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

The 250-300-m-thick Carajas Formation in the Carajas mineral 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 delta S-34-isotope ratios of TRIS are between +0.3 to +10.7 parts per thousand, with heavy data restricted to pyrrhotite-free samples. The data suggest microbial sulfate reduction under, at least partially, sulfate-limiting conditions with later overprint by migrating solutions. The black shale is affected by pronounced low-temperature potassium metasomatism (K2O/Na2O > 100; up to 10 wt.% K2O as adularia) related to diagenetic processes at <100 degrees 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 (2 sigma) 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). Molybdenum-isotope patterns suggest mixing between a clastic end member, with about 0.2 parts per thousand delta Mo-98/95 for continental input (T-DM of 2.8-3.1 Ga according to Nd-isotope data), and 0.9 parts per thousand delta Mo-98/95 for a hydrogenous component. Black-shale samples from the bottom of the BIF sequence have heavy Mo-isotope composition of up to 1.8 parts per thousand delta Mo-98/95. The significant Mo-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. (C) 2013 Elsevier B. V. All rights reserved.