New rock and mineral data from the Li (Sn, W, Nb, Ta)-deposit Cínovec-jih

 

Karel Breiter, Zuzana Korbelová, Vojtěch Šešulka, Sven Honig

Geoscience Research Reports 49, 2016, pages 113–121
Map sheets: Petrovice (02-14)

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Published online: 12 August 2016

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Abstract

The Sn-W (Li, Nb, Ta) deposit Cínovec/Zinnwald is situated at both sides of the Czech-German border in the eastern part of the Krušné hory/Erzgebirge Mts. The deposit is genetically linked to intrusion of late Variscan highly fractionated granite, the latest evolutionary stage of a volcano-plutonic system of the Teplice caldera. The ore-bearing albite-zinnwaldite granite forms a cupola-like mostly hidden body with an elliptical N-S elongated outcrop (1.4 ? 0.3 km). Its upper part to the depth of 750 m is formed by albite-zinnwaldite granites, while the deeper part with biotite granites. In the uppermost part of the pluton (the “canopy”) to the depth of ˜300 m, the granite is altered, crosscut with flat quartz-zinnwaldite veins and partly greisenized (Fig. 1).
Historical mining of quartz-zinnwaldite veins with cassiterite and zinnwaldite between 1378 and 1990 provided about 8-10 millions of metric tons of Sn + W ore. Increase in the prices of the high-tech metals and especially Li recently sparked a new wave of interest in the both parts of the Cínovec ore deposit: in Germany (Nessler - Seifert 2015), and in the Czech Republic (GEOMET company, this article). The company GEOMET started in 2014 a new borehole campaign in order to authenticate results of the old exploration and to summarize Li-resources in the southern part of the Cínovec deposit. The borehole CIS-2 was chosen for detailed petrological and mineralogical study (Tab. 1).
A Stockscheider (marginal pegmatite) forms several dm to 2.5 m thick layer at the contact between granite cupola and surrounding rhyolite. The upper part of the cupola to the depth of 200-250 m under the contact is formed by fine-grained albite-zinnwaldite granite composed of 46% quartz, 38% albite, 4 % relict of K-feldspar, 5% zinnwaldite, 5% sericite (alteration product of Kfs), and 0.2% fluorite. This granite crystallized in situ from volatile-enriched melt.
Mica free-granite (typically composed of 33 % quartz, 38 % albite, 28 % Kfs and max. 1 % mica) with intercalations of feldspatite forms ˜200 m thick zone beneath the albite-zinnwaldite granite. Fluids enriched in fluorine and metals released during crystallization of these feldspar-rich rocks migrated upward forming irregular bodies of quartz-zinnwaldite greisen dispersed within the albite-zinnwaldite granite.
Li-bearing tri-octahedral mica from granites and greisens should be termed as zinnwaldite. In addition to about 3.5 wt.% Li2O, it contains also ca 1 wt.% Rb2O (Tab. 2). Along the borehole CIS 2 upwards, the contents of Si, Li, and Rb systematically increase and the contents of divalent elements decrease (Fig. 3).
Among the oxide ore minerals (Fig. 2), cassiterite (Tab. 3, Fig. 4a), columbite (Tab. 4, Fig. 4b) and scheelite (Tab. 5) are common, while minerals of the microlite/pyrochlore group (Tab. 6) are rare. In terms of the debate on future exploitation of the deposits, the most significant mineralogical findings are as follows: (i) the main carrier of tungsten is scheelite, (ii) the main parts of Nb and Ta presented in the greisen form very fine-grained columbite, only minor parts are hosted in cassiterite.
 

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