Use of seismic reflection profiling for 3D-modeling of mineralized regions; A case study from Pyhäsalmi, Finland

Understanding the geological structure in 3D is essential for studying tectonic evolution and for mineral exploration. Often only limited amount of data is available and models are regularly based mainly on near surface observations. The geophysical methods are needed to follow the features mapped in the surface to the depth to fully understand the geological structures and tectonic setting of the study area. Many widely used geophysical methods, such as electromagnetic measurements, loose their resolution with depth. The seismic reflection profiling, or preferably 3D-seismic studies, provide an important tool for geophysical-geological modeling as high resolution seismic images down to the 5 km depth can easily be achieved with this technique. We present the results of the on-going 3D-modeling effort from Pyhäsalmi, Finland. Pyhäsalmi is volcanic hosted massive sulphide deposit that has been actively mined since 1950's. Currently mine is ~1.5km deep. The existing infrastructure with depth enables the economic exploitation of new deep ore reserves. This is motivating modeling and exploration in the area. Interpretation of six seismic reflection profiles acquired at 2007 is one important part of the modeling effort as they provide information beyond the exploration depths. As typical for seismic data acquired in hard rock environment, the profiles are not easy to interpret due to lack of continuous reflectors. The acoustic logging in three drill holes gave us the constraints of reflectivity. Mafic volcanites provide good acoustic contrast to any other rock type of the area, and thus the units containing mafic components are probable cause of reflective structures. The reflective units also coincidence with the positive anomalies in magnetic maps. We used the texture and intensity of reflectivity instead of individual reflectors to model the geological structures of the area. Near the surface (>500m) the structures are known to be sub-vertical. Seismic profiles show us, that generally they turn to almost subhorizontal orientation with depth. The folding with approximately north-south oriented fold axis explains both observations and enables the mapping of the key horizon for mineral exploration over a wide area. These results need to be confirmed with the geological data, especially the data from the oriented drill core and information from the recent deep exploration drilling. The high amplitude reflectors in one of the seismic lines have recently been linked with high grade zinc mineralization, and thus seismic method has already proved to be valuable tool in 3D-modeing and also an important complementary method for mineral exploration.