Different metals concentrated at different depths in the same ore body. The zoning pattern reveals fluid temperatures and chemistry from 1.6 billion years ago.
A 1.65 billion-year-old chemical blueprint
In the Proterozoic rocks of northwest Queensland lies a world-class mineral deposit with a peculiar structure. The Mount Isa ore bodies contain immense quantities of both copper and lead-zinc-silver, but they are not uniformly mixed. Instead, they are spatially separate within the same host rock, a 1-kilometer-thick formation of carbonaceous dolomitic siltstones and shales known as the Urquhart Shale. This distinct separation shows the physical and chemical conditions that existed deep in the Earth's crust more than 1.65 billion years ago.
The lead-zinc-silver ores are found as more than 30 stacked, stratiform lenses in the upper 650 meters of the Urquhart Shale. Below this, and slightly offset, is a massive, irregular body of silica-dolomite rock that contains the copper mineralization. The entire system is located next to a major structure, the Mount Isa Fault, which acted as a conduit for the mineralizing fluids. These were not two separate mineralizing events happening at different times, and two products of a single, massive hydrothermal system.
A geologic thermometer
The zoning is a direct result of a temperature gradient in the hot, metal-rich brines that circulated through the rock. The copper ore, primarily composed of the mineral chalcopyrite, precipitated from hotter fluids, with temperatures estimated between 300 and 350 degrees Celsius. These fluids moved through fractures, altering the host shale into the brecciated silica-dolomite rock that now contains the copper.
As the fluids moved away from the main conduits and cooled, the chemistry changed. At lower temperatures, estimated between 120 and 240 degrees Celsius, the conditions became favorable for precipitating lead and zinc sulfides—the minerals galena and sphalerite. This process formed the finely layered lead-zinc-silver deposits further from the main heat source. The result is a large-scale chemical zonation where copper is concentrated in the hotter, core zones and lead-zinc is deposited in the cooler, more distal parts of the system. By studying the distribution of these minerals today, geologists can reconstruct the plumbing and temperature profile of this ancient deep-earth system.
💡Fun Facts
The lead-zinc-silver deposits were discovered first, in 1923 by prospector John Campbell Miles. The enormous copper orebody was only found decades later during development of the lead mine.
The Mount Isa system is one of the largest networks of underground mine development in the world.
The main copper mine, Enterprise, is Australia's deepest underground mine, reaching a depth of 1.9 kilometers.
The Urquhart Shale that hosts the minerals has organic matter, which is believed to have helped trigger the chemical reactions that caused the metals to precipitate out of the hot brines.
While the active mine is not open to the public, the Outback at Isa visitor center offers extensive exhibits. It is open Monday to Friday from 8:30am to 4:30pm, and on weekends from 7:30am to 2:00pm.
Admission
Admission fees apply for specific experiences like the Hard Times Mine tour and the Riversleigh Fossil Discovery Centre.
Accessibility
The Outback at Isa visitor center and its exhibits are generally accessible. The surrounding region is rugged outback terrain.