Pegmatite Formation Research Articles

Geology and geochronology of neoarchean anorogenic magmatism of the Keivy structure, Kola Peninsula

Anorogenic magmatic complexes were formed during protoplatformal evolution of the Keivy structure. This evolution ended with development of aluminous schists, which were derived by deep disintegration and redeposition of the rocks from the lower parts of the sequence and surrounding of the structure. The anorogenic rocks of the region are represented by the following magmatic complexes: gabbro-labradorite-latite-monzonite-granites; ophitic gabbro and gabbrodiabases; quartz syenite-alkaline granites; alkaline and nepheline syenites. The magmatic activity of this period, starting from the emplacement of gabbrolabradorite massifs and ending with alkaline and nepheline syenite bodies, was caused by ascent of mantle asthenolith, which destructed the Earth’s crust basement in this area. The anorogenic magmatism of the Keivy structure lasted for no more than few or few tens of million years. The granitoid subcomplex of the gabbro-labradorite-latite-monzonite-granite complex is dated at 2674 ± 6 Ma, which is comparable with an age of alkaline granites of the Ponoy and Beliye Tundry massifs (2673 ± 6 Ma). The considered complexes are separated in time by intrusion of amphibole-biotite plagiomicrocline granites with an age of 2667 ± 8 Ma. Gabbrolabradorites of the Shchuch’e Ozero and Tsaga massifs have close ages (2663 ± 7 and 2668 ± 10 Ma, respectively, Bayanova, 2004), but were formed earlier than granitoids (Bayanova, 2004). Formation of alkaline syenites of the Sakharijok I Massif, which finalized the Neoarchean anorogenic magmatism of the region, falls in the same interval. During Paleoproterozoic transformations, the rocks of the Keivy structure were sheared and uranium was introduced in the contact zones of the alkaline granite massifs, which caused formation of palingenetic melts and subsequent formation of pegmatites in the outer contact zones of the granite bodies.

Hydrothermal Alteration, Fluid Inclusion and Stable Isotope Studies of the North Roby Zone, Lac des Iles PGE Mine, Ontario, Canada

AbstractThe Archean mafic–ultramafic complex of Lac des Iles, Ontario, Canada, hosts economic platinum group elements (PGE)‐Au‐Cu‐Ni mineralization in the Roby Zone. All lithologies in the North Roby Zone have been affected by hydrothermal alteration. The alteration products include talc (the most dominant mineral), anthophyllite, serpentine, actinolite, tremolite, chlorite, hornblende, zoisite, clinozoisite, epidote and sericite. In the altered rocks, light rare earth elements (La, Ce, Nd, Sm), Pb, Rb, Ba, Cs, S and possibly Y have been added by hydrothermal solution whereas Eu and heavy rare earth elements (Yb, Gd, Dy, Er) remained immobile. There are five types of fluid inclusions in the pegmatitic plagioclase with homogenization temperature and salinity ranging from 240°C to 445°C and 15.37 to 48.52 wt% equivalent NaCl, respectively. The δ18O and δD of talc range form 6.2‰ to 6.9‰ and −28‰ to −48‰, respectively. δ18O and δD water in equilibrium with talc during the hydrothermal alteration suggest a modified source for the hydrothermal solution. Microthermometry and stable isotope studies suggest that high temperature–high salinity fluid was diluted by, and mixed with, low temperature–low salinity meteoric solution. This mechanism precipitated the hydrothermal assemblage and redistributed trace elements during and after pegmatite formation in the North Ruby Zone.

The nature of fluids during pegmatite development in metamorphic terrains: Evidence from Hamadan complex, Sanandaj-Sirjan metamorphic zone, Iran

In the Sanandaj-Sirjan zone of metamorphic belt of Iran, the area south of Hamadan city comprises of metamorphic rocks, granitic batholith with pegmatites and quartz veins. Alvand batholith is emplaced into metasediments of early Mesozoic age. Fluid inclusions have been studied using microthermometry to evaluate the source of fluids from which quartz veins and pegmatites formed to investigate the possible relation between host rocks of pegmatites and the fluid inclusion types. Host minerals of fluid inclusions in pegmatites are quartz, andalusite and tourmaline. Fluid inclusions can be classified into four types. Type 1 inclusions are high salinity aqueous fluids (NaCl eq >12 wt%). Type 2 inclusions are low to moderate salinity (NaCl eq <12 wt%) aqueous fluids. Type 3 and 4 inclusions are carbonic and mixed CO 2 -H 2 O fluid inclusions. The distribution of fluid inclusions indicate that type 1 and type 2 inclusions are present in the pegmatites and quartz veins respectively in the Alvand batholith. This would imply that aqueous magmatic fluids with no detectable CO 2 were present during the crystallization of these pegmatites and quartz veins. Types 3 and 4 inclusions are common in quartz veins and pegmatites in metamorphic rocks and are more abundant in the hornfelses. The distribution of the different types of fluid inclusions suggests that CO 2 fluids generated during metamorphism and metamorphic fluids might also contribute to the formation of quartz veins and pegmatites in metamorphic terrains.

