Abstract Pan-African volcanic cauldrons of the Saldania Belt, along the South African West Coast have been named the Postberg and Saldanha centres. Outcrops of subvolcanic and volcanic rocks in the Trekoskraal area indicate the presence of a third volcanic centre that is mostly hidden just off shore. In coastal outcrops of the Porphyry Bay area, porphyritic varieties of the Trekoskraal Granite are unconformably overlain by silicic crystal tuffs and ignimbrites that are, in turn, intruded by the Porphyry Bay microgranite. New zircon U-Pb dates for the rocks include the Trekoskraal Granite (535.4 ± 0.9 Ma), the overlying Trekoskraal crystal tuff (536.1 ± 1.3 Ma), the intruding Porphyry Bay microgranite (534.9 ± 1.1 Ma) and a late, leucomicrogranite dyke (523.1 ± 4.7 Ma). Age and field relationships indicate that the Trekoskraal Granite was emplaced at shallow depth and unroofed to form the substrate for the overlying, folded Trekoskraal tuffs and ignimbrites, within an interval of ≤1.5 Myr, during regional deformation. Inherited zircon in the Porphyry Bay rocks has a ca. 839 Ma peak that represents the only inherited age common to all three units. The youngest concordant detrital (inherited) zircons in the basal tuff and the Porphyry Bay microgranite have ages of ca. 570 Ma, suggesting the presence of Ediacaran metasediments in the anatectic source region. The chemical scatter in major-oxide parameters for the S-type rocks in this area, both within and between different units, is consistent with heterogeneity inherited from metasedimentary magma sources. Such differences show that the Cape Granite Suite (CGS) S-type magmas formed from various metasedimentary units with distinct ages and somewhat different sediment sources, perhaps at different crustal depths. The late-stage (523.1 Ma) leucomicrogranite indicates that minor crustal melting continued for at least 10 Myr after the main phase of CGS magmatism. The field and age relationships underline the close spatial and temporal relationships between volcanic and near-surface plutonic activity, related to the CGS, in this part of the Saldania Belt.
Abstract The growing demand for critical and strategic materials, coupled with environmental concerns, has renewed interest in mine tailings. However, limited geochemical and mineralogical data on these repositories constrain their effective reprocessing and the evaluation of environmental risk. The present study serves to present an integrated geochemical and process mineralogical characterisation of a tailings impoundment derived from historical antimony mining in the Murchison Greenstone Belt, with emphasis on antimony recovery and environmental assessment. Five vertical auger holes were drilled according to a site-specific grid spacing of 180 m, in order to obtain material for study. Antimony (Sb) concentrations range from 3 740 to 12 125 ppm, while gold (Au) contents range from 90 to 2 200 ppb. Major oxides are dominated by SiO2 (41.7 to 51.9 wt%), MgO (9.6 to 19.0 wt%) and Fe2O3 (6.0 to 11.1 wt%). An FEI 600F Mineral Liberation Analyser was employed to determine the modal and relative mineralogy, mineral associations, Sb deportment, liberation characteristics, and grain-size distributions. Sb occurs predominantly in sulphide phases associated with silicate and carbonate minerals, and these sulphide phases are preferentially concentrated at the bottom of the tailings impoundment. In contrast, Sb oxides are more concentrated near the surface, probably due to oxidative alteration. Integrated geochemical and mineralogical results demonstrate an economically significant resource of Sb, with an estimated 94 457 tonnes of contained Sb, which is predominantly hosted in recoverable sulphide phases. The tailings are characterised by a range of coarse to fine particle sizes with varying degrees of locking among the Sb minerals present. Based on the current data, it is considered that further milling may be required to achieve sufficient liberation of Sb phases and to improve metal recovery efficiency. Flotation and bulk alkaline leaching present promising and environmentally benign routes for recovering Sb from fine-grained, low-grade ores. Geochemical indices such as geo-accumulation index, as well as enrichment factor, were calculated and applied to evaluate levels of Sb enrichment and associated potential environmental risk represented by the samples collected. Elevated geo-accumulation index and enrichment factor values confirm strong Sb and arsenic (As) enrichment. Acid-base accounting results indicate that the tailings are non-acid-generating due to the abundance of carbonate minerals, which buffer acidity and restrict metal mobility. The limited oxidation potential and stable neutral to slightly alkaline characteristics of the Murchison Greenstone Belt Sb tailings are indicative of low environmental risk, supporting the potential for comparatively safe and efficient resource recovery. However, inhalation and ingestion of Sb- and As-laden dust emanating from the tailings, by residents and livestock in surrounding communities, remain an area of concern. This aspect should also be carefully considered prior to commencing tailings retreatment.
