Trace-element and multi-isotope geochemistry of Late-Archean black shales in the Carajas iron-ore district, Brazil
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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
Original language | English |
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Journal | Chemical Geology |
Volume | 362 |
Issue number | SI |
Pages (from-to) | 91-104 |
Number of pages | 14 |
ISSN | 0009-2541 |
DOIs | |
Publication status | Published - 20 Dec 2013 |
ID: 119288975