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SUPERGENE JAROSITE FORMATION WITHIN SMECTITE AND ILLITE HYDROTHERMAL ZONES FROM LIMNOS ISLAND, NORTHEAST AEGEAN SEA, GREECE
Abstract
Smectite and illite occurrences extended over an area of about 5 km2 in the Southern parts of Limnos, island, Northeast Aegean sea, Greece, were found and studied. The smectite as well as the illite occurrences are alteration products of volcanic rocks principally trachyandesites, dacites and dacitoids. The study of the thirty samples collected from five fault zones was carried out using XRPD, SEM, SEM-EDS, techniques and ICP bulk rock chemical analyses for major, trace and rare earth elements. The extended alteration of the parent rocks resulted from the circulation of hydrothermal fluids through faults and fractures. The hydrothermally altered rocks are enriched in Al2O3 and LOI and depleted in SiO2 and REE relatively to the parent rocks. Smectite and illite were detected in the altered volcanic rocks forming small alteration zones. Smectite rich zones and illite rich zones are found in the Southern parts of Limnos island. In the smectite rich zones the amount of smectite, usually nontronite and in some cases montmorillonite, decreases radically with the increase of the distance from the fault and it is absent in the parent rocks, 5-10 meters away from the fault. In the illite rich zones, occasionally, smectite becomes the dominant phase while illite and quartz are present. Supergene alteration was observed in the hydrothermally altered rocks of the smectite and illite rich zones as it is suggested by the presence of jarosite, natrojarosite, pyrite and traces of barite and gypsum. Pulverized jarosite and natrojarosite are formed due to the presence of aqueous sulfate derived from supergene oxidation of hydrothermal pyrite. Usually, relatively not well formed 1-10$\mu$m jarosite and natrojarosite crystals are found in minor concentrations. The concentrations of jarosite and natrojarosite are significantly increased along fractures and fault gauges rich in smectite. As a result the amounts of K2O, F2O3 and LOI are significantly increased. The rather abundant jarosite observed along fractures and within fault gouges is also derived from the oxidation of pyrite.
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References12
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