Journal of African Earth Sciences (and the Middle East)最新文献

South Africa has more than 70 per cent of the coal resources of Africa, and coal forms the back-bone of the South African industry. In terms of the norms generally accepted for the Carboniferous coals of the Northern Hemisphere, South African coal has long been regarded as “abnormal”. However, these apparent abnormalities can be adequately explained in terms of the petrography which in turn reflects the conditions of peat formation and the subsequent metamorphism under a steep palaeogeothermal gradient.

In common with other Gondwana coals South African coals are generally rich in material transitional between vitrinite and inertinite (sensu stricto). This transition material is partly reactive during technological processes like carbonisation, and is therefore regarded as semi-reactive. South African coals are generally low in sulphur, nitrogen and phosphorus, and in the case of the first two the contents are dependent on maceral composition and rank. On account of the low basisity of the coal ash the ash fusion temperatures are generally high and this is an advantage in most areas of coal utilisation.

A review is given of adverse geological conditions affecting coal exploitation, and of methods that can possibly be used to recognise and predict of even eliminate these conditions for purposes of mine planning.

Maputaland, the coastal plain on the eastern sea-board of South Africa, is underlain by late Mesozoic to Quaternary sequences. It lacks any major mineral wealth, apart from heavy minerals in the Holocene coastal dunes. Mining of these dunes is a controversial environmental issue whilst the whole area is not suitable for industrialization. Development options for Maputaland therefore are largely limited to the rational and sustainable utilization of its renewable natural resources. The development and improvement of the region's agricultural output and eco-tourism industry are the best options to pursue in order to provide much needed employment for a rapidly growing and already impoverished rural community. The region has a high biodiversity due to its being a tropical-subtropical transition zone and to east-west variations in geology and climate. These have been the reasons for the establishment of various conservation areas. Future improvements to the agriculture and the reserves depend upon important geological input both on the environmental side, in order not to interfere with natural long-term changes, as well as on the applied side, particularly concerning the development of an infrastructure for the eco-tourism industry.

The dolomite rocks in South Africa coincide for the most part with the so-called Pretoria-Witwatersrand-Vereeniging complex which comprises the industrial heart of the country. Urban development will, therefore, also be concentrated in this area, with the inevitable establishment of townships on the dolomite. This development has great impact on the dolomitic environment and vice versa.

The most important environmental aspects which influence Man's occupation of the dolomite environment include the formation of sinkholes and dolines (compaction subsidences) and also the potential pollution of the dolomite aquifers. These aquifers consist of weathered, blanketing materials overlying the bedrock with sometimes shallow water levels, high permeabilities, storativities and transmisivities. All these factors contribute to the excellent groundwater resource in the dolomite rocks as well as to their high pollution potential.

The environmental hazards associated with development of dolomite land is discussed with reference to loss of life, financial implications and efforts to reduce the impact of development. The importance of pre-emptive and appropriate land management is stressed. A clear understanding of the dolomite environment is, therefore, essential.

Much research has been conducted on these banded iron formations (BIF) over the last 15–20 years. This contribution seeks to provide an overview of old and new facts and critical discussion on the latest ideas regarding the origin of these sediments in the early Proterozoic. The recently suggested stratified ocean water model and a new stratified lake water model are compared using new evidence of a stratigraphic, major and trace element, stable isotope and REE nature. It appears that any hypothesis on the genesis of these Transvaal Supergroup rocks will have to satisfactorily account for at least the following:

• 1.

  i) A constant supply of enough Fe and Si over at least several hundred thousand years.

• 2.

  ii) A macro- and a micro-cyclicity. The latter is the most basic building stone and any hypothesis that can not explain these phenomena must be considered unrealistic.

• 3.

  iii) The stratigraphic and isotopic evidence for heterogeneity of the waterbody.

• 4.

  iv) The REE and trace element evidence for contributions from different sources.

• 5.

  v) The distribution of organic carbon in rock facies and minerals.

• 6.

  vi) The difference between Proterozoic and present day atmospheres and surface waters.

• 7.

  vii) The fossil record of the early Proterozoic, and coupled to this the role that chelation, complex formation, stable colloids and co-precipitation played in weathering, transportation and deposition of Fe.

• 8.

  viii) Factors influencing or controlling cyclicity.

• 9.

  ix) The role that atmospheric and crust-mantle evolution plays in producing most large BIF-deposits over a time span of about 500 Ma from the late Archaean into the early Proterozoic.

• 10.

  x) The concomitant evidence provided by early Proterozoic paleosols.

• 11.

  xi) The fact that several large Proterozoic BIF deposits are immediately preceded by platform carbonates.

Finally, the two important ore districts, Sishen in the Northern Cape Province and Thabazimbi in the Central Transvaal, are dealt with. The general geology, mineralogy and genesis of these very high-grade major deposits are presented. New information on ore morphology and new evidence on multiple epigenetic enrichment of BIF-protore are presented and discussed. The modern and specialised ore-mining, -processing and blending techniques at Sishen are explained.

