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Journal of Geosciences

Valenta J, Rapprich V, Stárková M, Skácelová Z, Fojtíková L, Staněk F, Balek J; Vol. 59, issue 3, pages 169 - 181The Permian pyroclastic deposits on the Principálek Hill SW of Vrchlabí (Czech Republic) were investigated by means of geological mapping and geophysics. The pyroclastic rocks are exposed in several small coherent outcrops, yet many interpretations are based on debris. The preserved textures enable reconstruction of eruptive styles. The volcanism started with phreatomagmatic eruptions documented by the fine-grained tuff with accretionary lapilli at the base. Subsequently, the activity changed to phreato-Strombolian/Surtseyan producing lapilli-tuffs and lapillistones. These events were followed by a Strombolian phase as evidenced by ill-sorted scoriaceous tuff-breccias with volcanic bombs. The uppermost unit consists of welded lava agglutinates and basaltic lavas suggesting a Hawaiian style of eruption. One of the basaltic feeder necks is exposed in a small abandoned quarry.Geophysical surveys were carried out over the extent of pyroclastic deposits. These included gravimetry, magnetometry, and DC resistivity tomography. The magnetic data indicate the occurrence of basaltic dykes in the central part of the pyroclastic deposits. The DC tomography confirms the presence of dykes, enables estimation of the pyroclastic deposits thickness, and provides evidence for the size and position of the maar-diatreme. Gravity data yield an insignificant low associated with the diatreme, which is likely a product of low rock-density contrast between the diatreme and country rocks. We argue that this is due to short time gap between sedimentation and volcanic activity and also related to conjoint burial and diagenetic history of the diatreme and the surrounding country rocks.

http://www.jgeosci.org/rss.php?ID=jgeosci.170 2014/07/12 - 22:21

Coubal M, Adamovič J, Málek J, Prouza V; Vol. 59, issue 3, pages 183 - 208The Lusatian Fault in the northern Bohemian Massif is one of the most prominent products of the latest Cretaceous to Paleogene thrusting in the Alpine foreland in Europe. Its fault plane dips to the N to NE, typically separating crystalline units in the N from Upper Paleozoic and Cretaceous units in the south. Crystalline units in Lusatia consist of Proterozoic to Lower Paleozoic rocks epi- to mesozonally metamorphosed during the Variscan Orogeny, cut by Cadomian and Variscan granitoid plutons. Anatomy of the fault was studied in outcrops and in a series of test trenches, and the course of fault trace in complex topography was used to determine the dip of the fault. Based on a detailed analysis of brittle structures accompanying the main fault, the Lusatian Fault Belt can be further subdivided into the fault core, zone of wall-rock brecciation, and the damage zone which also includes large-scale dismembered blocks and flanking structures. Other components of the fault belt originated in spatial association with the Lusatian Fault, during its formation (e.g., the drag zone and bedding-plane slips in the footwall-block sediments) or later, during its multi-stage evolution.A general dependence of fault belt architecture on the orientation of the fault plane to the acting stress can be demonstrated. With the NNE-SSW subhorizontal maximum principal stress calculated for the thrusting episode at the Lusatian Fault, less complex structures appear where the dip of the fault is shallower, and more complex structures including thicker damage zone and drag zone were formed where the fault was steeper. In flat fault segments, competent members of the footwall-block sedimentary package, whose bedding planes were sub-conformable to the fault plane, were sheared during the thrusting and dragged to near-surface levels as dismembered blocks. A progressive eastward increase in the fault dip angle is associated with the appearance of the zone of flanking structures in which the degree of rotation is a function of the fault dip angle. In the eastern part of the fault (fault dip angle of ˜60°), the same stress acted at a high angle to the fault plane producing a “bulldozer effect”. Limbs of the flanking structures were overturned, and a parallel, more gently inclined Frýdštejn Fault was initiated as a structure more advantageous for slip movement.

http://www.jgeosci.org/rss.php?ID=jgeosci.174 2014/07/12 - 22:21

Števko M, Uher P, Ondrejka M, Ozdín D, Bačík P; Vol. 59, issue 3, pages 209 - 222The quartz veins with primary fluorapatite, xenotime-(Y), monazite-(Ce) to monazite-(Nd), uraninite, and secondary florencite-(Ce) and goyazite occur in Lower Devonian metavolcano-sedimentary sequence of the Gelnica Group, Gemeric Superunit, the Central Western Carpathians (eastern Slovakia). They represent an example of hydrothermal REE-U mineralization. Fluorapatite forms parallel bands of columnar crystals (≤ 3 cm) in massive quartz. Monazite-(Ce) to (Nd) shows a near end-member composition with very small amounts of cheralite and huttonite components. Widespread xenotime-(Y) forms colloform aggregates or irregular aggregates in association with fluorapatite and monazite; the Y contents vary between 0.77 and 0.63 apfu. Uraninite electron-microprobe U-Pb dating gave the average age of 207 ± 2 Ma (n = 16, 2σ), which is consistent with formation of the U mineralization in the Gemeric Superunit (e.g., Kurišková uranium deposit) during early Alpine hydrothermal activity.

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Plášil J, Sejkora J, Škoda R, Škácha P; Vol. 59, issue 3, pages 223 - 253Uraninite and the supergene minerals from the Červená hydrothermal uranium vein (Jáchymov ore district, Czech Republic) were studied. These supergene minerals represent alteration products of the joint weathering of uraninite and hypogene sulfide minerals, connected to the acid-mine drainage (AMD) systems. The complex geochemistry of the hypogene mineralization provided a unique environment for formation of chemically diverse supergene phases. Among other features, the weathering system is characterized by the high activity of Cu2+ and REE, which control the composition of the resulting supergene minerals: commonly occurring are Cu-dominant uranyl sulfates of the zippeite group (pseudojohannite, Cu-rabejacite), Cu-dominant uranyl silicates (cuprosklodowksite) or Y- and REE-containing uranyl sulfate mineral sejkoraite-(Y). The high activity of Cu2+ and REE is also reflected by the fact that both elements enter minerals, which are nominally Cu- or REE-free (marécottite, rabejacite, tyuyamunite, and compreignacite). The alteration association was evaluated with regard to the crystal-chemical properties of each mineral using the bond-valence approach, documenting distinct evolutionary trends during weathering.

http://www.jgeosci.org/rss.php?ID=jgeosci.171 2014/07/12 - 22:21

Han C, Xiao W, Zhao G, Su B, Sakyi PA, Ao S, Wan B, Zhang J, Zhang Z, Wang Z; Vol. 59, issue 3, pages 255 - 274The Chinese Altai-East Junggar collage in southern Altaids is one of the largest and most important metallogenic provinces in China. It is composed of five major types of Middle to Late Paleozoic metal deposits: (1) VMS Cu-Pb-Zn, (2) porphyry Cu-Au, (3) magmatic Cu-Ni-sulfide, (4) skarn Cu-Mo-Fe and (5) orogenic Au. Tectonically, the development of these metal deposits was closely associated with accretionary and convergent processes that occurred along the southern margin of the Central Asian Orogenic Belt (CAOB). The formation of the deposits involved three main stages, briefly described as follows: (i) Those formed during extensional back-arc volcanism along the Paleozoic active margin of the CAOB. They are Late Devonian to Early Carboniferous polymetallic volcanogenic massive sulfide deposits, together with some broadly contemporaneous Fe-Cu skarns, located in the accreted Qiongkuer-Talate Terrane in the western Altai; (ii) Carboniferous to Permian terrane accretion and arc magmatism, resulting in widespread metalliferous ores of different types such as copper-bearing porphyries and Alaskan-type Cu-Ni-PGE zoned ultramafic bodies developed in arcs in the Buerjin-Ertai and Erqis terranes, and Cu-Fe skarns formed in the Erqis flysch basin; (iii) Continuing accretion in the Permian leading to the development of the Dulate arc in the southern Altai associated with the formation of Cu-Mo skarns and orogenic-type gold vein systems. The Chinese Altai-East Junggar collage typically demonstrates the various classic metalliferous ores formed during the processes of subduction-accretion and arc generation.

