Apatitites – texture and mineralogy

The apatitite clasts exhibit diverse textures ranging from apparently folded clasts (Fig. 4a; Fig. 5a) through elongated and boudinaged types (Fig. 4b) to schlieren-like (Fig. 5b) sub-parallel varieties. The clasts consist of apatite prisms (Fig. 6a) which range in size from ~100–500 μm × 10–25 μm. In some instances grain boundaries are highlighted by thin films of brown Fe-oxides/hydroxides or extremely fine-grained amorphous mixtures of unidentifiable minerals containing C, Mg, Si, S, Cl, Ca, Fe and Cu. Although individual prisms typically exhibit undulose extinction there is no common optical orientation within a given clast (Fig. 6b). In some of the smaller clasts, and at the terminations of boudinaged clasts, the apatites are rarely coarsened into anhedral grains with uniform extinction. Allotriomorphic granular textures typical of sub-solidus coarsened cumulates are not present.

Fig. 4. Visible light scans of thin sections of Good Hope carbonatite illustrating: (a) folded bands of apatite prisms; (b) boudinaged apatitite clasts. Note that these carbonatites have not been subjected to metamorphism or regional tectonic deformation. (ap = apatitite; c = carbonate)

Fig. 5. Crossed polarised light images of: (a) band of ‘folded’ apatite prisms illustrating the subparallel orientation of the prism axes and the anisotropic heterogeneity: (b) crossed polarised light image of deformed and elongated apatitite schlieren occurring as sub-parallel bands in a carbonate matrix with a large opaque pyrochlore (pcl) crystal.

Fig. 6. Typical texture of apatite in the apatitite clasts in: (a) plane polarised light; and (b) crossed polarised light.

The apatite prisms have no internal mineral or fluid inclusions. The pyrochlores present in the clasts have apparently crystallised prior to the prismatic apatite as they lack prismatic apatite inclusions (see below). Deflection of apatite prisms around pyrochlores is present but not common and the orientation of prisms are typically not disturbed by the presence of pyrochlore.

The apatite in addition to the strong blue–green ultraviolet fluorescence exhibits cathodoluminescence (CL). Figure 7a illustrates a hot cathode CL image of apatite in a portion of a folded clast and shows the presence of green luminescent cores mantled by thin blue–green rims. Figure 7b illustrates a cold cathode CL image of apatite which exhibits the common blue CL of primary apatite occurring in carbonatites (Mitchell, Reference Mitchell and Coulson2014). The marginal CL evident in the ultraviolet fluorescence and hot cathode CL is present but not well defined. The CL spectrum (Fig. 7c) is dominated by complex Sm, Pr and Dy emission (550–650 nm) together with a broad blue–green band (400–500 nm) attributed to Eu or Ce, and a weak Nd emission in the infra-red (850–900 nm). Mitchell (Reference Mitchell and Coulson2014) has noted that the CL spectra of apatite from carbonatites consist of two groups: (1) those with a broad 400–500 nm band in the blue region of the spectrum (Verity, Howard Creek, Big Beaver House); and (2) those lacking this band (Fen, Oka, Cargill). The reasons for this dichotomy have not yet been established. Good Hope apatite belongs to group 1, whereas apatite derived from Prairie Lake heterogeneous carbonatites belongs to group 2 (Mitchell, Reference Mitchell and Coulson2014).

Fig. 7. (a) Hot cathode luminescence image of folded apatite prisms illustrating green cores and thin blue green margins; (b) cold cathode luminescence image of apatite in apatitite clasts showing the typical blue CL of carbonatite apatite; (c) CL spectrum of apatite (see text for discussion).

Carbonatites containing folded apatitites of the type found at Good Hope have not been described previously. The folding, boudinage and schlieren textures are not due to external tectonic deformation by events subsequent to crystallisation of the carbonatites, as neither Good Hope nor Prairie Lake have been metamorphosed and are emplaced in a stable craton. The folds shown in Fig. 4a are not analogous to folded apatite described by Chakhmouradian et al. (Reference Chakhmouradian, Reguir, Kressall, Crozier, Pisiak, Sidhu and Yang2015) from the Aley carbonatite as this intrusion, in common with other carbonatites in the British Columbian Cordillera, have been subjected to metamorphism and tectonic deformation subsequent to their emplacement (Mitchell et al., Reference Mitchell, Chudy, McFarlane and Wu2017).

