Progressive Melt Fractionation as the Primary Control on the Formation of Rare-Metal Pegmatites: Evidence for Continuous Granite–Pegmatite Evolution in the Central Kalba Ore District, Eastern Kazakhstan

Abstract

Rare-metal pegmatites of the Kalba–Narym belt (Eastern Kazakhstan) represent an important source of Li, Cs, Ta, Nb, Be, and associated critical metals. Despite extensive geological investigations, the relationships between granites, pegmatites, and late-stage alteration products within the Central Kalba ore district remain insufficiently constrained. This study integrates whole-rock geochemistry and muscovite trace-element data to evaluate regional fractionation trends and rare-metal enrichment within the granite–pegmatite system. The dataset comprises 29 whole-rock samples, including Phase I and Phase II granites, pegmatites, greisens, and hornfels, together with 11 muscovite separates from the Akhmetkino, Yubileynoye, Bakennoye, and Asubulak ore fields. Geochemical evolution was assessed using granite-normalized multi-element patterns and the Cs–Rb, K/Rb–Cs, Li–Rb, Li–Cs, Rb/Sr–Cs, and Ta–Cs relationships. Muscovite compositions were additionally compared with published datasets from the Totoral (Argentina) and Gatumba (Rwanda) pegmatite districts. The results reveal systematic enrichment in Li, Rb, Cs, Nb, and Ta accompanied by depletion in Sr and Ba from granites to the most evolved pegmatites. The strongest geochemical relationship is recorded by the Rb/Sr–Cs system (R² = 0.805), whereas Ta and Cs show only weak correlation (R² = 0.062). Muscovite compositions display decreasing K/Rb and K/Cs ratios and increasing Rb and Cs concentrations from Akhmetkino through Yubileynoye and Bakennoye to Asubulak, defining a regional fractionation sequence consistent with whole-rock geochemistry. The most evolved muscovites overlap compositional fields characteristic of highly fractionated LCT pegmatites. The geochemical patterns identified in both whole-rock and muscovite datasets indicate progressive melt evolution across the Central Kalba district. Late-stage alteration and volatile-rich mineral assemblages record additional fluid-related processes, although their quantitative contribution to rare-metal redistribution remains uncertain. The results identify Cs, Rb/Sr, K/Rb, and K/Cs as useful indicators of relative pegmatite evolution and provide new constraints on the development of rare-metal granite–pegmatite systems in Eastern Kazakhstan.

1. Introduction

Rare-metal granitic pegmatites constitute one of the most important sources of critical raw materials required for modern technologies and the global energy transition. Lithium, cesium, tantalum, niobium, beryllium, and tin are essential components of rechargeable batteries, electronic devices, renewable energy systems, and advanced alloys. Among the different pegmatite classes, the LCT (Li–Cs–Ta) family represents the principal source of lithium and tantalum worldwide and includes numerous economically significant deposits in Canada, Brazil, Africa, Australia, and Central Asia [1,2,3,4,5,6,7]. Consequently, understanding the processes responsible for rare-metal enrichment in LCT pegmatites remains a major objective of economic geology.

Despite decades of investigation, the origin of rare-metal pegmatites remains a subject of active debate. The classical model considers LCT pegmatites as products of extreme fractional crystallization of granitic magmas, resulting in progressive enrichment of residual melts in incompatible elements and volatile components [1,2,3]. Alternatively, some authors have emphasized the role of partial melting of fertile crustal sources and the influence of tectonic setting on the generation of pegmatitic melts [8]. Although both mechanisms may produce highly evolved melts enriched in Li, Rb, Cs, Nb, and Ta, the relative importance of fractional crystallization and crustal melting remains controversial in many rare-metal provinces worldwide [2,8,9]. Resolving this issue requires integrated geological, mineralogical, and geochemical investigations capable of evaluating the genetic relationships between granites and associated pegmatites.

One of the most widely used approaches for evaluating pegmatite evolution is based on the behaviour of incompatible trace elements such as Li, Rb, Cs, Nb, Ta, Sr, and Ba. Progressive enrichment of Li, Rb, Cs, Nb, and Ta coupled with depletion of Sr and Ba is generally interpreted as evidence of advanced magmatic fractionation [2,3,5,10]. While whole-rock geochemistry provides information on large-scale differentiation trends, mineral chemistry is increasingly recognized as a more sensitive indicator of melt evolution. In particular, muscovite has proven to be an effective recorder of fractionation processes owing to systematic variations in Li, Rb, Cs, and K/Rb ratios [5,9]. Studies from Argentina, Namibia, Rwanda, and other rare-metal pegmatite provinces have demonstrated that muscovite chemistry can reliably distinguish different evolutionary stages of pegmatite systems and can serve as an exploration tool for lithium mineralization [5,6,9,10].

The Kalba–Narym belt of Eastern Kazakhstan represents one of the largest rare-metal provinces of the Central Asian Orogenic Belt and hosts numerous occurrences and deposits of Li, Ta, Nb, Be, Sn, and associated rare metals [11,12,13,14,15,16,17,18,19]. Formation of the belt was associated with extensive Late Carboniferous–Early Permian magmatism that accompanied post-collisional tectonic evolution of the Altai region [11,20,21,22,23,24,25,26,27,28]. Rare-metal pegmatites are spatially and genetically associated with granites of the Kalba batholith and occur within several ore districts characterized by different degrees of magmatic evolution and rare-metal specialization [11,12,13,14,19,28]. Recent investigations have significantly improved understanding of the geology, mineralogy, geochronology, metallogeny, and classification of pegmatites within the Kalba–Narym belt [11,12,13,19,21,22]. In particular, the comprehensive classification proposed by Mizernaya et al. [11] demonstrated the existence of a continuous spectrum of pegmatite types ranging from barren and beryl-bearing pegmatites to highly evolved spodumene-, albite-, and lepidolite-bearing varieties.

