Pegmatite Field Research Articles

Columbite U-Pb Geochronology of Kalu’an Lithium Pegmatites in Northern Xinjiang, China: Implications for Genesis and Emplacement History of Rare-Element Pegmatites

The Kalu’an-Azubai pegmatite field, one of the most important rare-metal metallogenic regions in China, contains a large number of pegmatite dikes belonging to spodumene and lepidolite subtypes. Columbite-group minerals (CGMs) collected from three spodumene subtype pegmatites (No. 802, No. 803, and No. 805 pegmatites) were analyzed for major element contents using EPMA (electron probe micro-analyzer) and dated using LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometer). The crystallization ages of the CGMs from No. 802, No. 803, and No. 805 pegmatites are 209.5 ± 1.4 Ma (2σ), 198.3 ± 2.0 Ma (2σ), and 224.3 ± 2.9 Ma (2σ), respectively. Oscillatory zoning and/or sector zoning along with the associated mineral assemblages suggest that the dated columbite is of magmatic origin. The crystallization ages of the columbite grains thus represent the emplacement ages of the Li pegmatites. Therefore, our dating results indicate that there were three emplacement events of the Li-rich pegmatite-forming melts in a timeframe of ~30 Ma. In combination with previous studies, we conclude that the Li pegmatites were formed before the Be-Ta-Nb pegmatites (~194–192 Ma), which precludes the genesis of rare-metal pegmatites via fractional crystallization of a granitic magma in the Kalu’an-Azubai region.

A Ta,Ti–rich oxide mineral assemblage from the Nancy beryl–columbite–phosphate granitic pegmatite, San Luis, Argentina

An assemblage of tantalite-(Mn), tantalian rutile, tapiolite-(Fe), titanowodginite, ferrotitanowodginite, and hydroxycalciomicrolite occurs in the Nancy granitic pegmatite, San Luis range, Argentina. The Nancy beryl-type, beryl–columbite–phosphate subtype of LCT (Li-Cs-Ta) rare-element pegmatite was emplaced in the Paleozoic Conlara pegmatitic field. The assemblage occurs at the core margin of the pegmatite, forming an irregularly shaped, 18 by 6 cm nodule. The chemical composition of tantalite-(Mn) shows median Ta# [= (Ta/(Ta + Nb) apfu (atoms per formula unit)] and Mn# [= (Mn/(Mn + FeT) apfu] values of 0.57 and 0.64, respectively; Ti, U and Zr show maximum and [median] contents of: 3.37 [1.25] wt.% TiO2, 0.58 [0.24] wt.% UO2, and 0.72 [0.50] wt.% ZrO2. The unit-cell parameters indicate a moderately ordered structure. Tantalian rutile occurs as anhedral grains replacing tantalite-(Mn), associated with hydroxycalciomicrolite. Its chemical composition shows moderate to high Ti contents, with a maximum and [median] of 64.77 [38.67] wt.% TiO2. The proportion of Ta is very high, with 49.67 [39.59] wt.% Ta2O5. Tapiolite-(Fe), with 82.49 [81.86] wt.% Ta2O5, 2.51 [2.33] wt.% Nb2O5, 0.94 [0.79] wt.% TiO2, and 13.31 [13.18] wt.% FeO, has uniform Ta# and Mn# values, 0.95 and 0.09, respectively. Titanowodginite shows Ta# values ranging from 0.82 to 0.88, whereas in ferrotitanowodginite it ranges from 0.88 to 0.94. The Mn# value is similar in titanowodginite (0.51–0.64), and decreases in the ferrotitanowodginite (0.04 to 0.41). These minerals form a replacement sequence of tantalite-(Mn). Hydroxycalciomicrolite occurs in two generations: I and II. The dominant A cation is Ca, with a median value of 14.39 wt.% CaO. The MnO content, with a median of 1.16 wt.% MnO, is relatively constant. The amount of UO2 is usually below 3 wt.%, but locally attains 6.9 wt.%, and exceptionally 43.6 wt.%, in irregular rims that show a low analytical total, giving compositions that depart from the expected stoichiometry; it is clearly a subsolidus phase. In the more plausible explanation for the evolution of this assemblage, the magmatic crystallization of tantalite-(Mn) was followed during the early subsolidus stage by its partial replacement by tantalian rutile + tapiolite-(Fe) + titanowodginite + ferrotitanowodginte, associated with hydroxycalciomicrolite I, and later, by hydroxycalciomicrolite II produced by the influx of a late Fe–Ti–Ca-bearing fluid phase likely entering the pegmatite from the wall rocks.

