A mineral pump for gold in the earth's crust
To form an ore deposit, gold must be enriched thousands to millions of times more than it occurs on average in the Earth's crust (which is only about 1 mg per ton of rock). In nature, very few minerals, namely arsenopyrite (arsenopyrite) and löllingite (arsenic iron), are known to have such enrichment factors for gold. However, despite its enormous importance, the state of this "invisible" gold and the cause of its emplacement in these sulfides remains one of the greatest mysteries in the history of ore deposit exploration.
An international and interdisciplinary scientific consortium, including chemist Prof. Thomas Doert, has been able to unravel this mystery. They have demonstrated the nature of gold uptake by these minerals, revealing the fundamental mechanism that drives these "mineral pumps" on an atomic scale. By combining high-resolution experiments - conducted at the European Synchrotron Source (ESRF) - and physicochemical modeling, the team found that gold is taken up into these minerals at the +2 oxidation state. This is made possible by a redox reaction between the liquid and the mineral, which causes gold to bind to arsenic and leads to the formation of the atomic cluster AuAsnS6-n (image). "This universal gold-arsenic coupling mechanism explains how these sulfides can capture and later release gold in large quantities, controlling both the concentration and distribution of gold in
different types of hydrothermal deposits. The new conceptual model opens perspectives for finding novel sources of gold and other valuable and critical metals hidden in iron sulfide minerals, and can thus contribute to an improvement in processing and recycling of metal ores in our "metal-hungry" society," explains Thomas Doert.