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Mineralogy and Geochemistry of the Bitu-Dzhida multiphase intruded massif of Li-f rare-metal granites (Northern Mongolia)

Zolboo Ts.1, Perepelov A.B.1, Tatarnikov S.A.1, Antipin V.S.1, Kanakin S.V.2

1A.P. Vinogradov Institute of Geochemistry sb ras, Irkutsk, Russia; 2Institute of Geology sb ras, Ulan-Ude, Russia

zolboo@igc.irk.ru

Rare metal mineralization associated with magmatic and post-magmatic processes of multiphase granite massifs evolution, is characterized by large reserves of mineral raw materials, but as a rule, scattered nature of the assignment of useful components. One of the most challenging for practical purposes is the geochemical type of Li-F granites with intrusive complexes of which the development of mineralization of Li, Rb, Sn, W, Be, Ta and Nb is associated. During the studying of these complexes important issues are to identify genetic interdependence between intrusive phases, to determine the mineralogical and geochemical features of rocks and factors of their potential ore content, processes of scattering and concentrating of useful components through the evolution of ore-magmatic systems.

The Bitu-Dzhida multiphase intrusion of Li-F granites located in the southern spurs of Khamar-Daban Ridge. It is localized in the Cambrian metamorphic sequence (the crystalline schist of the Bitu-Dzhida suite). The massif was discovered in 1933 [5] and was studied through geologic survey and prospecting for Li, Rb, Ta, Nb (1954-1960) as well as through thematic scientific research [2, 3]. New geological and geochemical surveys of the massif were carried out in 2004 in Russia and in 2008 in Mongolia. Previously, by means of K-Ar - dating the massif age was determined as the Permian - Triassic – 262-218 Ma [2]. According to the new data [6] the time of intruding of the 1st initial phase of granites the Bitu-Dzhida intrusion is regarded as the Late Carbonaceous (C2) that is 311 ± 10 Ma.

By the results of recent investigations we can distinguish three main phases of intruding of granite magma. The 1st phase includes small outcrops of Pl-Kfs-Qtz-Bt middle-grained and porphyry granites; the 2nd phase consists of Qtz-Kfs-Pl-Bt leucocratic ones. And, finally, amazonite-albite-zinnwaldite (Amz-Ab) rare-metal granites refer to the 3rd phase.

The rocks of all the three intrusive phases belong to the group of plumasite granites of Li-F geochemical type. Although, their substantial features essentially differ. A common feature for the granites of all intrusive phases of the massif is their high level of concentration of Li and F. The composition of the rocks of the early phase differs from the granites of the 2nd and 3rd phases in lower silica content and the lowest total alkalinity. The granites of the 1st phase have the most differentiated REE spectrum allocation (LaN/YbN 10.6-16.3), whereas the 2nd phase rocks show a decrease in REE fractionation (LaN/YbN 3.6-6.6) resulting from an essential enrichment by heavy elements of the spectrum and possess more considerable EuN deficiency (Eu* 0.18-0.29 vs 0.49-0.65 as compared to the rocks of the 1st phase). The Amz-Ab leucogranites of the final 3rd phase are characterized by a sharp LREE depletion (La/Yb<1) and a deep Eu-minimum (Eu*≤0.05).

Recent advances in our investigation have continued by microprobe analyses of the Bitu-Dzhida massif rocks, which helped to identify the composition of the rock-forming minerals and the mineral-concentrators: Nb, Ta, LREE, HREE, Th, U, Pb, Sn, Li – allanite, columbite, tantalite, sinhizit, monazite, cassiterite, xenotime, furdite, changbait, zinnwaldite, protolitionite, Li-muscovite.

The rock-forming minerals of the granites are represented by feldspar, quartz and mica. Evolution of their composition indicates a genetic alliance between the rocks of all three intrusive phases. The feldspar of the 1st and 2nd intrusive phases granites mainly consist of albite, microcline and rare oligoclase (An 11-17). There are solely albite and microcline in the 3rd intrusive phase. Another group of the rock-forming minerals of the Bity-Dzhida massif granites is mica. The types of mica and evolution of their compositions in the rocks of the intrusive phases essentially differ too. In the granites of the 1st and 2nd phases mica is represented by ferriferous biotite and muscovite. There was no significant level of F and Li detected in biotites and muscovite. On the contrary, in the granites of the 3rd phase mica is solely composed of the Li-F varieties. The content of F in Li-mica varies from 2 to 8 wt.%, the content of Li in such micas could not been estimated by chemical analyses but was determined due to empirical formulae [7] depending on the contents of F and Si. Among the Li-micas protolitionite and zinnwaldite were identified. In the amazonite-albite granites of the 3rd intrusive phase Li-F micas contain a considerable concentrations of Na2O and ZnO, that can be adduced as an important feature of special conditions of their crystallization.

The occurrence of a wide variation of accessory minerals in the rocks of the massif , such as carbonates, phosphates, silicates and oxides of series of elements, such as Sn, TR, Pb, Ta, Nb, Th and U shows the significant variations in crystallization conditions of Li-F magma and the potential ore content of the latest intrusive phases.

Recent advances show that isotope features for granites of the 1st and 2nd intrusive phases, as 87Sr/86Sr(t) (0.705312-0.706187), 143Nd/144Nd(t) (0.512088-0.512290), 206Pb/204Pb(t) (17.761-17.961), 207Pb/204Pb(t) (15.454-15.491), 208Pb/204Pb(t) (37.426-37.587), are similar. At the same time, the isotope features of the granitiods of the 3rd intrusive phase are notable for increasing radiogenic isotope system of component 87Sr/86Sr(t) (1.100426-1.135334) and for reducing isotope system of 206Pb/204Pb(t) (17.208-17.480). So, we can conclude that the source of Li-F granitoid melt was enriched in Rb, Th and depleted in U. These data correspond with the model of forming the initial Li-F granitoid melts at the lower levels of the ancient Precambrian continental crust with an average model age TDM2 = 1260 Ma and the maximal = 1600 Ma.

References

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2. Kosals Ya.A. Geochemistry of amazonite granites. Publishers of the Institute of geology and geophysics, SB USSR Academy of Sciences. Issue 219. Novosibirsk: Nauka. 1976. 190p.

3. Koval P.V. Petrology and geochemistry of albited granites. Novosibirsk: Nauka. 1975. 258p.

4. Kovalenko V.I., Kostitsyn Yu.A., Yarmoluk V.V., Budnikov S.V., Kovach V.P., Kotov A.B., Salnikova E.B., Antipin V.S. Magma sources and the isotopic (Sr and Nd) evolution of Li-F rare-metal granites. Petrology. 1999. V.7. №4. P.383-409.

5. Naletov P.I., Shalaev K.A., Deulya T.T. geology of Dzhida ore area. Publishers of VSGU. Issue. 27. Irkutsk. 1941. 282p.

6. Perepelov A.B., Tatarnikov S.A., Dril S.I., Antipin V.S., Vladimirova T.A., Sandimirova G.P. Geochemical features, magma sources and age of the Bity-Dzhida multiphase intrusive of Li-F granites (Khamar-Daban). Abstracts of 1st International Geological Congress “Granites and evolution of the Earth: geodynamic position, petrogenesis and ore content of granitoids”. Ulan-Ude. Publisher of Buryat Scientific Center SB RAS. 2008. P.291-293.

7. Tischendorf G. On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representation // Mineralogical Magazine. 1997. V. 61. P. 809-834.

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