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The Las Minas area corresponds to an exhumed geothermal system considered a proxy for the deep part of the nearby Los Humeros active geothermal system. The stratigraphic succession is made up of: Palaeozoic-Miocene granitoids, a thick Jurassic- Cretaceous carbonate succession, Neogene lava flows and volcano-sedimentary deposits. Linked to a Miocene magma intrusion, marble and skarn rock-volumes developed by contact metamorphism and geothermal fluid flow. Faults are arranged in SW- and NNW-striking systems. These controlled the morphological evolution and favored Neogene-Quaternary dyke emplacement. Faulting gave rise to a tectonic depression where lacustrine sediments and pyroclastics deposited. Skarn rocks are mainly located at fault intersections and along pre-existing discontinuities, suggesting the role of bedding and/or fractures in channeling deep fluids. Results give inputs for exploration at depth of Los Humeros geothermal system.

We review and refine the geological setting of an area located nearby the Tyrrhenian seacoast, in the inner zone of the Northern Apennines (southern Tuscany), where a Neogene monzogranite body (estimated in about 3 km long, 1.5 km wide, and 0.7 km thick) emplaced during early Pliocene. This magmatic intrusion, known as the Gavorrano pluton, is partially exposed in a ridge bounded by regional faults delimiting broad structural depressions. A widespread circulation of geothermal fluids accompanied the cooling of the magmatic body and gave rise to an extensive Fe-ore deposit (mainly pyrite) exploited during the past century. The tectonic setting which favoured the emplacement and exhumation of the Gavorrano pluton is strongly debated with fallouts on the comprehension of the Neogene evolution of this sector of the inner Northern Apennines. Data from a new fieldwork dataset, integrated with information from the mining activity, have been integrated to refine the geological setting of the whole crustal sector where the Gavorrano monzogranite was emplaced and exhumed. Our review, implemented by new palynological, petrological and structural data pointed out that: (i) the age of the Palaeozoic phyllite (hosting rocks) is middle-late Permian, thus resulting younger than previously described (i.e., pre-Carboniferous), (ii) the conditions at which the metamorphic aureole developed are estimated at a temperature of c. 660 °C and at a depth lower than c. 6 km, (iii) the tectonic evolution which determined the emplacement and exhumation of the monzogranite is constrained in a transfer zone, in the frame of the extensional tectonics affecting the area continuously since Miocene.

Comparison between fossil and analogue active geothermal systems permit to obtain key-parameters to define a conceptual model of the area under exploration. The approach is based on structural, kinematic, and fluid inclusions analyses. The fossil system is investigated to describe the distribution of the hydrothermal mineralization as witness of the fluid flow through geological structures and bodies, at depth. Structural and kinematic data (to define the preferential direction of fluid flow) are collected in structural stations and by scan lines and scan boxes on key outcrops. Distribution, length, width of fractures, and hydrothermal veins bring to evaluate permeability in the fossil system and, by analogy, in the deep roots of the active system. Fluid inclusions analysis shed light on density, viscosity, and temperature of the paleo-fluids. Data integration provides the hydraulic conductivity. In active geothermal systems, fieldwork is addressed to paleo-stress analysis with data from recent faults (

This study is part of the H2020-GEMex Project, a collaboration between Europe and Mexico for investigating super-hot geothermal systems. The novelty of this work is in its integrated methodological approach which bridges the past to the present and contributes to the reconstruction of conceptual models of deep geothermal systems, thus favoring appropriate strategies for deep exploration. The Las Minas mining area (Veracruz State) where Cu-Fe-Au ore-deposits have been historically exploited, is considered as an exhumed Neogene geothermal system analogous to the deeper part of the Los Humeros geothermal field (Puebla State). We used an integrated approach, which involved structural and kinematic, petrological, geochemical, geochronological and fluid inclusion analyses of the pre-volcanic Paleozoic-Cretaceous succession, intruded by Miocene-Pliocene granitoids and mafic dykes. Our emphasis was on the (paleo) fluid-rocks interaction and the related structural control. The results indicate that saline and hypersaline fluids (up to 70 wt. % NaCl equiv.) of magmatic derivation and aqueous-carbonic fluids issued during metamorphic or metasomatic reactions with T up to 600-650°C were channeled along and in the intersection between the NW- and NE-trending faults and their damage zones. The faults are characterized by strike-slip to oblique-slip/normal movements. Fluids were also stored in carbonate rock-volumes at the contact with the granitoids. The distribution of the ore-deposits and hydrothermal alteration related to the interaction of geothermal fluids and hosting rocks indicates the importance of conduits (such as fractures and faults) in the reservoirs. Hence, understanding the relationships between ore-deposits, hydrothermal alteration and structures can increase our knowledge of the present geothermal fluids' deep circulation in the Los Humeros geothermal field.