Despite substantial progress in understanding the geological evolution of the Kalba–Narym pegmatite province, several fundamental questions remain unresolved. Previous studies have focused primarily on individual deposits, regional metallogeny, mineralogical characteristics, or geochronological constraints [11,12,13,19,20,23,24]. However, the geochemical relationships between granites, pegmatites, and metasomatic rocks have not been evaluated at the scale of the Central Kalba ore district using a unified dataset. Furthermore, it remains unclear whether the principal pegmatite fields represent independent magmatic systems or successive stages of a common evolutionary lineage derived from a single granite–pegmatite system. The relative contribution of progressive melt fractionation and late-stage metasomatic processes to rare-metal enrichment also remains insufficiently constrained.

This study addresses these questions through an integrated investigation of granites, pegmatites, greisens, and host rocks from the Central Kalba ore district, combined with new trace-element data for muscovite from the Akhmetkino, Yubileynoye, Bakennoye, and Asubulak ore fields. Particular attention is paid to the behaviour of Li, Rb, Cs, Nb, Ta, Sr, and Ba, as well as to mineral-chemical fractionation indicators such as K/Rb ratios. Unlike previous studies focused on individual deposits or pegmatite fields, this work integrates whole-rock and mineral-chemical datasets across the entire Central Kalba ore district, providing a regional-scale assessment of granite–pegmatite evolution and rare-metal enrichment. The objectives of the study are to (1) evaluate the genetic relationship between granites and rare-metal pegmatites of Central Kalba, (2) assess the role of progressive melt fractionation in rare-metal enrichment, and (3) establish an evolutionary model for the principal pegmatite fields of the district. The results provide new constraints on the petrogenesis of LCT pegmatites in Eastern Kazakhstan and contribute to a broader understanding of rare-metal pegmatite formation in post-collisional tectonic environments.

2. Materials and Methods

2.1. Study Area and Sampling Strategy

The study was conducted within the Central Kalba ore district of the Kalba–Narym rare-metal belt (Eastern Kazakhstan), one of the principal rare-metal provinces of the Central Asian Orogenic Belt [11,18,20,21,22,23,24,25,28,29]. Sampling focused on four representative rare-metal pegmatite fields (Akhmetkino, Yubileynoye, Bakennoye, and Asubulak), which collectively represent different stages of LCT-type pegmatite evolution [11,13,19].

The analytical database consists of two independent datasets.

The first dataset comprises 29 whole-rock samples, including: 17 pegmatite samples; 2 greisenized rocks; 2 hornfelsed metasedimentary host rocks; 3 Phase I granite samples; 5 Phase II granite samples, including one two-mica granite.

The second dataset comprises 11 muscovite separates collected from pegmatites representing different stages of rare-metal specialization within the four principal ore fields.

Sampling was designed to capture the complete evolutionary sequence of the granite–pegmatite system, from relatively primitive granites to highly fractionated rare-metal pegmatites and associated metasomatic rocks. Geographic coordinates of all sampling locations were recorded during field investigations and are presented in

Table 1. Locations of pegmatite bodies and associated greisenized and hornfelsed zones in the Central Kalba ore district.

N	Rock	Latitude	Longitude
1	Pegmatite (Akhmetkino)	82.8029	49.5127
2	Pegmatite (Tochka)	82.5242	49.6761
3	Pegmatite (Medvedka)	82.3742	49.6989
4	Pegmatite (Aldai)	82.7039	49.6665
5	Pegmatite (Yubileynoye)	83.0827	49.5465
6	Pegmatite (Bakennoye)	82.9675	49.6379
7	Pegmatite (Asubulak)	83.0158	49.544
8	Pegmatite (Ognevskoye)	82.9786	49.6867
9	Pegmatite (Cherdoyak)	83.9399	48.8122
10	Greisen (Akhmetkino)	82.8033	49.5099
11	Greisen (Medvedka)	82.3763	49.6961
12	Greisen (Asubulak)	83.0184	49.5490
13	Greisen (Bakennoye)	82.9739	49.6357
14	Greisen (Cherdoyak)	83.9589	48.8070
15	Hornfel (Akhmetkino)	82.8031	49.5116
16	Hornfel (Tochka)	82.5278	49.6751
17	Hornfel (Bakennoye)	82.9708	49.6351

.

Whole-rock samples were collected from fresh, unweathered outcrops. Muscovite concentrates were hand-picked from representative pegmatite zones and carefully cleaned prior to analysis to minimize contamination by associated minerals.

2.2. Mineralogical Investigations

Mineralogical investigations were carried out using optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS).

SEM studies were performed using a JSM-6390LV scanning electron microscope (JEOL Ltd., Tokyo, Japan) equipped with an Oxford Instruments INCA Energy Penta FET X3 EDS system at the VERITAS Centre of Excellence, D. Serikbayev East Kazakhstan Technical University.

