Pegmatite Quartz Research Articles

The First (and Second) Known Occurrences of Bazzite in Canada – The Quadeville Rose Quartz Quarry, Ontario, and the Bugaboo Castles Aquamarine Deposit, British Columbia – Description and Crystal Structure

Abstract Bazzite, ideally Be3Sc2Si6O18, has been discovered for the first time at two granitic pegmatite localities in Canada: the Quadeville Rose Quartz quarry, Ontario (Bz-ON/CMNMC 90604), and the Bugaboo Castles aquamarine deposit, Purcell mountains, British Columbia (Bz-BC/CMNMC 90725). Bazzite from both Canadian localities occurs as colorless, hexagonal prismatic to acicular crystals up to 0.6 mm in length. The crystal chemistry and structure of the two samples, Bz-ON and Bz-BC, was determined and compared with bazzite from other known occurrences. The average composition of Bz-ON is Be2.99(Sc1.39Mg0.37Fe3+0.10Fe2+0.09Al0.06Mn0.01)Σ2.02Si5.99O18·[Na0.47(H2O)], and that of Bz-BC is Be3.00(Sc1.43Fe2+0.28Mg0.20Al0.05Fe3+0.03Mn0.01)Σ2.01Si6.00O18[(Na0.46Cs0.01)Σ0.47(H2O)]. Bazzite from the Quadeville Rose Quartz pegmatite contains Mg = 0.33–0.41 (avg. 0.37) apfu, Fe3+ = 0.00–0.18 (avg. 0.10) apfu, and Fe2+ = 0.05–0.13 (avg. 0.09) apfu, whereas bazzite from Bugaboo Castles is more Fe2+-rich, with Fe2+ = 0.19–0.38 (avg. 0.28) apfu, Mg = 0.16–0.22 (avg. 0.20) apfu, and Fe3+ = 0.00–0.08 (avg. 0.03) apfu. Both samples have low Al, Mn, Ca, and Cs contents and, compositionally, are more similar to bazzite from alpine fissures and orogenic pegmatites than that from more common anorogenic NYF pegmatites. Bazzite from Canada is hexagonal, P6/mcc, with unit cell parameters a = 9.52741(14), c = 9.19326(15) Å for Bz-ON and a = 9.54396, c = 9.16495(20) Å for Bz-BC, respectively. The structure is composed of SiO4 tetrahedra that share corners to form Si6O18 rings perpendicular to the c axis. Each Si6O18 ring is linked via corners to a 12-membered ring of edge-sharing BeO4 tetrahedra and AO6 octahedra (A), resulting in channels parallel to the c axis (2a and 2b sites). Voids in the channels contain H2O (2a) and large alkali cations (Na, 2b). Both Bz-ON and Bz-BC have fully occupied 2a sites and 2b sites which are 50% occupied by Na. Increased substitution of the smaller cations Mg ([6]r = 0.72 Å) and Fe3+ ([6]r = 0.65 Å) for Sc ([6]r = 0.75 Å) results in significant changes in the bazzite structure, including a decrease in the <A–O> distance and decreased distortions (compression) in the AO6 and SiO4 tetrahedra. Bazzite from Quadeville and Bugaboo Castles are late-stage minerals, the product of breakdown of beryl and/or bertrandite and a Sc-bearing mineral. At Bugaboo Castles, bazzite is the result of alteration of Sc-rich spessartine (up to 3000 ppm) and primary beryl by late-stage OH-bearing fluids. At Quadeville, the source of Sc for bazzite is less evident and suggested to be Sc-bearing columbite-group minerals (up to 0.23 apfu Sc), or primary ferromagnesian minerals.

