Search

Archaean greenstone remnants comprising mainly metamorphosed basaltic lavas and lesser metasedimentary interlayers are components of the granite-gneiss-migmatite basement terrane of the central and northern parts of the Kruger National Park (KNP), South Africa's premier wildlife nature reserve. Most of the greenstone remnants represent extensions into the Kruger Park of better developed and exposed greenstone belts that exist beyond the borders to the west, including the Giyani and Murchison greenstone belts. Geological mapping in the national park presents challenges as exposures are poor and discontinuous, with the dense bush and wildlife adding to the difficulties. Satellite remote sensing provided an ideal solution and in this study Google Earth imagery proved to be especially helpful. The imagery provides a synoptic overview of the terrane making it possible to establish the outline of greenstone remnants irrespective of the dense bush cover. Google Earth imagery furthermore provided a preliminary impression of a hitherto unknown, yet sizeable, greenstone belt remnant in a remote area south of the Olifants River in the central section of the KNP. Referred to as the Olifants River Greenstone Belt in this study the remnant occurs as a major Type 2 interference fold resembling a mushroom-shaped diapir with a broad bulbous core fringed by laterally flattened skirts. Images are presented that illustrate various components of this imposing structure. Google Earth images furthermore display the general character of the Archaean granite-greenstone basement and the Palaeoand Mesoproterozoic intrusive dyke swarms on the northeastern flank of the Kaapvaal Craton. [ABSTRACT FROM AUTHOR]

Sandy, microbial-mat-laminated sediments are common in estuarine and tidal environments of the Palaeoarchean Moodies Group (ca. 3.22 Ga); they are interspersed with numerous expressions of mafic to intermediate (sub-) volcanism, including sills, stockwork dykes, lavas, and air-fall tuffs. We describe abundant fluid-escape structures up to 6 m in height associated with this facies in the Saddleback Syncline of the central Barberton Greenstone Belt. The fluid-escape conduits fed small sand volcanoes during prolonged and/or recurring discharge of gases, liquids, and solids. They are filled by sand, sericitic clay, and fine-grained organic matter of former microbial mats. In comparison to the mean composition of adjacent beds of identical composition, the conduits are enriched in Fe, Cr, Ti, and Mg. This suggests that fluid-escape was not only a consequence of overpressure buildup from decaying microbial mats in the shallow subsurface or of water-level fluctuations but also due to periodic or continuous release of hydrothermal fluids circulating in the thermal aureole above the cooling Lomati River Sill of Moodies age. Such an inference is also supported by textures characteristic of in-place argillaceous and sericitic alteration and by Raman spectroscopy of carbonaceous matter (RSCM) indicating temperatures ca. 50 to 100°C above the regional maximum metamorphic temperature of 320 to 380°C. Pre-compaction carbonate and/or silica cementation also preserved the abundant carbonaceous laminae interpreted as benthic microbial mats. Analogue recent hot spring deposits suggest that surficial hydrothermal activity in the medium-energy siliciclastic tidal zone would have significantly boosted microbial growth. [ABSTRACT FROM AUTHOR]

Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the posttectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event. [ABSTRACT FROM AUTHOR]

Greenstone belt dynamics are a key to understanding the formation and evolution of continental crust but the scarcity, commonly poor exposure, strong deformation, and high metamorphic grades of many Archaean supracrustal rocks preserved in greenstone belts often hinder geological insights. Exceptionally, the largely siliciclastic Palaeoarchaean (ca. 3.22 Ga) Moodies Group, uppermost unit of the Barberton Greenstone Belt (BGB) in South Africa and Eswatini, allows a detailed understanding of regional depositional processes and stratigraphic architecture due to its low degree of deformation, considerable thickness, generally good exposure, and diverse sedimentary facies. We here investigate the Powerline Road Syncline (PRS) and the adjacent Maid-of-the-Mists Syncline (MMS), two east-west-trending, tightly to isoclinally folded, locally northwardly overturned synclines in Moodies Group strata in the central BGB. The northern and southeastern margins of the PRS and the southern to southwestern margin of MMS are bounded by the bedding-parallel 24-Hour-Camp Fault. Along this fault, Moodies Group strata and stratigraphically underlying units of the Auber Villiers Formation of the upper Fig Tree Group structurally overlie lower Fig Tree strata above Onverwacht Group strata. Stratigraphic architecture and palaeocurrent analysis of Moodies Group strata in both synclines document a northeastward (depositionally downdip) facies transition from proximal fan delta conglomerates to fluvial- to coastal-plain and estuarine sandstones in which thick foresets likely represent subtidal channel fills. The overall deepening-and fining-upward trend in this sequence, corresponding to an increasing mineralogical maturity of sandstones, is partially obscured by local hydrothermal alteration which was contemporaneous with sedimentation, minor volcanism, and sill intrusion. Moodies Group conglomerates and sandstones were largely shed from the rising Onverwacht Anticline to the southwest. Lithologies and stratigraphy in the PRS-MMS region closely resemble strata of the lower Moodies Group in the Sadddleback Syncline, located nearby to the north but across the Inyoka Fault. If the 24-Hour-Camp Fault was part of a group of extensional faults accompanying the rise of the Onverwacht Anticline and of tonalitic-trondhjemitic-granodioritic plutons around the BGB margins at ca. 3 224 Ma, it would have acted as one of numerous basin-bounding normal faults during early Moodies time. This would place the deposition of Moodies Group strata in the PRS-MMS region in a supra-detachment fault environment which was subsequently tightly folded. [ABSTRACT FROM AUTHOR]

Ultramafic-mafic layered complexes are important but not-well studied components of Archaean granitoid-greenstone terranes. In the vicinity of the Barberton Greenstone Belt (BGB), at least 27 such complexes are intimately associated with the supracrustal succession. The petrogenesis of one of these layered bodies, the Stolzburg Complex (SC), is explored, together with its relationship to the surrounding Barberton volcanic succession. Previous models for the origin of Barberton layered complexes proposed a variety of mechanisms, such as single chamber subvolcanic sills, ponded lavas, and alpine-type tectonites. In contrast, the present work suggests that emplacement mostly occurred as sheeted sills of crystal slurries into the country rocks. Unlike the subvolcanic sills model, whereby each complex grew through repetitive magma injection and differentiation in a single chamber, the preferred model regards the layered bodies as ‘stacks’ of discrete intrusions, where each magmatic unit represents a distinct sill. Through comparison of trace element geochemistry (i.e., trace element ratios and patterns), the Lower and Upper divisions of the SC are inferred to be petrogenetically related, but compositionally distinct from the enveloping Nelshoogte volcanic rocks. The trace element geochemistry of the Lower and Upper divisions of the complex is indistinguishable. While the SC ultramafic rocks display an Al-undepleted character, Nelshoogte metavolcanics can be classified as Al-depleted komatiites and komatiitic basalts. [ABSTRACT FROM AUTHOR]

A new lithostratigraphic framework based upon a review of historic data, field mapping and remote sensing, including aerial photography, high-resolution airborne aeromagnetic and radiometric data, is proposed for the Archaean geology along the southeastern margin of the Kaapvaal Craton, South Africa. A synthesis of new and existing data reveals that previously accepted lithostratigraphic schemes require complete revision, with reinterpretations identifying multiple major shear zones and previously unidentified granitoid successions along the margin of the craton. In this new lithostratigraphic framework, lithologies of the Southern Syncline previously correlated with the Nsuze Group of the Pongola Supergroup, are redefined as greenstone lithologies associated with the Ilangwe Greenstone Belt. The geology of the Nkandla region can be subdivided into five distinct geophysical domains including: (i) an extension of the Ilangwe Greenstone Belt, (Domain 1) which is subdivided into; a lower volcanic succession, the Thathe Formation, comprising pillow and amygdaloidal volcanics; the adjoining Sabiza Formation, comprising pillow volcanics exposed in the southeast of the study area; the volcano-sedimentary Mtshwili Formation, which overlies the Thathe and Sabiza formations, consisting of quartz (sericite) schist, phyllite, metavolcanics and iron formation; the Nomangci Formation, which occurs as a region of highly deformed quartz-kyanite-sericite schists, and the Simbagwezi Formation, which comprises maroon to green phyllites and schists in the north of the study area. (ii) granitoids of the Impisi Granitoid Suite (Domain 2) which border the greenstone succession to the north, intruding the Nomangci and Simbagwezi formations. (iii) a southern complex of sheared granitoids termed the Umgabhi Granitoid Suite (Domain 3), which intrudes the Thathe, Sabiza and Mtshwili formations. (iv) The two remaining domains, comprise the Mesoproterozoic Mfongosi and Ntingwe Groups (Domain 4) and Mesoarchaean volcano-sedimentary sequences of the Pongola Supergroup (Domain 5). [ABSTRACT FROM AUTHOR]

