Mineral chemistry of magnetite from magnetite-apatite mineralization and their host rocks: Examples from Kiruna, Sweden and El Laco, Chile

Magnetite-apatite deposits, sometimes referred to as Kiruna-type deposits, are major producers of iron ore that dominantly consist of the mineral magnetite (nominally [Fe²⁺Fe³⁺₂]O₄). It remains unclear whether magnetite-apatite deposits are of hydrothermal or magmatic origin, or a combination of those two processes, and this has been a subject of debate for over a century. Magnetite is sensitive to the physicochemical conditions in which it crystallizes (such as element availability, temperature, pH, fO₂, and fS₂) and may contain distinct trace element concentrations depending on the growing environment. These properties make magnetite potentially a useful geochemical indicator for understanding the genesis of magnetite-apatite mineralization. The samples used in this study are from precisely known geographic locations and geologic environments in the world class districts of Kiruna and the Atacama Desert and their associated, sometimes hydrothermally altered, host rocks. Trace element analyses results of magnetite from the Kiruna area in the Norrbotten region of northern Sweden, and the El Laco and Láscar volcanoes in the Atacama Desert of northeastern Chile, were evaluated using mineral deposit-type and magmatic vs. hydrothermally derived magnetite discrimination diagrams. The objectives of this study are to critically evaluate the practical use and limitations of these discrimination diagrams with the goal of determining if the trace element chemistry of magnetite can be used to resolve if magnetite-apatite deposits form in a hydrothermal or magmatic environment, or a combination of those two processes. The results of this study reveal that the magnetite from Kiruna have relatively low trace element concentrations and are homogeneous. There is little chemical variation between the ore, the hydrothermally altered host rocks, and related igneous intrusives from Kiruna, which may be a result of pervasive post-formation alteration due to extensive metasomatism and later greenschist facies metamorphism that has affected the region. Distinct chemical variations do exist, however, in the magnetite at El Laco between magnetite from the Laco Sur magnetite-apatite ore and host andesite, and a dacite sample from the nearby Láscar volcano. Accessory magnetite from the host andesite and Láscar dacite volcanic rocks have relatively high trace element concentrations, typical of magnetite crystallizing from a melt and; with overall trace element concentrations higher than the magnetite-apatite ore, or the Kiruna magnetites. The Laco Sur ore magnetite has low overall low trace element concentrations and displays growth zoning defined by incompatible elements (Si, Ca), compatible elements (Mg, Al, and Mn), LILE (Sr) and HFSE (Y, Nb, Ce and Th) with each element displaying similar geochemical trends. While the trace element concentrations of the ore magnetite at Laco Sur has cores that are enriched in incompatible element; similar to magnetite that is known to have formed from hydrothermal fluids (such as magnetite from iron-rich skarns), the trace element zoning patterns suggests that the magnetite crystallized from a volatile-rich iron-oxide melt