# Dating multiple fractionation trends in pegmatites – Implications for the genesis of a large lithium pegmatite province in West-Africa

Marieke van Lichtervelde, Oscar Laurent, Prince Ofori Amponsah, Isaac Iwan E. Williams

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Marieke van Lichtervelde, Oscar Laurent, Prince Ofori Amponsah, Isaac Iwan E. Williams. Dating multiple fractionation trends in pegmatites – Implications for the genesis of a large lithium pegmatite province in West-Africa. Chemical Geology, 2025, 691, pp.122935. 10.1016/j.chemgeo.2025.122935. hal-05390717

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Research Paper

# Dating multiple fractionation trends in pegmatites – Implications for the genesis of a large lithium pegmatite province in West-Africa

Marieke Van Lichtervelde<sup>a, *,</sup> , Oscar Laurent<sup>a</sup> , Prince Ofori Amponsah<sup>b</sup> , Isaac Iwan E. Williams<sup>c</sup>

<figure><img src="image_2.png" /></figure>

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ARTICLE INFO

Editor: Marco Fiorentini

Keywords:

Lithium pegmatites

Columbite-group minerals

U-Pb dating

Rayleigh fractionation

Ghana

A B S T R A C T

The race for lithium supply for the green energy transition has brought Li pegmatites, the primary source of Li in the world, to the forefront. Their origins and mechanisms of formation are still debated, and geochemical tools to distinguish granite-related from anatectic pegmatites are still needed. In the current study, we combine two classical approaches, i.e., alkali fractionation in micas coupled to Nb-Ta-oxide geochronology, to date multiple fractionation trends in pegmatites from a large lithium pegmatite province in Ghana. We investigated distinct fractionation trends in the different pegmatites and we timeframed these trends by dating columbite-group minerals associated with primary magmatic micas. Distinct Rayleigh-type Rb/Cs fractionation paths were modeled using different starting Rb and Cs melt compositions that can be attained by direct partial melting of the meta-sedimentary country rocks. Columbite-group mineral dating reveals that each of these fractionation paths occurred at different ages spanning ca. 140 Myr. Moreover, our data shows that the oldest pegmatites correspond to poorly fractionated, Li-rich, Nb-Cs-Ta-Sn-poor pegmatites, whereas the youngest are the most fractionated, Cs-Ta-rich pegmatites (up to levels of the major Lithium-Cesium-Tantalum pegmatites worldwide). The proposed model of formation for this large Li-bearing province involves at least three distinct stages of partial melting and melt extraction at ca. 2170, 2080 and 2030 Ma. The produced melt batches evolved independently through fractional crystallization, the spodumene-bearing pegmatites being the least fractionated, and the “true LCT” pegmatites the most fractionated.

# 1. Introduction

With a demand which is called to be multiplied by thirty by the middle of the century for the needs of the green energy transition, lithium is experiencing a global rush to secure supplies for batteries. Among the prospected lithiniferous areas, West Africa is promising with a number of pegmatite fields which are under investigation (e.g., Wilde et al., 2021 for Mali, Adams et al., 2023 for Ghana, Attourabi et al., 2024 for Niger; see also Goodenough et al., 2025). Those pegmatites all have the particularity to be extremely enriched in spodumene, the primary Li ore in the world.

An increasing number of studies have shown that minimum lithium concentrations in evolved granitic melts for Li-minerals saturation necessitate a two-step model involving either two stages of partial melting, and/or fractional crystallization of the produced anatectic

melts (Ballouard et al., 2023; Knoll et al., 2023; Koopmans et al., 2024). For example, Koopmans et al. (2024) have modeled the genesis of Li pegmatites by a two-step melting model. First, partial melting of fertile metasediments produced Li-enriched granites. In a second step, the Li contents necessary to trigger Li-minerals saturation were reached by fractional crystallization of the granitic melts, or subsequent partial melting of the granitic rocks. However, whether rare-element Li-Cs-Ta (LCT) pegmatites are the product of extreme fractionation of fertile granitic plutons (London, 2018, 2022), or of direct partial melting of country rocks (Deveaud et al., 2015; Shaw et al., 2016; Villaros and Pichavant, 2019), is still debated. One way to investigate that question is to date fractionation trends in pegmatites, i.e., to relate distinct fractionation trends to distinct episodes of crystallization. This relies on geological fields where several pegmatites, from sterile to strongly mineralized, can be encountered, and datable minerals, e.g. columbite-

<footnote><sup>a</sup> Geosciences Environnement Toulouse, Univ Toulouse, Centre National de Recherche Scientifique, Institut Recherche et Développement, 14 Avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote><sup>b</sup> Department of Earth Science, School of Physical and Mathematical Sciences, University of Ghana, P.O. Box LG58, Legon-Accra, Ghana</footnote>

<footnote><sup>c</sup> Atlantic Lithium Ltd., Level 17, Angel Place, 123 Pitt Street, Sydney, NSW 2000, Australia</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: marieke.vanlichtervelde@get.omp.eu (M. Van Lichtervelde).</footnote>
group minerals (CGM), can be found as early magmatic minerals even in the least evolved pegmatites, in association with primary magmatic micas.

