Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola)

: The Angolan alkaline–carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected. rhabdophane-(Ce) Ce-La

The Monte Verde alkaline-carbonatite complex, located between Huambo and Kwanza Sul provinces, is one of the least studied carbonatite complexes in Angola. As a few studies have reported, Figure 1. Geological map of Monte Verde complex obtained by joining our data with those of [23]. Path and roads are marked as dotted lines. The Angolan town of Cambambe is located on the left side of the map.

Materials and Methods
Sampling and mapping of the Monte Verde complex were done in a 2014 field campaign. A total of 95 samples of plutonic alkaline rocks, alkaline dykes, breccia, and fenitized granites were selected and collected. About 76 polished thin sections were prepared and studied in order to identify mineral phases and textural features using transmitted and reflected optical microscopy with plane polarized light. The most representative regions were selected for further analysis by SEM-EDS.
A total of 21 selected samples were studied in detail with SEM-BSE-EDS. Qualitative analyses and detailed characterization of mineral textures were done using a Quanta ESEM Quanta Q-200 FEI XTE 325/D8395 Scanning Electron Microscope (Thermo Fisher Scientific, Waltham, MA, USA) coupled with an INCA Energy 250 EDS microanalysis system, located at the Scientific and Technological Centres of the University of Barcelona. Operating conditions were 20 keV, 1 nA beam current, and 10 mm working distance.
Seven samples were selected in order to quantify Nb and REE bearing phases using a JEOL JXA-8230 electron microprobe (JEOL USA, Peabody, MA, USA) with five wavelength dispersive spectrometers (JEOL USA, Peabody, MA, USA), also located at the Scientific and Technological Centres of the University of Barcelona. Operating conditions used were the following: pyrochlore, 20 kV and 14.4 nA; Na-Ti-Zr-REE silicates, 20 kV and 14.7 nA; perovskite and loparite, 20 kV and 14.9 nA; REE carbonates and phosphates, 20 kV and 6.5 nA. For all elements, the counting time was 10 s peak and 10 s background except for Ce, La, Ba (20 s peak and 20 s background), and Na and F (30 s peak and 30 s background). The acquisition method used was two background measurements at each side of the peak. The beam diameter was 5 µm. Standards and lines used are listed in Table 1. A total of 681 analyses were obtained.

Monte Verde Carbonatite Structure
The Monte Verde complex displays a concentric semi-circular structure defined by alkaline rocks surrounded by fenitized Precambrian granites. Colluvial and alluvial deposits cover the central and north-western area, hiding most outcrops. No intrusive carbonatite bodies occur, and only a few small outcrops of carbonatite breccia have been distinguished. However, carbonatitic rocks are very common in most of the other sub-volcanic Angolan complexes, such as Tchivira, Bonga, Virulundo, or Bailundo [26,27,31,32].
The complex presents a central unit comprised of two plutonic alkaline rock types, according to their nepheline and feldspar contents ( Figure 1). Nepheline syenites are made up of K-feldspar and lesser nepheline (Figure 2a), whereas andradite-rich nepheline syenites in the north-eastern zone have high andradite-schorlomite contents. In addition, three different foidolites (urtites, ijolites, and melteigites) appear at the centre of the complex in contact with the nepheline syenites, with higher contents of feldspathoids, such as nepheline, sodalite, or cancrinite, and an absence of feldspar (Table  2). An external aureole of fenitization surrounds these plutonic rocks ( Figure 1). Fenitization processes have extensively replaced the granites (Figure 1), and the alteration is more intensive near the carbonatite and plutonic alkaline rocks. Dominant K-feldspar stained by hematite gives a rosereddish tone to these rocks, which strongly differentiates them from the unaltered Precambrian host rock.
Magmatic breccia outcrops occur among fenitized granites. Carbonatite and alkaline breccias can be distinguished according to the composition of the groundmass. Carbonatite breccias mainly contain fenitized granite xenoliths of variable size, ranging from microscopic to metric dimensions with an angular shape, suggesting an absence of assimilation by the carbonatite mesostase. The mesostase corresponds to a calciocarbonatite, as calcite is the major phase with lesser apatite, pyrite, and pyrochlore (Table 2).
Alkaline breccia (Figure 2b) outcrops are scarce and located among fenitized rocks surrounding the central alkaline intrusion. Two main types of alkaline breccias can be distinguished based on the ratio of host rock fragments with respect to the mesostase. In one case, the proportion of fragments is dominant over that of the mesostase, and in another, the opposite is true. Both breccia may concentrate Nb and REE minerals.
Four types of alkaline dykes of metric to decametric width cut all the above-mentioned rocks ( Figure 1). Sodalite trachyte, nepheline trachyte, tinguaite, and latite dykes present porphyry textures with K-feldspar, augite, and nepheline as phenocrysts. Trace amounts of Na-Ti-Ce-Zr sorosilicates of the eudialyte and rinkite groups, REE-bearing oxides, such as those of the perovskite-loparite series, and REE carbonates and phosphates occur as disseminations in the rock.
Pervasive quartz alteration with Fe-oxides fills late fractures in the south and south-eastern ring area occupied by the fenites. At central and north-western regions, alluvial and colluvial deposits form a thick cover with clay deposits and rock fragments from alkaline and fenitized units.

