IMAGE-D3.1 - Final report on: understanding from deep drilling and exhumed systems

This report summarizes the results of the studies carried out for the achievement of the objectives described at page 14 of 40 of the DoW, i.e.: the description of the geometric characteristics of the structural conduits and traps where geothermal fluids are channeled and stored, by studying exhumed geothermal systems to be compared to the active ones (Task 3.1). We studied also the geochemical features of the paleofluids (Task 3.1)

The main target of WP3 is to provide information on the geological processes and processes controlling the relationships between the geological structures and fluid flow. To follow this, we have applied the classical approach of structural geology and geochemistry of paleofluids and of hydrothermal minerals. The field structural study produced new geological maps and highlighted the kinematics and geometry of the shear and extensional fractures, as later on described. The geochemical studies focused on the boundary conditions (P-T and geochemical properties) of reservoirs of high-temperature magmatic geothermal systems in both study areas, the exhumed Elba Island (Italy), for the continental crusta, and Geitafell (Iceland) for the oceanic crust. The geochemical study developed through: 1) rock sampling for laboratory analyses in key-areas comprising a selections of core-samples from bore-holes drilled in the Geitafell area and 2) mineralogical and petrographic studies and laboratory analyses of fluid inclusions, isotopes and mineral-chemistry on selected samples. Fluid inclusions, isotopes and minerals represent, in fact, unique geological archives to reconstruct the to derive the pressure-temperature and geochemistry-time evolution of luid circulations in fossil high-temperature magmatic-hydrothermal systems, to obtain information on water-rock interaction between host rock and fluids and to characterize the fluid source(s) that flowed in the fossil systems.

The study had benefitted of the scientific and fruitful integration between the Italian and Islandic teams both during the field and laboratory studies.

In this report therefore the results of the activities carried out during the first and second year of the IMAGE project. We have started the investigation of the exhumed and active geothermal systems in Italy and Iceland, as representative of continental and oceanic geothermal systems, respectively. As it concerns Italy, the eastern Elba Island is considered as the fossil exhumed analogue of the active Larderello geothermal field. As it concerns Iceland, Geitafell is the exhumed analogue of the Krafla active geothermal system.

Please note the Icelandic collection of samples and data was influenced by the date of beginning of the project (2 November 2013), in a period not favorable to fieldwork. Thus, the first collection of data was provided in the second half of May 2014, In Iceland.

The results here below summarized derive from the work of: Caterina Bianco, Massimiliano Ciacci, Martina Zucchi (PhD students, the latter two are IMAGE-funded scholarships), Alfredo Caggianelli, Rosa Anna Fregola, Domenico Liotta, Andrea Brogi (researchers) of Bari University, Gu.mundur Omar Fri.leifsson of HS-Orka, Helga Margrét Helgadóttir of ÍSOR, Giovanni Ruggieri, Andrea Dini, Andrea Orlando, Chiara Boschi (researchers, CNR-IGG), Valentina Rimondi and Laura Chiarantini (post-doc student, IMAGE funded scholarship, CNR-IGG).

In synthesis our studies allow us to state: 

a) the analysis of exhumed geothermal systems give us fruitful information on what it is at depth in present geothermal fields whether the tectonic and stratigraphic context are strictly comparable;

b) the study of the relationships between fractures and mineralization indicate that transfer/transform faults and normal faults are the main structural conduits for channeling hydrothermal fluids;

c) the structural traps depends on the original permeable characteristics and geometry of the geological bodies that, in the continental crust, are affected by folds;

d) The analysis of the paleofluids by means of fluid inclusion studies indicated that supercritical conditions can be reached in the continental crust where T and P are sufficient;

e) The results from laboratory tests indicate that the composition of the palaeohydrothermal fluids can be reached simulating the leaching of wall-rock minerals by a saline fluid.

We believe that the collection and analysis of data was successful completed and it followed the schedule. We are confident to have reached our goals. The text is followed by the annex containing the cited figures and tables. The two papers already published with some of the results hereafter summarized are also included at the end of this report.