Global Fe-O isotope correlation reveals magmatic origin of Kiruna-type apatite-iron-oxide ores - PubMed (original) (raw)

Global Fe-O isotope correlation reveals magmatic origin of Kiruna-type apatite-iron-oxide ores

Valentin R Troll et al. Nat Commun. 2019.

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.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1

Sample overview map. a Global map showing the different locations of origin for apatite-iron-oxide ore and reference samples. b A close-up view of the main part of the Fennoscandian Shield showing the sample locations for magnetites from Sweden

Fig. 2

Fig. 2

Images of apatite-iron oxide ores in this study. False-color BSE images of massive magnetite ore samples from a, b Kiruna; c, d Grängesberg; e, f Bafq; and g, h El Laco. Kiruna, Grängesberg, and Bafq magnetite samples are homogeneous and commonly lack zonation or signs of alteration. El Laco (g, h), is exceptional in this respect as for some samples intra-crystal zonation is observed. As a supplement, regular greyscale BSE images for these samples are provided in Supplementary Fig. 1

Fig. 3

Fig. 3

Iron isotope results. Shown is a the distribution of iron isotopes in magnetites from the Kiruna and Grängesberg districts, El Laco, and the Bafq district from this study, and b our data together with available literature data,,,. Reference fields for common hydrothermal and magmatic magnetites are shown for comparison,–,,,. Magnetites from apatite-iron oxide ores show a clear distinction from low-temperature or hydrothermal magnetites and overlap with the layered intrusions and volcanic reference magnetites (i.e. in the magmatic reference field). Data from Wang et al. show the effects of a progressive transgression from ortho-magmatic processes to hydrothermal fluid evolution from originally higher to lower δ56Fe values and an originally magmatic fluid may thus evolve into a hydrothermal fluid. One hydrothermal sample from the highly altered, remobilized, and recrystallized Mineville deposit in the USA (δ56Fe = −0.92‰, δ18O = −0.79) is not shown for simplification. Ve-Di samples represent vein and disseminated magnetites

Fig. 4

Fig. 4

Oxygen isotope results. a Oxygen isotopes of magnetite samples from Kiruna, Grängesberg, El Laco, and the Bafq district from this study are compared to reference samples from layered igneous intrusions, recent volcanic magnetites, and low-temperature or hydrothermal ore deposits as well as b data from the Chilean Iron Belt, the Pea Ridge and Pilot Knob deposits. The range of typical igneous magnetite is outlined in the reference box. The majority of magnetite samples from apatite-iron oxide ores plot within the reference field for common magmatic δ18O-values and overlap with magnetite values from recent volcanic rocks and layered intrusions. The low-temperature or hydrothermal reference suite, together with low-temperature magnetite literature data,, plot dominantly to the left of the magmatic magnetite field, with only one exception, a magnetite from the Fe-skarn deposit at Dannemora. This particular outlier comes from a part of the deposit (Konstäng) which itself represents a geochemical anomaly within the Dannemora deposit. Our values for El Laco overlap with results from previous studies,,,,

Fig. 5

Fig. 5

Distribution of Fe and O isotope values of magnetite samples used in this study. a The various magnetite samples can be divided into three groups according to their Fe–O isotope composition; (i) high-temperature magmatic magnetites, (ii) hydrothermal magnetite samples, and (iii) low-temperature magnetite samples. b Most of the magnetite compositions of the apatite-iron-oxide ores in this study lie within, or near, the reference field for igneous magnetite, and overlap with the plutonic and volcanic magnetite samples analysed as reference suite. See also Supplementary Note 1 and Supplementary Fig. 2 for a detailed assessment of temperature-dependent equilibrium compositions. Reference field for common igneous and hydrothermal magnetites are based on literature data,,,,,,

Fig. 6

Fig. 6

Schematic representation of magmatic stages for Kiruna-type apatite-iron-oxide ores from this and other studies, and from the analyzed reference materials. Stages II and III comprise ortho-magmatic ore formation: with decreasing temperature and on-going crystallization in the melt, the volatile/fluid pressure will increase and magmatic fluids are being expelled into the surrounding rocks. Below ~600 °C (towards the end of stage III), the magmatic-derived volatile pressure may begin to decrease, allowing progressively more of available external fluids into the system that initiate hydrothermal activity (<400 °C). Massive apatite-iron oxide ores appear to commence crystallization in the ortho-magmatic stages (Stages II and III), whereas vein and disseminated magnetites formed mainly during Stage IV (hydrothermal precipitation and replacement). This implies that the commonly observed hydrothermal signals in apatite-iron oxide ores are late-stage products that are results of syn- to post-magmatic hydrothermal processes active during the cooling of the volcanic system, or in some cases possibly reprecipitation during later overprints

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