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New geochemical data demonstrate the similarity between Neoarchaean TTGs found in the middle crust of the Zimbabwe Craton and the lower crust of the Northern Marginal Zone of the Limpopo Belt. Hence the Neoarchaean segment of the Zimbabwe Craton provides a complete crustal section—from lower crustal granulites to the supracrustal rocks of the greenstone belts. Mantle activity recorded in the basalts of the greenstone belts is interpreted as plume volcanism, setting a context for the process of crustal generation. However, published isotopic data and new trace element data reported here show that magma genesis in Zimbabwe Craton in the Neoarchaean incorporated up to 20% of older crust, indicating that not all the granitoids are juvenile. This paper seeks to identify the trace element composition of the parental melts to the TTGs of the Zimbabwe Craton, define the petrological processes by which they were generated and set limits on possible petrological models for the process of crust generation. The least contaminated TTGs have low SiO2 and moderately fractionated trace element patterns. Calculations indicate that they were derived by the partial melting of a garnet amphibolite source at ca 10 kb depth. Rare intrusive diorites represent a low fraction melt of an enriched mantle source derived from refertilised depleted mantle. These observations lead to a model in which the source material of the TTGs was basaltic and the thermal energy to melt the basalts came from continued basaltic and komatiitic magmatism. Partial melts of old felsic crust were incorporated into the TTG magmas. The resulting 'enriched' TTGs provided a fertile lower crust from which an extensive suite of late tectonic granites could be generated. These data do not support a modern-style plate tectonic model for the growth of the Zimbabwe Craton in the Neoarchaean. [ABSTRACT FROM AUTHOR]

Data-driven methods have increasingly been applied to solve geoscientific problems. Incorporation of data-driven methods with hypothesis testing can be effective to address some long-standing debates and reduce interpretation uncertainty by leveraging larger volumes of data and more objective data analytics, which leads to increased reproducibility. In this study, lithogeochemical data from regionally persistent Archean shale units were aggregated from literature, with special reference to the Kaapvaal Craton of South Africa—namely, shales from the Barberton, Witwatersrand, Pongola, and Transvaal Supergroups—and the Belingwe and Buhwa Greenstone Belts of the Zimbabwe Craton. We examine the feasibility of using machine-learning algorithms to produce a geochemical classification and demonstrate that machine learning is capable of accurately correlating stratigraphy at the formation, group, and supergroup levels. We demonstrate the ability to extract highly useful scientific findings through a data-driven approach, such as geological implications for the uniqueness of the sediment compositions of the Central Rand and West Rand Groups. We further demonstrate that when lithogeochemistry and machine-learning algorithms are used, only about 50 samples per geological unit are necessary to reach accuracy levels of around 80%–90% for our shale samples. Consequently, for many traditional tasks, such as rock identification and mapping, some expensive analyses and manual labor can be replaced by an abundance of cheaper data and machine learning. This approach could transform large-scale geological surveys by enabling more detailed mapping than currently possible, by vastly increasing the coverage rate and total coverage. In addition, the aggregation of historical data facilitates data reuse and open science. These results justify the need to bridge data- and hypothesis-driven techniques for the stratigraphic correlation and prediction of rock units, which can improve the accuracy of the inferred stratigraphic correlation and basin setting. [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]

Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth's history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle. [ABSTRACT FROM AUTHOR]

Neoarchaean greenstone belts of the Zimbabwe craton host microbialite-bearing limestone successions that range in age from ~2.82 to 2.68 Ga. The best-preserved successions, according to Raman geothermometry, are situated in the Bulawayo and Belingwe greenstone belts where they have been subjected to lower greenschist facies metamorphism with a peak temperature of ~350 °C. Elsewhere, such as in the Masvingo belt, peak temperatures reached ~540 °C, giving rise to marbles. The carbonate rocks consist mainly of calcite and, to a lesser extent, dolomite and contain variably admixtures of siliciclastic detritus. Samples with the least amount of siliciclastic detritus have shale-normalized rare earth element and Y distribution patterns that indicate open marine conditions, with positive La, Eu, and Gd anomalies, superchondritic Y/Ho ratios, and depleted light rare earth elements relative to the heavy rare earth elements. The δ13С VPDB values are −2.0 to 2.1 ‰, and δ18O VPDB values cluster in the range of −20 to −10 ‰. Strontium isotope ratios vary in concert with siliciclastic detritus. The lowest initial 87Sr/86Sr ratio of 0.70155 is close to the depleted mantle value at the time of deposition, suggesting minimal contribution from evolved felsic crust to the Neoarchean ocean. Only samples from the Masvingo belt show radiogenic Sr isotope ratios (0.70202–0.70340) indicative of an epicratonic setting with exposed felsic basement. For the rest, open marine conditions prevailed in a proto-cratonic setting characterized by mantle-derived greenstone volcanism. Carbonate sedimentation on the Zimbabwe craton thus predate the fundamental rise in the marine 87Sr/86Sr Sr isotope signal in the late Neoarchaean. [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]

