EBSD Investigation of High-Temperature Magnetite from Apatite-Iron-Oxide Deposits: Implications for the Formation of Giant Kiruna-Type Deposits

Abstract: European iron production is to a large extent dependant on massive Kiruna type apatite-iron ore deposits. In this contribution, high-temperature magnetite samples from apatite-iron-oxide deposits are investigated by means of Electron Backscattered Diffraction. However, the origin of Kiruna-type deposits is still unresolved. Although magmatic processes are likely, it is not clear how small-scale processes can form giant Kiruna-type deposits. The sample suite consists of magnetite samples from six global apatite-iron-oxide deposits: the famous Kiirunavaara deposit and the Malmberget deposit, both located in northern Sweden, the Grängesberg deposit in south-central Sweden, the iconic El Laco deposit in north-eastern Chile, the Bafq deposit in central Iran, and the Varena deposit in south Lithuania. Fe-O systematics has been conducted to complement existing δ18O and δ56Fe isotope data and ensure magmatic origin of the samples from the Malmberget deposit (n=6) and the Varena deposit (n=2). This is the first effort to characterise magnetite samples from apatite-iron-oxide deposits utilising EBSD. In total, twelve EBSD maps have been produced. Evaluation of the EBSD data have been performed to quantify the preferred orientation of the magnetite crystals. Four deposits, with Kiirunvaara being the prime example, shows no preferred alignment of the magnetite crystals. Whereas the El Laco samples exhibits a strong preferred alignment of {111}. The EBSD data from magnetite samples in equilibrium with a magmatic source indicate that apatite-iron-oxide deposits are formed in both intrusive and extrusive environment and that magmatic crystal accumulation is a key process in aggregating magnetite to form large and even giant Kiruna-type deposits.