These conditions are met in the Cape Coast pegmatite (CCP) province in southern Ghana, where several pegmatite fields outcrop over more than 100 km along the coast. One of these is the Ewoyaa pegmatite field which is being prospected and developed by Atlantic Lithium Ltd. The current JORC compliant mineral resource estimate for the Ewoyaa Deposit is 36.8 Mt. @ 1.24 wt% Li₂O. The deposit consists of tenths of pegmatite dykes about 10 to 30 m thick, with spodumene as the

dominant Li mineral occurring from border to core of the pegmatite intrusions. The Ewoyaa field is located 80 km away from the Winneba and Mankwadze pegmatite groups, which were recently investigated by Adams et al. (2023) and turned out to be made of two distinct pegmatite fields with different emplacement structures, internal zoning, mineral assemblages and geochemistry. All three fields (Ewoyaa, Winneba and Mankwadze) count several tens to hundreds of subvertical pegmatite bodies and can therefore be considered as multi-intrusion deposits. The pegmatites show various styles of mineralization, mineralogical assemblages and internal organizations, from unzoned to complexly zoned.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Geological map of the West African craton with study localities. E = Ewoyaa, W = Winneba, M = Mankwadze.</figcaption></figure>
The site therefore offers a perfect setting for dating multiple fraction-
ation trends in pegmatites.

The fractionation degree of pegmatites is commonly determined
through fractionation indicators such as alkali (K/Rb or K/Cs) ratios in
micas and potassic feldspar (Černý et al., 1985; London, 2022). Alkali
elements in muscovite are particularly reliable because they increas-
ingly partition into that mineral upon fractionation with decreasing
cation size from Cs to Li. K and Rb are well accommodated (Kd > 1) in
mica structural sites, whereas the small Li and the large Cs cations are
incompatible, with smaller partition coefficients (Kd < 1). Rubidium
and Cs can attain 6 wt% Rb₂O and 2 wt% Cs₂O in muscovite, which is
measurable by electron probe microanalysis (EPMA), but low concen-
trations are more common in poorly fractionated pegmatites, and laser
ablation (LA) ICP-MS analyses are necessary to get precise alkali ratios
(Van Lichtervelde et al., 2008). U—Pb dating of CGM is favored by low
initial Pb concentrations and relatively high U concentrations in the
order of 100–1000 ppm. Since the first development of U—Pb CGM
dating in solution by Romer and Wright (1992), in-situ CGM dating has
been widely used to date rare-element granites and pegmatites, in which
it is commonly the only datable (i.e. non-metamict) U-bearing mineral
(Smith et al., 2004; Melcher et al., 2015; Van Lichtervelde et al., 2017;
Legros et al., 2019; Sun et al., 2023).

In this study, we have performed both trace element analyses of
micas and U—Pb dating of associated CGM by LA-ICP-MS, allowing a
direct link between the fractionation degree of the host pegmatite and its
crystallization age. The site also gave us the opportunity to distinguish
different types of Li-bearing pegmatites, and to test the hypothesis of an
origin by partial melting vs. crystallization from a common parent
reservoir.

## 2. Geological setting of the lithium pegmatites of southern Ghana

The CCP province occurs in southern Ghana between Accra and
Takoradi. It is part of the Paleoproterozoic Baoule-Mossi domain or
Birimian Terrane of the Leo-Man Shield (Fig. 1), which is the southern
part of the West African Craton (see the WAC geology synthesis by
Baratoux et al., 2024 and references therein). The Baoule-Mossi domain
consists of volcanic and volcano-sedimentary units intercalated or
juxtaposed with deep-water basins originally formed of sediments
metamorphosed under greenschist facies or higher grade (Jessell et al.,
2012; Amponsah et al., 2016; Masurel et al., 2022). The subsequent
metamorphic stage was accompanied by partial melting, driven by
decompression and local heating, which led to abundant migmatisatio-
nation (Ganne et al., 2014; Baratoux et al., 2023). This Birimian super-
group was intruded by granitoids of intermediate granodioritic to felsic
leucogranitic compositions (Forson et al., 2024; Kwayisi et al., 2024).