Folded apatitite similar to the Good Hope clasts are apparently rare as most accumulations of apatite in carbonatites consist of subhedral crystals with allotriomorpic granular rather than prismatic textures, although when present prisms tend to be large (0.1–15 mm) isolated euhedral to subhedral crystals. An apatite carbonatite with folded bands of apatite has been noted from the Ngualla carbonatite (Tanzania) by Witt et al. (Reference Witt, Hammond and Hughes2019). This carbonatite has not been metamorphosed or deformed tectonically. Unfortunately, only a macroscopic description was presented by Witt et al. (Reference Witt, Hammond and Hughes2019) without accompanying textural or mineralogical data. Deformed apatitite clasts have been recognised (this work) from the St. Honoré (Québec) carbonatite which are analogous to the Good Hope folded clasts. In this example (Supplementary Fig. S1, see below) apatite prisms plus pyrochlore are deflected around large magnetite crystals. Clearly, on the basis of their texture prismatic apatitite clasts must have a different genesis and/or subsolidus re-equilibration history to granular types (see below).

Apatite – composition

The apatite-group mineral comprising the apatitite clasts is close in composition (3.6–3.8 wt.% F; 41.2–41.5 wt.% P₂O₅; 54.9–55.0 wt.% CaO) to ideal Ca₅(PO₄)₃F and is thus fluorapatite. The only minor element detectable by quantitative EDS is Sr (~0.9–1.1 wt.% SrO). Compositional zoning is not evident in back-scattered electron images.

Of the trace elements (ppm) present the most notable are: Si 108–572; Mn 79–396; Sr 7030–13,000; Y 139–439; Ba 25–59; Th 3.8–17; La 589–1280; Ce 1830–3940; Pr 262–571; Nd 1450–2740; Yb 5.2–12. The full data set is given in Supplementary Table S1 (see below). Representative data, only for apatite cores are given in Table 1, as the rims evident in CL are too small for analysis with the analytical system employed. Figure 8 illustrates chondrite-normalised (CN) rare earth element (REE) distribution patterns for Good Hope apatite. Although the patterns show a general depletion in REE abundance from La–Lu two distinct segments are evident with an inflection at Nd–Sm: a relatively flat segment from La–Nd; and a smoothly decreasing segment from Sm–Yb. The former is a consequence of these apatites having relatively low La/Nd_CN ratios (0.73–1.14) in comparison to apatite from other carbonatites which exhibit smooth REE distribution patterns from La–Yb [e.g. Alnö 1.8–2.9; Fen; 1.80; Jacupiranga 1.58; Hornig-Kjarsgaard (Reference Hornig-Kjarsgaard1998); Kaiserstuhl 2.94–4.04; Wang et al. (Reference Wang, Marks, Wenzel, Von Der Handt, Keller, Teiber and Markl2014); Belaya Zima 1.53–1.77; Doroshkevich et al. (Reference Doroshkevich, Veksler, Klemd, Khromova and Izbrodin2017)]. These data are interpreted to imply that the Good Hope apatites are relatively depleted in La, Ce, Pr and Nd. Another notable difference is their paucity in Si (108–572 ppm) compared to other carbonatite apatite [2000–2288 ppm Kaiserstuhl; 560–1494 ppm; Wang et al. (Reference Wang, Marks, Wenzel, Von Der Handt, Keller, Teiber and Markl2014); Doroshkevich et al. (Reference Doroshkevich, Veksler, Klemd, Khromova and Izbrodin2017)].

Fig. 8. Chondrite-normalised (Boynton, Reference Boynton and Henderson1985) REE distribution patterns of apatite in four different apatitite clasts. Samples (a) VAL2 and (b) VAL4A are from near-surface exploration pits. Data for VAL2 are for the folded clast shown in Figs 3 and 8. Samples (c) 337 and (d) 540 are different levels (depth in metres) from 60° and 50° inclined drill cores in the complex. These data show that REE distributions are very similar regardless of location in the complex.

Table 1. Representative trace-element compositions (ppm) of apatite.*

* Good Hope: VAL2, VAL4A and 337 prismatic apatite; 540 coarse grained anhedral apatite. Prairie Lake: PX-A apatite in pyroxene apatitite; W-A apatite in wollastonite apatitite. Blanks indicate below limit of detection.