This study presents new field and petrological observations combined with geochemical data on a range of komatiitic to tholeiitic volcanic rocks from the ca. 3.48 Ga mid-lower Komati Formation type-section of the Barberton Greenstone Belt, South Africa. A range of mafic-ultramafic rocks is identified across a 1.44 km profile, leading to the proposition of a new preliminary volcanic architecture for the mid-lower Komati Formation type-section. Major, trace and rare earth element (REE) data in conjunction with Lu-Hf isotopic constraints indicate that the tholeiites, newly recognized highmagnesium basalts, basaltic komatiites and komatiites in the volcanic sequence have a primitive mantle signature with no geochemical affinity to Archaean or modern-day supra-subduction zone boninites. The whole rock initial εHf values of spinifex and massive komatiite flows in the lowermost part of the Komati type-section are negative, ranging between -1.9 and -3.1, whereas the second overlying spinifex and massive flow unit records positive initial εHf values between +0.5 and +4.7. A new geodynamic model involving crustal contamination of the mafic-ultramafic lavas is proposed for the Barberton mid-lower Komati Formation type-section, involving mantle plume-crust interaction. The new observations and data indicate that the komatiites erupted as a result of a mantle plume from a hot (>1 600oC) mid-Archaean mantle, in which the earliest volcanic flows were variably affected by crustal contamination during their ascent and eruption. The possibility of incorporation of lower crustal material and/or recycled crust residing in the mantle source region cannot be excluded. This indicates that modern-style plate tectonic processes, such as subduction, may not have been a requirement for the formation of the 3.48 Ga Barberton komatiite suite, with implications for the hydration state, geodynamic processes and secular thermal evolution of the Archaean mantle. [ABSTRACT FROM AUTHOR]

The ~3.22 Ga Moodies Group, Barberton Greenstone Belt (BGB), South Africa, provides a unique window into Archaean sedimentary, magmatic and ecological processes. In the central BGB, a regional mafic complex, consisting of a genetically related major mafic sill, a peperitic dyke stockwork, and extensive basaltic lava flows affected thick quartzose sandstones of the Moodies Group. We argue that epithermal hydrothermalism associated with this magmatic event occurred, at least in part, syndepositionally and in places destroyed, in other places preserved the abundant benthic microbial mats in terrestrial- and coastal-facies sandstone of this unit. We differentiate four principal types of hydrothermal alteration: (1) Sericitization resulted from ubiquitous feldspar breakdown; (2) iron-oxide alteration replaced the original matrix by fine-grained iron oxide; (3) silicification replaced matrix and most nonsilica grains by microcrystalline silica and locally preserved kerogenous microbial mats; and (4) hydraulic fracturing at shallow depth brecciated consolidated Moodies Group sandstone and created closely spaced, randomly oriented fractures and quartz-filled veins. Because stockwork intrusion locally interacted with unconsolidated water-saturated sediment and because the dykes connect the sill with the mafic lava but also follow zones of structural weakness, we suggest that hydrothermalism associated with this magmatic event occurred syndepositionally but was also - within the resolution of radiometric age data - contemporaneous with tight regional folding. We conclude that microbial organisms in Paleoarchaean coastal (tidal, estuarine) environments may have been formerly widespread, possibly even abundant, but are nearly nowhere preserved because they were easily degradable. Preservation of Early Archaean microbial mats in a thermal aureole in the central BGB was controlled by the "just right" degree of heating and very early hydrothermal silicification. [ABSTRACT FROM AUTHOR]

The Stolzburg domain to the south of the Barberton Greenstone Belt preserves evidence for a 3.23 Ga subduction- collision tectonic event. Garnet amphibolite greenstone remnants have previously yielded conventional thermobarometric P-T estimates of 12 to 15 kbar at 600 to 650°C, 8 to 11 kbar at 650 to 700°C and 7.5 to 8.5 kbar at 560 to 640°C from, respectively, the Inyoni shear zone along the western margin of the Stolzburg domain, the central part of the domain and from the Tjakastad schist belt on the boundary with the main body of the Barberton Greenstone Belt. Pseudosection calculations constrain the stability conditions of the peak metamorphic assemblages at the three localities to be 10 kbar at 675 to 690°C, ~10 kbar at 700°C and ~7 and 10 kbar at 660°C respectively. Although it is possible that the peak metamorphic assemblages may be displaced to somewhat lower conditions if Mn is considered in the calculations, these estimates are generally in good agreement with existing estimates, and confirm that the Stolzburg domain exposes an intact mid- to lower-crustal section that was metamorphosed in a relatively cool environment at 3.23 Ga. Our results do not support previously documented higher-pressure conditions, and we contend that the mineral assemblages used to derive these estimates can equally reflect the conditions determined here. The presence of albite-epidote inclusion assemblages in garnet indicates that the likely prograde path involved a component of heating at depth, which is typical of subduction-collision environments and markedly different from the heating-burial paths expected for sinking greenstones in a vertical tectonic model. [ABSTRACT FROM AUTHOR]

Interpretations of the structural/tectonic evolution of the Barberton Greenstone Belt (BGB) and its surrounding granitoid rocks remain controversial, with proponents for both horizontal thrust-accretion (plate tectonic) and partial convective overturn (vertical tectonic) models. Here, an area of complex folds that was used to support the operation of plate tectonic-derived gliding and overthrust nappe tectonics is re-investigated in detail and placed within the broader structural development of the BGB and surrounding granitoid domains via a re-analysis of structures, and geochronological, stratigraphic and metamorphic data across the whole of this important geological terrain. The results of detailed field mapping show that the complex folds, which occur on the northern limb of the 20 km wavelength, vertically plunging, Onverwacht Anticline, do not represent a re-folded, originally recumbent, isoclinal fold, as previously interpreted. Instead, the folds represent a moderately shallow east-plunging fold train that formed from a single episode of deformation. Fold asymmetry is consistent with formation during originally north-side-up reverse shear on bounding faults, consistent with the offset direction required to explain the faultrepeated slices of Mendon Formation + Fig Tree Group rocks that uniquely occur across the northern limb of the Onverwacht Anticline. More broadly, a review of the BGB and surrounding granitoid rocks show that formation was likely through two discrete, ~120 Ma long, episodes of mantle upwelling, or plume, magmatism, each of which led to crustal melting and partial convective overturn (PCO), a tectonic mechanism that arises from the gravity-driven interaction between dense, upper crustal greenstones and partially melted, more buoyant, granitoid-dominated middle crust. The first mantle upwelling episode, at 3 530 to 3 410 Ma, commenced with long-lived eruption of ultramafic-mafic lavas of the Sandspruit, Theespruit, Komati, and lower Hooggenoeg formations (3 530 to 3 470 Ma). Heat from this magmatic event gave rise to partial melting of the crust that, combined with fractionation of mafic magma chambers produced widespread felsic magmatism at 3 470 to 3 410 Ma (upper Hooggenoeg Formation and Buck Reef Chert), the latter parts of which were accompanied by the formation of D1 dome-and-keel structures via PCO in deeperlevels of the crust represented by the Stolzburg Domain in the far southwest part of the belt. The second mantle upwelling, or plume, episode commenced at 3 334 to 3 215 Ma with the eruption of ultramaficmafic lavas of the Kromberg, Mendon and Weltevreden formations. Heat from this magmatic event gave rise to renewed partial melting of the crust that, combined with fractionation of mafic magma chambers, produced widespread felsic magmatism at 3 290 to 3 215 Ma. A second, longer-lived and more complex, multi-stage episode of PCO (D2-D4) accompanied deposition of the Fig Tree and Moodies groups from 3 250 to 3 215 Ma. Late D5 deformation accompanied emplacement of the Mpulizi and Piggs Peak batholiths at ca. 3.01 Ga, as previously identified. The Inyoka and Kromberg faults, which separate domains with distinct structural styles, represent neither terrane boundaries nor suture zones, but rather axial faults that separate deformed but generally inward-facing greenstone panels that sank inwards off rising granitoid domains that surround the BGB. [ABSTRACT FROM AUTHOR]