The Kolmozero deposit: a unique Li source in the European Arctic of Russia

Tens to hundreds of pegmatite veins can accumulate within a pegmatite field. However, only a limited number of pegmatite veins can be practically valued. Hence, there is a problem of identifying rare metal pegmatites at the stage of geological prospecting and appraisal. Geochemical indicators are most impartial, reproducible and highly informative for these purposes. The Kolmozero lithium deposit is located in the Neoarchaean metagabbro-anorthozites of the Patchemvareksky massif in the junction zone of the two major regional structures of the Archaean age, i.e. the Murmansk block and the Kolmozero-Voronya Greenstone Belt of the Kola Peninsula. It is shown that pegmatites of the Kolmozero deposit with industrially valued rare metal mineralization (Be, Ta, Nb, Li) are poor in large-ion lithophile elements (LILE) (Sr and Ba) and in high charged high field strength elements (HFSE) (REE, Th, Zr). They also have low values of fractionation indexes – Mg/Li and Zr/Hf. The defined geochemical indicators have been applied to estimate feldspar pegmatites and muscovite-feldspar pegmatites that are spatially associated with rare metal pegmatites of the Kolmozero lithium deposit. The feldspar (beryllium-bearing) and muscovite-feldspar (beryllium-niobium-tantalum) pegmatites have rare metal mineralization and, like albite-spodumene pegmatites, are rich in Li, Nb, Ta, Be and depleted by Sr, Ba, Zr, Y, REE. Contents of ore elements (Li, Nb, Ta, Be) content increases from feldspar to muscovite-feldspar and albite-spodumen pegmatites. It is accompanied by a decrease in values of fractionation indexes of Zr/Hf and Mg/Li, as well as a decrease in contents of light, medium and heavy lantanoids Zr, Ba and Sr.

Non-Metamict Aeschynite-(Y), Polycrase-(Y), and Samarskite-(Y) in NYF Pegmatites from Arvogno, Vigezzo Valley (Central Alps, Italy)

At Arvogno, Vigezzo valley in the Central Alps, Italy, pegmatite dikes are unique in the scenario of a tertiary alpine pegmatite field because they show marked geochemical and mineralogical niobium–yttrium–fluorine features. These pegmatites contain AB2O6 aeschynite group minerals and ABX2O8 euxenite group minerals as typical accessory minerals including aeschynite-(Y), polycrase-(Y), and samarskite-(Y). They are associated with additional typical minerals such as fluorite, Y-dominant silicates, and xenotime-(Y). The Y–Nb–Ti–Ta AB2O6 and ABX2O8 oxides at the Arvogno pegmatites did not exhibit any textural and compositional features of oxidation or weathering. They are characterized by low self-radiation-induced structural damage, leading to the acquisition of unit-cell data for aeschynite-(Y), polycrase-(Y), and samarskite-(Y) by single-crystal X-ray diffraction. Aeschynite-(Y) and polycrase-(Y) crystals allowed for both to provide space groups whereas samarskite-(Y) was the first crystal from pegmatites for which cell-data were obtained at room temperature but did not allow for the accurate determination of the space group. According to the chemical compositions defined by Ti-dominant content at the B-site, the cell parameters, respectively, corresponded to polycrase-(Y), aeschynite-(Y), and the monoclinic cell of samarskite-(Y). Emplacement of Alpine pegmatites can be related to the progressive regional metamorphic rejuvenation from east to west in the Central Alps, considering the progressive cooling of the thermal Lepontine Barrovian metamorphic dome. Previous studies considered magmatic pulses that led to emplace the pegmatite field in the Central Alps. As an example, the pegmatites that intruded the Bergell massif were aged at 28–25 millions of years or younger, around 20–22 m.y.

