Structurally controlled Cu-Zn-(Co-Pb) mineralization in the Neoproterozoic Ombombo subgroup, Kaokoland, Namibia

Abstract

The Dolomite Ore Formation (DOF) is a Cu-Zn-(Co-Pb) 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 in order to determine lateral facies variations in the DOF horizon, and the rocks immediately above and below this horizon. The goal was to constrain mechanisms of mineralization for this regionally geochemically anomalous unit, and to compare and contrast the DOF horizon with other sediment-hosted copper occurrences in the Otavi Mountainland of northern Namibia, in order to test whether the DOF horizon might be a viable exploration target. The DOF dips about 60⁰ to the north, averages <1m to 6m in thickness, and thickens towards the east. The horizon is an organic-rich (1.2-1.8% TOC), shaley, ankeritic dolostone that overlies interbedded dolostones and recessive siltstones (both with <1% TOC) in the east and interbedded arkose sandstones and recessive siltstones in the west. It contains pyrite with minor chalcopyrite, sphalerite, and galena with trace catterite. Supergene alteration resulted in formation of chalcocite and late-stage, sometimes 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 (560-550 Ma). The veins display little to no alteration other than weak silicified halos. 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 DOF occurs at a deeper stratigraphic level than the base metal deposits of the Otavi Mountainland to the southeast, and it is likely that the DOF represents a deep expression of the types of mineralizing systems that produced these deposits. Results of this study suggests that carbonaceous strata in the Ombombo Subgroup that are cut by late Damaran-aged faults could have the potential for concentrating base metal sulfide or oxide minerals.