Sc- and REE-bearing ixiolite and associated minerals from the Sosedka pegmatite vein in the Malkhan pegmatite field, central Transbaikal region

Composition and localization of REE mineralization in miarolitic pegmatites and its role in the pegmatite formation were studied at the Malkhan gem deposit (jewel-quality tourmaline, morganite, danburite, and hambergite) in the central Transbaikal region. The chemical composition of Ti-, Ta-, Nb- and REE-bearing minerals, their relationships with rock-forming and accessory minerals indicate that two geochemically specialized stages of pegmatite formation are distinguished. The early stage gave rise to the crystallization of quartz-feldspar aggregates including K-feldspar block zone with Sc and REE mineralization. The rare-metal (Li, Cs, F, B, Be) albite-lepidolite-cleavelandite complex with pockets of gem mineralization was formed at the late stage.

Dualite, Na30(Ca,Na,Ce,Sr)12(Na,Mn,Fe,Ti)6Zr3Ti3MnSi51O144(OH,H2O,Cl)9, a new zircono-titanosilicate with a modular eudialyte-like structure from the Lovozero alkaline Pluton, Kola Peninsula, Russia

Dualite has been found at Mount Alluaiv, the Lovozero Pluton, the Kola Peninsula in peralkaline pegmatoid as sporadic, irregularly shaped grains up to 0.3–0.5 mm across. K-Na feldspar, nepheline, sodalite, cancrinite, aegirine, alkaline amphibole, eudialyte, lovozerite, lomonosovite, vuonnemite, lamprophyllite, sphalerite, and villiaumite are associated minerals. Dualite is yellow, transparent or translucent, with conchoidal fracture. The new mineral is brittle, with vitreous luster and white streaks. The Mohs hardness is 5. The measured density is 2.84(3) g/cm3 (volumetric method); the calculated density is 2.814 g/cm3. Dualite dissolves and gelates in acid at room temperature. It is nonfluorescent. The new mineral is optically uniaxial and positive; ω = 1.610(1), ɛ = 1.613(1). Dualite is trigonal, space group R3m. The unit cell dimensions are a = 14.153(9), c = 60.72(5) A, V = 10533(22) A, Z = 3. The strongest reflections in the X-ray powder pattern [d, A (I,%)(hkl)] are as follows: 7.11(40)(110), 4.31(50)(0.2.10), 2.964(100)(1.3.10), 2.839(90)(048), 2.159(60)(2.4.10, 0.4.20), 1.770(60)(2.4.22, 4.0.28, 440), 1362(50)(5.5.12, 3.0.42). The chemical composition (electron microprobe, H2O calculated from X-ray diffraction data) is as follows, wt %: 17.74 Na2O, 0.08 K2O, 8.03 CaO, 1.37 SrO, 0.29 BaO, 2.58 MnO, 1.04 FeO, 0.79 La2O3, 1.84 C2O3, 0.88 Nd2O3, 0.20 Al2O3, 51.26 SiO2, 4.40 TiO2, 5.39 ZrO2, 1.94 Nb2O5, 0.58 Cl, 1.39 H2O,-O = 0.13 Cl2; they total is 99.67. The empirical formula calculated on the basis of 106 cations as determined by crystal structure is (Na29.79Ba0.1K0.10)Σ30(Ca8.55Na1.39REE1.27Sr0.79)Σ12 · (Na3.01Mn1.35Fe 0.87 2+ Ti0.77)Σ6(Zr2.61Nb0.39)Σ3 (Ti2.52Nb0.48)Σ3(Mn0.82Si0.18)Σ1(Si50.77Al0.23)Σ51 O144[(OH)6.54(H2O)1.34·Cl0.98]Σ8.86). The simplified formula is Na30(Ca,Na,Ce,Sr)12(Na,Mn,Fe,Ti)6Zr3Ti3 MnSi51O144 (OH,H2O,Cl)9). The name dualite is derived from Latin dualis (dual) alluding to the dual taxonomic membership of this mineral, which is at the same time zirconosilicate and titanosilicate. The crystal structure is characterized by two module types (alluivite-like and eudialyte-like) alternating along a threefold axis with a doubled c period relative to eudialyte and close chemical affinity to rastsvetaevite (Khomyakov et al., 2006a) and labyrynthite (Khomyakov et al., 2006b). According to the authors’ crystal chemical taxonomy of the eudialyte group, the new mineral belongs to one of three subgroups characterized by a 24-layered structural framework. Dualite is a mineral formed during the final stages of peralkaline pegmatite formation. The type material of dualite is deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.