http://www.jgeosci.org/rss.php?ID=jgeosci.173 2014/07/12 - 22:21

Yuan Q, Cao X, L&uuml, X, Yang E, Wang X, Liu Y, Ruan B, Mohammed-Abdalla-Adam M; Vol. 59, issue 3, pages 275 - 291We report the petrology, whole-rock geochemistry, zircon LA-ICP-MS U-Pb chronology and zircon Hf isotopic data of Daxigou granitoids (western part of the Kuluketage Block, NW China) to evaluate their likely petrogenesis and tectonic setting. Zircons from syenogranite can be divided into two groups: 1) those that display oscillatory zoning and high Th/U ratios (average = 1.38), implying their magmatic origin and 2) those that exhibit weak zoning and extremely high U and Pb contents but low Th/U ratios (average = 0.35), resembling zircons that experienced hydrothermal alteration. The zircon LA-ICP-MS, U-Pb dating of the two groups of zircons yielded weighted mean ages of 1830 ± 12 Ma (MSWD = 0.78) and 1798 ± 21 Ma (MSWD = 1.6) respectively.The Daxigou granitoids belong mostly to normal-K and sodium-rich metaluminous calc-alkaline type, systematically enriched in LREE and large ion lithophile elements (LILE, e.g., K, Ba and Rb), but significantly depleted in high field strength elements (HFSE, e.g., Ti, P, Nb, Ta and U). Their εHf(t) values and two-stage Hf model ages range from -7.16 to -5.03 and 2.69 to 2.76 Ga, respectively. Taken together, it is suggested that Daxigou granitoids are of I-type affinity and that they were derived by partial melting of a Neoarchaean TTG (e.g., Tuoge Complex) rocks in a continental-arc environment. These new data, combined with previous regional geological studies, demonstrate that a series of Palaeoproterozoic (c. 2.0-1.8 Ga) tectono-magmatic events occurred in Kuluketage Block during the assembly of Columbia.

http://www.jgeosci.org/rss.php?ID=jgeosci.172 2014/07/12 - 22:21

Plášil J; Vol. 59, issue 2, pages 99 - 114Oxidation-hydration weathering of uraninite, the most common U-bearing mineral in nature, comprises various physical and chemical processes that lead to the destruction of the fluorite-type structure of uraninite where U is present as tetravalent. This results in replacement of uraninite by weathering products containing U in hexavalent form, i.e. as uranyl ion, UO22+. The final assemblage of the weathering products, uranyl minerals, and their compositions depend on the various factors, namely the composition of the primary minerals and percolating oxidizing fluids that cause the alteration. The knowledge of such processes and stabilities of the uranium minerals is of the great interest namely due to demand for U as the energy source. During the past decade there has been substantial progress in understanding the mineralogy, crystallography and thermodynamics of uranyl minerals and thus a substantially improved understanding of the weathering processes themselves. This review aims to summarize the state-of-art of the current knowledge on uranium-related topics as well and identify some of the important questions that remain unanswered. The text is dedicated to Jiří Čejka on occasion of his 85th birthday anniversary. Jiří greatly contributed not only to the spectroscopy and mineralogy of uranyl minerals, but also to the questions pertaining their origin and stability. Many important issues were addressed, even if briefly, in the pioneering book “Secondary Uranium Minerals” by Čejka and Urbanec (1990) which has served, for a long-time, as a guide for beginning uranium mineralogists.

http://www.jgeosci.org/rss.php?ID=jgeosci.163 2014/04/16 - 20:00

Krivovichev SV; Vol. 59, issue 2, pages 115 - 121Orange platy crystals of K2Na8(UO2)8Mo4O24[(S,Mo)O4], the first known uranium molybdosulfate, have been prepared by high-temperature solid-state reactions. The structure (monoclinic, C2/c, a = 24.282(4), b = 12.1170(18), c = 13.6174(17) Å, β = 106.33(1)°, V = 3845.0(9) Å3, Z = 4) has been solved by direct methods and refined to R1 = 0.058 on the basis of 2837 unique observed reflections. The structure consists of a three-dimensional heteropolyhedral framework based upon the [(UO2)4O4(Mo2O10)] layers parallel to (010). The layers are formed by linkage of chains of edge-sharing UO7 bipyramids running along the c axis via the Mo2O10 dimers of edge-sharing MoO6 octahedra. Linkage of the layers into a three-dimensional framework is provided by the MoO4 and SO4 tetrahedra that share corners with the MoO6 octahedra. The Na+ and K+ cations reside in framework cavities in between the uranyl molybdate layers. The structure of K2Na8(UO2)8Mo4O24[(S,Mo)O4] is yet another complex derivative of the U2MoO8 structure type.

http://www.jgeosci.org/rss.php?ID=jgeosci.166 2014/04/16 - 20:00

Gurzhiy VV, Tyumentseva OS, Kornyakov IV, Krivovichev SV, Tananaev IG; Vol. 59, issue 2, pages 123 - 133Single crystals of two new uranyl selenates K3(H3O)[(UO2)4(SeO4)6(H2O)4]∙5H2O (I) and K2.5(NO3)0.5[(UO2)2(SeO4)3(H2O)]∙4H2O (II) have been prepared by room-temperature evaporation from aqueous solution of uranyl nitrate, selenic acid, potassium carbonate and (for the compound I) carbamide. The crystal structure of I has been solved by direct methods [monoclinic, P21/m, a = 12.001(3), b = 13.613(3), c = 13.753(3) Å, β = 109.187(4)°, V = 2122.0(8) Å3 and Z = 2] and refined to R1 = 0.029 (wR2 = 0.084) for 4865 reflections with |Fo| ≤ 4σF using least-square methods. The crystal structure of II has been solved by direct methods [monoclinic, С2/с, a = 20.290(4), b = 10.380(2), c = 21.436(4) Å, β = 103.446(3)°, V = 4391.0(13) Å3 and Z = 4] and refined to R1 = 0.027 (wR2 = 0.066) for 7944 reflections with |Fo| ≤ 4σF using least-square techniques. The structures of I and II are based upon the [(UO2)2(SeO4)3(H2O)2]2- and [(UO2)2(SeO4)3(H2O)]2- layers, respectively, consisting of UO7 pentagonal bipyramids sharing corners with SeO4 tetrahedra. Potassium cations induce curvature of the uranyl selenate layers, which is mediated by the interlayer water molecules, hydronium ions and nitrate groups. The topology of the 2D units in the structure of I is novel for the structural chemistry of uranyl selenates.

http://www.jgeosci.org/rss.php?ID=jgeosci.165 2014/04/16 - 20:00

Krivovichev SV, Burns PC; Vol. 59, issue 2, pages 135 - 143A new uranyl sulfate, [Co(H2O)6]3[(UO2)5(SO4)8(H2O)](H2O)5, has been synthesized using mild hydrothermal methods. The structure (monoclinic, P21/c, a = 27.1597(14), b = 9.9858(5), c = 22.7803(12) Å, β = 106.520(1)°, V = 5923.2(5) Å3, Z = 4) has been solved by direct methods and refined on the basis of F2 for all unique reflections to R1 = 0.056, calculated for the 9124 unique observed reflections (|Fo| ≤ 4σF). It contains five symmetrically distinct uranyl pentagonal bipyramids and eight sulfate tetrahedra that link via the sharing of vertices between uranyl polyhedra and sulfate tetrahedra, resulting in sheets parallel to (001). Adjacent sheets are linked by hydrogen bonds to Co2+(H2O)6 octahedra and H2O groups located in the interlayer. The uranyl sulfate sheet contains four- and five-connected uranyl pentagonal bipyramids and three and four-connected sulfate tetrahedra. The sheet may be constructed using modules from related structures involving pentagonal bipyramids and tetrahedra, and is readily described using a nodal representation. In general, the uranyl sulfate sheets in [Co(H2O)6]3[(UO2)5(SO4)8(H2O)](H2O)5 are more rigid than the structural units typically found in comparable uranyl molybdates, which involve the sharing of vertices between uranyl pentagonal bipyramids and molybdate tetrahedra.

