H. De Bruiyn

The Ventersdorp supergroup: an overview

Abstract The Ventersdorp Supergroup is one of the least deformed late Archaean-early Proterozoic lowgrade metamorphosed supracrustal sequences in the world and can serve as a model for tectonic, geochemical and volcanological evolution. The development of the Ventersdorp Supergroup on the Kaapvaal Craton was initiated by the outflow of lava of kmatiitic affinity during an epoch of crustal extension. The Ventersdorp sequence comprises three groups namely the Klipriviersberg Group at the base followed by the Platberg Group and Pniel Sequence. The Klipriviersberg Group consists of mafic volcanics whilst the Platberg Group is composed of sediments deposited in grabens followed by a bimodal volcanic suite with intermixed sediments and tuff towards the top. The presence of stromatolitic cherty limestone and mature sedimentary material indicates an abatement in tectonic activity towards the end of Platberg times. The uppermost Pniel Sequence overlies the Platberg Group with a marked unconformity and consists of an arenaceous unit, followed by a mafic to intermediate volcanic sequence. Geochemistry and isotopes indicate a mantle origin for some of the volcanics, whilst crustal contamination probably also contributed to the evolution of the Ventersdorp volcanics. A full account of the literature pertaining to this important supergroup and the specific area of impact is also given.

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

Abstract The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

High temperature ash flow–wet sediment interaction in the Makwassie Formation, Ventersdorp Supergroup, South Africa

Abstract The Late Archaean Ventersdorp Supergroup in South Africa constitutes a supracrustal succession consisting of mafic lava at the base that is overlain by a clastic wedge deposit, known as the Kameeldoorns Formation, which formed in grabens that were the result of extensional tectonics. Overlying these units is an intermediate to felsic ash-flow deposit, the Goedgenoeg and Makwassie Formations, that are now represented by feldspar and quartz-feldspar porphyries. At T’Kuip in the Northern Cape Province, spherical structures or clasts varying greatly in size occur at the contact of the Makwassie Formation with the underlying sedimentary Kameeldoorns Formation. The larger clasts have cavities of different shapes and are regarded to have developed as a result of the deposition of a high temperature ash-flow onto water saturated sediments.

Alteration of the Allanridge Formation of the Ventersdorp Supergroup near Douglas, Northern Cape Province.

The Allanridge Formation of the Ventersdorp Supergroup in the Northern Cape Province consists of andesite to basaltic andesite. The properties of these rocks are described by aid of detail from two localities near Douglas (Kalkdam and Katlani), where seven lava flows can be discerned. Greenschist metamorphism has affected all of the lava flows and sporadic sulphide mineralization has also occurred. The intensity of sulphide mineralization is controlled by porosity so that the amygdaloidal bases and tops of flows are more affected than the massive parts, hence producing a conspicuous bleaching. Thus light amygdaloidal lavas (LA) that are sporadically mineralised by sphalerite, galena and chalcopyrite can be distinguished from less altered dark amygdaloidal (DA) lavas. Sulphides are present in altered amygdales of the LA together with quartz, chlorite and calcite. These minerals were introduced by means of hydraulic fracturing and brecciation features are common. Sphalerite tends to dominate over galena in the amygdaloidal lava flows, while the opposite is true in the breccia zones. Mass transfer calculations reveal that the net mass loss for LA and DA was 7 to 20 % and 3 to 5% respectively. Enrichment/depletion diagrams illustrate that LA is relatively enriched in SiO2, Al2O3, K2O, P2O5, Rb, Ba, Cr and Zn, while the DA is relatively enriched in MnO, Fe2O3, Nb, Cr, V, Co and Zn with respect to unmineralized lava flows. The alteration of DA may be regarded as propylitic, while changes in LA involved potassium metasomatism also. Two generations of mineralization can be discerned. The source of the saline fluids capable of transporting metals is probably in the banded iron formations of the Griquatown and Kuruman Formations. Channelways for fluid migration were provided during extensive deformation.

Picrite from the Katse area, Lesotho: evidence for flow differentiation

Abstract A basalt flow in the Katse area of Lesotho displays composite features with tholeiitic characteristics along its margins and a picritic composition in the centre. Geochemical modelling of the magma and the composition of microscopic melt inclusions indicate that the primary magma was olivine-rich. The chemical variation of the flow is evidence for fractionation of olivine from the magma, causing a trend towards tholeiitic compositions. The derived lava flow was modified by flow differentiation so that olivine concentrated progressively towards the centre, owing to differential flow velocity. Evidence from geochemical modelling corroborates these conclusions.

