The optimal windows for seismically-enhanced gold precipitation in the epithermal environment

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 (similar to 1-5 ppb) at saturated liquid pressures between 20 and 100 bar (210 degrees C < T-sat <310 degrees C). We show that if hydrothermal fluids reach this optimal threshold for gold precipitation at a temperature near 250 degrees C, small adiabatic pressure drops (similar to 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. (C) 2016 Elsevier B.V. All rights reserved.