http://www.jgeosci.org/rss.php?ID=jgeosci.164 2014/04/16 - 20:00

Plášil J, Kampf AR, Kasatkin AV, Marty J; Vol. 59, issue 2, pages 145 - 158Bluelizardite (IMA 2013-062), Na7(UO2)(SO4)4Cl(H2O)2, is a new uranyl sulfate mineral from the Blue Lizard mine, San Juan County, Utah (USA). It was found in a sandstone matrix and is associated with chalcanthite, copiapite, ferrinatrite, gypsum, kröhnkite, johannite, and several other new, unnamed Na- and Mg-containing uranyl sulfates. Bluelizardite is a supergene mineral formed by the post-mining weathering of uraninite. The mineral is monoclinic, C2/c, with a = 21.1507(6), b = 5.3469(12), c = 34.6711(9) Å, β = 104.913(3)°, V = 3788.91(17) Å3 and Z = 8. Crystals are blades up to 0.4 mm long, flattened on {001}, elongated parallel to [010] and exhibiting the forms {100}, {001} and {111}. Bluelizardite is pale yellow and has a yellowish white streak. It has good cleavage on {001} and uneven fracture. The Mohs hardness is estimated at 2. The calculated density based on the empirical formula is 3.116 g/cm3. Bluelizardite exhibits bright yellow-green fluorescence under both long- and short-wave UV radiation. The mineral is optically biaxial (-), with α = 1.515(1), β = 1.540(1) and γ = 1.545(1) (measured with white light). The measured 2V is 48(2)° and the calculated 2V is 47.6°. Mineral does not exhibit any dispersion or pleochroism. The optical orientation is X = b, Y = a, Z = c*. The empirical formula of bluelizardite is Na6.94(U1.02O2)(SO4)4.00Cl0.94O0.06(H2O)2 (based on 21 anions pfu). The Raman spectrum is dominated by the symmetric stretching vibrations of the uranyl (UO22+) group and sulfate tetrahedra and by the O-H stretching and bending vibrations of the H2O molecules. The eight strongest powder X-ray diffraction lines are [dobs Å(I )(hkl) ]: 17.08(52)(002), 10.31(60)(200), 5.16(100)(mult.), 4.569(22)(402,-114), 4.238(23)(-115, 310, 008), 3.484(27)(-602,-604,-2•0•10), 3.353(28)(mult.), 3.186(36)(mult.). The crystal structure of bluelizardite (R1 = 0.016 for 4268 reflections with Iobs > 3σI) is topologically unique among known structures of uranyl minerals and inorganic compounds. It is based upon clusters of uranyl pentagonal bipyramids and sulfate tetrahedra. Two uranyl pentagonal bipyramids are linked through the two vertices of SO4 groups. The remaining three vertices of each UO7 bipyramid are occupied by SO4 groups, linked monodentately. The eight independent Na+ cations are linked through the Na-O bonds along with hydrogen bonds (involving H…O and H…Cl bonds) into a 3D framework.

http://www.jgeosci.org/rss.php?ID=jgeosci.159 2014/04/16 - 20:00

Cooper MA, Hawthorne FC, Karpenko VY, Pautov LA, Agakhanov AA; Vol. 59, issue 2, pages 159 - 168Metahewettite was encountered in hypergene crusts in Paleozoic carbon-silica schists included in the carbon mélange matrix at Hodzha-Rushnai-Mazar in southern Kirgizia. Schist outcrops are marked by multicolored yellow, orange, brown and green crusts of vanadates and sulfates of chalсoalumite group, volborthite, V-bearing phosphates, Cr-V-bearing members of alunite subgroup, members of the pascoite group and vanadium-bronze oxides, including metahewettite. Metahewettite is acicular with individual crystals up to 1 mm in length, and forms radial aggregates 2-3 mm diameter, or flattened aggregates in a narrow fissures. Crystals are dark-brown to reddish-brown with a golden sheen. The crystal structure of metahewettite, Ca(V5+6O16)(H2O)3, monoclinic, a = 12.208(5), b = 3.6011(15), c = 18.358(7) Å, β = 118.538(8)°, V = 709.0(8) Å3, Z = 2, A2/m, was refined to an R1 index of 2.4 % based on 1047 unique observed (Fo > 4σF) reflections. Electron-microprobe analysis (EDS) showed no detectable constituents apart from Ca and V, and the scattering from each site in the structure is conformable with the ideal composition Ca(V6O16)(H2O)3. There are three V sites in the structure with scattering in accord with their complete occupancy by V. The V(1) site is [5]-coordinated by O2- anions with a <V-O> distance of 1.823 Å and a [2 + 3] arrangement of vanadyl <1.67 Å> and equatorial (<1.925 Å>) bonds. The V(2) and V(3) sites are coordinated by O2- anions with <V-O> distances of 1.934 and 1.916 Å and [2 + 2 + 2] and [1 + 4 + 1] arrangements of vanadyl <1.652 Å>, equatorial <1.906 Å> and trans <2.237 Å> bonds, respectively. The V(1) square pyramids share edges and vertices to form chains extending parallel to b with a repeat distance of 3.6 Å. The V(2) and V(3) octahedra share edges and vertices to form ribbons also extending parallel to b. The chains and ribbons link by sharing polyhedron corners to form sheets of V polyhedra parallel to (001). These sheets are linked by interlayer Ca that occupies two interstitial Ca sites, and by (H2O) groups.

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Ulrych J, Adamovič J, Krmíček L, Ackerman L, Balogh K; Vol. 59, issue 1, pages 3 - 22Dykes of the Late Cretaceous to Early Tertiary (79.5 ± 3.5 to 60.7 ± 2.4 Ma) melilitic rock series of the Osečná Complex and the Devil’s Walls dyke swarm, including ultramafic lamprophyres - polzenites - of Scheumann (1913) occur dispersed in the entire Upper Ploučnice River basin in northern Bohemia. Polzenites and associated melilitic rocks are characterized by the mineral association of olivine + melilite ± nepheline, haüyne, monticellite, phlogopite, calcite, perovskite, spinels and apatite. New data on their mineral and chemical compositions from original Scheumann’s localities (the Vesec, Modlibohov, Luhov types) argue against the abolition of the group of ultramafic lamprophyres and the terms ‘polzenite’ and ‘alnöite’ by the Le Maitre (2002) classification. Marginal facies and numerous flat apophyses of the lopolith-like body known as the Osečná Complex show an olivine micro-melilitolite composition (lamprophyric facies). The porphyritic texture, chemical composition and the presence of characteristic minerals such as monticellite and phlogopite point to their affinity with ultramafic lamprophyres - polzenites of the Vesec type. Melilite-bearing olivine nephelinites to olivine melilitites (olivine + clinopyroxene + nepheline + melilite ± haüyne and spinels with apatite) form a swarm of subparallel dykes known as the Devil’s Walls. The Scheumann’s non-melilite dyke rock “wesselite”, spatially associated with polzenites and often erroneously attributed to the polzenite group, is an alkaline lamprophyre of monchiquite to camptonite composition (kaersutite + phlogopite + diopside + olivine phenocrysts in groundmass containing clinopyroxene, phlogopite, haüyne, analcime, titanian magnetite, apatite ± glass/plagioclase). First K-Ar data show Oligocene ages (30.9 ± 1.2 to 27.8 ± 1.1 Ma) and an affinity to the common tephrite-basanite rock series.

http://www.jgeosci.org/rss.php?ID=jgeosci.158 2014/03/02 - 07:02

Esteve S, Enrique P, Galán G; Vol. 59, issue 1, pages 23 - 40The camptonites of Platja Fonda are intrusive into Cambro-Ordovician metasediments and late Variscan granitoids. They appear as decimetre to centimetre thick sub-horizontal dykes following a fracture system, with dilation nearly orthogonal to their strike. The largest dykes have zoned structure, with chilled margins at both borders grading to porphyritic camptonites towards the centre, whereas the latest thinner dykes are intrusive into the porphyritic camptonites and their texture is almost aphyric. The macrocrysts of the porphyritic camptonites are interpreted as phenocrysts according to their chemistry and conditions of formation with respect to the crystals in the groundmass. They are mainly of kaersutite and of subordinate diopside-augite. Whole-rock major-element compositions reveal the existence of two types of camptonites: the predominant, sodi-potassic and the subordinate, slightly potassic. Other geochemical differences between the two types are minimal. The potassic camptonites are found as isolated dykes, but the potassic tendency exists in the latest thinner dykes that cross-cut earlier sodi-potassic camptonites and show small amount of normative leucite. Both types do not seem to be related by fractional crystallization and could represent slightly different alkaline melts, which would have been formed by partial melting of a metasomatized mantle. Increasing participation of the incongruent breakdown of calcic amphibole and phlogopite during the genesis of the most potassic camptonites is suggested. These camptonites in general, and specifically the potassic ones, have the lowest SiO2, the highest CaO concentrations and the lowest Mg numbers of all known camptonites in the Catalan Coastal Ranges.

