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Article

Chemical Composition and Petrogenetic Implications of Eudialyte-Group Mineral in the Peralkaline Lovozero Complex, Kola Peninsula, Russia

by
Lia Kogarko
1,* and
Troels F. D. Nielsen
2
1
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia
2
Geological Survey of Denmark and Greenland, 1350 Copenhagen, Denmark
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(11), 1036; https://doi.org/10.3390/min10111036
Submission received: 23 September 2020 / Revised: 15 November 2020 / Accepted: 16 November 2020 / Published: 20 November 2020
(This article belongs to the Special Issue Study of the Eudialyte Group Minerals)
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Abstract

:
Lovozero complex, the world’s largest layered peralkaline intrusive complex hosts gigantic deposits of Zr-, Hf-, Nb-, LREE-, and HREE-rich Eudialyte Group of Mineral (EGM). The petrographic relations of EGM change with time and advancing crystallization up from Phase II (differentiated complex) to Phase III (eudialyte complex). EGM is anhedral interstitial in all of Phase II which indicates that EGM nucleated late relative to the main rock-forming and liquidus minerals of Phase II. Saturation in remaining bulk melt with components needed for nucleation of EGM was reached after the crystallization about 85 vol. % of the intrusion. Early euhedral and idiomorphic EGM of Phase III crystalized in a large convective volume of melt together with other liquidus minerals and was affected by layering processes and formation of EGM ore. Consequently, a prerequisite for the formation of the ore deposit is saturation of the alkaline bulk magma with EGM. It follows that the potential for EGM ores in Lovozero is restricted to the parts of the complex that hosts cumulus EGM. Phase II with only anhedral and interstitial EGM is not promising for this type of ore. Nor is the neighboring Khibiny complex despite a bulk content of 531 ppm of Zr. Khibiny only has interstitial and anhedral EGM. The evolution of the Lovozero magma is recorded in the compositions EGM up through a stratigraphy of 2400 m in Phase II and III of the complex, and distinct in elements like rare earth elements (REE), Sr, Ba, Th, U, Rb, Mn, Fe. The compositional evolution reflects primarily fractional crystallization processes within the magma chamber itself in combination with convective magma flow and layering by precipitation of minerals with different settling velocities. The suggested mechanism for the formation of the EGM deposits is flotation of very small, suspended EGM crystals in the convective magma and concentration below the roof of the magma chamber. Phase III EGM is enriched in total REE (1.3%) and in HREE (Ce/Yt = 8.8) and constitutes a world class deposit of REE in the million tons of Phase III eudialyte lujavrites.

1. Introduction

High-field strength elements (HFSE) including the rare earth elements (REE) are strategic and critical for high-tech industry and green technologies. Their consumption and economic importance is progressively growing up. REE are for the most part mined in a restricted number of carbonatite-related deposits in which the REE are hosted in a large group of carbonate- and phosphor-bearing minerals including, bastnaesite, parisite, and monazite. A very different resource for HFSE (e.g., Zr, Hf, Nb, Ta, REE) is large-volume alkaline syenitic complexes rich in Eudialyte-Group Mineral (EGM) and other HFSE bearing minerals. The aim of this contribution is to improve our understanding of the formation of HFSE resources in large alkaline complexes in order to define more effective exploration criteria for EGM-rich magmatic systems.
EGMs are complex zirconium silicates and typomorphic [1,2,3,4] for highly evolved agpaitic alkaline complexes. Agpaitic alkaline rocks have a bulk rock molar (Na + K)/Al > 1. Zircon is the typomorphic zirconium mineral in lesser alkaline miaskitic systems with an agpaitic index below 1.
More than 70 agpaitic intrusive complex are known world-wide, including the large Lovozero and Khibina complexes (Kola Peninsula, Russia), Ilimaussaq intrusion (Greenland), Pilanesberg (South Africa), or Poços de Caldas (Brazil). All show extreme enrichment in sodium (Na), volatiles (F, Cl, S), and have HFSE hosted in chemically complex minerals such as EGM, mosandrite-(Ce), keldyshite, lavenite, catapleiite, vlasovite, a. o. The very large tonnage of EGM-rich rocks in, e.g., the Lovozero complex suggests that it is one of the largest resources of Zr, REE, and other HFSE on earth and that it, as well as comparable large-volume alkaline complexes, may hold the HFSE resources for the future.
Studies of Rb-Sr, Sm-Nd, U-TH-Pb, Lu-Hf isotopic systems and of O, H [5,6,7,8,9] suggest that peralkaline magmatic systems are generally closed and inhibit the separation and loss of volatiles and the rare HFSE metals, i.e., REE, Zr, Hf, Nb, Ta, Th, U, Sr, Ba to late developed fluid phases [10]. The highly alkaline systems remain closed to these components leading to an extended interval of crystallization to very low temperatures [11,12,13]. This allows sub-liquidus mineral reactions, re-equilibration of parageneses, and a gradual transition from the magmatic to auto-metasomatic and hydrothermal stages. Residual melt and brines would concentrate ore components, e.g., in pegmatites. But such late accumulations would only form more local and smaller volume deposits, e.g., pegmatites and metasomatic zones. Large tonnage REE and HFSE deposits in alkaline complexes require large volumes of silicate melt to form and to be profitable scavenged at large scale. They would therefore likely form in the magma chamber during the crystallization of the bulk silicate melt.
The Eudialyte complex, hereafter Phase III, in the uppermost part of the Lovozero complex is rich in HFSE with layers composed of up to 95 vol. % EGM [1,14]. They are seemingly formed by classic accumulation of liquidus crystals of EGM and mat formation processes during the crystallization of bulk liquid in the magma chamber [15,16,17]. The EGMs of Lovozero show a wide variation in composition and therefore offer the possibility for the present study of compositional and morphological evolution of EGM and genesis of EGM ores.

