Why is it that gold deposits are found? The reaction of gold is well-known. There seems little reason to think that it should be concentrated rather than spread throughout the Earth's crust. A group of international geochemists has now discovered that gold can be found in concentrated forms with arsenic. This is what explains the majority of gold deposits. This could also explain why arsenic poisoning has been a problem for many gold miners. After recent publication*, this work will be presented at the Goldschmidt conference.For its purity and stability, gold has been treasured for millennia. It is also rare enough that it retains its value. The World Gold Council estimates that all of the gold mined worldwide would fit in a cube measuring 20x20x20 meters. Its beauty is what makes it so valuable. However, it also reacts well with other substances because it is one the most inert metals on the Periodic Table. Why should gold be combined in sufficient quantities to mine?While some gold can be found in the form of gold nuggets (the stuff that prospectors dream about), a significant amount of gold is bound up with other minerals. Iron- and arsenic-containing mineral minerals such as pyrite or arsenopyrite are known to have a connection to gold. These minerals act like a sponge and can concentrate gold up to a million-fold more than what is available in nature. These minerals chemically bind the gold, making it invisible to the naked eye.The European Synchrotron (ESRF), Grenoble, France, produced intense X-rays beams that allowed the scientific team to study the reaction of gold-concentrating minerals. These can be used to probe chemical bonds between the metal and the mineral.The researchers discovered that gold can be stabilized in mineral structures if the mineral is rich in arsenic. This happens by binding directly to arsenic. If the concentration of arsenic is low, however, gold does not enter the mineral structure and forms weak gold-sulfur bond with the mineral surface.Lead researcher Dr. Gleb Pkrovski, Directeur de Recherche CNRS, GETCNRS-University of Toulouse Paul Sabatier, said:Our results indicate that arsenic is responsible for the high concentration of gold. This arsenic-driven pump explains how iron sulfides can capture large amounts of gold and release it. It also controls ore deposit distribution. It will allow you to locate new sources of precious metals and iron sulfides that are arsenic-containing. It could also allow us to control chemical reactions. If we can improve gold processing, then we can recover more of it.This new model shows why gold is often found in combination with arsenic. Dr Pokrovski said:It has been known for centuries now that gold contains arsenic. This has led to severe health problems for miners. We now know the atomic details of what happens and can see if we can prevent it.France is well aware of the noxious link between gold and arsenic. This includes the Salsigne mine in Carcassonne. It was the largest gold mine in Western Europe and also the largest arsenic producer in the world at one point. Although it was closed in 2004, the environmental effects of arsenic poisoning still exist in the area.University of Ottawa Dr. Jeffrey Hedenquist commented that "Geologists, as well as prospectors, have known for a long time that gold can be associated avec arsenic rich minerals. Over the past few decades, others have quantified this association. The team of Dr. Pokrovski has now provided a detailed explanation for this association. It is caused by an atomic-scale attraction of gold and arsenic. This marriage is arranged by certain minerals.###This comment is made by Dr Hedenquist and is not part of the work.
