Pegmatites as Hosts of Critical Metals: From Petrogenesis to Mineral Exploration

The increasing demand for critical metals in green energy technologies has recently renewed the interest of the scientific community in granitic pegmatites, as this rock type constitutes one of the main sources of Li, Rb, Cs, Be, Nb, Ta, Sn, and REEs [1]. Inspired by the rising economic importance of pegmatites, this Special Issue of Minerals is a collection of articles centered on the topic of “Pegmatites as Hosts of Critical Metals: From Petrogenesis to Mineral Exploration”. This Special Issue is both timely and relevant, addressing renewed fundamental questions that continue to challenge our understanding of pegmatite formation, while also reflecting the surge of exploration programs unfolding worldwide [2].

Pegmatites are crustal igneous rocks with unique textural and geochemical characteristics, such as extreme crystal sizes and pronounced mineral zoning. The nature of pegmatite-forming melts, the pressure–temperature conditions under which they crystallize, and the timing of fluid exsolution are key factors controlling mineralization processes in pegmatites, yet these aspects remain debated [3,4]. Current petrogenetic models propose that pegmatite crystallization occurs when granitic magma is undercooled below the equilibrium liquidus, leading to suppressed nucleation and enhanced crystal growth rates [3,5,6,7]. These undercooling conditions promote the formation of megacrysts and extreme internal fractionation, which can ultimately result in critical-metal mineralization [8]. The pegmatite melt may be derived through advanced magmatic differentiation of a parent granitic melt, direct ± multistage anatexis of crustal rocks, or a combination of these processes [9,10]. Given the small size of pegmatitic bodies and the limited number of pegmatite occurrences that host economic concentrations of critical metals, it is essential to develop improved petrogenetic models, more rigorous analytical methods, and innovative geochemical tools addressing the pegmatites that can guide mineral exploration [11].

This Special Issue brings together studies spanning new descriptions of pegmatite occurrences and mineral assemblages, geochronology, fluid inclusion interpretations of fluid evolution, experimental simulations probing unresolved aspects of pegmatite petrogenesis, and the development of innovative prospectivity indicators for critical mineral resources. Collectively, the seven papers offer complementary perspectives, presenting an engaging snapshot of state-of-the-art methods, approaches, and enduring questions in pegmatite science.

The paper by Wise et al. [12] featured in this Special Issue has already raised interest in the academic and Li exploration communities. The study examined a large collection of muscovite crystals from different types of granitic pegmatites to test the usefulness of the K-Rb-Li systematics as a tool for distinguishing mineralogically simple bodies from pegmatites with potential Li mineralization [13]. The authors found that muscovite from common pegmatites exhibits low degrees of fractionation with high K/Rb ratios (ranging from 618 to 25) and typically low Li contents (<200 ppm). Muscovite crystals from Be-Nb-Ta-P-enriched pegmatites have generally lower K/Rb ratios and variable Li contents between 5 and >1700 ppm. On the other hand, muscovite from moderately to highly fractionated Li-rich pegmatites exhibits a wide range of K/Rb ratios and Li contents, depending on whether spodumene, petalite or lepidolite is the dominant type of Li aluminosilicate mineral. Lastly, REE- to F-enriched pegmatites may contain muscovite with K/Rb ratios between 691 and as low as 4, and Li concentrations between 19 and 15,690 ppm. The authors concluded that the proposed limits of K/Rb values and Li concentrations for identifying spodumene- or petalite-bearing pegmatites as part of an exploration program are reliable for Group 1 (LCT) pegmatites derived from S-type parental granites or anatectic melting of peraluminous metasedimentary rocks. The thresholds are not recommended for application to Group 2 (NYF) pegmatites affiliated with anorogenic to post-orogenic granitoids with A-type geochemical signatures or those derived by the anatexis of mafic rocks that generated REE- and F-rich melts.

Bilodeau and Baker [14] investigated nucleation delay in felsic melts by applying a modified model based on classical nucleation theory [15,16] to a natural hydrous peraluminous pegmatite composition and testing it against the results of crystallization experiments at 630 MPa pressure and temperatures between 650 and 1000 °C. The experimental results were compared with an established theoretical nucleation delay model, showing good agreement (within a factor of 5) for quartz and moderate agreement (within a factor of 10) for sodic feldspar. The authors demonstrated the potential of the model to predict nucleation delay, which is promising for the quantification of the nucleation delay of quartz and feldspar in natural felsic melts.

Krenn and Husar [17] combined previous fluid inclusion work [18] with new analyses to propose an ambitious regional evolution model for Permian pegmatites and their fluids in the Eastern Alps. To avoid overprinting from Cretaceous high-pressure metamorphism, their study focused on fluid and solid inclusions trapped in non-recrystallized accessory pegmatite minerals such as garnet, spodumene, tourmaline and beryl. Fluid inclusion microthermometry, micro-Raman spectrometry, and host-mineral compositions for three newly documented pegmatite fields provided a robust framework for reconstructing a generalized fluid evolution scenario of Group 3 anatectic pegmatites in the Austroalpine Basement. Pressure–temperature conditions and fluid evolution were recorded by inclusions trapped during two stages of garnet growth: peritectic cores formed during muscovite dehydration melting at 670–700 °C and 6–8 kbars, and outer rims formed during buoyant uprise of the fractionating melt at temperatures ≤ 670 °C and 4–5 kbars. The results revealed diverse fluid chemistries linked to regional-scale compositional variations in the sedimentary pile and highlighted the significant role of water-rich melts along deep crustal faults. The ultimate timing of post-entrapment inclusion modifications remained uncertain as the pressure estimates were compatible with both original Permian magmatic conditions and post-Permian high-pressure metamorphism.

