2011

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ABOUT SPECIFIC FEATURES OF KIMBERLITE PIPES FORMED IN VARIOUS GEODYNAMIC CONDITIONS

N.N. Zinchuk

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

Mirny, Russia, nnzinchuk@rambler.ru

Geodynamic factors essentially effect the development of secondary processes both in kimberlite bodies in whole and in their individual parts. It is known that during the process of kimberlite magma intrusion a “breakthrough” of overlying rocks usually forms, which can change both the shape and the incline of contacts at different levels. Quite often paired shafts may arise. Depending on intrusion impulse, ratio of liquid and gas phases in magma, as well as composition of overlapping rocks, a various in size and aggregate condition zone of close to contact fragmentation emerges, accompanied by changing of quantities and sizes of fractures, as well as the direction of fissuring in the rocks surrounding kimberlite bodies. Heterogeneity of kimberlite bodies both along the vertical and lateral lines depends to a great degree on composition and properties of those rocks, including their various stability in relation to secondary (and iteratively secondary) alterations. Besides, kimberlite bodies in some cases undergo alterations under the influence of much later basic magma intrusion with formation of trappean sills, occurrence of which is accompanied by sharp increase of fissuring of not only surrounding rocks but of a kimberlite body itself. All these tectonic factors effect the hydrodynamic conditions first of all, stipulating subsequent transformation of kimberlite bodies. Intrusion of kimberlite magma, in the opinion of many researchers, took place in the result of sedimentary thick layer “shooting”. That is why not gases create channels, but hardening on its way portion of magma, performing the role of peculiar “shells”, which can total to several. They interchange with strongly compressed gas, ejections of which lead to dilution of lateral rocks, causing their collapses. Prior to breaking through sedimentary rocks the “shell”, pushed by gases, should pass through already existing fracture in the platform’s basement, emerged as a result of the fault’s tectonic activity. With disappearance of gas constituent further ascent of magma takes place under the pressure of the melt itself which infills (if it does not cool) the hole produced by the “shell”. After emission of gas, sharp pressure decay (hence the temperature) water solutions from hosting rocks rushed into the kimberlite body. As a result hydrothermas were generated, which were saturated with carbon dioxide, escaping from the cooling magmatic melt. Degassing of magma, as it is known, may last for whole epochs. Not only tectonic situation during the kimberlite melt intrusion but hosting rocks to a considerable degree effected the formation of modern rocks infilling explosion pipes. Thus, for example, in the Yakutian diamondiferous province (YDP) these were the hosting rocks that determined formation of carbonates and serpentine on kimberlite, but not more rich in silica stratified silicates (talc and smectite), as well as pyroaurite as a rock-forming mineral. Together with this at some sites and in individual bodies conditions for emerging of definite quantities of talc (at depth) and brucite (eastern body of pipe Udachnaya and individual blocks in pipe Yubileynaya and others) appeared. All this was specified by hydrodynamic conditions during formation and subsequent transformation of kimberlite rocks of each of these pipes. And all this together point to the fact that kimberlite bodies were formed as a result of repeated impact on initial rock of various factors which settled in minerals composing these bodies at present. Chemical and mineral specific features of kimberlite bodies were effected by: compositions of the intruded magma, xenoliths and hosting rocks, intensive impact of postmagmatic hydrothermal and exogenous processes, intrusion of the basic composition magma and others. It is very complicated to study the influence of all of these factors on kimberlite rocks, as this process is multidisciplinary. However during mathematical processing of chemical analysis data it is necessary to consider and apply information about geological setting of pipes, mineral composition and properties of kimberlites proper and components composing them.

Formation of kimberlite bodies, all their subsequent history is closely related with tectonic situation of each of existing kimberlite fields. The shape, size, and modern composition of kimberlite bodies depend much on specific geological structure of the territory. Especially great is the influence of tectonics on all subsequent processes which have transformed the rocks of kimberlite pipes to the modern state. All of them affected rocks via physical-chemical factors. Kimberlite bodies are usually composed by breccias (autolithic, tuff breccias, or simply by breccias) and tuffs. That is why by chemical composition they can not be compared to any volcanic rocks. Xenoliths, the content of which sometimes amounts to 20% and more, in most of kimberlites are small in size and practically can not be separated completely by a mechanical method when preparing samples for chemical analyses. Xenoliths are represented both by kimberlites of early generations and related to them mantle rocks (peridotites, lherzolites and others), as well as by rocks of crystalline basement and hosting sedimentary formations. The latter in many cases prevail over the rest of xenoliths, which complicates petrochemical investigations greatly. Significant contribution into the study of petrochemistry of kimberlite formations was made by geochemical investigations of elements-admixtures. As with petrogenic components there arose a number of difficulties which were caused by the following reasons: a) both the initial minerals and xenoliths underwent significant alterations, which could not fail to effect redistribution in rock of all its constituents: b) chemical composition (including elements-admixtures as well) depends not only on composition of initial magma but on xenoliths as well, which contain typical for certain types of rocks elements-admixtures. That is why chemical composition in many respects is determined by properties of elements proper, which in the process of initial rock reworking and further alterations can be mobile to various degree. As far as there are no quite inert elements, than it is important to study relative inertia, which can change with transition of rock to other physical-chemical conditions.

