5.1. Timing of protolith deposition and reworking of the Manganese Formation of the Northern Borborema Province
According to Santos et al. (2021), the Canindé do Ceará Complex spessartite-quartzite layers are Mn-rich metamorphic products derived from Mn-siliciclastic beds deposited in a redox-stratified marine setting along with Mn-rich marls and organic-rich pelites. The youngest detrital population found in the spessartite-quartzite (sample Ocr-14) from the present study yielded concordant (> 90%) 207Pb/206Pb ages between 2123 ± 18 and 2145 ± 18 Ma, which can be interpreted as the maximum sedimentation age of the Mn-rich protolith. Similar to sample Ocr-14, the graphite-bearing pelitic gneiss (sample Ocr-29) that hosts the manganese ores also yielded maximum sedimentation age in the Orosirian (207Pb/206Pb age of 2156 ± 7 Ma). This age is very close to those reported for the spessartite-quartzite and implies that the graphite-bearing pelitic gneiss was deposited synchronically with the Mn-rich rocks, confirming a previous hypothesis of Santos et al. (2021). Based on a detailed chemical and petrographic study, these authors suggested that these graphite-bearing pelitic gneiss are remnants of organic-rich black shales deposited on a euxinic-anoxic seafloor and were spatially related to Mn oxy-hydroxide-rich layers deposited under oxic surface water conditions. This black shale-hosted association makes the Manganese Formation of the Northern Borborema Province a potential metamorphic analog of the Francevillian manganese deposits in Gabon (Leclerc and Weber, 1980; Gauthier-Lafay and Weber, 2003).
Regionally in the Northern Borborema Province, more specifically in the Algodões and Troia/Serra das Pipocas greenstone-like terrane, which neighbors the Canindé do Ceará Complex and also hosts Mn-rich silicate rocks, ages range between 2156 and 2123 Ma. This is marked by arc-related mafic and intermediate volcanic-plutonic rocks (Fetter, 1999; Martins et al., 2009; Costa et al., 2015; Sousa et al., 2019). Although mafic and intermediate rocks have not been reported in the Lagoa do Riacho drillhole logs, the close spatial and temporal relationship of these units allow us to infer that the volcanic members may have been potential source-rocks, providing manganese to sediments in an early oceanic basin (most likely a back-arc basin) that was subsequently deformed and metamorphosed in a continent-continent collisional setting. However, the early opening of this oceanic basin was estimated at ca. of 2236 ± 55 Ma by Martins et al. (2009). An evolving convergent setting is also supported by the large proportion of zircon ages close to the depositional age of the sediment (Cawood et al., 2012).
Multiple collisions around 2100 − 2000 Ma are recorded in several areas of the South American (Brito Neves et al., 2011) and African continents (Baratoux et al., 2011) and mark the assembly of the Atlantica Supercontinent, part of Nuna/Columbia (Zhao et al., 2002). This collisional phase is recorded in the zircons of this study and accounts for the metamorphism imprinted in the Mn-rich rocks. For example, twelve concordant zircon grains from the spessartite-quartzite (sample Ocr-14) yielded an 206Pb/238U upper intercept age at 2028 ± 9.5 Ma, interpreted as a metamorphic event, very similar to the upper intercept yielded by metamorphic zircons from sample FDM-14 (2023 ± 6.3 Ma). However, we believe that the metamorphic reworking of this Mn-rich protolith may have started earlier, at ca. 2099 Ma, according to a 206Pb/238U upper intercept age of metamorphic overgrowths of sample Ocr-29 zircons. Thus, this metamorphic phase may be used as a first approach to unveil the minimum age for the Mn-rich protolith deposition. To the east of the Canindé do Ceará Complex, in the neighboring Jaguaretama Complex, a migmatite paragneiss petrographically similar to that described in our study (e.g., samples FDM-14 and Ocr-29) and that also hosts Mn-rich rocks, yielded a U-Pb zircon age of 2046 Ma interpreted as the timing of the high-grade metamorphism of the sedimentary protolith (Calado et al., 2019). In addition, further south to the study area and in the same Mn mineralization trend of the Northern Borborema Province, migmatized orthogneisses hosting manganese silicate rocks yielded a U-Pb isochron at ca. 2046 Ma (Gomes, 2013). This age was interpreted as of a metamorphic event related to migmatization.
