Trace and rare earth element geochemistry of the Oligocene Nikopol
stratiform manganese oxide-hydroxide ores, Ukraine
Abstract
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.