Kimberlite-hosted diamond deposits of southern Africa: A review

Following the discovery of diamonds in river deposits in central South Africa in the mid nineteenth century, it was at Kimberley where the volcanic origin of diamonds was first recognized. These volcanic rocks, that were named “kimberlite”, were to become the corner stone of the economic and industrial development of southern Africa. Following the discoveries at Kimberley, even more valuable deposits were discovered in South Africa and Botswana in particular, but also in Lesotho, Swaziland and Zimbabwe.A century of study of kimberlites, and the diamonds and other mantle-derived rocks they contain, has furthered the understanding of the processes that occurred within the sub-continental lithosphere and in particular the formation of diamonds. The formation of kimberlite-hosted diamond deposits is a long-lived and complex series of processes that first involved the growth of diamonds in the mantle, and later their removal and transport to the earth's surface by kimberlite magmas. Dating of inclusions in diamonds showed that diamond growth occurred several times over geological time. Many diamonds are of Archaean age and many of these are peridotitic in character, but suites of younger Proterozoic diamonds have also been recognized in various southern African mines. These younger ages correspond with ages of major tectono-thermal events that are recognized in crustal rocks of the sub-continent. Most of these diamonds had eclogitic, websteritic or lherzolitic protoliths.In southern Africa, kimberlite eruptions occurred as discrete events several times during the geological record, including the Early and Middle Proterozoic, the Cambrian, the Permian, the Jurassic and the Cretaceous. Apart from the Early Proterozoic (Kuruman) kimberlites, all of the other events have produced deposits that have been mined. It should however be noted that only about 1% of the kimberlites that have been discovered have been successfully exploited.In this paper, 34 kimberlite mines are reviewed with regard to their geology, mantle xenolith, xenocryst and diamond characteristics and production statistics. These mines vary greatly in size, grade and diamond-value, as well as in the proportions and types of mantle mineral suites that they contain. They include some of the world's richest mines, such as Jwaneng in Botswana, to mines that are both small and marginal, such as the Frank Smith Mine in South Africa. They include large diatremes such as Orapa and small dykes such as those mined at Bellsbank, Swartruggens and near Theunissen. These mines are all located on the Archaean Kalahari Craton, and it is apparent that the craton and its associated sub-continental lithosphere played an important role in providing the right environment for diamond growth and for the formation of the kimberlite magmas that were to transport them to the surface.     

Ocean current intensification during the Cretaceous oceanic anoxic event 2 – evidence from the northern Tethys

Mesozoic Oceanic Anoxic Events (OAEs) are expressions of major physical oceanographic changes at times of perturbation of the global carbon cycle. A northern Tethyan record of OAE2 is preserved in expanded Cenomanian–Turonian pelagic limestone sections (Seewen Formation) in Eastern Switzerland. The new carbonate carbon‐isotope stratigraphy extracted from these limestones demonstrates that the OAE2 is condensed in all the studied successions and only the onset of the δ¹³C excursion (5.0‰) is present. The condensed interval is characterized by dissolution features, which are filled by a glauconite quartz sandstone. This bed is overlain by a well‐sorted sandstone with intercalated limestone pebbles (Götzis Member), which can be compared with palimpsest sands forming today along current‐swept shelves. The wide distribution of this thin sandstone layer within OAE2 indicates that an intense, erosive, east‐west trending shelf current was active during the highest sea level and most extreme carbon‐cycle perturbation of the OAE2.     

Melt water driven stream and groundwater stage fluctuations on a glacier forefield (Dammagletscher,Switzerland)

In many mountain regions, large land areas with heterogeneous soils have become ice‐free with the ongoing glacier retreat. On these recently formed proglacial fields, the melt of the remaining glaciers typically drives pronounced diurnal stream level fluctuations that propagate into the riparian zone. This behaviour was measured on the Damma glacier forefield in central Switzerland with stage recorders in the stream and groundwater monitoring wells along four transects. In spite of the large groundwater stage variations, radon measurements in the near‐stream riparian zone indicate that there is little mixing between stream water and groundwater on daily time scales. At all four transects, including both losing and gaining reaches, the groundwater level fluctuations lagged the stream stage variations and were often damped with distance from the stream. Similar behaviours have been modelled using the diffusion equation in coastal regions influenced by tidal sea level variations. We thus tested the ability of such a model to predict groundwater level fluctuations in proglacial fields. The model reproduced several key features of the observed fluctuations at three of four locations, although discrepancies also arise due to non representative input data and model simplifications. Nevertheless, calibration of the model for the individual transects yielded realistic estimates of hydraulic diffusivities between the stream and groundwater monitoring wells. We conclude that studying diurnal groundwater fluctuations can provide important information about the subsurface hydrology of alpine watersheds dominated by glacier melt. Copyright © 2012 John Wiley & Sons, Ltd.     

