Structural control of the emplacement of the Portrerillos porphyry copper, central Andes of Chile

Abstract

The Potrerillos porphyry is located in the Late Eocene–Oligocene metallogenic belt of porphyry copper deposits of Chile, Cordillera de Domeyko, central Andes. It is formed of a porphyritic biotite–hornblende monzonite that was emplaced at less than 1000 m depth. The intrusion has a cylindrical vertical form at depth, whereas its uppermost section resembles a mushroom whose stem is inclined 20–25° to the northwest. The location of the Potrerillos porphyry coincides with a major facies and east–west thickness change in Jurassic to Late Cretaceous marine sedimentary and Tertiary volcanic host rocks. Structurally, the porphyry was emplaced at the intersection between the northeast-oriented Potrerillos fold-and-thrust belt, and a system of northwest-trending strike-slip faults of the Ciénaga fault system. Both structural elements are related to a sinistral regional transpression that was active during the Middle to Late Eocene. The north–northeast trending Sierra Castillo-Agua Amarga fault system was trench-linked with an oblique subduction along the Peru–Chile trench in the southwestern part of South America at this time. One of the main structural elements of the Potrerillos fold-and-thrust belt is the eastward verging Potrerillos Mine reverse fault. We demonstrate a syntectonic relationship between the Potrerillos porphyry emplacement and the onset of the Potrerillos Mine fault. The control at the shallowest levels of the Potrerillos porphyry emplacement was studied by means of a structural analysis of folds, cleavage and deformed oolites in the country rock. This analysis suggests that the porphyry occupied the core of an anticline. The form of this anticline was modeled as a fault-propagation-fold taking into account a 200 m displacement along the basal fault and a local shortening of 45.5% occurred. This model is compatible with a limited displacement along the Potrerillos Mine fault under hot, ductile conditions. The remaining displacement of 1600 m would have taken place afterwards under cold, brittle conditions. The Potrerillos porphyry was emplaced during a transpressional regime within the Potrerillos fold-and-thrust belt. The fact that the cleavage does not show a concentric distribution around the Potrerillos porphyry, discounts the possibility of a forced emplacement of the porphyry, pushing aside its wall rocks. This is reinforced by the deformed oolites which do not show a strain gradient from outside towards the edge of the body. The clockwise transection of the fold axes by the cleavage and the southeast direction of displacement along the Potrerillos Mine fault are interpreted as related to the regional sinistral transpression related to the Sierra Castillo Fault movement.