Pegmatite Formation Research Articles

Mineralogy of columbite-group minerals from the rare-element pegmatite dykes in the East-Qinling orogen, central China: Implications for formation times and ore genesis

The East-Qinling pegmatite district located in the North Qinling terrane is the largest one in the Qinling orogen, Central China. It is famous for its rare-element (REL) reserves, but its genesis is unraveled. We present in-situ major- and trace-element chemistry and U-Pb dating analysis of columbite-group minerals (CGM) and wodginite from representative Li-, Be-mineralized and barren pegmatite dykes. The mineralogical researches reveal that Li-mineralized pegmatite formed from a highly-evolved fluxes-rich magma, experiencing a magmatic stage at undercooling condition and a post-magmatic stage involving disturbance by alkali liquid or hydrothermal fluid. The lithium mineralization is accomplished by fractional crystallization after emplacement, alkali-fluid replacement or overprint of two Li-rich magmas. CGM Fe-Mn fractionation and REE distribution patterns with tetrad effect are considered as indicators to identify generations of CGM and trace the evolution of pegmatite formation. The CGM and zircon U-Pb ages suggest that the REL pegmatites formed in 422–410 Ma (Period I) and 399–384 Ma (Period II) and an early pegmatite magma activity occurred in 447–443 Ma. The late Ordovician-early Silurian and Period II pegmatite magmas stem from partial melting of North Qinling unit in collision and post-orogenic extensional settings, respectively, whereas Period I pegmatites formed in a weakening convergence setting by anatexis of fertile metasedimentary rocks or extremely fractional crystallization of granitic magma. The early-Paleozoic evolution of the North Qinling terrane contributes to the formation of large-scale East-Qinling REL pegmatites on the aspects of magma production and subsequent emplacement by crustal thickening, crustal reworking and dynamic extension during crust uplift.

Evolution of pegmatite ore-forming fluid: The Lijiagou spodumene pegmatites in the Songpan-Garze Fold Belt, southwestern Sichuan province, China

The late to post-tectonic Lijiagou spodumene pegmatites are located in the Ke’eryin orefield, which lies in the Triassic central Songpan-Garze Fold Belt (SGFB) in southwestern Sichuan Province, China. Fluid inclusion petrography, microthermometry and Raman spectroscopic analyses were carried out on quartz and spodumene in granites and four types of pegmatite in the Ke’eryin orefield. A Hydrothermal Diamond Anvil Cell (HDAC) was used to observe the microthermometric behavior of crystal-rich inclusions. Five types of fluid inclusions were identified. Crystal-rich inclusions (Type 1) in spodumene pegmatites were trapped at high temperature (658 to 741 °C) and high pressure (>4.7 kbar). Other inclusion types were found in granite and all types of pegmatite. Triphase CO2-H2O (CO2 + V + L, Type 2) inclusions show medium homogenization temperatures (324 to 339 °C), medium trapping pressures (2.09 to 2.54 kbar) and low to moderate salinities (3.1 to 11.3 wt% NaCl equiv). Biphase aqueous inclusions (L + V, Type 3) show similar homogenization temperatures (319 to 347 °C), and low to moderate salinities (5.5 to 18.0 wt% NaCl equiv). Monophase liquid CO2 (Type 4) inclusions have very low homogenization temperatures (15.4 to 19.1 °C). Monophase liquid H2O (Type 5) inclusions have moderate ice melting temperatures (–18.6 to –9.8 °C), and moderate salinities (18.4 to 18.5 wt% NaCl equiv). The various types of fluid inclusion reveal the P–T conditions of pegmatite formation, which records the transition from a magmatic to a hydrothermal environment and indicate that the ore-forming fluid resulted from the unmixing of magmatic melt. Fluid immiscibility is the key mechanism responsible for formation of the Lijiagou spodumene pegmatites and lithium mineral precipitation occurred from a high to medium temperature CO2-rich fluid. The absence of temperature and salinity gradients between granite and progressively more distal and evolved pegmatite compositions is inconsistent with formation of pegmatite magmas by fractional crystallization of granitic magmas, but consistent with separate anatectic origins of each type of pegmatite magma.

U-Pb dating and composition of columbite from Vishteritsa: Implication for timing of granite magmatism and rare-element granitic pegmatites in the Western Rhodopes, Bulgaria

The economic significance of pegmatites as a source of strategic rare metals for high-tech products and green energy motivated the present study on Ta–Nb oxides from Vishteritsa rare-element beryl–columbite LCT pegmatites of the Rila–West Rhodopes Batholith in the Western Rhodopes, Bulgaria. Here, we present the first U/Pb age data from columbite with application of the LA–ICP–MS U–Pb technique and a new X36 columbite standard reference material. The obtained Concordia age of 47.57 ± 0.32 Ma with a small spread of the individual 206Pb/238U ages between 45 and 51.3 Ma argues for Early Eocene magmatism and pegmatite formation. The host granite of the rare-element pegmatites is dated 51.94 ± 0.61 Ma with LA–ICP–MS U–Pb technique on zircon and suggests a fertile Early Eocene magmatic period in the Western Rhodopes. EPMA data for the composition of the columbite is used to refine the formula of the mineral (Mn0.554Fe0.427U0.006)0.987(Nb1.826Ta0.085Ti0.116)2.03O6 and define it as columbite-(Mn). Application of the in-situ LA–ICP–MS data technique establishes a series of typical trace elements (Ti, U, Zr, Hf, Y, W, and Zn) that are usually found in content above 500 ppm. The studied columbite is enriched in heavy rare earth elements (HREE sum: 306–697 ppm) and depleted in light REE and Eu. These geochemical characteristics are collectively interpreted as evidence for crystallization from highly fractionated fluid-rich magma. High UO2 content reaching 0.89 wt. % is characteristic for the Vishteritsa columbite. The decrease of U proximal to cracks and in outer crystal zones documents U-mobility during overprinting hydrothermal processes.