2. Geology

The peralkaline Lovozero complex is one of the earth’s largest layered agpaitic nepheline syenite-complexes. It is part of the Devonian Kola ultramafic–alkaline and carbonatitic province (KACP) which includes thousands of cubic kilometers of mafic to peralkaline and carbonatitic magma [5,18] (Figure 1). The Lovozero complex was emplaced centrally in Kola peninsula in Archaean basement and into a northwesterly striking tectonic zone that hosts a preserved E-W-trending belt of Paleozoic lithologies. The complex is rectangular in outline, has an area of 650 km2, and exposes a 2400 m stratigraphic succession of undersaturated felsic plutonic rocks. The complex is depicted as trough-shaped with a feeding channel in the southwestern part of the intrusion [19]. The complex has attracted special interest because of enrichment in loparite-(Ce) and EGM, two rare minerals that are enriched in rare earths elements (REE), niobium, tantalum, zirconium, hafnium, and radioactive raw materials. Syenites, the largest of the KACP complexes, the Khibina alkaline complex, are exposed no more than 5 km west of Lovozero [20]. Khibiny is the host of major apatite-group mineral deposits.
The alkaline rocks of Lovozero divide into three distinct phases. Phase I is preserved in the lowermost parts of the complex and as xenoliths throughout the complex. Phase I is composed of poikilitic, even-grained feldspathoidal syenite. Their bulk composition is miaskitic, i.e., have an agpaitic index (molar (Na2O + K2O)/Al2O3) > 1. The rock-forming minerals are K-Na feldspar, nepheline, nosean, aegirine-diopside, and magnesio-riebeckite with accessory ilmenite, magnetite, titanite, apatite group mineral and mosandrite (Ce).
Phase II, also known as the differentiated complex, is formed by repeated three-layer successions composed of urtite, followed by foyaite and lujavrite. Urtite is a syenite with a nepheline to pyroxene ratio > 0.7, foyaite is a nepheline-rich syenite, and lujavrite is a green peralkaline syenite rich in alkali pyroxene and feldspathoids. The transitions between the three rock types are mostly gradational. Sharp contacts only occur between the urtite and underlying lujavrite of the previous three-layers succession. The layered units of Phase II range in thickness from a few centimeters to hundreds of meter and sum to a total of app. 2000 m. The bulk compositions of Phase II nepheline syenites have an agpaitic index > 1 and are more alkaline than rocks of Phase I. The rock-forming minerals are nepheline, microcline, sodalite, aegirine, and arfvedsonite with accessory phases typomorphic for peralkaline syenites including EGM, lamprophyllite, lomonosovite, murmanite, apatite-group mineral, loparite-(Ce), villiaumite, titanite, sodalite, and lorenzenite (Table 1). Cumulus loparite-(Ce) locally appear concentrated into stratigraphically well constrained thin layers.
The nepheline syenites of Phase III comprise 15–18% of the volume of the Lovozero complex and forms the upper sheet-like part of the Lovozero. The complex contains more than 120 pegmatites. The contact to the Phase II is conformable and sharp with dips shallowing toward the center of the complex. The eudialyte lujarites are exposed in the summits of the Lovozero Massif, have a maximum thickness of 450 m, of which an increasing part is lost to erosion toward the southeast. Phase III is composed of repeated units of urtite, juvite, eudialyte foyaite, and leucocratic, mesocratic, and melanocratic eudialyte lujavrites [1]. Juvite is a strongly nepheline- dominated rock with ~20 modal % alkali feldspar.
A coarse layering is developed in the eudialyte lujavrites. Their bulk rock compositions are very alkaline with an agpaitic index of ~1.50 and they are rich in high field strength elements (HFSE) such as zirconium that reaches a bulk rock maximum of 1.66 wt. % ZrO2 [8]. Bodies of porphyritic lujavrite are located at the contact to the underlying Phase II syenites. They are believed to be partly quenched melts or mushes of eudialyte lujavrite magma. The eudialyte lujavrites host a suite of later intrusive lithologies including veins of lovozerite and murmanite and lamprophyllite porphyritic lujavrite, (about 2 vol. % of Phase III). They are interpreted as residual melts of Phase III are up to several kilometers in length and up to 50 m wide. Other late intrusive bodies are composed of poikilitic sodalite syenite and tawite (sodalite dominated rock). Titanite, apatite-group mineral, and amphibole-rich rocks in the eastern part of the complex are formed in a reaction between the Phase III magma and host rocks in the roof of the Lovozero complex [1].
The main rock-forming minerals of eudialyte-lujavrites are nepheline (23%), aegirine (17%), alkali-amphibole (15%), microcline (20%), and EGM (about 25%). EGM occurs as euhedral crystals and grain cores, a distinctive characteristic that sets EGM of Phase III apart from the EGMs of Phase II. The common accessory minerals include lamprophyllite, lomonosovite, murmanite, loparite-(Ce), lovozerite, pyrochlore supergroup minerals, and sodalite. The modal proportion of EGM crystals increases significantly to 30–40 vol. % in upper parts of Phase III. These EGM-rich rocks of ore grade are found in the apical part of the eudialyte complex as layers and meter-size lenticular bodies. Horizons defined as eudialyte ore are almost monomineralic with up 95 vol. % EGM with minor nepheline, aegirine, microcline, and arfvedsonite. EGM ores are a significant resource for especially the heavy rare earths elements (HREE), zirconium, and hafnium. In addition, the ores are also enriched in manganese, niobium, scandium, uranium, and thorium.

