The Precambrian Manganese deposits of the Serra dos Carajás have been studied only in the Igarapé Azul Mine, mainly under geochemistry and mineral resources point view. Stratigraphically, previous work has included these deposits in the Archean Grão Pará Group that include volcanic rocks and banded iron formations or in the fine siliciclastics deposits of the Lower Member of the Paleoproterozoic Águas Claras Formation. Facies and stratigraphic analysis of this pelitic succession carried out in the Igarapé Azul Mine exposures and eleven cores that include the core number 706 of the Azul Manganese Project, dating to 2.1 Ga (age obtained by diagenetic pyrite), confirmed the Águas Claras Formation as host of the Manganese ore. Cores were distributed in a 25 km long SE-NW oriented section, which includes the plateau N4 (N4 Domain), the Azul Igarapé Mine (Azul Domain) and the outcrops in the Águas Claras River (Águas Claras Domain). In relation to the other segments, the Azul Domain is beheaded in the Carajás Fault, previously interpreted as an uplifted block. Despite the structural discontinuities, hydrothermalism and / or low degree of metamorphism (?), the sedimentary facies are preserved allowing the sedimentary processes and the paleoenvironmental interpretations. Two depositional successions were identified, separated by an unconformity. The lower succession include two facies associations (AF) representatives of marine platform settings: 1) offshore (AF1), characterized by intercalations of very fine sandstones with massive to laminated pelites, forming small-scale (a few cm) fining upward cycles, with scoured to flat base, interpreted as distal low density turbidite linked to the deceleration of turbulent flows; and 2) offshore-transition to shoreface (AF2) consisting in fine sandstone and manganesiferous siltstone with even parallel lamination and massive bedding, cross- and undulated laminated siltstones, low-angle truncated and pinch-and-swell laminations indicative of oscillatory and combined flows (storms). Carbonaceous pelite with total organic carbon content (TOC) up to 1% indicates predominantly anaerobic conditions. The medium- to coarse-grained sandstones of the upper succession (AF3) exhibit tabular (planar) and even parallel cross stratifications, with pelite and quartz pebbles in the foresets and bottomsets, represent the migration of bed forms during incision of braided channels on the marine platform. The primary Mn precipitation is associated with the oxide-hydroxide form through the oxidation of Mn+2 above the redox interface under favorable oxygenation conditions in the offshore transition-shoreface zone, simultaneously deposition of facies influenced by waves. Meter-scale coarsening upward cycles formed by Manganesiferous siltstone/Mn oxide laminae and siltstone/sandstone with wave structures suggest recurrence of Mn precipitation during the deposition of the Águas Claras Formation. It is possible that more consistent model for primary deposition of Mn in the Igarapé Azul Mine is related to the dominant precipitation of oxides-hydroxides on shallow marine platform at the offshore transition-shoreface zone, and a carbonate phase subordinate related to offshore zone. This provides a new perspective for the evolutionary understanding for Precambrian seas without correlation with modern analogues, in the Amazon Craton.
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