From late March to mid-June 2020, the mining fronts of the northern area in the 11th horizon of the Trepca Stan Terg mine, 745 m below the surface, went through an extraordinarily bournonite-rich zone, with very sharp cogwheel-shaped crystals (cyclic twins) of good size. The species had been first first reported in this historical Balkans deposit in 1937 (1), seven years only after the onset of the modern mining activities. The bournonite has therefore continuously occurred in Trepca, although always in a modest and sparse way: lately in 2014-2016, several specimens, among the most significant to this day, had been found, featuring 0.5 to 1 cm crystals set on pyrite. The last year in 2019, small bournonite crystals were found, for most of them small and very dull, associated with elongated arsenopyrite needles. In the last months, several types of specimens were found, almost all of them cleaned with diluted hydrochloric acid to remove the carbonate layer that coated the sulphides: the beginning of the find in late March - early April stood out from the rest for amazing combinations of 1 to 2.5 cm bournonite crystals set in a very airy way on the tips of large pyrite pseudomorph after pyrrhotite blades. The find continued in May-June with specimens showing a more ‘classic’ architecture, in the form of plates sometimes reaching a large size (25-30 cm) and bearing pluricentimetric bournonite crystals, rarely up to 5-6 cm long. Most of the specimens display tarnished crystals oxidized during the acid etching process showing then a blackened or dimpled appearance, but in the best cases, the crystals that probably initially had a particularly good surface state ended up perfectly lustrous, with very attractive grey color and metallic luster. A few specimens show a gravity-deposited coating of sulphide microcrystals which chemical nature has yet to be determined. This discovery now elevates Trepca Stan Trg to the level of the very best European localities for the species, among the Herodsfoot mine in Cornwall, United Kingdom, the Altenkirchen district and Harz (Neudorf) deposits in Germany, Pribram in Czech Republic, Baia Sprie in Romania. In France, let’s mention among others the La Mure and Mésage alpine mines in the Alps, the Pontgibaud and Les Malines mines in the Massif Central and Saint-Pons in the southeastern Alps.

(1) Gagarin, G. and Pavlović, S. 1937, Bournonite from Trepca mine, Jugoslavia. Geological Record of the Balkan peninsula.

Lucas ETIENNE

Trepca: a surprising metallic deposit...
The metallogenic belt of Trepca encompasses several Pb-Zn-Ag deposits and about 40 mines, among which 7 are nowadays operating. Stan Terg is by far the most important deposit, both from the economy and mineralogy points of view: the vast majority of the collection specimens come from the eponymous mine. The deposit formation is related to a late alpine magmatic event, evidenced by the intrusion during the Oligocene-Miocene of granodiorite plutons and andesite pipes in low grade metasedimentary rocks. The Stan Trg mineralized bodies replace some metacarbonate rocks and are developped along a 100-200 m in diameter phreatomagmatic pipe, eroded in surface and kilometric-scale downdip. The limestones host rocks are not always metasomatized and the sulphide mineralization can be, or not, associated with calcic skarns.