The miarolitic pegmatites from the Königshain: a contribution to understanding the genesis of pegmatites

In this paper, we show that the crystallization of miarolitic pegmatites at Konigshain started at about 700°C, in melts containing up to 30 mass% water. Such high water concentration at low pressures (1–3 kbar) is only possible if the melts are peralkaline. Such peralkaline melts are highly corrosive, and reacted with the wall rock—here the granite host—forming the graphic granite zone, in part via a magmatic–metasomatic reaction. With cooling, the water concentration in some melt fractions increased up to 50 mass% H2O. The melt-dominated system ends below 600°C and passes into a fluid-dominated system, the beginning of which is characterized by strong pressure fluctuations, caused by the change of OH and CO3 2− in the melt, to molecular water and CO2. We note two generations of smoky quartz, one crystallized above the β–α-transition of quartz (≈573°C), and one below, both of which contain melt inclusions. This indicates that some melt fraction remains during at least the higher-temperature portion of the growth of minerals into the miarolitic cavity, contradicting the view that minerals growing into a pegmatite chamber only do so from aqueous fluids. We show that the Konigshain miarolitic pegmatites are part of the broad spectrum of pegmatite types, and the processes active at Konigshain are representative of processes found in most granitic pegmatites, and are thus instructive in the understanding of pegmatite formation in general, and constraining the composition and characteristics of pegmatite-forming melts.

Ramanite-(Cs) and ramanite-(Rb): New cesium and rubidium pentaborate tetrahydrate minerals identified with Raman spectroscopy

This study of melt and fluid inclusions in graphic pegmatite quartz from the Island of Elba, Italy has identified silicate melts and coexisting aqueous fluid from the final stage of pegmatite formation, which contain extremely high concentrations of boron (up to 19.8 mass% B2O3), cesium (>5 mass% Cs2O), and rubidium (>0.5 mass% Rb2O). Several boron-bearing minerals have been identified in melt and fluid inclusions, including sassolite (H3BO3) and santite (KB5O8·4H2O). In addition, we have found two new minerals observed as daughter mineral phases, and these have been accepted by the IMA, Commission on New Minerals, Nomenclature and Classification (CNMNC) as ramanite-(Cs) (2007-007), monoclinic β-cesium pentaborate tetrahydrate (CsB5O8·4H2O) with space group C2/c, and ramanite-(Rb) (2007-006), orthorhombic rubidium pentaborate tetrahydrate (RbB5O8·4H2O) with space group Aba2. At the moment, given sample constraints, we cannot directly determine some of the properties traditionally reported for new minerals; however, both phases have Raman spectra identical to spectra obtained on synthetic Cs and Rb pentaborates, which were studied for comparison. Although difficult to recognize optically, ramanite-(Cs) and ramanite-(Rb) have distinctive Raman spectra allowing simple and definitive identification from other translucent phases found in many hypersaline fluid inclusions. Although currently only identified from a limited number of occurrences, given the availability of a definitive means of identification, we suspect that ramanite-(Rb) and ramanite-(Cs) may in fact be common, at least in boron-rich pegmatites.

Luminescence of Eu2+ in feldspars from muscovite pegmatites

The data on photoluminescence (PL) that precisely detects Eu2+ centers and X-ray luminescence (XL) were compared for plagioclases and potassium feldspars in 21 samples from muscovite pegmatites of the Mama region. The Eu contents determined in 10 samples vary from 10−4 to 10−6 wt %. Europium occurs mainly as bivalent species that replaces Sr2+, Ca2+, and Ba2+. Eu is gained in the products of early crystallization, and its relative amounts decrease by an order of magnitude in the course of pegmatite formation down to complete disappearance in late generations of feldspars. It is shown that Eu2+ can be detected in XL spectra, and the Eu2+ content can be determined in qualitative terms, for instance, by the intensity of radiation band 400–420 nm in plagioclase.

