Abstract

Gem-quality (cabochon) ruby-bearing occurrences (here termed PAR-1 and PAR-5) located near Paranesti, north eastern Greece have been systematically studied for the first time in this paper. Tectonically, the occurrences are located within the Nestos Shear Zone (NSZ). The NSZ separates two distinct geological units. The Rhodope Terrane is a heterogeneous unit of gneisses, mafic, ultramafic, and meta-sedimentary rocks in the hanging wall. The footwall Pangaion-Pirin Complex consists of marbles and acid gneisses of a Mesozoic carbonate platform on pre-Mesozoic continental basement. In this paper, a range of petrographic and geochemical techniques were used to determine (1) any similarities and differences to other mafic-ultramafic hosted ruby deposits worldwide; (2) distinctive geochemical fingerprints for Paranesti; and (3) the likely P-T conditions of formation.

Detailed petrographic and whole-rock analyses utilizing ICP-MS, XRF, and XRD have found the Paranesti corundum to be of a mafic/ultramafic protolith with approximately 40 wt.% SiO2, 16 wt.% Mg, 11000 ppm Cr, and 440 ppm Ni. EMPA major element analysis determined the mineral inclusions within the corundum grains to be picotite and hercynite spinels. Pargasite is the dominant amphibole within the corundum-bearing amphibole schist host. The surrounding non-corundum bearing chlorite schist mainly comprises clinochlore. Petrographic examination of the mineral assemblages within the corundum-bearing schists revealed strong fracturing and alignment (parallel to the main regional foliation) of the corundum grains and margarite reaction rims around the corundum. The surrounding non-corundum amphibolites also contain anorthite, along with relict sillimanite, kyanite, and chlorite/muscovite/epidote overprinting. Detailed LA-ICP-MS trace element analysis of the color range of corundum from the two occurrences showed the corundum to be mainly of metamorphic origin, though pale rubies from PAR-5 suggest some metasomatic influence.

The corundum displays distinctive geochemical locality signatures, with a combination of high Cr (average 2300 ppm with 15% sample points on core positions >5000 ppm and maximum 8600 ppm); high Si (average 1400 ppm with 40% over 1500 ppm and maximum 2500 ppm), low Mg (average 30 ppm), and very low V, Ti, and Ga. Based on the literature for similar occurrences, and the mineral assemblages observed at Paranesti, the estimated P-T conditions of corundum formation are <7 kbar and <750 °C, similar to the mafic African amphibolite-hosted rubies. This study has found the Paranesti occurrences to be most similar to the Winza, Tanzania ruby deposit, whilst there are some similarities to other high-Cr ruby deposits, primarily the Fiskenæsset, Greenland and metamorphic amphibolitic schist hosted African deposits. The Paranesti corundum most likely formed during regional amphibolite facies metamorphism which created the Nestos Shear Zone, along with a lesser influence (primarily observed in the PAR-5 occurrence) of more localized metasomatism. Subsequent multiple greenschist facies retrogression of the occurrences resulted in the current-day host amphibole-chlorite schist assemblages.

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