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Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Mineralogy-geochemical specificity kimberlitic off Yakutia with the different productivity

Rotman A. Ya. *, Ivanov А.S. **, Pomazansky B.S.*

* Geo-Scientific Research Enterprise, ALROSA, Mirny, Russia

** Botuobinsky geological prospecting expedition ALROSA, Mirny, Russia

RotmanAYa@alrosa.ru, arotman@mail.ru

 

The kimberlite appearances (diatrems, more rare veins, dykes and sills) are the main source of diamonds and represent a characteristic magmatic formation of ancient platforms. In Yakutian kimberlitic province (a northeast part of the Siberian platform) they are connected mainly with intense manifestations of middle paleozoic (D3–C1 367-345 Мa) and mesozoic (T 245-215 Ma, J3 160-149 Ma), taking various spatial positions. The first include industrially diamondiferous kimberlites. The basic volume kimberlitic rocks is presented volcanic brecсias (kimberlite brecсia – KB, autholithic kimberlite brecсia - AKB) and, in a smaller measure, massive porphyritic kimberlite (hypabyssal kimberlite - HK) with a varying is qualitative-quantitative spectrum diamond indicator minerals - Cr-pyropes, chromites and picroilmenite. 

The chemical composition of kimberlites is characterised high magnesium more often at almost equal quantities SiO2 and MgO (SiO2/MgO=1,1-1,2, sometimes 1,0 and less – (pipes Internationalnaya, Aihal, Udachnaya-east), high maintenance of Cr and Ni, high values of relations Mg/Fe and Ni/Co. The kimberlytes of the region and, first of all, industrially diamondiferous bodies on the parity of oxides of K and Ti, Al and Ca,  and also on the parity of Zr and Nb, Nb and La, Ce and Sr correspond to kimberlites of the group I of South Africa. Thus in middle paleozoic kimberlite fields with industrial diamond’s higher values MgO/FeOtotal (3,7-5,8), than in those mesozoic age (1,6-2,8) are fixed. Stand apart kimberlites of the Nakynsky field in which, as well as in separate pipes Daldyn-Alakitsky diamond area, it is marked rather low - less than 0,5 мас. % - maintenance TiO2 at the lowered values of La/Yb. Behaviour К2О is ambiguous with appreciable variations of concentration both in middlle paleozoic, and in mesozoic kimberlites at frequent raised potassium in productive bodies (Mir, Dachnaya, Internationalnaya, Botuobinskaya, Njurbinskaya, Aihal, Udachnaya). Sometimes positive correlation of diamond and potassium is found out, that was marked by Vasilenko V.B. with colleagues [Vasilenko, etc., 1997]. As concentrator K2O serves phlogopite which quantity considerably differs both between pipes, and in tubes between crystallisation products melting different intrusive phases. The main feature of the chemical composition of kimberlites is the antagonism of oxides Ca and Mg, their carriers reflecting share a carbonate (mainly calcite) and olivine/serpentine.

Geochemical specificity is expressed in higher maintenances Ni, Cr, Li and PGE in industrially diamondiferous kimberlites, and Co, Mn, V, Ti, Sc, Be, Ga, Ba, Sr, Zr, Nb, Ta, REE, Zn and Sn - in poorly diamondiferous and not diamondiferous displays at approximately equal quantities Rb, Pb, Mo, Th and U.

Results of studying of correlation dependences of chemical compounds of minerals-accompanying of diamond (MAD), the maintenance and properties (morphology varieties, colours of a photoluminescence and other) diamonds representative selections from 18 kimberlitic bodies of a southern part Yakutian kimberlitic province with using clustaring (R and G methods) and factorial analyses show to a high and significant factors of correlation between average frequencies of occurrence of noted characteristics in communities of pipes which are narrowly localised within this or that kimberlitic fields. With the help cluster (k-means) the analysis chemical compounds pyropes selection tests (more than 10 000 grains) are divided on 24 cluster groups (CG), composition of picroilmenite (more than 10 000 grains) - into 16 CG by a technique described earlier [Ivanov, 2004]. Frequency of occurrence of groups accessory minerals of diamond, and also the basic forms of crystals and colour of a photoluminescence of diamond along with conditional level of efficiency kimberlitic rocks is defined.

For pyrope the ultrabasic paragenesis Daldynsky kimberlitic fields raised maintenance Ca that it is possible to explain is characteristic, being based on existing high positive correlation Са with Ni in this area, higher PT-conditions of crystallisation pyrope concerning those Mirninsky and Alakit-Marhinsky fields. The important difference of average composition of CG pyrope eclogite paragenesis from kimberlitic pipes of the Alakit-Marhinsky field under maintenance Ni and Ti-presence of six groups of composition in the field of joints with diamonds. They have higher level Na-content, than other groups. Little a bit more low average maintenance Na is fixed in pyropes Mirninsky and Daldynsky kimberlites.

Average compositions similar clusters groups picroilmenite also have the prominent features in everyone kimberlitic a field. Among picroilmenite from kimberlite the Mirninsky field the greatest quantity (6) CG with raised maintenances Cr2O3 concerning other fields is noted.

Kimberlitic bodies of Daldyn-Alakitsky area find out appreciable differences on morphology, colour, photoluminescence and other properties of diamonds. The general quality sign of diamonds of kimberlitic bodies of these fields is predominance of blue, yellow-green photoluminescence above frequency of appearance of crystals with of pink-lilac colour luminescence.

