Abstract
Kiruna-type apatite-iron-oxide ores are key iron sources for modern industry, yet their origin remains controversial. Diverse ore-forming processes have been discussed, comprising low-temperature hydrothermal processes versus a high-temperature origin from magma or magmatic fluids. We present an extensive set of new and combined iron and oxygen isotope data from magnetite of Kiruna-type ores from Sweden, Chile and Iran, and compare them with new global reference data from layered intrusions, active volcanic provinces, and established low-temperature and hydrothermal iron ores. We show that approximately 80% of the magnetite from the investigated Kiruna-type ores exhibit δ56Fe and δ18O ratios that overlap with the volcanic and plutonic reference materials (> 800 °C), whereas ~20%, mainly vein-hosted and disseminated magnetite, match the low-temperature reference samples (≤400 °C). Thus, Kiruna-type ores are dominantly magmatic in origin, but may contain late-stage hydrothermal magnetite populations that can locally overprint primary high-temperature magmatic signatures.
Highlights
Kiruna-type apatite-iron-oxide ores are key iron sources for modern industry, yet their origin remains controversial
The discussion revolves around a direct magmatic origin from volatile- and Fe–P-rich magmas or high-temperature magmatic fluids[1,5,18,31], versus a purely hydrothermal one, where circulating, metal-rich fluids replace original host rock mineralogy with apatite-ironoxide mineralizations at medium to low temperature[4,19,28,29,30,32]
An ortho-magmatic origin is generally understood to be either formation by direct crystallization from a magma or from hightemperature magmatic fluids (e.g. ≥800 °C), or via hightemperature liquid immiscibility and physical separation of an iron oxide-dominated melt from a silicate-dominated magma, where the former may subsequently crystallize as a separate body[17,18,33,34,35,36,37,38]
Summary
Kiruna-type apatite-iron-oxide ores are key iron sources for modern industry, yet their origin remains controversial. Hydrothermal processes, in turn, encompass transport and precipitation, including replacement-type reactions, by means of aqueous fluids at more moderate to low temperatures (typically ≤400 °C)[27,29,30,32] Both the magmatic and the hydrothermal hypotheses are supported in part by field observations, textural relationships, and mineral chemistry, petrological field evidence and chemical trends of major and trace elements have frequently been interpreted in different ways[5,14,16,17,18,19,20,21,22,28,29,30,31,32]. To date no systematic stable isotope study employing several distinct Kiruna–type apatite iron oxide ore deposits is available and, importantly, no systematic comparison with accepted magmatic and hydrothermal rock and ore suites has previously been presented in the literature
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