The CCP province is part of the Eburnean orogeny whose meta-
morphic peak is dated between 2137 ± 8 Ma and c. 2060–2050 Ma
through the WAC (Agyei Duodu et al., 2009; Kone et al., 2020;
Amponsah et al., 2023). In Ghana, younger metamorphic ages ranging
from 2005 ± 26 Ma to 1898 ± 11 Ma (White et al., 2014) were also
found in the Birimian metavolcanic rocks. Granitoids in the CCP prov-
ince are represented by the metaluminous Winneba biotite granite,
dated around 2139 ± 6 Ma (Agyei Duodu et al., 2009; Peterson et al.,
2016; Forson et al., 2024).

The CCP province consists of at least three pegmatite fields. The
pegmatites from the Ewoyaa field are part of the unzoned, albite-
spodumene type of the rare-element pegmatites. By contrast, the Win-
neba and Mankwadze pegmatite fields reveal more conventional LCT
characteristics with complex zoning and mineralogy, abundant Nb-Ta-
Sn mineralization and strong Cs enrichment in micas (Adams et al.,
2023). For the three fields, the silicate minerals consist of alkali feld-
spars albite and microcline, quartz, muscovite, with minor garnet, beryl
and tourmaline. The Ewoyaa pegmatites are up to 30 m in thickness and
can be followed up to 500 m along strike on drill cores. They intrude and

are intercalated with meta-sedimentary rocks (meta-greywackes,
quartzites, phyllites and schists). The host schists are made of micas-
chists, staurolite schists and garnet schists. The dykes are unzoned but
gradual change in textures occurs from very coarse to aplitic grain sizes.
The Winneba and Mankwadze pegmatites rarely reach 30 m in thick-
ness. The Winneba area consists of meta-sedimentary and meta-volcanic
Birimian rocks, whereas amphibolite, hornblende-biotite-quartz schist
and quartz-hornblende-actinolite schist (so-called “greenstones”) make
up the country rocks at Mankwadze (Kesse, 1985; Agyei-Duodu et al.
2009; Nyame et al., 2021).

In terms of mineralization, lithium mainly occurs as large tapered
crystals of spodumene, but it also forms Li-phosphates of the triphylite-
lithiophilite type that may occur as large (about 10 cm) crystals. Spod-
umene is ubiquitous at Ewoyaa, it is very abundant at Winneba, but it is
encountered in only one pegmatite at Mankwadze. Nb-Ta-Ti-Sn oxides
are rarely observed at Ewoyaa, and the few crystals that could be dated
are commonly smaller than 100 µm (Fig. 2). In contrast, Nb-Ta-Ti-Sn
oxides are widespread at Winneba and Mankwadze and can reach a
centimeter in size, forming sedimentary placers in places. The Winneba
pegmatites mostly carry CGM and microlite, whereas CGM, wodginite-
group minerals (WGM) and cassiterite are widespread in the Man-
kwadze pegmatite field. The rarity of Sn minerals in the Winneba field
and of Ti minerals in the Mankwadze field is reflected in the CGM
chemistry where Ti > Sn at Winneba and Sn > Ti at Mankwadze. Based
on this dichotomy, Adams et al. (2023) have suggested that the two
pegmatite fields are independent and have distinct origins.

## 3. Methods

### 3.1. Sample selection

We have selected 6 samples from Winneba and 9 samples from
Mankwadze based on the descriptions and results of our previous study
(Adams et al., 2023). For Ewoyaa, about fifty core samples of Atlantic
Lithium Ltd. (AIM: ALL, ASX: A11, GSE: ALLGH, OTCQX: ALLIF) were
examined, and 15 of them were selected for CGM and/or mica investi-
gation. A short description of these samples is given in Table S1. Thin
and thick sections were prepared by Thin Section Lab in France and
observed by optical microscopy to select the least altered samples. The
main selection criterium was the grain size, i.e., CGM crystals above 40
µm and mica crystals above 100 µm to be analyzed by laser ablation.
Textural features and genetic relationships with other minerals were
used to select the primary magmatic phases (see Fig. 2).

