The North Break Zone of the Late Precambrian Otavi carbonate platform sequence in Namibia

Abstract

Abstract provided by author:

The main objective of this study was to characterize the North Break Zone of the Otavi Mountain Land, Namibia in terms of stratigraphy and petrography and to investigate its relationship with the Tsumeb ore body and other mineralized prospects in the immediate vicinity of Tsumeb

The Late Proterozoic Otavi carbonate, platform sequence is famous for its base metal deposits. The North Break Zone is a stratabound zone of sporadic mineralization, brecciation and silicification occurring in the lower part of lithozone T6 of the Huttenberg Formation (Tsumeb Subgroup). It intersects the pipe-like Tsumeb Cu-Pb-Zn-Ag ore body at a depth of about 900m below surface. Where the North Break Zone intersects the Tsumeb ore body large massive ore associated with calcitized dolomite, dolomite breccia as well as feldspathic sandstone lenses occur. These features extend along strike and dip outside the normal dimensions of the Tsumeb ore body. The genesis of the Tsumeb ore body is poorly understood. The conventional model is that meteoric fluids circulated through the so-called North Break Zone paleo-aquifer, dissolving carbonate and giving rise to solution collapse and eventually the creation of the Tsumeb karst pipe. However, no direct evidence is available to support this model. This study was devised to critically evaluate the relationship, between the North Break Zone and formation of the Tsumeb ore body

The study entailed field mapping, detailed sampling of the stratigraphic sequence and ore bodies, white light, reflected light, UV/blue light and cathodoluminescence petrography. Cathodoluminescence proved to be the most effective petrographic tool for differentiating various carbonate phases

The North Break Zone is defined as a 10 to 14m thick chert free oolitic to intraclastic dolomitic grainstone, stromatolite and mudstone unit, in which discontinuous lenses of mineralized secondary quartz are present. It is interbedded with dark grey cherty micritic dolomite of lithozone T6 of the Huttenberg Formation. Minor calcification, Cu-Pb-Zn mineralization and manganese and iron enrichment are associated with the quartz-rich bodies. The mineralized quartz bodies are only present up to 2. 5km to the west and 2. 6km to the east of the Tsumeb ore body

The petrographic study indicated that (1) the paragenetic sequence of carbonate alteration, precipitation of new carbonate phases and mineralization is virtually identical in all Cu-Pb-Zn occurrences and (2) that the mineralization is closely associated with Mn-bearing brightly luminescent (CL) carbonates. Earlier Cu-Pb-Zn sulphide mineralization is associated with Mn-bearing bright red luminescent sparry dolomite (dolomite IIIB). Late stage Cu-arsenate, oxide and silicate mineralization is associated with an episode of Mn-bearing bright yellow luminescent calcite (calcite II) which also causes dedolomitization of the associated dolomites. A very simple paragenetic model of mineralization is proposed. The earliest is defined by pre-mineralization calcite (calcite-1) vein formation with associated dedolomitization. This phase is followed by deposition of kerogen - luminescent Mn-bearing dolomite IIIB - quartz and Cu-Pb-Zn sulphides representing the main mineralization event. It is followed by a late mineralization event composed of Mn-bearing calcite (calcite II) with associated Cu-arsenates, oxides and silicates. Supergene alteration is represented by the precipitation of very late stage non-luminescent Mn and Fe-poor calcite (calcite III) and quartz without any associated Cu-Pb-Zn mineralization

The sequence of mineralization is explained by the evolution of a single hydrothermal fluid, from relatively cold to hot and then back to cold, during a major period of fluid migration through the carbonate platform sequence. The North Break Zone probably never acted as a paleo-aquifer for fluids that formed the Tsumeb ore body. Rather hydrothermal fluids moved from the Tsumeb ore body into the North Break Zone. Hydrothermal fluids may have been derived from the Damara orogen to the south of Tsumeb during a period of tectonic loading and thrust deformation