North of 60 Mining News - The mining newspaper for Alaska and Canada's North

The formation of bonanza-grade gold veins like those running through the ore at Pretium Resources Inc.'s Brucejack Mine in Northern British Columbia, Canada has long baffled scientists. Given the time it would take the mineralizing fluids carrying minute concentrations of gold to form the thick veins of the precious metal, they simply should not exist. Yet, these seemingly impossible ultrahigh-grade seams of gold are found at places like Brucejack, Ballarat in Australia, Serra Palada in Brazil, and Red Lake in Ontario.

Taking clues from the electrochemical reaction that causes milk to coagulate when it goes sour, Anthony Williams-Jones, a professor of earth and planetary sciences at McGill University in Quebec, and PhD student Duncan McLeish have solved the mystery of how these hyper-enriched gold veins are formed.

In simplified terms, gold veins are formed as hot water dissolves gold atoms and redeposits the precious metal as this enriched fluid flows through cracks in the earth's surface. Typically, this redeposited gold is microscopic but sometimes form veins several centimeters thick. Given the low concentrations of gold in the fluid, it should take millions of years for such veins to form. The cracks, however, usually seal up in days, months, or years.

"The paradox of bonanza gold deposits is that there is simply not enough time for them to form, they should not exist, but they do!" the pair of McGill researchers explained.

Using a powerful electron microscope to observe particles in thin slices of gold-enriched rock from the Brucejack Mine, Williams-Jones and McLeish discovered that gold colloid, a fluid reminiscent of milk, is the hidden actor filling cracks in the earth with thick layers of gold.

"Milk is a colloid that consists of butterfat particles that remain suspended in water because they repel each other, like the negative ends of two magnets. When the milk goes sour, however, the surface charge breaks down and the particles clump together to form a jelly. In a similar way, gold colloids consist of charged nanoparticles of gold that repel each other. When the charge breaks down, however, the gold particles 'flocculate' to form a jelly," the researchers explained.

This gold jelly gets trapped in the cracks of rocks, and voila, bonanza-grade gold veins are formed.

These gold jelly-producing colloids have a distinctive red color that is unlike a non-colloid fluid containing dissolved gold, which is clear.

The McGill researchers were able to produce the first images of small veins of gold colloid particles at the nanoscale, demonstrating how the process works in nature.

"Our findings solve the paradox of 'ultrahigh-grade' or 'bonanza' gold formation, which has frustrated scientists for over a century," they said.

Knowing how the process works is expected to help mineral exploration and mining companies discover new bonanza gold deposits similar to Brucejack.

Duncan McLeish

McGill University Professor Anthony (Willy) Williams-Jones and Pretium Resources geologist Joel Ashburner study a gold-bearing vein coming to the surface on the Brucejack property.

"Genetic studies of Canada's most fertile metallogenic districts – such as the one we have just completed at Brucejack – are required to improve our understanding of how world-class mineral deposits form, and thereby develop more effective strategies for their exploration," said Williams-Jones and McLeish.

The McGill professor and PhD student suspect that the same colloidal processes that deposited thick veins of gold at Brucejack played a role in more typical gold deposits but will need to find suitable material to test their hypothesis.

More clues as to where to look for this material may turn up as the team continues investigating Brucejack to better understand the reasons why colloid formation and flocculation occurred on the scale observed and reconstruct the geological environment of these processes.

EDITOR'S NOTE: Brucejack, sour milk and bonanza gold veins was originally published in the May 26, 2021 edition of Metal Tech News.

Author Bio

Shane Lasley, Publisher

Over his more than 16 years of covering mining and mineral exploration, Shane has become renowned for his ability to report on the sector in a way that is technically sound enough to inform industry insiders while being easy to understand by a wider audience.

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