Scientific publications

The article presents the results of a field and mineralogical study of a large pegmatite lens identified in the upper part of the Paleoproterozoic (1.77–1.75 Ga) Ropruchey gabbrodolerite sill (Kokkarevo area, Onega Basin, Karelian Craton). The pegmatite lens is about 3 m thick, exhibits differentiated internal structure, and represents a typical example of pegmatite bodies developed at the second sill level of the Ropruchey gabbrodolerite complex. Mineralogical mapping using optical and scanning electron microscopy with EDS analysis revealed a consistent succession of mineral assemblages: from contact metasomatites through K-feldspar-albite pegmatites to albite-quartz central zones with myrmekites, and terminating epidote-quartz miarolitic cavities. Analysis of mineral composition variations in both pegmatites and host gabbrodolerites allowed us to propose a model for the evolution of the fluid magmatic system: from crystallization of the gabbroic melt through emplacement of the fluid-saturated pegmatite melt and metasomatic reworking of the contact zone, followed by hydrothermal REE-F-Cl mineralization. The obtained data significantly refine the formation conditions of pegmatites in Paleoproterozoic mafic intrusions of the Onega Basin, and confirm the primary contamination of the mafic melt with substantial enrichment in H₂O, F, and Cl. High chlorine contents up to 0.5 % are recorded in amphiboles from both albitite zones and the gabbrodolerites themselves. Further evolution of the intrusive complex proceeded in a closed magmatic system, while the discovery of parisite and allanite indicates REE specialization of the final crystallization stage. The dates are relevant for understanding the mechanisms of rareelement concentration in fluid-magmatic systems of mafic composition.