• New lithological, whole-rock geochemical and U-Pb geochronological constraints provided for the West Laverton Greenstone Belt (WLGB) in the Kurnalpi Terrane. • Greenstones erupted sub-aqueously onto rifted sedimentary basement in three volcanic events between 2854 ± 4 and 2717 ± 4 Ma, from a depleted mantle source. • Comparisons with better-characterised successions in the west Kalgoorlie Terrane indicate a common ca. 2.96–2.69 Ga stratigraphic framework, supporting an autochthonous setting. • The Yilgarn Craton reveals remarkable > 2.71 Ga lithostratigraphic similarities with the Zimbabwe and Slave Cratons, supporting NeoArchean ancestory. The time-interval between 2.8 and 2.7 Ga is associated with the worldwide appearance of thick tholeiite-komatiite successions. However, their origin and affiliation, if any, is poorly understood. The Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, is dominated by the 2.7 Ga Kalgoorlie and Kurnalpi terranes, both of which consist of well-preserved mafic-ultramafic volcano-stratigraphy. On a local scale, these supracrustal sequences permit reconstruction of volcano-sedimentary environments, which provides important constraints to the local conditions of magmatism and/or deposition. Regionally, these observations can be used to test correlations between greenstone belts, constrain basin architecture and the extent of volcanism. Studies of this nature provide important insights to global, Archean volcano-sedimentary processes. Here we present an integrated field, geochemical and U-Pb geochronological study from the well-preserved West Laverton Greenstone Belt in the Kurnalpi Terrane and compare it to the relatively well constrained Kalgoorlie Terrane. The sequence comprises three volcanic 'events' consisting dominantly of low-Th tholeiitic basalt and komatiite derived from depleted mantle sources. Magmatism commenced after 2854 ± 4 Ma and ceased by 2717 ± 4 Ma. Prominent banded iron formations separate volcanic cycles, indicating an overall quiescent subaqueous setting. The West Laverton Sequence is correlated with volcanic events in the neighbouring Kalgoorlie Terrane, where pre-existing basement with remarkably similar character is documented near Norseman, over 360 km away. Both terranes contain basal paragneiss (2.96–2.93 Ga) and epiclastic sediments (∼2.93–2.85 Ga) that are overlain by three variably preserved and widespread tholeiite-dominated volcanic events, here defined as the 2.81–2.71 Ga 'Goldfields Tholeiitic Super Event'. These correlations support an autochthonous setting for the Kalgoorlie and Kurnalpi terranes with the eruption of volcanic tholeiites onto submerged continental crust. Voluminous tholeiitic volcanism at 2.8–2.7 Ga was a global event. The Zimbabwe and Slave cratons preserve remarkably similar basement overlain by thick basaltic units, that are compositionally similar to the 'Goldfields Tholeiitic Super Event'. This correlation indicates that a globally significant rifting event at 2.8–2.7 Ga affected much of the continental crust extant. [ABSTRACT FROM AUTHOR]

Abstract: The Neoarchaean Epoch nickel sulphide (NiS) deposit, discovered in 1970 and subsequently mined from 1972 to 1998, has been previously interpreted to be a magmatic ore system. However, on the basis of field, petrographic and analytical data obtained during the initial exploration and evaluation stages, integrated with limited new analytical data (SEM and ore microscopy), we propose that the Epoch NiS may be of hydrothermal origin. Key features that have prompted a re-appraisal of the genesis of the Epoch mineralisation, include: a) ore lenses are irregularly distributed along foliation fabrics of the host talc-carbonate rocks and as such no relationship to basal ultramafic/mafic units; b) the thickness of the ore lenses are highly variable from tens of centimetres to several metres; c) ore mineralogy is characterised by dominant millerite–pyrite–chalcopyrite assemblages; and d) high Pd/Pt ratios. Although our hydrothermal model is somewhat speculative, we suggest that on present evidence there is little doubt that the Epoch NiS lacks the features of typical magmatic sulphide deposits. More geochemical and isotopic data are needed to test our model. [Copyright &y& Elsevier]

