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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

New isotopic (Rb-Sr, Sm-Nd) and geochemical (ICP-Ms) data on kimberlites of the different emplacement phases from the Udachnaya pipe (Yakutia)

Egorov K.N.

Institute of the Earths crust SB RAS, Irkutsk, Russia

egorov@crust.irk.ru

 

The Udachnaya kimberlite pipe is one the largest pipe in the world as to reserves and size of the primary diamond deposits of Yakutia. Its ore structure consists of two double multi-phase pipes Udachnaya-Eastern and Udachnaya-Western, four structurally conjugated satellite blindkimberlite bodies and six kimberlite veins. The Early Triassic dolerite dykes and Cretaceous vein of K-trachytes are structurally conjugated with occurrences of kimberlite magmatism (Egorov et al., 1988; 1989). These rocks form a unified series: dolerites-trachydolerites-trachytes characterized by gradual change of mineralogo-petrographic and petrochemical peculiarities from initial to later members. Occurrence of kimberlite and basic rock magmatism is related to system of disjunctive dislocations forming the regmatic network: orthogonal (sublatitudinal and submeridional systems) and diagonal (north-western and north-eastern systems).

The first stage of the kimberlite magmatism occurrence in the region of the Udachnaya ore complex is related to the formation of powerful north-eastern system of rupture dislocations controlling spatially the majority of prepipe kimberlite veins and satellite blindkimberlite bodies. Kimberlite veins confined to the north-eastern fault zone consist of carbonatizated magnophyric kimberlites with variable (5-7%) quantity of phlogopite. The ground mass of vein kimberlites are characterized by calcite pseudomorphs after olivine, phlogopite laths, microlites of calcite, grains of perovskite and apatite.

The highest concentrations of all elements (LILE, HFSE, and REE) are characteristic of vein kimberlites in comparison with those of other phases of emplacement (Fig.). Vein kimberlites are characterized by negative anomalies of U, Zr, Hf and poorly differentiated gentle slope of the distribution specter of middle and hard REE. Judging by isotopic composition of kimberlites (εNd=4.2, 87Sr/86Sr(t)=0.7050) its mantle source corresponds to the moderately depleted mantle (Tabl.). The model age of TNd(DM) enrichment of the kimberlite mantle source is equal to 651 m.y.

The kimberlite veins are cut by satellite blindkimberlite bodies structurally related to north-eastern, rarely to sublatitudinal system of rupture dislocations. They are exposed at the depth of 5-20 m from the present-day surface. The bodies are composed of carbonatizated kimberlite breccia containing autoliths, fragments of vein kimberlite and xenogenic material.

Massive monticellite and monticellite-olivine kimberlites composing mainly the peripheral zones and stock-like bodies (at deep horizons) in the Udachnaya-Eastern and Udachnaya-Western pipes were emplaced after kimberlite veins and satellite bodies (Egorov, Bogdanov, 1991; Egorov et al., 1991). These kimberlite are characterized by obviously zonal olivine with high Fe(Fe+Mg) content (12-14 mol. %) and high concentration of CaO. Monticellite exhibits low mg number and high content of Al, Ti and Na. Unlike the kimberlite of the pre-pipe veins the geochemical composition of monticellite kimberlite of the early emplacement phases is characterized by lower values of LILE, HFSE concentration and particularly hard REE (Fig.). They exhibit the distinct negative anomalies of Rb, Zr, Hf and Y.

The stock-like and vein bodies composed of micaceous and mica kimberlites were formed at the same stage (Egorov et al., 1986). The mica kimberlites with pyroxene groundmass (Udachnaya-Eastern pipe) and high sphene content (Udachnaya-Western pipe) are presented in the form of fragments in kimberlite breccias (Egorov et al., 1991). The age of fragment of the vein unmodified mica kimberlite from the Udachnaya-Eastern pipe obtained by Rb-Sr method, corresponds to 3525 m.y. (Maslovskaya et al., 1982). The mica kimberlites are characterized by more differentiated character of the distribution specter of rare incoherent elements (Fig.). They exhibit sharp positive anomalies of Nb and Ta as well as negative anomalies of Th and U. Weak  positive anomalies of Zr and Hf, and so right with slope the specter character from Nd to hard REE are the characteristic property of them (Fig.). According to the obtained isotopic data (εNd=-4.8, 87Sr/86Sr(t)=0.7078) the mantle source of mica kimberlite corresponds to the enriched mantle of the EM11-type (Tabl.). The model age of TNd(DM) enrichment of the mantle source of the mica kimberlite is more ancient and equal to 924 m.y.

