| Keywords |
vein, fault, fluid flow, mineralisation, fracture sealing, quartz, calcite, epidote, fieldwork, duplex |
| Abstract |
Geothermal reservoirs require high permeability to sustain fluid flow, which is often controlled by fracture networks. Mineral precipitation in veins within these fractures records interaction between fluids and deforming crust. Understanding vein formation processes in an exhumed geothermal system is fundamental to understanding fluid flow and efficiently exploiting modern geothermal systems in similar tectonic settings. The Caleta-Coloso transtensional duplex, part of the Atacama Fault Zone in Chile, is an analogue structural setting for a large proportion of geothermal systems. The duplex, formed almost entirely in granodirotic host rock, is bounded by two sinistral NNW-SSE strike-slip faults, and contains later ~NW-SE and ~E-W transtensional to extensional faults. Fractures in the fault damage zones contain a mineral assemblage of chlorite-epidote-quartz-calcite, recording a cooling geothermal system. Through analysis of mechanical damage and mineralogical data collected via linear transects and outcrop surface mapping, we characterize structural controls on episodic flow to help identify where may be most prospective in terms of permeability and fluid flux. Measurements of vein intensity suggest that areas within the duplex have undergone more brittle deformation than areas outside, with greater connectivity also suggesting higher paleo-permeability. There is a large degree of variability, however, with greatest paleo-permeability in a localised dilational jog near a major bounding fault, where each successive vein phase has exploited existing structures (crack-seal) but also creates linkage structures. Successive precipitation phases (i.e. fluid flow events) often form in different parts of the damage zone, suggesting earlier phases have significantly reduced permeability and/or increased rock strength. Vein thickness is used as a proxy for fluid flux, indicating the amount of sealing by precipitate. Nearest the bounding faults, high total thicknesses of all vein phases, alongside evidence of hydrobrecciation and crack-seal structures associates with epidote-bearing phases, suggest that ongoing high-temperature, high-pressure hydrothermal fluid flux has occurred consistently and over long timescales in these areas. In the duplex centre (dominated by few, thick, single-phase calcite veins), sporadic low-temperature high-flux fluids capable of propping open fractures succeeded short-lived, low-flux, higher-temperature fluid flow. Outside of the duplex it appears that fewer, less connected fracture orientations remained open, but potentially for longer periods. Ongoing related work aims to further consider vein microstructure to quantify mineral precipitation rates and pressure/temperature conditions to help constrain the longevity of these geothermal systems. |