Scientists have been puzzled by the formation of rare hyper-enriched gold deposits for over a century.

Examples of these ultra-high-grade or ‘bonanza’ deposits occur in places like Ballarat in Australia, Serra Palada in Brazil, and Red Lake in Ontario.

Gold deposits form when hot water flows through rocks, dissolving very small amounts of gold and concentrating it in cracks in the Earth’s crust, usually at levels invisible to the naked eye.

In rare cases, the cracks are transformed into veins of solid gold centimetres thick.

To fill a single centimetre-wide crack with gold would take centuries via this process, whereas these cracks on ‘bonanza’ deposits typically seal in years, months, or even days.

‘Like soured milk’

 

Studying the ‘Brucejack’ mine in northwestern British Columbia, McGill Professor Anthony Williams-Jones learned that these gold deposits form much like soured milk.

“Milk consists of little butterfat particles that are suspended in water because they repel each other, like the negative ends of two magnets,” Williams-Jones says.

“When the milk goes sour the surface charge breaks down, and the particles clump together to form a jelly.

“It is the same with gold colloids, which consist of charged nanoparticles of gold which repel each other, but when the charge breaks down, they ‘flocculate’ to form a jelly.

“This jelly gets trapped in the cracks of rocks to form the ultra high-grade gold veins.”

A ‘colloid’ is a mixture in which microscopically dispersed insoluble particles are suspended throughout another substance.

‘Flocculation’ occurs when  colloidal particles come out of suspension to form small lumps.

 

How does this benefit gold exploration?

At Brucejack, the next step will be to better understand the reasons why colloid formation and flocculation occurred on the scale observed and reconstruct the geological environment of these processes, Williams-Jones says.

These results are important to the mineral exploration and mining industry in Canada and around the world, he says.

“Now that we finally understand how bonanza deposits form, mineral exploration companies will be able to use the results of our work to better explore for bonanza deposits.

“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.”

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