It should be noted that similar negative εNd values for the fragments of vein mica kimberlites from the Udachnaya pipe are first obtained unlike the all known εNd isotopic ratios for the kimberlites of the Yuakutian diamondiferous province as a whole.

In the following stages of the Udachnaya ore node formation the main volume of kimberlitic material occurred (according to succession of the phases emplacement) in the form of brecciated ovoid kimberlite, taxite in the Udachnaya-Western pipe and protoclastic and deuteroporphyric kimberlites, tax kimberliteite in the Udachnaya-Eastern pipe (Egorov et al., 1991). The final phases of the brecciated kimberlites emplacement possessed a great penetration capacity and more frequent reached the upper levels of the pipes (Egorov, 1985). When rising they actively disintegrated the early phases of massive porphyric kimberlites.  Small in volume post-pipe dyke and vein formations composed of olivine-monticellite kimberlites occur among the latest phases of the kimberlite emplacement (Egorov, Bogdanov, 1989; Kornilova et al., 1998). They differ from the early monticellite variety in composition of olivine, monticellite, spinel and perovskite of ground mass. Geochemical composition of the monticellite kimberlites of the late phases of emplacement is characterized by the lowest values of LILE, HFSE and REE concentrations (Fig.). They exhibit the distinct negative anomalies of Zr and Hf.

 

Fig. Distribution specters of rare incoherent elements in kimberlite vein (spec. Zh-2), in fragment of mica kimberlite (spec. 78-186 UW), in fragment of monticellite kimberlite of the early emplacement phase (spec. 218/470-1UE), in post-pipe veins of kimberlite (spec. 222/407UE, 222/403UE).

 

 Table. Nd and Sr isotopic composition of the mica kimberlite fragment (spec. 78-186 UW) from the Udachnaya-Western taxite and  kimberlite of the vein (spec. Zh-2) (spec. Zh-2) (2).

 

[Sm]

[Nd]

147Sm/144Nd

143Nd/144Nd

εNd

TNd(DM)

[Rb]

[Sr]

87Rb/86Sr

87Sr/86Sr

(87Sr/86Sr)t

ppm

ppm

1

11.78

81.26

0.08806

0.512390

-4.8

924

132.1

858.3

0.44557

0.71012

0.707836

2

22.50

173.7

0.0781

0.512573

4.2

651

200.7

1581.1

0.37791

0.70696

0.705076

 

Note. Sm-Nd and Rb-Sr isotopic data of the mica kimberlite were obtained at ECI SB RAS (Irkutsk). Sm-Nd isotopic data of the vein kimberlite were performed at IGGD RAS (St. Petersburg). Rb-Sr isotopic measurements of the vein kimberlite were performed at IG KSC RAS (Apatity).

 

 References

Egorov K.N. Contact relationships of kimberlites of different phases of emplacement in composite pipes (in Russian) // Izvestiya of AS USSR. 1985. Ser. geol. No 9. P. 23-35.

Egorov K.N., Kornilova V.P., Safronov A.F. et al. Mica kimberlite from the Udachnaya-Eastern pipe (in Russian) // Doklady AS USSR. 1986. Vol. 291. No 1. P. 199-202.

Egorov K.N., Vladimirov B.M., Zaborovsky V.V. et al. About discovery of the K-trachyte vein in the region of the Udachnaya kimberlite pipe, Yakutia (in Russian) // Doklady AS USSR. 1988. Vol. 298. No 1. P. 186-189.

Egorov K.N., Vladimirov B.M., Zaborovsky V.V. et al. Mesozoic alkaline-basaltic subintrusive series of the Daldyn River basin (Yakutia) (in Russian) // Izvestiya of AS USSR. 1989. Ser. geol. No 7. P. 32-42.

Egorov K.N., Bogdanov G.V. Monticellite from the kimberlites of Yakutia (in Russian) // Zapiski VMO. 1991. Issue. 2. P. 78-87.

Kornilova V.P., Egorov K.N., Safronov A.F. et al. Monticellite kimberlite from the Udachnaya pipe and some aspects of the kimberlite melt evolution (in Russian) // Otechestvennaya geologiya. 1998. No. 6. P. 48-51.

Maslovskaya M.N., Egorov K.N., Kolosnitsyna T.V. et al. Isotopic composition of strontium, Rb-Sr absolute age and rare alkalines in micas of the kimberlites of Yakutia (in Russian) // Doklady AS USSR. 1982. Vol. 266. 1. P. 451-455.

Egorov K.N., Bogdanov G.V. Geology, Petrology and mineral composition of the Udachnaya ore complex (Yakutia) // In 5th Int. Kimberlite Conf.: Ext. Abst., Brazil. 1991. P. 498-500.