2010

Comparative Characteristics of Kimberlites of Various World Platforms

Zinchuk N.N.

Western-Yakutian Scientific Center of Republic Sakha (Yakutia) Academy of Sciences, Mirny, Russia;

nnzinchuk@rambler.ru 

     Kimberlite rocks – is a complicated heterogeneous system consisting of various proportions of mantle and crust material. Mantle material is represented by association of deep-seated minerals and xenoliths of mantle rocks, as well as by melted part of mantle material which composes mesostasis. Xenogranules and xenoliths of hosting rocks, as well as re-entrained from them dissolved material form the crust constituent. African geologists divide kimberlites into two groups. The first one has alkalescent composition, contains little titanium and other incoherent elements. The second group is characterized by increased content of potassium, titanium and rare earths. Usually these are micaceous rocks, containing fine grains of clinopyroxene and К-richterite within mesostasis. The number of kimberlite bodies of the second group on the African platform is increased. Kimberlites of this group are not revealed on the Siberian and East-European platforms yet. Broad variations of mineral composition are inherent to kimberlites of the Siberian and African platforms, while diamondiferous kimberlites of East-European platform have more common composition.

     Kimberlites differ from lamproites by decreased content of titaniferous phlogopite, potassium-bearing richterite, the finest isolations of clinopyroxene (diopside), as well as such rare minerals as wadeite, priderite and some others. Apatite and chrome-spinellid П are present in mesostasis of lamproites in increased quantities, and in more basic varieties leucite is revealed as well. Between kimberlites and lamproites there is gradual conversion via intermediate differences of diamondiferous rocks – kimberlite-lamproites. Kimberlite-lamproites have been revealed by us from commercially diamondiferous targets in pipe Madzhgavan (Central India). The rock of the said pipe differs from typical kimberlite rocks by increased content of titanium, potassium and phosphorus. High-titaniferous phlogopite and anatase serve as the source of titanium in kimberlite-lamproite rocks of pipe Madzhgavan; phosphorus is concentrated in the finest grains of apatite, and potassium – in phlogopite. From classical diamondiferous lamproites the rock of pipe Madzhgavan differs by decreased content of potassium and absence of rare typomorphic minerals – priderite, potassium richterite and some others.

      Our earlier studies have shown that the hosting kimberlite bodies’ environment (composition and mechanical properties of hosting rocks) plays an exclusively large role in formation of the modern kimberlites image. Investigation of material composition of kimberlites occurring in environment with various chemical-mechanical properties makes it possible to distinguish several basic material-indication models of diamondiferous vulcanites and a whole number of petrochemical factors, which have been formed with participation of hosting diatremes rocks. The first type – hosting rocks are composed by terrigene-carbonate sedimentary complexes. Kimberlite rocks in them are subjected to strong carbonatization of xenogenous and infiltration nature. This type of kimberlites is typical of the Siberian platform and serpentine and calcite are the main rock-forming minerals in them. Depending on mineralization of buried waters it is possible to distinguish two subtypes within this type: the first one is developed in conditions when hosting rocks contain high-mineralized waters of brines type, and the second one – when waters are poorly mineralized. The first subtype is general in the central part of the Siberian platform, and the second one is common in all kimberlite regions and fields of the northern part of the platform. The distinction features are concluded in less intensive development of pneumatolytic-hydrothermal and hypergene mineralization and significantly smaller number of mineral phases in the second subtype. The second type unites kimberlites, hosting terranes of which are composed by poorly lithified argillaceous-quartz siltstones and sandstones. A lot of disintegrated xenogenic material represented by quartz and argillaceous constituent is present in composition of kimberlites. Instead of traditional carbonatization and serpentinization in kimberlites broadly developed is saponitization. This type is common for kimberlite diatremes of East-European platform. The third type model – well lithified sandstones, siltstones, argillaceous shales or igneous and metamorphic formations: granites, norites, dolerites, felsites, gabbro, crystalline shales and gneisses, serve as hosting the pipes rocks. Kimberlites in this type are relatively poorly altered; increased quantity of “fresh” olivine of both the first and the second group is present; poorly altered well devitrified basic mass is preserved. This type of kimberlites is reported on the greater part of African continent pipes. Here kimberlites are less of all contaminated by debris of hosting rocks, and therefore they are more close to initial composition, though among African kimberlites one can frequently observe rocks with increased content of silica due to assimilation of hosting acidic (sour) formations. Lamproites of Australia can be referred to the fourth petrochemical model, which is characterized by high concentrations of potassium and titanium together with decreased ferruginous and increased magnesia properties. The source of lamproite magma was represented by high-metasomatised rocks of upper mantle, composed mainly by titaniferous phlogopite and some other minerals of metasomatic genesis. Dependence of petrochemical and the whole number of other kimberlite features on composition and mechanical properties of hosting rocks should be taken into special account during forecasting and prospecting of primary diamond deposits. Prospecting of kimberlite rocks in new regions of all ancient platforms of the world must be performed with consideration of those possible changes, to which initial maternal formations depending on composition of hosting sedimentations can be subjected. Under influence of the latter chemical composition of kimberlites may become sufficiently different from usual composition of these rocks, owing to which chemical criterion of typification may loose its diagnostic role. Physical properties (magnetic, density, electrical and other) change differently in kimberlite rocks with different intensity and directivity of secondary alterations, which should also be considered during employment of this or that geophysical method. All these should be taken into account in heavy-concentrate and geochemical methods of diamond deposits’ prospecting as well.