http://www.jgeosci.org/rss.php?ID=jgeosci.161 2014/03/02 - 07:02

Štemprok M, Dolejš D, Holub FV; Vol. 59, issue 1, pages 41 - 68Variscan lamprophyres occur in the greisen tin-, tungsten- and molybdenum-ore district of Krupka in the Eastern Krušné hory/Erzgebirge (KHE). They belong to a bimodal dyke suite of aplites, felsic porphyries, microgranites and mafic dykes associated with late Variscan tin-bearing granites and include minettes, kersantites and spessartites, while vogesite reported earlier has not been confirmed. One altered mafic dyke is interpreted as original microdiorite. All lamprophyres are basic to intermediate rocks (47.3-56.9 wt. % SiO2) with shoshonitic to ultrapotassic composition (3.1-7.5 wt. % K2O). The high concentrations of MgO (4.7-11.4 wt. %), molar Mg/(Mg + Fe) ratios (0.56-0.74) and abundances of compatible elements (350-800 ppm Cr, 130-360 ppm Ni) indicate that lamprophyres represent primary mantle melts that underwent no or little fractionation or contamination, and high LREE/HREE ratios point to magma formation in the stability field of garnet peridotite. In addition, high contents of potassium and LILE (50-370 ppm Li, 150-920 ppm Rb, 750-3100 ppm Ba) indicate metasomatic enrichment of the upper mantle prior to partial melting. The LILE-HFSE-REE patterns indicate involvement of slab components (subducted siliciclastic and carbonate sediments). Strong enrichment in U (6-29 ppm) and Th (17-75 ppm) is another characteristic feature of lamprophyres from the Eastern KHE and elsewhere in central Europe, and it is consistent with the metasomatic transport via oxidized saline fluids from the slab to the mantle wedge. The lamprophyres in the Krupka district were variably greisenized in the vicinity of granite greisens and Sn-W hydrothermal veins and their original minerals were replaced by an assemblage of lithian phlogopite, topaz, fluorite, apatite and titanium-bearing phases. During alteration, they were strongly depleted in Na2O, CaO, Sr and Ba, moderately depleted in REE, and enriched in Li, Rb, Cs, Sn and F. By contrast, Al and Zr behaved as immobile elements and their abundances indicate overall mass loss of 10-18 % during greisenization consistent with the formation of porosity, which facilitated the hydrothermal dissolution-precipitation reactions. The spatial association of greisens and lamprophyres suggests that the greisenizing fluids migrated along similar geological structures, which were previously accessible to the mantle-derived media (melts and/or fluids). On a local scale, the lamprophyre dykes intersected by greisen veins provided geochemical or rheological barrier, which favoured the cassiterite deposition. The timing of lamprophyre dykes also indicates that the mantle metasomatism beneath the KHE area occurred before the late Variscan granitic magmas were generated.

http://www.jgeosci.org/rss.php?ID=jgeosci.156 2014/03/02 - 07:02

Couzinié S, Moyen J-F, Villaros A, Paquette J-L, Scarrow JH, Marignac C; Vol. 59, issue 1, pages 69 - 86Mg-K mafic intrusive rocks are commonly observed during the late stages of the evolution of orogenic belts. The Variscan French Massif Central has many outcrops of these rocks, locally called vaugnerites. Such magmas have a mantle-derived origin and therefore allow discussion of the role of mantle melting and crust-mantle interactions during late-orogenic processes. In the Southern Velay area of the French Massif Central, LA-ICPMS U-Pb dating on zircons and monazites from three vaugnerites and four coeval granites reveals that the two igneous suites formed simultaneously, at c. 305 Ma. This major igneous event followed after an early, protracted melting stage that lasted for 20-30 My and generated migmatites, but the melt was not extracted efficiently and therefore no granite plutons were formed. This demonstrates that widespread crustal anatexis, melt extraction and granite production were synchronous with the intrusion of vaugneritic mantle-derived melts in the crust. The rapid heating and subsequent melting of the crust led to upward flow of partially molten rocks, doming and collapse of the belt.

http://www.jgeosci.org/rss.php?ID=jgeosci.155 2014/03/02 - 07:02

Krmíček L, Halavínová M, Romer RL, Vašinová Galiová M, Vaculovič T; Vol. 59, issue 1, pages 87 - 96Mineral/matrix and mineral/mineral partition coefficients were determined for clinopyroxene and phlogopite phenocrysts and their very fine-grained (chilled) groundmass from a Variscan calc-alkaline (agpaitic index = 0.6) lamprophyre dyke of minette composition from the Bohemian Massif (Křižanovice, Teplá-Barrandian Unit). This dyke is characterized by high MgO (8.9 wt. %; Mg-number = 69) at intermediate SiO2 content (53 wt. %). The partition coefficients (D) between clinopyroxene and matrix were determined for 23 elements by laser ablation-inductively coupled plasma-mass spectrometry. Calculated phlogopite/matrixD values are very low, except for heavy rare earth elements (HREE) that range between 0.9 and 1.1. This suggests that HREE can be concentrated in clinopyroxene during crystallization from lamprophyre melt. In phlogopite, only 15 elements had contents above their respective detection limits. Phlogopite/matrix partition coefficients are on average higher than 1 (DBa = 1.1, DRb = 1.7 DTi = 1.5) and extremely low (≥ 0.02) for light rare earth elements (LREE). On the other hand, phlogopite/matrixD values for the majority of HREE could not be determined. Taken together, during simultaneous crystallization of clinopyroxene and phlogopite phenocrysts, Th, Zr, Hf, Y and LREE are preferentially partitioned into clinopyroxene and Ba with Rb and Ti into phlogopite.

http://www.jgeosci.org/rss.php?ID=jgeosci.160 2014/03/02 - 07:02

Anderson MO, Lentz DR, McFarlane CRM, Falck H; Vol. 58, issue 4, pages 299 - 320The internal zonation and chemical evolution of Nb-Ta oxides and muscovite have been characterized in the Moose II pegmatite, Northwest Territories, Canada, to distinguish primary magmatic mineralization from that formed during late-stage metasomatic processes. In addition, muscovite associated with Nb-Ta oxides was examined in order to assess the evolution of the pegmatite melt and the nature of the late-stage fluids.Detailed mapping shows that the studied body (430 × 61 m) is a highly fractionated, irregularly zoned, spodumene-subtype rare-element pegmatite dominated by coarse-grained to megacrystic albite, K-feldspar, and spodumene, with intergranular assemblages of quartz, K-feldspar, albite, spodumene, muscovite, and minor amblygonite-montebrasite. Monomineralic core zones (quartz and amblygonite-montebrasite), aplitic albite ‘pods,’ and units characterized by phyllic alteration occur in the central portions of the pegmatite.Columbite-tantalite minerals occur throughout the pegmatite, excluding the quartz cores, and patterns of internal zoning include: (1) normally zoned ferrocolumbite with early Ta-Ti-rich, and later Nb-W-rich overgrowths; (2) oscillatory zoned Ti-rich ferrotantalite with Nb-rich patchy replacement; (3) reversely zoned ferrocolumbite, with a Ta-rich cores and (4) complexly zoned Ti-rich ferrotantalite with abundant late Nb- and Ta- rich patches and sieve textures. The textures and chemical patterns demonstrate an evolution from columbite to tantalite, whereby the Ta/(Ta + Nb) ratio increased with progressive fractionation (0.13-0.59) but the Mn/(Mn + Fe) ratio remained nearly constant (0.15-0.31). The chemical evolution of the Nb- Ta oxides from columbite to tantalite is consistent with crystallization from a magmatic to late-stage magmatic Na-rich melt, with a sequence of crystallization progressing from those of the wall zone, to the first intermediate zone, and finally the late aplitic albite zones. Minor remobilization of Nb, and to a lesser extent Ta, was responsible for some of the replacement features found in the Nb-Ta oxides. Textural observations and trace-element analyses of fine-grained, secondary muscovite found throughout the pegmatite indicate hydrothermal metasomatism by a late F- and Nb-rich vapor-like (“supercritical”) fluid.

