For decades, scientists have been puzzled about the formation of rare hyper-enriched gold deposits

For decades, scientists have been puzzled about the formation of rare hyper-enriched gold deposits in places like Ballarat in Australia, Serra Palada in Brazil, and Red Lake in Ontario. While such deposits typically form over tens of thousands to hundreds of thousands of years, these “ultra high grade” deposits can form in years, months, or even days. How do they form so quickly?

McGill professor Anthony Williams-Jones from the Department of Earth and Planetary Sciences and PhD student Duncan McLeish used examples of these deposits from the Brucejack Mine in northwestern British Columbia and PhD student Duncan McLeish found that these gold deposits form similar to sour milk . When the milk becomes sour, the butterfat particles clump together into a jelly.

Questions and Answers with Anthony Williams-Jones and Duncan McLeish

What did you want to find out?

Scientists have long known that gold deposits form when hot water flows through rocks, dissolving tiny amounts of gold and concentrating it in cracks in the earth’s crust on levels invisible to the naked eye. In rare cases, the cracks turn into veins of solid gold that are inches thick. But how do fluids with such low gold concentrations produce rare, ultra-high grade gold deposits?

What did you discover

Our results solve the “ultra high grade” or “bonanza” gold formation paradox that has frustrated scientists for over a century. The paradox of Bonanza’s gold deposits is that they simply don’t have enough time to form. They shouldn’t exist, but they do!

Since the gold concentration in hot water is very low, very large amounts of liquid have to flow through the cracks in the earth’s crust in order to deposit degradable gold concentrations. This process would take millions of years to fill a single inch wide crack with gold, while these cracks typically seal in days, months, or years.

Using a powerful electron microscope to observe particles in thin slices of rock, we discovered that a liquid, similar to milk, forms bonanza gold deposits. Milk is made up of small butterfat particles that are suspended in water because they repel one another, like the negative ends of two magnets. When the milk becomes acidic, the surface charge breaks down and the particles clump together into a jelly. The situation is similar with gold colloids, which consist of charged nanoparticles of gold that repel one another. However, when the cargo collapses, it “flocculates” into a jelly. This jelly is trapped in the cracks of rocks to form the ultra-high gold veins. The gold colloids are distinctly red and can be made in the laboratory, while solutions of dissolved gold are colorless.

Why are the results important?

We have provided the first evidence of the formation and flocculation of gold colloids in nature and the first images of tiny veins of gold colloid particles and their flocculated aggregates on the nanoscale. These images document the process by which the cracks are filled with gold and show how the integration of millions of these tiny veins enlarges how bonanza veins are formed.

How will this discovery affect the mining industry?

Our results are important to the mineral exploration and mining industries in Canada and around the world. After we finally understand how bonanza deposits form, mineral exploration companies can use the results of our work to better explore bonanza deposits and gold occurrences. Genetic studies of Canada’s most fertile metallogenic areas – like the one we just completed at Brucejack – are needed to improve our understanding of the origins of world-class mineral resources and, thus, to develop more effective strategies for their exploration.

What’s next for this research?

We suspect that the colloidal processes that occurred in Brucejack and other bonanza gold systems may also have formed more typical gold deposits. The challenge will be to find suitable material to test this hypothesis. With Brucejack, the next step is to better understand the causes of colloid formation and flocculation on the observed scale and to reconstruct the geological environment of these processes. We also prepared gold colloids in the lab to simulate what we discovered at Brucejack.

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About this study

“Colloidal transport and flocculation are the cause of gold overenrichment in nature” by Duncan F. McLeish, Anthony E. Williams-Jones, Olga V. Vasyukova, James R. Clark, and Warwick S. Board was published in Proceedings of the Board National Academy of Sciences of the United States of America.

DOI: https: //.doi.org /10.1073 /pnas.2100689118

About McGill University

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