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

The origin and fate of organic carbon in graphite-manganese bearing rocks and implications for the Lomagundi-Jatuli Event

“…In some cases, the conversion of organic matter to graphite has often been the result of changes in temperature and pressure during regional metamorphism. In our study, the correlation of graphitisation temperatures with those of the host rocks 30 provides evidence for a primary organic reservoir rather than a secondary or external reservoir for these graphite-bearing sequences. For example, it is expected that the same P-T metamorphic conditions would be imprinted in pelitic metasedimentary rocks associated with genetically related organic matter from an ancient basin involved in a continental collision.…”

Paleoproterozoic manganese oxide precipitation in oxic seawater surface and reductive enrichment in anoxic seafloor

“…Because the mineral assemblage of the manganese-bearing rocks from the Borborema Province is mainly composed of calc-silicate minerals (e.g., garnet, pyroxene, and minor carbonate), primary silicic marlstones or likely a silicate-rich chemical precursor are considered a potential protolith for the manganese-rich rocks. Thus, a first hypothesis is that the mineral phases precursors are believed to be kutnohorite-like carbonate minerals (Johnson et al, 2016;Santos et al, 2021). In the present study, this assumption comes from a textural relationship where kutnohorite-like carbonate occurs as inclusions in the garnet-spessartine crystals and manganese-pyroxene, suggesting a pre-metamorphic precipitation (Johnson et al, 2016;Polgári et al, 2012;Santos et al, 2021).…”

“…Thus, a first hypothesis is that the mineral phases precursors are believed to be kutnohorite-like carbonate minerals (Johnson et al, 2016;Santos et al, 2021). In the present study, this assumption comes from a textural relationship where kutnohorite-like carbonate occurs as inclusions in the garnet-spessartine crystals and manganese-pyroxene, suggesting a pre-metamorphic precipitation (Johnson et al, 2016;Polgári et al, 2012;Santos et al, 2021). After that, metamorphic spessartine garnet and manganese pyroxene minerals would develop by isochemical decarbonation and dehydration reactions during prograde metamorphism, according to the following reactions:…”

“…Thus, the Lagoa do Riacho manganese deposit was studied as a representative site for drill-core sampling and analysis. At this deposit, post-metamorphic changes related to the supergene enrichment of silicate manganese minerals are recorded mainly at the top of the drill cores and evidenced by the presence of late oxides and hydroxides (e.g., pyrolusite and cryptomelane) (Santos et al, 2021). The manganese mineralization in the Lagoa do Riacho manganese deposit is hosted within the Canindé do Ceará Complex, which is subdivided into (1) paragneisses and schist layers, interbedded with marbles, quartzites, calc-silicate rocks, amphibolites, and manganese-and iron formations; and (2) orthogneisses of granitic to tonalitic composition and subordinate amphibolites (Fetter et al, 2000;Castro, 2004;Torres et al, 2007;Costa and Palheta, 2017) (Fig.…”

“…Normally Mn oxide shuttling to anoxic or eventually euxinic waters results in high Mo enrichment in the bottom sediments (Algeo and Tribovillard, 2009). Such high Mo EF is found in the manganese-rich silicate rocks (EF= 20 to 360), which is believed to have been deposited in an anoxic/euxinic setting according to Santos et al (2021). The ubiquitous dissemination of sulfide minerals (e.g., cobaltite and pyrite), as well as biogenic graphite (pyrolyzed remnants of organic matter; Fragomeni and Pereira, 2013) in these manganese-rich rocks, supports this interpretation.…”