2011

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SPECIFIC FEATURES OF SOME MINERALS’ TRANSFORMATION IN THE PROCESS OF KIMBERLITES ALTERATION

N.N. Zinchuk

West-Yakutian Scientific Center of Republic Sakha (Yakutia) Academy of Sciences,

Mirny, Russia, nnzinchuk@rambler.ru

In our previous works we generalized original and literary material on geological structure, material composition, and diamondiferousness of the greater part of primary diamond deposits of kimberlite type. In spite of sufficiently detailed study of geology and material composition of these unique rocks, alteration specific features of composing them minerals both in hydrothermal and hypergene conditions are investigated poorly so far. During alteration of kimberlites both rock-forming minerals and mantle components contained in rocks undergo sufficient transformations. We studied in more details the behavior of these minerals in the course of kimberlite rocks’ weathering (erosion), which to some extent is similar with their transformation under the action of pneumatolytic-hydrothermal processes. Earlier performed investigations revealed increase in the kimberlite crust of weathering of a number of diamonds with blue coloring or covered with film brown pelitomorphic neoformations, differing by structure and composition from kelyphite rims on other primary minerals, on pyrope in particular. Investigations of recent years also indicated, in the course of kimberlite rocks’ weathering, significant instability of basic associated to diamond minerals: pyrope, picroilmenite, pyroxenes and olivine. Olivine and pyroxenes in greatly altered kimberlites are practically completely replaced by secondary formations (serpentine and montmorillonite), garnets of pyrope row corrode greatly; picroilmenite is less altered, and chrome-spinellids are most stable. Regularities of pyrope and picroilmenite alteration have been studied in more details by us with the help of a complex of modern research methods on the crust of weathering section of pipe XXIII-rd CPSU Party Congress in Malo-Botuobinsky diamondiferous region of the Siberian platform.

By chemical and structural-energy properties diamond accessory minerals represent a group of the most abyssal parageneses. Thermodynamic limits of their stability (by temperature – to 1200° C, and by pressure – to 40·10⁸Pa) provide the ability of broad isomorphic miscibility. Specific conditions of genesis and evolution of kimberlite melt up to the moment of kimberlite pipe formation determine high degree of imperfection for all primary minerals. Thus, for pyrope, block structure of crystals is typical. Structural etching of polished surfaces of oval pyrope grains indicated the existence of epigenetic zoning by imperfection degree of structure – they are more imperfect along the periphery and perfect in nucleus. Zoning is clearly expressed in most deep-seated pyropes of various hues of violet, and it is often not traced in less deep-seated orange pyropes. Block structure and increased imperfection of the whole grain are typical for the latter. In whole, imperfection increases in the row of violet – red – orange pyropes. In rare cases pyropes with strips of plastic deformation occur. In kimberlites there are practically no pyrope grains without fissures, the degree of fissuring herein increases from violet pyropes to orange ones and is related with their alterations during postmagmatic stage. Imperfection (faultiness) is also increased by structures of solid solution disintegration, syngenetic inclusions of chlorite, picroilmenite, olivine, pyroxenes, and other phases creating a stress field in matrix around them due to variation in thermal dilatation coefficients. Picroilmenite, being one of the most common minerals of kimberlite, is paragenically related with orange pyropes and characterized by extremely high degree of structure imperfection. Possessing excessive energy, related with structure defects, part of picroilmenite grains undergoes recrystallization in favorable conditions, as a result of which equilibrium aggregate of grains’ faultlessness forms. Since this process rarely comes to the end, inequigranular aggregates occur with complicated intergranular boundaries. The latter are concentration locations of all possible defects in crystals and therefore possess excessive energy. Structures of solid solution disintegration and inclusions of diverse phases should also be referred to defects of picroilmenite structure.