The origin of Xuefengling rare metal pegmatites and implications for ore prospecting in West Kunlun Orogen, China

The Xuefengling rare metal pegmatite deposit is located in the West Kunlun orogenic belt, Northwest China. It is one part of the Bailongshan orefield, which is located 5 km southeast of the Bailongshan deposit, and it is composed of the Xuefengling, Xuepen, Longmenshan and Shuangya ore segments. The Xuefengling pegmatite and the Bailongshan pegmatite have similar zoning characteristics, and the pegmatite-hosted rare metal mineralization is approximately 206–213 Ma. On the basis of detailed observations and characterization of the textures and mineral assemblages of pegmatites in the Xuefengling deposit, four zones are identified: a quartz–spodumene (QS) zone, quartz–albite–muscovite (QAM) zone, quartz–albite–tourmaline (QAT) zone, and block albite–quartz pegmatite (BAQ) zone. We use the textures and major and trace element compositions of muscovite and columbite-group minerals from the Xuefengling pegmatite to illustrate both the existing state of rare elements and the origin of the Xuefengling pegmatite. Our results show that from Li-poor (QAM and QAT zones) to Li-rich pegmatites (QS zone), the K/Rb and K/Cs ratios of muscovite gradually decrease from 41.6 to 10.9 and 2276 to 208, respectively, and the Mn# values of columbite-group minerals increase from 0.35 to 0.49, which are similar to the progressive evolution trend identified in Bailongshan pegmatites. However, the lower K/Rb and K/Cs ratios of muscovite from the Xuefengling pegmatites than those from the Bailongshan pegmatites indicate that the Xuefengling pegmatites may have formed from the lower degrees of fractional crystallization of the same parental magma chamber, leading to the Bailongshan ore-field pegmatites gradually strengthening their evolution from east to west.

Lithium enrichment of the magmatic-hydrothermal fluid in albite-spodumene pegmatite from Lijiagou, Eastern Tibetan Plateau: Evidence from fluid inclusions

Albite-spodumene pegmatites, which are remarkable resources of Li worldwide, may experience both magmatic-hydrothermal transition and sub-solidus hydrothermal alteration during their formation and evolution. However, fluids involved in the different stages of the evolution of a spodumene-bearing pegmatitic systems are not well defined, especially at the magmatic-hydrothermal transition stage. In the Lijiagou deposit, western Sichuan Province, fluid inclusions of a typical albite-spodumene pegmatite were studied to determine the degree of magmatic-hydrothermal fluid Li enrichment and the potential for lithium mineral crystallization at the magmatic-hydrothermal transition stage.Micro-petrographic analyses identified different types of fluid inclusions of either primary, pseudo-secondary or secondary origin and inclusions containing devitrified silicate glass. The presence of these inclusions suggests that the pegmatite underwent both the magmatic-hydrothermal transition stage and a post-crystallization hydrothermal process over its evolution. The results of fluid inclusion LA-ICP-MS analyses show that the aqueous ± carbonic fluids represented by the pseudo-secondary type 1a and 1b inclusions carry significant amount of Li, with Na and Li predominating the cation budget of these pegmatitic fluids. The aqueous ± carbonic fluids exhibit typical magmatic-hydrothermal fluid compositional characteristics and exhibit high abundance of Li in this pegmatitic system at the magmatic-hydrothermal transition stage. The relative abundance of Li to Na and K lies within the uppermost range of those reported from lithium-cesium-tantalum pegmatite systems. This is consistent with the elevated incorporation of Li in pegmatitic quartz from the albite-spodumene pegmatite. The compositional characteristics of the fluids at the magmatic-hydrothermal transition stage may be used to imply the Li mineralization potential of the pegmatitic system.

Quartz chemistry fingerprints melt evolution and metamorphic modifications in high-purity quartz deposits

