Research Article Structural and Mineralogical Characterization of a Fossil Hydrothermal System Located at the Outermost Front of the Southern Apennines Fold-and-Thrust Belt Paolo Fulignati , 1 Fabrizio Agosta , 2 Claudia Belviso , 3,4 Giacomo Prosser , 2 Antonio Lettino , 3,4 Angela Vita Petrullo , 2 and Francesco Cavalcante 3,4 1 Dipartimento di Scienze della Terra, Università di Pisa, Pisa 56126, Italy 2 Dipartimento di Scienze, University of Basilicata, Potenza 85100, Italy 3 Istituto di Metodologie per l’Analisi Ambientale–CNR, Tito Scalo 85050, Italy 4 Istituto di Struttura della Materia–CNR, Area di Tito Scalo, PZ 85050, Italy Correspondence should be addressed to Francesco Cavalcante; francesco.cavalcante@imaa.cnr.it Received 12 November 2018; Accepted 30 January 2019; Published 28 April 2019 Academic Editor: Fabien Magri Copyright © 2019 Paolo Fulignati et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Aiming at investigating the hydrothermal circulation along the eastern flank of the Vulture volcano, along the outermost edge of the southern Apennine fold-and-thrust belt (ftb), we studied the fossil hydrothermal alteration that mineralized a transtensional fault that crosscuts volcanoclastic rocks in the Rapolla area. On the basis of structural, mineralogical, and fluid inclusion data, three main stages of activity of the hydrothermal system are documented. Stage 1 was produced by the circulation of fluids having low-pH conditions (pH ≈ 3-4) and relatively high-SO 4 2- activity, as testified by the hydrothermal alteration mainly carried out by the alunite group minerals (particularly jarosite), which is typical of an advanced argillic alteration facies. Hydrothermal fluids were characterized by a high temperature of about 200 ° -210 ° C. These hot fluids altered and mineralized the matrices of pyroclastic rocks and sealed both burial-related and fault-related fracture networks. Later hydrothermal circulation (Stage 2) was recorded by opal A-rich veins present both within and outside the fault zone. The fluids responsible of opal A precipitation were characterized by lower temperature conditions, probably lower than 100 ° C. Current goethite mineralization takes place along the main slip surfaces of the study high-angle fault zone due to low temperature (<30 ° C) underground water circulation. This study highlights that a high-temperature hydrothermal system developed in the past within the transtensional fault zone of the Rapolla area when a high thermal anomaly was present. If we take into account that this area is still affected by a heat flux positive anomaly (90 mW/m 2 ), we may infer that it has the potentiality to be considered an interesting site for future exploration devoted to the finding of medium-enthalpy geothermal resources at depth. 1. Introduction Hydrothermal systems cause the redistribution of both energy and mass in response to circulating H 2 O fluids and form in response to thermal perturbations among which the magma-induced thermal anomalies are the most frequent [1]. A hydrothermal system is comprised of the following components: a heat source, a permeable reservoir in which fluids can flush and trigger an active convective circulation, a recharge system, and an impermeable cover, respectively. The rocks affected by a hydrothermal fluid circulation undergo to a variety of alteration processes due to the insta- bility of the primary mineralogical assemblages, which tend to reequilibrate by forming new minerals stable under the new conditions [2]. As a consequence, the different alteration mineralogical assemblages are primarily controlled by the physical-chemical conditions of the hydrothermal fluids. High-temperature geothermal systems are quite common in areas characterized by active extensional tectonic environ- ments [3–5]. In the Central Mediterranean region, a very Hindawi Geofluids Volume 2019, Article ID 1840436, 14 pages https://doi.org/10.1155/2019/1840436