Tourmaline and quartz in the igneous and metamorphic rocks of the Tashisayi granitic batholith, Altyn Tagh, northwestern China: Geochemical variability constraints on metallogenesis

This study examines five rock units related to the Tashisayi chrysoberyl-bearing granitic batholith, Altyn Tagh, NW China: biotite granite, two-mica granite, muscovite granite, granitic pegmatite, and the surrounding biotite plagiogneiss. Three types of tourmalines and quartz are described, occur in the biotite plagiogneiss (Tur-1, Qtz-1), granitic rocks (two-mica granite [Tur-2a, Qtz-2a] and muscovite granite [Tur-2b, Qtz-2b]), and granitic pegmatite (Tur-3 and Qtz-3; tourmaline and quartz coexisting with chrysoberyl). Zircon UPb dating of the muscovite granite and granitic pegmatite yielded ages of 490–486 Ma for the Tashisayi batholith.The major-(by EPMA) and trace-element contents (by LA-ICP-MS) of the tourmaline and trace-elements (by LA-ICP-MS) and O-isotope (SIMS) of quartz indicates that: 1) The tourmalines are alkali schorl (Fe-rich tourmaline) and foitite; 2) tourmalines and quartz from the pegmatite have similar elemental concentrations and substitution mechanisms as tourmalines and quartz in granitic units of the Tashisayi batholith, but different from those of tourmalines and quartz in the biotite plagiogneiss, especially for Li, Be; 3) LA-ICP-MS analyses show that Qtz-1 in the biotite plagiogneiss has lower Ge/Ti values, lower Li, Be, Rb concentrations than Qtz-2a, Qtz-2b, and Qtz-3. Moreover, Qtz-2a, Qtz-2b, and Qtz-3 have a good positive correlation, consistent with whole-rock compositions, indicating crystal fractionation process in the formation of pegmatite, the chrysoberyl-bearing pegmatite is derived from a more evolved fluid than granitic rocks, thus, Li, Be are enriched by magmatism; 4) The O-isotopic compositions of Qtz-1 (+0.03 to +1.90‰) are consistent with a metamorphic water source, whereas the quartz in the two-mica granite (Qtz-2a; +8.85 to +9.58‰), in the muscovite granite (Qtz-2b; +9.27 to +9.93‰), and in the granitic pegmatite (Qtz-3; +14.8 to +17.8‰) are derived from the magmatic water or fluids from granite.

Experimental constraints on the formation of pegmatite-forming melts by anatexis of amphibolite: A case study from Evje-Iveland, Norway

The Evje-Iveland pegmatite field in Norway contains pegmatites that are known for their rare scandium mineralization. The petrogenesis of these pegmatites has been debated in the literature for nearly a century. Hypotheses for the origin of the pegmatite-forming melt have included either anatexis of the host amphibolite in vapor-absent conditions, wherein scandium is scavenged from the host amphibolite; or magmatic differentiation, wherein scandium is concentrated through magmatic processes. In order to test the hypothesis that the pegmatite-forming melt was sourced from the host amphibolite, partial melting experiments on the host amphibolite have been performed. These experiments were performed at temperatures ranging from 700 to 1064 °C and pressures between 400 and 550 MPa in a piston-cylinder apparatus. The solidus of the host amphibolite has been determined to be approximately 900 °C at 500 MPa and is significantly higher than the temperature of pegmatite formation. Partial melting of <40% can produce glasses that are broadly granitic in composition and are aluminum- and sodium-rich; however, they are less siliceous than the Evje-Iveland pegmatites. These glasses are also scandium- and REE-poor, and have REE patterns similar to leucosomes in vein-type migmatites, produced at low pressures, but dissimilar to the Evje-Iveland pegmatites. The results of these experiments are thus inconsistent with the hypothesis that the Evje-Iveland pegmatites or, by extension, other rare-element pegmatites, are the result of direct anatexis alone of common metamorphic rock such as amphibolites. It is proposed that the formation of the Evje-Iveland pegmatites is the result of partial melting of a scandium-rich ultramafic or mafic complex, differentiation of that partial melt, and emplacement of that melt into the host amphibolite. Thus, the pegmatite-forming melt may represent the final stages of magmatic differentiation, which is the preferred model for the formation of the Evje-Iveland pegmatites.

