Mineral Chemistry of Gangue Minerals in the Kiirunavaara Iron Ore

Abstract: The iron-ore of Kiirunavaara is one of the largest magnetite-apatite mineralisation in the world. It is well known for its purity and size and its origin has been the subject of an intense debate for over a century. The composition of the most important minerals from an economic point of view, magnetite and apatite, has been well studied but little is known about the composition and role of the silicate minerals.
Nowadays, even the silicates have been targeted for investigations, both in Kiirunavaara and in similar deposits elsewhere, e.g., Sierra la Banderra, Chile. The growing interest derives mainly from the importance of the gangue minerals in ore processing but also because they might hold the key to the origin of the ores.
In this study, a large scale microprobe investigation of over 250 analyses has been carried out mainly focussing on silicates within the ore body but also to some extent on sulphates, phosphates and carbonates. Most of the analysed grains belonged to either the amphibole group or the mica group. Amphiboles and micas are by far the most abundant silicates within the ore body.
Through (EMPA) analysis of 198 separate mineral grains, this investigation led to the broadest and most updated characterisation of silicates within the Kiirunavaara ore body. It also briefly looks at setting and chemistry of sulphates, phosphates, and carbonates.
This study indicates that silicates may have been formed during shifting periods of crystallization and chemical environments, and that the mineral assemblage and composition do not oppose a magmatic origin.