BCGS Technical Talks: Scott Napier

Title: 3D Time Domain Inversion of Ground Electromagnetic Data and Applications to VMS Exploration; The Torres Deposit, a New Greenfield Discovery in the Iberian Pyrite Belt The Iberian Pyrite Belt (IPB) is one of the most important VMS provinces worldwide, with more than 90 known deposits. Local stratigraphy and structural complexity are extremely important to exploration and frequently critically important to the design and interpretation of exploration surveys. This case study demonstrates, through the story of an important and recent new mineral discovery, the Torres deposit, that the use of ground Time Domain Electromagnetic (TDEM) data in combination with 3D inversion modelling can play a critical role in exploration in this setting and can advance exploration outcomes. In 2009 a dataset of ground-based SQUID TEM data was collected on the Portuguese side of the IPB, by a previous operator, in a mineral concession within about 20km of the giant VMS deposit complexes of Aljustrel and Neves Corvo. The modelling and interpretation methodology used at that time employed 1D inversion (EM1DTM) and plate and host response modelling (LEROI) which failed to produce a discrete target in the area, despite broadly strong EM responses in the ground EM data. In 2022 Mira Geoscience was contracted to provide a 3D inversion of the legacy ground TDEM data. The outcome of the 3D modelling was interpreted to be a very interesting, localized conductivity bright spot, in a favourable structural position, and associated with a very broad circular gravity high of approximately 1 mgal amplitude. The EM/gravity target was tested in the 2023 season and, although drilling results failed to intersect mineralization, the 2023 drill results featured a number of interesting outcomes including intriguing volcanic VSC rocks with interesting hydrothermal alteration indexes, interpreted to be the correct age and type for hosting ore. Furthermore, the rocks were a co-located with a very strong offhole-type response in Downhole EM (DHEM) data. The 2023 drilling data allowed for the construction of geological sections by Almina geologists which were then converted to a 3D geological model. The 3D gravity modelling at the position of the offhole indicated that a high-density orebody, at the indicated depth would explain the observed co-located gravity anomaly if the strike extent was significantly increased from the DHEM model. In 2024 a follow-up hole, drilled on the ground TEM, DHEM and gravity model anomalies made the discovery of what is now referred to as the Torres deposit. The Torres deposit is the first major greenfield discovery in the IPB in 13 years and one of the first, if not the first, known example where 3D TEM inversion played a critical role in the discovery of the ore body.