Wall rock alteration and lithogeochemical haloes associated with the sediment-hosted Rosh Pinah Zn-Pb-Ag deposit in the Pan African Gariep Belt, southwestern Namibia

Abstract

Limitations on the life-of-mine of the sediment-hosted Rosh Pinah deposit urged exploration for extensions to existing ore and similar new deposits in the Gariep Belt. Identification and quantification of hydrothermal wall rock alteration and the extent of dispersion haloes of ore indicator elements were considered supportive to this cause. The results from detailed lithogeochemical traverses on the C Mine and Eastern Orefield, coupled with mineralogical studies and the interpretation of a large (n > 400) multielement data set of the mine area, showed that the extent of wall rock alteration and associated element dispersion is limited. Barium has shown the most extensive dispersion in argillaceous, arkosic and dolomitic-arkosic wall rocks with anomalous values evident up to 100m into the footwall and 50m into the hanging wall of the ore zone. Alteration of detrital K-feldspar clasts by Ba-rich hydrothermal fluids provided a visual, qualitative index, which exceeds the limits of the elemental halo by 10-20m. Zinc and Pb, as well as Fe and Mn, were considerably less effective as indicator elements. Indices devised for the various lithologies of the Rosh Pinah deposit, such as the Barium Alteration Index and Combined Index, as well as modifications to previously proposed indices are based on variations in Fe(t), MnO, MgO, Al 2 O 3, SiO 2 , BaO, and K 2 O. The indices are aimed at minimizing the influence of sedimentologically induced variation in chemistry and maximizing those superimposed by hydrothermal alteration. These quantitative parameters allowed the successful distinction between unaffected and hydrothermally altered wall rocks, as well as an indication of proximity to the ore zone. The results confirmed the limited extent of alteration demonstrated by single element distribution, but indicated that a slightly broader zone has been affected. The limited alteration of feldspathic, dolomitic and siliciclastic sediments is a function of the physico-chemical properties and evolution of the mineralizing fluids. It indicates that the fluids were near neutral to weakly acidic, whereas high concentrations of Ba in the ore and wall rocks required the fluids to be reduced (H 2 S-dominant). These physico-chemical properties of the fluids resulted in rapid precipitation of the metal load in response to a variety of processes such as cooling, increases in pH, dilution or addition of H 2 S. Consequently, mineralization has proven to be distinctly concentrated within a favourable horizon, with only limited vertical metal dispersion. SOUTH AFRICAN JOURNAL OF GEOLOGYWALL ROCK ALTERATION AND LITHOGEOCHEMICAL HALOES, ROSH PINAH ZN-PB-AG DEPOSIT 120 database is available on request from the corresponding author. Petrological and textural relationships have been studied by optical microscopy from polished thin sections of the wall rocks and the different ore types. Back-scattered scanning electron microscope (SEM) images were obtained using a LEO 1430 VP SEM with a Centauros detector mounted below pole-piece at the University o...

A review of the Pan-African, Neoproterozoic Rosh Pinah Zn-Pb deposit, southwestern Namibia

“…(Laing, 1996). Barium, which forms the most extensive dispersion halo (Rozendaal et al, 2004), is present within the Ba-rich, ore-bearing carbonates and is highly anomalous in proximity to the ore-zone contact rocks. Lithological units of the Rosh Pinah Formation have been affected by widespread and pervasive hydrothermal alteration in the form of carbonatization and silicification.…”