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 Paleoproterozoic high- carbon schungite-like rocks of the Mlynkovsky area (Pravoberezhnyi district of the Krivoy Rog-Kremenchug structural formation zone). Their princi- pal structural, material and genetic similarities with stratified metamorphosed clay-siliceous schungite-bearing rocks of Karelia are established. A characteristic feature of schungite-like rocks of the Mlynkovsky area, that distinguishes them from other metamorphosed sediments of the Early Pre- cambrian 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 metamorphosed rocks in the Early Precambrian is of exceptional interest for recon- struction the primary sedimentatary conditions. Numerous biogenic formations (microfossils), which are closest (but not identical) in their morpho- logical characteristics to cyanobacteria of the genus 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 Mlyn- kovsky area with the schungite rocks of Karelia in many ways, 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, indicates that the shungite of Karelia is not a unique phenomenon. This serves as an indication of the possibility of identifying new manifestations of shungite-like rocks in Ukraine in significantly different geological situations that are not typical for 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.