The first results of the SEM study of banded iron ores of the Uda-Shantar basin in the Russian Far East are described. They show that the ores contain various microbial remains with Fe-Mn mineralization.

The formation of manganese rocks and ores occurred during the whole geological history of the Earth. Five metallogenic epochs (Early to Middle Proterozoic, Late Proterozoic, Early to Middle Paleozoic, Late Paleozoic, and Meso-Cenozoic) and 7 very important phases (Early, Middle, and Late Proterozoic, Early to Middle Paleozoic, Late Paleozoic, Late Mesozoic, and Meso-Cenozoic) can be distinguished. The phases of manganese ore genesis at many stratigraphic levels are closely related to the global climatic and tectonic reconstructions (the breakup of the continent of Gondwana and periods of glaciations and aridization) and biotic events (mass extinction of organisms). Based on carbon isotopic composition in manganese carbonates, participation of oxidized organic carbon is established.

The paper presents results of the detailed study of phosphorites from manganiferous beds of the Chiatura deposit. The relatively high-grade (P2O5 20–28%) phosphorites are represented by various rocks ranging from the variety dominated by massive phosphates with a rare aleuritic admixture of quartz and feldspar grains to rocks mainly composed of terrigenous material with phosphates in the matrix. Phosphates make up the matrix of various organic remains: differently preserved diatom algae and microbial species. Some relatively large organic remains (in particular, sponge spicules) are typically composed of iron minerals (with manganese admixture) rather than phosphates. Manganese ores comprise phosphorite fragments composed of phosphatized cyanobacterial mat. Phosphorites of the Chiatura deposit were likely formed in a shallow-water zone away from the continental land.

Ferromanganese nodules (pisolites) form accumulations in basal layers of Pliocene-Quaternary clayey sections of Far East Russia and Vietnam. They are composed of minerals that are in common for both these regions (authigenic vernadite, feroxyhyte, goethite, halloysite, and terrigenous quartz) and minerals that are characteristic of either the northern (authigenic hollandite, lithiophorite, and bernessite) or southern (authigenic alumophorite, lepidocrocite, ferrihydrite, gibbsite, and terrigenous ilmenite) regions. Pisolites are considered to be microbial colonies with Mn and Fe oxides frequently forming biomorphs. The growth of the colonies was accompanied by dying off and mineralization of microorganisms successively from the central toward the peripheral parts of the nodules. The formation of metalliferous pisolites was linked to the oxidizing geochemical barrier developed at the interface between compact sedimentary clays and the underlying porous readily permeable weathered products of basalts.

A detailed investigation of microstructures of the phosphorites from Christmas Island under a scanning electron microscope coupled with an analysis of their chemical composition revealed that both their structure and composition are quite similar to those of the granular and microgranular phosphorites on present-day continents, as well as those of the phosphorites on some of the Pacific guyots. Their composition, together with their geological position and interrelationships with the surrounding rocks, proves that the ornitogenic hypothesis based on the presumed guano accumulation followed by its transformation into phosphatic rock is not compatible with the field observations. Meanwhile, the problem of the source of the Fe and Mn impregnation in the phosphorites remains unresolved and needs further investigations.

The compositions, structures, and microstructures of different types of phosphorites and poorly phosphatized rocks from low atolls in the near-equatorial part of the Western Indian Ocean are described. The rocks were examined under optical and scanning microscopes using microprobing techniques and etching of selected samples with weak solvents as well as with the help of chemical analyses. It is proved that phosphorites have been formed owing to the uneven phosphatization of the primary carbonate rocks; the degree of their phosphatization ranges from traces to 40% P2O5. In the phosphorites, numerous organic remains were encountered; they included fragments of plankton, debris of tortoise shells, and coccoidal and filamentous bacteria-like formations. It is suggested that the phosphorites were formed due to the high local biological productivity over the outer edges of coral reefs and are not related to guano accumulation or to endoupwelling.

Phosphatized biogenic limestones and phosphorites with initial Fe-Mn mineralization dredged from the summit surface of the Kammu Seamount ( Milwaukee Seamounts, northwestern Pacific) are studied. The rocks are largely composed of nannofossils and planktonic foraminifers with an admixture of benthic foraminifers, bryozoans, and other organic remains, presumably including bacterial ones. The nannofosssil and foraminiferal assemblages indicate the Quaternary age of the sediments, and their phosphatization is consistent with the phosphatization age determined previously based on nonequilibrium uranium (within the limits of 1 My). The age of phosphatization and the Fe-Mn mineralization in the sediments from Pacific seamounts that young implies the dependence of these ore-forming processes on oceanic environments favorable for ore accumulation rather than on their age.