The geochemistry and petrogenetic relationships of two granites and their inclusions in the Keimoes Suite of the Namaqua mobile belt, South Africa

Abstract The Colston and Straussburg plutons consist of two dissimilar granites belonging to the Proterozoic (1100–1200 Ma) Keimoes Suite along the eastern margin of the Namaqua mobile belt. The intrusives and their enclaves were investigated to establish their genetic relationships. The peraluminous Colston granite exhibits S-type characteristics while the Strausaburg intrusive shows marked similarity with I-type granites. One set of quartz-rich melanocratic inclusions are present in the Colston granite. The corresponding melanocratic enclaves in the Straussburg pluton are comprised of a porphyritic type and a non-porphyritic type, while leucocratic inclusions are also abundant. Field relationships, mineralogical and chemical compositions as well as REE-patterns, point to a strong genetic relationship between the granites and their enclaves. Genetic models involving fractional crystallization, wall-rock assimilation, two-magma mixing, and minimum melt-restite mobilisation, were considered to explain the nature and origin of the inclusions and the host granites, and the observed REE-characteristics make most of the models unlikely. It is concluded that the quartz-rich melanocratic inclusions in both plutons formed as products of partial batch melting from a common source, possibly a basic or intermediate gneiss or granulite, under relatively high XCO 2 conditions. After the intrusion and consolidation of the first melts, water-undersaturated granitic magmas intruded as a secondstage, incorporating the first melt products as melanocratic inclusions. The different crystallization conditions of the melanocratic inclusions and the host granites are highlighted by the difference in chemical composition of the biotites, as for instance illustrated in Ti-Al diagrams. The biotite is accompanied by hornblende in all the rock types with the exception of the Colston granite and the leucocratic inclusions of the Straussburg granite. The leucocratic enclaves in the Straussburg granite are interpreted as depleted restite material which formed at a pressure near 5 kbar, and which was transported to the surface by the granite magma. When the compositions of the granitic rocks are compared with the experimental results for the haplogranitic (Ab-Or-An-Q) system, they prove to be quite near the expected minimum melt compositions for a total pressure of 5 kbar.

The composition of descloizite-mottramite in relation to the trace-element distribution of Pb, Zn, Cu and V in the Otavi Mountain Land, South West Africa/Namibia

Abstract The carbonate rocks of the Otavi Group (ca. 800 Ma) in the northeastern part of South West Africa/Namibia are known to contain Pb, Zn and Cu sulphide deposits with the mining activities at Tsumeb, Kombat and Berg Aukas being the most prominent. Vanadium mineralisation, associated with karst development, is often associated with the sulphide deposits, but occurrences without a sulphide association are also present. The distribution of Pb, Zn, Cu and V in the dolomite and limestone of the Otavi Mountain Land delineates geochemical regions which coincide with the above mineralisation. This coincidence is also manifest in the type of V mineralisation; where high concentrations of Zn occur in the country rocks, descloizite predominates, whilst mottramite occurs when the Cu values are higher.

The geology and petrochemistry of the Mashikiri Formation along the Olifants River Section, Kruger National Park, South Africa.

The volcaniclastic sediments and nephelinite lavas of the Mashikiri Formation, Lebombo Group, have been well documented in the Pafuri region and in the Sabi region, Zimbabwe. The well-exposed section along the Olifants River in the Kruger National Park provides cogent additional information. The Shishwayini Beds at the base of the succession represents volcaniclastic deposits on the Tshipise Sandstone Member of the Clarens Formation. The rest of the Mashikiri Formation in the Olifants River section is nephelinite in which nepheline is accompanied by olivine, clinopyroxene, devitrified glass, katophorite and opaque oxides. Chemical modelling shows that the nephelinites were derived by partial melting of mantle peridotite enriched in incompatible trace elements and by subsequent fractionation of pyroxene and olivine. Crustal contamination did not play a significant role in derived magma composition. The Olifants River section defines a third source of undersaturated parent magma distinct from the two sources previously identified from outcrops to the north by other authors.

Isotopic relationships of epigenetic Pb-Zn mineralisation in the Ventersdorp Supergroup near Douglas, Northern Cape Province

Hydrothermally altered and mineralised lavas of the ~2.7 Ga Ventersdorp Supergroup were sampled at two localities near Douglas (Kalkdam and Katlani). Propylitic alteration has affected some of the lavas (the dark coloured amygdaloidal type), while others (the light coloured amygdaloidal type), show signs of potassium metasomatism as well, representing at least two mineralisation events. A Rb/Sr age of 2014±38 Ma was obtained on clinochlore from the amygdales and whole rock samples. This is interpreted to be the age of the first hydrothermal alteration and main mineralisation of the lavas. Variable isotopic values for Sr and Pb suggest a complex provenance for the hydrothermal fluids, whilst a narrow range of δ34S in galena (−3.4 to −6.3‰) indicates a specific source region. Four fluid inclusion populations are dominated by a brine population (Tm = −12.9°C) with a model density of 1.07g/cm3 (Th = 117°C). The temperature of the fluid must have been ~270°C according to the chlorite geothermometer of Cathelineau. The temperature of entrapment implies a pressure of roughly 2 kb when the slope of the relevant isochore (1.07g/cm3) is taken into account. Fluids seem to have migrated along channelways that were provided during extensive fracturing such as along the Griquatown fault zone. This migration probably propagated from rocks of the Transvaal Supergroup (Griquatown and Kuruman Formations) into the underlying Ventersdorp lavas.

Secondary ZnPbCu minerals, an aid to mineral exploration in the Aggeneys area, Bushmanland, South Africa

Abstract A close spatial association exists between secondary and primary CuPbCu minerals in the Aggeneys area, Bushmanland, South Africa. A wide variety of metamorphic and non-metamorphic indicator mineral assemblages occur within this area which may successfully be utilized in mineral exploration programmes. The mode of occurrence, associations and parageneses of secondary silicates, oxides, sulphates, phosphates, carbonates, carbonate-sulphates and chlorides are described, while their importance as indicators of mineralization is stressed.