Trace-element and multi-isotope geochemistry of Late-Archean black shales in the Carajás iron-ore district, Brazil

摘要

The 250-300-m-thick Carajás Formation in the Carajásmineral province, northern Brazil, consists of banded iron formation (including giant high-grade iron-ore deposits) and minor black shale, overlying a thick pile (2-3 km) of about 2.75-Ga-old metabasalt. Carbonaceous shale with pyrite- and locally pyrrhotite-rich patches from drillcore of the Serra Sul exploration project has up to 29 ppm Mo; iron-speciation analysis indicates essentially ferruginous and for some samples likely euxinic depositional conditions. Positive δ34S-isotope ratios of TRIS are between+0.3 to+10.7‰, with heavy data restricted to pyrrhotite-free samples. The data suggestmicrobial sulfate reduction under, at least partially, sulfate-limiting conditions with later overprint bymigrating solutions. The black shale is affected by pronounced low-temperature potassium metasomatism (K2O/Na2O N 100; up to 10 wt.% K2O as adularia) related to diagenetic processes at b100 °C, and low-grade metamorphic overprint. We studied a 20-cm-black-shale drillcore interval from the middle part of the Serra Sul BIF sequence in detail. Five samples with the most euxinic signature give a Re-Os regression of 2710 ± 38 Ma (2σ) with an initial ratio of ?0.37 ± 0.40 (MSWD = 3.3), whereas the full data set (n = 11), including black-shale samples from the top and bottom of the BIF sequence, gives a regression of 2661 ± 110 Ma (MSWD = 121). Molybdenumisotope patterns suggest mixing between a clastic end member, with about 0.2‰ δ98/95Mo for continental input (TDM of 2.8-3.1 Ga according to Nd-isotope data), and 0.9‰ δ98/95Mo for a hydrogenous component. Black-shale samples from the bottom of the BIF sequence have heavy Mo-isotope composition of up to 1.8‰ δ98/95Mo. The significantMo-isotope fractionation is either the result of an early and transient "whiff of oxygen" at 2.7 Ga, or the result of hydrothermal fluid overprint.