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Anderson MO, Lentz DR, McFarlane CRM, Falck H; Vol. 58, issue 4, pages 299 - 320The internal zonation and chemical evolution of Nb-Ta oxides and muscovite have been characterized in the Moose II pegmatite, Northwest Territories, Canada, to distinguish primary magmatic mineralization from that formed during late-stage metasomatic processes. In addition, muscovite associated with Nb-Ta oxides was examined in order to assess the evolution of the pegmatite melt and the nature of the late-stage fluids.Detailed mapping shows that the studied body (430 × 61 m) is a highly fractionated, irregularly zoned, spodumene-subtype rare-element pegmatite dominated by coarse-grained to megacrystic albite, K-feldspar, and spodumene, with intergranular assemblages of quartz, K-feldspar, albite, spodumene, muscovite, and minor amblygonite-montebrasite. Monomineralic core zones (quartz and amblygonite-montebrasite), aplitic albite ‘pods,’ and units characterized by phyllic alteration occur in the central portions of the pegmatite.Columbite-tantalite minerals occur throughout the pegmatite, excluding the quartz cores, and patterns of internal zoning include: (1) normally zoned ferrocolumbite with early Ta-Ti-rich, and later Nb-W-rich overgrowths; (2) oscillatory zoned Ti-rich ferrotantalite with Nb-rich patchy replacement; (3) reversely zoned ferrocolumbite, with a Ta-rich cores and (4) complexly zoned Ti-rich ferrotantalite with abundant late Nb- and Ta- rich patches and sieve textures. The textures and chemical patterns demonstrate an evolution from columbite to tantalite, whereby the Ta/(Ta + Nb) ratio increased with progressive fractionation (0.13-0.59) but the Mn/(Mn + Fe) ratio remained nearly constant (0.15-0.31). The chemical evolution of the Nb- Ta oxides from columbite to tantalite is consistent with crystallization from a magmatic to late-stage magmatic Na-rich melt, with a sequence of crystallization progressing from those of the wall zone, to the first intermediate zone, and finally the late aplitic albite zones. Minor remobilization of Nb, and to a lesser extent Ta, was responsible for some of the replacement features found in the Nb-Ta oxides. Textural observations and trace-element analyses of fine-grained, secondary muscovite found throughout the pegmatite indicate hydrothermal metasomatism by a late F- and Nb-rich vapor-like (“supercritical”) fluid.

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McKeough MA, Lentz DR, McFarlane CRM, Brown J; Vol. 58, issue 4, pages 321 - 346In northern Saskatchewan, granitic pegmatites intruded Early Paleoproterozoic Wollaston Group metasedimentary rocks and interfolded granitoids that unconformably overlie Late Archean gneisses, all of which have been subjected to deformation during the protracted 1.86 to 1.77 Ga Trans-Hudson Orogeny. The U-Th ± REE-Y-Nb pegmatite intrusions and fracture-controlled U mineralization characterize the occurrences at Kulyk Lake, Eagle Lake, and Karin Lake properties in the Wollaston Domain. The pegmatites are moderately to highly evolved, ranging from mineralogically simple to complex types. These are rare-earth element class, NYF pegmatites (Nb-Y-F), and are interpreted to have formed in a late syn- to post-collisional tectonic setting. The complex-type pegmatites are hybridized, due to metasomatic interaction with the host rocks and therefore are locally crudely zoned. Saturation of U-Th ± REE-Y-Nb occurred at the margins (predominantly border and wall zones) of the hybridized pegmatites. Partial melts were generated at depth, and then coalesced as they intruded to higher structural levels during exhumation of this orogen. This agrees with U-Pb geochronology of these granitic pegmatites, which constrains them between peak- and late-metamorphic events of the Trans-Hudson Orogeny. The age constraints and relatively high-T partial melting conditions (˜750 °C) confine the pegmatite melt-forming conditions to an early deformational event (1835-1805 Ma) that was overprinted by high-T retrograde metamorphism at c. 1770 Ma. Field relationships, textures and geochemical variations provide strong evidence that the U, Th, REE ± Y- Nb phases in the studied pegmatites were progressively enriched through extreme fractionation effects of which are evident throughout multiple pegmatite injections. In addition, volatiles and other fluxing components further enriched U, Th, REE ± Y- Nb during complex hybridization reactions between pegmatite melt and wallrock, up to the final stages of pegmatite emplacement.

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McKeough MA, Lentz DR, McFarlane CRM, Brown J; Vol. 58, issue 4, pages 321 - 346In northern Saskatchewan, granitic pegmatites intruded Early Paleoproterozoic Wollaston Group metasedimentary rocks and interfolded granitoids that unconformably overlie Late Archean gneisses, all of which have been subjected to deformation during the protracted 1.86 to 1.77 Ga Trans-Hudson Orogeny. The U-Th ± REE-Y-Nb pegmatite intrusions and fracture-controlled U mineralization characterize the occurrences at Kulyk Lake, Eagle Lake, and Karin Lake properties in the Wollaston Domain. The pegmatites are moderately to highly evolved, ranging from mineralogically simple to complex types. These are rare-earth element class, NYF pegmatites (Nb-Y-F), and are interpreted to have formed in a late syn- to post-collisional tectonic setting. The complex-type pegmatites are hybridized, due to metasomatic interaction with the host rocks and therefore are locally crudely zoned. Saturation of U-Th ± REE-Y-Nb occurred at the margins (predominantly border and wall zones) of the hybridized pegmatites. Partial melts were generated at depth, and then coalesced as they intruded to higher structural levels during exhumation of this orogen. This agrees with U-Pb geochronology of these granitic pegmatites, which constrains them between peak- and late-metamorphic events of the Trans-Hudson Orogeny. The age constraints and relatively high-T partial melting conditions (˜750 °C) confine the pegmatite melt-forming conditions to an early deformational event (1835-1805 Ma) that was overprinted by high-T retrograde metamorphism at c. 1770 Ma. Field relationships, textures and geochemical variations provide strong evidence that the U, Th, REE ± Y- Nb phases in the studied pegmatites were progressively enriched through extreme fractionation effects of which are evident throughout multiple pegmatite injections. In addition, volatiles and other fluxing components further enriched U, Th, REE ± Y- Nb during complex hybridization reactions between pegmatite melt and wallrock, up to the final stages of pegmatite emplacement.

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Vrána S, Janoušek V, Franěk J; Vol. 58, issue 4, pages 347 - 378A detailed petrological and geochemical study of the Blanský les Granulite Massif (BLGM; Moldanubian Zone of the Bohemian Massif) provides evidence for processes and mineral assemblages previously unrecognized, or not fully appreciated, in the South Bohemian HP-HT Granulite Complex. It also underlines the “patchwork”-like nature of the BLGM, caused by distinct protolith compositions as well as the differences in occurrence, or preservation, of mineral assemblages, reaction textures, and/or variable changes in chemical compositions. Such a complexity reflects superposition of several processes operating from the eclogite-facies metamorphic peak to the final emplacement, as well as variable resistance of individual rock types with distinct metamorphic fabrics. Even on a thin section scale, domains preserving evidence for early phases of eclogite-facies metamorphism, including original textural relations and largely intact HP-HT mineral assemblages, occur in close proximity to domains dominated by phases produced during later decompression. Consistent high-pressure estimates, 2.2-2.3 GPa (GASP), are newly obtained for S1/S2 fabrics in the prevailing (type 1) felsic granulites, and correlate well with the recent finds of rare omphacite relics in the mafic granulites. It indicates that much of the HP history of the felsic and mafic rock types was shared and took place under eclogite-facies conditions.Three important metamorphic processes leading to chemical changes in felsic granulites are described. (1) The small bodies of Kfs-dominated hyperpotassic granulites with minor Grt (or, rarely, Di) are fairly common and provide direct evidence for operation of non-eutectic HP melting. These occurrences are accompanied by compositionally complementary residual (type 2) felsic granulites, rich in Qtz and Ky and partly depleted in alkalis with Zr. Owing to their Ca-poor composition, type 2 granulites were not involved in some of the decompression reactions, such as formation of plagioclase rims around kyanite and garnet breakdown, and thus their mineral assemblages remained effectively “frozen” during decompression. They further imply that the partial melting and genesis of the hyperpotassic melts took place under eclogite-facies conditions, (2) Another case of syn-metamorphic modification is represented by a local limited base-cation loss during superimposed amphibolite-facies de-alkalization, accompanying fluid flux along S3 shear domains (formation of feldspar-free Qtz-Sill bands). (3) On a microscale, the retrogression reactions, often incomplete, were clearly associated with transfer of ions. The felsic Moldanubian HP-HT granulites were assumed by several authors to have undergone only limited modification during peak metamorphism, most notably loss of Cs, Th and U. Clearly, some of them may have suffered rather significant changes, such as perceptible loss of alkalis or even relatively immobile elements (e.g., zirconium), compared to the likely primary protolith(s) composition.

