Geology and metamorphism of the neoproterozoic Mangabal Complex: An example of Ni[sbnd]Cu-PGE mineralized intrusion in the Goiás Magmatic Arc, central Brazil

Abstract

Inserted in the context of the Goiás Magmatic Arc, the Mangabal Complex represents a partially metamorphosed mafic-ultramafic intrusion, which is part of a cluster of neoproterozoic layered bodies formed during the Brasiliano Orogeny in central Brazil. The Mangabal Complex is composed of two exposures, the northern and the southern members, which exhibit the similar igneous mineralogy, crystallization sequence and were affected by the same metamorphic conditions. The stratigraphy of the Mangabal Complex can be divided into three main zones: i. the Lower Mafic Zone (LMZ), composed of adcumulatic norite and, forming the basal portion of the southern member; it is composed of adcumulatic norite; ii. the Ultramafic Zone (UZ), characterized by dunite and harzburgite cumulates, and iii. the Upper Mafic Zone (UMZ), consisting predominantly of norite, with restricted interlayeres of feldspathic dunite-orthocumulate. Petrographic data reveal a crystallization sequence for the Mangabal Complex formed by Olivine + Cr-Spinel > Olivine + Orthopyroxene > Orthopyroxene + Plagioclase > Clinopyroxene. In the olivine cumulus, the forsterita (Fo) content (Fo 78–85) suggest a moderately primitive parental magma. The NiCu-PGE mineralization occurs as disseminated sulfides, located in the UMZ, UZ and LMZ cumulate rocks, or forming massive to net-textured bodies in the metamorphic rocks of the UMZ. The main sulphide assemblage is composed of pyrrhotite + pentlandite + chalcopyrite ± pyrite, with minor pyrite + millerite. Disseminated sulphides are present in igneous rocks as well as in the metamorphic rocks that still partially preserve their original primary texture. This fact, together with the occurrence of abundant olivine and chromium-bearing spinel cumulates, suggests a fertile magma for the Mangabal Complex. However, the highest S, Ni, Cu, and PGE contents correlate positively with and are related to amphibolite and calc-silicate rocks associated with shear zones, indicating that metamorphism and deformation processes were critical in promoting the remobilization of sulphides and their deposition along shear zones. The primary mineral assemblage was largely replaced due to high-pressure upper-amphibolite facies metamorphism. The paragenesis representing metamorphic peak conditions is composed of kyanite + orthoamphibole + hornblende, which reveals pressures of approximately 8.5 kbar and a temperature up to 750 °C. The metamorphic mineral assemblage, together with the proximity to the São Luis de Montes Belos shear zone, suggests that kyanite crystallization in the Mangabal Complex and similar layered intrusions worldwide might be linked to the development of major crustal-scale structures.