Pyrochlore-Supergroup Minerals and Their Relation to Columbite-Group Minerals in Peralkaline to Subaluminous A-Type Rare-Metal Granites with Special Emphasis on the Madeira Pluton, Amazonas, Brazil

Abstract

Niobium (Nb) and tantalum (Ta) are quoted as “strategic” or “critical” elements for contemporaneous society. The main sources of Nb and Ta are minerals of the pyrochlore supergroup (PSGM) and the columbite group (CGM) mined from different magmatic lithologies. Textures and chemical compositions of PSGM and CGM often provide key information about the origin of NbTa mineralization. Therefore, we decided to carry out a detailed study of the relations between the PSGM and CGM and their post-magmatic transformations, and the Madeira peralkaline pluton (Brazil) is an ideal object for such a study. Textures of the PSGM and CGM were studied using BSE imaging and SEM mapping, and their chemical compositions were determined using 325 electron microprobe analyses. Pyrochlore from the Madeira granite can be chemically characterized as Na, Ca-poor, U- and Pb-dominant, and Sn- and Zn-enriched; REE are enriched only during alteration. Two stages of alteration are present: (i) introduction of Fe + Mn, with the majority of them consumed by columbitization; (ii) introduction of Si and Fe, and in lesser amounts also Pb and U: Si, Pb, and U incorporated into pyrochlore, iron forming Fe-oxide halos around pyrochlore. During both stages, F and Na decreased. In the case of a (nearly) complete pyrochlore columbitization, U and Th were exsolved to form inclusions of a thorite/coffinite-like phase. In contrast to altered pyrochlores from other localities, pyrochlore from Madeira shows a relatively high occupancy of the A-site. Although Madeira melt was Na-, F-rich, contemporaneous crystallization of cryolite consumed both elements and pyrochlore was, from the beginning, relatively Na-, F-poor.

1. Introduction

Niobium and tantalum, together with some other chemical elements used for modern technologies, are quoted as “strategic” or “critical” elements for contemporaneous society [1,2], and the search for new sources (mineral deposits) of these elements is supported in different ways worldwide [3].

The main sources of Nb and Ta are minerals of the pyrochlore supergroup (PSGM) and the columbite group (CGM) mined from carbonatites [3,4,5], to a lesser scale, also from peralkaline syenites, alkaline granites, and NYF-type pegmatites (Nb-rich varieties), and peraluminous LCT-type pegmatites (Ta-rich varieties). Mainly, the PSGMs represent a chemically diversified group of minerals with several approved and many other even officially unapproved or only hypothetical endmembers [6,7].

Textures and chemical compositions of PSGM and CGM often provide key information about the origin of Nb-Ta mineralization. Fresh PSGM and products of their hydrothermal alteration have been repeatedly studied in detail [8,9,10,11,12,13,14,15,16,17,18,19,20]; however, most studies focus on only one location, and especially on pegmatites. Poor information about the mutual relations between PSGM and CGM exists, i.e., about their time and matter relations. In most cases referred, CGMs are the relatively older primary phase, and PSGMs are formed later during their hydrothermal alteration; such relation has been reported from pegmatites at Separation Rapids, Ontario [21], La Canalita, Spain [22], Maršíkov, Czech Republic [23], from the Orange River pegmatite belt, South Africa [24], or from Liešťany, Slovakia [17]. The opposite case, i.e., older primary PSGM replaced with younger CGM, has been reported from non-pegmatitic environments such as the Beauvoir peraluminous granite, France [10], Saint Honore alkali syenite, Canada [14], or Yenisei Ridge carbonatite, Russia [25]. Another such case is also the Madeira albite granite in Brazil [12,26]. The association of primary PSGM and CGM, together with a later replacement of both by a late PSGM population, was found in Greer Lake pegmatite, Canada [27] or in Varutrask pegmatite, Sweden [28].

The Madeira peralkaline A-type pluton (Amazonas, Brazil), containing abundant pyrochlore and columbite in different stages of hydrothermal alteration, is an ideal object of study of the PSGM/CGM relation. Brief information about pyrochlore from the Pitinga mine was given by Bastos Neto et al. [29,30] and Minuzzi et al. [12]. These authors distinguished here, based on the nomenclature of the time, pyrochlore, uranpyrochlore, plumboan uranpyrochlore, and uranoan plumbopyrochlore, all varieties locally replaced with columbite (dominantly columbite-Fe). Recently, Hadlich et al. [26] provided a detailed description of pyrochlore as potential uranium ore, including a discussion of pyrochlore U enrichment, and Bollaert et al. [31] studied nanoscale processes during hydrothermal alteration of Madeira pyrochlore.

To obtain a more general view of pyrochlore diversity and its time/elemental relation to columbite, we analyzed, together with 25 Madeira samples, also samples from subaluminous A-type plutons at Orlovka (Siberia) and Cínovec (Erzgebirge, Czech Republic), and for comparison evaluated also published data from peraluminous rare-metal granites at Beauvoir (France) and Podlesí (Erzgebirge, Czech Republic).

2. Geology of the Studied Plutons and Samples

Simplified geological maps and sections of the studied plutons are included in Supplementary Materials. A list of major lithological units in the studied plutons is attached as Table S1.

The Madeira pluton, together with the associated Agua Boa and Europa plutons, form the Madeira Suite (1824–1818 Ma), situated in the central part of the Amazon Craton, Brazil (0.754° S, 60.106° W), intruding the 1889–1888 Ma Iricoumé Group volcanic basement [32,33,34]. The Madeira pluton (60 km²) consists of four rock types (Costi et al. [33,34]): (i) metaluminous porphyritic amphibole-biotite “rapakivi granite” (1824 ± 2 Ma), (ii) metaluminous biotite granite (1822 ± 2 Ma), (iii) a sheet-like body of metaluminous hypersolvus alkali feldspar granite (Figure 1a), and (iv) the central body of peralkaline F, Sn, Nb, Th, REE and Zr-rich albite granite in two facies. The magmatic “core facies” (Figure 1b) is composed of quartz, albite, K-feldspar, aegirine, annite, lepidolite, and cryolite with disseminated cassiterite, zircon, pyrochlore, thorite, and REE-fluorides. At depth, it contains two major zones of veins and pockets mainly composed of massive cryolite, besides layers of intragranitic cryolite-rich pegmatites [35]. The outer shell of the albite granite, the “border facies” (Figure 1c), underwent pervasive hydrothermal alteration; it is now slightly peraluminous and contains common fluorite instead of cryolite. The hypersolvus and albite granites are interpreted as coeval, with the U/Pb age of 1818 ± 2 Ma [33,34].

The contents of Nb and Ta in the albite granite reached 800–1400 and 100–280 ppm, respectively; the Nb/Ta value varied in the range of 5–9. Pyrochlore is the dominant carrier of Nb and Ta, supplemented with some columbite. Pyrochlore and columbite from 18 representative samples of the core albite and hypersolvus granites (150 and 40 spots) were analyzed within this study.

The Khangilai pluton hosting the Orlovka Ta deposit is located 140 km SE of the city of Chita, Siberia, Russia (51.055° N, 114.834° E). It is a highly fractionated, only slightly peraluminous (A/CNK = 1.05–1.17), P-, Fe-, Mg-, Ca-poor, and Li, Rb, Nb, Ta, Sn, W-enriched magmatic system of Jurassic age. While the central part of the pluton is formed by barren biotite and two-mica granites, the cupola-like body of the western satellite (i.e., the Orlovka deposit) is formed from the known base upwards by (i) biotite granite, (ii) Li-phengite granite, (iii) Li-muscovite granite, (iv) albite-amazonite-lepidolitetopaz granite with flat banded pegmatite/aplite bodies, and (v) bodies of quartz-poor, and topaz, beryl-rich greisen along the upper contact with the metasedimentary country rocks [36,37].

The lepidolite granite and layered aplite/pegmatite bodies are enriched in columbite–tantalite and microlite. The contents of Nb and Ta at the deposit reached 25–270 and 5–450 ppm, respectively, at Nb/Ta = 0.5–5. Microlite and columbite–tantalite from representative samples of lepidolite granite and banded bodies were analyzed (22 and 34 spots, respectively).

The Cínovec subvolcanic granite pluton is located inside the Altenberg-Teplice caldera in the Eastern Erzgebirge, directly at the Czech/German border (50.72° N, 13.76° E). The Altenberg-Teplice caldera is the largest complex of A-type rhyolites and granites in Variscan Europe, covering about 500 km², 314–312 Ma in age [38,39,40]. After the caldera collapse, small plutons of A-type topaz-bearing biotite granites intruded the rhyolite sheet. The intrusions of biotite granites were followed by the emplacement of a number of cupolas and small subvolcanic stocks of albite-zinnwaldite granites with tin mineralization. The structure of the Cínovec ore-bearing system is well known due to borehole CS-1, 1596 m in depth (50.7332° N, 13.7653° E; [41,42]) providing unique material for petrological study. The deeper part of the granite body, below the depth of 736 m, is formed by different facies of albite-biotite granites, while the upper part is formed by albite-zinnwaldite granite. The uppermost part of the granite cupola is greisenized and mineralized with cassiterite and wolframite. A zone of mica-free and feldspar-rich lithologies, localized below the greisen bodies, is supposed to represent the source area of mineralizing fluids. A characteristic feature of zinnwaldite granites is the increase in the modal contents of zircon, monazite, thorite, xenotime, uraninite and Nb-Ta and REE minerals. The contents of Nb and Ta in the granite cupola reached 40–115 and 7–54 ppm, respectively, at Nb/Ta = 2–7. Both microlite (in the uppermost part of the cupola) and pyrochlore (in deeper parts of the pluton) appear only sporadically; the main Nb, Ta-hosts are CGM and rutile [43,44,45,46].

To check the general difference in the shape and composition of PSGM and associated CGM between the studied A-type granites and common strongly peraluminous rare-metal granites, we used published data from the Beauvoir granite (France) and Podlesí granite (Czech Republic) for comparison. The strongly peraluminous (ASI = 1.3–1.4), P, F, Li, Rb, Nb, Ta and W-enriched Beauvoir rare-metal granite pluton is located in the northern part of the Massif Central, France (46.179° N, 2.953° E). Borehole GPF1, 900 m deep, allowed us to study the vertical evolution of this pluton over a section more than 700 m long [47,48]. Microlite and CGM data from Beauvoir [10,49] are shown for comparison. CGM data from the peraluminous P-, F-rich Podlesí granite stock (Western Erzgebirge, Czech Republic, 50.43° N, 12.78° E) are shown as an example of CGM composition in the case where pyrochlore/microlite have not been saturated [50,51].

