Lithostratigraphy, mineralisation and geochemistry of the Rosh Pinah Zn-Pb-Ag deposit, southern Namibia

The demand for metals is expected to increase steadily in the next decades to sustain population and economic growth globally as well as supply the materials required for the global energy transition. Zinc as a base metal is a crucial component in construction (galvanised steel) and transport industries as well as for energy infrastructure. To meet projected Zn demands current mineral system models require updating to aid in exploration and mineral discovery success. The Gariep Belt is a fertile Pan-African metallogenic belt that hosts three known Zn resources at Skorpion, Gergarub and Rosh Pinah in Namibia. Currently the Skorpion Zn Mine is on care and maintenance. The Rosh Pinah Zn-Pb-Ag deposit (19.94 Mt @ 7.38 wt.% Zn, 1.83 wt.% Pb and 27.7 g/t Ag) is hosted in carbonaceous argillites, siltstones, carbonates and volumetrically minor volcaniclastic rocks of the Neoproterozoic Rosh Pinah Formation. Despite decades of production and exploration and academic research at and around the deposit, its genesis remains unclear. Specifically, unanswered questions remain concerning the timing of sulphide mineralisation, structural controls on the mineralised bodies, the origin and nature of the carbonates, and significance of barium in the mineralising system. Application of new analytical techniques (e.g., in-situ LA-ICP-MS) and careful study of the deposit spatially and paragenetically provide the opportunity to answer some of these outstanding questions and refine our understanding of the Rosh Pinah mineral system. The research presented in this thesis focusses on the geology of the Western Orefield at Rosh Pinah, specifically: (1) lithofacies types and distribution, (2) metal zonation and distribution of alteration types, and (3) characterisation of the carbonates at the Rosh Pinah deposit and regionally in the Rosh Pinah area. The thesis aims to improve mineral exploration efforts in the area through a better understanding of the key components of the Rosh Pinah mineral system. The lithofacies architecture study, which combined surface and underground observations, drill core logging data and petrography (including transmitted light, scanning electron microscopy (SEM) and cathodoluminescence (CL) reveals that the sedimentary sequence hosting the mineral deposit, the Ore Equivalent Horizon (OEH), displays a complex set of fine-grained units, carbonates and volcaniclastic rocks in an arkose dominated sequence with gradational facies boundaries indicating deposition in a relatively small and isolated/restricted, structurally controlled depocenter. Work on metal distribution at Rosh Pinah demonstrates that highest Zn, Cu and Fe contents within mineralised zones are located adjacent to structures that served as conduits for metal bearing fluids, especially where juxtaposed with carbonate rocks. Study of the mineralogical and textural relationships of mineralised rocks utilising petrographic microscopy (transmitted and reflected light, CL, raman spectrometry) suggests that the deposit evolved from an early mineralisation stage that resulted in deposition of diagenetic pyrite and barite followed by a main stage of pyrite, sphalerite, galena, barite and minor chalcopyrite precipitation, coincident with silicification, potassic alteration, and dolomitisation as well as additional Ba enrichment. Several vectors to mineralised zones have been established based on element enrichment (Ba and Sr) vs depletion (Na and K), especially in fine-grained siliciclastic rocks. Delineation of relatively subtle alteration associated with mineralisation points to sedimentary facies and structural architecture controlling favourable positions for hydrothermal fluid flow.