The optimal windows for seismically-enhanced gold precipitation in the epithermal environment |
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| Institution: | 1. Department of Geology, Universidad de Chile, 8370450 Santiago, Chile;2. Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Santiago, Chile;3. Department of Earth Sciences, Swiss Federal Institute of Technology, ETH Zentrum NO, 8092 Zürich, Switzerland;4. Department of Structural and Geotechnical Engineering, Pontificia Universidad Católica de Chile, 7820436 Santiago, Chile;5. School of Environment, The University of Auckland, Auckland, New Zealand;6. GEOMAR, Helmholtz-Zentrum für Ozeanforschung Kiel, Kiel, Germany;7. Departamento de Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile;1. Université de Ouagadougou, Burkina Faso;2. Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherche pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F-31400 Toulouse, France;3. ONG-D Le Soleil dans la Main asbl, 48, Duerfstrooss, L-9696 Winseler, Luxembourg;4. IFAN Cheikh Anta Diop, Dakar, Senegal;5. B2Gold Corp., 595 Burrard Street, Vancouver, BC V7X 1J1, Canada;6. GF Consult bvba, Antwerpsesteenweg 644, 9040 Gent, Belgium;1. IC2MP, Université de Poitiers, UMR 7285, Bât. B35, 6 rue Michel Brunet, TSA 51106, 86073 Poitiers cedex 9, France;2. Institut P'' - UPR 3346 Département Physique et Mécanique des Matériaux, SP2MI - téléport 2 - Bd Marie et P. Curie, BP 30179, 86962 Futuroscope Cedex, France;3. IMPMC, UMR 7590, Université Pierre et Marie Curie, Institut de Recherche pour le Développement, Muséum d''Histoire Naturelle, 4 Place Jussieu, 75252 Paris, cedex 05, France;4. OSUR - Géosciences Rennes, UMR CNRS 6118, Campus de Beaulieu Université de Rennes 1, Avenue Général Leclerc, 35042 Rennes, cedex, France;1. Grupo Recursos Minerales, Dpto. Ciencias de la Tierra, Universidad de Zaragoza, c/ Pedro Cerbuna 12 (Edificio Geológicas), 50009 Zaragoza, Spain;2. Department of Geological Sciences, Swedish Museum of Natural History, Frescativagen 40, Box 50007, SE-104 05 Stockholm, Sweden;3. CADIC-CONICET, c/ Houssay 200, 9410 Ushuaia, Tierra del Fuego, Argentina;1. School of Geosciences, University of Sydney, NSW 2006, Australia;2. Bare Rock Geological Services Pty Ltd, Fremantle, WA 6160, Australia;3. School of Earth Sciences, University of Western Australia, Crawley, WA 6009, Australia;4. Geology Department, Lakehead University, 955 Oliver Rd, Thunder Bay, Ontario, P7B 5E1, Canada |
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| Abstract: | Epithermal gold (Au) deposits result from the combination of a sustained flux of metal-rich fluids and an efficient precipitation mechanism. Earthquakes may trigger gold precipitation by rapid loss of fluid pressure but their efficiency and time-integrated contribution to gold endowment are poorly constrained. In order to quantify the feedbacks between earthquake-driven fracturing and gold precipitation in the shallow crust, we studied the gold-rich fluids in the active Tolhuaca geothermal system, located in the highly seismic Southern Andes of Chile. We combined temperature measurements in the deep wells with fluid inclusion data, geochemical analyses of borehole fluids and numerical simulations of coupled heat and fluid flow to reconstruct the physical and chemical evolution of the hydrothermal reservoir. The effect of seismic perturbations on fluid parameters was constrained using a thermo-mechanical piston model that simulates the suction pump mechanism occurring in dilational jogs. Furthermore, we evaluated the impact of fluid parameters on gold precipitation by calculating the solubility of gold in pressure (P)–enthalpy (H) space. The reconstructed fluid conditions at Tolhuaca indicate that single-phase convective fluids feeding the hydrothermal reservoir reach the two-phase boundary with a high gold budget (~ 1–5 ppb) at saturated liquid pressures between 20 and 100 bar (210 °C < Tsat < 310 °C). We show that if hydrothermal fluids reach this optimal threshold for gold precipitation at a temperature near 250 °C, small adiabatic pressure drops (~ 10 bar) triggered by transient fault-rupture can produce precipitation of 95% of the dissolved gold. Our results at the active Tolhuaca geothermal system indicate that subtle, externally-forced perturbations – equivalent to low magnitude earthquakes (Mw < 2) of a hydrothermal reservoir under optimal conditions – may significantly enhance gold precipitation rates in the shallow crust and lead to overall increases in metal endowment over time. |
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