Griqualand West Research Articles

Paraburdoo spherule layer (Hamersley Basin, Western Australia): Distal ejecta from a fourth large impact near the Archean-Proterozoic boundary

We report the discovery of the Paraburdoo spherule layer, which consists entirely of replaced impact-produced melt spherules. It is distal ejecta from a large impact close to the Archean-Proterozoic boundary in the Hamersley Basin of Western Australia. The spherule layer occurs in the Paraburdoo Member of the Wittenoom Formation and was deposited ca. 2.57 Ga (date via U-Pb age interpolation) in a deep shelf environment. The layer consists of microkrystites rich in plagioclase and ferromagnesian crystals, replaced by K-feldspar and a phlogopite-like sheet silicate, respectively. The skeletal textures indicate rapid cooling of a melt with mafic to ultramafic composition and suggest oceanic target rocks. The Paraburdoo spherule layer is 2 cm thick, normally graded, and was probably deposited by suspension settling, as there is no evidence of reworking; it is strikingly similar to the Reivilo spherule layer in the Griqualand West Basin (South Africa), appears to provide a third impact-related high-resolution stratigraphic correlation between these two basins, and points to a high frequency of large impacts around the Archean-Proterozoic boundary.

Textural and knowledge-based lithological classification of remote sensing data in Southwestern Prieska sub-basin, Transvaal Supergroup, South Africa

TM data and other medium spatial resolution satellite data are used in geological and lithological-mineralogical classification on regular basis, although their usefulness is limited because of relatively coarse spectral resolution. In this contribution, we provide an example for the application of TM data for classification of rocks and minerals within the Neoarchean sedimentary and volcanic basin of Griqualand West, South Africa. An improved methodology is introduced that results in significantly higher classification accuracy. The TM multispectral image and Principal Component analysis (PCA) image of the test area were individually combined with textural features and then classified individually using a maximum-likelihood supervised classification (MLC). Subsequently, the two classified images were integrated, compared and re-classified in a knowledge-based system (KBS) using the generalized supplementary geological map on 1:250,000 scale. With this method, the accuracy was improved from 54.3% to 83.2%, when compared to the former supervised classification. The method integrates the spectral and textural features, greatly contributing to the precision of the lithological classification, mapping and prospecting, in extensive areas where field work is limited by time and cost constrains.

Isotopic constraints on the Late Archean carbon cycle from the Transvaal Supergroup along the western margin of the Kaapvaal Craton, South Africa

Few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago. Stable carbon isotopic measurements of shallow stromatolitic carbonates (∼0‰ VPDB) and basinal carbonate minerals (−6‰) in iron formation have been used to infer a strong isotopic depth gradient in Archean ocean basins. From new diamond drill cores obtained by the Agouron Drilling Project from the Griqualand West structural basin in the Northern Cape Province, South Africa, we present δ 13 C data from carbonates and organic matter that offer fresh insights into the Late Archean carbon cycle. Three drill cores cover the development, progradation, and ultimate demise (by drowning) of the Campbellrand carbonate platform (ca. 2590–2500 Ma); one captures the platform top shallow marine and intertidal paleoenvironments, the other two run through slope and basinal sections deposited adjacent to the platform margin, increasing in water depth (likely to >1 km). Both shallow and deep-water carbonates precipitated on the seafloor consistently show δ 13 C values around −0.5‰, incompatible with a strong Late Archean isotopic depth gradient. A mathematical model suggests that these isotopic data are consistent with a reduced biological pump, increased dissolved inorganic carbon in seawater due to higher atmospheric P CO2, or both. Certain horizons do show distinct isotopic variability. Such areas are commonly shaly, and they tend to be organic and/or iron rich. Strong C-isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter. In sediment-starved areas where iron formation developed, siderite tends to be 13C-depleted, sometimes by as much as −14‰. These observations suggest a carbon cycle in which iron respiration played a conspicuous role. Carbon isotope ratios from organic matter in shales are commonly >1‰ lighter than stratigraphically contiguous carbonates, but there is no clear water depth trend in the organic carbon isotopic data. Taken as a whole, the δ 13 C of organic matter can be explained by several non-unique sets of processes, including different autotrophic mechanisms of carbon fixation, heterotrophic recycling (including fermentation and methanotrophy), and post-depositional diagenesis. The most striking feature is the occurrence of organic δ 13 C values <−40‰, a feature that appears to be commonplace in Late Archean successions. Framed in the context of carbon cycle isotopic mass balance, both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen.

