A unique Mesozoic greenalite deposit as a window into the genesis of iron formations through time

Abstract Greenalite, a ferrous Fe-clay, has been increasingly regarded as the primary precipitate during hydrothermal vent fluid–seawater mixing in early anoxic oceans, leading to widespread deposition of Fe- and Si-rich sediments, namely, iron formations (IFs). However, due to elevated surface oxygen levels, both greenalite and IFs have not been documented in Phanerozoic oceans. Here, we report a rare Triassic sedimentary greenalite-dominated deposit analogous to IFs, the Huimin Fe deposit, SW China. This deposit is hosted in mafic volcanic rocks formed in a deep-water setting within a localized, redox-stratified basin with hydrothermal venting. The Fe ores are laminated, composed mainly of greenalite and siderite, stilpnomelane, and apatite, and locally dominated by hematite. Greenalite is the earliest-formed mineral and occurs as nanoparticles with features of primary sediments, indicative of a primary origin. Rare earth element analysis suggests that the greenalite was likely formed during vent fluid–seawater mixing and deposited at a vent-distal site. Diagenetic alteration of greenalite produced 13C-depleted siderite and stilpnomelane, whereas secondary oxidation formed hematite. Our study highlights that restricted, anoxic basins with hydrothermal vents held potential to form young greenalite IF analogues. Moreover, these findings reinforce the emerging view that greenalite is the primary mineral of IFs and provide new insights into its postdepositional fate, such as the origin of 13C-depleted siderite, thereby offering new constraints on the origin of IFs.