The mineralogical investigations focused on identifying ore and gangue mineral assemblages, rare and critical elements in pegmatite systems [1,2,3,30,31], textural relationships, mineral succession, and mineral hosts of rare and critical elements. Particular attention was given to Li-bearing micas, spodumene, tourmaline, cassiterite, columbite–tantalite, pollucite, beryl, fluorite, and phosphate minerals.

The obtained mineralogical information was used to constrain the degree of pegmatite evolution and to support interpretation of geochemical trends.

2.3. Geochemical Analyses

Whole-rock and mineral samples were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at the VERITAS Centre of Excellence (EKTU).

Samples were crushed, pulverized, and digested using a multi-acid digestion procedure prior to analysis. The analytical program included major rare-metal indicators and trace elements relevant to pegmatite evolution, including Li, Be, Rb, Cs, Sr, Ba, Nb, Ta, Sn, Ga, Ge, W, Y, Zr, Hf, U, Th, and rare-earth elements commonly used as indicators of pegmatite evolution and rare-metal specialization [1,2,3,30].

Analytical quality control included certified reference materials, duplicate analyses, procedural blanks, and internal laboratory standards. Data quality was evaluated through comparison with certified values and repeated measurements.

Whole-rock and mineral datasets were treated independently throughout the study. Mineral-separate analyses were not combined with whole-rock data in statistical calculations, regression analyses, or petrogenetic interpretations.

2.4. Geochemical Data Processing and Statistical Analysis

To evaluate melt evolution and rare-metal specialization, a series of established geochemical fractionation indicators were applied, including Cs–Rb, K/Rb–Cs, K/Cs–Cs, Rb/Sr–Cs, and Ta–Cs [1,2,3,5,9,30,32].

These parameters are widely used in studies of LCT-type pegmatites because they are highly sensitive to progressive enrichment of incompatible elements during fractional crystallization.

Multi-element diagrams were normalized to the least evolved Phase I granite, which was used as an internal reference representing the parental magma composition. Additional primitive mantle-normalized diagrams were constructed using normalization values of Sun and McDonough (1989) [33].

Log–log least-squares regression analysis was applied to all principal geochemical relationships. Regression equations, coefficients of determination (R²), and 95% confidence intervals were calculated to quantify fractionation trends and evaluate their statistical significance.

To place the Central Kalba pegmatites within a global context, muscovite compositions were compared with published datasets from the Totoral pegmatite field (Argentina) [5,34] and the Gatumba pegmatite field (Rwanda) [10]. Comparative fields shown in were compiled from these published datasets and represented using convex-hull envelopes.

3. Geological Setting of the Central Kalba Granite–Pegmatite System

The Central Kalba ore district is located within the Kalba–Narym rare-metal belt of Eastern Kazakhstan, which forms part of the Irtysh–Zaisan fold system in the Central Asian Orogenic Belt [11,12,13,19,20,25]. In the context of this study, the geological significance of the region is determined primarily by the spatial, temporal, and genetic relationships between the Kalba granitoids and the associated LCT-type rare-metal pegmatites.

The Kalba–Narym Batholith is mainly composed of granitoids of the Kalba and Monastyrsky complexes. The Kalba Complex represents the principal magmatic unit associated with rare-metal pegmatite formation in the Central Kalba district. It includes earlier granodiorite–granite assemblages and later more evolved granite–leucogranite phases (Figure 1). Available U–Pb geochronological data indicate Early Permian magmatism, with emplacement ages of approximately 295 ± 1 Ma for the early Kalba granodiorite–granite phase, 287 ± 1 Ma for the late Kalba granite–leucogranite phase, and 284 ± 2 Ma for the Monastyrsky leucogranites. These ages define a relatively short-lived magmatic evolution during which progressively more fractionated granitic melts were generated.

Rare-metal pegmatites of Central Kalba are spatially associated with granitoids of the Kalba Complex and are commonly localized in apical parts and exocontact zones of granitic intrusions (Table 1, Figure 1). This geological position is important for interpreting the pegmatites as late-stage derivatives of evolved granitic melts. The close association between pegmatite bodies, evolved leucocratic granites, greisenized zones, and hornfelsed host rocks suggests that pegmatite formation occurred during the late stages of granite crystallization, when residual melts enriched in incompatible elements and volatile components were extracted from the crystallizing magma system.

Structural control played a major role in the emplacement of both granitoids and rare-metal pegmatites. The Kalba–Narym belt is dissected by northwest-trending and sublatitudinal fault systems, which acted as long-lived magma- and fluid-conducting zones. These structures provided pathways for the ascent and lateral migration of evolved residual melts and controlled the localization of pegmatite fields within the Central Kalba district. Therefore, the distribution of pegmatite bodies is interpreted not simply as a regional metallogenic feature, but as the result of structurally controlled melt extraction from a fractionating granitic system.

The locations and coordinates of the main deposits are shown in Figure 2 and

Table 2. List of rare metal deposits of Central Kalba, East-Kazakhstan.

