Origin of ooids, peloids and micro-oncoids of marine ironstone deposits in Western Siberia (Russia)

Abstract

This study investigates the authigenically formed Late Cretaceous and Paleogene marine ironstones in Western Siberia (Russia) based on petrographic and spectroscopic investigations. In the Upper Cretaceous Kuznetsovo, Ipatovo, Slavgorod, Gan'kino and Paleogene Lyulinvor Formations of the West Siberian ironstone basin, there are three main ore horizons, known as the Narym, Kolpashevo, Bakchar layers. The marine succession predominantly comprises ooidal or peloidal ironstones, glauconitolites, glauconitic rocks, sandstones, siltstones, claystones and gritstones. The ooidal ironstone exhibits predominantly abiogenic precipitates with subordinate microbial signatures, including micro-oncoids, relicts of lipids, carbohydrates and microfilaments. Iron-rich ooids, peloids and micro-oncoids formed primarily by adsorption of iron and occasionally by microbial iron-oxidising and sulphate-reduction actions. The abiogenic formation of ooid and peloid depends on physico-chemical conditions of environment during ion adsorption. While berthierine-goethite in ooid cortex formed in an oxic or dysoxic environment, siderite laminae therein represent an anoxic condition. Microbial mediation influenced the precipitation of berthierine and goethite, pyrite, siderite, greigite, pyrrhotite, wurtzite, barite, As-Ni-Co-Fe sulphide, and probably monazite. Siderite, pyrite, and to a lesser extent greigite and pyrrhotite formed exclusively within organic remains. Goethite with a high content of phosphate within micro-oncoids probably formed with the mediation of microbial activities. Bacterial microfilaments and Raman peaks of lipids and carbohydrates strongly support a bacterial origin of micro-oncoids. Abundant intraclasts at the base of the Ipatovo Formation that marks the Coniacian-Santonian boundary, and those within the Lyulinvor Formation that marks the Palaeocene-Eocene boundary, correspond to active tectonics, which provided an enhanced supply of metal-rich nutrients. Distribution of trace elements in the main iron-rich minerals supports a hydrothermal source of iron and other metals for iron-rich ooids and peloids in Western Siberia. The study of the Meso-Cenozoic marine ironstones provides insights into the biotic or abiotic processes involved in case of Precambrian banded iron formations (BIFs).