Migration of geothermal fluids in extensional terrains: the ore deposits of the Boccheggiano-Montieri area (southern Tuscany, Italy)

Abstract

An integrated study based on fluid inclusion, d 18 O 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.% NaCl equiv. , 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.% NaCl equiv. . 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 d 18 O 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 d 18 O 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.

Investigating fossil hydrothermal systems by means of fluid inclusions and stable isotopes in banded travertine: an example from Castelnuovo dell’Abate (southern Tuscany, Italy)

“…15), thus markedly different from the (calculated) much heavier O isotope composition of the water that fed the same system in the past (δ 18 O = + 8.4 to +11.6 ‰; Gasparrini et al 2013) and from the water that deposited the Castelnuovo dell'Abate travertine (δ 18 O = + 0.9 to +7.4 ‰; this study). Accordingly, hydrothermal calcite veins developed around the geothermal anomalies of southern Tuscany generally indicate an O composition in fossil fluids heavier than meteoric water (Liotta et al 2009). Concerning the Castelnuovo dell'Abate and Bagni San Filippo fossil hydrothermal systems, inputs of heavy O isotopes from a magmatic source water could be reasonably excluded based on the low salinities of the fluid inclusions in calcite, which suggest a meteoric origin of the original waters feeding the Castelnuovo dell'Abate system.…”

Tectonic control on travertine and calcareous tufa deposition in a low-temperature geothermal system (Sarteano, Central Italy)

“…Crustal thinning was accompanied by a widespread anatectic magmatism (Serri et al 1993;Peccerillo 2003), which gave rise to the emplacement of granitoids at shallow depth (Dini et al 2005) and widespread hydrothermal systems affecting the host rocks (Tanelli 1983;Tanelli et al 1991;Dini 2003;Boschi et al 2009;Liotta et al 2010). Magmatic bodies that are not completely cooled are responsible for geothermal anomalies occurring at present in the Larderello-Travale and Monte Amiata geothermal areas (Batini et al 2003), as part of a more generally extensive geothermal anomaly that characterizes the whole of southern Tuscany (Fig.…”

“…The deposition of travertine and calcareous tufa can be related to bicarbonate-rich saline waters flowing from thermal springs and resulting from deep circulation in a generally extensive geothermal reservoir; their rise is favoured by the occurrence of interconnected brittle structures (Bruhn et al 1994;Sibson 1996;Evans et al 1997;Caine & Forster 1999;Bellani et al 2004;Brogi 2004Brogi , 2008aLiotta et al 2010) giving rise to highly permeable, fractured rock masses (Caine et al 1996). However, it is known that if saline hydrothermal fluids flow through porous media, permeability is time dependent because the sealing of voids tends to occur over time across the zone of the pressure drop (e.g.…”

Hydrogeochemical processes controlling water and dissolved gas chemistry at the Accesa sinkhole (southern Tuscany, central Italy)

“…In the study area large outcrops of Mesozoic limestone deposits occur and are able to host large volume of meteoric waters that interact with deep-originated CO 2 -H 2 S-rich fluids favoring the formation of sinkholes and dolines (Nisio et al, 2007;Caramanna et al, 2008;Liotta et al, 2010). The funnel-shaped Lake Accesa, originated by karstic collapse, is located at the intersec- (Merciai, 1933;Magny et al, 2007;Negri et al, 2008;Vanniere et al, 2008) (Fig.…”

“…An enhanced hydrothermal activity in this area was interpreted as related to a heat flow anomaly (Gianelli et al, 1997;Batini et al, 2003;Bellani et al, 2004) and extensional structures coeval with the emplacement of granitoids at depth in the Late Miocene, exposed or drilled during mining exploitation (Serri et al, 1993;Dini et al, 2005;Benvenuti et al, 1994). Hydrothermal fluid patterns are currently controlled by Early Pliocene-Present high angle normal to strike-slip faults giving rise to local tectonic depressions (Bertini et al, 1991;Carmignani et al, 1994;Jolivet et al, 1998;Rossetti et al, 2011;Brogi et al, 2005;Liotta et al, 2010). Lake Accesa (42°59' N; 10°53' E; 157 m asl) is a waterfilled sinkhole located 5 km from the town of Massa Marittima (southern Tuscany, central Italy) and 12 km west of the Tyrrhenian Sea coast, located at the southern border of Colline Metallifere mining district, which is marked by oredeposits and widespread hydrothermal mineralization (Serri et al, 1991) (Fig.…”