Metallogenetic Mn-model of the Rhyacian-aged Buritirama Formation, Carajás domain (Amazon Craton).

Abstract

The Mn-bearing Buritirama Formation is composed of clastic and chemical metasedimentary rocks representing a Rhyacian platformal basin (ca. 2.18–2.06 Ga) that limits the northern border of the Archean-Paleoproterozoic Caraj ́ as domain with the Paleoproterozoic Bacaj ́

a domain in the southern Amazon Craton, northern Brazil. The Buritirama Formation is divided into three main stratigraphic units. The Lower and Upper units consist of a basal quartzite followed by carbonate–silicate rocks (marls), dolomitic marbles (dolostone) and minor biotite schist while the Intermediate Unit consists of Mn-carbonate rocks topped by a supergene Mn-oxide deposit (Cenozoic age). These rocks were deformed and metamorphosed during the Transamazonian event (ca. 2.06 Ga). Geochemical data from dolomitic marbles (Lower Unit) exhibit negative Ce anomalies, high Y/Ho ratios (ca. 48) and seawater-like REE + Y patterns that reinforce an open platform setting for the Buritirama basin. Positive

δ13C values (+3.2‰ to +5.0‰ V-PDB) for dolomitic marbles are considered to be an expression of the Loma- gundi Carbon Isotopic Excursion. The metallogenesis of Mn-carbonate rocks is evaluated based on redox- sensitive trace elements (e.g. Zn and Ni), PAAS-normalized REE + Y patterns and δ13C values (− 2.56‰ to

+0.15‰ V-PDB) and point out to a classical multi-stage redox-controlled Mn-model involving: i) a primary

hydrothermal fluid source related to ocean vents and/or normal faults; ii) microbially-mediated reduction of Mn- oxides particles at the ocean bottom; and iii) reactions between Mn2+ and CO3

2− (organic matter and seawater

related), giving rise to diagenetic Mn-carbonates. The Rhyacian age, Mn deposits, and presence of Lomagundi- like δ13C allows to correlate the Buritirama Formation with the Aguas ́ Claras Formation within the Caraj ́

as domain further south, thus establishing an important metalliferous event that was up to now largely overlooked within the southern Amazon Craton.