Cryogenesis

As known, all mineral formations are subdivided into endogenous (magmatic and metamorphic) and exogenous (or sedimentary). Among the latter the following groups can be distinguished in accordance with conditions of formation and stages of lithogenetic process: weathering, transfer, continental sediment accumulation, basin sediment accumulation and aftersedimentation transformation of deposits (10). Each of these groups can be further subdivided into classes of mineral deposits (according to the origin of the mineral inclusions or by the nature of the rock-forming process) and subclasses (by the mechanism of the formation of the deposits).

Mineral formations of the weathering group are directly connected with development of relatively thin weathering crusts of cryogenic type and provide mineral masses for a number of other types of mineral deposits (placer, sedimentary and the like). The mineral formations of this group are a result of physical, chemical and biochemical weathering of bedrock and washing by surface and ground water; or of redeposition of part of these products.

Eluvial sources of mineral deposits of this group comprise the residual and redeposited products of physical weathering, the intensity of which exceeds that of chemical weathering in conditions of cryolithic genesis. Physical weathering gives rise to the formation of thick rock waste-fine earth and block-fragmental deposits, block eluvium and sandy-aleurolite deposits. On one hand, these are deposits suitable as construction materials and widely developed in the permafrost regions (rock waste, sand, very silty sandy, and clay rich materials for wall, foundation and road construction, etc.) and glass-ceramic raw material (sand, clay); on the other hand, there are eluvial placer deposits of cryogenic type. Eluvial placers of gold and diamonds, cassiterite-columbite, platinum and the like which may be formed with the participation of the cryogenic factor, are found at present in the permafrost regions and are intensely transformed under the influence of cryogenic processes.

Presumably one can describe the deposits of iron, copper, uranium etc., of the permafrost regions, which are highly typical of humid conditions, as infiltration deposits. A key role is played by geochemical and mechanical barriers, i.e. situations that hamper the migration of chemical elements thus leading to their accumulation and concentration.

The specific nature of weathering in the permafrost regions manifests itself in the processes of near-surface change of mineral deposits. For instance, according to N.A. Shilo (21), transformations of sulphide deposits in the permafrost zone have various distinct features. Thus, with substantial changes of the profile of the weathering crust, the 'iron cap' has a smaller thickness than in similar conditions outside the permafrost regions. The presence of hydroxides and hydrosilicates with relics of primary sulphides is typical of the profile and this is evidence of the underdevelopment of the crust and stagnation of the weathering processes at an intermediate stage. Due to the restricted water exchange and slow leaching of sulphates they are present in greater quantity to depths of hundreds of metres in the north-east regions of Russia, Yakutia and the Kola Peninsula (Kol'skiy Polvostrov).

The sources of mineral deposits of the transfer and continental accumulation group of sediments are most strikingly represented by placer deposits, the formation of which occurs with accumulation of ore minerals in the coarse-grained deposits during displacement of the weathering crust material.

Talus and proluvial deposits form as a result of downslope movement of loose fragmental material under the influence of gravitational forces, thermal erosion, solifluction, mudflows, rock streams and the like. As with eluvial placers, frost heaving of fragmental material can take place in these deposits thus causing nonuniform distribution of valuable minerals across the section. However, commercial-scale concentrations of mineral deposits in the talus and proluvial placers of the permafrost regions have not been found.

As shown in the works of N.A. Shilo, the cryogenic type of lithogenesis is characterized by displacement of the concentration zone of placer deposits from slopes towards rivers and seas - which is the reason for the wide development of alluvial placers within the limits of the permafrost regions (placer deposits of Aldan, Yakutia, Chukotka, etc). The alluvial placers known at present are predominantly concentrated and confined to valleys of the northern rivers, while beyond the permafrost regions they are less distinct.