In the case of homogeneous ultrabasic rocks, with the purpose of defining the sequence of revealing individual phases of intrusion, variation analysis was developed, based on the ratios of titanium and titanium sum, ferric and ferrous iron which are constant for these rocks under definite PT-conditions. In many cases it is hard to apply this method successfully for kimberlite rocks of the Siberian platform because initial (unaltered or poorly altered) rocks occur in them very seldom, which allowed us dividing kimberlite formation into two sub-formations – kimberlitic and apokimberlitic (the number of secondary minerals often may reach 90-95% of the rock’s volume in the latter). Due to the mentioned difficulties more complicated calculations are used in petrochemical investigations of kimberlites in modern conditions, among which factor analysis is of significant importance as one of variation methods of rocks study. The factor analysis is carried out on the basis of correlation matrices with application of principal components’ method. As an example we shall try to determine some factors which correspond to individual formations in kimberlite bodies, utilizing at this the values of the first three factors total dispersion of which usually does not exceed 60%. For this purpose samples from three kimberlite pipes of Yakutia were selected for calculating correlation dependence. Thus, on one of the key horizons of pipe Udachnaya (190 m), basing on the bulk chemical analyses’ calculations two lines of components were received, responding to positive and negative factor loads. Water, oxides of magnesium, titanium, silicium, phosphorus, iron, and manganese respond to positive values. Oxides of carbon, sodium, potassium, sulfur, and aluminum are antipodes. The first factor characterizes serpentinization and magnetization of initial rocks, the second emphasizes their mica formation and carbonatization. Analysis of constructed by chemical analyses’ data of samples on one of the key horizons of pipe Udachnaya, dendrograms of correlative relations between rock-forming components (taking some rare components into account) allowed making the following conclusions: a) more close relation is observed of magnesia with water, and less close – with silica, which is caused by presence of also free hydrated oxide of magnesium – brucite, besides serpentine; b) correlative relations of secondary components were revealed, such as TiO₂ andP₂O₅ and more faint with MnO; c) ferric oxides are poorly related with the whole previous complex of components, which testifies about some independence of magnetite formation process on serpentinization of initial rocks; d) all other components are completely separated from the previous ones, which testifies about absence of direct relationship of their origination with brucite formation and serpentinization of initial rocks; e) correlation relationship of alumina with potassium oxide indicate to formation of micas by them, and lime with carbon dioxide – to formation of calcite by them. Sodium oxide, possibly, is related with sprayed about the rock shortite and weak connection of SO₃ and calcium testifies about formation of sulfates (gypsum in the first turn). In different way correlation relations are implemented between components in kimberlites of pipes Komsomolskaya and Krasnopresnenskaya, significant part of which underwent radical alterations under the influence of the intruded basic magma with formation of thick, nearly horizontal dolerite sills with much smaller apophyses. Calculations were made on samples selected along vertical for the whole kimberlite body and individually for the rodingite zone. Characteristic features of components distribution by correlation relations are as follows: a) like in pipe Udachnaya close relationship of calcium oxide with CO₂, which confirms formation of calcite by them; b) alkalis belong to the same packet, and rodingites also include P₂O₅ and water, to which though insignificant but still positive relationship with calcium carbonate is inherent; c) TiO₂ is related with Р₂О₅, and both – with pair MgO – water (concerning rodingite close relationship of titanium oxide with magnesia, as well as with silicium-iron-aluminous layer is observed). Thereby one can make the following assumptions for these two pipes: a) in spite of diopside formation the main mass of lime is still related with carbonate; b) during the process of rodingite formation phosphorus more gravitates to alkalis and deviates from titanium; c) alumina (argil) is already poorly related with alkalis (due to strong development of chloritization) and attaches to silica; d) definite relationship of ferric oxides with silicates exists; e) magnesium correlates with water, however in close to contact formations one can observe its more close relationship with titanium. Separation of phosphorus from titanium is the main thing in dendrogram of rodingite in comparison with the same for rocks composing a kimberlite body into the whole. High correlation relations of sulfide sulphur with alumina are explained by transformation of mica into chlorite with formation of iron, pyrite, and other compounds at the expense of liberation from their structure. Mica and especially chlorite are low-stable minerals in comparison with magnetite and ilmenite. Factor analysis makes it possible to reveal general features of initial rocks and trace their alteration in the process of all postmagmatic transformation. All this is registered according to the change of correlation characteristics which reflect to significant degree the behavior of each of rock-forming and rare components in new physical-chemical conditions, in which kimberlite bodies occur after intrusion of magmatic melt.

Key words: kimberlite pipes, magma, geodynamics, petrochemistry, ultrabasic rocks, correlation, factor analysis, postmagmatic and hypergene alterations.