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In the Pyhäsalmi case study, the seismic data is used in direct targeting of shallowly dipping mineralized zones in a massive sulfide ore system that was deformed in complex fold interference structures under high-grade metamorphic conditions. The Pyhäsalmi volcanic-hosted massive sulfide (VHMS) deposit ([Formula: see text]) is located in a Proterozoic volcanic belt in central Finland. Acoustic impedance of Pyhäsalmi ore ([Formula: see text]) is distinct from the host rocks ([Formula: see text]), enabling its detection with seismic reflection methods. Drill-hole logging further indicates that the seismic imaging of a contact zone between mafic and felsic volcanic rocks possibly hosting additional mineralizations is plausible. Six seismic profiles showed discontinuous reflectors and complicated reflectivity patterns due to the complex geology. The most prominent reflective package at 1–2 km depth was produced by shallowly dipping contacts between interlayered felsic and mafic volcanic rocks. The topmost of these bright reflections coincides with high-grade zinc mineralization. Large acoustic impedances associated with the sulfide minerals locally enhanced the reflectivity of this topmost contact zone which could be mapped over a wide area using the seismic data. Seismic data enables extrapolation of the geologic model to where no drill-hole data exists; thus, seismic reflection profiling is an important method for defining new areas of interest for deep exploration.

The Vihanti-Pyhäsalmi district is located at the border zone between the Archaean craton in the northeast and the Palaeoproterozoic Svecofennian domain in central Finland. It is part of the Savo belt and contains one operating and four past mines and several uneconomic VHMS (Volcanic Hosted Massive Sulphide) deposits. Regional, semi-regional and mine scale 3D models of the Vihanti-Pyhäsalmi area were built in order to understand the formation of the VHMS deposit and accordingly map the ore critical geological formations and structures. In addition to the 3D modelling of the present day geology, structural geological inference was used to build 3D geological models visualizing the main deformational stages in past. The used data and information consisted of geological maps and reports, drill core logs, lithogeochemistry, reflection seismic profiles, potential field measurements and electromagnetic surveys. The geophysical 2D and 3D inversion were used to assist the 3D geological modelling seeking for continuity of the surficial structures especially in areas with only few or no drill core data. As a result, regional and semi-regional 3D models of the Vihanti-Pyhäsalmi area were obtained and new zones with VHMS exploration potential were identified in the vicinity of the Pyhäsalmi mine.

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