In this paper, statistical quartz chemistry is combined with petrographic observations to improve our understanding of how metamorphism reworks trace elements of igneous quartz. We elucidate the key processes involved in the formation of the pegmatite-type high-purity quartz deposits in Tysfjord, Northern Norway, where produced quartz has the second-best chemical purity in the world. The Tysfjord pegmatites are the largest known intra-plutonic Nb-Y-F type, comprising Paleoproterozoic and Devonian generations. The Paleoproterozoic pegmatites show characteristics of Caledonian deformation and can be divided into a strongly-deformed type and a less-deformed type: primary internal zonation is replaced by quartz core and K-feldspar-rich intermediate zones surrounded by sheared granitic wall zones; quartz has undergone dynamic recrystallization and contains very few fluid inclusions. The Devonian pegmatites are undeformed and the quartz is rich in healed, fluid-inclusion-bearing microcracks.Trace elements in the pegmatite quartz are dominated by Al, Ti, Li, Ge and B, and total concentrations of the five elements are 29 ± 9 μg/g, 35 ± 10 μg/g and 38 ± 12 μg/g for the strongly-deformed, less-deformed and undeformed types, respectively. The quartz chemistry of the Devonian pegmatites is largely heterogeneous and shows significantly increasing Ge and B contents from early- to late-stage quartz, reflecting the fractionation enrichment of Ge and B in the evolved melts. The quartz chemistry of the Paleoproterozoic pegmatites has become homogeneous and is dominated by either Ti or Li and Al, resulting from the metamorphic modifications during the Caledonian overprint. The modifications on quartz took place via volume diffusion or dynamic recrystallization equilibrium, resulting in a resetting of Ti (at low TiO2 activity) and losses of Li and Al. Therefore, the less-evolved pegmatite-forming melts derived from the Paleoproterozoic magmatism and the moderate metamorphic modifications during the Caledonian orogeny are important for the formation of the high-purity quartz deposits. This study manifests that quartz chemistry has the potential to fingerprint igneous and metamorphic mineralization processes in tectonomagmatic terranes.

Quartz chemistry of lithium pegmatites and its petrogenetic and economic implications: Examples from Wolfsberg (Austria) and Moylisha (Ireland)

Trace element concentrations in quartz were determined from two major LCT pegmatite occurrences in Europe to test the applicability of quartz as a pathfinder mineral for Li mineralized pegmatites. The Wolfsberg spodumene pegmatite deposit and pegmatites throughout the wider Austroalpine Unit (Austria), and the Moylisha spodumene pegmatite deposit (SE Ireland), present two distinctly different geological histories. Spodumene pegmatites at Wolfsberg are associated with late Permian lithospheric extension that formed the Austroalpine Unit Pegmatite Province spanning the Eastern Alps. They were metamorphosed at up to eclogite-facies conditions during the Alpine orogeny. Contrastingly, the simple and spodumene pegmatites at Moylisha form a late Silurian to early Devonian NE-SW-striking pegmatite belt that intruded the East Carlow Deformation Zone along the SE margin of the late Caledonian S-type Leinster Batholith. All analyzed pegmatites contain assemblages including K-feldspar, albite, quartz, muscovite ± spodumene. LA-ICP-MS analysis shows quartz from spodumene pegmatites in both regions is distinguishable from that in simple pegmatites by higher concentrations of Al, Li, Ge and B, whereas simple pegmatite quartz contains higher Ti. Increasing concentrations of Al, Li and Ge and decreasing Ti in quartz from simple pegmatites (to leucogranites in the Austroalpine Unit) to spodumene pegmatites reflects increasing degree of fractionation, resulting from either magmatic differentiation or separately generated but increasingly fractionated melts. Chemical profiles through individual spodumene pegmatite bodies show relatively little chemical variation, consistent with Li saturation through most of their crystallization history. Principal component analysis of quartz data shows that high Ge, Be and B concentrations characterize quartz in pegmatites from the Austroalpine Unit whereas high Al and Li concentrations characterize quartz in pegmatites from Moylisha. Concentrations of >100 μgg−1 Al and >30 μgg−1 Li in pegmatite quartz represent an important threshold for potential spodumene mineralization, which may also be indicated by host rocks (e.g., mica schist) metasomatized by fluids expelled by pegmatites during emplacement and/or crystallization and generating a chemical halo. Retention of pegmatite chemical signatures at Wolfsberg supports the robustness of quartz as an effective pathfinder tool for Li mineralized pegmatites in regions that have been affected by high-pressure metamorphism. LA-ICP-MS of quartz in soil and stream sediments may also be a useful pathfinder in pegmatite provinces where Li-rich quartz is not too diluted by other quartz.