AGE AND COMPOSITION OF THE EARLY PALEOZOIC MAGMATIC ASSOCIATIONS AND RELATED RARE-ELEMENT PEGMATITES IN THE SOUTH-EASTERN PART OF THE SANGILEN BLOCK, TUVA-MONGOLIAN MASSIF

The article presents new data on ages (U-Pb zircon dating, SIMS SHRIMP-II) and chemical compositions of rocks from gabbro-granitic and granite-leucogranitic magmatic associations. These rocks preceded the formation of Li-enriched spodumene pegmatites of the Tserigiyngol-Burchin ore cluster (Russian: ЦБРУ), one of the main clusters in the South Sangilen pegmatite belt (SSB) located in the Tuva-Mongolian massif being a part of the Central Asian Fold Belt. We investigated the rocks from the Upper Tserigiyngol, Uchuglyk and Temenchulu plutons, and pegmatites from two neighbouring fields. We distinguish three impulses of granitic magmatism (517±7, 508±7, and 488±6 Ma), which are attributed to different stages of the Early Paleozoic collision orogeny (520-480 Ma). The period when the Li-enriched pegmatites were formed (494±7 Ma) is close to the magmatism impulse at 488±6 Ma. Differences are discovered in compositional and isotopic (Sm-Nd) features of granites dominating at the following stages of collisional orogeny: (1) early collision (517±7 Ma) – I-type granites, eNd(T)=0–1.5, TNd (DM-2st)=1.2–1.1 b.y., and (2) late collision (488±6 Ma) – A-2-type granites, eNd(T)=–3.0…–1.6, TNd (DM-2st)=1.5–1.4 b.y., which are due to different sources. Our study shows that facies transitions are absent between the late-collision granites (488±6 Ma) and the spodumene pegmatites from the Tserigiyngol-Burchin ore cluster (494±7 Ma), although these rocks are close in age. In terms of geochemical features, the spodumene pegmatites from the cluster are strongly different from both the late-collision granites and spodumene pegmatites from other SSB fields, including the large Tastyg lithium deposit. We have analysed the role of interactions between the crustal and mantle materials in the formation of granitoid sources in the Tserigiyngol-Burchin ore cluster, and described their evolution in time and the influence on the pegmatite rare-element specialization.

Fluid and Solid Inclusions in Host Minerals of Permian Pegmatites from Koralpe (Austria): Deciphering the Permian Fluid Evolution during Pegmatite Formation

Fluid inclusions (FIs) and associated solids in host minerals garnet, tourmaline, spodumene, and quartz from six pegmatite fields of Permian origin at Koralpe (Eastern Alps) have been investigated. Although pegmatites suffered intense Eoalpine high-pressure metamorphic overprint during the Cretaceous period, the studied samples originate from rock sections with well-preserved Permian magmatic textures. Magmatic low-saline aqueous FIs in garnet domains entrapped as part of an unmixed fluid together with primary N2-bearing FIs that originate from a host rock-derived CO2-N2 dominated high-grade metamorphic fluid. This CO2-N2 fluid is entrapped as primary FIs in garnet, tourmaline, and quartz. During host mineral crystallization, fluid mixing between the magmatic and the metamorphic fluid at the solvus formed CO2-N2-H2O–rich FIs of various compositional degrees that are preserved as pseudo-secondary inclusions in tourmaline, quartz, and as primary inclusions in spodumene. Intense fluid modification processes by in-situ host mineral–fluid reactions formed a high amount of crystal-rich inclusions in spodumene but also in garnet. The distribution of different types of FIs enables a chronology of pegmatite host mineral growth (garnet-tourmaline/quartz-spodumene) and their fluid chemistry is considered as having exsolved from the pegmatite parent melt together with the metamorphic fluid from the pegmatite host rocks. Minimum conditions for pegmatite crystallization of ca. 4.5–5.5 kbar at 650–750 °C have been constrained by primary FIs in tourmaline that, unlike to FIs in garnet, quartz, and spodumene, have not been affected by post-entrapment modifications. Late high-saline aqueous FIs, only preserved in the recrystallized quartz matrix, are related to a post-pegmatite stage during Cretaceous Eoalpine metamorphism.

The physical and chemical evolution of magmatic fluids in near-solidus silicic magma reservoirs: Implications for the formation of pegmatites

