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.
Size and Scale
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.
Formation and Geology
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.
In terms of geologic time, the Mt Isa cluster falls within a sweet spot for clastic hosted Pb-Zn deposits. The chemistry of the ocean was intermediate between the acid Archaean ocean, and the oxygenated Neoproterozoic – Phanerozoic ocean. The first oxygenation event was from 2.4 – 1.85 billion years ago and saw the deposition of the Banded iron formation. In addition to the reduction in acidity the ocean was getting less salty with values again intermediate between Archaean and modern. The cluster of deposits at Mt. Isa is dated at 1.65 – 1.635 billion years, right in the middle of this long period of oceanic chemical change.
The lead-zinc(+-silver) deposits are hosted in dolomitic and variably carbonaceous, pyrite rich, black shale. They occur as stacked lenses of sulfide minerals, in some cases 80 metres thick. They are considered contemporaneous with their hosts with the representative Black Star deposits dating to ~ 1.655 billion, and the host Urquart shale dating to 1.653 ± 0.007 billion years ago. The mine sequence is one kilometer thick and hosts 20 distinct clusters of lead-zinc mineralization. The sediments dip at 65° into the earth from surface.
The copper mineralization is thought to have post dated the lead-zinc deposition by 100 million years, and is hosted in a very different, brecciated (broken-up) siliceous-dolomitic-vein complex that has a chalcopyrite-pyhrrotite-pyrite metallogeny. Like South Australia, and the Northern territory, the area was subjected to intense metasomatism (a type of metamorphosis) from 1.5 billion years to 1.4 billion years ago, which resulted in the emplacement of numerous IOCG deposits including Olympic Dam. This activity didn’t leave behind gold but instead resulted in the formation of numerous copper sulfide deposits at Mt Isa. These copper deposits are much deeper than the Pb-Zn deposits.
Despite the discovery of high-grade copper mineralization at Mt Isa, the copper was ignored for many years and only briefly mined during World War 2. That changed as the price of copper rose and the lead-zinc was gradually depleted. Given the extensive infrastructure at Mt Isa it made economic sense to develop the deep copper deposits.
The Enterprise Mine is the most recently developed copper asset and at 1,900 meters is Australia’s deepest mine.
References and Further Reading
Mount Isa Mines (Company Website)
Leach, D. L., Bradley, D. C., Huston, D., Pisarevsky, S. A., Taylor, R. D., & Gardoll, S. J. (2010). Sediment-hosted lead-zinc deposits in Earth history. Economic Geology, 105(3), 593-625. Link (Subscription Required)
Finlow-Bates, T., & Stumpfl, E. F. (1980). The copper and lead-zinc-silver orebodies of Mt Isa mine, Queensland: products of one hydrothermal system. Annales de la Société géologique de Belgique. (PDF)