Bushveld Complex

Well preserved supracrustal basin-fills including the oldest large depository (Witwatersrand) known makes the Kaapvaal craton an ideal example for studying controls on first-and second-order stratigraphic cyclicity. The duration of the three basins studied (Witwatersrand, Ventersdorp and Transvaal), from c. 3.0 to c. 2.0 Ga is almost twice that of the entire Phanerozoic period, from which the paradigms of classical sequence stratigraphy have been derived. Amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through Earth history. This does not appear to be borne out by the Precambrian record studied here; instead, a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first-and second-orders is preferred. First-order cycles within the c. 3.0-2.0 Ga period on Kaapvaal are synonymous with the entire evolution of a supracrustal basin-fill defined by a specific basin-type; second-order cycles define basin-fill packages of unique character, which reflect large scale shifts in the accommodation-sedimentation balance. Early to Middle Archaean evolution of the Kaapvaal craton encompassed amalgamation of a mosaic of subdomains, to produce an early south-southeastern nucleus by c. 3.1 Ga. Accretion of composite terranes from both north and west followed from c. 3.0-2.7 Ga; this was contemporaneous with evolution of the c. 3.0-2.8 Ga Witwatersrand basin. The high-grade Limpopo terrane was amalgamated with the Kaapvaal craton between 2.95 and 2.69 Ga, although a younger age for this orogensis is also possible. The Witwatersrand first-order depositional cycle was controlled largely by subduction-related tectonic loading and had a duration of c. 190 My. The c. 5 My first-order cycle of the c. 2.7 Ga Ventersdorp basin was essentially controlled by thermal uplift-induced extensional subsidence, and that of the c. 650 My Transvaal basin by extensional and subsequent thermal subsidence. It is noteworthy in the three basins examined that first-order cycles controlled by plate tectonics (Witwatersrand and Transvaal basins) were about two orders of magnitude longer relative to the Ventersdorp bplume tectonicQ cycle; second-order cycles also varied greatly, from c. 100 My to c. 1 My. Within the Witwatersrand and Transvaal basins, subsidence allowed extensive seaways to develop, within which classical sequence stratigraphic systems tracts can be recognized, reflecting different stages of shoreline shifts. Within the short-lived Ventersdorp 0037-0738/$ -see front matter D (P.G. Eriksson).

Four poikilitic pyroxenites of the Bastard Unit, all being relatively low in osmium concentrations, define a Re-Os isochron age of 2043 š 11 Ma (with initial 187 Os= 188 Os D 0.151), which is in accordance with the normally cited crystallization age of the Bushveld Complex. The isochron fit demonstrates an excellent radiogenic Os isotopic homogeneity of this portion of the complex at the time of crystallization and excludes any later disturbance of the Re-Os isotope system. Initial 187 Os= 188 Os ratios of platinum-group element (PGE) enriched sulfides and whole rocks of the directly underlying Merensky Reef are highly radiogenic and variable (with values between 0.168 and 0.181). Separates of chromite grains and interstitial phases of Critical Zone chromitite layers show initial 187 Os= 188 Os signatures ranging from the near-chondritic mantle values of lower group chromitites (i.e. ¾0.120) to 0.137 for middle group chromitites and to 0.150 for the upper group 2 chromitite layer. Osmium concentrations of interstitial phases exceed those of the associated chromite in all layers by up to 30 fold, showing the presence of highly PGE-enriched interstitial sulfides. Equal or very similar initial 187 Os= 188 Os signatures of the interstitial minerals and the cumulus chromite grains together with their coupled increasing and decreasing trends in this ratio clearly demonstrate that interstitial sulfides and associated cumulus chromite of the different chromitite layers scavenged their osmium from the same or similar sources. Therefore, a PGE enrichment of the interstitial sulfides by a primary magmatic process, such as precipitation of immiscible sulfide droplets during magma mixing appears more likely than a scenario involving fluid or melt transport of PGE into these layers. Such a primary magmatic PGE enrichment process is also preferred for the formation of the Merensky Reef.

The Woodlands Formation (uppermost Pretoria Group) of eastern Botswana overlies thick quartzites of the Sengoma Formation (Magaliesberg Formation) and comprises a lower unit of interbedded mudrocks and fine-grained recrystallised quartzitic sandstones,