AbstractAs ascending magmas undergo cooling and crystallization, water and fluid-mobile elements (e.g., Li, B, C, F, S, Cl) become increasingly enriched in the residual melt until fluid saturation is reached. The consequential exsolution of a fluid phase dominated by H2O (magmatic volatile phase or MVP) is predicted to occur early in the evolution of long-lived crystal-rich “mushy” magma reservoirs and can be simulated by tracking the chemical and physical evolution of these reservoirs in thermomechanical numerical models. Pegmatites are commonly interpreted as the products of crystallization of late-stage volatile-rich liquids sourced from granitic igneous bodies. However, little is known about the timing and mechanism of extraction of pegmatitic liquids from their source. In this study, we review findings from thermomechanical models on the physical and chemical evolution of melt and MVP in near-solidus magma reservoirs and apply these to textural and chemical observations from pegmatites. As an example, we use a three-phase compaction model of a section of a mushy reservoir and couple this to fluid-melt and mineral-melt partition coefficients of volatile trace elements (Li, Cl, S, F, B). We track various physical parameters of melt, crystals, and MVP, such as volume fractions, densities, velocities, as well as the content in the volatile trace elements mentioned above. The results suggest that typical pegmatite-like compositions (i.e., enriched in incompatible elements) require high crystallinities (&amp;gt;70–75 vol% crystals) in the magma reservoir, at which MVP is efficiently trapped in the crystal network. Fluid-mobile trace elements can become enriched beyond contents expected from closed-system equilibrium crystallization by transport of MVP from more-evolved mush domains. From a thermomechanical perspective, these observations indicate that, rather than from melt, pegmatites may more likely be generated from pressurized, solute-rich MVP with high concentrations of dissolved silicate melt and fluid-mobile elements. Hydraulic fracturing provides a mechanism for the extraction and emplacement of such pegmatite-generating liquids in and around the main parental near-solidus mush as pockets, dikes, and small intrusive bodies. This thermomechanical framework for the extraction of MVP from mushes and associated formation of pegmatites integrates both igneous and hydrothermal realms into the concept of transcrustal magmatic distillation columns.

Formation of miarolitic-class, segregation-type pegmatites in the Taishanmiao batholith, China: The role of pressure fluctuations and volatile exsolution during pegmatite formation in a closed, isochoric system

Abstract The Taishanmiao granitic batholith, located in the Eastern Qinling Orogen in Henan Province, China, contains numerous small (mostly tens of centimeters in maximum dimension) bodies exhibiting textures and mineralogy characteristics of simple quartz and alkali feldspar pegmatites. Analysis of melt inclusions (MI) and fluid inclusions (FI) in pegmatitic quartz, combined with Rhyolite-MELTS modeling of the crystallization of the granite, have been applied to develop a conceptual model of the physical and geochemical processes associated with the formation of the pegmatites. These results allow us to consider the formation of the Taishanmiao pegmatites within the context of varios models that have been proposed for pegmatite formation. Field observations and geochemical data indicate that the pegmatites represent the latest stage in the crystallization of the Taishanmiao granite and occupy ≤4 vol% of the syenogranite phase of the batholith. Results of Rhyolite-MELTS modeling suggest that the pegmatite-forming melts can be produced through continuous fractional crystallization of the Taishanmiao granitic magma, consistent with the designation of the pegmatites as a miarolitic class, segregation-type pegmatites rather than the more common intrusive-type of pegmatite. The mineral assemblage predicted by Rhyolite-MELTS after ~96% of the original granite-forming melt had crystallized consists of ~51 vol% alkali feldspar, 34 vol% quartz, 14 vol% plagioclase, 0.1 vol% biotite, and 1 vol% magnetite, similar to the alkali feldspar + quartz dominated mineralogy of the pegmatites. Moreover, the modeled residual melt composition following crystallization of ~96% of the original melt is similar to the composition of homogenized MI in quartz within the pegmatite. Rhyolite-MELTS predicts that the granite-forming melt remained volatile-undersaturated during crystallization of the batholith and contained ~6.3 wt% H2O and ~500 ppm CO2 after ~96% crystallization when the pegmatites began to develop. The Rhyolite-MELTS prediction that the melt was volatile-undersaturated at the time the pegmatites began to form, but became volatile-saturated during the early stages of pegmatite formation, is consistent with the presence of some inclusion assemblages consisting of only MI, while others contain co-existing MI and FI. The relationship between halogen (F and Cl) and Na abundances in MI is also consistent with the interpretation that the very earliest stages of pegmatite formation occurred in the presence of a volatile-undersaturated melt and that the melt became volatile saturated as crystallization progressed. We propose a closed system, isochoric model for the formation of the pegmatites. Accordingly, the Taishanmiao granite crystallized isobarically at ~3.3 kbar, and the pegmatites began to form at ~734 °C and ~ 3.3 kbar, after ~96% of the original granitic melt had crystallized. During the final stages of crystallization of the granite, small pockets of the remaining residual melt became isolated within the enclosing granite and evolved as constant mass (closed), constant volume (isochoric) systems, similar to the manner in which volatile-rich melt inclusions in igneous phenocrysts evolve during post-entrapment crystallization under isochoric conditions. As a result of the negative volume change associated with crystallization, pressure in the pegmatite initially decreases as crystals form, and this leads to volatile exsolution from the melt phase. The changing PTX conditions produce a pressure-induced “liquidus deficit” that is analogous to liquidus undercooling and results in crystal growth as required to return the system to equilibrium PTX conditions. Owing to the complex closed system, isochoric PVTX evolution of the melt-crystal-volatile system, the pressure does not decrease rapidly or monotonically during pegmatite formation but, rather, gradually fluctuates such that at some stages in the evolution of the pegmatite the pressure is decreasing while at other times the pressure increases as the system cools to maintain mass and volume balance. This behavior, in turn, leads to alternating episodes of precipitation and dissolution that serve to coarsen (ripen) the crystals to produce the pegmatitic texture. The evolution of the pegmatitic melt described here is analogous to that which has been well-documented to occur in volatile-rich MI that undergo closed system, isochoric, post-entrapment crystallization.

