Mineral texture and composition of magnetite in the SahavaaraIOCG deposit, Pajala area: a mineral processing approach

Abstract: Stora Sahavaara is one of several iron (Cu-Au) deposits of economic potential in the Pajala-Kolari area and it is currently developed for mining by Northland Resources Inc. These deposits are hosted by 2.3-2.0 Ga Karelian greenstones and occur as magnetite-rich lenses at certain stratigraphical positions and often in association with carbonate rocks. They contain various amounts of skarn minerals with serpentine, pyroxene, and amphibole as the most important gangue minerals. The Stora Sahavaara iron occurrence was previously considered as belonging to a broad group of magnetite-dominated, Ca-Mg- and Mg-silicate skarn-hosted deposits, which occur throughout northern Sweden, within a Karelian volcano-sedimentary domain. Several deposits considered to be of the same type also occur and have been well studied in the Finnish part of the Lappland, in the Kolari district. These deposits, which are interpreted to be epigenetic Fe-Cu + Au deposits, occur within the same structures as the Stora Sahavaara deposit. They are suggested to belong to the increasingly important group of iron-oxide-copper-gold (IOCG) deposits. A characteristic feature of the Stora Sahavaara magnetite is the high Mgcontent of its concentrate which is suggested to be lattice bound. In somecases the recovery (magnetite) is poor, or the concentrates are not clean,which is clearly indicated by a high SiO2 content in the concentrate. Also,significant inclusions/intergrowths of sulphides in magnetite grains couldcause the final iron ore concentrate to have high S content, for example.Alternatively, the grade may be lower than predicted due to silicate or other gangue carrying finely disseminated magnetite in some textural ore types. The general purpose of this work is to apply the mineralogy and geology of the Stora Sahavaara deposit in order to better understand the metallurgical processing characteristics of the ore. For the mineral processing of the ore and the properties and quality of the final products, the texture of magnetite, its impurities and its chemistry is of great importance. According to the data presented, the principal ore minerals of the Stora Sahavaara magnetite deposit are Mgt-Po-Py-Ccp. The main ore mineral magnetite contains high amounts of Mg, bound to the crystal lattice only in cases where coarse-grained textures of the first generation occur and where serpentine skarn dominates. The dominant sulphide is pyrrhotite which exists in both monoclinic and hexagonal crystal forms. The monoclinic crystal form of pyrrhotite is the principal factor behind some S-bearing concentrates as it turns to report with magnetite. All other metal oxides such as SiO2+Al2O3, TiO2 and vanadium, which have upper acceptable limits in the Direct Reduction Ironmakers (DRI) pellets, which is the pellet towards which Northland's efforts are primarily directed, are within acceptable ranges. This study shows that the Stora Sahavaara magnetite composition is suitable for the DRI type of pellets, most especially considering blending. It gives a detailed characterization of the magnetite, and will hopefully serve as a useful manual during the mining process.