For decades, scientists have been puzzled by the formation of rare super-enriched gold deposits in places such as Ballarat in Australia, Serra Palada in Brazil and Red Lake in Ontario. Although such deposits usually form over tens or hundreds of thousands of years, these high-grade deposits can form over years, months, or even days. By studying examples of these deposits from the Brucejack mine in northwest British Columbia, scientists have found that these deposits of gold form like curdled milk.
Scientists have long known that gold deposits are formed when hot water flows through rocks, dissolving small amounts of gold and concentrating it in cracks in the earth’s crust at levels invisible to the naked eye. In rare cases, cracks transform into veins of pure gold several centimeters thick. But how do fluids with such low gold concentrations create rare ultra-high grade gold deposits?
The new discovery resolves the paradox of the formation of ultra-high quality gold, or the gold mine, which haunted scientists for more than a century. The paradox of gold-mine gold deposits is that there simply shouldn’t be enough time for them to form, and they shouldn’t exist, but they do exist! Because the concentration of gold in hot water is very low, very large volumes of liquid must pass through cracks in the earth’s crust to deposit useful gold concentrations. This process takes millions of years to fill a one-centimeter-wide crack with gold, whereas these cracks usually close in days, months, or years.
Using a high-power electron microscope to observe particles in thin slabs of rock, we found that gold deposits form from a liquid very similar in possible function to milk. Such “milk” is saturated with gold colloids, which are composed of charged gold nanoparticles that repel each other, but when the charge is destroyed, they flocculate, forming “jelly”. This “jelly” gets stuck in cracks in rocks, forming streaks of ultra-high gold. Colloids of gold are bright red in color and can be made in the laboratory, while solutions of dissolved gold are colorless.
The researchers obtained the first evidence of the formation of gold colloids and flocculation in nature, as well as the first images of small veins of colloidal gold particles and their flocculated aggregates at the nanoscale. These images document the process of filling the cracks with gold and, magnified by combining millions of these small veins, show how the gold veins are formed.
Now that scientists understand how gold deposits are formed, exploration companies can use the results of this work to better explore gold deposits. The researchers suspect that the colloidal processes that took place in Brucejack and other gold systems could also lead to the formation of more typical gold deposits. The challenge is to find suitable material to test this hypothesis.