Lithium in the Subsoil of Ukraine. Part 2. Mineralogy of Lithium-Bearing Objects: Quartz

The second part of the publication "Lithium in the subsoil of Ukraine" highlights the mineralogy of rock-forming quartz from lithium-bearing objects of Ukraine: chamber pegmatites of the Korosten pluton, Perzhansky ore district, rare-metal granites and pegmatites of the Azov region, rare-metal pegmatites of the Inhulsky megablock, hydrothermalites of the Nagolny Ridge (Donetsk basin). In all these objects, quartz is characterized mostly from the point of view of its typomorphic value. The conclusion was as follows: the habit of quartz crystals has a typomorphic value, but cannot be described for all genetic situations by one simple scheme of the evolution of simple forms; so the main typomorphic information is "hidden" in the middle of quartz crystals. Point defects of its crystal structure are of greatest importance, primarily Al-O– centers and hydrogen defects, which are the most common in the mineral. They are best studied in crystals of chambered pegmatites and hydrothermal veins of the Nagolny ridge. The dependence of these defects on the action of external factors has been clarified. In particular, in chambered pegmatites, in contrast to crystal-bearing veins, a paradox occurs: with a decrease in the temperature of quartz growth, which is accompanied by an increase in the concentration (activity) of Li in the environment of mineralization, the number of lithium-hydrogen defects in the late zones of quartz decreases. It is assumed that in productive pegmatites this is due to the competing action of lithium micas — the main Li absorbers, the role of which increased with a decrease in the temperature of the solutions and an increase in their pH. In the quartz of rare-metal pegmatites, a high concentration of Al-Li centers has been established, which reaches a maximum in the quartz of the central zones and is therefore a criterion for the degree of differentiation of pegmatites. The luminescent properties of quartz from the petalite pegmatites of the Inhulsky megablock, caused by superimposed metasomatic processes and metamorphism, are unusual.

In Situ Trace Elements in Quartz and K-Feldspar from Felsic Igneous Rocks: A Titanium-in-K-Feldspar Geothermometer for Natural Magmatic Systems

Abstract Scanning electron microscope, cathodoluminescence (CL) imaging and laser ablation–inductively coupled plasma–mass spectrometry analyses were conducted on coexisting pairs of quartz and K-feldspar from 14 samples of various types of felsic igneous rocks. Difference of the concentration of trace elements in quartz and K-feldspar among plutonic, pegmatitic and volcanic rocks is closely related to the rock-forming process and P–T condition. In general, a decreasing Ti concentration and increasing Al, Li, and Ge concentrations from plutonic to pegmatitic quartz suggest a higher degree of magmatic fractionation and lower crystallization temperature. Sensitive elements to magma differentiation in K-feldspar such as Ba, Sr, and LREE show a decreasing trend from various granitic rocks to pegmatite, while Rb, Cs, Li, Ge, and P exhibit increasing trends. The K-feldspar from various types of felsic igneous rocks typically shows similar CL textures but relatively higher luminescence intensity compared with coexisting quartz. Both quartz and K-feldspar phenocrysts in the volcanic rhyolite have a bright rim and a dark core in CL images, corresponding to bimodal Ti concentrations. Among all samples, Ti concentrations in both quartz and coexisting K-feldspar positively correlate with their CL intensities, suggesting the activation of Ti-impurity in these minerals leads to increased CL intensity. Meanwhile, there is a good positive correlation between Ti concentrations in quartz and those in K-feldspar with an R2 value of 0.86. It is considered that Ti concentrations in the both minerals are mainly temperature dependent at relatively constant pressure on basis of a fair aTiO2 restriction. Here, for the first time, we calculated a titanium-in-K-feldspar (TitaniKfs) thermometer in the form of log (XTi, kfs/aTiO2) = −(3430 ± 268)/T(K) + (5.081 ± 0.298) for natural felsic magma systems. The Ti contents of K-feldspar (in ppm by weight) increase exponentially with reciprocal T at temperatures ranging from 500°C to 800°C, at 200–300 MPa. An application of this thermometer to granitic rocks from Xinjiang and Inner Mongolia in China fits well with other geothermometers. In addition, the present TitaniKfs thermometer is expected to be particularly useful in determining the temperature condition of K-feldspar-bearing extraterrestrial materials such as lunar rocks.

IR-SPECTROSCOPIC STUDY OF WATER AND HYDROGEN DEFECTS IN INDUSTRIAL QUARTZ TYPES OF KARELIA (RUSSIA)

The results of an IR spectroscopic study of the content of water and hydrogen defects in hydrothermal and pegmatite quartz from quartz-vein occurrences in Karelia (North-West Russia), considered as a promising source of high-purity quartz raw materials, are presented. It is shown that the main amount of water in the analyzed quartz is in molecular form, and OH groups associated with aluminum, lithium, and boron trace elements in the quartz lattice are also identified. Granular quartz with the lowest content of molecular water and OH-groups is the most preferred for use as a high-purity quartz raw material.