### 3.2. BSE imaging and EPMA analyses

Before the laser ablation (LA) inductively coupled plasma mass
spectrometry (ICP-MS) analyses, scanning electron microscopy under
backscattered electron mode (BSE) and electron probe micro-analyses
(EPMA) were completed on micas and CGM. BSE images were used to
locate the LA spots and to estimate the size of the spot depending on the
grain size and texture. They were also used to avoid mineral inclusions
during ablation. EPMA was used to characterize the mica and CGM
composition and to avoid areas where mixed mineral phases occur, e.g.,
microlite or cassiterite in CGM. EPMA were performed using a Cameca
SX-Five electron microprobe at the Raimond Castaing Centre of the
University of Toulouse. EPMA procedure follows that of Adams et al.
(2023): 15 kV, 10 and 20 nA for micas and CGM, respectively; 1 and 2
µm electron beam for CGM and micas, respectively; 10 s counting time
for all elements except F (30 s); signals F (Kα), Al (Kα), Si (Kα), K (Kα), Ca
(Kα), Ti (Kα), Mn (Kα), Fe (Kα), Na (Kα), Mg (Kα), Rb (Lα), Cs (Lα), Ba
(Lα), Cr (Kα), Sn (Lα), Ta (Mα) and Nb (Lα); standards topaz (F), Al₂O₃
(Al), Wollastonite (Si, Ca), Sanidine (K), MnTiO₃ (Mn, Ti), Fe₂O₃ (Fe),
Albite (Na), MgO (Mg), BaSO₄ (Ba), Cr₂O₃ (Cr), Rb, and Cs Glasses, SnO₂
(Sn), Nb and Ta metals. In micas, EPMA revealed that Al is homogeneous
at the crystal scale, which validates its use as an internal standard for the
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Representative images of samples for the three studied localities. A: photographs of sample blocks (width = 48 mm). B: scan pictures of thin sections (width = 48 mm). C: representative BSE pictures of columbite-group minerals showing their association with coarse primary magmatic muscovite. D: BSE pictures of CGM internal textures. From left to right: Ewoyaa (samples EW3–1 and EW8–3), Mankwadze (samples MKW2B and MKW6), Winneba (sample WTF4). Ap = apatite; CGM = columbite-group minerals; ms = muscovite; spd = spodumene.</figcaption></figure>

LA-ICP-MS analyses.

### 3.3. LA-ICP-MS analyses

Trace elements in micas and U—Pb isotopes in CGM were measured using an Elemental Scientific Instruments (ESI) NWRfemto solid-state femtosecond laser ablation system set in UV mode (257 nm wavelength; 235 fs) coupled to a Thermo Element XR high-resolution, sector-field ICP-MS at the Service ICP-MS of Observatoire Midi-Pyrénées (OMP-UAR831). In both cases, ablation was performed in the in-house, dual-volume, fast-washout ablation cell (<1 cm³ effective volume) flushed with a He carrier gas (0.5–0.7 L/min depending on the analytical session) to which was admixed a Ar make-up gas (0.8–0.85 L/min depending on the analytical session) downstream of the ablation cell before introduction into the plasma. A typical run consisted of 15–20 s of background signal acquisition, followed by 20–40 s of ablation and 10 s of washout. Details about data acquisition parameters and instrument optimization are provided in the Supplemental Table S2.

#### 3.3.1. Trace elements in micas

More than 300 trace element analyses of micas were completed in spot or line mode, using a 20 µm laser beam diameter, a repetition rate of 10 Hz and an energy density of ca. 2.5 J.cm⁻². The raw signal intensities

were processed using Iolite v4.0 software. NIST SRM610 was used as primary reference material for correction of instrumental drift and calculation of elemental sensitivities, through conventional standard-sample bracketing; and NIST SRM612 was used as a secondary, validation standard. Aluminum was used as internal standard for the correction of relative sensitivity between mica and NIST glass, as determined by EPMA on the same thin section (see above) and mostly on the same crystals. The results on NIST SRM612 show that the reproducibility (2 standard deviation, SD) is better than 5 % relative for all elements except Li, B, P, Ti and Fe (between 5 and 15 % relative) and that the results are accurate within 2 SD uncertainty. In the analyzed micas, the rare-earth elements, Zr, Hf and Th are below detection limit. Anomalous analyses with high concentrations of these elements are attributed to the presence of micro-inclusions of zircon or monazite and were discarded. Lithium concentrations were verified on spodumene which displayed ~8 wt% Li₂O.

Twenty two masses were measured by LA-ICP-MS in micas, and the full dataset is available in Supplemental Table S3, although only Rb and Cs, and to a minor extent Ta, are used for the current study. The full dataset allows illustrating the reliability of the LA-ICP-MS measurements, first by comparing minor element concentrations with EPMA data, and second, by checking homogeneity in the concentration of all trace elements. However, it is out of the scope of that study to discuss all
elements.

