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

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Highlights

  • Manganese-bearing strata are hosted in the ca. 2.27 Ga Azul red beds.

  • Both sedimentary and tectonic mechanisms controlled the manganese enrichment.

  • A model of accumulation similar to oil migration is envisaged.

  • Tectonic perturbations are linked to the ca. 2.0 Ga Transamazonian Orogeny.

Abstract

Manganese is a redox-sensitive element that was widely deposited during the early Paleoproterozoic. This was caused by the oxygen catastrophe of the Great Oxidation Event (GOE) that raised the oxygen levels in the hydrosphere-atmosphere system, as well as an aftermath of Paleoproterozoic glaciations. In addition, manganese was widely remobilized during Paleoproterozoic collisional events, which affected most of the successions worldwide. In this study, we investigated the manganese-bearing deposits in the Azul Formation (ca. 2.27 Ga) of the Carajás Basin in the southeastern Amazonian Craton, Brazil. The facies analysis indicates that manganese is hosted in offshore strata (i.e., intervals of red beds) and was deposited during the transgression of the Azul Sea onto the Carajás Protocontinent. The structural analysis shows that the manganese-bearing succession is tightly deformed, and chemical and mineralogical investigations revealed that manganese oxides are enriched near the fault zones. The deposit is characterized by a diversity of manganese-bearing minerals, including cryptomelane, pyrolusite, spessartite, and todorokite. The results allowed us to propose an accumulation model for the Azul manganese-bearing succession. The manganese was widely precipitated as oxide along the marine platform, above the redoxcline interface (i.e., in suboxic or intermediate conditions), which controlled the manganese deposition. Subsequently, the remobilization of manganese from rhodochrosite-enriched strata, which was formed during diagenesis, under hydrothermal conditions allowed for the reprecipitation of this metal in the form of oxides in the discontinuities within the succession. Manganese oxides migrated through faults and accumulated in low-strain zones and in intervals with high porosity and permeability within the host rock (e.g., sandy laminations and beds), as also observed in the migration of hydrocarbons. Thus, it is highlighted that the large-scale accumulation of manganese was only possible due to favorable conditions, involving an interplay between sedimentary and tectonic controls. It is important to note that we are not disputing that supergenic processes actuated the enrichment of the manganese ore, instead, we are simply proposing that the hypogene mechanism was a crucial mechanism for the accumulation of manganese in the Paleoproterozoic Azul red beds.

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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