Zinc-rich and copper-bearing amphiboles from sulfide-free ore occurrences of the Pelagonian Massif, Republic of North Macedonia

This paper describes specific features of isomorphism of unusual amphiboles containing up to 23 wt% ZnO and up to 1.3 wt% CuO from sulfide-free metasomatic rocks of the "Mixed Series" metamorphic complex, Pelagonian Massif, Republic of North Macedonia, in which chalcophile elements (Pb, Cu, Zn, Sb, As) occur in high concentrations in the form of oxides and oxysalts. The studied amphiboles belong to six mineral species: magnesio-riebeckite, ferri-winchite, F-dominant analogue of ferri-winchite ("ferri-fluoro-winchite"), ferri-barroisite, ferri-nyboite, and a zinc-rich amphibole related to clino-suenoite. The main substitution schemes are Zn -> Mg and Na + Fe3+ -> Ca + Mg. The amphibole related to clino-suenoite has been studied in the most detailed way. Its empirical formula is (Mg2.99Zn2.47Mn1.26Fe0.303+Ca0.07)(sigma 7.09)(Si7.89Fe0.063+Al0.05O22)(OH)(2), and it is the most Zn-rich amphibole among those that have ever been described in literary sources. This amphibole is characterized by single-crystal and powder X-ray diffraction, IR spectroscopic and optical data. The role of high barium activity in the formation of sulfide-free ores of chalcophile elements has been substantiated. A possible mechanism of sulfur immobilization can be described by the scheme S2- + 2O(2) <-> SO42-; SO42- + Ba2+ -> BaSO4 down arrow.