Geochemical characteristics and significance of apatite from the Koktokay pegmatitic rare-metal deposit, Altay, Xinjiang

磷灰石可以记录和保存岩浆和热液活动的信息。可可托海伟晶岩型稀有金属矿床磷灰石发育，为研究该矿床伟晶岩成岩成矿过程提供了优良的条件。已有对可可托海伟晶岩型稀有金属矿床磷灰石的研究集中在其稀土元素特征，较少讨论其对伟晶岩成岩成矿过程的制约。本文选取可可托海伟晶岩型稀有金属矿床富矿伟晶岩脉（3号脉）和相对贫矿伟晶岩脉（1号、2b号和3a号脉）中的磷灰石作为研究对象，进行磷灰石岩相学和地球化学研究。岩相学分析表明，磷灰石主要与钠长石、石英、白云母、锰铝榴石等伴生。EPMA分析显示，磷灰石F含量为3.67%~4.41%，Cl含量小于0.67%，较低的Cl含量表明伟晶岩熔体出溶的流体Cl含量较低；大部分磷灰石MnO含量为4.67%~8.71%，但2b号脉磷灰石MnO含量变化较大（1.23%~14.28%），这是由于2b号脉磷灰石具有分带结构，暗示其遭受后期热液作用，促使磷灰石溶解-再沉淀，导致MnO含量发生较大变化。LA-ICP-MS分析显示，贫矿伟晶岩脉磷灰石的稀土元素含量较低（<180×10^-6）；相反，富矿伟晶岩脉磷灰石的稀土元素含量较高（>700×10^-6），并具有明显的四分组效应（TE_1-3平均值为1.7）。1号脉和3a号脉磷灰石均显示轻稀土元素富集，反映其形成过程中有含Cl热液的参与。3号脉磷灰石显示强烈Eu负异常和Ce正异常，而2b号脉磷灰石显示强烈Ce负异常和中等Eu负异常，这种Eu、Ce异常的差异可能与岩浆-热液阶段大量流体出溶密切相关。磷灰石的沉淀将导致热液中HF含量的降低，促使磷灰石周围铌钽矿结晶和Nb、Ta进入磷灰石中。可见，在伟晶岩形成过程中，磷灰石并非保持稳定，其分带结构和主微量成分变化记录了后期热液活动，暗示后期热液活动对伟晶岩的成矿具有重要作用。;Apatite records and preserves the information of magmatic and hydrothermal processes. There is much apatite in Koktokay pegmatitic rare-metal deposit, which provides excellent conditions for studying the diagenetic and metallogenic process of pegmatite in the deposit. Previous studies on apatite from the deposit have focused on the characteristics of rare earth elements, and less on its constraints on the diagenesis and mineralization of pegmatite. In this paper, apatite from rare-metal enriched pegmatite (No.3) and relatively barren pegmatite (No.1, No.2b, and No.3a) of the Koktokay pegmatitic rare-metal deposit is selected as the research object to study their petrography and geochemistry. Petrographic analysis shows that the apatite is mainly associated with albite, quartz, muscovite, and spessartine. EPMA analysis shows that the F content of apatite is 3.67%~4.41%, and the Cl content is less than 0.67%. The low Cl content indicates that the Cl content of the fluid exsolved from pegmatite melt is low; The MnO content of most apatite is 4.67%~8.71%, but the MnO content of apatite from No.2b pegmatite changes greatly (1.23%~14.28%), which is due to the zoning structure of the apatite, suggesting that it was affected by later hydrothermal process, which promoted apatite dissolution and reprecipitation, resulting in great changes in MnO content. LA-ICP-MS analysis shows that the rare earth element content of apatite from the relatively barren pegmatite is low (<180×10^-6); On the contrary, the content of rare earth elements in apatite of the enriched pegmatite is high (>700×10^-6) and has obvious REE tetrad effect (the average value of TE_1-3 is 1.7). The apatite from No.1 and No.3a pegmatite shows enrichment of light rare earth, reflecting the participation of bearing Cl hydrothermal fluid in their formation. The apatite of No.3 pegmatite shows extreme negative Eu anomaly and positive Ce anomaly, while the apatite of No.2b pegmatite shows strong negative Ce anomaly and medium negative Eu anomaly. The difference between Eu and Ce anomalies may be closely related to a large number of fluids dissolved by the melt in the magmatic-hydrothermal stage. The precipitation of apatite leads to the decrease of HF content in hydrothermal fluid, promotes the crystallization of columbite-tantalite group mineral around apatite, and the entry of Nb and Ta into apatite. Therefore, apatite is not stable during the formation of pegmatite, and its zoning structure and changes of major and trace composition record the later hydrothermal process, suggesting that the process plays a vital role in the mineralization of pegmatite.