Of a certain importance among placer deposits are glacial placers, associated with morainic or fluvioglacial deposits. Some diamond-bearing and metalliferous glacial deposits are worked on a commercial basis. So are the gold-bearing moraines of Alaska, diamond-bearing moraines of Brazil, fluvioglacial placer of gold in New Zealand, platinum placers of Canada, etc. Among biogenic deposits of the permafrost regions, the most developed are peats, usually poorly degraded, with well-preserved vegetation residues and peaty or humic sandy silty to clay materials, and deposits of bogs and marshlands. These deposits, as a rule, are characterized by a higher content of hydrogen compounds (methane, hydrogen sulphide), vivianite and bog ores.

The group of sedimentary mineral deposits is subdivided into lacustrine, marine and oceanic. Their formation occurs in different ways: mechanically, physically, chemically and biochemically. Mechanically formed sedimen tary deposits are mainly represented by gravel, sand and clay. A major role in the accumulation of sediments within the permafrost regions is played by terrigenous material and this serves as a source of construction material in different deposits. Of great practical importance are lacustrine and marine sands of monomineral and polymictic varieties. Marine sands are well graded, homogeneous and useful for construction purposes. Sandy sediments in lakes are limited, being represented by discontinuous bands and lenses of littoral sands. They are poorly graded and to a significant extent covered with silt.

Chemical-biological sedimentary deposits of lakes are rather widely developed in the permafrost regions, since, for instance, there are some 400 000 lakes over the territory of the European Russian North, characterized by a cryogenic type of lithogenesis. Among the clay minerals of lacustrine sediments (both recent and fossil lacustrine-glacial varved clays) the most developed are hydromicas, chlorite and mixed-laminated formations of the type mica-montmorillonite and chlorite-montmorillonite. The area of montmorillonite development is often the largest. Principal types of the recent formations in lakes are peaty and algal sapropels. Among iron forms in sediments ferrous iron prevails; its increased content is associated with simultaneous increase of residual organic matter. Lacustrine (Fe-Mn) and lacustrine-bog ores are widely developed and typical of the lacustrine deposits of the European Russian North.

Typical lacustrine ores (concretions of crustal, globular and irregular shape) are characterized by a significant manganese content (over 4%) and iron (up to 10%) and are mainly represented by iron hydroxides with inclusions of manganese hydroxides. The size of Fe-Mn nodules does not remain constant, varying both laterally and vertically. Usually, manganese and C02 content increase towards the top of the section and in the direction of shallow water, taking the form of manganese hydroxide and calcium rhodochrosite - manganous calcite. Iron and phosphorus in this case are concentrated in the lower parts of the section.

Typical lacustrine-bog ores, the formation of which is associated with acidic water of peats, have smaller concretions, massive structure, higher iron content (up to 50%) with insignificant content of phosphorus (up to 2%). Mineralogically, they are represented by iron hydroxides and ferriferous vermiculite-montmorillonites. Sometimes, concentrations of vivianite crystals are observed.

Chemical and biochemical sedimentary deposits of the northern seas and oceans are practically unexplored. The specific nature of their formation is likely to be associated with the low positive temperatures of the bottom sediments as well as higher solubility and washing out of part of the carbonates and authogenous silicates. The main minerals of clays of both recent and Pleistocene bottom sediments of the polar seas are hydromicas and montmorillonite.

