What happens when a mine is depleted?

Open-pit mines are often expanded until the mineral resources are exhausted or until an increasing ratio of overburden to ore makes further mining uneconomical. When this happens, exhausted mines are sometimes converted to landfill sites for the disposal of solid waste. However, some form of water control is often necessary to prevent the mine shaft from becoming a lake, if the mine is located in a climate of considerable rainfall or if any of the layers of the shaft forming the mine border on productive aquifers. In Germany and adjacent countries, several former opencast mines have been deliberately converted into artificial lakes, forming areas such as the Lusatian Lake District, the Central German Lake District or the Upper Palatinate Lake District.

A special concern in the formation of these lakes is the acid mine drainage. Most pit walls are generally mined at an angle less than vertical. The waste rock falls out as the shaft gets deeper, so this angle is a safety measure to prevent and minimise damage and danger from rock falls. However, this depends on how weathered the rocks are and the type of rocks involved.

It also depends on the amount of structural weaknesses in the rocks, such as faults, shears, joints or foliations. At the side of the pit there is usually a haulage road that forms a ramp for trucks transporting ore and waste rock. Waste rock is transported to a dump. Waste rock dumps can be piled on the surface of the active pit or in pits that have already been mined.

But if we were to run out of a mineral, i.e. exhaust our reserves, it would probably not be because there was none left in the Earth. The problem would be that the processes used to extract it have become too expensive, difficult or damaging to make mining worthwhile. Even then, as mining technology advances, previously inaccessible minerals will become available and lower-producing minerals will be processed more efficiently.

Open-pit mining involves the removal of soil and rock (overburden) above a layer or seam (especially coal), followed by the extraction of the exposed ore. Metal mining provides the necessary elements for energy supply, communication and transport, among others. The increasing adoption of green technologies, such as electric vehicles and renewable energy, will also increase the demand for metals. However, the lifespan of the average mine is much shorter than the timescales of mineral deposit formation, suggesting that metal mining is unsustainable on human timescales.

Moreover, some research suggests that known primary metal supplies will be exhausted in about 50 years. Here we present an analysis of global metal reserves that suggests that primary metal supplies will not run out on this timescale. On the contrary, we find that world reserves of most metals have not declined significantly relative to production over time. This is the result of the replenishment of depleted reserves through increased delineation of known deposits as mineral exploration progresses.

We suggest that environmental, social and governance factors are likely to be the main source of risk to the supply of metals and minerals in the coming decades, rather than direct reserve depletion. This could lead to increased conflict over resources and a decline in the conversion of resources into reserves and production. Geologically scarce mineral resources could be exhausted within a few decades or a century. At first glance, sustainability and mineral resource development appear to be in conflict.

Mining depletes finite resources and is therefore, strictly speaking, inherently unsustainable. For example, there is only a finite amount of copper in the earth's crust, and each unit of copper extracted increases the fraction of the total copper resource base that is in use. Therefore, it can be argued that if we continue to mine, we will eventually exhaust the available supply of minerals.

David Gerula
David Gerula

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