Composition and crystallization conditions of rare-metal dyke rocks in Eastern Kazakhstan

Sokolova E.N., Smirnov S.Z., Khromykh S.V.

Novosibirsk State University; V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

ekaterina@igm.nsc.ru

Magmatic complexes with related rare-metal mineralization are widespread within the Greater Altay (the area including North-West of Gorny Altay, Mogolian and Chinese parts of Altay Mountains). In the Kazakhstan rare-metal magmatic rocks and pegmatites compose a part of large batholiths of Kalba-Narym complex and some of dyke belts. Rare-metal rich granitic dykes were found in two separate belts – Chechek and Akhmirovka, in the North-Eastern part of Irtysh folded zone situated at the border of Altay Mountains near Ust-Kamenogorsk city in the Eastern Kazakhstan.

The dyke belts consist of rare-metal rich felsic rocks. They were formed at the end of Irtysh folded zone evolution and are confined to the fault zones cross-cutting its major tectonic structures. The dykes cut Devonian gabbroid intrusion and metamorphic rocks. Radiogenic ages of single dykes (265-280 Ma) show that they were formed later than Kalba-Narym batoliths and pegmatites (280-290 Ma) [1, 2]. According to the geological position the dykes were joined into the single complex along with dykes of basic and intermediate composition. However no dykes of basic compositions were found within the studied belts. The appearance of magmatic complexes with the same geochemical specialization (enrichment of rare metals), which were formed separately in time within the same region indicate complex history of rare-metal magmatism at the Permian – Triassic boundary and rise a question about the magma sources and their evolution in time and space.

Geology and geochemistry of the dyke belts

The dyke belts are several kilometers long. Single dyke thicknesses are about 1-2 m. Chechek belt contains big amount of dykes striking to North-East. Akhmirovka belt is situated about 20 km to North-West from Chechek belt and contain several dykes oriented similarly to the Chechek ones.

The dykes are composed of microgranite, granite-porphyry and felsite-porphyry. Major minerals of phenocrysts are quartz, albite, K-feldspar and muscovite. The groundmass is composed of the same minerals.

The major compositional features of the studied rocks are high concentrations of rare lithophyle elements (Li, Rb and Cs, up to 4000 ppm in total) and fluorine (up to 1.5 wt %). This makes them similar to rocks of the ongonite group [3]. Detailed geochemical study revealed significant variations in rare metal, fluorine and total REE contents of the rocks. The rocks of Chechek belt divide into two groups. The first one has high concentrations of rare metals (up to 2500 ppm) and low total REE (3-15 ppm) (Fig. A,B). These rocks show highest concentrations of F (up to 1.4 wt %) and P₂O₅ (up to 0.35 wt %) (Fig. C). High concentrations of F and P in these rocks result in appearance of accessory topaz and apatite. The second group is composed of rocks with lower rare element (up to 1000 ppm), F (up to 0.45 wt %) and higher total REE (40-100 ppm) contents. The rocks of these two types differ in La_N/Yb_N. The first group show negative slope with La_N/Yb_N 3-5, while the second one have shallower or flat slopes with La_N/Yb_N 0.7-3. The rocks of Akhmirovka belt differ from those of Chechek one by higher rare metal (up to 4000 ppm) and total REE (110-180 ppm) contents with positive REE slopes (La_N/Yb_N 0.3-0.4). F content is similar to that of the Chechek (Fig. A,B,C).

Melt and fluid inclusion study

Quartz phenocrysts of the dyke rocks contain melt (MI) and fluid inclusions (FI). The phenocrysts from Chechek and Akhmirovka belts contains similar FI and MI assemblages. At room temperature MI consist of aggregate of silicate daughter minerals. The studied inclusions often contain fluid segregation. The absence of glass in the MI indicate that their entrapment and subsequent cooling occur under hypabyssal conditions and was slow enough to provide complete crystallization of the entrapped melt. MI sizes vary significantly and reach 30-50 mkm. Large and medium size MIs are surrounded by decrepitation haloes of tiny fissures and fluid inclusions. Primary FIs are composed of low-salinity (1,5-8 wt% NaCleq) aqueous solution and gas bubble.

