School of Geosciences (ETDs)

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Browsing School of Geosciences (ETDs) by Keyword "Cu and Fe isotopes"

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    Trans-crustal and temporal perspectives of Palaeoproterozoic porphyry copper deposit formation (Haib deposit, Namibia)
    (University of the Witwatersrand, Johannesburg, 2023) Jogee, Bibi Ayesha
    Porphyryِcopperِdepositsِareِtheِworld’sِprimaryِsourceِofِcopperِwhichِisِanِessentialِcriticalِ metal in the green energy sector. These deposits are most often temporally found within the Phanerozoic Eon and occur principally at convergent plate boundaries. The Haib porphyry copper deposit is a rare example of a Palaeoproterozoic porphyry copper deposit. The Haib porphyry copper deposit is hosted by the rock types of the Richtersveld Subprovince. The Richtersveld Subprovince consists of an older volcano-sedimentary sequence, the Orange River Group (ORG), which was intruded by an extensive composite cogenetic suite of plutonic rocks, the Vioolsdrif Intrusive Suite (VIS). The Vioolsdrif Intrusive Suite is subdivided into several subunits, known as the Vuurdood Subsuite (mafic-ultramafic), Gaobis Diorite, Khoromus porphyry, Goodhouse Subsuite (Granodiorite) and the Ramansdrif Alkali Granite. These subsuites are variably composed of diorites, granodiorites, monzogranites, quartz- feldspar porphyry granodiorites and leucogranites as well as poorly-described clinopyroxenites and gabbros (named the Kokerboom Intrusion in this study). The age-range for the Richtersveld Subprovince is between approximately 1910 and 1865 Ma. The mineralization occurs as disseminations, blebs and veins in the feldspar porphyry andesites and quartz-feldspar porphyry granodiorites. The age of mineralization is bracketed at 1884ِ ±ِ 2ِMa. The Haib porphyry copper deposit is in close vicinity to clinopyroxene and amphibole-rich cumulates, which host novel sulphide mineralization, and present lines of evidence suggesting these are the deeper parts of the porphyry copper system. The mineralization is characterized into two main phases based on drill-core sample descriptions, petrographic observations, mineral liberation maps and isotopic analyses. Previous studies show that garnet and/or amphibole fractionation is related to the petrogenesis of porphyry copper deposits. The Haib porphyry copper deposit shows evidence for amphibole fractionation as opposed to garnet fractionation, which is supported by petrological and geochemical observations in this study. The Haib porphyry copper deposit also has limited ranges in δ65Cuِ andِ δ 56Fe signatures supporting exposure at deeper crustal levels of an arc system. Typical cumulates from deep crustal arc sections (arclogites) are coarse-grained, show cumulate textures and are composed of garnet-clinopyroxene-amphibole-iron-titanium oxides and in some cases plagioclase. The mafic-ultramafic cumulates such as the clinopyroxenites observed in the Kokerboom Intrusion, in close proximity to the Haib porphyry copper deposit, are coarse-grained, are composed of clinopyroxene-amphibole-iron-titanium oxides and consist of non-cotectic proportion of mineral phases. They also comprise a high modal abundance of pyroxene (>70 %) and show distinct cumulate textures. This study has demonstrated that the clinopyroxene and amphibole-rich cumulates which belong to the ultramafic end member of the Kokerboom Intrusion form part of the Vuurdood Subsuite. This conclusion was made through the analysis of lithological characteristics, mineral and alteration assemblages, major and trace element geochemistry and U-Pb age data. Various lines of evidence from this study indicate that the Haib porphyry copper deposit represents the deeper levels of the porphyry system. These lines of evidence include the lack of shallow alteration assemblages (propylitic and argillic alteration), the presence of early dark micaceous veins, the coarse-grained nature of the Vioolsdrif Intrusive Suite (clinopyroxenites, granodiorites, diorites, gabbros), which is indicative of slow cooling at mid-crustal batholith depths, and the presence of clinopyroxene and amphibole rich cumulates of the Kokerboom Intrusion. The clinopyroxenites are mafic, dense, and usually occur at mid- to lower-crustal levels. Geothermobarometry conducted on rocks of the Vioolsdrif Intrusive Suite yield anapproximate depth of crystallization between ~4-12 km which forms part of the mid-crustal levels of an arc system.