Abstract: The Palaeoproterozoic Magondi Supergroup lies unconformably on the Archaean granitoid-greenstone terrain of the Zimbabwe Craton and experienced deformation and metamorphism at 2.06–1.96Ga to form the Magondi Mobile Belt. The Magondi Supergroup comprises three lithostratigraphic units. Volcano-sedimentary rift deposits (Deweras Group) are unconformably overlain by passive margin, back-arc, and foreland basin sedimentary successions, including shallow-marine sedimentary rocks (Lomagundi Group) in the east, and deeper-water shelf to continental slope deposits in the west (Piriwiri Group). Based on the upward-coarsening trend and presence of volcanic rocks at the top of the Piriwiri and Lomagundi groups, the Piriwiri Group is considered to be a distal, deeper-water time-equivalent of the Lomagundi Group. The Magondi Supergroup experienced low-grade metamorphism in the southeastern zone, but the grade increases to upper greenschist and amphibolite facies grade to the north along strike and, more dramatically, across strike to the west, reaching upper amphibolite to granulite facies in the Piriwiri Group. Carbonates form prominent horizons in the lower Lomagundi Group, occur in the Deweras Group as thick packages in the northern part of the basin, but form only thin discontinuous beds elsewhere, and are rare in the Piriwiri Group. Beds of anhydrite and sulphate pseudomorphs are relatively common in the Deweras Group, and also occur in the Lomagundi Group. found extreme enrichment in 13C in carbonates of the Lomagundi Group, with an average δ13C value of +8.2‰ VPDB. Subsequent work in the Magondi Basin has shown that high δ13C carbonates are also present in the continental rocks of the underlying Deweras Group. The initiation of the Deweras rift is not well constrained geochronologically, but it may have started as early as 2.26Ga, and was followed by deposition of the lower part of the Lomagundi Group on a passive continental margin. Assuming that the deepening trend in the upper Lomagundi Group and upward-coarsening trend in the Piriwiri Group reflect subsidence in a back-arc and subsequent foreland basin setting, with sediment derivation from an approaching volcanic arc, the age of the onset of Magondi deformation at ca. 2.0Ga provides an upper age limit for sedimentation. Combining all available data for the Magondi Basin, the Limpopo Belt between the Zimbabwe and Kaapvaal cratons, and the Kheis Belt on the southwestern margin of the Kaapvaal craton, a new tectonic model is presented for the assembly of the Kaapvaal and Zimbabwe cratons at ca. 2.1–2.0Ga. These cratons along with the Archaean cratons in West Africa and South America record an assembly of a large continent at the time when Archaean cratons in North America and Fennoscandia experienced extension and breakup. Carbonates of the Magondi Basin, the Kheis Belt, and the northern margin of the Kaapvaal craton therefore reflect the carbon isotope composition of the open ocean at 2.2–2.1Ga and provide further evidence that the δ13C values reaching to +10‰ VPDB and higher record a seawater signal rather than local diagenetic or closed basin conditions. The emerging picture is that Earth experienced dramatic tectonic reorganization between 2.1–2.0Ga, which likely influenced ocean circulation and redox state and potentially ended the conditions that promoted high burial of organic matter with sediments during the Lomagundi excursion. [Copyright &y& Elsevier]

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]

Gold mineralization at the Viceroy Mine is hosted in extensional veins in steep shear zones that transect metabasalts of the Archaean Arcturus Formation. The gold mineralization is generally made up of banded or massive quartz carrying abundant coarse arsenopyrite. However, most striking is a distinct suite of Au-Bi-Te-S minerals, namely joseite-A (Bi4TeS2), joseite-B (Bi4Te2S), hedleyite (Bi7Te3), ikunolite (Bi4S3), 'protojoseite' (Bi3TeS), an unnamed mineral (Bi6Te2S), bismuthinite (Bi2S3), native Bi, native gold, maldonite (Au2Bi), and jonassonite (AuBi5S4). The majority of the Bi-Te-S phases is characterized by Bi/(Se+Te) ratios of <1. Accordingly, this assemblage formed at reduced conditions at relatively low fS2and fTe2. Fluid-inclusion thermometry indicates depositional temperatures of the main stage of mineralization of up to 342°C, in the normal range of mesothermal, orogenic gold deposits worldwide. However, melting temperatures of Au-Bi-Te phases down to at least 235°C (assemblage (Au2Bi + Bi + Bi7Te3)) imply that the Au-Bi-Te phases have been present as liquids or melt droplets. Furthermore, the close association of native gold, native bismuth and other Bi-Te-S phases suggests that gold was scavenged from the hydrothermal fluids by Bi-Te-S liquids or melts. It is concluded that a liquid/melt-collecting mechanism was probably active at Viceroy Mine, where the distinct Au-Bi-Te-S assemblage either formed late as part of the main, arsenopyrite-dominated mineralization, or it represents a different mineralization event, related to rejuvenation of the shear system. In either case, some of the gold may have been extracted from pre-existing, gold-bearing arsenopyrite by Bi-Te-S melts, thus leading to an upgrade of the gold ores at Viceroy. The Au-Bi-Te-S assemblage represents an epithermal-style mineralization overprinted on an otherwise mesothermal (orogenic) gold mineralization. [ABSTRACT FROM AUTHOR]