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Vrána S, Janoušek V, Franěk J; Vol. 58, issue 4, pages 347 - 378A detailed petrological and geochemical study of the Blanský les Granulite Massif (BLGM; Moldanubian Zone of the Bohemian Massif) provides evidence for processes and mineral assemblages previously unrecognized, or not fully appreciated, in the South Bohemian HP-HT Granulite Complex. It also underlines the “patchwork”-like nature of the BLGM, caused by distinct protolith compositions as well as the differences in occurrence, or preservation, of mineral assemblages, reaction textures, and/or variable changes in chemical compositions. Such a complexity reflects superposition of several processes operating from the eclogite-facies metamorphic peak to the final emplacement, as well as variable resistance of individual rock types with distinct metamorphic fabrics. Even on a thin section scale, domains preserving evidence for early phases of eclogite-facies metamorphism, including original textural relations and largely intact HP-HT mineral assemblages, occur in close proximity to domains dominated by phases produced during later decompression. Consistent high-pressure estimates, 2.2-2.3 GPa (GASP), are newly obtained for S1/S2 fabrics in the prevailing (type 1) felsic granulites, and correlate well with the recent finds of rare omphacite relics in the mafic granulites. It indicates that much of the HP history of the felsic and mafic rock types was shared and took place under eclogite-facies conditions.Three important metamorphic processes leading to chemical changes in felsic granulites are described. (1) The small bodies of Kfs-dominated hyperpotassic granulites with minor Grt (or, rarely, Di) are fairly common and provide direct evidence for operation of non-eutectic HP melting. These occurrences are accompanied by compositionally complementary residual (type 2) felsic granulites, rich in Qtz and Ky and partly depleted in alkalis with Zr. Owing to their Ca-poor composition, type 2 granulites were not involved in some of the decompression reactions, such as formation of plagioclase rims around kyanite and garnet breakdown, and thus their mineral assemblages remained effectively “frozen” during decompression. They further imply that the partial melting and genesis of the hyperpotassic melts took place under eclogite-facies conditions, (2) Another case of syn-metamorphic modification is represented by a local limited base-cation loss during superimposed amphibolite-facies de-alkalization, accompanying fluid flux along S3 shear domains (formation of feldspar-free Qtz-Sill bands). (3) On a microscale, the retrogression reactions, often incomplete, were clearly associated with transfer of ions. The felsic Moldanubian HP-HT granulites were assumed by several authors to have undergone only limited modification during peak metamorphism, most notably loss of Cs, Th and U. Clearly, some of them may have suffered rather significant changes, such as perceptible loss of alkalis or even relatively immobile elements (e.g., zirconium), compared to the likely primary protolith(s) composition.

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Akiska S, Demirela G, Sayili S; Vol. 58, issue 4, pages 379 - 396Three main Pb-Zn ± Cu deposits (Handeresi, Bağırkaçdere, and Fırıncıkdere) occur south of Kalkım in the southeastern part of the Biga Peninsula (NW Turkey). Massive and disseminated Pb-Zn mineralization consists of ore veins in fault zones and replacement bodies in carbonate layers in metamorphic rocks. Garnet (˜Ad64Gr36), pyroxene (˜Di30Hd45Jo25), epidote and rare tremolite-actinolite are found as gangue minerals in ore zones. The garnets often show oscillatory zoning and correspond to grossular, andradite and grandite in composition. The pyroxene minerals are manganiferous hedenbergite, johannsenite and diopside. The Mn/Fe ratios of the pyroxene range between 0.3 and 0.9 (mean 0.5). Such compositions of the garnet and pyroxene and gangue assemblages are typical of distal Pb-Zn skarns. The ore minerals are pyrite, sphalerite, chalcopyrite, galena, magnetite, hematite, and arsenopyrite. The mean lead isotopic values for galena are 206Pb/204Pb ˜ 18.760, 207Pb/204Pb ˜ 15.689, and 208Pb/204Pb ˜ 38.935. The comparison of these isotope data with the host-rock samples (graphite schist, andesite and granodiorite) indicates that part of lead could have been leached from the schists. The δ34SVCDT values for galena, sphalerite, and chalcopyrite are -1.1 to +1.5 ‰ (mean: -0.2 ‰), 0.7 to +2. 1 ‰ (mean: +0.7 ‰), and -0.6 to +1.5 ‰ (mean: +0.4 ‰), respectively. Lead isotope data and δ34SVCDT ratios close to 0 ‰ indicate the formation of ores by magmatic processes.Taken together, the Oligo-Miocene magmatism in the area is taken responsible for the polymetallic mineralization. However, the metamorphic rocks and their protoliths may have contributed part of the lead.

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Akiska S, Demirela G, Sayili S; Vol. 58, issue 4, pages 379 - 396Three main Pb-Zn ± Cu deposits (Handeresi, Bağırkaçdere, and Fırıncıkdere) occur south of Kalkım in the southeastern part of the Biga Peninsula (NW Turkey). Massive and disseminated Pb-Zn mineralization consists of ore veins in fault zones and replacement bodies in carbonate layers in metamorphic rocks. Garnet (˜Ad64Gr36), pyroxene (˜Di30Hd45Jo25), epidote and rare tremolite-actinolite are found as gangue minerals in ore zones. The garnets often show oscillatory zoning and correspond to grossular, andradite and grandite in composition. The pyroxene minerals are manganiferous hedenbergite, johannsenite and diopside. The Mn/Fe ratios of the pyroxene range between 0.3 and 0.9 (mean 0.5). Such compositions of the garnet and pyroxene and gangue assemblages are typical of distal Pb-Zn skarns. The ore minerals are pyrite, sphalerite, chalcopyrite, galena, magnetite, hematite, and arsenopyrite. The mean lead isotopic values for galena are 206Pb/204Pb ˜ 18.760, 207Pb/204Pb ˜ 15.689, and 208Pb/204Pb ˜ 38.935. The comparison of these isotope data with the host-rock samples (graphite schist, andesite and granodiorite) indicates that part of lead could have been leached from the schists. The δ34SVCDT values for galena, sphalerite, and chalcopyrite are -1.1 to +1.5 ‰ (mean: -0.2 ‰), 0.7 to +2. 1 ‰ (mean: +0.7 ‰), and -0.6 to +1.5 ‰ (mean: +0.4 ‰), respectively. Lead isotope data and δ34SVCDT ratios close to 0 ‰ indicate the formation of ores by magmatic processes.Taken together, the Oligo-Miocene magmatism in the area is taken responsible for the polymetallic mineralization. However, the metamorphic rocks and their protoliths may have contributed part of the lead.

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Anderson MO, Lentz DR, McFarlane CRM, Falck H; Vol. 58, issue 4, pages 299 - 320The internal zonation and chemical evolution of Nb-Ta oxides and muscovite have been characterized in the Moose II pegmatite, Northwest Territories, Canada, to distinguish primary magmatic mineralization from that formed during late-stage metasomatic processes. In addition, muscovite associated with Nb-Ta oxides was examined in order to assess the evolution of the pegmatite melt and the nature of the late-stage fluids.Detailed mapping shows that the studied body (430 × 61 m) is a highly fractionated, irregularly zoned, spodumene-subtype rare-element pegmatite dominated by coarse-grained to megacrystic albite, K-feldspar, and spodumene, with intergranular assemblages of quartz, K-feldspar, albite, spodumene, muscovite, and minor amblygonite-montebrasite. Monomineralic core zones (quartz and amblygonite-montebrasite), aplitic albite ‘pods,’ and units characterized by phyllic alteration occur in the central portions of the pegmatite.Columbite-tantalite minerals occur throughout the pegmatite, excluding the quartz cores, and patterns of internal zoning include: (1) normally zoned ferrocolumbite with early Ta-Ti-rich, and later Nb-W-rich overgrowths; (2) oscillatory zoned Ti-rich ferrotantalite with Nb-rich patchy replacement; (3) reversely zoned ferrocolumbite, with a Ta-rich cores and (4) complexly zoned Ti-rich ferrotantalite with abundant late Nb- and Ta- rich patches and sieve textures. The textures and chemical patterns demonstrate an evolution from columbite to tantalite, whereby the Ta/(Ta + Nb) ratio increased with progressive fractionation (0.13-0.59) but the Mn/(Mn + Fe) ratio remained nearly constant (0.15-0.31). The chemical evolution of the Nb- Ta oxides from columbite to tantalite is consistent with crystallization from a magmatic to late-stage magmatic Na-rich melt, with a sequence of crystallization progressing from those of the wall zone, to the first intermediate zone, and finally the late aplitic albite zones. Minor remobilization of Nb, and to a lesser extent Ta, was responsible for some of the replacement features found in the Nb-Ta oxides. Textural observations and trace-element analyses of fine-grained, secondary muscovite found throughout the pegmatite indicate hydrothermal metasomatism by a late F- and Nb-rich vapor-like (“supercritical”) fluid.

