Magmatic and structural controls on mineralization in the Paleocene magmatic arc between 22°40' and 23°45' south latitude, Antofagasta, II region, Chile

Abstract

Geochemical and structural data from the Paleocene magmatic arc in a part of northern Chile constrain magmatism and metallogenesis in the Sierra Gorda, Sierra del Buitre, Lomas Bayas, and San Crist6bal districts. The Paleocene magmas were primarily derived from the mantle which was enriched prior to and/or during ascent by crustal materials. The Nd and Sr isotopes have ranges of 1.06 to 3.34 and 0.703888 to 0.707142, respectively. La/Sm ratios increase and Eu/Eu* ratios decrease from the mafic to felsic phases but these ratios do not correlate with Si0₂ content within the districts. The subtle isotopic heterogeneity and lack of REE systematic behavior within the districts indicate that multiple sources and/or petrogenetic processes generated this rock suite. The geochemical data do not predict metal type among the districts; the spatial relationship, however, of a quartz-eye porphyry with gold mineralization implies consanguinity. Tne petrogenesis of the quartz-eye porphyry is distinct from that of the other Paleocene rocks. The investigated area is characterized by epithermal mineralization that is constrained to two discrete episodes: 1) 65-60 Ma when polymetallic veins, silver rich in the Sierra Gorda and Sierra del Buitre districts, and copper-rich veins in the Lomas Bayas district were emplaced during normal convergence, and 2) 60-55 Ma when shear veins, copper-rich in the Sierra Gorda district and gold-rich in the San Cristóbal district, formed during oblique convergence. Copper mineralization occurred in the Sierra Gorda district during the transition between these episodes. The structural geology of the Chilean continental crust preserves a record of plate interactions along a consuming margin and provides constraints for convergence angle of the oceanic plate. A comparison with other Paleocene-Eocene deposits in northern Chile and southern Peru as well as with Jurassic copper deposits to the west and Oligocene porphyry copper deposits to the east shows that the emplacement of large, magmatism-related ore deposits is principally controlled by change in plate motion. Optimum conditions occur when a change to oblique convergence allows the development of trench-linked strike-slip faults. The paucity of known large ore deposits in the investigated area is due to the lack of extensive margin-parallel strike-slip faults that did not develop because of short-lived oblique convergence as well as the relative magmatic inactivity during changes in plate motion.