Rb and Cs concentrations in micas have to be determined with the highest accuracy in order to get reliable Rb/Cs ratios. In particular, we checked that the gaps that exist between the various Rb/Cs fractionation series (cf. Fig. 3A) are not related to errors in the LA-ICP-MS analyses. The Rb/Cs ratios vary by up to a factor of three between the different series, which can be attributed neither to small variations of the Al₂O₃ internal standard with respect to the real value (as typical Al₂O₃ variations in a given sample are <10 % relative), nor to a poor precision on Rb and Cs quantification by LA-ICP-MS analyses (as typical internal uncertainties on the Rb/Cs ratio are ≤10 % relative, and typical bulk reproducibility based on repeated analyses of NIST SRM612 is ≤5 %

relative; see Table S3). We conclude that our analytical set is precise and reliable especially for Rb and Cs.

### 3.3.2. U-Pb dating in columbite- (and wodginite)-group minerals

CGM U-Pb dating was performed using LA spot diameters of 10 or 20 µm (see below), a repetition rate of 8 Hz and energy density of ca. 2.5 J.cm⁻². Trace element concentrations of Mn, U, Pb and Th were also measured using MnO wt% concentrations measured by EPMA as internal standard. The raw signal intensities were processed using Iolite v4.0 software. Despite the active development of CGM standards by the community (Che et al., 2015; Legros et al., 2019; von Quadt et al., 2019; Galliski et al., 2021; Yang et al., 2024; Qing et al., 2024), there is no

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** A. Fractionation series of the CCP based on Rb/Cs systematics in micas. The three graphs correspond to the Rb/Cs vs. Cs plot with progressive close-up upward. The points can be sorted into 4 series with distinct Rayleigh fractionation trends. B. Calculated Rayleigh models for series 1 and series 3 + 4, where C₀ are the starting compositions and Kd are the partition coefficients. Ms. = muscovite.</figcaption></figure>
conventionally accepted, widely-used international standard at this date, except Coltan139 which is not fully homogeneous (Yang et al., 2023). Most CGM crystals used as reference materials are not isotopically homogeneous and may contain small quantities of common lead. As an example, coltan X36 (von Quadt et al., 2019; Galliski et al., 2021) was regularly measured during our different sessions, and it turned out to display ages dispersed between the two published ages reported for CGM crystals from the Gigante pegmatite from which it has been extracted (see Galliski et al., 2021: Table 2), i.e., 335 ± 9 Ma and 354.8 ± 1.7 Ma. The average value of our analyses displays a Concordia age of 340.7 ± 3.0 Ma [n = 54, MSWD = 2.8]. This likely reflects in-homogeneity of our X36 crystal. Furthermore, complex matrix effects between different compositional end-members of the CGM group cannot be excluded (Yang et al., 2024).

For these reasons, we have employed a similar strategy as routinely applied for e.g. U—Pb dating of carbonates (Roberts et al., 2017; Guil- long et al., 2020) or apatite (Rocher et al., 2024; Mathieu et al., 2025) by LA-ICP-MS, i.e. using NIST SRM610 as a primary reference material for the correction of instrumental drift and mass bias; and repeatedly measuring CGM reference materials of known age to assess matrix ef- fects arising from laser-inducted Pb/U element fractionation (which, in theory, are mitigated by the use of the femtosecond laser). In our case, the CGM secondary reference materials employed were X36 (see above) and ISSIA10CT4 from Ivory Coast, which was previously dated by ID- TIMS and SIMS by Legros et al. (2019). Our NIST SRM610-normalized data for ISSIA10CT4 yield an intersect $^{206}$Pb/$^{238}$U age of 2028 ± 13 Ma [n = 29, MSWD = 2.4] which is within the uncertainty of that of 2045 ± 2 Ma obtained by Legros et al. (2019). These results point to the absence of significant matrix-dependent Pb/Pb and U/Pb fractionation with our setup, as also noticed by Melcher et al. (2015) who standard- ized their CGM dating with zircon (see also Melleton et al., 2022).

More than 270 analyses of CGM were completed with a spot size of 10 or 20 µm, depending on the crystal size. Some crystals were analyzed with the two spot sizes and no difference was noticed in the results. CGM was dated on both thin and thick sections but this did not impact the results either. The crystal size is also not determinant. At Ewoyaa, very small (<100 µm) crystals were successfully analyzed and may display concordant ages. One sample with wodginite-group minerals (WGM) was also dated and displayed similar ages as CGM, therefore validating the use of wodginite for U—Pb dating.

