You can find out more about terms used in mining here.
Open cast mining
Essentially, overburden is removed, the exposed ore is recovered, and the pit is filled, progressively, by the next tranche of overburden.
The ratio of overburden to ore may be 10:1, but can vary widely.
The working footprint of an opencast mine like Macraes in Otago would typically be up to 300 ha although the total area mined over time would be much greater than that.
Open cut mining
A pit is dug, progressively deeper as the ore, technical considerations, consents and economics allow.
When complete, the pit is left as a lake for community use, or modified as agreed in the consent conditions. This is the method adopted at the Martha gold mine at Waihi.
The footprint of an open cut mine is similar to an open cast mine.
Sea bed mining
The technologies for extracting resources off the seafloor, e.g., ironsands, rock phosphate, precious and base metal sulphides, are very different to that deployed on land, with the added challenges of working at depth and often by remote control. Material has to be scooped off the seafloor and pumped via a pipe to the surface to a purpose-built vessel. Separation of ore from waste rock is carried out on board, and the waste rock is then returned to the seafloor via a pipe to minimise sediment dispersion. Any project proposal is subject to independent scrutiny that establishes the environmental impacts and remediation requirements specific to that proposal
In all metal mines and many coal mines there will be a processing facility to upgrade/refine the ore into a saleable product. At the Macraes mine for example, the processing plant crushes, grinds and chemically treats the ore to recover the gold. Any chemicals used are neutralised before discharge. All water (including from rainfall) in the process plant area is collected and treated, and recycled where possible. Any discharge is monitored and controlled to meet volume and quality standards required in RMA consent conditions.
Requirements for rehabilitation are set out in consent conditions. An opencast mine will rehabilitate land progressively as mining advances, usually returning to grazing which is typically more productive than pre-mining.
Mining companies are required to put in place bonds as insurance for the regulator, in the event the company fails.
The bond amount is typically based on an engineer’s peer reviewed assessment of the cost of closure.
An underground mine will normally have a surface footprint of 5-10 ha.
There are a multitude of underground mining techniques to extract the maximum amount of ore in a safe manner.
The percentage of ore recovered depends on safety considerations.