3. Samples and Analytical Method

Most of the samples in this study were collected from seven drill cores (numbers 469, 904, 521, 178, 144, 272B and 905) that cover a complete stratigraphic section through the Lovozero complex (~2400 m) except for the middle part of Phase II. Hand samples completed the vertical section of Lovozero.
The EGM analyses were made using Cameca SX50 and CAMECASX 100 electron microprobes with four wavelength-dispersive spectrometers at the Natural History Museum, London, and at Vernadsky Institute, Moscow. Operating conditions were in both cases an accelerating voltage of 15 kV and a 20 nA probe current. A combination of natural minerals, synthetic compounds, and pure metals, which included synthetic NaNbO3, SrTiO3, CaTiO3, ZrSiO4 and individual REE-doped glasses for the major components were used as standards.
Trace element analyses were performed in Frankfurt University, Max Plank Institute in Mainz, Germany, and Vernadsky Institute, using Thermo Scientific Element 2 ICPMS instruments coupled with a Resonetics Resolution M-50 excimer laser. The laser spot size varied from 20 to 60 µm. NIST glasses and Zircon 91500 were used as standards. To minimize the influence of later rimming and equilibrations of first crystallized EGM, only the cores of EGM grains and crystals were analyzed. The full data set includes analyses from 650 grains in 213 samples.

4. Results

4.1. Petrographic Characteriatics of EGM

Detailed accounts of the petrography of the lithologies of the Lovozero complex can be found in Vlasov et al. [14], Gerasimovsky et al. [1], and Bussen and Sakharov [22]. The rock-forming paragenesis remains the same throughout Lovozero, and includes nepheline, potassium feldspar, aegirine, and alkali-amphibole. Only the relative proportion and the compositions of the rock-forming minerals vary. The present work focusses on the petrographic relations and compositions of EGM.
Eudialyte is not present in Phase I lithologies, which therefore will not be described further. In the lower miaskitic part of Phase II the accessory phases include zircon, manganese-rich ilmenite, mosandrite-(Ce), keldyshite, lavenite, titanite, apatite group minerals, and no EGM. Zircon is the mineral that hosts the Zr. EGM is found higher in Phase II in association with agpaitic minerals such as lorenzenite, lamprophyllite, villiaumite, murmanite, and lomonosovite. In these rocks, EGM occurs as interstitial and anhedral grains. The size of EGM grains varies from tens to 1–2 mm. Individual EGM grains display very complex zoning that shows well in back-scattered electron images (Figure 2).
EGM occurs as euhedral crystals throughout the overlying Phase III eudialyte lujavrites. The crystals are 2–4 mm in size, increasing to 5 mm in eudialyte “ore” (Figure 3 and Figure 4). The EGM crystals of Phase III show sector and oscillatory zoning [1]. In the upper part of Phase III the EGM grains of a given sample have a very uniform size and some contain fragments of small, broken crystals (Figure 5). They are especially common in the part of eudialyte ores in the uppermost part of the complex.

4.2. Compositions of EGM

EGM is a complex mineralogical group with many compositional endmembers [23]. Lovozero EGM is dominated by “common” eudialyte, with proportions of raslakite (Na15Ca3Fe2+3(Na,Zr)3Zr3(Si,Nb)(Si25O73)(OH,H2O)3(Cl,OH)), taseqite (Na12Sr3Ca6Fe3Zr3Nb(Si25O73)Cl2(O,OH,H2O)3), and sergevanite Na15(Ca3Mn3)(Na2Fe)Zr3(Si26O72)(OH)3·H2O) [24].
Calculated average concentrations of Sr, Ba, Rb, REE (La, Ce, Yb), Th, U, Mn, Fe in EGMs of Phase II and Phase III rocks are shown in Table 2, Table 3, Table 4 and Table 6, and chondrite normalized REE spectra, ∑LREE versus ∑HREE concentrations, and spidergrams normalized to primitive mantle are shown in Figure 6, Figure 7 and Figure 8.
The EGM of the Phase II is REE enriched and reaches ~0.8 wt. % and has from c. 600 to 1700 ppm La and from c. 100 to 180 ppm Yb corresponding to chondrite normalized ratios of ~2500 to ~7000 c. and ~300 to ~1100, respectively (Figure 6). In ∑LREE versus ∑HREE (Figure 7) the analyses of Phase II EGM are divided into two well-defined groups with a small number of compositions in between. EGMs of individual samples of Phase II appear to have very similar ratios, though with variations in ∑REE. Compared to the Phase II the EGMs of the Phase III have elevated ∑REE contents reaching 1.8 wt. %. and have from c. 1100 to 3400 ppm La and from 200 to ~3000 ppm Yb corresponding to chondrite-normalized ratios of ~5000 to ~15000 and ~800 to ~1100, respectively (Figure 6). In ∑LREE versus ∑HREE (Figure 7) Phase III EGMs, except for a small number of outliers, form a suite with a steep trend similar to that of the HREE-rich Phase II EGMs. Average EGM of Phase III has Ce/Yb = 8.88 and 15.23 in EGM of the Phase II (Table 6).
The EGM composition shows continuous upwards decrease in Fe and a complementary increase in Mn from Phase II to the Phase III. The Mn/Fe ratios (apfu) increase upward from 0.76 in the Phase II to 0.87 in EGM of Phase III (Figure 9, Table 2 and Table 4, average values in Table 5 and Table 6). Shilling et al. [25] considered Mn/Fe ratio to be the most suitable fractionation indicator. The difference in Mn/Fe ratio in EGMs of Phase II and Phase III is insignificant. Consequently, the values of the Mn and Fe distribution coefficients of surrounding minerals mainly affected this ratio. Trace element compositions normalized to primitive mantle (Figure 8) show very similar patterns for all EGM though with overall higher concentrations in the EGMs of Phase III. Nb, Ti, and Hf increase from 1872, 1237, 963 to 3242, 1455, 2076, respectively. U and Th are enriched in EGM of the eudialyte complex. The spidergrams show expected peaks at Zr and Nb, and low peaks at Sr, Pb, Ba and Rb. In contrast to most other elements Pb, U, Th, Ba, Rb, Cs show up to 10 variation in normalized values.