Abstract The Goedgenoeg Formation is here described as a separate formation from the Makwassie Formation of the Platberg Group, Ventersdorp Supergroup. The Goedgenoeg Formation is visually distinguished from the overlying Makwassie Formation by the occurrence of green dacitic large-feldspar porphyritic ash flows, compared to the much paler coloured rhyolitic quartz-porphyritic ash flows of the overlying Makwassie Formation. Both formations also contain dark green andesitic lavas, which cannot easily be distinguished. The two formations usually occur together, but either may be absent. Goedgenoeg Formation dacite samples contain highly variable amounts of magmatic and metamorphic zircon xenocryst grains ranging from 3.1 to 2.7 Ga in age, but the magmatic autocrysts formed during extrusion are now dated at 2 723 ± 4 Ma on four samples of the Goedgenoeg Formation. This is within error of the published Makwassie Formation 2 720 Ma ± 2 Ma extrusive age on four samples. The Goedgenoeg Formation is widespread in the Platberg Group depository in the Witwatersrand and Kimberley terranes of the Kaapvaal Craton as documented by boreholes, but is poorly exposed. Exploration boreholes intersected it in the Bothaville Gap, between the Free State and Klerksdorp goldfields and westwards to Vryburg and Kimberley. The type section is defined in borehole VE1 and supplementary sections in boreholes LL1, BES1 and DHK1, all in the Bothaville Gap and stored at the Orkney core yard of Harmony Gold Mining Company Ltd. Together they show the main characteristics and lithological diversity of the Goedgenoeg Formation and how it is distinguished from the Makwassie Formation. Three poorly exposed and weathered outcrop localities are also documented. The volcanic succession varies greatly in thickness, with maxima of more than 3 000 m in major grabens. Coeval dykes and possible volcanic correlates occur in the Swaziland Terrane of the Kaapvaal Craton. The Goedgenoeg Formation overlies the sedimentary Kameeldoorns Formation, which forms the base of the Platberg Group. Minor sedimentary units may occur close to the lower contact and near the upper contact with the overlying Makwassie Formation quartz porphyries. The volcanic rocks have undergone greenschist grade burial metamorphism and generally minor alteration. Their geochemical composition includes basaltic andesite, andesite lavas and high-and low-Zr groups of dacitic feldspar porphyry ash flows. The volcanic rocks are, geochemically ascribed to a calc-alkaline volcanic arc in a continental margin setting. Magma source signatures suggest an enriched mantle source with added continental material, probably related to subduction. The Goedgenoeg Formation volcanic rocks are easily distinguished from the underlying Klipriviersberg Group and the overlying Allanridge Formation on geochemical diagrams such as Ti-Zr-P and P/Ti vs Zr/P. They overlap geochemically to some extent with rocks of the Makwassie and Rietgat formations, but the Ti/Zr ratio allows partial discrimination. The origin of the Goedgenoeg Formation and Ventersdorp Supergroup may be ascribed to either a long-lived plume beneath the Kaapvaal Craton, or to subduction of an Archaean ocean beneath the craton before the 2.7 Ga continent-continent collision with the Zimbabwe Craton, which formed the Limpopo Belt. The Goedgenoeg Formation should now generally be distinguished from the Makwassie Formation on maps and borehole logs.
Abstract Manganese (Mn) is vital for steel and battery production, requiring an improved understanding of ore textures to optimise extraction. This study investigates the Paling Pan ferruginous manganese (Fe-Mn) deposit in the Postmasburg Manganese Field (PMF), where ores, hosted in the Gamagara Formation, unconformably overlie Campbellrand dolostones. We integrate geology, geochemistry (major oxides), mineralogy, and machine learning (ML) to classify ore textures (massive, vuggy, layered, conglomeratic) and predict manganese oxide (MnO) grades. These textures exhibit distinct geochemical signatures that reflect the depositional environment, diagenesis, and supergene processes. Depth profiles show MnO enrichment in karstic depressions, with braunite and bixbyite correlating strongly with Fe2O3 and SiO2, while partridgeite and hollandite influence Mn distribution. ML analysis (Random Forest, XGBoost, Gradient Boosting) of drill-core geochemical data achieved an F1-score of 0.8682 for texture classification and 0.9983 coefficient of determination (CoD) for MnO prediction. Results demonstrate that MnO distribution is controlled by primary sedimentary processes and secondary alteration. This data-driven approach enhances geochemical modelling, enabling better resource evaluation in the PMF. The study advances understanding of Mn mineralisation processes and establishes an integrated ML-geological framework for exploring sedimentary FeMn deposits, with direct applications for optimising exploration and mining strategies in similar deposits globally.
