Hydrogeochemistry of the Tsumeb Deposit: Implications for Arsenate Mineral Stability

The Tsumeb base-metal deposit contained one of the most diverse examples of mineralogical paragenesis ever observed within a single mineral deposit (Keller 1977). The deposit hosted approximately 307 minerals and 232 of those minerals are most likely formed in the oxidation zone. Of the total number, 69 minerals were first described from the deposit. Arsenic minerals show the greatest diversity in the Tsumeb deposit: 63 arsenates, 6 arsenites, and 7 arseno-sulfate minerals (see Appendix 1). Typically, As content was around 1% in the ore zone, and was intermittently produced as a by-product (white As oxide). Mineralization is hosted in the Otavi dolomite. The main ore body is a pipe that comprises of massive peripheral ores, manto-style ores, and disseminated and stringer ores. These ores were subjected to extensive oxidation not just from surficial surface weathering but also along deep-seated permeable faults that developed complex secondary mineral assemblages at depth. Due to the karstic nature of the host dolomite, there has been considerable water flow through the deposit and during operations into the mine workings, even during early mining. As such, water chemistry within the mine has a varied composition reflecting the different areas of the mine, water source, and geochemical reactions with host rock and the ore. In addition to water, which has been locally enriched from sulfide oxidation, saline and dilute water can be observed in the mine. With such a complex mineralogy and paragenesis, it is possible to describe the geochemical conditions that influenced the mineral evolution of the deposit and predict interactions with groundwater. The extent to which current mine water reflects mineral paragenesis and the observed As-mineral assemblage in the mine is reviewed and used to provide an understanding of the formation of the oxide zone and the geochemical conditions at the time of formation compared …