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.
Highlights
Amethyst is a quartz variety often used in jewellery and occurs in varying shades of violet colors [1,2]
This study focuses on five amethyst deposits which are related to the Tertiary volcanic rocks of Greece
Altered volcanic rocks in Greece host amethyst veins in association with various amounts of adularia, calcite, smectite, chlorite, sericite, pyrite, zeolites, analcime, barite, fluorite and goethite/lepidocrocite in the veins
Summary
Amethyst is a quartz variety often used in jewellery and occurs in varying shades of violet colors [1,2]. Minerals 2018, 8, 324 range from bluish violet to purple-violet and red-violet and its origin has been controversial for a long time [2,3,4,5,6]. According to Cox [3] as cited in Fritsch and Rossman [4] the purple color in amethyst is due to O2− → Fe4+ inter valence charge transfer, which absorbs light in the middle of the visible region. The Fe4+ ion, at the Fe4+ site (e.g., FeO4 ) is formed from Fe3+ by the action of ionizing radiation and is important for the coloration of amethyst [2]. Is remains controversial if the Fe4+ site is substitutional, e.g., [3], or interstitial [5]
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