Contribution of magmatic and tectono-metamorphic processes in the genesis of the Ni-Cu sulfide ores from Mangabal I and Mangabal II, Goiás, Brazil

Abstract

The Mangabal I and Mangabal II layered complexes are part of a set of mafic-ultramafic synorogenic intrusions emplaced during the Brasiliano orogenic Cycle in central Brazil. In this work, new petrographic and scanning electron microscopy data of Ni-Cu sulfides of both complexes are presented. A well-preserved, igneous cumulate textured rock has pyrrhotite-pentlandite and pyrrhotite-pentlandite-chalcopyrite assemblages as intercumulus droplets. This rock has pyrrhotite with iron contents between 64.9 and 65.5%, pentlandite with nickel contents between 14.6 and 25.3%, and chalcopyrite with copper contents between 29.1 and 31.5%. The rocks intensely transformed by metamorphism, which do not preserve igneous texture, have pyrrhotite-pentlandite, pyrrhotite-pentlandite-chalcopyrite, and pyrrhotite-pentlandite-chalcopyrite-pyrite assemblages, all of which accompanied with varying amounts of rutile. These assemblages have pyrrhotite with iron contents between 59.1 and 62.1%, pentlandite with nickel contents between 15 and 40%, and chalcopyrite with copper contents between 32.9 and 36.6%. The pentlandite-pyrite association, as well as venules composed of pyrite or chalcopyrite, constitute phases of replacement and remobilization of pre-existing sulfides. Based on mineralogy, texture and base metal content, the sulfide assemblages found in the rocks intensely transformed by metamorphism are similar in both complexes and they differ from the sulfide assemblages found in the igneous textured rock. Therefore, it is suggested that the tectono-metamorphic processes related to the Brasiliano Cycle triggered remobilizations and chemical changes in the Ni-Cu sulfide ores from the Mangabal I and Mangabal II complexes.