The Kwekwe district comprises the central part of the Archean Midlands greenstone belt of Zimbabwe and represents one of few Archean gold provinces in the world hosting gold deposits associated with massive antimony mineralization. The deposits typically occur along late Archean transcrustal deformation zones. Mineralization is characterized by an early Au-As stage 1, followed by a pronounced Au-Sb stage 2, and a low-grade Sb-bearing stage 3. Four gold deposits within the Kwekwe district have been studied. The Primrose and Globe and Phoenix gold deposits display typical features of Archean orogenic lode gold systems such as fluid inclusions with low salinity, mixed aqueous-carbonic fluids, formation temperatures between 300° and 400°C, and a common stable isotope composition of fluid and mineral precipitates. Deposits of this type formed in the brittle-ductile crustal transition zone at 1.5 to 3.0 kbars. In contrast, gold mineralization at Jojo and especially the Indarama gold deposits probably formed at lower temperatures (<300°C) and from dominantly aqueous, early moderate- to late high-salinity fluids. At Indarama, microthermometric studies indicate stibnite-gold mineralization at temperatures between 120° and 230°C at about 0.8 to 1.8 kbars. Low formation temperatures and extremely low δ34S values of pyrite (-52‰) and stibnite (-36‰) of the latest mineralization stage at Indarama reveal reactivation of the hydrothermal system within a near subsurface, organic-rich epithermal or hot spring-type environment during the late Archean or even early Proterozoic. Such low δ34S values have not been reported previously from gold deposits in Archean greenstone belts. We propose that hydrothermal activity and gold mineralization has not only taken place at different crustal levels but also at different times. [ABSTRACT FROM AUTHOR]

Abstract: Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25–3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth. PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment. δ98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1–3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing. Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans. [Copyright &y& Elsevier]

We report comprehensive trace element and Sr-isotope data for microbial carbonates from the Archaean Mushandike limestone, Masvingo Greenstone Belt, Zimbabwe. The stromatolites have very coherent REE+Y patterns and share the essential shale-normalised characteristics of other Archaean marine precipitates (positive La and Gd anomalies, absence of a negative Ce anomaly and a strongly superchondritic Y/Ho ratio). Mixing models constrain the maximum amount of shale contamination to 0.25–1% and calculated detritus-free carbonate REE+Y systematics require precipitation from seawater. In terms of light-REE over heavy-REE depletion, however, the studied samples are very different from all other known Archaean marine precipitates. In shale-normalised plots, the Mushandike samples yield a negative slope. A very restricted, regional input source of the dissolved load is indicated because normalisation with locally occurring tonalite gneiss REE+Y data yields a pattern closely resembling typical shale-normalised Archaean marine chemical sediments. The disappearance of a negative Eu anomaly when patterns are normalised with local tonalite gneiss strengthens this interpretation.Sr-isotope ratios are strongly correlated with trace element contents and ratios, which explains the modest scatter in Sr-isotope ratios as representing (minor) clastic contamination. Importantly, even the least contaminated samples have very radiogenic initial 87Sr/86Sr ratios (0.7184) implying Sr input from an ancient high Rb/Sr source, such as the early Archaean gneisses of south-central Zimbabwe. A local ancient (3.5–3.8 Ga) source is also indicated by previously published Pb-isotope datasets for the Mushandike stromatolites. This is entirely compatible with the occurrence of 3.7–3.8 Ga zircons in quartzites and metapelites from comparably old greenstone belts within less than 150 km of the studied locality.Comparison of the Pb-isotope ratios of the Mushandike stromatolites with 2.7 and 2.6 Ga old stromatolites from the neighbouring Belingwe Greenstone Belt demonstrates differences in initial isotope composition that relate to the extent of exchange with the open ocean. The development of numerous basins on old continental crust, with water masses variably restricted from the open ocean, suggests a lack of strong vertical topography on this late Archaean craton. [Copyright &y& Elsevier]