5. Discussion

5.1. Arrival of EGM on Liquidus

The timing of crystallization of EGM relative to that of the liquidus paragenesis of its host, and the compositional variations of the EGMs provide constrains to the genesis of large deposits of EGM with high concentrations of valuable HREE. In Phase II of Lovozero, EGM is an interstitial and poikilitic phase. It has anhedral grain shapes and shows significant and non-systematic chemical zonation (Figure 2). This type of EGM is typical for all parts of Phase II, with modal proportion of EGM, lamprophyllite, murmanite, and lomonosovite increasing upwards toward Phase III. The petrographic relations suggest that the anhedral EGM grains crystallized from late stage melts in Phase II crystal mushes, and that local geochemical variation, coexisting with HFSE-bearing accessory phases, and diffusion of HFSE account for the compositional variability in, e.g., ∑LREE/∑HREE (Figure 7).
The petrographic relations are very different for EGM in Phase III and the eudialyte deposits. EGM grains are euhedral (Figure 3 and Figure 4), and some are composed of amalgamated fragments of EGM crystals (Figure 5). The EGM crystals are understood as liquidus crystals that crystallized in equilibrium with the lujavritic liquidus paragenesis, that includes nepheline, aegirine, and alkali-amphibole. Support for EGM as liquidus phase in melt of Phase III and eudialyte ores is provided by the porphyritic lujavrites. In these partly quenched rocks very small euhedral EGM crystals are enclosed other liquidus phases (Figure 10).
Although the ∑LREE/∑HREE ratios and concentrations vary and outliers occur, the ratios of Phase III EGMs are in the 2–3 range and suggest a general equilibration with a single and large volume of melt. That is a characteristic of liquidus phases, and the main control on the formation of EGM ores in Lovozero is therefore, suggested to be saturation of agpaitic magma in components necessary for nucleation of, liquidus eudialyte i.e., in the melt with 1–2% ZrO2. Such concentrations are found in quenched and homogenized melt inclusions in the minerals of EGM ores and experiments [13].

5.2. Genesis of the Lovozero EGM Ore

The timing of arrival of EGM on the liquidus in the Lovozero complex is here estimated from the relative volumes of the phases of the complex, and assuming a continuum of fractionation and melt evolution. Phase III comprises the last 15 vol. % that crystallized in Lovozero, and the bulk magma apparently saturated with EGM after ~85% of the initial volume of Lovozero had solidified. Following Gerasimovsky et al. [1] the initial ZrO2 content of the bulk liquid Lovozero complex was 0.35 wt. %. With crystallization and evolution of Lovozero, the ZrO2 content is modelled to have reached 1.66 wt. %. in Phase III [7]. This corroborates the experimentally estimated concentration of 1.4–2 wt. % ZrO2 needed to bring EGM on the liquidus of alkaline agpaitic magma [13], and thus a classic fractional crystallization scenario.
Crystals of EGM form mats in the uppermost parts of in Phase III. The density of eudialyte is in the order of 2.8. The agpaitic magma of Ilimaussaq intrusion is and a proxy for that of Lovozero has a modelled density of ~2.3. It does not seem reasonable to assume the EGM mats to be simple floatation cumulates. The small euhedral crystals with sizes in the hundredths of a millimeter, in particular in the porphyritic lujavrites, would be in suspension and be carried along in convective magma. The here preferred genesis is therefore floatation of tiny crystals of EGM to the top of Phase III melt volume, where they are envisaged to have stuck together and re-crystallize to larger crystals in a type of ripening process (Figure 4). A similar mechanism for the accumulation and concentration of heavier apatite-group mineral and lighter nepheline was envisaged in the model proposed for the Genesis of Apatite ores in the Khibiny complex [26].
The EGMs of Phase III lujavrites and eudialyte ores have elevated HREE contents compared to EGMs of Phase II. The anhedral Phase II EGMs are confined to interstitial volumes of melt which evolved relative to the contemporaneous bulk liquid. Interstitial EGM would start to crystallize in mushes of Phase II, before the arrival of EGM on the liquidus of the bulk liquid of Phase III. The EGMs of Phase II would be trapped in small volumes of melt and not be able to scavenge HFSE elements from a larger bulk magma. They would have compositions controlled by the bulk composition of the interstitial melts, and as suggested in Figure 7, the paragenesis of coexisting HFSE-bearing phases.
On the contrary we suggest that the euhedral and idiomorphic liquidus crystals of EGM of Phase III crystalized in a large convective volume of melt in equilibrium with the rock-forming liquidus paragenesis and layering in Phase III lujavrites. They also formed after crystallization of loparite-(Ce) and formation of the loparite ores. Loparite-(Ce) is very strongly enriched in LREE and the effect would be a relative enrichment in HREE over LREE in melts of Phase III.
Consequently, a required condition for the formation of a EGM deposit with high concentration of HREE is the saturation of alkaline magmas with respect to EGM at a stage in the magmatic evolution when the ∑LREE/∑HREE ratio was already lowered because of the fractionation of ∑LREE-rich phases, e.g., loparite-(Ce). In addition, equilibration and scavenging from a large volume of melt would favor higher concentrations of REE in EGM. If the concentration of the elements needed for saturation are significantly lower at the liquidus of the melt, then the crystallization of the ore mineral would occur at sub-liquidus conditions in small and possible partly isolated volumes of interstitial melt. Such a scenario would not facilitate the formation and accumulation of the EGM. It seems evident that the potential for viable EGM ores in the large Lovozero complex, is restricted to Phase III in which EGM crystallized as the liquidus phase in the lujavritic magma. From this follows that anhedral and interstitial EGM of the neighboring Khibiny complex EGM is unlikely to form eudialyte ore deposits, despite a bulk concentration of 531 ppm Zr [27].