Origin of two distinct multiple-sulfur isotope compositions of pyrite in the 2.5 Ga Klein Naute Formation, Griqualand West Basin, South Africa

Quadruple sulfur isotope ratios ( 32S / 33S/ 34S/ 36S) were measured by a high precision SF 6 method for early diagenetic pyrite in the 2.5 Ga upper Prieska facies of the Griqualand West Basin from the Agouron drill core GKP01. Measured Δ 33S (≈ δ 33S–0.515δ 34S) values range between −2.3 and +8.5‰ with no apparent meter-scale stratigraphic trend for 70 m section. It was found that early diagenetic pyrites occur in two texturally and isotopically distinct populations such that bulk rock isotope data would reflect mixing of these two sulfur pools. Nodular and layered pyrite yield mostly negative (down to −2.3‰) and relatively constant Δ 33S values over a few tens of cm scale. Disseminated pyrites, in contrast, yield positive (0.0–8.5‰) Δ 33S values that are heterogeneous in sub-cm scales. The former pyrite sulfur was likely derived from seawater sulfate through microbial sulfate reduction, and the latter preferentially sampled aqueous H 2S-S 0 reservoirs in late Archean oceans. Relatively constant Δ 33S values for layered pyrite suggest Archean sulfate reservoirs had residence times of a few million years. Most data yield a well-defined Δ 36S/Δ 33S ratios of −0.9, suggesting predominance of a single S-MIF source reaction. Four analyses of a two cm length sample (306.5 m) yield Δ 36S/Δ 33S slope of −1.6. This slope is consistent with the relationship expected for a symmetry-breaking isotope effect during the S + S 2 reaction, but deviations from the main Archean array (i.e., Δ 36S/Δ 33S = −0.9) may also result from various combinations of SO photolysis and SO 2 oxidation. High spatial resolution-high precision SF 6 measurements of Archean pyrite provide significant new insights into the origin of Archean sulfur mass-independent fractionation (S-MIF) and the late Archean sulfur cycle.

Correlating multiple Neoarchean–Paleoproterozoic impact spherule layers between South Africa and Western Australia

Well-studied successions in the Griqualand West Basin (South Africa) and the Hamersley Basin (Western Australia) that both straddle the Archean–Proterozoic boundary have been correlated on the basis of numerous lithostratigraphic similarities. Each succession hosts a minimum of three impact spherule layers, implying a minimum of three large impacts by extraterrestrial objects over a time span of about 140 million years. The occurrences of the youngest spherule layers in both basins in comparable stratigraphic positions in banded iron formations that have already been correlated suggest the youngest pair of layers were formed by a single impact. Close similarities in stratigraphic setting, age, and petrographic characteristics of the spherules, as well as the restriction of distinctive irregular melt particles to only these layers, suggest the oldest layers in the two basins were likewise formed by a single impact. In contrast, the middle layers on the two continents differ significantly in both their stratigraphic positions and the textures of the spherules, suggesting they are products of different impact events. These results suggest that using impact spherule layers to establish a global network of high-resolution stratigraphic markers for early Precambrian successions may be an achievable goal, but careful comparisons between potentially correlative layers will be necessary to achieve it.

Paleomagnetism of a Neoarchean-Paleoproterozoic carbonate ramp and carbonate platform succession (Transvaal Supergroup) from surface outcrop and drill core, Griqualand West region, South Africa

Detailed paleomagnetic study across the Archean-Paleoproterozoic boundary interval preserved in the Ghaap Group, Transvaal Supergroup, of the Kaapvaal craton reveals a complex series of viscous and thermo-chemical magnetic overprints. Despite this complex history, a potential primary single-polarity remanence direction was revealed at high-temperature demagnetization steps in about 10% of orientated drill core samples only. This component (declination = 132°, inclination = 69°) was not identified in any of the outcrop sites. Overprint directions include a present geomagnetic field remanence and a very well developed thermo-chemical overprint (declination = 359°, inclination = 54°), which is believed to be associated with a ∼2.0 Ga regional thermal event and the development of extensive epigenetic Pb–Zn deposits. At higher levels of demagnetization, two shallow west-directed components appear to be related to the extrusion of the Ongeluk Formation lava at 2.22 Ga (declination = 268°, inclination = −20°; eastern parts of the studied region) and to the post-1.92 Ga Kheis orogeny (declination = 272°, inclination = 16°; exclusive to the western margin of the study area), respectively.