Basic geochemical features of the studied rare-metal granites are shown in Figure 2: Madeira albite granite is usually peralkaline (ASI = 0.8–1.1), granites from Orlovka and Cínovec are mostly subaluminous (ASI = 1.0–1.1) with only limited peraluminous altered facies, while the Podlesí and Beauvoir granites are strongly peraluminous (ASI equals 1.2–1.3 and 1.3–1.5, respectively, Figure 2a). All granites are moderately to strongly enriched in Nb and Ta; the Nb/Ta value is well and negatively correlated with peraluminosity. While Ta is relatively strongly enriched during the fractionation of subaluminous to peraluminous melts, i.e., the Nb/Ta value distinctly decreases (up to <1 in some cases), only small changes in Nb/Ta values appear in peralkaline melts (Figure 2b). The peralkaline Madeira pluton is, in comparison with other studied RMGs, conspicuously enriched in U, Th, Pb, and Zn (Figure 2c,d), i.e., elements tending to be hosted by the pyrochlore supergroup minerals.

To compare our pyrochlore/microlite data with the widest possible range of rocks hosting PSGM, we also included diagrams with published data from carbonatites [4,5,9,14,18,25], nepheline syenites and related rocks [9,19,52,53], alkaline granites [19], kimberlites [54], NYF pegmatites [13,20,55,56], LCT pegmatites [8,11,16,17,22,23,57], and the Katugin Ta-Nb-Y-Zr-cryolite deposit (Transbaikalia), in some aspect the most similar granite to the Madeira pluton worldwide [15,58]. This will allow us to compare the chemical features of pyrochlore from all its important formation environments, i.e., crystallization and alteration.

For a list of all studied samples, see Table S2.

Figure 2. Whole-rock geochemical characteristic of the studied rare-metal granites: (a) aluminum saturation index (ASI = Al/(Na + K + 0.5Ca) mol) vs. Nb/Ta (by weight); (b), Nb vs. Ta by weight; (c), U vs. Th by weight; (d), Pb vs. Zn by weight. Data from Raimbault et al. [48] (Beauvoir), Badanina et al. [36] (Orlovka), and authors’ data (Madeira, Cínovec and Podlesí). Values for deeper crust from Rudnick and Gao [59].

Figure 2. Whole-rock geochemical characteristic of the studied rare-metal granites: (a) aluminum saturation index (ASI = Al/(Na + K + 0.5Ca) mol) vs. Nb/Ta (by weight); (b), Nb vs. Ta by weight; (c), U vs. Th by weight; (d), Pb vs. Zn by weight. Data from Raimbault et al. [48] (Beauvoir), Badanina et al. [36] (Orlovka), and authors’ data (Madeira, Cínovec and Podlesí). Values for deeper crust from Rudnick and Gao [59].

3. Methods

3.1. SEM Microscopy

Images of pyrochlore/microlite and columbite in back-scattered electron (BSE) mode and their semi-quantitative elemental spectra were acquired using a TESCAN Vega 3XMU scanning electron microscope (SEM) (TESCAN ORSAY HOLDING, Brno, Czech Republic) equipped with a secondary electron detector (SE), a back-scattered electron detector (BSE) and a micro-analytical system for energy dispersive analysis (EDS) Oxford Instruments Ultim Max 65 with a SDD (silicon drift detector) (Oxford Instruments, Abingdon, UK). An accelerating voltage of 20 kV was applied for the acquisitions of microphotographs and also for analytical acquisitions. The absorbed current for acquisitions of microphotographs was set to optimal values to obtain images of the best possible quality at higher magnifications. For the determinations of chemical composition with EDS, the absorbed current was set to achieve the optimum gain/yield of the detector.

The qualitative elemental maps were provided by a Vega 3XMU scanning electron microscope from Tescan Orsay using an energy-dispersive spectrometer from Oxford Instruments Ultim Max 65 with a SDD (silicon drift detector). In our case, the operating conditions were as follows: accelerating voltage 20 kV, beam diameter 0.4 μm, working distance 15 mm, resolution 2048 × 2048 pixels, scan life time 3 h. The elements were mapped on the following lines: Na K_α1,2, Si K_α1, Ca K_α1, Fe K_α1, Nb L_α1, Pb M_α1, Th M_α1, U M_α1.

3.2. Automated Mineralogy (TIMA)

A TESCAN Integrated Mineral Analyzer (TIMA) based on a TESCAN MIRA FEG-SEM platform in the demonstration facility of TESCAN ORSAY HOLDING in Brno, Czech Republic, was used for automated mineralogical, modal, and textural analyses. This included a collection of backscattered electron (BSE) and energy dispersive (EDS) data on a regular grid of 10 μm point spacing. One thousand counts per pixel were acquired using the high-resolution mode. An acceleration voltage of 25 kV and a beam current of 10 nA were used during the data acquisition. The individual points were grouped based on a similarity search algorithm, and areas of coherent BSE and EDS data were merged to produce segments (i.e., mineral grains). Individual spectra from points within each segment were summed. Data from each segment were then compared against a classification scheme to identify the mineral and assign its chemistry and density. The results were plotted as a map showing the distribution of minerals within each individual sample [60].

3.3. Electron Probe Microanalysis (EPMA)

The compositions of the PSGM and the associated CGM were determined using a Jeol JXA-8230 electron microprobe operated in a wavelength-dispersive mode and housed at the Institute of Geology of the Czech Academy of Sciences, Praha. All details of measurement parameters are referenced in Table S3. The counting times on each peak were optimized for individual elements according to their expected concentrations. X-ray lines and background offsets were selected to minimize interference. The PRZ correction procedure (XPP method metal/oxide [61]) was applied. Empirically determined correction factors were applied to the overlapping X-ray lines. The detection limits are included in Table S3.

Empirical formulae of pyrochlore were calculated on the basis of Nb + Ta + Ti + Sn + W + Si + Al + Mg + Sb + As = 2 at the B site according to Atencio et al. (2010) [6], those of columbite on the basis of six oxygen atoms.

3.4. Remark on Mineral Names

The nomenclature of both pyrochlore group minerals (PGM, a part of the pyrochlore supergroup of minerals, PSGM) and columbite group minerals (CGM, a part of the columbite supergroup of minerals, CSGM) has undergone a fundamental change recently. From the viewpoint of this article, the changes in the CGM nomenclature [62] are relatively minor: the traditional names ferrocolumbite and manganocolumbite should be replaced with columbite-Fe and columbite-Mn. In the case of PGM, the changes are essential [6,7], and the assignment of a correct name to chemical analysis has often become a complex process [63].

The general formula of the pyrochlore-supergroup minerals (PSGM) is A₂B₂X₆Y [7]. In this formula, A typically is a large eight-fold coordinated cation (Na, Ag, Ca, Mn, Sr, Ba, Fe²⁺, Pb²⁺, Sn²⁺, Sb³⁺, Bi³⁺, Y, REE, Sc, U, Th) or a vacancy (□) or H₂O. B is typically a six-fold coordinated high field-strength cation (Ti, Mn, Ta, Sb, W, Sn⁴⁺, Zr, Hf), but also V⁵⁺, Fe³⁺, Si, Al, or Mg. X typically is oxygen, but subordinate F or OH^– are also possible. Y typically is an anion (O, OH^–, F), H₂O, □, or a very large cation (K, Rb, Cs). The IMA mineral names are composed of a root name (in our case, pyrochlore or microlite) and two prefixes. The first prefix refers to the dominant anion of the dominant valence (or H₂O or □) at the Y site, while the second prefix refers to the dominant cation of the dominant valence (or H₂O or □) at the A site [6]. The applied principle “dominant cation of the dominant valence” has the undesirable consequence that often the most represented cation is nomenclaturally omitted, and important information about the evolutionary processes is sometimes lost [63].

Even on the basis of our analyses involving 29 chemical elements, the assignment of a correct IMA-compatible name to each analysis is not easy since the amounts of OH^–, H₂O, and □ are unknown and cannot be determined using EPMA. Other minor discrepancies may arise from other than the assumed oxidation state of Sn, Fe, etc. In the text below, we follow these rules: prefix “fluoro” is used if the content of fluorine at site Y is equal to, or greater than, 0.5 apfu; thus, F is undoubtedly dominant at Y. Prefixes “natro”, “calcio”, and “plumbo” are used if the content of the cations of specific valence at site A is greater than the theoretically possible maximum of (OH + H₂O + □). In other cases, we use the root name alone. In a few cases, Fe, Mn, and U may also be dominant cations of dominant valence; if so, the names ferro-, mangano-, and uranopyrochlore are taken as appropriate. However, these names are not IMA-approved mineral names.

Due to the wide chemical variability of the PSGM and the IMA nomenclature rules, we often found that (i) closely located analyses within the same mineral grain would imply different names, and (ii) even the significantly dominant cation at site A is often not taken into account in the name. Since our paper is primarily focused on changes in chemical composition during PSGM crystallization and alteration, we need to account for the presence of dominant or unusual elements in the description. Therefore, in the text, we use names consisting of a root name with a prefix that takes into account the dominant elements present (except Nb and Ta), like “Na, Ca, F-microlite” or “Pb, U-pyrochlore”. Nevertheless, the correct IMA mineral names for all analyses are typed in Table S4. The most common compositions correspond to fluoro-keno-pyrochlore, oxo-keno-pyrochlore, keno-pyrochlore, fluoro-plumbo-pyrochlore and keno-plumbo-pyrochlore at Madeira, fluoro-natro- and fluoro-calcio-microlite at Orlovka, and keno-microlite and keno-pyrochlore at Cínovec.

Abbreviations of mineral names in the text and Figures are as follows [64]: Ab = albite, Ann = annite, Clb = columbite, Clb-Mn = columbite-Mn, Cof = coffinite, Crl = cryolite, Cst = cassiterite, Flr = fluorite, Gn = galena, Hem = hematite, Kfs = K-feldspar, Mic = microlite, Pcl = pyrochlore, Px = pyroxene, Py = pyrite, Qtz = quartz, REE-F = fluorides of REE, Sch = scheelite, Sp = sphalerite, Thr = thorite, Ttl-Fe = tantalite-Fe, Wlf = wolframite, Xnt = xenotime, Znw = zinnwaldite, Zrn = zircon.

