Migration of geothermal fluids in extensional terrains: the ore deposits of the Boccheggiano-Montieri area (southern Tuscany, Italy) |
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Authors: | Domenico Liotta Giovanni Ruggieri Andrea Brogi Paolo Fulignati Andrea Dini Isabella Nardini |
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Institution: | 1. Dipartimento di Geologia e Geofisica, Università di Bari, Via Orabona 4, 70125, Bari, Italy 2. CNR-Istituto di Geoscienze e Georisorse, Via La Pira 4, 50121, Florence, Italy 3. Dipartimento di Scienze della Terra, University of Siena, Via Laterina 8, 53100, Siena, Italy 4. Dipartimento di Scienze della Terra, University of Pisa, Via S.Maria 53, 56100, Pisa, Italy 5. CNR-Istituto di Geoscienze e Georisorse, Via Moruzzi 1, 56125, Pisa, Italy
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Abstract: | An integrated study based on fluid inclusion, δ18O composition and structural analyses was carried out on a Pliocene fossil hydrothermal system, located to the South of the
present active Larderello geothermal field, in the Boccheggiano-Montieri area. The study area is typified by mineralized cataclastic
levels related to Late Oligocene–Early Miocene thrust surfaces, and to the following two generations of normal faults of Miocene
and Pliocene ages, respectively. Within the damage zone of the Pliocene Boccheggiano fault, the mineralization is mainly made
up of quartz and pyrite. Quartz + Pb–Zn sulfides, or quartz + Pb–Zn sulfides + fluorite + carbonates assemblages occur instead
in the older cataclastic levels. Two generations of liquid-rich fluid inclusions were recognized in quartz and fluorite: the
first one, with homogenization temperatures ranging between 172 and 331°C and salinity between 0.0 and 8.8 wt.% NaClequiv., records the early stage of hydrothermal activity. The second generation of fluid inclusions documents a later stage, with
homogenization temperature from 124 to 288°C and salinity from 0.2 to 1.9 wt.% NaClequiv.. Fluid inclusions analyses also indicate that mixing of fluid with distinct salinities and/or temperatures was a widespread
process during the early stage, and that fluid temperatures decreased moving from the Boccheggiano fault toward the more distal
and older cataclastic levels. The δ18O values of water in equilibrium with hydrothermal quartz, which range from −5.7 to −0.1‰, are related to the circulation
of meteoric water mixed with saline water that leached the evaporite level and enriched in δ18O through water–rock interaction, and/or with magmatically derived fluids. Results indicate that the damage zone of the Pliocene
Boccheggiano fault represented the main channel for the flow of meteoric water, which was heated at depth, then mixed with
high salinity fluids, and finally ascend to infiltrate along the older cataclastic levels. Our results, based on fluid inclusions,
oxygen isotopic compositions and structural analyses indicate that a single fluid flow path run through the damage zone of
the Boccheggiano fault and the older cataclasites, which were thus hydraulically connected. |
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