1976), Boudreau and McCallum (1992) and Boudreau and Meurer (1999) considered that an upward infiltration process of a volatile-rich fluid was responsible for the transport and precipitation of the PGE from the footwall, and thus represent the “uppers” theory. But there are also other models (e.g. by Willmore et al. 2000) which combine both theories, thereby suggesting that the PGE were scavenged from the footwall by a fluid that subsequently reacted with the magma at the crystal-magma interface to produce a primary sulfide precipitate. Although the exact process responsible for the PGE mineralization remains unclear, there evidently exists a strong relation between the PGE and the BMS mineralization.

PhD thesis Inga Osbahr 3. Sample Description and Petrography

3. Sample Description and Petrography 3.1. Merensky Reef The samples investigated originate from four fresh drill cores, SD124 D1 and SD 134 D3, from the farm Styldrift in the western Bushveld (Fig. 3.1). Drill core US200 D6 and US186 D4 stem from the farm Umkoanesstad in the eastern Bushveld (Fig. 3.1). In both cases, samples were taken from the economically mineralized part of the Reef. The mining width generally encompasses material from 30 cm above the top chromitite stringer to 1.5 m below the top chromitite. The cores were drilled by Anglo American Platinum in 2008.

The investigated drill cores show a normal Merensky Reef profile, as displayed in Figure 2.3 and described above. Drill core SD124 (western Bushveld) consists of a ca. 2.5 m thick pegmatoidal feldspathic pyroxenite with small lenses of harzburgite in the lower part. Drill core SD124 has one 5 mm thick lower chromitite stringer and two 5 mm and 10 mm thick upper chromitite stringers. The footwall consists of a poikilitic anorthosite while the hanging wall consists of feldspathic pyroxenite followed by gabbronorite (Fig. 3.1). The mineralization is tightly confined to the area of and around the upper chromitite stringers. Drill core SD134 (western Bushveld) shows mineralization mainly confined to a ca. 50 cm thick pegmatoidal feldspathic pyroxenite which is bound by a 5 mm thick lower chromitite stringer and two 5 mm and 10 mm wide upper chromitite stringers. The footwall consists of poikilitic anorthosite and the hanging wall consists of feldspathic pyroxenite followed by gabbronorite (Fig. 3.1).

The lithological succession in the drill cores from the eastern Bushveld coincide with the lithology from Schwellnus et al. (1976) from Lebowa, and show a normal Reef facies. Drill core US200 (eastern Bushveld) consists of a ca. 50 cm pegmatoidal feldspathic pyroxenite, overlain by a by a 7 mm thick chromitite stringer and a 75 cm thick feldspathic pyroxenite followed by norite. A sulfide stringer occurs 25 cm above the chromitite stringer. The footwall consists of gabbronorite, which also contains some PGE-poor sulfide stringers (Fig. 3.1). The mineralization is distributed around the chromitite and the sulfide stringers in the pyroxenite. Drill core US186 (eastern Bushveld) shows a ca. 60 cm thick pegmatoidal feldspathic pyroxenite overlain by a ca. 1 m thick feldspathic pyroxenite. The transition between both units is characterized by a 7 mm thick lower chromitite stringer. A one cm thick upper chromitite stringer occurs about 50 cm above the lower chromitite stringer. The footwall consists of feldspathic pyroxenite (Fig. 3.1). The mineralization is distributed around the chromitite stringers.

PhD thesis Inga Osbahr 3. Sample Description and Petrography

The important difference between the stratigraphy of the drill cores from the eastern and western Bushveld is that the pegmatoidal feldspathic pyroxenite hosts the PGE mineralization in the western Bushveld, while in the eastern Bushveld the feldspathic pyroxenite hosts the main PGE mineralization and the pegmatoidal feldspathic pyroxenite forms the immediate footwall of the reef.

Disseminated sulfides occur throughout the Merensky Reef and the immediate wall-rocks, showing an increasing grain size and modal content in the pegmatoidal feldspathic pyroxenite. The BMS usually form coarse-grained irregular granular aggregates of chalcopyrite, pentlandite, pyrrhotite, and minor pyrite. Occasionally pentlandite occurs as exsolution flames in pyrrhotite, or as coarse-grained granular aggregates intimately intergrown only with pyrrhotite. Cubanite and platinum group minerals (PGM) are accessory minerals. In addition, abundant troilite forms exsolution lamellae in pyrrhotite in samples from drill core SD134. The modal content of the sulfides varies from 1 to 5 vol. % in the norite, gabbronorite and feldspathic pyroxenite and reaches almost 8 vol. % in the sulfide-rich layers of the pegmatoidal feldspathic pyroxenite of drill core US200. This sulfide assemblage is made up of ca. 45 % pentlandite, 25 % chalcopyrite and 30 % pyrrhotite (Table 9.1), however the highest concentration of chalcopyrite occurs ca. 5 cm and 15 cm below the chromitite stringers in the medium and the wide reefs, respectively. On account of pyrrhotite the modal amount of pyrite increases from the bottom to the top in drill core US200. The grain size varies from a few µm in the feldspathic pyroxenite to around 1 cm in the pegmatoidal feldspathic pyroxenite, whereby the majority of BMS occur in the pegmatoidal and feldspathic pyroxenite units, while the norite and gabbronorite units are almost barren of BMS.

Pyroxenite

Two types of pyroxenite occur in the sequence of the Merensky Reef, the “normal” feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite. The units are characterized by orthopyroxene contents of 72-91 vol. % and up to 10 vol. % of clinopyroxene. Both units are mineralogically similar and only differ in the sizes of the orthopyroxene grains, which varies between 2-4 mm in diameter in the feldspathic pyroxenite and up to several cm in length in the pegmatoidal feldspathic pyroxenite. Clinopyroxene usually occurs as inclusion in orthopyroxene and only rarely as a matrix mineral. The other main component in these units is xenomorphic plagioclase, which has a similar grain size as the pyroxene and normally occurs as an interstitial mineral. In all investigated drill cores, the feldspathic pyroxenite and pegmatoidal feldspathic pyroxenite units contain most of the BMS, which reach up to 5 vol. % in pyroxenite. Disseminated chromite grains

PhD thesis Inga Osbahr 3. Sample Description and Petrography and accessory phlogopite with grain sizes up to 1 and 3 mm, respectively, are frequently present.

Norite and gabbronorite

The associated norite and gabbronorite comprise 60-67 vol. % orthopyroxene and up to 35 vol. % plagioclase with similar grain sizes as in the feldspathic pyroxenite. Minor components in all investigated units are phlogopite, disseminated sulfides and chromite as well as rare rutile and olivine.

Pyroxene anorthosite and poikilitic anorthosite

The difference between the feldspathic and poikilitic anorthosite is the size of the plagioclase and the pyroxenes within this unit. They can be up to 3 mm and 2 cm in size in the feldspathic and poikilitic anorthosite, respectively. The unit of the anorthosite is characterized by a plagioclase content of 80-100 vol. % and can contain up to 20 vol. % pyroxene, mainly orthopyroxene.

Harzburgite

Harzburgite lenses occur in the pegmatoidal feldspathic pyroxenite of drill core SD124. The harzburgite is characterized by pyroxene contents of up to 65 vol. %, up to 20 vol. % olivine and up to 10 vol. % of plagioclase. Olivine and plagioclase show average grain sizes of ca. 5 mm whereas the pyroxenes reach up to 1.5 cm in diameter. The olivine grains are usually or at least partly replaced by serpentine minerals. Disseminated sulfides, chromite and phlogopite are also present.

Chromitite

Chromitite either occurs in the form of thin stringers at the top and bottom of the Merensky Reef, or is disseminated throughout most of the units. The chromitite stringers consist, up to 95 vol. %, chromite. The grain sizes of the idiomorphic to hypidiomorphic chromite usually range between 100-200 µm, but may reach up to 750 µm in diameter. The chromite grains are embedded in a matrix of plagioclase with minor pyroxene and phlogopite. Inclusions of magnetite, sulfides and/or rutile are occasionally found in the chromites.

PhD thesis Inga Osbahr 3. Sample Description and Petrography

Fig. 3.1: Merensky Reef stratigraphy of the drill cores SD124 and SD134 from the western Bushveld and US200 and US186 from the eastern Bushveld. Highlighted in red is the geochemically investigated part of the Reef.

PhD thesis Inga Osbahr 3. Sample Description and Petrography

3.2. UG2 The mineral composition of the anorthosite, feldspathic pyroxenite, pegmatoidal feldspathic pyroxenite and norite is similar to those occurring in the Merensky Reef (Chapter 3.1). In drill core SD124 from the western Bushveld, the UG2 main seam is a chromitite seam, ca. 65 cm thick, which is located at around 710 m. The footwall consists of norite containing a 10 cm thick lens of pyroxene anorthosite at the immediate contact with the footwall. The UG2 hanging wall consists of feldspathic pyroxenite with several chromitite stringers (“leader” chromitite seam), which reach 1 mm to several cm in thickness. A sulfide mineralized zone is located in the hanging wall, above the UG2 chromitite, in immediate contact with the chromitite main seam and reach 1 m in thickness (Fig. 3.2). The modal content of the sulfides is much lower (< 1%) and their grain size much smaller (1-4 mm) than in the Merensky Reef. The sulfides are made up of ca. 60 % pentlandite and 40 % chalcopyrite, and accessorily pyrite and PGM occur. A second mineralized zone with an additional chromitite stringer occurs ca. 4 m above the main chromitite seam. The mineralogy of the second mineralized zone is different from that of the first zone. Chalcopyrite and pentlandite occur in the lower mineralized part, while the second mineralized part additionally contains abundant troilite and cubanite. Very small amounts of pyrrhotite were found in the lower mineralized part, however, in the upper mineralized part, abundant troilite, a late magmatic FeS phase, was detected. Additionally, cubanite, which exsolves from chalcopyrite below 200 °C, occurs in the upper mineralized zone. In the chromitite main seam and the “leader” chromitite seam, sulfides generally are absent. The UG2-chromitite comprises 80-95 vol. % compact chromite with a grain size of 1 mm. The chromite is embedded in a matrix of xenomorphic plagioclases.

In the eastern Bushveld, the main UG2 chromitite seam occurs at 946 m and is ca. 55 cm thick. The UG2 hanging wall consists of feldspathic pyroxenite, the footwall consists of a ca. 30 cm thick harzburgite, followed by feldspathic pyroxenite. As already described for drill core SD124 the hanging wall contains several “leader” chromitite stringers with a thickness of some mm. The mineralized zone of drill core DT46 is located in and ca. 1 m above the UG2 chromitite main seam (Fig. 3.2). The sulfides are mainly pentlandite, chalcopyrite, pyrrhotite and pyrite, which occur in the UG2 as well. They reach a grain size of up to 1 cm and a modal content of up to 1 %, which is rather uncommon for the UG2 section.

PhD thesis Inga Osbahr 3. Sample Description and Petrography

Fig. 3.2: UG2 stratigraphy of the drill cores SD124 and DT46 from the western and eastern Bushveld, respectively.

Noticeable is a strong alteration in drill core DT46 that occurs mainly in the units of the harzburgite. The alteration shows various grades, but is strictly related to the olivine in the harzburgite and only secondarily to clino- and orthopyroxene (Fig. 3.3). The alteration consists mainly of talc, magnesite, dolomite and calcite which were detected by XRD and shows beige to brown colors. Fig. 3.4 shows the process of olivine being replaced by the alteration products, ranging from only low affection at the rim (Fig. 3.4 a) to total replacement, but unaffected core (Fig. 3.4 c) and total replacement of olivine (Fig. 3.4 d) (pers. communication A. Josties, 2012). Horizontal channels are recognizable in which the alteration fluid came through. Different zones in these channels leads to the assumption of multiple impulses of fluid input. Some cataclastic cracks that are filled with alteration minerals and small fragments of pyroxene run through the lower sections of the feldspathic pyroxenite. These cracks, as well as cracks that occur in chromites and kink bands in plagioclase, argue for a slight brittle tectonic deformation has taken place (pers. communication A. Josties, 2012).

PhD thesis Inga Osbahr 3. Sample Description and Petrography

Fig. 3.3: Lithological profile of drill core DT46 with alteration grades (Josties 2012)

Fig. 3.4: Various grades of the alteration process: a) Alteration only at the rim of the olivine; b) proceeding alteration of olivine; c) altered olivine with unaffected core; d) olivine totally replaced by the alteration (images by A. Josties). 27

PhD thesis Inga Osbahr 4. Analytical Methods

4. Analytical Methods 4.1. Transmitted and reflected light microscopy A total of 79 thin sections were prepared by the University of Würzburg and the Federal Institute of Geoscience and Resources (BGR), in order to undertake the petrography of the Merensky Reef and the UG2 samples.

4.2. Electron microprobe analyses (EMPA) In-situ major element analyses of BMS, chromites and silicates were conducted by wavelength-dispersive X-ray spectrometry (WDX) using an electron microprobe type JEOL JXA-8200 superprobe at the GeoZentrum Nordbayern (GZN). The thin sections were sputtered with a graphite layer to enable electric conductivity. When analyzing BMS, the superprobe operates with a 20 kV acceleration voltage, a probe current of 20 nA and a focused beam with a counting time for peak and background of 20 s and 10 s, respectively. When analyzing silicates and chromites an acceleration voltage of 15 kV and a probe current of 15 nA was used; for PGM analysis an acceleration voltage and a probe current of 25 kv and 25 nA, respectively, was used. Beam size and counting time remain constant. The detection limit of the microprobe is 0.1 wt. % and analyses with totals between 99-101 wt. % were taken in consideration for further examination. Standards used for calibration are listed in Table 4.1. For qualitative analyses energy-dispersive X-ray spectroscopy (EDX) was used.

Table 4.1: Standards and analyzer crystals used for electron microprobe analyses

PhD thesis Inga Osbahr 4. Analytical Methods

Element Standard Analyzer crystal

Fe Fe2O3 LIF Ni NiO LIF Cu chalcopyrite LIF

S FeS2 PETH Co metallic Co LIF

Cr Cr2O3 LIF PGE (Pt, Pd, Rh, metallic Pt, Pd, Rh, Pd, Rh, Os, Ru Os, Ir, Ru) Os, Ir, Ru (PETJ) Pt, Ir (LIF) As GaAs TAP Te PbTe PETJ Si wollastonite TAP

Ti TiO2 LIF

Al Al2O3 TAP

Mn MnSiO3 TAP Mg MgO TAP Ca wollastonite PETJ Na albite TAP K orthoclase PETJ F apatite TAP Cl vanadinite PETH

4.3. Laser ablation-ICP-MS The content of trace elements in BMS, silicates and chromites were determined using LA- ICP-MS analyses at the GeoZentrum Nordbayern of the University of Erlangen-Nürnberg. The LA-ICP-MS system consists of a laser (UP193FX, Eximer, New Wave Research, coupled with a quadrupole ICP-MS (Agilent 7500i). Laser ablation is the most versatile in-situ solid sampling technique for ICP-MS. The focusing characteristics of the laser allow investigations of small areas, local microanalysis and spatially resolved studies, e.g. of trace elements in minerals, rocks and volcanic glasses, isotope ratios, bulk analysis of pressed powder briquettes and many others. The signal intensity is directly proportional to the amount of the ablated material, which is transported to the ICP-MS.

PhD thesis Inga Osbahr 4. Analytical Methods

Quantification of the signals occurs via appropriate standard reference material. Repeated standard measurements were made throughout a full analytical day for checking an eventually instrumental drift. The standards were measured several times, first calculated as an unknown sample before being taken over as a calibration standard, to determine the reproducibility and the accuracy of the results. The count rate obtained for a particular ion is compared to a calibration plot to yield the concentration for a respective element in the studied sample (c.f. Homepage University of Erlangen).

56 polished thin sections of four drill cores were analyzed with the main focus on PGE, Au and Ag concentrations. The PGE (99Ru, 101Ru, 103Rh, 105Pd, 108Pd, 189Os, 193Ir, 195Pt) as well as 197Au, 107Ag, 185Re, 59Co, 60Ni, 61Ni, 63Cu were determined in BMS, chromites and silicates. For external calibration, the following standard reference material was used: Po724 B2 SRM (synthetic pyrrhotite, Memorial University Newfoundland) for Pd, Rh, Pt, Ru, Os, Ir and Au;

MASS-1 polymetal sulfide (USGS) for Co, Ag and Cu and (Fe, Ni)1-xS for Ni, Re and S (Wohlgemuth-Ueberwasser et al. 2007). LA-ICP-MS measurements were conducted using a spot size of 50 µm in diameter, a laser frequency of 15 Hz and 0.32 GW/cm2 (irradiance) and a fluence of 1.62 J/cm2. For smaller grains a 35 µm spot was used. The carrier gas consists of a mixture of 0.65 l/min helium and 1.06 l/min argon. Acquisition time was 20 s for the background and 20 s for the mineral analysis. Signal quantiﬁcation was carried out by GLITTER (Van Achterbergh et al. 2000) using sulfur as an internal standard for BMS, as determined by means of the electron microprobe, chromium for chromite and silicon for the silicates.

Various interferences caused by the carrier and plasma gas Ar lead to falsified results for Ru, Rh and Pd if not corrected. It is necessary to measure some elements (Ru, Pd) for more than one isotope, because of the existing argide interferences which cannot be solved, due to the given instrumentation at the GZN; these interferences can only be circumvented by choosing the appropriate isotope. 99Ru is interfered by 63Cu36Ar and 59Co40Ar, 101Ru by 63Cu38Ar, 65Cu36Ar and 61Ni40Ar, 103Rh by 63Cu40Ar, 65Cu38Ar and 67Zn36Ar, 105Pd by 65Cu40Ar and 108Pd by 68Zn40Ar. 108Pd is additionally interfered by 108Cd. Due to the extremely low relative isotopic abundance of 36Ar (0.337%) and 38Ar (0.063%), their effect on 99Ru, 101Ru and 103Rh is negligible. Only a few counts per second for Zn were recorded for the sulfides in this study; a significant influence can therefore be excluded. However, a remarkable impact occurs for CuAr on 103Rh and 105Pd when analyzing chalcopyrite. The argide-unaffected 108Pd for chalcopyrite can be used, which nonetheless requires a correction for the elemental interference from 108Cd. A PGE-free hydrothermal

PhD thesis Inga Osbahr 4. Analytical Methods chalcopyrite from Messina (South Africa) was analyzed to check the 63Cu40Ar and the 65Cu40Ar production; the results vary from 4-5 ppm for 103Rh and 6-8 ppm for 105Pd. Since the interference of 61Ni40Ar leads to an overestimation of 101Ru in pentlandite, 99Ru was used instead to obtain the correct concentration. Numerous measurements on pentlandite, over a long time period and for different samples, yield ~0.5 ppm as the lowest concentration for Ru, whereby it is presumed that this originates solely from the Ni-argide. This concentration always lies within the 1б error margin for every analysis. For Co-rich minerals the interference 99Ru by 59Co40Ar has to be corrected. Considering that the pentlandites in this study contain a maximum of only up to 0.5wt.-% Co, a correction is therefore negligible. The detection limit for each element analyzed in the BMS is summarized in Table 4.2.

Table 4.2: Average detection limits in ppb for each element analyzed in BMS by LA-ICP-MS. SD124 D1 101Ru 103Rh 105Pd 189Os 193Ir 195Pt 197Au Pn 74 25 94 86 25 55 35 Po 73 16 65 93 26 58 37 Chp 49 12 46 58 16 36 25 Py 20 5 21 31 8 19 11 SD134 D3 101Ru 103Rh 105Pd 189Os 193Ir 195Pt 197Au Pn 29 12 140 39 12 21 14 Po 30 6 26 29 8 20 13 Chp 22 5 20 23 6 15 10 Py 27 6 25 29 8 20 13 US200 D6 101Ru 103Rh 105Pd 189Os 193Ir 195Pt 197Au Pn 60 16 79 79 22 50 30 Po 56 11 42 69 22 40 25 Chp 42 13 47 58 16 36 23 Py 17 4 16 20 5 12 8 US186 D4 101Ru 103Rh 105Pd 189Os 193Ir 195Pt 197Au Pn 33 8 36 41 11 27 17 Po 27 6 27 34 8 23 14 Chp 25 6 25 30 8 20 13 Py 10 2 10 13 4 9 5

The reproducibility of the measurements was checked by analyzing and calculating the SRM as unknown sample at least four times (ca. every 30th analyses). Only measurements with the correct results were used for external calibration. The reproducibility for SRM is < 9 % and the accuracy, tested by ablating the PGE SRM FeNiS standard, is < 15 %. 11 sessions were conducted on the PGE distribution in BMS. Figs. 4.1 and 4.2 show the measured and accepted values of the PGE, Ag and Au of the reference material (Po724 and

PhD thesis Inga Osbahr 4. Analytical Methods mass-1) and their uncertainty values. Generally, the measurements at the GeoZentrum agree between 95.23 and 101.35 % with the accepted values. Two sessions agree between 92.8 and 100.17 % and one session between 99.40 and 108.50 %. Frequently, it is Ag that shows the maximum deviation. In all sessions the measured element contents of the standard are within the error bars of the standard accepted values even for sessions which only agree within 92 or 108 %. Fig. 4.1 shows e.g. session three, which is representative for all sessions that agree within 95 and 100 %. Fig. 4.2 shows e.g. session six, which is representative for all sessions which agree between 92 and 108 %.

60.00

55.00

50.00

45.00 Standard accepted GZN measured

40.00

35.00

30.00 Ru Rh Pd Os Ir Pt Au Ag

Fig. 4.1: Comparison of accepted standard values of PGE Po724 and mass-1 (red line) and measured values at the GeoZentrum Nordbayern (dashed blue line) of Ru, Rh, Pd, Os, Ir, Pt, Au and Ag. Third session (26.05.2010); error bars are indicated.

60.00

55.00

50.00

45.00 Standard accepted GZN measured

40.00

35.00

30.00 Ru Rh Pd Os Ir Pt Au Ag

PhD thesis Inga Osbahr 4. Analytical Methods

Fig. 4.2: Comparison of accepted standard values of PGE of Po724 and mass-1 (red line) and measured values at the GeoZentrum Nordbayern (dashed blue line) of Ru, Rh, Pd, Os, Ir, Pt, Au and Ag. Sixth session (14.04.2011); error bars are indicated.

Re and Ni (FeNiS) and Co and Cu (mass-1) agree between 93.22 and 106.56 % in all sessions. Re and Ni show a maximum deviation of 70.72 and 84.35 %, respectively. The relative standard deviation (RSD %) is generally below 10 % with the exception of Re (≤ 62 %) and in one session Ni (≤ 48.10 %) and Cu (28.10 %). According to these data the LA-ICP-MS measurements are regarded as accurate and precise.

4.4. Electric pulse disaggregation (EPD) and Hydroseparation (HS) The PGM distribution and composition were determined using Electric Pulse disaggregation (EPD) and hydroseparation (HS) methods with additional scanning electron microprobe analyses. The analyses were conducted by CNT-Mineral Consulting Inc. in St. Petersburg. Furthermore, these methods deliver detailed information about grain size, mineral associations and recovery of precious metals (PGM, Ag and Au). Samples were processed by EPD through a 1 mm sieve and afterwards through a 71 µm and a 50 µm sieve. After crushing, the samples, containing magnetic parts, were processed through wet magnetic separation and subsequently cleaned using ultrasound sedimentation. The crushing products were hydroseparated in order to produce monolayer polished sections from the heavy metal concentrates of each size fraction of every sample. These polished sections were investigated by scanning electron microprobe (Camscan-4DV, Link An-10000) in order to document the relationship between the rock-forming and accessory minerals such as precious minerals and sulfides. For more information about this method see Cabri et al. (2008).

4.5. Ni-fire assay Gold and PGE analyses were conducted by Anglo American’s Technical Solutions - Research (Johannesburg, South Africa) and Actlabs (Activation Laboratories Ltd., Ontario, Canada). Representative samples, covering the entire section of the mineralized part of the Merensky Reef, were taken from the four drill cores (US200, US186, SD124 and SD134) in order to undertake bulk rock chemical analyses. Each sample with a length of 16 to 25 cm of halved 3.5 cm diameter drill cores were pulverized and analyzed for Pd, Pt, Rh and Cu, Ni, Cr and S, using the Ni-sulfide fire assay method and Leco CNS for sulfur. Additional sulfur and IPGE analyses of samples which were below the detection limit in the 20 cm samples were conducted on smaller samples (7 cm in length) from drill core US200. The basic procedure for Ni-fire assay involves mixing the sample (ca. 30 g) with flux, which is a combination of sodium carbonate (soda ash-Na2CO3), sodium borate (borax-Na2B4O7), 33

PhD thesis Inga Osbahr 4. Analytical Methods nickel powder, sulfur and silica. In order to properly flux the sample, Ni, Cu, Cr and S content should be determined in advance. The flux and sample are heated at 1000 °C to 1100 °C. Nickel sulphide droplets were formed which scavenge the PGE. The resulting Ni sulphide button is dissolved in concentrated HCL. The residue of insoluble sulfide contains the PGE and Au and can then be irradiated for subsequent INAA (Instrumental Neutron Activation Analysis) (more information in Hoffmann and Dunn 2002). At Actlabs detection limits are 2 ppb for Os, 0.1 ppb for Ir, 5 ppb for Ru, 0.2 ppb for Rh, 5 ppb for Pt, 2 ppb for Pd, 0.5 ppb for Au and 0.01 wt. % for S. At Anglo American’s Research Laboratories the detection limit for Pt, Pd and Rh is 0.01 ppm and for S 0.005 wt. %.

PhD thesis Inga Osbahr 5. Mineral Chemistry

5. Mineral Chemistry 5.1. Merensky Reef The mineral chemistry of the minerals from the western and eastern Bushveld are described as one, since no significant variation in mineral chemistry was observed between them. If differences occur they are mentioned separately in this chapter.

5.1.1. Orthopyroxene

Orthopyroxene is, with the exception of anorthosite, the most common mineral in all units investigated. The composition of the orthopyroxene generally is enstatitic (En77-83) with a wollastonite component of up to 5.92 % (Fig. 5.1). The MgO and FeO content ranges between 25.7 and 31.2 wt. % and 10.7 and 16.9 wt. %, respectively, while the CaO content ranges between 0.40 and 5.92 wt. %. The sum of MnO, TiO2 and Cr2O3 is between 0.61 and 1.06 wt. %. The #Mg (#Mg=Mg/Mg+Fe2+) varies between 0.74 and 0.85. The orthopyroxene shows fine parallel lamellae (Fig. 5.3 a), which are thought to be due to exsolution during cooling. Frequently, the idiomorphic to hypidiomorphic orthopyroxene builds up triple junctions due to recrystallization.

Fig. 5.1: Composition of orthopyroxene in the ternary CaSiO3, MgSiO3 and FeSiO3 system after Morimoto (1988). 24 analyses of SD124 samples (western Bushveld) and 13 analyses of US200 samples (eastern Bushveld).

5.1.2. Clinopyroxene

Clinopyroxene is a minor component in the pyroxenite and norite and an accessory mineral in the other units. Clinopyroxene occurs as inclusions in orthopyroxene or as oikocrists (Fig. 5.3 b). The composition reaches from diopside to augite, according to the classification of (Morimoto 1988) (Fig. 5.2).

