Harilaos Tsikos

Deconstructing the Transvaal Supergroup, South Africa: implications for Palaeoproterozoic palaeoclimate models

Abstract Current correlations between the Pretoria and Postmasburg Groups of the Transvaal Supergroup are shown to be invalid. The Postmasburg Group is also demonstrated to be broadly conformable with the underlying Ghaap Group and therefore considerably older (∼2.4 Ga) than previously supposed. The new stratigraphy documents an extensive (100 Ma) and continuous cold-climate episode with a glacial maximum at the Makganyene Formation diamictite. Iron formations of the underlying Asbesheuwels and Koegas Subgroups and overlying Hotazel Formation have similar origins, related, respectively, to the onset and cessation of the glacial event. This interpretation of the Transvaal Supergroup stratigraphy has significant implications for various Palaeoproterozoic environmental models and for the timing of the development of an oxygenated atmosphere.

A sugilite-bearing assemblage from the Wolhaarkop breccia, Bruce iron-ore mine, South Africa: Evidence for alkali metasomatism and 40Ar-39Ar dating

An occurrence of the alkali-rich, Fe-Mn silicate sugilite [KNa2(Fe3+, Mn3+, Al)2Li3Si12O30] is reported from the Wolhaarkop manganese-rich chert breccia at the Bruce iron-ore mine, Northern Cape Province, South Africa. The upper portion of the breccia is dominated by braunite, albite, K-feldspar and aegirine, and locally hosts irregular-shaped void-fills (generally <5 cm) consisting of both zoned and unzoned assemblages of alkali-rich mn-silicates. the zoned void-fills are characterised by marginal needles of a mineral with norrishite-like composition [kmn3+2LiSi4O10(O)2] penetrating into a persistent wall zone that comprises tabular serandite [NaMn2Si3O8(OH)], albite, K-feldspar and lesser granular sugilite. The cores of larger vugs are occupied by masses of interlocking armbrusterite [K5Na6Mn3+Mn2+14(Si9O22)4(OH)10.4H2O] and/or fibrous sugilite. Unzoned assemblages contain a similar mineralogy, comprising albite, K-feldspar, quartz, sugilite, norrishite, serandite, armbrusterite and minor witherite, strontia- nite and kentrolite [PbMn2O2(Si2O7)]. Similarities to sugilite-bearing assemblages at the Wessels mine in the Kalahari Manganese Field suggest that a single regional hydrothermal alteration event in the vicinity of the unconformity between the Olifantshoek and Transvaal Supergroups was the probable genetic cause for both occurrences. 40Ar-39Ar dating of fibrous sugilite, however, yields an age of 620.2 ± 3.3 Ma (2σ) that is distinctly younger than the ages reported for the assemblages from the Kalahari Manganese Field.

Glendonites (pseudomorphs after ikaite) in late carboniferous Marine Dwyka beds in Southern Africa

Glendonites are carbonate pseudomorphs after monoclinic crystals of ikaite (calcium carbonate hexahydrate: CaCO 3 .6H 2 O). Ikaite forms at present within organic-rich marine or brackish sediments that are at near freezing temperatures and decomposes to a mush of calcium carbonate and water when the temperature rises above 5°C. These unusual pseudomorphs were first recognised in southern Africa in 1972 as a rare component in marine shales just above the top of the Dwyka Group in the Great Karoo basin in South Africa. The crystal casts are rhomb-shaped in cross-section, show well-developed pyramid faces and are up to 19cm long. The glendonites occur in two forms: 1. as hollow or ochre-filled casts of ikaite crystals in concretions in fossiliferous marine Dwyka shales at Blaauwkrantz near Kimberley and also in the Ganigobis Shale at Zwartbas on the Orange River. 2. as carbonate pseudomorphs after ikaite, in concretions within shaly siltstone in the upper part of the marine glacial boulder mudstone at De Kalk (adjacent to Blaauwkrantz). The concretions are unweathered and the crystal casts are filled with a hard dark brown granular carbonate. Each granule encloses one or more stellate aggregates of zoned calcite crystals with an outer layer of brown carbonate. Electron microprobe analysis shows that the stellate crystals are pure calcite CaO: 55% (by wt.), Mg: 0.19% and the enclosing brown carbonate layer is Fe/Mg/Mn-enriched calcite (FeO+MgO+MnO: 6.18%, CaO: 48.5%). The presence of glendonite demonstrates that the fossiliferous host sediments accummulated under near freezing, highly alkaline, orthophosphate-rich bottom water conditions. These stressful conditions help to explain the observed severely limited species diversity of the associated fossil assemblage.

Prehnite from the Kalahari manganese field, South Africa, and its possible implications

Prehnite is described herein from the N’Chwaning II mine, Kalahari manganese field for the first time. The habit and composition are atypical for the species. The mineral occurs as elongated, prismatic orthorhombic yellow-orange crystals arranged in sprays and/or aggregates. It is associated with hydroxyapophyllite, datolite, ferroan inesite, calcite and pectolite. SEM, microprobe and XRF analyses reveal typical chemical compositions in terms of SiO2, Al2O3 and CaO values, but the mineral has very low FeO and Na2O contents, and MnO (±0.4 wt%) is an order of magnitude higher than published values for prehnite. Stable (H and O) isotope data for structurally-bound H2O are consistent with a meteoric origin for the precipitating fluid. The existence of prehnite in the Kalahari manganese field provides evidence in support of low-temperature (150–250°C) formation of the mineral and possible metamorphism in the north-westernmost portion of the field, but its importance in terms of processes involving metal upgrading in the Kalahari manganese ores is uncertain.