The structural/stratigraphic development of the Sishen South (Welgevonden) iron ore deposit, South Africa, as deduced from ground gravity data modelling

The Palaeoproterozoic Transvaal Supergroup in the northern Cape Province of South Africa hosts high grade (>60%Fe) hematitic and specularitic iron and manganese mineralisation. The mineralisation is primarily exposed along the western margin or in the central part of a major interference fold structure known as the Maremane dome. The bulk of South Africa's iron ore production is currently provided by two active mines, Kumba Resources megaton Sishen iron ore mine on the northern end of the dome, and the smaller Assmang Beeshoek mine on the southern closure. The undeveloped Sishen South deposit, which contains ∼400 Mt of high-grade hematite ore, is located ∼9 km towards the southwest of Postmasburg. It forms a linear extension of the Beeshoek mine and as such contains comparable laminated, breccia and conglomeratic iron mineralisation. The Transvaal Supergroup sediments were deposited in two distinct fault-controlled basins on the Archean Kaapvaal craton. Basement rocks in the western Griqualand West basin are formed by a carbonate platform sequence (Campbellrand Subgroup), which is overlain by a thick sequence of banded iron formation of the Asbesheuwels Subgroup. A major unconformity separates these rocks from the red-bed Gamagara Formation, which is correlated with the base of the Olifantshoek Supergroup. Diamictites and lavas of the Postmasburg Group have been thrust over these units from the west. The mineralisation process is related to regional scale supergene iron enrichment of Kuruman banded iron formations along the unconformity. During regional tectonism associated with the Kheis orogeny (2400–1700 Ma) the Maremane dome was uplifted, and the basal dolomite units thus exposed to erosion and karstification. The overlying banded iron formations collapsed into karst sinkhole structures where leaching of silica and enrichment of hematite occurred. Although the exact mechanism of the mineralising process remains unclear, karstification preserved components of the original concordant laminated and massive ores. These ores were subsequently eroded and deposited as conglomeratic and gritty ores down-dip of the Maremane dome in alluvial fan systems. A thin veneer of recent calcrete or Tertiary Kalahari clay beds masks the subsurface geology and most of the orebodies, thus ground geophysical gravity surveys proved excellent exploration tools to penetrate the surficial cover. These surveys were used extensively for delineation drilling and to deduce the subsurface geology and structural development of the area. Finally Sishen South orebodies were modelled with ModelVision software in an attempt to obtain an exploration template for the surrounding area.

The geology and geochemistry of the Palaeoproterozoic Makganyene diamictite

The Palaeoproterozoic Earth experienced a global glacial event at ∼2400 Ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans. The Transvaal Supergroup in the Griqualand West Basin, South Africa, hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes. The focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic Makganyene Formation, in order to constrain its palaeoenvironmental settings. The Makganyene Formation forms the base of the Postmasburg Group and has been regarded as resting on an erosive regional unconformity throughout the Northern Cape Province. Systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal. The Makganyene Formation is, in fact, conformable with underlying formations of the Koegas Subgroup in the deep southern Prieska basin and rests on an unconformity only on the shallow Ghaap platform to the north-east. The Makganyene Formation displays lateral facies changes that reflect the palaeogeography of the study area, and the advance and retreat of ice sheets/shelves. Geochemical investigations of glacial strata of the Makganyene Formation demonstrate that underlying banded iron formations of the Transvaal Supergroup acted as the main clastic source for the diamictite detritus. Geographic variations in bulk composition of the diamictites correlate well with field observations, and show that sorting processes were controlled largely by the morphology of the palaeobasin. Carbon isotope results emphasize the transitional nature of the Makganyene Formation in terms of the environmental conditions that resulted in widespread global glaciation in the Palaeoproterozoic. On the basis of the above geological evidence, it is proposed that the Transvaal Supergroup in the Northern Cape Province represents a continuous depositional event that lasted approximately 250 Ma and hence provides a unique opportunity for assessing the transitional changes experienced by the Palaeoproterozoic Earth.

Carbonic fluid inclusions in Paleoproterozoic carbonate-hosted Zn-Pb deposits in Griqualand West, South Africa

This paper reports the discovery of carbonic CH4-dominated fluid inclusions in two carbonate-hosted zinc-lead deposits in Griqualand West, South Africa. The primary carbonic inclusions were identified in coarse crystalline quartz of the main mineralisation stages at the Pering and Bushy Park deposits, hosted by Neoarchean dolostones of the Campbellrand Subgroup of the Transvaal Supergroup. The composition of the carbonic inclusions was determined from their low temperature phase behaviour during microthermometry and was confirmed with Raman microspectometry. It provides evidence for the presence of a highly reactive reducing agent (CH4) that may have been important during the mineralisation process. The carbonic inclusions and their coexistence with saline aqueous fluid inclusions allow the determination of the fluid temperature (200 to 240°C) and pressure (0.8 to 1.5 kbar) during mineralisation, suggesting that ore formation took place at 2.8 to 4.8 km depth under lithostatic pressure conditions. This is deeper than widely believed for Mississippi Valley-Type deposits, with which the Pering and Bushy Park deposits show many similarities.