№	Name	Major Commodity	Latitude	Longitude
1	Bakennoye	Sn, Be	49.41719	82.59417
2	Belogorskoye	Sn, Be	49.28145	83.75933
3	Asubulak	Be	49.54126	83.01611
4	Akhmetkino	Sn, Be	49.51155	82.80314
5	Yubileinoye	Sn, Be	49.24700	83.41587

. The studied pegmatite fields—Akhmetkino, Yubileynoye, Bakennoye, and Asubulak—represent different stages of LCT-type pegmatite evolution. Akhmetkino is dominated by less evolved beryl-bearing pegmatites, whereas Yubileynoye and Bakennoye contain more evolved spodumene-bearing and albite-rich varieties. Asubulak represents the most evolved stage and is characterized by highly fractionated albite–lepidolite and lepidolite-bearing pegmatites. This spatial and mineralogical sequence is consistent with progressive enrichment of residual melts in Li, Rb, Cs, Ta, Nb, and volatile components during fractional crystallization.

The internal evolution of the pegmatite system is expressed by a transition from relatively barren or weakly mineralized microcline–albite pegmatites to beryl-, spodumene-, and lepidolite-bearing assemblages. Greisenization, albitization, muscovitization, and related metasomatic processes locally modified the primary magmatic assemblages and redistributed some mobile elements. However, these processes are interpreted as superimposed late-stage modifications rather than independent controls on the regional-scale geochemical evolution of the system.

Pegmatites of the Kalba–Narym region are characterized by superposition of multistage metasomatic processes, including microclinization, albitization, muscovitization and silicification. These processes resulted in the formation of complex mineral assemblages involving a wide range of lithium- and cesium-bearing minerals such as spodumene, lepidolite, rubellite, amblygonite, pollucite, cleavelandite, and others [23].

The principal gangue minerals include quartz, K-feldspar, and muscovite, whereas the main ore minerals are cassiterite, tantalite, columbite, beryl, spodumene, and pollucite. In economically significant ores, average contents of the main ore components reach approximately Ta 125 g/t, Nb 117 g/t, Li₂O 0.1–0.3 wt.%, and Sn about 0.06 wt.%. Mineralogical investigations revealed a wide variety of tantalum-bearing minerals, including columbite–tantalite, manganotantalite, ixiolite, and microlite. In terms of mineralogical characteristics, Kalba pegmatites show close similarities to several well-known foreign pegmatite deposits, such as Bernic Lake (Canada), Koktokay (China), pegmatite fields of Zimbabwe, and the Kolmogorskoye deposit (Russia) [26,27,35,36,37,38].

Thus, the geological setting of the Central Kalba ore district provides a framework for interpreting the studied pegmatites as products of progressive differentiation of the Kalba granitic magmatic system. The combination of Early Permian granitic magmatism, emplacement in apical and exocontact zones, structural control by major fault systems, and systematic mineralogical evolution from less evolved to highly fractionated pegmatites supports the interpretation of a genetically related granite–pegmatite system.

4. Results

4.1. Multi-Element Geochemical Patterns Normalized to the Least Evolved Phase I Granite

Multi-element geochemical patterns for granites, pegmatites, greisens, and hornfels normalized to the least evolved Phase I granite from the Akhmetkino occurrence are presented in Figure 3,

Table 3. Trace-element composition of pegmatites, granites, and host rocks of the Kalba–Narym Belt (East Kazakhstan). All concentrations are given in ppm. Analytical method: ICP–MS.