Chemical Evolution of Nb-Ta Oxides and Cassiterite in Phosphorus-Rich Albite-Spodumene Pegmatites in the Kangxiwa–Dahongliutan Pegmatite Field, Western Kunlun Orogen, China

The Kangxiwa–Dahongliutan pegmatite field in the Western Kunlun Orogen, China contains numerous granitic pegmatites around a large granitic pluton (the Dahongliutan Granite with an age of ca. 220 to 217 Ma), mainly including barren garnet-, tourmaline-bearing pegmatites, Be-rich beryl-muscovite pegmatites, and Li-, P-rich albite-spodumene pegmatites. The textures, major element contents, and trace element concentrations of columbite-group minerals (CGM) and cassiterite from three albite-spodumene pegmatites in the region were investigated using a combination of optical microscopy, SEM, EPMA and LA-ICP-MS. The CGM can be broadly classified into four types: (1) inclusions in cassiterite; (2) euhedral to subhedral crystals (commonly exhibiting oscillatory and/or sector zoning and coexisting with magmatic cassiterite); (3) anhedral aggregates; (4) tantalite-(Fe)-ferrowodginite (FeSnTa2O8) intergrowths. The compositional variations of CGM and cassiterite are investigated on the mineral scale, in individual pegmatites and within the pegmatite group. The evolution of the pegmatites is also discussed. The variation of Nb/Ta and Zr/Hf ratios of the cassiterite mimics the Nb-Ta and Zr-Hf fractionation trends in many LCT pegmatites, indicating that these two ratios of cassiterite may bear meanings regarding the pegmatite evolution.

Geochemistry and rare-metal bearing potentials of pegmatites of Gbugbu, Lema and Bishewa areas of North Central Nigeria

The authors acknowledge the immense supports received from the management of the Nigerian Geological Survey Agency (NGSA). Mr. Olufemi Ogedengbe, a retired Director of Economic Geology Department, NGSA was exceptionally supportive in all aspects of this research. Abstract The pegmatites of Gbugbu, Lema and Bishewa areas belong to the pegmatite belt of North-Central Nigeria which has been prospected for minerals since the early 1940’s. Detailed geological mapping of the pegmatite bodies and sampling of muscovite extracts were carried out, while major, trace and rare earth elements geochemistry were achieved using ICP-OES and ICP-MS. Variation plots of major and trace elements were used to determine the geochemical characteristics of these pegmatite bodies. Structural features of the study area suggest fracture patterns in the Nigerian Basement rocks controlled the distribution of the pegmatites on a regional NW-SE, N-S and NE-SW and minor E-W trends. The pegmatites are peraluminous with major and trace elements pattern indicating similarities with granitic clan of igneous rocks. Qualitative assessment of the pegmatites using K/Rb vs Cs, K/Cs vs Rb, K/Rb vs Rb/Sr, variation plots suggest the majority of the pegmatites are mineralized with Na/K ratio>1 and belong to the rare metal bearing class with a compositional variation suggesting regional zonation of the pegmatites. Furthermore, these plots suggest the Lema pegmatite field is highly evolved and belong to the Li-Be-Ta Type (III) and Li-Cs-Be-Ta Type (IV) while majority of those in Gbugbu and Bishewa fields belong to the muscovite bearing Type (I) and Be bearing Type (II). It is concluded from this study that the pegmatites of Gbugbu, Lema and Bishewa pegmatite fields have the potentials for rare metal mineralization with alkali and rare alkali fractionation playing significant roles in the mineralization processes. Furthermore, field evidence and Rb vs Sr plot, suggests the pegmatites were emplaced at variable depths of between 20-30km indicating that the Gbugbu and Bishewa pegmatite fields have been extensively eroded and are close to the roots of the primary mineralization exposing mainly the interior pegmatites of the Types I-II. On the other hand, the Lema field which suffered lesser degree of erosion has Type III-IV pegmatite still remaining at the center of a regionally zoned pegmatite body. The significance of this study is that the pegmatites of Gbugbu, Lema and Bishewa areas have potentials for economic mineralization of tantalite, wolframite, cassiterite and columbite. Keywords: Pegmatites, Pnuematolysis, Albitization, Rare-metal, Mineralization, Peraluminous DOI : 10.7176/JEES/9-3-15 Publication date :March 31 st 2019

Structure of the Granitic Pegmatite Field of the Northern Coast of Portugal—Inner Pegmatite Structures and Mineralogical Fabrics