Besides the metamorphic record discussed above, S-type granitic bodies from the Canindé do Ceará Complex provide U-Pb zircon ages of ca. 2070 Ma (Garcia et al., 2014; Costa and Palheta, 2017). Furthermore, these S-type granites yield very low Th/U ratios (< 0.03), a common feature of granites generated by the melting of meta-sedimentary rocks (Rubatto, 2017). Thus, these S-type granites strongly resemble the intrusive leucocratic biotite-granite from drillhole Ocr-1 (sample Ocr-26). It also yielded low Th/U ratios (< 0.03) and a 206Pb/238U Concordia age at 2009 ± 4.5 Ma, which was interpreted as the crystallization age of the leucocratic biotite-granite. Therefore, given the intrusive nature of this rock and the cross-cutting relationship with the Mn-rich rocks, we firmly believe that this age, close to 2.0 Ga., may also account for a good definition of minimum age the Mn-rich protolith deposition.
In summary, the 2000–2100 Ma time span is here interpreted as a long-lived period of metamorphic reworking of the Mn-rich protolith that occurred during the Transamazonian/Eburnean orogeny. Figure 9 (adapted from Santos et al. 2021) is a sketch of the most likely Mn-rich protoliths, their metamorphic products and the timing of deposition and metamorphic reworking.
5.2. A new piece in the puzzle and connection with the Paleoproterozoic manganese metallogenesis in South America and Africa
Our new geochronological dataset allows us to place the Mn-rich rocks of the Northern Borborema Province in the Rhyacian, and, similarly to other African and South American manganese deposits, to draw some comparisons between these rocks. By far, the African continent hosts the most extensive manganese deposits, e.g., those from the Kalahari manganese field in the BIF-hosted Hotazel Formation in South Africa (Tsikos and Moor, 1997; Tsikos et al., 2003). It is believed that such manganese formation was deposited in a retro-arc basin along the Kaapvaal craton margin during the Rhyacian at approximately 2200 Ma (Beukes et al., 2016).
Other African deposits known worldwide are hosted in the Francevillian Group. Besides the age between 2200 − 2050 Ma (Bouton et al., 2009; Bros et al., 1992; Horie et al., 2005; Thiéblemont et al., 2009), such deposits hold some similarities with the Mn-rich succession of the Northern Borborema Province. For example, the stratigraphic filling of the manganese formations of the Francevillian Group, which was deposited adjacent the Congo Craton, also consists of interbedded black shales and siliciclastic lenses and accumulations of manganese-carbonate rocks (Gauthier-Lafaye and Weber, 2003; Préat et al., 2011). Thus, the well-preserved Francevillian sedimentary protoliths would be examples of likely precursors for the metamorphosed Mn-rich rocks of Northeast Brazil. For example, the presence of spessartite-quartzite (siliciclastic remnant) and graphite-bearing pelitic gneiss (black shale remnant) closely related to Mn-rich rocks gives support to this point of view (Santos et al., 2021). Furthermore, at this time (2200 − 2050 Ma), it is believed that the Congo Craton was linked to the São Francisco Craton in Brazil (Weber et al., 2016; Rigoni Baldim and Oliveira, 2021). As the Borborema Province had a connection with the São Francisco Craton in the Paleoproterozoic, by association, the rocks studied here may have shared a common paleogeography at that time.
In the Kasai Block (Congo Craton), there is a group of Paleoproterozoic manganese formations in the so-called Kisenge-Kamata series (Katanga, D.R. Congo) (De Putter et al., 2018) that strongly resembles the Northern Borborema Province Mn-rich succession. The available geochronological dataset indicates that this manganese formation was deposited between 2.0-1.9 Ga. (Delhal et al., 1986; André, 1993; De Putter et al., 2018), which is a slightly younger age than that presented in this study. However, similar to the Mn-rich rocks of the Northern Borborema Province, Kisenge-Kamata series contains spessartite-rich rocks interlayered with oxidized manganese ores (De Putter et al., 2018). Spessartite is developed during the prograde metamorphism of Mn-carbonate plus an aluminosilicate at the greenschist facies (Santos et al., 2021; Nyame, 2001). Both terranes underwent metamorphism during the Eburnean/Transamazonian orogeny in the Paleoproterozoic, explaining the presence of metamorphic products in these deposits. Additionally, this may also give support to a Paleoproterozoic connection between these terranes. Another African deposit of similar age, lithology and metamorphism to those described in our study are the Nsuta Mn-rich successions in the Ashanti orogenic system from the Birimian terrane (Mücke et al., 1999; Nyame, 2001). In this deposit, several granitic bodies of U-Pb crystallization age ~ 2.17 Ga intrude these Mn-rich rocks, providing a minimum depositional age for the manganese sequence (Mücke et al., 1999). Furthermore, Goto et al. (2021) have recently reported a Re-Os isochron age of 2235 ± 64 Ma for phyllite and manganese ore samples from the Nsuta manganese field, confirming a Rhyacian age. The relationship with this deposit is also meaningful because the Borborema Province occupies a strategic position between the Amazonian-West African and São Francisco-Congo cratons. In this sense, some authors (e.g., Klein and Moura, 2008; Neves, 2011; Costa et al., 2018; Grenholm, 2019; Grenholm et al., 2019) also suggest linking the Borborema Province with the West African Craton during the Paleoproterozoic.