PhD thesis Inga Osbahr 5. Mineral Chemistry

The MgO and FeO content of the clinopyroxenes ranges between 15.4 and 27.3 wt. % and 4.64 and 13.3 wt. %, respectively. The CaO content shows a broad range from 4.43 -22.8 wt.

%. Al2O3 can reach up to 2.58 wt. % and the sum of MnO, TiO2 and Cr2O3 is between 0.61 and 1.47 wt. %. The #Mg varies between 0.82 and 0.92, while the #Fe and #Al vary between 0.08 and 0.18 and 0.27 and 0.81, respectively.

Fig. 5.2: Composition of clinopyroxene in the ternary CaSiO3, MgSiO3 and FeSiO3 system after Morimoto 1988. 3 analyses of SD124 samples (western Bushveld) and 11 analyses of US200 samples (eastern Bushveld).

a) b) Fig. 5.3: a) Lamellae in orthopyroxene, due to exsolution during cooling; b) Unit of the pegmatoidal feldspathic pyroxenite with orthopyroxene and interstitial plagioclase and inclusions of clinopyroxene (US200-7a).

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.1.3. Plagioclase

Plagioclase is the main component in the poikilitic and pyroxene anorthosite, and it occurs as a main component in the norite, feldspathic pyroxenite and pegmatoidal feldspathic pyroxenite and as an interstitial mineral in the chromitite stringers (Fig. 5.5 a and b). The composition of the plagioclase in these units covers a range from andesine to anorthite (An44 - An91) in the western and eastern Bushveld (Fig. 5.4). CaO ranges from 9.04 wt. % to 18.7 wt. % and Na2O from 0.97 to 6.51 wt. %, while K2O reach a maximum of 0.40 wt. %. FeO and MgO reach up to 0.36 and 0.08 wt. %, respectively. The content of the plagioclase, which occur in the range of chromitite stringers or in the range of larger amounts of sulfides is more An-rich (An80) than that of plagioclase, which occurs below or above the chromitite stringers or in sulfide-depleted areas (An58-An65).

Fig. 5.4: Composition of plagioclase in the ternary NaAlSi3O8, CaAl2Si2O8 and KAlSi3O8 system after Okrusch and Matthes (2005). 16 analyses of SD124 samples (western Bushveld) and 11 analyses of US200 (eastern Bushveld).

a) b) Fig. 5.5 a) Unit of the pyroxene anorthosite with plagioclase and interstitial orthopyroxene and phlogopite (SD124- 30); b) Interstitial plagioclase in the chromitite stringer (SD124_14-2).

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.1.4. Phlogopite

Phlogopite occurs as an accessory mineral (< 1 vol. %) in almost all investigated units. The hypidiomorphic phlogopite is interstitial and reaches several mm in grain size. Phlogopite is mostly associated with chromites and sulfides (Fig. 5.6 a and b). MgO and FeO vary between 18.1 and 21.5 wt. % and 7.58 and 11.1 wt. %, respectively. Al2O3 ranges between

13.0 and 14.5 wt. % and K2O between 8.84 and 9.59 wt. %. The phlogopite contains significant amounts of TiO2 ranging from 0.30-4.95 wt. %. F and Cl reach up to 1.07 and 0.49 wt. %, respectively. The #Mg ranges from 0.76-0.90 and is higher, if the phlogopite is associated with the chromitite stringers or with sulfides.

a) b) Fig. 5.6: a) Chromitite stringer with associated phlogopite, parallel nicols (US200-8); b) with crossed nicols.

5.1.5. Olivine

Olivine (Fig. 5.7 a) is the main component of the harzburgite and its composition ranges between the Mg-endmember forsterite (Mg2SiO4) and the Fe-endmember (Fe2SiO4) fayalite. However, most of the olivine is in the process of being replaced to serpentine (Fig. 5.7 b).

The olivine of the harzburgite investigated has a Mg-rich composition of Fo77-78Fa22-23. The content of MgO and FeO has a limited range between 40.9 and 41.7 wt. % and between 19.6 and 20.8 wt. %, respectively. Their #Mg lies between 0.80 and 0.81. Small amounts of MnO (0.22-0.27 wt. %) and NiO (0.42-0.54 wt. %) were detected as divalent cations replacing Mg and Fe, respectively (Deer et al. 1992).

PhD thesis Inga Osbahr 5. Mineral Chemistry

a) b) Fig. 5.7: a) Unit of the gabbronorite showing orthopyroxene, olivine, chromite and plagioclase (SD124-19); b)The unit of the harzburgite showing olivine, which is in the process of being replaced by serpentine (SD124-29).

5.1.6. Chromite

Compact chromite (around 90 vol. %) occurs in the lower and upper chromitite stringers (Fig. 5.9 a-c), while it is disseminated in the units above, below and between the chromitite stringers. Little compositional variations may occur between the disseminated chromites and the chromites of the stringers. Cr2O3 concentration is higher in the chromitite stringers than in the disseminated chromites, while the Fe concentration in the chromites of the stringers is lower than in the disseminated chromites (Fig. 5.8).

In the lower chromitite stringer the Cr2O3 concentration of the chromites ranges from 39.4-

43.0 wt. %, and the upper chromitite stringer shows maximum Cr2O3 concentrations of up to 46.1 wt. %. The disseminated chromite, between the chromitite stringers, has lower concentrations of 32.9-40.9 wt. %. The FeO concentration in chromites reaches 31.8 wt. % and 35.0 wt. % in the lower and upper chromitite stringer, respectively, while the disseminated chromites reach 37.3 wt. % (Fig. 5.8).

The Al2O3 and MgO concentrations of chromites increase from the lower chromitite stringer

(13.9 wt. % Al2O3 and 6.8 wt. % MgO) to the upper chromitite stringer (19.0 wt. % Al2O3 and 8.8 wt. % MgO) (Fig. 5.8).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.8: Concentration of a) Cr2O3 and FeO b) Al2O3 and MgO in chromites of the lower and upper chromitite stringer and of disseminated chromites in the unit of the pegmatoidal feldspathic pyroxenite between the chromitite stringers of the Merensky Reef of drill core SD124. No scale on x-axis!

PhD thesis Inga Osbahr 5. Mineral Chemistry

a) b)

c) Fig. 5.9: a) Unit of the pegmatoidal feldspathic pyroxenite with interstitial plagioclase and a 1 mm thick chromitite stringer (SD124-14_2); b) Chromites of the lower chromitite stringer of drill core SD124 (SD124-30) c) Transition between the lower chromitite stringer and the feldspathic pyroxenite in drill core US200 ( US200-9).

Base-metal sulfides

Base-metal sulfides mostly occur in the units of the feldspathic pyroxenite and pegmatoidal feldspathic pyroxenite in immediate contact with silicates (Fig. 5.18 a) or frequently with disseminated chromites (Fig. 5.18 b). They are rare in the norite and gabbronorite and hardly any sulfides occur in the anorthosite or chromitite stringers. The disseminated sulfides are chalcopyrite, pentlandite, pyrrhotite, and minor pyrite.

5.1.7. Pentlandite (Ni, Fe)9S8

Three types of pentlandite occur in the units of the Merensky Reef. The main type occurs as coarse-grained pentlandite, which is intimately intergrown with chalcopyrite, pyrrhotite and pyrite (Fig. 5.18 a). Frequently, pentlandite is intergrown only with pyrrhotite (Fig. 5.18 c), while chalcopyrite and pyrite are absent in these aggregations. Rarely pentlandite occurs as exsolution flames in pyrrhotite (Fig. 5.18 d). The composition of pentlandite plots in the stability field of pentlandite in the ternary Fe-Ni-S system at 400 and 600 °C (Fig. 5.13). The S content is very constant and varies between 31.6 and 34.6 wt. %, while Fe varies between 30.3 and 39.5 wt. % and Ni between 26.5 and

PhD thesis Inga Osbahr 5. Mineral Chemistry

38.7 wt. %, respectively. The pentlandites investigated may contain up to 4.30 wt. % Co, which generally ranges between 0.01 and 2.27 wt. %. For example, in drill core SD134 of the western Bushveld, the highest Co contents are in the samples of the feldspathic pyroxenite. An increasing trend of Co concentration in pentlandite is noticeable from the lower chromitite stringer (SD134-19) to the unit of the feldspathic pyroxenite (SD134-5, -6 and -14), which is located above the upper chromitite stringer (SD134-15) (Fig. 5.10).

1.2

1.0 SD134-5

0.8 SD134-6 0.6 SD134-14

Co (at %) (atCo SD134-15 0.4 SD134-16 0.2 SD134-18 0.0 SD134-19 23 23.5 24 24.5 25 25.5 26 Ni (at %)

Fig. 5.10: Ni vs. Co in pentlandite of drill core SD134 (western Bushveld) in at %.

Whereas, in drill core US186 of the eastern Bushveld, no increasing or decreasing trend of Co concentration in pentlandite is evident (Fig. 5.11). However, the highest Co concentration occurs in the uppermost portion of the pegmatoidal feldspathic pyroxenite (US186-11), while lower contents occur in the range of the lowermost portion of the pegmatoidal feldspathic pyroxenite (US186-12 and -13). The samples, including the lower (US186-6) and upper chromitite stringer (US186-8) contain similar Co concentrations (Fig. 5.11).

PhD thesis Inga Osbahr 5. Mineral Chemistry

3 US186-3

US186-4 2 US184-5

Co (at%)Co US186-8 1 US186-11 US186-12 0 US186-13 25 26 27 28 29 30 Ni (at%)

Fig. 5.11: Ni vs. Co in pentlandite of drill core US186 (eastern Bushveld) in at %.

Co concentration in pentlandite are up to 3.3 wt. % in the eastern Bushveld and are thus higher when compared to those of the western Bushveld (maximum of 1.0 wt. %).

The Fe/S ratio is between 0.92 and 1.15, and the Fe/Ni ratio, which reflects the Fe-Ni substitution, between 0.82 and 1.59; however, the lowest Fe/Ni ratio of ca. 0.83 occurs in the pentlandites of drill core US186. Frequently a significant amount of Ag can be detected in pentlandites. Generally, the pentlandites from the eastern Bushveld (US200 and US186) have slightly higher Ni contents (average ca. 36.02 wt. %) than the pentlandites from the western Bushveld (average ca. 29.6 wt. %) (SD124 and SD134). It is noticeable that in the western Bushveld the Fe content is higher than the Ni content, conversely in the eastern Bushveld, the Ni content is higher than the Fe content (Fig. 5.12).

In drill core SD134, the Fe concentration increases slightly from the lower to the upper chromitite stringer, while Ni decreases (Fig. 5.12). In drill core US186, the Ni concentration increases and the Fe content decreases upwards from the feldspathic pyroxenite to the upper chromitite stringer (Fig. 5.12).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.12: Fe and Ni concentration in pentlandite samples from the feldspathic pyroxenite, lower and upper chromitite stringer and the pegmatoidal feldspathic pyroxenite of drill core SD134 (western Bushveld) and US186 (eastern Bushveld).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.13: Composition of pentlandite in the Merensky Reef plotted in the ternary system of Fe-Ni-S at a) 400 °C after Craig and Scott (1976) and b) 600 °C after Barnes and Lightfoot (2005).

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.1.8. Chalcopyrite (CuFeS2)

Chalcopyrite occurs, mostly intergrown with pentlandite, pyrrhotite and pyrite (Fig. 5.18 a and c), or frequently as single residual grains mainly associated with the chromitite stringers. The composition of chalcopyrite fits in the stability field of iss in the Cu-Fe-S ternary system of 400 °C (Fig. 5.16), as its exsolution from iss does not start below 550 °C (Fig. 5.17). The S content in chalcopyrite only shows a limited variation between 33.5 and 35.1 wt. %, however, in thin section SD134-16, S contents are lower, and range between 32.8 and 33.36 wt. %. Fe and Cu contents vary between 30.6-33.6 wt. % and 33.1-35.2 wt. %, respectively.

5.1.9. Pyrrhotite (FeS-Fe1-xS) and troilite (FeS)

While pyrrhotite occurs, mostly intergrown with pentlandite, chalcopyrite and pyrite (Fig. 5.18 a and f), or frequently only with pentlandite (Fig. 5.18 c), troilite occurs only as exsolution in pyrrhotite (5.18 e). These two phases were identified using the electron microprobe. Due to their similar composition, it was hard to clearly distinguish between pyrrhotite and troilite, hence additional XRF-analyses were conducted. However, both pyrrhotite and troilite were only detected in drill cores SD134 and SD124 from the western Bushveld. The composition of pyrrhotite and troilite plots in their stability field in the Cu-Fe-S ternary system at 600 °C (Fig. 5.17). The Fe content in pyrrhotite and troilite ranges between 56.4 and 62.8 wt. % and 62.2 and 65.7 wt. %, respectively. The S content ranges from 36.1- 39.8 wt. % in pyrrhotite and from 34.4-38.5 wt. % in troilite.

Pyrrhotite (Fe1-xS) exhibits a variable iron content which is shown in Fig. 5.14 and Fig. 5.15. Pyrrhotite reveals lower Fe contents in samples containing the chromitite stringer (SD134- 19), due to substitution of Fe with Cr. In the upper chromitite stringer no pyrrhotite analyses were available. In the eastern Bushveld, the Fe depletion in chromite-rich samples (US186-5 and US186-8) is not as obvious as in those from the western Bushveld. However, the pyrrhotite from the eastern Bushveld shows generally lower Fe content than those from the western Bushveld.

PhD thesis Inga Osbahr 5. Mineral Chemistry

51.5 51.0 50.5 SD134-5

50.0 SD134-6 49.5 SD134-13

Fe (at %) Fe (at 49.0 SD134-14 SD134-16 48.5 SD134-18 48.0 SD134-19 47.5 48 49 50 51 52 53 S (at %)

Fig. 5.14: S vs. Fe in pyrrhotite in at % in drill core SD124 (western Bushveld). Sample SD134-19 contains a chromitite stringer and thus, reveals a lower Fe content due to substitution with Cr.

47.9

47.7 US186-3 47.5 US186-4 47.3 US186-5

Fe (at %) Fe (at 47.1 US186-8

46.9 US186-11 US186-12 46.7 US186-13 46.5 52.2 52.4 52.6 52.8 53 53.2 53.4 S (at %)

Fig. 5.15: S vs. Fe in pyrrhotite in at % in drill core US186 (eastern Bushveld).

5.1.10. Pyrite (FeS2)

Pyrite is an accessory mineral and occurs intimately intergrown with pentlandite, chalcopyrite and pyrrhotite (Fig. 5.18 f). The composition of pyrite is also shown in the Cu-Fe-S ternary system at 600 °C (Fig. 5.17). The S content varies between 51.9 and 54.2 wt. %, while the Fe content ranges from 46.8-50.7 wt. %.

5.1.11. Cubanite (CuFe2S3)

The composition of cubanite is shown in the Fe-S-Cu ternary system at 400°C (Fig. 5.16). Rare cubanite occurs only in a few of the investigated units. It commonly occurs as intimately

PhD thesis Inga Osbahr 5. Mineral Chemistry oriented intergrowth with chalcopyrite. Cubanite is thought to exsolve from chalcopyrite at temperatures below 220 °C (Anthony et al. 2011). The S content varies between 34.7 and 34.9 wt. %, while Fe and Cu range from 42.3 to 42.3 wt. % and from 23.5 to 23.8 wt. %, respectively.

Fig. 5.16: Composition of chalcopyrite and cubanite at 400°C after Cabri 1973. Blue squares are chalcopyrite samples; red squares are cubanite samples. Atomic proportion in %.

Fig. 5.17: Composition of chalcopyrite, cubanite, pyrite, pyrrhotite and troilite plotted in the Cu-Fe-S ternary system at 600°C. Chalcopyrite plots in the field of the iss before its exsolution at 550 °C after Cabri (1973). Atomic proportion in %.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.18: a) Mineral aggregation of pentlandite (Pn, cream-white), chalcopyrite (Ccp, yellow-greenish), pyrrhotite (Po, grayish) and PGM (blue-grey) (Reflected light photomicrograph) (SD134-15_1); b) Sulfide in immediate contact with chromite (Chr) (US186-11); c) Coarse-grained mineral aggregates of pentlandite (cream-white) and pyrrhotite (grayish) without chalcopyrite (Reflected light photomicrograph) (US186-6); d) Exsolution flames of pentlandite in pyrrhotite (US186-11); e) Exsolution of troilite (Tr) in pyrrhotite (SD134-5); f) Mineral aggregation of pentlandite (cream-white), pyrrhotite (brownish), chalcopyrite (yellow-greenish) and pyrite (blue-white) (Reflected light photomicrograph) (SD124-13).

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.1.12. Platinum-group minerals

As a representative example for PGM, analyses of samples from drill core US200 were analyzed by EPD and HS. In the western Bushveld PGM were analyzed by electron microprobe, but it was difficult to identify the PGM due to their very small grain size (ca. 10 µm) and a probable PGM loss during preparation. In drill core US186, PGM were detected only sporadically. Several PGM were identified in the chromitite stringers, while they were hard to find in the units of the feldspathic pyroxenite. Due to the small grain size of the PGM, contamination occurred (as was expected) during measurements from the adjacent sulfides or silicates, and hence the identification was complicated, and totals sometimes are below 100 wt. % in samples from the western Bushveld. Samples for PGM analyses, from the drill core US200, were taken from three different lithologies, namely the lower chromitite stringer (US200-9), the sulfide-rich feldspathic pyroxenite (US200-7a), and from the contact between the pegmatoidal pyroxenite and the gabbronorite (US200-11) (Fig. 6.3). Most PGM were individual grains (~ 70 %) or intergrown with pentlandite or chalcopyrite (~ 20-25 %). The remaining PGM (~ 5 %) were associated with silicate minerals (Table 5.1). Inclusions of PGM in silicates or chromite grains were not detected (Rudashevsky 2011). The PGM assemblage of the samples from core US200 is dominated by Pt-rich-minerals, namely cooperite/braggite [(Pt,Pd,Ni)S], moncheite [Pt(Te,Bi)2], rare laurite (RuS2), isoferroplatinum (Pt3Fe), gold and minor sperrylite (PtAs2). In addition, rustenburgite (Pt3Sn) and an unnamed phase with the composition (PtSnS) were identified. Most of the PGM were detected in the chromitite stringer (94 PGM plus 4 gold grains) and in the sulfide stringer above (40 PGM plus 10 gold grains), whereas only 4 PGM grains were found in the pegmatoidal feldspathic pyroxenite unit (Table 5.1). The PGM (n=94) in the vicinity of the chromitite stringer are dominated by cooperite (50 %) and moncheite (23 %) followed by the unnamed PtSnS mineral (11 %), rustenburgite (7 %), sperrylite (4 %), braggite (2 %), native gold (4 grains), isoferroplatinum and niggliite (one grain each). In the sulfide stringer (n PGM+gold= 50), cooperite dominates (58 %) followed by native gold (20 %), moncheite (18 %) and one grain each of braggite and the unnamed PtSnS. The five grains from the pegmatoidal feldspathic pyroxenite are isoferroplatinum and moncheite (two grains each) and native gold. The precious metal grains are mostly irregular in shape with an average grain size of between 30-40 µm in diameter (Rudashevsky 2011). Altogether 33 PGM grains were identified in the polished sections of the samples from the western Bushveld (SD124, SD134). The PGM assemblage comprises cooperite, braggite, laurite, sperrylite, moncheite, isoferroplatinum and hollingworthite (Table 5.2).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Table 5.1: Type and association of PGM in sample US200, eastern Bushveld (Rudashevsky 2011)

Lower chromitite stringer number L Lp Pn Ccp Po Py Silicate (US200-9) Cooperite 47 29 8 4 4 Moncheite 22 7 10 1 9 2 Braggite 2 1 1 1 Isoferroplatinum 1 1 Rustenburgite 7 1 17 8 Niggliite 1 Sperrylite 4 4 4 unnamed mineral (PtSnS) 10 10 3 Au 4 3 1 1 Total 98 38 53 6 30 0 0 3 Sulfide-rich feldspathic pyroxenite (US200-7a) Cooperite 29 20 1 2 3 4 1 Moncheite 9 2 1 3 1 3 1 Braggite 1 1 unnamed mineral (PtSnS) 1 1 Au 10 4 2 2 4 1 Total 50 26 4 5 11 5 4 2 Pegmatoidal pyroxenite (US200-11) Moncheite 2 1 1 Isoferroplatinum 2 1 1 (Au,Ag) 1 1 Total 5 3 0 0 0 1 0 1 L = Liberated (free) precious metal; Lp = more than one precious metal completely liberated (free); in contact with pentlandite (Pn), chalcopyrite (Ccp), pyrrhotite (Po), pyrite (Py), silicates

Table 5.2: Type and association of PGM in samples from samples SD124 and SD134 (western Bushveld).

Upper chromitite stringer number L Lp Pn Ccp Po Py Silicate Laurite 8 1 3 1 1 2 Braggite 5 1 2 2 Sperrylite 6 3 2 1 Cooperite 4 2 2 Isoferroplatinum 3 1 2 Hollingworthite 1 1 Total 27 0 2 10 9 1 0 5 Lower chromitite stringer Moncheite 4 1 2 1 Cooperite 1 1 Isoferroplatinum 1 1 Total 6 2 3 1 L = Liberated (free) precious metal; Lp = more than one precious metal completely liberated (free); in contact with pentlandite (Pn), chalcopyrite (Ccp), pyrrhotite (Po), pyrite (Py), silicates

PhD thesis Inga Osbahr 5. Mineral Chemistry

Laurite (RuS2) In reflected light, laurite is white with a touch of bluish grey. It forms a complete solid solution with ehrlichmanite (OsS2) and a limited solid solubility exists with IrS2. It mostly occurs as inclusion in sulfides or at silicate-sulfide boundaries (Fig. 5.19). The laurite is hypidiomorphic- idiomorphic and can reach grain sizes up to 70 µm, sometimes even 200 µm.

a) b)

c) d) Fig. 5.19: a) Laurite in pentlandite with contact to chalcopyrite (SD134-15_2, reflected light in air); b) Laurite located at the sulfide-silicate boundary (SD134-15_2, reflected light in air); c) Laurite at a sulfide-silicate boundary (SD134-15_1, BSE-image); d) Laurite as inclusion in sulfide (US186-5).

In the system of Ru-Os-Ir, the samples from the eastern Bushveld are slightly more depleted in Os and Ir compared to those from the western Bushveld (Fig. 5.20 a). However, all samples show similar proportions of Os and Ir. The system of Ru-Fe-Os+Ir reveals a strong Fe-enrichment for the samples from the eastern Bushveld (Fig. 5.20 b). The composition of laurite shows a broad range in Ru composition from 20.0-52.2 wt. % (Fig. 5.20 b). The S content varies from 38.4-47.7 wt. %. Significant amounts of Os and Ir were detected with concentrations between 3.26-7.40 wt. % and 0.99-3.16 wt. %, respectively. The laurite, especially in drill core US186, reaches high Fe concentration with up to 29.3 wt. %. Small amounts of Pd and As were detected with up to 1.42 and 2.86 wt. %, respectively.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.20: a) Laurite in the Ru-Os-Ir system. Samples from drill core SD124 and US186 show higher Ru concentrations while samples from SD124 show higher Os-Ir concentrations; b) Laurite in the Ru-Fe-Os+Ir system. Samples from drill core US186 reveal a higher Fe component compared to samples from the western Bushveld. Atomic proportion in %.

Braggite ((Pt,Pd)S) Braggite is white in reflected light, very slightly bireflectant and lacks pleochroism, whereas it is distinctly anisotropic in air. It forms an extensive solid solution series with vsotskite (PdS). Braggite was found in the chromitite stringers of the western Bushveld and in the chromitite- and sulfide-rich areas of the eastern Bushveld. It is one of the most abundant platinum group minerals found in the western Bushveld samples. The here detected braggite occurs intergrown with chalcopyrite or as liberated grains. It has irregular shaped grain boundaries and reaches grain sizes between 24.3 and 61.0 µm (Fig. 5.21 a and b).

a) b) Fig. 5.21: a) Braggite at the sulfide-silicate boundary (SD124-14-1); b) Braggite intergrown with chalcopyrite (US200-9).

Braggite samples fit in the field of the braggite series, however, two samples from drill core SD124 show an extremely high Ni-Fe-Cu component and do not plot in the braggite series field (Fig. 5.22). This is interpreted to be due to contamination of adjacent minerals (sulfides) during the measurement. The Pt content in the braggite varies between 56.8 and 79.6 wt. % while the Pd content range from 4.2-18.4 wt. %. The S content is very constant between 53

PhD thesis Inga Osbahr 5. Mineral Chemistry

15.6 and 17.9 wt. %. Braggite contains significant amounts of Ni (0.03-5.31 wt. %). Fe and Cu concentrations are thought to be due to contamination from adjacent sulfides.

Fig. 5.22: Braggite and cooperite samples in the Pt+Ir-Pd+Rh-Ni+Fe+Cu system from the eastern and western Bushveld. Atomic proportion in %.

Cooperite (PtS) In reflected light cooperite shows a pleochroism, which ranges from white or creamy white to bluish white in air and is strongly anisotropic. Cooperite was detected in the chromitite stringers of the samples from the western Merensky Reef samples, and is the most abundant PGM in the eastern Bushveld, in the areas rich in chromitite and sulfides. It mostly occurs as liberated (free) grains or intergrown with BMS or other precious metals (Fig. 5.23 a and b). Cooperite occurs as irregular shaped grains with grain sizes between 0.8 and 79.3 µm.

a) b) Fig. 5.23: Cooperite in the western Bushveld a) Sulfide aggregation with inclusion of laurite and cooperite (SD124-14_2, reflected light in air); b) Cooperite at the sulfide-silicate boundary (US186-5, BSE-image).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Cooperite is part of the braggite series (Fig. 5.22), however the Pt content ranges between 69.5 and 84.7 wt. % and Pd between 1.99-2.71 wt. %. It contains up to 3 wt. % Ni and Fe, while the S content remains constant between 12.4 and 14.7 wt. %

Sperrylite (PtAs2) Sperrylite is white in air with a faint bluish tint in oil in reflected light. It is the most abundant arsenide found in the Merensky Reef and was only detected in the upper chromitite stringer of drill core SD134 of the western Bushveld, and in the chromitite stringer of drill core US200 of the eastern Bushveld. It occurs associated with sulfides (Fig. 5.24 a) or as inclusions in precious metals, e.g. rustenburgite or moncheite (Fig. 5.24 b). Sperrylite reaches grain sizes up to 150 µm.

a) b) Fig. 5.24: a) Sperrylite as inclusion in sulfide (SD134-15_1, BSE-image); b) Sperrylite in association with moncheite, rustenburgite and chalcopyrite (US200-9).

Sperrylite has constant Pt and As concentrations with 54.3-56.2 wt. % and 39.1-40.0 wt. %, respectively. Small amounts of Rh, Os and Ir were detected, however, their total concentration does not exceed 1 wt. %.

