Andrew Watson reports that the mine is located in the Mount Isa inlier of Queensland, Australia, the Mt Isa deposits represent the largest accumulations of lead, zinc and silver in the world. The first deposits were discovered in 1923 as the result of lucky whack of a horseshoe and the area has been mined continuously since then. There are 11 years of known reserves remaining. The location is literally in the middle of nowhere and the challenges overcome to create a multi-decade mining camp has created a strong sense of local pride. In addition, the remoteness of the deposit has given rise to several national innovations including Australia’s Royal Flying Doctor Service.
Mt Isa is the largest example of a Sedimentary Exhalative or SEDEX deposit. These are formed as hot, metal-rich brines percolate through sea-floor sediments. The figure below demonstrates the scale of Mt Isa in comparison to other SEDEX deposits and the distribution of known Lead-Zinc deposits over time. Note that Mt Isa (represented by the blue bar) contains an estimated of 90,000,000 tonnes combined lead-zinc (Pb-Zn) metal. (Not 90 million tonnes of ore, but contained metal!)
In addition to the lead and zinc in the deposits, there are 300 million tonnes of ore grading at 2.8% Cu, and additional resources containing of lower grade material. Finally there is the small matter of 150 g/t Ag or 5 ounces of silver per tonne that is associated with some of the Mt. Isa Sedex deposits. While not a single deposit, they are close enough in age, lithology and form to be considered a mining camp and part of single genetic event.
The Mt Isa deposits are hosted in what are known as “sag basins” where the Earth’s crust has been stretched and then “sags” and fills with sediments. In the Mt. Isa basin, the oldest sediments are dolostones and fluvial sandstones, with minor black shales, passing into turbidites, more black shales and finally stromatolitic dolostones. This sediment record indicates initial deposition in a shallow water and near shore environment followed by deeper water ocean environment and ending with a shallow-water environment as the basin filled.
Sedex deposits form at low temperatures (100°-200°C) distal to magmatic heat sources. They are composed of layer after layer of galena (Lead Sulfide), sphalerite (Zinc Sulfide) and iron sulfides such as pyrite and pyhrrotite in shales or turbidites. Unlike the other big source of lead-zinc mineralization, carbonate hosted Mississippi Valley Type Deposits, they do not require a source of H2S to precipitate mineralization and the deposition processes can run for a very long time.
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