The Bulk Fluid Compositions of Inclusions in Quartz from Selected Rare Metal Pegmatites in Siberia

D. K. Hallbauer Dept. of Geology, University of Stellenbosch, Private Bag X1, Matieland, 7602, Rep. South Africa

V. Ye. Zagorsky Institute of Geochemistry, P.O. Box 4019, Irkutsk-33, Russia, 6644033

B. M. Shmakin Institute of Geochemistry, P.O. Box 4019, Irkutsk-33, Russia, 6644033

Quartz is the most informative and convenient mineral for a study of the mineral forming medium in pegmatites since it is deposited at all stages of the pegmatite formation process and is enriched in fluid inclusions as a rule. More than 50 quartz samples from different types of miarolitic pegmatites of Siberia, Russia, and Kazakhstan were studied with regard to their bulk fluid inclusions by capillary ion analysis (CIA). The aim of the investigation was to identify and quantify discriminating ionic species in the bulk fluid compositions to advance the understanding of pegmatite formation and develop a possible prospecting tool for the identification of suitable pegmatite deposits.

The studied pegmatites differ in mineral composition, inner structure and kind as a gem raw material. Crystal-bearing pegmatites of Akzhailyau deposit, Kazakhstan, are connected with an upper Palaeozoic complex of alaskites and leucogranites within a granitic pluton. These pegmatites have predominantly quartz -K-feldspar composition and simple zonal structure. All other pegmatites are of Mezozoic age.

Gem bearing pegmatites in Eastern Transbaikalia (Borschovochny ridge, Adun-Chelon) have been known since the end of 19th century. The inner structure and composition of pegmatites are close to those in the Akzhailyau deposit. The Adun-Chelon deposit is the classic representative of topaz-beryl miarolitic pegmatites. The majority of pegmatites are surrounded by porphyritic biotite granites in the in the eastern part of the concentric Adun-Chelon granite massif, an area of about 70 km2.

The newly discovered Malkhan field of miarolitic pegmatites in Central Transbaikalia is the largest source of gem tourmaline in Europe and Russia. The field is situated at the downward bulge of the roof between two massifs composed of biotite and two-mica granites. The country rocks are Proterozoic amphibolites, metamorphosed diorites and quartz diorites. The ratio of rock-forming feldspars in pegmatites varies to a considerable extent but highly productive gem bearing bodies are typically enriched in albite, coloured tourmaline, lepidolite and tantalum minerals. They sometimes contain petalite and polluzite. Gem topaz, beryl, danburite, gambergite occur in cavities, together with tourmaline. The Malkhan and Adun-Chelon pegmatites are classified as subraremetal miarolitic ones. Separate quartz samples from similar topaz and tourmaline deposits of Borschovochny ridge have been included in this study.

The Vodorazdelnoye deposit in the Menza pegmatite field, Central Transbaikalia, is presented by a vein-like, zonal K-feldspar-albite body intruded into Palaeozoic biotite granites. Fine-grained quartz-albite pegmatite prevails in the outer zones, whereas the inner zone is characterised by coarse-grained quartz-K-feldspar-clevelandite pegmatite with Li-micas, beryl, spodumene and tantalum minerals. There are many large miarolitic cavities with rock crystal, beryl-clevelandite druzes and polluzite in the central zone. Colourless, light-rose and light-green, transparent morganite crystals are the main gem stones, but several "pockets" which appear to be filled by clay material contain gem quality, polychrome tourmaline.

The Vodorazdelnoye mine is regarded as an example of miarolitic facies of rare metal pegmatites. It is accompanied by barren pegmatites and quartz veins with schorl and muscovite. Results of fluid inclusion studies obtained so far allow to relate the bulk fluid composition in pegmatites to the type of their gem mineralization, the extend of differentiation and gem productivity.

For their bulk fluid composition, clean samples of quartz, usually fragments of quartz crystals, were selected, hand-picked for possible intergrowths, and washed in de-ionized (MQ) water. Samples of about 1 g mass were then prepared and crushed in an agate mortar in the presence of 2.5 ml of MQ water containing 0.05 mM tetrabutylammoniumhydroxide to prevent the adsorption of cations onto fresh fracture surfaces of quartz, and filtered. Anionic as well as cationic species in the filtrate were determined by capillary ion analysis using both the hydrostatic injection as well as the electromigration injection modes.

From their anionic fluid composition, two large groups of pegmatite quartz could be identified, i) fluids with a dominance of HCO3- (from the Malkhan pegmatite field) and, ii) fluids containing a balanced mixture of Cl- and HCO3- (form the Adun-Chelon deposits). The cationic composition of the first group is dominated by Na, Ca and Li with smaller amounts of K, Mg and Zn. The fluids from the Adun-Chelon field are characterised by Na as the dominating cation with variable smaller proportions of K, Mg, Li and Zn. Some samples contain traces of Rb. Pegmatite quartz that is macroscopically inter grown with tourmaline also contains traces of boric acid in its fluid.

So far only one sample from Malkhan was found to contain traces of La in the bulk fluid.