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

Abstract

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