Shallow-water hydrothermal venting and the formation of Precambrian ironstones: Insights from the terminal-Paleoproterozoic Xuanlong ironstones in North China Craton

Abstract

Xuanlong ironstones occur as stromatolitic or ooidal, hematite-rich, concordant bodies in the Paleoproterozoic Chuanlinggou Formation of North China. Continental-derived iron was traditionally suggested as the ironstone source. However, widespread hydrothermal discharge chimney structures have been discovered. These structures are interpreted as fluid-flow pathways resulting from the ejection of hydrothermal fluids into unconsolidated sandy sediments in a shallow subtidal zone. Simple tubular and curviplanar structures outlined by the occurrence of Fe-rich silicified sediments are well-developed in the underlying sandstones. Honeycomb-like cavities and possible fluid-flow channel textures attest to primary porosity during the formation of the ironstones, indicating that hydrothermal fluids play a key role in the formation of ironstones. This study provides an unequivocal example of Paleoproterozoic shallow-water hydrothermal activity for the first time, which is of great significance in understanding the genesis of granular or Superior-type IFs in general.