Paper No. 318-4
Presentation Time: 9:00 AM-6:30 PM
STRUCTURALLY CONTROLLED CU-ZN-PB-(CO) MINERALIZATION IN THE OMBOMBO SUBGROUP, KAOKOLAND, NAMIBIA
The Dolomite Ore Formation (DOF) is a Cu-Zn-Pb-(Co) horizon within the Neoproterozoic Ombombo Subgroup of the Kaoko Belt in northern Namibia. The horizon was mapped and sampled along its approximately 30-kilometer strike length. The DOF dips about 60⁰ to the north, averages <1m to 6m thick, and thickens towards the east. The DOF is organic-rich (0.7-1.8% TOC), shaley, ankeritic dolostone that overlies interbedded dolostones and recessive siltstones (both with <1% TOC) to the east and interbedded arkose sandstones and recessive siltstones towards the west. It contains pyrite with minor chalcopyrite, sphalerite, and galena with trace catterite. Evidence of Supergene alteration resulted in formation of chalcocite and late-stage, sometime zinc-enriched carbonate minerals. Although not obvious in outcrop, two drill holes through the DOF indicate that sulfides occur primarily within crack-and-seal quartz veins that likely formed during the Damaran Orogeny (550-540 Ma). The veins display little to no alteration other than weak silicified halos, which is supported by carbon and oxygen isotopic analyses across the DOF. Pyrite and pyrite ± chalcopyrite have sulfur isotopic values of -4 to +9‰ suggesting thermochemical reduction of sulfate probably derived from Neoproterozoic marine sulfate. Analysis of kerogen in the DOF suggests the rocks are overmature and given the Cu-Zn-Pb sulfide assemblage it is likely that the hydrothermal fluids had temperatures between 200 and 300⁰C. The mineralized DOF shares several similarities with mineral deposits in the Otavi Mountain Land to the east, including Tschudi, Kombat, Tsumeb, and Berg-Aukas deposits. The DOF occurs at a deeper stratigraphic level than these deposits and it is likely that the DOF represents a deep expression of these types of deposits.