The information on mining being promoted by Far North Mayor Wayne Brown is (so far) silent on the potential downsides of mining. A recent media release from Forest and Bird highlighted concerns about longterm pollution caused by toxic mine tailings, a mixture of water, chemicals and crushed rock. The Forest and Bird media release says that dams near the Waihi gold mine hold 40 million tonnes of toxic waste *. It also says that about 18 tonnes of waste is created for one gold ring. Other estimates the Newsletter has seen range from 20 to 720 tonnes, depending on the type of rock being mined and the concentrations of the gold.
The Northland mineral exploration promotion booklet says there are approximately 1.5 million ounces of gold (about 40 million grams) in Northland, at concentrations of around 5 – 6 grams/tonne. Mining this would produce at least 8 million tonnes of toxic tailings. This would have a volume of approximately 2.5 million cubic metres, depending on the specific gravity of the waste. It would take holding ponds covering an area of about 2km long by 1km wide and 1 metre deep to hold this waste – the equivalent of 12,500 Rawene school swimming pools, or 250 rugby fields buried a metre deep. There are also large amounts of waste rock (“strip” material dug out to get at the mineral-bearing ore) that does not contain commercially usable minerals and is not chemically treated. This material is sometimes used to build retaining walls for the tailings, and may also contain toxic substances.
* [In 2005 the Martha mine at Waihi produced 4.6 million grams (US) of gold from 2.5 million tonnes of rock.]