5.3. Consequences for Layered Agpaitic Complexes

The evolutionary trend recorded in the EGM compositions is consistent with a continuous and closed system evolution of Lovozero intrusion and feeder system exemplified, e.g., in parageneses and clinopyroxene compositions [7]. The same is suggested for kakortokites of the Ilimaussaq intrusion based on the stratigraphic variation in EGM compositions and core to rim fractionation during in situ fractionation in crystal mushes [15].
Much information has, however, emerged over the past decade and two very different models are advocated for the development of igneous layering in Ilimaussaq, including: (1) A classic closed system evolution with layering caused by gravitational accumulation, and e.g., crystal mat formation [16,26,28,29,30]; and (2) a crystallization model based on repeated replenishment of the magma chamber [31]. The latter model, however, require that the magmas or mushes evolved along evolutionary trends similar to those observed in Ilimaussaq. Such models relocate observed evolution to a not known or specified feeder chamber. The need for such an unspecified feeder chambers for the Ilimaussaq intrusion rests on petrofabric (CDS) studies, sometimes sharp contacts between repeated cyclic layers, and structural characteristics interpreted as the result of turbulent accumulation of new pulses of inclusion-bearing agpaitic mush in the kakortokites [31]. It is also argued that the classic gravitational settling (or flotation) mechanisms cannot explain the petrofabrics and the repetition of 29 tripartite cycles of black, red, and white kakortokites.
These models, however, appear to exclude liquidus crystallization under the roof and subsequent gravitational accumulation of crystal mushes in the floor of the magma chamber. Magmas cool through roof and walls and are better insulated at floor by accumulated crystal mush. Cooling would in most cases be most efficient at the roof and cause crystallization and mush formation. The mushes, that would be variable affected by interaction with rafts of roof rocks, sink to the floor, and simulate being new magma pulses, although being the result of processes within the magma chamber itself. Such processes are well described from e.g., the Skaergaard intrusion [32]. At present, there is no indisputable evidence to suggest that the kakortokites of the Ilimaussaq intrusion should be anything but the result of crystallization of magma and density-controlled distribution of solids, crystal mushes, and melts within a magma chamber. The same may apply to the Lovozero complex and following Gerasimovsky et al. [1] and Kogarko et al. [17] there is no compelling evidence to suggest that the Lovozero complex represents rocks crystallized from multitude of replenishment events.

6. Conclusions

The conclusions of the study are:
  • Eudialyte (EGM) of the Lovozero complex is enriched in HREE. EGM of the ore-bearing Phase III of the complex reaches an average of 1.3 wt. % total REEs (max 1.8), and a Ce/Yb = 8.88. In addition, EGM-ores are strongly enriched with Zr and many other HFSE elements commodities.
  • The petrography shows that the time of crystallization of EGM relative to liquidus paragenesis changes upward and as the Lovozero complex crystallizes. The interstitial, anhedral EGM crystals in all of Phase II indicate that the bulk magma was not yet saturated in components needed for nucleation of EGM. Only after crystallization, about 85% of the volume of the initial magma was saturated and nucleation of EGM was reached and EGM changed the role to become a cumulus phase.
  • Saturation of the bulk magma that leads to EGM nucleation is a prerequisite for eudialyte ores. The ores formed as a result of the suspension and upward transportation of very small crystals and subsequent amalgamation growth below the roof of the magma chamber. The process compares to that suggested for the Khibina apatite-group mineral deposits [26].
  • EGM of the neighboring Khibiny complex is anhedral, interstitial, and unlikely to form eudialyte ore deposits, despite a bulk concentration of 531 ppm Zr.

Author Contributions

All scientific conceptualization, analytical work, and synthesis of the results have been carried out by the first author (L.K.), who also prepared an earlier version of the manuscript; T.F.D.N. revised the structure and wording of the manuscript, added resource perspective introduction, added petrographic definitions in description of Lovozero complex, added LREE/HREE calculation and illustration (Figure 7), added comparisons and general perspectives to the Discussion. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Federation represented by the Ministry of education and science of Russia № 075-15-2020-802.

Acknowledgments

We are grateful to Glasatova I.A. from GEOKHI RAS for her help in work with the database on the obtained eudialyte analyses.

