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Minerals 2018, 8(8), 324; https://doi.org/10.3390/min8080324

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

1
Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, 15784 Athens, Greece
2
Faculty of Geology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
3
GeoRessources, Faculté des Sciences et Technologies, UMR Université de Lorraine, 54506 Nancy, France
4
Institute of Mineralogy, TU Bergakademie Freiberg, 09599 Freiberg, Germany
5
6 Kairi str., 15126 Athens, Greece
6
Department of Earth Sciences, St. Francis Xavier University, 5005 Chapel Square, Antigonish, NS B2G 2W5, Canada
7
Maccaferri Hellas Ltd., 13674 Athens, Greece
*
Author to whom correspondence should be addressed.
Received: 19 June 2018 / Revised: 20 July 2018 / Accepted: 25 July 2018 / Published: 28 July 2018
(This article belongs to the Special Issue Mineralogy of Quartz and Silica Minerals)
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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. View Full-Text
Keywords: amethyst; volcanic rock; hydrothermal alteration; gemstones amethyst; volcanic rock; hydrothermal alteration; gemstones
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Voudouris, P.; Melfos, V.; Mavrogonatos, C.; Tarantola, A.; Gӧtze, J.; Alfieris, D.; Maneta, V.; Psimis, I. Amethyst Occurrences in Tertiary Volcanic Rocks of Greece: Mineralogical, Fluid Inclusion and Oxygen Isotope Constraints on Their Genesis. Minerals 2018, 8, 324.

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