4. Results

All analyses of PSGM from the Madeira albite granite correspond to pyrochlore (pyrochlore group of minerals according to IMA, PGM) and those from Orlovka to microlite (microlite group of minerals, MGM). At Cínovec, both microlite and pyrochlore were encountered. Columbite-group minerals (CGM) were found in all localities, but their relation to PSGM is diverse.

4.1. Pyrochlore and CGM from the Madeira Albite Granite

In the Madeira core albite granite, pyrochlore is part of a diverse assemblage of minor and accessory minerals composed of cryolite (1–6 vol%), fluorite (0–0.8 vol%), zircon (0.5–2 vol%), riebeckite (0.25–2.45 vol%), magnetite (0.15–0.78 vol%), thorite (0.1–0.35 vol%), pyrochlore (0.11–0.38 vol%), columbite (0.05–0.23 vol%), cassiterite (0.15–0.34 vol%), and some sulfides (namely galena and sphalerite), native Bi, and REE-fluorides in accessory amounts. The border albite granite contains minor columbite with rare secondary pyrochlore, while both strongly altered pyrochlore and columbite were found in the hypersolvus granite.

Pyrochlore forms euhedral to subhedral crystals or their fragments, scarcely homogeneous or only faintly mottled (Figure 3a), more often distinctly zoned in BSE (Figure 3b–f and Figure 4a–d). The crystals most often have a core and 2–3 distinct outer zones. Zoning is conditioned mainly by changes in Pb and U contents. The relative width and the order of BSE-bright and dark zones are irregular. Alteration usually begins in the core, which is often replaced with columbite (Figure 3f and Figure 4a,d). Nevertheless, columbite can also replace pyrochlore in the outer zones (Figure 4b,f) or from the edges (Figure 4c–e). Gradually, alteration affects the entire pyrochlore crystal: besides columbite domains, zones with different intensities of Pb, U, or Si incorporation are formed (bright vs. dark areas, Figure 5a–d). Simultaneously, Fe oxides form rims or halos around the remnants of pyrochlore grains (Figure 5b,d,e). Scarcely, we found euhedral crystals almost fully replaced with columbite and thorite/coffinite, and only small relicts of pyrochlore (Figure 5f).

In the border facies of albite granite, pyrochlore is rare, and in all cases it is formed secondarily by the transformation of columbite. U-rich secondary pyrochlore forms thin coatings on the surface of columbite grains and small inhomogeneous granular aggregates in interstices of columbite grains, while Si, U-rich secondary pyrochlore replaces relatively larger domain within columbite aggregates (Figure 6b).

Only scarce, strongly altered pyrochlore crystals enriched in Pb, U, Ti, and Ce were found in the hypersolvus granite (Figure 5b,e).

Primary-looking aggregates of fine-grained, tabular CGM crystals with variable #Mn values (Figure 6a) were found in the border albite granite. In the core albite granite, CGM (columbite-Fe to columbite-Mn) were commonly found as a secondary product of pyrochlore alteration (Figure 4a–f and Figure 5a–c,f) and only rarely as 1–10 μm sized irregular admixtures in cassiterite; no single primary-looking CGM grains were found in the core albite granite or in the hypersolvus granite.

In the first step, we analyzed the contents of a wide set of chemical elements that are potentially able to enter the pyrochlore lattice: W, Nb, Ta, As, Sb, Si, Sn, Th, U, Zr, Al, Bi, Ce (representing the REEs), Y, Sc, Ca, Pb, Zn, Mg, Fe, Mn, Sr, Ba, Rb, Cs, K and F in all selected samples. In the case of K, Rb, Cs, As, Sb, Bi, and Zn, these are elements only rarely mentioned in pyrochlore [65,66] but abundantly represented in the composition of albite granite, which is why they were also monitored. The contents of As, Al, Mg, Sr, K, Rb, and Cs were found to be below the detection limits in nearly all cases. Thus, in the second analytical session, we excluded these elements and analyzed other REEs (La, Pr, Nd, Sm, Dy, Er, Yb) instead. All analyses are included in Tables S4 and S5, and some typical analyses are in

Table 1. Typical analyses (wt%) and empirical formulae (apfu) of pyrochlore from Madeira. Rem.: W, Sb, Cs, Rb, and K in all cases below the detection limits (b.d.l.); n.a.—not analyzed.

Sample	PHR160	PHR160	PHR163	PHR163	PHR128	PHR171	PHR245	PHR191	PHR191	PHR247	PHR174
Crystal	3	3	3	3	3	5	6	1	1	2	5
Spot	20	455	63	64	138	479	124	30	31	492	449
Type	1	1	1	1	2	2	2	3	3	3	4
Nb2O5	44.23	46.49	45.95	48.21	47.84	49.06	53.45	35.53	29.7	31.72	33.77
Ta2O5	4.31	4.12	5.15	4.71	5.38	5.22	5.48	4.26	6.77	3.88	4.28
As2O5	b.d.l.	n.a.	0.04	b.d.l.	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	n.a.
ZrO2	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	0.44	0.76	0.64	n.a.
SnO2	3.51	3.81	3.38	4.18	3.45	2.68	3.52	1.82	0.28	0.42	b.d.l.
TiO2	0.70	0.54	0.50	0.33	0.35	0.33	0.23	0.71	1.78	b.d.l.	1.11
MgO	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	0.01	0.01	n.a.	n.a.
Al2O3	b.d.l.	n.a.	b.d.l.	b.d.l.	0.01	n.a.	b.d.l.	0.13	0.97	n.a.	n.a.
SiO2	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	2.56	11.31	1.76	7.36
Na2O	0.99	2.15	1.80	1.98	b.d.l.	0.48	1.67	1.03	0.99	0.98	b.d.l.
CaO	0.18	0.17	0.28	0.33	0.57	0.67	2.84	0.52	0.21	b.d.l.	0.01
ThO2	0.59	1.92	1.39	5.32	2.88	3.50	9.89	0.46	0.12	0.05	9.48
UO2	12.55	10.50	10.89	5.98	6.55	3.64	0.86	11.96	24.51	10.86	22.75
Bi2O3	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	n.a.
La2O3	n.a.	0.46	n.a.	n.a.	n.a.	0.50	n.a.	n.a.	n.a.	0.22	b.d.l.
Ce2O3	0.97	1.91	0.85	1.43	1.74	2.09	1.42	2.42	1.18	1.46	b.d.l.
Pr2O3	n.a.	0.21	n.a.	n.a.	n.a.	0.19	n.a.	n.a.	n.a.	0.29	0.02
Nd2O3	n.a.	0.90	n.a.	n.a.	n.a.	0.80	n.a.	n.a.	n.a.	0.46	0.11
Sm2O3	n.a.	0.34	n.a.	n.a.	n.a.	0.34	n.a.	n.a.	n.a.	0.09	b.d.l.
Dy2O3	n.a.	0.19	n.a.	n.a.	n.a.	0.69	n.a.	n.a.	n.a.	0.11	0.02
Er2O3	n.a.	b.d.l.	n.a.	n.a.	n.a.	0.10	n.a.	n.a.	n.a.	0.03	0.05
Yb2O3	n.a.	b.d.l.	n.a.	n.a.	n.a.	0.10	n.a.	n.a.	n.a.	b.d.l.	b.d.l.
Y2O3	0.38	0.41	0.49	1.29	1.01	0.98	1.37	0.04	0.05	0.19	0.07
Sc2O3	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	n.a.
PbO	21.79	17.85	20.2	16.29	17.28	15.37	3.67	24.24	8.06	35.32	2.48
BaO	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.03	b.d.l.	0.75	0.36	b.d.l.	b.d.l.
SrO	b.d.l.	n.a.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	0.26	0.08	n.a.	n.a.
MnO	0.16	0.17	0.15	0.04	0.18	0.39	0.13	0.29	0.16	0.02	3.13
FeO	0.23	0.74	0.20	0.26	0.98	0.33	0.34	4.60	4.32	4.61	2.72
ZnO	0.04	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.01	b.d.l.	0.05	0.07	b.d.l.	b.d.l.
F	1.37	0.51	2.79	3.40	0.71	b.d.l.	2.79	0.11	0.15	b.d.l.	b.d.l.
Feq.	−0.58	−0.24	−1.17	−1.43	−0.30		−1.17	−0.05	−0.06		0.00
Total	91.42	93.76	92.89	92.32	88.63	87.50	86.49	92.14	91.78	93.09	87.37
Nb	1.732	1.747	1.735	1.744	1.748	1.780	1.775	1.5	0.908	1.627	1.240
Ta	0.102	0.093	0.117	0.103	0.118	0.014	0.109	0.108	0.125	0.120	0.095
As			0.002
Zr								0.020	0.025	0.035
Sn	0.121	0.126	0.113	0.133	0.111	0.086	0.103	0.068	0.008	0.019
Ti	0.046	0.034	0.031	0.02	0.021	0.020	0.012	0.050	0.091		0.068
Mg								0.001	0.001
Al					0.001			0.014	0.077
Si								0.239	0.765	0.199	0.548
Sum B	2.001	2.000	1.998	2.000	1.999	2.000	1.999	2.000	2.000	2.000	2.000
Na	0.166	0.346	0.292	0.307		0.075	0.238	0.186	0.130	0.215
Ca	0.017	0.015	0.025	0.029	0.049	0.057	0.224	0.052	0.016		0.001
Th	0.012	0.036	0.026	0.097	0.053	0.064	0.165	0.010	0.002	0.001	0.175
U	0.242	0.194	0.202	0.106	0.118	0.065	0.014	0.248	0.369	0.274	0.411
Bi
La		0.014				0.015				0.009
Ce	0.031	0.058	0.026	0.042	0.051	0.061	0.038	0.083	0.029	0.061
Pr		0.006				0.006				0.012
Nd		0.027				0.023				0.019
Sm		0.010				0.009				0.004
Dy		0.005				0.018				0.004
Er						0.003				0.001
Yb						0.003
Y	0.018	0.018	0.022	0.055	0.043	0.042	0.054	0.002	0.002	0.012	0.003
Sc
Pb	0.508	0.399	0.454	0.351	0.376	0.332	0.073	0.609	0.147	1.079	0.054
Ba						0.001		0.027	0.01
Sr								0.014	0.003
Mn	0.012	0.012	0.011	0.003	0.012	0.027	0.008	0.023	0.009	0.002	0.215
FeO	0.017	0.052	0.014	0.018	0.066	0.022	0.021	0.359	0.244	0.437	0.185
ZnO	0.003							0.004	0.004
Vacancy A	2.974	0.869	0.928	0.992	1.232	1.253	1.165	0.383	1.035	0.000	0.955
Sum A	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.081	2.000
F	0.375	0.133	0.737	0.861	0.182		0.648	0.032	0.031
O	6.134	6.147	6.102	6.029	5.920	5.837	5.883	6.620	5.762	7.158	6.300
Nb + Ta	1.833	1.840	1.852	1.847	1.866	1.894	1.884	1.608	1.033	1.747	1.334
Mn/(Fe + Mn)	0.411	0.191	0.438	0.142	0.158	0.551	0.280	0.059	0.036	0.005	0.538
Ta/(Nb + Ta)	0.055	0.051	0.063	0.056	0.063	0.060	0.058	0.067	0.121	0.069	0.071

,

Table 2. Typical analyses (wt%) and empirical formulae (apfu) of the columbite group of minerals from Madeira. Rem.: Sb, As, Zr, Bi, Ce, Sr, Mg, Cs, Rb, K, and Na in all cases below the detection limits (b.d.l.); n.a.—not analyzed.