The Kaapvaal and Zimbabwe cratons host some of the earliest evidence for life. When compared to the contemporaneous East Pilbara craton, cherts and other metasedimentary horizons in southern Africa preserve traces of life with far greater morphological and geochemical fidelity. In spite of this, most fossiliferous horizons of southern Africa have received relatively limited attention. This review summarises current knowledge regarding the nature of early life and its distribution with respect to environments and ecosystems in the Archaean (>2.5 Ga) of the region, correlating stratigraphic, sedimentological, geochemical and palaeontological understanding. There is abundant and compelling evidence for both anoxygenic photosynthetic and chemosynthetic biomes dominating Palaeoarchaean-Mesoarchaean strata dating back to around 3.5 Ga, and the prevalence of each is tied to palaeoenvironmental parameters deducible from the rock record. Well-developed, large stromatolites characteristic of younger Mesoarchaean-Neoarchaean sequences were probably constructed by oxygenic photosynthesisers. Isotopic evidence from the Belingwe greenstone belt and the Transvaal Supergroup indicates that both a full sulphur cycle and complex nitrogen cycling were in operation by the Mesoarchaean-Neoarchaean. The Archaean geological record of southern Africa is thus a rich repository of information regarding the co-evolving geosphere and biosphere in deep time. [ABSTRACT FROM AUTHOR]

The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates. This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism. [ABSTRACT FROM AUTHOR]

Layered ultramafic-mafic complexes are a common component of the stratigraphically uppermost part of the Onverwacht Group of the Barberton Greenstone Belt. Associated with the Mendon Formation in the south and the Weltevreden Formation in the north, they represent an assemblage of thick differentiated flows and shallow synvolcanic intrusions ranging in composition from dunite to gabbro. U-Pb zircon dating of gabbro from the Sawmill and the Mundt's Concession ultramafic complexes from the northern part of the Barberton Greenstone Belt yielded ages of 3 258 ± 8 Ma and 3 244 ± 11 Ma, respectively. The ultramafic complexes are thus regarded to have been emplaced during a magmatic flare-up in the final stage of Weltevreden Formation volcanism, post-dating ultramafic magmatism in the southern part of the belt by several millions of years and thus suggesting diachronous evolution of the Onverwacht Group in the Barberton Greenstone Belt. [ABSTRACT FROM AUTHOR]

The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly known. Here we report zircon and rutile in situ U-Pb ages and zircon Hf isotopic data from a variety of supracrustal rocks. The zircon cores from a metamafic amphibole-bearing gneiss from the DKGB give a protolith age of 3 441 ± 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3 211 ± 16 Ma. The 176Hf/177Hft values for all zircon domains give a tight cluster around 0.280596 ± 0.00006 (2 SD). U-Pb analyses of zircon for an amphibolite intercalated with thin calc-silicate layers from the WGB give a single crystallisation age of 3 230 ± 3 Ma, but the Hf isotope ratios of these zircon grains define two different populations. The first population yields 176Hf/177Hf~3.23 Ga = 0.28064 ± 0.00004, corresponding to eHf~3.23 Ga = 2.4 ± 1.9 (2SD) and Hf model ages between ca. 3.51 to 3.30 Ga. These are Hf isotope characteristics for zircons from a relatively juvenile source extracted from a depleted mantle source ca. 0.28 to 0.07 Ga prior zircon crystallisation. The second population yields 176Hf/177Hf~3.23 Ga = 0.28093 ± 0.00004 with eHf~3.23 Ga = 8.1 ± 1.3 (2SD). These Hf data combined with the 206Pb/207Pb ages lead to isotope ratios that lie above those of Depleted Mantle. The unusually high Hf isotope signature for the cores of the zircons from the WGB amphibolite most likely represent a contribution from an early highly depleted mantle source. A rutile in situ U-Pb age of 3.085 Ga from a recrystallised quartzite indicate that the rocks from the DKGB experienced slow cooling following the 3.21 Ga metamorphic event or (partial) resetting due to elevated geothermal gradient caused by the ca. 3.1 Ga intrusions of the Vrede Granitiod Suite. The latter interpretation is preferred because ~145 Ma of slow cooling from the amphibolite facies conditions of peak metamorphism to the blocking temperature for mass diffusion of Pb in rutile is unlikely. While the Zr-in-rutile temperature of ca. 710°C at 7 kbar for DKGB most likely records the peak temperature of the ~3.23 to 3.21 Ga event. The trace element concentrations of the metamorphic rutile grains within the quartzite of the DKGB indicate that the source rock was enriched in Cr. Either due to silification during hydrothermal alteration of the (ultra)mafic country rock or during deposition in an atmosphere that allowed for chromite grains to be part of the sediment. [ABSTRACT FROM AUTHOR]

The paper reviews published and unpublished geological data pertaining to the structural and metamorphic controls, rock types, characteristic features, source, and timing of hypozonal orogenic gold mineralization in the Giyani Goldfield. The Giyani Goldfield includes the NW domain of the >3.0 Ga Giyani greenstone belt (GGB) at the northern edge of the Kaapvaal Craton and the southern retrograde hydrated domain of the juxtaposed Southern Marginal Zone (SMZ) of the ca. 2.72 Ga Limpopo Complex (LC). Mineralization at all gold mines and gold prospects of the Giyani Goldfield is structurally controlled and closely associated with the Hout River shear zone (HRSZ) and associated smaller shear zones suggesting a specific tectonic setting. This tectonic setting is the direct consequence of thrusting the SMZ of the LC against and over the adjacent GGB at the position of the steeply north-dipping (south verging) HRSZ during the exhumation stage of the ca. 2.72 to 2.69 Ga Limpopo orogeny followed by regional retrograde hydration of the southern part of the SMZ at ca. 2.68 to 2.62 Ga. This tectonic setting offers an explanation for a deep-seated crustal source for gold and for the concentration of orogenic gold mineralization within specific structural features located within the Giyani Goldfield. This tectonic setting also explains the lithological, structural and metamorphic complexity, metasomatic alteration and post-peak metamorphic timing of gold mineralization. Finally, it provides important clues with regards to a crustal source for gold mineralizing fluids and the identification of new potential targets for gold exploration in the Giyani Goldfield. [ABSTRACT FROM AUTHOR]