In terms of data processing, analyses showing elevated concentra- tions in $^{204}$Pb (initial Pb) were treated carefully, but most of them do not impact the weighted mean $^{207}$Pb/$^{206}$Pb ages. Common lead does not exceed 714 ppm. The $^{206}$Pb, $^{207}$Pb, $^{235}$U and $^{238}$U signals were used to detect metamict zones. U contents can reach 45,000 ppm, and concen- trations above 10,000 ppm are commonly associated with younger ages and high uncertainties. Data from two samples (WIN4A and EW5–4) were discarded from the final results based on such anomalous data.

All Concordia age calculations, weighted mean averages, intercept ages and plotting of concordia and weighted mean diagrams were per- formed using IsoplotR (Vermeesch, 2018). In general, we have used the $^{207}$Pb/$^{206}$Pb dates for age calculation, as these are the most robust for Precambrian times and a priori least affected by matrix effects; but upper intercept or Concordia ages were calculated for some samples for which the data distribution allowed so. Outlier rejection from the final age calculation for each sample was based on data concordance, pres- ence of initial Pb, signal quality (presence of inclusions, stability of isotopic ratios...) and, finally, anomalous $^{207}$Pb/$^{206}$Pb ages compared to the main sample population. The full analytical set is presented in Supplemental Table S4 and a synthesis of the CGM dating results is presented in Supplemental Table S5. Supplemental Fig. S1 shows se- lections of BSE images and Isoplot graphs for each sample.

### 3.4. Rayleigh fractionation model

Rb/Cs fractionation paths were modeled in muscovite, based on a

model of fractional crystallization calculated using the Rayleigh equation

$$C_{sol} = Kd. [C_0. [1 - F]^{D-1}]$$ where

- $C_{sol}$ is the concentration of an element (here Rb and Cs) in the mineral (here muscovite);

- Kd is the muscovite-liquid partition coefficient for the element of interest;

- $C_0$ is the initial concentration of the element of interest in the liquid at the onset of crystallization (therefore it is also the composition of the anatectic melt at the time of its emplacement);

- F is the fraction of crystallization;

- D is the global partitioning for the element of interest; it takes into account the solid fraction and partition coefficient for each mineral present during the crystallization.

For simplicity, D and Kd are supposed constant throughout the crystallization, which is a correct first approximation since the mica species (i.e., muscovite only and no Li-muscovite nor biotite) and the mineralogy do not change significantly through the crystallization of the pegmatite dykes (see also Hulsbosch et al., 2014).

For each of the two trends plotted in Fig. 3B (series 1 and series 3 + 4 of Fig. 3A), we iteratively fit eq. [1] to the data by estimating the $C_0$, D and Kd parameters. The best fits are obtained for muscovite-melt $Kd_{Rb} = 1.75$ and $Kd_{Cs} = 0.1$, which gives D values of 0.7 for Rb and 0.04 for Cs, corresponding to a muscovite fraction of 40 %. The Rb (1.75) and Cs (0.1) muscovite-melt Kd are consistent with literature (Pichavant et al., 2024). The first trend (series 1) is modeled using $C_0[Rb] = 800$ ppm and $C_0[Cs] = 130$ ppm, whereas the second trend (series 3 + 4) is modeled using $C_0[Rb] = 2200$ ppm and $C_0[Cs] = 250$ ppm.

## 4. Results

### 4.1. Fractionation degree of cape coast pegmatites

Muscovite is a common mineral in the CCP. It occurs as fine- to coarse-grained flakes associated with the primary magmatic albite, microcline, and spodumene crystals (Table S1; Fig. 2). It is euhedral and generally unaltered. Therefore, the mica composition reliably represents its primary content, especially for Li, Rb and Cs which are highly mobile with fluid interactions.

Rb/Cs ratios are moderately variable within crystals and samples, but they vary significantly from one sample to the other and from one locality to the other (Fig. 3). The large and precise analytical set (see the Supplemental Material for a detailed scrutiny of data precision) allows distinguishing different Rb/Cs fractionation paths. At Ewoyaa, at least two series can be distinguished. Series 1 includes samples EW10–26 → EW1–3 → EW11–4 → EW4–2 → EW5–3. Series 2 is represented by samples EW4–6 → EW3–3 → EW9–3. Muscovite from Winneba and Mankwadze follows similar but discontinuous fractionation paths, with two distinct series. Series 3 shows Cs contents below 300 ppm and high Rb/Cs, and series 4 has Cs contents between 300 and 2800 ppm and low Rb/Cs.