Sample	PHR128	PHR128	PHR159	PHR159	PHR160	PHR163	PHR191	PHR242	PHR245	PHR245	PHR247	PHR174
Crystal	3	1	1	1	1	3	1	7	1	1	2	6
Spot	139	144	44	45	10	62	27	156	120	122	37	446
WO3	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.52	b.d.l.	b.d.l.	b.d.l.	b.d.l.
Nb2O5	73.47	70.28	76.62	71.68	73.71	73.63	61.06	75.37	67.83	67.05	66.96	70.89
Ta2O5	4.39	4.56	1.83	6.46	4.74	5.94	5.16	0.62	6.19	6.51	5.16	6.99
As2O5	b.d.l.	b.d.l.	b.d.l.	0.07	0.04	0.03	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	n.a.
SnO2	0.46	2.13	0.23	0.43	0.47	0.15	1.69	0.56	2.17	2.04	2.59	0.13
TiO2	1.32	0.48	0.80	0.39	0.22	0.34	5.18	1.65	1.38	1.39	2.22	1.12
Al2O3	0.03	0.01	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.12	b.d.l.	0.02	0.04	0.05	n.a.
SiO2	b.d.l.	0.28	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.20	0.01	0.15	0.22	0.10	b.d.l
CaO	0.01	b.d.l.	b.d.l.	0.02	0.05	b.d.l.	0.02	b.d.l.	b.d.l.	0.05	b.d.l.	b.d.l.
ThO2	0.03	0.08	b.d.l.	0.07	0.42	0.18	b.d.l.	b.d.l.	0.10	0.07	0.01	0.03
UO2	b.d.l.	0.25	0.05	0.09	b.d.l.	0.04	2.01	0.05	0.19	0.77	1.03	b.d.l.
Y2O3	b.d.l.	b.d.l.	0.02	b.d.l.	0.12	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.02	0.02	0.02
Sc2O3	b.d.l.	0.01	0.01	0.01	0.03	b.d.l.	0.02	0.04	0.01	b.d.l.	0.03	n.a.
PbO	b.d.l.	0.30	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.18	0.02	0.34	0.41	0.16	b.d.l.
BaO	b.d.l.	b.d.l.	0.02	b.d.l.	b.d.l.	0.04	0.02	0.01	0.02	b.d.l.	b.d.l.	b.d.l.
MnO	0.90	9.82	10.66	13.79	13.73	11.91	12.23	7.14	7.95	7.04	4.88	11.05
FeO	19.53	11.17	10.03	6.27	6.63	8.41	10.71	13.68	13.02	13.48	15.53	9.40
ZnO	0.12	0.46	0.44	0.26	0.17	0.30	0.14	0.14	0.11	0.11	0.15	b.d.l.
F	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.45	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.
Feq.						−0.19
Total	100.25	99.83	100.71	99.54	100.31	101.52	98.74	99.81	99.48	99.18	98.87	99.78
W								0.008
Nb	1.887	1.837	1.942	1.876	1.905	1.894	1.613	1.917	1.787	1.778	1.769	1.850
Ta	0.068	0.072	0.028	0.102	0.074	0.092	0.082	0.009	0.098	0.104	0.082	0.110
As				0.002	0.001	0.001
Sn	0.010	0.049	0.005	0.010	0.011	0.003	0.039	0.013	0.050	0.048	0.060	0.003
Ti	0.056	0.021	0.034	0.017	0.009	0.015	0.228	0.070	0.060	0.061	0.098	0.049
Al	0.002	0.001					0.008		0.002	0.003	0.003
Si		0.016					0.011	0.001	0.009	0.013	0.006
Ca				0.001	0.003		0.001			0.003
Th		0.001		0.001	0.005	0.002			0.001	0.001
U		0.003	0.001	0.001			0.026	0.001	0.002	0.010	0.013
Y			0.001		0.004					0.001	0.001	0.001
Sc		0.001			0.001		0.001	0.002			0.001
Pb		0.005				0.005	0.003		0.005	0.006	0.002
Ba						0.001
Mn	0.043	0.481	0.506	0.676	0.665	0.574	0.605	0.340	0.392	0.350	0.242	0.540
FeO	0.928	0.540	0.470	0.304	0.317	0.400	0.523	0.644	0.635	0.661	0.759	0.454
ZnO	0.005	0.020	0.018	0.011	0.007	0.012	0.006	0.006	0.005	0.005	0.006
F						0.081
Ta/(Nb + Ta)	0.035	0.038	0.014	0.051	0.037	0.046	0.048	0.005	0.052	0.055	0.044	0.056
Mn/(Fe + Mn)	0.044	0.471	0.518	0.690	0.677	0.589	0.536	0.346	0.382	0.346	0.241	0.544

,

Table 3. Typical analyses (wt%) and empirical formulae (apfu) of pyrochlore and microlite from Orlovka and Cínovec. Rem.: Sb, Zr, Y, Sr, Zn, Cs, Rb, and K in all cases below the detection limits (b.d.l.); n.a.—not analyzed.

Locality	Orlovka				Cínovec
Sample	O253	O369	O369	O369	5423	5423	5427	5436	4689	4690B
Crystal	3	2	2	2	1	1	2	1	6	1
Spot	204	185	186	194	413	220	4	13	406	7
WO3	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	8.39	4.02	2.58	4.28
Nb2O5	4.39	17.69	22.85	19.92	8.00	7.89	14.02	23.62	33.37	36.88
Ta2O5	72.44	58.28	49.65	55.12	59.16	61.93	6.19	14.72	7.10	6.23
As2O5	b.d.l.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	0.32	0.59	n.a.	b.d.l.
SnO2	1.30	1.31	1.78	0.75	0.96	1.33	0.55	0.39	b.d.l.	0.10
TiO2	0.06	0.07	0.05	b.d.l.	1.84	1.57	0.93	1.22	2.22	3.16
MgO	b.d.l.	b.d.l.	b.d.l.	b.d.l.	n.a.	b.d.l.	0.07	b.d.l.	n.a.	0.04
Al2O3	0.02	b.d.l.	0.04	0.28	n.a.	0.10	0.24	0.47	n.a.	1.04
SiO2	b.d.l.	b.d.l.	b.d.l.	0.83	b.d.l.	b.d.l.	11.13	5.91	5.39	8.13
Na2O	4.58	5.26	5.18	5.37	0.07	0.03	0.09	0.35	0.12	n.a.
CaO	8.39	9.68	8.72	9.82	1.45	2.31	3.26	2.71	3.35	6.31
ThO2	b.d.l.	0.06	0.27	0.15	0.55	0.58	b.d.l.	b.d.l.	0.08	b.d.l.
UO2	5.31	2.86	5.10	1.25	11.39	10.85	38.21	3.71	23.14	12.60
Bi2O3	b.d.l.	b.d.l.	b.d.l.	0.01	n.a.	0.14	b.d.l.	19.16	n.a.	b.d.l.
La2O3	n.a.	n.a.	n.a.	n.a.	0.03	n.a.	n.a.	n.a.	b.d.l.	n.a.
Ce2O3	b.d.l.	b.d.l.	0.19	0.12	1.05	1.41	n.a.	n.a.	b.d.l.	n.a.
Pr2O3	n.a.	n.a.	n.a.	n.a.	0.21	n.a.	n.a.	n.a.	b.d.l.	n.a.
Nd2O3	n.a.	n.a.	n.a.	n.a.	0.46	n.a.	n.a.	n.a.	0.05	n.a.
Sm2O3	n.a.	n.a.	n.a.	n.a.	b.d.l.	n.a.	n.a.	n.a.	b.d.l.	n.a.
Dy2O3	n.a.	n.a.	n.a.	n.a.	b.d.l.	n.a.	n.a.	n.a.	b.d.l.	n.a.
Er2O3	n.a.	n.a.	n.a.	n.a.	b.d.l.	n.a.	n.a.	n.a.	0.21	n.a.
Yb2O3	n.a.	n.a.	n.a.	n.a.	b.d.l.	n.a.	n.a.	n.a.	0.25	n.a.
Y2O3	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.32	b.d.l.
Sc2O3	0.05	0.02	0.02	0.03	n.a.	0.03	0.15	0.01	n.a.	0.03
PbO	0.14	0.49	0.72	0.43	2.79	2.13	0.49	5.46	b.d.l.	4.59
BaO	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.22	0.03	0.62	2.57	1.06	5.53
MnO	0.01	0.10	0.04	0.02	0.19	0.06	0.15	0.15	4.15	0.69
FeO	0.05	b.d.l.	b.d.l.	0.03	0.71	0.79	0.88	2.97	3.75	0.68
F	2.31	2.93	2.59	3.12	b.d.l.	b.d.l.	0.30	b.d.l.	b.d.l.	0.32
Feq.	−0.97	−1.23	−1.09	−1.31			−0.13			−0.14
Total	98.08	97.52	96.21	95.94	89.09	91.17	85.84	88.04	87.13	90.44
W							0.189	0.087	0.054	0.085
Nb	0.178	0.655	0.837	0.708	0.337	0.321	0.552	0.894	1.219	1.360
Ta	1.769	1.298	1.094	1.178	1.499	1.515	0.147	0.335	0.156	0.081
As							0.014	0.026		0.012
Sn	0.047	0.043	0.058	0.023	0.036	0.048	0.019	0.013		0.014
Ti	0.004	0.005	0.003	0.000	0.129	0.106	0.061	0.077	0.135	0.118
Mg							0.009	0.000		0.001
Al	0.002		0.004	0.026		0.011	0.024	0.046		0.096
Si				0.065			0.970	0.495	0.436	0.222
Sum B	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
Na	0.798	0.835	0.814	0.818	0.013	0.006	0.015	0.057	0.019	0.014
Ca	0.807	0.849	0.757	0.827	0.145	0.223	0.304	0.243	0.290	0.051
Th		0.001	0.005	0.003	0.012	0.012			0.001
U	0.106	0.052	0.092	0.022	0.236	0.217	0.741	0.069	0.416	0.012
Bi						0.003		0.414		0.053
La					0.001
Ce			0.006	0.003	0.036	0.046
Pr					0.007
Nd					0.015				0.001
Sm
Dy
Er									0.005
Yb									0.006
Y									0.014
Sc	0.004	0.002	0.001	0.002		0.003	0.011	0.001		0.020
Pb	0.003	0.011	0.016	0.009	0.070	0.052	0.012	0.123		0.011
Ba					0.008	0.001	0.021	0.084	0.034	0.016
Mn	0.001	0.007	0.003	0.001	0.015	0.005	0.153	0.109	0.284	0.622
Fe				0.002	0.055	0.059	0.064	0.208	0.253	0.290
vacancy A	0.277	0.243	0.306	0.313	1.411	1.373	0.679	0.692	0.689	0.911
Sum A	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
F	0.656	0.759	0.664	0.775			0.082
O	6.405	6.370	6.352	6.237	5.766	5.790	6.556	6.198	6.468	5.871
Nb + Ta	1.947	1.953	1.932	1.886	1.836	1.836	0.699	1.229	1.375	1.441
Mn/(Fe + Mn)	0.139	1.000	1.000	0.357	0.213	0.078	0.705	0.344	0.528	0.682
Ta/(Nb + Ta)	0.908	0.665	0.567	0.625	0.816	0.825	0.210	0.273	0.113	0.056

