The Luanga deposit, Carajas Mineral Province, Brazil: Different styles of PGE mineralization hosted in a medium-size layered intrusion

摘要

The Luanga Complex, located in the eastern portion of the Carajas Mineral Province, is part of a cluster of PGE-mineralized layered intrusions, grouped into what is known as the Serra Leste magmatic suite. The Luanga deposit, the largest PGE deposit in South America, has two distinct styles of PGE mineralization. The first type, termed as Sulfide Zone, consists of a 10-50 m thick interval with disseminated base metal sulfides (pendandite > pyrrhotite > > chalcopyrite) located along the upper contact of the intrusion's Ultramafic Zone. The Sulfide Zone extends along the entire length of the intrusion (similar to 3 km) and hosts the bulk of PGE resources of the Luanga Complex (i.e., 142 Mt at 1.24 ppm Pt + Pd + Au and 0.11% Ni). The second type of PGE mineralization, termed as low-S-high-Pt-Pd Zones, consists of 2-10 m thick stratabound PGE mineralization within a sequence of interlayered ultramafic and mafic cumulates located above the Sulfide Zone. Host rocks of the low-Shigh-Pt-Pd Zones consist mainly of sulfide-and chromite-free harzburgite and orthopyroxenite. These mineralized rocks do not show any distinctive texture or change in modal composition. The Sulfide Zone and low-Shigh-Pt-Pd Zons have distinct PGE distribution. The Sulfide Zone has Pt/Pd ratios of 0.52 and a positive correlation between PGE and S. The low-S-high-Pt-Pd Zones have Pt/Pd ratios of 1.2 and depletion in IPGE relative to primitive mantle. The platinum-group minerals (PGM) observed in the Sulfide Zone are predominantly Pt-Pd bismuthtellurides, stanides and arsenides, mainly enclosed within sulfide minerals. In contrast, the PGM observed in low-S-high-Pt-Pd Zones are mainly Pt-arsenides, stannides and antimonides, mostly enclosed within alteration silicates. Differences in texture, geochemistry and PGM assemblage between these mineralization styles suggest that they originated from distinct geological processes. The Sulfide Zone was formed by a major event of segregation of an immiscible sulfide liquid, whereas the low-S-high-Pt-Pd Zones formed by a sulfide liquid saturation followed by sulfur loss during post-magmatic alteration. The identification of PGE-rich layers in rocks without sulfides or chromite at the Carajas Mineral Province is important as these may have been overlooked during previous exploration programs.