Journal of Geosciences, 53 (2008), 1–16 DOI: 10.3190/jgeosci.017 www.jgeosci.org Original paper Unusual mineralization with niobian titanite and Bi-tellurides in scheelite skarn from Kamenné doly quarry near Písek, Moldanubian Zone, Bohemian Massif Stanislav HOUZAR 1* , Jiří LITOCHLEB 2 , Jiří SEJKORA 2 , Jan CEMPÍREK 1 , Jaroslav CÍCHA 3 1 Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic; [email protected] 2 Department of Mineralogy and Petrology, National Museum, Václavské náměstí 78, 115 79 Prague 1, Czech Republic 3 Prácheň Museum, Velké náměstí 114, 397 24 Písek, Czech Republic * Corresponding author Clinopyroxene-garnet scheelite skarn containing accessory niobian titanite, native bismuth and Bi-tellurides of the Bi 2 X and Bi 5 X 3 series, together with Fe- and Fe-As sulphides, is associated with the contact of chondrodite marble with melanocratic granite–quartz syenite (durbachite). Hedenbergite-rich vein in this marble can be considered as an additional type of skarn. The presence of accessory niobian titanite is typical, in particular, of the veins. In addition to increased Nb contents (≤ 10.6 wt. % Nb 2 O 5 ; ≤ 0.164 apfu Nb) and locally Ta (≤ 4.07 Ta 2 O 5 ; ≤ 0.038 apfu Ta), it is relatively rich in Al (≤ 0.253 apfu). Locally it contains elevated Fe, Sn, Zr and F contents. Negative correlations of Al + F with R 4+ and Al + (Nb 5+ , Ta 5+ ) with R 4+ accompanied by low contents of Na in titanite correspond to the substi- tutions Al(OH,F)(TiO) -1 and Al(Nb,Ta)Ti -2 . Niobian titanite has anomalous composition if compared to skarn mineral assemblages worldwide. The Bi-(sulpho-) telluride with empiric formula Bi 1.98 (Te 0.66 S 0.34 Se 0.02 ) Σ1.02 is a member of the Bi 2 X series, whose members were described from numerous localities as unnamed minerals. In contrast to the publis- hed data this phase contains increased sulphur contents up to 0.33 apfu. The second Bi-(sulpho-) telluride with empiric formula (Bi 4.51 Fe 0.40 Cd 0.03 Pb 0.02 Sb 0.01 As 0.01 ) Σ4.98 (Te 1.58 S 1.36 Se 0.08 ) Σ3.02 is probably a S-rich member of the Bi 5 X 3 series. In the evolution of the skarn it is possible to distinguish stage I responsible for the formation of the older Mg- and younger Ca-skarns, and stage II, characterised by increased Fe contents. The scheelite-sulphide mineralization was formed during a third (retrogression) stage followed by brittle deformation. Emplacement of siderite ± quartz veins represents the fourth (terminal) stage of mineralization. Keywords: niobian titanite, Bi tellurides, scheelite, sulphides, electron microprobe, marble, skarn, Moldanubian Zone Received: 30 January 2008; accepted 5 March 2008; handling editor: S. Vrána 1. Introduction Titanite CaTiOSiO 4 , including varieties rich in (Al, OH, F) components, is a typical and common mineral in calc-silicate rocks of various genetic types. In contrast, Nb-rich titanite (> 0.5 wt. % Nb 2 O 5 ) is rather rare, being restricted mainly to peralkaline rocks and pegmatites. Its occurrence in calc-silicate rocks – including skarns – is exceptional. The Bi-tellurides on the other hand can be typical accessory minerals in skarns but so far they were considered very scarce in skarns of the Bohemian Massif. Within the Bohemian Massif, skarns of variable ge- netic types and various mineral assemblages represent characteristic rock type especially in the Moldanubian Zone. Besides the regionally metamorphosed skarns (e. g. Němec 1991; Pertold et al. 1997), there are also smaller bodies of contact skarns restricted to contacts of marbles with silicate rocks (Suk 1974; Fišera et al. 1986; Houzar 1998; Drahota et al. 2005). Unusual mineraliza- tions with niobian titanite and Bi-tellurides were recently discovered in a skarn developed at the contact between chondrodite marble and potassic melanocratic granite to quartz syenite (durbachite) at the Kamenné doly quarry near Písek. This unusual assemblage is in focus of the current contribution. 2. Geological setting The Písek area belongs to the Moldanubian Zone, the presumed high-grade root of the Variscan orogen in Bo- hemian Massif (Dallmeyer et al. 1995). It features leuco- cratic migmatites and orthogneisses, together with minor granulite occurrences accompanied by ultramafic rocks, representing the Podolsko Complex. In addition there are minor domains of graphite quartzite and marble enclosed in biotite gneiss belonging most likely to the Varied Unit (Fig. 1). The metamorphic complex was intruded by gran- itoid plutons, including amphibole-biotite granodiorite (Červená type), by ultrapotassic porphyritic amphibole- biotite melagranite to quartz syenite (durbachite), and
Unusual mineralization with niobian titanite and Bi-tellurides in scheelite skarn from Kamenné doly quarry near PÃsek, Moldanubian Zone, Bohemian Massif
Jun 23, 2023
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