Moncheite Pt(Te,Bi)2 Moncheite is bright grayish white in reflected light and shows a weak bireflectance in air. It forms an extensive solid solution with merenskyite (PdTe2) and melonite (NiTe2). Moncheite is the only telluride detected in the samples investigated and occurs in the lower chromitite stringer of drill core SD134 of the western Bushveld, and is very common throughout all units in the eastern Bushveld. It occurs as inclusion in sulfides, intergrown with BMS, chromite or other precious metals or as a single grain (Fig. 5.25 a). The irregular shaped grains can reach 200 µm in length and can develop a very specific shape (Fig. 5.25 b).

PhD thesis Inga Osbahr 5. Mineral Chemistry

a) b) Fig. 5.25: a) Moncheite as inclusion in sulfides (SD134-19a, BSE-image); b) Moncheite intergrown with pyrrhotite (US200-7a).

Moncheite forms a solid solution with the mineral insizwaite (PtBi2) - (Fig. 5.26). The Bi concentration shows a broad range between 8.3 and 23.9 wt. %, while the Pt and Te concentrations range only between 38.9 and 40.1 wt. % and between 37.5 and 47.4 wt. %, respectively. A mixing with the mineral merenskyite (PdBi2) is not observed in the investigated samples, since the Pd concentration is below the detection limit.

Fig. 5.26: Moncheite in the Pt-Te-Bi system. Samples from drill core SD134 show a broad range in the Ni component. Atomic proportion in %.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Isoferroplatinum (Pt3Fe) In reflected light isoferroplatinum is bright white. It is isotropic and thus shows no bireflectance. Isoferroplatinum was only detected in the lower and upper chromitite stringer of drill core SD134 from the western Bushveld, and some grains were found in the chromitite stringer and norite of drill core US200. The irregular shaped grains reach up to 40 µm in diameter and occur as single grains or as inclusions in BMS, or associated with BMS (Fig. 5.27). This mineral is an almost pure Pt alloy with a Pt concentration of up to 90 wt. % and a Fe concentration of ca. 10 wt. %.

Fig. 5.27: Isoferroplatinum as inclusion in pentlandite (US200-9).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Rustenburgite (Pt, Pd)3Sn Rustenburgite is a characteristic mineral in the chromitite stringer area of the eastern Bushveld. Six grains were detected which usually occur as intergrowths with other PGM (Fig. 5.28 a and b) such as cooperite, moncheite, sperrylite and an unnamed PtSnS mineral (see below), or with chalcopyrite, rarely as single grains. The grains of the rustenburgite are irregular shaped and reach grain sizes between 13.4 and 37.4 µm. The Pt content in the rustenburgite ranges between 54.8 and 75.5 wt. %, Pd between 6.4 and 23.4 wt. % and the Sn content varies from 17.6-20.5 wt. %.

a) b) Fig. 5.28 a) Rustenburgite intergrown with PtSnS and chalcopyrite (US200-9) b) Rustenburgite intergrown with moncheite, chalcopyrite and sperrylite (US200-9).

Unnamed Platinum mineral phase PtSnS Several grains were found in the chromitite stringer and sulfide-rich areas of the eastern Bushveld. The grains occur in association with other precious metals as rustenburgite, cooperite or native gold or with chalcopyrite (Fig. 5.28 a). Until now, this phase in association with gold was described only in the PGE-ores of the Great Dyke, Zimbabwe (Oberthür et al. 2003). This PGM shows an irregular grain shape and its size ranges between 16 and 31 µm. The composition of this unnamed phase is ca. 56.3 wt. % Pt, 33.9 wt. % Sn and 9.1 wt. % S.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Native gold (Au, Ag) Gold is a characteristic precious metal mineral, with 14 grains detected in the eastern Bushveld. It occurs intergrown with pentlandite and chalcopyrite (Fig. 5.29 a) or with PGM such as cooperite or the unnamed PtSnS mineral, rarely with orthopyroxene or as a single grain. The grains show an irregular shape and reach grain sizes between 16.5 and 117.8 µm (Fig. 5.29 a and b). The average composition of native gold is 83.8 wt. % Au and 15.8 wt. % Ag.

a) b) Fig. 5.29 a) Native gold intergrown with chalcopyrite (US200-7a); b) Native gold as liberated grain (US200-7a).

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.2. UG2 The mineral chemistry described in the following is based on the electron microprobe analyses of drill core SD124 from the western Bushveld.

5.2.1. Orthopyroxene

Orthopyroxene is a common mineral in the feldspathic pyroxenite (Fig. 5.30 a) and an accessory mineral in the chromitite seam. The composition of the orthopyroxene is mainly enstatitic (En78.5-84) with a wollastonite component of up to 6.24 % (Fig. 5.31 b). The MgO and FeO concentration varies from 28.5-31.9 wt. % and 8.73-12.8 wt. %, respectively. The

Al2O3 and CaO contents are up to 1.61 wt. % and 3.15 wt. %, respectively. The sum of MnO,

Cr2O3 and TiO2 is between 0.45 and 1.01 wt. %. The #Mg varies from 0.81-0.87. As with the pyroxenes from the Merensky Reef, the pyroxenes from the UG2 also show fine parallel lamellae, due to exsolution during cooling. It is noticeable that the pyroxenes and the plagioclases of the feldspathic pyroxenite in the immediate vicinity of the UG2-chromitite are intensely altered to sericite and talc (Fig. 5.30 b and c).

a) b)

c) Fig. 5.30: a) The chromitite stringer of the second mineralized zone of the UG2 (SD124-UG2-11_2); b) Orthopyroxene and plagioclase in the process of being replaced by talc and sericite (SD124-UG2-38a); c) Orthopyroxene intensely altered to sericite and talc in the UG2 main seam (SD124-UG2-39a).

5.2.2. Clinopyroxene

PhD thesis Inga Osbahr 5. Mineral Chemistry

Clinopyroxene is an accessory mineral in almost all units and the few identified grains are plotting in the field of augite, according to the classification of Morimoto (1988) - (Fig. 5.31 a). The dominant components MgO and CaO range between 19.8 and 20.9 wt. % and 12.3 and

12.5 wt. %, respectively. FeO content ranges from 4.53-5.32 wt. %, while Al2O3 reaches up to

4.29 wt. %. The sum of TiO2, MgO and Cr2O3 in clinopyroxenes from the UG2 reaches up to

1.73 wt. %, while a significant amount of 0.57-0.85 wt. % Na2O3 may occur.

Fig. 5.31: Composition of a) clinopyroxene and b) orthopyroxene in UG2 samples (SD124) in the ternary CaSiO3, MgSiO3 and FeSiO3 diagram after Morimoto (1988).

5.2.3. Plagioclase

The plagioclase forms the matrix of the chromitites and appears as an intercumuls phase in the feldspathic pyroxenite. The composition ranges from labradorite to bytownite (An49-79)

(Fig. 5.32). CaO ranges from 9.23-16.2 wt. % and Na2O3 from 2.33-5.78 wt. %, while K2O reaches up to 3.06 wt. %. FeO and MgO reach a maximum concentration of 0.47 and 0.23 wt. %, respectively. The plagioclase in the unit of the pegmatoidal feldspathic pyroxenite is less An-rich (An50-68), while the anorthite content increases from the lower part of the UG2 (An58) to the middle part

(An78), and decreases towards the upper part of the UG2 chromitite (An49). The plagioclase in the chromitite is intensely altered to sericite (Fig. 5.30 b).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.32: Composition of plagioclase in UG2 samples (SD124) in the ternary system NaAlSi3O8, CaAl2Si2O8 and KAlSi3O8 after Okrusch and Matthes (2005).

5.2.4. Phlogopite

Phlogopite occurs accessorily in all units of the UG2. The concentration of MgO and FeO ranges between 21.1 and 22.3 wt. % and 4.08 and 5.66 wt. %, respectively. K2O and Al2O3 vary from 8.42-9.30 wt. % and 12.8-14.9 wt. %, respectively. Significant amounts of TiO2 of up to 5.04 wt. % occur in phlogopite, while F and Cl reach up to 0.30 and 0.28 wt. %, respectively. The #Mg ranges from 0.88-0.90.

5.2.5. Olivine

Frequently, small lenses of harzburgite occur in the pegmatoidal feldspathic pyroxenite, the olivine shows a higher degree of serpentinization when compared to those of the Merensky

Reef (Fig. 5.33). The composition of the olivine is Fo82-86Fa14-18. The MgO and FeO content ranges from 44.7 to 47.8 wt. % and from 14.1 to 16.3 wt. %, respectively. The #Mg is between 0.85 and 0.89. Small amounts of Mn and Ni were detected with 0.13-0.25 wt. % and 0.31-0.48 wt. %, respectively.

Fig. 5.33: Olivine is in the process of being replaced by serpentine in the second mineralized zone, above the UG2 main seam (SD124-UG2-10).

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.2.6. Chromitite

Three types of chromites were distinguished on the basis of their chrome content that decreases upwards in the drill core of the UG2 (Fig. 5.35 a-d). The first type occurs in the

UG2 main seam and shows the highest Cr2O3 contents (42.0-50.0 wt. %). The second chromite type occurs in the UG2 leader chromitite seam and has somewhat lower Cr2O3 contents, than the first type, ranging between 39.8-44.8 wt. %. The third chromite type occurs in the chromitite stringer of the second mineralized zone ca. 4.5 m above the UG2 main seam (see chapter 3.2), with the lowest Cr2O3 contents of the three chromite types, ranging between 35.6 and 41.4 wt. %. The FeO content of these three chromite types shows a broad range from 25.7-38.8 wt. %, but Fig. 5.34 reveals an upward FeO increase, while Cr2O3 concentration decreases slightly upward from 43.2 wt.% to 39.1 wt.% (Fig. 5.34).

The Al2O3 and MgO concentrations in chromite decrease upwards and reach 10 wt. % and ca. 17.1 wt.% in the UG2 main seam, respectively. In the UG2 leader chromitite seam Al2O3 and MgO in chromite reach 18.3 wt. % and 8 wt. %, respectively, and the chromites of the chromitite stringer reach 16.5 wt. % Al2O3 and 7.4 wt. % MgO (Fig. 5.34). Chromites often form triple points with adjacent sulfides (Fig. 5.35 d), due to recrystallisation and are often associated with rutile or ilmenite.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.34: Concentration of a) Cr2O3 and FeO b) Al2O3 and MgO in chromite samples of the chromitite stringer of the second mineralized zone above the UG2, the UG2 leader chromitite seam and the UG2 main seam of drill core SD124.

PhD thesis Inga Osbahr 5. Mineral Chemistry

a) b)

c) d) Fig. 5.35: Chromites of the leader chromitite seam with interstitial plagioclase (SD124UG2-39); a) parallel nicols b) Triple points of chromites in the UG2 main seam (SD124UG2-47); c) Triple points of chromites with associated sulfide in the UG2 main seam (SD124UG2-44); d) Rutile associated with chromite (SD124UG2-38).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Base-metal sulfides

Sulfides are rare and only small grains (<1 mm) were detected in the UG2 chromitite. The most abundant sulfides are pentlandite and chalcopyrite, while pyrrhotite is almost absent in the UG2 chromitite. The absence of pyrrhotite from the chromitites is probably due to a reaction between sulfides and chromites. Additional Fe partitions into chromite, while sulfur is liberated; see Naldrett and Lehmann (1988):

4Fe2O3 (chr) + FeS -> 3Fe3O4 (chr) + 0.5S2

Additional cubanite and troilite were found in the second mineralized zone ca. 4.5 m above the UG2 main seam (see chapter 3.2). In contrast to the Merensky Reef sulfides, the sulfides in the UG2 have a much smaller grain size (< 1 mm) and aggregations of different sulfides are uncommon. More often the BMS occur as small individual grains associated with chromites (Fig. 5.35 c) or pyroxenes.

5.2.7. Pentlandite (Ni, Fe)9S8

The composition of pentlandite plots in the stability field of pentlandite in the ternary Fe-Ni-S system at 400 and 600 °C (Fig. 5.36 a and b). The S content ranges from 31.7-34.3 wt. %, while Fe and Ni show a broader range and vary from 24.9 to 40.9 wt. % and 24.7 to 39.0 wt. %, respectively. The Ni content in the pentlandites of the UG2 main seam decreases towards the chromitite stringer of the second mineralized zone, while Fe increases from the main seam to the chromitite stringers above.

The pentlandite of the UG2 main seam show higher Ni than Fe concentrations. The pentlandite from the UG2 leader chromitite seam, which is in immediate contact at the top of the main seam, reveals Ni and Fe concentration showing a very disordered pattern (Fig. 5.37). The Ni concentration in pentlandite of the chromitite stringer, of the second mineralized zone, is higher than the Fe concentration (Fig. 5.37). According to the wide range of the Fe and Ni contents, the Fe/Ni ratio varies from 0.61-1.66 while Fe/S range from 0.76-1.23.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.36: Composition of UG2 pentlandite plotted in the ternary Fe-Ni-S system at a) 400 °C after Craig and Scott (1976) and b) 600 °C (Barnes and Lightfoot 2005).

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.37: Concentration of Ni and Fe in pentlandite samples of the chromitite stringer of the second mineralized zone above the UG2 main seam, the UG2 leader chromitite seam and the UG2 main seam of drill core SD124.

Pentlandite contains significant Co contents that range from 0.42-1.29 wt. %. The pentlandite from the UG2 leader chromitite seam (SD124-38 and -41) shows higher Co concentration (up to 1 at. %) than pentalandite from the the main seam (up to 0.5 at. %) (SD124-44 and - 47). However, the pentlandite from the second mineralized zone has Co concentration of 0.4 - 0.6 at. % but lower Ni concentrations than the other samples (Fig. 5.38).

1.2

1.0

0.8 SD124-10

SD124-11 0.6

SD124-38 Co (at%)Co 0.4 SD124-41 SD124-44 0.2 SD124-47

0.0 0 10 20 30 40 Ni (at%)

Fig. 5.38: Ni vs. Co in pentlandite of drill core SD124 (eastern Bushveld) in at %.

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.2.8. Chalcopyrite (CuFeS2)

Chalcopyrite mostly occurs as single grain in immediate contact with all three types of chromite, the UG2 main seam, the UG2 leader chromitite seam and the chromitite stringer of the second mineralized zone (Fig. 5.39). The composition of chalcopyrite plots in the stability field in the ternary Cu-Fe-S system at 400 °C (Fig. 5.40). S, Cu and Fe concentration in chalcopyrite ranges from 32.3-35.04 wt. %, 30.9-34.6 wt. % and 29.3-32.0 wt. %, respectively. The Ni content in chalcopyrite may be up to 0.74 wt. %, rarely up to 3.06 wt. %.

Fig. 5.39: Chalcopyrite with filled fractures of chromite (SD124-UG2-10, reflected light).

5.2.9. Cubanite (CuFe2S3)

Cubanite only occurs in the chromitite stringer of the second mineralized zone. Cubanite shows a very constant range of S, Cu and Fe (Fig. 5.40). The S content in the cubanite varies from 34.3-35.3 wt. %, while Cu and Fe range from 22.6-23.7 wt. % and 39.4-42.6 wt. %, respectively.

Fig. 5.40: Composition of troilite, chalcopyrite and cubanite in the ternary Cu-Fe-S system at 600 °C after (Cabri 1973). Atomic proportion in %. 69

PhD thesis Inga Osbahr 5. Mineral Chemistry

5.2.10. Troilite (FeS)

Troilite, like cubanite only occurs in the chromitite stringer of the second mineralized zone and the UG2 leader chromitite seam. The Fe content ranges from 61.7-64.9 wt. % and S from 36.0-36.5 wt. %, thus the composition plots in the stability field of troilite in the ternary Cu-Fe-S system at 400°C (Fig. 5.40). Troilite in chromite-rich samples (UG2-10) is Fe-depleted due to the Cr substitution. However, troilite in chromite-poor samples of (UG2-11) show a higher Fe concentration (Fig. 5.41).

51.0

50.5

50.0

UG2-10 Fe Fe (at%) UG2-11 49.5

49.0 49.0 49.5 50.0 50.5 51.0 S (at%)

Fig. 5.41: S vs. Fe in at % in pyrrhotite from drill core SD124.

5.2.11. Platinum-group minerals

PGM in the UG2 of the western Bushveld were identified by electron microprobe and the PGM of the eastern Bushveld by a mineral liberation analyzer (MLA). In the western Bushveld, the PGM identified are braggite, laurite, isoferroplatinum and cooperite. Braggite and laurite were found in the UG2 main and leader chromitite seam. In the second mineralized zone only isoferroplatinum was detected. For optical properties see the description of PGM in chapter 5.1. In the eastern Bushveld Complex, the most abundant PGM are laurite, braggite, cooperite, and sperrylite. Five thin sections were analyzed for PGM (three in the UG2 main seam, one below and one above the UG2 main seam. Below the UG2 main seam (DT46-21), 19 grains of a Pt-alloy were detected. One grain of sperrylite was detected in the lowermost section of the UG2 main seam (DT46-13). The middle part of the UG2 main seam (DT46-12) contains 20 grains of PGM and is dominated by laurite (45 %), cooperite (15 %), braggite (10 %), Pt-

PhD thesis Inga Osbahr 5. Mineral Chemistry

Rh sulfide (10 %) followed by one grain each of moncheite, sperrylite, Pt-alloys and PGE- sulfarsenide (Table 5.3). Thin section DT46-11(middle part of the UG2 main seam) contains nine PGM grains and is dominated by laurite (33 %) and moncheite (22 %), followed by one grain each of cooperite, Pd-alloy, PGE-sulfarsenide and ferroplatinum (Table 5.3). Above the UG2 main seam (DT46-6) three grains of braggite and one grain each of sperrylite, ferroplatinum and laurite were detected (Table 5.3).

Table 5.3: Type and number of PGM grains identified in samples DT46 (UG2-eastern Bushveld)

DT46-6 DT46-11 DT46-12 DT46-13 DT46-21 Laurite 1 3 9 Braggite 3 2 Cooperite 1 3 Pt-Rh-sulfide 2 Sperrylite 1 1 1 Ferroplatinum 1 1 1 19 Moncheite 2 1 Pd-alloy 1 PGE- sulfarsenide 1 1 Total 6 9 20 1 19

Braggite ((Pt,Pd)S) Braggite was found in the chromitite stringers of the eastern Bushveld, and in drill core SD124 of the western Bushveld, and is one of the most abundant platinum group minerals found in the Merensky Reef. The braggites were located at sulfide-silicate boundaries and reach grain sizes up to 50 µm. Braggite from the UG2 fit in the field of the braggite series (Fig. 5.42); however, three samples from thin section UG2-44 show an extremely high Ni-Fe-Cu component and do not fit in the braggite series field. That is possibly due to contamination of adjacent minerals (sulfides) during the measurement. The Pt content in the braggites varies between 22.4 and 77.6 wt. %, while the Pd content range from 1.93-39.0 wt. %. The S content varies from 14.1- 29.7 wt. %. Braggite contains significant total amounts of up to13 wt. % of Ni, Fe and Cu, however, in two cases this component reaches 40 wt. % which is probably due to contamination from the adjacent sulfides.

PhD thesis Inga Osbahr 5. Mineral Chemistry

Fig. 5.42: Braggite in the Pt+Ir-Pd+Rh-Ni+Fe+Cu system. Atomic proportion in %.

Isoferroplatinum (Pt3Fe) Isoferroplatinum was only detected in the second mineralized part, above the UG2 main seam. This mineral is an almost pure Pt alloy and consists of a Pt component between 86.8 and 89.8 wt. %, and a Fe component of between 9.66 and 10.7 wt. %.

Laurite Laurite occurs in all investigated chromitite stringers and is the most abundant platinum group mineral identified in the UG2. The laurite is hybidiomorphic-idiomorphic and reaches grain sizes of up to 70 µm. In the Ru-Os-Ir system (Fig.5.20 a), the UG2 samples show a strong Ru enrichment compared to the Os and Ir component. The Ru-Fe-Os+Ir system (Fig. 5.20 b) reveals only a small amount of Fe. The two laurite grains show a similar composition: Ru ranges from 46.9- 48.9 wt. % and the S content varies from 37.5-38.2 wt. %. Significant amounts of Os and Ir were detected with concentrations between 3.02-6.50 wt. % and 5.07-5.91 wt. %, respectively. Small amounts of Pd and As were detected with up to 1.74 and 2.98 wt. %, respectively.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

6. Analytical Results – Merensky Reef 6.1. Whole-rock chemistry Representative samples, covering the entire section of the mineralized part of the Merensky Reef, were taken from the four drill cores (farm Styldrift and Umkoanesstad) in order to undertake bulk rock chemical analyses. Each sample with a length of 16 to 25 cm of halved 3.5 cm diameter drill cores was pulverized and analyzed for Pd, Pt, Rh, Cu, Ni, Cr and S. The whole-rock data of all drill cores analyzed by Anglo American are listed in Table 6.1 a-d. Depending on the location of the maximum concentration of the PGE in the Merensky Reef, it is called “top” loaded if the maximum concentration is found within the upper chromitite layer and its immediate vicinity, or “bottom” loaded if it occurs within the lower chromitite stringer and its immediate vicinity.

6.1.1. Western Bushveld

In drill core SD124 all investigated elements show a maximum concentration in the range of the upper chromitite stringer at 666.75 m, hence, drill core SD124 reveals a top loaded mineralization (Fig. 6.1). In the unit of the pegmatoidal feldspathic pyroxenite, all analyzed elements show one to three further concentration peaks with much lower contents than those observed in the upper chromitite stringer. The maximum concentration peaks of Ni, Cu, S and Cr are mostly consistent with that of Pt, Pd and Rh, however, the base-metal peaks are somewhat displaced and occur 25 cm above the PGE concentration peaks (Fig. 6.1). Pt concentration reaches a maximum of up to 10 ppm in the upper chromitite stringer and 4 ppm in the pegmatoidal feldspathic pyroxenite. Pd and Rh reach up to 4 ppm and 0.8 ppm in the upper chromitite stringer, respectively, and 1 ppm and 0.2 ppm in the pegmatoidal feldspathic pyroxenite, respectively (Fig. 6.1; Table 6.1 a).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.1: Stratigraphic variation of whole-rock Pt, Pd, Rh, Ni, Cu, S and Cr of drill core SD124 (western Bushveld). Note the different units: ppm for the PGE and wt. % for the base-metals, S and Cr.

In drill core SD134 all analyzed elements reveal one concentration peak in the range of the pegmatoidal feldspathic pyroxenite at 613 m, or in the range of the upper chromitite stringer at 612.6 m (Fig. 6.2). Only Cr shows three concentration peaks, the first one in the range of the lower chromitite stringer, the second peak in the range of the upper chromitite stringer and a third one 50 cm above the upper chromitite stringer. The distribution of Pd and Rh revealed a top-loaded mineralization, whereas Pt shows a “homogenous” distribution, since it is neither top nor bottom loaded. Ni and S show a similar distribution trend as Pd and Rh, whereas Cu follows exactly the trend of Pt (Fig. 6.2).

The Pt concentration increases in the range of the lower chromitite stringer and reveals its maximum concentration peak (10 ppm) in the pegmatoidal feldspathic pyroxenite. Pt is still enriched at the upper chromitite stringer (8 ppm) and decreases steadily towards the middle of the feldspathic pyroxenite at 612 m, where concentrations are below the detection limit. Pd and Rh show a similar distribution as Pt, but their concentration peaks are located in the upper chromitite stringer and reach 5 and 0.8 ppm, respectively. The concentration of Pd and Rh decreases steadily above the upper chromitite stringer, and is below detection in the upper part of the feldspathic pyroxenite (Fig. 6.2; Table 6.1 b).

Fig. 6.2: Stratigraphic variation of whole-rock Pt, Pd, Rh, Ni, Cu, S and Cr of drill core SD134 (western Bushveld). Note the different units: ppm for the PGE and wt. % for the base-metals, S and Cr.

6.1.2. Eastern Bushveld

The profile of drill core US200 reveals two concentration peaks for Pt, Pd and Rh in the range of the lower chromitite stringer and the sulfide stringer, the latter of which occurs 74

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef instead of the upper chromitite stringer (Fig. 6.3). Nickel and Cu reveal three concentration peaks. The first one is in the range of the lower chromitite stringer. The second peak, which exhibits the maximum Ni and Cu concentration, is located about 30 cm above the sulfide stringer and is displaced when compared to the maximum concentration of Pt, Pd and Rh. The third Ni and Cu peak occurs about 40 cm above the second concentration peak, at the transition from the pegmatoidal feldspathic pyroxenite to norite. Due to the location of the maximum concentration of Pt, Pd and Rh, drill core US200 shows a top loaded mineralization (Fig. 6.3).

The Pt concentration increases in the range of the pegmatoidal feldspathic pyroxenite and reaches 3 ppm at the first concentration peak at the lower chromitite stringer. Platinum reaches a maximum concentration of 5 ppm at the sulfide stringer, which occurs instead of the upper chromitite stringer. Between the chromitite stringer and the sulfide stringer, the concentration decreases to 1 ppm. Between the feldspathic pyroxenite and norite, the concentration of all PGE decreases below the detection limit. Palladium shows contents of 1.75 ppm at both concentration peaks, and it decreases to 0.8 ppm between them. Rhodium has 0.1 ppm at the first concentration peaks, and reaches a maximum concentration of 0.35 ppm at the sulfide stringer (Fig. 6.3; Table 6.1 c).

Fig. 6.3: Stratigraphic variation of whole-rock Pt, Pd, Rh, Ni and Cu of drill core US200 (eastern Bushveld). Note the different units: ppm for the PGE and wt. % for the base-metals. Ellipses show the three different US200 lithologies, namely the lower chromitite stringer, the sulfide-rich feldspathic pyroxenite, and the pegmatoidal pyroxenite, investigated for PGM by Rudashevsky (2011).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

In drill core US186, Pt and Rh show two concentration peaks, one in the range of the lower and one in the upper chromitite stringer (Fig. 6.4). Palladium is homogenously distributed between the lower and upper chromitite stringer but has its maximum in the range of the upper chromitite stringer. The maximum concentration of Ni and Cu are displaced compared to the PGE concentration peaks. The peaks are located ca. 25 cm below the lower chromitite stringer (Fig. 6.4).

A classification of top or bottom loaded is ambiguous in drill core US186, since Pt has similar contents in the lower and upper chromitite stringer. Pd is homogenously distributed and Rh has a maximum concentration in the range of the lower chromitite stringer. However, the total concentration of all PPGE obtains a more bottom loaded mineralization.

Platinum shows around 4.5 ppm and 4 ppm in the range of the lower and upper chromitite stringer, respectively. It decreases to a concentration of ca. 1 ppm between the two stringers. The Pd concentration varies between 1 and 1.5 ppm in the range of the lower chromitite stringer and the upper stringer (Fig. 6.4). Rhodium reveals a maximum concentration of 0.7 ppm in the lower and chromitite stringer and ca. 0.3 in the upper. In the feldspathic pyroxenite Rh reaches ca. 0.1 ppm. The uppermost part of the pegmatoidal feldspathic pyroxenite at ca. 552 m contains small amounts of Pd and Pt and the maximum concentration peaks of Ni and Cu (Fig. 6.4; Table 6.1 d).

