Fractures, fluid flow and inherited structures in geothermal systems: Inputs from the Fe-ore deposits of eastern Elba Island (Northern Apennines, Italy)

Geothermal systems in terrains affected by polyphase deformation have reservoirs witha 3D geometry that is always difficult to predict. In this paper we describe a fossil exhumedgeothermal system exposed in eastern Elba Island that developed in a polyphase folded andfaulted setting, which can help us to understand how geothermal fluids circulate in geologicalbodies with inherited structures. Geothermal circulation at Elba allowed the deposition ofFe-ore deposits (haematite/magnetite and pyrite) and altered rock volumes, which representtracers of the palaeo-fluid flow. Normal and oblique-slip faults dissected a polyphase foldedmetasiliciclastic succession and produced a secondary permeability in the range of 5 × 10-13to 5 × 10-16 m2. From the permeable fault zones acting as feeder conduits, geothermal fluidspermeated the hydraulically connected metasiliciclastic rock bodies previously deformed bytwo generations of folds. Geothermal fluids followed the already defined geometry, thus givingrise to apparent folded mineralized levels. Fluid migration into the metasiliciclastic rocks waspossible due to their chemical aggression, which favoured the dissolution and reprecipitationof quartz, and Fe-oxide and sulphide deposition. Renewed fluids maintained their chemicalproperties (pH value and temperature, mostly). This conclusion provides inputs forreconstructing geothermal conceptual models and evaluating the geothermal potentiality ofexploitable areas developing in similar geological settings.