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

authored by

Xiuqing Yang, Jingwen Mao, Simon W. Poulton, Aiguo Dong, Ting Liang, Dachuan Wang, Xusheng Zhang

Abstract

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 ε_Nd(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.

Organisation(s)

Institute of Mineralogy

External Organisation(s)

Chang'an University
Chinese Academy of Geological Sciences (CAGS)
University of Leeds
China University of Geosciences

Type

Article

Journal

Gondwana Research

Volume

93

Pages

218-226

No. of pages

9

ISSN

1342-937X

Publication date

05.2021

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Geology

Sustainable Development Goals

SDG 14 - Life Below Water

Electronic version(s)

https://doi.org/10.1016/j.gr.2021.02.008 (Access: Closed)