Update on carbonatites of South Africa and Namibia

Brief descriptions are given of 43 carbonatite occurrences with emphasis on information that has come to light since 1967. This includes previously unpublished data on Keikamspoort, Welgevonden, Goudini, Mickberg, Weltevrede, Garub, Bokiesbank, Keishohe, Teufelskuppe, and Kaukausib. The descriptions attempt to summarize data like location, size, geology, mineralogy, geochemistry, and geochronology of each carbonatite occurrence, if available. They are grouped in five age groups showing little overlap, ranging from early Proterozoic to Tertiary, and it is concluded that the subcontinent does not show a pattern of repeated alkaline-carbonatitic activity at the same place. Some doubtful and discredited cases are also mentioned. -Autho

A review of South African research on volcanic rocks and mantle processes, 1987-1991

Although South African scientists are seldom concerned with active volcanism, some other topics falling under the dual ambit of the International Association of Volcanology and Chemistry of the Earth's Interior are of long-standing importance in South African geology, for example plateau basalts, ignimbrites, komatites, kimberlites and carbonatites. Most of this research is orientated towards the petrographic and geochemical characteristics of lavas, intrusives and deep-seated xenoliths in order to attempt to understand processes in ther upper mantle of the Earth. -from Autho

Carbon and oxygen isotope variations in southern African carbonatites

The C and O isotope composition of 54 carbonate samples from 18 carbonatite localities throughout southern Africa was determined. Sample frequency distributions indicate a dependence of the δ13C and δ18O values of southern African carbonatites on their carbonate mineralogy. The calcite carbonatite samples fall into the range -8‰ to -4‰ for δ13C and + 6‰ to +10‰ for δ18O, which is defined as the primary igneous carbonatite field (Taylor et al., 1967). Those samples with higher δ13C and δ18O values are either dolomite/ankerite carbonatites or contain dolomite/ ankerite, possibly indicating the involvement of different magmatic or hydrothermal phases in the carbonatite formation. Emplacement level, type of country rock intruded and secondary alteration by e.g. hydrothermal fluids, seem to be the most important parameters which affected the C and O isotope patterns found in southern African carbonatite complexes. Magmatic processes like fractional crystallisation and liquid immiscibility are of minor importance in controlling C and O isotope ratios. An alteration model of at least two stages is proposed for the evolution of the C/O isotope patterns observed in southern African carbonatites.The first stage involved magmatic-metasomatic processes during or shortly after the formation of carbonatites. These processes affected both the C and O isotope composition of carbonatites by exchange with a H2O/CO2-rich fluid and led to higher δ13C and δ18O values when compared to primary igneous values. The elevated 18O content of carbonatite carbonates was introduced by a second stage (or more stages) of post-magmatic, hydrothermal alteration and/or interaction with ground water at low (<200-250°C) temperatures. These alterations seem to be accompanied by a growing mineralogical diversity, which reflects, to some extent, the degree of alteration affecting the carbonatites. Information on stable isotope heterogeneities of the igneous sources of southern African carbonatites is obscured. Small scale C isotope heterogeneities within the igneous sources of carbonatites may be found, provided that the effects of a first stage, magmatic-metasomatic alteration can be eliminated. Changes in δ18O values of carbonatites by sub-solidus second stage alteration processes within the crust further complicate an estimation of stable isotope variations of igneous carbonatite sources. © 1997 Elsevier Science Limited.Articl

Geology at Stellenbosch: a legacy of Scottish Professors

[No abstract available]Articl

The Stukpan carbonatite in the Orange Free State Goldfield

The Stukpan Complex is interpreted to be an oval pipe-like body, 3.1 × 1.7 km in diameter, unconformably overlain by Karoo strata and a dolerite sill. One borehole penetrated an intrusive contact between fine-grained amphibole-rich calciocarbonatite and an alkaline amphibolite derived by fenitization from andesitic Ventersdorp lava. A second borehole encountered calciocarbonatite sills as well as red fenite and brecciated quartzite belonging to the Welkom Formation of the Central Rand Group. Geochemical logs show consistently high Sr contents in the carbonatite but low P, Nb, Zr, and REE. Despite erratic Cu and Ni values and sporadic sulphides no indications of economic mineralization have been found. Rb-Sr mineral separate geochronology yielded a well-constrained isochron age of 1354 ± 11 Ma for the carbonatite and confirmed a Ventersdorp (2700 Ma) age for the alkaline amphibolite. -from Author