Plutonic Alkaline Rocks
The central intrusion of Monte Verde is made up of foidolite. They present a phaneritic holocrystalline texture, often symplectitic, with major nepheline, mafic minerals, very minor sodalite, and no K-feldspar. Urtite, the most abundant rock type, contains major nepheline, up to 95 modal %. It contains kalsilite exsolutions and is partly replaced by small amounts of sodalite and altered to secondary cancrinite. Aegirine augite (partly overgrown by kaersutite rims and arfvedsonite), titanite, ulvöspinel, and pyrrhotite with pentlandite exsolutions are accessory minerals. At the same location, melteigites have the same texture and similar composition as urtites, with major nepheline but display an agpaitic texture and contain higher contents of aegirine augite, kaersutite, pyrrhotite, ulvöspinel, and, locally, phlogopite-tetraferriphlogopite.
Minor ijolite outcropping at the central intrusion show intermediate compositions between urtite and melteigite. Aegirine augite and diopside are the major phases intergrown with nepheline and lesser phlogopite-annite. These ijolites have accessory amounts of reddish to brownish garnets of the andradite-schorlomite solid solution. In addition, scarce subhedral rosenbuschite grains (up to 1 mm in length), overgrown by rinkite rims, replace titanite and augite ( Figure 3a). Rare britholite-(La) anhedral crystals of a few hundred microns in length also occur replacing titanite grains. A late association of calcite and radial aggregates of synchysite crystals, less than 10 µm in length, replaces the ensemble.
At the northwestern sector, andradite-bearing nepheline syenites intrude fenitized granites ( Figure 1). K-feldspar is the most abundant mineral and is intergrown with nepheline (the last, strongly replaced by fine cancrinite) and with minor aegirine augite. The accessory Ca-bearing garnet has a high andradite-schorlomite component and forms euhedral grains partly replaced by finegrained zircon, calcite, and locally, a very fine intergrowth with an HFSE and REE mineral, probably corresponding to fergusonite-(Nd). In addition, an unknown primary silicate (probably eudialyte, up to 1 mm in diameter) is completely substituted by a complex fine-grained intergrowth of calcite, albite, zircon, pyrochlore, and wöhlerite ( Figure 3b).

Fenitized Granites
Eburnean fenitized granites comprise the most extensive unit of the Monte Verde carbonatite complex, roughly forming a ring shape of hectometric width surrounding the central alkaline intrusion. Fenitization is intimately associated with hydrothermal fluids involved during carbonatite formation, leading to the formation of alkaline rocks mostly at the contact between carbonatite and host rock [40,41]. Fenitization intensity progressively decreases from the contact with the intrusions of carbonatitic or alkaline rocks, so distal granites may preserve primary compositions and textures. At Monte Verde, fenitization is mainly K-rich as K-feldspar and biotite are pervasive. However, Na alteration made up of albite, richterite, nepheline, and sodalite occurs overprinting the K-rich alteration. At the same time, Na alteration was replaced by quartz and calcite.
Fenitized granites from the outermost areas present remnants of primary plagioclase with late fine microcline rims indicative of incipient K-fenitization. At the easternmost ring, fenites also contain fine-grained aegirine augite, in addition to biotite and pyrite. These minerals can be veined and replaced by Mn-rich ankerite followed by a complex mineral sequence with REE, HFSE, and LILE, including strontianite, calcioancylite-(Ce), barite, galena, Th-rich bastnäsite-(Ce), fine-grained rutile (Figure 3c), Th-and Ce-rich pyrochlore, and daqingshanite-(Ce) (Figure 3d). Most of these minerals occur in anhedral crystals, less than 30 µm in diameter. Quartz and calcite late veins replace all these minerals. Weathering of micas produced mixtures of vermiculite and smectite, and pyrite is replaced by goethite.
Highly fenitized granites from the innermost areas contain two generations of K-feldspar: the first generation has very fine-grained hematite inclusions whereas a second type does not contain Fe oxides. Late silicification produces interstitial quartz between the feldspar grains, as well as veinlets. Quartz is accompanied by zircon, Ba-and Sr-rich pyrochlore crystals less than 40 µm in diameter (Figure 3e), and tiny fibrous crystals of rhabdophane-(Ce), up to 50 µm in length ( Figure 3f).

