Investigating Borehole TDIP Response in the Ivrea-Verbano Zone (ICDP-DIVE project):Linking Chargeability to Mineral Distribution from SEM and MicroCT Data

As part of a multidisciplinary effort to characterize the deep continental crust, two scientific boreholes were drilled in the Ivrea Verbano Zone (IVZ, Western Alps, Italy), one of the few near-complete continental crustal sections exposed on Earth's surface (Pistone et al. 2020). The boreholes were drilled within the Drilling the Ivrea Verbano ZonE (DIVE) project, supported by the International Continental Scientific Drilling Program (ICDP-5071; Li et al. 2024; https://gfzpublic.gfz.de/pubman/item/item_5037328) . Among various well log measurements, time-domain induced polarization (TDIP) logs with two electrode spacings (16″ and 64″) were collected in both wells, from which chargeability data is inferred. The boreholes intersect a wide range of lithologies hosting sulfides and oxides, either disseminated or concentrated along veins and fractures, which represent potential sources of chargeability. A set of eleven samples from these boreholes were analyzed using both scanning electron microscopy (SEM) and micro-computed tomography (μCT). The following mineralogical and microstructural characteristics have been evaluated so far: the type and abundance of metallic minerals (expressed as volume and area fractions); the perimeter-to-area and surface-to-volume ratios and the preferred orientation of these conductive phases. These parameters were compared to the TDIP response signal at the corresponding depths of the borehole, resulting in the following findings:

i) Borehole chargeability is not necessarily proportional to the abundance of metallic minerals;

ii) The total surface area (which is high for fine grain sizes) plays a dominant role over the total volume fraction of metallic minerals;

iii) The shape preferred orientation of conductive phases appears to be a key factor influencing the measured chargeability;

iv) The presence of other mineral phases, such as graphite, may mask or amplify the response of metallic minerals depending on their structural relationship.

While no deterministic relationship has been identified at this stage, this work outlines a potential path to improve the interpretation of TDIP data in mineralized systems and to define complementary yet efficient tools for assessing the economic potential of mineral deposits.

References

Li, J., E. Caspari, A. Greenwood, et al. 2024. “Integrated Rock Mass Characterization of the Lower Continental Crust Along the ICDP‐DIVE 5071_1_B Borehole in the Ivrea‐Verbano Zone.” Geochemistry, Geophysics, Geosystems 25 (12): e2024GC011707. https://doi.org/10.1029/2024GC011707.

Pistone, Mattia, Luca Ziberna, György Hetényi, Matteo Scarponi, Alberto Zanetti, and Othmar Müntener. 2020. “Joint Geophysical‐Petrological Modeling on the Ivrea Geophysical Body Beneath Valsesia, Italy: Constraints on the Continental Lower Crust.” Geochemistry, Geophysics, Geosystems 21 (12): e2020GC009397. https://doi.org/10.1029/2020GC009397.