Formations on the northwestern flank of the Barberton Greenstone Belt have hosted over 85% of all the gold recovered from the ca. 3550 to 3000 Ma Barberton Supergroup since early discoveries in 1872. This sector of the greenstone belt also happens to coincide with a complex tectonic architecture resulting from successive stages of folding and faulting superimposed onto a complex lithostratigraphy. Of particular importance has been the influence of two diapiric granitoid intrusions that caused added structural complexity following their emplacement ca. 3227 to 3250 Ma. Of these the larger Kaap Valley Pluton invaded the area north of present day Barberton town causing the separation of the greenstones into a northern arm (Jamestown Schist Belt) and a southern sector which remained attached to the main greenstone belt (Moodies Hills). The ballooning pluton produced vertical as well as horizontal flattening stresses, the latter reactivating earlier high-angle faults and resulting in subhorizontal strike-slip movements, particularly along the Barbrook Fault Zone, which acted as a right-lateral strike-slip fault. Formations north of this fault were buckled, following progressive deformation in the region known as the Sheba Hills, into major synclinal folds (Eureka and Ulundi Synclines) with folded axial planes that dip steeply to the south, southeast or east. The second granitoid intrusion (Stentor Pluton), which has been extensively modified by subsequent magmatic events, caused significant flattening of greenstone belt rocks in the northeastern part of the Barberton Greenstone Belt (Three Sisters region) as well as in other areas rimming the granitic body. Combined, the two plutons produced a wide range of interference and reactivated structures particularly affecting a triangular region extending from the Jamestown Schist Belt into the area occupied by the New Consort Gold Mine and areas to the east. This paper attempts to outline, in the simplest manner, the geological and structural evolution of the main gold-producing region of the Barberton Goldfield. The principal aim is therefore to highlight the structural influence of the diapiric plutonism and the manner in which the plutons contributed significantly to the horizontal reactivation of pre-existing regional faults, which in turn, resulted in the progressive deformation of a heterogeneous lithological terrane. [ABSTRACT FROM AUTHOR]

Documenting evidence of fossil microbial life on early Earth is made difficult by the paucity of suitable Archean sedimentary rocks, their common metamorphic overprint, the small outcrop areas, and the small size of the objects of interest. Although a large number of putative microfossils dating back as far as 3700 Ma have been described, the syngeneity and biogenicity of many occurrences is debated, and some of the proposed fossils have been found to be either contaminants or abiotic artefacts. The ~3200 Ma Moodies Group of the Barberton Greenstone Belt (BGB), South Africa, contains locally abundant and remarkably well-preserved microbial mats which show indirect evidence of photosynthetic activity. They also contain microstructures which strongly resemble remains of microbial cells. Detailed morphological and geochemical analyses, however, show that these structures mostly represent fragments of volcanic tephra. Our study demonstrates that opaque microstructures within microbial mats can potentially be misidentified as microfossils even when a strict protocol is followed. It also posits the question to which degree volcanic air-borne fertilization contributed to the remarkable growth rate, high mechanical tenacity and wide extent of these oldest tidal microbial mats in siliciclastic environments. [ABSTRACT FROM AUTHOR]

Early Palaeoarchaean granite clasts in the basal conglomerate of the Moodies Group, Barberton Greenstone Belt, South Africa, belong to the oldest true granites on Earth and document that intracrustal melting occurred early during generation and stabilization of continental crust. Zircons from granite-gneiss clast MD6 of the Moodies conglomerate analyzed on SHRIMP II yielded a magmatic crystallization age of 3564.5 ± 2.2 Ma and contained two older xenocrysts with 207Pb/ Pb206 ages of 3602 ± 3 and 3634 ± 3 Ma, respectively. These xenocryst ages are almost identical to zircon ages for the oldest tonalite-trondhjemite components of the Ancient Gneiss Complex (AGC) in Swaziland. Hf-in-zircon isotopic analyses on the dated zircons yielded predominantly negative εHf(t) values (-2.3 to +0.7), supporting an origin through melting of older felsic crust for the MD6 granite, as predicted by earlier melting experiments. We suggest that the oldest granite clasts in the Moodies conglomerate were derived from crustal-melt granites in the AGC or in the southern Barberton greenstone terrane that rose to high crustal levels and were eroded during an unroofing event that affected large areas of the southeastern Kaapvaal craton during a major, ca. 3.2 Ga tectono-metamorphic event. This is why the source rocks of these granite clasts can no longer be found. [ABSTRACT FROM AUTHOR]

Gold occurs in a variety of ancient geological environments in South Africa, the most well-known being the unique Witwatersrand Basin in the central part of the Kaapvaal Craton. Outside of the Witwatersrand Basin significant quantities of gold have been recovered from Archaean greenstone belts of the 3500 to 2700 Ma Kaapvaal Craton as well as from various occurrences in the 2600 to 2200 Ma Transvaal Supergroup. Very little gold has been found in younger formations. Archaean greenstone belts in South Africa are characterised by substantial sequences of ultramafic (komatiitic) volcanic rocks generally overlain by mafic volcanic successions which, in turn, are overlain in some cases by greywacke-shale sequences and younger arenaceous lithologies. Highly sheared and carbonate altered ultramafic zones, together with the immediately surrounding country rock, host some of the most important gold deposits on the Kaapvaal Craton. In the northern part of the Barberton Greenstone Belt, shearing tends to be most intense along the contact of the ultramafic schists of the Zwartkoppie Formation and the overlying greywackeshales, the contact being marked by the presence of a chert "bar" or banded iron formation (BIF) horizon. Gold mineralisation occurs below, above and/or within the more competent and fractured chert/BIF marker horizon. The locus of gold mineralisation is typically controlled by the rheological characteristics of the wallrocks in the proximity of the shear systems, e.g. the ductile greywacke-shale strata are tightly folded with sub-parallel mineralised shear zones, while competent arenaceous rocks have yielded in a brittle manner resulting in high-angle mineralised fractures. Extensive carbonate alteration in ultramafic rocks is often present along, and in the vicinity of, shear zones where potassium alteration results in a vivid green coloration due to the formation of fuchsite (Cr-mica) and where zones of silicification and quartz veining are closely associated with gold, arsenic and antimony mineralisation. Where mafic rocks act as more competent and fractured hosts to mineralisation, carbonate alteration results in the breakdown of ferromagnesian minerals and the liberation of considerable quantities of silica, manifested as silicification and as mineralised quartz veins. The breakdown of mafic minerals results in "bleaching" of the rocks as carbonate minerals replace ferromagnesian minerals, but may also result in the development of green, brown or black schists depending on the composition of the host rocks. One or more of the above alteration rock types and mineralisation styles is present in virtually all of the greenstone-hosted gold deposits of the Kaapvaal Craton. Wherever available, geochemical data is consistent with alteration fluids of relatively uniform composition but the style of the resultant mineralisation is ultimately dependent on the nature of the host rock.Greenstone gold deposits are irregularly distributed, both in terms of their size and distribution. Only about 1% of greenstone deposits can be regarded as successful mines, while the vast majority remain as showings. Geographically, these gold deposits are preferentially located along litho-structural features where both fluid flow and wallrock conditions were suitable for mineralisation. These criteria are restricted to major structural nodes, significant lineaments, or lithological units, with the result that the stronger mineralising systems tend to be concentrated in finite gold "camps" which host the larger deposits and numerous styles of mineralisation, e.g., the Sheba-Fairview Complex on the refolded arcuate Eureka and Ulundi synclines. Gold hosted in BIFs is a widespread association, but rarely economic except where the nature of the mineralisation allows for economic extraction, e.g., in the Kraaipan Greenstone Belt, where the volume of BIF is large enough to contain the mineralised fracture system. Gold in the Paleoproterozoic Transvaal Supergroup occurs in the lowermost Black Reef Formation quartzites as detrital accumulations in conglomerate channels, but more abundantly as epithermal veins in the dolomite of the overlying Malmani Subgroup and shale of the Pretoria Group as vertical cross-cutting veins and sub-horizontal reefs emplaced along parting planes. The quartz-carbonate-sulphide veins differ from Archaean ores in that gold rarely impregnates the wallrocks; they contain significant carbon which impedes gold recovery, and they contain silver, copper, arsenic, bismuth, lead and zinc in addition to gold. [ABSTRACT FROM AUTHOR]