Again About the "Magmatic" Nature of Topaz Crystals From Chamber Pegmatites of Volyn (Ukrainian Shield)

Various aspects of the genesis of primary fluid inclusions (0.01-1.0 sometimes up to 2 mm) with a large number of mineral inclusions in topaz crystals from chamber pegmatites of Volyn were analyzed. The data could be interpreted in two fundamentally different ways. The first argues for crystals grown in a magmatic melt; the second for an aqueous solution, with a density close to critical. The essence of the discrepancy is the reliability of the identification of the nature of mineral phases in the primary inclusions, if they are crystals captured during growth (xenogenic) or daughter crystals from the fluid. The xenogenic origin of the phases is indicated by the following observations: 1) The location of the mineral inclusions on the growing faces of the topaz crystals depends on the orientation of the crystal’s axis [001] relative to the horizontal plane. It determines the faces on which small mineral phases could be deposited from an aqueous suspension during the growth of topaz crystals. The studied crystals are dominated by individuals in which the mineral inclusions are located on the growing faces {011}, {021}, (001) (and others) of the crystal head. During growth, they were approximately in an upright position. 2) The filling of primary fluid inclusions is not constant. The volume of mineral phases in the inclusions varies from 40 to 95%, often 70-75%, the rest of the volume is gas and aqueous solution. Liquid-gas (liquids ˂ 40%) inclusions without or with &lt; 5% solid phases are very rare. In addition, the ratio between the volumes of different mineral phases in the inclusions is not constant. 3) Light rims (Becke lines) around the inclusions record a change in the refractive indices (caused by a different chemical composition) of topaz when inclusions are acquiring the equilibrium form of the negative crystal. 4) The xenogenic nature of the mineral phases of the primary fluid inclusions in topaz is indirectly confirmed by the value of the fluid pressure (260-300 MPa)of the magmatic melt (determined by the method of homogenization of these inclusions), as it denies the possibility of chamber pegmatite formation at depths of 9-11 km. Thus, the peculiar mineral inclusions were deposited on the face of growing topaz crystals of small mineral phases from a turbid aqueous suspension, which boiled violently. We conclude that topaz crystals in chamber pegmatites of Volyn grew in aqueous solution at a temperature of 380-415ºС and a pressure of 30-40 MPa.

Granite Systems with Rare-Metal Pegmatites

For most rare-metal pegmatite fields, two generations of granitic pegmatites are documented, namely, beryll-bearing (often with tantaloniobates and muscovite, which are inseparable from vein granites of the leucogranite complex), and Na-Li (Li, Ta, Cs, Be, and Sn) (LCT-pegmatites). The latter, in turn, subdivide into (i) a “multicomponent rare-metal type” with pegmatite zoning and main mineralization (large crystals of spodumene, tantalates, beryll, cassiterite, pollucite, petalite, and amblygonite) in the central parts of the respective bodies (Koktogai, Bernic Lake, Bikita, Karibib, Varutrask, Vishnyakovskoe, and others) and (ii) “albite-spodumene pegmatites” that compose multiple extended dikes that are devoid of pegmatite zoning and are grouped into fields of up to 10–15 km or more in length (Kings Mountain, Zavitinskoe, Gol’tsovoe, Kolmozero, Polmos-Tundrovoe, Tastyg, and others). For a number of deposits (e.g., Zavitinskoe, Vasin-Myl’k, and Shukbyul’), the occurrence of multicomponent rare-metal pegmatites in the “heads” of feeding dikes of “albite–spodumene pegmatites” is established. Any attempt to identify parent granites for “albite–spodumene pegmatites” is a priori futile, because these are not pegmatites, but granites, although specific ones (in fact, spodumene granites of the Allakha type). The purely terminological correction, granite instead of pegmatite, surprisingly has both scientific and forecasting implications. The scientific implications imply that, first, the problem about parent granites is solved and, second, the “pegmatite” status of “albite–spodumene pegmatites” no longer debated. These pegmatites correspond to the particular, spodumene–rare-metal–granite stage in the history of magmatism of certain pegmatite-bearing areas (the end of this stage was marked by the formation of true multicomponent pegmatites that are characteristic of only this stage). Regarding the forecast implications, it can be supposed that such multicomponent deposits as Koktogai, Bernic Lake, Bikita, Karibib, and Vishnyakovskoe are underlain by a suite of dikes comprised by spodumene-bearing rare-metal granites (i.e., an independent Li deposit), and multicomponent pegmatites are differentiates of these granites.

Crystal chemistry and microfeatures of gadolinite imprinted by pegmatite formation and alteration evolution

