Browsing by Subject "Petrology"

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    Alkaline igneous rocks of the coastal belt, south of Luderitz, South West Africa : a petrological study
    (1973) Marsh, Julian Saville
    The Luderitz Alkaline Province, as it is at present known, comprises the subvolcanic central complexes of Drachenberg, Pomona, and Granitberg. An attendant dyke swarm strikes NE-SW and crops out between the latitudes of 27°00' and 27°30' S. Stratigraphic indications (now confirmed by a K/Ar age from Granitberg) are that the Luderitz Province is early-Cretaceous in age and therefore older than the Klinghardt phonolites (Eocene) as well as the smaller melilitite and nephelinite intrusions. Granitberg is a circular foyaite complex, in the centre of which is preserved a large fragment of sedimentary rocks that originally formed the roof of the intrusion. The foyaites have been emplaced into the feldspathic sandstones and dolomites of the Bogenfels Formation, and three major intrusive phases can be recognised. The first phase produced chilled nepheline syenites beneath the roof of the intrusion. These chilled rocks grade downwards into coarse-grained foyaites. The second phase was the emplacement of the Inner Foyaite which crystallized as a cylindrically zoned plug, capped by a zone of layered, laminated, and xenolithrich foyaites. The third phase was the emplacement of the Outer Foyaite, into which the Roof Zone and the Inner Foyaite foundered. The Outer Foyaite is zoned with a miaskitic core, and an agpaitic outer zone.
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    Petrology and petrography of samples from two Algoa Basin cores
    (1973) Stewart, Allan Graham; Fuller, A O
    Ninety-four samples from two cores drilled in the Uitenhage Group of the Algoa Basin were analysed. The group of sedimentary rocks comprises the Enon, Kirkwood and Sundays River Formations and consists of continental (Enon and Kirkwood Fms.) and marine (Sundays River Fm.) deposits typical of a transgression sequence into an intermontane valley. Only the Kirkwood and Sundays River Formations were intersected in the boreholes but in one - AD 1/68 'basement' of Bokkeveld Group is reached, while the other borehole, CO 1/67, did not reach 'basement'. The Kirkwood Formation sediments are typically red shales interbedded with drab, fine sandstones (wackes) while the Sundays River Formation is typically drab fine to very fine silty sandstones and gray shales of a marine delta environment. Grain size analyses utilizing wet sieving and pippeting methods showed that two populations of grains are dominant, namely fine to very fine silty sand and clay size material. From the granulometric analyses statistical parameters (e.g. mean, standard deviation, skewness, kurtosis) were calculated and plotted in various combinations. Distinctive trends are thus revealed indicating a mixing of the two sediment populations in varying proportions. Compaction has resulted in an increase with depth in specific gravity (which varies from about 2,5 to +2,6 gm cc⁻³). Studies of the heavy mineralogy reveals a dominance of garnet (two varieties, colourless and pink) with zircon, sphene, rutile and some others in considerably lesser amounts. Opaque grains are also present, sometimes in dominant amounts. X-ray diffraction analyses of the <2 μm clay fraction showed that illite is dominant in both the marine and terrestrial deposits and that chlorite is abundant to infrequent, while montmorillonite is more prevalent in the continentally deposited rocks. Selected clay samples were photographed with both the Transmission and the Scanning Electron Microscopes. Thin sections of the arenaceous samples reveal that those which are carbonate cemented are relatively free from matrix, while those which are uncemented are matrix-rich. This latter situation can be ascribed to the breakdown, after burial, of the commonly occurring rock fragments which frequently constitute about one-third of the sandstones. The problem of red beds is considered, and the red pigmentation found in the Kirkwood Formation is believed to be due to oxidation of iron after deposition of the sediment. Intense weathering in the upland source area is not a suitable explanation for the formation of the Kirkwood Formation red beds. The basin as a whole is considered with attention being focussed on the provenance areas (believed to be the Cape and Lower Karroo Supergroups), the dispersal, depositional environments and the lithification and diagenesis. Finally, the economic potential is briefly considered and some suggestions for future research are put forward.
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    The petrology of the Kielder sulphide bodies and their wall rocks : district of Prieska, N. Cape, South Africa
    (1981) Gorton, Richard Kalmer; Patterson, Colin
    Recently discovered base metal (Zn>Cu>>Pb) massive sulphide deposits in the Northern Cape district of Prieska occur at Kielder 12 kilometres north-west of a similar Zn-Cu deposit, the Copperton mine. Three massive sulphide bodies known as K3, Kl and K6 occur as stratabound massive sulphide lenses within granulite grade quartz-feldspar gneisses, basic granulites and amphibolites. Extensive exploratory core-drilling provided specimens of the massive sulphides, their disseminated pyritic haloes and the enclosing wall rocks in an area of poor outcrop with extensive calcrete and sand cover and, in places,in situ Karoo Super-group Owyka System varved shales.Geothermometry and geobarometry using garnet-biotite, garnetcordierite,garnet-hypersthene,and the FeS content of sphalerites showed a gradual metamorphic gradient from east to west, with the K3 area suffering P-T conditions of 695⁰ C and 6. 0 Kbars; the Kl area 686⁰ C and 5.8 Kbars and the K6 area 590⁰ C and 5.6 Kbars.
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    Why was silcrete heat-treated in the Middle Stone Age? An early transformative technology in the context of raw material use at Mertenhof Rock Shelter, South Africa
    (Public Library of Science, 2016) Schmidt, Patrick; Mackay, Alex
    People heat treated silcrete during the Middle Stone Age (MSA) in southern Africa but the spatial and temporal variability of this practice remains poorly documented. This paucity of data in turn makes it difficult to interrogate the motive factors underlying the application of this technique. In this paper we present data on heat treatment of silcrete through the Howiesons Poort and post-Howiesons Poort of the rock shelter site Mertenhof, located in the Western Cape of South Africa. In contrast to other sites where heat treatment has been documented, distance to rock source at Mertenhof can be reasonably well estimated, and the site is known to contain high proportions of a diversity of fine grained rocks including silcrete, hornfels and chert at various points through the sequence. Our results suggest the prevalence of heat treatment is variable through the sequence but that it is largely unaffected by the relative abundance of silcrete prevalence. Instead there is a strong inverse correlation between frequency of heat treatment in silcrete and prevalence of chert in the assemblage, and a generally positive correlation with the proportion of locally available rock. While it is difficult to separate individual factors we suggest that, at Mertenhof at least, heat treatment may have been used to improve the fracture properties of silcrete at times when other finer grained rocks were less readily available. As such, heat treatment appears to have been a component of the MSA behavioural repertoire that was flexibly deployed in ways sensitive to other elements of technological organisation.