The arcuate Saldania Belt marks the intersection of two high-angle orogenic systems that record the final-stage amalgamation of southwest Gondwana and subsequent tectonics along Gondwana's southern margin in the Late Ediacaran to Mid Cambrian. The northerly trending western Saldania Belt represents the southernmost extension of interior belts of the West Gondwana Orogen. The belt records the oblique, low-angle convergence and closure of oceanic realms between the Kalahari Craton and South American cratonic blocks between >550 and 520 Ma. The lithological and structural inventory is that of a fore-arc realm situated along the southwestern leading edge of the Kalahari Craton in which folded, low-grade metamorphic metaturbidites of the fore-arc basin (Malmesbury group) overlie imbricated, m & eacute;lange-like rocks of a structurally lower accretionary complex (Swartland complex). Oblique subduction below the Kalahari Craton resulted in sinistral, strike-slip partitioned transpression (folding and strike-slip shearing) of the fore arc. Uniformly low grades of metamorphism and the preservation of ca. 540 Ma subaerial volcanics indicate only very limited burial of rocks. The soft collisional event and late-stage, short-lived (540 to 530 Ma) peak of plutonism and volcanism of the Cape Granite Suite relate to the opening of a slab window during ridge subduction or detachment of the subducting slab. The southern Saldania Belt, in contrast, was located along the southern edge of the Kalahari Craton and Gondwana. Its evolution was closely linked to the paleo-Pacific convergent margin of the Terra Australis. Earlier passive-margin sedimentation of the Kaaimans and Kango groups and the Gamtoos complex in the late Ediacaran was reversed during the onset of northward subduction below the Kalahari Craton at ca. 530 Ma, leading to the formation of a north-verging, retro-arc, foreland fold and thrust belt. The timing of deformation along the southern Saldania Belt broadly coincides with the final amalgamation of Gondwana along the western belt. The two high-angle belts converge and are deflected against each other in the region that later accommodated the location of the Permo-Triassic Cape Syntaxis. Unconformably overlying rift-type sedimentary rocks of the Klipheuwel and Kansa groups across the Saldania Belt indicate regional extensional processes along the southern margin of Gondwana in the Mid-to Late-Cambrian (<515 Ma) and prior to the deposition of the extensive, Mid-Ordovician (ca. 470 Ma) to Early-Carboniferous Cape Supergroup.
The Keis Supergroup comprises a geological succession dating from the Palaeoproterozoic to Mesoproterozoic eras, with an estimated thickness of approximately 28 km, inferred from interpretations of structurally complex geology. It consists of predominantly metasedimentary and subordinate metavolcanic rocks exposed between Olifantshoek, Upington, Marydale and the Korannaberg Mountains. It is subdivided into four major lithostratigraphic groups: the Elim-, Olifantshoek-, Groblershoop-and Wilgenhoutsdrif groups. The Elim Group comprises upward-fining fluvial to shallow marine sediments of the Mapedi Formation, overlain by stromatolitic carbonates and arenites of the >1 916 Ma Lucknow Formation. The Olifantshoek Group lies unconformably on the Elim Group and consists of the conglomeratic Neylan Formation and the volcanic Hartley Formation (similar to 1 916 Ma), succeeded by fluvial arenites of the Volop-and Top Dog formations. The Groblershoop Group represents a similar to 14 km-thick package of upward-coarsening marine shelf deposits consisting of arenites and schists, interpreted as progradational passive margin successions that were deposited prior to 1 290 Ma. The uppermost Wilgenhoutsdrif Group includes quartz arenites, mafic and felsic volcanic rocks, serpentinites, carbonates and phyllites, deposited in a forearc or back-arc basin, with felsic lavas dated at similar to 1 290 Ma. The Keis Supergroup defines the Kheis Terrane and records sedimentation along the western margin of the Kaapvaal Craton, from post-rift to forearc basin development, and preserves the effects of crustal shortening during the Kheis Orogeny (similar to 1 290 to 1 114 Ma). Structural, geophysical and provenance data indicate that the Kheis Terrane, formed as a distinct crustal domain as a result of the collision between the Rehoboth-and Kaapvaal cratons, prior to overprinting during the Namaqua Orogeny. The Keis Supergroup provides a critical window into early Mesoproterozoic basin evolution and tectonic reworking at the interface between the Kaapvaal Craton and the Namaqua-Natal Belt.
