Why Are There Gold Deposits at All? Scientists Discover the Answer

Gold Microparticles

Scanning electron microscopy photomicrograph showing gold microparticles (bright color) deposited with pyrite crystals from a hydrothermal solution in a laboratory experiment. Such experiments simulate the formation of gold deposits in nature. Credit: Maria Kokh and Gleb Pokrovski

Why are gold deposits found at all? Gold is famously unreactive, and there seems to be little reason why gold should be concentrated, rather than uniformly scattered throughout the Earth’s crust. Now an international group of geochemists has discovered why gold is concentrated alongside arsenic, explaining the formation of most gold deposits. This may also explain why many gold miners and others have been at risk from arsenic poisoning. This work is presented at the Goldschmidt conference, after recent publication.

Gold has been prized for millennia, for its purity and stability. It’s also rare enough to retain its value — the World Gold Council estimates that all the gold ever mined in the world would fit into a 20x20x20-meter (66x66x66-foot) cube. It is valued for its beauty, but also because it is one of the most inert metals in the whole Periodic Table, it doesn’t easily react with other substances. So why should gold come together in sufficient quantity to mine — why are there gold deposits at all?

While some gold is found as gold nuggets, the stuff of prospectors’ dreams, an appreciable amount is bound up with minerals. Gold is known to be related to iron- and arsenic-containing minerals, such as pyrite and arsenopyrite. These minerals act sort of like a sponge, and are capable of concentrating gold up to a million times more than is found elsewhere in nature, such as in the hot spring waters that transport the gold. This gold becomes chemically bound in these minerals, so it is invisible to the naked eye.

Super Pit Gold Mine

A giant gold deposit in W. Australia (Super Pit). Pyrite and arsenopyrite are the major ore minerals concentrating invisible gold together with arsenic. Credit: Gleb Pokrovski

The scientific team studied the action of the gold-concentrating minerals using the intense X-ray beam produced by the European Synchrotron (ESRF) at Grenoble in France, which can probe the chemical bonds between the mineral and gold.

They found that when the mineral is enriched with arsenic, gold can enter the mineral structural sites by directly binding to arsenic (forming, chemically speaking, Au(2+) and As(1-) bonds), which allows gold to be stabilized in the mineral. However, when the arsenic concentration is low, gold doesn’t enter the mineral structure but only forms weak gold-sulfur bonds with the mineral surface.

Lead researcher, Dr. Gleb Pokrovski, Directeur de Recherche at CNRS, GET-CNRS-University of Toulouse Paul Sabatier said:

“Our results show that arsenic drives the concentration of gold. This arsenic-driven gold pump explains how these iron sulfides can massively capture and then release gold, so controlling ore deposit formation and distribution. In practical terms, it means that it will make it easier to find new sources of gold and other precious metals, which bind to arsenic-containing iron sulfides. It may also open the door to controlling the chemical reactions, and if we can improve gold processing, we can recover more gold.”

The new model identifies just why gold tends to be found with arsenic. Dr. Pokrovski continued:

“It has been known for centuries that gold is found with arsenic, and this has caused severe health problems for gold miners. Now we know what happens at an atomic level, we can begin to see if there’s anything we can do to prevent this.”

The noxious link between arsenic and gold is well-known in France and elsewhere in the world, including at the Salsigne mine near Carcassonne. This was one of Western Europe’s largest gold mines, and the world’s largest arsenic producer at one time. It closed in 2004, but the environmental consequences of the arsenic pollution still persist in the region.

Dr. Jeffrey Hedenquist, University of Ottawa, commented that “Geologists as well as prospectors have long known that gold can be associated with arsenic-rich minerals, and over the past few decades others have quantified this association. The findings of Dr. Pokrovski and his team now help to explain why we see this association, caused by an atomic-scale attraction between gold and arsenic, with this marriage arranged by the structure of certain minerals.”

14 Comments on "Why Are There Gold Deposits at All? Scientists Discover the Answer"

  1. Clyde Spencer | July 4, 2021 at 5:41 pm | Reply

    “Gold has been prized for millennia, for its purity …”

    I don’t know what is meant by this. Gold is never found in nature as 100% gold. It is usually alloyed with significant silver, and minor amounts of copper, iron and other elements such as arsenic. Native gold is typically around 80-90% gold.

    Native electrum (Ag-rich)and amalgam (Hg-rich) can contain less than 50% gold. Native gold is anything but pure!

    • An alloy is just a mixture. A compound involves chemical bonds – for example, water is not a mixture of gaseous oxygen and gaseous hydrogen.

      • I might have worded that last statement badly (it’s late enough here to qualify as being really early, lol).

  2. Sounds like a hairs breadth from making sponges that extract gold from ocean water.

  3. From the heading to the content the announcement is repeated 5 times
    Please one heading and then get into it

  4. Pierre Plauzoles | July 8, 2021 at 3:52 pm | Reply

    Gold is also often associated with other platinum-group elements, which are collectively known as “PGEs” (gold and silver are both in the group).

  5. Pierre Plauzoles | July 8, 2021 at 3:55 pm | Reply

    Gold’s specific gravity also acts as a frequent concentrator.

  6. In fact so Clyde. It is even labor intensive to purify it. The fact remains, we would both love to own 10lbs of the stuff

  7. Interesting read.

  8. Why does gold react the way it does to Mercury. Are there any concrete answers to explain the attraction.

  9. James Mcglothlin | July 10, 2021 at 7:19 pm | Reply

    You never answered the question. You gave a brief rundown of how gold is found with arsenic …actually twice!
    You still haven’t said anything about why isn’t there a layer of gold crust or why its sometimes found as nuggets and in streams. I just wish you would have addressed those questions

  10. The purity in comparison to any other “metals” being used at that particular time could be used to qualify that statement

  11. There has to be gold in Antarctica. Tons of it.

  12. 20x20x20 METER cube is a typo. Much more gold than that.

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