The Indarama lode gold deposit is hosted by vertically-dipping basalt in the Late Archaean Midlands Greenstone Belt of Zimbabwe. Major deformation events at 2.68 and 2.58 Ga established a complex array of fractures. A limited range of orientations of this fracture network opened towards the end of the younger deformation event, creating a lode pattern where 92% of mineralised veins dip at less than 50°, mainly to the E and W, and most strike directions are represented. A clustered distribution of poles to the quartz–carbonate veins indicates a constrictional stress field at the time of vein opening where σ1 and σ2 were near horizontal, (directed NNW–SSE and ENE–WSW, respectively), and σ3 was near vertical. 3-D Mohr circle analysis demonstrates that σ2 was approximately 67% of σ1 (the stress ratio) and that the driving pressure ratio (R′) was approximately 0.4, reflecting the role of fluid pressure, mean stress, and the maximum shear stress in controlling conditions of fracture opening. [Copyright &y& Elsevier]

Controversy exists as to whether the 2.7 Ga Belingwe greenstone belt, Zimbabwe, is autochthonous or allochthonous. In this study we report direct evidence for an autochthonous continental setting for the Belingwe greenstone belt. Garnet and clinopyroxene xenocrysts were discovered in the fresh ultramafic komatiites. Major and trace element compositions of these xenocrysts suggest that they originated from mafic lower crust, presumably garnet granulite in composition, at a low temperature of ∼600 or ∼700 °C. Furthermore, in a komatiitic basalt sample, we have discovered orthopyroxene and clinopyroxene xenocrysts that may have originated from lithospheric mantle beneath the continental margin. Discoveries of these xenocrysts indicate that the komatiite magma was transported to the surface and quenched rapidly enough to prevent the complete melting of continental fragments. [ABSTRACT FROM AUTHOR]

Late Archaean (∼2.68 Ga) clastic sedimentary rocks of the Shamvaian Group form a shear zone-bounded lithotectonic unit in the Midlands greenstone belt (MGB) of Zimbabwe. The sedimentary rocks are wedged in between mid-Archaean granitoid gneisses and late Archaean mafic and bimodal volcanic rocks. The ≥2000-m thick sedimentary sequence consists of marine turbidite deposits overlain by high-energy shelf sandstone, fluvial braid-plain pebbly sandstone and alluvial fan conglomerate. Deposition took place in a northward-deepening basin with fluvial systems draining a source dominated by granitoid gneiss and banded chert. Basin formation was associated with a west-directed thrusting event, and the basin fill was incorporated into the thrust stack soon after deposition. The Shamvaian Group is similar to modern foreland and forearc successions and possibly accumulated during collision of a continental fragment with an island arc-like complex. [Copyright &y& Elsevier]