Magmatic Breccia with an Alkaline Groundmass
This type of breccia contains carbonatite fragments, thus suggesting the presumable presence of a deeper carbonatite intrusion, despite the fact that it does not crop out at the surface. These breccias tend to contain abundant Nb and REE minerals. Two varieties of breccias with alkaline mesostase were defined: a) clast-supported alkaline breccias with calciocarbonatite and fenitized granite fragments and b) matrix-supported breccia with scarce calciocarbonatite grains but a major alkaline groundmass.
In clast-supported alkaline breccias, calciocarbonatite fragments are made up of major coarse calcite, with apatite grains (up to 10 modal %) locally arranged in bands, thus suggesting the existence of magmatic bedding in the original photolith; pyrite (replaced by goethite) and alkaline amphiboles of the richterite-ferrorichterite series can be very abundant, up to about 20 modal %. Euhedral zoned fluornatropyrochlore crystals may be found as inclusions in the calcite or amphibole crystals ( Figure  4a). These zoned crystals may reach up to 40 µm in diameter. A secondary association of Sr-, Ba-, REE-carbonates and sulphates replaces the primary assemblages by filling late microfractures and as patches. Strontianite, barite, and rutile are the most abundant phases in this stage, and they are cut or overgrown by radial aggregates of fine-grained (less than 20 µm in length) crystals of synchysite-(Ce), calcioancylite-(Ce), and burbankite ( Figure 4b). The compositions of the subsolidus pyrochlore generations depend on the accompanying minerals. Pyrochlore scattered in calcite is replaced by thin rims (up to ten microns) of secondary U-and Th-bearing pyrochlore, whereas pyrochlore included in amphiboles is veined by a late association of Sr-late pyrochlore, quartz, and raphdophane-(Ce). In matrix-supported alkaline breccia, calciocarbonatite clasts are scarce and surrounded by a reaction rim of fine-grained biotite, K-feldspar, albite, calcite, and trace ilmenite, pyrite, pyrrhotite, REE-carbonates, and pyrochlore. The abundance of alkali-bearing minerals in these rims suggests a strong interaction with the alkaline mesostase. Some of the calciocarbonatite fragments may display up to 50 modal % anhedral fluorite crystals, which may contain inclusions of rounded calcite grains suggesting its simultaneous magmatic crystallization in equilibrium with calcite ( Figure 4c). Fluornatropyrochlore, sphalerite, and pyrrhotite are common minerals found as euhedral inclusions in both fluorite and calcite crystals. Late fine-grained calcioancylite-(Ce) and synchysite-(Ce) also occur as anhedral interstitial grains, up to 20 µm in length, scattered across the carbonate.
Two types of alkaline groundmass are observed in the alkaline magmatic breccia: 1. Quartz-syenitic: which tends to be clast-supported and is comprised of reddish K-feldspar and albite with late interstitial quartz and ilmenorutile. Late veinlets contain a new generation of quartz along with hematite, synchysite-(Ce), and barite ( Figure 5a, Table 2). 2. Foidolitic: which tends to be groundmass-supported, and feldspathoids (nepheline and sodalite, more or less replaced by cancrinite) are the most abundant minerals, replacing Kfeldspar and albite. In addition, fluorapatite, anhedral biotite, calcite, richteriteferrorichterite and fluornatropyrochlore occur as minor components. Pyrochlore occurs as euhedral inclusions inside all the above-mentioned minerals, suggesting its early crystallization from the melt. Nepheline appears to be associated with zircon. Cancrinite could have an important metallogenetic role, as it often appears intergrown with fluorite, calcioancylite-(Ce), burbankite, and secondary pyrochlore ( Figure 4d). Late sodalite replaces albite and nepheline. A new generation of U-and Ti-rich pyrochlore is associated with sodalite and replaced earlier pyrochlore types. Because of its U content, it presents intense fracturing due to metamictization processes ( Figure 4e). Rare rounded ilmenorutile is also overgrown by Mn-rich ilmenite, secondary fine-grained Na-rich-calciopyrochlore, pyrrhotite, and galena ( Figure 5b, Table 2).

Breccia with Carbonatite Groundmass
Breccia with major calciocarbonatite groundmass contain angular fragments of fenitized granites, although these are partly assimilated by the carbonatite matrix ( Figure 6, Table 2). Idiomorphic millimetric calcite is the major phase of the groundmass, reaching up to 80 modal%, with minor aegirine augite. In addition, scarcer fluorapatite displays Sr-and Si-rich rims and may contain fine-grained calcite locally, as well as U-Ti-rich pyrochlore inclusions. U-Ti-pyrochlore also grows as hipidiomorphic crystals of millimetric size among calcite. They are replaced by patches of a late Ba-and Si-rich, U-poor pyrochlore generation (Figure 4f). Scarce burbankite of very fine grain size replaces calcite following the cleavages.

