Browsing by Author "Reid, A M"

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    A comparative geological study of some major kimberlite pipes in the Northern Cape and Orange Free State
    (1982) Clement, Cameron Roger; Reid, A M
    In the first part of this thesis the petrological status of 'kimberlite' is defined and mineralogical and textural classifications which allow systematic documentation and collation of these complex rocks are evaluated. Quantitative mineralogical classification based on the modal abundances of the matrix minerals is advocated. A textural classification is adopted within which different textures are related to different modes and conditions of kimberlite emplacement. In the subsequent and major part of the thesis the results of a comparative geological study of six diamondiferous kimberlite pipes are reported. The pipes concerned are the De Beers, Wesselton, Dutoitspan, Bultfontein, Finsch and Koffiefontein occurrences (collectively termed the KIMFIK pipes).
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    Gahnite and its formation in the context of regional metamorphism and mineralization in the Namaqualand metamorphic complex
    (1988) Hicks, Judith Anne; Reid, A M; Moore, J M
    Gahnite (ZnAl₂O₄) is commonly associated with sulphide mineralization in metamorphosed massive sulphide deposits, and also occurs in marbles, pegmatites and quartz veins. Its formation has been attributed to the breakdown of Zn-staurolite or desulphidation of sphalerite during metamorphism. The stability of zinc-rich spinels under a wide range of metamorphic conditions in a variety of lithologies results in its persistence in rocks where many other prograde, high temperature minerals and sulphides have been altered. Thfs has resulted in various investigations into its use in exploration and potential for determining metamorphic parameters. With the interest in finding new ore bodies and in determining the metamorphic history and mineralogy in Namaqualand, some gahnite-bearing localities have been investigated in this study.
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    Geochemistry and mineralogy of selected Atlantic Ocean basalts
    (1980) Le Roex, Anton Powter; Erlank, A J; Reid, A M
    Bulk rock compositional variations in lavas from four localities in the Atlantic Ocean are evaluated quantitatively in terms of fractional crystallisation and partial melting models. Samples studied are from the Mid-Atlantic Ridge at 36°49 'N (FAMOUS), from the Islas Orcadas fracture zone on the Southwest Indian Ocean Ridge, from the Conrad fracture zone on the America-Antarctica Ridge and from Bouvet Island, situated at the southernmost tip of the Mid- Atlantic Ridge and in the vicinity of both the fracture zones mentioned above. Bulk rock major and trace element variations in selected basalts from the FAMOUS area, in conjunction with a detailed study of the chemistry of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationships between the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. The NiO content of these phenocrysts in the former three basalt types is low, or a given Mg Mg+Fe²⁺ atomic ratio of the mineral, relative to the phenocrysts in the olivine basalts. Consideration of the Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase- pyroxene basalts but, in addition, distinguishes the picritic basalts from the former three basalt types.
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    Sugilite and associated metamorphic silicate minerals from Wessels Mine, Kalahari manganese field
    (1988) Dixon, Roger; Reid, A M; Moore, J M
    Sugilite, a purple mineral belonging to the milarite group, occurs in the lower manganese orebody at Wessels Mine, in the Kalahari Manganese Field. This orebody was formed in the lowermost manganiferous horizon of the Hotazel Member of the Voelwater Jasper Formation, part of the volcanogenic sedimentary iron formation of the Griqualand West Sequence. At Wessels Mine, which is located northwest of Kuruman in the northwestern Cape Province, evidence of a widespread and pervasive hydrothermal event which took place around 1300 Ma is found in the form of upgrading of the manganese-ore horizons and the formation of zoned silicate-mineral assemblages. The presence of unusual minerals such as glaucochroite, iron akermanite, xonotlite and hydrogarnets of various types constrains the main phase of metamorphism to between 400 and 450 °C in a low pressure, hydrous environment with XCO2 ≤ 0,02. All the minerals which occur in these assemblages are described and discussed in terms of their chemistry and formation.
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    Surface textures on pyrope, picroilmenite and chrome diopside from Kimberlite
    (1981) Garvie, Owen George; Reid, A M
    The examination and interpretation of surface features on pyrope, picroilmenite and chrome diopside from kimberlite gives valuable information on the history of these minerals both during and after kimberlite emplacement. Descriptions and scanning electron photo-micrographs of surface features observed on these mantle-derived minerals from thirty-four kimberlites having a global distribution are presented. Careful observations established the age relationships between the different surface features on each of the minerals studied, and enabled the compilation of a classification scheme of these surface features. This proposed classification scheme is based on the chronological sequence of formation of th.e surface morphological characteristics observed on these mantle-derived minerals and separates thoie which are formed during kimberlite emplacement (primary features), from those resulting ffom in-situ weathering and diagenetic etch processes (secondary features). The mineralogy, structure and mode of formation of kelyphite are discussed in relation to kimberlite emplacement. Pyrope encrusted by kelyphite is commonly found in most garnet peridotites, some eclogite xenoliths from kimberlite, as well as a few garnet-bearing serpentinites. Although the relative abundance of kelyphite-encrusted garnet varies from one kimberlite to another, kelyphite shells are most commonly developed on mauve garnets which are most probably derived from peridotite and are least commonly found on orange varieties from eclogites.