Fossil vs. Active Geothermal Systems: A Field and Laboratory Method to Disclose the Relationships between Geothermal Fluid Flow and Geological Structures at Depth

Comparison between fossil and analogue active geothermal systems permit to obtainkey-parameters to define a conceptual model of the area under exploration. The approach is basedon structural, kinematic, and fluid inclusions analyses. The fossil system is investigated to describethe distribution of the hydrothermal mineralization as witness of the fluid flow through geologicalstructures and bodies, at depth. Structural and kinematic data (to define the preferential directionof 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 inthe fossil system and, by analogy, in the deep roots of the active system. Fluid inclusions analysisshed light on density, viscosity, and temperature of the paleo-fluids. Data integration provides thehydraulic conductivity. In active geothermal systems, fieldwork is addressed to paleo-stress analysiswith data from recent faults (<2 Ma), to compare with local focal mechanisms. By this, indications onthe present fluid pathways are given. The main advantage resides in obtaining parameters normallygot after drilling, thus contributing to strengthen the strategy of exploration, de-risking unsuccessfulboreholes.