The Karoo Supergroup of southern Africa offers a valuable record of Gondwana's late Palaeozoic to early Mesozoic evolution over similar to 120 million years of geological history. This review synthesizes advances in sedimentology, stratigraphy, palaeontology, and basin analysis in the main Karoo Basin since 2000, highlighting key lessons and ongoing challenges. Significant developments include: (1) a refined flexural foreland basin model and the recognition of extensional overprinting in the Early Jurassic; (2) palaeoclimate reconstructions revealing non-linear shifts, particularly in the upper Karoo, challenging previous ideas on gradual aridification; (3) revised biozonation and expanding fossil records demonstrating ecosystem resilience during mass extinctions (end-Permian, end-Triassic); and (4) an improved chronostratigraphic framework through volcanic ash and detrital zircon geochronology and magnetostratigraphy, facilitating pan-Gondwanan correlations. However, critical unresolved questions remain regarding the precise nature and timing of formation boundaries; the relative roles of tectonics versus climate in facies changes; within-basin heterogeneity, sediment source links, and detailed correlation with global events. Future progress requires high-resolution stratigraphy integrating new geochronological data through denser sampling across critical boundaries; AI-assisted facies analysis and remote sensing applications to address correlation challenges and basin heterogeneity; expanded palaeontological and ichnological surveys; and synthesis of tectonic models with lessons from the sedimentary record. Coupled basin evolution models linking sedimentology, geochemistry, and geochronology are essential to resolve drivers of stratigraphic architecture. Assessments of resource potential (groundwater, CO2 storage, mineralisation) must consider the impacts of dolerite intrusion on basin-fill compartmentalisation. Revitalised institutional oversight by the South African Committee for Stratigraphy (SACS) is needed to formalise units and standardise frameworks. The Karoo's significance extends beyond Gondwana, offering insights into responses to supercontinental dynamics and fragmentation, climatic extremes, and biological crises. The Karoo remains a global deep-time laboratory for understanding tectonic-climatic-biotic interactions, but ongoing stratigraphic refinement is essential for unlocking additional Earth system insights and resource potential in southern Africa.
The Karoo Large Igneous Province (LIP) is mainly situated in South Africa but extends into the surrounding countries and Antarctica. The present area of outcrop in southern Africa is at least 140 000 km(2), and the original lava field may have covered two million km(2). Added to this is an extensive system of subvolcanic sills and two major dyke swarms. In South Africa, the lava stratigraphy is divided into the Drakensberg and Lebombo groups that have maximum thicknesses of 1.5 and 8 km, respectively. On the broadest scale, the Karoo LIP can be divided into two sub-provinces, and we refer to these geographically and compositionally distinctive magmatic suites as the Drakensberg and Lebombo sub-provinces. This minor modification of previously proposed schemes results in a mutually compatible stratigraphic and geochemical provinciality, that can be extended outside South Africa. Peak eruption and emplacement occurred within 2 Myr to 300 ka at similar to 183 Ma, with magmatism in the Lebombo sub-province having a longer duration (similar to 6 Myr). Compositionally, the Lebombo sub-province is bimodal in terms of SiO2, being largely confined to the 4 to 5 km-thick sequence in the 600 km long Lebombo Monocline. The mafic magmas are compositionally diverse and can be divided into high-and low-Ti types. All of the lavas and intrusions in the Drakensberg sub-province are low-Ti whereas the Lebombo sub-province has both high-and low-Ti types, with the low-Ti types in the two sub-provinces exhibiting distinct trace element signatures. Trace element and Nd and Sr isotope composition is highly variable for fixed intervals of MgO, and this reflects derivation from a heterogeneous mantle, with the much more diverse high-Ti types having a significant component from the subcontinental lithospheric mantle (SCLM) and the much more uniform low-Ti types potentially including plume-derived components, although the exact nature of the magma sources remains uncertain. The Drakensberg sub-province was emplaced in an intracratonic setting and the Lebombo sub-province is rift-related, which explains the greater thickness, limited areal extent, and the presence of basal nephelinites in the latter. The rift-related rhyolites probably formed by decompression melting of underplated basalt as the crust thinned.
Abundant evidence of impact cratering is found on the surfaces of all solid bodies in the solar system, although on Earth this evidence is more restricted owing to the effects of an active geosphere. Investigating impact structures on Earth is important not only from the perspectives of understanding Earth's geological history and the economic significance of some impact structures, but also because of the demonstrable threat large impacts pose to life on Earth. Besides the formation of a crater, the transfer of exceptional amounts of energy into Earth's crust during an impact creates distinctive deformation features and rock types that aid in identifying impact sites, including where the original crater form has been lost through erosion or is deeply buried. South Africa contains a small but diverse sample of impact structures in which impact processes can be investigated. The countrys four confirmed impact structures-Tswaing, Kalkkop, Morokweng and Vredefort-have ages between similar to 200 000 years and 2023 million years, and diameters that range from <1 km to 250 km. They provide an excellent sample of the size range, different morphological types, and different levels of preservation of Earth's impact structure inventory. Whereas Tswaing is one of Earth's best-preserved small impact craters, Morokweng and Vredefort are significantly eroded, thus providing insight into the deep-level processes caused by impact. Vredefort is Earth's largest and most deeply eroded impact structure, and its formation was integral to preservation of the Witwatersrand gold deposits. Morokweng is one of the few terrestrial impact structures preserving a thick, differentiated, impact melt sheet; it is also the only impact structure in which fragments of the impacting meteorite have been recovered from within a melt sheet. Tswaing and Kalkkop preserve rare long-term palaeoenvironmental records for the interior of South Africa, spanning the last several hundred thousand years. Additionally, the South African Archaean rock record also hosts a number of impact spherule beds within 2.5 to 3.3 Ga sedimentary sequences that constitute parts of the earliest impact record known on our planet.