Mn-rich graftonite, ferrisicklerite, staněkite and Mn-rich vivianite in a granitic pegmatite at Soè Valley, central Alps, Italy

AbstractMn-rich graftonite, (Ca,Mn2+)(Fe2+,Mn2+)2(PO4)2, ferrisicklerite, Li1–x(Fe3+,Mn2+)PO4, manganoan apatite, (Ca,Mn2+,Fe2+Mg)(PO4)3Cl, staně kite, Fe3+Mn2+O(PO4) and Mn-rich vivianite, (Fe2+)3(PO4)2·8H2O, occurring in a granitic pegmatite at Soè Valley (central Alps, Italy) were characterized by powder and single-crystal X-ray diffraction (XRD) and electron microprobe analyses. Geochemically, the Mn-rich graftonite phases are poorly evolved Fe/Mn-phosphates of rare-earth elements-lithium (REE-Li) granitic pegmatites. The assemblage Mn-rich graftonite + ferrisicklerite + staněkite has rarely beendocumen ted in pegmatites. Inthe Soè Valley pegmatite, ferrisicklerite forms exsolution lamellae with Mn-rich graftonite associated with manganoan apatite and staněkite. Graftonite is associated with Mn-rich vivianite. Powder and single-crystal XRD data indicate that the unit-cell volume of graftonite increases as a function of Mn2+ content. Staněkite shows a distinctly smaller unit-cell volume with respect to previously reported staněkites, probably due to reduced Mn2+. Vivianite with significant Mn2+ has a unit-cell volume similar to nearly Mn-free vivianite. The formation of Mn-rich graftonite and manganoan apatite is related to destabilization of Mn-rich almandine and biotite during pegmatite formation. Ferrisicklerite forms exsolution lamellae along the 010 cleavage planes of Mn-rich graftonite, whereas staněkite forms by alterationof ferrisicklerite and Mn-rich vivianite due to circulation of late-stage hydrothermal fluids.

Phase Relations Involving Hydrous Silicate Melts, Aqueous Fluids, and Minerals

The importance of magma degassing and separation of magmatic fluids (aqueous as well as other components) has been recognized in many geologic observations at least since Niggli (1912; see also Morey and Niggli 1913; Bowen 1928; Morey 1957). Improved analytical techniques in recent years have permitted micron scale examination of melt and fluid inclusions inside magmatic minerals. In these inclusions, concentrated aqueous solutions and extreme enrichment of melts and fluids in volatile components (S, CO2, B2O3, Cl, F, and metals) have been observed (Roedder 1992; Webster 1997; Thomas et al. 2000; Kamenetsky et al. 2004). Pegmatite formation reflects the role of exsolution of aqueous fluid from silicate melt (Jahns and Burnham 1969) at the magmatic–hydrothermal transition (e.g., Veksler 2004). Fluid–gas immiscibility is clearly involved in formation of pegmatites, and for some other fluids originating in magmatic or metamorphic environments. Certain natural fluid systems rich in volatile components are immiscible and others show completely mixed compositions at geological temperatures and pressures (e.g., Roedder 1984; hydrocarbons, Pedersen and Christensen 2007; geothermal and submarine hydrothermal systems, Arnorsson et al. 2007, Foustoukos and Seyfried 2007; ore-forming fluids, Hedenquist and Lowenstern 1994; Heinrich 2007a; mixed metamorphic fluids, Trommsdorff and Skippen 1986; Heinrich 2007b; and magmatic fluids, Webster and Mandeville 2007). This has encouraged the idea that certain common natural melt–fluid–gas systems pass from immiscible (with multiple coexisting fluids) to miscible behavior (a single homogeneous fluid) with changing pressure and/or temperature (increasing or decreasing) or changing composition (see recent summary by Veksler 2004). The term supercritical is applied to any solution (liquid or solid) occurring at a temperature or pressure above its critical point. In the case of supercritical fluid this corresponds to conditions where liquid …

High-Pressure Melting of Eclogite and the P-T-X History of Tonalitic to Trondhjemitic Zoisite-Pegmatites, Munchberg Massif, Germany

