Evidence of “oil-like” manganese remobilization in the ca. 2.27 Ga Azul red beds of the Carajás Basin, Amazonian Craton, Brazil: An interplay among sedimentary and tectonic controls
Introduction
The accumulation of manganese in the Precambrian sedimentary successions is associated with, in many cases, multiple factors from specific paleoenvironmental conditions to post-depositional processes (Roy 1997, 2006, 2006; Tsikos et al., 2003; Sekine et al., 2011; Jones, 2011; Jones et al., 2013; Johnson et al., 2016). Although peaks of deposition of this metal occurred during the Neoarchean and Neoproterozoic eras, manganese deposits were mainly deposited in the early Paleoproterozoic Era (Roy, 2006; Maynard, 2010). The widespread deposition of manganese in this era is generally associated with the emergence of oxygenated earth onset of the Great Oxidation Event (GOE) at ca. 2.45 Ga (Laznicka, 1992; Glasby, 1997; Roy 1997, 2006, 2006; Bekker et al., 2004; Sekine et al., 2011; Johnson et al., 2013).
Importantly, manganese, a redox-sensitive element, is commonly used as a proxy to indicate the amount of oxygen in the atmosphere-hydrosphere system (Sekine et al., 2011; Johnson et al., 2013). The primary deposition of manganese was triggered when the atmospheric O2 became higher than ca. 10−2 times the present atmospheric level (PAL) (Sekine et al., 2011). Broadly, manganese was deposited at the transition point between reducing and oxidizing shallow marine waters (i.e., in a suboxic/intermediate environment), whereas the deep waters with euxinic conditions allowed for the deposition of thick deposits of black shale (Force and Cannon, 1988). Furthermore, manganese-bearing successions are generally enriched and controlled by secondary processes such as tectonic mechanisms, which frequently cause the distinction between these processes to become very difficult (Jones et al., 2013; Ghosh et al., 2015).
In the Carajás Basin, which is considered as a relic sedimentary basin, situated in southeastern Amazonian Craton in Brazil (Fig. 1), large manganese deposits are hosted in a dominantly siliciclastic succession named the Azul Formation, anteriorly considered as a part of the Águas Claras Formation (Araújo Filho et al., 2020; Araújo et al., 2021). Although manganese ore has been extensively explored in the past 50 years, the stratigraphic setting and the sedimentary mechanisms involved in the deposition of this metal are still uncertain. While some studies suggest that the origin of these deposits is linked to predominantly supergenic processes through episodic precipitation occurring throughout the Cenozoic Era (Vasconcelos et al., 1994; Ruffet et al., 1996), authors of other studies have indicated the presence of some structural controls on the manganese deposits (Pinheiro, 1997; Silva, 2006). However, an integrative model that unravels the role of each of these processes and the mechanisms involved in manganese deposition in the succession of the Azul Formation has not been conceived yet.
Moreover, the relationship between the Azul manganese deposits and the events occurring during the early Paleoproterozoic Era has not been discussed as well, and its exact controls on manganese deposition and enrichment still remain uncertain. Partially, this problem results due to imprecise stratigraphic settings and poor age constraints that hindered accurate paleoenvironmental reconstructions. Importantly, the Azul succession may be a significant archive of events occurring at that time period, mainly regarded as the evolution of the primitive atmosphere-hydrosphere system. The discovery of the Paleoproterozoic glacial diamictite strata of the Serra Sul Formation revived the possibility that these units can record a geological history more complex than presumed.
In this study, we carried out a combination of sedimentological, stratigraphical, tectonic, chemical, and mineralogical investigations on the Azul manganese-bearing succession of the Carajás Basin to address the origin of and the mechanisms involved in the enrichment of manganese in this unit. Our results throw more light on the evolution of this part of the Amazonian Craton in the early Paleoproterozoic Era and allowed to insert the Carajás manganese deposit within a context of regional and global scale events, including paleoclimatic, tectonic and paleoenvironmental changes intrinsically related to that time period.
Section snippets
Geological background
The Carajás Province is one of the most important Archean–Paleoproterozoic domains of the Amazonian Craton, where several world-class mineral deposits are located (Dall’Agnol et al., 2013; Docegeo, 1988). Located in the southeastern part of this craton, this province has been inserted into the Amazonia Central Geochronological Province and is surrounded by the youngest rocks from the Statherian to the Neoproterozoic ages (Tassinari and Macambira, 2004). The Carajás Province is compartmented in
Materials and methods
Sedimentological and stratigraphic analyses in the drill cores and outcrops crossing the Azul Formation (ca. 250 m thick) and a weathering mantle (ca. 50 m thick) was undertaken (Supplementary Material S-1). They are located in the Azul manganese mine, situated in the central area of the Carajás Basin. A detailed sedimentary log of each studied drill core and outcrop was obtained. Classical procedures of facies analysis, including the recognition of stratigraphic surfaces and lithofacies, and
Description
The Azul manganese-bearing succession is composed strictly by siliciclastic rocks which constitutes an uninterrupted interval with a thickness of approximately 250 m, which encompasses a major part of the Azul Formation (Fig. 3). The rhythmite is the only lithofacies that constitute this succession and is composed by millimeter to centimeter intercalation between fine-grained sandstone and mudrock (Fig. 4, Fig. 5a). The color of the rock varies between red, gray black and white. The
Sedimentary environment and model of primary manganese deposition
The manganese in the Azul Formation was deposited during the transgression of the Azul Sea (i.e., in a shallow marine environment) into the protocontinent, as already previously admitted (Araújo et al., 2021). The locus of deposition of manganese was at the interface between the oxidizing and reducing conditions (i.e., the interface that controls the manganese deposition). The water above the redoxcline interface was possibly supersaturated with respect to manganese oxides, which precipitates
Final remarks
This envisaged model of the Azul manganese-bearing succession, which involves a miscellaneous process, is considered for the first time here. Although rhodochrosite has been considered by some authors as the protore of the Azul deposits and secondary manganese enrichment with the supergene process being recurrently debated and studied (e.g., Valarelli et al., 1978; Bernardelli and Beisiegel, 1978; Beauvais et al., 1987; Vasconcelos et al., 1994; Ruffet et al., 1996; Costa et al., 2005), minor
Author statement
RA: Writing – original draft; Conceptualization; Methodology; Writing – review & editing; Investigation. LC: Writing – review & editing; Validation; Investigation; Methodology. MS: Writing – review & editing; Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are very grateful to the Vale S.A. for making the drill cores available to study; the Geological Survey of Brazil (CPRM) for providing support through the Área de Relevante Interesse mineral de Carajás project. This paper is a part of the PhD thesis of the first author, who is grateful to the Post-graduate Program in Geology and Geochemistry (PPGG/UFPA). We also extend our gratitude to François Gauthier-Lafaye for help in the geochemical analyses; Rômulo Angélica for support in
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