The Olympic Dam ore deposit was formed in a volcanic environment about 1,500 million years ago.
At that time, huge blob-like bodies of molten rock (magma) were rising, or intruding, from the deep, hot interior of the Earth into the upper part of the Earth's crust. Many magma intrusions cooled and crystallised before reaching the surface to form bodies of solid granite. In some cases magma reached the surface before solidifying, and was erupted explosively and violently from volcanoes. The crystallising magmas released super-hot, water-rich fluids that contained dissolved iron together with copper, gold, silver, and uranium. These fluids were under extreme pressure, and caused fracturing of the crust in a process known as brecciation. They also deposited into the brecciated crust red coloured hematite (an oxide of iron) along with crystals of a range of ore minerals containing copper, gold, silver and uranium.
Hundreds of millions of years later the area was eroded flat by the souring action of glaciers, then covered by thick layers of flat-lying sedimentary rocks (mostly shales). The shales were deposited onto the floor of an ocean. Much later, the land was uplifted above sea level, and the shales eroded to form the current land surface. At present, the ore body is 350 m below the surface and buried by these shales.
The ore body was discovered in 1975 when exploration drill holes intersected rocks rich in copper and uranium bearing minerals. Olympic Dam is estimated to contain about 2,320 million tonnes of ore. On average, each tonne of ore is estimated to contain about 13 kg copper, 0.4 kg of uranium, and 0.5 grams of gold. It is the biggest known uranium ore body in the world, and the 6th biggest known copper ore body. It is also one of Australia's most significant gold and silver resources.