Conflicts of Interest

The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Minerals 10 01036 g001 550
Figure 1. Geological map of the Lovozero intrusion with inset showing location of Lovozero (star) within the Kola Peninsula. Modified from Gerasimovsky et al. [1].
Figure 1. Geological map of the Lovozero intrusion with inset showing location of Lovozero (star) within the Kola Peninsula. Modified from Gerasimovsky et al. [1].
Minerals 10 01036 g001
Minerals 10 01036 g002 550
Figure 2. Backscatter image of xenomorphic and interstitial Eudialyte Group of Mineral (EGM) (grey) in foyaite of the Phase II. All other phases including pyroxene, alkali feldspar, and nepheline are black. Field of view 1.5 mm.
Figure 2. Backscatter image of xenomorphic and interstitial Eudialyte Group of Mineral (EGM) (grey) in foyaite of the Phase II. All other phases including pyroxene, alkali feldspar, and nepheline are black. Field of view 1.5 mm.
Minerals 10 01036 g002
Minerals 10 01036 g003 550
Figure 3. Eudialyte lujavrite, with idiomorphic crystals of EGM. Transmitted light, field of view 30 mm.
Figure 3. Eudialyte lujavrite, with idiomorphic crystals of EGM. Transmitted light, field of view 30 mm.
Minerals 10 01036 g003
Minerals 10 01036 g004 550
Figure 4. EGM ore composed of idiomorphic crystal, from the uppermost part of the Phase III. Transmitted light, field of view 25 mm.
Figure 4. EGM ore composed of idiomorphic crystal, from the uppermost part of the Phase III. Transmitted light, field of view 25 mm.
Minerals 10 01036 g004
Minerals 10 01036 g005 550
Figure 5. Backscatter electron image of an EGM crystal from ore containing composed of fragments of EGM crystals (Field of view 1 mm).
Figure 5. Backscatter electron image of an EGM crystal from ore containing composed of fragments of EGM crystals (Field of view 1 mm).
Minerals 10 01036 g005
Minerals 10 01036 g006 550
Figure 6. Distribution of rare earth elements (REE) in Lovozero EGM of Phase II (black) and of Phase III (red).
Figure 6. Distribution of rare earth elements (REE) in Lovozero EGM of Phase II (black) and of Phase III (red).
Minerals 10 01036 g006
Minerals 10 01036 g007 550
Figure 7. ∑LREE versus ∑HREE in EGM. Black dots: Phase II; red dots: Phase III.
Figure 7. ∑LREE versus ∑HREE in EGM. Black dots: Phase II; red dots: Phase III.
Minerals 10 01036 g007
Minerals 10 01036 g008 550
Figure 8. Spidergram for EGM of differentiated (black) and eudialyte (red) complexes of Lovozero.
Figure 8. Spidergram for EGM of differentiated (black) and eudialyte (red) complexes of Lovozero.
Minerals 10 01036 g008
Minerals 10 01036 g009 550
Figure 9. Variation of Fe and Mn in differentiated (black) and eudialyte (red) complexes.
Figure 9. Variation of Fe and Mn in differentiated (black) and eudialyte (red) complexes.
Minerals 10 01036 g009
Minerals 10 01036 g010 550
Figure 10. Backscattered electron image of a porphyritic EGM (white) lujavrite (polished thin section).
Figure 10. Backscattered electron image of a porphyritic EGM (white) lujavrite (polished thin section).
Minerals 10 01036 g010
Table
Table 1. Mineral formulae for the tipomorphic minerals of Lovozero.
Table 1. Mineral formulae for the tipomorphic minerals of Lovozero.
NameCNMMN/CNMNC Approved Formula
Barytolamprophyllite(BaK)Ti2Na3Ti(Si2O7)2O2(OH)2
Belovite-(Ce)NaCeSr3(PO4)3F
Cerite-(Ce)(Ce,La,Ca)9(Mg,Fe3+)(SiO4)3(SiO3OH)4(OH)3
LomonosoviteNa6Na2Ti2Na2Ti2(Si2O7)2(PO4)2O4
Loparite-(Ce)(Na,Ce,Sr)(Ce,Th)(Ti,Nb)2O6
LorenzeniteNa2Ti2O3(Si2O6)
Mosandrite-(Ce)(Ca3REE)[(H2O)2Ca0.5☐0.5]Ti(Si2O7)2(OH)2(H2O)2
MurmaniteNa2Ti2Na2Ti2(Si2O7)2O4(H2O)4
Nenadkevichite(Na, ☐)8Nb4(Si4O12)2(O,OH)4·8H2O
Nordite-(Ce)Na3SrCeZnSi6O17
Steenstrupine-(Ce)Na14Ce6Mn2+2Fe3+2Zr(PO4)7Si12O36(OH)2·3H2O
Vitusite-(Ce)Na3Ce(PO4)2
VuonnemiteNa6Na2Nb2Na3Ti(Si2O7)2(PO4)2O2(OF)
International Mineralogical Association (2020) [21].
Table
Table 2. Representative electron microprobe (EMP) compositions of EGM from Phase II.
Table 2. Representative electron microprobe (EMP) compositions of EGM from Phase II.
Sample382383385389390392393399401402
Nb2O51.091.140.721.770.791.060.670.480.480.88
TiO20.330.370.621.330.410.380.410.480.480.48
ZrO212.3312.8115.909.9013.6212.8114.4114.5513.6513.59
HfO20.200.230.350.140.280.320.340.200.250.28
ThO20.000.000.000.000.000.000.000.100.100.10
UO20.000.000.000.000.000.000.000.160.100.10
