TY - JOUR

T1 - Iron isotope constraints on the metal source and depositional environment of the Neoproterozoic banded iron- and manganese deposits in Urucum, Brazil

AU - Huang, Qingyu

AU - Viehmann, Sebastian

AU - Walde, Detlef H.G.

AU - Li, Weiqiang

N1 - Funding information: We thank Max Lechte for help with the compilation of Fe isotope data in literature. This study is supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences ( XDB26020101 ) and Natural Science Foundation of China (No. 41622301 ) to WL. SV greatly acknowledges Berhard Bühn and Michael Bau for organizing a field trip to the Urucum area within the joint project between the Deutsche Forschungsgesellschaft (DFG) and the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). We also greatly acknowledge the support of Adriana Zapparoli and her colleagues from the Vale Company which gave access to the mining area and the drill core samples, and a continuing helpful and pleasant cooperation. We thank Max Lechte for help with the compilation of Fe isotope data in literature. This study is supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB26020101) and Natural Science Foundation of China (No. 41622301) to WL. SV greatly acknowledges Berhard B?hn and Michael Bau for organizing a field trip to the Urucum area within the joint project between the Deutsche Forschungsgesellschaft (DFG) and the Brazilian Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico (CNPq). We also greatly acknowledge the support of Adriana Zapparoli and her colleagues from the Vale Company which gave access to the mining area and the drill core samples, and a continuing helpful and pleasant cooperation.

PY - 2021/9

Y1 - 2021/9

N2 - The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ56Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ56Fe values with increasing manganese contents. The low δ56Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.

AB - The Urucum area of Brazil hosts a series of Cryogenian ironstones intercalated by oxide-dominated manganese layers. The Urucum iron and manganese formations (IF-MnF) are among the largest sedimentary iron and associated manganese deposits of the Neoproterozoic, however, the depositional model and the source of metals for the IF-MnF in this area are highly controversial. In this study, we performed systematic Fe isotope analysis on fresh and geochemically characterized drill core samples of the Urucum iron and manganese formation deposited in the center of the ancient Urucum graben system. The samples have a large variation in Fe isotope composition, with a δ56Fe range of −2.04‰ to +0.75‰, and exhibit a general trend of decreasing δ56Fe values with increasing manganese contents. The low δ56Fe values of the IF and MnF samples reflect Rayleigh fractionation processes of contineous partial oxidation of aqueous Fe(II) prior to deposition at the sampling site. Using a mixing model and previously published Nd isotope data on the same samples, we estimated that benthic (i.e., porewaters released from submarine sediments in the Urucum basin) Fe fluxes provided 7–50% of total Fe in the Urucum IF-MnF, and the rest of Fe source was from low-temperature hydrothermal vents. Based on combined Fe and Nd isotope data of the Urucum IF-MnF, we propose that low-temperature hydrothermal fluids and benthic fluxes of pore waters were mixed and transported by an upwelling current. The fluid subsequently experienced partial oxidation during the transportation process and became enriched in light Fe isotopes. In the Urucum graben basin, the iron- and manganese-rich oxides deposition occurred progressively under increasingly oxidizing conditions, and such process could have operated repeatedly to produce the alternation of iron and manganese formations. The chemical sediments of the Urucum IF-MnF deposits thus reflect the existence of a sharp redox gradient in the marine environment during the late Cryogenian period.

KW - Bacterial iron reduction

KW - Iron formation

KW - Iron isotope

KW - Manganese formation

KW - Neoproterozoic

KW - Snowball Earth

KW - Urucum

UR - http://www.scopus.com/inward/record.url?scp=85127808489&partnerID=8YFLogxK

U2 - 10.1016/j.chemer.2021.125771

DO - 10.1016/j.chemer.2021.125771

M3 - Article

AN - SCOPUS:85127808489

VL - 81

JO - Geochemistry

JF - Geochemistry

SN - 0009-2819

IS - 3

M1 - 125771

ER -