In the South American Platform, some Paleoproterozoic (2.07 to 1.86 Ga; Cabral et al., 2019) metamorphosed black shale-hosted manganese mineralization occur in Morro da Mina, Southern Quadrilátero Ferrífero, strongly resembling the Northern Borborema Province Mn-rich succession. Besides field associations, petrographic relationship with black shales metamorphosed to the amphibolite facies, the Morro da Mina manganese ores are composed of spessartine, pyroxmangite-rhodonite, and tephroite with widespread dissemination of graphite. The Serra do Navio manganese deposit (Chisonga et al., 2012) in the Amazonian Craton also represents this very same petrographic association and was deposited between 2.2-2.0 Ga. Other important Brazilian manganese deposits include the Buritirama and Azul mines (Salgado et al. 2019; Araújo et al., 2021). The depositional age of the Buritirama Formation has been constrained to 2.3 and 2.1 Ga and its manganese ores are chrono-correlated to those of the Serra do Navio (Guiana Shield, Amazon Craton) and Nsuta (Birimian succession, West African Craton) (Salgado et al. 2019). All these Brazilian manganese deposits have been affected by the Transamazonian orogeny, which explains why they are all metamorphosed.
5.3. A major control on the Paleoproterozoic manganese deposition?
It is understood that for Mn oxides to accumulate in large quantities in the geological record, large amounts of oxygen are required in the ocean-atmosphere system. This process occurs because manganese has a high redox potential and can only be oxidized by O2 or superoxide-derived species (Calvert and Pedersen, 1996; Post, 1999; Tebo et al., 2005; Roy, 2006; Johnson, 2015). The largest global manganese accumulations are known to have occurred during the Great Oxidation Event (GOE) in the Paleoproterozoic and the chemical signals of the environmental changes triggered by GOE are recorded through a prominent positive carbon isotope (δ13C) excursion that is usually interpreted as the result of an oxygen-rich atmosphere establishment in the Lomagundi-Jatuli Event (LJE; Karhu and Holland, 1996; Bekker et al., 2006; Kump et al., 2011; Bekker and Holland, 2012; Canfield et al., 2013; Bekker, 2014) (Fig. 10A). Although Kirschvink et al. (2000) suggest that manganese deposition is driven by high bio-productivity levels from glaciation aftermath and, in this sense, supporting a causal link between manganese precipitation and climatic-biological crisis, additional mechanisms leading to massive manganese oxidation are still not well understood.
As stated here and in other studies, the African and Brazilian geological settings have undergone the Eburnean and Transamazonian orogenies, respectively (Alkmim and Marshak, 1998; Rosa-Costa et al., 2006; Feybesse et al. 2006; Vasquez et al. 2008; Brito Neves., 2011; Baratoux et al., 2011; Weber et al., 2016; Grenholm, 2019; Klein et al., 2020). This event may have enabled the opening and closing of specific depositional sites and also may have provided the metal source, which ultimately led to manganese accumulation on a global scale. Furthermore, it should be stressed here that some authors (e.g., Klein and Moura, 2008; Neves, 2011; Costa et al., 2018; Grenholm, 2019; Grenholm et al., 2019) stated that both continents (South America-Africa), nowadays separated in distinct continents by the Atlantic Ocean, may have shared common paleogeography known as Atlantica Supercontinent during the Paleoproterozoic. Thus, looking from an integrated perspective, the geological record of these world-class manganese deposits (2.2–1.9 Ga), such as those of the Francevillian and Birimian successions of the African continent and the corresponding members in South America (e.g., Quadrilátero Ferrífero, Serra do Navio; Northern Borborema Province) may suggest a link between global tectonics and manganese formations (Fig. 10B).
In addition to some authors (e.g., Roy, 2006; Beukes et al., 2016) who discuss the role of major climatic and tectonic changes in the sedimentary manganese metallogenesis, Campbell and Allen (2008) argue that the erosion of super mountains built up via supercontinent cycles may have provided nutrients to the oceans, leading to cyanobacterial bloom and ultimately a photosynthetic O2 production. This relation is vital since peak deposition of manganese formations coincides with The Great Oxidation Event (GOE – Fig. 10A). In support to this association, there are other huge manganese accumulations in the Late Neoproterozoic during the so-called Neoproterozoic Oxygenation Event (NOE; Och and Shields-Zhou, 2012). This time is also marked by an oxygen overshoot induced by intense photosynthetic biological activity (Och and Shields-Zhou, 2012) from the Gondwana’s super mountain erosion. In summary, this association between tectonism and metallogeny may not be casual and more studies are necessary to solve this conundrum.