SiO249.4049.9948.4449.6248.2748.8348.7249.3548.9848.54
Al2O30.000.000.000.000.000.000.000.200.220.16
Y2O30.210.250.520.220.560.630.570.690.680.35
La2O30.580.500.290.500.390.440.370.190.240.37
Ce2O31.261.240.631.230.810.930.780.660.630.74
Pr2O30.280.170.150.150.150.150.150.150.150.15
Nd2O30.540.520.290.470.400.480.470.390.260.35
CaO7.968.144.768.817.507.486.706.387.077.14
MgO0.000.000.000.000.000.000.000.050.050.05
MnO3.092.922.423.031.822.271.702.932.763.27
FeO1.681.573.090.804.344.024.751.220.951.10
SrO1.861.660.983.020.921.120.771.361.561.93
BaO0.520.350.070.690.040.140.050.290.280.24
Na2O16.7116.7016.9714.3915.9616.0515.8415.7014.5915.67
SO30.540.370.270.430.150.160.220.180.200.24
Cl0.540.501.120.501.251.361.241.141.150.76
Total99.1299.4397.5997.0097.6698.6398.1696.8594.8396.49
O=Cl0.120.110.250.110.280.310.280.260.260.17
TOTAL99.0099.3297.3496.8997.3898.3297.8896.5994.5796.32
A.P.F.U.
Nb0.260.270.170.410.190.250.160.110.110.21
Ti0.130.140.250.520.160.150.160.190.190.19
Zr3.133.224.132.493.553.293.733.723.523.52
Hf0.030.030.050.020.040.050.050.030.040.04
Th0.000.000.000.000.000.000.000.010.010.01
U0.000.000.000.000.000.000.000.020.010.01
Si25.7425.7325.8325.5925.8125.7525.8425.8925.8925.79
Al0.000.000.000.000.000.000.000.120.140.10
Y0.060.070.150.060.160.180.160.190.190.10
La0.110.090.060.100.080.090.070.040.050.07
Ce0.240.230.120.230.160.180.150.130.120.14
Pr0.050.030.030.030.030.030.030.030.030.03
Nd0.100.100.060.090.080.090.090.070.050.07
Ca4.444.492.724.874.304.233.813.594.004.06
Mg0.000.000.000.000.000.000.000.040.040.04
Mn1.361.271.091.320.821.010.761.301.241.47
Fe0.730.681.380.351.941.772.110.540.420.49
Sr0.560.500.300.900.290.340.240.410.480.59
Ba0.110.070.010.140.010.030.010.060.060.05
Na16.8816.6717.5414.3916.5516.4116.2915.9714.9516.14
S0.210.140.110.170.060.060.090.070.080.10
Cl0.480.441.010.441.131.221.111.011.030.68
The basis for APFU calculations Si + Nb = 26.
Table
Table 3. Representative ICPMS trace element analyses of EGM from Phase II.
Table 3. Representative ICPMS trace element analyses of EGM from Phase II.
Element (ppm)01-101-201-302-102-202-31610-031610-04463-041610-06
Sc10210410811712012011912179111
Ti1331137412571267133312761340141912071318
Mn8518870485817632866281448306865776097894
Co0.160.170.150.20.230.180.180.2–0.2
Cu0.851.081.061.211.141.081.271.130.751.24
Rb6.747.617.517.828.557.918.528.932.857.52
Sr2880287228412773281527382837293732732632
Y1487147114831453147714041307141410451324
Nb2600246921682112217721192128198813272090
Ba222021151615251745915
La1703162715361462151214501438142911781435
Ce3049294327872634278227052543261223242602
Pr349337315304315322303306273304
Nd1455143713111312131513221292132111451256
Sm364347336322331328316336283308
Eu1041039592969894957888
Gd309302274267297288270268249259
Tb52505046474545433743
Dy270262269253256244241243204238
Ho55505248525045473946
Er14513013412413013211612195116
Tm20191918191917181316
Yb13112011811611711210911683108
Lu15151414141413141113
∑REE8021774273107012728371286842696960106832
Ce/Yb23252423242423222824
Hf107510671039105810461069111211237891126
Ta287270181184187193217222127188
Pb3131262326243127525
Th20211819201919172818
U25242223222122223922
Table
Table 4. Representative EMP compositions of EGM from Phase III.
Table 4. Representative EMP compositions of EGM from Phase III.
Sample7781532901373304779336335
Nb2O50.760.670.750.990.510.770.710.730.61
TiO20.630.530.60.580.650.480.520.490.67
ZrO214.2813.4213.5313.2613.8113.0313.3513.3714.22
HfO20.000.270.290.410.000.270.260.220.00
SiO249.4950.0949.9548.8450.1949.3949.8150.2750.04
Al2O30.200.000.000.160.230.140.150.110.16
Y2O30.690.70.630.70.560.570.850.330.34
La2O30.360.190.340.190.280.180.330.150.27
Ce2O30.710.630.570.60.510.580.660.380.66
Pr2O30.000.150.150.150.000.000.000.000.00
Nd2O30.430.310.370.370.40.320.350.410.34
Sm2O30.230.000.000.000.000.000.120.170.00
Gd2O30.110.000.000.000.260.000.000.260.32
CaO8.287.247.38.027.977.688.458.657.93
MgO0.040.000.000.070.100.040.010.050.05
MnO1.862.31.992.131.973.201.912.052.02
FeO3.423.153.893.134.924.343.245.274.76
SrO0.831.491.352.190.510.630.850.520.69
BaO0.110.100.110.010.110.000.100.180.09
Na2O16.2416.9816.1816.2415.4115.8216.1415.2214.92
K2O0.280.30.380.250.290.210.280.220.29
SO30.260.180.260.180.280.180.200.250.30
Cl1.431.401.401.581.311.321.441.421.39
