Lithium-rich granite in the Lysina-V1 borehole in the southern part of the Slavkov Forest, western Bohemia

 

Veronika Štědrá, Tomáš Jarchovský, Pavel Krám

Geoscience Research Reports 49, 2016, pages 137–142

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Published online: 12 August 2016

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Abstract

Three core boreholes were drilled in the Slavkov Forest to document in situ bedrock lithology and chemistry in each of three contrasting forest catchments (Lysina, Na Zeleném and Pluhův Bor). This contribution provides a primary documentation of the core from the drillhole LY-V1 located in the Lysina catchment (Fig. 1), draining water from the Lysina-type granitic domain in the Lesný-Lysina (-Kynžvart) Massif. It forms a minor massif containing the Li-Rb rich granites belonging to the younger intrusive complex (YIC) of the Karlovy Vary Composite Pluton. The whole magmatic system belongs to the Krušné hory-Fichtelgebirge plutonic complex, late Variscan in age.
The LY-V1 is a 30.3 m deep borehole with complete core recovery. Field documentation, rock samples and new geochemical data show the whole section comprises several facies of highly evolved Li-F rich granite of the Lysina type varying in grain size, proportion of porphyroclasts and matrix, and feldspar ratios (Fig. 2). The central part of the core shows disseminated haematite pink pigmentation of K-feldspars related to the oxidic mineralization along the system of brittle fractures. An abridged description of variations of the Lysina granite along the LY-V1 borehole is given in Tab. 1.
Microscopy indicates variable proportions of K-feldspars, sodic plagioclase, quartz, Li-mica, muscovite, topaz, and accessory minerals apatite, fluorite, zircon, monazite, and Fe-Ti opaque phases. The upper 28 metres of the core exhibit strong signs of surface alteration accompanied by intense sericitization and hematitization, and the whole drill core displays effect of autometamorphism. The latter process was described from the Krušné hory Mts. for other Li-rich Sn-bearing granites as the mechanism of penetrative mineral changes throughout granitic bodies, e.g., albitization and alteration of both feldspars, mainly K-feldspar, replacing them partially by kaolinite and sericite, the influence on the ion exchange in Li-micas, and epi-magmatic replacement of topaz by micas and associated precipitation of minor newly formed fluorite and apatite. Li-rich mica (probably polylithionite) is a member of the Li-biotite-zinnwaldite series. Textures of fluorescent primary and secondary major and accessory minerals are indicated by cathodoluminescence images on Fig. 4. The normative mineral composition in the analysed samples has been computed using the EVOLGRA programme (Dolejš - Štemprok 2001, Tab. 3.).
When plotted on the binary classification diagram of Debon and Le Fort (1983), samples show a wide range of composition from prevailing adamellites (quartz monzonites) to minor leucogranites (in the deepest part of the drill hole at the depth of 28.0-30.3 m), and alkali-feldspar granite (less silicic cumulates enriched in K2O at depth of 22.3 and 26.0 m). Chemical characteristics of eight samples (Tab. 2) confirm affinity of the heterogeneous granite from LY-V1 to the Li-Rb rich S-type monzogranites (adamellite) to leucocratic granites typical of the youngest members of the YIC.
 

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