Anhedral single crystals of apatite in the Prairie Lake heterogeneous carbonatites, in marked contrast to Good Hope apatite, in common with many other carbonatite apatites (Hornig-Kjarsgaard, Reference Hornig-Kjarsgaard1998; Wang et al., Reference Wang, Marks, Wenzel, Von Der Handt, Keller, Teiber and Markl2014; Doroshkevich et al., Reference Doroshkevich, Veksler, Klemd, Khromova and Izbrodin2017), exhibits smooth chondrite-normalised REE distribution patterns with La/Nd_CN 1.26–2.0 (see fig. 3 in Wu et al., Reference Wu, Mitchell, Li, Zhang and Yang2017). Apatite in allotriomorphic granular apatitite cumulates occurring as megaxenoliths in the Prairie Lake heterogeneous carbonatite exhibit smooth La–Lu distribution patterns with La/Nd_CN ratios ranging from 1.36–1.53 and 1.85–2.01 for pyroxene apatitite and wollastonite apatitite, respectively (Fig. 9). These data further emphasise the unusual textural and geochemical character of the Good Hope apatite relative to that in the main carbonatites of the Prairie Lake complex.

Fig. 9. Chondrite-normalised (Boynton, Reference Boynton and Henderson1985) REE distribution patterns of apatite from (a) wollastonite apatitite and (b) pyroxene apatitite xenoliths in the Prairie Lake heterogeneous carbonatites. Note that these apatite do not exhibit the La–Nd depletion evident in the Good Hope apatites illustrated in Fig. 8.

Pyrochlore paragenesis

Pyrochlore is the only primary Nb-bearing mineral in the apatitite clasts. The pyrochlore occurs primarily as euhedral single crystals which exhibit a wide range in size from ~50 μm to 1 mm, with the majority ranging from 300–600 μm (Figs 5, 10–11). Large aggregations of pyrochlore with allotriomorphic granular textures are not present, although clusters of two to three intergrown euhedral-to-subhedral 30–100 μm crystals are relatively common. The pyrochlores exhibit a very wide range of characters depending on: the abundance of inclusions of apatite and calcite; the extent of compositional zoning; and replacement by other Nb-bearing minerals. Note that apatites included in pyrochlore occurs as small (<15 μm) inclusions of irregular amoeboid shape and prismatic apatite is rarely present: both these and anhedral calcite are possibly secondary inclusions.

Fig. 10. Back-scattered electron images at the same scale of pyrochlore (white) enclosed in apatite in different apatitite clasts illustrating the extremely wide range in size of the crystals. Note that the majority of pyrochlores are single euhedral crystals. (A = apatite; pcl = pyrochlore; D and dol = dolomite). Scale bar = 1 mm with 100 μm divisions.

Fig. 11. False-colour back-scattered electron images of pyrochlores in two different apatitite clasts demonstrating that individual clasts differ with respect to the size and compositional zoning of the pyrochlores. (a) Na–Ca pyrochlores with columbite-(Fe); (b) pyrochlores with A-site deficient Sr, Ba and Fe enriched cores (white) and Na–Ca pyrochlore margins. (pcl = pyrochlore; apa = apatite; carb = carbonates). Scale bar = 1 mm with 100 μm divisions.

The majority of the pyrochlores show only minor resorption although some crystals appear to have been broken prior to incorporation in their host apatitite clasts (Fig. 11a). Euhedral and broken single crystals can be found with a given clast. Pyrochlores are also common as single crystals disseminated in the ferrodolomite and late-stage hydrothermal quartz-bearing carbonatites. Some of these pyrochlores are derived from the disaggregation of apatitite clasts whereas others are primary minerals which have apparently crystallised from the diverse later intruded carbonatites. Such pyrochlores are commonly smaller than those in the apatite clasts, exhibit complex oscillatory compositional zoning and can host inclusions of quartz or magnesio-arfvedsonite. Description of the paragenesis and composition of all pyrochlores in the Good Hope carbonatites is beyond the scope of this work, and will be the subject of a subsequent manuscript.