Alkaline Dykes
Several swarms of alkaline dykes crosscut all the central intrusive rocks (carbonatites, alkaline breccias, and fenitized granites). According to their composition, four main types of dykes can be distinguished: sodalite-feldspar bearing trachytes, tinguaites, latites, and nepheline trachytes. The latter contain remarkable agpaitic mineral associations enriched in HFSE and REE.
Although scarce, Na-K-Ca-Ti-Zr minerals of the eudialyte group are recognized in augite-rich nepheline trachyte dykes. An early eudialyte generation develops poikilitic crystals, up to 1 mm in diameter, intergrown with aegirine augite and nepheline (Figure 7c) associated with late sodalite and replaced by Nb-rich wöhlerite. A second generation of eudialyte replaces the above crystals or occurs in veinlets filled with late calcite and rare burbankite. Secondary eudialyte may form euhedral zoned crystals reaching up to 1 mm in diameter and have strong Mn and Nb enrichments (Figure 7d).
Late generations of albite and calcite have special interest as they are often associated with REE minerals, filling late veins or interstices between nepheline, sanidine, aegirine augite, titanite, and cancrinite-sodalite. Fine carbocernaite and britholite-(Ce) (less than 20 µm) grow with albite. Locally, late calcite precipitates with ancylite-(Ce) and calcioancylite-(Ce) and may overgrow rosenbuschitemosandrite together with very fine burbankite aggregates. In addition, tiny veins of albite and acicular synchysite-(Ce) surround them.

Mineral Chemistry
The mineral composition of the most significant Nb-and REE-phases has been studiednamely, the members of the pyrochlore, perovskite, rinkite, wöhlerite, and eudialyte groups. Results are shown at Supplementary material. Unfortunately, due to the fine-grained size of some of the REE carbonates, it was not possible to obtain good results. The earliest pyrochlore is restricted to the carbonatite breccia unit, the calciocarbonatite fragments, and the feldspathoid-bearing mesostase of foidolite breccias. This pyrochlore commonly displays a homogenous composition. The B-site is mainly occupied by Nb, although Ta and Ti contents can be significant in pyrochlore from the foidolitic mesostase in the breccias, where low Zr, Fe, and Si enrichments have also been recorded (less than 0.05 a.p.f.u.). The A-position mainly contains Na and Ca, ranging from 0.64 to 1 and 0.86 to 1 a.p.f.u., respectively. Na proportion is higher in alkaline groundmass than in carbonatite fragments. Sr, Ce, and Th are very low, whereas U was not detected (Figure 8).
Type I pyrochlore in the fluorite-bearing carbonatite xenoliths shows the lowest REE and HFSE contents. Regarding the Y-site, F is the only anion occupying this position. Type I pyrochlore should be classified as fluornatropyrochlore to fluorcalciopyrochlore according to the criteria of Atencio et al. [42]. Pyrochlore II occurs in the alkaline matrix of breccia. According to the composition of primary pyrochlore, two subtypes can be recognized. IIa subtype grows within the nepheline-sodalite groundmass and contains remarkable UO2 and ThO2 contents, up to 4.08 and 10.54 wt.%, respectively. These elements substitute Na and Ca in the A-site of the pyrochlore structure. SrO and BaO contents are nearly 2.5 wt.%, whereas Ce2O3, the dominant LREE, is less than 1 wt.%. The B-site is mostly occupied by Nb, whereas in the Y-site, F is lower than in the earlier fluornatropyrochlore. The second subtype (IIb) occurs in the alkaline groundmass associated with richterite, as fine rims overgrowing zoned primary pyrochlores. It displays a similar composition to the IIa subtype; however, it does not contain Na and UO2 contents are higher (up to 6.13 wt.%). In both subtypes, Ca is the major cation in A; therefore, they could be classified as hydroxycalciopyrochlore [42].
Pyrochlore III only occurs in the carbonatite mesostase of the breccia. There are significant UO2 contents in the A-site, major Nb in the B-site, but no F in the Y position have been recorded. Three compositional varieties are identified based on A-site occupancy. The first one has high UO2 contents (12.39-16.92 wt.%) with high Ce2O3 and major vacancies but no Na and low Ca contents. However, the highest UO2 contents have been recorded into the second subtype, up to 23.07 wt.%, which also includes high BaO amounts and vacancies. Finally, the third subtype is relatively depleted in UO2 (only up to 7.60 wt.%), with low ThO2, whereas vacancies are dominant in the A-site.
Pyrochlore IV is located in the carbonatite xenoliths of the breccia, with syenitic mesostase overgrowing or replacing fluornatropyrochlore in association with late quartz and REE minerals.
This secondary pyrochlore has the highest SrO contents, up to 0.29 wt.% although Ca is still the major cation at the A-site and vacancies are predominant. F may reach 4.22 wt.%.
The latest type VI pyrochlore concentrates in highly fenitized granites and is intergrown with zircon crystals. Two subtypes were recognized: the first one contains high Ba and Sr at the A-site (up to 0.35 and 0.17 a.p.f.u.) and vacancies, but no Na and Ca. On the other hand, a second subtype crosscuts subtype I and has remarkable ThO2 and UO2 enrichments, up to 8.24 and 8.25 wt.%, respectively. Nb is predominant at the B site, whereas the Y-site is fully occupied by OH (Figure 8).
Therefore, III, IV, V, and VI generations can be defined as kenopyrochlore according to Atencio [42], as vacancies are predominant at the A-site.

