Metamorphism in IOCG Systems: Contact Metamorphism in the Candelaria-Punta del Cobre District, Chile

Abstract

Near Copiapó, Chile, volcanic-dominated rocks of the Early Cretaceous Punta del Cobre Formation host Fe-oxide(-Cu-Au) (“IOCG”) mineralization with varied development of metamorphic mineral assemblages. Western occurrences, such as the Candelaria deposit, lie within the metamorphic aureole of the composite Copiapó batholith (110-120 Ma), while other, more easterly deposits in the district lack evidence of high-grade metamorphism. This allows comparison of features seen at Candelaria, but not at the other deposits, that may have resulted from contact metamorphism. Petrography and scanning electron microscopy of Candelaria samples reveal early penetrative, planar fabrics overprinted by texturally and mineralogically recrystallized assemblages. High-temperature metamorphism of iron-enriched protoliths led to the development of biotite, garnet, (Mg,Fe,Mn) amphiboles, and cordierite-bearing assemblages in the Punta del Cobre formation. Metamorphic silicates overgrowing and crosscutting deformed, recrystallized ore minerals suggests that shear-zone deformation and contact metamorphism occurred during and after mainstage Cu-sulfide and Fe-oxide deposition. Peak greenschist to amphibolite-facies contact metamorphism occurred at P<4 kbar and 730-815±50℃. New U-Pb geochronology of garnets spans 115.8±3.7 Ma to 87.5±3.7 Ma and broadly coincides with magmatism and later alteration in the district. The contact metamorphosed mineralization at Candelaria resembles nominally syn-metamorphic mineralization elsewhere in the world, supporting post-mineralization metamorphic interpretations at these other locations.