Вестник МГТУ. 2019, Т. 22, № 1.

Сорохтин Н. О. и др. Первая находка алмаза на полуостровах Средний и Рыбачий. УДК 551.24+553.81 N. O. Sorokhtin, N. E. Kozlov, V. Yu. Kalatchev The first find of diamond on the Sredny and Rybachy Peninsulas in the north-eastern Baltic Shield The regular features of the region's spatial-temporal evolution have been discussed. Based on geodynamic analysis and the time scale of the crust formation in the eastern Baltic Shield, it is defined, that the most promising search areas for diamond-bearing rocks are intersections of deep lithospheric faults and discovered belts of possible diamond-bearing kimberlitic magmatism. The paper suggests the most promising search areas for diamond-bearing kimberlitic explosion pipes that can be united in the single diamond-bearing Norwegian- Mesensky belt. Prospecting for diamonds was carried out in the area of the Rybachy and Sredny Peninsulas in 2005-2007. Detailed decoding of large-scale aerial photographs and in situ measuring of fractures in the northern margin of the Sredny Peninsula allowed defining prospective intersection nodes of deep faults. Heavy mineral concentrate has been sampled here and indicated diamonds and associated minerals. They can suggest either native wash-out origins, or immediate scouring of Paleo-marine terraces that used to be rich in these minerals. No magmatic explosion pipes of the kimberlitic composition have been found here. It can be due to a lack of geological data on the region obtained by large-scale geophysical methods. The diamond finds have made it possible to recommend enhancing such investigations and paying more attention to this issue. We take into account that the Riphean sedimentary complexes of the Rybachy and Sredny Peninsulas are formations of the passive continental margin at that time and the watershed of the Musta-Tunturi Range is close to the study area. Based on that, we suggest that the matter transfer was not significant, just first km or, probably, first tens of km. Therefore, bedrocks of diamond-bearing explosion pipes should be prospected for close to the location of the diamond find. Key words: diamond, moissanite, chrome diopside, chrome spinellid, Rybachy Peninsula, Sredny Peninsula Article info: received 26.01.2018 Introduction The issue of prospecting for diamonds in the North-East of the Baltic Shield has been discussed for a long time [1-5]. This interest is quite reasonable, since back in the late IX century a diamond grain (or several grains, according to some sources) was found by Ch. Rabot (with Ch. Vellain, according to some sources) on the Paz River (Paatsjoki in Finnish, Pasvikelva in Norwegian). Noteworthy, further attempts at finding diamonds here or in the area adjacent to the north-western Murmansk region have failed. Researchers of the Geological Institute KSC RAS, including the authors of this paper, have studied and prospected the area with a full set of geophysical, lithological, mineralogical, petrographical and geochemical methods. Combined with results of geodynamic analysis, these methods allowed justifying potential diamond placers in the southern and south-eastern Kola Peninsula. Thus, new possible kimberlitic fields have been detected on the Tersky coast, i. e. the Pulongskoye and Snezhnitskoye fields [2]. After years of searching, just several diamond grains have been found here between the Pyalitsa and Babya Rivers [1]. Besides, one diamond grain has been identified in the central Kola Peninsula, using the heavy concentrate analysis o f a sample from the Elnjok river-bed deposits in the White Tundra area to the east of Lovozero. It has extended the area of possible diamond placers [2]. Yet, there were no reasons to include north-western parts of the region in this target area, and the find of diamonds near the Paz River remained underestimated. Based on general concepts [5], the authors of the work have doubted this approach and continued prospecting for diamonds in the north-west of the Kola region. Finally, the search has become a success, as described in this paper. Materials and methods To forecast areas of the diamond-bearing magmatism in the north-eastern Baltic Shield, the main task is to study general patterns of the spatial and temporal evolution of the region. In this regard, we should note that one of the latest peaks of tectonic activity in the study area was in the late Ordovician, about 480-450 Ma ago, when the Paleo-Atlantic Iapetus Ocean closed. At that time, the Grampian-Caledonian Geosyncline changed the regime of its development. The setting of subduction zones replaced the regime of the passive continental margin along the north-western coast of the European platform from Scotland, through Norway to the Spitsbergen Islands. In the early and middle Devonian (390-370 Ma ago), there was a collision between continental lithospheric plates that rimmed the Iapetus Paleo-ocean. By the early Devonian, the North-American continent had collided with the East European continent. As a result, a single continental lithospheric plate was produced. At the same time, the Caledonian fold belt formed, stretching from Scotland to the north of Scandinavia and Spitsbergen Islands and marking the continental collision line. The Caledonian Geosyncline Complex with ophiolites was pushed out of the geosyncline and thrusted over Archaean 36