Wolframoixiolite – new accessory mineral from subalkaline zinnwaldite granites of Severniy massif (Chukotka, Russia)
Alekseev V.I., Marin Yu.B.
Saint-Petersburg State mining institute named after G.V.Plekhanov (technical university), Saint-Petersburg, Russia
In the reports at the previous conferences (Alekseev, 2008; 2009) it was stated that the cause of wide spread occurrence of rocks rich in rare elements in the Severny massif are the intrusion of subalkaline leucogranites of lithium-fluoric geochemical type and associated metasomatic transformation of biotite granites. The problem of selection of minerals which are capable to be a benchmark in regional petrological and metallogenic investigations has arisen because of the convergence of chemical characteristics of the granites of earlier and late complexes (Alekseev, 2005). Wolframoixiolite – new accessory mineral from subalkaline rare-metal granites it is offered to use as such benchmark in Northeast region.
At present wolframoixiolite – (Fe,Mn,Nb)(Nb,W,Ta)O4 (Ginzburg et. al., 1969) is discredited as a mineral species but it is widespread (common) mineral in rare-metal pegmatites of Canada, Czechia, China, Italy, Madagascar, Slovakia, Thailand, Ukraine as well as in topaz granites of Algeria, France, Kazakhstan, Morocco and Siberia. Our investigations let us to discover wolframoixiolite in zinnwaldite granites and ongonites of the Severniy pluton which is about 30 sq. km in area (Chukotka) where this mineral is one of the common accessory minerals.
In composition the investigated mineral (Fe0.46Nb0.31Mn0.14Sc0.05)0.96(Nb0.40W0.30Ta0.15Ti0.15)1.00O4.00 is more similar to wolframoixiolites from rare-metal granites of Algeria, France, Kazakhstan, Morocco (Marignac et al., 2001; Letnikov, 2008). High ratios Ta2O5/Nb2O5, FeO/MnO as well as the absence of Sn are specific features of the Chukchi wolframoixiolite composition. Similarity of crystal-chemical characteristics of wolframoixiolites is confirmed by the trend of their composition changes distinguished earlier by P. Černý (2007) (Fig. 1).
Fig. 1. Compositions of the wolframoixiolite (1) and niobian wolframite (2) from subalkaline zinnwaldite granites of Severny massif, Chukotka.
Being included in zinnwaldite, topaz, quartz, microcline and albite long prismatic wolframoixiolite grains (1-35 x 5-150 μm) (Fig. 2) constantly contain xenomorphic inclusions of niobian ferberite (Fe0.62Mn0.20Nb0.12 Sc0.04)0.98(Nb0.23W0.71Ti0.06)1.00O4.00 which are similar in composition to wolframite from rare-metal pegmatites of the Písek region, Czech Republic (Černý P. et al., 2007). Relatively low Nb content and the absence of Sn and Ta are characteristics of wolframite composition.
The Chukchi wolframoixiolite is peculiar in morphology and composition and represents a metastable variety of ixiolite. Its formation is associated with crystallization of Nb, Ta, W- enriched ongonite melt. Wolframoixiolite and niobian wolframite are key indicators of subalkaline rare-metal magmatism in investigated region.
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