Overburden is the term used to describe what is left of the mined rock after the zinc-lead ore has been removed. In reality, it is the naturally occurring rock and soil found in the region. It is not subject to any chemical processes at the mine.

For every tonne of ore mined at MRM, there is five times more overburden disposed as waste material within an area called the Overburden Emplacement Facility (OEF).

The overburden includes some topsoil, clay, and a variety of rocks. Each of these 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.

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 with coloured paint and tape to differentiate between the ore and overburden. The geologists do more than the minimum standards required by industry guidelines to make sure all ore and overburden is categorised accurately.

The overburden is categorised into two types of rock. The majority of the overburden is non-acid forming (NAF) materials. These are types of rock which are chemically stable and will not generate any by-products which could impact on the environment. Only NAF material is used on the external faces of all mine structures to ensure any runoff draining into the McArthur River system has no acid forming potential.

Potentially-acid forming (PAF) materials are the types of rock that when exposed to air and water, can generate an acidic water, soluble metals or salts that could impact on the environment. Studies show that 20% of the total overburden at MRM could be PAF material.

For this reason, each section of ore and the different types of rock are scooped up separately by the excavators used to dump it into the back of large trucks. The trucks also identify whether they carry ore, PAF or NAF material. How they are categorised determines where they deposit their loads.

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 encapsulated in the OEF.

Any clay extracted from mining is also put to use around the mine 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 the mine’s requirements.

All topsoil moved during earthworks on site is stockpiled and used in the progressive rehabilitation of mine facilities.

Other areas of interest

2011 Factsheet - Overburden