Gold enrichment in active geothermal systems by accumulating colloidal suspensions

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Abstract

The origins of high-grade hydrothermal ore deposits are debated, but active geothermal systems provide important clues to their formation1,2. The highest concentrations of gold are found in geothermal systems with direct links to island arc magmatism3,4,5,6,7. Yet, similar concentrations have also been found in the absence of any input from arc magmas, for example, in the Reykjanes geothermal field, Iceland8. 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 liquids8,9. 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.

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Figure 1: Location of the Reykjanes geothermal system where the Mid-Atlantic Ridge emerges on Iceland.
Figure 2: Trace metal concentrations of the deep geothermal brines at Reykjanes compared with typical mid-ocean ridge black smoker fluids from the East Pacific Rise.
Figure 3: Schematic representation of the deep geothermal reservoir showing conditions leading to the accumulation of gold in the deep liquids.

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Acknowledgements

We thank ÍSOR and HS Orka Ltd for field support during the sampling of RN-12, 19, and 21. Financial support for this work was provided by HS Orka Ltd and by GEOMAR − Helmholtz Centre for Ocean Research Kiel and an NSERC Discovery grant to M.H. F. Óskarsson, G. Ó. Friðleifsson and B. Steingrímsson are thanked for their helpful discussions during the sampling and for insight on the discharge history of the wells. M. Olafsson assisted with the collection of the surface samples. U. Westernströer and K. Bremer assisted with the trace metal analyses.

Author information

All authors participated in the fieldwork, the design of the study, and the analysis of the data. M.H. and V.H. prepared the manuscript with D.G.-S. and K.L.B. The sampling programme was managed by V.H., together with ÍSOR and HS Orka staff. K.L.B. provided the downhole sampler and oversaw the deployments. D.G.-S. conducted the laboratory analyses. All authors discussed the results and contributed to the writing of the paper.

Correspondence to Mark Hannington.

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The authors declare no competing financial interests.

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Hannington, M., Harðardóttir, V., Garbe-Schönberg, D. et al. Gold enrichment in active geothermal systems by accumulating colloidal suspensions. Nature Geosci 9, 299–302 (2016) doi:10.1038/ngeo2661

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