and

Table 4. Typical analyses (wt%) and empirical formulae (apfu) of the columbite group of minerals from Orlovka and Cínovec. Rem.: Sb, As, Zr, Si, Bi, Ce, Y, Pb, Cs, Rb, K, and Na in all cases below the detection limits (b.d.l.); n.a.—not analyzed.

Locality	Orlovka			Cínovec
Sample	O-222	O-222	O-353	13	13	13	4972A	4972A	5427
Crystal	1	1	7	2	2	2	1	1	2
Spot	197	198	169	3	4	5	6	7	3
WO3	b.d.l.	b.d.l.	10.15	1.34	0.35	8.20	4.00	3.96	2.66
Nb2O5	69.96	40.29	19.86	64.87	60.25	44.13	55.93	56.36	51.94
Ta2O5	10.09	41.37	45.28	10.26	16.04	23.58	16.95	16.82	25.28
SnO2	0.03	b.d.l.	0.15	0.39	0.35	0.61	0.33	0.25	0.16
TiO2	0.19	0.17	0.63	2.08	2.33	3.20	1.75	1.86	1.14
MgO	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.01	b.d.l.	b.d.l.	b.d.l.	b.d.l.
Al2O3	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.01	0.01	b.d.l.	0.03	b.d.l.
CaO	0.01	0.07	0.00	0.04	0.04	0.01	b.d.l.	b.d.l.	0.02
ThO2	b.d.l.	b.d.l.	0.15	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.
UO2	0.05	0.11	0.09	0.08	0.09	0.14	0.02	b.d.l.	0.05
Sc2O3	0.03	0.06	0.13	0.68	0.95	1.41	0.84	0.77	0.63
BaO	b.d.l.	b.d.l.	0.02	0.09	0.13	0.20	0.07	0.13	0.11
MnO	13.07	11.70	7.93	12.70	14.08	10.20	7.61	7.47	9.91
FeO	7.48	5.87	9.21	7.93	5.38	8.44	12.01	11.97	8.57
ZnO	0.04	b.d.l.	b.d.l.	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.
F	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	0.05	b.d.l.
Feq.								−0.02
Total	100.98	99.64	93.60	100.45	100.01	100.12	99.52	99.64	100.46
W			0.205	0.020	0.005	0.134	0.063	0.062	0.043
Nb	1.830	1.229	0.699	1.706	1.620	1.256	1.542	1.550	1.464
Ta	0.159	0.759	0.959	0.162	0.259	0.404	0.281	0.278	0.428
Sn	0.001		0.005	0.009	0.008	0.015	0.008	0.006	0.004
Ti	0.008	0.008	0.037	0.091	0.104	0.152	0.080	0.085	0.053
Mg			0.001
Al					0.001	0.001
Ca	0.001	0.005		0.002	0.003	0.001			0.002
Th			0.003
U	0.001	0.002	0.001	0.001	0.001	0.002			0.001
Sc	0.001	0.004	0.009	0.034	0.049	0.077	0.045	0.041	0.034
Ba			0.001	0.002	0.003	0.005	0.002	0.003	0.003
Mn	0.641	0.669	0.523	0.626	0.709	0.544	0.393	0.385	0.523
FeO	0.362	0.331	0.600	0.386	0.267	0.444	0.613	0.609	0.447
ZnO	0.002
F								0.010
Ta/(Nb + Ta)	0.080	0.382	0.578	0.087	0.138	0.243	0.154	0.152	0.226
Mn/(Fe + Mn)	0.639	0.669	0.466	0.619	0.726	0.550	0.391	0.387	0.539

. Selected relations among chemical elements are shown in Figure 7, Figure 8 and Figure 9.

After a brief confrontation of chemical data with BSE images, we divided analyzed pyrochlores into four types. Types 1–3 include pyrochlores from the core granite; type 4 includes pyrochlore from the border facies:

(1) Fresh or only slightly altered type 1 pyrochlore from the core albite granite (typically crystals like those in Figure 3a–d); (2) moderately altered type 2 pyrochlore from the core albite granite (typically crystals from Figure 3e,f and Figure 4); (3) strongly altered type 3 pyrochlore from the core albite granite and hypersolvus granite (typically grains from Figure 5); and (4) secondary type 4 pyrochlore from the border albite granite (Figure 6b). Typical analyses are shown in Table 1, and some relations among elements are shown in Figure 7. Type 1 pyrochlore displays high sums of Nb + Ta at site B (>1.8 apfu), i.e., it is poor in Ti and Si, relatively rich in Na at site A (0.2–0.8 apfu) and in F at site Y (0.15–0.6 apfu). Type 2 pyrochlore is depleted in Nb + Ta (1.5–1.8 apfu) and Na (mostly 0.05–0.20 apfu) and relatively enriched in REE (up to 0.1 apfu sum of REE). The strongly altered type 3 pyrochlore is poor in Nb + Ta (1.8→1.0), REE, Y, and Na, free of F, and simultaneously but irregularly enriched in Si (up to 0.8 apfu), Pb (up to 1.4 apfu) and sometimes also in Al.

The contents of U and Sn varied irregularly regardless of the pyrochlore type from zero to 0.25 apfu and 0.18 apfu, respectively. Type 4 pyrochlore includes inhomogeneous Si-rich (0.6–1.3 apfu), Th-, U-, and vacation-rich (A = 0.6–1.0) pyrochlores replacing columbite in the border albite granite.

Among the REEs, Ce is the most abundant, usually reaching ca. 1 wt% Ce₂O₃ in fresh primary crystals and up to 5 wt% Ce₂O₃ in altered type 2 pyrochlore. In strongly altered type 3 pyrochlore, its contents are again reduced to 1–3 wt% Ce₂O₃. The contents of other REEs are distinctly lower: max. 1 wt% La₂O₃, Nd₂O₃, and Dy₂O₃, 0.4–0.5 wt% Sm₂O₃, and Pr₂O₃, and 0.25 wt% Er₂O₃. The chondrite-normalized distribution curve is usually rugged but with generally equal values for the analyzed LREE and HREE in type 1 pyrochlore (Figure 8a). In altered type 2 and 3 pyrochlores, the HREE contents are lower, and the curve becomes clearly LREE-dominant (Figure 8b–d). Secondary type 4 pyrochlore is REE-poor (Figure 8d).

CGM in the core albite granite (ca. 55 analyses, Table 2) are represented by columbite-Fe and columbite-Mn originating by a replacement of pyrochlore. The Mn/(Fe + Mn) (further #Mn) value varied between 0.2–0.7, while Ta/(Nb + Ta) (further #Ta) is low, around 0.05, throughout the whole dataset (Figure 9). Among minor elements, the most common are Sn (max. 4.2 wt% SnO₂), U (max. 2.8 wt% UO₂), and Ti (max. 2.5 wt% TiO₂).

Primary-looking columbite in the border albite granite has #Mn = 0.37–0.54 and #Ta = 0.05–0.06. Among minor elements, only Ti (1–2.4 wt% TiO₂) and Sn (0.1–0.7 wt% SnO₂) exceed the limits of detection. Thus, primary-looking columbite is poorer in Sn than columbite originated through pyrochlore replacement, which immediately incorporated Sn liberated from decomposed pyrochlore.

In general, the low Ta/Nb values in all CGM mimic those in pyrochlore, while the fluctuating Mn/Fe values do not correlate with pyrochlore composition and indicate locally different conditions for Mn and Fe transport during alteration.

4.2. The Distribution of PSGM Across the Madeira Pluton

The vast majority of the Madeira pluton is composed of the core facies of albite granite. According to the position within the pluton, our samples can be divided into three groups: (i) borehole 00/400W in the eastern part of the pluton close to the transition to the border albite granite (PHR159, 160, 161 and 163), (ii) borehole 250N/950W in the northwestern part of the pluton near the contact with the rapakivi granite (PHR242, 243, 244 and 245), and (iii) samples from outcrops in the northern and central parts of the pluton (PHR82A, 127, 128, 171 and 246). All samples from borehole 00/400W contain fresh pyrochlore relatively rich in Na, F, and Th, with stable Pb contents around 20 wt% PbO, while borehole 250N/950W contains mostly moderately altered pyrochlore poor in Na, enriched in Ca and Ce, and mostly depleted in Pb. Another difference between the two boreholes is the common appearance of sulfides in 250N/950W. All samples from the outcrops contain moderately to strongly altered pyrochlore poor in Na, Ca, F, and Th, distinctly enriched in Si, and with generally high but variable contents of Pb, REE, Y, and Fe showing a strong negative correlation between Pb and Y.

The border albite granite (PHR174) is the only granite containing secondary type 4 pyrochlore originated by a replacement of older columbite.

Altered pyrochlore from the hypersolvus granite (PHR191) is enriched in Ti (3–4 wt% TiO₂), U (10–13 wt% UO₂), REE, Ba, and Fe.

