Carbonate-silica gemstone deposit Vučkovica (central Serbia) – Geologic properties, genetic processes and deposition age.

Abstract

Carbonate-silica veins and masses in Vučkovica locality in central Serbia, classified as Kraubath type deposit, despite their thickness, are not economically significant as magnesite deposit due to silica content well above 0.3%. What makes this mineralisation economically insignificant as magnesite deposit – heavy silification of magnesite – is what enables its use as a gemstone. The regional hydrothermal activity along deep faults of the Sava-Vardar zone of central Serbia had caused the alteration of serpentinite and formation of carbonate-silica mineralisation. Unlike other deposits of the same type in this ophiolite belt, here the depth of the erosional level provides an opportunity to explore previously unknown features of these mineralisations, such as veins' trending, dip direction, dip angle and thickness, and somewhat different relationships between carbonate and silica constituents – namely predominance of magnesite with minor content of green dolomite and chalcedony. X-ray diffraction analyses have shown that carbonate minerals present are predominantly magnesite with minor dolomite, and that silica is fully crystallised despite its colloform structure. Energy-dispersive X-ray fluorescent (EDXRF) analysis indicated the presence of significant content of Nickel, which can cause the green colour of dolomite. Optical microscopy has shown that the precipitation process is carried out continuously between cryptocrystalline magnesite and crystalline dolomite, and that silica crystallite (fibre) size varies between cryptocrystalline and microcrystalline. As a gemstone deposit, Vučkovica can be considered small and with complicated internal setting, but with significant depth span, as is typical for Kraubath type deposits. Present gemstone types are considered "semi-precious", of low to moderate economic value, but with very pleasing aesthetic properties.