Rare metal mineralization at the Salpeterkop carbonatite complex, Western Cape Province, South Africa

The Salpeterkop complex is an eroded central volcano of Late Cretaceous (66 Ma) age with a sediment-filled crater remnant 1 km in diameter associated with radial and concentric carbonatite/K-trachyte dykes as well as K-trachyte, breccia and olivine melilitite plugs. The sedimentary country rocks have been domed. Geochemical anomalies for Ti, Y, Zr, Nb, Th, V, Zn and Mo, based on a core-drilling programme, were found to outline a mineralized zone coinciding with the crater rim, especially on the western and southern sides. The rim rocks are volcanic breccias intimately mixed and veined by decomposed, silicified and ferruginized carbonatite and carbonatitic tuff. A large variety of very fine-grained minerals have been identified in a groundmass consisting mainly of goethite and chalcedony. The principal Nb ore mineral is niobian brookite occurring as microscopic disseminated grains of several morphological types. High-Nb (12% Nb2O5) and low-Nb (8% Nb2O5) brookites have been distinguished. Some of them contain inclusions of ilmenorutile. Delicate brookite crystal aggregates crystallized in situ. No discrete V and Zn minerals could be identified but Th occurs as thorianite and thorite, Zr occurs in eudialyte and an unidentified KNb silicate and Y occurs as xenotime. Nepheline and eudialyte are probably xenocrysts from an unexposed body of nepheline syenite. It is concluded that mineralization took place in three overlapping stages: 1. i)primary magmatic, with concentration of Nb, Zr, Th, V in both carbonatite and associated silicate rocks; 2. ii)mesothermal, with oxidation and introduction of Fe, Ti, REE, Mo and F; 3. iii)epithermal, with hydration and dissolution of carbonates, removal of Ca and Mg, redistribution of Y, Ti, Nb, Zn and deposition of Si, Ba, Mn and other mobile elements. The suggestion is made that the preservation and extent of the third stage, which may be linked to fumarolic activity, distinguishes the multi-element mineralization at Salpeterkop from other carbonatite complexes. Secondary alteration in the weathered zone merged with the third stage, but is less important. © 1995.Articl

Volcanoes of the Antarctic Plate and Southern Ocean

This volume attempts to present both a comprehensive overview of the south polar and subpolar volcanic provinces, and summary data on the status of knowledge of each volcano or volcano group. This broad region, covering roughly 6% of the Earth's surface, is infrequently visited and yet truly international. Its geoscience literature is dispersed in many journals, monographs, symposium volumes, and expedition reports, in many nations, and we hope that this summary will make it more widely available. The preparation of the volume has been a project of the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) through its Working Group on Antarctic Volcanology. It represents a substantial update and expansion of IAVCEI's 1960 Catalogue of Active Volcanoes for Antarctica (edited by W.H. Berninghausen and M. Neumann van Padang)

Aeolian processes and landforms in the sub-Antarctic: preliminary observations from Marion Island

Sub-Antarctic Marion Island has a hyperoceanic climate, with cold and wet conditions and consistently strong wind velocities throughout the year. Recent observations recognized the increasing role of aeolian processes as a geomorphic agent, and this paper presents the first data for transport by aeolian processes on a sub-Antarctic island. Data were collected through an intensive and high-resolution measurement campaign at three study sites using Big Spring Number Eight sediment traps and surface sediment samplers in conjunction with an array of climatic and soil logger sensors. Observed aeolian landforms are megaripples, and the data suggest that aeolian processes are also modifying solifluction landforms. The sediment traps and sediment samplers collected wind-blown scoria at all three study sites, and the annual (horizontal) aeolian sediment flux extrapolated from this preliminary data is estimated at 0.36–3.85 kg cm−2 y−1. Importantly, plant material of various species was trapped during the study that suggests the efficiency of wind for the dispersal of plants in this sub-Antarctic environment may be underestimated. This paper advocates long-term monitoring of aeolian processes and that the link between aeolian processes and synoptic climate must be established. Furthermore, wind as a means to disperse genetic material on Marion Island should be investigated