 On a ratio gabitus forms of crystals of diamonds 3 groups of bodies are allocated. In the first group comparable maintenances of diamonds octahedral, transitive between octahedral and rombododecahedral gabitus, laminar rombododecahedral are observed at the least quantity dodecahedral. All get to this group practically known industrial and semicommercial deposits Daldynsky and Alakit-Marhinsky kimberlitiс fields. In the second group comparable quantities of octahedrons, transitive forms, laminar rombododecahedral at domination dodecaedroids. For the third group the comparable quantity of transitive forms, laminar and roundish rombododecahedrons is characteristic at domination octahedron crystals. Groups 2 and 3 are presented unproductive kimberlitic pipes. For the investigated bodies of the Alakit-Marhinsky field it is noticed, that high positive correlation with diamond concentrations find out CG of ferrimagnetic picroilmenite Р3, Р4, and Р6. Occurrence such picroilmenite in highly productive bodies and correlations with frequencies of occurrence of diamonds defined gabitus or photoluminescences can be connected with possibility of growth of diamonds in conditions concerning raised fugitive oxygen [Palianov, etc., 2000]. Only in highly productive kimberlite there are groups pyrope, allocated on known classifications [Dawson J.B., Stephens, 1975, etc.] G17, G18 and G20 (G17 concerns to lherzolitic, G18 - to diamond it is low - and mildly-chromic dunite-harzburgites, and G20 - to lherzolitic high chromic paragenesis).

For the Daldynsky field high and significant factors of correlation between groups of pyrope composition G5, G12 and G21, G16 and occurrence of diamonds in the form of laminar rombododecahedrons, crystals with an orange photoluminescence are defined (G5 concerns to eclogitic to paragenesis, G12 - to lherzolitic it is low - chromic low titanic paragenesis, G21 - to diamond high chromium dunite-harzburgites paragenesis, and G16 represents verlitic paragenesis. Significant correlation of frequency of occurrence clusters groups ferrimagnetic picroilmenite Р6 and Р7 which meet from 1 to 6 grains on one hundred in casual samples, and level of efficiency of bodies on diamonds is appreciable.

Among kimberlite bodies of the Mirninsky field stand apart high diamond grade pipe Mir and Internationalnaya, XXIII congress and Dachnaya. High maintenances octahedron diamonds, transitive between oktahedrical and rombododecahedritical gabitus are peculiar to them at the minimum maintenance laminar and roundish rombododecahedron, pink and pink-lilac colours of photoluminescence, and also the raised maintenances eclogite pyropes G7 and G10 and granates of all groups of composition of chromic ultrabasic paragenesis G20-G23, switching on diamond. In samples of diamonds of these bodies, along with the high maintenance octahedron crystals and prevalence of a pink-lilac photoluminescence, rather raised maintenance of diamonds with zone distribution of colour of a photoluminescence, and also with uncertain yellow colour of a luminescence is noted. Kimberlite Mir pipe contains the greatest quantity of pyropes. Among them CG eclogite paragenesis G5, G6 and G8 with lowered value Na2O from joints with diamonds and groups G12 and G11 from megacrystals with raised values TiO2  prevail. In a Mir pipe rather raised maintenance of diamonds with a violet photoluminescence, prevalence ferrimagnetic picroilmenite  cluster groups P4 is noted.

In kimberlite underproductive bodies of the Mirninsky field (vein An-21, pipe Amakinskaya, Tayognaya) which high maintenances of diamonds in the form of laminar and roundish rombododecahedrons with blue, whitish, green, yellow and violent colours of a photoluminescence are characteristic, prevalence paramagnetic not chromic groups picroilmenite CG Р8 and Р11, pyrope G13 high titanic paragenesis, and also G14 and G17 the ultramafic and G18 diamond poorly chromic dunite-harzburgites paragenesis is noted. The high (+0.96) coefficient of correlation for frequencies of occurrence picroilmenite cluster group P2 and group G16 granates verlite paragenesis is indicative.

Kimberlite the studied displays find out is qualitative-quantitative variations of mineralogy-geochemical characteristics. Frequency of occurrence of some CG of minerals-accompanying of diamond is in direct or return significant correlation communication with defined crystal morphology and physical properties of diamonds only in communities of bodies defined kimberlitic fields. It is characterizes as a territorial unit with specific features of processes deep mineral formation. Presence of minerals-accompanying of diamonds with raised maintenance Ti and Fe in process diamond formation possibly promotes formation of crystals dodecahedron gabitus, is more rare laminar rombododecahedron.

 

References

Vasilenko V. B, Zinchuk N.N., Kuznecova L.G.Petrochemical models of diamond deposits of Yakutia. Novosibirsk: Nauka. Siberian Enterprise of the Russian Academy of Sciences, 1997. 574 p.

Ivanov A.S.  Technique of statistical processing of structures typemorfic minerals kimberlitic  pipes  by searches of diamonds in Yakutia. Applied geochemistry. Issue. 5. Moscow, 2004.  P. 45 - 56.

Paljanov J.N., Falcon А.Г., Hohrikov A.F.Paljanova G. A, Borzdov J.M., Sobolev N.V. Crystallisation of diamond and graphite in the DREAM - a fluid at Р-Т parametres natural diamond formation. Reports AS. Т. 375. №3. 2000. P. 384 - 388. 

Dawson J.B., Stephens W.E. Statistical classification of garnets from kimberlites and xenoliths. J. Geol. 1975. Vol. 83. № 5. P. 589-607.