Zmeny chemickeho slozeni kremene behem greisenizace, predbezne vysledky ze zapadnich Krusnych hor

Occurrence of so-called "vein greisens" is one of characteristic features of the Variscan peraluminous granites in the western part of the Krušné hory/Erzgebirge area (Nejdek-Eibenstock Pluton, Horní Blatná body). The "veins" actually represent steeply dipping zones consisting of tens to hundreds of individual roughly parallel cm- to dm-thick stringers of metasomatic greisen reaching total thickness to several meters, and lengths of hundreds of meters. They mostly consist of quartz and Li-bearing mica with some topaz and cassiterite. Greisens of this type were mined at Přebuz, Rolava and Horní Blatná since the 15^th century until 1945.
The aim of this study is to distinguish magmatic and hydrothermal quartz in greisen, i.e. to differentiate relics of the original magmatic quartz from quartz originated hydrothermally during greisenization. We studied a series of samples from the historic mine Streitpingen situated in the Horní Blatná granite body near the village of Potůčky. The parent rock of greisens is medium-grained alkali-feldspar granite composed of 40 vol.% quartz, 29 vol.% albite, 20 vol.% perthitic K-feldspar and 9 vol.% Li-enriched biotite with small amount of topaz, apatite, rutile, monazite and zircon (Tables 1, 2). The zone of greisenization is up to 5 m thick and enriched with quartz (87 vol.%) and topaz (9 vol.%). The content of mica decreased to 3 vol.% and both feldspars disappear. Greisen is penetrated by monomineralic up to 10 cm thick quartz veins (> 97 vol.% quartz) with abundant tiny cavities. Veins are composed of clear long columnar crystals (5 mm diam., up to 5 cm long) which are coated with a thin layer (˜ 1 mm) of milky white quartz.
For comparison, we analyzed also quartz from quartz-tourmaline fillings of miarolitic cavities (5-10 cm diam.) in granite, traditionally called as "tourmaline suns", which are widespread in the central and northern parts of the Nejdek-Eibenstock Pluton (Schust et al. 1970).
Internal structure of analyzed quartz grains visualized by cathodoluminescence (CL) is shown in Figs 1 and 2. Trace elements in quartz were analyzed using laser-ablation ICP-MS (for detail of the method used see Breiter et al. 2013). Average contents of the analyzed element in selected quartz crystals are shown in Table 3, all the individual analyses of selected elements are shown in Fig. 4. Contents of Al, Ti, and Li across selected crystals are given in Fig. 5; position of these profiles is shown in Figs 2b, c and 3.
Aluminum, Li, and Ti are the most abundant trace elements in quartz. The content of Al in phenocysts of magmatic quartz and quartz from tourmaline suns corresponds roughly to 400 ppm (range 300-500 ppm), whereas only 115-270 ppm were found in the greisen quartz. The Al-contents in hydrothermal quartz strongly fluctuate: the clear domains contain 50-340 ppm Al, while 500-2400 ppm were detected in cloudy crystal cores, and 1100-4800 ppm Al in the milky white crystal rims. Content of Ti in magmatic quartz fluctuates between 40 to 100 ppm with extreme values up to 200 ppm near the margins of some crystals. Quartz from tourmaline suns contains 10-40 ppm Ti, while quartz from the greisen approximately 20 ppm Ti, and the hydrothermal quartz from veins generally contains < ppm Ti. The contents of Li in magmatic quartz and quartz from tourmaline suns range between 30-50 ppm. Contents of Li in quartz from greisen and clear domains of hydrothermal quartz are rather smaller, approximately 10-30 ppm. The highest Li-values in the range of 70-170 ppm were found in the milky hydrothermal quartz. The contents of Li and Al reveal positive correlation.
Our preliminary research has shown that quartz originated during metasomatic greisenization differs from the primary magmatic quartz by lower intensity of cathodoluminescence and significantly lower concentrations of trace elements Al, Ti, and Li.

References

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