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Browsing by Author "Laakso, Viveka"

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  • Laakso, Viveka (2019)
    Siilinjärvi mine in Finland is the only mine within the European Union producing phosphate rock, a critical raw material for the European Union. With the current mining plans, the production is estimated to continue until 2035. The extent of the ore deposit and new locations for open pits are currently being investigated to ensure continuation of the mining operations also after 2035. The Siilinjärvi carbonatite-glimmerite deposit has been intruded by multiple waste-rock diabase dykes crosscutting the deposit and by a tonalite-diorite intrusion, creating a complex geological setting. To study the depth and lateral extent of the deposit, the diabase dykes, tonalite- diorite, major zones of weakness and the geophysical anomalies related to these features, active-source 2D reflection seismic, Ground Penetrating Radar (GPR) and magnetic surveys were conducted at the Siilinjärvi mine site in fall 2018 as part of the H2020 Smart Exploration project. Understanding the locations of the waste rocks and fracture and shear zones is crucial for mine planning and optimising the production prognoses. The interest of this study is in particular on imaging the sub-horizontal diabase dykes, whose locations and continuation are harder to predict. The focus area of this study is in the southern end of the Särkijärvi pit and the area just south of the pit, where the well-known geology of the pit can be used to constrain the interpretation. Processing of the reflection seismic data focused especially on the static corrections and the methods used to improve the signal-to-noise ratio. This was done so that the processed data could serve as a reference for new processing methods, focused on these aspects, developed within the Smart Exploration project and planned to be tested with the Siilinjärvi data. The static corrections were constrained by the limited number of first-break picks clear enough for picking from the data. In addition to the bandpass filtering, suppressing the S-wave arrivals was found to be crucial for increasing the signal-to-noise ratio, particularly in the near subsurface which is the main interest area of this study. The GPR and magnetic data were processed with standard processing workflows. The lateral and depth extent of the Siilinjärvi carbonatite-glimmerite deposit, the large-scale sub-horizontal waste- rock dykes and the major zones of weakness are imaged with the active-source reflection seismic data. The ore deposit is associated with a complex reflectivity pattern due to the intruded diabase dykes and tonalite-diorite, and the fracture and shear zones. The interpreted diabase dykes correlate with a large-scale sub-horizontal waste-rock dyke model created from the production drilling data as part of the Smart Exploration project, supporting the continuation of the sub-horizontal diabase dykes south of the pit. With GPR data, the smaller- scale sub-horizontal dykes within the shallow subsurface (<30 m) are imaged. The GPR data correlates with a detailed waste-rock dyke model created as part of the Smart Exploration project from the southern end of the Särkijärvi pit based on geological mapping, GigaPan images and a 3D photogrammetry model. The reflection seismic, GPR and magnetic data have very different scales and these different data are suitable for different purposes in mineral exploration and mine planning at Siilinjärvi. The carbonatite-glimmerite ore is associated with elevated magnetic total field values and at a larger scale the deposit could possibly be followed with magnetic surveys. With reflection seismic method, the large geological structures can be imaged at depth, and the data could be used for detailed planning of a new open pit. The higher resolution GPR measurements could then be implemented in the operating phase of the mine in a more routine manner to aid creation of reliable production prognoses.