The Zr-Ti mineralization in carbonatites of the Samchampi alkaline carbonatite complex, Assam, India

Viladkar S.G.*, Sorokhtina N.V.**, Senin V.G.**

* Carbonatite Research Centre, Amba Dongar Kadipani, India; ** Vernadsky Institute of geochemistry and analytical chemistry, Moscow, Russia

The Zr-Ti minerals are widespread in carbonatites of a many alkaline provinces - Kola Peninsula, Polar Siberia, Ukrainian, Canada, Brazil et al. We have found calzirtite and zirconolite in calcite carbonatites of the Samchampi alkaline complex (India). The Samchampi-carbonatite alkaline igneous intrusion is a roughly circular and stock-like body cropping out in Archaean of the Mikir Hills massif of Assam (eastern India). The Samchampi complex is composed of syenitic fenite, magnetite-perovskite-apatite rock, phoshatic rock, ijolite-melteigite, carbonatites of varied dimensions, chert breccia, nepheline syenite, alkali pyroxenite, phonoliteand volcanic tuff [1].

Carbonatites the first generation are presented with olivine-phlogopite sovite and sovite with pyrochlore. The second generation’s carbonatites are olivine-biotite sovite, beforsite and silicocarbonatites and they are changed metasomatically often. Carbonatites forms discontinuous dykes and veins intruding silicate rocks of massif.

Fig.1 (a) backscattered electron images of euhedral zirconolite showing chemical zoning and (b) replacement of calzirtite (Cz) with zirconolite (Zr) and baddeleyite (Bd) from the Samchampi complex.

Table 1 Microprobe analyses of zirconolite, calzirtite and baddeleyite (wt.%)

Sovite of the second generation with Zr-Ti mineralisation has been investigated in our studies. Carbonatite is represented with a medium-grained, magnetite-rich variety, which consists of aegirine, biotite, orthoclase, fluorapatite, ilmenite, baddeleyite, hematite, calzirtite, zirconolite, chevkinite-(Ce) and sulphides. Calzirtite is a first Zr-Ti mineral of this association, it forms small (up to 100 µm), euhedral grains, which are replied with zirconolite and baddeleyite (fig.1). The zirconolite occurs also as small (up to 50 µm) idiomorphic zoned crystal.

Table 2 Microprobe analyses of magnetite, ilmenite and chevkinite-(Ce) (wt.%)

Representative microprobe analyses of Zr-Ti minerals are listed in Table 1, 2. The chemical composition of zirconolite grains is inhomogeneous (table 1). Single crystals are commonly zoned with respect to REE, Ca, and Nb. The composition of zirconolite from the Samchampi has been plotted in terms (according to isomorphic scheme (Th⁴⁺ + U⁴⁺) + Ti⁴⁺ = REE³⁺ + (Nb⁵⁺ + Ta⁵⁺) [2]) together with data of different deposits for comparison (fig. 6). Chemical compositions for zirconolites from the Samchampi carbonatites follow the general trend typical of syenites and differ from those recorded for this mineral from the intrusive carbonatites.

Chevkinite-(Ce) is the only REE-bearing accessory mineral in carbonatites of the Samchampi massif (fig.3). The minerals of the chevkinite group are widespread in pegmatites of peralkaline granites and syenites, mafic gabbros, fenites, granulite facies gneisses, but occur occasionally in carbonatites [3]. Chevkinite-(Ce) forms a thin corona around ilmenite and magnetite grains. Apparently the chevkinite-(Ce) was formed by hydrothermal alteration of ilmenite or magnetite in the late stage of carbonatite.

Calzirtite, zirkelite and baddeleyite are primary Ti-Zr minerals of the Samchampi carbonatites. During the transformation of the calzirtite, Zr was liberated to produce baddeleite and zirconolite. Thus, hydrothermal alteration of carbonatites in the Samchampi massif causes evolutional changes of mineral associations. The trends of the transformation of primary Ti and Zr-minerals can be presented as: calzirtite - baddeleyite + zirconolite and ilmenite (magnetite) - chevkinite-(Ce).

Research covered by the RFFI grant

References:

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2 Bellatreccia F., Della Ventura G., Williams C.T., Lumpkin G.R., Smith K.L., Colella M. Non-metamict zirconolite polytypes from the feldspathoid-bearing alkali-syenitic ejecta of the Vivo volcanic complex (Latinum, Italy) // Eur. J. Mineral. 2002. V. 14. P. 809-820.

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