On the coastline of Northern Portugal, metamorphic formations and pegmatites were the subject of structural analysis with the main goal of understanding Variscan kinematics and related pegmatite intrusion. This study also aims to discriminate, select and characterize relevant aspects of the structure and the paragenesis of pegmatites, well exposed as a result of coastal erosion, justifying its inclusion in the geological heritage of the Northern coast of Portugal. The pegmatite bodies show distinctive internal and external structures that are attributable to different modes of emplacement and subsequent deformation. The pegmatitic implantation in the areas of Moledo and Afife occurs in an intragneissic and perigranitic environment, for the first area, and perigneissic and perigranitic environment, for the second. In Pedras Ruivas predominates the implantation into an exo-gneissic to exo-granitic domain. The Moledo veins show evidence of multiphase open/filling, revealing positions, shapes, attitudes, sizes and internal structures that change as a function of the host lithology and host structure, but mainly due to the dilation and the cycles number of local telescoping. The structural analysis of the pegmatite bodies allows the deduction of a local fulcrum of expansion that hypothetically overlaps a hidden stock of parental granite. In Afife and Pedras Ruivas, some pegmatitic lenses are specialized and mineralized in Li, Cs and Ta, with spodumene and tantalite ± cassiterite. Spodumene occurs as giant crystals, centimetric to pluri-decimetric in length, which mark very clearly the structures of in situ or in flow crystallization inside the pegmatites (primary structures) and also the secondary structures resulting from deformation. The geometric analysis of fabrics helps the individualization of well-defined stages of progressive evolution of the deformation of the pegmatites, allowing its correlation with major D2–D3 episodes of regional Variscan deformation.

Особенности гранитовых систем с редкометалльными пегматитами

There are usually two generations of granitic pegmatites revealed in most of rare-metal pegmatite fields: beryl-bearing ones, often with tantalum-niobate, muscovite, inseparable from vein-shaped granite leucogranite complex, and sodium-lithium (Li, Ta, Cs, Be, Sn) pegmatites (or LCT-pegmatites), which are subdivided into «complex rare-metal» ones, with the pegmatite zonation and the major mineralization (large crystals of spodumene, tantalates, beryl, cassiterite, pollucite, petalite, amblygonite) in central parts of their body (Koktogay, Bernick Lake, Bikita, Karibib, Varutresk, Vishnyakovskoye, etc.), and «albite-spodumene» ones, composing numerous extensive pegmatite dykes without zoning, grouping in their fields as long as up to 10—15 km and more (Kings Mountain, Zavitinskoye, Goltsovoye, Kolmozero, Polmos Tundras, Tastygskoye, etc.). In some cases (Zavitinskoye, Vasin-Mylk, Shuckbjul), there are established complex rare-metal pegmatites occurring in «heads» of conveying dykes of «albite-spodumene pegmatites». The search of «parent granites» for «albite-spodumene pegmatites» is meaningless, because they are not pegmatites, but granites, although special ones (in fact, they are spodumene granites of the alakhinsky type). The terminological, as it seems to be, readjustment: «granite, not pegmatite», has its scientific ( a ) and prognostic ( b ) consequences. Firstly, there is resolving ( a ) the problem of parent granites, objections to the pegmatite status of «albite-spodumene pegmatites» become removed. This type of pegmatites occupies in history of magmatism over some pegmatite-bearing areas its independent, spodumene-rare-metal-granite stage, concluded by formation of their own — true — complex pegmatites. And secondly ( b ): it may be assumed that under such complex pegmatite fields as Koktogay, Bernick Lake, Bikita, Karibib, Vishnyakovskoye, there are occurring series of dikes of spodumene-rare-metal granites (i. e., Li ore deposits), which are differentiates of complex pegmatites.

Metallogenesis of the Totoral LCT rare-element pegmatite district, San Luis, Argentina: A review

The Totoral Pegmatite District (TPD) is the southernmost rare-element LCT pegmatite field of the Pampean Pegmatite Province. The TPD produced intermittently in the last 60 years Ta-Nb ore minerals, spodumene, beryl and ceramic raw materials. It is located in the southern part of the Eastern Pampean Range of San Luis province. It was developed during the late stage of accretion of the Famatina terrane to the West Gondwana in the Lower Paleozoic (≈450 Ma). The parental S-type leucogranites and rare-element pegmatites of REL-Li subclass form three groups aligned NNE. The leucogranites were originated by muscovite (±incipient biotite) dehydration melting of preferably metapelites (±metagreywackes) of the Pringles Metamorphic Complex (PMC). The resultant bimodal suite of S-type muscovite-tourmaline and muscovite-biotite leucogranites show major, trace elements, and Pb-Ba ratios compatible with both low-T and higher-T collisional leucogranites. These leucogranites were emplaced after regional metamorphism in the upper part of the mica schists unit of PMC at 640–725 °C and ≈400–500 MPa, and they fractionated to their associated pegmatites as is supported by the spatial association, similar age and fractionation trends of leucogranites and pegmatites. The regional integrated pegmatite zoning shows the sequence: leucogranite, pegmatitic leucogranite, barren-transitional to beryl-type, beryl-columbite-phosphate subtype, albite-spodumene type, complex-type spodumene-subtype and albite type rare-elements pegmatites. This zonation follows a path towards decreasing pressure of emplacement. The crystallization of pegmatites was triggered by the rapid undercooling provoked by the thermal contrast due to the fast forced emplacement in the hosting mica schists and the H2O and fluxes content of the melts that produced nucleation delay and high crystal growth rate of the minerals. The Li-bearing pegmatites have genetic links with the higher-T leucogranites. The emplacement of the pegmatites was facilitated by a shear zone, and they show synkinematic ductile-state deformation ascribed to the late stage of the Famatina terrane accretion. Later on, most of them were tectonically affected in brittle state by the diastrophism attributed to the westward accretion of the Cuyania terrane.