TRACE ELEMENTS IN QUARTZ: INSIGHTS INTO SOURCE AND FLUID EVOLUTION IN MAGMATIC-HYDROTHERMAL SYSTEMS

AbstractQuartz trace elements record information about fluid evolution as well as metal migration and precipitation. Here, we summarize most of the reported (including this study) quartz trace element data (N = ~4,600) generated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on various textural types and paragenetic stages of quartz in I-type porphyry-epithermal (Cu-Mo-Au-Ag-Te) and S- and A-type granitegreisen (Sn-W and rare metal) systems in the world. The results show that Li versus Al diagrams, combined with Ti-Ge-As-Sb contents, can be used to decipher the source and evolution of fluids in magmatic-hydrothermal systems. In I-type porphyry-epithermal systems, magmatic quartz has low Li/Al ratios from 0.001 to 0.173 (N = 483) with a mean of 0.039 ± 0.032. Hydrothermal quartz has progressively higher Li and Al concentrations that are dominated by cooling along fluid pathways. Quartz evolves from Ti rich to Ge rich from early to late stages in porphyry hydrothermal veins and is As and Sb rich in epithermal veins. In S- and A-type granite-greisen systems, magmatic quartz has high Li/Al ratios from 0.007 to 0.502 (N = 604) with a mean of 0.130 ± 0.063 and from 0.009 to 0.327 (N = 325) with a mean of 0.126 ± 0.065, respectively. Hydrothermal quartz has progressively lower Li and Al concentrations that are dominated by fluid-rock reactions and cooling along fluid pathways. Quartz evolves with decreasing Ti concentrations from magmatic to hydrothermal stages. Ge is abundant in pegmatite quartz in S-type systems. Variations in pH or precipitation rate along fluid pathways have a small influence on Li/Al ratios. The variation of quartz trace elements with elevation in individual systems suggests that they can be used as a vector to guide exploration in magmatic-hydrothermal systems.

Texture and Trace Element Geochemistry of Quartz: A Review

Quartz is one of the most abundant minerals. Used in a variety of materials, it preserves geological history and reflects alteration conditions. Data were collected (>2400 data points) from more than 40 ore deposits to understand its internal texture and geochemistry. Cathodoluminescence imaging is a technique for examining the internal texture of quartz that may reveal information about the crystal’s origin and evolution. The dominant trace elements in quartz lattice are Al, P, Li, Ti, Ge, K, and Na. These, combined with internal texture, can distinguish quartz from different origins and can differentiate between different types of ore deposits, as each type of ore deposit has its own unique CL characteristics. Therefore, Al did not correlate with cathodoluminescence (CL) in epithermal Au-Sb-Hg, Carlin-type Au, epithermal Ag, or shale-hosted Zn deposits. Epithermal base metal and porphyry-Cu-type deposits were intermediate, and Mississippi-Valley-type, epithermal Au-Ag, and porphyry Cu-Mo deposits were characterized for Al correlation with CL. Furthermore, Gigerwald, Rohdenhaus, and Westland deposits had Li/Al ratios less than one, suggesting that H (as hydroxyl substituting for oxygen) completed the charge. However, trace elements (i.e., Ge, Sb, Ti, and Al), sector zoning, and resorption surfaces were vital parameters to differentiate between magmatic and hydrothermal quartz. Additionally, titanium and aluminum were the most important trace elements. Their values could be used to differentiate between different quartz types. Among them, hydrothermal and pegmatitic quartz were characterized by lower temperatures and Ti concentrations. Rhyolitic quartz was characterized by the lowest Al, the highest temperatures, and lower Al/Ti ratios. Aluminum, Li, and H were most important in hydrothermal and metamorphic quartz, but magmatic quartz was generally enriched with Ti.

