Abstract
A vast accumulation of manganese occurred in the Paleoproterozoic and it is closely related to the Paleoproterozoic Great Oxidation Event. Although the largest Mn deposits are located in the African continent, relevant deposits that potentially correlate with the African ones are found in South America, specifically in the Brazilian territory. The Borborema Province in Northeast Brazil hosts several Mn-rich meta-sedimentary sequences, containing up to 40 wt.% MnO. These sequences are composed of oxidized and manganese silicate lenses that alternate with spessartite-quartzite layers. This succession is hosted by graphite-bearing pelitic gneisses, is metamorphosed to the amphibolite facies, and is intruded by granitic bodies. Although some preliminary studies report Palaeoproterozoic ages for these meta-sedimentary sequences, the age at which the Mn-rich sequences were deposited has not yet been determined. We investigate in this study the Lagoa do Riacho Manganese Deposit, a representative member of the Northern Borborema Province Mn-rich sequences. Not only was the age of the Mn-rich protolith deposition determined by LA-SF-ICP-MS zircon U-Pb geochronology, but also the timing of the metamorphic reworking of the Mn-rich succession was inferred. The youngest detrital zircon population from a meter-sized bed of spessartite-quartzite interlayered with an oxidized manganese ore from drillhole Ocr-1 yielded a maximum depositional age close to 2130 Ma. Similarly, a gneiss hosting the manganese mineralization at the base of drillhole Ocr-1 also yielded a maximum depositional age in the Rhyacian, close to 2156 Ma. We infer from the U-Pb dating of metamorphic overgrowths on zircon from the Mn-bearing sequences and host rocks and the U-Pb crystallization age of an intrusive leucocratic granite that the minimum age of the Mn-rich succession to be 2023-2106 Ma. The metamorphic reworking of the Mn-rich succession is time-equivalent to the Transamazonian/Eburnean orogeny. Thus, these constraints point to a Rhyacian age (ca. 2100-2200 Ma) for the deposition of the Mn-rich protolith, chrono-correlated with other world-class manganese deposits, such as those from the Franceville Basin in Gabon.
A preserved early Ediacaran magmatic arc at the northernmost portion of the Transversal Zone central subprovince of the Borborema Province, Northeastern South America
