Magmatic origin of giant 'Kiruna-type' apatite-iron-oxide ores in central Sweden - PubMed (original) (raw)

Magmatic origin of giant 'Kiruna-type' apatite-iron-oxide ores in central Sweden

Erik Jonsson et al. Sci Rep. 2013.

Abstract

Iron is the most important metal for modern industry and Sweden is by far the largest iron-producer in Europe, yet the genesis of Sweden's main iron-source, the 'Kiruna-type' apatite-iron-oxide ores, remains enigmatic. We show that magnetites from the largest central Swedish 'Kiruna-type' deposit at Grängesberg have δ(18)O values between -0.4 and +3.7‰, while the 1.90-1.88 Ga meta-volcanic host rocks have δ(18)O values between +4.9 and +9‰. Over 90% of the magnetite data are consistent with direct precipitation from intermediate to felsic magmas or magmatic fluids at high-temperature (δ(18)Omgt > +0.9‰, i.e. ortho-magmatic). A smaller group of magnetites (δ(18)Omgt ≤ +0.9‰), in turn, equilibrated with high-δ(18)O, likely meteoric, hydrothermal fluids at low temperatures. The central Swedish 'Kiruna-type' ores thus formed dominantly through magmatic iron-oxide precipitation within a larger volcanic superstructure, while local hydrothermal activity resulted from low-temperature fluid circulation in the shallower parts of this system.

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Figures

Figure 1

Figure 1

(A). Overview map of Fennoscandia with the Grängesberg mining district (GMD), the Bergslagen province, and the Kiruna-Malmberget mining district indicated. (B). Geological map over the main ore zone in the GMD. (C). Vertical section (line X–Y in panel B) through the main ore body of the GMD. The ore zone extends downwards at a 70–80° dip to the SE. Black horizontal lines are adits. Modified from Geological Survey of Sweden (SGU) datasets and A. Hallberg, SGU.

Figure 2

Figure 2. Chondrite-normalised REE diagram of host rocks and iron-oxide ores from drill cores 717 and 690.

Chondrite values for normalisation after Sun & McDonough. The iron-oxide ores and the moderately altered host rocks share similar REE concentrations and patterns with the regional bedrock and recent subduction zone volcanic rocks. The hydrous altered host rocks (marked with hatched pattern), in turn, are enriched in REE relative to moderately altered host rocks and recent subduction zone volcanic rocks.

Figure 3

Figure 3. Parts (A) and (B) show the oxygen isotope data for two drill cores (Numbers 690 and 717) that traverse the main ore zone at Grängesberg between 570 and 670 m below the surface (see Fig. 1).

Shown are the oxygen isotope compositions of the host rocks, quartz separates, massive magnetites and VeDi-magnetites, the latter including magnetite from hematite ore. All oxygen data are reported in standard δ18O-notation relative to SMOW after Hoefs. The δ18O ranges for the mantle and arc-andesites are after Bindemann and Taylor. Range of igneous magnetites after Taylor.

Figure 4

Figure 4. Magnetite δ18O values from GMD compared to other volcanically-hosted iron ore deposits.

For reference, magnetites in equilibrium with MORB [red box], the range for typical ‘ortho-magmatic’ magnetites after Taylor [pale pink box] and magnetite in equilibrium with an evolved rhyolite with a δ18O of 10‰ (the demarcation between I-type (<10‰) and S-type (>10‰) magmas) are shown. The GMD magnetites plot dominantly above the +0.9‰ demarcation and in the field of ‘ortho-magmatic’ magnetites after Taylor35), and satisfy equilibrium with magma or magmatic fluids at magmatic temperatures (~800–1000°C). A small fraction of the GMD data (n = 2), however, is more consistent with formation from a low-temperature fluid regime. The cut-off point for this is calculated to be +0.9‰ in magnetites, because fractionation factors determine that samples < +0.9‰ cannot be in equilibrium with either a magma or a magmatic fluid at high temperatures (≥ 800°C). Magnetites with values lower than +0.9‰ are calculated to have been in equilibrium with a high- δ18O (likely meteoric) fluid at temperatures of ≤400°C.

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