Origin of ironstones of the Udokan Cu deposit (Siberia, Russia): A key study using SEM and LA-ICP-MS

Iron is an important element involved in the formation of sedimentary-hosted copper deposits. Its accumulation in sedimentary basins and subsequent redistribution ultimately affect the position of copper mineralization. The Udokan copperstone deposit contains considerable Fe reserves as magnetite and hematite. The Fe oxides form disseminated and layered aggregates in both Cu-bearing horizons and host metasandstones. The layered aggregates of Fe oxides contain detrital (zircon, chromite and apatite) and authigenic (a Th silicate) components. The ironstones bear chemical features both of clastic and chemical sediments. The enrichment of ironstones in Cr, Zr and Hf is due to the presence of relics of detrital chromite and zircon. The presence of rare earth elements (REEs), U, and Th in composition of ironstones is a result of chemical precipitation/sorption. According to the LA-ICP-MS data, Al, Mg, and V are the most significant trace elements of magnetite, whereas Ti and V are hosted in hematite. The extremely high Ti, Cr and Cu contents of magnetite are associated with inclusions of minerals of these elements. In hematite, the high Ti contents are due to inclusions of rutile or titanite. The correlations between the trace elements in Fe oxides reflect the mineral assemblages characteristic of two stages of sediment transformation: (i) earlier diagenetic with the formation of a hematite-rutile assemblage and early Cu sulphides and Th and REE mineralization and (ii) later metamorphic, which is related to the formation of magnetite metacrysts and recrystallization of Cu sulphides. The Fe-rich sediments formed at a river–sea geochemical barrier as a result both of sedimentation of clastic Fe oxides and flocculation of dissolved iron. The maturation of gel and lithification of sediments were accompanied by desorption of metals that accumulated in the ferruginous sediments. Some of them formed the authigenic phases (like Th), while others were isomorphically incorporated in other authigenic minerals (e.g., Nb, Ta and Sn in rutile).