SUMMARY The zoisite-pegmatites provide an excellent example of theonset of metabasite melting at eclogite-facies conditions.Such conditions can be expected in a down-going oceanicslab as well as in collision zones.The newly produced meltin a metabasaltic rock of MORB composition is tonalitic(to trondhjemitic) and shares some characteristics withthe TTG series or with adakites. The amount of melt,forming at a temperature of less than 7508C, however,is small, as a result of the combined effect of relatively lowtemperature and restricted availability of water, and themelt does not separate from the rock unit. Only if themelt fraction increases will the melt segregate and mightmigrate upwards, influencing melting in the overlyingmantle wedge. Because of the poor outcrop situation ofthe Mu«nchberg area in the Variscan chain of CentralEurope, it is unfortunately impossible to give an estimateof the amount of melt produced. This will be a crucialquestion to solve: at what degree of melting, at whattemperature in a given composition andat what geological(mechanical) conditions the melt will effectively separate.Our data suggest the following model: high-pressurepartial melting and pegmatite formation occurred at con-ditions only slightly above the water-saturated solidus andcan be a fluid-conserving or fluid-recycling process.Any fluid released from hydrous minerals such as zoisite,epidote or amphibole is trapped in small fractions offully to nearly water-saturated siliceous melt (for truedehydration melting, a free fluid phase never exists).Neither the fluid nor the siliceous melt will separate andmigrate into the overlying rock units. During uplift, cool-ing and crystallization, the water, which is stored in thesiliceous melt, is again liberated and encounters a dehy-drated eclogite-facies assemblage. Here, it hydrates theeclogite-facies assemblage, leading to crystallization ofhydrous minerals such as amphibole or mica. This modelof fluid conservation or fluid recycling implies also thatthe trace element budget of the whole-rock does notchange during melting and subsequent crystallization.The pegmatites from the Hangendserie of the Mu«nchbergarea still preserve some trace element signatures of theirindividual protoliths, which means that the partial meltsdid not mix andwere not homogenized. Element transportoccurs onlyona restricted scale of probably much lessthanthe present thickness of the Hangendserie.The coarse grain size of the pegmatitic rocks is, how-ever, a good condition to preserve textural disequilibriumand to record different stages of the uplift and coolinghistory. Reheating is indicated in the pegmatites by partialresorption of zoisite and crystallization of a second andthird generation with distinctly different composition.This observation is in line with the idea that upliftoccurred during the collision event and shortly afteror simultaneously with stacking of the nappes; that is,in a short time frame, which is in many cases close to thetime resolution of available geochronometric methods.

SHRIMP U–Pb Zircon geochronology of the Altai No. 3 Pegmatite, NW China, and its implications for the origin and tectonic setting of the pegmatite

The “Altai No. 3 granitic pegmatite” in NW China is the largest pegmatite in the world that has been found in an orogenic belt; it forms one of the largest muscovite deposits in Asia and is most famous for its concentric-ring structure. Although it has been mined for more than 70 years, the age of formation and mineralization of the pegmatite has not been well determined, despite numerous previous geochronological studies. Here we present new results of SHRIMP U–Pb zircon dating. Three weighted mean 206Pb/ 238U ages of 220 ± 9 Ma, 198 ± 7 Ma and 213 ± 6 Ma were obtained for pegmatite zones 1, 5, and 7, respectively. The analyses were made on clear, recrystallized domains and overgrowth rims of large, opaque high-U zircon grains. We interpret the age of 220 ± 9 Ma for zone 1 to approximately represent the initial time of formation for No. 3 pegmatite. An additional date of 213 ± 2 Ma was determined on zircon from a small subordinate vein formed in the late-stage phase of the No. 3 pegmatite. This age set the late age of emplacement of the pegmatite. The time span of 213 to 198 Ma may reflect late stage hydrothermal alteration. In addition, a weighted mean 206Pb/ 238U age of 277 ± 6 Ma was obtained from small, clear, transparent low-U zircon grains which are more probably xenocrysts. Our new age data indicate that the pegmatite has no genetic relationship with the wall granitic rocks (409 Ma), but is likely related to the early Mesozoic (220 Ma) granitic magmatism that was just recognized. The No. 3 pegmatite was formed in an early Mesozoic anorogenic extensional regime. A stable tectonic setting was undoubtedly required for the formation of the large pegmatite.

Low-rhenium molybdenite by metamorphism in northern Sweden: Recognition, genesis, and global implications

