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M.Sc. thesis (Diplom)

THE ORIGIN OF IRON AND LEAD MINERALIZATION ON LEEUWBOSCH AND CORNWALL, THABAZIMBI DISTRICT, LIMPOPO PROVINCE, SOUTH AFRICA - INSIGHTS FROM FLUID INCLUSION MICROTHERMOMETRY AND STABLE ISOTOPE MASS SPECTROMETRY

 

A geochemical investigation of high-grade iron ore and lead deposits on the farms Leeuwbosch and Cornwall, located N of Thabazimbi, Limpopo Province, South Africa was carried out as part of a diploma thesis in cooperation with Aquila Steel South Africa. The structurally controlled hydrothermal mineralizations are hosted by dolomitic carbonates of the Neoarchaean to Palaeoproterozoic Malmani Subgroup (Transvaal Supergroup) and have recently become a target of exploration. The first geochemical study of one of South Africa’s largest historic lead deposits, located on the Leeuwbosch prospect is presented. High grade hematite iron ore deposits are abundant on both of the prospects as well as in the surrounding areas. The thesis provides a review of the regional historical, structural and sedimentological record with a focus on the Neoarchaean and Proterozoic development of the Transvaal Basin sediments. Based on recent geological maps, aeromagnetic survey data and structural evidence the age of mineralization can be constrained between 2054 and 1930 Ma which coincides with mineralization ages for the Thabazimbi deposit. The detailed petrographic study of wall rocks, ore petrography, fluid inclusion microthermometry of quartz and calcite as the main gangue minerals and stable isotope analyses (δ13C, δ18O) of siderite, calcite and wall rocks reveal the hydrothermal nature of the mineralizations and the physicochemical properties of ore-forming brines. The identified fluids are very similar to those found in Palaeozoic Mississippi-Valley-Type (MVT) deposits and show characteristically variable total salinities and contents of CaCl2 or MgCl2 in addition to NaCl which are interpreted to be the result of fluid mixing. The developed hydrothermal mineralization model incorporates three major stages of ore formation. The first stage involved fluid-mixing of diagenetic, high-salinity brines and surface-derived meteoric fluids in a burial depth of ca. 6 to 8 km at temperatures of c. 190 to 260 °C and resulted in the precipitation of galena, pyrite and siderite in veins and fractures as well as in disseminated-type stratabound bodies. During the second stage large quantities of iron ore which were leached from the overlying Penge Formation precipitated along fault zones and in dissolution cavities within the dolomitic carbonates of the Malmani Subgroup. The precipitation process involved the mixing of supergene fluids with high-salinity diagenetic brines as well as carbonate metasomatism and extensive dissolution of the wall rocks. Iron ores formed in a depth of c. 4.5 to 7 km at temperatures from c. 160 to 220 °C. Since the Cretaceous the formation of weathering surfaces and karstification played an important role in the exposure and supergene upgrade of iron ore mineralizations.

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