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Abstract An integrated study involving petrographical, mineralogical and geochemical investigations proposes the origin of the Bakchar ironstone deposit by upward migration of a mixture of Fe-rich brine and hydrothermal fluid through marine sediments. Trace element concentrations of the ironstone deposits rule out continental derivation of Fe, while high REE concentrations indicate mixed hydrogeneous and hydrothermal source. Three main types of ironstone within the Bakchar deposit are characterized by distinct mineralogical assemblage of authigenic minerals corresponding to oxic, methanic and weakly sulfidic dysoxic conditions. Changing redox conditions of the depositional environment, methane seepage in warm seawater and availability of iron influx possibly determined the final mineralogy of three varieties of ironstones. The association of siderite with ferrimagnetic sulfides like greigite and pyrrhotite in type-1 ore indicate anaerobic oxidation of methane with limited bacterial sulfate reduction. The ironstone deposits of Narym, Kolpashevo and Bakchar are contemporaneous with geological events like ocean anoxic event 3 (OAE-3), Cretaceous-Paleogene (K-Pg) boundary and Palaeocene-Eocene Thermal Maximum (PETM), respectively, all corresponding to warm seawater conditions. Graphical abstract Image 1 Highlights • Trace element concentrations rule out continental derivation of Fe in Bakchar ironstone. • Ironstone formed in oxic, methanic and weakly sulfidic, dysoxic conditions. • The association of siderite with ferrimagnetic sulfides indicates AOM with limited BSR. • The ironstone deposits are contemporaneous with warming events. [ABSTRACT FROM AUTHOR]

Glauconite occurs either as unaltered greenish or as altered brownish variety in Upper Cretaceous-Palaeocene sediments in the southeastern corner of Western Siberia. Studied section within the Bakchar iron-ore deposit includes Ipatovo, Slavgorod, Gan'kino and Lyulinvor formations, which are represented by sandstones, siltstones, claystones and oolitic ironstones of coastal-marine facies. The origin of unaltered glauconite is explained by the “verdissement theory”. Transgressions during Lower Coniacian, Santonian and Campanian favored the formation of unaltered glauconites in dysoxic to anoxic conditions. Subaerial exposure of glauconite resulted in leaching of potassium, oxidation of iron and formation of iron hydroxides in Upper Coniacian, Maastrichtian and Palaeocene. Glauconite ultimately converts to leptochlorite and hydrogoethite by this alteration. Abundant microscopic gold inclusions, besides sulphides, sulphates, oxides and silicates characterize this glauconite. Mineral inclusions include precious, rare metals and non-ferrous metals. The concentration of gold in glauconite may be as high as 42.9 ppb. Abundant inclusions of various compositions in glauconites indicate enrichment of marine sediments in precious and non-precious metals. While major element composition of glauconites is affected by subaerial exposure, the broadly similar micro-inclusions in both altered and unaltered varieties are possibly related to the comparatively immobile nature of REE and trace elements. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ooidal ironstone deposits were formed by both abiogenic and biogenic processes. • Physico-chemical conditions of the porewater regulated abiogenic precipitates. • Microbial mediation is required for the authigenic mineral formation. • Some mineral inclusions within oncoids formed by the iron-reducing bacterial activity. • Abundant intraclast is indicative of tectonic events. • Mineral geochemistry supports the hydrothermal source of iron and other metals. 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). [ABSTRACT FROM AUTHOR]

• Mineral composition of the bottom part of the peat deposit was studied. • This part of deposit is characterized by a change in the environmental conditions. • Clay minerals of peat layers have been inherited from the underlying loam layer. • Geochemistry of peat layer is different from the organo-mineral sediments and loam. • Authigenic sulfides have formed in the reducing conditions of the basal peat. This paper is devoted to a study of the nature and origin of the mineral matter in the Vasyugan Mire in Western Siberia. Based on mineralogical and geochemical analyses, new results were obtained concerning the mineral composition within the bottom part of the peat deposit for different landscape zones in the Vasyugan Mire. This part of the peat deposit represents a transition sequence from peat through organo-mineral sediments (OMS) to basal loams and is characterized by a gradient change in the physicochemical and environmental conditions with depth. In the same sequence, pH and peat ash yield increase, while moisture decreases. The clay minerals identified in the basal layers of the mire, consisting of loam, OMS and peat, include illite, chlorite, kaolinite and smectite. All these clay minerals found in the OMS and peat layers are believed to have been inherited with little change from the underlying loam layer during the initial transient stage of mire development. That this is indeed the case is strongly suggested by the extraordinary correspondence in the mean content of over 40 trace elements between the OMS and loam layers. In the overlying peat layer, however, some elements, including U, W, Sb, Sn, Mo, Zn, Cd, Nb, Ta, Br and B, are present in higher concentrations than in the OMS and loam. Scanning electron microscope observations suggest that this can be at least partly accounted for by the precipitation of sulfide phases in the strongly reducing and acidic environment of the peat, although whether by inorganic or microbial processes is unknown. The age of the Vasyugan Mire within the Holocene succession is discussed and it is concluded that it developed either during the Atlantic Period (8 ka–6 ka BP) or during the upper part of the Sub-Boreal Period (4 ka BP). [ABSTRACT FROM AUTHOR]