Perovskite-Loparite-(Ce)
Perovskite occurs in nepheline trachyte dykes associated with late cancrinite alteration, where it is overgrown by loparite-(Ce), Nb, and REE-rich titanite. Perovskite shows significant contents of Ce and La but also Na (up to 0.09, 0.08, and 0.22 a.p.f.u., respectively). However, Nb and Sr contents (corresponding to lueshite and tausonite components) are not significant and may reach up to 0.02 and 0.09 a.p.f.u.
Loparite-(Ce) contains about 0.40 a.p.f.u. of Na and significant Ce, La, and Nd enrichments reaching up to 0.16, 0.12, and 0.02 a.p.f.u., respectively ( Figure 9). Therefore, the evolution trend observed from perovskite to loparite-(Ce) describes Na, REE, Nb, and Th enrichments, as has also been reported in the alkaline complexes of the Kola Peninsula [44,45].  [44]. Arrows indicate evolution trends from early perovskite to secondary loparite-(Ce).

Eudialyte
Two types of eudialyte have been distinguished in the nepheline trachyte dykes cutting the central intrusion. The first eudialyte type, of magmatic origin, is intergrown with early nepheline and aegirine augite. High CaO and SrO (up to 15.5 and 2.68 wt.%) and low MnO contents ranging from 2.30 to 2.51 wt.% were recorded ( Figure 10). The Z-site is fully occupied by Zr, whereas Nb2O5 can reach up to 2.70 wt.%. However, significant amounts of Cl (1.48-1.95 a.p.f.u.) can enter the X-position.
Secondary eudialyte occurs, filling late veins cutting nepheline dykes. It displays concentric zoning, with a core of similar composition to that of eudialyte I but surrounded by Mn-and Nb-richer rims (Figure 10). In addition, rims have high Ce and La contents (1.55 and 1.61 wt.% Ce2O3 and La2O3, respectively) replacing Na. The X-site of the rims is poor in Cl in comparison to the cores.

Na-Zr-Ti Sorosilicates
Minerals of this group have complex general formulas with high HFSE and REE as well as Mn, Fe, Na, and Ca [46][47][48], and include the wöhlerite and rinkite groups.
Rinkite group minerals occur in nepheline trachyte dykes and melteigite rocks as accessory phases.

REE Carbonate Group
Some REE carbonates were found in nepheline trachyte crosscutting both plutonic alkaline rocks and fenitized granites but also in calciocarbonatite fragments of clast-supported breccia of alkaline groundmass. Commonly, they are related to the late stages of the complex formation. •

Evolution of the Monte Verde Complex
Although the amount of geological data is relatively restricted because of the limited size of the outcrops, textural patterns can help to reconstruct the formation history of the complex.
Breccias with carbonatite matrix have fragments of carbonatites and fenitized granites, but the alkaline breccias include fragments of carbonatites. In addition, alkaline dykes crosscut all the existing units. Therefore, the intrusion of alkaline magmas seems to have occurred after the crystallization of the main carbonatite bodies.
The association of carbonatites and alkaline rocks is a well-known fact and has also been documented in the currently active natrocarbonatite volcano at Oldoinyo Lengai in Tanzania. In this case, the existence of magma mingling between a carbonatitic and an undersaturated alkaline magma is assumed [55]. In addition, very recent studies about Italian carbonatite alkaline complexes display an immiscible separation of silicate and carbonatite liquids [56,57].