The chemostratigraphy of a Paleoproterozoic MnF- BIF succession -the Voelwater Subgroup of the Transvaal Supergroup in Griqualand West, South Africa

The Paleoproterozoic Voelwater Subgroup in Griqualand West, South Africa, is an exceptionally well preserved succession of banded iron formation, braunite lutite and shallow marine limestone/dolostone. A detailed chemostratigraphic study was carried out on this succession, resulting in a comprehensive set of whole rock geochemical data, including stable isotope (δ13Ccarb, δ13Corg, δ18Ocarb) and 87Sr/86Sr isotope data. The results are in excellent agreement with previous investigations and reveal trends that correspond closely to mineralogical variations and reflect systematic changes in the depositional environment. Total inorganic carbonate contents (TIC) and δ13Ccarb values increase systematically from manganiferous iron formation at the base of the succession to dolostones that mark the top of the Voelwater Subgroup. These first order trends transgress across all lithologies and mirror the interplay of synsedimentary carbonate formation (utilizing dissolved marine bicarbonate), and Fe-Mn-rich carbonate formation during suboxic early diagenesis. First order trends are modulated by second order variations associated with lithological changes in the lower portion of the Voelwater Subgroup, constituted by three symmetrical cycles of iron formation and braunite lutite deposition. These second order trends, delineated by TIC and δ13Ccarb, as well as δ18Ocarb and major element concentrations (Mn, Fe, SiO2) can be attributed to repeated physicochemical changes in the MnF-BIF depositional environment. Variations in 87Sr/86Sr isotope composition do not relate to lithological changes. Instead, carbonate-rich and/or Sr-rich samples of MnF and BIF with well-preserved early diagenetic carbonate textures define a tight cluster with 87Sr/86Sr signatures of ~0.703 to ~0.706, in good agreement with Paleoproterozoic oceanic crust. Carbonate- and Sr-poor BIF samples with markedly more radiogenic 87Sr/86Sr signatures (~0.711 to ~0.722), in contrast, show evidence of extensive post-depositional recrystallization, apparent effects of dia- or epigenetic fluid flow. Chemostratigraphy and lithostratigraphy of the Voelwater Subgroup are in good agreement with models that predict a layered Paleoproterozoic ocean, with a hydrothermally-dominated Fe-Mn-rich deep water mass, overlain by a mass of oxygenated shallow marine water. A facies distribution model for the Voelwater Subgroup is proposed, taking into consideration not only the presence of these two isotopically distinct water masses, but also abundance and transport of organic carbon and shallow marine carbonate muds, as well as the transformation of primary iron- and manganese oxihydroxide precipitates into carbonate minerals during early suboxic diagenesis.

Using impact spherule layers to correlate sedimentary successions: a case study of the Neoarchean Jeerinah layer (Western Australia)

Spherule layers constitute the only trace of impacts by large extraterrestrial bodies in the early Precambrian. Strata in the Hamersley Basin (Western Australia) contain layers from at least three impacts; the older two date to ~2.63 and ~2.54 Ga and occur in the Jeerinah and Wittenoom Formations respectively. A third formation containing one spherule layer, the Carawine Dolomite, is restricted to an area of the basin where the Wittenoom Formation is absent. Upon discovery, the Carawine and Wittenoom layers were attributed to the same impact, but the subsequent discovery of the Jeerinah layer raised questions about this interpretation. To test it, petrographic characteristics of spherules and irregular particles in the Jeerinah layer at Hesta were quantified by point counting. Minor amounts of quartzo-feldspathic sand are present in both the Jeerinah and Carawine layers and their textural characteristics were also quantified for comparative purposes. The textures of the impact melt particles in the Jeerinah and Carawine layers are indeed very similar to one another and differ from those in the Wittenoom layer. The quartz in the Jeerinah and Carawine layers is broadly similar, but its textural characteristics are more variable. Moreover, the feldspar grains in the Jeerinah layer appear to have a more volcanic provenance than those in the Carawine layer. All of the quartz shows textures typical of plutonic to regionally metamorphosed basement rocks instead of shock metamorphism, affirming its earlier interpretation as detrital sand rather than impact ejecta. Correlations have also been proposed between Hamersley layers and an impact spherule layer in the roughly coeval Monteville Formation (Griqualand West Basin, South Africa) that is locally rich in quartzose sand. Petrographically, the melt population in the Monteville layer is remarkably similar to those of the Jeerinah and Carawine layers, including similar diagenetic histories. The quartzose detritus is again broadly similar but not identical to that of the Hamersley layers. In summary, our petrographic data are consistent with the Jeerinah, Carawine and Monteville layers all being products of a single large impact. However, the quartzo-feldspathic detritus appears to have multiple sources, consistent with its being locally derived detritus. These results suggest petrographic analysis can be used to test correlations in well-dated Precambrian successions that contain impact spherule layers.

Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America

Carbonate units of the Chocolay Group in the Lake Superior area, USA and the Gordon Lake Formation of the correlative upper Huronian Supergroup, Ontario, Canada were deposited after the last Paleoproterozoic glacial event and an episode of intense chemical weathering. The Huronian Supergroup contains at the base ∼2.45 Ga volcanics and is intruded by the 2.22 Ga Nipissing diabase dykes and sills while the Chocolay Group is bracketed in age between ∼2.29 and 2.20 Ga. The Lomagundi (∼2.22–2.1 Ga) carbon isotope excursion started after the Paleoproterozoic glacial epoch and before a plume breakout event at 2.22 Ga. Therefore, the Chocolay and Upper Huronian carbonates were deposited either before or during the onset of the Lomagundi event. Notably, thin carbonates of the basal Gordon Lake Formation and thick carbonate succession of the Kona Dolomite in the northeastern exposures of the Chocolay Group record δ 13C values as high as +9.5‰ versus V-PDB. Similar to other successions deposited during the Lomagundi event, both units contain pseudomorphs and molds after sulfates. This observation suggests that seawater sulfate contents rose dramatically in association with the onset of the Lomagundi event and the rise of atmospheric oxygen. Carbonates in the western and southern exposures of the Chocolay Group (Randville and Bad River dolomites, and Saunders Formation) previously assumed to be equivalent to the Gordon Lake Formation and Kona Dolomite have carbon isotope values close to 0‰. Based on basin analysis, we infer that these carbonate units were deposited during a negative carbon isotope excursion after the Lomagundi event started and are slightly younger than the Kona Dolomite and Gordon Lake Formation. This interpretation implies that the carbonate platform in the Great Lakes area transgressed to the west over shallow-marine and fluvial deposits. The negative carbon isotope excursion in the Lake Superior area might correspond to similar δ 13C values of the Mooidraai Dolomite in the Griqualand West Basin, South Africa supporting correlation between Paleoproterozoic successions of North America and South Africa and the notion of three global glaciations in the Paleoproterozoic Era. Carbonates of the Mille Lacs Group (Trout Lake, Glen Township, and Denham formations) in Minnesota have δ 13C values ranging from −1.2 to +2.5‰. Combined with geochronologic constraints, these data suggest that these units were deposited after the Lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized ∼2.0 Ga passive margin in the Lake Superior area.

Asteroid impact ejecta units overlain by iron-rich sediments in 3.5–2.4 Ga terrains, Pilbara and Kaapvaal cratons: Accidental or cause–effect relationships?

The significance of temporal and spatial associations between asteroid/comet impact ejecta units and overlying iron-rich sediments, including banded iron-formation (BIF), jaspilite and ferruginous shale, observed in the Pilbara Craton, Western Australia and the Barberton Greenstone Belt (BGB), Kaapvaal Craton, South Africa, is considered. Such associations include (1) 3470.1 ± 1.9 Ma impact spherule units and tsunami-type breccia overlain by jaspilite in the Antarctic Chert Member, central Pilbara Craton; (2) 3258 ± 3 Ma impact spherule unit (S2) in the BGB overlain by BIF, jaspilite and ferruginous shale; (3) 3243 ± 4 Ma impact unit (S3) in the BGB overlain by iron-rich sediments (Ulundi Formation); (4) two formations of BIF and ferruginous shale at the base (Nimmingarra Iron Formation) and lower part (Paddy Market Formation) of the Gorge Creek Group, central Pilbara Craton, which overlie 3235 ± 3 Ma felsic volcanics along a boundary correlated with the BGB-S3 unit; (5) 2629 ± 5 Ma impact spherule unit and tsunami-type deposit overlain by BIF and ferruginous shale (Marra Mamba Iron Formation), central Pilbara Craton; (5) 2481 ± 4 Ma spherule unit intercalated at the lower part of the Dales Gorge Iron Member of the Brockman Iron Formation, Hamersley Basin, and possible equivalents in the Kuruman Formation, western Transvaal. No significant thicknesses of iron-rich sediments are known to overlie carbonate-hosted impact ejecta units, including the Bee Gorge Member (BGM) of the Wittenoom Formation (2561 ± 8 Ma), Carawine Dolomite (eastern Hamersley Basin, Pilbara Craton), Monteville Formation and Reivilo Formation (Griqualand West Basin, Transvaal Group), or the Graensco–Vallen spherule occurrence (southwest Greenland). The juxtaposition between impact ejecta units and overlying BIF and jaspilite may be accidental or purely related to preservation of these units in below-wave-base environments. Alternatively, this association may hint at enrichment of seawater in soluble iron under low-oxygen atmosphere/hydrosphere conditions due to enhanced denudation of mafic volcanic terrains uplifted/exposed through impact-induced tectonic movements and/or impact-triggered mafic volcanic and hydrothermal activity. The general absence of iron-rich sediments above carbonate-hosted impact ejecta units may be attributed to lack of Fe enrichment or to pH and E h conditions unsuitable for the precipitation of iron oxides. The near-absence of BIF in high-energy shallow water environments overlying impact ejecta units may reflect retardation of colloidal/chemical sedimentation in such environments. A systematic association between impact ejecta and iron-rich sediments, if confirmed, may yield useful stratigraphic tracers in the search for impact signatures in early Precambrian terrains.