4.3. Chemical Zoning of Pyrochlore Crystals at Madeira

The majority of pyrochlore crystals from the Madeira albite granite show a clear zoning in BSE. This zoning usually consists of 3–5 distinct zones in relatively fresh crystals (Figure 3b,c,f) but is sometimes noticeable even after strong alteration (Figure 4b,d,f). To identify the chemical basis of the zonal texture, we conducted a detailed chemical mapping of three selected crystals, two with clear primary zoning (Figure 10 and Figure 11) and one strongly altered (Figure 12).

The subhedral pyrochlore crystal from sample PHR160 (Figure 10) is associated with nearly euhedral zircon and embedded in a large albite crystal (orange in Figure 10 Na). Later, under the action of F-rich fluids along the pyrochlore–albite interface, albite was replaced by cryolite (bright yellow in Figure 10 Na) and pyrochlore by columbite (yellow in Figure 10 Fe). Internal zoning of pyrochlore is displayed by the negatively correlated Th-U pair and subordinately by Nb and Pb.

The euhedral pyrochlore crystal (PHR82A, Figure 11) is embedded in a coarse-grained cryolite crystal with no signs of mutual reaction. The central part of the pyrochlore crystal is replaced with columbite along narrow cracks. Internal zoning is affected by variations in all measured elements: as expected, Nb negatively correlates with Si, while U and Pb correlate positively. Th generally follows the trend of Nb in the crystal center but not in the rims. Ca correlates negatively with U and Th, being replaced with both. Na is slightly enriched along the rims and, although it tends to be typical of primarily magmatic pyrochlore, it is most likely the result of diffusion from the surrounding cryolite in this case.

A strongly altered but morphologically still automorphic pyrochlore crystal from sample PHR247 (Figure 12) is in 85% converted to columbite (light blue in Figure 12 Nb). Only 10% remain in the form of U, Pb, (Th,Na)-enriched pyrochlore. The rest is a U, Th-silicate, bright in BSE as it hosts elements released from altered pyrochlore and being not compatible with newly formed columbite-Fe.

The elemental maps of the crystals generally confirm, as already indicated by individual spot analyses, the significantly negative correlation of U and Th and a predominantly positive correlation between Nb and Th. Generalized zoning (Figure 10 and Figure 11) shows a relatively Nb,Th-poorer, U-enriched core, a U-poor, Nb,Th-enriched inner rim, and again a Nb,Th-poor, U-enriched outer rim.

4.4. Microlite and CGM from Orlovka

Microlite was found together with CGM, wolframite, thorite, monazite, pyrite, and fluorite in the lepidolite granite forming the uppermost part of the granite cupola and in flat, layered aplite/pegmatite bodies crossing the lepidolite granite. In both rock types, microlite forms mostly subhedral to euhedral crystals with distinct zoning (Figure 13a,b) and only subordinate signs of alteration. Alteration is mainly manifested by the formation of zones of high porosity or by the diffuse washing out of some elements along cracks (darkening in the BSE, Figure 13b). Anhedral grains showing strong alteration (dissolution) were found only scarcely (Figure 13c).

Chemical composition corresponds to fluoro-natro microlite or fluoro-calcio microlite (Figure 7). The site B is occupied almost exclusively by Ta and Nb (Ta + Nb > 1.9 apfu). Microlite from Lpd-Toz granite is rather richer in Ta than that from lined rock (Ta/(Nb + Ta) = 0.90–0.95 vs. 0.52–0.83); the first variety with a small Ti admixture (around 0.04 apfu), while the second variety with Sn (around 0.04 apfu). The A site is occupied with mostly nearly equal contents of both Na (0.79–0.84 apfu) and Ca (0.54–0.85 apfu) with some admixture of U (max. 0.1 apfu) and Sr (max. 0.04 apfu in the Ta-richer variety) or Pb (0.02 apfu in their second variety). Fluorine dominates at site Y (mostly > 0.6 apfu). The contents of all REEs are usually lower than the EPMA detection limits.

CGM form (i) subhedral, distinctly zoned crystals with Ta-enriched rims (Figure 13d,e) or (ii) oblong to irregular aggregates, homogeneous in BSE, sometimes associated with wolframite (Figure 13f). The compositions of CGM show a distinct trend from columbite-Mn to tantalite-Fe with only a small overlap to tantalite-Mn (Figure 9a). While CGM of the second type correspond to tantalite-Fe in all cases, crystals of CGM of the first type correspond mostly to columbite-Mn with only thin rims of tantalite-Fe. Tantalite-Fe of the second type is enriched in W (up to 0.3 apfu), while low admixtures of Sn and Ti (both < 0.05 apfu) were found in both CGM types.

4.5. Microlite, Pyrochlore and CGM from Cínovec

Within the Cínovec Li-Sn-W deposit, microlite and pyrochlore were scarcely found in two different sites of the deposit, in distinct mineral assemblages. Microlite (Ta/(Nb + Ta) = 0.69–0.86) was found in zinnwaldite granite in the highest part of the granite cupola, no deeper than 150 m below the upper contact associated with CGM, zircon, xenotime, monazite, cassiterite, scheelite, pyrite, fluorite, bastnaesite, and Bi minerals. Microlite forms rare homogeneous subhedral grains (Figure 14a), more often anhedral, strongly altered grains replaced with cassiterite, scheelite, or columbite (Figure 14b,c). In addition to Nb and Ta (Nb + Ta mostly 1.5–1.8 apfu), site B is occupied with Ti (max. 0.18 apfu), W and Sn (both max. 0.06 apfu), and locally also Si (max. 0.58 apfu). Calcium and U (both mostly 0.2–0.3 apfu) with some Na, REE, and Pb are located at site A, while max. 0.2 apfu F was found at site Y. The contents of REE are generally low, usually with a predominance of HREE over LREE (Figure 8e).

Pyrochlore rarely occurs in the upper part of the granite cupola together with microlite (depth < 150 m, Ta/(Nb + Ta) = 0.24–0.34) and somewhat more often in deeper parts of zinnwaldite granite (depths of 700–735 m, Ta/(Nb + Ta) = 0.06–0.22) in an assemblage of CGM, zircon, xenotime, cassiterite, bastnaesite and REE-fluoride; and also in the underlying biotite granite in an assemblage with Nb,Ta-rutile, zircon, xenotime, and monazite. The B-site is occupied, along with Nb and Ta (0.80–1.45 Nb + Ta apfu), by Si (max. 0.8 apfu), Ti and W (max. 0.2 apfu), and some Sn and Al. Calcium (max. 0.57 apfu), U (max. 0.40 apfu), Ba (max. 0.42 apfu), and Fe (max. 0.29 apfu) dominate at the A site, and water and/or vacancies in the Y site. The REE distribution curves are variable, generally showing the dominance of LREE (Ce and Pr) over HREE (Figure 8e). There is no statistical difference in A- and Y-site occupancy between pyrochlores from the upper and the deeper portions.

Unlike PSGM, CGM are present throughout the known vertical profile of zinnwaldite granite down to the depth of 735 m, including all greisen bodies. The principal host of Nb and Ta in the underlying biotite granite is Nb,Ta-enriched rutile. CGM appear in two distinct forms: (i) euhedral to subhedral, zoned crystals, often with irregular, inhomogeneous overgrowths (Figure 14e), and disseminated aggregates of small, needle-like crystals (Figure 14f). CGM and PSGM are spatially strictly separated, and their mutual relationship cannot be determined. The only grains found to be at mutual contact (Figure 14d) are both replaced with scheelite and cannot be interpreted.

In the CGM, Nb always prevails over Ta (Ta/(Nb + Ta) = 0.05–0.30), while the Mn/Fe values change from columbite-Fe to columbite-Mn (Mn/(Fe + Mn) = 0.20–0.85). While the Ta/(Nb + Ta) values in granite and greisen fluctuated with no relation to the depth, the Mn/(Fe + Mn) values correspond well to the petrological change from altered granite of the cupola to generally fresh zinnwaldite granite at ca. 300 m depth (from 0.5–0.7 to 0.20–0.35). In greisen, higher Mn/Fe values, i.e., columbite-Mn, were found in the southern cupola, while columbite-Fe prevails in the central cupola along borehole CS-1. Among other elements, the contents of W slightly increase with depth, while Ti, Sn, and Sc vary randomly.

5. Discussion

5.1. Chemical Changes During PSGM Alteration: Generally vs. Locally Typical Elements

Lumpkin et al. [8,9] defined the alteration of pyrochlore and microlite as a complex process, generally decreasing the contents of Nb, Ta, Na, Ca, F and sums of A-site cations, and increasing the contents of U, Pb, Bi, Sb, Fe, Mn, Sr, Ba, water and other exotic elements. These authors [8,9] distinguished two stages of alteration in many cases: the first, a high-temperature one, immediately following the full crystallization of the rock (especially the loss of Na and F, and possibly a gain of REE, Sr, Ba, Fe), and the second, a low-temperature one, occurring near the actual Earth’s surface (<100 °C) and distinguished by a loss of all A-site cations, and a strong gain of A- and Y-site vacancies.

In the Madeira core albite granite, two stages of pyrochlore alteration can be reliably distinguished, each having a specific chemical character. The first stage is characterized by the strong contribution of Fe and Mn and, to a lesser extent, of Ca and REE. While Fe and Mn are mainly bound to newly formed columbite, and their contents increase only slightly in pyrochlore, Ca and REE are incorporated exclusively in pyrochlore. Along with this, the contents of F, Na, and Th decrease, and the amount of A-site vacancies increases (for values see

Table 5. Means of the contents of selected elements (wt%) and sums of A-site cations (apfu) in the studied pyrochlores and microlites. Remarkably high values are highlighted in bold; b.d.l.—below the detection limit.

