Geology, petrology and geochronology of the Lago Grande layered complex: Evidence for a PGE-mineralized magmatic suite in the Carajas Mineral Province, Brazil

摘要

The Lago Grande and Luanga mafic-ultramafic complexes are part of a cluster of PGE-mineralized layered intrusions located in the Carajas Mineral Province (CMP) in the Amazonian Craton. The Lago Grande Complex is a NE-trending medium-size (12-km-long and average 1.7-km-wide) layered intrusion consisting mainly of mafic cumulate rocks (Mafic Zone) and minor ultramafic cumulates (Ultramafic Zone). Geological sections indicate that igneous layers are overturned, such that the Ultramafic Zone overly the Mafic Zone. The Ultramafic Zone, about 4 km long and 500 m wide, comprises an up to 250 m-thick sequence of interlayered harzburgite and orthopyroxenite at the base and orthopyroxenite at the top. The Mafic Zone consists of a monotonous sequence of gabbroic rocks with an estimated thickness of up to 1000 m in the central part. Primary igneous minerals of the Lago Grande Complex are partially replaced by metamorphic assemblages that indicate temperatures up to the amphibolite fades of metamorphism. This metamorphic alteration is heterogeneous and characterized by an extensive hydration that largely preserves primary textures and bulk chemical composition.