Unusual mineralization with niobian titanite and Bi-tellurides in scheelite skarn from Kamenné doly quarry near Písek, Moldanubian Zone, Bohemian Massif

Abstract

Clinopyroxene-garnet scheelite skarn containing accessory niobian titanite, native bismuth and Bi-tellurides of the Bi2X and Bi5X3 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. % Nb2 O5; ≤ 0.164 apfu Nb) and locally Ta (≤ 4.07 Ta2O5; ≤ 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 R4+ and Al + (Nb5+, Ta5+) with R4+ accompanied by low contents of Na in titanite correspond to the substitutions A1(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 Bi1.98 (Te0.66 S0.34 Se0.02)∑1.02 is a member of the Bi2X series, whose members were described from numerous localities as unnamed minerals. In contrast to the published data this phase contains increased sulphur contents up to 0.33 apfu. The second Bi-(sulpho-) telluride with empiric formula (Bi4.51 Fe0.40 Cd0.3Pb0.02 Sb0.01 As0.01)∑4.98 (Te1.58 S1.36 Se0.08)∑.02 is probably a S-rich member of the Bi5X3 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.