6.4 Operational management and treatment
At most sites management of mine tailings and waste rock and treatment of mine drainage is likely to be required to ensure that trace metals and suspended solid loads in discharge waters (or at a downstream point of interest) do not exceed acceptable levels.
Naturally occurring Fe oxyhydroxide can provide an extremely effective mechanism of removing As and Sb from mine drainage, reducing the need for further treatment. Specifically, exposure of low-As waste rock to allow oxidation of Fe minerals will enhance the formation of Fe oxyhydroxide and increase the capacity of the overall site for As adsorption. The following Fe-bearing minerals are of most relevance in hard-rock gold mines:
- Pyrite. Minor acidification associated with pyrite oxidation is normally not an issue because of the high ANC of surrounding rocks. Pyrite typically accompanies most hard-rock gold, but some deposits are notably poor in pyrite. In this case, acidification is desirable as minor acidification can help to limit dissolving of As minerals;
- Iron-bearing carbonates. Ankerite and/or siderite commonly occur in gold deposits and in the immediately adjacent waste rock. Oxidation of these carbonate minerals produces Fe oxyhydroxide. Ankerite (commonly Fe-bearing dolomite) is the most soluble, so that Fe oxyhydroxide forms rapidly. Siderite is less soluble, and finer grain size may be needed for effective Fe oxyhydroxide formation for As and Sb attenuation; and
- Chlorite. This Fe-bearing silicate mineral is commonly present in host rocks, and will produce some Fe oxyhydroxide. The oxidation process is slow, and volumes are small, so the attenuation effects are minor only.
If Fe oxyhydroxide is required to reduce dissolved As discharge, the following information is required in the early stages of mine development:
- Confirmation of presence of pyrite (>2%) in waste rock and/or tailings;
- Confirmation that pyrite oxidation is occurring, with subsequent precipitation of Fe oxyhydroxide; and
- Confirmation that sufficient Fe oxyhydroxide is being formed to reduce dissolved As by at least one order of magnitude (typically to <0.1 mg/L dissolved As).
An active or passive treatment system may be required if the above Fe oxyhydroxide formation and adsorption does not occur spontaneously.
6.4.1 Prevention and mitigation
Waste rock management
Despite the fact that opencast gold mines produce very large amounts of waste rock, most of this waste rock is barren host rock with negligible amounts of sulphide minerals and As, therefore management of the waste rock is primarily focused on minimising the discharge of suspended solids (Appendix B). However, relatively minor amounts of sulphide-bearing rock from near the mineralised zone may be included in the waste rock and if this mineralised rock is sulphide-rich, separate management of this rock as PAF may be required (see Chapter 4).
Appropriate management of mine tailings is critical in ensuring that mine drainage meets acceptable water quality limits. Figure 25 highlights simple techniques that should be taken into account when managing drainage from mine tailings at hard-rock gold mines. As stated above, the method of ore processing is critical in influencing the release of As and Sb in mine drainage. Where no oxidation processes are used, the release of As and Sb in the mine drainage can be minimised by mine tailings being saturated with water to exclude oxygen, and minimising the flow of water through these tailings. Where oxidation processes are used, water incursion into the tailings piles should be minimised by diverting surface streams and constructing low-permeability caps on the surface of piles to exclude rainwater. Where Fe minerals are present, in particular pyrite and siderite, the formation of natural Fe oxyhydroxide will remove As and Sb present in the mine drainage, potentially to levels that do not require further treatment. However, the formation of natural Fe oxyhydroxide is only able to be determined when mining proceeds and as such requires that appropriate monitoring takes place during mine operations.
Figure 25. Summary of mitigation strategies for environmental issues at a West Coast gold mine. The principal strategy is to remove As and Sb from discharge waters via adsorption to Fe oxyhydroxide. If insufficient natural Fe oxyhydroxide formation occurs, a treatment system is required.