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Study could shine path to hidden treasures

The geometry of cracks in rock formed when earthquakes rip along fault lines could point the way to untapped mineral reserves.

Paul Stenhouse, of The Australian National University, is looking for links between the twists and turns in the fractures and the amount of economic minerals deposited there by water.

Hot, salty water forced along fractures in the wake of earthquakes can dump metals ranging from gold to lead, says Stenhouse, a PhD candidate at the Research School of Earth Sciences.

"As fluids move up through the fault system, metals precipitate,” he says. “We want to understand what controls these flow systems in the hope of identifying new mineral deposits."

Exploration geologists have long known that it takes large volumes of fluid, in the order of 20 Sydney Harbours, to pass through one of these fracture systems to form an economic mineral deposit.

"They also know that the distribution of fluid flow depends on the geometry of the faults," Stenhouse says.

He says rich economic mineral deposits form in systems with irregular geometry. "They occur in areas of structural complexity, such as bends and bifurcations," he says.

However, using these features as an exploration tool has so far been a black art. "Noone has quantified the difference in fluid flux between different structural features for use as a reliable predictive tool," he says.

Stenhouse is studying a big, 75-million-year-old fault system in Oman and a 15-million-year- old system in Switzerland to put more science into the process.

The fault systems, consisting of calcite veins in limestone host rock, are exposed, so Stenhouse can trace them with satellite images before mapping them in more detail on the ground.

He has collected samples along the fault lines from regions of differing geometry, applying in the field skills honed as a recreational rock climber.

Back at the laboratory at the ANU, he is comparing the oxygen and carbon isotopic composition of the veins with that of the host rock.

Veins from segments of the fault that hosted large volumes of water have isotopic signatures differing greatly from those of the host rock. "But in areas without much fluid flow, the vein takes on the isotopic signature of the host rock," he says.

By correlating the flow volume with the geometry of the fault system, Stenhouse hopes to come up with a way to identify the most promising features for exploration.

"Maybe this will give us a guide to ranking targets, and, hopefully, reduce the discovery time," he says.

Although the fault systems under investigation in Oman and Switzerland are outcropping, the method could also work on buried systems, which could be mapped using geophysical methods.

"This method has big potential in Australia, especially in ‘greenfields’ regions which have not yet had significant exploration," he adds.

The work comes as existing deposits are being mined out, and companies search for new ones.

Stenhouse undertook his undergraduate studies at the University of Otago, and worked for the New Zealand science agency, GNS Science.

Later, as an exploration geologist, he worked across the world, from Vietnam to Alaska.