Gahnite, garnet and magnetite compositions of metamorphosed sediment-hosted Zn-Pb-(CuAg) deposits of the Mesoproterozoic Nova Brasilândia Group: Vectors for SEDEX deposits with Broken Hill-type affinities in the western Amazonian Craton, Brazil

Stratiform and stratabound Zn-Pb-Ag sulfide deposits hosted in siliciclastic sedimentary rocks constitute some of the world's largest accumulations of base metals. This research paper is the first to describe the DM-1 Zn-(PbAg) and PQ Zn-(CuAu) sulfide deposits, located about 20 km apart in the Mesoproterozoic Nova Brasilândia belt at the southwest margin of the Amazonian Craton. These deposits occur in a NW-SE trending sequence of metamorphosed siliciclastic rocks. The Zn-rich mineralization in both deposits occurs as massive, semi-massive and banded sulfide orebodies concordant with the layering of the country rocks. The DM-1 deposit (1.2 Mt at 12 wt% Zn) has up to 30 m thick zones of sulfide ore consisting predominantly of sphalerite, galena, pyrrhotite and pyrite while the PQ deposit (5 Mt at 5.6 wt% Zn) contains up to 20 m thick zones of sulfide ore consisting mainly of pyrite, pyrrhotite, and sphalerite along with minor chalcopyrite. The host rocks of both deposits consist of schist, gneiss and quartzite with variable modal proportions of garnet, sillimanite, gahnite, magnetite and biotite. These host rocks are up to 50 m thick and commonly show gradational contacts with country rocks. Country rocks, Zn-rich orebodies and their host rocks were overprinted by a post-ore upper amphibolite facies regional metamorphic event.