The focus of this synthesis of Cenozoic regolith addresses the neotectonic context and changing palaeo-environmental conditions within the interior of southern Africa. The long-term geomorphological development of the southern African subcontinent since the early Cretaceous has resulted in the erosion of kilometres of rock, much of that cover being removed during the erosion cycle that formed the African Surface. Geological deposits representing that long period of weathering and erosion by drainage systems are rarely preserved. Continued landscape incision by Post-African erosion cycles during the Cenozoic did leave more widespread surficial deposits representing a variety of sedimentary environments and weathering regimes. The development of the Vaal-Orange drainage system and its residual fluvial terrace deposits provides a framework for the pulsed uplift of the interior that triggered episodes of river incision. Epeirogenic movement along axes of relative uplift across the subcontinent, drainage disruption and the subsidence and infilling of the Kalahari Basin are addressed. The Pleistocene sedimentary cover including widespread aeolian deposits in the arid Kalahari region and older windblown deposits extending eastward from the Vaal and Orange valleys, mass movement and colluvial cover associated with slope development, calcareous tufa deposits and karst processes, peat accumulation and lacustrine sediments are described. Weathering profiles and pedogenic duricrust profiles represent hiatus events in the long record of landscape development. The characteristics, occurrence and formation of calcrete, silcrete-dorbank, ferricrete and lateritic profiles is described. The changing palaeo-climatic conditions spanning the terminal Mesozoic and Cenozoic periods are outlined to contextualise the controls on geomorphological processes and accretion of geological materials on land surfaces of different ages across the interior region.
Kimberlites are the deepest derived magmas on Earth, and together with the mantle xenoliths and xenocrysts they carry, provide an unprecedented look into the composition and evolution of the mantle from the Archaean to Cenozoic. Although superficially similar to kimberlites, olivine lamproites of the Kaapvaal Craton, previously known as Group II kimberlites, are highly micaceous and represent a distinct magmatic episode to kimberlites. Both kimberlites and olivine lamproites are derived by small degrees of partial melting in the upper convective asthenospheric mantle, which then traverse the non-convective, sub-continental lithospheric mantle (SCLM) en route to Earth's surface. The major element compositions of the parent magmas are modified during complex melt-SCLM interactions, and this leads to significant diversification of the magmas. During their ascent, they also entrain diamonds, and consequently, kimberlites and olivine lamproites form primary volcanic ore deposits on emplacement near the surface. These volcanic pipes, similar to 3 km in depth and several hundred metres in diameter, formed from predominantly juvenile volatile-driven eruptions. In most cases across the Kaapvaal Craton, kimberlite pipes have been significantly eroded post-emplacement and the diamonds within them have been transported toward the west coast, forming secondary alluvial diamond deposits along the way. Diamond exploration and evaluation relies on kimberlite indicator minerals, e.g., garnet, ilmenite, and clinopyroxene, which are mantle xenocrysts from the SCLM transported to the surface in kimberlite and olivine lamproite magmas. The composition of these minerals is controlled by the pressure-temperature (P-T) conditions at which they last equilibrated. The majority of diamonds form at relatively low temperatures (900 to 1200 degrees C) but high pressures (4 to 7 GPa). These conditions are met in the SCLM and this region is termed the 'diamond window'. A smaller proportion of diamonds have a sub-lithospheric origin likely forming in the mantle transition zone (MTZ). Thermobarometry is the approach used to constrain the P-T conditions of indicator minerals based on their chemistry. This provides a means to map the composition and thermal state of the SCLM to depths of similar to 200 km, and to assess the extent of a possible diamond window sampled by individual kimberlites. Ultimately, kimberlites and olivine lamproites, along with their mantle cargo, are invaluable in our understanding of the deep Earth and are economically valuable as diamond deposits.
