Vein complex of the Biraya carbonatite deposit

Koneva A.A., Konev A.A., Vladykin N.V.

Vinogradov Institute of Geochemistry SB RAS, Irkutsk, Russia;

vlad@igc.irk.ru

 

               The Biraya carbonatite dyke and associated rare-metal - REE deposit are situated in the East Siberia (116°50’E, 57°50’N) at the eastern margin of Vitim-Patom highland. The dyke stretches with some discontinuities over 20 km. It is presumably of mid-Paleozoic age. The dyke is hosted by Precambrian plagioschists, ortho-amphibolites and granite-gneisses and is crosscut by upperpaleozoic granites. The total thickness of the dyke combined with fenites varies from 10 to 300 m.  The carbonatites are presented mainly by ferrodolomite, and, to a lesser degree, by calcite varieties, which often grades into fenites through their carbonatized varieties. Albitization of fenites and associated monazite mineralization there widely occur. Hydrothermal vein mineralization of peculiar composition develops about carbonatite body among fenites. By prevailing rock-forming minerals there have been distinguished cordylite, benstonite, norsethite, toernebohmite, dolomite and barite vein rock types.

               Mineral composition of vein rocks is varied, especially in cordylite rock-type. Cordylite rocks were noted in some veins 3-10cm in thickness. One of the best-studied vein with thickness 6-7cm lays accordingly to fenites layering some 50m from carbonatite body. The vein has zoning - in the center it is light brown with cordylite prevailing, in salbands amphiboles and other dark-colored minerals occur. Benstonite and norsethite rocks usually form combined thin veins 1-10cm in thickness. The edges of such veins are composed by benstonite and the central part  - by norsethite. They differ in color - white benstonite and yellowish-red norsethite zones. Their mineral composition is quite diverse and includes some new and rare minerals. Toernebohmite veins are composed mainly by calcite and toernebohmite with insignificant admixture of other minerals. Barite veins are striped with layers of different composition and color - black, white, yellow - composing vein of 7cm thick among fenites. The stripes are due to segregation of various minerals - barite, allanite, aegirine-diopside, amphibole, monazite in thin layers. Ferro-dolomite veins are most abundant and thick (up to 1m). Sometimes they differ little from dolomite carbonatites of the main dyke body, though containing more diverse mineralization.

 

Table 1. Mineral composition of the Biraya vein rocks.

 

               Chemical analyses show the rocks to have high contents of Ba, Sr and TR. The minimal contents of the elements were reported for ferro-dolomite veins. Other rock types contain 17-43w.% of sum of these elements. The cordylite rocks are equally enriched in Ba, Sr and TR. The benstonite-norsethite rocks are rich in Ba and to a lesser degree in Sr and REE. The barite veins are enriched in Ba and TR, and toernebohmite ones - in TR and partially in Sr and don’t contain Ba. A characteristic feature of vein rocks is high content of  Nb  - up to 1,3% (with Ta less 20 ppm),  and high contents of LREE.

               The mineralogical study of one of the vein revealed a new mineral with a novel structure, biraite-(Ce), ideally Ce₂Fe²⁺(CO₃)(Si₂O₇), named after locality – the Biraya deposit (Konev et al., 2005). Electron microprobe and wet chemical analyses gave the following empirical formula based on 10 O+F: (Ce_1.01La_0.57Nd_0.25Pr_0.09Sm_0.02Ca_0.07Na_0.02Ba_0.01)_Σ=2.04(Fe_0.60Mg_0.25Mn_0.11Ti_0.01)_Σ=0.97(CO₃)_0.99[Si_1.97(O_6.87F_0.17)_Σ=7.04]. A structural investigation revealed the unique crystal-chemical nature of the mineral, which is characterized by the presence of both disilicate and carbonate groups. On further investigation there was found also the Mg-rich variety of biraite, up to the equal (50% Fe + 50% Mg) site occupancy.

The further study of mineralogy of the Biraya vein rocks revealed abundant TR-Nb-Ba-Sr minerals, many of them being supposedly new mineral species. Following are listed preliminary formulae of some minerals under study from the Biraya deposit.

 

X1                                       (Ce,Ca)₂(Al,Fe³⁺)₂[SiO₄]₃

X2                                       Ca₃Ce₄MgFe³⁺₃Al₅[SiO₄]₈[SiO₃OH]₄

X3                                       (Nd₃Ca)₄Nb₅Si₂O₂₂

X4                                       (Ca₃Ce)₄Nb₅Si₂O₂₁

X5                                       BaFe²⁺₂Nb₂Si₂O₁₁(OH,Cl)

niobochevkinite  Ce₄Fe²⁺Fe³⁺(Nb,Ti)₂Si₄O₂₂

b-1                                       CeNb₂Si₂O₁₀

b-2                                       CeNbSiO₆

b-3                                       CeFe³⁺SiO₅

Sr-benstonite                     (Sr,Ba)₆Ca₆(Ca,Mg)(CO₃)₁₃

cordylite-(La)                    (Ca,Na)Ba(La,Ce)₂(CO₃)₃F

 

The authors are grateful to the RFBR for financial support through grant 10-05-00122.

 

References:

Konev A., Pasero M., Pushcharovsky D., Merlino S., et al. Biraite-(Ce), Ce₂Fe²⁺(CO₃)(Si₂O₇), a new mineral from Siberia with a novel structure type // Eur. J. Mineral. 2005. Vol. 17. P. 715-721.