Fig. 6.4: Stratigraphic variation of whole-rock Pt, Pd, Rh, Ni and Cu of drill core US186 (eastern Bushveld). Note the different units: ppm for the PGE and wt. % for the base-metals.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

The western Bushveld reveals a Pt/Pd ratio between 0.2 and 16. Drill core SD124 shows a Pt/Pd ratio of 16 in the range of the lower chromitite stringer, while the remaining parts of the drill core reveal a maximum Pt/Pd ratio of 6 but generally between 1.5 and 4. The entire section of the investigated drill core reveals a slightly increasing Pt/Pd ratio from the lower to the upper portion of the Reef (Fig. 6.5).

Drill core SD134 reveals the highest Pt/Pd ratio of 5 in the range of the lower chromitite stringer. The remaining parts of the drill core exhibit a Pt/Pd ratio of ≤ 3. A slightly increasing Pt/Pd ratio occurs from the footwall to the hanging wall of the Merensky Reef (Fig. 6.5).

In the eastern Bushveld both drill cores reveal a tendency to an increasing Pt/Pd ratio from the bottom to the top of the Reef. Drill core US200 shows a Pt/Pd ratio of 3 in the immediate vicinity of the sulfide stringer and is otherwise between 0.6 and 2 (Fig. 6.5).

Drill core US186 exhibits Pt/Pd ratios between 3 and 5.5 in the range of the chromitite stringers. The footwall of the Reef reveals Pt/Pd ratios <1, while the hanging wall reveals Pt/Pd ratios around 1.5 (Fig. 6.5).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.5: Pt/Pd ratio of the Merensky Reef of the western Bushveld (SD124 and Sd134) and eastern Bushveld (US200 and US186).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

The Cu/Pd ratio increases from the bottom to the top in all four drill cores. In drill core SD124 the Cu/Pd ratio ranges between 200 and 3100 in the Merensky Reef and its footwall with the highest Cu/Pd ratio in the lower chromitite stringer. The Merensky Reef hanging wall rocks reach Pd/Cu ratio of 16500 (Fig. 6.6). Drill core SD134 shows a similar distribution of the Cu/Pd ratio throughout the drill core, but has a lower Cu/Pd ratio than drill core SD124. The Merensky Reef and its footwall rocks reach a Cu/Pd ratio of 300 -1600, with the highest Cu/Pd ratio around the lower chromitite stringer. The hanging wall rocks reveal a Cu/Pd ratio between 7500 and 16000 (Fig. 6.6). The drill cores from the eastern Bushveld show the lowest Cu/Pd ratio in the Merensky Reef (between the lower and upper chromitite stringer). The Cu/Pd ratio reaches values between 200 and 1300 in drill core US200 and US186, respectively. A higher Cu/Pd ratio exists in the footwall rocks of both drill cores with a ratio of up to 5000 and 10000 in drill core US186 and US200, respectively. The Cu/Pd ratio in the hanging wall of the Merensky Reef reaches up to 20000 in both drill cores from the eastern Bushveld (Fig. 6.6).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.6: Cu/Pd ratio of the Merensky Reef of the western and eastern Bushveld Complex.

The mantle-normalized patterns of samples from drill core US200 reveal that almost all samples are enriched in PGE compared to the mantle, however, the PPGE are more enriched than the IPGE. Sample US200-9 stems from the chromitite stringer and is more 80

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef enriched in PGE than the other samples. Sulfide- and chromite-depleted samples, such as US200-10_2, even reveal depleted Ni and IPGE contents compared to the mantle, however, Cu is relatively enriched in all samples (Fig. 6.7).

Fig. 6.7: PGE, Ni, Cu and Au contents of the different samples of drill core US200, with depletion of Ni and IPGE in comparison to the PPGE, Cu and Au, normalized to the mantle after McDonough and Sun (1995).

Chondrite-normalized pattern of the different samples of drill core US200 show upward trending curves from Os to Pd. However, most of the samples show a slight depletion in Ir and Pd (Fig. 6.8). For example, Naldrett and Duke (1980) and Barnes et al. (1985) described this type of normalized pattern as typical for the PGE mineralization produced by magmatic scavenging of PGE through segregating sulfides. Only chromite and sulfide-rich samples (US200-7a and US200-9) are at least enriched in PPGE compared to the chondrite, whereas all samples are depleted in IPGE compared to the chondrite.

Fig. 6.8: PGE, Ni, Cu and Au contents of the different samples of drill core US200 with a slight depletion in Ir, normalized to C-1 chondrite after McDonough and Sun (1995).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Table 6. 1: Whole-rock data of Pt, Pd, Rh and Au in ppm and Cu, Ni, Cr2O3 and S in wt. % analyzed by Anglo American. a) SD124 D1 Styldrift Merensky Total No. From (m) To (m) (m) Lithotype Pt Pd Rh Au Cu Ni Cr2O3 S 13 666.000 666.250 0.25 Feldsp.Px 0.05 0.03 b.d.l. 0.01 0.01 0.07 0.41 0.01 14 666.250 666.500 0.25 Feldsp.Px 0.87 0.31 0.10 0.11 0.07 0.18 0.45 0.25 15 666.500 666.740 0.24 Feldsp. Px 5.69 1.96 0.37 0.38 0.17 0.36 1.45 0.68 16 666.740 667.000 0.26 Peg.Feld.Px 10.34 3.63 0.85 0.42 0.12 0.29 1.80 0.57 17 667.000 667.260 0.26 Peg.Feld.Px 2.97 1.62 0.22 0.13 0.06 0.19 0.53 0.25 18 667.260 667.520 0.26 Peg.Feld.Px 0.74 0.52 0.05 0.17 0.06 0.13 0.51 0.11 19 667.520 667.780 0.26 Peg.Feld.Px 2.70 0.89 0.13 0.19 0.05 0.17 0.42 0.22 20 667.780 668.040 0.26 Peg.Feld.Px 1.19 0.67 0.06 0.14 0.04 0.15 0.35 0.16 21 668.040 668.300 0.26 Peg.Feld.Px 0.26 0.21 0.00 0.04 0.02 0.12 0.37 0.08 22 668.300 668.560 0.26 Peg.Feld.Px 0.59 0.41 0.04 0.10 0.03 0.16 0.34 0.14 23 668.560 668.820 0.26 Peg.Feld.Px 0.14 0.11 0.00 0.03 0.01 0.14 0.36 0.03 24 668.820 669.080 0.26 Peg.Feld.Px 0.84 0.28 0.20 0.01 0.01 0.12 0.79 0.02 25 669.080 669.35 0.27 Peg.Feld.Px 0.16 0.01 b.d.l. 0.01 b.d.l. 0.04 0.41 0.01 26 669.35 669.60 0.25 Poik.An. 0.01 0.01 b.d.l. 0.01 b.d.l. b.d.l. 0.03 0.01 27 669.60 669.85 0.25 Poik.An. 0.01 0.01 b.d.l. 0.01 b.d.l. b.d.l. 0.01 0.16

b) SD134 D3 Styldrift Merensky Total No. From (m) To (m) (m) Lithotype Pt Pd Rh Au Cu Ni Cr2O3 S 11 611.07 611.33 0.26 Feldsp.Px. 0.06 0.03 b.d.l. 0.05 0.03 0.10 0.41 0.09 12 611.33 611.59 0.26 Feldsp.Px. 0.03 0.03 b.d.l. 0.02 0.02 0.08 0.41 0.03 13 611.59 611.86 0.27 Feldsp.Px. 0.06 0.04 b.d.l. 0.03 0.02 0.07 0.41 0.01 14 611.86 612.13 0.27 Feldsp.Px. 0.07 0.42 0.14 0.13 0.05 0.13 0.42 0.23 15 612.13 612.40 0.27 Feldsp.Px. 1.87 1.34 0.45 0.23 0.09 0.20 1.05 0.41 16 612.40 612.66 0.26 Feldsp.Px. 2.85 2.07 0.53 0.31 0.11 0.24 0.84 0.63 17 612.66 612.96 0.30 Peg.Feld.Px 8.10 5.42 0.80 0.51 0.16 0.35 1.21 0.88 18 612.96 613.25 0.29 Peg.Feld.Px 9.69 2.19 0.34 2.46 0.18 0.23 0.48 0.61 19 613.16 613.41 0.25 Poik.An. 0.35 0.07 b.d.l. 0.01 0.01 0.01 0.47 0.01 20 613.41 613.66 0.25 Poik.An. 0.02 0.01 b.d.l. 0.01 b.d.l. b.d.l. 0.02 0.01 21 613.66 613.91 0.25 Poik.An. 0.01 0.02 b.d.l. 0.01 b.d.l. b.d.l. 0.01 0.01

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

c) US200 D6 Lebowa Merensky

No. From (m) To (m) (m) Lithotype Pt Pd Rh Au Cu Ni 2 917.490 917.690 0.20 Norite 0.07 0.05 b.d.l. 0.05 0.05 0.11 3 917.690 917.890 0.20 Norite 0.08 0.04 b.d.l. 0.07 0.07 0.15 4 917.890 918.060 0.17 Feldsp.Px. 0.12 0.06 b.d.l. 0.13 0.12 0.24 5 918.060 918.230 0.17 Feldsp.Px. 0.22 0.19 b.d.l. 0.12 0.10 0.24 6 918.230 918.400 0.17 Feldsp.Px. 2.76 1.45 0.15 0.43 0.16 0.40 7 918.400 918.570 0.17 Feldsp.Px. 5.02 1.76 0.36 0.53 0.10 0.27 8 918.570 918.740 0.17 Feldsp.Px. 1.08 0.89 0.10 0.08 0.02 0.09 9 918.740 918.910 0.17 Peg.Feld.Px 2.90 1.84 0.13 0.40 0.08 0.15 10 918.910 919.080 0.17 Peg.Feld.Px 0.35 0.33 0.05 0.06 0.03 0.06 11 919.080 919.280 0.20 Gabbronorite 0.02 0.03 b.d.l. 0.01 0.01 0.03 12 919.280 919.480 0.20 Gabbronorite 0.03 0.02 b.d.l. 0.02 0.01 0.03 13 919.480 919.680 0.20 Gabbronorite 0.02 0.01 b.d.l. 0.01 0.01 0.03 14 919.680 919.880 0.20 Gabbronorite 0.04 0.03 b.d.l. 0.01 0.01 0.02 15 919.880 920.080 0.20 Gabbronorite 0.01 0.01 b.d.l. 0.01 0.01 0.03 16 920.080 920.280 0.20 Gabbronorite 0.03 0.02 b.d.l. 0.02 0.01 0.03

d) US186 D4 Lebowa Merensky

No. From (m) To (m) (m) Lithotype Pt Pd Rh Au Cu Ni 6 551.020 551.200 0.18 Feldsp.Px. 0.94 0.17 0.05 0.05 0.02 0.07 7 551.200 551.380 0.18 Feldsp.Px. 1.67 1.14 0.10 0.27 0.08 0.25 8 551.380 551.540 0.16 Feldsp.Px. 3.92 1.36 0.26 0.14 0.03 0.19 9 551.540 551.700 0.16 Feldsp.Px. 0.77 1.14 0.09 0.22 0.05 0.49 10 551.700 551.865 0.16 Feldsp.Px. 4.44 1.06 0.68 0.95 0.14 0.66 11 551.865 552.040 0.17 Peg.Feld.Px. 0.84 0.95 b.d.l. 0.39 0.16 1.21 12 552.040 552.210 0.17 Peg.Feld.Px. 0.54 0.54 b.d.l. 0.54 0.23 4.09 13 552.210 552.385 0.17 Peg.Feld.Px. 0.18 0.25 b.d.l. 0.10 0.03 0.68 14 552.385 552.585 0.20 Peg.Feld.Px. 0.02 0.02 b.d.l. 0.02 0.01 0.09 15 552.585 552.785 0.20 Feldsp.Px. 0.01 0.01 b.d.l. 0.01 0.01 0.05 16 552.785 552.985 0.20 Feldsp.Px. 0.01 0.01 b.d.l. 0.01 b.d.l. 0.03 Abbreviations: Feldsp.Px. = Feldspathic pyroxenite, Peg.Feld.Px. = Pegmatoidal feldspathic pyroxenite, Poik.An = Poikilitic anorthosite.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

6.2. PGE in base-metal sulfides 6.2.1. Stratigraphic variation of PGE

Trace PGE contents in pentlandite, chalcopyrite, pyrrhotite and pyrite were analyzed by LA- ICP-MS. A total of 557 analyses were conducted. The average values are listed in Table 6. On the detection of micro-inclusions of PGM, individual analyses were removed from the average values. Examples for time-resolved spectra of laser analyses with and without a PGE-rich inclusion in BMS are given in Figure 6.9. This paragraph describes the stratigraphic variation of the PGE in BMS. The concentrations of PGE in BMS are outlined in the paragraphs 6.22 and 6.23.

Fig. 6.9 Time-resolved spectra for laser analysis of pyrrhotite a) without a PGE-rich inclusion b) with a PGE-rich inclusion. A stratigraphic variation of PGE contents in BMS exists in the four drill cores. Palladium, Rh, Ru and Ir in pentlandite show a top loaded mineralization, and thus the maximum concentration peak is in the range of the upper chromitite stringer in the samples of drill core 84

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

SD124 from the western Bushveld. However, the pegmatoidal feldspathic pyroxenite, which is located between the upper and lower chromitite stringer, is also enriched with up to 250 ppm Pd, 100 ppm Rh and 45 ppm Ru. Platinum and Os show higher concentrations in the range of the pegmatoidal feldspathic pyroxenite than in the upper chromitite stringer. The lower chromitite stringer and its host rocks were not analyzed, due to the absence of sulfides (Fig. 6.10). Pyrrhotite and chalcopyrite show one distinct concentration peak for Os and Pt in the range of the upper chromitite stringer while the other elements show a more irregular distribution pattern.

In drill core SD134, two pronounced concentration peaks occur at 613.00 m and 612.75 m for most PGE in the middle and upper part of the pegmatoidal feldspathic pyroxenite. Drill core SD134 from the western Bushveld shows a top loaded mineralization for Pd and Os, a homogenous distribution of Rh and Pt (same concentrations in the lower and upper chromitite stringer), and a bottom loaded mineralization for Ru and Ir (higher concentrations in the lower chromitite stringer, but significant concentrations in the upper chromitite stringer as well) (Fig. 6.11). Pyrrhotite shows a bottom loaded mineralization for Pd, Rh and Ru while Pt, Os and Ir are homogenously distributed between the lower and upper chromitite stringer. Chalcopyrite hardly contains any Pd, Rh and Pt. However, Ru, Os and Ir show significant concentrations in the pegmatoidal feldspathic pyroxenite between the lower and upper chromitite stringer.

In drill core US200 from the eastern Bushveld, all PGE in pentlandite show a maximum concentration in the range of the pegmatoidal feldspathic pyroxenite, which is located below the lower chromitite stringer. A further concentration peak occurs for Rh in the range of the lower chromitite stringer, while the other PGE (Pd, Ru, Pt, Os and Ir) show an additional concentration peak at the sulfide stringer ca. 25 cm above the lower chromitite stringer (Fig. 6.13). Pyrrhotite and chalcopyrite show a similar distribution of PGE as pentlandite.

The maximum Pd concentration in pentlandite from drill core US186 is located in the pegmatoidal feldspathic pyroxenite below the lower chromitite stringer. Platinum, Rh, Ru, Os and Ir show a concentration peak in the pegmatoidal feldspathic pyroxenite as well, but further concentration peaks occur in the range of the lower and upper chromitite stringer (Fig. 6.15). The PGE in pyrrhotite are slightly enriched in the lowermost part of the pegmatoidal feldspathic pyroxenite, and are close to the detection limit in the remaining parts of the drill core. 85

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Chalcopyrite shows a maximum concentration for Pd in the lowermost part of the pegmatoidal feldspathic pyroxenite, and further enrichments occur in the lower and upper chromitite stringer. Rhodium reveals two concentration peaks directly below the lower and upper chromitite stringer, while Pt, Ru, Os and Ir show one significant concentration peak in the range of the lower chromitite stringer.

6.2.2. PGE concentration in base-metal sulfides (western Bushveld)

In the following section the PGE concentration in BMS, which were found to occur in all lithological units in the drill cores of the eastern and western Bushveld, is described. The Pd-, Rh-, Ru-, Os-, Ir- and Pt- distributions in BMS are separately described, due to the different content and sometimes different distribution behavior.

The stratigraphic variation of Pd, Rh, Ru, Os, Ir and Pt in the base-metal sulfides pentlandite, pyrrhotite and chalcopyrite of the samples from drill cores SD124 and SD134 of the western Bushveld is shown in Figures 6.10 and 6.11. Pentlandite shows the highest PGE concentration. Chalcopyrite and pyrrhotite have similar PGE concentrations, and despite very low PGE concentrations a distribution trend occurs in the different lithologies from the respective drill cores. The mentioned values in the following chapter are average values, for the sake of simplicity.

6.2.2.1. SD124

The PGE concentrations for pentlandite, pyrrhotite and chalcopyrite of drill core SD124 are given in Figure 6.10 and Table 6.2.

Pentlandite

The Pd and Rh concentration in pentlandite usually ranges from 32 to 321 ppm and from 0.05 to 250 ppm, respectively, in the range of the pegmatoidal feldspathic pyroxenite. A maximum concentration of 1750 and 1400 ppm for Pd and Rh, respectively, occurs in the upper chromitite stringer.

Ruthenium shows considerable concentrations of up to 40 ppm in pentlandite in the pegmatoidal feldspathic pyroxenite, with a maximum concentration of 60 ppm in the upper chromitite stringer.

Osmium reveals a maximum concentration of 8 ppm in the uppermost part of the pegmatoidal feldspathic pyroxenite, while it reaches 4 ppm in the lower part of the pegmatoidal feldspathic pyroxenite and the upper chromitite stringer. In the remaining samples Os usually ranges between 0.43 and 2.5 ppm.

Iridium concentration ranges between 5 and 20 ppm and shows one significant concentration 86

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef peak of up to 100 ppm at the upper chromitite stringer.

In the samples from drill core SD124, pentlandite shows considerable amounts of platinum: 12 ppm at the first concentration peak and 27 ppm at the second concentration peak, but generally less than 2 ppm.

Pentlandite grains from sample SD124-13, in immediate contact with chalcopyrite have higher Pd contents (133-191 ppm) than isolated grains (51 ppm).

Pyrrhotite

Pyrrhotite hardly contains any Pd and Rh. The maximum concentrations are 2 and 3 ppm for Pd and Rh, respectively.

At the first concentration peak in the lower part of the pegmatoidal feldspathic pyroxenite, pyrrhotite has up to 20 ppm Ru and 3 ppm Os. At the second concentration peak in the middle part of the pegmatoidal feldspathic pyroxenite, pyrrhotite has a maximum concentration of 35 ppm and 7 ppm for Ru and Os, respectively. The remaining parts of the drill core reveal between 0.26 to 5.21 ppm Ru and between 0.18 and 2.83 ppm in pyrrhotite.

Iridium, which usually ranges between 5 and 15 ppm in pyrrhotite, shows only one concentration peak of 70 ppm in pyrrhotite from the upper chromite stringer.

Some pyrrhotite grains may contain up to 30 ppm Pt on average in the upper chromitite stringer, while the remaining parts of the drill core reveal concentrations close to the detection limit.

Chalcopyrite

Pd and Rh show a highly irregular distribution pattern, with several concentration peaks in the pegmatoidal feldspathic pyroxenite, the upper chromitite stringer and even above the stringer. At these concentration peaks, the Pd and Rh content is up to 4 ppm, and decreases below the detection limit between the peaks.

Ruthenium, Os and Ir only show one concentration peak in the pegmatoidal feldspathic pyroxenite, with concentrations of up to 10 ppm for Ru and Ir, respectively, and 6 ppm for Os. The remaining samples are below the detection limit. Chalcopyrite can contain significant amounts of Pt (up to 10 ppm) in the lowermost part of the pegmatoidal feldspathic pyroxenite, while the remaining samples are below the detection limit.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.10: The stratigraphic variation of PGE contents in pentlandite, pyrrhotite and chalcopyrite throughout drill core SD124. Whole-rock Pt and Pd contents are shown for comparison of the maximum concentration in the BMS and the whole-rock. A sudden drop in PGE contents in pentlandite occurs at the transition to the poikilitic anorthosite footwall due to lack of sulfide in the samples. The highest contents of PGE in pentlandite occur in the range of the upper chromitite stringer and in the pegmatoidal feldspathic pyroxenite.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Table 6.2: Average PGE values in BMS of drill core SD124 in ppm.

Pentlandite Pyrrhotite Ru Rh Pd Os Ir Pt Ru Rh Pd Os Ir Pt SD124-13 n=3 n=4 Average 2.75 41.26 125.07 0.36 0.94 1.67 0.69 1.71 2.22 0.21 0.32 3.91 SD124-14_1 n=6 Average 6.51 4.27 29.68 0.96 0.85 0.44 n.a. n.a. n.a. n.a. n.a. n.a. SD124-14_2 n=2 n=2 Average 57.8 236.25 321.5 3.76 18.92 4.92 2.67 0.51 0.01 1 22.09 16.26 SD124-15 n=7 n=6 Average 30.27 1,423.29 1,753.43 0.43 96.51 6.11 0.65 1.61 0.12 0.18 66.41 28.27 SD124-15a_1 n=8 n=6 Average 25.32 449.11 635.38 0.93 18.94 6 8.38 0.66 b.d.l. 0.69 9.53 4.09 SD124-15a_2 n=9 n=2 Average 23.96 22.91 207.56 7.56 8.35 1.1 5.21 3.01 b.d.l. 2.22 4.34 0.37 SD124-19 n=2 n=2 Average 43.4 227.5 317 5.91 3.5 1.62 1.87 0.29 b.d.l. 0.65 1.03 0.48 SD124-20 n=5 Average 23.19 13.8 177.4 3.23 1.56 b.d.l. n.a. n.a. n.a. n.a. n.a. n.a. SD124-21 n=7 Average 5.9 11.39 239.24 1.32 7.63 b.d.l. n.a. n.a. n.a. n.a. n.a. n.a. SD124-23b n=8 n=5 Average 10.57 8.64 224 2.5 1.45 9.82 19.14 0.77 0.13 2.83 2.7 1.23 SD124-24 n=8 n=3 Average 25.97 8.83 167.38 4.38 2.72 11.78 19.95 0.19 0.08 3.12 3.24 0.86 SD124-27 n=7 n=3 Average 1.17 0.05 32.66 0.11 0.17 0.06 0.26 b.d.l. 0.13 b.d.l. b.d.l. b.d.l. SD124-28 n=2 Average 0.76 0.2 22.4 0.01 0.03 b.d.l. Chalcopyrite Ru Rh Pd Os Ir Pt SD124-13 n=1 Average 1.7 2.38 3.28 0.56 1.1 0.5 SD124-14_2 n=2 Average 0.07 1.31 4.43 b.d.l. b.d.l. 0.16 SD124-15 n=2 Average b.d.l. 0.52 1.25 b.d.l. b.d.l. 0.15 SD124-15a_1 n=2 Average 0.14 0.75 1.17 0.39 2.53 1.43 SD124-15a_2 Average n.a. n.a. n.a. n.a. n.a. n.a. SD124-19 n=4 Average 0.08 1.43 2 b.d.l. b.d.l. b.d.l. SD124-23b n=4 Average 11.49 1.28 3.57 5.65 7.1 14.5 SD124-24 n=3 Average 0.09 0.77 0.25 b.d.l. 7.09 9.97 SD124-27 n=2 Average 0.07 0.55 1.29 b.d.l. b.d.l. b.d.l. b.d.l.: below detection limit; n.a.: not analyzed

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

6.2.2.2. SD134

All PGE concentrations for pentlandite, pyrrhotite and chalcopyrite of drill core SD134 are shown in Figure 6.11 and Table 6.3.

Pentlandite

In drill core SD134 pentlandite has 200 ppm Pd at its first concentration peak at the lower chromitite stringer, and increases steadily to 550 ppm Pd in the range of the upper chromitite stringer. All elements are below the detection limit above the upper chromitite stringer, except Ru and Os.

Rhodium shows two concentration peaks of 90 ppm and 80 ppm in the lower and upper part of the pegmatoidal feldspathic pyroxenite, respectively, and in between the Rh concentration decreases to 40 ppm.

Ruthenium reaches a maximum concentration of 50 ppm at the lower chromitite stringer. It decreases to 20 ppm at the upper chromitite stringer and to around 10 ppm in the range above the upper chromitite stringer.

Osmium and Ir reveal concentrations of 7 ppm and 11 ppm, respectively at the lower chromitite stringer, while Os increases to 18 ppm at the upper chromitite stringer and Ir decreases to 8 ppm. Above the upper chromitite stringer, both elements have contents close to the detection limit.

Pentlandite contains up to 4 ppm Pt in the range of the pegmatoidal feldspathic pyroxenite.

Pyrrhotite

Pyrrhotite contains 70 ppm Pd, 30 ppm Rh and 35 ppm Ru at the first concentration peak in the vicinity of the lower chromitite stringer, while in the remaining parts of the drill core pyrrhotite shows Pd and Rh concentrations below the detection limit.

The Ru concentration steadily decreases from the lower chromitite stringer (35 ppm) to the upper chromitite stringer (8 ppm), and below the detection limit in the feldspathic pyroxenite.

The Os concentration reaches up to 8 ppm and remains constant at the first and second concentration peak. Usually the Os concentration ranges between 2 and 4 ppm.

Iridium in pyrrhotite show two concentration peaks. It reaches 6 ppm and 10 ppm, respectively. The remaining samples from the pegmatoidal feldspathic pyroxenite have Ir concentrations below 4 ppm.

Platinum concentration is below 2 ppm in pyrrhotite.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Chalcopyrite

Palladium and Rh show no significant concentration peaks and are ≤ 5 and ≤ 2.5 ppm, respectively, in chalcopyrite.

Ruthenium, Os and Ir reveal one concentration peak in the pegmatoidal feldspathic pyroxenite, with significant concentrations of 15 ppm, 11 ppm and 30 ppm, respectively. The remaining samples contain ≤ 2 ppm of the respective elements.