The Mesoproterozoic Pilanesberg Complex (PC) is one of the world’s biggest and best-preserved complexes of intrusive and extrusive felsic alkaline and peralkaline igneous rocks and belongs, together with smaller centres of alkaline silicate and carbonatitic rocks that are mainly concentrated to its east, to the Pilanesberg Alkaline Province (PAP). Recent work on this province has varied from age dating to mineralogy and geochemistry to paleomagnetism. New U-Pb dating on the silicate rocks converges on intrusive ages of ca. 1.4 Ga, but the carbonatite complexes need revisiting in view of their more scattered and up to 150 Myr younger ages. The new ages present a problem for paleomagnetic results, with different apparent palaeopoles for rocks of broadly the same age. The PC has been found to contain a highly peralkaline agpaitic ignimbrite unit, the Beacon Heights Phonolite, previously classified as a tinguaite sheet. Mineralogical investigations on the peralkaline units have led to the discovery of the mineral pilanesbergite, and have shown the importance of varying levels of water activity on the mineral assemblages. Enrichments of rare elements (“critical minerals”) are modest in the PC, and the carbonatites within the province may prove to be more prospective.
This paper provides an update of Chapter 16 in the Geology of South Africa (2006) and sumniarizes recent advances in understanding the Mesoproterozoic Namaqua-Natal Province (NNP). The NNP forms a major orogenic belt along the southern and southwestern margins of the Kaapvaal Craton, that developed between similar to 1.55 and 0.96 Ga during the assembly of Rodinia, It comprises two main parts: the Namaqua Sector in the west and the Natal Sector in the east, separated by Phanerozoic cover. In South Africa, the Namaqua Sector is a composite of five tectonostratigraphic entities separated by major thrusts and shear zones. The Richtersveld Subprovince in the west contains 1.9 to 1.86 Ga, arc-related volcanic and plutonic rocks partly reworked during the Namaqua orogeny. The Bushmanland Subprovince in the south is dominated by high-grade supracrustal gneisses (1.21 to 1.13 Ga), granitic orthogneisses (similar to 1.21 to 1.12 Ga), abundant late- to post-tectonic granites (similar to 1.10 to 1.04 Ga) and minor mafic intrusions, accompanied by crustal heting, at -1.03 Ga. The Kakamas Domain, also composed of granulite-facies paragneisses (similar to 1.22 Ga), granitic orthognekses (similar to 1.23 to 1.15 Ga), and granites (-1.12 to 1.08 Ga), was thrust south-westward over the Richtersveld and Bushmanland Subprovinces along the Lower Fish River - Onseepkans Thrust Zone, East of the Kakamas Domain, the Areachap Terrane comprises 1.29 to 1.22 Ga island-are volcanic and sedimentary rocks and juvenile granitoids generated after collision with the Kaapvaal-Rehoboth Craton at -1.21 Ga. The easternmost Kaaien Domain represents a foreland thrust complex on the craton margin containing a possible back-arc volcanic sequence in the Wilgenhoutsdrif Group. Deformation in the Namaqua Sector involved several phases. Western Richtersveld Subpovince rocks preserve an older, Paleoproterozoic (-1.89 Ga), greenschist-grade folding event (D1). The Mesoproteroloic Namaqua Orogeny was polyphase, with an initial extended period of isoclinal folding and thrusting under high grade metamorphic conditions, accompanied by a penetrative gneissic foliation (D2 at similar to 1.20 to 1.12 Ga), followed by dome-and-basin type refolding (D3) and regional dextral shearing (D4 at -1.00 to 0.96 Ga). The latter was associated with the emplacement of the Orange River Pegmatite Belt into D2 and Di structures. Two contrasting geodynamic models have been proposed to explain the evolution of the Namaqua Sector. The traditional accretionary model interprets it as a collage of previously unrelated arc terranes successively juxtaposed with the Kaapvaal Craton, later affected by mantle delamination and localised thermal overprinting (metamorphism). The alternative continental back-arc model proposes long-lived (1.2 to 1.0 Ga) crustal extension and heating of a pre-existing crustal block in a continental back-are setting, generating the widespread high-temperature/low-pressure metamorphism and voluminous granite magmatism that typify the Namaqua Sector. The Natal Sector comprises, from north to south, the Tugela, Mzumbe and Margate terranes. Over the past two decades, new geochronological datasets have refined the timing of key events but have not significantly changed the overall model. Juvenile island arcs formed south of the Kaapvaal Craton through southward subduction of the "Tugela Ocean before 1210 Ma. Subsequent northeast directed closure led to obduction of the Tugela oceanic arc terrane onto the craton's southern margin and accretion of the Mzumbe and Margate arcs around 1150 +/- 20 Ma (D1), accompanied by high grade metamorphism, polyphase granite intrusion and minor mafic magmatism. Continued northeast-southwest convergence produced steep ductile sinistral shear belts in the southerly two terranes (D2) but not in the rigid, craton-underlain Tugela Terrane. The 102 phase was accompanied by extensive A-type granitoid magmatism (Oribi Gorge Suite). The Natal Sector is still regarded as an accretionary assemblage of juvenile Mesoproterozoic terranes.
