Late Proterozoic outer shelf manganese and iron deposits at Otjosondu (Namibia) related to the Damaran oceanic opening

Abstract

The Otjosondu manganese mining area is located at the eastern exposed extent of the inland branch of the late Proterozoic Damara orogen. In spite of intense deformation and a high-grade metamorphic overprint, the sedimentological interpretation of the manganeserich rocks and their associated lithologies has led to a paleoenvironmental model which gives insight into ore genesis. At least two manganese silicate ore horizons (manganese formations) are associated with banded iron-formations. This sequence is sandwiched between supermature metaquartzarenites interpreted as a shoreface deposit derived from a cratonic basement and deposited under high-energy conditions.Manganese ore varieties and their country rocks have been classified using their texture, mineralogy, and geochemistry and have thus been interpreted in sedimentological and facies terms. The evaluation of the vertical facies sequences, their lateral distribution and their contact relationships renders a spatial reconstruction of the environmental setting for the manganese ores possible. The development of facies from a near-shore clastic regime to manganese-bearing hemipelagic sediments and ultimately to pelagic manganostones and ironstones reflects a zonation on a marine shelf. The facies evolution indicates transgressive conditions on a large scale onto a tectonically stable but subsiding shelf, starting with the transgressive quartzarenites overlying an unconformity to the pre-Damara basement, followed by chemical sediments of the ore-bearing horizon and overlying carbonates, and ending up with pelitic schists. On a smaller scale both transgressive and regressive conditions prevailed during manganese precipitation. Small-scale sedimentary cycles are interpreted as due to glaciogene-influenced fluctuations of the sea level.The source of manganese and iron is proposed to have been situated in the deeper oceanic parts of the associated basin. Initial spreading in the Khomas sea and the formation of oceanic crust is documented by tholeiitic volcanics and associated base metal orebodies representing more proximal manifestations of the exhalative activity. These base metal sulfide deposits are associated with pyrite-bearing graphitic pelagics and this was the first site to partition iron and other base metals from manganese. Upwelling along the shelf edge was involved and additional separation of Mn and Fe was due to a change in Eh conditions associated with a redox interface intersecting with the outer shelf environment. The evolution of the Khomas sea provided the ore-forming solutions and also initiated the subsidence of the passive shelf margin. Fluctuations in sea level were due to the basin development on one hand and glacial-interglacial cycles on the other. The latter fluctuations controlled the amount of terrigenous input into an otherwise pelagic regime.