Pt reaches a maximum concentration of 0.5 ppm in the pegmatoidal feldspathic pyroxenite, and is otherwise below the detection limit.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.11: The stratigraphic variation of PGE contents in pentlandite, pyrrhotite and chalcopyrite throughout drill core SD134. Whole-rock Pt and Pd contents are shown for comparison of the maximum concentration in the BMS and the whole-rock. The highest PGE contents in pentlandite occur in the range of the upper chromitite stringer and in the pegmatoidal feldspathic pyroxenite.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Table 6.3: Average PGE values in BMS of drill core SD134 in ppm. Pentlandite Pyrrhotite

Ru Rh Pd Os Ir Pt Ru Rh Pd Os Ir Pt SD134-5 n=9 n=4

Average 1.16 0.6 3.99 0.42 0.09 0 0.18 0 0 0.23 0.07 0 SD134-6 n=5 n=6

Average 1.74 0.38 3.61 0.35 0.09 0.06 0.5 0.07 0.07 0.36 0.15 0.07 SD134-8 n=3

Average 13.23 0.66 4.08 0.96 0.15 b.d.l. n.a. n.a. n.a. n.a. n.a. n.a. SD134-11 n=6

Average 0.68 0.61 13.05 0.67 0.03 b.d.l. n.a. n.a. n.a. n.a. n.a. n.a. SD134-14 n=4 n=6

Average 15.48 3 38.38 2.32 0.22 b.d.l. 4.19 0.06 0 0.82 0.06 0.04 SD134-15_1 n=3 n=5

Average 17.37 24.7 131 1.68 0 b.d.l. 7.34 0.02 0 1.6 0 0.02 SD134-15_2 n=3 n=5

Average 26.27 79.46 548.6 18.67 9.04 1.73 6.27 0.16 0.05 7.81 1.16 0.67 SD134 -16 n=5 n=4

Average 19.38 37.94 366.4 5.55 6.88 3.94 15.34 0.31 0.02 3.65 10.86 1.89 SD134-18 n=4 n=6

Average 19.51 90.85 187.75 8.45 7.87 2.25 22.44 29.42 71.24 7.73 3.99 0.72 SD134-19 n=7 n=8

Average 50.73 49.07 230.71 7.2 11.12 3.67 34.21 1.48 0.2 5.81 6.39 1.55 SD134-19a n=10

Average 49.62 38.24 229.4 7.09 10.15 2.45

Chalcopyrite

Ru Rh Pd Os Ir Pt

SD134-5 n=7

Average 0.04 0.89 2.47 0.04 0.05 0.03

SD134-6 n=3

Average 0.05 1.17 2.8 0 0 0.01

SD134-14 n=1

Average 1.01 1.14 2.31 0.36 0.11 0.03

SD134-15_1 n=4

Average 0.04 0.69 1.85 0.06 0 0

SD134-15_2 n=5

Average 7.81 0.73 3.75 11.33 6.41 0.38

SD134 -16 n=5

Average 15.7 1.52 1.81 0.55 30.19 0.33

SD134-18 n=6

Average 1.17 1.18 2.69 1.31 0.99 0.22

SD134-19 n=2

Average 0.03 0.64 1.62 0 0 0.02 b.d.l.: below detection limit; n.a.: not analyzed

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

A comparison of mantle-normalized PGE spectra in different BMS is shown in Figure 6.12. Pentlandite has enriched PPGE, except Pt, compared to the IPGE in all units of the drill cores of the western Bushveld.

The pentlandite in the chromitite stringers and in the pegmatoidal feldspathic pyroxenite is always more enriched in PGE than pentlandite in the feldspathic pyroxenite (Fig. 6.12 a-b). Compared to pentlandite, pyrrhotite and chalcopyrite show a more variable PGE pattern. Pyrrhotite shows a pattern where Pd is depleted, and Ir, Pt and Rh may be slightly enriched compared to the other elements (Fig. 6.12 c-d). In chalcopyrite, the IPGE are mostly depleted compared to Rh and Pd, whereas Pt always shows a strong depletion (Fig. 6.12 e- f).

Fig. 6.12: Mantle-normalized patterns of the three BMS (pentlandite, pyrrhotite, chalcopyrite in the drill cores of the western Bushveld (SD124, SD134). a-b) Pentlandite has enriched PPGE values, except Pt, in all units compared to the IPGE. c-d) Pyrrhotite and chalcopyrite show a more variable PGE pattern compared to that in pentlandite. Palladium is depleted in most pyrrhotite samples, while Ir is enriched (e-f) Chalcopyrite mostly is depleted in Pt compared to the other PGE (e-f).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

6.2.3. PGE concentration in base-metal sulfides (eastern Bushveld)

The drill cores from the eastern Bushveld have a PGE concentration peak at 919.05 m (US200). A second concentration peak occurs in the range of a sulfide stringer at 918.45 m, one meter above the lower chromite stringer. Platinum and Rh also have a concentration peak at the lower chromite stringer at 918.70 m in the samples from drill core US200. All PGE concentrations for pentlandite, pyrrhotite and chalcopyrite are shown in Figure 6.13 and Table 6.4.

6.2.3.1. US200

Pentlandite

Palladium in pentlandite only shows a single concentration peak of 700 ppm at 919.05 m in the pegmatoidal feldspathic pyroxenite. The concentration decreases steadily to 200 ppm at the sulfide stringer and reaches concentrations below the detection limit ca. 20 cm above the sulfide stringer.

Rhodium in pentlandite usually ranges between 0.5 and 18 ppm and has a maximum concentration of 55 ppm at the first concentration peak in the pegmatoidal feldspathic pyroxenite and 30 ppm at the second concentration peak at the lower chromitite stringer.

Ruthenium in pentlandite usually is between 2.46 and 12.68 ppm, but has a maximum concentration of 70 ppm and 25 ppm at the first and the second concentration peaks, respectively. Besides the two concentration peaks the concentration is close to the detection limit.

Osmium, which normally ranges between 0.39 and 2.89 ppm, reaches 11 ppm in pentlandite at the first concentration peak in the pegmatoidal feldspathic pyroxenite, and 4 ppm in the range of the sulfide stringer.

Iridium is up to 14 ppm at the first concentration peak and 5 ppm at the second concentration peaks in the range of the sulfide stringer. The remaining parts of the drill core contain around 4 ppm Ir, or are below the detection limit.

Platinum in pentlandite, which is usually below 4.68 ppm, reaches 9 ppm at the first concentration peak and 8 ppm in the sulfide stringer.

Pyrrhotite

Palladium in pyrrhotite mostly is below the detection limit. Rhodium, Pt and Ir show a maximum concentration peak in the range of the lower chromitite stringer, where they reach

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

16 ppm, 3.5 ppm and 10 ppm, respectively. In the remaining parts Rh is below 4 ppm and Ir is ≤ 1 ppm. Platinum usually is below the detection limit.

The maximum Ru concentration is ca. 2 ppm and is located in the range of the sulfide stringer. The remaining contents in pyrrhotite mostly are ≤ 0.5 ppm.

Osmium shows a maximum concentration in the range of the sulfide stringer, up to 1.4 ppm, and a further concentration peak in the pegmatoidal feldspathic pyroxenite, with up to 1 ppm. The Os concentration in the remaining samples is below the detection limit.

The concentration of all PGE below the lower chromite stringer and above the upper stringer decreases rapidly and is usually below the detection limit.

Chalcopyrite

Palladium in chalcopyrite is enriched in the sulfide stringer of the gabbronorite where it has a maximum concentration of 3 ppm. Two further concentration peaks occur at the lower chromitite stringer and 20 cm above the sulfide stringer, with up to 2 and 1.5 ppm, respectively.

Rhodium reveals a maximum concentration of 4 ppm in chalcopyrite ca. 20 cm above the sulfide stringer, and is below 2 ppm in the remaining parts of the drill core.

Ruthenium and Pt in chalcopyrite are close to the detection limit in all samples of the drill core.

Osmium and Ir show one concentration peak (1.6 ppm and 4 ppm, respectively) in chalcopyrite at the sulfide stringer. The Os and Ir concentration in the remaining parts of the drill core are below the detection limit.

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.13: The stratigraphic variation of PGE contents in pentlandite, pyrrhotite and chalcopyrite throughout drill core US200. Whole-rock Pt and Pd contents are shown for comparison of the maximum concentration in the BMS and the whole-rock. In drill core US200 the highest PGE contents in pentlandite occur in the range of the pegmatoidal feldspathic pyroxenite. 97

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Table 6.4: Average PGE values in BMS of drill core US200 in ppm.

Pentlandite Pyrrhotite

Ru Rh Pd Os Ir Pt Ru Rh Pd Os Ir Pt

US200-5_2 n=13 n=4

Average 2.46 0.53 4.21 0.41 0.14 0.17 0.1 0.21 0.39 0.09 b.d.l. b.d.l.

US200_6 n=5 n=1

Average 2.06 0.71 6.39 0.39 0.06 d.l. b.d.l. b.d.l. b.d.l. 0.22 0.01 b.d.l.

US200-7a n=6 n=2

Average 16.59 5.3 150.83 2.42 0.28 0.04 b.d.l. 3.37 b.d.l. 0.07 0.29 b.d.l.

US200-8 n=16 n=3

Average 23.91 12.97 168.31 4.08 5.42 7.71 2.2 0.46 b.d.l. 1.3 0.97 0.09

US200-9 n=2 n=1

Average 1.18 29.05 358.5 0.37 1.83 3.16 b.d.l. 0.17 0.02 b.d.l. b.d.l. 0.22

US200-10_1 n=3 n=4

Average 6.08 17.15 512.5 0.49 0.38 3.66 b.d.l. 15.99 0.33 b.d.l. 9.76 3.56

US200-11 n=4 n=2

Average 69.44 54.93 711.67 11.26 13.8 8.8 0.4 0.02 b.d.l. 0.97 0.24 b.d.l.

US200-12 n=3 n=5

Average 1.8 7.07 129.67 0.77 1.73 4.68 b.d.l. 0.19 0.01 b.d.l. 0.15 0.03

US200-15 n=7 n=4

Average 12.68 23.79 80.3 2.81 3.59 10.47 0.3 0.02 0.06 0.65 0.09 0.07

Chalcopyrite

Ru Rh Pd Os Ir Pt

US200-5_2 n=3

Average 0.07 0.35 0.45 b.d.l. b.d.l. b.d.l.

US200-6 n=1

Average b.d.l. 3.41 0.09 b.d.l. b.d.l. b.d.l.

US200-7a n=3

Average d.l. 0.08 1.53 0.81 0.03 0.03

US200-8 n=6

Average 2.15 1.14 0.63 1.61 3.43 0.26

US200-9 n=3

Average 0.17 0.79 1.24 0.07 b.d.l. 0.17

US200-10_1 n=2

Average d.l. 1.03 1.8 b.d.l. b.d.l. 0.07

US200-11 n=3

Average 0.14 0.77 0.77 b.d.l. 0.14 0.08

US200-12 n=2

Average 0.11 1.74 2.83 b.d.l. b.d.l. 0.17

US200-15

Average n.a. n.a. n.a. n.a. n.a. n.a. b.d.l.: below detection limit; n.a.: not analyzed

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6.2.3.2. US186

All PGE concentrations of pentlandite, pyrrhotite and chalcopyrite are shown in Figure 6.15 and Table 6.5. Pentlandite

The PGE in pentlandite shows three concentration peaks, in the pegmatoidal feldspathic pyroxenite, in the lower and in the upper chromitite stringer, respectively.

Palladium in pentlandite shows a maximum concentration (between 500 and 600 ppm) in the pegmatoidal feldspathic pyroxenite. Above the lower chromitite stringer, the concentration decreases to a maximum of 50 ppm and decreases further below the detection limit above the upper chromitite stringer.

Rhodium in pentlandite reaches 40 ppm in the pegmatoidal feldspathic pyroxenite, 60 ppm in the lower and 130 ppm at the upper chromitite stringer. Between the concentration peaks Rh reaches up to 40 ppm.

The Ru content in pentlandite is 30 ppm at the first concentration peak in the pegmatoidal feldspathic pyroxenite and between 15 and 25 ppm at the chromitite stringers, but usually ranges between 2 and 17.50 ppm.

Osmium concentrations in pentlandite are ca. 13 ppm at the first concentration peak and reach 6 ppm at the lower chromitite stringer and 4 ppm at the upper chromitite stringer. The remaining samples have concentrations between 0.5 and 5 ppm.

Iridium in pentlandite has 9 ppm at the first concentration peak in the pegmatoidal feldspathic pyroxenite and reaches 5 and 2 ppm at the lower and upper chromitite stringers, respectively. The remaining parts of the drill core reveal concentrations below 2 ppm Ir in pentlandite.

Platinum shows a similar trend when compared to Rh. At the first concentration peak, Pt in pentlandite reaches 6 ppm and around 4.5 ppm at the chromitite stringers. Between these concentration peaks, Pt decreases to 1 ppm.

Usually the concentrations of the PGE are very similar within one thin section. But in the range of chromitite stringers, significant differences were observed. In two cases pentlandite contains varying Pd and Rh concentrations within or close to a chromite stringer. One example is shown in Figure 6.14. Pentlandite grains within a chromite stringer reveal higher Rh contents (around 120 ppm) but lower Pd contents (13 ppm) than those grains, which occur close to a chromite stringer (Rh=17 ppm and Pd=107 ppm; Fig. 6.14).

PhD thesis Inga Osbahr 6. Analytical Results-Merensky Reef

Fig. 6.14: Varying Pd and Rh concentrations in pentlandite within or close to a chromitite stringer. At the top left: A sulfide grain within the chromitite stringer. At the bottom left: A sulfide grain close to the chromitite stringer. The variation of Pd and Rh contents in pentlandite depends on the location in the Merensky Reef. For instance, pentlandite in chromitite stringers shows higher Rh contents (120 ppm in average) and lower Pd contents (13 ppm in average) than pentlandite in the country rocks of the chromitite stringer, which contains higher Pd contents (107 ppm in average) and lower Rh contents (ca. 17 ppm in average).

Pyrrhotite

Pyrrhotite in drill core US186 hardly contains any PGE. The PGE only reveal one concentration peak in the lowermost part of the pegmatoidal feldspathic pyroxenite and are below the detection limit in the remaining samples of the drill core. Palladium usually is ≤ 1 ppm, while Rh and Pt reach up to 4 ppm, respectively. Ruthenium shows a significant concentration of up to 30 ppm. Osmium and Ir reach 5 and 4 ppm, respectively, at the concentration peak.

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Chalcopyrite

Palladium in chalcopyrite shows a significant concentration with up to 28 ppm in the lowermost part of the pegmatoidal feldspathic pyroxenite. Further concentration peaks occur in the lower and upper chromitite stringer of 5 and 3 ppm, respectively.

Rhodium in chalcopyrite shows three concentration peaks. The first one (1 ppm) occurs at the lowermost part of the pegmatoidal feldspathic pyroxenite. At both, the second and third, concentration peaks, which occur just below the two chromitite stringers, Rh concentration reaches 4 ppm.

Ruthenium, Os, Ir and Pt show the same distribution, with a concentration peak in the lowermost part of the pegmatoidal feldspathic pyroxenite, and a maximum concentration at the lower chromitite stringer. At the first concentration peak, Ru reaches 5 ppm, while Os, Ir and Pt each reach 2 ppm. Ruthenium has a maximum concentration at the second concentration peak of up to 42 ppm, while Os and Ir each reach 7 ppm. Chalcopyrite contains up to 12 ppm Pt at the lower chromitite stringer.

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Fig. 6.15 The stratigraphic variation of PGE contents in pentlandite pyrrhotite and chalcopyrite throughout drill core US186. Whole-rock Pt and Pd contents are shown for comparison of the maximum concentration in the BMS and the whole-rock. The highest contents of Pd in pentlandite occur in the pegmatoidal feldspathic pyroxenite. Rh, Ru and Pt show the highest contents in the range of the lower and upper chromitite stringer.

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Table 6.5: Average PGE values in BMS of drill core US186 in ppm. Pentlandite Pyrrhotite Ru Rh Pd Os Ir Pt Ru Rh Pd Os Ir Pt US186-3 n=13 n=5 Average 1.8 0.27 3.93 0.33 0.08 0.07 0.8 0.02 b.d.l. 0.39 0.29 0.06 US186-4 n=5 n=3 Average 3.09 0.38 6.3 1.02 0.71 0.39 0.4 b.d.l. b.d.l. 0.7 0.5 0.07 US186-5 n=4 Average 16.6 128.9 13.1 2.29 2.1 4.52 n.a. n.a. n.a. n.a. n.a. n.a. US186-6 n=2 n=1 Average 16.79 15.42 30.45 3.95 2.36 0.66 b.d.l. b.d.l. b.d.l. 0.22 0.01 b.d.l. US186-8 n=4 n=2 Average 25.05 61.9 60.48 6.2 4.9 5.28 0.1 0.04 0.15 0.26 0.22 0.17 US186-9 n=13 Average 0.43 0.04 492 0.03 b.d.l. b.d.l. n.a. n.a. n.a. n.a. n.a. n.a. US186-11 n=7 n=4 Average 6.25 43.12 620.67 13.2 9.05 5.95 b.d.l. 0.33 0.38 0.04 d.l. 0.04 US186-12_1 n=7 n=2 Average 14.5 17.04 243.29 4.77 2.86 4.06 11 3.09 b.d.l. 4.79 3.59 2.18 US186-12_2 n=4 n=5 Average 27.38 8.77 251.25 4.98 2.04 3.22 30 1.71 0.12 4.01 2.72 3.01 US186-13 n=8 n=4 Average 17.5 7.96 109.55 3.82 1.28 3.11 2.7 1.18 0.82 1.39 1.03 3.3

Chalcopyrite Pyrite Ru Rh Pd Os Ir Pt Ru Rh Pd Os Ir Pt US186-3 n=2 n=6 Average 0.03 0.32 1.22 b.d.l. b.d.l. 0.33 0.2 0.01 0.03 0.03 0.01 b.d.l. US186-4 n=6 n=3 Average 0.06 0.44 1.25 0.13 0.1 0.03 0 0.05 b.d.l. 0.02 0.05 b.d.l. US186-5 n=2 n=1 Average 0.05 0.84 1.96 b.d.l. b.d.l. 0.07 23 8.39 0.13 2.39 1.59 26.3 US186-6 n=1 Average b.d.l. 3.41 0.09 b.d.l. b.d.l. b.d.l. n.a. n.a. n.a. n.a. n.a. n.a. US186-8 n=2 Average 42.8 1.03 5.12 8.91 7.48 11.53 n.a. n.a. n.a. n.a. n.a. n.a. US186-11 n=7 n=2 Average 0.26 0.17 1.07 0.04 0.05 0.06 0.2 0.01 0.13 0.6 0.21 0.02 US186-12_1 n=8 n=8 Average 2.26 0.91 1.59 1.71 1.89 1.08 2.2 0.03 0.04 2.2 1.19 0.76 US186-12_2 n=4 Average 0.09 0.58 1.35 b.d.l. 0.06 0.41 n.a. n.a. n.a. n.a. n.a. n.a. US186-13 n=4 n=4 Average 5.61 0.73 28.44 1.83 0.46 1.77 9.5 2.08 2.53 1 1.3 9.95 b.d.l.: below detection limit; n.a.: not analyzed

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A comparison of mantle-normalized PGE spectra in the three BMS (Pentlandite, pyrrhotite and chalcopyrite) is shown in Figure 6.16. Pentlandite has enriched PPGE, except Pt, compared to the IPGE in all units of the two drill cores. The pentlandite in the chromitite stringers show the highest PGE concentrations compared to the pyroxenites and gabbronorite (Fig. 6.16 a-b). Pyrrhotite and chalcopyrite show a more variable PGE pattern compared to that of pentlandite. In most pyrrhotite samples Pd and Pt are depleted while Ir and Rh are slightly enriched compared to the other PGE (Fig. 6.16 c-d).

In chalcopyrite IPGE and Pt are mostly depleted compared to Pd and Rh (Fig. 6.16 e-f) except in the sulfide stringer of drill core US200 and the lower chromitite stringer of drill core US186, where Ir and Pt are enriched compared to Pd, but not to Rh.

Fig. 6.16: Mantle-normalized patterns of the three BMS (pentlandite, pyrrhotite, chalcopyrite) in the drill cores of the eastern Bushveld. a-b) Pentlandite has enriched PPGE values, except Pt, in all units compared to the IPGE. Pyrrhotite and chalcopyrite show a more variable PGE pattern compared to that in pentlandite. c-d) In the most pyrrhotite samples Pd is depleted (e-f) In most of the chalcopyrite samples IPGE and Pt are depleted compared to Pd and Rh.

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6.2.4. Silver and gold concentration in BMS (eastern and western Bushveld)

The analysis of gold in the BMS shows contents mostly below the detection limit in all units of the four drill cores. The highest Au concentration in BMS is 0.07 ppm and occurs in the chalcopyrite of the pegmatoidal feldspathic pyroxenite in drill core US200.

Silver only was detected in larger amounts in the eastern Bushveld samples. Chalcopyrite has significant amounts of silver in the pegmatoidal feldspathic pyroxenite at 552.20 m (30 ppm) and at the upper chromitite stringer at 551.39 m (40 ppm) in the samples from drill core US186.

Pentlandite in drill core US186 reaches up to 9 ppm Ag at the lower chromitite stringer, while pyrrhotite and pyrite have only 1-2 ppm. Ag distribution shows an inverse trend in chalcopyrite and pentlandite throughout the samples from drill core US186 (Fig. 6.17).

Fig. 6.17: Ag concentration shows an inverse trend in chalcopyrite and pentlandite of drill core US186.

Silver concentration is lower in chalcopyrite from drill core US200 compared to that from drill core US186. In drill core US200 Ag in chalcopyrite reaches 19.80 ppm at 919.05 m in the middle of the pegmatoidal feldspathic pyroxenite and 5 ppm at 918.45 m in the range of the sulfide stringer. The maximum concentration of silver reaches 1.8 ppm in pentlandite and pyrrhotite, respectively, in drill core US200.

The BMS from the western drill core showa a maximum of 1-2 ppm Ag.

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6.3. Summary The whole rock chemical distribution patterns of Pt and Pd in the Merensky Reef sequences show distinct mineralization sequences with usually one well-defined peak of Pt and Pd in the western Bushveld or two peaks in the eastern Bushveld.

In the drill cores of the western Bushveld and US200 of the eastern Bushveld, the Cu trend follows that of Pd, Pt and Rh, however, drill core US200 and SD124 reveal an offset pattern regarding the concentration peak of Cu and Ni which are displaced to the PGE concentration peaks. Drill core US186 also exhibits an offset pattern; Ni and Cu are displaced to the PGE peaks and are located 25 cm below the PGE concentration peaks. The similar distribution patterns of PGE with Cu and Ni are a consequence of the fact that Pd is mainly hosted by sulfides, especially pentlandite. Another relationship exists between the PGE and Cr, since the concentration peaks of the PGE mostly occur in the range of the chromite stringers.

Pentlandite in drill core SD124 contains high concentrations of Pd and Rh, with average concentrations of up to 1800 ppm and 1400 ppm, respectively. In the other three drill cores maximum Pd concentration ranges from 550 to 700 ppm, and Rh concentration from 54 to 130 ppm. Maximum contents of Ru range from 27 to 69 ppm in all drill cores. Osmium and Ir reveal a similar range from 8 to 19 ppm and 9 to 14 ppm, respectively. However, in drill core SD124 Ir shows much higher concentrations of up 96 ppm in pentlandite. Platinum has the lowest contents in pentlandite, with maximum concentrations ranging from 4 to 12 ppm in the different drill cores.

Pyrrhotite generally has lower PGE concentrations than pentlandite. The maximum concentration of Pd in pyrrhotite from the four drill cores usually ranges from 0.5 to 2 ppm, but reaches up to 71 ppm in pyrrhotite from drill core SD134. Rhodium ranges from 3 to 29 ppm and Ru from 2 to 34 ppm. Osmium and Ir show a similar range from 1.30 to 7.81 and 4 to 11, respectively. However, Ir in pyrrhotite from drill core SD124 has higher concentrations with up to 66 ppm. Platinum concentrations range between 2 to 28 ppm, similar to or even higher than that in pentlandite.

Chalcopyrite usually shows a PGE concentration between 0.05 to 11 ppm for all drill cores. However chalcopyrite from drill core US186 (eastern Bushveld) shows higher concentration of Pd (28.24 ppm), Ru (42.80 ppm) and Pt (11.5 ppm).

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Pyrite mostly has PGE concentrations below the detection limit, although pyrites from drill cores SD124 and US186 have significant concentrations of Ru (12-23 ppm), Rh (8 to 29 ppm) and Pt (5 to 26 ppm) in the range of the upper chromite stringer.

The whole-rock PGE distribution in the drill cores SD134 and SD124 corroborate former studies which describe similar distributions throughout the Merensky Reef in samples from the Styldrift farm (drill cores SD22-3 and SD46) in the western Bushveld (Fig. 1a; e.g. Naldrett et al. 2009). Furthermore, the PGE distribution patterns throughout the Merensky Reef in the drill cores of the present study from the eastern Bushveld, with maximum PGE+Au concentration of 6 and 9 ppm at the lower and upper chromitite stringer, respectively, are similar to “typical” Merensky Reef profiles as reported by Mitchell and Scoon (2007) – their channel AF, Winnaarshoek. Notably, the PGE+Au concentrations in the Merensky Reef profiles at Winnaarshoek are slightly higher (6 and 12 ppm) at the lower and upper chromitite stringer, respectively (Mitchell and Scoon 2007).

The maximum Pd concentration (700 ppm) measured in pentlandite in drill core SD134 is similar to that (600 ppm) from Rustenburg published by Godel et al. (2007), but the concentration of Pd in pentlandite in drill core SD124 is distinctly higher (1400 ppm). The concentrations of the other PGE in pentlandite are in fair agreement with data reported by Godel et al. (2007) from the Rustenburg area. Furthermore, chalcopyrite contains most of the Ag.

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7. Analytical Results-UG2 7.1. Whole-rock chemistry 7.1.1. SD124 (western Bushveld)

The drill core SD 124 UG2 D1 was divided into 18 pieces, each with a length of between 16 to 22 cm. Whole-rock data are listed in Table 7.1 a. Figure 7.1 shows that the contents of

Cr2O3, Pt, Pd , Rh, Ni and Cu start increasing at the transition between the anorthosite to the UG2. The Pt, Pd and Rh contents show similar distribution trends. The concentration peaks of Pt, Pd and Rh are located at the top and the bottom of the UG2 main chromitite seam, whereas the maximum contents of Pt (7.0 ppm), Pd (2.5 ppm) and Rh (1.6 ppm) occur at the bottom of the UG2. At the top of the UG2 main chromitite seam the concentration reach 5 ppm Pt, 2 ppm Pd and 1.5 ppm Rh. In the middle of the UG2 main chromitite seam, the contents of Pt, Pd and Rh decrease to 2.5 ppm Pt, 1 ppm Pd and 0.5 ppm Rh. The UG2 leader chromitite seam, above the main seam, contains between 1.1 to 2.3 ppm Pt, 0.38 to 0.79 ppm Pd, 0.07 to 0.35 ppm Rh, 0.020 to 0.030 ppm Au, 700 to 1100 ppm Ni and 20 to 60 ppm Cu. Above the leader seam the concentration of Pt, Pd and Rh decreases below the detection limit (Fig. 7.1, Table 7.1 a). The content of sulfur is below the detection limit (≤ 0.005 wt. %) in most of the units. An increasing content of S (0.013 wt%) was only found in the norite at 711.30 m. Gold is interpreted to be associated with sulfur, since both show the same compositional trend. The Au content is about 0.01 ppm within all units, except in the feldspathic pyroxenite. Au also shows a concentration peak of 0.04 ppm in the norite at 711.30 m (Fig. 7.1).

Fig. 7.1: The stratigraphic variation of whole-rock Pt, Pd, Rh, Ni, Cu and Cr2O3 throughout drill core SD124. The PGE concentration is enriched at the bottom and top of the UG2 chromitite and reveals significant amounts of PGE in the middle part of the UG2 and in the several chromitite stringers that occur above the UG2 main seam.

