Iron-manganese mineralization in Eocene deposits of Parashka skyba (Skybov nappe, Ukrainian Carpathians)

Abstract

Formulation of the problem. Modern advances in oceanographic studies of sedimentogenesis and orogenesis open up opportunities for in-depth study of rocks by fold-nappe systems, including the Carpathian region. Review of previous research. On the territory of the Carpathians in flysch complexes researchers have mainly noted the findings of carbonates of manganese and iron nodules.At the same time, flysch complexes contain iron-manganese oxide mineralization. The purpose of the work was to study the mineral and chemical composition, morphological types of aggregates and the origin of iron-manganese mineralization (IMM) among the Eocene deposits of the Parashka skyba of Skybov nappe in the middle course of the river Opir. Methods used: Sedimentological, mineralogical, petrogeochemical and X-ray diffractometric analyzes of wall rocks and ore minerals. Results: The ore mineralization is associated with the Paleocene to Eocene transition strata, which is represented by coarse-grained turbidites and grainites of the Yamna suite and variegated fine- and medium-grained turbidites and hemipelagites of the Manyava suite. IMM are deposited in a main ore bed, ore crusts and scattered in the wall rocks. It is synchronously sedimented with deep-water deposits of the lower bathyal. The оre bed is composited of concretionary, sinter, brecciated, earthy and sooty aggregates. Concretionary morphotypes have concentric-zonal structure, which is formed by shells with different mineral composition and structural features. Crystalline secretions are observed in the nucleus. The ore substance is represented by X-ray amorphous compounds of iron and manganese hydroxides, among which are minerals of hydrothermal (pyrolusite, psilomelan, todoroquite and bersenite) and hydrogenic (vernadite, buserite) origin. Shells often have columnar, dendrіtic and colomorphic structures typical of hydrothermal formations. Differentiation of mineralization by mineral composition causes significant fluctuations in metal contents in ore aggregates (from 3 to 11.5% of Fe oxides and from 2 to 10% of Mn) and host rocks (from 1.4 to 11.5% of Fe oxides and from 0.12 to 12% Mn). This confirms the endogenous origin of ore-bearing fluids. On the diagram of the ratios of the main components of iron-manganese formations of different zones of modern oceans by E. Bonatti, the geochemical composition of the ore mineralization of the Nad’yamne variegated horizon falls into the fields of both hydrothermal and hydrothermal-hydrogen origin. Mineralogical and geochemical features of ore mineralization indicate that the main source of metals on the seabed were hydrothermal vents. Direct deposition of ore mineral phases and deposition of oxide and hydroxide compounds from metal-enriched bottom waters occurred as a result of reaction of hydrothermal fluids with seawater. The formation and functioning of the hydrothermal system was likely caused by the intensification of tectonic movements of the Laramian phase on Paleocene-Eocene boundary, which also led to restructuring and deepening of the Carpathian paleobasin. An important role in these processes was played by faults in the flysch foundation, which served as channels for circulation of hot fluids. The source of heat and metals could be deep magma, according to authors. Scientific novelty. Iron-manganese mineralization in the Eocene rocks of the Nad’yamne variegated horizon was characterized for the first time, IMM morphotypes, structural-textural, mineralogical and geochemical features of ore formations and sedimentation and petrographic features of the content rocks were described. The studied features make it possible to propose a hydrothermal model of mineral formation. Practical significance. The studied features of IMM correlate well with modern and recent formations of pelagic sedimentation. The presence of iron-manganese mineralization in the Eocene Nad’yamne variegated horizon expands the cognitive aspects of the study of the Outer Carpathian basin of the Tethys Ocean and will contribute to the study of other variegated horizons and their geochemical specialization.