We modeled Rayleigh fractionation trends for two possible paths (see also Hulsbosch et al., 2014). A first path corresponding to the Nb-Ta-Cs-poor Ewoyaa samples (series 1) can be modeled by 10–90 % fraction- ation using Rb and Cs starting compositions of 800 and 130 ppm, respectively. A second path corresponds to Nb-Ta-Sn-mineralized sam- ples of Winneba and Mankwadze (series 3 and 4) and can be modeled using 2200 ppm Rb and 250 ppm Cs as starting compositions, with 10–80 % fractionation required for Winneba and > 95 % for Mankwadze (Fig. 3B). Series 2 follows a path intermediate between these two.

### 4.2. Age of pegmatites

In the CCP, CGM are intergrown with primary magmatic silicates
albite, microcline, muscovite and spodumene and show sector/oscilla-
tory zoning consistent with a magmatic origin (Fig. 2). Therefore, by
dating CGM we could date directly the crystallization of the pegmatite
dyke. CGM crystals are found in all pegmatites, although rare and small
in Ewoyaa samples. CGM from the three fields occupy distinct chemical
ranges, from mostly ferrocolumbite at Ewoyaa to manganotantalite at
Mankwadze (Fig. 4B). The Winneba and Mankwadze CGM have been
recognized as being part of distinct chemical groups by Adams et al.
(2023). BSE images from the three fields show simple to complex zoning

(Fig. S1; Table S1) with local alteration by U-rich microlite. Two samples
have particularly high U contents that likely render the CGM metamict,
which impacted their ages. The resulting dates were not considered for
interpretations (Fig. 4A).

The individual ²⁰⁷Pb/²⁰⁶Pb dates show moderate to no variability within single crystals and among different crystals of a single sample (Fig. S1), allowing to define crystallization ages for each pegmatite sample (Fig. 4). However, these ages vary according to locality, petrological characteristics and/or fractionation series (as defined based on mica Rb and Cs data). Accordingly, three main episodes of pegmatite crystallization are determined. The Ewoyaa CGM from series 2 are the oldest (2173 ± 21 Ma), while series 1 samples from the same locality display a younger age (2076 ± 15 Ma). The latter is identical within uncertainty to that of the Winneba CGM, including samples from series 3 (2085 ± 16 Ma). Lastly, the Mankwadze CGM, mainly from series 4, are the youngest (2033 ± 8 Ma). It is interesting to note that the whole dataset (N = 283) displays an average age of 2060 ± 10 Ma (Table S5) which is close to the CGM age reported by Melcher et al. (2015) in the Kokobin area, located further north in Ghana (2080 ± 3 Ma by TIMS).

**5. Discussion**

Each fractionation series as defined in Fig. 3 can be related to an
episode of CGM mineralization (Fig. 5). Pegmatites from series 1 and 2
(Ewoyaa), which are poorly zoned but strongly mineralized in Li, with
low Nb-Ta-Cs contents and high Rb/Cs ratios, are the oldest and
emplaced as two distinct episodes spanning ~100 Ma. Their muscovite
crystals have Ta contents below 60 ppm. The youngest series 4 (Man-
kwadze) is the most fractionated with low Rb/Cs ratios in micas close to
those of the giant LCT Tanco pegmatite (Van Lichtervelde et al., 2008),
moderate Li mineralization and abundant Nb-Ta-Sn mineralization.
Their muscovite shows Ta contents above 70 ppm (an indicator of
economically important Ta mineralization; Möller and Morteani, 1987)
and up to 250 ppm. These are well zoned and can be qualified as “true”
LCT pegmatites.

Altogether, the dataset unequivocally shows that the CCP define
several groups characterized by distinct fractionation paths and crys-
tallized at significantly different ages, emplaced over 140 ± 29 Myr
(based on the age difference between series 2 and 4). With such a time
span, the origin and fractionation degree of the CCP cannot be related to
the fractional crystallization of a single granitic intrusion, which would
occur on much shorter timescales (<10 Myr). Strikingly, none of the
pegmatite groups seems to be temporally connected to the local granitic
intrusion dated at 2139 ± 6 Ma.