Re–Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore deposits using the model age or isochron approach. But, molybdenite has a new niche in the greenschist- to granulite-facies metamorphic environment. Re–Os ages for metamorphic molybdenite may be used to construct regional metamorphic histories. Age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single, petrographically-characterized molybdenite occurrences. In this study, twelve geologically distinct molybdenite-bearing samples from two small Mo districts in northern Sweden trace a 150 m.y. Paleoproterozoic Svecofennian metamorphic history from ∼1900 to 1750 Ma. These data reveal a little-known, widespread and protracted, Late Svecofennian anatexis in northern Sweden. The Kåtaberget Mo–(Cu, F) deposit is located in the Moskosel granite batholith north of the economically-renown Skellefte district. Four different molybdenite samples from outcrop at Kåtaberget indicate an intrusion age of 1895 ± 6 Ma with the formation of later pegmatite–aplite at 1875 ± 6 Ma. The Allebuoda (Björntjärn) and Munka Mo–(W) deposits in the Rappen district are represented by three outcrop and five drill core samples of molybdenite-bearing aplite–pegmatite–granite. These two deposits were previously described as intrusion-related Climax-type Mo mineralization. Re–Os ages for molybdenites from these deposits range from 1865 to 1750 Ma and, significantly, Re concentrations are markedly low, extending to the sub-ppm level. Age agreement within the deposits is conspicuously lacking, whereas, with one exception, age agreement within any single sample (geologic occurrence), as established by analysis of additional molybdenite separates, is very good. These data, together with fundamental geologic observations discussed in this paper, suggest that Mo–(W) mineralization in northern Sweden is not intrusion-related, but the local product of episodic melting of Archean–Paleoproterozoic supracrustal gneisses related to the Svecofennian orogeny. Petrographic traverses across the boundary between widespread, foliation-parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation. Dehydration breakdown of zircon-rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine, post-deformational biotite plus sulfides, oxides, hydrothermal zircon and fluorite, all associated with microcline-dominant leucosomes. This process has profound implication for the traditional leucogranite, intrusion-related genesis attributed to the broad classification of Mo–W–Sn–base and precious metal mineralization (e.g., South Mountain Batholith, Nova Scotia; Okiep, Namaqualand, South Africa; Mactung, Yukon; Pogo–Liese, Tintina, Alaska; Carajás and Goiás–Rio Tocantins, Brazil; New England Batholith, NSW, Australia; Bergslagen, Sweden; Nevoria, Western Australia; Alpeinerscharte, Austria; Erzgebirge, Germany; Sardinia–Corsica Batholith). In addition to biotite, metallogenic contributions (e.g., Mo, W, Sn, U, Bi, Cu, Pb, Zn, Fe, Ni, Co, Au, Ag, Te, As, Sb, REE) in various combinations may also be controlled by breakdown of amphibole. In effect, the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite. In the absence of focusing structures (e.g., shear zones, sheeted vein development), this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades. Low Re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss (Mo–W) or muscovite schist (Sn–W).

K-Ar Ages of Granitic Magmatism and Related Pegmatite Formation at the Umanotani-Shiroyama Mine, Shimane Prefecture, SW Japan and Their Bearings on Cooling History

Abstract. The Umanotani-Shiroyama pegmatite deposits, the largest producer of K-feldspar and quartz in Japan, are of typical granitic pegmatite. Ilmenite-series biotite granite and granite porphyry, hosting the ore deposits, and biotites separated from these rocks yielded K-Ar ages ranging from 89.0 to 81.4 Ma and 95.2 to 93.7 Ma, respectively. Muscovite and K-feldspar separated from the ore zone yielded K-Ar ages with the range of 96.2 to 93.1 Ma and 87.3 to 80.7 Ma, respectively. Muscovites from quartz-muscovite veins in the ore zone and in the granite porphyry yielded K-Ar ages of 90.4 and 76.3 Ma, respectively. K-feldspar is much younger in age than coexisting muscovite. It is noted that the K-Ar ages of biotite separates and the whole-rock ages are identical to those of muscovite and K-feldspar in the ore zone, respectively. These time relations, as well as field occurrence, indicate that the formation of the pegmatite deposits at the Umanotani-Shiroyama mine is closely related in space and time to a series of granitic magmatism of ilmenite-series nature. Using closure temperatures of the K-Ar system for biotite and K-feldspar (microcline), cooling rate of the pegmatite deposits is estimated to be about 82d̀C/m.y. at the beginning, but slowed down to about 15d̀C/m.y. in the later period.

Emplacement of polygeneration pegmatites in relation to Sveco-Norwegian contractional tectonics: examples from southern Norway