Quartz chemistry of granitic pegmatites: Implications for classification, genesis and exploration

Quartz from 254 pegmatites representing eight pegmatite fields and provinces worldwide was investigated by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) to determine concentrations of trace elements Al, Ti, Li, Ge, B, Be, Rb, Na, K, Ca, P, Ga, Sb, Zn and U. A total of 271 new analyses combined with 535 published LA-ICP-MS quartz chemistry data were evaluated with binary and ternary trace element discrimination plots and principal component analysis (PCA). The classifications applied for discrimination of pegmatite types include the widely applied NYF(Nb-Y-F) - LCT(Li-Cs-Ta) classification and the new RMG (pegmatites derived from residual melts of granite magmatism) - DPA (pegmatites as direct products of anatexis) grouping. Pegmatites of both classifications can be well distinguished via Al-Ti, Al-Li and Al/Ti-Ge/Ti binary trace element plots and the Ti - Al/10 - 10*Ge ternary diagram. PCA applied to Al, Li, Ti, Be, B, Ge and Rb contents in quartz allowed to further distinguish between anatectic DPA-1 (Li-enriched DPA) and granite-pluton-derived RMG-1 (Li-enriched RMG) pegmatites. Some pegmatite fields and provinces (Hagendorf-Pleystein, Oxford County) are distinguishable by region-specific Li, Ge and Al contents. The results imply that the chemistry of pegmatite quartz is mainly controlled by the origin (source rock chemistry) of pegmatite melts and, to a much lesser extent, by the geodynamic setting of the pegmatite fields and provinces. Chemically primitive NYF and DPA-2 type pegmatites contain quartz with the lowest total trace-element contents and lowest internal-pegmatite trace-element variation, making it potentially suitable for high-tech application. Pegmatite quartz containing >30 μgg-1 Li and >100 μgg-1 Al is strongly indicative of economic spodumene/montebrasite mineralization and, thus, serves as a strong Li-mineralization pathfinder mineral. Quartz with >5 μgg-1 B may be a potential indicator for gem-quality tourmaline mineralization.

EPR characterization in natural quartz samples of a newly discovered hydrogen related defect and already known germanium related defects

The role of hydrogen in the formation and modification of defects in quartz is well known, but still to be clarified is its involvement in trapping and recombination properties of quartz on which its dating characteristics are based. In this work, we studied the thermal stabilities of EPR signals in natural pegmatitic quartz using an X-band spectrometer. We also observed at least two previously unreported EPR signals in quartz, one of them possessing a hydrogen hyperfine structure with large coupling constant. Its thermal stability is intermediate between the well-known [GeO4]- and [GeO4/Li+]0 centres, while the other newly reported signals possess a thermal stability comparable to the latter. The sample has been further studied with a Q-band spectrometer to enhance the resolution of the spectrum. This allowed to confirm, without having to fit or simulate the overlapped signals, the g-factors reported in the literature for Ge related centres, which are strongly overlapped in the X-band. We performed simulations to extrapolate the g-factors and coupling constants of the observed components. We carried out pulse annealing experiments to determine the trap depth of the centres originating such signals. The hydrogen related centre signal has been simulated with the g-tensor's main components being g1 = 2.0007, g2 = 1.9974 and g3 = 1.9929, with coupling constants A1 = 233 MHz, A2 = 235 MHz and A3 = 239 MHz, and trap depth of 1.14 eV.

Geology of the Macao Special Administrative Region (China)

ABSTRACT A 1:12,000 geological map of the Macao Special Administrative Region has been produced through detailed field work supported by petrographic, mineralogical, geochronological and geochemical data obtained in previous studies. This map aims to represent a reliable tool to understand the geological evolution of the region and for management of the territory. The geology of Macao is dominated by two groups of Jurassic granitic rocks belonging to an intrusive suite located along the coast of Southeast China: Macao Group I (MGI: 164.5 ± 0.6 to 162.9 ± 0.7 Ma) and Macao Group II (MGII: 156.6 ± 0.2 to 155.5 ± 0.8 Ma), including the associated microgranite, aplite and pegmatite dikes and quartz veins. Remnants of the metasedimentary wall-rock are present as Devonian xenoliths enclosed within the granites. Younger Jurassic to Cretaceous andesite to dacite dikes (150.6 ± 0.6 to <120 Ma) intrude the granitic rocks. Additionally, Quaternary sedimentary deposits cover the older lithologies.

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.