AbstractGadolinite [REE2Fe2+Be2Si2O10] is a common mineral in certain types of rare element and rare earth element (REL-REE) pegmatites. Changes in pegmatite environment during and after gadolinite formation may be devised by studying its crystal-chemical properties and a thorough observation of microfeatures in the mineral matrix. Post-crystallization processes in pegmatite might trigger alteration mechanisms in gadolinite like in other REE-rich pegmatite minerals, whereby various late-magmatic or metasomatic events may affect mineral chemistry. Three gadolinite samples originating from various pegmatite occurrences in southern Norway offer an excellent opportunity in studying post-crystallization evolution of the pegmatites; by determining their crystallographic, chemical, and micro-textural features, imprints of the related processes in the pegmatites have been characterized in this study. Relevant mineral information was collected in recrystallization experiments of fully or slightly metamictized gadolinite samples and subsequent XRD analyses. Micro-Raman spectroscopy, electron microprobe analysis (EMPA), and scanning electron microscope–backscattered electron–energy-dispersive X-ray spectroscopy (SEM-BSE-EDS) analyses were employed to retrieve micro-chemical properties and related micro-textural features of the mineral matrix. With a reference to the gadolinite supergroup, a general alteration path can be envisaged outlining the pegmatite evolution and suggesting the occurrence of the secondary REE mineral phases: altered gadolinite domains prove Ca enrichment with a tendency toward the hingganite composition, while a slight fluorine increase and sporadic secondary fluorite occurrence imply a significant role of fluorine as a complexing agent in the dissolution-reprecipitation mechanism of metasomatic alteration in the mineral. Micro-Raman spectra show improved vibration statistics for the altered gadolinite domains, which could be linked to the substitution of rare earth elements (REE) by Ca and a possible increase of structural ordering within the gadolinite structure, being at the same time an indication of structural healing of metamictized domains by metasomatic processes. A study of microfeatures in the complex silicates like gadolinite proves to be an excellent tool to trace post-crystallization processes in a pegmatitic environment. With a slight redistribution of radionuclides during an alteration in gadolinite, a moderate precaution has to be taken when selecting gadolinite for U-Th-Pb dating.

Episodes of fast crystal growth in pegmatites

Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length. Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. Growth rates and conditions, however, remain poorly constrained. Here we investigate quartz crystals and their trace element compositions from miarolitic cavities in the Stewart pegmatite in southern California, USA, to quantify crystal growth rates. Trace element concentrations deviate considerably from equilibrium and are best explained by kinetic effects associated with rapid crystal growth. Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10−6–10−7 m s−1 to 10−5–10−4 m s−1 (10-100 mm day−1 to 1–10 m day−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together.

Lithium and Nd isotopic constraints on the origin of Li-poor pegmatite with implications for Li mineralization

Although volumetrically minor in the upper continental crust, pegmatites host lots of rare-metal mineralization (e.g., Li, Be). Pegmatites and associated mineralization were generally thought to originate from extreme differentiation of a parent granite system. We propose an alternative model to interpret the genesis of pegmatite and factors controlling Li mineralization based on combined Nd and Li isotopic studies on the Li-poor pegmatites (barren) from the Qinghe pegmatite field of the Altai orogen (NW China). The Qinghe pegmatites show significantly varied Nd isotopic compositions (εNd(t) = −4.3 ~ −1.7), which overlap the surrounding schists (−6.5 ~ −1.5), but lower than the granites (−2 ~ +2.9) of the orogen. These data, together with the low REE (3.3–41 ppm) and absence of biotite in the pegmatites, suggest they were not derived from extreme differentiation of a parent granite as traditionally thought, but from low degrees of partial melting of the schists involving muscovite dehydration melting under amphibolite facies conditions. The pegmatites are characterized by heavy Li (δ7Li = 4.1–14.5‰) and low Li abundance (3.6–50 ppm), which contrast to that of local schist (δ7Li = +0.9 ~ +3.0‰, Li = 24–123 ppm) and granite (δ7Li = +0.9 ~ +3.0‰, Li = 24–70 ppm), and also to that of global Li-rich pegmatites (δ7Li = −1.0 ~ +10‰, Li >500 ppm). Modeling study indicates that Li-poor pegmatites always have heavy Li signature compared with Li-rich pegmatites due to the abundance of biotite in the source. The formation of Li-rich pegmatite with Li mineralization is attributable to the distinct source characteristics with abundant fluxing components like Li, Na, CO32−, and HCO3− and sufficient alumina-rich clay phases, and the fluxing components probably originated from evaporate interlayers in the source.

Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity

This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

Formation features and the pegmatite-controlling role of the thrusts and reverse faults in the Mamskaya muscovite province

Research subject . The Mamskay muscovite province (MMP) is located on the border between the Siberian platform and the Baikal orogenic area, near the boundary fault. The MMP hosts numerous regional and local thrust and reverse faults, which are typical of compression zones. This article aims to present a generalized description of the largest faults in the study area, to show their position in the regional structure of the province, as well as to consider their relationship with the main MPP tectonic stages, pegmatite and mica formation. Materials and methods . The study was conducted on the basis of literature and archive data, as well as those obtained during the authors’ long-term research. The correlation between the formation stages of the MMP reverse faults and the dynamics of the compression zone formation was investigated using the methods of physical modelling at the Laboratory for Tectonophysics, Institute of the Earth’s crust of the Siberian Branch of the Russian Academy of Sciences. Results. A comparison of experimental and field observations revealed the following six major stages in the evolution of the MMP fault tectonics: sedimentation, pre-pegmatite inversion or collisional-folded, early pegmatite, late pegmatite, post pegmatite and neotectonic. These stages were linked with the processes of folding, magmatism, pegmatite and mica formation. Conclusions . It was established that the formation of reverse faults and thrusts in the MMP resembled the process of fracturing in compression zones. The pegmatite-controlling role of the thrusts and reverse faults is determined by their age relations with the stages of the pegmatite and mica formation process. The main types of the mica-rich pegmatite fields in the MMP fault zones, which were mapped during geological survey studies, were distinguished and characterized.

