Interaction between high-temperature magmatic fluids and limestone explains ‘Bastnäs-type’ REE deposits in central Sweden

The presently increasing demand for rare earth elements (REE), particularly in high-tech and “green energy” applications, has led to global interest in the distribution, origins and formation conditions of REE deposits. The World’s first hard-rock REE sources, the polymetallic deposits of Bastnäsfältet in Bergslagen, central Sweden, were also the place of the original discovery of several REE and many of their host minerals. Similar deposits with high concentrations of REE occur along a > 100 km corridor in the region and they share a number of geological and mineralogical features; all comprising Palaeoproterozoic, skarn-hosted magnetite-REE mineralisation of ambiguous origin. Here we report oxygen isotope data for magnetite and quartz, and oxygen and carbon isotope data for carbonates from ten of these classic deposits, to model and assess their mode of origin. Combined with existing geological observations, the isotope results support an origin in a c. 1.9 Ga shallow-marine back-arc, sub-seafloor setting, where felsic magmatic-sourced, high-temperature fluids reacted with pre-existing limestone interlayers, leading to localised skarn formation and magnetite-REE-mineral precipitation. These findings help us to better understand the geological processes that have produced economic REE mineralisation and may assist future exploration for these critical commodities.

Bastnäs-type deposits are characterised by skarn-hosted magnetite-REE mineralisation, with or without additional polymetallic assemblages 11,15,17 . These deposits occur as clusters along the so-called "REE-line" 18 ; a more than 100 km long, narrow belt in the western Bergslagen region of variably Na, K and/or Mg altered, c. 1.90-1.88 G.y. old felsic metavolcanic rocks with intercalated marble layers (Fig. 1). This volcano-sedimentary sequence formed during magmatic activity in what has been interpreted as a shallow-marine, but principally continental back-arc setting 19 . Both the magnetite-REE ore bodies and the host rocks have been affected by later, polyphase deformation and greenschist to amphibolite facies metamorphism at c. 1.85-1.80 Ga during the Svecokarelian orogeny [20][21][22][23] . In the Bergslagen ore province, where base metal mineralisations and iron oxide ores predominate [23][24][25][26][27] , the REE-line represents both a geological anomaly and a zone of significant concentration of in general light REE, but also yttrium, and the heavy REE 16 . The REE minerals in these deposits are dominantly silicates [allanite-and dollaseite-group minerals, cerite-(Ce), fluorbritholite-(Ce), törnebohmite-(Ce)], and the carbonate bastnäsite-(Ce), besides numerous low-volume accessory phases 16,17 . The skarns hosting the magnetite-REE mineralisations are calc-silicate aggregates, typically amphibole-dominated, featuring actinolitic to tremolitic, as well as anthophyllitic compositions [28][29][30] . Magnetite was always the main ore mineral mined in these deposits, besides the REE mineralisations, but copper as well as cobalt ores have also been locally exploited 29,30 .
The genesis of the Bastnäs-type REE deposits has been critically debated with respect to their formation process, and their temporal relationships to host rocks and to other mineralisation types 11,[15][16][17][18]28,30,31 . Their genesis, and that of the associated skarn iron ores and extensive host rock alteration, was originally interpreted to be related to large-scale, so-called "magnesia metasomatic" processes, generated by abundant granitoid intrusions during the waning stage of regional (Svecokarelian) metamorphism 15,27 . Later studies have rendered this theory obsolete 17,18,31,32 , and they mostly revolve around an essentially syn-volcanic hydrothermal formation scenario at c. 1.90-1.88 Ga, or somewhat later. Here we employ mineral oxygen and carbon isotopes to unravel the