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7.1.2. DT46 (eastern Bushveld)

Drill core DT46 was divided into 13 pieces, each with a length of between 20 and 24 cm. Whole-rock data analyzed by Anglo American are listed in Table 7.1 b. Additional whole-rock analyses with smaller pieces (ca. 6 cm in length) were conducted by Actlabs. In drill core DT46 the distribution of Pt, Pd, Rh, Ni and Cu follows exactly the same trend (Fig. 7.2). All elements reveal at least two concentration peaks, one at the bottom and one at the top of the UG2 chromitite main seam. All elements investigated usually show a higher concentration close to the bottom of the UG2. Ni shows two further concentration peaks ca. 50 cm below and above the UG2 chromitite seam (Fig. 7.2).

Platinum reaches 4 ppm in the lower range and 3.5 ppm in the upper part of the UG2 and decreases to 2 ppm between them. The concentration decreases below the detection limit below and above the UG2 chromitite seam. Palladium and Rh show a similar trend as Pt. Palladium and Rh reach concentrations of 6 ppm and 0.9 ppm in the lower part of the UG2, respectively, and 4 ppm and 0.6 ppm in the upper part respectively. In the middle of the UG2 seam the concentration of Pd decreases to 2 ppm and the Rh concentration decreases to 0.4 ppm (Fig. 7.2; Tab. 7.1b).

Fig. 7.2: The stratigraphic variation of whole-rock Pt, Pd, Rh, Ni and Cu throughout drill core DT46. The PGE concentration is enriched at the bottom and top of the UG2 chromitite and reveals significant amounts of PGE in the middle part of the UG2 as well.

The mantle-normalized patterns of the samples from drill core DT46 reveal that almost all samples show the same mantle patterns, with an increase from Os to Pd but a negative anomaly in Pt (Fig. 7.3). The samples DT46-0-Dt46-16 reflect the downward stratigraphy with 109

PhD thesis Inga Osbahr 7. Analytical Results-UG2 samples DT46-0 and -6 are located above the UG2 main chromitite seam, samples DT46-9 to -13 are located within the UG2 main chromitite seam and samples Dt46-14 and -16 are located below the UG2. All samples are enriched in PGE compared to the mantle, however, the PPGE show a higher enrichment than the IPGE. The samples that stem from within the UG2 (e.g. DT46-12, purple line) are more enriched than samples from below or above the UG2 (e.g. DT46-14_1, dark blue line or DT46-0, light blue line). Ni, Cu and Au show a lower enrichment compared to the PGE. However, Ni is depleted compared to the mantle in most of the samples (Fig. 7.3).

10000.00

1000.00 DT46-0

DT46-6 100.00 DT46-9 DT46-12

10.00 DT46-13

Sample/Mantle DT46-14_1 DT46-14_2 1.00 DT46-16

0.10 Ni Os Ir Ru Rh Pt Pd Au Cu

Fig. 7.3: PGE, Ni, Cu and Au contents of the different samples of drill core DT46 with a depletion of Ni and an enrichment of PPGE compared to the IPGE, Ni and Au; normalized to the mantle after McDonough and Sun (1995).

The sample numbers for the chondrite-normalized patterns are the same as for the mantle- normalized patterns. The different samples of drill core DT46 show upward trending curves from Os to Pd, however, most of the samples show a slight depletion in Os compared to the other IPGE and Pt compared to the other PPGE (Fig. 7.4). Naldrett and Duke (1980) and Barnes et al. (1985) described this type of normalized pattern as typical for the PGE mineralization produced by magmatic scavenging of PGE through segregating sulfides. The samples are enriched in PPGE and Cu but depleted in IPGE, Ni and Au compared to chondrite. Only the sample from the middle part of the UG2 main chromitite seam (DT46-12, purple line) is enriched in IPGE as well compared to chondrite (Fig. 7.4).

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100.00

DT46-0

10.00 DT46-6 DT46-9 1.00 DT46-12 DT46-13 DT46-14_1 Sample/Chondrite 0.10 DT46-14_2 DT46-16 0.01 Ni Os Ir Ru Rh Pt Pd Au Cu

Fig. 7.4: PGE, Ni, Cu and Au contents of the different samples of drill core DT46 with a depletion of Ni, Cu, Au and IPGE compared to the PPGE; normalized to C-1 chondrite after McDonugh and Sun (1995).

The Pt/Pd ratio is much higher in the western Bushveld (between 0.9 and 4.3) than that (0.2- 1.5) in the eastern Bushveld (Fig. 7.5). In the western Bushveld (SD124) the highest Pt/Pd ratio (3.0) occurs in the UG2 main seam and the “Leader” chromitite stringers above the main seam. The footwall of the UG2 chromitite reveals a Pt/Pd ratio between 1.2 and 2.0 (Fig. 7.5). In the eastern Bushveld (DT46) a Pt/Pd ratio with up to 1.5 exists above and below the UG2 main seam. The UG2 main seam reveals a Pt/Pd ratio between 0.6 and 1.1 (Fig. 7.5).

Fig. 7.5: Pt/Pd ratio of the UG2 of the western Bushveld (SD124) and eastern Bushveld (DT46) with a higher Pt/Pd ratio exists in the western Bushveld. 111

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In drill core SD124 the Cu/Pd ratio ranges between 100 and 1000 in the footwall and shows a decrease in the UG2 main seam, with a Cu/Pd ratio of 8-100. The UG2 leader chromitite seam generally reveals a Cu/Pd ratio around 180, but increases to 450 at 708.90 m. Drill core DT46 shows a similar Cu/Pd ratio trend as drill core SD124. The harzburgite, being the footwall of the UG2, shows a Cu/Pd ratio of up to 2000, while the UG2 main seam only has a Cu/Pd ratio between 250 and 350. The Cu/Pd ratio increases above the UG-2 main seam to a ratio 500-1200 (Fig. 7.6).

Fig. 7.6: Cu/Pd ratio of the UG2 of the western and eastern Bushveld. Cu-Pd ration decreases from the bottom to the top of the UG2 profile.

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Table 7.1: Whole-rock data of Pt, Pd, Rh and Au in ppm and Cu, Ni Cr2O3 and S in wt. %. a) SD124 D1 Styldrift UG-2 Chromitite

No. From (m) To (m) (m) Lithotype Pt Pd Rh Au Cu Ni Cr2O3 Total S 1 708.18 708.34 0.16 Feldsp. Px. 0.56 0.19 0.01 0.003 0.06 0.63 0.005

2 708.34 708.50 0.16 Feldsp. Px. 0.67 0.23 0.01 0.003 0.06 1.04 0.005

3 708.50 708.66 0.16 Feldsp. Px. 0.68 0.29 0.01 0.004 0.06 15.79 0.005

4 708.66 708.82 0.16 Feldsp. Px. 1.10 0.38 0.02 0.004 0.07 0.95 0.005

5 708.82 708.98 0.16 Feldsp. Px. 0.13 0.11 0.03 0.005 0.08 0.65 0.005

6 708.98 709.14 0.16 Feldsp. Px. 0.12 0.15 0.02 0.003 0.07 16.65 0.005

7 709.14 709.30 0.16 UG2 leader seam 1.99 0.79 0.35 0.02 0.006 0.11 22.87 0.005 8 709.30 709.47 0.17 Feldsp. Px. 2.31 0.68 0.42 0.01 0.006 0.11 13.50 0.005 9 709.47 709.66 0.19 Feldsp. Px. 0.51 0.17 0.07 0.01 0.002 0.07 31.22 0.005 10 709.66 709.87 0.21 UG2 main seam 5.12 1.85 0.85 0.01 0.003 0.12 31.57 0.005 11 709.87 710.08 0.21 UG2 main seam 2.50 0.89 0.52 0.01 0.002 0.13 37.22 0.005 12 710.08 710.30 0.22 UG2 main seam 7.02 2.45 1.56 0.01 0.002 0.11 29.95 0.005 13 710.30 710.50 0.20 Leuconorite 0.12 0.09 0.05 0.01 0.001 0.02 0.35 0.005 14 710.50 710.70 0.20 Leuconorite 0.07 0.04 0.03 0.01 0.002 0.03 0.55 0.005 15 710.70 710.90 0.20 Leuconorite 0.07 0.05 0.01 0.01 0.003 0.03 0.24 0.005 16 710.90 711.10 0.20 Leuconorite 0.07 0.05 0.01 0.001 0.02 0.26 0.005

17 711.10 711.30 0.20 Leuconorite 0.05 0.04 0.04 0.002 0.02 0.33 0.013

18 711.30 711.50 0.20 Leuconorite 0.06 0.03 0.01 0.003 0.04 0.25 0.005

b) DT46 D1 Lebowa UG-2 Chromitite

No. From (m) To (m) (m) Lithotype Pt Pd Rh Au Cu Ni

1 944.77 945.00 0.23 Peg. Feld.Px 0.12 0.10 0.04 0.012 0.024

2 945.00 945.24 0.24 Peg. Feld.Px 0.02 0.09 0.01 0.005 0.070

3 945.24 945.48 0.24 Peg. Feld.Px 0.22 0.14 0.13 0.05 0.007 0.017

4 945.48 945.69 0.21 UG2 main band 3.35 3.73 0.59 0.17 0.127 0.260

5 945.69 945.91 0.22 UG2 main band 2.21 2.02 0.42 0.08 0.064 0.180

6 945.91 946.13 0.22 UG2 main band 3.50 4.67 0.65 0.21 0.156 0.280

7 946.13 946.35 0.22 UG2 main band 3.79 6.05 0.89 0.28 0.150 0.250

8 946.35 946.55 0.20 Harzb. 0.51 0.42 0.25 0.18 0.017 0.038

9 946.55 946.75 0.20 Harzb. 0.38 0.36 0.02 0.010 0.150

10 946.75 946.95 0.20 Harzb. 0.06 0.04 0.01 0.007 0.110

11 946.95 947.15 0.20 Harzb. 0.04 0.03 0.01 0.006 0.130

12 947.15 947.35 0.20 Harzb. 0.03 0.03 0.01 0.004 0.150

13 947.35 947.55 0.20 Harzb. 0.05 0.10 0.01 0.001 0.190

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7.2. PGE in base-metal sulfides The trace PGE contents in pentlandite, chalcopyrite and pyrrhotite were analyzed by LA-ICP- MS with a total of 102 analyses. The average values are listed in Table 7.2 and 7.3.

7.2.1. SD124 (western Bushveld)

Drill core SD124 has no measurable BMS in the UG2 chromitite seam, thus hardly any PGE in BMS were analyzed. The mineralized zone, however, is in immediate contact above the UG2 main chromitite seam. The chromitite seam itself contains only a few small chalcopyrite grains, which do not contain any PGE. However, troilite was detected in the upper mineralized part of the drill core. In the area above the chromitite seam the PGE distribution in BMS is similar to that in the Merensky Reef. Pentlandite contains most of the PGE, primarily Pd, while chalcopyrite and troilite hardly contain any PGE (Table 7.2).

Pentlandite Palladium, Pt and Rh show at least two concentration peaks, while Os, Ir and Ru seem to reveal only one concentration peak (Fig. 7.7). Palladium and Rh show a concentration peak of ca. 350 ppm and 140 ppm in the range of a chromitite stringer of the UG2 leader chromitite seam at 709 m. A further Pd concentration peak with up to 80 ppm is located in another stringer of the leader seam, ca. 25 cm above the UG2 (Fig. 7.7). However, Rh, Os and Ir have their maximum concentration ca. 30 cm above the UG2 main chromitite seam. Rhodium reaches 200 ppm, Os reaches 15 ppm and Ir up to 150 ppm. Platinum and Ru have concentration peaks of up to 12 ppm and 125 ppm, respectively, ca. 50 cm above the UG2 in the range of a further chromitite stringer (Fig. 7.7). The analyses described above provide a good overview of the distribution of the PGE within the UG2 and the immediate contact. However, for a more detailed description more analyses have to be done in the future.

Chalcopyrite Palladium in chalcopyrite in samples above the main chromitite seam reaches up to 25 ppm, whereas maximum concentration of ca. 80 ppm was detected in the range of the uppermost chromitite stringer of the UG2 leader chromitite seam. However, in the same thin section, chalcopyrite grains with only 8 ppm Pd were detected. The remaining samples contain no more than 3 ppm Pd (Fig. 7.7). Rhodium in chalcopyrite has one concentration peak with up to 62 ppm in the uppermost chromitite stringer of the leader seam. In the same part of the drill core Ru and Ir each 114

PhD thesis Inga Osbahr 7. Analytical Results-UG2 reaches maximum concentration of 3 ppm, respectively. The remaining samples contain less than 0.2 ppm Ru and Ir, while Os is close to the detection limit and reach maximum concentrations of 0.06 ppm in almost all samples (Fig. 7.7).

Fig. 7.7: The stratigraphic variation of Pt, Pd, Rh, Os, Ir and Ru in pentlandite and chalcopyrite throughout drill core SD124. Pentlandites within the UG2-chromitite main seam were too small to be analyzed, thus only pentlandites above the UG2 main seam were investigated. Table 7.2: Average PGE values in pentlandite and chalcopyrite of drill core SD124 Pentlandite

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Thin section n Ru Rh Pd Os Ir Pt Au Ag UG2-11_1 9 40.20 32.70 172.49 5.57 8.12 1.11 b.d.l. 2.51 UG2-38 1 3.91 136.00 355.00 b.d.l. 22.80 1.21 <0.036 4.63 UG2-39b 4 132.41 930.25 100.02 8.71 40.39 754.10 b.d.l. 4.60 UG2-41_2 2 6.81 206.00 7.29 14.50 151.00 0.24 b.d.l. 0.46 UG2-42 3 4.78 0.24 62.62 0.35 0.04 b.d.l. 0.08 0.45 UG2-44 2 0.46 3.85 15.70 0.27 0.27 6.08 b.d.l. 8.02

Chalcopyrite Thin section n Ru Rh Pd Os Ir Pt Au Ag UG2-38 4 2.77 20.18 73.91 0.17 3.24 b.d.l. b.d.l. 0.51 UG2-39b 3 b.d.l. 3.99 0.07 b.d.l. b.d.l. 0.05 b.d.l. 0.11 UG2-41_2 2 b.d.l. 4.12 1.94 0.12 0.02 0.11 0.07 4.99 UG2-42 1 0.21 3.46 0.34 b.d.l. b.d.l. b.d.l. b.d.l. 1.20 UG2-44 1 b.d.l. 4.13 25.90 b.d.l. b.d.l. b.d.l. 0.04 8.29

7.2.2. DT46 (eastern Bushveld)

Pentlandite Palladium in pentlandite is the only element investigated that shows two concentration peaks, one peak in the range of the lower part of the UG2 main seam and the other peak in the upper part. Thus the Pd concentration peaks in pentlandite coincide with the concentration peaks of the whole-rock Pd. The Pd concentration in the lower and upper part of the UG2 reaches up to 800 and 950 ppm, respectively, and decreases to ca. 200 ppm in the middle part of the main seam. Rhodium, Os, Ir and Ru reveal a maximum concentration in the upper part of the UG2. Rhodium reaches up to 140 ppm, Os 20 ppm, Ir 22 ppm and Ru 175 ppm. The maximum concentration of Pt coincides with that of Pd and reaches ca. 6 ppm. In the upper part of the UG2 the Pt concentration in pentlandite reaches ca. 4 ppm (Fig. 7.8, Table 7.3).

Chalcopyrite In chalcopyrite almost all PGE are close to the detection limit. Only one grain reveals very high concentrations of Ru (632 ppm), Rh (452 ppm), Os (59 ppm) and Pt (17 ppm). However, this is more likely to be the result of micro inclusions in the chalcopyrite (Fig. 7.8, Table 7.3).

Pyrrhotite The PGE concentrations in pyrrhotite are similar to those of chalcopyrite. The concentrations are mostly close to the detection limit and only some individual grains contain significant concentrations of up to 4 ppm Pd or 12 ppm Ru. One pyrrhotite grain contains a micro inclusion which is composed of 1014 ppm Au and 142 ppm Pt (Fig. 7.8, Table 7.3).

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Fig. 7.8: The stratigraphic variation of Pd, Rh, Pt, Os, Ir and Ru in pentlandite, chalcopyrite and pyrrhotite throughout drill core DT46. The pentlandites are enriched in PGE all over the UG2, however, the maximum concentration peaks of the PGE are located in the uppermost part of the UG2 and for Pd and Pt in the lowermost part of the UG2 as well. No pyrrhotite could be identified within the UG2 chromitite main seam.

Table 7.3: Average PGE values in BMS of drill core DT46 Pentlandite

Thin section n Ru Rh Pd Os Ir Pt Au Ag 117

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DT46-6 5.00 2.62 23.31 222.92 1.18 1.57 3.70 0.02 0.47 DT46-7 3.00 5.90 118.63 948.67 2.71 5.34 4.00 b.d.l. 0.65 DT46-9 4.00 178.68 138.53 180.28 20.61 22.81 2.34 b.d.l. 2.24 DT46-12 13.00 4.51 79.71 817.54 2.23 8.14 6.14 b.d.l. 0.66 DT46-13 5.00 0.63 58.00 158.64 0.18 0.84 3.33 0.09 2.59

Chalcopyrite

Thin section n Ru Rh Pd Os Ir Pt Au Ag DT46-6 3.00 0.06 4.10 0.06 b.d.l. b.d.l. b.d.l. 0.01 0.80 DT46-7 1.00 0.23 4.20 0.03 0.19 0.62 0.09 0.02 1.05 DT46-9 3.00 210.71 153.30 0.47 59.60 0.94 5.78 0.09 1.23 DT46-12 8.00 0.29 3.61 0.18 0.05 0.09 0.13 0.08 1.28 DT46-13 3.00 0.12 4.11 b.d.l. 0.17 b.d.l. b.d.l. 0.09 4.19

Pyrrhotite

Thin section point Ru Rh Pd Os Ir Pt Au Ag DT46-2 2 b.d.l. 0.06 b.d.l. 0.14 b.d.l. b.d.l. b.d.l. 0.10 DT46-6 4 0.39 0.15 0.04 0.16 0.15 0.24 b.d.l. 0.32 DT46-12 5 4.64 0.23 0.06 0.78 0.55 0.32 b.d.l. 0.41 DT46-13 11 0.42 0.09 2.98 0.03 b.d.l. 19.16 338.07 35.95

A comparison of mantle-normalized PGE spectra in different BMS is shown in Figure 7.9. Pentlandite shows an upward trending curve from Os to Pd, with a strong depletion in Pt in drill SD124 and Dt46 from the western and eastern Bushveld, respectively. All samples show enriched PPGE values, except Pt, compared to the IPGE. The pentlandite in the UG2 chromitite main seam is more enriched in PGE than in those samples from above and below the UG2 chromitite (Fig. 7.9 a-b).

Chalcopyrite shows a more variable PGE pattern compared to that of pentlandite in both drill cores. The pattern shows an increase from Os to Pd, however, there is a strong depletion in Ir and Pt (Fig. 7.9 c-d).

The PGE patterns in pyrrhotite are only available in drill core DT46 from the eastern Bushveld. Two of the three samples show a slight increase from Os to Ru and then a decrease from Ru to Pd thus compared to pentlandite and chalcopyrite. The IPGE are generally enriched compared to the PPGE in pyrrhotite (Fig. 7.9 e).

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Fig. 7.9: Mantle-normalized patterns of the three BMS (pentlandite, pyrrhotite and chalcopyrite) in the drill cores of the western (SD124) and eastern (Dt46) Bushveld. a-b) Pentlandite has enriched PPGE values in all units compared to the IPGE; c-d) Chalcopyrite shows a more variable PGE pattern compared to that in pentlandite; e) In pyrrhotite samples IPGE are enriched compared to the IPGE.

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7.3. Summary The most significant difference concerning the bulk chemical PGE concentration in the UG2 of the western and eastern Bushveld is that in the western Bushveld the Pt concentration is much higher (max. 7 ppm) than the Pd concentration (max. 2.5 ppm), whereas in the eastern Bushveld the Pd concentration (max. 6 ppm) is higher than the Pt concentration (max. 4 ppm). All elements in both drill cores follow the same distribution trend.

The PGE concentration in BMS of the UG2 is similar to that of the Merensky Reef, thus pentlandite in both hosts most of the PGE, while pyrrhotite and chalcopyrite hardly contain any PGE. The PGE distribution in the BMS of the UG2 from the western and eastern Bushveld reveals two significant differences. The first one is the location of the BMS, which occur in the former in the area above the UG2 main seam and in the latter within the UG2 main seam. The western Bushveld reveals e.g. lower Pd (80 ppm) and Rh (20 ppm) concentration in pentlandite than the eastern Bushveld (900 ppm Pd and 140 ppm Rh). The different distribution is thought to be the result of a hydrothermal alteration that influenced the BMS distribution. However, the hydrothermal alteration which was found to occur mostly in the unit of the harzburgite does not influence the PGE concentration in the BMS, since pentlandite of both drill cores still hosts most of the PGE. However, the alteration may be responsible for the sulfide-bearing UG2-chromitite, which was found in the eastern Bushveld (pers. communication A. Josties, 2012).

It should be noted that dominantly Pt-rich PGM are present in the ores, which supports that a bimodal distribution of the various PGE, i.e. Pd and Rh occurring mainly in sulfides (see Fig. 9) and Pt in the form of discrete PGM.

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8. Geochemistry 8.1. Merensky Reef 8.1.1. Whole-rock trace element geochemistry

The following paragraph discusses correlations between PPGE, S, Ni, Cu and Cr. Palladium, Rh and Pt reveal a positive correlation, however only the correlation diagram between Pd and Pt is shown below (Fig. 8.1 a) in order to demonstrate the positive correlation within the PPGE group. In this chapter the total of the PPGE was always taken instead of every single PPGE in order to avoid confusion with so many diagrams. The correlation diagrams reveal positive correlations among almost all elements, except PPGE vs. Cr. Whole-rock IPGE data are not available for most of the samples and are not described here. Samples with chromium contents above the detection limit are rare and thus not always representative. The correlation diagrams point out differences between the eastern and western Bushveld as well as differences between the units. The correlation matrix (Table 8.1) shows the correlation coefficient of every pair plotted in the correlation diagrams in the western and eastern Bushveld, respectively. Very good positive correlations are described by a correlation coefficient of > 0.641. Values between 0.514 and 0.641 are termed as good positive correlations and below 0.514 as poor correlation. The same scheme is used for negative correlations.

PPGE vs. S, and Cu, Ni vs. S Figure 8.1 shows the correlation between Pt and Pd, between the PPGE and S and between S and Cu and Ni. The PPGE group reveals a positive correlation. An example is given in Figure 8.1a. Pd vs. Pt shows a very good positive correlation in the eastern and western Bushveld, respectively and in all units. The samples containing a chromitite stringer show the highest PPGE concentrations of all rock types, while the pegmatoidal feldspathic pyroxenite mostly is more enriched than the feldspathic pyroxenite. The unit of the norite is depleted in PPGE. The correlation between PPGE and sulfur displays a positive correlation in the eastern and western Bushveld, respectively (Fig. 8.1 b). The base-metals versus S also reveal a positive correlation, although Cu shows a better positive correlation with S in the western and eastern Bushveld (Fig. 8.1 d, Table 8.1), respectively, than with Ni (Fig. 8.1 c, Table 8.1). However, the correlation is positive in every unit between PPGE and S and between S and Ni and Cu.

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Fig. 8.1: Whole-rock correlation diagrams of PPGE vs. S and correlation of S, Ni and Cu; a) Pt vs. Pd; b) S vs. PPGE; c) Ni vs. S; d) Cu vs. S.

PPGE vs. Cu, Ni and Cr2O3 The correlation between the PPGE and Cu and Ni is similar (Fig. 8.2 a, b). The correlation is positive, but shows a broad distribution. The chromitite stringer samples again show the highest PPGE concentrations, but similar Ni and Cu contents to the units of the pegmatoidal feldspathic pyroxenite and the feldspathic pyroxenite. The norite is depleted in PPGE and shows a medium Ni and Cu concentration (Fig. 8.2 b, c).

The correlation between Ni and Cu is positive in all units of the eastern and western Bushveld, with the exception of some outliners in the pegmatoidal feldspathic pyroxenite of the eastern Bushveld, which show higher Ni contents than the remaining data points (Fig. 8.2 c).

The positive correlation between the PPGE and Cr is poorly developed in the eastern and western Bushveld (Fig. 8.2 d). For most of the units the chromium content is consistent with increasing PPGE contents. The chromitite stringers contain the highest chromium and PPGE concentrations. However, it should be noted that only 5 samples with chromium contents above the detection limit were analyzed for the eastern Bushveld, thus their correlation is regarded as non-representative.

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Fig. 8.2: Whole-rock correlation diagrams of PPGE vs. Cu, Ni and Cr. a) Cu vs. PPGE; b) Ni vs. PPGE; c) Cu vs. Ni; d) Cr2O3 vs. PPGE.

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8.1.2. Trace element correlation in pentlandite

Correlations between the PGE in pentlandite are described in the following paragraph. Due to their coherent behavior during magmatic processes, the PGE are subdivided into the PPGE-group (Pt, Pd, Rh) and the IPGE-group (Os, Ir, Ru) - (e.g. Mungall 2005). Thus, a positive correlation is expected within the respective subgroups, i.e., PPGE and IPGE. However, in the investigated rock types, a positive correlation only exists between Pd and Rh in the PPGE group (Fig. 8.3) and within the IPGE group (Fig. 8.4). The correlation coefficients of the correlation diagrams for the eastern and western Bushveld are listed in the correlation matrix (Table 8.2).

PPGE A positive correlation between Pd and Rh (Fig. 8.3 a) exists in pentlandite from the eastern and western Bushveld. Platinum does not correlate with Pd or Rh in pentlandite (Fig. 8.3 b, c). The pentlandites in the pegmatoidal feldspathic pyroxenite show the highest PPGE concentrations (up to 1000 ppm Pd and Rh, respectively and 20 ppm Pt), while in the feldspathic pyroxenite and the norite they have similar PPGE concentrations, ranging from 1-900 ppm Pd, 1-100 ppm Rh and 0.01-20 ppm Pt in the eastern and western Bushveld Complex, respectively (Fig. 8.3). The pentlandites that are located in the chromitite stringers are enriched in PPGE (1-100 ppm Pd, 10-500 ppm Rh and 3-10 ppm Pt) compared to the feldspathic pyroxenite and norite but show lower concentrations than the pentlandites in the pegmatoidal feldspathic pyroxenite as is revealed in the correlation diagrams Fig. 8.3.

Fig. 8.3: Correlation diagrams of the PPGE group in pentlandite a) Rh vs. Pd b) Rh vs. Pt c) Pd vs. Pt. 124

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IPGE In contrast, the IPGE group reveals a positive correlation between Os, Ir and Ru in pentlandite from all four drill cores (Fig. 8.4). The correlation coefficients (Table 8.2) indicate a better correlation of Os vs. Ir (Fig. 8.4 a) and Os vs. Ru (Fig. 8.4 b) in the eastern Bushveld than in the western Bushveld. However, the correlation between Ru and Ir (Fig. 8.4 c) is better in the western Bushveld than in the eastern Bushveld (Fig. 8.4 and Table 8.2). The IPGE concentrations in the pentlandites of the different rock types are very similar and range between 0.1-50 ppm for Os, 0.05-80 ppm for Ir and 0.3-90 ppm for Ru. However, the pentlandites in the chromitite stringers are slightly enriched compared to the pentlandites in the pegmatoidal feldspathic pyroxenite, the feldspathic pyroxenite and the norite.

