Phosphate is reported to be subject to “high supply risk” by the EU Commission (European Commission 2017). At present, the Siilinjärvi mine in Finland is the only mine in the EU producing phosphate. Optimising the productivity of the Siilinjärvi mine is crucial to address the demand for phosphate within the EU. The current production prognosis of the mine is to the end of 2035. To improve the prognosis of the mine, an exploration program is being undertaken to investigate the extent of the deposit and possible locations for new pits. The main area of interest is the area south of the current Särkijärvi pit. Exploration drilling is limited in this area due to obstacles created by infrastructure of the mine, including the factory area and gypsum pile. To address this, 3D passive source seismic, 2D active-source reflection seismic, Ground Penetrating Radar (GPR) and magnetic surveys were conducted at the Siilinjärvi mine site as part of the H2020 Smart Exploration project.
This study focuses on two of the acquired active-source seismic reflection profiles, SM2 and SM3. The aim of the study is to determine the depth and lateral extent southern continuation of the deposit in the area south of the Särkijärvi pit, next to the gypsum pile, and create a 3D model of the Siilinjärvi deposit based on the obtained results. In addition, obtaining information on waste rocks and zones of weakness, such as shear and fracture zones, is also of interest as this information is critical for mine planning. The main focus for seismic data processing was to improve the signal-to-noise ratio. Strong amplitude S-waves and unclear first-breaks were limitations found in the data. As a consequence, in addition to bandpass filtering, seismic line SM2 required a combination of attenuation and muting to supress the impact of S- waves. Seismic line SM3 had a lower data quality in comparison to that of SM2. The suppression of S- waves had a negative impact on the near-surface reflections along SM3 and therefore was not carried out. The GPR and magnetic data were processed using standard workflows.
The active-source seismic survey was successful in determining the depth and the lateral extent of the southern continuation of the Siilinjärvi deposit. A 3D model of the deposit was created based on the obtained seismic images. This model expands on the previous model and indicates that the carbonatite- glimmerite deposit expands towards the W, beneath the gypsum pile. This information can be used as a guide for future drilling in the area. In addition, information was obtained on zones of weakness and the waste-rock dike network. Sub-horizontal to gently dipping reflections observed in the seismic data were interpreted as diabase dikes. On a smaller scale, GPR measurements detected shallower near-surface features which are also interpreted to possibly be dikes. For some features, a correlation could be made between the various geophysical measurements. The carbonatite-glimmerite deposit was found to be associated with elevated magnetic total field (nT) values.
