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Mining & Resources

Tectonics and implications for gold prospectivity in Victoria

22 Jul 2020, by Amy Sarcevic

Victoria’s rich orogenic gold endowment has helped make the State what it is today. But much of its potential is yet to be discovered, with historic goldfields mainly found through surface geochemistry (panning), where mineralised levels are serendipitously exposed.

Even the most superficial data supports this belief. Of the all-time ~190,000 tonnes of gold mined, globally, Victoria has contributed more than 2600 tonnes. This represents 1.5 percent of the world’s gold from only 0.15 percent of the world’s land mass. Or just 0.03 percent when you consider the small land mass occupied by the state’s existing productive goldfields.

That makes Victoria’s goldfield rocks 2 orders of magnitude (100 x richer in gold) than the global average.

Digging deeper with magnetic and gravity data, experts like Ross Cayley, Rob Duncan and Phil Skladzien of the Geological Survey of Victoria say there’s even more reason to think the state is a barely tapped treasure trove for mineral explorers.

As well as large swathes of unexplored greenfield space, they believe existing brownfield sites may have far more to offer – hidden by widespread surficial cover and few near-surface indications of mineralisation at depth.

“Along strike, north of the golden triangle – Bendigo, Ballarat and Ararat (including Fosterville) – prospective basement rocks are covered by younger rocks and sediments,” Senior Geophysicist Phil Skladzien said.

“This has discouraged mineral exploration in the past, but this region has excellent potential.

“Using modern geophysical data – such as gravity and magnetics – and novel modelling techniques, we can peer through the cover sequences and, by proxy, interpret the underlying geology of these rocks of interest.”

Economic Geologist Rob Duncan said geochemical data – a key component of the modern explorer’s technical toolkit – is also important for orogenic gold systems, which tend to be “cryptic” in nature.

“Orogenic gold systems can be tricky to discover, as the fluid that generates them isn’t that chemically different to the rock itself – meaning that alteration footprints are generally subtle,” Rob said.

“There often aren’t a lot of clues and, as a mineral explorer, you need to be savvy when generating and ranking targets worthy of additional testing.

“To understand why Victoria is so well-endowed we need to evaluate evidence preserved in the rock record. This helps us see if a sequence of rocks has seen a gold-bearing fluid, identify what structures and rock types were likely sites for gold precipitations, and the timing of gold mineralisation with respect to deformation events.

“In Victoria’s orogenic gold systems, structural controls – such as faults and fold hinge intersections – are key predictors of gold mineralisation. This means that linking deposit-scale structures with a larger tectonic framework is crucial.”

Senior Geologist Ross Cayley said the monotonous appearance of the goldfield geology across Victoria can also be misleading to explorers with minimal local experience.

“Rocks in Victoria – around Bendigo, as one key example – don’t look particularly special or different to the other rocks in the region – but they have exceptional orogenic gold endowment,” Ross said.

“Why some of these rocks host rich gold – while virtually identical-looking rocks nearby host virtually none – has been one of the big puzzles facing geologists for decades.

“We are finally starting to get to the bottom of this mystery, and delivering this predictive capacity is a big step forward for gold exploration in the State.

“Partly because of these past challenges, somewhere along the way people have forgotten about the possibility of success in these sorts of deposits.

“Sites like Fosterville have reminded us of what world-class gold fields can look like in Victoria, and how hidden they might be.”

With an estimated total gold endowment of 75 million ounzes in Victoria’s northern undercover rocks alone, the experts believe the complex data modelling is worth all of the effort.

“It’s certainly an exciting prospect and explains lot of recent ground pegging in the region,” Rob concluded.

Rob Duncan, Phil Skadzien and Ross Cayley are due to speak at the Victoria Gold Mining and Exploration Forum – held as a virtual event on 27 August 2020.
Hear more from them on the advanced data modelling and geoscientific techniques they are using to suss potential gold endowment in the state.
Learn more and register.

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