Locality	Rock/Mineral	Li	Be	P	K	Ga	Ge	Rb	Sr	Y	Nb	Sn	Cs	Ba	Ta	W
Akhmetkino	pegmatite	85.3	53.5	786	17,940	24.68	1.45	463.9	34.99	1.84	93.4	43.94	47.28	121.6	62.34	0.40
Akhmetkino	pegmatite	95	45.2	657	19,280	22.18	1.20	514.5	41.51	2.08	79.66	52.42	57.23	122.6	95.71	0.37
Akhmetkino	pegmatite	272	51.5	673	13,370	16.87	1.42	199.7	35.31	3.93	70.43	17.18	11.80	119.6	47.08	1.19
Akhmetkino	pegmatite	4354	42.1	519	12,060	24.39	1.59	303.0	31.53	0.98	60.22	51.27	21.46	171.1	20.94	0.25
Akhmetkino	pegmatite	8050	19.83	619	5316	26.46	1.24	228.0	30.35	0.73	55.98	43.33	17.04	214.8	13.56	0.27
Akhmetkino	hornfelsed metasediments	1399	8.29	10,830	42,870	20.43	1.42	786.9	100.2	24.23	15.76	212	132	258.7	2.93	1.67
Akhmetkino	hornfelsed metasediments	177.3	1.77	779	34,990	16.75	1.30	81.4	148.7	18.70	8.81	22.97	12.62	490.9	1.36	0.95
Bakennoye	pegmatite	2022	1.77	3618	3039	34.93	1.18	469.4	63.06	4.03	19.08	29.50	113.2	1.65	0.35	0.0014
Bakennoye	pegmatite	2002	2.23	4928	34,310	33.42	1.22	1284	86.91	2.62	26.97	41.91	120.1	2.83	1.68	0.0022
Bakennoye	pegmatite	3260	12.90	5239	22,830	24.63	1.43	1166	271.10	2.89	86.51	25.47	124.2	0.86	1.45	0.0014
Bakennoye	pegmatite	1036	3.06	7020	50,690	22.22	1.38	1294	80.34	4.73	14.69	23.48	119.6	1.12	161.00	0.0015
Bakennoye	greisenized hornfels	860	1.66	531	46,120	20.16	1.12	143.6	119.20	4.58	14.65	3.90	514.2	8.30	22.08	0.0022
Bakennoye	greisenized hornfels	282	1.18	1643	30,310	18.71	1.44	112.3	267.40	14.91	19.36	3.28	323.6	8.34	7.07	0.0017
Yubileynoye	pegmatite	1042	2.80	581	78,800	83.3	2.00	748	90.6	8.04	40.22	93.4	49.4	105.4	10.16	20.54
Yubileynoye	pegmatite	646	2.52	456	81,800	142.2	2.27	2015	108.23	3.06	112.17	194	100.47	55.80	41.12	39.75
Asubulak	pegmatite	2166	1.21	3531	4544	10.97	2.34	354.1	46.61	0.35	37.84	14.29	101.2	110.7	7.91	0.18
Asubulak	pegmatite	151.7	4.74	924	1767	20.25	4.10	71.4	33.94	1.64	4.77	6.89	37.8	124.7	10.19	0.18
Asubulak	pegmatite	6470	2.60	3357	37,810	31.0	2.56	3169	25.73	0.60	1940	80.51	500.3	113.3	248.7	2.44
Asubulak	pegmatite	3930.6	2.17	1865	9785	16.28	3.41	748.1	39.24	0.70	398.04	22.17	164.0	117.8	54.78	0.59
Asubulak	pegmatite (albite–lepidolite)	10,680	4.77	323	77,380	54.41	3.73	6017	52.64	0.45	60.12	126.2	4176	113.0	34.97	1.23
Asubulak	lepidolite pegmatite	9300	5.20	100	73,640	79.27	4.68	9568	20.81	1.77	66.40	140.6	4758	110.9	51.33	1.38
Bakennoye	granite (I phase)	268.6	2.46	994.95	31,513.33	17.02	1.07	155.78	133.6	12.67	16.73	12.72	12.72	315.77	315.77	0.59
Yubileynoye	granite (II phase)	58.15	1.61	6678	66,455	21.46	2.31	1209.95	248.5	8.27	7.395	9.5	9.5	230.2	230.2	11.855
Asubulak	granite (phase II)	70.2	2.06	806	28,780	11.86	1.22	99.72	86.54	4.671	6.391	5.932	5.932	175.3	175.3	0.4752
Belogorskoye	granite (II phase)	43.68	1.286	1508.4	28,426	16.45	0.94228	146.8	167.16	12.414	12.0416	11.322	11.322	537.68	537.68	0.4751
Akhmetkino	granite (II phase)	127.9	3.046	1020.64	35,722	16.95	0.72854	219.94	121.78	8.1638	13.6358	15.388	15.388	337.4	337.4	0.6240
Akhmetkino	granite (I phase)	12.64	2.64	130.87	1772.6	15.722	0.138	50.572	79.22	5.836	1.47	11.734	11.734	236.8	236.8	0.92
Yubileynoye	granite (I phase)	369.5	1.657	1083	43,790	17.78	1.32	150.3	152.9	15.24	14.02	3.243	3.243	10.37	10.37	0.002985
Yubileynoye	two-mica granite (II phase)	66	1.07	785	40,510	26.97	0.94	189.9	307.8	15.09	10.29	6.62	6.62	366.5	366.5	0.81

. Compared with the reference composition, most Phase II granites exhibit elevated Rb, Nb, Th, Pb, P, and Zr values, whereas Sr and Ba show comparatively lower enrichment factors (Figure 3a). Variations among individual granite samples remain moderate, and the geochemical patterns of the two granite phases largely overlap.

Pegmatites, greisens, and hornfels display substantially greater compositional variability (Figure 3b). Rare-metal pegmatites are characterized by strong enrichment in incompatible elements, particularly Cs, Rb, Nb, and P. The highest enrichment levels are observed in pegmatites from the Bakennoye and Asubulak ore fields. Greisens exhibit similarly elevated Cs and Rb concentrations together with pronounced enrichment in K and P. In contrast, hornfels are characterized by distinct multi-element patterns, including comparatively high concentrations of Ti, Yb, Lu, and several rare-earth elements.

The normalized patterns reveal systematic geochemical differences between granites, pegmatites, greisens, and hornfels and provide a basis for evaluating compositional relationships within the Central Kalba granite–pegmatite system.

4.2. Whole-Rock Geochemical Fractionation Trends

The relationships between Cs, Rb, and K/Rb ratios are shown in Figure 4. The K/Rb ratio is widely used as an indicator of magmatic fractionation because Rb behaves as an incompatible element and progressively substitutes for K during melt evolution. The Cs–Rb diagram defines a positive logarithmic trend described by a coefficient of determination of R² = 0.719 (Figure 4a). Both granite and pegmatite samples follow the same general trend, characterized by increasing Cs concentrations with increasing Rb contents. The least evolved granites occupy the lower-left part of the diagram, whereas pegmatites from the Bakennoye and Asubulak ore fields are concentrated in the upper-right portion, corresponding to the highest Cs and Rb concentrations.

The K/Rb–Cs diagram shows a negative logarithmic relationship with R² = 0.609 (Figure 4b). K/Rb ratios decrease systematically with increasing Cs concentrations. Granite samples are characterized by relatively high K/Rb values and low Cs contents, whereas pegmatites from the Asubulak and Bakennoye ore fields exhibit lower K/Rb ratios and higher Cs concentrations. Most samples fall within the 95% confidence envelopes of the regression models.

