Geochemistry of alkaline magmatism, Burpala massif

Sotnikova I.A., Vladykin N.V.

A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk

vlad@igc.irk.ru

 

The Burpala massif belongs to unique rare-metal alkaline occurrences. The massif includes the intrusion of the central type (250 sq. km., 287 Ma).

The scheme of the massif’s magmatism is the following:

Rocks of the main facies: dykes of shonkinites à nepheline syenites à alkaline pulaskites à quartz syenites à Rocks of veined facies: nepheline and alkaline syenites à rare metal pegmatites à nepheline-sodalite mariupolites à alaskites and alkaline granites à dyke of apatite-fluorite rocks à veins of carbonates.

The pegmatites of the massif are associated with Zr, Nb, REE and Y deposits.

Average REE contents in alkaline silicate rocks of the main intrusive phases and dyke alkaline rocks of the Burpala massif are two times higher as compared with the Clarke level in syenites (500-4000 ppm) (Fig. 1.1). All spectra lines demonstrate a slight inclination that indicates the primary high contents of heavy REE rather than a slight differentiation of rocks. The rare-metal pegmatites from the Burpala massif are marked by a great dispersion of REE total, which varies from 500 ppm up to 36000 ppm that can be accounted for high alkalinity of pegmatites.

Following the REE spectra and considering the main minerals- REE concentrators we divided pegmatite bodies into 6 groups. Fig. 1.2 shows TR spectra of the first group of pegmatites. We can distinguish three varieties of pegmatites: 1 – with eudialyte, 2 - with Zr-silicates of lovenite-seidozerite group, 3 - with catapleite. All three pegmatite varieties are high alkaline with high values of Kagp, that leads to crystallization of Zr-silicates instead of zircon. Spectra lines starting from Eu demonstrate horizontal character with a slight increase towards final REE members.

Fig. 1. Distribution  of rare earth elements in rocks of  the Burpala massif

Symbols: 1 – syenites of the main phase, 2 – pegmatites of the 1^st  group, 3 – pegmatites of the 2^nd group, 4 – pegmatites of the 3^rd group, 5 – pegmatites of the 4^th group, 6 – pegmatites of the 5^th group, 7 – pegmatites of the 6^th group, 8 – apatite-fluorite rocks, carbonatites.

The pegmatites of the second group are marked by significant content of astrophyllite and variable, insignificant concentrations of loparite and Zr-silicates. This variety shows a sinusoid-like ТR spectrum line (Fig. 1.3). Total REE concentration is comparable with that in pegmatites of the first group.

Pegmatites of the third group are characterized by significant concentrations of Mn-ilmenite. Loparite and zircon-silicates are found in addition to manganese-ilmenite. ТR spectrum of these pegmatites (Fig. 1.4) demonstrates an insignificant inclination and high REE total.  Eu fractionation in them is almost completely lack and spectrum line after Eu is almost horizontal.

Loparite and Zr-silicates are the main REE concentrators in pegmatites of the fourth group. Spectrum ТR is presented on Fig. 1.5. The spectrum is characterized by Sm anomaly, which is almost on one line with Eu.

Loparite and ilmenite in cases pyrochlore are the main concentrators in the pegmatites of the sixth group. Spectra ТR (Fig. 1.6) have specific features. In the beginning there is a sharp drop of spectrum line up to Eu, then there is a sharp rise of a spectrum from Eu to Gd and further almost horizontally up to Lu.

Pegmatites of the fifth group are characterized by leucocratic, essentially albite composition. Rare metal minerals are scarce and ТR total is low. REE spectrum is different from other varieties (Fig. 1.7). In the beginning from La to Sm the spectrum line is almost horizontal, a positive Eu fractionations is observed later and further from Gd to Lu the spectrum line is horizontal again. Eu fractionation is associated with its accumulation in albite components of pegmatites.

The latest occurrence of the massif is apatite-fluorite vein containing magnetite, pyroxene and mica. The thermobarogeochemical studies indicate temperature of fluorite formation as 560^oС and that of apatite as over 800oС. REE spectrum is characterized by a smooth inclination with positive Gd anomaly, the reason of which is not clear to us (Fig. 1.8).

            In addition we found the vein of calcite carbonatite with sulfides in host rocks and a vein of calcite-brewsterite carbonatites in the center of the massif. These carbonatites are characterized by REE spectrum, which is similar to the spectrum of apatite-fluorite rocks (Fig. 1.8).

Fig. 2. Distribution spectra of rare elements in rocks of the Burpala massif. Symbols: 1 – rocks of the main phase, 2 – mariupolites, 3 – pegmatites, 4 – phenites, 5 apatite-fluorite rocks, carbonatites.

As a whole REE spectra in the studied early rocks of the massif and pegmatites are characterized by the a great similarity that indicates their genetic relationship. The distribution of rare elements on the spider-diagrams also verifies the genetic association of all rocks of the Burpala massif (Fig. 2).

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This study was financially supported by RFBR grant 09-05-00116.