Banded Ca-carbonate veins in travertine deposits are efficient recorders of the compositional fluctuations of geothermal fluids flowing (or flowed) from deep reservoirs up to the surface, within fault zones. In this view, these veins represent key tools for decoding those factors that influenced the geochemical variations. We have analyzed veins developed in fractures channeling geothermal fluids forming travertine deposits. The studied veins cut a fossil travertine fissure ridge, near the Larderello geothermal area (Iano area, southern Tuscany) where geothermal fluid circulation is favored by NE-trending strike-to-oblique-slip faults and their intersections with NW-trending normal ones. U-Th dating indicates that fluid circulation occurred from (at least) 172 ka to 21 ka. In this time span, the geothermal fluid changed in composition, and the banded Ca-carbonate veins recorded these variations in terms of mineralogical and stable isotope composition and temperature ( T ) of deposition. We also documented for the first time the occurrence of Mn-rich black tree-shaped structures within the veins. Mineralogy coupled with stable and clumped isotope measurements allows the reconstruction of some features (i.e., crystal texture, temperature, and CO2 origin) and the inference of the processes (i.e., pH, T, and pCO2 variations) that have controlled the fluid evolution through time. Multiple-stage and one-stage deposition processes have played an important role in modifying the stable isotope composition of banded Ca-carbonate veins; temperature coupled with pCO2 also influenced their mineralogical composition. Interpreted in the context of the tectonic setting, the data show that the NW-trending faults have mainly controlled travertine deposition. Their intersection with NE-trending faults, interpreted as transfer faults, highlights the important role of transfer zones in channeling the geothermal fluids.

Estimating values of permeability (k), efficient porosity (P) and hydraulic conductivity (K) by analysing field outcrops as analogue of geothermal reservoirs, is a timely theme useful for predictions during geothermal exploration programs. In this paper we present a methodology providing k, P and K values, based on geometric analysis of quartz-tourmaline faults-vein arrays hosted in micaschist exposed in south-eastern Elba Island (Tuscan Archipelago, Italy), considered as the analogue of rock hosting the so-called “deep reservoir” in the Larderello geothermal field. The methodology is based on the integration among structural geology, fluid inclusions results and numerical analyses. Through a detailed structural mapping, scan lines and scan boxes analyses, we have reconstructed three superposed faulting events, developed in an extensional setting and framed in the Neogene evolution of inner Northern Apennines. Geometrical data of the fault-veins array were processed by reviewing the basic parallel-plate model equation for k evaluation. Fluid inclusion analyses provided those salinity and pressure-temperature values necessary for defining density and viscosity of the parent geothermal fluids. Then, permeability, density and viscosity were joined to get hydraulic conductivity (K). Permeability is estimated between 5 × 10 −13 and 5 × 10 −17 m 2 with variations among the different faults generation, while the hydraulic conductivity is encompassed between 1.31 × 10 −8 and 2.4 × 10 −13 m/s. The obtained permeability and hydraulic conductivity values are comparable with those from several geothermal areas, and in particular from the Larderello geothermal field. The main conclusion is that the proposed integrated approach provides a reliable methodology to obtain crucial values, normally obtained after drilling, for developing numerical flow models of geothermal fluid path in active geothermal systems by field and laboratory analyses of analogue, exhumed, geothermal systems.