Abstract The immense size of the Bushveld Igneous Complex (BIC) allows for a diversity of different parental magma types. Restricting ourselves to its mantle-derived (ultra-)mafic parents, with a special emphasis on the Rustenburg Layered Suite (RLS), these are first reviewed before presenting field, petrographic and bulk rock geochemical data on 35 new mafic-ultramafic sill samples, from a supra-Machadodorp Mb transect through the uppermost Pretoria Group. As observed by others, these sills can be subdivided into (1) more altered and evolved metadolerites, tentatively linked to low-Ti (LT) lavas of the Dullstroom Formation, (2) generally fresher ‘boninitic’ and melanoritic LT sills that are convincingly recognised as initial B1 magmas for the RLS, and (3) dunitic to harzburgitic sills that are likely basal cumulates to either of the above two groups. Focusing more on bulk rock geochemistry, including published data on parental basic magmas, we caution against the liberal use of parental candidates, concluding that only a very specific B1 composition, with a very strong lithospheric component, is certainly parental to the RLS, while an additional asthenospheric high-Ti (HT) parent still needs to be more confidently linked to it. In fact, we speculate on the possibility for (1) a solitary B1 parent for the entire RLS, (2) its generation from a uniquely TTG melt enriched and orthopyroxenitic sub-cratonic lithospheric mantle, and (3) a scatter of lesser HT magma injections to have mainly avoided the RLS in both space and time.
The Steinkopf Gneiss represents remnant Palaeoproterozoic crust in the western part of the predominantly Mesoproterozoic Namaqua Sector of the Namaqua-Natal Metamorphic Province. The Steinkopf Gneiss is typically migmatitic with the development of leucosomes, which are seemingly of different ages, based on field observations such as the intensity of deformation and cross-cutting contacts. In the study area, the Steinkopf Gneiss is associated with major Mesoproterozoic granitic units such as the pre-to syntectonic Little Namaqualand Suite and Concordia Granite, the late-to posttectonic Spektakel Suite, as well as some pegmatites in the Orange River Pegmatite Belt. Previous studies have suggested a significant contribution of crustal-derived melts in the formation of all these units, however, potential sources of such crustal-derived melts have not been identified. The current study investigates the possibility that the migmatites in the Steinkopf Gneiss represent potential source materials to these melts. In order to do so, U-Pb zircon and monazite age data are provided for two leucosomes which, based on field evidence, represent some of the oldest and youngest leucosomes, respectively, in the Steinkopf Gneiss. The zircons display two distinctly different textural sectors, which each renders unique ages and Th/U values. Brightly zoned areas are associated with a weighted mean age of 1 799 +/- 3 Ma, which is considered to represent the crystallisation age of the Steinkopf Gneiss. Transgressive unzoned dark areas are associated with a weighted mean age of 1 146 +/- 8 Ma, which is considered to represent the timing of anatexis in the Steinkopf Gneiss during which new felsic melts were generated. Weighted mean monazite ages of 1 028 +/- 3 Ma and 1 035 +/- 3 Ma in the two leucosomes respectively are considered to represent their crystallisation ages. These results suggest that the Steinkopf Gneiss might have acted as a source to new anatectic melts between about 1 146 Ma and 1 030 Ma; these melts might have migrated unknown distances into the surrounding areas and contributed to the formation of new granitic crust including the Little Namaqualand and Spektakel suites, the Concordia Granite, and some of the pegmatites in the Orange River Pegmatite Belt. Such anatectic events might have occurred incrementally throughout this period but it is not possible to determine how many times, since each incremental event will partly or completely destroy evidence of earlier events.