In the course of diagenesis of marine sediments a substantial change of clay minerals is observed (beidellitization and montmorillonitization of hydromicas) as well as formation of a complex of authigenic minerals which are further redistributed and become concretions at certain local sites. According to the investigations presented by I.D. Danilov (7), in such cases iron sulphides, vivianite, iron-manganese compounds are formed with insignificant amounts of carbonates. Vivianite concretions, as a rule, have a ball shape, and a rough hummocky surface and diameter up to 5 mm. Their occurrence gives evidence of higher organic matter content of sediments and of the presence of a very reducing medium at the stage of diagenesis. Fe203 content in vivianite concretions is as high as 40%, while that of P2Os, 20%. Concretions of iron sulphides are typically oval, globular and ellipse-like shapes with diameters up to 10 mm. The core of concretions is usually pyrite surrounded by black amorphous matter consisting of colloidal ferric sulphide (hydrotroillite). The chemical composition of iron sulphide concretions of polar basins differs greatly from that of concretions occurring in warm-water sea ooze. In cores of all concretions Si02 content is high (60%) at the expense of quartz admixtures. A1203 content is on the average about 4% varying between 1 and 9%, that of iron sulphide varies between 9 and 25%, and sulphur as sulphide from 10 to 30%. The diffused organic matter admixture makes up 0.5 to 1% on the average. The character of these concretions is evidence of the oxidizing medium maintained by diffused organic matter. Iron oxide and iron-manganese concretions are encountered in the recent bottom ooze of polar seas as well as in coastal marine sediments (recent and Pleistocene) with sandy and sandy-gravelly composition, having small amounts of degrading organic matter. Usually, concretions have a globular shape, small size (2-5 mm) and are more or less uniformly distributed without substantial concentrations. The Fe203 content in iron-manganese concretions varies between 4 and 19%, and MnO between 2 and 18%.

The group of mineral deposits associated with post-sedimentation cryogenic transformation can be subdivided into deposits of epi- and syn-cryogenic classes which correspond to the basin and continental epi-cryogenic and continental syncryogenic and palaeocryoeluvial strata of sedimentary rocks.

The formation of cryogenic deposits proper, of cryolites, is noteworthy and among them the most widely represented are structure-forming ice, recurring ice wedges and injection and massive ice beds. An important feature of the freezing and formation of sedimentary deposits of epi-cryogenic strata is the occurrence of crystalline hydrates (calcite, mirabilite, gypsum, etc.) which precipitate at negative temperatures as poorly soluble compounds, as well as the formation of a variety of authigenic minerals and monoxide compounds (pyrite, siderite, marcasite, hydromica, montmoril-lonite, vivianite, carbon dioxide, methane, etc.). Cryopegs and gas hydrate deposits are also very typical of the permafrost regions.

Cryopegs are highly salinized underground waters having negative temperatures, which often lie below the base of the permafrost, although there are other types: intra- and above-permafrost cryopegs. Such subsurface water is formed as a result of downwards exclusion at freezing of easily soluble compounds with formation of thick zones containing sodium chloride (more rarely calcium and magnesium chloride) brines. An intensified concentration occurs below the freezing front - cryogenic concentration. The thickness of the cryopeg layer varies from several tens to several hundreds of metres, and salinity of the water ranges from 30 to 300 g l"1. Cryopegs are widely developed in the shelf area of the northern seas, the arctic islands, the Siberian platform, etc.

Gas hydrates or gases in the hydrate form belong to the solid crystalline hydrates and are represented by compounds of natural gases and water and have a complex structure. Their formation requires a certain range of temperatures and pressure, which creates conditions for the interaction between gas and water molecules such that there occurs a specific grid of crystalline hydrate with transition of the compound into the solid hydrate state. This process often leads to large accumulations of gas hydrate deposits. Beyond the permafrost regions gas hydrate deposits occur at a depth often exceeding 1 km. In the permafrost regions they lie below the base of the permafrost and may occur within the permafrost sequence. A tentative estimate of possible depth of occurrence of natural gases in the hydrate state (P-t-H diagram) is presented in Fig. 6.4. As shown in the diagram, within the limits of the permafrost regions the range of depth for gas hydrates is dependent on the thickness of the frozen zone, since permafrost reduces the temperature of the underlying unfrozen rocks. Recently drilling of boreholes in West Siberia showed widespread shallow deposits of gas hydrates. Usually gas deposits arise in the presence of gas hydrates even without a lithologically impermeable cover. This is associated with the fact that gas hydrate concentrations usually occurring in the roof of gas deposits (nearer to the permafrost) serve as an impermeable barrier for stream-like leakage of p 1CT5, Pa

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