Mineralogical, fluid inclusion and oxygen isotope studies for the evolution of pegmatites in Igangan area, southwestern Nigeria

The fluid characterisation and oxygen isotope studies of pegmatites in the Igangan area in southwest Nigeria was carried out to delineate the source and evolution of mineralising fluid, and to determine the crystallisation temperature of the pegmatites that are associated with I-type metaluminous granites. The geochemical analysis was carried out using ICPMS, the mineralogical analysis was carried out with the aid of a Scanning Electron Microscope. In contrast, microthermometry measurements were carried out on pegmatitic quartz using a Linkam THMSG600 Conventional and laser fluorination was used to determine the oxygen isotope composition of selected minerals from the pegmatites.The geochemical analysis revealed a low concentration of Ta, Nb, W, Cs and low mineralisation potential for the pegmatites; while the mineralogical analysis of accessory spinels revealed a high concentration of Zn. Two types of hydrocarbon/CO2 free aqueous inclusions were observed; which were Primary/pseudo primary (Type 1) inclusion with 0–11 wt % NaCl eq. salinity and secondary (Type 2) inclusions with a salinity range of 15–22 wt % NaCl eq. The density of the inclusions ranged from 0.57 to 1g/cc. δ18O values in quartz varied from 9.88 to 14.63‰ with an average of 13.13‰; feldspar, 8.9–13.85‰ with an average of 11.9‰ and tourmaline, 10.63–11.96‰ with an average of 11.40‰. Homogenisation temperature ranged from 175 to 370 °C with all inclusions homogenising into the liquid phase.Bivariate plot of salinity against Thtotal reveals the possibility of magmatic-meteoric mixing with Na/Ca rich fluids in the pegmatite evolution and equilibrium temperature calculations yield values of 496–685 °C and 392–713 °C for the quartz–tourmaline pair and the quartz-feldspar pair, respectively. The pegmatites are muscovite class LCT pegmatites with low rare metal mineralisation potential and crystallised from a H2O–NaCl (±CaCl2) fluid which was preserved in the primary/pseudo-primary inclusions.

Optical and Chemical Study of Quartz from Granitic Pegmatites

Of the nine quartz samples collected from the Contu - Negovanu granitic pegmatites (Southern Carpathians, Romania), three were separated under binocular lenses and turned into fine grained powders, for X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) investigation techniques, whilst the other six samples were prepared in thin slides for electron microprobe analyzer (EMPA) and optical investigations, using a Babinet compensator and a Rayleigh interferometer. The results show that the quartz samples belonging to the Contu-Negovanu granitic pegmatites are chemically very pure, showing an expected high participation of silica and only some remarkable contents of Fe, derived from iron oxide micro-inclusions finely disseminated in the quartz grains. The minor and trace elements distribution suggests that different crystallization conditions influenced the quartz composition, thus resulting some rather high contents of Ba, Rb, Sr and some REE contents (Ce) in the pegmatite quartz crystallized from alkali- and alkaline-earth-rich fluids, while the metamorphic quartz appears to be highly pure, showing no such presence. The analyzed quartz samples present birefringence parameters within the limits of other quartz samples and a decrease of the linear birefringence of quartz with the light wavelength increase is emphasized by the data obtained using the channeled spectrum.

Mixing of magmatic-hydrothermal and metamorphic fluids and the origin of peribatholitic Sn vein-type deposits in Rwanda