Recent studies have demonstrated that there is a strong link between sites of concentrated magmatism and crustal deformation zones. Magmatism is often described as being syn-kinematic where one or more increments of intrusion punctuate deformation with successive generations of injections being progressively deformed. Pegmatite formation in the Mesoproterozoic of south Norway has been considered as post-kinematic in nature relative to Sveconorwegian (Grenvillian) deformation (1.13 Ga to ∼0.85 Ga) during accretion of the SW margin of Baltica. This paper presents structural observations from mapping of some pegmatites in the Bamble Terrane of south Norway demonstrating that the pegmatites are structurally related to Sveconorwegian fold geometries associated with peak metamorphism at approximately 1.14 Ga and are kinematically related to overthrust geometries associated with the initial overthrusting phase of the Porsgrunn-Kristiansand Fault when the Bamble Terrane docked with the underlying Telemark Terrane. Local fold geometries suggest a strain regime where the maximum contractional axis is sub-horizontal and oriented NW-SE consistent with the regional kinematics. The pegmatites display a systematic deformation pattern consistent with deformation in the isoclinal fold limbs. Pegmatites are also deformed into asymmetric antiformal folds above reverse-shear structures and are also cut by these structures thus showing a thrust geometry. The undeformed pegmatites are emplaced in sub-horizontal fractures and suggests sub-horizontal contractional axis related to overthrusting which is identical to the strain regime of the folding. The evidence suggests that the pegmatites are syn-tectonic and were injected into reverse shear-related fractures. Deformation was progressive and incremental with longer periods of ductile deformation at low strain rate punctuated by shorter periods of fracturing and pegmatite injection at high strain rate. This work therefore implies an intimate spatial and temporal relationship between deformation and magmatism during crustal accretion on the western margin of Fennoscandia.

Ordovician meta-pegmatite garnet (N-W Ötztal basement, Tyrol, Eastern Alps): preservation of magmatic garnet chemistry and Sm–Nd age during mylonitization

Meta-pegmatites and meta-pegmatite-hosted garnet from three localities in the western/northern Ötztal basement, Tyrol (Eastern Alps), have been analysed for major and selected trace element chemistry and for Sm–Nd isotope systematics. The garnets have frequently retained a euhedral shape, even though they are now set in a strongly foliated matrix consisting of dynamically recrystallized quartz and feldspar. They are characterized by high Fe and Mn contents (up to 29 mol% spessartine) and a simple major element zoning with decreasing MnO and increasing FeO from core towards the rim, suggesting a single-phase crystallization and primary, magmatic chemistry. Chondrite-normalized REE patterns display a strong enrichment of HREE relative to LREE, and pronounced negative Eu anomalies. Based on ID-TIMS analysis, neodymium concentration in all but one of the optically clean garnet separates varies between 0.12 and 5.5 ppm, at concomitant high Sm/Nd ratios (1.2–4.9). LA-ICP-MS analysis revealed very strong within-grain variation both for Nd concentration (<0.1–6.3 ppm) and Sm/Nd ratio (<1–27.4). Garnet-whole rock or garnet-feldspar Sm–Nd isochron ages range between 445±3 and 473±3 Ma, with a distinct concentration of the age data in the 455–462 Ma time interval. These ages indicate pegmatite formation in different areas of the Ötztal basement during Middle to Late Ordovician time, related to the widespread generation of crustal melts towards the end of the long-lived Early Palaeozoic magmatic event. The presence of granites and associated pegmatites is a distinctive feature mainly of the north-western parts of the Austroalpine basement in the Eastern Alps and typical for the so-called “Caledonian” era.

The source and transport of arsenic in a bedrock aquifer, New Hampshire, USA

The geographic distribution of elevated groundwater As concentrations in a fractured silicate bedrock aquifer in central New Hampshire correlates with the presence of pegmatites which border late Devonian granites and intrude metasedimentary rocks. As concentrations in the pegmatites averaged 9.6 mg/kg, which is much higher than the associated granites (0.24 mg/kg) and metasedimentary rocks (0.8 mg/kg). As was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low As concentrations and pegmatites with high As concentrations. Arsenic behaves similarly to B, which was also concentrated into these late stage fluids. Arsenopyrite (FeAsS) with an oxidation reaction rim of scorodite (FeAsO 4·2H 2O) was observed in aquifer materials. Elevated As concentrations have been observed to occur in other New England locations in pelitic metasediments intruded by anatectic plutons. It is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating As in crystalline aquifer materials, which can then cause local As enrichment of groundwaters. Groundwater As concentrations ranged from 26 nmol/l to 5300 nmol/l with a median value (210 nmol/l) that is more than 30 times higher than the median for groundwaters from all of New Hampshire (6.5 nmol/l). High chloride concentrations (>1 mmol/l), resulting from road salt contamination of recharge waters, suggest that groundwaters are most likely young (<50 years). A systematic relationship was observed between pH and dissolved As and Fe concentrations. All waters with elevated As concentrations (>670 nmol/l or ∼50 μg/l) have very low Fe concentrations (<18 μmol /l) and high pH (>7); samples with low As (<340 nmol/l) have variable Fe concentrations, but all occur at low pH (<7). pH-dependent Fe oxyhydroxide adsorption of As oxyanions is consistent with the observations. At pH>∼7, Fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb As. At pH <∼7, Fe oxyhydroxide formation is slow and depends on dissolved O 2 availability, however the resultant Fe oxyhydroxides have a positive net surface charge, and appear to adsorb As readily.

