The quartz-albite "snow balls" and sericite "hearts" of Li-F granites. Etyka, Eastern Transbaikalia.

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Unfortunately, I do not have time to update my blog very frequently but finally, I am finishing this post in a week before St. Valentine's Day. This is my pleasure to share things I find incredibly wonderful and beautiful. The post focuses on the interesting and unusual textures and minerals observed in the rare-metal Li-F-rich granites.
During the last semester I had an opportunity to study thin-section samples of Li-F granites, Etyka massif located in the Eastern Transbaikalia (Fig. 1). Granites form a small intrusive body of Jurassic age which is a part of Hangilayskiy ore cluster. There are several W, Sn, Be, Ta deposits related to plutons of peraluminous (Al/(2Ca+Na+K)>1), subalkali ((Na2O+K2O)>8 wt. %) granites (Syritso et al., 2011, Zaraisky et al., 1997). Due to relatively low melt viscosity, saturation with the volatile elements (F, H2O) and enrichment in some rare elements (Li, Be, Rb, Ta, W, Nb) (Fedkin, 2000) the mineral composition and textural features of Li-F granites are particularly interesting for study.

Figure 1. The location of the Etyka granite massif, Eastern Transbaikalia.

The most part of the pluton is composed of coarse-grained microcline-albite granites (Fedkin, 2000), where microcline often has light green color (amazonite) due to the impurities of lead, divalent iron and some other elements (Seltmann et al., 1999). These granites are characterized by the presence of mica siderophyllite-polythionite and polylithionite-trilithionite series - zinnwaldite, lepidolite (vol. 10% ). Topaz is relatively common here and occurs in the amount of 5-10 vol. % , the fluorite is less abundant (vol. 1% ).

A well-known feature of the Etyka granites is a poikilitic texture formed by rounded quartz grains (2-3 mm in size) containing numerous inclusions of predominantly albite. Frequently albite is oriented concentrically inside the quartz grains. Such forms is often referred to the "snowball" textures (Fig. 2a, b).

a)

b)

Figure 2. a - Poikilitic quartz grains with elongated albite crystals inside. "Snowball" texture. b -  the position of extinction ; XPL,  45x zoom magnification.

The formation of these "snow ball"-textures apparently occured in the final stages of the process, in the subsolidus conditions of crystallization. The later alteration caused the occurence of the fine-grained aggregate of sericite which is frequently found inside the quartz in the form of concentric, circular shapes. Such processes are most likely related to the beginning of metasomatic replacement. Sericite aggregates could be found in the quite interesting shapes of rings often resembling the "hearts" (Fig. 3).

Figure. 3. Sericite "heart" in the center of the "snowball". XPL,  60x zoom magnification.

As mentioned above, the granite massif Etyka distinguished by the presence of significant amount of topaz. Mineral forms crystals of irregular shape, fills the cavities between the minerals formed earlier. Topaz is well recognized by its high relief, which helps to find the mineral with closed down iris diaphragm (Fig. 4a). In XPL the mineral has a low birefringence (Fig. 4b), the maximum color in thin section (30μm) - white, the first order.

Figure 4a. Xenomorphic topaz grain. Iris diaphragm is almost closed. PPL, 60x zoom magnification.

Figure 4b. The same topaz grain. XPL, 60x zoom magnification.

This particular crystal has a low interference colors and remains extinct during the rotation of the microscope stage. This allows to indirectly determine the value of 2V (axial angle) and the optical sign of the biaxial mineral (Fig. 5a, b, c).

Figure 5a. Topaz conoscopic figure. Axial angle ~60°.

Figure 5b. Quartz compensator inserted. Positive optic sign.

Figure 5c. The scetsh of the optic axis in convergent light with inserted compensator.

The band of isogyre is approximately matches 2V angle of 60 °, the optic sign is positive. The diagram 2V -  Al2[SiO4] F2 mol. % (Tröger, 1971) helps to determine the approximate content of the molecule Al2[SiO4]F2 in topaz -  80 mol. %  (20 mol. % of topaz-(OH), respectively). The mineral was formed, most probably, in the final stages of crystallization, it could be concluded that the residual melt was rich in fluorine.

Lithium mica occurs in the rock in the amount of 5-10 % and located mainly in the interstitium between the crystals of quartz, albite, potassium-sodium feldspar. The optical determination of species of lithium mica group was skipped. The most characteristic feature of the mineral, which distinguishes it, for example from muscovite, is its low birefringence (Fig. 6).

Figure 6. The crystal of lithium-fluoric mica, inclusions of acessory minerals. XPL, 60x zoom magnification.

For the comparison, the appropriate photo of muscovite from pegmatite veins of the Middle Urals is given here (Fig. 7).

Рис. 7. The muscovite crystal, inclusion of acessory mineral. Pegmatite, the Middle Urals. XPL, 60x zoom magnification.

Li-F granite massif Etyka is highly differentiated pluton. It was forming along with simultaneous accumulation of volatile components in the residual melt. The high saturation of volatile components such as water, carbon dioxide, phosphorus and fluorine lowers the solidus and liquidus temperatures. The pluton was discribed by the rhythmically layered textures, the occurrence of which is the result of crystallization in supercooling conditions (Fedkin, 2000). That appearence of the textures in this conditions were confirmed experimentally by Fedkin A.V. (Fedkin et al., 2002). Apparently, all of these factors were involved in the formation of the mentioned above textuers of lithium-fluoric granites.

REFERENCES

1. Fedkin A., Seltmann R., Bezmen N., Zaraisky G., 2002. Experimental testing of line rocks in Li-F granites: evidence from superliquidus experiments with F and P added. Bulletin of the Czech Geological Survey 77: 113–125. 
2. Seltmann R., Zaraisky G.P., Aksyuk A.M., Fedkin A. V., Shatov V. V. 1999. Amazonite in layered granites of the Orlovka and Etyka Та deposits and its relation to magmatic-hydrothermal ore deposition. International Symposium "Physico-chemical aspccts of endogenic geological processes" devoted to the 100-anniversary of D.S. Korzhinskii. Moscow , 136--138.
3. Tröger, W. E., 1971. Optische Bestimmung der gesteinsbildenden Minerale. Teil 1: Bestimmungstabellen.
4. Syritso, L.F., Badanina, E.B., Abushkevich, V.S., Volkova, E.V., Gazizova D.G., 2011. Orudenenie (Ta, Nb, Li, Cs, W, Sn), svazanoe s plumazitovymi redkometal'nymi granitami: usloviya i mehanizmy formirovaniya koncentracii. Materialy Vserossiiskoi Konferencii, Moskva, IGEM RAS, 25-26 October 
5. Fedkin A.V. 2000. Geohemicheskaya evolucia i rassloennost' litii-ftoristyh granitov tantalovyh mestorozhdenii Orlovka i Etyka Vostochnogo Zabaikalia. Autoreferat of dissertation, Moscow, p.161.
6. Zaraisky G. P., Seltmann R., Shatov V. V., Aksyuk A. M., Shapovalov Yu. B., Chevychelov V. Yu., 1997: Petrography and geochemistry of Li-F granites and pegmatite-aplite banded rocks from the Orlovka and Etyka tantalum deposits in Eastern Transbaikalia, Russia. Mineral Deposits, Papunen (ed.). Balkema, 695–698.