Magnetotelluric Exploration of Zn Pb Mineralization in Stonepark, Irish Midlands

ABSTRACT To meet the increasing demand for raw materials and to foster the energy transition, advances in exploration techniques are required that extend capabilities to deeper and/or more densely populated areas while having a low environmental and societal impact. Although active electromagnetic (EM) geophysical techniques have a long tradition in mineral exploration, magnetotellurics (MT) as a passive EM method has recently attracted more attention in this context, as it naturally offers a wider range of survey depths while being even less invasive. Here, we present the results of an MT study conducted at the Stonepark Zn–Pb deposit, an exploration project hosted in Mississippian carbonates and volcanics in the Irish Orefield. The survey was carried out in late 2022, using a novel experimental layout of 33 five‐component broadband MT stations deployed in concert with 75 two‐component electric field only stations. Resulting data exhibit strong EM noise but reasonable MT transfer functions could be obtained in the frequency range of 10 4 –10 −2 Hz by using a combination of the robust remote reference (RR) technique, notch filtering and physical and statistical pre‐selection thresholds. Quality of full MT and E‐field stations is the same demonstrating the applicability of the hybrid surveying approach also in noisy environments. The electrical conductivity structure at Stonepark revealed by 2D and 3D inversion shows the host rocks as high resistivity material (500–5000 Ωm) typical of limestones with little lateral variations in resistivity across the survey area. Although the MT models do not provide a direct image of economically relevant mineralized zones, they are in excellent agreement with resistivity measurements on rock samples from nearby drill holes. This is remarkable because laboratory measurements are made on centimetre‐sized samples, whereas MT soundings sample resistivities over volumes of 10–100 s of meters. Integration of the MT results with the drill hole data, a 3D geological model of the Stonepark mineral system and a reprocessed seismic profile allowed the continuation of the horizons to the south where they are sparsely documented by drill holes. In particular, the MT data revealed alteration of volcanic material in the upper 200–800 m as well as a subvertical conductive feature that is spatially coincident with a hidden fault zone inferred from the seismic data, suggesting that the fault zone may be fluid enriched.