Australasian Science: Australia's authority on science since 1938

Ancient Oceans of Gold Point to Potential New Deposits

Tasmanian geologists have estimated how gold concentrations in the ocean have varied over the past 3.5 billion years.

The research, published in Earth and Planetary Science Letters, found that three billion years ago there was ten times the gold in the ancient oceans compared with now. “This was a time when the world’s greatest gold ore deposits were formed in South Africa in the Witwatersrand Basin,” said Prof Ross Large of The University of Tasmania.

“Over the next 400 million years, gold remained high in the oceans and many other important deposits formed, including the Golden Mile in Western Australia.”

Large said that the concentration of gold in the oceans indirectly relates to gold ore generation in the shallow crust. “This means peak times of gold in the oceans correspond to the best times in Earth history for gold ore formation.”

So why was there so much gold in the oceans back then? “Firstly, there was far more volcanic activity, and gold was carried in microparticles in the volcanic magmas and volcanic gases from deep in the Earth and erupted on the Earth’s surface.

“Erosion then transported the gold along with other related elements arsenic, nickel, antimony, tellurium and mercury into the oceans. The very ancient oceans were therefore enriched in gold but highly toxic.”

Following this period, gold levels in the oceans fell to an all-time low. This helps to explain why so few gold deposits are present during the Proterozoic eon.

However, gold returned to the oceans in a third period of Earth history starting 550 million years ago. Large says that the solubility of gold is strongly affected by the amount of oxygen dissolved in the seawater, “so when oxygen increased during the explosion of life in the oceans in the Cambrian period, gold also gradually increased and ultimately reached a maximum 525 million years ago.

“During this third period many major gold deposits formed in the shallow crust, including Bendigo and Ballarat in Victoria, Beaconsfield in Tasmania and Cracow in Queensland,” Large said.

“This new gold time series curve not only tells geologists the time periods to explore for deposits, but also informs the ongoing scientific debate on how oxygen has changed in the atmosphere and oceans through time.”