The pyrochlores in the apatitite clasts exhibit many similar textural and compositional characteristics although these differ between individual clasts (Figs 11,12). All pyrochlores exhibit compositional zoning in back-scattered electron images. This can range from a few very thin bands of higher average atomic number in an otherwise uniform crystal (Fig. 12a) and more rarely to complex zoning from an irregular high average atomic number core to multiple growth bands of differing composition (Fig. 12b). The pyrochlores range from essentially alteration-poor varieties to those with minor or near complete replacement by fine-grained (<20 μm) anhedral crystals of either columbite-(Fe) (Fig. 12c) and/or fersmite (Fig.12c), with some crystals replaced by both minerals in fine-grained intergrowths. Columbite-(Fe) is Ti- and V-bearing with up to 6 wt.% TiO₂ and 3.5 wt.% V₂O₅. Fersmite is near stoichiometric CaNb₂O₆ with minor TiO₂ (2–3 wt.%) and FeO_T 0.6–3.2 wt.%). Note that discrete crystals of these niobates are not present in the apatitites or the associated carbonatites.

Fig. 12. False-colour back-scattered electron images illustrating the range of compositional zoning occurring in Good Hope pyrochlores from apatitite clasts. (a) Relatively uniform Na–Ca pyrochlore with thin oscillatory zoning resulting from Sr, Ba and Fe enrichment; (b) pyrochlore with an A-site deficient Sr, Ba and Fe enriched core (white) and zones of Na–Ca pyrochlore (orange) and Sr–Ba–Fe pyrochlore (yellow) margins; (c) small anhedral columbite-(Fe) crystals (yellow) replacing Na–Ca pyrochlore (orange); (d) anhedral fersmite (yellow) replacing Na–Ca pyrochlore (orange). Scale bar indicates 100 μm.

Pyrochlore composition

Pyrochlore-group compounds have the ideal structural formula A ₂B ₂O₆Z. In carbonatites pyrochlores are not stoichiometric and contain cation vacancies (Atencio et al., Reference Atencio, Andrade, Christy, Gieré and Katashov2010); typical compositions are expressed as [(Ca,Na,Sr,Ba,Ce,U⁴⁺)_2–x(Nb,Ti,Ta,Fe³⁺)₂O₆(OH,F)_1–y⋅zH₂O]. Hydroxyl anions and H₂O cannot be analysed by EDS and F determination is not exact due to significant overlaps of the FKα peak with diverse low-energy X-ray peaks (particularly FeLα). Pyrochlore compositions obtained by both wavelength-dispersive and energy-dispersive electron beam methods are generally expressed on a volatile-free basis with the number of B-site cations fixed at 2.0 (Hogarth et al., Reference Hogarth, Williams and Jones2000), as unlike A-site cations these are considered not to be subject to leaching during weathering and/or hydrothermal activity (Lumpkin and Ewing, Reference Lumpkin and Ewing1995). The A-site deficiency is commonly used as a measure of alteration or metamictisation (Nasraoui and Bilal, Reference Nasraoui and Bilal2000; Zurevinski and Mitchell, Reference Zurevinski and Mitchell2004). Representative compositions of Good Hope pyrochlores in apatitite clasts are given in Table 2. All of the pyrochlores in the Good Hope apatitites are poor in Ti and Ta. As Nb > Ti >> Ta (atoms per formula unit) they are classified as pyrochlore sensu stricto (Hogarth, Reference Hogarth1977; Atencio et al., Reference Atencio, Andrade, Christy, Gieré and Katashov2010).

Table 2. Representative compositions (wt.%) of pyrochlore.*

* Σ_cat = total cations; A site = total cations in this site; ΣA = A-site deficiency. Elements not detected at <0.4 wt.% oxide are: Mg; Al; Cl; Mn; REE; Pb. Blanks in the table indicate not detected.

Typically the commonest, earliest forming and least replaced pyrochlores (Fig. 12a) are Na–Ca pyrochlore that is poor in Sr and has only minor (5–20%) A-site deficiency (Table 2, comps. 1–3). Thin oscillatory bands of higher average atomic number are present in some crystals which have increased Sr and Ba contents and lower Na (Table 2, comps. 4–5), but are not otherwise markedly different. Less commonly, pyrochlores exhibit strong compositional zoning (Fig. 12b) with typically irregular cores of Na–Ca depleted material with high average atomic number consisting of A-site deficient (>50%) pyrochlore enriched in Sr, Ba, Fe, Si and U (Table 2; comp. 6–8). These cores are typically overgrown by, or are gradational into, Na–Ca pyrochlores without A-site deficiency. The example shown in Fig. 12b illustrates further syntaxial overgrowth of Sr-rich pyrochlore followed by Na–Ca pyrochlore.

