Investigating the potential of systematic optical petrography in a geometallurgical context : A case study on boulder characterization from Rajapalot property, Finland

Sammanfattning: Geometallurgy describes a holistic approach to mining (integrating geological, metallurgical, geotechnical, environmental, and more parameters) with the goal to improve the efficiency and sustainability of a mining operation. The potential of systematic optical petrography in a geometallurgical context was investigated in this study, as well as how petrography may be useful across disciplines in several blocks of the mining value chain. In a case study, twelve boulder samples (non-mineralized and mineralized) from the Rajapalot exploration property in Finland were characterized by detailed optical microscopy and scanning electron microscopy (SEM) with the aim to delineate the origin of the respective boulders. This allowed to demonstrate some benefits and challenges of systematic optical petrography in the mining value chain and geometallurgical programs. The study was performed through the geology department at Mitta AB in Luleå, Sweden, using rock samples supplied by Mawson Gold Ltd, Finland. The Rajapalot Au-Co property lies a few km east of the Rompas Au-U property, both owned by Mawson Gold Ltd and located in the Paleoproterozoic Peräpohja belt in northwest Finland. When determining the boulders’ origin, geochemical gradients of the Peräpohja belt lithostratigraphy; oxidized sodic rocks in the Kivalo group and reduced potassic rocks in the Paakkola group were crucial factors. The mineralization style of the Rompas Au-U association and the Rajapalot Au-Co association (including the “Palokas” Fe-Mg type and the “Rumajärvi” K-Fe type) were further crucial to classify the mineralized boulders. Results show that the rock types (and suggested origin) vary between boulder samples. Most samples are presumed to originate from the Paakkola group, one sample from the Kivalo group, and a few samples are inconclusive. Furthermore, the samples impact on the mining value chain was discussed based on the results of the petrographic analysis. A method description for a more systematic way of examining rock material is presented, including for example quantitative parameters such asmodal mineralogy and grain size distribution. However, the applicability of this method description needs further study.