Structural characteristics of the Kimamyaki graphite ore occurrence (Karelia, Northern Ladoga region)

The ore body of the Kimamyaki graphite occurrence, exposed by an old mine working, is a zone of intense shearing and carbonization of the Lower Proterozoic Sortavala series metavolcanics. Application of tectonophysical methods revealed numerous shear-type parageneses of subplanar structural elements (PSSEs). Their presence convincingly proves that the ore-controlling structure was shaped by deformational shear. The PSSEs were formed under various deformation regimes and, judging by the variability of shear angle values, under variable thermodynamic conditions. This indicates that the orecontrolling structure was developing with interruptions over a prolonged period, with the tectonic settings changing over time. The similarity between the PSSEs at the Kimamyaki ore occurrence and in the early orogenic rocks of the Kaalama clinopyroxenite- gabbronorite-diorite complex (not older than 1.9 Ga) intruding the Lower Proterozoic Sortavala and Ladoga series deposits suggests the ore-controlling structure was initiated in the post-Kalevi time and formed in the Svecofennian (1.95–1.85 Ga BP) and, possibly, post-Svecofennian (1.8–1.7 Ga BP) periods of tectono-metamorphic reworking. The multiple activation of most subplanar anisotropy elements of the rocks, changes in their mineral composition, and, above all, intense graphitization in the shear dislocation zone indicate a high fluid permeability of the ore-controlling structure and its feeder role. The isotopic composition of carbon from the Ladoga graphites points to its endogenous origin, mantle source and graphite minerogenesis through pyrolysis of juvenile hydrogen-methane fluid. The structural and developmental features of the ore controlling structure, its being epigenetic from the host rocks, and the isotopic composition of carbon in the graphites corresponding to that of the mantle suggest that the main factors responsible for the discrete localization of graphite in heterogeneous geological formations of the North Ladoga structural-facies zone were shear deformation and fluid carbonization.