The Nikopol manganese deposit is one of the world’s largest deposits among the sedimentary manganese deposits. The Nikopol Oligocene basin is located between the Azov crystalline massif and the Ukrainian shield. Nikopol Ore horizon is traced in a thickness varying from several cm to 4.5 m and a single stratum from the west to the eastwards to about 250 km and separated to three different units; carbonate, mixed carbonate-oxide, and oxide ore. The oxide ores can contain the concretion or earthy masses bigger than 25 cm, sometimes with remnants of carbonate or carbonate-oxide textures. The manganese oxide-hydroxide ores were analyzed for major oxides, trace and rare earth elements (REE) using ICP-MS. The PAAS-normalized REE patterns of the Nikopol manganese oxide ores have similar trends and show MREE and HREEs enrichments. The Ce/Ce* values of manganese oxide-hydroxide ores collected from the study area vary from 0.88 to 1.43, indicating that ore-forming rocks were primarily marine chemical or biogenic deposit. The Eu/ Eu* anomalies of the manganese oxide-hydroxide ores are close to 1. The Y concentrations vary from 9,1 to 47,1 ppm and show negative Y anomalies. Both geochemical and Pb isotope data indicate that the Nikopol manganese oxide-hydroxide ores formed rapidly within oxic/suboxic seawater as reflected by Ce anomalies close to 1 in low-oxygen fugacity in source of the hydrothermal fluids, volcanogenic input or hydrothermal contributions to seawater. Also, our results point out that the metal was transported from both a hydrothermal source in deeper water and terrestrial sources.

The Chiatura deposit is considered one of the world’s largest deposits of metallurgical grade manganese, yet its geochemistry is poorly known, which is the focus of this paper. The Oligocene sedimentary manganese deposit is located in the Chiatura region of central Georgia and formed on stable crystalline basement in a restricted arm of the Paratethys during a transgressive-regressive cycle. The average main trace element contents of the samples are 3944 ppm Ba, 946 ppm Sr, 511 ppm Ni, 150 ppm Zn, 94 ppm V, 84 ppm Cu, and 55 ppm Co. The trace element concentrations of the manganese oxide ore are generally low and indicate an enrichment assemblage of V, Co, Ni, Cu, Zr, Ba, As, Cd, Pb, and Zn. The total rare earth element (REE) concentrations of the deposits vary from 83 ppm to 521ppm, with a mean of 199 ppm. All manganese-oxide samples have negative Ce anomalies (mean 0.58) and generally positive Eu anomalies. All geochemical data indicate that the Chiatura manganese-oxide ores formed rapidly within oxic seawater as reflected by the negative Ce anomalies. The mixed oxide and carbonate ores formed at deeper-water depths compared to the oxide-hydroxide ores in the Chiatura region. Also, our results point out that the metals were transported from both terrestrial sources (Pb isotopic data) and a hydrothermal source in deeper water (chemical discrimination diagrams). The metals associated with the Mn-oxide ores likely formed syngenetically through microbially mediated mineralization at water depths deeper than the area where the carbonate-ore deposits formed.