Conditions of polylithionite formation by experimental data

Vasil'ev N.V.^*, Zaraisky G.P.^**, Schuriga T.N.^***.

*-The Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements Moscow, Russia.

** - The Institute of Experimental Mineralogy (IEM RAS), Chernogolovka, Russia.

*** - N.M. Fedorovsky Russian Institute of Mineral Materials, Moscow, Russia.

Lithium is very important component of granites, syenites and nepheline syenites. In contrast to the widespread occurrence of lithian muscovite and lepidolite in granitic pegmatites, polylithionite K(Li₂Al)Si₄O₁₀(F,OH)₂ is restricted to alkalic and peralkaline paragenesis. The well-known localities of polylithionite are the Ilimaussaq massif in Greenland; Mount St. Hilaire in Canada; the Lovozero massif in Kola Peninsula, Dara-i-Pioz massif in Altai, and in Tajikistan region. Polylithionite stage was one of the main metasomatic stages of Ulug-Tanzek rare metal deposit formation (Grechishev et al. 1998) and the polylithionite greisens are complex Li-Ta-Nb-REE ores.

At the previous stage of our works, during studying the equilibrium of Kfs - Lepidolite reaction, the wide field of polylithionite stability is established at LiF, SiO₂ saturated conditions. All experiments have been carried out in hydrothermal conditions (T=400бС, P = 1.0 kbar) in presence of HF solutions. Then we decided to investigate the interaction of LiF saturated solutions with two types of rocks from Ulug-Tanzek rare-metal deposit (samples 1-Rib and 268/85).

Experiments by duration of 2 weeks were carried out in autoclaves in sealed big platinum capsule (10x100 mm). Small open Pt capsules (5x25 mm) with rock powder, SiO₂, LiF, natural columbite (Ta₂O₅ ≈ 8-10 wt. %) were placed on a bottom of a big platinum capsule, filled with solution. HF concentration of solution was 0,1 m. As a result of metasomatic interaction with a solution by diffusuion way the rock in small open capsule was transformed into quartz-polylithionite greisen. The metasomatic column received in the small capsule was taken and studied by a microprobe analysis. Figure 1 shows photo (BSE) of new-forming polylithionite in experimentally obtained quartz-polylithionite metasomatite.

Then polylithionite zone (2-3 mm length) was selected for the ICP-MS/AES analysis for major and trace elements (Table 1). The sample of natural Ulug-Tanzek polylithionite greisen (PolGr) was analyzed for comparison.

Concentration of lithium in fluorine rich solutions is controlled by excess of poorly soluble LiF. Presence of even small amount of lithium (0.01 m) in hydrothermal solution strongly reduces area of quartz-topaz greisen stability and expands the area of quarts-Li-mica greisen. The saturated by LiF solutions produce quartz-polylithionite metasomatites. Polylithionite replaces not only Kfs, but it effectively replaces albite with removing sodium. Small amount of residual sodium produces cryolite Na₃AlF₆ in our experiments. Simultaneously precipitation of Ta and Nb in metasomatic zones of greisen column is observed. It is shown that Ta and Nb in conditions of experiments can be gained in a column by HF solutions and be precipitated in greisen.

Our experiments shows that polylithionite can easily replace feldspars and/or other phases like riebeckite and topaz. Presence of Li and F volatiles and SiO₂ saturation are necessarily. They play a key role for polylithionite formation.

Table 1. Comparison of chemical composition of Ulug-Tanzek rocks before and after experiment.

Acknowledgements: Authors thank the Russian Foundation for Fundamental Research (No. 08-05-00835) and the President RF Grant of Leading Research Schools (SS-3763.2008.5) for the financial support.

References:

Grechishev O.K., Sherbakov Yu. O., Obolenskiy A.A.. The geochemical trend and the evolution regularities of the Ulug-Tanzek rare-metal deposit (Tuva). // Large and unique deposits of rare and noble metals: 101-109. Nauka. 1998. (in Russian).