Evidence of 'oil-like' manganese remobilization in the ca. 2.27 Ga Azul red beds of the Carajas Basin, Amazonian Craton, Brazil: An interplay among sedimentary and tectonic controls

摘要

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 Caraj acute accent as 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 Caraj acute accent as 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.