The Barberton Greenstone Belt (BGB) in South Africa is one of the few well-preserved, albeit deformed and complex volcano-sedimentary remnants from the Paleoarchean, and thus an excellent locality to study the formation and evolution of the early Earth's crust. Due to the significant amounts of resources, especially gold in shear zones, the BGB has been extensively studied by geologists for almost 100 years. While the surface geology is well known, only a few geophysical studies have been conducted to investigate the deeper architecture of the BGB and its granitoid surroundings. Here we describe the results of a Magnetotelluric (MT) survey that was conducted over two field seasons to image the subsurface electrical conductivity distribution of geological units of the southern BGB, and to locate dykes, faults and shear zones that are imprints of subsequent tectonic processes. Specifically, mineralization along the shear zones is predicted to reveal high electrical conductivities, in contrast with highly resistive adjacent mafic to ultramafic rocks. The MT station layout of our survey was planned to allow for 2D and 3D interpretation, although it was expected that the 2D inversion models might not be adequate to reveal the expected complex subsurface geology of the BGB and its surrounding region. However, both 2D and 3D inversion results show electrically conductive structures that appear to correlate well with surface traces of known fault zones such as the Inyoka-Saddleback fault system. High resolution 2D conductivity images along selected profiles suggest that some faults might continue further south into the granitoids of the Mpuluzi batholith, implying that the batholith was emplaced along faults (Inyoka-Saddleback fault system and/or Komati Fault), or that a younger fault cuts across the pre-existing batholith. This is contrasted by 3D models that reveal deep-reaching (>10 km) resistive structures beneath the intrusive bodies within the BGB and surrounding batholiths. These results suggest that the granitoids are not disrupted by shear zones, and may imply that episodes of predominant magmatic emplacement have affected the BGB in large parts. A network of conductive faults, especially in the central part of the BGB, suggests that tectonicprocesses along shear planes have also shaped the BGB, and may have provided pathways for fluids creating zones of gold mineralization. [ABSTRACT FROM AUTHOR]

The geology, structure and subsurface mass distribution in the Fort Rixon-Shangani (FRSh) granite-greenstone terrain, Zimbabwe craton, has been investigated using regional aeromagnetic and gravity data, as well as density-constrained gravity modelling along four traverses/profiles. Processed aeromagnetic data reveal sets of north-northwest-trending dykes and faults, west-northwest-trending faults and a major north-northeast-trending shear zone bounding the FRSh greenstone belt in the southeast. The distribution of dykes is more widespread and continuous than previously recognised, while concealed mafic and ultramafic horizons or extensions are revealed. Cross-cutting relationships and magnetic anomaly displacements provide relative age constraints for the various dykes and associated faults in the area, with the north-northwest-trending dyke set being the youngest and the north-northeast-trending set the oldest. There has been repeated movement along the faults over time. The regional Bouguer gravity anomaly map shows that the Fort Rixon (FR) greenstone belt is marked by a 30 mGal positive anomaly,,elative to a background of -120 mGal over the surrounding tonalitic gneisses. Both the Bouguer anomaly and first vertical gravity gradient maps suggest a smaller body than the current known outcrop, and this is confirmed by gravity modelling which shows a reduced extent of the greenstone belt lithologies with depth. The maximum anomaly appears to be significantly offset to the southwest, suggesting continuation of greenstone belt lithologies beyond surface exposures in to the southwest. The porphyritic Nalatale granite intrudes and bisects the FRSh greenstone belt, and is associated with a gravity low (~7 mGal amplitude), which appreciably reduces the FR greenstone belt positive anomaly in the northeast. The regional setting suggests a western granite-greenstone contact that may be explained by doming, an eastern contact that is marked by the Irisvale-Lancaster strike-slip shear zone, and a northern contact with an intrusive and post-tectonic granite. Interpretation of the gravity data by 2 1/2D modelling along three profiles shows that the FR greenstone belt has a boat or v shape and a depth extent of 3.0 to 4.0 km. The thinnest part of the greenstone belt occurs in the northeast near the Nalatale granite, suggesting that the pluton underlies much of the greenstone belt in the area. The Nalatale pluton is on average 2.5 km thick under the greenstone belt but includes a root up to 4.5 km thick, and a steep contact with the tonalitic gneisses to the east. It is probably a wedge-shaped pluton that was emplaced along steeply oriented fractures in the brittle crust. The models show that the Nalatale pluton has a shallow outward dip with steep contacts at depth. These results, together with the surface and subsurface disposition of the various geological units, suggest that the greenstone belt underwent deformation by granite intrusion through diapiric-type geotectonic processes. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic Mako Belt in eastern Senegal is characterised by gold-bearing quartz veins. These are superimposed on the regional Birimian structural complex consisting of volcano-sedimentary and intrusive rocks in an Early Proterozoic greenstone belt. In the Mako Belt the multi-stage deformed quartz veins and hydrothermally altered protoliths are characterised by distinctive microstructures and CL properties. The high-grade Au-bearing quartz veins contain one or more generations of hydrothermal quartz. Brecciation of the veins is indicated by cemented fragments of zoned quartz. Gold in the mineralised zones occurs either as inclusions in pyrite or as native gold. Mineralisation typically occurs in vein systems which are brecciated, laminated, and/or show crack-seal textures. Microstructures indicate syn- to late-tectonic mineralisation. Au-mineralised veins contain quartz, carbonate, muscovite, fuchsite, tourmaline, and chlorite. Fluid inclusion studies reveal early, highly saline aqueous inclusions followed by main-stage aqueous-carbonic inclusions of low salinity and finally aqueous low-salinity inclusions. Petrographic observation and microthermometric data suggest that fluid inclusions in main stage quartz were trapped after phase separation from a heterogeneous H2O-CO2 fluid. Gold deposition occurs over a temperature range of 220 to 320°C and pressures of 1.4 to 2.75 kbar (~5 to 10 km depth). The late-stage quartz carbonate vein corresponds to pressures of 0.75 to 1.25 kbar and depths of 3.0 to 4.5 km. A fluid pressure drop due to fracture failure is likely to have triggered Au precipitation. The Mako Belt Au deposits are comparable to a large number of orogenic lode gold deposits in the West African Craton. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses articles in the issue on topics including Greenstone belts, Granites, and Archaean geodynamics.

U-Pb geochronological data from the northern and western extensions of the Oudatan-Gorouol volcano-sedimentary belt (OGB) on 3 samples of granodiorite gneiss (sample BF 1144), migmatitic gneiss (sample BF_1299), and granite clast from a metaconglomerate (sample BF_1147) gave the oldest U-Pb crystallisation ages for outcropping Palaeoproterozoic rocks yet identified in the West African Craton (WAC). The U-Pb concordant crystallisation ages of 2253 ± 9 Ma and 2253 ± 15 Ma for samples BF_1144 and BF_1299, respectively likely represent the age of crystallisation of parent granodiorite gneiss and may constrain the age of a gneissic protolith on which the OGB basin was deposited, or an early group of granitoids that was emplaced in the hinterland during formation of the OGB basin. A concordant age of 2255 ± 26 Ma for sample BF 1147 is interpreted as the age of crystallisation of a granite pluton that was eroded from a site nearby before becoming part of the Dembam Member, and constrains the maximum age of the Birimian Supergroup in the OGB. The older Pb-Pb ages on zircons from the granodiorite gneiss, migmatitic gneiss and granite clast are in the range ~2.34 to ~2.27 Ga, and are interpreted as inherited from pre-existing zircon cores. They are similar to Pb-Pb ages reported as inherited zircon ages from the Dabakala tonalitic gneiss (in Cote d'Ivoire). [ABSTRACT FROM AUTHOR]