Pyrochlore Evolution in Monte Verde
Pyrochlore is a ubiquitous mineral in carbonatites worldwide and can be formed in different generations, which can be used to interpret the sequence of processes during carbonatite crystallization, as established in the Tchivira model [25].
Primary pyrochlore from Monte Verde formed early in the carbonatite crystallization sequence. Its Y site is fully occupied by F, similar to the magmatic pyrochlore of the Tchivira complex [25], owing to the high F activity in the magma chamber. This interpretation is also compatible with the abundant coeval crystallization of this pyrochlore with fluorite and magmatic calcite, as observed in some carbonatite fragments in the carbonatitic magmatic breccia. By contrast, the Bonga carbonatite breccias do not contain F-bearing pyrochlore [30]. This difference could record a different behaviour of F during explosive processes, or a different F content in the primitive magma. The absence of fluorite in Bonga is more suggestive of lower F contents in the Bonga primitive carbonatitic magma by comparison with the Tchivira or Monte Verde carbonatitic magmas. In this sense, it is interesting to take into account that undersaturated alkaline rocks seem to be absent in Bonga, while mixtures with lamprophyric magmas are present [30]. Finally, the A-site contains mostly Na and Ca in similar proportions, as is the case of Tchivira and Bonga, thus reflecting a high Na activity during the early stages of crystallization of the carbonatite magma.
Five secondary pyrochlore types were identified, replacing and overgrowing magmatic pyrochlore. Na and Ca commonly decrease in these pyrochlores, and thus vacancies, Sr, Ba, U, Th, and REE increase to balance charges. This evolution trend observed in all Angolan carbonatites [25][26][27][28][29][30][31]33]. Na decrease can be related to the decrease of Na activity in the magma, which can be associated with the loss of alkalis during fenitization processes [29]. This deficit of alkaline cations in the A position allows for the incorporation of REE, HFSE, and LILE. The enrichment in these elements can be the result of their fractionation in residual fluids. However, LILEs and REE can also be released from the structures of primary carbonates during replacement of primary calcite-which can accommodate these elements at high temperatures [58]. However, as reported by [25,26], Ba and Sr may also (or at least partly) come from the feldspars of the original host granites, carried by the fluids involved in the aforementioned fenitization processes. In any case, these hydrothermal or carbothermal fluids should be enriched in HFSE, LILE, and REE. Therefore, the successive generations of pyrochlore became enriched in these elements. Hence, type II and III record high UO2 contents, positively correlated to significant Ta2O5 enrichment, although Ta is lower than in other carbonatite complexes, such as U-Ta rich pyrochlore in the Sokli complex [59] or in other Angolan carbonatites [25,26,33].
Type II pyrochlore in alkaline rocks from Monte Verde also records a slight Zr and Si enrichment, as it grew into an environment that is mostly made up of primary and secondary silicates including zircon.
Fenitized granites can concentrate pyrochlore V, which shows the highest Th enrichments. In addition, type VI pyrochlore with high Ba and Sr contents is associated with feldspar in highly fenitized areas, thus suggesting (again) that at least part of the cations in pyrochlore from fenites (and perhaps in carbonatites) could originate from the host rock.