The fluid sources of granite-related Sn-quartz vein deposits are commonly obscured by fluid mixing or fluid-rock interactions. As a result, fluid inclusions, minerals and isotopes in these veins indicate an intermediate composition between magmatic and metamorphic, but the degree of mixing between these endmembers is currently unquantified. This study presents a novel quantitative approach to assess the degree of mixing between magmatic-hydrothermal and external metamorphic fluids in the formation of peribatholitic Sn-quartz veins. In particular, fluid mixing in the Sn-mineralized Rwamagana-Musha-Ntunga pegmatite-quartz vein field in East Rwanda has been evaluated by the following four methods: quartz stable isotopes, muscovite geochemistry, fluid inclusion microthermometry and LA-ICP-MS, and geochemical modelling.The quartz stable isotope data (δ18O: +13.1 to +15.8‰ V-SMOW; δD: −27.6 to −59.7‰ V-SMOW) cannot uniquely differentiate between a metamorphic fluid origin or an initial magmatic hydrothermal fluid origin with subsequent metamorphic fluid mixing or host-rock interaction. However, granitophile element concentrations in magmatic muscovite from pegmatites and hydrothermal muscovite from associated Sn-quartz veins are equally high, indicating a close genetic link (Rb: 530–8740 ppm, Li: 110–1990 ppm, Sn: 87–810 ppm, Cs: 62–420 ppm). Primary H2O-CO2-N2-NaCl medium saline magmatic fluid inclusions in quartz of pegmatites (∼12.7 wt% NaCleq) and H2O-CO2-(N2)-NaCl low saline fluid inclusions in barren metamorphic quartz veins (∼4.9 wt% NaCleq) were analyzed by LA-ICP-MS. These results show an enrichment in Li, Rb, Sn and Cs for the magmatic fluid, while the metamorphic fluid is characterized by low granitophile element concentrations and high Sr and Ba contents. The expected Rb-Cs and Rb-Sn signature of the Sn-quartz vein muscovite was modelled using the measured fluid endmember compositions, confirming mixing between magmatic and metamorphic fluids in the formation of the veins. The quantification suggests that the hydrothermal Sn-quartz vein fluid contains 5–80% of an external metamorphic fluid component.

A NEW INVESTIGATION OF VIETNAM’S BERYL

The paper presents briefly new investigation of Vietnam beryl group, especially some characteristics of regional geology of Song Chay granite massif and new find of big crystals of beryl in them. The green beryl is found in quartz - mica - tourmaline, mica - feldspar – quartz pegmatite bodies in the south part of Song Chay granite - gneiss massif near Na Chi and Tan Nam villages, Quang Binh district, Ha Giang province.

Evaluation of alkali–silica reaction potential of quartz-rich rocks by alkaline etching of polished rock sections

Rock sections mainly composed of quartz (coming from orthoquartzite, quartz metagreywacke, pegmatite quartz, vein quartz, chert, and flint) were subjected to experimental etching in 1 M NaOH solution at 80 °C. After 4 h–14 days, the rock sections were analysed using a scanning electron microscopy (SEM) combined with energy-dispersive spectrometer. Representative areas were documented in backscattering mode and analysed using fully automatic petrographic image analysis (PIA). Specific features connected to alkaline etching were observed on the surface of the rock sections: dissolution gaps, accentuation of pre-existing microcracks, etched out parts of aggregates, and the formation of altered silica-rich layer, and alkali–silica gels, all influencing the greyscale spectrum of the BSE microphotographs. Using SEM-PIA, it was possible to quantify the area affected by alkaline etching (etched area) and to express it as a percentage of the total area analysed. A very good correlation was found between the etched area and alkali–silica reaction potential of investigated rocks measured according to an accelerated mortar bar test.

Microscopy and Cathodoluminescence Spectroscopy Characterization of Quartz Exhibiting Different Alkali–Silica Reaction Potential

Different quartz types from several localities in the Czech Republic and Sweden were examined by polarizing microscopy combined with cathodoluminescence (CL) microscopy, spectroscopy, and petrographic image analysis, and tested by use of an accelerated mortar bar test (following ASTM C1260). The highest alkali-silica reaction potential was indicated by very fine-grained chert, containing significant amounts of fine-grained to cryptocrystalline matrix. The chert exhibited a dark red CL emission band at ~640 nm with a low intensity. Fine-grained orthoquartzites, as well as fine-grained metamorphic vein quartz, separated from phyllite exhibited medium expansion values. The orthoquartzites showed various CL of quartz grains, from blue through violet, red, and brown. Two CL spectral bands at ~450 and ~630 nm, with various intensities, were detected. The quartz from phyllite displayed an inhomogeneous dark red CL with two CL spectral bands of low intensities at ~460 and ~640 nm. The massive coarse-grained pegmatite quartz from pegmatite was assessed to be nonreactive and displayed a typical short-lived blue CL (~480 nm). The higher reactivity of the fine-grained hydrothermal quartz may be connected with high concentrations of defect centers, and probably with amorphized micro-regions in the quartz, respectively; indicated by a yellow CL emission (~570 nm).