The Mesoarchaean Pongola Supergroup is an exceptionally well-preserved succession of volcanic and sedimentary rocks that extends for 270 km close to the eastern margin of the Kaapvaal Craton in South Africa and Swaziland. It represents a volcano- sedimentary continental or epicontinental deposit that is one of the most extensive coherent Archaean terrains in the world, and the oldest of this extent. Its characteristics are unique amongst supracrustal terrains of this age and are unlike those of other Archaean greenstone belts. Its unique nature is that it marks the transition in southern African crustal development from preceding early greenstone belts, such as observed in the Barberton and Nondweni greenstone belts, to late Archaean basin formation on stable continental crest. The volcano-sedimentary succession of the Pongola Supergroup is divided into the lower, predominantly volcanic, Nsuze Group, which contains subsidiary but nonetheless important sedimentary formations, and the upper, dominantly sedimentary, Mozaan Group, which also contains several volcanic successions of highly variable forms and thicknesses. A basement high separates the Pongola Supergroup into a Northern Domain, which is relatively undeformed, and the Southern Domain, which is strongly deformed and folded. The focus of this paper is on the occurrence, field relations (with detailed maps), description and correlation of the volcanic rocks in both the Nsuze and Mozaan Groups in all areas of the Pongola Supergroup, focusing on 11 key areas in South Africa and Swaziland. The compositional range from basalt to rhyolite is remarkable, and this paper focuses on the distribution of different magma types and styles of eruption throughout the belt, including the first record of ultramafic compositions in the Pongola Supergroup. Ultramafic pyroclastic rocks in the Southern Domain contain highly vesicular fragments that may explain the absence of lava flows of the same composition. Volcanic styles transgress the entire length of the belt, with subaerial sheet flows and minor pillow development in the southern Nkandla region through to a possible continental marginal setting with both subaqueous and subaerial volcanism in the south of the Northern Domain and, finally, into a bimodal suite of felsic and mafic to intermediate volcanic rocks in the northernmost regions. Felsic ignimbrite volcanic rocks with abundant lithophysae are present in the central region. A 770 m-thick section of volcanic rocks that includes andesite and dacite compositions as well as fragmental and volcaniclastic units, including an ultramafic volcaniclastic deposit, is observed in a borehole section of the Tobolsk Formation of the Mozaan Group, that is completely overlain by Karoo sediments in the east. Cyclical episodes of volcanism, many of which were of an explosive nature, were interspersed with periods of rapidly deposited coarse-grained sediments in alluvial fans and braided rivers close to the continental margin. Carbonate formation, together with the development of stromatolites, took place during periods of volcanic quiescence, or reduced volcanic activity, in highly protected surrounds and in intertidal zones. Synchronous volcanism, sedimentation and deep erosion gave rise to the marked lateral variations in both subaerial and subaqueous deposits and marked lithologic variability. The largely volcaniclastic (with a high proportion of ultramafic component) Ndikwe Formation in the Southern Domain is enigmatic with contrasting suggested settings of an early phase of Pongola formation, or a later stage nappe structure, with a possible Mozaan Group time equivalence. No single tectonic setting, on modern day analogues, can explain all features of the Pongola volcano-sedimentary belt. Current evidence for the Nsuze Group is most consistent with a continental arc migrating into a continental rifted environment to the north. The overlying Mozaan Group is inferred to have formed in a cratonic basin following thermal relaxation after the preceding cycles of voluminous volcanism. Dedication: This paper is dedicated to the late Professor Don Hunter and Professor Peter Matthews of the University of Natal (now the University of KwaZulu-Natal) who provided the inspiration and insight to unravel the complexities of the Pongola rocks over a period of more than 30 years. [ABSTRACT FROM AUTHOR]

Komatiites occur in many Archaean and Proterozoic greenstone belts metamorphosed to greenschist or upper anaphibolite facies conditions. However, komatiites have so far not been reported from high-grade metamorphic environments (upper granulite facies conditions). Here we report the occurrence of Mg-, Cr- and Ni-rich ultramafic rocks (26 to 31% MgO, 2800 to 3800 ppm Cr, 1400 to 1800 ppm Ni) with unfractionated PGE patterns (8.15 to 12.36 ppb Pt, 6.17 to 13.49 ppb Pd, Pd/Ir ~2 to 6) in the high- grade polymetamorphic Central Zone of the Limpopo Belt, South Africa. The composition of the rocks overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, except that the Central Zone samples are markedly enriched in highly incompatible trace elements and show negative Nb anomalies. Based on these data, we interpret the rocks to represent contaminated komatiites of Archaean age - the first such manifestation in the Limpopo Belt and in a high to ultra-high grade (≥900°C, 10 kbar) metamorphic environment. [ABSTRACT FROM AUTHOR]

Pillow basalts from the Witkop Formation of the Nondweni granitoid-greenstone terrain preserve evidence of Palaeoarchaean sea-floor alteration and high-grade metamorphism. Post-depositional sea-floor alteration preferentially affected volcanic glass and led to a marked contrast in compositions and mineral assemblages of the cores and rims of the pillows. The pillow rims have elevated Al2O3, TiO2, Fe2O3, Zr, Nb, Nd, Th concentrations and lower SiO2, Na2O, Sr contents, relative to pillow cores. Igneous clinopyroxene and plagioclase is preserved exclusively in pillow cores, whereas the pillow rims consist of a metamorphic mineral assemblage of garnet-hornblende-plagioclase-chlorite-quartz. The whole rock δ18O values as high as 9-6‰ suggest that hydrothermal alteration of the pillow basalts initially occurred under low-temperature conditions. δ18O values as low as 2.8%o may indicate water/rock interaction at a later stage under high-temperature conditions during metamorphism. Following sea-floor alteration, the pillow basalts were deformed and metamorphosed under amphibolite facies conditions that led to formation of hornblende-defined foliation and syntectonic garnet growth in pillow rims. Garnet-hornblende-plagioclase-quartz geothermo-barometry from pillow rims reveal peak metamorphic conditions of -6.5 kbar, 600 to 650°C. P-T pseudosection analysis also confirms that contrasting mineral assemblages in pillow cores and rims developed during metamorphism principally due to the difference in major element concentrations in these domains. P-T pseudosection analysis also predicts an isobaric cooling of the basalts along an anticlockwise P-T path. The P-T estimates indicate burial to a depth of ~17 to 20 km post-tectonic to hydrothermal alterations. The occurrence of high-grade mineral assemblages in the basalts was observed only in the vicinity of the intrusive Mvunyana granodiorite. The presence of migmatitic gneiss with steeply dipping foliations at their contact indicate that metamorphism of basalts was concomitant with intrusion of the granodiorite. Hence the time-frame of metamorphism of the Witkop Formation pillow basalts can be constrained from the crystallization age of the Mvunyana granodiorite, which is ~3.29 Ga. These observations along with the high geothermal gradient (30°C/km) recorded during peak metamorphism, and the post-peak near-isobaric cooling along an anticlockwise P-T path, are consistent with a magmatic arc setting. [ABSTRACT FROM AUTHOR]

This paper discusses the deformation pattern of the Murchison Greenstone Belt, which is located in the northeastern Kaapvaal Craton, South Africa. The belt strikes northeast to southwest, is surrounded by gneisses and granitoids, and contains Meso-Archaean metavolcanics and metasediments. It is narrow, strongly deformed, and deeply rooted in the centre with bulk keel geometry. Regional fabrics consist of a sub-vertical foliation at a low angle to the belt strike and bears steeply plunging lineations. From available data and our observations, we constructed a map of the foliation trajectories. The map reveals the following: the deformations are distributed, both the belt and the northern bounding gneisses are deformed together in a ductile way, and the plutons were emplaced during regional deformation. The overall structural pattern, together with the existence of contemporaneous intrusive and extrusive rocks outcropping at the surface, supports a vertical tectonic model, with burial of the upper crustal rocks within the underlying weak material during horizontal crustal shortening. The occurrence of leucogranite intrusions is consistent with the melting of buried sediments belonging to the belt at depth. Finally, ages available on syn-kinematk granitoids (2.97 to 2.77 Ga) suggest a long-lasting tectonic process. [ABSTRACT FROM AUTHOR]