On heating under atmospheric pressure the majority of MIs were decrepitated due to a high internal fluid pressure. Homogenization experiments were conducted in autoclave under external water pressure 2 kbar by a quenching method. Stepwise heating within 660-625⁰С with quench at each step was applied in order to register behavior of inclusions on heating. Melt inclusions homogenize into silicate melt at temperatures ≤ 625⁰C. Low crystallization temperature is apparently caused by highly evolved melt compositions with high concentrations of fluorine and water, which decrease solidus temperature.

Chemical composition of homogeneous and re-melted MI was studied by EDS and EMP analyses. The quenched MI glasses contain moderate silica (68 – 70 wt%) and elevated F (up to 1.5 wt%) and phosphorus (P₂O₅ up to 0,4 wt%). Assuming that deficiency of analytical totals of EMP analyses is due to water content about 4-7 wt%. The majority of glasses are peraluminious (A/CNK =1,1-1,4) with sodium predominating over potassium. In general compositions of melt inclusions correspond to the bulk rocks. It is important to note that compositional differences in the bulk rock compositions is reflected by MI glasses (Fig. C,D). For instance MIs from phenocrysts of rare-metal enriched rocks of Chechek belt have the highest P and F content.

Fig. A - Chondrite normalized REE spectrum of rocks of Chechek and Akhmirovsky dyke belts, на которых видно выделение трах типов пород. B –Histogram of Li+Rb+Cs sum distribution in rocks. C, D – phosphorus and fluorine content in rocks (C) and melt inclusions (D).

1 – rocks of Akhmirovsky dyke belt, 2 – low rare-metal rocks of Chechek dyke belt, 3 – high rare-metal rocks of Chechek dyke belt. In the fig. D – composition of melt inclusions in the corresponding rocks.

Discussion

The data obtained in this study indicate that Chechek and Akhmirovka dyke belts in general have similar geochemistry and are built up of the ongonitic dykes. However on the basis of REE geochemistry and rare metal contents the dyke rocks are divided into three different groups. This indicates that in spite of similar geochemical character their magmatic sources have separate differentiation history. The difference between Chechek and Akhmirovka rock compositions may be related to differentiation of spatially divided magmatic sources. However within Chechek belt compositional variations can be explained by intrusions of magma from the sources located at different geological levels.

The study of melt inclusions revealed that quartz phenocrysts crystallized at relatively low temperatures (~625°C) and high fluid pressure. The crystallizing magma was saturated in water and fluid phase coexisted with the silicate melt. In the course of magma ascent some large MI decrepitated. This resulted in appearance of fissure haloes containing inclusions of magmatic aqueous fluid phase. The similarity of MI and bulk rock compositions, however, indicates that there was no significant melt crystallization after the MI entrapment before the dyke emplacements. Thus the formation of the dyke belts is related to relatively shallow magma sources.

Variations in compositions of rare-metal rich dyke belts indicate complex history of the final stages of rare metal magmatism in the North-Western part of the Greater Altay. The data obtained indicate that the formation of dykes and their source evolution take place under different conditions, fluid regime and probably within relatively long time period.

This work was supported by RF President grant for young scientists МК-1753.2012.5.

References:

1. Dovgal’ V.N., Tribunsky E.N., Sabotovich S.A., Distanova A.N. Geological and compositional features of rare-metal lithium-fluorine granitoids of Altay // Russian Geology and Geophysics. 1995. Vol. 38, № 11, p.1807-1841.

2. Maslov V.I., Kozlov M.S., Dovgal’ V.N., Distanova A.N. Complex of ongonites and lithium-fluorine granites in south-west Altay // Petrology. 1994. Vol. 2, № 3, p.331-336.

3. Pushko E.P., Shamanaeva V.V., Stapanov A.V., Shipovalov Yu.V. About finding of Li-F granite-porfires (analogs of ongonites) in Kalba-Narym ore belt // Geology, geochemistry and mineralogy of rare element deposits. 1978. Vol.5, p. 3-19.