Gemstone deposits of Lece volcanic complex (South Serbia)

Gem raw materials do not have a long tradition in Serbia, neither concerning their geological exploration nor their utilization. Nevertheless, few relatively modest exploration campaigns discovered number of gemstone deposits and occurrences grouped into several regions. One of the most important regions is Lece volcanic complex which is situated in the southern part of Serbia and covers an area which exceeds 700 km². This volcanic complex, formed as a result of Tertiary volcanic (intermediate) activity, is a part of Serbo-Macedonian metalogenic province, i.e. Lece–Chalkidiki metalogenic zone. It comprises mostly andesite rocks and their pyroclastic equivalents.
   Gemstone deposits of Lece volcanic complex became the subject of interest after the World War II, although on the basis of certain archeological finds, we can assume that the Ancient Romans beside gold exploited amethyst and agate as well. The first modern explorations were carried out during 1970`s and at the beginning of 1980`s, when several deposits with calculated reserves were defined. The exploration was continued in 2002 and 2003. Laboratory analyses – at first micropetrographic, were followed by chemical and gemological (refractive index). Apart from the above mentioned investigations, in order to establish whether silica minerals have real gem quality gemstone processing (lapidary) was conducted.
   This paper deals with explored deposits (having reported reserves according to Serbian laws). There are two basic types of deposits: primary (hydrothermal) and secondary (sedimentary).
   Rasovača deposit. Precious minerals in this deposit occur in the same fracture zone together with metallic ore mineralization of Pb, Zn, Ag and Au (Lece underground mine). It is a quartz-brecciated fracture zone, with hydrothermal (epithermal) mineralization, a few kilometers long. Numerous intensive tectonic movements made space for the circulation of hydrothermal solutions which deposited not only galena, sphalerite, pyrite and gold but gem minerals as well. Precious silica minerals are represented by amethyst, amethyst-agate, and agate. Red jasper appears only in small quantities. Amethyst is characterized by a fine dark purple colour. Chalcedonic agate is represented by concentric bands of grey, bluish, brown, purple and red chalcedony.
   Bučumet deposit. In the succession of andesite lava flows and pyroclastic material, silica masses formed as plate-like ore bodies. These masses are result of depositing silica around thermal springs and geysers. This type of deposit is known as siliceous sinters or geyserites and represents second type of primary gem deposits in this volcanic complex. Siliceous mass, represented by fibrous chalcedony, granular quartz and relict opal, has very heterogeneous colour varieties. Basically, a very wide range of colours appears in short range. Chalcedony is represented by dominantly mixed and uniform colour varieties of white, bluish, gray, brown, red and black colour. Jasper is yellowish-brown to reddish-brown.Vrtače and Kameno rebro deposits. These deposits belong to the group of secondary deposits – placer type deposits. While Vrtače is an eluvial deposit in pyroclastic material with partially preserved primary ore body, Kameno rebro is a completely delluvial deposit formed beyond the volcanic complex in the surrounding Proterozoic metamorphic complex. Gem minerals which occur in these two deposits are of the same type as in the Bučumet. It is assumed that the material in the deposit of Kameno rebro mostly originates from the eroded part of Bučumet deposit.
   Apart from the above mentioned deposits with defined reserves, there are also numerous insufficiently explored occurrences, mostly placer ones (eluvial, delluvial, proluvial and alluvial). These occurrences are mostly concentrated out of the volcanic complex, i.e. on its eastern rim.