40Ar/39Ar isotopic evidence for mid-Devonian post-metamorphic pegmatite emplacement in the Marianske Lazne Complex, Bohemian Massif, Central European Hercynides

Large (2-3 cm) crystals of ferroan pargasite in a narrow pegmatite vein that cross-cuts amphibolite in the Marianske Lazně Complex yield 4 0 Ar/ 3 9 Ar regression ages of 378.7 ′ 3.5 and 379.5 ′ 4.3 Ma: these date the time of cooling through 550-500 °C soon after emplacement and crystallization. With this and associated veins also cutting flexural-type folds and a fracture cleavage, both tectonothermal activity in, and uplift of the host metamorphic assemblage into the upper crust must have taken place by, or near, a time corresponding to the Eifelian-Givetian boundary (mid-Devonian). The age of pegmatite formation corresponds, within errors, with the time of closure of the Sm-Nd isotopic system in eclogites elsewhere in the domain (377 ′ 7 Ma). This points to the crustal uplift having been rapid. Isotopic evidence also indicates a considerable time gap between the tectonothermal - igneous activity so prominently expressed in the Marianske Lazně Complex and the superimposed effects of early Carboniferous tectonothermal activity.

Preservation of Permo–Triassic low‐pressure assemblages in the Cretaceous high‐pressure metamorphic Saualpe crystalline basement (Eastern Alps, Austria)

AbstractMetapelites and intercalated metapegmatites of the Saualpe crystalline basement, which forms part of the Austroalpine nappe complex in the Eastern Alps, display a polyphase tectonometamorphic history. Here, we focus on the evolution that these rocks underwent prior to Cretaceous (eo‐Alpine) high‐pressure metamorphism and related penetrative deformation. Geothermobarometry on coarse‐grained porphyroclastic parageneses (garnet–biotite–muscovite–plagioclase–sillimanite–quartz), which occur as relics in kyanite–garnet, two‐mica gneiss, yielded 600 °C/0.4 GPa. Results from a corundum‐bearing lithology suggest that higher temperatures may have been reached in very restricted areas. The matrix of these rocks displays intense recrystallization during a pressure‐dominated metamorphic overprint. Microstructures and mineral chemistry indicate that this low‐pressure metamorphism was the first significant metamorphic imprint in these rocks. Mineral relics in all metapelitic rock types reflect low‐pressure conditions for this interkinematic crystallization phase.The distribution, macroscopic and microscopic observations and the mineralogical composition of intercalated metapegmatites point to regionally elevated temperature conditions during their emplacement. Therefore, pegmatite formation is correlated with mineral formation in metapelites. Sm–Nd‐dating of magmatic garnet from the pegmatite gneiss yielded 249 ± 3 Ma, which is interpreted to represent the age of pegmatite‐emplacement and low‐pressure metamorphism in the metapelites. Since the pegmatites are overprinted by mylonitisation and high‐pressure metamorphism, this Permo–Triassic age also sets an upper age‐limit to the eclogite facies metamorphic event, which affected considerable parts of the Saualpe crystalline basement.

The evolution of the Palim granite in the Bastar tin province, Central India

The Palim granite, hosted by the metasedimentary country rocks in the Bastar tin province, is a heterogeneous pluton that comprises hornblende granite, biotite granite and two-mica granite. Spherical inhomogeneous surmicaceous enclaves occur within the granites with coarse grained cores of muscovite mantled by finer muscovite-quartz-biotite (± sillimanite) rims. Geochemical features imply that the granites are highly evolved and geochemically distinct. Petrographic and geochemical considerations point towards a transition from metaluminous I-type hornblende-bearing granite in the south to peraluminous volatile-enriched S-type like lithologies (biotite and two-mica granites) towards north. Modeling of highly incompatible elements such as Nb and Cs, implies 31 to 33% assimilated fractional crystallization of a melt with an initial composition close to that of the hornblende granite to form the two-mica granite. Hornblende geobarometry, plagioclase-hornblende thermometry (in hornblende granite) and phengite barometry (in two-mica granite), yield P-T estimates of 5–7 kb/725°–760 °C, and 6 kb/700 °C, respectively. The study further implies that a genetic link exists between granite magmatism and the formation of tin pegmatites in the region. The preponderance of peripheral pegmatites to the north-east of the Palim granite is regarded a result of outward crystal-melt fractionation and tectonic tilting of the pluton.