The oxidation state of vanadium in natural and synthetic Fe-Ti oxides is determined using high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). Eleven natural magnetite-bearing samples from a borehole of the Main Magnetite Layer of the Bushveld Complex (South Africa), five synthetic Fe oxide samples, and three natural hematite-bearing samples from Dharwar supergroup (India) are investigated. V K edge spectra were recorded on the ID26 beamline at the European Synchrotron Radiation Facility (Grenoble, France), and the pre-edge features were used to determine the local environment and oxidation state of vanadium. In the case of the magnetite samples (natural and synthetic), we show that vanadium is incorporated in the octahedral site of the spinel structure under two oxidation states: ?III and ?IV. The variations of the pre-edge area are interpreted as various proportions in V 3? and V 4? (between 9.5 and 16.3% of V 4? ), V 3? being the main oxidation state. In particular, the variations of the V 4? /V 3? ratio along the profile of the Main Magnetite Layer seem to follow the crystallization sequence of the layer. In the case of the hematite samples from India, the pre-edge features indicate that vanadium is substituted to Fe and mainly incorporated as V 4? (between 40 and 72% of V 4? ). We also demonstrate the potentiality of HERFD-XAS for mineralogical studies, since it can filter out the unwanted fluorescence and give better resolved spectra than conventional XAS.

Metapelitic rocks in the aureole beneath the Bushveld Complex preserve evidence for both high-and low-a H 2 O anatexis. The aureole is characterized by an inverted thermal structure in which suprasolidus rocks potentially interacted with an H 2 Orich volatile phase derived from underlying, dehydrating rocks. At lower grade (T 5 700 C) the rocks contain fibrolite mats and seams that record local redistribution of volatiles. Incongruent reactions consuming biotite produced small quantities (51 mol %) of liquid and peritectic cordierite that remained trapped within the mesosome. Larger volumes of melt (3±4%), preserved as coarse-grained discordant leucosomes, were produced by congruent melting following a structurally focused influx of H 2 O. Subhorizontal volatile-phase flow was concentrated within thin ($10 mm) metapsammite horizons that are preserved as stromatic quartz±sillimanite veins. Upward migration occurred along steep fibrolite seams that are subparallel to a variably inclined foliation. Discordant leucosomes are concentrated within antiformal fold closures of quartz±sillimanite veins and along the axial planar schistosity. Closer to the contact (T 4 725 C), volatile-phase-absent, biotite-consuming melting and melt extraction produced coarse-grained garnet±cordierite granofels. At the contact, leucodiatexites devoid of peritectic phases suggest effective segregation of melt from an underlying source. Migmatitic metapelites and their lower-grade stratigraphic equivalents have similar bulk-rock oxygen isotope values, consistent with very limited volatile-phase infiltration and precluding the Bushveld Complex magmas as the source of the volatiles.

The Normal Reef Subfacies of the Swartklip facies of the Merensky Reef along the western limb of the Rustenburg Layered Suite, exposed on Northam Platinum mine, displays lateral and vertical variations in its inter-chromitite lithologies, content in platinum-group elements and grade, and the mode of sulfide occurrence. Reef greater than 300 cm in thickness consists of a footwall of mottled anorthosite, overlain by a granular orthocumulate chromitite layer (the lower chromitite of the reef), followed by a pegmatitic feldspathic dunite, a 5-to 10-cm-thick massive, plagioclase-poor dunite, a heterogeneous pegmatitic harzburgite, succeeded by a coarse-grained pegmatitic pyroxenite, collectively comprising a pre-Merensky cyclic unit. The upper pegmatitic pyroxenite is in turn overlain by the granular (upper) chromitite of the reef, occurring at the base of the Merensky Cyclic Unit, which is overlain by a homogeneous poikilitic orthopyroxenite. Disseminated and clustered pentlandite, pyrrhotite and chalcopyrite occur throughout the inter-chromitite pegmatite, but are not observed in the massive, plagioclase-poor dunite, which typically occurs approximately 100 cm above the lower chromitite. Thermal recrystallization and downward migration of a reconstitution front and mechanical erosion of the medium-grained heterogeneous pegmatitic harzburgite accompanied progressive thinning of the reef from an inter-chromitite reef greater than 300 cm in thickness. Breaching of the massive, relatively impermeable layer of massive dunite resulted in pronounced or accelerated thermomechanical erosion of the underlying pegmatitic dunite. Peaks in platinum-group-element content, situated just below the upper chromitite and just above the lower chromitite, merge with reef thinning, becoming indistinct in reef that is less than 160 cm thick. With further thinning, until the point where the two chromitites are juxtaposed or merged, the single peak in PGE content "migrates" into the poikilitic orthopyroxenite hanging-wall. A sulfide lag, consisting of up to 20% sulfide by volume, occurs in reef less than 160 cm in thickness. Its presence is especially evident near the point where the Regional Pothole subfacies becomes manifest, and the upper chromitite or base of the Merensky Cyclic Unit transgresses the lower chromitite. Spatial distributions and graphical correlations of sampling data, related to reef thickness, indicate that there is a strong positive correlation between ⌺PGE grams in the total reef sampled and the inter-reef pegmatite. A strong inverse relationship, relative to thinning, is observed between ⌺PGE reef grade and ⌺PGE grams. The ⌺PGE grams content of the upper or Merensky chromitite of the reef is remarkably consistent over the extent of Normal Reef of the Merensky cyclic unit exposed at Northam, indicating that it is a drape or post-erosional crystal cumulate over an eroded and recrystallized pre-Merensky cyclic unit. SOMMAIRE Le sous-faciès considéré "Banc normal" du faciès de Swartklip du banc de Merensky, le long de la séquence occidentale de la suite stratifiée de Rustenburg, telle qu'elle affleure à la mine Northam Platinum, fait preuve de variations latérales et verticales en types de roche intercalée entre le deux horizons de chromitite, en contenu et en teneurs en éléments du groupe du platine (EGP), et en aspects des sulfures présents. Où il est supérieur à 300 cm, le banc possède une base d'anorthosite mouchetée, que recouvre un niveau orthocumulatif de chromitite granulaire (la chromitite inférieure du banc) et, successivement, une dunite feldspathique pegmatitique, une dunite massive de 5 à 10 cm en épaisseur et à faible teneur en plagioclase, une harzburgite hétérogène pegmatitique, et ensuite une pyroxénite pegmatitique, le tout définissant une unité cyclique pre-Merensky. Vient ensuite la chromitite granulaire supérieure du banc, définissant la base de l'unité cyclique de Merensky, qui est recouverte d'une orthopyroxénite poecilitique homogène. Des grains de pentlandite, pyrrhotite et chalcopyrite sont disséminés ou regroupés partout §

