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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

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.