An international team of geochemists has discovered why gold is concentrated alongside arsenic, thereby explaining the formation of the majority of gold deposits. The Goldschmidt Geochemistry Conference findings may also help explain why many gold miners and others have been exposed to arsenic poisoning. Gold is renowned for its inert properties, and there appears to be little reason for it to be concentrated rather than uniformly distributed throughout the Earth’s crust.
For millennia, gold has been prized for its purity and stability. Additionally, it is scarce enough to retain its value – according to the World Gold Council, all gold ever mined would fit inside a 20x20x20-meter cube.
It is highly prized not only for its beauty, but also because it is one of the most inert metals in the Periodic Table, meaning it does not react readily with other substances. Therefore, why should gold aggregate in sufficient quantity to be mined – why are there even gold deposits?
While some gold is discovered in the form of gold nuggets, the stuff of prospectors’ dreams, a significant amount is bound up with minerals. Gold is known to be associated with minerals containing iron and arsenic, such as pyrite and arsenopyrite.
These minerals act similarly to sponges, concentrating gold up to a million times more than what is found naturally, such as in the hot spring waters that transport the gold. Due to the chemical bonding of the gold with these minerals, it becomes invisible to the naked eye.
The scientific team investigated the gold-concentrating minerals’ action by using an intense X-ray beam produced by the European Synchrotron (ESRF) in Grenoble, France, which can probe the chemical bonds between the mineral and gold.
They discovered that when a mineral is enriched with arsenic, gold can enter the mineral’s structural sites directly by covalently bonding with arsenic (forming Au(2+) and As(1-) bonds), allowing gold to be stabilised within the mineral. When arsenic levels are low, gold does not penetrate the mineral structure but instead forms weak gold-sulfur bonds with the mineral surface.
Dr Gleb Pokrovski, the lead researcher and Director de Recherche at CNRS’s GET-CNRS-University of Toulouse Paul Sabatier, stated, “Our findings indicate that arsenic is responsible for the concentration of gold. This arsenic-driven gold pump explains how these iron sulphides can capture and then release gold in large quantities, thereby controlling the formation and distribution of ore deposits. In practise, this means that new sources of gold and other precious metals that bind to arsenic-containing iron sulphides will be easier to discover. Additionally, it may pave the way for better control of chemical reactions, and if we can improve gold processing, we will be able to recover more gold.”
The new model elucidates why gold is frequently associated with arsenic. Dr Pokrovski went on to say, “For centuries, it has been known that gold is associated with arsenic, which has resulted in severe health problems for gold miners. Now that we understand what occurs at the atomic level, we can investigate whether there is anything we can do to prevent this.”
Arsenic’s poisonous relationship with gold is well-known in France and throughout the world, including at the Salsigne mine near Carcassonne. This was once one of the largest gold mines in Western Europe and the world’s largest producer of arsenic. Although it ceased operations in 2004, the environmental consequences of arsenic pollution continue to be felt in the region.
Dr Jeffrey Hedenquist of the University of Ottawa stated, “Geologists and prospectors have long known that gold can be associated with arsenic-rich minerals, and others have quantified this association over the last few decades.”
Dr Pokrovski’s and his colleagues’ findings now help to explain why this association exists, owing to an atomic-scale attraction between gold and arsenic, which is facilitated by the structure of certain minerals.
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