The Kaapvaal Craton (South Africa) was the host of several major magmatic events around 2000 Ma, including the Bushveld Complex, the world's largest known layered mafic intrusion (∼0.5-1 × 10 6 km 3 ). The Bushveld Complex has been the subject of numerous palaeomagnetic studies, which yielded a large spread in palaeomagnetic pole positions for the different Zones, and interpreted to indicate that the Bushveld Complex was emplaced and cooled over a time span of ∼50 million years. New palaeomagnetic data collected from 100 sites (996 drill-cores) from all Zones of the Rustenburg Layered Suite of the Bushveld Complex, yield exceptional palaeomagnetic results with high unblocking (HB) components carried by magnetite. Comparable palaeomagnetic poles from all Zones (mean pole: latitude = 19.2 • N, longitude = 030.8 • E, A95 = 5.8 • ) [correction made after online publication 22 September 2009: the mean pole values have been corrected] eliminates the previously noted large spread in poles, and this observation concurs with precise age data that constrain the time period of emplacement of the Bushveld Complex to a few million years at around 2054 Ma. Bedding-corrected HB components from all zones produced better directional groupings, which together with at least seven reversals, strongly points to a primary magnetic signature. This imply that cooling of the Bushveld Complex below the blocking-temperature of magnetite (<580 • C) occurred in a nearhorizontal position, and based on the maximum observed reversal rates in the geological history (∼5 Ma −1 ), we estimate a minimum cooling-interval for the Bushveld Complex of 1.4 million years.

We evaluate the 190 Pt-186 Os decay system, measured by laser ablation MC-ICPMS, as a useful geochronometric tool for direct dating of platinum-group minerals (PGM) in large mafic/ultra-mafic intrusions by analysing mineralised rocks from the geochronologically well constrained Bushveld Complex, S. Africa. Mixed PGM (laurite, cooperite, laurite-platarsite, sperrylite and Pt-Fe alloy) from the Merensky Reef yield a Pt-Os isochron age of 1995± 50 Ma (2σ, n = 48, MSWD= 1.16, initial 186 Os/ 188 Os= 0.119819± 0.000006). While this is 59 Myr younger than the U-Pb zircon age for the Merensky Reef (Scoates and Friedman, 2008) it is consistent with recent Re-Os ages for the Platreef. Considering the younger age recorded by both Re-Os and Pt-Os systems, we propose that the Pt-Os isochron age records a late, low temperature hydrothermal event affecting PGM and sulphides in the Platreef and Merensky Reef. Pt-rich phases may also yield single grain 190 Pt-186 Os model ages since initial 186 Os/ 188 Os can be well constrained. A Pt-Os model age of 2024 Ma± 101 Ma was calculated for a Merensky cooperite; a Pt-Fe alloy from the Onverwacht pipe and a Tweefontein Hill sperrylite give Pt-Os model ages of 2113± 106 Ma and 2042± 102 Ma, respectively. A Pt-Os isochron derived from all Bushveld units gives an age of 2012± 47 Ma (2σ, 186 Os/ 188 Os i = 0.119818± 0.000006) with an MSWD of 1.19, hence PGE mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the Pt-Os chronometer. Nonetheless, the potential for the Pt-Os system in dating events in PGE mineralised systems is evident and the agreement of single grain model ages in high Pt/Os phases with the Pt-Os and Re-Os isochron ages indicates their usefulness in the geochronology of these systems.