The Pantelleria Caldera Geothermal System- Data From The Hydrothermal Minerals

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Journal Article: The Pantelleria Caldera Geothermal System- Data From The Hydrothermal Minerals

This paper proposes, on the basis of petrographic and mineralogic data on cutting and cores from two deep wells ("Pantelleria 1" and "Pantelleria 2"), the first model of the active hydrothermal system of the island of Pantelleria. Phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes. The results of these studies emphasize differences between the intracaldera and pericaldera areas of the island. Within the 45 ka caldera there is a high-temperature (240-260°C at 600-800 m depth) active hydrothermal system with five zones of characteristic alteration minerals with increasing depth. Rocks are unaltered to a depth of 200 m, contain smectite and mixed-layer chlorite-smectite (C/S) between 200 and 380 m, chlorite, illite, chalcedony and quartz from 380 to 500 m, albite, adularia and saponite from 500 to 680 m, mixed-layer biotite-vermiculite from 680 m to the depth drilled (1100). Outside the caldera, but near the rim, a low-temperature and low-permeability (< 140°C) hydrothermal system is characterized by smectite, dolomite and ankerite at depths of 390 to 650 m, chlorite and calcite at 650-900 m, and mixed layers of chlorite-smectite, illite-smectite and iron carbonates (ankerite, siderite) from 900 m of the well at 1203 m. The superimposition of hydrothermal mineral assemblages is evidence for cooling in the hydrothermal system both inside and outside the caldera. We propose that a high-temperature hydrothermal system developed inside the caldera. In an early stage in the area surrounding the subvolcanic body, biotite isograd is reached and an alkali-metasomatism zone develops inside the body itself. This phase may also account for the development of a chlorite-albite-adularia zone extending to 400 m. A cooling phase (nearly 50°C) followed, resulting in the substitution of biotite by mixed-layer biotite-vermiculite and by the crystallization of Fe-rich saponite instead of chlorite, within the currently active reservoir. A cooling phase has also been identified in the well outside the caldera.

Paolo Fulignati, Giuseppe Malfitano and Alessandro Sbrana

Published Journal 
Journal of Volcanology and Geothermal Research, 1997

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Paolo Fulignati,Giuseppe Malfitano,Alessandro Sbrana. 1997. The Pantelleria Caldera Geothermal System- Data From The Hydrothermal Minerals. Journal of Volcanology and Geothermal Research. (!) .