From the above textural and compositional data it is apparent that many pyrochlores have had a complex evolutionary history and that more than one generation of pyrochlore is present. The suite has some similarities to pyrochlore populations in other carbonatites in which U–Ta-enriched pyrochlore of irregular habit is enclosed in oscillatory zoned pyrochlore (Walter et al., Reference Walter, Parsapoor, Braunger, Marks, Wenzel, Martin and Markl2018; Lee et al., Reference Lee, Lee, Garcia, Moutte, Williams, Wall and Kim2006; Zurevinski and Mitchell, Reference Zurevinski and Mitchell2004; Hogarth et al., Reference Hogarth, Williams and Jones2000; Chakhmouradian and Zaitsev, Reference Chakhmouradian and Zaitsev1999). However, note that extensively oscillatory zoned pyrochlores of the type illustrated in figure 2C of Hogarth et al., Reference Hogarth, Williams and Jones2000) or figures 2A and 2C of Walter et al. (Reference Walter, Parsapoor, Braunger, Marks, Wenzel, Martin and Markl2018) are not found at Good Hope.

Walter et al. (Reference Walter, Parsapoor, Braunger, Marks, Wenzel, Martin and Markl2018) and Lee et al. (Reference Lee, Lee, Garcia, Moutte, Williams, Wall and Kim2006) have suggested that the U–Ta enriched pyrochlore cores in Kaiserstuhl and Sokli carbonatites are resorbed xenocrysts which did not crystallise from the magma which crystallised oscillatory zoned pyrochlore overgrowths. Similar Good Hope pyrochlores might have had such an origin but they cannot be linked to any pyrochlores crystallising from co-genetic silicate magmas, as there is no geological evidence for the presence of such magmas i.e. xenoliths, consanguineous pyrochlore-bearing intrusions, interaction with, or derivation from, the main Prairie Lake occurrences of ijolites and carbonatites. In addition, the Good Hope Sr–Ba–Fe–U enriched cores are notably poor in U and Ta (<2 wt.% UO₂; <1 wt.% Ta₂O₅) relative to Kaiserstuhl and Sokli types (>20 wt.% UO₂; >9 wt.% Ta₂O₅). An alternative explanation for Ta, and by analogy Sr, Ba, Fe and U, enrichment in early crystallising pyrochlores has been suggested by Kjarsgaard and Mitchell (Reference Kjarsgaard and Mitchell2008), who note that Ta-rich pyrochlores are F poor and Ta poor whereas Na–Ca pyrochlores are F rich. Although quantitative compositional data for F are not available for Good Hope pyrochlores, semi-quantitative data suggest that the strongly A-site deficient pyrochlores have lower F (<3 wt.% F) contents than Na–Ca pyrochlores (<4 wt.% F) Thus, it is suggested that variations of F fugacity in the melt can also lead to the crystallisation of different varieties of pyrochlore.

Currently, oscillatory zoning of pyrochlores is considered to have magmatic origins (Hogarth et al., Reference Hogarth, Williams and Jones2000), and to be originating from supersaturation of magma with mineral end-members of different solubilities. Oscillatory zoning is considered to occur in undisturbed environments, whereas homogeneous crystals form in turbulent conditions. Thus, crystals exhibiting both homogeneous regions and oscillatory zoning might reflect either (or both) in situ growth with repeated changes in magma rheology from turbulence to quiescence or transport of growing crystals from one rheological regime to another.

The observations suggest the Good Hope pyrochlore suite has diverse origins with some pyrochlores formed by incorporation of Sr, Ba, Fe and U-rich pyrochlore into a magma (of higher F fugacity?) crystallising mainly homogeneous Na–Ca pyrochlore and/or oscillatory zoned pyrochlore with alternating conditions of turbulence and quiescence. Thus, pyrochlores with, or lacking, enriched cores can coexist with homogeneous and/or oscillatory zoned types. Many, but not all pyrochlores were then subject to alteration and replacement by fersmite and columbite-(Fe), with some altered crystals being returned to environments where homogeneous overgrowth of Na–Ca pyrochlore were resumed. Overall the suite represents growth under diverse rheological conditions and is undoubtedly a consequence of magma mixing.