Overburden is the natural rock and soil that sits above and around the ore body. It is not subject to any chemical processes at the mine but needs to be removed to allow access to the ore.
For every tonne of ore we mine, there are five tonnes of overburden which are moved to an area called the Overburden Emplacement Facility (OEF).
Overburden includes topsoil, clay, and a variety of rocks, each of which is stored in distinct parcels in order to reuse materials wherever possible. The OEF is designed to protect against any environmental impact.
Identifying the ore, overburden and the different types of rock within it, is important for both the mining operation and environmental protection.
Our geologists check each stage of mining using extensive studies, geochemical testing, maps, surveys and samples. They physically walk the face to be mined and mark it up to differentiate between the ore and overburden. Our geologists use above minimum industry standards to make sure all ore and overburden is categorised accurately.
We used to categorise the overburden in three ways – Alluvium (or clays), Non Acid Forming (NAF) and Potentially Acid Forming (PAF).
Now we know there are six categories of overburden. There are three types of NAF and two types of PAF as well as the Alluvium, which all need to be handled appropriately.
For this reason, each section of ore and overburden is separated by our excavators into trucks. Each truck is categorised to identify whether it carries ore, PAF or NAF material, and this determines where the load is deposited.
NAF material is used in developing all mine structure: the tailings storage facility, mine levee walls as well as the OEF. PAF material is isolated and fully enclosed in the OEF.
Any clay extracted from mining is also put to use around our site. A large amount is needed for the mine levee walls around the open pit. It is also used to seal the PAF cells. There is enough clay material on site to meet our full requirements.
All topsoil is stockpiled for use in progressive site rehabilitation.