Elsagheer et al. [19] described rare-metal pegmatites with a mixed NYF-LCT signature for the first time at Wadi Sikait in the Egyptian Nubian Shield. The Wadi Sikait pegmatites [20] include zoned and complex bodies with barren cores and outer wall zones highly mineralized with Nb, Ta, Y, Th, Hf, REE and U. The pegmatites contain mainly K-feldspar, quartz, micas (biotite, muscovite, zinnwaldite, lepidolite) and less albite as well as a wide range of accessory minerals, including garnet, columbite, fergusonite-(Y), cassiterite, allanite, monazite, bastnaesite (Y, Ce, Nd), thorite, zircon, beryl, topaz, apatite, and Fe-Ti oxides. The authors explained that the pegmatites and associated sheared granite represent highly differentiated peraluminous rocks typical of post-collisional rare-metal bearing granites, showing parallel chondrite-normalized REE patterns, enriched HREE relative to LREE contents and strongly negative Eu anomalies. The REE patterns showed an M-type tetrad effect, which is usually observed in strongly differentiated granites and is ascribed to hydrothermal fluid exchange. The Wadi Sikait pegmatites have mineralogical and geochemical characteristics of the mixed NYF-LCT family and exhibited non-CHARAC behavior due to hydrothermal effects. The authors also established a genetic relationship between the Wadi Sikait pegmatites and the surrounding sheared granite based on the similarities in their geochemical properties. The source magmas, which are believed to have been mostly derived from the juvenile continental crust of the Nubian Shield through partial melting, underwent advanced fractional crystallization. During a late hydrothermal stage, the exsolution of F-rich fluids transported some elements, locally increasing their concentrations to economic grades. The authors concluded that the origin of the studied pegmatites is consistent with that of the Wadi Zareib Li-pegmatite located in the central Eastern Desert of Egypt [21].

Sirbescu et al. [22] investigated dispersion mechanisms and pathways in metasomatic halos of fractionated LCT pegmatites and their shallow soil cover within forested, postglaciated terrain of Wisconsin, USA. Their study quantified the magnitude of soil geochemical anomalies and demonstrated that these anomalies can provide reliable indicators of lithium mineralization, in line with evidence from other regions [23,24]. Lithium (Li) and its pathfinder elements (Rb, B, Ga, and Sn) in soils overlying relatively thin pegmatite dikes and their metasomatized host rocks were analyzed with ICP-OES, revealing < 20 m-wide anomalies with up to 1400 ppm Li, 450 ppm Rb, 3100 ppm B, 40 ppm Ga, and 60 ppm Sn. These values exceeded those from the control-soil concentrations from nonmineralized granite and pegmatites. For prospecting, the study proposed thresholds of >100 ppm Li, Li + Rb + B + Ga + Sn > 420 ppm, and K:Rb > 275 in soils as indicators of proximity to mineralized dikes. However, soil thickness > 1 m or the presence of intervening glacial till tended to mask the hidden pegmatite bodies. The first occurrence of holmquistite to ferro-holmquistite, a lithium ortho-amphibole in the metasomatized exo-contacts of the Florence County, Wisconsin pegmatites, further emphasizes their economic potential. This greenfield pilot project led to additional Li-rich pegmatite discoveries within the district and highlighted the effectiveness of proximal sensors such as portable X-ray fluorescence (pXRF) or laser-induced breakdown spectroscopy (pLIBS) for soil exploration in North America [25,26].

Călin et al. [27] used electron micro-probe analysis (EMPA), polarized optical microscopy (POM), Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (p-XRD) to describe the first occurrence of triphylite in Li-bearing pegmatites from the Conțu-Negovanu area in Romania. In addition, the authors identified and described two new minerals, Fe-rich gatehouseite [28] and wolfeite [29], in pegmatites from the Conțu-Negovanu area, and calculated tentative empirical formulae for these secondary phosphate minerals using EMPA. The results indicate that these spodumene and triphylite-bearing pegmatites from the Southern Carpathian Mountains may be “upgraded from a lithium occurrence to a lithium ore deposit”.

Oitseva et al. [30] investigated the geochronology, mineralogy, and geochemistry of the Kvartsevoye Pegmatite, a Li-Ta-Nb mineralized system associated with plutons of the Khalba batholith, Eastern Kazakhstan [31]. This zoned field of variably differentiated granitic pegmatites hosts spodumene and tantalite–columbite mineralization localized in the quartz–albite–muscovite zone. Ar/Ar dating of muscovite yields ages ranging from 288 to 285 Ma, demonstrating a genetic link between the Kvartsevoye Pegmatite and granites of the Kalba complex, and suggesting that further promising rare-metal targets may be identified in the Kalba–Narym belt.

In summary, this collection of articles integrates various facets of ongoing fundamental and applied research on pegmatites. Spanning prospect-, district-, and regional-scale studies from Africa [19], Asia [30], Europe [17,27] and North America [22]; global-scale datasets [12]; and experimental advances in fundamental understanding of pegmatite petrogenesis [14], this Special Issue contributes to the much-needed arsenal of methods and approaches for identifying critical mineral reserves of the future.

As guest editors, we extend our sincere thanks to the authors for sharing their expertise, the reviewers for their thoughtful evaluations, and the editorial team for their support. It is our hope that the knowledge compiled in this Special Issue will serve as a foundation for future discoveries in pegmatite science and critical mineral exploration.