MULTI-PHASE SKARN MINERALIZATION ALONG A TRANSPRESSIONAL SHEAR ZONE, COPIAPÓ, CHILE

The Chilean Iron Belt hosts world-class iron oxide-copper-gold deposits that are typically spatially associated with the Atacama Fault System (AFS). Near Copiapó, clear fault branches no longer define the AFS, and mineralization in the Punta del Cobre district occurs east of the main system. This off-AFS location may be due to the lack of a controlling brittle structure, which is unusual for the fault system. Here we combine new mapping and structural measurements with geochronometric and laser fluorination stable isotope analyses to understand the magmatic, deformation, and mineralization history of this AFS-related shear zone.

The main branch of AFS near Copiapó is defined by a ~500-m-thick steeply NW-dipping shear zone that lacks brittle overprint. Zircon U-Pb ages document synkinematic emplacement of a tonalite in the shear zone at ~121 Ma. NW-dipping ultramylonitic fabrics in the tonalite strike ~5–10° clockwise of the shear zone boundary on average and have shallowly NE-plunging lineations. Kinematic indictors record oblique sinistral-reverse shear, but locally coaxial fabrics dominate, indicating an overall transpressional regime. The tonalite records a synkinematic endoskarn assemblage of act+ep+ttn+pl, but mylonitic microstructures are completely annealed. The shear zone is cut by an unstrained ~107 Ma diorite body that contains pervasive act+ep+adr+pl endoskarn mineralization. A minor ~103 Ma tonalite body in the shear zone lacks both strain and skarn-related alteration. A late adr+ep+cal+qz+mag skarn vein has a ~96 Ma andradite U-Pb age. Stable isotope analyses of the skarn vein yield δ¹⁸O values of +11.1‰ (qz), +5.4‰ (grt), and +5.7‰ (ep), indicating alteration by magmatic fluids at 410–470°C.

Together, these relationships document three pulses of skarn mineralization over a ~35 Myr period. The oldest is synkinematic with Early Cretaceous shearing. Shear zone activity overlaps in age with other sections of the AFS. The lack of brittle faulting is likely related to continued magmatism associated with the Copiapó batholith complex, which is younger than most arc plutons in the Coastal Cordillera. Postkinematic skarn mineralization along the AFS is unique to the Copiapó area, and magmatic fluids responsible for alteration were most likely derived from the Copiapó batholith.