Unraveling sedimentary precursors and metal enrichment of high-grade metamorphosed manganese-rich rocks from the Borborema Province, northeastern Brazil

The Paleoproterozoic Lagoa do Riacho manganese deposit in the Borborema Province is central to unravelling genesis of deposits, redox evolution and genetic relationship West African Craton. With contents averaging 35 wt%, unit lithologically subdivided into oxidized silicate ores a garnetite (garnet-rich rocks) subgroup. ore consists mainly pyrolusite, manganese-oxyhydroxide spessartine. has rhodonite, tephroite Garnetite contains spessartine minor quartz rhodonite. main host rocks include garnet quartzite graphite-bearing pelitic gneiss. composition carbonate inclusions spessartine-rhodonite Ca-Mn fractionation between core rim suggests all these manganese-rich probably formed from protoliths marl composition. Biogenic, syn-sedimentary graphite their indicates organic matter was deposited along with protolith. Within this general context through chemical-petrological relations, we outline an initial environment involving anoxic sediments' deposition (eg., matter-rich black shales, wackes) on seafloor. presence spessartine, peak metamorphic conditions reached amphibolite facies (6–7 kbars 550–700 °C). Amphiboles indicate later retrograde stages veins suggest possible hydrothermal stage after deposition. metamorphosed were upgraded medium-grade during exposure surface water. Our results indicated that generated sedimentary context, possibly marine setting contribution.