The Pilgangoora Lithium-Tantalum Deposit - Geological Overview and Evolution of Discovery

The Pilgangoora Lithium-tantalum pegmatite deposit with a total resource of 156.3 Mt grading 1.25% Li2O and 128ppm Ta2O5, is a globally significant hard-rock lithium-tantalum deposit. The deposit is located in the East Pilbara Terrane of the northwest Pilbara Craton in Western Australia. The Northwest Pilbara Craton is one of the world’s major lithium-tanatlum provinces with large scale lithium-caesium-tantlum bearing pegmatites located at Mt Francisco, Wodgina, Pilgangoora and Strelley. Pegmatites bearing columbite-tantalite at Mt. York were first described in government geological surveys in 1906. Subsequent interest in the pegmatites focussed on their tin and tantalum mineral potential, with small scale hardrock, eluvial and alluvial mining, chiefly in the period 1947-1978. Larger scale alluvial and eluvial mining of tin-tantalum was carried out over 1978-1982 and 1992-1996 by a number of junior companies. In May 2014, Pilbara Minerals acquired the Pilgangoora Project for its lithium potential and has since drilled over 1450 holes for approximately 120,000 metres. The Pilgangoora pegmatite intrusions crop out in a well exposed greenstone belt, with little weathering at surface. Exploration drilling programs along with detailed geological mapping of the Pilgangoora tenement group has provided a better understanding of the geological setting of the fractionated pegmatite intrusions within the East Strelley greenstone belt. This work has led to the recognition of some valuable exploration criteria that may be applied locally to locate additional resources and, longer term, may be used more strategically to review other pegmatite fields across the Pilbara region.

Interaction of crustal and mantle materials, sources of trace elements during the formation and evolution of Early Paleozoic Li-rich granite-pegmatite systems in southeastern Tuva

We present new data on the age, composition, and environments of formation of granites of the Kystarys complex and the associated Li-rich rare-element pegmatites of the South Sangilen pegmatite belt including the large Tastyg lithium deposit. It has been established that they formed during the Early Paleozoic collisional orogeny in the Tuva-Mongolian massif at the Cambrian-Ordovician boundary. The granites of the Kystarys complex are moderately alkaline high-K rocks and are enriched in Zr, Nb, Y, and REE; therefore, they are classified as postcollisional, transitional to within-plate (A-type). The spodumene pegmatites of the South Sangilen pegmatite belt are similar to the above granites in age and isotopic and geochemical parameters, which suggests a paragenetic relationship between these rocks. Pegmatites form several pegmatite fields within the belt, which differ in trace-element signatures. In addition to predominant Li, Cs, and Ta, specific to all spodumene pegmatites (LCT family), pegmatites of two fields have high contents of Nb, Y, REE, and Zr, which are indicator elements of NYF family pegmatites. It has been established that the formation of spodumene pegmatites with combined LCT-NYF geochemical signatures was preceded by the intrusion of dikes of monzogabbro with the geochemical characteristics of OIB and of alkali aegirine granites and by the formation of associated metasomatites enriched in Zr, Nb, Y, and REE. Based on the geological, mineralogical, and geochemical data, we substantiate the hypothesis of the formation of Li-bearing granite-pegmatite melts from a mixed source resulted from the influence of fluids of an alkaline igneous complex of mantle genesis on the crustal protolith.

