Tectonic setting, structures, and Au-Cu mineralization age of the Indiana deposit: An example of ore deposit formation controlled by Andean transverse faults, Atacama region, Chile

Abstract

The Indiana Deposit corresponds to a Cu-Au (Mo-Co) fault-vein deposit located in the Central Andes Coastal Cordillera Belt (similar to 27 degrees S). It is hosted by strongly altered volcanic and intrusive rocks located between the NNE-striking central and principal branches of the Atacama Fault System (AFS) in northern Chile. The Indiana Deposit is part of a 10 km-long and 5 km-wide mine district that includes the Iron-Copper bearing Cerro Negro Norte (CNN) deposits and the Cu-Au Galleguillos vein system, termed the CNN-Indiana District in this study. The district provides insights into the mid-Cretaceous structural and lithological control of iron, copper and gold mineralization in northern Chile.Mineralization in the Indiana Deposit is spatially and temporally related to NW- and ENE-striking fault-vein systems, which are kinematically unrelated to the AFS. Three main mineralized ENE-striking, hundred-meter-long structures have been identified in Indiana. Structural and kinematic data suggest that mineral precipitation took place at dilation zones (e.g. extensional jogs, transtensional fault terminations) generating steeply-plunging ore shoots, and reactivating pre-existing structures (NW fault-vein system).Sinistral transtensional kinematics observed in ENE-striking fault-vein structures postdate the activity of the margin-parallel Atacama fault system, which has for long been regarded as accommodating the strike-slip component of plate convergence during the Jurassic to middle Cretaceous neglecting the role of Andean Transverse Faults (ATF), such as the ones recognized in this study. Sinistral transtension along ENE-striking faults gave rise to a local NE-striking maximum principal stress, different from that arising from displacement along the main margin-parallel faults, which in turn is consistent with far field stresses at that time.The CNN-Indiana district evolved from an early Ca-Na high temperature system related to Iron-bearing deposits to a brittle mineralized fault-vein copper-gold-bearing quartz-sericite (k-feldspar-calcite) corridor associated with a lower temperature environment along the Costal belt.Re-Os ages yielded 109-108 Ma obtained along the mineralized ENE-striking structures in Indiana Deposit and are slightly younger than those obtained from similar deposits associated with deformation along the AFS, which is consistent with our field observations indicating that at least part of the strike slip displacement on transverse faults hosting mineralization outlasts that of the AFS.