In the zone of hypergenesis, where conditions for volume diffusion are practically absent, diffusive phenomena come to the first plan, related with defects and imperfections of lattice, since the diffusion activation energy on them is significantly lower than the volume diffusion activation energy. One- and two-dimensional defects of the lattice, to a smaller degree of stress field around three-dimensional defects, serve as conductors of mineral material for penetration into ambient medium. And this diffusion determines morphology of corrosive surfaces: channels and fissures of etching, depressions and caverns, positive block relief, which forms during etching of boundaries between blocks, and others. Formation of trachytic (hair-like) channels of etching and much larger tapered depressions may be considered characteristic feature of pyrope weathering. They are simultaneous structural failures – dislocations, and owing to excessive energy provide increased migration of crystal material over them. Formation of tapered depressions is related by us with processes of polygenesis occurrence during annealing in magmatic melt. These processes lead to prevailing concentration of dislocations in peripheral part of oval pyrope grains, while the internal part remains low-defective. Such zoning is clearly distinguished for garnets from kimberlite crust of weathering and less – for grains of the mineral, altered by pneumatolytic-hydrothermal processes. Corrosive fissuring of primary minerals, related with adsorption decrease of their stability (resistance), is another characteristic specific feature of weathering. Garnets in the crust of weathering are more fissured than in kimberlite, that is why their significant part goes to fine particle classes during processing of samples. In the zone of hypergenesis the so called cuboids are formed, having usually knobby surface and pyramidal relief, emphasizing their block structure. Increased faultiness of orange pyropes and more regular distribution of dislocations in the volume of crystals make them less stable to weathering than violet and red, which leads to depletion of pyropes’ color gamma in upper parts of weathering profiles in comparison with compact kimberlite.

Owing to much greater resistance to agents of weathering picroilmenite grains corrode insignificantly, corrosive relief is observed only on total reflection from close to pinacoid surfaces. As a result of rather high faultiness of most picroilmenite grains their corrosive relief becomes random. Contrast corrosive relief often develops here, resembling emery during intensive development. Close to pinacoid surfaces are more resistant to corrosion and flat oval and trigonal pits form on them. Such picture of etching stresses anisotropy of dislocations distribution and their concentration in parallel to pinacoid layers. Effect of adsorption decrease of resistance (stability) results in chipping of finest scales from the surface of grains. Recrystallized picroilmenite aggregates behave in somewhat different way in the process of weathering and secondary transformation. In the process of alteration such grains corrode uniformly, their corrosive relief is clear, reflecting the surface symmetry, sculptures are distributed evenly. In whole picroilmenite aggregates are less resistant to processes of both hydrothermal and hypergene alteration than monolithic crystals. Besides this stability of picroilmenite sufficiently depends on the size of the grains which undergo alterations. Quite often leucoxene is formed on the surface of ilmenite grains, the quantity of which significantly increases in the upper part of weathering profiles.

Thus, carried out by us investigations revealed different degree of mantle minerals’ resistance to processes of hydrothermal-pneumatolytic and hypergene alteration. Basic primary minerals of kimberlites by degree of resistance (stability) increase form the following row: pyroxenes - olivine – pyrope – picroilmenite – chrome-spinellids. Stability in this row is related with structural-energy specific features of minerals, for various genetic groups therewith it is determined by character and degree of structure faultiness. Pneumatolytic-hydrothermal and hypergene transformation of kimberlite primary minerals occurs by way of diffusion material removal along dislocations and dislocation block boundaries, as activation energy for them is significantly decreased. Broadly developed at this are events of adsorption stability (resistance) decrease, resulting in dispersion of crystal material and respectively to increase of transformation (erosion) rates. Enrichment of upper horizons of kimberlites’ crust of weathering by violet garnets is related with various resistance of different color varieties of a mineral during this process. Increased stability of picroilmenite and chromespinellids relative to pyrope results in alteration of their ratio for benefit of ore minerals in crust of weathering formations in comparison with unweathered kimberlites.

Key words: kimberlites, mantle minerals, pneumatolytic-hydrothermal and hypergene processes, stability (resistance), imperfection (faultiness), corrosion.