NEW DATA ON HIGH-CARBONACEOUS SHUNGIT-LIKE ROCKS OF MLYNKOVSKII AREA (KRIVOI ROG-KREMENCHUG ZONE, UKRAINIAN SHIELD)

Abstract

The paper presents the results of mineralogical, petrographic, electron microscopy, isotopic and X-ray structural studies of Paleoprotero-zoic high-carbon schungite-like rocks of the Mlynkivska area (Pravoberezhnyi district of the Kryvyi Rih-Kremenchuk structural formation zone). Their principal structural, material and genetic similarities with stratified metamorphosed clay-siliceous schungite-bearing rocks from Karelia are established. A characteristic feature of schungite-like rocks of the Mlynkivska area, that distinguishes them from other metamorphosed sediments of the Early Precambrian of the Ukrainian Shield, is the extremely low degree of metamorphism corresponding to the zeolite facies. This is evidenced by the weak crystallization of the siliceous (opal-chalcedony) matrix of rocks and the presence in the rocks of zeolites and a carbonaceous substance that has not reached the graphitization stage. The discovery of such weakly metamor-phosed rocks in the Early Precambrian is of exceptional interest for reconstruction of the primary sedimentatary conditions. Numerous biogenic formations (microfossils), which are closest (but not identical) in their morphological characteristics to cyanobacteria of the ge-nus Lyngbiopsis ambigolaevis sp.nov. Oscillatoriacea family, were found in the rocks. The fine-grained carbon-opal-chalcedony matrix of rocks and the presence of microfossils in them indicate the biogenic-chemogenic nature of their primary matter. Despite some differences in the geological settins and the conditions of sedimentation, the similarity of the carbonaceous rocks of the Mlynkivska area with the schungite rocks from Karelia in many parameters, including the characteristic close association of fine-grained siliceous and carbona-ceous matter as a result of crystallization and polymerization of a primary homogeneous gel precipitate - organo- silicate complex, indi-cates that the shungite from Karelia is not a unique phenomenon. This serves as an indication of the possibility of identifying new manifes-tations of shungite-like rocks in Ukraine in significantly different geological situations that are not typical for the deposits in Karelia. The need for systematic studies of various groups of carbon-bearing rocks of the Ukrainian Shield with the parallel development of methods for their rational use is emphasized.

Keywords

• paleoproterozoic
• ukrainian shield
• schungite
• schungite-bearing rocks
• high-carbonaceous rocks