The structure and subsurface configuration of the Masvingo Greenstone Belt (MGB) in the Zimbabwe Craton (ZC), were investigated using aeromagnetic and gravity data. Gravity data from 158 new and 202 old gravity stations were combined to produce a Bouguer gravity map covering the greenstone belt and surrounding granites. The greenstone belt and ultramafic complexes are characterized by Bouguer gravity highs, whereas the granitic terrain are lows, with the lowest values over the post-volcanic plutons. Gravity modeling, constrained by surface geology and density measurements, was carried out on a profile crossing the greenstone belt. The modeling results indicate a depth extent of around 6 km for the dense greenstone belt with a thickness of about 8.5 km for the adjacent younger granite plutons, which are significantly less dense. Granite-greenstone contacts along the profile are near-vertical. Magnetic maps compiled fro the Zimbabwe Geological Survey national aeromagnetic data set show the bulk of the greenstone belt to have low magnetic anomaly values, but with high magnetic anomaly values along its northern and western margins over ultramafic units and banded iron formations. The younger granites generally have higher magnetic anomaly values compared to both the greenstone belt and the older gneisses. In particular, a pronounced east-northeastern-trending broad magnetic high is observed over the Charumbira pluton, and its edges mark the boundaries with the greenstone belt to the north, and the Limpopo belt to the south; with the latter characterized by low magnetic values. Several linear, small amplitude magnetic lows, which are mostly due to faults, trend northwest to southeast and north to south, with some revealing an apparent dextral movement. Derivative maps for both gravity and magnetic data bring out some of the linear features and the edges of the greenstone belt and granite plutons. Shallow and deep depth slices of the magnetic field allow separation of high and low frequency anomalies; such as the east-northeast-trending Limpopo dyke swarm of Karoo age. The information obtained from modeling, derivative maps and depth slicing suggest a trough-shaped configuration for the MGB, which is consistent with other known greenstone belts on the ZC and elsewhere. This information is inconsistent with an intracratonic, extensional rift environment for emplacement of the greenstone belt. [ABSTRACT FROM AUTHOR]

The Madibe greenstone belt is the easternmost belt of the northern Kraaipan terrane, located in the Kimberley Block of the Kaapvaal Craton. It comprises a mature Middle to Late Archaean volcanic island arc succession that was welded onto the evolving western margin of the Kaapvaal Shield. The lithologies strike in a north-south direction and represent an assemblage of metamorphosed and strongly deformed volcanic and minor sedimentary rocks, now represented by phyltites, quartz-sericite schists, quartz-chlorite schists, amphibolites, talc-carbonate schists and handed iron formations. We report a SHRIMP U-Pb age of 3082.5 ± 5.9 Ma for an intermediate metavolcanic rock. Ages of zircon grains within a volcano-sedimentary rock define ages around 3.43, -3.18 and, -3.09 Ga and are as young as -3.04 Ga, suggesting the presence of older, mature continental crust. Age relationships of rocks from the Madibe greenstone belt and the Murchison greenstone belt suggest that the Kaapvaal Shield apparently experienced an episode of rapid crustal growth at around 3090 Ma along both the northern and western margins. [ABSTRACT FROM AUTHOR]

Andalusite is found in kyanite-bearing siliceous rocks belonging to the Nondweni Group in the Ilangwe Greenstone Belt, along the southeastern margin of the Kaapvaal Craton. Textural relationships of the minerals indicate that kyanite growth post-dated and replaced pre-existing andalusite. The kyanite overgrowth is interpreted to result from tectonic loading associated with the tectonic emplacement of the Mesoproterozoic Natal Belt lithologies onto the Kaapvaal Craton. The andalusite probably grew in a thermal aureole during the emplacement of Archaean granitoids. [ABSTRACT FROM AUTHOR]

Evidence for the existence of Neoarchaean greenstone rocks in the western part of the Kaapvaal Craton is provided by ID-TIMS and SHRIMP U-Pb age data on zircons from supracrustal rocks of the Amalia greenstone belt. Although all the units described from the Amalia greenstone belt might not be coeval, an age of 2750.1 ± 4.6 Ma found for two samples appears to be the best estimate for the deposition of this part of the Amalia greenstone belt sequence. These samples also contain younger zircons, the majority of them yielding a 1099 ± 32 Ma upper intercept age. This date of ∼ 1.1 Ga might indicate that these rocks have been affected by the crustal thickening and magmatism event coeval with the peak of metamorphism during the Namaqua orogeny ∼1.1 Ga. These data provide the first evidence for the development of a Neoarchaean greenstone sequence in the western part of the Kaapvaal Craton. They also demonstrate that the Namaqua orogeny affected rocks within the craton itself. [ABSTRACT FROM AUTHOR]

Quartzites from the Patsoana area, part of the Nondweni greenstone belt, South Africa, comprise coarse-grained (1) banded graphite-bearing black and white quartzite, (2) greenish to beige finely-banded quartzite and (3) massive sulphide-bearing quartzite. The quartzites show distinctive Ti and Al-contents (LA-ICPMS analysis) that are highest for the finely-banded quartzite varieties with intense blue cathodoluminescence (CL) and lowest structural water as measured by FT-IR analysis. The quartzites underwent contact metamorphism, but stable oxygen isotope ratios (16.7‰ to 19.2‰) suggest formation at low temperatures as chert. Complex quartz zoning in CL combined with trace element analysis points at a 2-step evolution after primary formation with (1) prograde recrystallisation of a finer-grained siliceous lithotype accompanied by extensive hydrothermal alteration resulting in high Ti-contents, and (2) retrograde reduction of trace elements by diffusion and by the final formation of pure secondary quartz. Secondary quartz veinlets formed at lower temperatures. The study testifies the local remobilisation of titanium in solution as well as diffusion in quartz at high temperatures. Laser Raman analysis of dispersed graphite shows high crystallinity in most of the quartzites and points at formation temperatures of >600°C; poorly organised graphite was mainly found in the finely-banded and greenish quartzite and formed during retrogression. Very high concentrations of paramagnetic [TiO4/Li+] and [TiO4/H+] defect centres were found in the finely-banded quartzite, whereas paramagnetic [AlO4] centres are assumed to be largely decayed with time. The perseverance of the Ti-related paramagnetic defects are explained by a long-term position of the Nondweni greenstones in a stable part of the crust. [ABSTRACT FROM AUTHOR]

The Makoppa Dome, located in the far northwestern part of the Kaapvaal Craton, constitutes one of the least well-exposed Archaean granite-greenstone terranes in southern Africa. For this reason it has received very little attention in the past and its characteristics have largely gone undocumented. This study presents new field, petrological, geochemical and single zircon dating results for a suite of granitoid rocks that comprises foliated trondhjemitic gneisses and homogeneous monzogranitic and granodioritic phases, all of which post-date remnants of Archaean greenstone belts metamorphosed to amphibolite facies. Three separate granitoid types are recognized on the basis of their distinctive field as well as major, trace and rare-earth element characteristics. Confirmation of the three granite types follows from new ID-TIMS and LAM-ICP-MS dating of single zircons extracted from samples collected on the dome. The oldest granitoid rocks so far recognized are represented by the Vaalpenskraal trondhjemitic gneisses, two samples of which yielded ages that are identical within error and are dated at 3013±11 Ma and 3034±64 Ma, respectively. A single sample of homogeneous granitoid, referred to as Makoppa monzogranite, yielded an age of 2886+3/-2 Ma, while a further two samples of homogeneous granodiorite, referred to as Rooibokvlei granodiorite, are representative of the youngest granitic event recorded on the dome and yielded ages of 2777±35 Ma and 2797±2 Ma, respectively. The granitoid rocks of the Makoppa Dome have many features in common with those of the Kraaipan-Amalia terrane on the western side of the Kaapvaal Craton, as well as those of the Murchison-Giyani-Pietersburg terrane on the northern side, supporting suggestions that these rocks developed in, or adjacent to, a crescent-shaped magmatic arc that began forming at ∼3.1 to ∼3.0 Ga around the northern, northwestern and western rim of the Kaapvaal Craton. Continent-arc collision followed in the period up to ∼2.8 Ga, the time of emplacement of the Rooibokvlei granitoids on the Makoppa Dome and the 2791±8 Ma Mosita granitoids in the Kraaipan terrane. These ages closely approximate the age of the ∼2.79 Ga Gaborone Granite Complex in neighbouring Botswana and may represent the proto-magmatic stage leading to the development, on thickened crust, of the A-type Gaborone granite suite and Kanye volcanic formation, followed, in turn, by the rift-related ∼2.71 Ga volcanism of the Ventersdorp Supergroup. [ABSTRACT FROM AUTHOR]

