Metamorphic and Metasomatic Kyanite-Bearing Mineral Assemblages of Thassos Island (Rhodope, Greece)

Abstract

The Trikorfo area (Thassos Island, Rhodope massif, Northern Greece) represents a unique mineralogical locality with Mn-rich minerals including kyanite, andalusite, garnet and epidote. Their vivid colors and large crystal size make them good indicators of gem-quality materials, although crystals found up to now are too fractured to be considered as marketable gems. The dominant lithology is represented by a garnet–kyanite–biotite–hematite–plagioclase ± staurolite ± sillimanite paragneiss. Thermodynamic Perple_X modeling indicates conditions of ca. 630–710 °C and 7.8–10.4 kbars. Post-metamorphic metasomatic silicate and calc-silicate (Mn-rich)-minerals are found within (i) green-red horizons with a mineralogical zonation from diopside, hornblende, epidote and grossular, (ii) mica schists containing spessartine, kyanite, andalusite and piemontite, and (iii) weakly deformed quartz-feldspar coarse-grained veins with kyanite at the interface with the metamorphic gneiss. The transition towards brittle conditions is shown by Alpine-type tension gashes, including spessartine–epidote–clinochlore–hornblende-quartz veins, cross-cutting the metamorphic foliation. Kyanite is of particular interest because it is present in the metamorphic paragenesis and locally in metasomatic assemblages with a large variety of colors (zoned blue to green/yellow-transparent and orange). Element analyses and UV-near infrared spectroscopy analyses indicate that the variation in color is due to a combination of Ti4+–Fe2+, Fe3+ and Mn3+ substitutions with Al3+. Structural and mineralogical observations point to a two-stage evolution of the Trikorfo area, where post-metamorphic hydrothermal fluid circulation lead locally to metasomatic reactions from ductile to brittle conditions during Miocene exhumation of the high-grade host-rocks. The large variety of mineral compositions and assemblages points to a local control of the mineralogy and fO2 conditions during metasomatic reactions and interactions between hydrothermal active fluids with surrounding rocks.

Editorial for Special Issue “Mineralogy and Geochemistry of Gems”

“…The stable oxygen and hydrogen isotopic composition of both quartz varieties suggest a mixing of magmatic and meteoric (and/or marine) fluids. Large (up to several centimetres) and vividly coloured Mn-rich minerals (kyanite, green andalusite, garnet -grossular, and spessartine-and red-epidote) of "near" gem-quality from Thassos Island (Rhodope, Greece) are described by Tarantola et al [35]. They also show that the orange colour of kyanite from Thassos is due to Mn 3+ ; this is the second reported occurrence (after Loliondo, Tanzania) of such kyanite.…”

Gemstones of Greece: Geology and Crystallizing Environments

“…Microprobe analyses for orange kyanite indicate MnO up to 0.1 wt. % contributing to the coloration of the crystals [19] and up to 1.5 wt. % FeO.…”

“…In crystalline rocks, two groups of gemstones are distinguished: those formed during regional metamorphism-metasomatism and those associated with late alpine-type fissures [14][15][16]18,19,[27][28][29] (Figures 1 and 2). The first group includes ruby, sapphire, jadeite "jade" (jadeitite), Mn-andalusite, spessartine, Mn-grossular, Mn-zoisite/clinozoisite, Fe-Mn-bearing kyanite and rhodonite.…”

“…Kyanite in blue, green, yellowish to orange colored crystals up to 20 cm long, is found in quartz ± feldspar boudins intercalated within metapelites at Trikorfo, Thassos Island (Figure 6a-d) [17,19,47]. Orange kyanite also occurs in association with spessartine, and muscovite.…”

“…% (Table 2). Mn 3+ substituting for Al 3+ in the mineral formula is responsible for the dark green color [19,47].…”