Waste rock and tailings
The mining process generates two byproducts:
- Waste Rock: rock that is non-mineralised, or mineralised rock which contains insufficient gold to process economically
- Tailings: the slurry that remains once the gold and silver have been extracted from the crushed ore at the processing plant
The predominant rock type at the Waihi area is andesite, and it varies from a completely weathered material (i.e. soil) through to ‘fresh’ rock. Ignimbrite is also present and its composition also varies from completely weathered to ‘fresh’. There are smaller quantities of alluvium and ashes (sands, silts and clays) that mantle the pit area.
A special feature of the waste rock is that some of it contains sulphide and is capable of producing acid drainage when exposed to oxygen and water. This rock is referred to as potentially acid forming (PAF) rock. Waste rock without the potential to form acid drainage is referred to as non acid forming (NAF) waste rock. Special design and operational procedures are used to control acid drainage generation. As a contingency, provision is made to collect and treat any fluid that is generated.
Tailings are a slurry of around 30% solids and are deposited from the crest of the waste rock embankment via a series of spigots. Fresh tailings are deposited above water onto previously placed tailings.
The material is predominantly silt size, but also contains clay and sand sized particles. Immediately adjacent to the point of deposition into the pond the tailings contain a higher proportion of sand, because the sand particles settle out more quickly, while on the other side of the pond they contain a higher proportion of clay. Consequently, the physical characteristics of the tailings vary laterally across the pond, and this forms a tailings ‘beach’ adjacent to the crest.
The tailings have low permeability, and as they consolidate the permeability reduces to the extent that they are generally less permeable than the natural soils that underlie the embankment. The reducing permeability means that seepage from the tailings mass decreases significantly with time. Following consolidation, the permeability of the tailings reduces by more than a factor of 10 compared with its permeability after initial deposition, to become generally less than 10-9 metres per second. The permeability of the consolidated tailings is very low and is equivalent to that required for the liner of a modern landfill.
In many mines throughout the world, waste rock and tailings are disposed of separately to form waste dumps and tailings dams. At Waihi the situation is unusual, in that waste rock is used to form engineered embankments that are progressively raised in order to ‘keep up’ with tailings production. This allows for sufficient storage capacity to ensure that the water within the tailings pond will be contained, even in extreme rainfall events. Storage capacity is provided for a 1200 mm rainstorm, (the Probable Maximum Precipitation), plus an additional 1.0 metre minimum freeboard.
There are two tailings storage facilities, known as Storage 2 and Storage 1A. Construction of Storage 2 commenced in 1987, and has reached its final crest height. It was decommissioned from tailings deposition in 2005. A substantial database spanning more than twenty years demonstrates that the performance of Storage 2 has been very good. The design of Storage 1A is based largely upon the design of Storage 2. While there are some technical differences, the description of Storage 2 can also be applied to Storage 1A.
A number of environmental considerations have been taken into account in planning and designing the waste rock embankments and tailings storage facilities:
- identification of the best location for the tailings storage facilities, taking into account the status of the land with respect to ownership, geology, archaeological features, flora and fauna
- the design and construction of the embankments, considering short and long term structural stability
- ways that potentially significant adverse effects such as noise, dust, visual effects, acid drainage, and cyanide can be avoided, remedied and mitigated
- the surface and subsurface drainage systems required to ensure that potentially contaminated water can be collected and managed
- the water management and water treatment facilities required to ensure that there are no significant adverse effects on the surrounding rivers and streams
- the requirements for rehabilitation and closure of the site.
To address these issues, advice has been sought from a team of experts including archaeologists, geologists, hydrogeologists, engineers, geochemists, rehabilitation consultants and landscape architects.