Type	Madeira Pcl 1	Madeira Pcl 2	Madeira Pcl 3	Madeira Pcl 4	Cínovec Mic	Cínovec Pcl	Orlovka Mic
n	56	54	69	4	8	14	21
WO3	b.d.l.	b.d.l.	b.d.l.	b.d.l.	b.d.l.	3.20	b.d.l.
Ta2O5	4.86	4.93	4.80	2.96	56.51	9.94	65.29
Nb2O5	47.66	48.69	36.64	20.09	6.91	30.16	9.71
SnO2	2.89	3.34	1.67	b.d.l.	0.86	0.13	1.00
TiO2	0.39	0.55	0.89	1.02	1.83	4.32	0.22
Al2O3	b.d.l.	0.02	0.24	b.d.l.	0.30	0.44	0.13
SiO2	0.11	0.14	3.23	15.81	1.06	6.86	0.27
Na2O	1.93	0.55	0.43	0.00	0.05	0.69	4.68
CaO	0.44	0.86	0.42	0.06	2.34	4.04	8.53
ThO2	3.60	3.03	1.37	16.36	0.39	0.50	0.08
UO2	6.42	7.09	10.30	20.95	11.72	15.31	3.56
Ce2O3	1.38	2.23	1.36	0.05	0.97	0.01	0.04
Y2O3	0.75	0.97	0.22	0.27	0.00	0.65	b.d.l.
PbO	19.22	13.10	25.84	1.66	2.49	1.08	0.34
BaO	b.d.l.	b.d.l.	0.08	b.d.l.	0.85	4.53	b.d.l.
SrO	0.03	0.04	0.07	b.d.l.	b.d.l.	0.37	0.36
MnO	0.13	0.25	0.37	0.80	0.14	0.77	0.03
FeO	0.42	1.16	3.05	0.87	0.77	3.60	0.04
ZnO	0.03	0.16	0.39	0.04	b.d.l.	0.01	b.d.l.
F	1.67	1.26	0.34	0.70	0.02	0.09	2.59
Total	92.63	89.38	92.17	82.27	87.88	87.21	96.96
A site	1.07	0.84	1.35	0.81	0.70	1.17	1.68

).

In the second stage, the system was further enriched in Fe, Pb, and Si to a substantial extent. Iron (and Mn) further entered the pyrochlore lattice but concentrated on rims of Fe oxides/hydroxides around remnants of pyrochlore grains. During this stage, F, Na, and Th in pyrochlore were further lowered, followed by a decrease in Nb, Sn, Y, and REE and an increase in Si, Al, U, Zn, and especially Pb. The contents of Ta were generally stable throughout the whole process, varying between 4.0–5.5 (several spots up to 7) wt% Ta₂O₅.

The two above-defined distinct stages of pyrochlore alteration are in good agreement with the two alteration stages recently defined from Madeira pyrochlore by Bollaert et al. [29]. Secondary pyrochlore, in the border albite granite facies, is characterized by unusually high Si, Th, and U contents.

The common alteration trend of pyrochlore/microlite is towards an increase in the proportion of vacancies A and Y sites [8,9], i.e., towards kenopyrochlore/kenomicrolite. This is not the case of Madeira albite granite. The occupancy of site A was low already in primary pyrochlore (mean at 1.07 apfu); it decreased to 0.84 apfu in the first step but then increased surprisingly to 1.35 apfu in the second alteration stage. Here, pyrochlore absorbed a substantial amount of Pb and U, and also a small amount of Zn, during the alteration and partial columbitization via the process of dissolution-reprecipitation; the occupancy of site A during the alteration remained the same or even increased (Figure 15a,b). While some of the studied samples contain sulfides, including galena and sphalerite, the second alteration stage can be attributed to the hydrothermal sulfide stage of the deposit evolution. No chemical processes potentially attributed to near-surface weathering (i.e., the second alteration stage of Lumpkin [6,7]) were found.

While Zn is a typical local minor element for Madeira pyrochlore and columbite, Sc plays a similar role in the Nb-Ta oxides at Cínovec [68]. At Orlovka, no locally specific element has been identified.

5.2. Time Relation Between PSGM and CGM

The solubilities of columbite/tantalite, wodginite, and pyrochlore/microlite in natural melts are similar, and it seems that the availability of elements like Na, Ca, Fe, Mn, Sn, and Ti, along with the excess of alumina or alkalis, controls the crystallization of Nb,Ta-minerals at mineral deposits of different types [69]. Based on the published observations in pegmatites, CGM usually precede PSGM, the latter often arising from hydrothermal alteration of the former. Such a relationship has been observed in NYF pegmatites [13,55] as well as in LCT pegmatites [11,16,17,23]. The opposite relationship, i.e., the replacement of PSGM by younger hydrothermal columbite, was observed in different rock types like the Beauvoir rare-metal granite [10], Saint Honoré alkali syenite [14], Penchenga and Miaoya carbonatites [18,25], in all cases in non-pegmatitic environments, and in the Madeira pluton [12]. At the Katugin deposit, primary magmatic pyrochlore and CGM crystallized simultaneously from a F-supersaturated melt [15]. The occurrence of CGM as a single Nb,Ta host and its association with Nb,Ta-rutile are quite common [51], whereas the occurrence of PSGM as a single Nb,Ta host is rarer and restricted to peralkaline rocks and kimberlites, being rarely encountered in NYF pegmatites [20].

In all observed cases in the Madeira hypersolvus and core albite granites, the CGM (columbite-Fe and columbite-Mn) are formed as products of the replacement of primary pyrochlore. The replacement usually started as thin stringers in the crystal core (Figure 3f) and continued with a more massive replacement of any of the outer zones (Figure 4b), possibly also with a dissolution/precipitation on the crystal surface (Figure 3d and Figure 4c). The final stages of the transition represent almost fully columbitized pyrochlore crystals (Figure 4f and Figure 5f).

A different evolution is observed in the border facies where columbite forms aggregates of primary tabular crystals. During their alteration, thin pyrochlore coatings on the columbite surface and fillings of interstices also originated, along with inhomogeneous Si,Th,U-dominated porous colander-shaped matter replacing the whole columbite volume.

At Orlovka and Cínovec, we observed different intensities of microlite/pyrochlore alteration, but we never observed its replacement with columbite.

5.3. Chemistry of PSGM vs. Associated CGM

The concentrations of Ta in Madeira pyrochlore are generally low and stable at 0.10–0.12 apfu. Due to the fluctuation in Nb contents from 1.7–1.8 apfu in fresh crystals to 1.2–1.6 apfu in strongly altered ones, the Ta/(Nb + Ta) value slightly varied from 0.05 to 0.10. In columbite, this value varied mostly between 0.03 and 0.06, which corresponds to the values in fresh pyrochlore (compare Figure 7a and Figure 9a).

On the other hand, Fe and Mn contents in pyrochlore and the Mn/(Fe + Mn) values for both minerals varied substantially. The contents of Fe and Mn in fresh pyrochlore are often negligible but increase to as much as 0.4 apfu and 0.06 apfu in altered grains, respectively, keeping a strong predominance of Fe over Mn in all cases. The analyses of columbite, in contrast, revealed a wide range of Mn/(Fe + Mn) values from columbite-Fe to columbite-Mn (Mn/(Fe + Mn) = 0.05–0.75). A narrower range (0.37–0.56) was found in the case of primary columbite from the border albite granite.

Generally, columbite is slightly Ta-depleted and Mn-enriched if compared with associated pyrochlore (crystals from Figure 3 and Figure 4). It has to be, however, mentioned that the relative enrichment of pyrochlore in Fe may be a very late event related to Fe-rich hydrothermal fluid permeating granite and crystallizing hematite and/or limonite halos around some pyrochlore grains (Figure 5b,d,e).

At the Orlovka deposit, #Ta values in two varieties of microlite (Ta/(Nb + Ta) = 0.90–0.95 in layered zinnwaldite pegmatite/aplite and 0.52–0.83 in lepidolite granite) are conformable with those in associated CGM: tantalite-Fe with #Ta of 0.57–0.65, and columbite-(Mn) with #Ta of 0.1–0.5. In both cases, #Ta values in microlite are distinctly higher than those in associated CGM, similar to the majority of pegmatites [27]. Both microlite types are Fe-, and Mn-poor, often below EPMA detection limits, which makes the computation of the Mn/(Fe + Mn) values pointless. On the other hand, the #Mn in CGM correlated clearly negatively with #Ta (Figure 9a), which is rather unusual [70].

The contents of Ti, Sn, and W in secondary columbite roughly equal to, or only slightly exceed, those in associated pyrochlore. This indicates that these elements are only limited in their mobility, being released from pyrochlore and immediately incorporated into new columbite within the dissolution/precipitation process.

Uranium from decomposed pyrochlore entered the lattice of newly formed columbite only to a limited extent. A larger part was incorporated into residual U-enriched pyrochlore or formed small inclusions of a hydrated U, Si-rich phase in residual pyrochlore.

Among minor elements, Sc is a typical admixture in some columbites, especially those from phosphate pegmatites worldwide [71] and from greisenized granites in the Erzgebirge [51,66]. Scandium has rarely been reported from pyrochlore/microlite. In our samples from Cínovec and Orlovka, Sc in pyrochlore/microlite amounted at 0.37 and 0.07 wt% Sc₂O₃, respectively, which is ca. five times less than in associated columbite/tantalite. Pyrochlore and columbite from Madeira are Sc-free.

5.4. Silica in PSGM

Significantly elevated contents of Si were found only in the studied pyrochlore, not in microlite. At Madeira, altered pyrochlore of type 3 contains 2–5 (max. 14) wt% SiO₂, i.e., 0.24–0.52 (max. 0.97) apfu Si. Even higher Si contents (up to 16.3 wt% SiO₂) were found in secondary, strongly inhomogeneous type 4 pyrochlore. Here, however, the presence of submicroscopic inclusions of any SiO₂ phase cannot be ruled out.

At Cínovec, the contents of 5–10 wt% SiO₂ (0.35–0.7 apfu Si) were found. The contents of Si show a good negative correlation with Nb + Ta contents, supporting the accommodation of Si at site B (Figure 7b and Figure 15c).

According to a detailed structural study of Si-rich pyrochlore (up to 13 wt% SiO₂) from Mariupol, Ukraine (Dumaňska-Slowik et al. [53]), Si is not included in pyrochlore structure but forms microinclusions of amorphous SiO₂. In contrast, Chakhmouradian and Mitchell [52] interpreted the high negative correlation between Si and Nb + Ta + Ti in pyrochlore from Lovozero as indicative of Si incorporation at the B site. And also, Bindi et al. [57] reported 0.23 apfu Si incorporated at the B site in plumbomicrolite from Keivy, based on a detailed structural study. So, this partial issue calls for further structural investigation.

5.5. Pb-Dominant Pyrochlore/Microlite

Kenoplumbomicrolite, the Pb-dominant member of MGM, has been reported from the Ploskaya pegmatite, Kola Peninsula (up to 1.30 apfu Pb [57,72]), and from the Maria Elena pegmatite, Argentina (up to 1.167 apfu Pb [16]). Kenoplumbopyrochlore was found in NYF pegmatite in the Svalbard Archipelago, Norway (up to 0.906 apfu Pb [11]), in nepheline syenite at Lovozero (up to 0.875 apfu Pb [52]), and in the Khalzan Buregte pegmatite in Mongolia (96% of the Pb endmember [73]).

