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Fig. 6.4. Determination of the interval of probable deposition of natural gases in hydrate state (according to G.D. Ginzburg and Ya.V. Neizvestnov): 1-3 - equiweighted curves for formation of natural gas hydrates with different densities (1 -0.555 gem-3,2-0.6gem"3, 3 -0.7gem-3) with coordinates of pressure and temperature; I-V-distribution of temperatures over depth H for the areas of the Yenisei-Khatanga trough with varying thickness of the frozen layer.

free gas from the underlying deposits. Therefore, the zone joining the permafrost regions to the ocean has favourable conditions for the formation of gas hydrate covers and often serves as a reservoir accumulating large volumes of natural gas.

The syncryogenic class of mineral deposits of sedimentary rocks is most clearly represented by loess and loessified formations, with a higher content of carbonates, a prevailing concentration of secondary silty particles and predominance of limonite iron among hydroxide compounds. Loess rocks and the covering layer of silty-clays similar to them have a practical interest as a raw material for glass and ceramics production. They are developed in the course of formation of paleocryoeluvial strata. The syncryogenic rocks themselves have a higher content of cryolites (in the form of thick strata of syngenetic recurring ice wedges, several metres wide at the top and several tens of metres high), structure-forming ice and ice-cement, and also having an intense accumulation of humic matter across the whole section of the sequence with predominant concentration of iron monoxides, carbon dioxide and different hydrogen compounds.

As a conclusion it should be emphasized that the particular features of formation of mineral deposits in the permafrost regions have not been explored adequately and require further long-term investigations.

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