A Comparison of the Mica Geochemistry of the Pegmatite Fields in Southern Norway

A Comparison of the Mica Geochemistry of the Pegmatite Fields in Southern Norway

Chemical and spectroscopic characterization of tourmalines from the Mata Azul pegmatitic field, Central Brazil

Chemical and spectroscopic characterization of tourmalines from the Mata Azul pegmatitic field, Central Brazil

The Alvarrões-Gonçalo Li project: an example of sustainable lithium mining

Abstract. Across Europe, small scale deposits of critical materials and/or strategic elements, such as W, Li and Sn, are not viable to mine and process using the traditional large scale mining and processing technologies due to their geological characteristics. Project FAME (Flexible And Mobile Economic Processing Technologies, EC Horizon 2020 Grant Agreement No. 641650) was created in order to specifically address the technical and environmental challenges to the sustainable processing of low sulphide ores from pegmatites, greisens and skarns deposits. One of the deposits, Alvarrões-Gonçalo, is a Li pegmatite located at the Gonçalo pegmatite field (Central Portugal) that is being investigated on the scope of the concerned project. Currently, the pegmatites are only being exploited for ceramics, aggregates and ornamental purposes, however, there is exploitation potential for Li2O-rich ores. Froth flotation is being tested to produce a lepidolite concentrate. Results showed the feasibility to obtain a Li2O grade adequate for the metallurgical production of lithium compounds as Li2CO3. The lepidolite flotation rejects, normally stored in tailings dumps, are mainly composed by a mixture of feldspars, quartz and non-recovered lepidolite (though a very low content), which is, by itself, a very interesting raw material for ceramic purposes. Consequently, mining exploitation of lepidolite, as Li2O ore, from pegmatite could comply with the standards of “green mining”, reaching an almost “zero waste” exploitation, as it can be accomplished in the case of the mineral processing of the Alvarrões-Gonçalo lithium ore. This basic scenario could be improved by applying froth flotation also for feldspars/quartz separation, aiming at obtaining a high content feldspars concentrate (floated) and a quartz product (non-floated): the feldspars concentrate can be used to produce blends with different ratios feldspars/quartz for sanitary ware and tiles and quartz for other uses, such as glasses and “silica flour”.

The Mata Azul pegmatitic field, Tocantins/Goiás, central Brazil: geology, genesis and mineralization

In Goias and Tocantins States, Central Brazil, several granitic pegmatites were characterized and grouped for the first time. These pegmatites had been intensely explored by hand in the past, producing mainly gemstone varieties of tourmaline and beryl. Barren, beryl- and tourmaline-bearing pegmatites occur across an area of 2,000 km 2 where they intrude regional metasedimentary rocks and peraluminous granites. K‑feldspar (mostly altered to kaolin), quartz and mica (mainly muscovite) are the major minerals. The main accessory minerals are beryl, tourmaline, garnet, albite, Fe-Mn phosphate aggregates, and trilithionite. The paragneiss surrounding the barren pegmatites was affected by thermal metamorphism and later hydrothermal alteration, producing Ca-silicates, Ti-Nb-Y oxides and sulfides. Leucogranites of the Mata Azul Suite are peraluminous and syn- to post-orogenic with geochemical characteristics of the LCT granite-pegmatite group. LA-ICP-MS U-Pb geochronology in monazite yields an age of 519 ± 2.8 Ma. Additionally, U-Th-Pb chemical dating of uraninite reveals a maximum age between 500 and 560 Ma. These ages, the field relationships, the mineralogy and the geochemical data suggest that the granites of the Mata Azul Suite are the probable sources of the studied pegmatites. The mineral associations and the mineral chemistry are used to define the degree of fractionation of the pegmatites. We propose that the group of studied pegmatites represents a pegmatitic field, called the Mata Azul Suite Pegmatitic Field.

Beusite-(Ca), ideally CaMn22+(PO4)2, a new graftonite-group mineral from the Yellowknife pegmatite field, Northwest Territories, Canada: Description and crystal structure

ABSTRACTBeusite-(Ca), ideally Ca${\rm Mn}_{\rm 2}^{2 +} $(PO4)2, is a new graftonite-group mineral from the Yellowknife pegmatite field, Northwest Territories, Canada. It occurs in a beryl–columbite–phosphate rare-element pegmatite where it is commonly intergrown with triphylite–lithiophilite or sarcopside, and may form by exsolution from a high-temperature (Li,Ca)-rich graftonite-like parent phase. It occurs as pale-brown lamellae 0.1–1.5 mm wide in triphylite, and is pale brown with a vitreous lustre and a very pale-brown streak. It is brittle, has a Mohs hardness of 5, and the calculated density is 3.610 g/cm3. Beusite-(Ca) is colourless in plane-polarized light, and is biaxial (+) with α = 1.685(2), β = 1.688(2), γ = 1.700(5), and the optic axial angle is 46.0(5)°. It is non-pleochroic with X || b; Y ˄ a = 40.3° in β obtuse; Z ˄ a = 49.7° in β acute. Beusite-(Ca) is monoclinic, has space group P21/c, a = 8.799(2), b = 11.724(2), c = 6.170(1) Å, β = 99.23(3)°, V = 628.3(1) Å3 and Z = 4. Chemical analysis by electron microprobe gave P2O5 41.63, FeO 19.43, MnO 23.63, CaO 15.45, sum 100.14 wt.%. The empirical formula was normalized on the basis of 8 anions pfu: (Ca0.94Fe0.92Mn1.13)Σ2.99(PO4)2.00. The crystal structure was refined to an R1 index of 1.55%. Beusite-(Ca) is a member of the graftonite group with Ca completely ordered at the [8]-coordinated M(1) site.

