Ultramafic intrusions hosting Ni–Cu sulfide mineralization along a suture zone in the southwestern margin of the Amazonian craton, Brazil: Examples from Morro Sem Boné, Morro do Leme and their satellite intrusions

Abstract

This research paper describes several medium-to small-sized ultramafic intrusions located along the Mesoproterozoic Guaporé Suture Zone in the southwestern margin of the Amazonian craton. Apart from minor sheared zones, ultramafic rocks have igneous minerals and textures largely preserved. Metamorphic parageneses of metapelites and amphibolites of country rocks, as well as those from sheared zones of the ultramafic intrusions, are indicative of the amphibolite facies of regional metamorphism. Intersections of disseminated to massive sulfides consisting mainly of pyrrhotite and pyrite are common in the metamorphosed volcanic and sedimentary country rocks. The ultramafic intrusions consist mainly of olivine cumulates with high MgO contents (up to 45.9 wt%) and Mg# (up to 0.84), consistent with the composition of their most primitive olivine (Fo93). The crystallization sequence, dominated by olivine, orthopyroxene and chromite, together with ratios and patterns of incompatible trace elements of ultramafic cumulates, support the interpretation that they originated from a high-MgO magma partially contaminated with crustal rocks. Sulfide mineralization in the ultramafic intrusions comprises different styles of magmatic sulfides associated with cumulate rocks, including contact-type, stratabound-type and conduit hosted mineralization. Significant intersections of interstitial to net-textured sulfides consisting of pyrrhotite, pentlandite and chalcopyrire have variable but generally low Ni–Cu-PGE contents (e.g., ~20 m at 0.5 wt% Ni, 0.1 wt% Cu, and 0.5 ppm Pt+ Pd). The range of tenors of Ni, Cu, Pt and Pd, from moderate to very low, is interpreted as resulting from varying efficiency of equilibration of sulfide xenomelts incorporated from the sulfide-bearing country rocks. The Ni–Cu-PGE mineralized ultramafic intrusions are interpreted to belong to a network of magma conduits and staging magma chambers intruding the sulfide-bearing volcanic-sedimentary sequence. Our results indicate an additional prospective belt of high-MgO Ni–Cu-PGE sulfide mineralized intrusions located in a cratonic margin in Brazil.