Mineral chemistry of sphalerite and galena from Pb-Zn mineralization hosted by the Transvaal Supergroup in Griqualand West, South Africa

The relative and absolute abundance of minor and trace elements in ore-forming sphalerite and galena can yield important constraints regarding the metallogenesis of carbonate-hosted Pb-Zn deposits. This contribution presents an extensive set of mineral chemical data for sphalerite and galena from two deposits and several subeconomic occurrences of Pb-Zn mineralization in the Transvaal Supergroup on the Ghaap Plateau in Griqualand West. Significant differences are recognised between different localities and different styles of mineralization, but results obtained compare rather well with literature data of Phanerozoic Mississippi Valley-type deposits. Fe and Cd constitute the two most important minor constituents in sphalerite from the Ghaap Plateau. Average concentrations vary between 0.54 weight % and 3.21 weight % for Fe and 0.13 weight % and 0.67 weight % for Cd. A first order correlation exists between average Fe/Mn ratios in sphalerite and host rock lithologies, while variations in concentrations of Co (7 to 177ppm), Ni (9 to 21ppm), Ga (2 to 99ppm) can only tentatively be attributed to differences in mineralization style, host rock, and metal source. Compared to sphalerite, trace element contents, in galena are very low with only Cd and Ag occurring in concentrations that consistently exceed 10ppm. Concentrations of more than 1000ppm Ag are reported for structurally-controlled Pd-rich subeconomic breccia runs close to the Griquatown fault zone.

Ancient sub-seafloor alteration of basaltic andesites of the Ongeluk Formation, South Africa: implications for the chemistry of Paleoproterozoic seawater

Basaltic andesites of the Ongeluk Formation, Transvaal Supergroup in Griqualand West constitute part of a large flood volcanic province that extruded at ca. 2.22 Ga onto the partly submerged Kaapvaal Craton. Pods and short veins of megaquartz, with albite, epidote and traces of calcite and Cu–Ni–Co sulphides are closely associated with beds and pods of jasper and chert that are products of low-temperature seafloor alteration. Characterization of the megaquartz pods and veins yields evidence for an origin in a “passive” seafloor alteration regime. Fluid inclusion studies suggest the involvement of two aqueous fluid end members, one NaCl-dominated with salinity similar to modern seawater, the other Ca-dominated and with distinctly elevated salinity. The chemical composition of both fluids is akin to that of seawater, modified in its composition by interaction with the volcanic host rock. The low salinity fluid appears only little affected by fluid–rock interaction processes; the composition of the more saline Ca-rich fluid is more distinctly modified. The chemical composition of the two fluids has important implications for our understanding of the composition of ocean water during the Paleoproterozoic Era. Cl/Br ratios, widely regarded as being conservative in hydrothermal solutions, are significantly below those of present-day seawater, but remarkably similar to that predicted for Archean seawater. This observation suggests that Paleoproterozoic seawater was still buffered by vent fluids, and lacked sufficient organic matter to fractionate Cl from Br.

The geology and genesis of high-grade hematite iron ore deposits

This paper presents a first summary of results of an ongoing study, started some two years ago, of high-grade iron ore deposits in South Africa, India and Brazil, including a comparison with the rather well-studied deposits of the Hamersley Province in Australia. Large, high-grade hematite iron ore bodies hosted by Precambrian banded iron formations are the world's most important source of iron ore. Despite their great economic importance, the origin of these deposits has remained rather enigmatic. Results of preliminary investigations into the geological setting, petrography and geochemistry of BIF-hosted high-grade hematite iron ore deposits in South Africa, India, Western Australia and Brazil reveal distinct similarities, suggest ing a very similar mode of hydrothermal and supergene-modified hydrothermal origin for most BIF-hosted highgrade hematite ore deposits. Differences between the deposits of this type are largely the result of varying intensities of supergene modification in Late Cretaceous to Tertiary times. Notable exceptions are the deposits in the Griqualand West region of South Africa, including the giant Sishen deposit, that are regarded to be of ancient supergene origin.