Petrogenesis of syn-orogenic rare metal pegmatites in the Chinese Altai: Evidences from geology, mineralogy, zircon U-Pb age and Hf isotope

Recently, an increasing number of anatetic pegmatites have been confirmed. In contrast to the granitic pegmatite, the anatetic pegmatite has no relation to granite. It is therefore necessary to reacquaint the origin and classification of pegmatitic rocks. As one of the largest pegmatite provinces, the Chinese Altai provides a natural laboratory for pegmatite study. To date, the type, age, source and tectonic setting of the syn-orogenic pegmatites in the Chinese Altai remain unclear. In this work, the CMS (Chemical composition-Mineral assemblage-Structural geology) classification is primarily applied on the pegmatitic rocks in the Chinese Altai. Five types’ pegmatitic rocks, including metapegmatite, pegmatoid, pegmatite, pseudopegmatite and granite pegmatite, are identified by their CMS features. In addition, the geology, mineralogy, zircon U-Pb ages and Hf isotopes of six pegmatites are presented in this work. The pegmatites, numbered JMHB02, QBL02, TLT01, ALJK01, AMLG01 and TMLT01, are dated at 394.8 ± 4.0 Ma, 402.6 ± 5.5 Ma, 385.9 ± 3.5 Ma, 368.0 ± 4.0 Ma, 358.3 ± 4.6 Ma and 333.0 ± 6.0 Ma, respectively, indicating that they were formed during the syn-orogenic stage of the Altai orogenic process. They are exposed in the Qiongkuer domain in the southern Chinese Altai, juxtaposed with the late- and/or post-orogenic pegmatites in the same pegmatite field. No parental granite can be confirmed for all these pegmatites. Comparatively, the synorogenic pegmatites are scarce, and have lower mineralization grades than the late- and postorogenic pegmatites. Zircon Hf isotopic compositions of the syn-orogenic pegmatites have positive ɛHf(t) values of +2.06–+8.28 and old TDM2 ages of 1,234–860 Ma and are consistent with those of most of the syn-orogenic I- and S-granites, which indicates that the pegmatites and granites have a common immature crustal source from the early Paleozoic metasedimentary rocks. Compared to the post-orogenic pegmatites in the Central Altai domain, the syn-orogenic pegmatites in this work have larger positive ɛHf(t) values and younger TDM2 ages, indicating that more juvenile components were involved in their magma sources. Combining this study with previous works, we suggest that the anatexis of metasediments induced by decompression and injection of heat energy and mantle-derived materials under a forearc extensional setting led to the formation of the syn-orogenic pegmatites in the Chinese Altai.

Famatinian inner arc: Petrographical observations and geochronological constraints on pegmatites and leucogranites of the Comechingones pegmatitic field (Sierras de Córdoba, Argentina)

The Comechingones pegmatitic field comprises granitic pegmatites and leucogranites hosted by mylonitic rocks from the Guacha Corral shear zone. Pegmatites consist of variably sized, internally zoned bodies, with late replacement units. Leucogranites are fine-grained rocks that form small bodies with tabular-to lens-like shapes. Field relationships and petrographic observations suggest a synkinematic emplacement of both rock types with respect to the main deformational event of this shear zone. U-Pb LA-ICP-MS analyses on zircon crystals indicate crystallization ages of 473.8 ± 3.9 and 474.8 ± 12.2 Ma for the granitic pegmatites and leucogranites, respectively, suggesting that they originated coevally at around 474 Ma. This age provides a first-order time constraint on the deformation of the Guacha Corral shear zone. Using ages and pressure estimations, along with an average pressure gradient of 1GPa/35 km for the continental crust, an exhumation rate of approximately 0.1 mm/a and 10.5 km of total amount of exhumation were calculated for the time span between the early Ordovician and the late Devonian in this part of the Sierras Pampeanas. The inherited ages obtained in zircon crystals from the leucogranites display a distribution pattern that strongly resembles those of the metasedimentary rocks of the Pampean metamorphic belt, suggesting that equivalents of these rocks should have been involved in the melting processes that originated the leucogranites.

