Vol 10, No 3 (2016)

Arguments in favor of magmatic or metasomatic genesis of the Katugin rare-metal ore deposit are discussed. The geological and mineralogical features of the deposit confirm its magmatic origin: (1) the shape of the ore-bearing massif and location of various types of granites (biotite, biotite–amphibole, amphibole, and amphibole–aegirine); (2) the geochemical properties of the massif rocks corresponding to A type granite (high alkali content (up to 12.3% Na₂O + K₂O), extremely high FeO/MgO ratio (f = 0.96–1.00), very high content of the most incoherent elements (Rb, Li, Y, Zr, Hf, Ta, Nb, Th, U, Zn, Ga, and REE) and F, and low concentrations of Ca, Mg, Al, P, Ba, and Sr); (3) Fe–F-rich rock-forming minerals; (4) no previously proposed metasomatic zoning and regular replacement of rock-forming minerals corresponding to infiltration fronts of metasomatism. The similar ages of the barren (2066 ± 6 Ma) and ore-bearing (2055 ± 7 Ma) granites along with the features of the ore mineralization speak in favor of the origin of the ore at the magmatic stage of the massif’s evolution. The nature of the ore occurrence and the relationships between the ore minerals support their crystallization from F-rich aluminosilicate melt and also under melt liquation into aluminosilicate and fluoride (and/or aluminofluoride) fractions.

It is shown that the granitoids of the Main Kolyma Batholith Belt do not penetrate into the Upper Jurassic deposits of the Yana–Kolyma mesozoids. The applicability of U–Pb SHRIMP zirconometry to the Mesozoic period of geological history is subject to critical analysis. It is concluded that analytical limitations preclude the solution of the concordance–discordance alternative of the obtained datings in the Mesozoic U–Pb isotopic systems by computational approaches. The U–Pb isotopic system is established as highly sensitive to the superimposed processes. Based on the reconciliation of the U–Pb, Rb–Sr, Ar–Ar, and K–Ar geochronometric data, the granitoids of the Kolyma Main Batholith Belt intruded 170–160 Ma ago, while their isotopic systems were transformed 150–140, 135–125, and 100–80 Ma ago. The local SHRIMP zirconometry combined with other isotopic methods can be applied to date igneous rocks by the relic dates, to set the time of intrusion into their isotopic systems, and to predict thermal events not identified yet. The U–Pb SHRIMP zirconometry should not be considered as arbitrary.

Comprehensive lithogeochemical analysis is carried out for Norian sediments from the central part of the Kular–Nera shale belt. The sediments are represented by alternation of sandstones and siltstones. Petrochemical typification shows that the sandstones are subdivided into graywacke, arkose, and subarenite, while the siltstones belong to shale. The Norian sediments were mainly deposited in an oxidizing environment of a well-aerated basin in the distal parts of the Verkhoyansk passive continental margin. They are characterized by a high sorting of sedimentary material. The correlation between the rock-forming elements and the wide range of (La/Yb)_n ratios indicate that the sedimentation basin contained the disintegration products of both acid [(La/Yb)_n = 10.52–27.26] and mafic [(La/Yb]_n = 6.22–8.96)] rocks. The high values of the La/Sc and Th/Co ratios are typical of acid rocks. The low K₂O/Al₂O₃ values (0.16 ± 0.02) and high Zr/Sc ratio (10.1 ± 2.74) are typical of redeposited material. It is established that several provenances were involved in the formation of the Norian sediments in the central part of the Kular–Nera shale belt. Multicomponent clastics and well-sorted recycled material were supplied into the sedimentation basin.