Hydrothermal alteration in an exhumed cristal fault zone: Testing geochemical mobility in the Caleta Coloso Fault, Atacama Fault System, Northern Chile

Abstract

Crustal scale strike slip fault zones have complex and heterogeneous permeability structures, playing an impor-tant role in fluid migration in the crust. Exhumed faults provide insights into the interplay among deformationmechanisms,fluid-rock interactions and bulk chemical redistributions. We determined the whole-rockgeochemistryandmineralchemistryofthefaultcoreoftheCaletaColosoFaultinNorthernChile,inordertocon-strain the physical and chemical conditions that lead to strong hydrothermal alteration. The strike-slip CaletaColoso Fault core has a multiple-core architecture, consisting of alternate low strain rocks (protolith, weaklydeformed protolith and protocataclasites) and high-strain strands (cataclasites and discrete band ofultracataclasite) derived from a Jurassic tonalite. Hydrothermal alteration associated with fault-relatedfluid flow is characterized by a very low-grade association consisting of chlorite, epidote, albite, quartz and calcite.Chlorite thermometry indicates T-values in the range of 284 to 352 °C, no variations in mineral composition orT-values were observed among different cataclastic units. Mass balance and volume change calculations showsignificantly larger chemical mobility in the protocataclasites than in the cataclasite (and ultracataclasite). Thissuggests that fluid flow and chemical alteration are strongly controlled by deformation being protocataclasiterelatively more permeable than cataclasite. Chlorite precipitation and grain reduction in cataclasite (andultracataclasite) would reduce permeability acting as a barrier for fluid flow. Chemical mobility and volumechanges in the Coloso Fault core suggest different effective fluid/flow ratios during amalgamation of subsequentand subparallel deformation bands that finnally control the fracture-channeling allowing fault-related fluid–flowinto each of them.