Abstract
M.Sc. (Geology)
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 Iithozone
T6 of the Hoffenberg 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 HOffenberg 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 epigenetic 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 dolomitization of the associated
dolomites. A very simple paragenetic model of mineralization is proposed. The
earliest is defined by pre-mineralization calcite (calcite I) vein formation with
associated dolomitization. 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.