Kobolt i pyrit från Kiruna järnmalmsgruva

Sammanfattning: Summary: The Kiruna ore is situated in northernmost Sweden, and it is one of the largest iron ore deposits that is known in the whole world. Small amounts of cobalt occur in the ore. This paper is devoted to the study of the distribution of cobalt in the Kiruna ore. Results from studies under the polarizing microscope in both transmitted and reflected light in addition to chemical wholerock analyses, ore dressing tests and mineral analyses in a scanning electron microsocope (SEM) equipped with an energy dispersive X-ray system (EDX) indicate that most of the cobalt is found in solid solution in pyrite. No cobalt phases are present. The content of cobalt in chalcopyrite is below the detection limit of the EDX-system. Chemical analyses from 142 samples are used in the investigation, but polished sections were made from only eleven of the samples. The low content of cobalt in the pyrite and the overlapping peaks of cobalt and iron spectras make the EDX analyses extremely unreliable and most of the conclusions are based on the whole-rock analyses. Cobalt shows an uneven distribution in pyrite, and some samples are very poor in cobalt. Pyrite occurs mainly in fissure veins, but also in fault zones and as impregnations in both the ore and the country rock. It is found as euhedral to subhedral grains. A grainsize analysis indicates that 97% of the grains are smaller than 3 mm in diametre. However, the grain size varies both within and between individual samples. The investigation is restricted to the northern part of the Kiruna mine, where most of the pyrite is found. Most faults and fissure veins have a north-western strike but dip either in a south-western (dominating) or a northeastern direction. It has not been possible to demonstrate a concentration of cobalt to certain areas or certain levels in the mine. The low percentage of cobalt in the pyrite - maximum precentage 0.99 wt% - and the very irregular distribution of cobalt make mining operations for cobalt economically unprofitable.