• The Early Pliocene ooidal ores of the Azov–Black Sea region store high-REE contents. • The Fe3+-(oxy)hydroxides are main carriers of MREE and HREE. • LREE are mainly hosted by early-diagenetic hydrous phosphates. • The REEs stored in ooidal ironstones have a diagenetic source. This study is aimed at characterizing the distribution and speciation of the rare earth elements and yttrium (REE + Y) in ooidal ironstones from the Kamysh-Burun deposit (Kerch Peninsula), with implications for the depositional environments and contributions of different REE + Y carriers to the ore budget. The Lower Pliocene ooidal ironstone sequence of the so-called Kerch ores, up to 15 m thick, occupies an area of 28 km2. The ore sequence lies between Miocene – Lower Pliocene shell limestones and Upper Pliocene – Pleistocene sandy and clayey sediments and consists of horizontal ooidal ironstone beds composed mainly of goethite and X-ray amorphous Fe3+-(oxy)hydroxides intercalated with siderite and/or rhodochrosite beds. Iron is a main component in ooidal ironstones (50.03–66.19 wt% Fe 2 O 3 and ≤1.61 wt% FeO), as well as in fresh carbonate ores (~34 wt% FeO and ≤14.70 wt% Fe 2 O 3). The Kerch ores contain up to 4.59 wt% P 2 O 5 and show significant positive correlation between P and Fe (r = 0.75). The bulk ore samples and their coarse (1–10 mm) fractions are goethite-dominated and have similar phase, major- and trace-element, and REE compositions. The fine (<1 mm) fractions are poorer in goethite and P 2 O 5 but enriched in SiO 2, Al 2 O 3, Ti, Zr, Y, and Th due to greater percentages of Fe-saponite and fine detrital matter (including very scarce rutile, zircon, monazite, and xenotime). The average ΣREE values of 400 ppm in bulk ooidal ironstones and 405 ppm in the coarse fractions markedly exceed those in the Post Archean Australian Shale (PAAS). They have similar PAAS-normalized REE + Y patterns, with moderate enrichment in middle REE (MREE), minor Ce* (0.67–1.09), and moderate negative Y* anomalies (0.57–0.70). The fine ore fractions are markedly enriched in ΣREE (Xav = 858 ppm) mainly due to the presence of authigenic light REE phosphates of rhabdophane-tristramite series with Ce > La ≈ Nd ≈ Ca > Pr > Sm. The ΣREE content in fresh carbonate ores is below that in PAAS (Xav = 103 ppm), but approaches the latter in oxidized crusts (Xav = 178 ppm). The Kerch ironstones also contain REE-poor early-diagenetic Fe2+ phosphate vivianite (ΣREE Xav = 0.93 ppm) and Ca-Fe2+ phosphate anapaite (ΣREE Xav = 6.95 ppm) with typical seawater REE + Y distributions. The ooidal ironstones show MREE-enriched patterns with distinct negative Y* anomalies, which reveals Fe3+-(oxy)hydroxides as chief carriers of adsorbed REE (mainly MREE). The Kerch ironstones accumulated REE progressively due to hysteretic dissolution and precipitation of Fe3+-(oxy)hydroxides in oscillating redox conditions. The diagenetic source of REE stored in ooidal goethite ironstones was inferred from the PAAS-normalized values: Ce N /Ce N * vs Nd and Ce N /Ce N * vs Y N /Ho N discrimination diagrams. In general, the REE + Y budget of the Kerch ooidal ironstones mainly formed during early diagenesis and scavenging from pore water, with a minor contribution of siliciclastic inputs, in the absence of hydrothermalism. [ABSTRACT FROM AUTHOR]