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

A. R. Cabral; R. A. Creaser; T. Naegler; B. Lehmann; A. R. Voegelin; B. Belyatsky; J. Pasava; A. A. Seabra Gomes, Jr.; H. Galbiatti; M. E. Boettcher; P. Escher

doi:10.1016/j.chemgeo.2013.08.041

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

A. R. Cabral, R. A. Creaser, T. Naegler, B. Lehmann, A. R. Voegelin, B. Belyatsky, J. Pasava, A. A. Seabra Gomes, Jr., H. Galbiatti, M. E. Boettcher, P. Escher

Geology

35 Citations (Scopus)

Abstract

The 250-300-m-thick Carajás Formation in the Carajás mineral province, northern Brazil, consists of banded iron formation (including giant high-grade iron-ore deposits) and minor black shale, overlying a thick pile (2-3km) 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 29ppm 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 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 10wt.% K₂O as adularia) related to diagenetic processes at <100°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±38Ma (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±110Ma (MSWD=121). Molybdenum-isotope patterns suggest mixing between a clastic end member, with about 0.2‰ δ^98/95Mo for continental input (T_DM of 2.8-3.1Ga according to Nd-isotope data), and 0.9‰ δ^98/95Mo for a hydrogenous component. Black-shale samples from the bottom of the BIF sequence have heavy Mo-isotope composition of up to 1.8‰ δ^98/95Mo. The significant Mo-isotope fractionation is either the result of an early and transient "whiff of oxygen" at 2.7Ga, or the result of hydrothermal fluid overprint.

Original language	English
Journal	Chemical Geology
Volume	362
Issue number	SI
Pages (from-to)	91-104
Number of pages	14
ISSN	0009-2541
DOIs	https://doi.org/10.1016/j.chemgeo.2013.08.041
Publication status	Published - 20 Dec 2013

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Cabral, A. R., Creaser, R. A., Naegler, T., Lehmann, B., Voegelin, A. R., Belyatsky, B., Pasava, J., Seabra Gomes, Jr., A. A., Galbiatti, H., Boettcher, M. E., & Escher, P. (2013). Trace-element and multi-isotope geochemistry of Late-Archean black shales in the Carajas iron-ore district, Brazil. Chemical Geology, 362(SI), 91-104. https://doi.org/10.1016/j.chemgeo.2013.08.041

@article{593f6887ec5a483c8cb16f5e8286eff3,

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

abstract = "The 250-300-m-thick Caraj{\'a}s Formation in the Caraj{\'a}s mineral province, northern Brazil, consists of banded iron formation (including giant high-grade iron-ore deposits) and minor black shale, overlying a thick pile (2-3km) 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 29ppm 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 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 10wt.% K2O as adularia) related to diagenetic processes at <100°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±38Ma (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±110Ma (MSWD=121). Molybdenum-isotope patterns suggest mixing between a clastic end member, with about 0.2‰ δ98/95Mo for continental input (TDM of 2.8-3.1Ga 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 significant Mo-isotope fractionation is either the result of an early and transient {"}whiff of oxygen{"} at 2.7Ga, or the result of hydrothermal fluid overprint.",

keywords = "Serra Sul, Carajas, Brazil, Black shale, Re-Os, Mo isotopes",

author = "Cabral, {A. R.} and Creaser, {R. A.} and T. Naegler and B. Lehmann and Voegelin, {A. R.} and B. Belyatsky and J. Pasava and {Seabra Gomes, Jr.}, {A. A.} and H. Galbiatti and Boettcher, {M. E.} and P. Escher",

year = "2013",

month = dec,

day = "20",

doi = "10.1016/j.chemgeo.2013.08.041",

language = "English",

volume = "362",

pages = "91--104",

journal = "Chemical Geology",

issn = "0009-2541",

publisher = "Elsevier",

number = "SI",

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TY - JOUR

T1 - Trace-element and multi-isotope geochemistry of Late-Archean black shales in the Carajas iron-ore district, Brazil

AU - Cabral, A. R.

AU - Creaser, R. A.

AU - Naegler, T.

AU - Lehmann, B.

AU - Voegelin, A. R.

AU - Belyatsky, B.

AU - Pasava, J.

AU - Seabra Gomes, Jr., A. A.

AU - Galbiatti, H.

AU - Boettcher, M. E.

AU - Escher, P.

PY - 2013/12/20

Y1 - 2013/12/20

N2 - The 250-300-m-thick Carajás Formation in the Carajás mineral province, northern Brazil, consists of banded iron formation (including giant high-grade iron-ore deposits) and minor black shale, overlying a thick pile (2-3km) 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 29ppm 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 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 10wt.% K2O as adularia) related to diagenetic processes at <100°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±38Ma (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±110Ma (MSWD=121). Molybdenum-isotope patterns suggest mixing between a clastic end member, with about 0.2‰ δ98/95Mo for continental input (TDM of 2.8-3.1Ga 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 significant Mo-isotope fractionation is either the result of an early and transient "whiff of oxygen" at 2.7Ga, or the result of hydrothermal fluid overprint.

AB - The 250-300-m-thick Carajás Formation in the Carajás mineral province, northern Brazil, consists of banded iron formation (including giant high-grade iron-ore deposits) and minor black shale, overlying a thick pile (2-3km) 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 29ppm 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 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 10wt.% K2O as adularia) related to diagenetic processes at <100°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±38Ma (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±110Ma (MSWD=121). Molybdenum-isotope patterns suggest mixing between a clastic end member, with about 0.2‰ δ98/95Mo for continental input (TDM of 2.8-3.1Ga 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 significant Mo-isotope fractionation is either the result of an early and transient "whiff of oxygen" at 2.7Ga, or the result of hydrothermal fluid overprint.

KW - Serra Sul

KW - Carajas

KW - Brazil

KW - Black shale

KW - Re-Os

KW - Mo isotopes

U2 - 10.1016/j.chemgeo.2013.08.041

DO - 10.1016/j.chemgeo.2013.08.041

M3 - Journal article

SN - 0009-2541

VL - 362

SP - 91

EP - 104

JO - Chemical Geology

JF - Chemical Geology

IS - SI

ER -

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

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