Browsing by Author "Richardson, Steve"

Now showing 1 - 4 of 4
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    Determining the physico-chemical conditions on the early earth : Barberton scientific drilling project, South Africa
    (2011) Grosch, Eugene Gerald; Richardson, Steve
    Low-grade metabasites and hydrothermally altered ultramafic rocks comprise most of the ca. 3.5-3.2 Ga, Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. However, PT-constraints and detailed petrological information on the variably altered mafic-ultramafic rocks are sparse and the nature of greenschist facies metamorphism is poorly characterized. In addition, alteration and silicification associated with distinct emerald green, fuchsite-(Crmica)- carbonate-quartz alteration zones is commonly found beneath chert horizons in the mafic-ultramafic rocks of the Onverwacht Group of the BGB. The origin of this silicification is highly debated and has important bearing on the nature of Paleoarchean geodynamic models, crustal geothermal gradients, hydrothermal oceanic regimes, as well as potential early life-sustaining environments. Over the past four decades, widely varying interpretations have been reported for the origin of these silica-rich fuchsitic alteration zones in the BGB, ranging from very low temperature (less than 60oC) atmospheric weathering of komatiites (Lowe and Byerly, 1986; 1999); low-temperature (less than 125oC) seafloor alteration (Hofmann and Harris, 2008); to bedding-parallel ‘flaser-banded gneissesschists’ in oceanic ‘extensionaloverthrust glide planes’ (de Wit, 1982a; 1986a,b; de Wit et al., 1982b; 1987b; 2011). Consequently, these fuchsitic-alteration zones have been at the centre of much debate surrounding stratigraphy models and the possible operation of early Paleoarchean platetectonic processes in the BGB.
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    A geochemical study of diamonds from Cullinan diamond mine, South Africa
    (2005) Whitehead, Kerryn; Richardson, Steve; Gurney, John J
    The Cullinan kimberlite is a Group I kimberlite and is located in the northeastern region of the Kaapvaal Craton, South Africa. The kimberlite pipe has been dated at 1180 ± 30 Ma and intrudes the Bushveld Igneous Complex (2.05 Ga). This study explores the geochemistry of a suite of one hundred selected diamonds and their associated mineral inclusions. The majority of the diamonds described here are peridotitic (94%) and the remainder are eclogitic. The peridotic inclusions may be further subdivided into harzburgitic and lherzolitic parageneses.
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    A geochemical study of diamonds, diamond inclusion minerals and other mantle minerals from the Klipspringer kimberlites, South Africa
    (2000) Westerlund, Kalle; Gurney, John J; Richardson, Steve
    The Klipspringer kimberlites occur in the north-eastern part of the Kaapvaal craton which is poorly represented in kimberlite and mantle studies. The kimberlites have been dated at 148+-4 Ma and the current study presents the geochemical characteristics of their diamonds, diamond inclusion and mantle macrocrysts/bimineralic nodules. The Klipspringer lithosphere is dominated by eclogite and Iherzolite with harzburgite and dunite present to a lesser extent.
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    Sulphide Re-Os characterisation and nitrogen aggregation state of the Ellendale diamonds, Kimberley Province, Australia
    (2008) Smit, Karen Vena; Gurney, John J; Richardson, Steve; Le Roex, Anton; Shirey, Steve; Smith, Craig
    A suite of sulphide-bearing diamonds recovered from the Ellendale 4 and 9 pipes in the Ellendale diamond province of lamproite intrusions in north-western Australia have been investigated for their nitrogen aggregation state and the Re-Os isotope geochemistry of the sulphide inclusions. The Ellendale lamproites, dated at ca. 20 Ma (Allsopp et at., 1985) intrude the King Leopold Mobile Belt just south of the Kimberley craton and are thus an atypical, off-craton diamondiferous locality. The diamonds contain roughly equal proportions of peridotitic and eclogitic inclusions (Hall & Smith, 1984; Griffin et at., 1988; Jaques et at., 1989). The diamonds in this study range in size from 0.26 to 0.92 carats and are dominated, with the exception of one octahedron, by highly resorbed tetrahexahedroida.