TOTAL100.64100.1100.04100.05100.2799.1599.73100.72100.07
O=Cl0.320.320.320.360.300.300.320.320.31
TOTAL100.3299.7899.7299.6999.9798.8599.41100.4099.76
A.P.F.U.
Nb0.180.160.180.240.120.180.170.170.14
Ti0.250.210.230.230.250.190.200.190.26
Zr3.633.383.413.413.473.323.383.353.58
Hf0.000.040.040.060.000.040.040.030.00
Si25.8225.8425.8225.7625.8825.8225.8325.8325.86
Al0.120.000.000.100.140.090.090.070.10
Y0.190.190.170.200.150.160.230.090.09
La0.070.040.060.040.050.030.060.030.05
Ce0.140.120.110.120.100.110.130.070.12
Pr0.000.030.030.030.000.000.000.000.00
Nd0.080.060.070.070.070.060.070.070.06
Sm0.040.000.000.000.000.000.020.030.00
Gd0.020.000.000.000.040.000.000.040.05
Ca4.634.004.044.534.404.304.704.764.39
Mg0.030.000.000.060.080.030.010.040.04
Mn0.821.010.870.950.861.420.840.890.88
Fe1.491.221.511.241.911.711.412.261.85
Sr0.250.450.400.670.150.190.250.150.21
Ba0.020.020.020.000.020.000.020.040.02
Na16.4316.9916.2216.6115.4116.0416.2315.1714.95
K0.190.200.250.170.190.140.190.140.19
S0.100.070.100.070.110.070.080.100.12
Cl1.261.221.231.411.151.171.271.231.22
The basis for APFU calculations Si + Nb = 26.
Table
Table 5. Representative ICPMS trace element analyses of EGM from the Phase III.
Table 5. Representative ICPMS trace element analyses of EGM from the Phase III.
Element (ppm)1a1b5b9a8b2a3a3b3c4a
Sc75.2163.0669.1863.0853.9552.5647.548.1153.6547.76
Ti1302131312571081145911581104110612371177
Mn11,50711,83212,31513,01118,15919,26315,48815,57516,52520,349
Co81.817.694.86102.75––––––
Cu16.223.1919.3521.19––––––
Rb8.6815.338.785.667.817.465.396.447.227.1
Sr700476326185711810,43412,50111,67311,97112,4439446
Y2924338421433288485139993916392741912867
Nb2202220621772585351137153703387440513199
Ba197216523488677392639534600980
La1188150511071373214417512079182020311687
Ce2548320523202703438335383939360739463389
Pr367422306340564472497470508436
Nd1973199114601591270523662313232024602071
Sm653617443502872778744764815639
Eu217199138161291269257269278215
Gd718607387513913802769799833616
Tb13711569100184153150154163112
Dy6667314427031 099905895895959635
Ho14414786149236187187187199129
Er395395226413665506517507542339
Tm57563159977274727647
Yb382365196388648472487467501308
Lu49462449866062606439
∑REE949410,3997235904514,88512,33112,97112,39113,37610,661
Ce/Yb6.668.7711.836.976.767.58.087.737.8811.02
Hf1809155712291291242118541809187020721785
Ta198182145194303371387429423351
Pb9.212.9810.335.3911.5314.658.6510.910.2418.96
Th34.1331.6529.1119.4741.0636.8841.5737.6145.4240.66
U31.528.2830.6313.3730.9722.718.9623.3626.6426.33
Li5.845.314.67.54–3.03––––
Be0.781.213.481.272.698.9711.434.438.354.94
B87.43–––25.4138.4934.140.737.4646.89
V1.851.552.11.732.81.711.852.042.092.43
Ni4.3–4.695.78–––2.35––
Zn11.3822.8818.918.223.4154.7422.3521.7224.5158.37
Ga41.2415.3852.8644.4812.8612.6818.0414.6316.5512.22
Cs1.491.671.631.121.521.490.921.061.584.14
Table
Table 6. Average compositions of EGM in Phase II and Phase III.
Table 6. Average compositions of EGM in Phase II and Phase III.
Phase IIPhase III Phase IIPhase III
ElementppmppmElementppmppm
Sc85.9657.26Er156483
Ti12371455Tm2268.95
Co4.1874.25Yb143454
Cu1.5914.99Lu17.1158.64
Rb5.510.61∑REE603813,013
Sr37629828Ce/Yb15.238.88
Y15053811Hf9632076
Nb18723242Ta179293
Ba249891Pb17.7120.99
La11661980Th19.450.91
Ce21814032U23.6447.63
Pr265517Li–5.45
Nd11612475Be–5.61
Sm317762B–34.93
Eu97.51249V–2.63
Gd292760Ni–3.96
Tb51.04145Zn–69.38
Dy299851Ga–21.97
Ho58.61177Cs–2.33
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Kogarko, L.; Nielsen, T.F.D. Chemical Composition and Petrogenetic Implications of Eudialyte-Group Mineral in the Peralkaline Lovozero Complex, Kola Peninsula, Russia. Minerals 2020, 10, 1036. https://doi.org/10.3390/min10111036

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Kogarko L, Nielsen TFD. Chemical Composition and Petrogenetic Implications of Eudialyte-Group Mineral in the Peralkaline Lovozero Complex, Kola Peninsula, Russia. Minerals. 2020; 10(11):1036. https://doi.org/10.3390/min10111036

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Kogarko, Lia, and Troels F. D. Nielsen. 2020. "Chemical Composition and Petrogenetic Implications of Eudialyte-Group Mineral in the Peralkaline Lovozero Complex, Kola Peninsula, Russia" Minerals 10, no. 11: 1036. https://doi.org/10.3390/min10111036

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