The ~2.65 Ga old Shamvaian Group sedimentary rocks occur as a folded succession in the central part of the Bindura–Shamva greenstone belt of Zimbabwe. The strata comprise distinct, shear zone-bounded tectonostratigraphic units which may be stratigraphically arranged as follows. The lower part of the succession is represented by a transgressive, fining-upward sequence of alluvial fan conglomerate, overlain by fluvial braid-plain pebbly sandstone and marine shoreface sandstone. Detritus was derived from a mid-Archaean granitoid-gneiss terrain situated to the east. Sediment supply and subsidence rate must have been high. Shallow shelf sedimentation was followed by deep-water (sub-wave base) deposition by turbidity currents, giving rise to a thick succession of fine to coarse clastic material. The turbidite deposits were locally overlain by shallow-marine sandstone and fluvial to alluvial fan conglomerate. An upward increase in the abundance of intermediate and felsic volcanic clasts suggests an increase in the proximity of a volcanic terrain, such as a volcanic arc. Deposition was followed by layer-parallel shearing during thrust belt-style tectonism. Major shear zones developed preferentially along the contact between shallow- and deep-marine facies associations. Basin initiation may have been related to extensional tectonics, possibly on rifted continental crust, whereas later stages of basin history were characterized by compression, suggesting a foreland or fore-arc basin setting. Sedimentary facies, stratigraphy, and facies distribution are remarkably similar to some late Archaean sedimentary sequences of the Superior Province in Canada.Les roches sédimentaires du groupe de Shamvaian, âgées d'environ 2,65 Ga, se présentent comme une succession pliée dans la partie centrale de la ceinture de roches vertes de Bindura–Shamva au Zimbabwe. Les strates comprennent des unités tectonostratigraphiques distinctes, limitées par des zones de cisaillement; ces unités pourraient avoir la structure stratigraphique qui suit. La partie inférieure de la succession est représentée par une séquence transgressive à affinement vertical ascendant d'un conglomérat de cône alluvial, sur lequel repose un grès caillouteux de plaine de bras entrecroisés et un grès marin d'avant-plage. Les détritus proviennent d'un terrain granitoïde-gneissique (Archéen moyen) situé à l'est. Les taux d'apport de sédiments et de subsidence devaient être élevés. Une sédimentation de plate-forme peu profonde a été suivie d'une déposition en eau profonde (sous le niveau de base des vagues) par des courants de turbidité, donnant lieu à une succession épaisse de matériel clastique de fin à grossier. Les dépôts de turbidites ont été recouverts par endroits d'un grès marin d'eau peu profonde et d'un conglomérat de cône fluvial à alluvial. L'abondance des clastes intermédiaires et felsiques augmentant vers le haut suggère un rapprochement d'un terrain volcanique, tel un arc volcanique. La déposition a été suivie par du cisaillement parallèle aux couches au cours d'un événement tectonique de type ceinture de chevauchement. Des zones de cisaillement majeures se sont développées de façon préférentielle le long du contact entre les associations de faciès peu profonds et de grande profondeur marine. L'initiation du bassin peut être reliée à la tectonique d'extension, possiblement sur une croûte continentale en distension, alors que les stades postérieurs de l'histoire du bassin étaient caractérisés par de la compression, suggérant un environnement de bassin d'avant-pays ou d'avant-arc. Les faciès sédimentaires, la stratigraphie et la distribution des faciès sont remarquablement similaires à quelques séquences archéennes tardives de la Province du Supérieur au Canada.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Presents a study on the tectonic evolution of the Harare-Shamva greenstone belt in northern Zimbabwe focusing on sequential early stacking and pluton diapirism. Regional setting of the Harare-Shamva greenstone belt; Deformation and structure; Thermal constraints on regional metamorphism and doming; Age constraints; Tectonic reconstruction.

Rocks that make up the Mesoarchean (~3Ga) Buhwa greenstone belt (BGB) of the Zimbabwe craton divide into three associations: shelf, basinal, and transitional. Chert and iron formation exist within the three associations, allowing the unique opportunity to compare textural and geochemical attributes of iron- and silica-rich rocks from distinct positions within a single basin. We analyzed major-, trace-, and rare-earth-elements, and silicon and oxygen isotopes of chert and iron formation from the shelf, basinal, and transitional associations within the BGB. Samples possess elemental signatures consistent with formation from mixed seawater and hydrothermal fluids (La SN /La SN * > 1; Pr SN /Yb SN < 1; Y/Ho = 28.2–44.4; Eu SN /Eu SN * = 1.8–2.8). Oxygen isotope values broadly overlap for samples from different associations (ranging from δ18O V-SMOW = 9.41 to 15.10‰ (2σ = 0.8‰)), with isotopic variation linked to the presence of oxide inclusions in some samples. Silicon isotope values by contrast, vary widely. Iron formations possess 30Si-depleted silicon isotope signatures (δ30Si NBS-28 = −2.5 to −0.5‰ (2σ = 0.05‰)), in line with previous studies. Basinal and transitional cherts possess 30Si-depleted silicon isotope signatures (δ30Si NBS-28 = −1.1 to −0.5‰ (2σ = 0.05‰)) that overlap with values measured from iron formation samples. Within the transitional association, cherts are not 30Si-enriched compared to iron formation, while chert in the basinal association is 30Si-enriched compared to iron formation. Depleted silicon isotope signatures could result from fractionation during adsorption, re-equilibration, post-depositional processes, or possibly microbial reduction processes. Silicon isotope heterogeneity recorded in chert preserved in different assemblages across the BGB may in part reflect a seawater signature. The range of silicon isotope values preserved within broadly co-eval rocks from the same assemblage may indicate that samples preserve local isotopic signatures of a heterogeneous reservoir. Future work focused on identifying basin-position dependent trends may inform interpretations of the compiled Precambrian silicon isotope record. • Archean iron formation samples from the Buhwa greenstone belt possess 30Si-depleted silicon isotope signatures. • Basinal and transitional cherts possess 30Si-depleted silicon isotope signatures , despite low iron and aluminum contents. • Silicon isotope values of cherts display inter-sample and intra- sample heterogeneity. • Cherts from the Buhwa greenstone belt may record local isotopic signatures of a heterogeneous reservoir. [ABSTRACT FROM AUTHOR]

Presents evidence of ensialic origin for the Ngezi group in the Belingwe greenstone belt in Zimbabwe. Refutation of a hypothesis of Archean oceanic plateau; Stratigraphy; Structure of the Manjeri Formation; Composition of ultramafic and mafic volcanic rocks.