Seismic Stratigraphic Constraints on Neoarchean - Paleoproterozoic Evolution of the Western Margin of the Kaapvaal Craton, South Africa

The Kaapvaal Craton is partially covered by the supracrustal sequences of remnants of the Witwatersrand, Ventersdorp and Transvaal-correlated Griqualand West Basins that span about 800 million years (~3.0 to 2.2 Ga). This study describes and interprets eight newly available seismic reflection profiles of the Ventersdorp and younger cover sequences, acquired by the vibroseis method to 6 seconds TWT (~ 17km), and totalling ~720km in length. New stratigraphic and structural features are identified across three main regions: the Kaapvaal Craton’s western margin, the craton’s northern margin, (southwards from the Thabazimbi-Murchison Lineament across the western extremity of the Bushveld Igneous Complex) and the craton’s interior. Across the craton’s western margin, the volcano-sedimentary succession is shown to thicken towards its edge from 0 km to >17km over 150km. The Ventersdorp Supergroup underlies the Transvaal Sequence right to the western extremity of the craton. Close to the margin, we recognise a normal fault, here named the Moshaweng fault, with >6km normal displacement and an unknown distance of strike-slip displacement. Three prominent unconformities are identified, as well as ~300% tectonic thickening of the Hartley Formation in response to east-verging, Paleoproterozoic (~1.8 Ga) thrusting across the craton. Folding and thrusting of Griqualand West and Olifantshoek Supergroups across the craton are also apparent. A tectonic model for the evolution of the craton’s western margin involves the formation of an Atlantic-type margin by extension, volcanism and rifting over ~70 Ma, followed by 160 Myrs of thermal subsidence. Subsequently, Paleoproterozoic (1.7 to 1.9 Ga) compression involved folding and thrusting eastward across the margin. Along the northern margin of the craton, the seismic profiles traverse a syncline which trends east to west over several hundred kilometers and involves mainly Transvaal-aged rocks and sporadic Bushveld outcrop. Evident from these profiles are discrete Ventersdorp half grabens, up to 3km in depth, with bounding listric normal faults that trend east to west, subparallel to the Thabazimbi-Murchison Lineament (TML), and downthrow predominantly to the north. These faults are truncated by the well-imaged Black Reef unconformity. Across the craton interior, horst and graben structures, involving the Witwatersrand and lower Ventersdorp Formations, dominate the seismic profiles. Bounding listric normal faults (one example with minimum throw of 9.5km), trend east to west and northeast to southwest and downthrow to the north, south, northwest and southeast. The Black Reef unconformity is again distinctive, as is a Paleozoic unconformity (the Beaufort unconformity). The orientation of boundary faults of Ventersdorp-aged half-grabens along the craton’s margins and interior, south of the TML, suggests a complex process of extension tectonism for ~70 Myrs that overlaps in time with northward thrusting of volcano-sedimentary sequences to the north of the TML. On a regional scale, the data presented are consistent with a model for the Kaapvaal Craton that resembles, both in space and timing, the evolution of the Argentine Atlantic margin. This margin developed during an initial 40 to 70 Ma period of regional extension across a ~600 km wide volcano-tectonic rift zone behind an active (Andean) subduction zone, followed by regional lithospheric cooling and subsidence. Our results also confirm that the Kaapvaal Craton was part of a larger Meso-Archean landmass and thus supports growing realisation that models of Archean continental growth rates based on the present day aerial extent of Archean cratons, represent only minimum values.

Tropical laterites, life on land, and the history of atmospheric oxygen in the Paleoproterozoic