Metallogenetic Potential of the Monte Verde Carbonatites
The greenfield exploration campaign carried out in Monte Verde found only small carbonatite outcrops (Figure 1), in most of the cases poorly mineralized with respect to pyrochlore. However, any discussion on the metallogenetic potential of Monte Verde complex must calculate its potential three-dimensional geometry. The carbonatite occurrences of the Monte Verde complex are restricted to carbonatite-bearing alkaline breccia bodies of decametric size hosted in fenitized granites. However, some of these small outcrops of carbonatite breccias occur up to 2 km away from the central alkaline plug. The distribution in concentric rims of these magmatic breccia outcrops suggests that the structure of Monte Verde could correspond to the inner part of a collapsed caldera, and the breccias could represent channels (ring-faults) of extrusion controlled by subvolcanic explosive processes. A similar mechanism was proposed for Alnö [60] to explain the existence of carbonatite ring dykes in Sweden. Hence, by assuming that the structure could correspond to a partially eroded caldera structure, the area with carbonatite potential should be expanded, and the potential for larger intrusions at depth would still persist. In fact, colluvial materials cover large areas around the core of the complex, and therefore, more distal hidden carbonatite rings could also be present beneath these colluvial sediments. Some of these dykes could be mineralized, as occurs at the nearby Bonga complex, where phoscorites and some but not all of the carbonatite ring dykes were found to be valuable metallogenetic prospects (only those enriched in pyrochlore-magnetite-apatite) [60]. Similar enriched ring dykes were described, for instance, in the Sarfartôq carbonatite complex in Greenland [61]. Therefore, a geophysical survey exploration could help to determine the possible existence of mineralized carbonatites or phoscorites at depth. In particular, the existence of different types of anomalies could be helpful to delimitate the 3D structure at depth and its economic potential [62,63]. These include radioactive anomalies-due to the high contents in Th and U in pyrochlore and other minerals, magnetic anomalies-due to the high magnetite content, or gravimetric anomalies-since large volumes of magnetite can create a noticeable density contrast, or time-domain electromagnetic anomalies-when conductive sulphide deposits are present. These methods have been proven to be effective to determine the 3D structure of carbonatites and alkaline intrusions in several places, such as in Mt Weld, Australia [64]; El Morro, Brazil [65]; Elk Creek, USA [66]; Alnö, Sweden [67], Gallinas Mountains in USA [68]; and Beldih in India [69].
Pyrochlore concentrates most HFSE and REE; however, it occurs in extremely low amounts, less than 1 modal % in most of the studied sections, which is lower than the pyrochlore content in carbonatite ring dykes or magnetite-apatite rocks of the Bonga complex [26,29]. Therefore, in addition to the reduced number of carbonatite outcrops in the area, the observed pyrochlore content is low in most of the cases. Moreover, the HFSE (including U, Th and Ta) and REE are mainly concentrated in secondary pyrochlore generations, which are heterogeneously distributed. In addition, these late pyrochlore generations occur as complex intergrowths, thus presenting an obstacle for the extraction of these elements in the ores.
Monte Verde carbonatites record extremely scarce REE minerals in the outcropping carbonatites, especially if we compare the proportion of these minerals with those found in the Virulundo, Tchivira, or Bonga massifs [5,[7][8][9]. Although some authors propose a magmatic origin for these elements and barite in other carbonatites (e.g., [70]), the textural evidence and the mineral association with quartz suggest a hydrothermal origin for them, as it was also suggested in Tchivira [25]. Other carbonatites worldwide, for instance, Italian carbonatites [56,57]; the Amba Dongar complex, India [71][72][73]; the occurrences from Sichuan Province, China [74][75][76]; or the Montviel deposit, Canada [77][78][79][80]; may display similar late REE mineral associations. This interpretation poses a problem since it is difficult to deduce how hydrothermal processes distribute REE minerals within carbonatite. However, in the case of Angolan carbonatites, the proximity to intrusive contacts seems to be a key factor [25]. In the case of Monte Verde, the possible existence of a magma chamber at a certain depth, necessary to support the idea of the collapse caldera (see Martí et al.,2008 [81] and references therein), the resulting hydrothermal alterations could be more developed on the flanks (through ring faults) and at the top of the magma chamber.
In addition, concentrations of industrial minerals such as apatite, magnetite, or barite are irrelevant, at least in the documented outcrops. However, the high fluorite contents associated with high pyrochlore contents in some xenoliths can be a strong indication of the occurrence of interesting mineralized bodies at depth, including both fluorite and pyrochlore resources. In Tchivira, the flanks of the carbonatite intrusions are strongly mineralized in fluorite and secondary generations of pyrochlore replacing primary carbonates along with quartz and ankerite. Contrastingly, fluorite and pyrochlore mineralization in the Monte Verde breccias suggests a different type of evolution, with fluorite as a primary magmatic product and associated with typical magmatic pyrochlore.
As carbonatite breccia very rarely crops out at Monte Verde, laterite deposits with high concentrations of heavy minerals (pyrochlore, magnetite, and apatite), such as those developed at the Bonga or Tchivira complexes [25][26][27], do not occur, at least, in the known outcrops.
In conclusion, although we dismiss any metallogenetic potential of the outcropping carbonatite breccia units, the studied rocks may point out the possible existence of an important mineralized carbonatite body at greater depths. Therefore, a geophysical study of Monte Verde could contribute to gathering valuable information on the extent of these potential resources.

