The geology of the Tsumeb Carbonate Sequence and associated lead zinc occurrences on the farm Olifantsfontein, Otavi Mountainland, Namibia

Abstract

Abstract provided by author:

A spatial relationship exists between debris flow breccia deposits, formed along the upper contact of the Maieberg limestone, and the loci of mineral occurrences in the overlying dolomite sequences on Olifantsfontein. Whole rock geochemistry indicates that these breccia deposits have been dolomitised and are associated with above-background base metal values. Such early structures may have acted as aquifers in channelling metalliferous brines, derived either from rocks underlying the Otavi Group or from the shale horizons in the Maieberg limestone, into the overlying carbonates. These brines, upon entering favourable structural zones, underwent chemical changes that resulted in the deposition of gangue minerals and associated sulphides

The carbonate succession on Olifantsfontein, constituting part of the Tsumeb Subgroup, is exposed mainly on the northern and southern limbs of an upright, open, gently-plunging syncline, the Harasib-Olifantsfontein Syncline. The base of the exposed succession is formed by limestone of the Maieberg Formation, which was deposited in quiet water during the waning stages of a glacial period. The limestone is in sharp contact with overlying dolomites which are characterized by abrupt facies changes ascribed to the onset of syndepositional tectonics. The upper contact of the limestone is in places disrupted by deposits of chaotic debris flow breccias which are confined to zones of northwest-trending faults. These deposits were triggered by proximal syndepositional east-west faulting that also caused subsidence of the platform and an increase in thickness of sediments southwards. Periods of stability, in which finely laminated muds were laid down, were often punctuated by renewed movement along the syndepositional faults resulting in overlapping generations of mass flow breccia deposits on the slope. The thickness of sediments affected by these facies changes increases from some 230 m on the present north limb of the Harasib-Olifantsfontein Syncline to about 740 m on the south limb. A return to stable conditions of sedimentation is reflected by deposition of continuous units of grainstones that form the upper part of the Elandshoek Formation

The grainstones of the Elandshoek Formation have suffered intense neomorphism and partial silicification during diagenesis, and are host to most of the lead-zinc occurrences on Olifantsfontein. This mineralisation is typically associated with silica veins that have created crackle breccias within the unit. The fluids responsible for this event had an average salinity of 23 equivalent weight per cent NaCl with homogenization temperatures of up to 215°C