Interplay between brittle deformation, fluid-rock interaction and mineralization in hydrothermal systems of the southern Andes

Abstract

The interactions between seismic activity, fluid flow, and mineral precipitation play a critical role in promoting the development ofgeothermal systems and the formation of ore deposits. Here we study the high enthalpy, metal-rich and active Tolhuaca geothermalsystem to (1) address how the interplay between structural and geochemical processes controls the physicochemical evolution ofthe hydrothermal reservoir and (2) analyze the transient effects of earthquake-triggered pressure perturbations on metal solubilityand mineralization. To achieve this, a comprehensive structural and mineralogical analysis at both field and drill core scales werecombined with geochemical data of borehole fluids and fluid inclusions. Results obtained from this study reveal that hydrothermalalteration modifies the response of rock to deformation at Tolhuaca, produces a vertical compartmentalization of the system andpromotes the development of a clay-rich low permeability zone. Moreover, our data indicate that the lifespan and thermal structureof this system were highly affected by the low-permeability zone developed on top. Thermodynamic modeling of gold and silicasolubility at Tolhuaca reveals that the optimum conditions for metal precipitation are reached at liquid-saturated conditions (Ts< 310°C). Below this temperature, small pressure changes triggered by transient fault-rupture results in a solubility drop of goldand silica that can reach several orders of magnitude. These pressure changes are likely to occur in specific structural elementssuch as dilational jogs, where pore volume is rapidly created by dilation during fault activity resulting in a transient fluid pressuredrop. Numerical simulations of displacement in dilational jogs indicate that a moderate magnitude earthquake (e.g., Mw=4) cancreate a volume of approximately 600 m3 within the jog, affecting at least an equivalent amount of fluid. Simulations of coupledheat and fluid flow indicate that post-seismic fluctuations in the fluid conditions occur at two time scales. First, an extremely fastpressure increase occurs at the jog center in less than one second until boiling pressure is reached. Then, the pressure drops andthe flashing front moves towards the host rock and declines in magnitude at the minute scale. Thus, our results give new insightson the magnitudes and timescales of the transient processes occurring during the evolution of hydrothermal system and theformation of ore deposits.