The aptly named Nextisa project is possibly the boldest attempt yet in the long and disappointing search for repetitions of the giant lead-zinc-silver deposits and copper deposits that gave rise to the city of Mt Isa.

In the late 1980s and early 1990s, widespread use of geophysical techniques and persistence led to the discovery of a number of major deposits under cover, such as Cannington (about 60m deep), Century (40m) and the iron oxide-copper-gold type Ernest Henry deposit (40m).

But none of the discoveries rival the Mt Isa deposits and exploration in the past 20 years has produced disappointing results.

Red Metal aims to make the breakthrough with a new approach, based on a pre-competitive regional report and data package released earlier this year by Geoscience Australia and the Geological Survey of Queensland.

Red Metal’s analysis of the new report took its focus to a previously unexplored area between 180 and 270km south-southwest of Mt Isa, where it identified an exploration target concealed by 400-600m of Cambrian limestone cover.

The company moved quickly to secure tenure over the area by applying for three tenements (EPMA 19249, 19259 and 19251) and christened it the Nextisa project.

The company believes the physical properties of its new target, combined with modern geophysical exploration methods, give it a real prospect of making a discovery even at the challenging depths.

The geological model behind Red Metal’s Nextisa project is straightforward.

The giant deposits of Mt Isa project are contained within shales deposited at the bottom of rift valley in Paleoproterozoic times.

The Mt Isa fault near the middle of the ancient rift valley and the shale basins that formed within it are the most important structures controlling the location of the Mt Isa mineralisation.

While there is still debate over when and exactly how the deposits were formed by the circulation of fluids, geologists in the area have long understood the importance of exploring along the Mt Isa fault.

The fault can be seen in outcrop, striking south-southwest of Mt Isa over a distance of almost 150km.

It then disappears under the sediments of the Georgina Basin.

Red Metals managing director Rob Rutherford told MiningNewsPremium his team saw in the pre-competitive data a structure heading down from Mt Isa and thought it was very significant.

“In fact, the data clearly shows the location of the Mt Isa fault, which is at the truncation of two different terrains,” he said.

“Previous regional data was too widely spaced and did not allow the exact location of the fault to be pinpointed.

“But we can now see clearly that it changes direction and heads southeast soon after it disappears under cover.

“Previous explorers have seen the Mt Isa fault outcropping towards the southwest and have headed in that direction, including MIM in the ‘80s and ‘90s.

But no-one has explored a large area to the east where we now know the fault is located.”

Rutherford said analysis of geophysical data also revealed shale basins adjacent to the Mt Isa fault over a strike length of 90km.

They have a magnetic signature with strong similarities to the shales hosting the Mt Isa deposits and are believed to represent an equivalent part of the rift valley sedimentary succession.

“We had a look for shale basins along the fault using gravity and magnetic data,” Rutherford said.

“They have a very distinctive magnetic and gravity texture.

“The shales themselves are quite magnetically flat and have low gravity, whereas the volcanics around them are magnetically high and have a gravity high.

“There are distinctive magnitude and textural differences between the volcanics and the shale basins.”

Modelling with the new magnetic data and correlation with the nearest drill holes suggests the shale basins are buried under cover of between 400 and 600 metres but Rutherford says it will not hamper exploration.

“Modern ground electromagnetic exploration techniques can easily penetrate this thickness, especially in the ground we will be exploring,” he said.

“The Mt Isa deposits are hosted in shales with an exceptionally high pyrite content, so they’re a wonderful conductor on a regional scale.

“Outside of these shales, there is not much pyrite at all, so the shales show up as a district-scale anomaly that potentially surrounds lead-zinc and copper deposits.

“By following the pyrite, which can do over strike lengths for about 10 kilometres, you can look within that for lead-zinc deposits and where the fault hits those lead-zinc deposits you can also get the copper mineralisation.”

Rutherford said the nearest drill hole to Red Metal’s Nextisa permits – drilled by MIM – intersected volcanics with a lot of talc alteration.

“Interestingly, that’s what you get in the footwall to the Mt Isa deposits, so that’s another good sign as well,” he said.

Red Metal has all the new ideas and technology it needs for an exciting exploration campaign at Nextisa, which will begin with a series of ground electromagnetic traverses about 4km along the Mt Isa fault.

But the company expects to wait another 12 months before its tenement applications are approved.

“It currently takes 18 months in Queensland for tenement applications to be approved, which is one of many problems for greenfield explorers at the moment,” Rutherford said.

“The other limiting factor for exploration access is the Heritage Act. Don’t get me started on that.”

The Nextisa search has a number of parallels with the conceptual exploration program that discovered Olympic Dam in the 1970s, some 350m under another Cambrian limestone.

The similarities are not lost on Rutherford.

“You have to take those sorts of bold moves to find the tier 1 deposits – the Olympic Dams of this world,” he said.

“There are lots of small projects in Australia and even Prominent Hill and Carrapateena are only one or only two million tonnes of copper.

“A lot of people have been talking up what are effectively third prizes … at today’s copper prices these things still fly but if you want discoveries that are going to keep the country in good shape for the next 50 to 100 years, you have to try some bold ideas.”