The Shurugwi greenstone belt (SGB) in the central Zimbabwe Craton is one of the major volcano-sedimentary belts in the craton, but its formation age and tectonic history have not been well constrained. For revealing the evolution of SGB, this study performed in-situ U–Pb dating of zircons from meta-sedimentary rocks of the Wanderer and Mont d'Or formations in SGB, granitoids/orthogneisses around SGB, and also whole-rock Re–Os isotopic analysis of chromitite in an ultramafic unit in SGB. Igneous zircons from granitoids which intruded into SGB were dated ca. 2.91 Ga, whereas those from the orthogneiss were dated ca. 3.01 Ga. The Re–Os isotopic analysis provided an intrusive age of ca. 3.0 Ga for the ultramafic unit that is unconformably covered by the Wanderer Formation. These ages indicate that the Wanderer Formation was deposited between 3.0 and 2.91 Ga and the SGB was tectonically emplaced over the basement with 3.01 Ga orthogneiss. By ca. 2.91 Ga. The depositional age of the Mont d'Or Formation is constrained between ca. 3.59 Ga and 3.0 Ga nonetheless the age range is still large. The age spectrum of the detrital zircons from the Mont d'Or Formation resembles that of the Wanderer Formation, sharing a dominant age cluster at ca. 3.59 Ga. For the Wanderer Formation, a significant time gap of nearly 600 million years between the depositional age and the 3.59 Ga major cluster in detrital zircon age spectra suggests that the SGB had a remarkable provenance which prevented large-scale supply of young detrital grains from tectono-magmatically active region. Image 1 • Zircon dating and Re–Os isotopic analysis for the Shurugwi greenstone belt (SGB). • ~3.59 Ga detrital zircons and 3.0–2.9 Ga chromitite in the SGB. • 3.02 Ga orthogneiss basement and 2.91 Ga post-kinematic granite intrusion. • Ca. 3.0 Ga continental rifting and subsequent collision by ca. 2.9 Ga. [ABSTRACT FROM AUTHOR]

This study addresses the Neoarchean to Paleoproterozoic thermal evolution from the Buhwa Greenstone Belt (BGB) along the contact zone between the Archean Zimbabwe Craton (ZC) and a granulite terrane of the Northern Marginal Zone (NMZ) of the Limpopo Complex. Available tectonic models suggest a thrusting of the hot NMZ over and onto the cold ZC, which possibly gave rise to local heating of the margin of the craton by a "hot-iron effect". Petrographic, pressure–temperature data, along with hornblende Ar–Ar ages for metabasites from the BGB were coupled with a discussion of the tectonics of this region. The metabasites typically occur interlayered with metasediments. Four types of metabasites occur, namely, calcic amphibole–epidote schist, amphibolite (garnet-bearing and garnet-free), calcic amphibole–clinopyroxene schist, and mafic granulite, indicating a varying metamorphic grade from epidote–amphibolite facies to granulite facies. The lower-grade calcic amphibole–epidote schists occur in the northwestern margin of the belt, whereas the higher-grade amphibolites and mafic granulites are present along the southeastern margin of the BGB adjacent to the granulite terrane. The increase in metamorphic grade towards the NMZ of the Limpopo Complex is supported by temperature estimates of <500 °C to >700 °C from northwest to south east. The results of our study are consistent with the pervailing tectonic model which considers the NMZ–ZC boundary as a major thrust-sense shear zone dipping southward, and the hot NMZ thrusted over the cold ZC. New hornblende Ar–Ar ages indicated two discrete thermal events recorded in the BGB rocks: 2525 ± 19 Ma from mafic granulite and 2020 ± 8 Ma to 1968 ± 10 Ma from amphibolites. The Neoarchean age probably corresponds to a cooling age after the peak metamorphism, indicating that the NMZ thrusting onto the ZC took place at >2525 Ma. The age of the thrusting of the NMZ is younger than that of the Southern Marginal Zone (SMZ) of the Limpopo Complex onto the Kaapvaal Craton (KC) (ca. 2.72–2.69 Ga). Therefore, the formation of the Limpopo Complex is not because of a simple collision of the KC and the ZC, but the SMZ-KC collision is more than 100 Myr earlier than the NMZ–ZC collision, suggesting multiple collision for the formation of the Limpopo Complex. In contrast, the younger Paleoproterozoic thermal event coincidental with the last high-grade metamorphic event within the Limpopo Complex might correspond to a thermal reactivation along the NMZ–ZC boundary possibly caused by the global ca. 2.0 Ga orogenic events. • Low-to high-grade metabasites in the Buhwa Greenstone Belt along the southern Zimbabwe Craton margin. • Increase of peak temperature from NW (<500 °C) to SE (>700 °C) within the belt. • 2525 Ma Ar/Ar age from mafic granulite indicates the timing of peak metamorphism and thrusting. • Paleoproterozoic (ca. 2.0 Ga) thermal overprint affected the southern margin of the Zimbabwe Craton. [ABSTRACT FROM AUTHOR]