Results and Interpretation
Representative, variably mineralised rock samples were both collected in the field and sourced from the collections of the Geological Survey of Sweden in Uppsala and the Swedish Museum of Natural History in Stockholm. The sample suite encompasses ten different Bastnäs-type deposits from within the REE-line, Bergslagen (Fig. 1), and comprises massive magnetite ore (from Bastnäsfältet, Myrbacksfältet, Östra Gyttorpsgruvan and Rödbergsgruvan), skarn and skarn ore with disseminated-type magnetite (from Knutsbogruvorna, Östanmossagruvan, Södra Hackspikgruvan, Johannagruvan, Malmkärragruvan and Bastnäsfältet) and magnetite-skarn-bearing banded iron formation (BIF) ore (from Högforsfältet; Table 1). Magnetite concentrates (n = 25) were separated from all the ore types and have δ 18 Fig. 2). However, they plot partly within the fields for previously analysed carbonates from iron oxide skarns and carbonates from granite-related (metasomatic) tungsten-molybdenum skarns 34 (Fig. 2). To test for possible process scenarios behind the observed shifts in carbonate isotopic compositions for the Bastnäs-type deposits, we calculated two different numerical models. Trajectories A-C in Fig. 2  where F is the molar fraction of oxygen that remains in the rock after de-volatilisation; α is the fractionation factor; and δ 18 O i and δ 18 O f are the initial and final oxygen isotope compositions of the rock, respectively 35 . This model is based on the so-called "calc-silicate limit", which assumes that all carbon is released as CO 2 while c. 60% of the oxygen remains in the rock if the reaction goes to completion 35 . Unlike what has locally been observed in other marble-skarn environments in Bergslagen (e.g. 36 ), the analysed carbonates from the Bastnäs-type deposits do not plot on the steep or near-vertical trends of pure thermal decarbonation (A-C in Fig. 2). These results suggest that such reactions alone cannot realistically have produced the observed isotopic distribution. Lines 1-3 in Fig. 2, in turn, represent binary mixing trajectories between the same non-mineralised, Palaeoproterozoic marine carbonates and a typical magmatic aqueous fluid composition (δ 18 O = +5.3 to +10.0‰, δ 13 C = −8.0 to −4.0‰) 35,37-41 , using different end-member compositions. Mixing is defined as: where R M is the isotope ratio of element X in a mixture of compositions A and B; X A and X B are the concentration of X in A and B, respectively; and f is the weight fraction of A defined as f = A/(A + B) 41 . The calculated magmatic fluid-carbonate mixing trajectories envelop essentially all of the analysed carbonates from the Bastnäs-type deposits (Fig. 2). Based on these results, we interpret that progressive CO 2 release during interaction between a magmatic-derived, aqueous hydrothermal fluid and local, marine carbonate horizons caused the observed shifts to lower δ 18 O and δ 13 C values in the primary (protolith) carbonates, as well as re-precipitation of secondary (hydrothermal) carbonates of similar compositions (cf. 35,[42][43][44] Fig. 3A). Higher-than-magmatic fluid δ 18 O values are explained by interaction with pre-existing carbonate rocks (Fig. 3A).
The oxygen isotope compositions of hydrothermal fluids in equilibrium with carbonates from these deposits were also calculated for temperatures of 200 to 600 °C (Fig. 3B), based on the fractionation factors of Zheng 48 and Sheppard & Schwarz 49 . Equilibrium fluid δ 18 O values in the range of primary magmatic waters, from +5.8 to +8.0‰, are only produced for the highest δ 18 O value (+10.0‰) of the analysed carbonates (Fig. 3B). The average (+7.4‰) and lowest (+5.8‰) carbonate δ 18 O values give significantly lower equilibrium fluid δ 18 O values. With decreasing temperature, they range from the lowermost limit of magmatic fluids down to +2.0‰ (Fig. 3B). Unlike the physically and chemically refractory magnetite, which is much more likely to retain its original chemical and isotopic composition (e.g. 50,51 ), the reactive carbonates are more easily affected by both high-temperature processes that could lead to, e.g., de-volatilisation, and low-temperature fluid overprinting, either during primary skarn and ore formation or during subsequent phases of regional metamorphism and granitoid intrusion. Such retrograde isotopic modification may be more marked in some carbonates than in others, explaining the variation in carbonate equilibrium fluid δ 18 O values (Fig. 3B) relative to the more limited, essentially magmatic range for magnetite fluids (Fig. 3A).
Overall, the gradual transition from higher to lower equilibrium fluid δ 18 O values observed in the combined magnetite-carbonate datasets suggests that they record the progressive evolution of a hydrothermal system that commenced with high-temperature, magmatic-dominated fluids, via decreasing temperatures and gradual input (dilution) from external, non-magmatic water sources.

