Obsahy galia, india a thalia v granitoidech Ceskeho masivu a jejich hlavnich horninotvornych mineralech

Elements Ga, In, and Tl of the 3rd group of the Periodic table belong in the Earth's crust among those that are rather scarce, and remain scattered forming only a few rare minerals. Although all three elements are interesting in terms of modern technologies, their behaviour in magmatic processes is known only insufficiently (Table 1). Approximately 25% of the world's known reserves of In are associated with highly fractionated granites (Schwarz-Schampera – Herzig 2002), so we decided to analyze the content of In and accompanying Ga and Tl in different types of Variscan granitoids of the Bohemian Massif and their rock-forming minerals to define trends in behavior of these elements during the magmatic differentiation.
Typical bulk-rock samples of peraluminous granites from the Moldanubicum and the western Erzgebirge, calc-alkaline granitoids of the Central Bohemian Pluton, K, Mg-rich melagranitoids (durbachites) of the Třebíč pluton and anorogenic granites from the central and eastern Erzgebirge were analyzed using ICP-MS in ACME laboratories in Vancouver. The contents of Ga, In, and Tl in trioctahedral micas and feldspars were determined by LA-ICP-MS at the Department of Chemistry, Masaryk University, Brno.
The contents of Ga, In, and Tl in bulk-rocks are shown in the Table 2, and in Fig. 1. The contents of Ga in granites are generally in a range of 15–30 ppm, with maxima in the strongly fractionated rocks up to 55 ppm. The contents of In are usually lower than 0.1 ppm, in fractionated granites about 0.2 ppm, max. up to 0.55 ppm In. The Tl-contents are commonly in a range of <1–6 ppm, in fractionated rocks 6–10 ppm, max. up to 14 ppm.
Vertical profile through the intrusion of rare-metal granite at Cínovec/Zinnwald along the borehole CS-1 illustrates the increase in the content of all the observed elements in the upper part of the cupola (Fig. 1). The content of In reaches its maximum of 0.35 ppm in a single analyzed sample of quartz-zinnwaldite greisen at a depth of 154 m. The distribution of Tl is less regular, because due to the crystal-chemical similarities between Tl and K it is strongly influenced by the variability of K-feldspar content along the drilling profile.
The contents of all three elements are always higher in mica than in coexisting feldspars (Table 3). Gallium reached 60 ppm in feldspars and 170 ppm in micas from strongly peraluminous rocks. Thallium may be enriched up to 55 ppm in K-feldspar and up to 70 ppm in mica. Indium was detected only in micas ranging from 0.2 to 0.8 ppm, with maximum in a greisen from Cínovec.
Our tentative study proved accumulation of Ga, In, and Tl in the late facies of strongly fractionated granitic systems in the Erzgebirge. These contents are similar in both the strongly peraluminous S-type granites (Nejdek pluton, Podlesí) and subaluminous A-type granites (Zinnwald). We conclude, that the contents of Ga, In, and Tl are governed by the degree of magma fractionation, and not by its geotectonic-geochemical affiliation.

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