The combination of rugged topography and climate predisposes the province of Natal to severe floods. Information available since 1856 shows that bridge and slope failures have been recorded in twenty out of twenty-five flood episodes. Bridge failures are caused mostly by geological factors. The mechanism of failure can be classified broadly into foundation failures and changes of river course. Scour and debris build-up have led to failures of foundations located in rock and alluvial sediments. In preparing and replacing bridges the aims have been to increase the area of waterway, increase foundation depths to reach more competent strata and lay protection along banks and abutments to counteract scour. Historically, slope failures have not been well documented but following the 1987/88 storms 223 slope failures were recorded. The classification of the failures allowed the mechanisms of failure to be ascertained, and general design considerations to be reviewed. In areas adjacent to the Drakensberg Mountains slope failures are part of a natural erosion cycle which may be accelerated in periods of heavy rain. Throughout Natal, hummocky ground adjacent to dolerite intrusions reveals the on-going history of failure caused by water ingress and the generation of high pore water pressures on the slip planes. Classic flows occurred throughout the Greater Durban area where residual sandy soils of the Natal Group sandstone became supersaturated. Slumping was common on steep terrain underlain by granite-gneiss in the Kwa-Zulu area. Shales of the Pietermaritzburg Formation are notoriously unstable, yet few failures occurred during the summer storms of 1987/88. Inadequate drainage was responsible for many failures, this was particularly so along the railways.

Palaeomagnetic, isotopic and chemical evidence confirm that the emplacement of the complex was achieved by multiple injections of magma into the chamber. Gravity, magnetic, resistivity and regional seismic data favour the emplacement of the separate limbs as inclined, wedge-shaped bodies without continuity through the geographic contre of the complex. Pre-Bushveld faulting, and the folding of floor rocks, may have exerted significant control on the distribution, thickness and lithology of the Rustenburg Layered Suite (RLS). Palaeomagnetic data imply that the layering was initially horizontal, at least down to the Curie temperature of magnetite. The concept of an Integration Stage (Wager and Brown, 1968) should be extended to include the last major episode of magma addition, and mixing with liquid residua, that gave rise to the upper part of the Main Zone and the Upper Zone. Superimposed on smaller-scale fluctuations of composition within individual units of cyclically banded cumulates is a broad pattern of mega-cyclicity that is traceable through the lowermost 2000 m of the RLS. Normal fractionation trends are here traceable through intervals 200–400 m thick, with intervening sequences of progressive reversal 50–300 m thick. Such major reversals culminate in olivine-rich cumulates. Isotopic inhomogeneity within individual cyclic units, and even within single samples, as well as distinctive textures indicative of resorption of earlier crystalline phases, support a model of periodic mixing of residual liquid-plus-crystal mushes with batches of fresh, primitive liquid of essentially the same lineage during growth of the Lower and Critical Zones. The Main Zone constitutes a discrete lineage. The northwestern sector is identified as a proximal facies of the Western limb, where its main feeder zone was located. Over a strike distance of ca. 170 km, both Upper and Lower Critical Zone cumulates grade to more feldspathic and chemically evolved sequences of the distal facies. Profiles showing the increase of Ti and Al in orthopyroxenes, with stratigraphic height, are inflected at the level where cumulus plagioclase enters the paragenesis. On this criterion it is also possible to distinguish proximal and distal norites. The close association of PGF with chromitites is emphasized, and progressive increase in the metal ratios (Pt+Pd+Rh)/(Ru+Os+Ir) is correlated with progressive evolution of Cr-spinel compositions with stratigraphic height. The close lithological, chemical and isotopic resemblance between the UG1 and overlying UG2 units poses the problem of explaining why the former is sub-economically mineralized, whereas the latter is a major orebody. Current genetic models for PGE mineralization, based on the control of liquid densities by plagioclase crystallization, may explain Merensky Reef-type orebodies, but are not of general application in exploration.

The Lesotho Highlands Water Project is situated in the Kingdom of Lesotho and the adjoining north-easthern part of the Orange Free State Province of the RSA in an areaunderlain by Triassic and Jurassic basalts of the Lesotho Formation and Triassic sandstones and mudrocks of the Karoo Sequence. The Project will consist of a series of dams and tunnels to convey water from the Lesotho Highlands to the industrial centre of the Republic of South Africa.

The geological setting of the project area and some engineering geological properties of the underlying stratigraphic horizons is discussed.

The geotechnical properties of the different rock types are discussed with special reference to their use as concrete aggregate. The durability of basalt is primarily determined by the amount of secondary smectite clays present in the rock. These clays occur as discrete grains or as intergrowths with other secondary minerals disseminated throughout the rock. The clays originate from the deuteric alteration of primary glass, pyroxene, olivine and rarely plagioclase and also occur as infillings of amygdales. The durability of the sedimentary rocks is a function of their dimensional change with variability in moisture content and also the degree of cementation between grains.