The Rustenburg Layered Suite is economically the most vital component of the Bushveld Complex, South Africa, and contains numerous, laterally persistent layers of chromitite. Here we investigate the petrogenesis of the Middle Group chromitites from three sections in the Eastern Limb, two located in the northern sector and one from the southern sector. Regional differences in the igneous stratigraphy are explained by each sector having developed as a separate intrusive compartment. The chromitites straddle the MG3 anorthosite, a regional marker that demarcates the boundary between the Lower Critical Zone (LCZ) and the Upper Critical Zone (UCZ). At Jagdlust and Maandagshoek (northern sector), the chromitites are thin and poorly defined and occur in a succession of seven lithological units with a composite thickness of 220 m. The pyroxenitic E (upper) unit (LCZ) contains the MG1-MG2 chromitites. The F through L units (UCZ), which consist of intercalated layers of pyroxenite, norite, and anorthosite, contain the MG3-MG5 chromitites, together with the subordinate Upper-Middle Group layers. At Tweefontein (southern sector), the study interval is restricted to four units with a composite thickness of <100 m and yet the chromitites are represented by multiple layers with thicknesses of up to 2 m. This region contains a major resource of chromium ore, together with byproduct platinum-group elements (PGE). The chromitites are exposed in a succession of open pits covering a strike length of approximately 40 km. Descriptions of the igneous stratigraphy are supported by electron microprobe analyses for the orthopyroxene and plagioclase in the host silicates. There are only minor compositional geochemical differences between the pyroxenites and norites, and upward fractionation trends are insignificant. The Sr initial ratio based on plagioclase separates from the silicates is tightly constrained (0.70605 +/- 0.00039). The absence of a regular cyclicity, as well as the sharply defined nature of internal contacts, is reconciled with a hypothesis based on successions of intrusive events. The magma was intruded into a crystalline igneous stratigraphy. Intrusion rates associated with emplacement of the thin magma sheets were insufficient to enable a magma chamber to develop. The lithological units developed from two parental magma-types derived from a deep crustal staging chamber. They do not represent magmatic (fractionation) cycles, as is commonly thought. The norites are related to the batch crystallisation of a relatively felsic basaltic magma, the anorthosites having formed in situ, from low degrees of partial melting and metasomatism. The pyroxenites accumulated from a relatively mafic basaltic magma, injected as sills which exploited competency contrasts between the substantially, crystalline layers of norite and anorthosite. The different thermal history of the magma sheets and sills is consistent with the relatively fine-grained nature of the norites as compared with the coarse-grained pyroxenites. Detailed field relationships, including the non-sequential nature of parts of the stratigraphy, are reconciled with each layer of chromitite having accumulated from a chromite slurry. This concept addresses the chromium budget issue by explaining how anomalous concentrations of chromite can form, including the Middle Group layers at Tweefontein which have a composite thickness of 10 to 12 m. The chromite slurries consisted of xenocrysts entrained in the staging chamber, further concentrated in conduits, and transported laterally by the pyroxenitic sills. The sills functioned as loci for repeated magma recharge and contained multiple slurries. The chromite slurries were sufficiently mobile as to have intruded and partially disaggregated the primary stratigraphy, including the MG3 anorthosite. In some instances, the anorthosite is sandwiched between the multiple MG2 and MG3 chromitites, and in other cases the anorthosite hosts individual layers of chromitite. The principal accessory phase in the MG1 and MG2 chromitites is orthopyroxene, consistent with the relationship between the chromite slurries and the mafic basaltic magma. The abundance of plagioclase in the MG3 chromitite reflects the dynamic nature of the intrusive process, the chromite slurry having liberated grains of plagioclase from the anorthosite. The composition of the chromite and the PGE mineralisation contained in the chromitites was established at depth. We find no evidence of magma mixing processes or of the cotectic crystallisation of chromite and plagioclase in the petrogenesis of the Middle Group chromitites.
The Neoproterozoic Damara Supergroup of northern Namibia is one of the best studied sedimentary successions hosting Cryogenian snowball Earth glaciogenic deposits, represented by the Sturtian Chuos Formation and the Marinoan Ghaub Formation. Despite the wealth of sedimentological and geochemical data from these units, including precise geochronological age constraints for the younger glacial interval, there are few robust palaeomagnetic constraints on their depositional latitude between 740 and 550 Ma. Herein we summarise natural remanent magnetisation (NRM) data, acquired through progressive thermal demagnetisation, from Damara Supergroup strata in the Kunene Zone of Kaoko orogenic foreland of northwestern Namibia, including Tonian red beds of the Nabis Formation, Cryogenian interglacial carbonates and quartzites of the Etoto and Ombaatjie formations, and early Ediacaran cap carbonates of the Keilberg Member. High-stability NRM components are carried by magnetite and hematite. A pre-folding north-down component ("A1") recorded in the Nabis Formation may record either diagenetic or Kaoko synorogenic magnetisation, while a similar post-folding north-down direction in carbonate units ("A2") was acquired during late Kaoko orogenesis, likely through chemical processes. This component is observed elsewhere in northern Namibia as well as in other West Gondwanaland cratons, pointing to widespread remagnetisation in the early Palaeozoic. Authigenic mineral growth responsible for this NRM component may have been the result of orogenic fluids related to regional sediment-hosted base-metal mineralisation (Central African Copper Belt Type Cu-Co-Ag and Mississippi Valley Type Pb-Zn-Cu mineralisation) or clay transformation during burial diagenesis. Additional moderate to high unblocking temperature components directed south-down ("B") or north-up ("C") most likely record late Palaeozoic to Mesozoic chemical remagnetisation during weathering, and a low-stability north-up component ("D") records modern weathering-related chemical remanence. These data expand the collection of known remagnetisation directions in northern Namibia, important for understanding the extent and nature of tectonic and remagnetisation events, and necessary for ascertaining the fidelity of primary palaeomagnetic poles.
