Early Proterozoic diamond-bearing kimberlites of Karelia and their formation peculiarities

Abstract

Early Proterozoic kimberlites of Karelia are among the most ancient diamond-bearing primary source rocks in the world. They compose the large (2.0 × 0.8 km) Kimozero body localized in the predicted Zaonezhskoe kimberlite field. The established and assumed occurrences of kimberlite magmatism are located within the Karelian craton, which was stabilized during the Early Archean. They are confined to the central part of a large geophysical anomaly detected by gravity, magnetic, seismic, and heat-flow studies and mark a deep-seated magma chamber. Kimberlite bodies occur within structural blocks bounded by zones of plicative-rupture dislocations. The Kimozero kimberlites form an extensive but thin saucer-like body cut by narrow quasi-cylindrical feeders and dikes. It consists of metamorphosed kimberlites, their breccias and tuffs with widely varying amounts of mica. The body includes fragmentary fine-layered crater formations. The rocks contain olivine and phlogopite phenocrysts in an extremely altered groundmass of serpentine, chlorite, calcite, mica, and ore minerals as well as indicator minerals of kimberlites, such as Cr-spinel, manganiferous ilmenite, Cr-diopside, and rare pyrope. About 100 diamonds were extracted from 12 samples (total weight 815 kg). The crystals are colorless resorbed octahedra and, more seldom, combined octahedra-dodecahedra and spinel twins with abundant green spots caused by natural irradiation, which often make the whole crystal surface green. The diamonds contain inclusions of Mg-rich orthopyroxene and pentlandite suggestive of peridotitic lithospheric mantle derivation and dating of the sulfide inclusion implies a late Archean mantle source. By petrochemistry, the rocks are classified as kimberlites. The Kimozero kimberlites differ from classical Phanerozoic ones in having higher Fe contents, low contents of alkalies and P2O5, and intense superimposed carbonate, magnetite, and amphibole mineralization. The saucer-like bodies with narrow feeders without developed diatremes have no analogs in Russia but are similar to the saucer-like kimberlite bodies in Canada (Fort a la Corne), India (Tokapal), and Central Africa (Bakwanga) and the West Kimberley lamproites in Australia. By analogy with these bodies and on the basis of some common petrographic features (presence of pyroclastics and specific amoeba-like autoliths, scarcity of fragments of the enclosing rocks, local reworking of the deposited matter), the Kimozero kimberlites are considered to be the products of subaerial volcanic central-type eruptions.

Diamond Exploration Potential of the Northern East European Platform

“…The kimberlites nearest the 'L' and 'S' areas, i.e., the Kimozero kimberlites, cannot be regarded as a possible source for the KIMs because of the almost complete absence of pyrope and Cr-diopside in the Kimozero kimberlite and the limitations imposed by the Onega Lake area on the Kimozero KIM distribution [48]. Also, KIMs obtained from the 'L' and 'S' area samples cannot have the same source due to geomorphologic differences.…”

Radio-colouration of diamond: a spectroscopic study

“…Diamond specimens with surficial spots, or even coats, of green or brown radio-colouration, are found in many kimberlites and alluvial deposits (Meyer et al, 1965;Vance et al, 1973;Wang et al, 2006;Rubanova et al, 2009;Ustinov et al, 2009). The green colouration is generally interpreted as being due to alpha radiation emanating from radioactive substances (i.e., minerals or fluids) located in direct contact with, or in close proximity to, the diamond surface.…”

Paleoproterozoic Kimozero kimberlite (Karelian Craton): Geological setting and geochemical typing

“…The orientation of the bedding provides unam biguous evidence for the anticline character of the structure, which allows us to reconsider an earlier model [6], according to which kimberlite as the "major layered body" fills the ring mulde narrowing towards the periphery and in the center.…”