In the Honalilar area (Denizli Basin, Turkey), the occurrence of banded Ca-carbonate veins and travertine deposits, represented by a dismantled fissure ridge-type depositional system, are the evidence of a middle-late Pleistocene exhumed, shallow, hydrothermal system. Their occurrence offers the best opportunity to: (i) reconstruct the fluid paths from the underground to the palaeo-surface, and (ii) analyse the role of fault zones in controlling the permeability and fluids circulation. Permeability developed in overstepping regional scale normal faults, with a slight left-lateral oblique-slip component. At the surface, faults favored the localization and development of a fissure ridge-type travertine deposit. At depth, the root of the hydrothermal system consists of W-E oriented fractures filled of up to 6.5 m thick Ca-carbonate veins, developed in a high dilatation zone. It corresponds to the step-over determined by the oblique-slip kinematics of the NW-striking main faults. The high dilatation step-over zone contrasted the progressive sealing induced by the concomitant Ca-carbonate deposition within the fractures, thus favoring permeability maintenance and fluids circulation for at least 200 ka. This evidence adds key inputs for predicting permeable volumes during geothermal exploration in areas affected by extensional tectonics. The main NW-oriented faults remained active even after the hydrothermal fluid flow, causing the dismantlement and progressive exhumation of the upper part of the hydrothermal system. © 2020 Elsevier Ltd

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Studies on hydrothermal fluids circulation and tectonic structures improve the geothermal exploration and ex- ploitation in Iceland. In this paper we present the results of an integrated study (structural, kinematic, fluid inclu- sions analyses) carried out on a fossil and exhumed geothermal system at Geitafell (SE Iceland), considered an analogue of the active Krafla geothermal system (NE Iceland). Our work is addressed to the relationships be- tween the main structures and the paleo-geothermal fluids circulation at the boundary between the magma chamber (with magma of gabbroic composition) and host rocks, with the aim to get information on those factors having controlled the hydrothermal fluids flow, its storage and the rocks-fluid interaction, at 1-2 km crustal depth. The structural study is mostly based on defining the architecture of mineralized brittle shear zones, where, by measuring length and width of mineralized fractures, permeability was computed in 1.3 × 10-14 m2. As regards the hydrothermal paleo-fluids, fluid inclusions in hydrothermal quartz and calcite, usually associated with epidote and chlorite (i.e., the typical alteration minerals found in Icelandic geothermal fields), re- corded a fluid circulation with temperature ranging from about 255 °C to 320 °C, and boiling at the highest tem- perature. Transient pressure drop, likely related to fracture opening and propagation during tectonic activity, triggered local boiling process testified by fluid inclusions in a calcite sample. During a late evolution the system progressively cooled down to 60-90 °C. Paleo-geothermal fluids were characterized by low-salinity meteoric water, although higher salinity fluids (up to 10.6 wt% NaCl equiv.) entered in the system and mixed with mete- oric fluid. In the end, based on results from fluid inclusions, derived fluid density, permeability values and model- ing of viscosity, hydraulic conductivity between 2.8 × 10-8 to 1.8 × 10-7 m/s, was also computed within the fluids-channelling brittle shear zones. We finally encourage the key-study of fossil geothermal systems as a tool for getting parameters commonly obtained after drilling in active systems.

Estimating values of permeability (k), efficient porosity (P) and hydraulic conductivity (K) by analysing field outcrops as analogue of geothermal reservoirs, is a timely theme useful for predictions during geothermal exploration programs. In this paper we present a methodology providing k, P and K values, based on geometric analysis of quartz-tourmaline faults-vein arrays hosted in micaschist exposed in south-eastern Elba Island (Tuscan Archipelago, Italy), considered as the analogue of rock hosting the so-called "deep reservoir" in the Larderello geothermal field. The methodology is based on the integration among structural geology, fluid inclusions results and numerical analyses. Through a detailed structural mapping, scan lines and scan boxes analyses, we have reconstructed three superposed faulting events, developed in an extensional setting and framed in the Neogene evolution of inner Northern Apennines. Geometrical data of the fault-veins array were processed by reviewing the basic parallel-plate model equation for k evaluation. Fluid inclusion analyses provided those salinity and pressure-temperature values necessary for defining density and viscosity of the parent geothermal fluids. Then, permeability, density and viscosity were joined to get hydraulic conductivity (K). Permeability is estimated between 5 × 10-13 and 5 × 10-17 m2 with variations among the different faults generation, while the hydraulic conductivity is encompassed between 1.31 × 10-8 and 2.4 × 10-13 m/s. The obtained permeability and hydraulic conductivity values are comparable with those from several geothermal areas, and in particular from the Larderello geothermal field. The main conclusion is that the proposed integrated approach provides a reliable methodology to obtain crucial values, normally obtained after drilling, for developing numerical flow models of geothermal fluid path in active geothermal systems by field and laboratory analyses of analogue, exhumed, geothermal systems.