We propose a multi-intrusion model where small batches of melt originated from the partial melting of fertile metasediments and evolved through fractional crystallization, resulting in distinct series of pegmatites (Fig. 5). The evolution from Li-rich to Li-poorer but Nb-Ta-Cs-richer pegmatites could be explained by an early melting at low degree and/or temperature involving Li-rich staurolite, whereas later melting at higher degree and/or temperature allowed the contribution of biotite and oxides. Several studies have investigated the question of the mineral contribution to the melting depending on melting degree and temperature (Bonzi et al., 2022; Knoll et al., 2023). Knoll et al. (2023) interpret their albite-spodumene pegmatite poor in other accessory minerals as the result of the first metamorphic-migmatitic event, when melting temperature was low and biotite melting was not possible. Staurolite could be the source of lithium (Dutrow et al., 1986), and Knoll et al. (2023) demonstrate that it can melt incongruently by forming sillimanite at 650–750 °C.

At Ewoyaa, pegmatitic muscovite at the contact with the host rock (sample SE10–26) hosts 1600 ppm Rb and 20 ppm Cs. With muscovite-melt partition coefficients of 1.75 for Rb and 0.1 for Cs, melt concentrations of 900 ppm Rb and 200 ppm Cs can be calculated for the primitive melt at the origin of the Ewoyaa pegmatite series. This is consistent with the C₀ values estimated above. As Rb and Cs are

<figure><img src="image_6.png" /><figcaption>Fig. 4. A. Summary of results of the CGM U-Pb dating. B. CGM composition in the Ta/(Ta + Nb) vs. Mn/(Mn + Fe) columbite classification diagram.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Model of pegmatite formation based on the 4 distinct Rb/Cs fractionation series and 3 obtained U-Pb ages on CGM. Ewoyaa series 1 and 2 (as determined in Fig. 3) derived from different melt batches with different compositions, possibly linked to various degrees of staurolite-dominant melting. Series 3 (Winneba) and 4 (Mankwadze) probably derived from the same source, but two diachronous episodes of melt extraction evolved through different degrees of fractional crystallization (FC).</figcaption></figure>

incompatible, these concentrations can be produced from the direct
partial melting of biotite-bearing metasediments containing a few tens
ppm of Cs and hundreds ppm of Rb, which is consistent with concen-
trations observed in metasedimentary country rocks analyzed by
Atlantic Lithium Ltd.

We therefore propose that the biotite- and staurolite-rich micaschists
of Ewoyaa were the first to melt, at ~2170 Ma. A second episode of
melting affected both the Ewoyaa and Winneba metasediments at
~2080 Ma. Finally, the last episode of melting affected a source similar
to that of Winneba, in conditions that allowed stronger contribution of
biotite and enrichment in Nb-Ta-Cs. Strong fractional crystallization of

such melt batches, certainly favored by elevated flux contents (e.g.,
fluorine), as shown by the strong Fe/Mn fractionation in CGM (Fig. 4B),
led to the formation of the “true” LCT pegmatites with complex zoning,
abundant Ta mineralization but moderate Li mineralization, during this
latest stage.

6. Conclusion

Our study allows to directly link pegmatite crystallization ages with
the degree of fractionation, through CGM dating associated with pri-
mary magmatic muscovite. It brings evidence that increasingly
fractionated pegmatites (from poorly fractionated, but spodumene-rich, to highly fractionated, “true LCT” pegmatites) emplaced as three main events over a period of 140 ± 29 Myr in the CCP province. We propose a model of Li-bearing pegmatite genesis through direct partial melting of fertile meta-sediments, at various melting rates and/or temperatures, which involved various contributions of staurolite, biotite, oxides, and possibly other rare-metal-bearing minerals, followed by moderate to strong fractional crystallization within each melt batch. The study also shows that high-grade lithium mineralization does not necessarily involve extensive fractional crystallization of the anatectic melts.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemgeo.2025.122935.

## CRediT authorship contribution statement

Marieke Van Lichtervelde: Writing – review & editing, Writing – original draft, Methodology, Investigation, Funding acquisition, Conceptualization. Oscar Laurent: Writing – review & editing, Writing – original draft, Methodology, Investigation. Prince Ofori Amponsah: Writing – review & editing, Investigation. Isaac Iwan E. Williams: Writing – review & editing, Funding acquisition, Data curation.

## Declaration of competing interest

We hereby declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This study benefited from the support of Atlantic Lithium Ltd. for access to their drill core samples and whole rock analyses, the WAXI team for geological context, the French National Research Agency project TRANSFAIR (ANR-21-01CE-0022-01) and the Institute for Research and Development for analytical funding. Special thanks go to S. Adams for his help with sampling, and to F. Maubé, M. Leisen, A. Marquet and P. de Parseval for their technical support during analytical work. We are grateful to Patrick Carr and Frank Melcher for their constructive reviews, and to Marco Fiorentini for his care in handling the manuscript.

## Data availability

Data will be made available on request.

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