Metallogenetic Potential of Alkaline Rocks of Monte Verde
Subsaturated alkaline rocks from alkaline massifs set in continental rifts usually show high contents of characteristic Zr-Ti-Na-Ca silicates termed agpaitic minerals [82]. However, alkaline massifs with no agpaitic phases and mainly comprised of zircon, titanite, and ilmenite are defined as miaskitic [83]. The agpaitic association is made up of more than 450 complex minerals [5,84,85] most of them being exclusive to one or two deposits. Rare elements tend to concentrate at the most subsaturated peralkaline agpaitic rocks [5,86]. On the other hand, ultramafic and mafic plutonic rocks do not usually present rare element economic mineralization-although they contain rare element carriers such as perovskite, apatite, and titanite [9].
Agpaitic complexes often contain rare exclusive phases with highly diverse compositions which reflect the degree of evolution of the alkaline magma [85]. The following descriptive terms are proposed for alkaline rocks according to their magmatic HFSE mineralogy: miaskitic, transitional agpaitic, agpaitic, and hyperagpaitic [85,87].
To evaluate the metallogenetic potential of Monte Verde, a study of agpaicity and alkalinity of the main mineral association was carried out. Alkaline dykes and plutonic rocks mainly show low agpaitic aegirine augite as the major clinopyroxene. Furthermore, the occurrence of Na-Zr-Ti silicates of the seidozerite supergroup (eudialyte, wöhlerite, rosenbuschite, rinkite, and mosandrite) as well as apatite, titanite, zircon and britholite-(Ce) suggest a low agpaitic association.
Eudialyte is a strategic mineral as it concentrates high Nb, Y, and REE. Eudialyte occurs at Monte Verde as a scarce phase, only found in nepheline trachyte dykes. In addition, andradite-bearing nepheline syenite contains pseudomorphic eudialite that is completely replaced by a secondary association of zircon, wöhlerite, pyrochlore, calcite, and albite. Eudialyte decomposition is described at the kakortokite series of the Illimaussaq complex and resulted in the formation of catapleiite and two phases (A1 and A2) where most of the REE are concentrated [88]. Eudialyte generally exhibits a Fe/Mn > 1 ratio, although the late generation is richer in Mn and REE than the magmatic one-a trend observed in hydrothermal eudialyte from other alkaline complexes worldwide [52,89,90]. However, REE2O3 contents measured at Monte Verde are much lower than those found in other alkaline complexes, such as Pilanesberg, South Africa [91]. No significant REE enrichments have been recorded neither in magmatic nor in hydrothermal eudialyte crystals in this study. Thus, eudialyte occurring at Monte Verde is not of high economic interest. Similarly, the scarcity of Na-Zr-Ti silicates of the seidorezite supergroup led to reject the economic importance of the outcropping rocks when compared to economically interesting complexes [92].
Loparite-Ce is the only alkaline phase with high agpaicity. At Monte Verde, loparite-(Ce) occurs as an extremely rare mineral and has low Na contents but significant amounts of Nb and REE (1.12-7.50 wt.% and 18.50-30.45 wt.% Nb2O5 and REE2O3, respectively) when compared to alkaline complexes of economic interest [50,86]. In the Lovozero massif, Kola Peninsula, loparite-(Ce) is the main ore that is currently being mined for its extremely high REE2O3 and Nb2O5 enrichments (up to 8-12 wt.% and 30-35 wt.% respectively) [86]. However, because of the scarcity of loparite-(Ce) at Monte Verde alkaline dykes, its economic potential for HFSE and REE is also low.
The contents of pyrochlore are very low in the alkaline rocks, and therefore, the main potential ores in these rocks are loparite and eudialyte. The potential for Ta, U, and Th is more reduced than in the carbonatites.
The activity of volatile elements (F and Cl) should be taken into account in order to assess the HFSE potential, as Cl is an important REE carrier, even more than F, in igneous rocks [93,94]. As mentioned, although mosandrite, rosenbuschite, and rinkite of Monte Verde contain very low F, eudialyte has significant Cl contents. However, the occurrence of large amounts of fluorite in carbonatite xenoliths in the breccias suggests that F activity was high in some stages of crystallization, at least in the carbonatites.
Moreover, as mentioned above, the outcropping area can be considered as the apical part of the complex, and therefore, the existence of mineralization (although scarce) in the dykes opens the possibility that the mineralization can also be present in their buried plutonic equivalents.

Conclusions
The Monte Verde complex presents an association of alkaline rocks and carbonatitic and alkaline magmatic breccias. The magmatic breccias draw circular outcrops around a central alkaline plug, which could be indicative of the existence of a collapse caldera structure of nearly 5 km radius, partly covered by colluvial sediments. Based on field and textural evidence, we conclude that the intrusions of the alkaline breccias and dykes were produced later than that of the carbonatite breccias.
Although the carbonatite breccias are poorly mineralized in pyrochlore, some carbonatite fragments coming from a deeper carbonatitic magma chamber are strongly mineralized in primary magmatic fluorite and pyrochlore, suggesting the presence of well-mineralized bodies at depth.
Despite the fact that outcropping alkaline undersaturated plutonic rocks are poorly mineralized in REE and Nb, the occurrence of some moderate to hyperagpaitic minerals in the dykes, including eudialyte, loparite-(Ce), and minerals of the rinkite, rosenbuschite, and wöhlerite groups, could also be indicative of critical element mineralization in the plutonic equivalents at depth. Moreover, the occurrence of later evolved eudialyte can also be indicative of a certain grade of mineralizing processes in the magma chamber. All these results encourage the geophysical exploration of the massif.