At Madeira, Pb is generally present in all forms of pyrochlore. Minuzzi et al. [12] published analyses with max. 33.67 wt% PbO. According to our data, the supposed primary pyrochlore of type 1 usually contains 15–25 wt% PbO (0.25–0.60 apfu Pb), and the altered type 3 pyrochlore (fluoro-plumbo-pyrochlore and keno-plumbo-pyrochlore) contains as much as 47 wt% PbO, i.e., 1.407 apfu Pb (Table 1). The Madeira albite granite probably has the highest known concentration of plumbopyrochlore in the world.

5.6. Comparison of Pyrochlore Supergroup Minerals from Different Igneous Lithologies

Principal chemical characteristics of pyrochlore and microlite from different lithologies are shown in Figure 7 and Figure 16. The occurrence of betafite is limited mainly to some carbonatites [18] and will not be discussed here.

Niobium and Ta are generally incompatible elements in silicate melts, and their contents increase with ongoing magmatic fractionation. Along with the increase in alumina saturation, i.e., with the transition from peralkaline to subaluminous and to peraluminous environment, the fractionation between Nb and Ta increases, and Ta becomes relatively enriched with respect to Nb. This is mineralogically expressed in the domination of pyrochlore and columbite-Fe in peralkaline granites and syenites (such as Madeira, Nechalacho [74], or Saint-Honoré [14]) and in subaluminous rare-metal granites (such as Cínovec), or even NYF-type pegmatites (Emeishan, China [20], Orange River, South Africa [24]), and in the dominance of microlite and Mn, Ta-enriched varieties of CGM in peraluminous granites like Beauvoir [10] or Penouta [75], and LCT pegmatites (Greer Lake [27] or La Canalita [22]) (compare Figure 7a and Figure 16a). In carbonatites, the contents of Ta in pyrochlore are consistently very low, with #Ta <0.03 (Catalao in Brazil [4,5] or the Yenisei Ridge, Russia [25]).

Reasons for Ta enrichment, although repeatedly studied, are still explained in different ways. While Linen [76] emphasized the combined effects of F and Li, several other authors [77,78,79] assume a major influence of increasing peraluminity, and Ballouard et al. [80] see the cause in the separation of melt and fluid during magmatic/hydrothermal transition. However, it should be emphasized that all referred experiments were carried out with Mn-endmembers (columbite-Mn, tantalite-Mn), i.e., without the presence of Fe. In contrast, natural systems usually contain Fe > Mn, which can significantly affect the results.

Na-, Ca- and F-dominated endmembers are the most common constituents of the magmatic pyrochlore, microlite, and betafite groups of minerals elsewhere. Nevertheless, the ratio between Na and Ca varies in different lithologies considerably (Figure 7e,h and Figure 16e,h): for example in the Katugin deposit, a Na, F-super enriched system generally the most similar to the Madeira pluton, magmatic natro-fluoro pyrochlore is strongly Na-dominant (0.8–1.3 apfu Na [15]), while pyrochlore from alkaline granites, most of excerpted carbonatites [4,5,25] and some pegmatites [9,20,22,23,56] contains ca. 1 apfu Ca at widely variable Na contents. Nevertheless, the majority of pegmatites contain Na-poor pyrochlore with strongly divergent Ca contents (<0.1 apfu Na, 0–1.4 apfu Ca) [11,13,16,17]. A depletion in F during the alteration is a common feature of PSGM from all lithologies (Figure 7h and Figure 16h), as already stated by Lumpkin et al. [8,9].

In our dataset (Figure 7e), microlites from Orlovka and Beauvoir are dominated by the Na-Ca composition; however, pyrochlores from Cínovec and especially those from Madeira are Na,Ca-poor, with much higher contents of Pb, REE, Y, U, and Th. These, with the exception of Pb, trivalent, and tetravalent elements, are not ideal for accommodation A- and B-sites in pyrochlore. Nevertheless, all these elements at Madeira, and to a lesser extent also at Cínovec, make up a substantial part of the composition of primary pyrochlore already, i.e., of crystals with no or only minimum signs of alteration. The strong U, Th, REE, and Y enrichment, together with the low Ca contents in Madeira A-type melt, probably supported the crystallization of Th, U, REE-rich pyrochlore directly from magma. Madeira melt was rich in Na and F, but these elements were preferentially requested by coevally crystallizing cryolite. As a result, primary pyrochlore mostly contains only 0.2–0.4 apfu Na and mostly <0.5 apfu F, i.e., less than could be expected, given the melt composition.

In the case of pyrochlore/microlite, their chemical composition is complicated by their ability to accept a number of non-formula elements into their crystal lattice. For example, in Sn- or Sn + W-bearing rare-metal granites, Sn and W carried in hydrothermal fluid are easily incorporated in PSGM: Sn at site B and W at site A [6], regardless of the grade of peralkalinity/peraluminity. Thus, 2–4 wt% SnO₂ were found in pyrochlore from Madeira and up to 1 wt% SnO₂ in microlite from Orlovka and Beauvoir (Figure 7d). At Cínovec, ca. 1 wt% SnO₂ and 1–2 wt% WO₃ were found in microlite from the upper part of the magmatic–hydrothermal systems, while pyrochlore from the deeper part of the system contains only <0.5 wt% SnO₂ but 4–6 wt% WO₃. Pegmatites often contain accessory to minor cassiterite but almost never wolframite. Therefore, it is not surprising that pyrochlore/microlite from pegmatites is often Sn-enriched [11,17,56] (Figure 16d). The high SnO₂ contents in pyrochlore from Madeira are consistent with the pronounced Sn specialization of the Madeira pluton and suggest that Madeira albite-granite has an affinity with pegmatite systems.

Silica is a typical constituent of strongly altered pyrochlores, especially in peralkaline granites (Madeira, Figure 7b,c) and NYF pegmatites (Figure 16b,c). A secondary origin of Si enrichment is confirmed by the simultaneous decrease in F contents [8,9,26,31], i.e., by the negative correlation between Si and F (Figure 7c and Figure 16c). Silica-enriched pyrochlore from Cínovec also contains up to 19 wt% Bi₂O₃, which is another typical minor element of greisen-type Sn,W deposits. Up to 29.8 wt% Bi₂O₃ have been reported from oxybismutomicrolite from the Malkhan pegmatite, Transbaikalia [66].

Elevated contents of Ba and Sr are typical for the bulk-rock composition of carbonatites and their pyrochlore [5]. Rather difficult is the explanation of elevated contents of Sr in microlite from Orlovka and Ba in pyrochlore from Cínovec. Both elements are typically absent from fractionated granites and were probably introduced to late fluids from the neighboring country rock at these deposits.

Of radioactive elements U and Th, U is much more abundant in pyrochlore/microlite. Its contents are usually highly variable also within a single locality or a single rock type. While pyrochlore from Madeira contains mostly 0.1–0.3 apfu U, pyrochlore from the Cínovec greisen and some NYF pegmatites contains up to 0.4 apfu U. Thorium relatively dominates only in calciopyrochlore from kimberlite (0.1–0.3 apfu Th, Figure 7f and Figure 16f). Pyrochlores from carbonatites are highly variable in this respect: from relatively Th-enriched (0.06 apfu Th and <0.01 apfu U) in Catalao, Brazil [5], to extremely U-enriched (0.5–0.8 apfu U and <0.01 apfu Th) at Miaoya, China [18].

Pb is one of the typical elements for late or altered pyrochlores/microlites, which is highlighted by its strong negative correlation with Na (Figure 7g and Figure 16g). Besides Madeira (up to 1.4 apfu Pb), similarly high values were found in pegmatites [72,73].

REE and Y pose another element group that can significantly affect the composition of pyrochlore. Enrichment in LREE is more abundant, reaching, for example, up to 15 wt% Ce₂O₃ at the Katugin deposit [15], 10 wt% Ce₂O₃ in the Bouzig alkali syenite and 10 wt% Ce₂O₃ in the Lovozero alkali syenite, and 1–4 wt% Ce₂O₃ in the Catalao carbonatite. The contents of Ce₂O₃ at Madeira only scarcely exceed 3 wt%. The highest Y values (up to 22 wt% Y₂O₃) have been reported from NYF pegmatite in the Svalbard archipelago [13]; max. 1.5–2 wt% Y₂O₃ were found at Madeira. Very low contents of both Y and REE are typical for microlite from strongly peraluminous S-type granites (Beauvoir [10]), while microlite from subaluminous A-type granites can be REE-enriched (upper part of the Cínovec deposit) or REE-poor (the whole Orlovka system).

6. Conclusions

Based on generalized knowledge of the composition of PSGM and CGM from Madeira and on a comparison with other localities with different lithologies, the following basic conclusions can be formulated:

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In a trend from peralkaline to peraluminous granitoids and from NYF to LCT pegmatites, with increasing aluminosity, dominant pyrochlore is replaced with microlite;

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In pegmatites, columbite is usually the prime crystallized Nb,Ta oxide, being later replaced with late pyrochlore/microlite to a variable degree. In other lithologies, including carbonatites, alkali syenites, and peralkaline to peraluminous granites, pyrochlore/microlite is dominantly a primary magmatic phase; pyrochlore may be later partly replaced with columbite. Microlite does not undergo columbitization;

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In the Madeira albite granite, pyrochlore is the sole primary Nb oxide in the core albite granite and the hypersolvus granite. Minor primary columbite occurs only in the border albite granite;

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Pyrochlore from Madeira can be chemically characterized as Na, Ca-poor, U- and Pb-dominant, and Sn- and Zn-enriched. The contents of Ta and Ti are always low; REEs are enriched only during alteration;

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Two stages of pyrochlore alteration are present at Madeira: (i) the introduction of Fe + Mn to the system, with the majority of these elements consumed by columbitization; (ii) the introduction of Si and Fe, in lesser amounts also Pb and U: Si, Pb, and U were incorporated into pyrochlore, while iron formed Fe-oxide halos around pyrochlore. During both stages, the contents of F and Na decreased. In the case of a (nearly) complete pyrochlore columbitization, U and Th were exsolved to form inclusions of a thorite/coffinite-like phase. In contrast to altered pyrochlores from other localities, pyrochlore from Madeira shows a relatively high occupancy of the A-site;

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Although Madeira melt was Na-, F-rich, contemporaneous crystallization of cryolite consumed both elements, and pyrochlore was relatively Na-, F-poor from the beginning.