Gold enrichment in active geothermal systems by accumulating colloidal suspensions

The origins of high-grade hydrothermal ore deposits are debated, but active geothermal systems provide important clues to their formation(1,2). The highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism(3-7). Yet, similar concentrations have also been found in the absence of any input from arc magmas, for example, in the Reykjanes geothermal field, Iceland(8). Here we analyse brine samples taken from deep wells at Reykjanes and find that gold concentrations in the reservoir zone have increased over the past seven years from an average of 3 ppb to 14 ppb. The metal concentrations greatly exceed the maximum solubility of gold in the reservoir under saturated conditions and are now nearly two orders of magnitude higher than in mid-ocean ridge black smoker fluids-the direct analogues of Reykjanes deep liquids(8,9). We suggest that ongoing extraction of brine, the resulting pressure drop, and increased boiling have caused gold to drop out of solution and become trapped in the reservoir as a colloidal suspension. This process may explain how the stock of metal in the reservoirs of fossil geothermal systems could have increased over time and thus become available for the formation of gold-rich ore deposits.