The distribution of samples along both diagrams follows the inferred evolutionary sequence from Phase I granites through Phase II granites to the most evolved pegmatites of the Asubulak ore field.

4.3. Geochemical Indicators of Melt Fractionation and Rare-Metal Specialization

The Li–Rb, Li–Cs, Rb/Sr–Cs, and Ta–Cs relationships are presented in Figure 5. The Li–Rb diagram displays a positive logarithmic trend with R² = 0.439 (Figure 5a). Increasing Rb concentrations are accompanied by increasing Li contents. Granite samples occupy the lower part of the diagram, whereas pegmatites from the Bakennoye and Asubulak ore fields are characterized by the highest Li and Rb concentrations.

The Li–Cs diagram also exhibits a positive logarithmic relationship (R² = 0.497; Figure 5b). Cs concentrations generally increase with increasing Li contents, although some scatter is observed among individual pegmatite fields. The highest Li and Cs values occur predominantly in the Asubulak pegmatites.

The strongest correlation is observed in the Rb/Sr–Cs diagram (R² = 0.805; Figure 5c), where the Rb/Sr ratio (rubidium-to-strontium ratio) is used as a measure of progressive enrichment in incompatible elements relative to Sr. Rb/Sr ratios increase systematically with increasing Cs concentrations, defining a well-developed logarithmic trend across granites, pegmatites, and metasomatic rocks. Samples from the Asubulak ore field occupy the uppermost part of the trend and display the highest Rb/Sr values.

In contrast, the Ta–Cs diagram shows a weak relationship (R² = 0.062; Figure 5d). Ta concentrations display considerable scatter over the entire range of Cs values, indicating substantial variability among individual samples.

4.4. Muscovite Geochemistry as an Indicator of Melt Fractionation

To evaluate melt evolution independently from whole-rock compositions, trace-element compositions of muscovite were examined using the K/Rb (potassium-to-rubidium) and K/Cs (potassium-to-cesium) systems (Figure 6). Muscovite was selected because its trace-element composition is less sensitive to variations in modal mineralogy and more directly reflects the chemical characteristics of the melt from which it crystallized.

The analyzed muscovites display systematic compositional differences among the principal pegmatite fields of the Central Kalba ore district. Muscovites from the Akhmetkino field are characterized by relatively low Rb and Cs concentrations and comparatively high K/Rb and K/Cs ratios. Muscovites from the Yubileynoye and Bakennoye fields occupy intermediate positions, whereas muscovites from the Asubulak field exhibit the highest Rb and Cs contents together with the lowest K/Rb and K/Cs ratios (Figure 6).

Both diagrams show a progressive decrease in K/Rb and K/Cs ratios with increasing Rb and Cs concentrations. The distribution of samples defines a continuous compositional array extending from the Akhmetkino field through Yubileynoye and Bakennoye to the Asubulak pegmatites. The lowest K/Rb and K/Cs values are observed in the Asubulak muscovites, which occupy the most evolved part of both diagrams.

This systematic distribution defines a regional fractionation sequence, in which muscovite compositions record progressively increasing melt evolution from the least evolved Akhmetkino pegmatites to the highly fractionated Asubulak pegmatites.

The studied muscovites plot within, or immediately adjacent to, the fractionation fields defined for rare-metal pegmatites from the Totoral district (Argentina) and the Gatumba district (Rwanda) [5,10]. In particular, the compositions of the Asubulak muscovites overlap the fields characteristic of highly fractionated LCT pegmatites, whereas the Akhmetkino muscovites occupy less evolved compositional domains. The observed mineral-chemical trends are consistent with the whole-rock geochemical relationships described above and provide an independent record of progressive enrichment in incompatible elements during pegmatite evolution.

5. Discussion

5.1. Evidence for Progressive Granite–Pegmatite Evolution

The granite-normalized multi-element patterns (Figure 3) reveal systematic geochemical differences between granites, pegmatites, and late-stage alteration products of the Central Kalba ore district. Relative to the least evolved Phase I granites, Phase II granites show enrichment in incompatible elements, particularly Rb, Nb, Th, Pb, and P, whereas pegmatites exhibit progressively higher concentrations of Cs, Rb, Nb, and P. Greisens display similar enrichment patterns, while hornfels are comparatively enriched in Ti and heavy rare earth elements. Although individual samples show some variability, the overall geochemical patterns are consistent with progressive chemical evolution from granites to rare-metal pegmatites [1,2,30].

Additional evidence for this evolutionary sequence is provided by the whole-rock fractionation indicators (Figure 4 and Figure 5). The positive correlation between Cs and Rb (R² = 0.719) indicates coupled enrichment of highly incompatible elements during magma evolution [1,2]. The strongest relationship observed in this study is represented by the Rb/Sr–Cs diagram (R² = 0.805), where increasing Cs concentrations are accompanied by progressively higher Rb/Sr ratios. Together, these relationships define a continuous compositional trend extending from Phase I granites through Phase II granites to the pegmatites of the Akhmetkino, Yubileynoye, Bakennoye, and Asubulak ore fields [11], with no major compositional gaps between the investigated groups.

