Amethyst Occurrences in Tertiary Volcanic Rocks of Greece: Mineralogical, Fluid Inclusion and Oxygen Isotope Constraints on Their Genesis

Abstract

Epithermally altered volcanic rocks in Greece host amethyst-bearing veins in association with various silicates, carbonates, oxides and sulfides. Host rocks are Oligocene to Pleistocene calc-alkaline to shoshonitic lavas and pyroclastics of intermediate to acidic composition. The veins are integral parts of high to intermediate sulfidation epithermal mineralized centers in northern Greece (e.g., Kassiteres–Sapes, Kirki, Kornofolia/Soufli, Lesvos Island) and on Milos Island. Colloform–crustiform banding with alternations of amethyst, chalcedony and/or carbonates is a common characteristic of the studied amethyst-bearing veins. Hydrothermal alteration around the quartz veins includes sericitic, K-feldspar (adularia), propylitic and zeolitic types. Precipitation of amethyst took place from near-neutral to alkaline fluids, as indicated by the presence of various amounts of gangue adularia, calcite, zeolites, chlorite and smectite. Fluid inclusion data suggest that the studied amethyst was formed by hydrothermal fluids with relatively low temperatures (~200–250 °C) and low to moderate salinity (1–8 wt % NaCl equiv). A fluid cooling gradually from the external to the inner parts of the veins, possibly with subsequent boiling in an open system, is considered for the amethysts of Silver Hill in Sapes and Kassiteres. Amethysts from Kornofolia, Megala Therma, Kalogries and Chondro Vouno were formed by mixing of moderately saline hydrothermal fluids with low-salinity fluids at relatively lower temperatures indicating the presence of dilution processes and probably boiling in an open system. Stable isotope data point to mixing between magmatic and marine (and/or meteoric) waters and are consistent with the oxidizing conditions required for amethyst formation.

On the Color and Genesis of Prase (Green Quartz) and Amethyst from the Island of Serifos, Cyclades, Greece

“…We find evidence for a major contribution of meteoric (and/or marine) water in case of the amethysts, thus recording a transition to more oxidizing conditions, as the formation of amethyst goes together with incorporation of Fe 3+ [3] into its structure. The required oxidizing conditions were probably a result of mixing oxidized meteoric or marine water with upwelling hydrothermal fluids [50,51]. Amethyst was deposited in the final stages of the skarn, at temperatures around 250 • C. This is in good agreement with Kievlenko [21], who stated that amethyst in garnet-pyroxene-magnetite skarn deposits crystalizes in late or re-opened fractures and cavities.…”

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

“…The certified reference materials NIST SRM 610 and 612 (concentrations from Jochum et al [57]) were used as external standards for calibration of all analyses; they were analyzed twice at the beginning and at the end for each kyanite sample, following a bracketing standardization procedure. 27 Al and 29 Si were measured with a dwell-time of 10 ms; the following 17 isotopes were measured with a dwell-time of 20 ms for each: 7 Li (limit of detection of 20 ppm), 23 Na (70 ppm), 24 Mg (1 ppm), 39 K (50 ppm), 43 Ca (6000 ppm), 47 Ti (20 ppm), 51 V (1 ppm), 53 Cr (15 ppm), 55 Mn (15 ppm), 57 Fe (150 ppm), 63 Cu (10 ppm), 66 Zn (10 ppm), 88 Sr (0.5 ppm), 107 Ag (2 ppm), 137 Ba (3 ppm), 197 Au (1 ppm), and 208 Pb (0.5 ppm). All data were reduced off-line with the limits of detection calculated using the commercial version of Iolite (Version 3.71, https://iolite-software.com) data reduction software [58] running with Igor Pro.…”

“…The geology of Greece is marked by a recent alpine belt, which results in outstanding localities for minerals and gemstones owing to high PT metamorphic conditions and subsequent magmatism and hydrothermalism, e.g., [19][20][21][22][23]. Among these localities, the island of Thassos, in the Rhodope Belt, represents a unique mineralogical locality for kyanite and uncommon varieties of manganiferous/manganian and magnesian silicate and calc-silicate minerals, including garnet, andalusite and epidote among others [19,[24][25][26][27][28][29][30][31].…”

Editorial for Special Issue “Mineralogy of Quartz and Silica Minerals”

“…The formation of such ordered particle arrays not only takes place in inorganic photonic structures in the geosphere, however is also important for nanotechnology and biological systems. Other formation environments for quartz and silica minerals are discussed by Müller et al [8], Voudouris et al [9] and Trümper et al [10]. Spectacular quartz crystals of various colours and habits were reported from a hydrothermal breccia of Berglia-Glassberget, Norway [8] and also in volcanic rocks in different occurrences of Greece [9].…”

“…Other formation environments for quartz and silica minerals are discussed by Müller et al [8], Voudouris et al [9] and Trümper et al [10]. Spectacular quartz crystals of various colours and habits were reported from a hydrothermal breccia of Berglia-Glassberget, Norway [8] and also in volcanic rocks in different occurrences of Greece [9]. Both papers try to reconstruct the specific conditions leading to the formation of the quartz crystals based on thorough mineralogical and geochemical analyses (trace elements, fluid inclusions, oxygen isotopes).…”