Conclusions
The new oxygen and carbon isotope data together with the generated numerical models demonstrate that the magnetite-REE-mineralised skarn assemblages from Bergslagen formed from high-temperature hydrothermal fluids of a predominantly magmatic origin. These fluids reacted with local, Palaeoproterozoic marine carbonate rocks and, over time, the hydrothermal system cooled and experienced influx of isotopically distinct (low-δ 18 O) water sources, such as seawater. Combined with available geological, geochronological and textural observations [15][16][17][18][19]23,[28][29][30][31][32] , the new results are most easily reconciled with a scenario involving sub-seafloor, felsic magmatic activity at c. 1.90-1.88 Ga, within a shallow-marine back-arc setting (Fig. 4). In this setting, magmatic-sourced, metal-and silica-rich hydrothermal fluids were introduced to, and reacted with limestone interlayers in an otherwise pyroclastic-dominated volcano-sedimentary succession, leading to skarn formation and magnetite-REE-mineralisation.
Our study provides new evidence for a magmatic origin of the World's original hard-rock source of REE, the Bastnäs-type REE deposits of central Sweden. These findings help us to better constrain the geological processes www.nature.com/scientificreports www.nature.com/scientificreports/ associated with formation of economic REE mineralisation, and will thus assist exploration for these critical commodities in the future. Specifically, the Bastnäs-type deposits represent a large-scale (>100 km) feature of high-grade REE concentration in the Bergslagen province (Fig. 1), but are currently unknown in the form of direct analogues from other locations globally. We propose that geological terranes elsewhere that constitute shallow-marine, sub-seafloor settings within continental back-arcs may be prospective for Bastnäs-type REE www.nature.com/scientificreports www.nature.com/scientificreports/ mineralisation. In such settings, evidence of extensive felsic magmatism combined with the presence of nearby carbonate horizons constitute key first-order exploration criteria (Fig. 4). Determining the cause of original REE enrichment in the magmatic systems that produced Bastnäs-type deposits, and whether or not it is unique to this tectonic setting, remains an important avenue for future research.

Methods
Mineralogical characterisation. Representative rock samples were cut and prepared as polished thin and thick sections, and subjected to transmitted and reflected polarised light microscopy and reconnaissance scanning electron microscopy with an energy dispersive scanning system (SEM-EDS). In selected cases, subsequent field emission electron microprobe analyses (FE-EPMA) were performed on a JEOL JXA-8530F Hyperprobe at the Department of Earth Sciences, Uppsala University.
To determine the composition of carbonate separates prior to stable isotope measurements, samples were subjected to powder X-ray diffraction (XRD) analysis at the Swedish Museum of Natural History, Stockholm. The samples were first crushed to a fine powder and placed in a silicon holder. They were then analysed using a PANalytical X'pert PRO automated diffractometer, utilising an acceleration voltage of 45 kV and a beam current of 40 mA. The 2θ angles were measured in the interval 5-70° for 11 minutes, and mineral identification was done off-line using the Highscore Plus software.
Stable isotopes. Stable isotope data were produced at the University of Cape Town, South Africa. Magnetite and quartz separates were analysed following the laser fluorination technique described in Harris & Vogeli 52 . Each sample was reacted in the presence of 10 kPa BrF 5 , after which purified O 2 gas was collected onto a 5 Å molecular sieve within a glass storage bottle. Carbonate separates were analysed using a conventional carbonate line, where purified CO 2 gas was extracted after reacting the samples with 100% phosphoric acid 53 . Oxygen isotope ratios of magnetite, quartz and carbonates, and carbon isotope ratios of carbonates, were measured off-line using a Finnigan Delta XP mass spectrometer in dual-inlet mode. For magnetite and quartz analysed using laser fluorination, the Monastery Garnet standard 54 was used to normalise the raw data and to correct for drift. Carbonates were analysed alongside the Namaqualand Marble standard 55 , and the raw data was corrected based on the calcite:dolomite ratio determined previously for each sample by XRD. The δ 18 O data are reported in per mil (‰) relative to the Standard Mean Ocean Water (SMOW) standard, and the δ 13 C data in per mil relative to the Pee Dee Belemnite (PDB) standard. Both types of analyses gave 2σ errors of 0.2‰. Bergslagen. These deposits are interpreted to have formed in a c. 1.9 Ga shallow-marine back-arc, sub-seafloor setting associated with extensive felsic volcanism and plutonism. High-temperature hydrothermal fluids, enriched in silica, iron and REE among other components, exsolved from a sub-volcanic magma and reacted with nearby interlayers of limestone and carbonate-bearing BIF. This led to skarn formation and magnetite-REE-precipitation within the carbonate units, while extensive hydrothermal alteration affected the surrounding volcanic host rocks. Over the life of the hydrothermal system there was progressive involvement of surface waters. See text for detailed explanation.