Independent support for this interpretation is provided by muscovite geochemistry (Figure 6). Muscovites from the Akhmetkino field are characterized by relatively high K/Rb and K/Cs ratios and lower concentrations of incompatible elements, whereas muscovites from the Asubulak field display lower K/Rb and K/Cs ratios together with elevated Rb and Cs contents. Muscovites from the Yubileynoye and Bakennoye fields occupy intermediate positions between these end members. This mineral-chemical trend closely parallels the whole-rock geochemical evolution inferred from Figure 3, Figure 4 and Figure 5 and defines a consistent regional fractionation sequence from Akhmetkino through Yubileynoye and Bakennoye to Asubulak.

The muscovite compositions from the Central Kalba ore district overlap, or occur immediately adjacent to, the fractionation fields defined for the Totoral pegmatite district (Argentina) [5] and the Gatumba pegmatite field (Rwanda) (Figure 6) [10]. In particular, muscovites from the Asubulak field occupy compositional domains comparable to those characteristic of highly fractionated LCT pegmatites, whereas muscovites from the Akhmetkino field plot within less evolved portions of the diagrams. Despite differences in age, tectonic setting, and magma source characteristics, these similarities suggest that comparable geochemical responses to progressive melt differentiation may have operated during pegmatite evolution [1,2,30].

Taken together, the granite-normalized geochemical patterns, whole-rock fractionation trends, and muscovite compositions define a coherent evolutionary sequence linking granites and rare-metal pegmatites within the Central Kalba ore district. These observations are consistent with progressive magmatic differentiation playing an important role in the evolution of the granite–pegmatite system [1,2,3,30]. The mechanisms responsible for rare-metal enrichment during this evolution are discussed in the “Section 5.2”.

5.2. Fractionation as a Major Control on Rare-Metal Enrichment

The geochemical relationships observed in the Central Kalba ore district suggest that progressive melt fractionation exerted an important control on rare-metal enrichment during granite–pegmatite evolution. This interpretation is supported by the systematic increase in Li, Rb, and Cs from the less evolved granites and pegmatites toward the more fractionated members of the investigated sequence. However, the variable strengths of the individual element correlations indicate that rare-metal distribution cannot be explained by a single process alone.

The Li–Rb and Li–Cs relationships show positive, although moderately dispersed, trends, with coefficients of determination of R² = 0.439 and R² = 0.497, respectively (Figure 5a,b). These relationships indicate that Li enrichment was broadly associated with the accumulation of Rb and Cs during melt evolution. Because all three elements are preferentially retained in evolved residual melts, their coupled increase is consistent with progressive differentiation of the granite–pegmatite system [1,2,3,30]. Nevertheless, the moderate R² values and the dispersion of individual samples suggest that Li concentrations were also influenced by additional factors, potentially including variations in mineral assemblages, local crystallization history, and late-stage redistribution.

The inverse relationship between the K/Rb ratio and Cs concentration provides further evidence of progressive differentiation (Figure 4b). The decrease in K/Rb with increasing Cs is consistent with the preferential enrichment of Rb and Cs relative to K in increasingly evolved melts. This trend is widely used as an indicator of fractionation in granitic and LCT pegmatite systems [3,9] because Rb substitutes for K in rock-forming minerals, whereas Cs generally remains more strongly concentrated in the residual melt until advanced stages of crystallization. In the Central Kalba dataset, the K/Rb–Cs relationship (R² = 0.609) therefore supports the interpretation of increasing melt evolution from the less fractionated granites and pegmatites toward the most enriched Asubulak samples [11].

The Rb/Sr–Cs relationship provides the clearest whole-rock geochemical response to this process (Figure 5c). Its comparatively high coefficient of determination (R² = 0.805) reflects the combined enrichment of incompatible Rb and Cs and the relative depletion of Sr during differentiation. Sr is preferentially incorporated into early-crystallizing feldspars, whereas Rb and Cs tend to remain in the residual melt [2,30]. The resulting increase in Rb/Sr therefore records the progressive separation of evolved melt from earlier crystallizing mineral assemblages. Within the investigated dataset, this ratio appears to be one of the most sensitive indicators of the degree of granite–pegmatite evolution.

In contrast, Ta shows no meaningful correlation with Cs (R² = 0.062; Figure 5d). This result is important because it indicates that Ta enrichment did not simply parallel the accumulation of highly incompatible alkali elements. Its distribution may instead have been influenced by the crystallization or local concentration of Ta-bearing phases, changes in melt composition, and variations in the availability of complexing components during the latest stages of pegmatite evolution [3,30]. Accordingly, whole-rock Ta concentrations may reflect both the degree of melt differentiation and mineral-scale controls that are not captured by Cs alone.

The combined relationships suggest that progressive fractionation was a major regional control on the enrichment of Li, Rb, Cs, and related incompatible elements in the Central Kalba pegmatites. At the same time, the moderate Li correlations and the weak Ta–Cs relationship show that individual rare metals responded differently during advanced melt evolution. The rare-metal specialization of the investigated pegmatites is therefore best interpreted as the product of progressive magmatic differentiation modified by element-specific mineralogical and late-stage geochemical controls [1,2,30].

5.3. Mineral Chemistry Constraints on Fractionation

Muscovite chemistry provides an independent mineral-scale perspective on the evolution of the Central Kalba pegmatites. Compared with whole-rock geochemistry, muscovite compositions are generally less affected by variations in modal mineral abundances and may therefore preserve a more direct record of element partitioning during melt evolution. For this reason, muscovite has been widely used as a geochemical indicator of fractionation in granitic and rare-metal pegmatite systems [5,9,32,39].

