TY - JOUR

T1 - The origin of early-Paleozoic banded iron formations in NW China

AU - Yang, Xiuqing

AU - Mao, Jingwen

AU - Poulton, Simon W.

AU - Dong, Aiguo

AU - Liang, Ting

AU - Wang, Dachuan

AU - Zhang, Xusheng

N1 - Funding Information: This work was supported by the Natural Science Foundation of China (Nos. 41972075 and 41702063), Natural Science Basic Research Plan in Shaanxi Province of China (2020JM-213) and Fundamental Research Funds for the Central Universities , CHD (Nos. 300102279202 and 300102279401 ). SWP acknowledges support from a Royal Society Wolfson Research Merit Award. AD acknowledges support from the Fundamental Research funds for the Central Universities ( 2652017048 ). We thank two anonymous reviewers for constructive comments on an earlier version of this paper. We are grateful to Prof. Houmin Li, Jian Sun and Dr. Jialin Wu for their constructive discussions. We also thank Minwu Liu and Mu Liu for assisting with EPMA, geochemistry and isotope analyses.

PY - 2021/5

Y1 - 2021/5

N2 - There is growing evidence that the earliest Paleozoic ocean experienced fluctuations in ocean redox chemistry, with anoxic ferruginous conditions being a prevalent feature at certain times. However, the general absence of early Paleozoic banded iron formations (BIFs) means that direct supporting evidence for ferruginous conditions inferred from geochemical proxy data is currently limited. Here, we describe an early-Paleozoic BIF (Tiande No.2) from the late Silurian Habahe Group in Altay, NW China. The BIF is hosted in meta-volcano-sedimentary rocks, and is characterized by alternating iron-rich and silica-rich laminae. In addition to abundant magnetite and quartz, Fe-silicates (garnet and amphibole) are dominant minerals in the BIF. Like most earlier Precambrian examples, the BIF horizons are dominated by high Si and Fe contents, but also contain significant Al, Ti and rare earth element concentrations, consistent with a dominant chemical origin, but with significant terrigenous contribution. We observe slightly positive Eu anomalies (1.17 to 1.57) and low εNd(t) values (−4.2 to −0.8) in BIF samples, indicating a mixed seawater and submarine hydrothermal source for the iron. The BIF also generally displays negative Ce anomalies, and homogeneous and slightly negative δ56Fe values. This suggests near-quantitative oxidation of dissolved Fe(II) from the water column, likely following upwelling into oxygenated surface waters as sea level decreased, in a region where the terrigenous input was low relative to the dissolved Fe2+ flux. The Tiande No.2 BIF is conformably bedded within schist, which also has high concentrations of Fe (up to 12.67 wt%), generally elevated Fe/Al ratios (>1.0), and no Ce anomaly, while magnetite in the schist has heavy and variable δ56Fe values. This suggests that the schist precursor, which likely comprised a significant contribution from deposited marine sediment, was also deposited from ferruginous waters, but with only partial oxygenation of dissolved Fe2+ in a dominantly anoxic water column. The occurrence of this late Silurian BIF and associated ferruginous rocks provides new constraints on the nature of ocean redox chemistry at this time, and supports continued dynamic Fe cycling and at least intermittent ferruginous deeper water conditions in this location during the late Silurian.

AB - There is growing evidence that the earliest Paleozoic ocean experienced fluctuations in ocean redox chemistry, with anoxic ferruginous conditions being a prevalent feature at certain times. However, the general absence of early Paleozoic banded iron formations (BIFs) means that direct supporting evidence for ferruginous conditions inferred from geochemical proxy data is currently limited. Here, we describe an early-Paleozoic BIF (Tiande No.2) from the late Silurian Habahe Group in Altay, NW China. The BIF is hosted in meta-volcano-sedimentary rocks, and is characterized by alternating iron-rich and silica-rich laminae. In addition to abundant magnetite and quartz, Fe-silicates (garnet and amphibole) are dominant minerals in the BIF. Like most earlier Precambrian examples, the BIF horizons are dominated by high Si and Fe contents, but also contain significant Al, Ti and rare earth element concentrations, consistent with a dominant chemical origin, but with significant terrigenous contribution. We observe slightly positive Eu anomalies (1.17 to 1.57) and low εNd(t) values (−4.2 to −0.8) in BIF samples, indicating a mixed seawater and submarine hydrothermal source for the iron. The BIF also generally displays negative Ce anomalies, and homogeneous and slightly negative δ56Fe values. This suggests near-quantitative oxidation of dissolved Fe(II) from the water column, likely following upwelling into oxygenated surface waters as sea level decreased, in a region where the terrigenous input was low relative to the dissolved Fe2+ flux. The Tiande No.2 BIF is conformably bedded within schist, which also has high concentrations of Fe (up to 12.67 wt%), generally elevated Fe/Al ratios (>1.0), and no Ce anomaly, while magnetite in the schist has heavy and variable δ56Fe values. This suggests that the schist precursor, which likely comprised a significant contribution from deposited marine sediment, was also deposited from ferruginous waters, but with only partial oxygenation of dissolved Fe2+ in a dominantly anoxic water column. The occurrence of this late Silurian BIF and associated ferruginous rocks provides new constraints on the nature of ocean redox chemistry at this time, and supports continued dynamic Fe cycling and at least intermittent ferruginous deeper water conditions in this location during the late Silurian.

KW - Altay

KW - Band iron formations

KW - Early-Paleozoic

KW - Fe isotopes

KW - Ferruginous conditions

UR - http://www.scopus.com/inward/record.url?scp=85101810296&partnerID=8YFLogxK

U2 - 10.1016/j.gr.2021.02.008

DO - 10.1016/j.gr.2021.02.008

M3 - Article

AN - SCOPUS:85101810296

VL - 93

SP - 218

EP - 226

JO - Gondwana Research

JF - Gondwana Research

SN - 1342-937X

ER -