Mineral texture and composition of magnetite in the Sahavaara IOCG 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 Mg
content of its concentrate which is suggested to be lattice bound. In some
cases 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 could
cause 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.