Browsing by Author "Kitt, Shawn"

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    Geological setting and evolution of the Omitiomire Cu deposit in the Southern Zone accretionary prism of the Damara Belt, Namibia
    (Stellenbosch : Stellenbosch University, 2017-12) Kitt, Shawn; Kisters, Alexander F. M.; Buick, Ian; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Shear Zone hosted Cu mineralisation in the Deep Level Southern Zone accretionary prism of the Damara Belt is associated with deformation and fluid flow related to the exhumation of the Mesoproterozoic Ekuja basement gneiss dome during the Pan-African convergence of the Congo and Kalahari Cratons. In terms of tectonic setting, metamorphic grade and age structure, the mineralisation has no real equivalent in Namibia and seems to defy current classification schemes of Cu deposits. This study aims to describe the geometry and controls of mineralisation and the sources of fluids and metals in Omitiomire Cu deposit, and constrain the internal dynamics and processes that led to the imbrication of 100 km2 slivers of basement gneisses with the overlying prism metasediments. The Omitiomire Cu deposit (137 Mt at 0.54% Cu) is hosted by a low-angle, late-Damaran (ca. 520 Ma) shear zone system, referred to as the Omitiomire Shear Zone (OSZ), that developed around an older (ca. 1100–1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. The chalcocite-dominated mineralisation is associated with biotite-epidote assemblages that formed through the progressive hydration of the original mafic rocks to amphibole-biotite gneiss and biotite-epidote schist during deformation and fluid infiltration along the OSZ. Stable isotope (O, H and S) data indicate upper-greenschist-to middle amphibolite-facies conditions, a low fluid-rock ratio and a local redistribution of sulfur during fluid flow and mineralisation. This points to the remobilisation by epigenetic fluid flow of an older Cu mineralisation event. The geometry of the mineralisation is controlled by the spatial coincidence of the OSZ and the inherent heterogeneity of the original intrusive amphibolite–tonalite breccia. The gently undulating, shallowly-east dipping orebody is composed of several mineralised lenses that are contained in the regional S2 fabric. High-grade ore shoots are parallel to a prominent N-S trending L2 stretching lineation and are correlated with the cumulative number and thickness of several thin quartz-biotite-epidote-chalcocite shear zones at the contacts of interleaved schists and felsic gneisses. The kinematics and the association of the Cu mineralisation with retrograde assemblages in the OSZ link the mineralisation with the exhumation of the Ekuja Dome. The timing of exhumation is constrained to between 526.4 ± 3.5 and 521.9 ± 3.6 Ma by 40Ar/39Ar dating of biotite from the OSZ. Peak metamorphic assemblages in amphibolite gneisses from the Ekuja dome record PT conditions of 8.5-9.15 kbar and 635-655 °C, which equates burial to ca. 35 km. In contrast, PT estimates and U-Pb xenotime age determinations in the overlying prism metasediments show that peak metamorphic conditions of 7-9.25 kbar and 640-675 °C were only reached at ca. 515 Ma. This suggest that exhumation of the Ekuja dome started some ~10 Ma before maximum burial and peak metamorphism was reached in the overlying prism sediments and reflects the complexities of subduction-exhumation processes in accretionary complexes. The results of this study highlight the striking similarities with basement dome hosted deposits in the Lufilian Arc of Zambia and suggest the existence of a contiguous convergent margin along the leading edge of the Congo Craton that was active to at least ca. 515 Ma.
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    Structural controls of auriferous quartz veins in the Karibib Area, southern central zone of the Pan-African Damara Belt, Namibia
    (Stellenbosch : Stellenbosch University, 2008-12) Kitt, Shawn; Kisters, Alex; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    Detailed geological mapping and a structural analysis of auriferous quartz veins were undertaken in the Karibib region of the Pan-African Damara belt in central Namibia. The study focuses on the formation and controls of quartz-vein sets and associated lodegold mineralization in heterogeneous, siliciclastic- and marble- dominated amphibolitefacies host rocks around the Navachab gold mine and adjacent areas. Two main arrays of shallowly-dipping quartz veins can be distinguished that form a conjugate set. Steep, bedding-parallel and high-angle cross-cutting veins also occur, but play a subordinate role for mineralization. The orientation of the main conjugate set and progressive deformation of these quartz veins indicate that veining occurred during the late stages of the main phase of NW-SE directed, subhorizontal shortening (D2) and associated NW-verging folding and top-to-the-NW thrusting. Cross-cutting relationships with plutonic rocks indicate a timing of ca. 540 Ma for the mineralization. The quartz veins sets show a consistent orientation irrespective of their location with respect to NE-trending, NW-verging first-order fold structures that were previously considered to be pertinent for the mineralization. The quartz vein sets also cross-cut different lithologies at high angles. This suggests that the regional strain (D2) was the first-order control of quartz vein formation. More localized lithological and/or structural controls played, however, an important factor for the formation of economicgrade mineralization. Thick and closely spaced quartz veins in steeply dipping rocks of the Navachab open pit form a more than 150m thick economic-grade vein swarm. In this structural situation and during layer-normal subhorizontal shortening, the host rocks experienced high extensional strains in a vertical direction, favouring the formation of subhorizontal extension fractures.