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

Abstract

Fault zones must be considered as complex and heterogeneous systems, withareas of high permeability that alternate with very low permeability bands. Strike-slip fault zones playan important role in fluid migration in the crust, and exhumed faults can provide insights into theinterrelationships of deformation mechanisms, fluid-rock interactions and bulk chemicalredistributions. We determined the mineral chemistry and whole-rock geochemistry of the damagezone and fault core of the Caleta Coloso Fault, a complex major crustal scale strike-slip fault inNorthern Chile, in order to constrain the physical and chemical conditions of fluids that lead to stronghydrothermal alteration. Caleta Coloso Fault consists of variably altered protocataclasites,cataclasites and discrete bands of ultracataclasite derived from a protolith of Jurassic tonalite.Hydrothermal alteration associated with fault-related fluid flow is characterized by a very low-gradeassociation composed by chlorite, epidote, albite, quartz and calcite. Chlorite thermometry indicatesT-values in the range of 284 to 352 °C (average temperature of 323 °C) and no differences inmineral composition or T-values were observed among different cataclastic rock types. Mass balanceand volume change calculations document that the major chemical mobility was observed inprotocataclasite, whereas cataclasite and ultracataclasite show smaller changes. This suggests thatfluid flow and chemical alteration post-dated the faulting, when the protocataclasite was relativelypermeable and the cataclasite and ultracataclasite acted as a barrier for fluid flow having a very lowpermeability due to extreme grain size reduction during cataclasis.