Fig. 8.4: Correlation diagrams of the IPGE group in pentlandite. a) Os vs. Ir b) Os vs. Ru c) Ir vs. Ru.

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IPGE vs. PPGE Platinum, the only element that does not correlate within its subgroup, also has a poor positive correlation with the IPGE, e.g. with Ir (Fig. 8.5 a). Table 8.2 indicates a poor positive or even negative correlation. The correlation between the totals of IPGE and PPGE is only poorly developed in most cases, as is indicated in Figure 8.5 b. The pentlandites in the pegmatoidal feldspathic pyroxenite and the chromitite have higher total PPGE concentration (up to 4000 ppm) than the pentlandites in the feldspathic pyroxenite or norite (up to 1000 ppm). The total IPGE concentration in the pentlandites is similar in all rock types and ranges between 0.6 and 100 ppm.

Fig. 8.5: Correlation diagrams between IPGE and Pt in pentlandite. a) Ir vs. Pt b) total of IPGE vs. PPGE.

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8.1.3. Summary

A good positive correlation exists between all whole-rock PPGE with Cu, Ni and Cr in the Merensky Reef samples from the western Bushveld Complex. The samples from the eastern Bushveld mostly reveal a good positive correlation, however, the correlations with chromium is poorly positive or negative (Fig. 8.1 and 8.2).

Table 8.1: Correlation matrix of whole-rock PPGE, Cu, Ni, Cr and S of samples from the Merensky Reef. Western B. Pt Pd Rh Cu Ni Cr S Pt 1 Pd 0.92 1 Rh 0.72 0.93 1 Cu 0.87 0.83 0.59 1 Ni 0.80 0.87 0.83 0.93 1 Cr 0.69 0.84 0.91 0.72 0.81 1 S 0.86 0.89 0.82 0.96 0.97 0.80 1

Eastern B. Pt Pd Rh Cu Ni Cr S Pt 1 Pd 0.83 1 Rh 0.90 0.65 1 Cu 0.67 0.73 0.63 1 Ni 0.53 0.63 0.62 0.85 1 Cr -0.19 0.34 0.07 0.65 0.23 1 S 0.79 0.92 0.96 0.86 1.00 0.18 1

The trace elements in pentlandite reveal a very good positive correlation within the PPGE between Pd and Rh, but not between Pd and Rh with Pt in the samples of the Merensky Reef. The PGE of the IPGE group exhibit a very good positive internal correlation. However, the PGE of the IPGE group do not correlate with the PPGE group (Fig. 8.3-8.5 and Table 8.2).

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Table 8.2: Correlation matrix of PGE in pentlandite of samples from the Merensky Reef. Western B. Pt Pd Rh Os Ir Ru Pt 1 Pd 0.15 1 Rh 0.09 0.89 1 Os -0.18 0.06 -0.10 1 Ir -0.15 0.56 0.42 0.65 1 Ru -0.01 0.31 0.32 0.59 0.70 1

Eastern B. Pt Pd Rh Os Ir Ru Pt 1 Pd 0.35 1 Rh 0.31 0.59 1 Os 0.21 0.41 0.73 1 Ir 0.41 0.49 0.65 0.82 1 Ru 0.15 0.10 0.83 0.83 0.41 1

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8.2. UG2 8.2.1. Whole-rock trace element geochemistry

The bulk chemical distribution of the PPGE, Cu, Ni and Cr reveals a similar distribution pattern compared to the Merensky Reef. In the following chapter, for the sake of simplicity, the total of the PPGE was taken instead of every single PGE. All correlation coefficients are listed in the correlation matrix (Table 8.3).

PPGE, Cu, Ni and Cr2O3

The correlation between Pt and Pd and between PPGE, Cu Ni and Cr2O3 is displayed in the correlation diagrams of Fig. 8.6. A very good positive correlation is exhibited by all PPGE in the samples from the eastern and western Bushveld (Table 8.3). Pd vs. Pt acts as a substitute for all PPGE (Fig. 8.6 a). The correlation of PPGE vs. Cu is very good in the eastern Bushveld but poorly developed in the western Bushveld (Fig. 8.6 b). PPGE vs. Ni show a positive correlation in the eastern and western Bushveld. However, the correlation is better in the western Bushveld (Fig. 8.6 c). Thus the correlation between Ni and Cu reveals a good positive correlation in the eastern Bushveld, but is poorly developed in the western Bushveld (Fig. 8.6 d).

The correlation of the PPGE with Cr2O3 of the samples from the western Bushveld reveals a good positive correlation, while the samples from the eastern Bushveld show a poorly developed positive correlation (Fig. 8.6 e). It is noticeable that only seven samples were available for chromium in the eastern Bushveld, compared to 17 samples in the western Bushveld, thus their correlation for the eastern Bushveld is regarded as non-representative. The UG2-chromitite always shows the highest PPGE contents (up to 10 ppm), while the feldspathic pyroxenite and the harzburgite show a maximum of 2 ppm PPGE, and the norite is depleted in PPGE compared to the other units, with a maximum content of 0.3 ppm PPGE (Fig. 8.6 b, c, e). It is noticeable that the UG2-chromitite shows significant concentrations of Ni and Cu in the eastern Bushveld (Fig. 8.6 b, c and d), which is in agreement with the high sulfide contents observed in the UG2-chromitite of the eastern Bushveld, as described in the chapter “Sample description”. The norite shows the lowest Cu, Ni and Cr2O3 concentrations (Fig. 8.6 b, c, e), which is in agreement with the depletion in sulfides and chromites described in the chapters “Sample description” and “Mineral chemistry” as well.

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Fig. 8.6: Whole-rock correlation diagrams of Cu, Ni and Cr2O3 vs. PPGE a) Pd vs. Pt; b) PPGE vs. Cu; c) PPGE vs. Ni; d) Ni vs. Cu; e) PPGE vs. Cr2O3.

8.2.2. Trace element correlation in pentlandite

PPGE As already described in paragraph 8.1, a good positive correlation is expected within the subgroups PPGE and IPGE, due to their coherent behavior during magmatic processes (Mungall 2005). This assumption was confirmed by the correlation diagrams displaying the correlation of the PGE in pentlandite for the samples from the Merensky Reef. However, this assumption cannot be confirmed for samples from the UG2, since Figure 8.7 reveals only a poor positive correlation as well as a negative correlation for the PPGE group in the eastern and western Bushveld (Table 8.4). For example, Pd vs. Pt shows the best correlation of all PPGE, and this is therefore in contrast to the Merensky Reef correlation diagrams. The correlation coefficients for all correlation diagrams are listed in Table 8.4. 130

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Fig. 8.7: Correlation diagrams of the PPGE group in pentlandite a) Rh vs. Pd; b) Rh vs. Pt; c) Pd vs. Pt.

IPGE In contrast to the PPGE group, the IPGE group reveals a better correlation (Fig. 8.8) in the samples from the western Bushveld and eastern Bushveld. Correlation coefficients indicate a very good positive correlation (Table 8.4) between all IPGE in the eastern Bushveld, and between Os and Ir in the western Bushveld, while the correlation between Ru and Os is poorly developed, and the correlation between Ru and Ir is even negative (Fig. 8.8 and Table 8.4). The pentlandites in the pegmatoidal feldspathic pyroxenite and chromitite show higher Os (up to 60 ppm), Ir (up to 100 ppm) and Ru (up to 400 ppm) concentrations than those in the feldspathic pyroxenite (max. 6 ppm Os, 10 ppm Ir, 10 ppm Ru).

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Fig. 8.8: Correlation diagrams of the IPGE group in pentlandite a) Os vs. Ir; b) Os vs. Ru; c) Ir vs. Ru.

PPGE vs. IPGE The IPGE group and Pt reveal poor positive and negative correlations (Fig. 8.9 a, Table 8.4). By correlating the IPGE versus PPGE, a poor positive correlation is expected, however, this assumption was not confirmed for Rh vs. Os and Rh vs. Ir. Both diagrams reveal a very good positive correlation in samples from the western and eastern Bushveld (Table 8.4), while Rh vs. Ru reveals a very good positive correlation only in samples from the eastern Bushveld. All remaining correlations between IPGE and PPGE reveal poorly developed positive or negative correlations (Table 8.4). The correlation between the total of PPGE vs. IPGE is better than in the Merensky Reef (Fig. 8.9 b).

Fig. 8.9: Correlation diagrams of a) Ir vs. Pt; b) total IPGE vs. total PPGE.

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8.2.3. Summary

The whole-rock trace elements Pd, Pt, Rh, Cu, Ni, S and Cr reveal a positive correlation between all elements in the investigated samples from the UG2.

Table 8.3: Correlation matrix of whole-rock PPGE, Cu, Ni and Cr of UG2 samples.

Western B. Pt Pd Rh Cu Ni Cr Pt 1 Pd 1.00 1 Rh 0.99 0.98 1 Cu 0.13 0.12 0.00 1 Ni 0.72 0.72 0.67 0.58 1 Cr 0.69 0.70 0.60 0.13 0.49 1

Eastern B. Pt Pd Rh Cu Ni Cr Pt 1 Pd 0.99 1 Rh 0.99 0.98 1 Cu 0.99 0.91 0.86 1 Ni 0.84 0.84 0.79 0.88 1 Cr 0.58 0.47 0.56 0.79 0.49 1

The correlation of the PPGE and the IPGE in pentlandite of the UG2 samples reveals very good positive correlations only within the IPGE group and between Rh and the IPGE of the western and eastern Bushveld.

Table 8.4: Correlation matrix of PGE in pentlandite of UG2 samples.

Western B. Pt Pd Rh Os Ir Ru Pt 1 Pd -0.27 1 Rh 0.33 -0.15 1 Os 0.43 -0.33 0.80 1 Ir 0.25 -0.39 0.94 0.82 1 Ru -0.16 0.34 -0.10 0.36 -0.22 1

Eastern B. Pt Pd Rh Os Ir Ru Pt 1 Pd 0.66 1 Rh -0.19 0.07 1 Os -0.26 -0.13 0.73 1 Ir -0.23 -0.08 0.77 0.98 1 Ru -0.32 -0.21 0.74 0.98 0.95 1

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The whole-rock correlation diagrams reveal similar correlations (continuously positive) between the PGE in the Merensky Reef and UG2 samples.

Comparison of the results of the correlation diagrams developed for the trace elements in pentlandite samples of the UG2 with those developed for trace element samples from the Merensky Reef reveals the absence of a positive correlation between Pd and Rh but not between Rh vs. Os and Rh vs. Ir in the UG2.

The correlation behavior described in chapter 8.1 and 8.2 indicates a coherent behavior of PGE in a sulfide liquid before mss or iss crystallization. In addition it is shown that the PGE behave differently during partitioning into the BMS.

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9. Mass Balance Calculation Trace element analyses of representative samples from drill core SD124 and SD134 (Western Bushveld) and six half-core samples (US200-7a, US200-8, US200-9, US200-10_1, US200-11, US200-12) from drill core US200 (eastern Bushveld) were used to conduct a mass balance calculation, in order to shed some light on the PGE distribution between the BMS throughout the drill cores (Table 9.2-9.4). To calculate the weight proportions of PGE in BMS, the following formula, which requires the whole-rock chemical data, the weight fraction of the BMS and the concentration of the element in the BMS, was used (Godel et al. 2007):

i i i (Psul ) = (FsulCsul /Cwr )*100

with Fsul as the weight fraction of the BMS, Csul as the concentration of the element i in the

BMS, and Cwr as the concentration of the element i in the whole-rock.

The weight fraction of the BMS is calculated for pentlandite: (Fpn=Niwr/Nipn), chalcopyrite

(FCcp=Cuwr/CuCcp), and pyrrhotite (Fpo=Swr-SCcp*FCcp-SPn*FPn/SPo), assuming that all Cu is hosted by chalcopyrite, Ni by pentlandite, and all S by these three sulfides (Godel et al. 2007). A total of 0.09 wt. % Ni was subtracted from the whole-rock Ni of the Merensky Reef samples and 0.05-0.08 wt. % Ni for the UG2 samples, since electron microprobe data revealed a significant amount of Ni in pyroxene and pyrrhotite. Pyrite is regarded as negligible due to its low modal content. The calculated proportions for the PGE in the BMS of drill cores US200, SD124 and SD134 of the Merensky Reef and for drill cores SD124 and DT46 of the UG2 are given in Table 9.1.

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Table 9.1: Sample number, depth, lithotype, modal content of sulfides and calculated proportion of pentlandite, pyrrhotite and chalcopyrite of the Merensky Reef and UG2 samples. Merensky Reef

SD124 Sulfide Calculated proportion Sample from to Lithotype (modal content) Pentlandite Pyrrhotite Chalcopyrite 14 666.25 666.5 Feld. Px 1.00 38.75 34.34 26.91 15 666.5 666.74 Feld. Px 2.00 34.25 40.89 24.87 16 666.74 667.00 Upper chr. 0.50 38.39 40.78 20.83 17 667.00 667.26 Peg. Px 1.30 44.25 32.20 23.55 18 667.26 667.52 Peg. Px 0.75 41.54 5.05 53.41 19 667.52 667.78 Peg. Px 1.50 38.52 38.52 22.96 20 667.78 668.04 Peg. Px 0.50 37.59 34.57 27.84 21 668.04 668.30 Peg. Px 0.50 43.65 32.44 23.91 22 668.30 668.56 Peg. Px 0.75 55.32 21.14 23.53 SD134 Sulfide Calculated proportion Sample from to Lithotype (modal content) Pentlandite Pyrrhotite Chalcopyrite 14 611.86 612.13 Feld. Px 1.50 58.76 18.51 22.73 15 612.13 612.40 Feld. Px 1.50 51.16 27.54 21.29 16 612.40 612.66 Feld. Px 2.00 40.74 41.46 17.81 U- 17 612.66 612.96 chr./Peg.Px 3.00 42.55 38.71 18.74 Peg.Px/L- 18 612.96 613.25 chr. 6.00 39.09 31.94 28.98 US200 Sulfide Calculated proportion Sample from to Lithotype (modal content) Pentlandite Pyrrhotite Chalcopyrite US200-7A 918.30 918.36 Feld. Px. 4 56.63 29.36 14.01 US200-8 918.45 918.52 Sulf. Stringer 7.5 53.49 25.00 21.51 US200-9 918.72 918.78 Lower Chr. 1.5 26.97 43.35 29.67 US200-10_1 918.80 918.86 Peg. Px. 1.5 3.02 61.67 35.30 US200-11 919.05 919.11 Gabbronorite 0.5 18.14 55.04 26.82 US200-12 919.30 919.37 Gabbronorite 2.5 59.37 31.30 9.32

UG2

DT46 Sulfide Calculated proportion sample from to Lithotype (modal content) Pentlandite Pyrrhotite Chalcopyrite DT46-6 945.50 945.56 Chromitite 0.50 25.42 28.44 46.14 DT46-9 945.70 945.76 Chromitite 0.50 13.66 10.65 75.69 DT46-12 946.09 946.15 Chromitite 0.50 26.37 31.65 41.98 DT46-13 946.24 946.30 Harzburgite 0.50 4.98 62.15 32.87 SD124 Sulfide Calculated proportion sample from to Lithotype (modal content) Pentlandite Pyrrhotite Chalcopyrite SD124-7 709.14 709.30 Feld. Px. <0.50 63.61 0.00 36.39 SD124-8 709.30 709.46 Feld. Px. <0.50 63.61 0.00 36.39 SD124-9 709.46 709.66 Feld. Px. <0.50 72.39 0.00 27.61 SD124-10 709.66 709.78 Chromitite <0.50 8.66 29.44 61.91 Abbreviations: Feld. Px. = Feldspathic Pyroxenite, Peg. Px. = Pegmatoidal feldspathic pyroxenite, UChr = Upper chromitite stringer, Lchr = Lower chromitite stringer.

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9.1. Merensky Reef The proportions for all PGE in BMS were calculated for the six samples (US200-7a, US200- 8, US200-9, US200-10, US200-11 and US200-12) of drill core US200 D6 due to additional whole-rock analyses of Os, Ir and Ru. For the samples from the western Bushveld, a calculation was only possible for Pd, Rh and Pt since the other whole-rock PGE, the IPGE, were mostly below the detection limit.

9.1.1. Eastern Bushveld (US200)

The eastern Bushveld drill core samples US200-7a, US200-8 and US200-12 are sulfide-rich (3-8 vol. % sulfides), whereas samples US200-9, US200-10 and US200-11 have lower sulfide contents (< 1 vol. % sulfides) or contain a chromitite stringer (US200-9). In the case of the three sulfide-rich samples, a high percentage of PGE is hosted by base-metal sulfides and the remaining amounts in PGM, whereas in the presence of a chromitite stringer or in case of relatively low sulfide contents, most of the PGE are located in discrete PGM (Table 9.2, Fig. 9.1).

The mass balance revealed that in the sulfide-rich samples (US200-7a, US200-8, US200- 12), pentlandite hosts between 62-100 % of the Pd, 55 to nearly 100 % of the Rh, most of the Ru (up to 90 %), 10-60 % of Os and most of the Ir (Table 9.2). Chalcopyrite contains up to 10 % of the Os, 25 % of the Ir, 4 % of the Ru and 0.35 % and 3 % of the Pd and Rh, respectively. Pyrrhotite contains up to 9 % of the Os, 11 % of the Ir and 4 % of the Ru (Table 9.2).

Less than 0.5 % of Pt is hosted by the base-metal sulfides (Table 9.2). Therefore, it is apparent that Pt is present in the form of discrete PGM such as cooperite, isoferroplatinum, sperrylite or moncheite, and Au occur as native gold. The silicates and chromite contain no detectable amounts of PGE, which is in accordance with results of Ballhaus and Sylvester (2000) and Godel et al. (2008).

In the sulfide-poor or/and chromite-rich samples (US200-9, US200-10, US200-11), pentlandite generally contains less Pd (13-42 %) than in sulfide-rich samples. Furthermore, pentlandite in the sulfide-poor samples contains less Rh (3-5 %), Ir (1 %), Os and Ru (0.5-5 %). Only exceptionally, one sulfide-poor sample (US200-11) was found to contain up to 90 % Rh and Ir, respectively. Chalcopyrite and pyrrhotite contain less than 2 % of the PGE (Table 9.2). The presence of many discrete PGM such as cooperite and braggite mainly in and in the vicinity of the chromitite stringer may explain the lower Pd concentration in pentlandite in the sulfide-poor samples compared to the sulfide-rich samples which contain less PGM (Table 9.2).

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In the sulfide-rich samples (US200-7a, US200-8, US200-12), 55-100 % of Pd, Rh and Ru as well as 11-68 % of Os and Ir are hosted by the BMS. In samples US200-9, US200-10 and US200-11, which are relatively poor in sulfides, only 0.5-40 % of Pd, Rh and Ru as well as less than 3 % of Os and Ir are hosted in BMS (Table 9.2), the remaining PGE are hosted in PGM such as cooperite, braggite and moncheite.

The mass balance calculated for Pt and Pd is in fair agreement with the PGM analyses of some samples of drill core US200 which revealed a high number of Pt-rich PGM in all samples, however, more Pd-rich PGM are detected in the range of the chromitite stringer compared to the sulfide-rich area.

Hardly any Os-, Ir- or Rh-rich PGM were detected, what leads to the assumption that the remaining concentrations of Os, Ir and Rh, which are not hosted by sulfides, may be present in the sulfides or chromites in the form of microinclusions.

Table 9.2: Calculated proportions in percent of each element in the BMS of drill core US200, using the mass balance model of Godel et al. (2007). Os Ir Ru Rh Pt Pd US200-7a S-rich Feld. Px. Pn 51.06 9.20 83.44 49.29 0.02 57.36 Chp 4.69 0.27 0.00 0.20 0.01 0.16 Po 0.90 5.78 0.00 18.98 0.00 0.00 all sulfides 56.65 15.25 83.44 68.47 0.03 57.52 US200-8 Peg. Px. Pn 61.04 82.57 90.13 90.13 2.58 91.62 Chp 10.73 25.54 4.13 3.51 0.04 0.16 Po 10.69 12.58 5.12 1.75 0.02 0.00 all sulfides 82.46 120.69 99.38 95.39 2.64 91.78 US200-9 Chromitite Pn 0.23 1.06 0.16 4.94 0.03 12.09 Chp 0.05 0.00 0.03 0.16 0.00 0.05 Po 0.00 0.00 0.00 0.05 0.00 0.00 all sulfides 0.28 1.06 0.19 5.15 0.03 12.14 US200-10 Peg. Px. Pn 0.04 0.32 0.29 2.99 0.02 23.90 Chp 1.78 0.00 0.00 2.32 0.01 1.09 Po 0.00 41.38 0.00 59.18 0.43 0.33 all sulfides 1.82 41.70 0.29 64.49 0.46 25.32 US200-11 Peg. Px. Pn 0.00 87.21 45.09 91.04 0.86 38.86 Chp 0.00 2.04 0.15 2.39 0.01 0.08 Po 0.00 6.88 0.89 0.12 0.00 0.00 all sulfides 0.00 96.13 46.13 93.55 0.87 38.94 US200-12 Gabbronorite Pn 10.05 35.75 7.19 48.79 1.29 91.38 Chp 0.00 0.00 0.08 2.09 0.01 0.36 Po 0.00 2.02 0.00 0.85 0.01 0.01 all sulphides 10.05 37.77 7.27 51.73 1.31 91.75

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Fig. 9.1: PPGE and IPGE concentration in pentlandite, whole-rock PPGE and IPGE concentrations and calculated proportions in percent of PPGE and IPGE between pentlandite and PGM throughout drill core US200 (eastern Bushveld).

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9.1.2. Western Bushveld (SD124, SD134)

In drill core SD124, between ca. 90 % of the whole-rock Pd is incorporated in pentlandite from samples which occur in the range of the chromite stringers (at around 667 m). In addition, pentlandite contains up to 100 % of the Pd in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite in immediate contact with the upper chromitite stringer, while the pentlandites in the samples from the lower part of the pegmatoidal feldspathic pyroxenite contain between 40 and 70 % of the whole-rock Pd (Fig. 9.2). The remaining Pd is hosted by discrete PGM, since chalcopyrite and pyrrhotite are almost devoid of PGE. Chalcopyrite and pyrrhotite contain a maximum of 2 % of the whole-rock Pd in samples, which stem from the immediate vicinity of the upper chromitite stringer (Fig. 9.2, Table 9.3).

Rhodium shows a similar distribution to Pd, however, almost 100 % of the whole-rock Rh is incorporated in pentlandite from the drill core SD124 samples occurring in the range of the upper chromitite stringer. Pentlandite of the remaining parts of the drill core, namely the pegmatoidal feldspathic pyroxenite, contains up to 40 % of the whole-rock Rh.

Chalcopyrite contains up to 6 % of the whole-rock Rh in samples from the chromitite stringer and the pegmatoidal feldspathic pyroxenite.

Pyrrhotite may contain up to 4 % of the whole-rock Rh in samples from the feldspathic pyroxenite above the upper chromitite stringer, but is devoid of Rh in the remaining samples of the drill core.

As revealed by Figure 9.2, in the lowermost part of the pegmatoidal feldspathic pyroxenite, around 5 % of the whole-rock Pt is incorporated into BMS, and 4 % is found in pentlandite. Thus most of the Pt is hosted by discrete PGM.

In the range of the upper chromitite stringer, up to 100 % of the whole-rock Pd and Rh is contained in BMS. In the remaining parts of the drill core, between 30-50 % of the whole-rock Pd and Rh can be contained in BMS, mostly in pentlandite. The BMS are devoid of Pt in almost all investigated samples (Table 9.3).

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Table 9.3: Calculated proportions in percent of each element in the BMS of drill core SD124, using the mass balance model of Godel et al. (2007). 666.25 m Pd Rh Pt 667.52 m Pd Rh Pt Pn 70.25 10.63 0.57 Pn 50.04 22.73 0.00 Po 1.73 4.61 1.18 Po 0.37 0.39 0.01 Chp 2.23 4.34 0.02 Chp 0.05 4.54 0.01 all sulfides 74.21 19.58 1.77 all sulfides 51.46 27.66 0.02 666.50 m 667.78 m

Pn 95.34 8.71 0.06 Pn 50.66 28.04 0.00 Po 0.02 0.04 0.02 Po 0.05 0.57 0.01 Chp 0.25 0.27 0.01 Chp 0.04 0.22 0.01 all sulfides 95.61 9.02 0.09 all sulfides 50-75 28.83 0.02 666.74 m 668.04 m

Pn 96.73 99.00 0.26 Pn 70.96 0.76 2.94 Po 0.00 0.59 0.96 Po 0.11 0.00 0.70 Chp 0.38 0.49 0.00 Chp 0.94 6.76 0.02 all sulfides 97.11 100.08 1.22 all sulfides 72.01 7.52 3.66 667.00 m 668.30 m

Pn 72.72 99.00 0.58 Pn 13.36 43.83 4.05 Po 0.00 0.83 0.33 Po 0.03 0.51 0.14 Chp 0.12 0.63 0.15 Chp 0.05 1.97 0.50 all sulfides 72.84 100.46 1.06 all sulfides 13.44 46.31 4.69 667.26 m

Pn 52.26 99.00 0.15

Po 0.00 1.16 0.03

Chp 0.68 3.55 0.96 all sulfides 52.94 103.71 1.14

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Fig. 9.2: Pd-, Rh- and Pt concentration in pentlandite and whole-rock and calculated proportions in percent of Pd, Rh and Pt between pentlandite and PGM throughout drill core SD124 (western Bushveld).

In the samples of drill core SD134, 40 % of the whole-rock Pd is incorporated in pentlandites from samples of the lower chromitite stringer and 60 % in pentlandites of the upper chromitite stringer. In samples above the upper chromitite stringer, the percentage of whole-rock Pd in pentlandite decreases upwards and more Pd is incorporated in discrete minerals.

In the samples which stem from the range of the lower chromitite stringer, chalcopyrite and pyrrhotite contain up to 16 and 6 % of the whole-rock Pd, respectively, while they are devoid of Pd in the remaining samples (Fig. 9.3).

Rhodium has a similar distribution to Pd. In the range of the chromitite stringers and the pegmatoidal feldspathic pyroxenite, 60 % to 80 % of the whole-rock Rh is contained in pentlandite. A sudden drop occurs in samples above the chromitite stringer, where only 1 % of the whole-rock Rh is contained in pentlandite. Chalcopyrite and pyrrhotite can contain up

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to 30 and 1 % in the vicinity of the lower chromitite stringer samples, while they contain hardly any Rh in the remaining samples of the drill core (Fig. 9.3).

The BMS in drill core SD134 are devoid of Pt in all investigated parts samples (Table 9.4).

Table 9.4: Calculated proportions in percent of each element in the BMS of drill core SD134 using the mass balance model of Godel et al. (2006 ).