Eight deep-seismic reflection profiles (16 seconds TWT; generated by the vibroseis method) are presented and interpreted. The profiles cover a total length of 500km and are projected into a common section that strikes northwest to southeast across the central part of the Kaapvaal Craton, including the Colesburg Magnetic Lineament (CML). The profiles clearly image the crust-mantle transition (Moho) between 11 and 13 seconds TWT (∼ 36 to 41km). The sections link some of the brittle Ventersdorp (∼2.7 Ga) faults near surface to more subhorizontal ductile shear zones and related features that transect the upper crust of the Kaapvaal Craton. Sharp shallow-dipping reflectors that predate the Ventersdorp faults subdivide the entire crust into four elongate panels (or slivers; >200km by 10 to 20km). These panels extend from below Vryburg in the west, across the CML, to the crystalline basement below the Witwatersrand Basin in the east. The upper-most crustal reflectors dip towards the west and indicate extensive east-directed tectonic accretion and emplacement of a ∼10km-thick allochthonous upper crustal panel comprising the Amalia-Kraaipan granite-greenstone terrane, across a Witwatersrand basement panel. The latter, in turn, is underlain by an elongate crustal panel that dips west from the surface near Welkom, reaching the lower crust below the centre of the section. The CML that subdivides the craton into an eastern block and a western block at surface is not a feature that can be easily linked to an underlying suture in the lower crust and mantle. Instead, this lineament has a shallow origin related either (1) to the 3.0 to 2.8 Ga Amalia-Kraaipan greenstone belt lithologies (that include extensive BIF. and/or (2) to possible thin back-thrust wedges of highly magnetic shales of the lower Witwatersrand sequences with (as previously suggested by Corner et al., 1990) shallow east-dipping magnetite-rich basement granites. The entire crustal section resembl... [ABSTRACT FROM AUTHOR]

Presents a study that investigated the nature and timing of metamorphism in aluminous metasediment rocks in order to contribute to the understanding of the nature of the tectonic process that shaped the Barberton greenstone belt. Geological setting of the Schapenburg schist belt; Metamorphism of the metaturbidite sequence; Mineral chemistry.

Provides information on a study that examined the geology of the southwestern extremity of the Barberton Greenstone Belt in South Africa. Methodology of the study; Results and discussion on the study; Conclusions.

Abstract -- The 3.48 Ga Komati Formation, famous for its spinifex komatiites, was mapped (1:5000 scale) to provide an objective base for reconstructing the magmatic and tectonic architecture. All outcrops were drafted on digital orthophotographs using GIS software. The best outcrops were mapped to determine the textural zoning and shapes of chill-bound cooling units and to distinguish volcanic, intrusive, and tectonic contacts. The Komati and overlying Hooggenoeg Formations compose a 6-km-thick section of submarine lava flows and intrusions. The section is overlain by c. 3.45 Ga dacitic volcanic rocks, and the Komati Shear Zone marks the base. The Komati Formation is divided into upper and lower members. The Lower Komati, 1.3 km thick, consists of layers of olivine komatiite (48%) and komatiitic basalt (52%). The Upper Komati, 1.8 km thick, consists predominantly of pillow lavas of komatiitic basalt (84%). Komatiites of the Lower Komati consist of massive (61%), spinifex (37%), and vesicular (2%) cooling units. Massive units are interpreted to be undifferentiated sheet flows (<50 m thick by 11 km in extent). Spinifex komatiites are concentrated in five distinct horizons that generally overlie thicker massive sheets. Spinifex cooling units occur as sheet flows (<8 m by >1 km) and thinner lenses that are analogous to pahoehoe flow lobes. Vesicular komatiite occurs as a sheet (is similar to 11 m by >1.8 km) and a channel (55 m by <0.3 km) with 20 - 25% vesicles, locally interlayered in zones with olivine spinifex. Interlayered with pillows in the Upper Komati, sheet flows of komatiitic basalt (5 - 15 m thick) have pyroxene-spinifex zones. Similar textures occur in locally crosscutting sills in the Lower Komati. Dykes and sills of komatiite, komatiitic basalt, diabase, tonalite, and wehrlite intruded the Komati Formation. Komatiitic dykes occur within cooling units and may be related to inflation of volcanic flows. Wehrlite dykes, compositionally similar to the ... [ABSTRACT FROM AUTHOR]

Abtract -- The Rhenosterkoppies Greenstone Belt, situated in the northern Kaapvaal Craton close to the Southern Marginal Zone of the Limpopo Belt, was studied by structural mapping, petrography, and geochronology. The belt consists mainly of a 300 - 500-m-thick alternating sequence of amphibolite and banded iron formation horizons, with intercalated talc-tremolite schist, rare metapelite, and strongly foliated sheets of mainly granodioritic gneiss. This mainly supracrustal sequence broadly defines an antiform with axis plunging shallowly to the east. Granodioritic gneisses are exposed in the centre of the antiform. Their contact with the supracrustal sequence is sheared. The gneisses have T[sub DM] model ages between is similar to 2950 and is similar to 3100 Ma, in agreement with T[sub DM] ages of gneisses from the northernmost Kaapvaal Craton and the Limpopo Belt Southern Marginal Zone in general. Four deformational phases were recognized, the most prominent of which produced a pervasive layer-parallel foliation and a consistent northeast-trending stretching lineation, and probably represents the regional north-vergent thrusting episode recognized elsewhere in the northern Kaapvaal Craton. The shearing occurred under upper greenschist to lower amphibolite-facies conditions. Upper amphibolite-facies relics are found throughout the greenstone belt. The shearing was dated directly at 2729 +/- 19 Ma by stepwise leaching Pb/Pb analysis of synkinematic titanite. Dating of a retrograde post-kinematic titanite-epidote paragenesis within the granitoid gneisses in the core of the anticline yielded 2743 +/- 21 Ma, placing a lower age limit of is similar to 2720 Ma on this tectonism. It was followed by the updoming of the anticline, recognizeable as a separate event as the main lineation is rotated by it. In the northernmost part of the greenstone belt, close to the Hour River Shear Zone, a local medium- to high-pressure, lower amphibolite-facies metamorphic episode associ... [ABSTRACT FROM AUTHOR]

Presents information on a fluid inclusion study initiated as a gold prospecting tool in the Birimian greenstone terrane of the northern Haute Comoe area (Ivory Coast) in Africa. Information on gold mineralization and ore fluid composition; Geological setting of the Haute Comoe area; Methods of investigation; Microthermometric results; Conclusions.

Presents an analysis of the various greenstone belts in the north and northeastern parts of the Kaapvaal Craton in South Africa, which were generally regarded to represent time equivalent of the early Archaean Barberton Belt. Findings that suggest the age data for the various greenstone belts of the craton were generally scare and imprecise; How single zircons were dated using a felsic metavolcanic rock of the Rubbervale Formation in the Murchison Greenstone Belt.

Analyzes and elaborates metamorphic grades of greenstone belts. Structure and stratigraphy of greenstone belts; Geophysics and the third dimension of greenstone belts; Rock types of the belts; Geochronology and isotopic signatures of greenstone belts; Tectonic environments and evolution of the belts; Conclusions.