The K/Rb–Rb and K/Cs–Cs relationships (Figure 6) reveal systematic compositional differences among the investigated pegmatite fields. Muscovites from the Akhmetkino field are characterized by relatively high K/Rb and K/Cs ratios and lower concentrations of Rb and Cs, whereas muscovites from the Asubulak field display lower K/Rb and K/Cs ratios together with elevated concentrations of incompatible elements. Samples from the Yubileynoye and Bakennoye fields occupy intermediate positions between these end members. Although individual analyses show some overlap, the overall distribution of data is consistent with progressively increasing degrees of pegmatite evolution [5,9,32].

The Central Kalba muscovite compositions overlap, or occur immediately adjacent to, the fractionation fields reported for the Totoral pegmatite district (Argentina) [5,6,7,8,9,40] and the Gatumba pegmatite field (Rwanda) [8]. In particular, muscovites from the Asubulak field plot within domains comparable to those characteristic of highly evolved LCT pegmatites [3], whereas Akhmetkino samples occupy less evolved portions of the diagrams. Although direct genetic comparisons are not appropriate because of differences in geological setting and source composition, the observed similarities suggest that broadly comparable geochemical responses to melt evolution may have occurred in these pegmatite systems.

Taken together, the muscovite data indicate systematic changes in Rb and Cs enrichment that are consistent with progressive melt evolution and increasing rare-metal specialization. While muscovite compositions alone cannot fully constrain pegmatite genesis, they provide independent mineral-chemical evidence supporting the interpretation of a regional fractionation sequence within the Central Kalba ore district [11].

5.4. Role of Volatile Components and Late-Stage Processes

Several features of the Central Kalba pegmatites indicate that volatile-rich melts and late-stage fluid activity accompanied the final stages of pegmatite evolution. Evidence for these processes includes the occurrence of greisenization, albitization, tourmalinization, and mineral assemblages containing fluorite, topaz, lepidolite, and zinnwaldite within the most evolved pegmatites [2,30].

The presence of these minerals suggests enrichment of volatile components during advanced stages of crystallization. Experimental and natural studies of rare-metal pegmatites have shown that components such as B, F, P, and H₂O may influence melt properties and facilitate the evolution of highly differentiated residual melts [2,8,30]. The association of volatile-bearing minerals with the most evolved pegmatites of the Central Kalba ore district is consistent with such processes.

Greisens and albitized rocks occurring in spatial association with evolved pegmatites indicate that fluid-assisted alteration accompanied the late stages of system development [11,41]. However, the available dataset does not permit direct evaluation of the contribution of fluids to rare-metal transport or precipitation. Therefore, these alteration assemblages are interpreted primarily as indicators of late-stage fluid activity rather than direct evidence for fluid-controlled rare-metal enrichment.

Overall, the available evidence suggests that volatile-rich melts and late-stage fluids contributed to the final evolution of the Central Kalba pegmatite system. Their influence is most clearly reflected in alteration processes and volatile-bearing mineral assemblages, whereas the regional geochemical enrichment trends identified in this study are more consistently explained by progressive magmatic differentiation [1,2,30].

The identified geochemical and mineral-chemical trends may also have practical significance for exploration within the Kalba–Narym belt. In particular, Cs concentrations, Rb/Sr ratios, and muscovite K/Rb and K/Cs values appear to reflect the degree of pegmatite evolution and may therefore provide useful indicators for the preliminary assessment of rare-metal potential in poorly studied pegmatite occurrences [9,39,41].

6. Conclusions

• Granite-normalized multi-element patterns and trace-element systematics reveal a continuous geochemical transition from Phase I granites through Phase II granites to the rare-metal pegmatites of the Central Kalba ore district. The investigated rock groups occupy overlapping compositional fields and define a coherent regional evolutionary trend without pronounced geochemical discontinuities.
• Progressive enrichment in Li, Rb, Cs, Nb, and Ta accompanied by depletion in Sr and Ba reflects increasing melt evolution within the granite–pegmatite system. Among the evaluated indicators, the Rb/Sr–Cs relationship provides the strongest fractionation signal (R² = 0.805), whereas the weak Ta–Cs correlation (R² = 0.062) suggests additional controls on Ta distribution during advanced stages of evolution.
• Muscovite compositions record the same regional trend observed in the whole-rock dataset. Decreasing K/Rb and K/Cs ratios together with increasing Rb and Cs concentrations define a sequence from the less evolved Akhmetkino pegmatites through Yubileynoye and Bakennoye to the highly fractionated Asubulak field. The most evolved samples overlap compositional domains reported from fractionated LCT pegmatites elsewhere.
• Greisenization, albitization, tourmalinization, and the occurrence of fluorite-, topaz-, lepidolite-, and zinnwaldite-bearing assemblages indicate that volatile-rich melts and fluid-assisted alteration accompanied the final stages of pegmatite evolution. However, the available data do not allow quantitative evaluation of the role of fluids in rare-metal redistribution.
• The observed geochemical and mineral-chemical relationships are most readily explained by progressive magmatic differentiation. Within the Central Kalba district, Cs concentrations, Rb/Sr ratios, and muscovite K/Rb and K/Cs values may be used as practical indicators of relative pegmatite evolution and rare-metal specialization. Further isotopic studies and quantitative modelling are required to test the proposed granite–pegmatite relationships and to distinguish source-related effects from late-stage processes.