611.59 m Pd Rh Pt Pn 0.79 0.00 0.01 Po 0.00 0.00 0.00 Chp 0.11 0.00 0.00 all sulfides 0.90 0.00 0.01 611.86 m Pn 2.24 0.40 0.16 Po 0.09 0.27 0.55 Chp 0.04 0.11 0.00 all sulfides 2.37 0.78 0.71 612.13 m Pn 5.95 0.34 0.02 Po 0.04 0.13 0.03 Chp 0.02 0.06 0.00 all sulfides 6.01 0.53 0.05 612.40 m Pn 13.29 0.43 0.02 Po 0.05 0.19 0.04 Chp 0.02 0.06 0.00 all sulfides 13.36 0.68 0.06 612.66 m Pn 59.49 60.46 0.05 Po 0.02 0.15 0.06 Chp 0.01 0.06 0.00 Fig. 9.3: Pd-, Rh- and Pt concentrations in pentlandite and 0.11 all sulfides 59.52 60.67 whole-rock and calculated proportions in percent of Pd, Rh and 613.16 m Pt between pentlandite and PGM throughout drill core SD134 (western Bushveld). Pn 48.76 84.23 0.13

Po 6.30 1.19 0.13 Chp 16.27 29.95 0.00 all sulfides 71.33 115.37 0.26

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A positive correlation exists between the Pd content in pentlandite and whole-rock Pd in the samples of the western Bushveld, which is absent in the samples from the eastern Bushveld (Fig. 9.4). This is in accordance with the results of the mass balance calculation and is also displayed by Figures 9.1-9.3, which show that the Pd concentration peak in pentlandite and that of whole-rock Pd are consistent in the western Bushveld but are displaced in the eastern Bushveld.

Fig. 9.4: Correlation diagram between whole-rock Pd content vs. Pd content in pentlandite. The western Bushveld samples reveal a positive correlation which is absent in the eastern Bushveld samples.

The mass balance calculations are in fair agreement with the analyzed data and former studies of the PGE distribution in BMS in other igneous layered intrusions (e.g., Godel et al. 2007 and Dare et al. 2010). For instance, Godel et al. (2007) calculated that 50-100 % of Pd and Rh are predominantly hosted by BMS, mostly pentlandite, in samples from the western Bushveld. Furthermore, they reported that 35-72 % of Os, Ir and Ru are hosted by BMS located in the silicate rocks, but only 0.2-12 % of Os, Ir and Ru are hosted in BMS in the chromitite layers. They further suggested that the Pt is hosted by PGM rather than by BMS.

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9.2. UG2 9.2.1. Western Bushveld (SD124)

In the following chapter, a mass balance was calculated of some samples of the UG2 to shed some light on the PGE distribution between pentlandite and PGM. It was only possible to calculate a mass balance for four samples of drill core SD124, since only a few PGE analyses of BMS were undertaken in this drill core. Furthermore, the calculation was possible only for the PPGE, since the remaining PGE are below the detection limit in the bulk analyses. The mass balance calculation reveals that only 3.6-5 % of the whole-rock Pd is incorporated in pentlandite, while the remaining whole-rock Pd is incorporated in PGM, especially braggite, since almost no whole-rock PGE are in pyrrhotite or chalcopyrite of drill core SD124. In the most upper part of drill core SD124 samples, 6 % of the whole-rock Rh is incorporated in pentlandite at ca. 709.15 m. At 709.30 m, around 15 % of the whole-rock Rh is incorporated in pentlandite, while pentlandites in the lowermost samples close to the UG2 incorporate no whole-rock Rh (Fig. 9.5 and Table 9.5). Almost all whole-rock Pt is incorporated in PGM, mostly braggite and isoferroplatinum, since the mass balance calculation revealed no Pt in pentlandite or the other base-metal sulfides (Fig. 9.5 and Table 9.5) - (see mass balance of drill core DT46).

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Table 9.5: Calculated proportions in percent of Pd, Rh and Pt in pentlandite of drill core SD124 using the mass balance model of Godel et al. 2006. Pd in pn Rh in pn Pt in pn 709.14 m 5.04 6.12 0.07 709.30 m 3.60 14.58 0.11 709.46 m 4.50 0.02 0.09 709.66 m 0.17 0.014 0.01

Fig. 9.5: Pd-, Rh- and Pt concentrations in pentlandite and whole-rock and calculated proportions in percent of Pd, Rh and Pt between pentlandite and PGM throughout drill core SD134 (western Bushveld).

9.2.2. Eastern Bushveld (DT46)

In drill core DT46, a mass balance was calculated for four samples, including all six PGE throughout the UG2 due to additional whole-rock analyses of Os, Ir and Ru. The calculated proportions of the BMS are listed in Table 9.1. The mass balance calculation revealed that between 35-60 % of the whole-rock Pd is incorporated in pentlandite (Fig. 9.6), whereas less than 0.8 % of the whole-rock Pd is incorporated in pyrrhotite or chalcopyrite (Table 9.6). In the upper part of the UG2 up to 50 % of the whole-rock Rh is included in pentlandite, whereas only 20-30 % of the whole-rock Rh is incorporated in pentlandite from the lower part of the UG2 (Fig. 9.). Less than 0.5 % of the whole-rock Rh is incorporated in pyrrhotite or chalcopyrite, however, in the uppermost part of the UG2, up to 10 % of the whole-rock Rh is incorporated in chalcopyrite (Table 9.6).

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Osmium, Ir and Ru show a similar distribution trend between pentlandite and PGM. In the upper part of the UG2, almost 50 % of the whole-rock Os, 20 % of the whole-rock Ir and 40 % of the whole-rock Ru is incorporated in pentlandite. In these samples pyrrhotite contains no whole-rock PGE and chalcopyrite only up to 10 % of the whole-rock Ru. In the lower part of the UG2 only 10 % of the Ir and Os, respectively, and no Ru is incorporated in pentlandite (Fig. 9.6), this leads us to the assumption that almost all IPGE in the lower part of drill core must be incorporated in PGM, since no PGE were detected in pyrrhotite or chalcopyrite (Table 9.6). No Pt was detected in one of the base-metal sulfides thus all Pt is included in PGM (Fig. 9.6 and Table 9.6), such as laurite and isoferroplatinum.

Fig. 9.6: PPGE and IPGE concentrations in pentlandite and whole-rock and calculated proportions in percent of PPGE and IPGE between pentlandite and PGM throughout drill core DT46 (eastern Bushveld).

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Table 9.6: Calculated proportions in percent of PGE in the BMS of drill core DT46, using the mass balance model of Godel et al. (2007). 945.50 m Pd Rh Pt Os Ir Ru Pn 41.49 29.73 0.03 8.66 7.96 2.89 Po / / / / / / Chp / / / / / / all sulfides 41.49 29.73 0.03 8.66 7.96 2.89 945.70 m Pn 29.38 49.82 0.18 53.27 20.03 42.09 Po 0.00 0.10 0.03 0.54 0.36 0.20 Chp 0.00 0.53 0.00 0.00 0.00 0.04 all sulfides 29.38 50.45 0.21 53.81 20.39 42.33 946.09 m Pn 57.16 33.67 0.63 4.32 10.66 1.89 Po / / / / / / Chp 0.01 9.81 0.08 9.27 0.05 8.96 all sulfides 57.17 43.48 0.71 13.59 10.71 10.85 946.24 m Pn 35.08 28.07 0.90 0.22 1.96 0.65 Po 0.00 0.06 0.02 0.81 0.33 0.88 Chp 0.00 0.07 0.00 0.03 0.02 0.04 all sulfides 35.08 28.20 0.92 1.06 2.31 1.57 946.6 m Pn / / / / / / Po 0.79 0.11 7.09 0.04 0.00 0.37 Chp / / / / / / all sulfides 0.79 0.11 7.09 0.04 0.00 0.37

A positive correlation exists between the Pd content in pentlandite and the whole-rock Pd in the samples of the eastern Bushveld but is absent in the samples from the western Bushveld (Fig. 9.7). This is in accordance with the results of the mass balance calculation and is also displayed by Figures 9.5 and 9.6, which show that the Pd concentration peak in pentlandite and that of the whole-rock Pd are consistent in the eastern Bushveld, but are heterogeneous in the western Bushveld.

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5 4 3 Western Bushveld

2 Eastern Bushveld Pd in wr wr [ppm] inPd 1 0 0 200 400 600 800 1000 Pd in pn [ppm]

Fig. 9.7: Correlation diagram between the whole-rock Pd concentration and Pd content in pentlandite. The eastern Bushveld reveals a positive correlation, which is absent in the western Bushveld.

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PhD thesis Inga Osbahr 10. Summary and Discussion

10. Summary and Discussion The bulk-rock chemical distribution of Pt and Pd in the samples from drill cores SD124 and US200 shows a top loaded mineralization, with the highest concentration of Pt and Pd in the range of the upper chromitite stringer, whereas Pt and Pd in the samples from the drill cores SD134 and US186 reveal a more homogenous distribution between the lower and upper chromitite stringer. The stratigraphic variation of whole-rock Pt and Pd (Figs. 6.1-6.4) in the Merensky Reef samples throughout the drill cores reveals maximum contents of Pt (10 ppm in the western Bushveld and 5 ppm in the eastern) and Pd (4-6 ppm in the western Bushveld and below 2 ppm in the eastern Bushveld) in the chromitite and sulfide stringers and their immediate vicinity. In these areas, the Pt content is always higher than that of Pd. However, apart from drill core SD134, Pt and Pd reach the same concentrations in samples which are located in the unit between the lower and upper chromitite stringers. In the drill core US186 samples, the Pd concentration is even higher than the Pt concentration in the area between the stringers. The LA-ICP-MS analyses of PGE in BMS reveal the highest PGE contents in pentlandite, while lesser amounts occur in pyrrhotite and hardly any in chalcopyrite. However, chalcopyrite contains most of the Ag. Pentlandite is the principal host of Pd and Rh, which are homogenously distributed in the crystal lattice, with up to 700 ppm Pd in the samples of the eastern Bushveld and up to 1750 ppm Pd in the western Bushveld. Rhodium reaches up to 130 ppm in the eastern Bushveld and up to 1000 ppm in samples from the western Bushveld, while Pd in pyrrhotite mostly is below the detection limit, and Rh reaches a maximum of 25 ppm. Pentlandite contains, to a lesser extent, Ru (60 ppm), Os (17 ppm) and Ir (14 ppm). Pyrrhotite may contain significant amounts of up to 33 ppm of Ru, Os and Ir, respectively (Figs. 6.10, 6.11, 6.13 and 6.15).

Comparing the concentration peaks of Pd and Pt (i) in whole rock and (ii) in pentlandite reveals distinct offset patterns, mainly for Pd, in the Merensky Reef. The concentration peak of Pd in pentlandite shows a displacement relative to the maximum concentration of Pd in whole rock in drill cores SD124, US200 and US186. In drill core US200, this phenomenon is also observed for Rh, Ru and Pt. A general sequence of “offset patterns” of PGE and BMS maxima, in the order from bottom to top, Pd in pentlandite → Pd in whole rock → (Cu, Ni, S) can be observed in cores SD124 and US200. The sequences of cores SD134 and US186 are more complex in detail, but partially show similar trends.

The bulk chemical PGE concentration in the UG2 differs in the Pd and Pt concentration in the investigated samples of the western and eastern Bushveld. In the western Bushveld samples, the Pt concentration is always higher than the Pd concentration. In the eastern Bushveld samples, the Pd concentration is mostly higher than the Pt concentration (Figs. 7.1 150

PhD thesis Inga Osbahr 10. Summary and Discussion and 7.2). But generally, the PGE distribution in BMS of all UG2 samples is similar to that in the Merensky Reef samples (Figs. 7.7 and 7.8).

In the Merensky Reef and most of the UG2 samples, the covariance of whole rock Pd, Pt and Rh concentrations is positive and significant between all PPGE. In pentlandite, a distinct positive correlation exists only between Pd and Rh, and within the IPGE group, a finding which may be related to the coherent behavior of these elements in a sulfide liquid prior to mss or iss formation, and a different behavior of the individual PGE during partitioning into the BMS.

Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100% of the Pd and Rh and smaller amounts (10-40%) of the Os, Ir and Ru. Chalcopyrite and pyrrhotite usually contain less than 10% of the whole rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100%), are present in the form of discrete platinum-group minerals (PGM) such as cooperite/braggite, sperrylite, moncheite and isoferroplatinum.

In the UG2 of the western Bushveld pentlandite hosts 5 % of the whole-rock Pd and 15 % of the whole-rock Rh while in the eastern Bushveld it is 40 % of the Pd and 30 % of the Rh. Less than 20 % of whole-rock Os, Ir and Ru is incorporated in the pentlandite of the UG2. Similar to the Merensky Reef, in the UG2 the remaining PGE and almost 100 % of the Pt are present in the form of discrete platinum-group minerals.

The platinum group minerals identified in drill core SD124 are dominated by laurite, braggite and cooperite, while drill core SD134 contains a more variable PGM occurrence, with more arsenides and tellurides such as sperrylite, moncheite and hollingworthite. Moncheites, cooperite and isoferroplatinum occur in the lower chromite stringer to a lesser extent, while sperrylite, laurite, isoferroplatinum and hollingworthite occur in the upper chromitite stringer. In the eastern Bushveld (US200), mainly braggite, cooperite and native gold were found, and minor moncheite, sperrylite, isoferroplatinum, laurite and rustenburgite were detected. The most abundant PGM were found in the chromitite stringer, followed by the sulfide stringer. In the UG2 of the eastern Bushveld, most of the UG2 grains were found in the middle part of the UG2 main seam and are dominated by laurite, braggite and cooperite followed by sperrylite, moncheite, Pt- and Pd-alloys.

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PhD thesis Inga Osbahr 10. Summary and Discussion

10.1. Whole-rock “Offset”-feature in the Merensky Reef Offset patterns have been described for some layered intrusions, including the Bushveld (Maier et al. 2010), Great Dyke (Wilson et al. 1989; Oberthür 2002 and 2011), Munni Munni (Barnes 1993b) and Stella intrusions (Maier et al. 2003). These authors reported offset patterns between the PGE distribution and the whole-rock base metals (Cu, Ni and S), and between whole-rock Pd with Pt. Mungall (2002), for example, interpreted such offsets to be a reflection of progressively decreasing diffusion rates of the chalcophile elements into the segregating sulfide liquid. However, Wilson et al. (1989) and Barnes (1993b) suggested models of fractional segregation of sulfide liquid being responsible for PGE offsets in the Great Dyke and Munni Munni intrusion. Another possible explanation for the offsets, e.g. in the Great Dyke, is that Pt and Pd were decoupled from the other base-metals by a Rayleigh fractionation process. In this case, it was suggested that the most strongly chalcophile elements preferentially concentrate in the first sulfide liquid, and became rapidly depleted in the residual liquid (Wilson et al. 1989; Oberthür 2011).

Indeed, the present study revealed a general sequence of distinct “offset patterns” of PGE and BMS maxima in the Merensky Reef, in the order from bottom to top, Pd in pentlandite → Pd, Pt in whole rock → (Cu, Ni, S) in whole-rock in cores US200 and SD124. The sequences of cores SD134 and US186 are more complex in detail, but partially show similar trends.

For example, in drill core US200, the peaks of Ni and Cu are ca. 25 cm above those of Pt, Pd and Rh (Fig. 6.3). Furthermore, the concentration peak of Pd in pentlandite is decoupled by 25 cm from the concentration peaks of the PPGE, Ni and Cu in whole rock. Notably, the PGE concentration peaks in pentlandite are neither consistent with the Ni and Cu nor with the Pd and Pt whole rock concentration peaks. The offset patterns observed in drill core US200 closely resemble those observed in the Main Sulfide Zone of the Great Dyke (e.g. Oberthür 2011). Rayleigh fractionation, therefore, is a possible explanation for the offset-feature in core US200. A similar offset pattern is observed for drill core SD124.

In drill core US186, the whole-rock base-metal peak appears at a lower stratigraphic level than the Pt and Pd peaks which follow 25 cm above. Thus, Rayleigh fractionation is not regarded as a likely explanation for the PGE distribution (i.e. offset pattern) in drill core US186, since the most strongly chalcophile elements (Pt and Pd) were not concentrated in the first sulfide liquid. Another explanation may be a variation on the R-factor (e.g. Naldrett 2004). The R-factor parameter records the mass ratio of silicate to sulfide melt. Therefore a low R-factor (R=100) describes a low silicate to high sulfide liquid ratio, while a high R-factor (R=100000) represents a high silicate to sulfide melt ratio. A low R-factor accordingly leads to a PGE-depletion in BMS, since PGE are in competition to be incorporated in various BMS, while a high R-factor leads to a PGE-enrichment in relatively sparse BMS. Additionally, the 152

PhD thesis Inga Osbahr 10. Summary and Discussion concentration of compatible trace metals in sulfide melts vary as a function of the partition coefficient (KD) and R (e.g. Barnes and Francis 1995). Thus, a change from an initial low R- factor situation (i.e. relatively low whole-rock Pt and Pd due to the melting of S-rich country rocks) to a high R-factor situation (which would explain the constant Ni and Cu concentrations and a significant Pt and Pd concentration increase towards the chromitite stringer) may explain the PGE and the other base-metal distribution in the samples from drill core US186. In the drill core US186 samples, the Pd in pentlandite concentration occurs 1 m below the Pd and Pt whole-rock concentration peak, but is, however, consistent with the whole-rock Ni and Cu concentration peaks. The absence of PGM in this part of the drill core probably is responsible for the Pd enrichment in pentlandite, since PGM require very low S concentration in the liquid to crystallize from a sulfide liquid (Barnes and Maier 2002).

Generally, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids, found at the base of the Merensky Reef sequence. A possible explanation for the offset patterns in drill core US200 and SD124 may be Rayleigh fractionation or in the case of drill core US186 different silicate magma/sulfide liquid mass ratios, e.g. R-factor. The resolution in the samples from drill core SD134 (western Bushveld) may have been hampered by the relatively thin mineralized zones.

10.2. Pd-enrichment in pentlandite The present study identified high concentrations of Pd in pentlandite and low concentrations in chalcopyrite, which is different to experimental and theoretical results which show that Pd has a stronger affinity to partition into the Cu-rich liquid (e.g. Li et al. 1996; Barnes et al. 2001; Naldrett 2004; Mungall et al. 2005) but are in fair agreement with previous studies of PGE in BMS (Godel et al. 2007; Holwell and McDonald 2007). Consequently, possible mechanisms concerning the incorporation of Pd in pentlandite will be discussed briefly.

At high temperatures (<1000 °C) an mss and a Cu-rich residual liquid were formed subsequently to the separation of a Fe-Ni-Cu sulfide liquid from a silicate magma. The PGE partition into the (Fe, Ni, Cu)-sulfide liquid rather than into the basaltic melt with a partition coefficient in the range of 102-105 (Bezmen et al. 1993; Fleet et al. 1991, 1996; Stone et al. 1990). At 950-840 °C an iss phase is thought to crystallize from the Cu-rich liquid (e.g. Dare et al. 2010). At temperatures below 650 °C, mss exsolves into pyrrhotite, and below 610 °C pentlandite becomes stable, the latter was formed through a reaction of a nearly pure mss

FeS composition and (Ni,Fe)3±xS (Craig and Scott 1976). At 557 °C, chalcopyrite becomes stable, followed by cubanite at 210 °C (Craig and Scott 1976). During the crystallization of

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PhD thesis Inga Osbahr 10. Summary and Discussion mss and iss, the PGE behave differently. Platinum, Pd and Cu behave incompatibly during mss crystallization and thus mainly partition into the sulfide liquid, with partition coefficients of 0.05-0.16 for Pt, 0.08-0.27 for Pd and 0.17-0.27 for Cu. Iridium, Os, Rh and Ru partition into mss, with partition coefficient for Ir and Rh ranging between 1.06-13 and 0.37 to 8.23, respectively (Li et al. 1996). The partition coefficient for Ru ranges from 1.8 to 14.5 (Mungall et al. 2005). Nonetheless, as already pointed out above, in the investigated samples from the Bushveld Complex as well as from other layered intrusions (e.g. Dare et al. 2010), Pd is enriched in pentlandite instead of chalcopyrite. Dare et al. 2010 conducted studies on samples from the Sudbury Complex, and suggested that the Pd enrichment in pentlandite is caused by diffusion of Pd from the nearby Cu-rich portion (iss and/or Pd-bearing PGM) into the pentlandite, and that a small quantity of Pd is believed to stem from the Pd that had originally partitioned into the mss, however Dare et al. (2010) detected maximum concentrations of 3 ppm Pd in pentlandite. They further described three types of pentlandite, with the highest Pd contents in the coarse-grained pentlandite and lower contents in pentlandite veinlets, while Pd in exsolution flames of pentlandite was below detection.

However, the Bushveld samples investigated in the present study mainly contain coarse- grained pentlandite intimately intergrown with chalcopyrite, pyrrhotite and pyrite and rare pentlandite exsolution flames in pyrrhotite. Our study revealed high Pd concentrations (up to 650 ppm) in the coarse-grained pentlandite that is in immediate contact with chalcopyrite and, in addition, significant Pd concentrations in the exsolution flames of pentlandite in pyrrhotite (up to 75 ppm; Figs. 10.1 a and 10.1 b). It is interesting to note that coarse-grained pentlandite which is not in immediate contact with chalcopyrite also contains significant amounts of Pd independent of the stratigraphic level (107 ppm in the upper chromitite stringer, or 30 ppm in the feldspathic pyroxenite below the upper chromitite stringer; Fig. 10.1 c). This may be explained by diffusion of Pd from the Cu-rich portion (Dare et al. 2010), but this process probably could be used only for small amounts of Pd in pentlandite as in the Sudbury samples. However, in our opinion, a more likely mechanism for the Bushveld samples is the partitioning of large amounts of Pd into mss during an earlier, still magmatic, stage.

The preference of Pd for pentlandite rather than for pyrrhotite is explained by its preferential Ni substitution. Essential for such a substitution is that the substituting metal can form the same hybridized orbital as the metal which it aims to substitute. For instance, Pd and Pt adopt a dsp2 configuration suitable for square planar sites (IV fold coordination) - (Evans 1966). Iron, Os, Ir, Ru and Rh prefer a d2sp3 configuration that is suitable for octahedral sites (VI fold coordination). Nickel adopts either a d2sp3 or dsp2 hybridization suitable for either an octahedral or tetrahedral coordination (VI fold or IV fold coordination) - (Evans 1966). Thus,

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PhD thesis Inga Osbahr 10. Summary and Discussion the dsp2 configuration of Ni is in accordance with that of Pd and Pt, rather than the d2sp3 configuration of Fe (Barnes et al. 2001).

The reason for the very low Pt-content in sulfides is its preference to partition into alloys. Li et al. (1996) and Barnes et al. (2001) showed that Pt and Pd have similar partition coefficients concerning mss and sulfide liquid. Under all experimental conditions they partition into the sulfide liquid, whereby the Pt partition coefficient between sulfide liquid and alloys phases is higher (10 to 130) than that of Pd (0.04-0.11). The latter is incompatible with regard to the alloy phases, at least under low-S conditions (Barnes et al. 2001).

Fig. 10.1: BSE-images of different types of pentlandite of the eastern Bushveld a) Exsolution flames of pentlandite in pyrrhotite containing 75 ppm Pd in pentlandite, Pd concentration in pyrrhotite is 0.38 ppm (US186- 11) b) Coarse-grained aggregation of pentlandite (600 ppm Pd), chalcopyrite (2.38 ppm) and pyrrhotite (0.13 ppm) (US186-11) c) Coarse-grained aggregation of pentlandite (108 ppm Pd) (cream-white), pyrite (<0.004 ppm Pd) (dark grey) and pyrrhotite (light grey) (US186-5). Abbreviations after Whitney and Evans (2010).

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PhD thesis Inga Osbahr 11. Conclusions

11. Conclusions 1. In the Merensky Reef, the highest whole-rock PGE contents are mostly found within and around the upper and lower chromitite stringer and locally in sulfide stringers. Pt and Pd are the dominant PGE in the Merensky Reef with higher Pt than Pd concentrations.

2. In the UG2, Pd and Pt are the dominant PGE as well. However, in the eastern Bushveld the Pd concentration tend to be higher than the Pt concentration what is contradictory to the western Bushveld.

3. In all investigated samples, Merensky Reef and UG2, positive bulk-rock chemical covariances of PGE concentrations occur within and between the IPGE and PPGE groups. The distinct positive covariance of the whole-rock PGE with Cu and Ni indicates a strong relation of the PGE to sulfides. In pentlandite, the PGE generally correlate positive within their subgroups. However, no positive correlation exists between Pt and any other PGE.

4. In the Merensky Reef and the UG2, pentlandite generally is the principal host of Pd and Rh and to a lesser extent of Ru, Os and Ir. Pyrrhotite can contain significant amounts of Ru, Os, Ir and Pt, but hardly any Pd or Rh. Chalcopyrite carries very low PGE contents but hosts most of the Ag.

5. In the Merensky Reef sequence, the concentrations of Pd and Rh in pentlandite are unevenly distributed. In the samples from the western Bushveld, the highest Pd and Rh contents in pentlandite are present in samples from the upper chromitite stringer. In contrast, in the samples from the eastern Bushveld, the highest Pd and Rh concentrations in pentlandite occur below the lower chromitite, in the pegmatoidal feldspathic pyroxenite unit.

6. Pt-dominant mineral species are the principal PGM in the Merensky Reef and the UG2; most common in the Merensky Reef are cooperite, braggite and moncheite, followed by rarer Pt-Fe alloy and rustenburgite. In the UG2 braggite, isoferroplatinum and laurite are the most common PGM.

7. Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts most of the Pd and Rh (up to 100 %) and smaller amounts (10- 40 %) of the Os, Ir and Ru.

In the UG2 generally only 5-40 % of the Pd, 15-30 % of the Rh and less than 20 % of Os, Ir and Ru are incorporated in pentlandite. Chalcopyrite and pyrrhotite, of both localities, usually contain less than 10 % of the whole rock PGE. The remaining PGE

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concentrations, and especially most of the Pt (up to 100 %), are present in the form of discrete PGM.

8. Pd concentrations in pentlandite are thought to be the result of large quantities of Pd partitioning into mss at an early stage or due to diffusion from the iss at a later stage.

9. In the Merensky Reef, the distribution patterns of whole rock Cu, Ni and S versus whole rock Pd and Pt show commonly distinct offsets. The general sequence of “offset patterns” of PGE and BMS maxima, in the order from bottom to top, is Pd in pentlandite → Pd in whole-rock → (Cu, Ni, S). The relationship is not that straightforward in general; some of the reef sequences studied, only partially, show similar trends. Especially the resolution in the samples from the western Bushveld may have been hampered by the relatively thin mineralized zones.

10. In general, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids at the base of the Merensky Reef sequence. A possible explanation for the offset patterns may be Rayleigh fractionation or different silicate magma/sulfide liquid mass ratios, e.g. R-factor.

11. Offset patterns are observed in the eastern Bushveld (e.g. Pd concentration in pentlandite is displaced when compared to the whole-rock Pd concentration). Pd peak concentration occurs in the pegmatoidal feldspathic pyroxenite which is located below the lower chromitite stringer (whole-rock Pd concentration peak). Post- magmatic processes such as selective diffusion may be a responsible mechanism.

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