Hydrogeochemistry of the Tsumeb deposit: implications for arsenate mineral stability.

The Tsumeb base-metal deposit in NE Namibia contained one of the most diverse examples of mineralogical paragenesis ever observed within a single mineral deposit. From approximately 307 minerals in total, arsenic minerals show the greatest diversity: 63 arsenates, 6 arsenites and 7 arseno-sulphates. Arsenic content was around 1% in the ore zone and intermittently produced as a by-product (white As oxide). Mineralisation is hosted in the Otavi dolomite. Due to the karstic nature of the host dolomite, there has been considerable water flow through the deposit, even during mining. There are strong geochemical relationships observed in the current mine water chemistry and the existing secondary mineral assemblage, which started with oxidation of sulphide minerals, such as tennantite, that generated not only metal ions but also protons that reduced groundwater pH. The resulting acidity was quickly neutralised in dolomite and secondary minerals were deposited in response to changes in solution chemistry. Assessment of current mine waters provides limited insight into the relative stability of the minerals. It appears that metal and As activity have decreased over time in water contacting the oxidised sulphide deposit. The general trend of increasing acidity in the environment since the Cretaceous may explain the higher Cl content of the mine waters but this is by no means certain.
The Tsumeb base-metal deposit in NE Namibia contained one of the most diverse examples of mineralogical paragenesis ever observed within a single mineral deposit. From approximately 307 minerals in total, arsenic minerals show the greatest diversity: 63 arsenates, 6 arsenites and 7 arseno-sulphates. Arsenic content was around 1% in the ore zone and intermittently produced as a by-product (white As oxide). Mineralisation is hosted in the Otavi dolomite. Due to the karstic nature of the host dolomite, there has been considerable water flow through the deposit, even during mining. There are strong geochemical relationships observed in the current mine water chemistry and the existing secondary mineral assemblage, which started with oxidation of sulphide minerals, such as tennantite, that generated not only metal ions but also protons that reduced groundwater pH. The resulting acidity was quickly neutralised in dolomite and secondary minerals were deposited in response to changes in solution chemistry. Assessment of current mine waters provides limited insight into the relative stability of the minerals. It appears that metal and As activity have decreased over time in water contacting the oxidised sulphide deposit. The general trend of increasing acidity in the environment since the Cretaceous may explain the higher Cl content of the mine waters but this is by no means certain.