Bitumen-rich lavas in the Lower Cretaceous of Copiapó, Atacama Region, Chile: petrochemistry and metallogenesis significance.

Abstract

The bitumen-rich andesitic horizon emplaced in the upper part (Pabellón Formation) of the Lower Cretaceous marine sequences in the Atacama Region (Chañarcillo Group) has been formerly considered a sill. The main evidence for interpreting this horizon as submarine lava flows includes: perfect concordance with both the overlying and underlying marine limestones along kilometers, the presence of autobreccias, tuffs and thin chert beds between the flows, the presence of endofossils in undisturbed positions within calcareous lenses that grade laterally to lavas or between successive beds, and the recognition of, at least, one emission center in Quebrada Cerrillos where the lavas reach 300 meters thick. Petrographically, the lavas are porphyritic andesites, derived from K-rich calc-alkaline magmas. Trace and minor element geochemistry point to an intraplate more than to a subduction related origin. These lavas are associated with a back-arc environment and were emplaced in a relatively thin continental crust during the Early Cretaceous. The presence of hydrothermal bitumen is a distinct feature of this horizon in many localities between Copiapó and Illapel. The bitumen occurs as the infilling of vesicles or veins, and is directly associated with metallic mineralisation, copper sulphides essentially. Organic geochemical results demonstrate that the bitumen originated from marine organic matter, migrating and being emplaced before the Cu-rich hydrothermal fluids. This association between bitumen and copper sulphides has become a regional metallotect in the Copiapó region. The same association has been found in numerous deposits along the Lower Cretaceous porphyritic lavas between Copiapó and Illapel, suggesting that this is a regional control associated to the evolution of the back-arc basin.