http://www.jgeosci.org/rss.php?ID=jgeosci.149 2013/12/17 - 20:55

McKeough MA, Lentz DR, McFarlane CRM, Brown J; Vol. 58, issue 4, pages 321 - 346In northern Saskatchewan, granitic pegmatites intruded Early Paleoproterozoic Wollaston Group metasedimentary rocks and interfolded granitoids that unconformably overlie Late Archean gneisses, all of which have been subjected to deformation during the protracted 1.86 to 1.77 Ga Trans-Hudson Orogeny. The U-Th ± REE-Y-Nb pegmatite intrusions and fracture-controlled U mineralization characterize the occurrences at Kulyk Lake, Eagle Lake, and Karin Lake properties in the Wollaston Domain. The pegmatites are moderately to highly evolved, ranging from mineralogically simple to complex types. These are rare-earth element class, NYF pegmatites (Nb-Y-F), and are interpreted to have formed in a late syn- to post-collisional tectonic setting. The complex-type pegmatites are hybridized, due to metasomatic interaction with the host rocks and therefore are locally crudely zoned. Saturation of U-Th ± REE-Y-Nb occurred at the margins (predominantly border and wall zones) of the hybridized pegmatites. Partial melts were generated at depth, and then coalesced as they intruded to higher structural levels during exhumation of this orogen. This agrees with U-Pb geochronology of these granitic pegmatites, which constrains them between peak- and late-metamorphic events of the Trans-Hudson Orogeny. The age constraints and relatively high-T partial melting conditions (˜750 °C) confine the pegmatite melt-forming conditions to an early deformational event (1835-1805 Ma) that was overprinted by high-T retrograde metamorphism at c. 1770 Ma. Field relationships, textures and geochemical variations provide strong evidence that the U, Th, REE ± Y- Nb phases in the studied pegmatites were progressively enriched through extreme fractionation effects of which are evident throughout multiple pegmatite injections. In addition, volatiles and other fluxing components further enriched U, Th, REE ± Y- Nb during complex hybridization reactions between pegmatite melt and wallrock, up to the final stages of pegmatite emplacement.

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Vrána S, Janoušek V, Franěk J; Vol. 58, issue 4, pages 347 - 378A detailed petrological and geochemical study of the Blanský les Granulite Massif (BLGM; Moldanubian Zone of the Bohemian Massif) provides evidence for processes and mineral assemblages previously unrecognized, or not fully appreciated, in the South Bohemian HP-HT Granulite Complex. It also underlines the “patchwork”-like nature of the BLGM, caused by distinct protolith compositions as well as the differences in occurrence, or preservation, of mineral assemblages, reaction textures, and/or variable changes in chemical compositions. Such a complexity reflects superposition of several processes operating from the eclogite-facies metamorphic peak to the final emplacement, as well as variable resistance of individual rock types with distinct metamorphic fabrics. Even on a thin section scale, domains preserving evidence for early phases of eclogite-facies metamorphism, including original textural relations and largely intact HP-HT mineral assemblages, occur in close proximity to domains dominated by phases produced during later decompression. Consistent high-pressure estimates, 2.2-2.3 GPa (GASP), are newly obtained for S1/S2 fabrics in the prevailing (type 1) felsic granulites, and correlate well with the recent finds of rare omphacite relics in the mafic granulites. It indicates that much of the HP history of the felsic and mafic rock types was shared and took place under eclogite-facies conditions.Three important metamorphic processes leading to chemical changes in felsic granulites are described. (1) The small bodies of Kfs-dominated hyperpotassic granulites with minor Grt (or, rarely, Di) are fairly common and provide direct evidence for operation of non-eutectic HP melting. These occurrences are accompanied by compositionally complementary residual (type 2) felsic granulites, rich in Qtz and Ky and partly depleted in alkalis with Zr. Owing to their Ca-poor composition, type 2 granulites were not involved in some of the decompression reactions, such as formation of plagioclase rims around kyanite and garnet breakdown, and thus their mineral assemblages remained effectively “frozen” during decompression. They further imply that the partial melting and genesis of the hyperpotassic melts took place under eclogite-facies conditions, (2) Another case of syn-metamorphic modification is represented by a local limited base-cation loss during superimposed amphibolite-facies de-alkalization, accompanying fluid flux along S3 shear domains (formation of feldspar-free Qtz-Sill bands). (3) On a microscale, the retrogression reactions, often incomplete, were clearly associated with transfer of ions. The felsic Moldanubian HP-HT granulites were assumed by several authors to have undergone only limited modification during peak metamorphism, most notably loss of Cs, Th and U. Clearly, some of them may have suffered rather significant changes, such as perceptible loss of alkalis or even relatively immobile elements (e.g., zirconium), compared to the likely primary protolith(s) composition.

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Akiska S, Demirela G, Sayili S; Vol. 58, issue 4, pages 379 - 396Three main Pb-Zn ± Cu deposits (Handeresi, Bağırkaçdere, and Fırıncıkdere) occur south of Kalkım in the southeastern part of the Biga Peninsula (NW Turkey). Massive and disseminated Pb-Zn mineralization consists of ore veins in fault zones and replacement bodies in carbonate layers in metamorphic rocks. Garnet (˜Ad64Gr36), pyroxene (˜Di30Hd45Jo25), epidote and rare tremolite-actinolite are found as gangue minerals in ore zones. The garnets often show oscillatory zoning and correspond to grossular, andradite and grandite in composition. The pyroxene minerals are manganiferous hedenbergite, johannsenite and diopside. The Mn/Fe ratios of the pyroxene range between 0.3 and 0.9 (mean 0.5). Such compositions of the garnet and pyroxene and gangue assemblages are typical of distal Pb-Zn skarns. The ore minerals are pyrite, sphalerite, chalcopyrite, galena, magnetite, hematite, and arsenopyrite. The mean lead isotopic values for galena are 206Pb/204Pb ˜ 18.760, 207Pb/204Pb ˜ 15.689, and 208Pb/204Pb ˜ 38.935. The comparison of these isotope data with the host-rock samples (graphite schist, andesite and granodiorite) indicates that part of lead could have been leached from the schists. The δ34SVCDT values for galena, sphalerite, and chalcopyrite are -1.1 to +1.5 ‰ (mean: -0.2 ‰), 0.7 to +2. 1 ‰ (mean: +0.7 ‰), and -0.6 to +1.5 ‰ (mean: +0.4 ‰), respectively. Lead isotope data and δ34SVCDT ratios close to 0 ‰ indicate the formation of ores by magmatic processes.Taken together, the Oligo-Miocene magmatism in the area is taken responsible for the polymetallic mineralization. However, the metamorphic rocks and their protoliths may have contributed part of the lead.

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Kapsiotis AN; Vol. 58, issue 3, page: 0The Vourinos Ophiolite Complex in north-western Greece represents a fragment of oceanic lithosphere emplaced after the Late Jurassic closure of the Neotethyan Pindos Ocean. This Ophiolite Complex consists mainly of a well-preserved mantle sequence dominated by peridotite and chromitite. Based on petrographic features, four distinct peridotite varieties can be distinguished in the Vourinos mantle suite: i) normal harzburgite (Opx ≤ 15 vol. %), ii) transitional harzburgite (Opx < 15 vol. %), iii) coarse-grained dunite and iv) fine-grained dunite. Chromian spinel morphology, based on DR# (degree of roundness) measurements, varies systematically from harzburgitic to dunitic rocks. Anhedral chromian spinel occurs in normal harzburgite (DR# < 0.40), whereas in the other studied peridotite types this mineral is more euhedral (DR# > 0.40). The Cr# [Cr/(Cr + Al)] in chromian spinel increases from normal harzburgite to coarse-grained dunite, varying between 0.47 and 0.84. Such a variation in chromian spinel composition indicates a multi-stage melting evolution for the Vourinos mantle suite. Chromian spinel from fine-grained dunite bears similarities in terms of Cr# and Mg# [Mg/(Mg + Fe2+)] with chromian spinel from the neighboring chromitites, which implies a common origin for both lithologies. Chromian spinel morphological and compositional data indicate that after chromitite and fine-grained dunite formation a remnant boninite melt could have invaded the adjacent peridotites, modifying their accessory chromian spinels or even crystallizing new chromian spinel grains. Further evidence for that episode of melt percolation is provided by a few harzburgitic rock samples, which may be locally enriched in LREE exhibiting U-shaped chondrite-normalized REE patterns. Such patterns are characteristic of interaction between the depleted mantle peridotite and hydrous boninitic melt. The studied peridotites are interpreted as refractory residues whose initial composition was locally modified by mantle metasomatism in the fore-arc region in a supra-subduction zone.