Raman spectroscopic identification of cookeite in the crystal-rich inclusions in spodumene from the Jiajika lithium pegmatite deposit, China, and its geological implications

Abstract. Cookeite usually occurs as a late alteration product in lithium–cesium–tantalum-type granitic pegmatite. Consequently, cookeite-bearing crystal-rich inclusions (CIs) in pegmatite are considered to be of secondary origin, which constrains our understanding of pegmatite formation. Thus far, no reported cookeite has produced a distinct Raman spectrum. However, the CIs hosted in spodumene in the Jiajika pegmatite deposit, China, contain a cookeite-like hydrous lithium–aluminum–silicate phase, yielding a distinct Raman spectrum. In electron microprobe analysis, focused ion beam scanning electron microscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), the average composition of this hydrous phase was determined as Li1.005(Al3.997Fe0.018)(Si3.086Al0.914)O10.076OH7.902F0.098, close to the International Mineralogical Association (IMA) formula of cookeite, (Al, Li)3Al2(Si, Al)4O10(OH)8. The distinct Raman peaks at 98, 167, 219, 266, 342, 382, 457, 592, 710, and 3640 cm−1 were consistent with those of natural cookeite recrystallized in a hydrothermal diamond-anvil cell. The peaks were ascribed to the crystallization of cookeite from the liquid trapped in the closed space during the spodumene crystallization, which occurred at relatively high temperature and pressure without incorporating the minor elements commonly present during alteration processes. These minor elements often obscure the Raman signals, primarily by fluorescence effects. This type of cookeite in CIs with distinct Raman signals is unusual and can indicate whether the cookeite crystallized from fluid trapped within the closed space of a primary inclusion. In such a case, the fluid can be considered a flux-rich hydrous melt in pegmatite formation models.

Fluid Characteristics and Evolution of the Zhawulong Granitic Pegmatite Lithium Deposit in the Ganzi‐Songpan Region, Southwestern China

The Zhawulong granitic pegmatite lithium deposit is located in the Ganzi‐Songpan orogenic belt. Fluid inclusions in spodumene and coexisting quartz were studied to understand the cooling path and evolution of fluid within albite–spodumene pegmatite. There are three distinguishable types of fluid inclusions: crystal‐rich, CO2–NaCl–H2O, and NaCl–H2O. At more than 500°C and 350∼480 MPa, crystal‐rich fluid inclusions were captured during the pegmatitic magma‐hydrothermal transition stage, characterized by a dense hydrous alkali borosilicate fluid with a carbonate component. Between 412°C and 278°C, CO2–NaCl–H2O fluid inclusions developed in spodumene (I) and quartz (II) with a low salinity (3.3–11.9 wt%NaCl equivalent) and a high volatile content, which represent the boundary between the transition stage and the hydrothermal stage. The subsequentNaCl–H2O fluid inclusions from the hydrothermal stage, between 189°C and 302°C, have a low salinity (1.1–13.9 wt%NaCl equivalent). The various types of fluid inclusions reveal the P–T conditions of pegmatite formation, which marks the transition process from magmatic to hydrothermal. The ore‐forming fluids from the Zhawulong deposit have many of the same characteristics as those from the Jiajika lithium deposit. The ore‐forming fluid provided not only materials for crystallization of rare metal minerals, such as spodumene and beryl, but also the ideal conditions for the growth of ore minerals. Therefore, this area has favorable conditions for lithium enrichment and excellent prospecting potential.

On the history of mineralogenic studies and development of Arctic Karelia

The paper focuses on the mineral resources of the Karelian Arctic. Arctic districts are linked to the White Sea. The White Sea is connected via the White Sea-Baltic Channell with the Baltic, Caspian and Black seas. All the above districts have state railways and highways connecting Karelia with South and North Russia. A retrospective review of the history of research and mastering of mineral resources is given. The current state of mineral raw base of the northern regions of Karelia is shown. The mineral potential of three areas in Arctic Karelia is of great value as there are many deposits and occurrences and a variety of mineral reserves and resources to be developed there. The Lopian epoch witnessed the formation of the deposits and ore occurrences of several ore formations: molybdenum-porphyry, gold-porphyry and others. Big noble-metal ore bodies in Olanga Group layered intrusions and similar but smaller-scale occurrences in the Kuzema lherzolite-gabbro-norite complex were formed in the Sumian epoch. The Svecofennian metallogenic epoch is mainly represented by rare-metal pegmatite formation and complex noble-metal occurrences in their aureoles and shear-zones. Garnet ores were formed in a favourable setting with medium-temperature high-pressure metamorphism and a great contribution of metasomatism associated with acid leaching under kyanite-muscovite- and quartz-muscovite-facies conditions. Kyanite is an industrial mineral used in the refractory and ceramic industries and aircraft motor production. In Russia, no kyanite ore deposits have been mined yet. Nizhnekotozero anorthosites from the Belomorian mobile belt are a new type of feldspar deposits. A pyroxenite-gabbro-alkaline formation with carbonatites is represented by the Yeletozero, Tikshozero and Vostochny massifs in North Karelia. It is proposed to develop a target program aimed at forming a state order for prospecting and appraisal, exploration deposits on precious and rare metals, industrial minerals.