Abstract

At Tsongoari-Omupokko, in the southern Kaoko Belt of northwestern coastal Namibia, several metamorphosed coticule, iron formation and massive barite horizons with minor base metal sulphides are hosted by Late Proterozoic clastic to chemical sediments of the Ugab Subgroup, Damara Sequence. The depositional conditions of the host rocks were variable with environments changing from playa lacustrine, to coastal or shallow marine, to submarine turbiditic and deep marine. Hydrothermal activity is confined to metapelitic and metapsammitic rocks above carbonate layers. The mineralised units of the Pb-Cu-Ba-Zn-Ag prospects vary considerably in their modal composition and to a lesser extent in their thickness and overall mineral content. Vertical changes of mineralogy and geochemistry indicate rhythmic exhalative cyclicity. Associated mica schist and feldspar-bearing quartzite host rock contains Mn- and Cu-bearing biotite and muscovite, Zn-bearing amphibole, hyalophane, fluorite and apatite.At Tsongoari West, mineralisation consists of euhedral spessartine-rich garnet and magnetite in a matrix of manganoan amphibole, quartz, mica and carbonate. Considerable amounts of primary galena and secondary Cu-carbonate occur at the base of the mineralised horizon at the transition to the meta-pelites.

At Tsongoari East and Omupokko, aggregates of galena, chalcopyrite, digenite and sphalerite occur in laterally and vertically consistent massive barite, magnetite and coticule horizons, which have a maximum thickness of 10m and a length of 4.5 kilometres. Secondary biotite and amphibole are similarly enriched in trace elements as at Tsongoari West. Lateral thinning of the mineralised horizons occurs over 10’s of metres. Sharp contacts of the mineralised horizons with the hanging wall and footwall suggest rapid transition from exhalative to detrital sedimentation. Barite and Mn-silicate mineralisation are regarded as the more distal exhalative facies types relative to a potential fluid discharge site and indicate variable oxidation and pH conditions during submarine hydrothermal activity. Field evidence points towards a laterally uniform hydrothermal-sedimentary depo-environment within the unfolded Tsongoari/Omupokko lithologies, where mineralisation was shaped by the relief of the sedimentary palaeo-surface. Fluid inclusion data from barite and quartz mineralised zones yielded medium temperatures (150° to 280°C) and medium salinity (3 to 7% NaCl-equivalent), characteristic of fluid compositions which are influenced by mixing with meteoric fluids during recrystallisation processes. Sulphur isotope ratios between 13 and 16‰ δ34S for sulphide and 26‰ for sulphate at Tsongoari East, and −7‰ δ34S for sulphide and 9.5‰ for sulphate at Tsongoari West indicate overall strong seawater sulphur input. During Damaran Orogeny (~550 to ~490 Ma), strong compressional nappe- to thrust-dominated deformation metamorphosed the mineralised sections to lower amphibolite-grade facies (~550 to ~610° C at ~6.5 to ~9kbar) as indicated by mineral assemblages in the coticules, without significantly effecting primary geochemical profiles.

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