Fosfatos de Fe e Mn primários e secundários em pegmatitos graníticos do Campo Pegmatítico Mata Azul, Jaú do Tocantins, TO, Brasil

No sul do Estado do Tocantins e norte do Estado de Goiás, pegmatitos graníticos estéreis e mineralizados são conhecidos e explorados há muito tempo, mas só recentemente foram caracterizados quanto aos aspectos mineralógicos, composicionais e genéticos e agrupados sob a denominação de Campo Pegmatítico Mata Azul. Em três desses corpos localizados no município de Jaú do Tocantins, com mineralização de berilo, fosfatos de Fe e Mn ocorrem sob a forma de nódulos ou massas centimétricas. Duas associações de fosfatos são diferenciadas. Na primeira delas, hospedada nos pegmatitos São Júlio e Pichorra, foram descritas três fases primárias, identificadas como zwieselita, sarcopsídio e fluorapatita. Processos hidrotermais alteram essas fases, formando rockbridgeíta, uma fase do subgrupo da alluaudita e heterosita. Na segunda associação, presente no pegmatito Fazenda Mesquita, não há sinal de fosfato primário e todas as fases presentes são secundárias. Rockbridgeíta, de cor verde a alaranjada, compõe boa parte do volume das amostras. Fosfosiderita e strengita aparecem como alterações da rockbridgeíta, às vezes preenchendo cavidades nas massas de fosfato. Nas duas associações, óxidos e hidróxidos de Fe e Mn, oriundos da oxidação intempérica, correspondem às últimas fases formadas, podendo constituir crostas que recobrem boa parte dos nódulos e das massas de fosfatos. Em algumas amostras das associações de fosfatos, cristais de uraninita, gahnita, almandina e columbita-(Fe) foram identificados.

Melts and fluids evolution in the process of crust and mantle formation in Paleo-Proterozoic (2,2—1,75 Ga ago). Stratigraphy and magnetic activity

Stratigraphy of metamorphic Precambrian complexes related to Paleo-Proterozoic is controversial. An additional sign permitting to correlate strata may be their material composition because in the Precambrian composition of deep fluids changed regularly. Then some regularity in changes of composition of magmatic and metamorphic rocks has to exist. The rocks produced during this time are dolomite and calcite. Quartzite, carbonaceous shist and graphite gneiss are also abundant. Only during this activation of Precambrian carbonatite volcanic activity became apparent. The author’s method has been used generalizing the special features of magmatic and metamorphic rocks of the Ukrainian shield, which allowed reconstructing the course of the deep process and composition of melts and fluids in the layer of melting. Subdivision of the shield into blocks of two types has been proposed: inherent blocks and the areas spatially attached by suture zones. The blocks differ by decisive role of the fault zones formed before granitization; the presence of gabbro-montsonite-trachite granites massifs formed before granitization (2,1—1,99 Ga ago); mutual transitions between intusive and palingenous granites; formation of anorthosite-granite massifs after granitization. Activation consists of two tectono-magmatic cycles separated by granitization. Composition of rocks attached to suture zones is effected by oxidized deep melts and fluids. We can make estimation on geodynamic process and composition of deep fluids in blocks by composition of magmatic rocks. In blocks fault zones are generated with multiple intrusion of melts along them, differentiated under the crust. Neutral-alkalescent aquatic-chloride-potassium fluids take part in differentiation of melts. Composition of fluids changes after granitization under the whole shield. Immediately after it the massifs of carbonatites and alkaline rocks are formed where alkaline ultrabasites are also present. In the territories attached to suture zones carbonate ferruginous layers appear, which include xenocrysts. In blocks in places of intersection of sub-longitudinal and sub-latitudinal zones, in relict foci mixing of melts takes place diluted by aquatic-chloride-potassium fluids and added deep carbonate-fluoride-sodium ones. Massifs of anorthosites-rapakivi-granites with titanium are formed. They are accompanied by pegmatite fields, albite-uranium metasomatites when joint participation of carbonate-fluoride-sodium and aquatic fluids is required.