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Velebil D, Zachariáš J; Vol. 58, issue 3, page: 0The stratiform deposit of mercury at Horní Luby near Cheb (Czech Republic) is hosted by Ordovician phyllites of Vogtland-Saxony Paleozoic Unit, in the Saxothuringian Zone of the Bohemian Massif. Ore is represented by cinnabar disseminated within the phyllite and by lenticular bodies rich in massive cinnabar (lenses with a thickness of up to 1 m and length along the strike varying from less than 1 m up to 20 m). Cinnabar is accompanied by pyrite and locally also siderite. The P-T history of the mineralization was deciphered by the study of fluid inclusions. Early fluids are represented by a homogenous H2O-CO2 fluid trapped in secretion quartz. This fluid underwent several heterogenization events starting from approximately 300 °C and continued down to 200-150 °C. Metamorphic quartz of secretion origin crystallized at ˜300 °C, while hydrothermal pyrite crystallized at 220-210 °C and cinnabar crystallized at 195-160 °C. The formation of the richest ore is associated with the replacement of metamorphic quartz in phyllites by the cinnabar in weakly alkaline solutions.The cinnabar is pure phase, free of admixture (Bi, Sb, Zn, Fe, and Cu). It is accompanied by minute blebs of Hg-bearing sphalerite (11-12 wt. % Hg) that might indicate earlier presence of the zincian metacinnabar and more complex metamorphic history of the ore. The primary source of mercury is thought to be Lower Paleozoic submarine volcanism. The formation of ore bodies is, however, associated with metamorphic mobilization of mercury during the late stages of the Variscan orogeny. Despite being relatively small, the mercury deposit at Horní Luby competed with the mercury mines in Idrija (Slovenia) and Almadén (Spain) in the 16th century. The production of mercury at Horní Luby is estimated to have corresponded to c. 10-30 % of the mercury production in the mines of Idrija and Almadén at that time. In addition to Venice, the mercury from Horní Luby was also supplied to Nurnberg, Antwerp and Lyons. In 1520-1540, the production of mercury from the Horní Luby mines was 6 to 15 tons per annum. The mines were abandoned in 1597. Attempts to reinstitute mining activity in the 17th, 18th and 19th centuries were not very extensive and always failed. The total production of the Horní Luby mines during the whole mining history is estimated to have been at least 200 tons of mercury. The major portion of this amount was extracted between 1520 and 1540, with a minor portion between 1560-1570.

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Han C, Xiao W, Zhao G, Su B-X, Sakyi PA, Ao S, Wan B, Zhang J, Zhang Z; Vol. 58, issue 3, page: 0Exploration of the Hulu Cu-Ni-Co sulfide deposit in Eastern Tianshan, Central Asian Orogenic Belt, has detected horizons of sulfide mineralization in pyroxenite and peridotite units. SIMS U-Pb zircon ages of the gabbro-hosting Cu-Ni-Co sulfide deposit indicate that the Hulu intrusion was emplaced at 282.3 ± 1.2 Ma (95% confidence level, MSWD = 3.30, n = 13). Osmium isotopic data suggest that the Hulu intrusion and associated Cu-Ni mineralization were derived from crustally-contaminated mantle melts. The intrusions clearly show island-arc geochemical signatures, such as negative Nb, Ta, Zr and Ti anomalies and enrichment in LILE. These geochemical tracers indicate that the Hulu mafic-ultramafic intrusions, along with the Cu-Ni deposit, formed as a result of subduction of oceanic crust in the Early Permian. The arc-related geodynamic setting of the Hulu Cu-Ni deposit is unusual for magmatic sulfide deposits. This new type of ore deposit offers an alternative perspective for Cu and Ni exploration.

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Pašava J, Zaccarini F, Aiglsperger T, Vymazalová A; Vol. 58, issue 3, page: 0Lower Cambrian Mo-Ni sulfidic black shales from the Huangjiawan mine (Guizhou Province, south China) have anomalous platinum-group elements (PGE) concentrations (up to ˜1 ppm in total). In order to identify principal PGE carriers, we used heavy mineral separates which were produced by innovative hydroseparation techniques. Subsequent detailed mineralogical study using electron microprobe did not result in the identification of discrete platinum-group minerals. Pyrite (grainy, not framboidal), millerite and gersdorffite that were identified in our heavy concentrate were analyzed for PGE and Re. We found that they contain the following concentrations of PGE and Re: pyrite (up to 490 ppm Pt, 390 ppm Pd and 220 ppm Rh), millerite (up to 530 ppm Pt, 430 ppm Pd and 190 ppm Rh) and gersdorffite (up to 410 ppm Pt and 320 ppm Pd; no Rh detected). Rhenium was detected only in grainy pyrite (up to 1060 ppm). It was found that despite anomalous PGE concentrations, the Mo-Ni black shales do not contain any platinum-group minerals and that the PGE are bound to pyrite and Ni-sulfides (millerite and gersdorffite).

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Han C, Xiao W, Zhao G, Su B, Ao S, Zhang J, Wan B; Vol. 58, issue 3, page: 0During the Late Paleozoic, extensive magmatism and associated ore deposits were developed in the Eastern Tianshan Orogenic Belt (the Central Asian Orogenic Belt, NW China). In order to better constrain the petrogenesis of the intrusions in the area, we performed major- and trace-element whole-rock geochemical analyses as well as in situ zircon U-Pb and Hf isotopic analyses from the Xiangshan, Luodong and Poshi batholiths. Voluminous 276-284 Ma ultramafic and mafic rocks (associated with magmatic Cu-Ni sulfide deposits of the same age) have variable Hf isotopic compositions (εHf (t) = −10.3 to +14.3), indicating an origin via the contamination of depleted mantle-derived magma by variable amounts of ancient lower crust. The large mafic-ultramafic complexes were emplaced most likely during closure of the ancient Tianshan Ocean, resulting in the formation of several magmatic Cu-Ni sulfide deposits in the Early Permian times.

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Kohút M, Trubač J, Novotný L, Ackerman L , Demko R, Bartalský B, Erban V; Vol. 58, issue 3, page: 0The Kurišková U-Mo deposit from the Gemeric Unit of the Western Carpathians (Slovakia) is an example of polygenetic deposit whose origin involved several events: endogenous, related to magmatism/volcanism, and exogenous, associated with precipitation from meteoric hydrothermal fluids in repeated tectonically-driven (fold & thrust and shear zones) channel ways penetrating the Permian Huta volcano-sedimentary complex. Sources of the U-Mo mineralization were multiple: (a) molybdenite was derived directly from juvenile hydrothermal fluids related to igneous activity, (b) the U mineralization formed from meteoric fluids circulating through altered and metamorphosed basaltic and rhyolitic volcanics intercalated by clastic sediments (sandstones and mudrocks), which interacted in an arid to humid climate with organic and carbonate substances within Permian basin. The principal ore-forming minerals are uraninite, coffinite, molybdenite and apatite with rare orthobrannerite and powellite. Two basic mineralization forms are present: (a) tabular - “stratiform like” and (b) stockwork intraformational and/or dislocation stockworks in shear zones. The Re-Os molybdenite dating confirmed crystallization from igneous source in Late Permian (Lopingian; 257.2 ± 3.0 Ma to 255.6 ± 3.7 Ma) for massive vein mineralization, whereas the superimposed U remobilization within shear zones occurred in the Triassic/Jurassic period. The Kurišková U-Mo deposit represents a polygenetic endo/exogenous hydrothermal deposit of the Permian/Paleo-Alpine age, with metals sourced in Permian volcanosedimentary rocks that were leached by shear zone-related meteoric fluids.

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Pertold Z; Vol. 58, issue 3, page: 0

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