     Industrial significance of primary diamond deposits is first of all determined by grade of this precious mineral in rocks. The quality of diamond crystals plays an important role at this: if the quality is good (increased size and high percent of gem stones) deposits with relatively low grades are mined profitably (for instance, kimberlite pipes of Lesotho). The world experience shows that profitably mined are primary deposits of diamonds containing tenth percent of carat per thousand tonne of ore (for instance, pipes of Kimberly region in South Africa), if the quality of diamonds if relatively high. In most of foreign deposits negative correlation between grade of diamonds and their quality is reported (pipes Finsch, Orapa, Argyle and others). Somewhat differ in this plan primary diamond deposits of the Siberian platform, where the reported regularity is expressed less definitely. Thus, for example, high-diamondiferous pipes Mir, International, Udachnaya, Aykhal, and others are characterized by perfect quality of diamonds. One should note at this that commercially diamondiferous pipes usually differ by diamond morphology. In most of investigated by us deposits crystals of octahedral and combinational habit prevail. It has been noted at this that general content of diamonds in kimberlites decreases according to the increase of quantity of rhombic dodecahedral crystals. Interesting specific feature has been noted in deposits of East-European platform (Arkhangelsk diamondiferous province) and lamproite pipe Argyle (Australia), where depending on size two groups of crystals stand apart: diamonds larger than 1.5 mm are represented by rhombic dodecahedra, and crystals less than 1 mm size – by plane-face octahedra. For a kimberlite or lamproite pipe to represent commercial interest it is necessary to have accumulation of significant resources of diamonds in it. However in regions with already developed industrial infrastructure bodies with small diamond resources but with their high grade and quality can be mined, which has already been reported during the development of small diatremes ХХIII CPSU Congress and Dachnaya in Malo-Botuobinsky diamondiferous region of the Siberian platform.

     Diamonds with ultrabasic association of solid phase inclusions (olivine, chromite, pyrope and others) prevail in overwhelming majority of kimberlite pipes. Only in individual bodies (pipes Orapa, Premier and others) eclogite association of primary indicator minerals plays the main part. Eclogite set of inclusions in diamonds (pyrope-almandine garnet, coesite, clinopyroxene and others) also dominate in lamproites of Australia. The content of basic indicator accessory minerals of diamond – pyrope and picroilmenite vary widely in kimberlites. The ratio of these minerals is also different: in kimberlites of magnesia type, as a rule, pyrope prevails over picroilmenite, and in magnesia-ferruginous kimberlites picroilmenite is usually the main one among indicator minerals. Most of magnesia kimberlites and lamproites of Australia are characterized by low content of indicator minerals.

     Despite different chemical composition all the said diamondiferous magmatites contain a complex of deep-seated minerals (garnet-pyrope, chrome-spinellid, picroilmenite and chrome-diopside), forming balanced association with diamond. Taking into account dependence of mantle magmatites composition (including diamondiferous ones) on composition of mantle rocks subjected to melting, as well as strong differentiation of upper mantle rocks, there is reason to suppose that volcanic formations, composition of which would differ from stated above, may be diamondiferous. This should be considered during prospecting of primary sources of mantle diamonds (especially in new regions). Kimberlites start to acquire the features of ultrabasic in the process of upper mantle rocks melting, among which high-magnesia ultrabasites of garnet dunites, harzburgites, lherzolites and their spinel analogue types prevail. As investigations of representative xenolith collections of deep-seated rocks of many kimberlite bodies of the Siberian, African and East-European platforms indicated, ultrabasic rocks being prevailing, are not the only constituent components of the upper mantle. It also contains basic (eclogites), alkalic (micaceous peridotites and glimmerites) and alkali-titaniferous (ilmenite peridotites, pyroxenites and their micaceous-ilmenite differences) differentiates. Alkaline and alkali-titaniferous formations belong to metasomatic derivatives of upper mantle. Mechanical mixing of melts with relicts of initial rocks and minerals of various sites of differentiated along vertical upper mantle results in formation of kimberlite magmas, which, having relocated in conditions of the Earth crust, form tubular (pipe), dyke, and sill-like bodies. All commercial accumulations of diamonds of mantle genesis are confined to volcanic pipes, seldom to dykes, closely associating with the first ones at a small distance from them.