Research Article| June 01, 2002 Tropical laterites, life on land, and the history of atmospheric oxygen in the Paleoproterozoic Nicolas J. Beukes; Nicolas J. Beukes 1Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa Search for other works by this author on: GSW Google Scholar Herman Dorland; Herman Dorland 1Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa Search for other works by this author on: GSW Google Scholar Jens Gutzmer; Jens Gutzmer 1Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa Search for other works by this author on: GSW Google Scholar Munetomo Nedachi; Munetomo Nedachi 2Department of Planetary Sciences, Kagoshima University, Kagoshima 890, Japan Search for other works by this author on: GSW Google Scholar Hiroshi Ohmoto Hiroshi Ohmoto 3Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Nicolas J. Beukes 1Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa Herman Dorland 1Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa Jens Gutzmer 1Department of Geology, Rand Afrikaans University, Auckland Park 2006, South Africa Munetomo Nedachi 2Department of Planetary Sciences, Kagoshima University, Kagoshima 890, Japan Hiroshi Ohmoto 3Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Publisher: Geological Society of America Received: 10 Oct 2001 Revision Received: 14 Jan 2002 Accepted: 18 Jan 2002 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (6): 491–494. https://doi.org/10.1130/0091-7613(2002)030<0491:TLLOLA>2.0.CO;2 Article history Received: 10 Oct 2001 Revision Received: 14 Jan 2002 Accepted: 18 Jan 2002 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Nicolas J. Beukes, Herman Dorland, Jens Gutzmer, Munetomo Nedachi, Hiroshi Ohmoto; Tropical laterites, life on land, and the history of atmospheric oxygen in the Paleoproterozoic. Geology 2002;; 30 (6): 491–494. doi: https://doi.org/10.1130/0091-7613(2002)030<0491:TLLOLA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The ca. 2.2 Ga Hekpoort paleosol of the Transvaal Supergroup in southern Africa has been considered a type example and the youngest iron-depleted paleosol formed under a reducing atmosphere in the early Precambrian. However, the mineralogical and geochemical data on recently acquired deep drill core intersections indicate that the Hekpoort paleosol represents part of an ancient lateritic weathering profile with an iron-depleted pallid lower zone and an iron-enriched lateritic upper zone. Previous studies of the paleosol took place in areas where only the lower pallid zone was preserved from erosion prior to deposition of cover beds. The laterite profile is comparable to that of modern tropical laterites formed under an oxic atmosphere in the presence of abundant terrestrial biomass. Revised stratigraphic correlation indicates that the Hekpoort laterite profile is a correlative to highly ferruginous laterite profiles of Wolhaarkop in Griqualand West. This information indicates that the oxygen-evolution curve, based on loss or retention of iron in paleosols, should be reexamined. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Rare earth elements and Nd isotopic compositions in banded iron-formations of the Griqualand West Sequence, Northern Cape Province, South Africa

Rare earth elements and Nd isotopic compositions in banded iron-formations of the Griqualand West Sequence, Northern Cape Province, South Africa

Paleomagnetic Constraints on Ages of Mineralization in theKalahari Manganese Field, South Africa

We report paleomagnetic data from samples spanning several grades of enrichment in the Kalahari manganese field, South Africa, in order to assess mineralogical aspects of the ore-forming stages, and also to date these stages through comparison to previously existing, well dated paleomagnetic results from the Kaapvaal-Kalahari craton. Our paleomagnetic study confirms a multistage evolution for the orebodies, with three distinct, ancient remanent directions preserved. An early diagenetic remanence direction (MAM-1), associated with dusty hematite inclusions (1–10 μm) that are omnipresent in the microcrystalline matrix of low-grade, Mamatwan-type ore, yields a tilt-corrected paleomagnetic pole (–08.2° N, 111.1° E, dp = 5.6°, dm = 11.1°; n = 6 specimens) that is similar to previous results from the immediately underlying Ongeluk lavas. A late diagenetic or weak metamorphic overprint (MAM-2), carried by recrystallized hematite (20–250 μm), within both Mamatwan- and Wessels-type ore, generates a paleomagnetic pole (present coordinates 12.1° N, 321.8° E, dp = 3.4°, dm = 6.0°; tilt-corrected 16.1° N, 317.8° E, dp = 3.4°, dm = 6.4°; n = 14 specimens) that resembles those from the ca. 1900 Ma Hartley lavas and Mashonaland sills. The MAM-2 overprint may be related to Kheis thrusting at 1750 to 1800 Ma as previously proposed or to magma-driven fluid migration during rifting as the Hartley-Mashonaland igneous event perforated the Kalahari craton. The third magnetic component observed in our sample suite (WESS) is restricted to high-grade Wessels-type ore, rich in high Fe hausmannite and coarser hematite (0.1–1.0 mm), in the immediate vicinity of north-trending normal faults. It yields a pole (54.4° N, 033.7°E, dp = 4.7°, dm = 9.1°; n = 7 specimens) that is similar to both the ca. 1250 and 1100 Ma portions of the Kalahari craton’s apparent polar wander path. Either of these ages would be in accordance with previous multigenetic models for the Wessels event and its regional crosscutting relationships. Our WESS paleomagnetic pole, combined with previous paleomagnetic results from the Sishen-Postmasburg region, temporally links Kalahari manganese field hydrothermal upgrading with east vergent thrusting in the Griqualand West foreland, during the early or medial stages of the late Mesoproterozoic Namaqua orogeny.