Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Zinc in diopside in marbles of southern part of the Moldanubian Zone (Czech Republic)


Vladimír Žáček, Radek Škoda

Geoscience Research Reports 50, 2017, pages 73–79
Map sheets: Vimperk (22-34)

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Published online: 29 June 2017

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The clinopyroxenes from marbles and associated calc-silicate rocks from the vicinity of Výškovice, Vnarovy and Sudslavice (Prachatice District, České Budějovice Region, Czech Republic, see Fig. 1) contain up to 0.89 wt.% ZnO (˜7,130 ppm Zn, 2.5 mol.% of petedunnite component), corresponding to an empirical formula of (Ca0.99Na0.01) (Mg0.74Fe0.21Zn0.024Mn0.02) (Si1.99Al0.01) O6. The marbles studied (Table 1) form lenses enclosed in biotite-sillimanite gneiss to migmatite of the Moldanubian Zone, Bohemian Massif, which underwent a polyphase metamorphic development at P = 0.4-0.8 Gpa, and T = 630-760 °C (e.g. Vrána et al. 1995).
The studied marbles are composed of major calcite, while minor is clinopyroxene (Di64-87 Hd11-35, Table 2), and even less abundant are wollastonite and phlogopite with scarce plagioclase, potassium feldspar, quartz and secondary actinolite. As accessory minerals were identified fluorapatite, titanite, zircon, rutile, pyrite, galena, and sphalerite (Figs 2 and 3).
Low S content 0.02 wt. % suggests that the amount of sulphide minerals is not exceeding ˜0.05 %. The mineral assemblage indicates the possible formation of the petedunnite component in clinopyroxene via the following reaction (modified after Essene - Peacor 1987): 2 CaSiO3 (wollastonite) + 2 ZnS (sphalerite) + 2 SiO2 (quartz) + O2 = 2 CaZnSi2O6 (petedunnite) + S2.
Analogous occurrences from various high-grade metamorphic units of the Bohemian Massif mostly contain Zn-bearing spinel as a main Zn-carrier, but in fact the Zn content in pyroxenes was mostly not determined (see e.g. Houzar - Novák 1995, Novák et al. 1997).
The content of Zn in clinopyroxene studied is the highest ever recorded in marbles in the Czech Republic, and one of the highest found in natural clinopyroxenes worldwide, with the exception of a unique Zn-deposit of Franklin, New Jersey, USA (Essene and Peacor 1987). It is close to the Zn-richest clinopyroxenes of Pb-Zn skarns of Åm¬meberg and Langban, Sweden, and the Nakatatsu mine, Japan, which contain up to ˜10,000 ppm Zn, whereas the clinopyroxenes from other localities display mostly concentrations not exceeding ˜1,500 ppm Zn (Huber et al. 2012, Fig. 4).
The experimental study of Huber et al. (2012) has shown that an increasing petedunnite component in pyroxene shifts its stability fields to higher pressures in a similar way as the jadeite component, and there is also negative correlation between the petedunnite component in clinopyroxene and fS2/fO ratio, as well. It is assumed that the main reason for Zn entering the clinopyroxenes and the absence of spinels in the studied marbles was due to the existence of a convenient protolith showing low Al/Si and fS2/fO ratios, high Si activity in fluids, and up to granulite facies metamorphism.