Geochemistry and fluid inclusions across a crustal strike-slip Mesozoic fault: insights of fluid-flow / rock interaction in the Atacama Fault System

Abstract

Faults architecture and their permeability related fractures play a first order role in fluid-flowmigration throughout the upper crust. Commonly, the interaction between fluid-flow migration and host rock isreflected as mineral precipitation in a vein mesh and/or as mineralogical changes (alteration) of the host rock. Often,however, the relationship between a fault zone and the fluid-flow passing through it is poorly understood. In order toimprove our understanding of this process we have chosen, as a case study, the Jorgillo Fault (JF), which lies withinthe Atacama Fault System, a trench-parallel large-scale structure developed within Mesozoic rocks of the presentdayCoastal Cordillera in northern Chile.The JF is represented as a ca. 18 km long NNW-SSE, in its southern end, to NW-SE, in its northern part, westwardconcave-shape sinistral strike-slip fault showing a maximum left-lateral displacement of about 4 km and asubvertical dip. The fault cuts through crystalline rocks of gabbric, dioritic and granodioritic composition. The JFcore is composed by a ca. 1 m wide cataclasite zone bounded by two fault gouge zones ca. 40 cm in average whileits minimum damage zone extension, based in field observations, is ca. 2 m wide each side of the core zone.A fault perpendicular transect was mapped and sampled in order to run XRF and XRD analyses of the fault core,damage zone and undeformed protolith. XRF analyses of the rocks revealed that contents of Al and Ca decreasewith increasing Si, while Na increases towards the fault core. Fujita et al. (2012) interpreted similar behavior inanalysis of rocks belonging to the Coloso Fault, which is genetically and spatially related to the JF, as compositionalchanges of plagioclase to albite-rich ones due to chloritic-propilitic alteration processes. In the damage zone, L.O.I.data increase towards the fault core but decrease inside the core in its cataclastic zone. This behavior of L.O.I. datais explained by the presence of H2O-rich clays (and gypsum) in the boundary of the fault core represented as faultgouge zones while in the cataclastic zone, the decrease in L.O.I, is explained by the presence of epidote minerals.XRD analyses show a background chloritic alteration. These preliminary analyses give an insight into the evolution ofthe fault zone in which, at an early stage a cataclasite-rich core is formed and, as deformation continues, a gougeboundedcore is developed. The JF is likely to have had an evolving permeability structure with time with the laterstage development of these gouge-rich zones acting as a fluid-barrier for hydrothermal fluids passing across the JF.Fluid inclusions of two quartz veins within the damage zone were analyzed. The results show primary L-V type FI,no boiling evidence, with a wide range of homogenization temperatures (Th) from 119 to 230 °C with variable lowsalinities of 0,5 – 9,3 (wt% NaCleq.). This is consistent with fluids found in epithermal systems.