Controls on stratabound copper mineralization at Klein Aub Mine and similar within the Kalahari copper belt of SWA/Namibia and Botswana

Abstract

Abstract provided by author:

The Middle Proterozoic (1300-950 Ma) Sinclair Sequence and its stratigraphic equivalents crop out intermittently in a belt which extends from central SWA/Namibia to Botswana. Due to its numerous stratabound Cu occurrences, it is here called Kalahari Copperbelt. The sequence probably formed in the inland branch of an intracratonic failed rift system, situated between the Congo and the Kalahari Cratons. Development started with an initial, mechanical rift phase probably in response to an underlying mantle plume and resulted in bimodal tholeiitic volcanism and strong extensional tectonism of the Nückopf and Grauwater Formations. During Doornpoort Formation times, continental red beds filled the narrowfault-bounded grabens with alluvial fan, braided stream, local evaporitic playa lake, and aeolian dune sediments. A widening of the basin, an overstepping of the graben shoulders and a marine transgression, caused the deposition of laterally extensive, mixed siliciclastic/carbonate sedimentation (Klein Aub Formation) in a tidal flat environment. This resulted from a later thermal subsidence phase. Both tectonically and sedimentologically the Sinclair Sequence heralds the subsequent development of the early Damara rifting. The area underwent three phases of deformation; (D1) syndepositional block faulting, possibly with a dextral strike-slip component; (D2) the main deformational event of the Damara Orogeny, producing large-scale folding and a regional cleavage; (D3) transpression, resulting in the development of a dextral, oblique-slip fault zone. Regional lower greenschist metamorphism affected the sequence and was accompanied by basalt alteration and a substantial depletion in Cu, Zn, Co, Mg, Na and K. Stratabound sediment-hosted Cu/Ag mineralization occurs in dark pyritic sediments at the redox interface between the red bed and marine unit. Precious elements (Au and PGE) are enriched in the initial acid volcanics at the base of the rift sequence and in the stratabound ores. Mineralization of the sediments was a two-phase event and produced disseminated, permeability - controlled ores during early diagenesis and fracture-hosted ores, superimposed on the earlier phase, during or after deformation. Metals were supplied by the selective leaching of Cu from underlying basalts and Au and PGE possibly from red beds and transported as chloride complexes in acid, oxidizing fluids