Gold is one of the most important metals to humanity, primarily for the economic value we place on it: Unlike most metals, which are used for a variety of purposes, most gold is used for jewelry. Although we know that this rare ore was likely formed in massive stellar blasts, scientists weren’t sure why there was far more gold in the Earth’s crust than scientific models suggest. Now, it appears a tweak of chemistry may be to blame: A variant of the chemical sulfur looks to be responsible for trapping much of the Earth’s gold in the upper skin of the Earth. The new research is published in the Proceedings of the Royal Academy of Sciences.
Heavy elements – anything heavier than hydrogen or helium – are formed toward the end of a star’s life. When a supermassive star cataclysmically explodes in a supernova explosion, it generates an incredible amount of energy – for a moment, outshining more than all the other stars in its host galaxy put together. It is during this moment that a few atoms of gold are formed, and all the gold we have ever discovered on Earth likely came from the cumulated result of millions of these supernovae across the universe.
Gold, being a very heavy element, should have mostly sunk down into the depths of the Earth’s mantle as the planet formed 4.54 billion years ago, but there is far more found in crustal deposits than we should expect from well-accepted planetary formation models. Gold makes its way to the surface through hot chemical solutions called hydrothermal fluids, normally in chloride or hydrogen sulfide compounds. This new research highlights the role of the trisulfur anion S3-, a negatively charged, electrically excited sulfur molecule, in bringing the illustrious metal to the surface of the Earth.
The trisulfur anion exists as a chemical solution within hydrothermal fluids under very high temperatures, between 200-700°C (393-1292°F), and up to 30 bars (435 pounds per square inch) of pressure. It definitely forms compounds with gold, but it was thought to be an insignificant carrier of it compared to hydrogen sulfide. To test its effectiveness at snatching up elemental gold, model fluid systems containing gold and various sulfur compounds were constructed; these were then subjected to a range of environmental conditions in which the temperature, pressure, pH and so on were altered.
Their study reveals that although hydrogen sulfide compounds containing gold are the most stable under moderate temperatures and pressure, as the environment became more extreme, the number of trisulfide anions dramatically increased, permitting far more gold to bond with them. Not only is the trisulfide anion remarkably good at chemically bonding with gold, but it appears to bond with gold more than any other so-called “sulfur-loving” metal, such as copper or platinum.
The researchers think that the trisulfide anion enhances the extraction of gold from deep-seated magma, before using its durable chemical properties to transport vast amounts of it up through the Earth and into the crust, where it can be mined by humans.
“Our findings show that one of the oldest metals, known from Antiquity, has yet to divulge all its secrets," says lead author Gleb Pakrovski in a statement. "We are just at the beginning of our understanding of geological fluids from the deep.”