Critiques the proposed continental extensional setting for the Archean Belingwe Greenstone Belt in Zimbabwe. Consistency of geochemical heterogeneity and sedimentology of the Manjeri Formation to the deposition of locally derived material.

Presents the authors' reply to the comments of T.M. Kusky and others on the article `Ensialic origin for the Ngezi Group, Belingwe greenstone belt, Zimbabwe.' Possible interpretation for the deepening-upward succession in the Manjeri; Arguments raised by T.M. Kusky et al.; Points of agreement.

Comments on the article `Ensialic origin for the Ngezi Group, Belingwe greenstone belt, Zimbabwe,' by Tom G. Blenkinsop, Christopher M. Fedo, and others. Dispute over the allochthonous oceanic plateau interpretation; Description of the basal Reliance Formation; Claims regarding the lack of resemblance between volcanic stratigraphy and oceanic crust.

Sandawana emeralds formed at the contact between greenstones of the Mweza Greenstone Belt and rare-element granitic pegmatites, which were intruded during the main deformation event that occurred 2.6 billion years ago at the southern border of the Zimbabwe craton. Subsequendy; a Na-rich fluid was injected along shear zones, causing albitization of the pegmatite and phlogopitization in the greenstone wall-rock. Coeval ductile deformation is indicated by boudinage and folding of pegmatites, by differentiated layering in associated amphibole-phlogopite schist, and by the presence of (micro) shear zones. The synkinematic growth of phlogopite, emerald, fluorapatite, holmquistite, and chromian ilmenorutile indicates enrichment of Na, K, Li, Be, F, P, Rb, Cs, Ta, and Nb in the emerald-bearing shear zone. This suggests that emerald formation is closely related to syntectonic K-Na metasomatism. In this process, microdine, oligoclase, quartz (from the pegmatite), and chlorite (from the greenstones) were consumed, in favor of albite (in the pegmatite), phlogopite, some new actinolite and cumming-tonite, holmquistite, fluorapatite, and emerald (at the contact and in the greenstone). Mass balance calculations indicate that a Na- and F-rich hydrous fluid must be involved in these alterations that ultimately caused emerald formation. The presence of small, isolated, highly saline brine inclusions in emerald supports this calculation. Apatite-phlogopite thermometry gives temperatures of 560-650°C, which is interpreted as the range for emerald formation. These temperatures imply contact metamorphic rather than regional metamorphic conditions. Because of the intimate spatial and temporal relationship with magmatic activity, the pegmatitic/hydrothermal nature of the involved fluid, and the near-magmatic temperatures of phlogopite and apatite formation, a magmatic source for the Na-rich fluids is very likely. The Sandawana data lead to a new model of emerald formation: It is a product of contact metasomatism between ultramafic rocks and rare-element pegmatites during a deformation event involving late-stage magmatic/hydrothermal activity channeled by shearing. This model does not fit into genetic classification schemes proposed in the literature, and it demonstrates that no single theory can be applied to all schist-type emerald deposits. Gem-quality emeralds can be formed in very different geologic settings, as long as basic conditions are fulfilled: namely, the availability of beryllium and chromium (± vanadium); means of transport to bring the elements together (fluids of magmatic, hydrothermal, metamorphic, or combined origin); conditions in which emerald may form as a stable mineral (temperatures of 300-600°C); and sufficient space to grow transparent and well-formed crystals. [ABSTRACT FROM AUTHOR]