Relict rock glaciers and protalus lobes in the British Isles: implications for Late Pleistocene mountain geomorphology and palaeoclimate

Many relict rock glaciers and protalus lobes have been described in mountainous areas of the British Isles. This paper reviews their distribution, chronology, supposed origin and development, and places the research within current investigations and knowledge. Rock glaciers and protalus lobes are located in a number of different topographic locations and settings. They developed at the base of steep cliffs following the catastrophic failure of rock faces, at the base of scree slopes following the gradual accumulation of rock debris and in association with glaciers. Protalus lobes probably developed in response to the permafrost creep of talus material while rock glaciers formed through the deformation and sliding of large bodies of buried ice. Rock glaciers probably developed, or were last active, during the Younger Dryas, although the possibility exists that some of these landforms are Dimlington Stadial in age. The development of protalus lobes during the Younger Dryas suggests that precipitation levels were low and permafrost was widespread during this time. The lack of rock glaciers (sensu stricto) in the British Isles compared with other mountain areas is believed to be a consequence of the rock type and relative scarcity of weathered debris for their formation rather than a lack of suitable sites or appropriate environmental conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

The geochemical composition of the terrestrial surface (without soils) and comparison with the upper continental crust

The terrestrial surface, the “skin of the earth”, is an important interface for global (geochemical) material fluxes between major reservoirs of the Earth system: continental and oceanic crust, ocean and atmosphere. Because of a lack in knowledge of the geochemical composition of the terrestrial surface, it is not well understood how the geochemical evolution of the Earth’s crust is impacted by its properties. Therefore, here a first estimate of the geochemical composition of the terrestrial surface is provided, which can be used for further analysis. The geochemical average compositions of distinct lithological classes are calculated based on a literature review and applied to a global lithological map. Comparison with the bulk composition of the upper continental crust shows that the geochemical composition of the terrestrial surface (below the soil horizons) is significantly different from the assumed average of the upper continental crust. Specifically, the elements Ca, S, C, Cl and Mg are enriched at the terrestrial surface, while Na is depleted (and probably K). Analysis of these results provide further evidence that chemical weathering, chemical alteration of minerals in marine settings, biogeochemical processes (e.g. sulphate reduction in sediments and biomineralization) and evaporite deposition are important for the geochemical composition of the terrestrial surface on geological time scales. The movement of significant amounts of carbonate to the terrestrial surface is identified as the major process for observed Ca-differences. Because abrupt and significant changes of the carbonate abundance on the terrestrial surface are likely influencing CO₂-consumption rates by chemical weathering on geological time scales and thus the carbon cycle, refined, spatially resolved analysis is suggested. This should include the recognition of the geochemical composition of the shelf areas, now being below sea level.     

Earthquakes in Switzerland and surrounding regions during 2011

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2011. During this period, 522 earthquakes and 92 quarry blasts were detected and located in the region under consideration. With a total of only 10 events with M _L????2.5, the seismic activity in the year 2011 was far below the average over the previous 36?years. Most noteworthy were the earthquake sequence of Sierre (VS) in January, with two events of M _L 3.3 and 3.2, the M _L 3.3 earthquake at a depth of 31?km below Bregenz, and the M _L 3.1 event near Delémont. The two strongest events near Sierre produced shaking of intensity IV.     

Soil column experiments to quantify vadose zone water fluxes in arid settings

For the determination of groundwater recharge processes in arid environments, vadose zone water fluxes and water storage should be considered. To better understand and quantify vadose zone processes influencing groundwater recharge, a soil column experimental setup has been developed that mimics arid atmospheric conditions and measures water and temperature fluxes in high temporal and spatial resolution. The focus of the experiment was on the determination of water infiltration, redistribution, evaporation and percolation under non-isothermal conditions. TDR rod sensors and a specific TDR “Taupe” cable sensor were used for water content measurements and allowed the infiltration fronts to be traced over the whole column length. Applying single irrigations of different amount and intensity showed the applicability of the experimental setup for the measurement of water movement in the unsaturated soil column.