Depositional age and environment of the Xinyu iron deposit in the Nanhua Basin, South China

Abstract

Deposition of Neoproterozoic iron formations (IFs) was typically associated with Cryogenian glaciations and the breakup of the Rodinia supercontinent. Neoproterozoic IFs in South China are distinct, however, as they lack evidence for glacial and volcanic influence. The correlation of these IFs with Neoproterozoic sedimentary successions and their depositional environment remain controversial. Here we present new geochronological and geochemical constraints on the depositional age and environment of the Xinyu IF, one of the largest Neoproterozoic IFs in the Nanhua Basin of South China. The Xinyu IF, intercalated with clastic rocks and not in direct contact with glaciogenic sedimentary rocks, suggests deposition in an open water environment during the Sturtian glacial period. The Xiafang Formation, which hosts the Xinyu IF, overlies the diamictite-bearing Gujia Formation and underlies the gravel-containing and manganese (Mn)-bearing of the Dashajiang Formation. The youngest three zircon grains of the Xiafang Formation give a weighted mean ²⁰⁶Pb/²³⁸U age of 694 ± 17 Ma, representing the maximum depositional age of this formation. The Xiafang Formation is, therefore, likely correlated to the Liangjiehe Member of the Fulu Formation in the western Nanhua Basin, and was likely deposited between 694 ± 17 and 691 ± 12 Ma. Shale-normalized rare earth element (REE) plus Y (REY) patterns of the Xinyu IF samples display light REE depletion (average (LREE/HREE)_SN = 0.37) relative to heavy REE, insignificant Eu anomalies (Eu* = 0.85−1.21), and no Ce anomalies (Ce* = 0.95−1.00). These geochemical characteristics suggest that the Xinyu IF was likely deposited in anoxic seawater in which Fe(II) was likely sourced from low-temperature hydrothermal fluids. Collectively, we suggest that the Xinyu IF is a Cryogenian IF deposited in open water during the Sturtian glaciation.