Prospecting for clay minerals within volcanic successions: Application of electrical resistivity tomography to characterise bentonite deposits in northern Sardinia (Italy)

• The ERT is applied to prospect for a bentonitic clay deposit.
• The genesis and geometry of the clayey body were tectonically controlled.
• The synthetic modelling was undertaken to assist in the reading of the field data.
• The reliability of ERT models was assessed by comparison with local borehole data.
• 3D resistivity models are reliable in the volume calculation of exploitable deposits.

Electrical resistivity tomography (ERT) is applied to prospect for and characterise a bentonitic clay deposit in northern Sardinia. Sardinian bentonites derived from the hydrothermal alteration of thick successions of pyroclastic flows and epiclastites are associated with the Oligo-Miocene calc-alkaline volcanic cycle. The alteration of these rocks is generally controlled by faults that control the local circulation of hydrothermal fluids. Two-dimensional ERT investigations were performed close to a faulted area to define the location, thickness and lateral continuity of the clayey body, and determine how it relates to faulting and stratigraphy. A line-based three-dimensional ERT data acquisition was carried out in a selected area to estimate the available clay reserves. The reliability of these resistivity models was assessed by comparison with local borehole data. Finally, the interpretation of the ERT results was optimised through synthetic modelling of the electrical resistivity imaging technique. The results define the extent and geometry of the bentonitic deposit with good accuracy and outline the scenarios where the ERT method may provide optimal results when prospecting for clay deposits.