A study of the berg Aukas-type Pb-Zn-V deposits in the Otavi mountain land, Namibia

• Main Author: Chadwick, Peter J
• Other Authors: Frimmel, Hartwig E
• Format: Dissertation
• Language: English
• Published: University of Cape Town 2016
• Subjects: Geology
• Online Access: http://hdl.handle.net/11427/22548

Bibliography: pages 131-138. === This study is focussed on the petrographic and isotopic aspects of the crystallization processes taking place in carbonate-hosted Berg Aukas-type Zn-Pb-V deposits of the Otavi Mountain Land in Northern Namibia. Numerous studies have shown that ore formation in Mississippi Valley-type deposits bears a close relationship with carbonate diagenesis and basin evolution. The application of isotope (Sr , O, C) techniques as regards to timing and mechanisms of ore-forming processes, as a possible tool in exploration, is especially emphasized in this study. Detailed petrographic and isotope-geochemical investigations were carried out in the Late Precambrian deposits of Berg Aukas, situated just north of the town of Grootfontein, in addition to the Odin Prospect situated to the east of Kombat Mine. Both ore deposits display many similarities, but also show differences concerning the facies-bound character of the host rock, which is related to the paleogeographic position and the tectonic framework. In spite of these differences, similar complex diagenetic textures and structures suggest a comparable diagenetic evolution. Isotope-geochemical analyses have been performed on samples of consecutive crystallization generations, or dolomite types. This was checked petrographically using cathodoluminescence, and sample homogeneity was monitored by means of X-ray diffraction. The results reveal significant isotopic trends, which are interpreted to reflect the diagenetic evolution of the respective basin. A consistent strontium isotope trend is noted, and in both cases studied, the later crystallization generations are enriched in radiogenic strontium, as compared to the earlier ones, which tend to approximate the strontium isotope composition of contemporaneous seawater. The increase in ⁸⁷Sr/⁸⁶Sr ratios in subsequent diagenetic generations is explained with the pore fluids evolving towards more radiogenic compositions with advancing diagenetic stages. The influence of basinal brines enriched in radiogenic strontium as a consequence of the reaction with Rb-bearing phases, especially detrital (clay) minerals hosted by underlying basement rocks, is considered to be most likely. Stable isotope investigations carried out on identical samples also show certain trends, though these are less pronounced compared to the strontium studies. In general, the later crystallization generations are depleted in the heavy isotope of carbon and oxygen, which is explained in terms of changing physico-chemical conditions of the system, in combination with changing fluid compositions during the course of diagenetic evolution. By combining petrographic and isotopic observations, the genetic hypothesis for the ore deposits at Berg Aukas and Odin can be significantly constrained. In both cases, a model is proposed based on basinal brine migration during advanced diagenetic stages. These brines were enriched in radiogenic strontium due to the interaction with basement rocks. Thus, the combination of petrographic and isotope-geochemical methods has proved to constitute a powerful tool in tracing the diagenetic evolution, and in an exploration sense may be used to identify imprints of fluid circulation, which mobilized and concentrated metals during various stages of basin evolution.