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Thorite, Thorogummite and Xenotime-(Y) Occurrence in Ditrau Alkaline Intrusive Massif, East Carpathians, Romania

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Thorite, Thorogummite and Xenotime-(Y) Occurrence in Ditrau Alkaline Intrusive Massif, East Carpathians, Romania THE PUBLISHING HOUSE GEONOMY OF THE ROMANIAN ACADEMY Research article THORITE, THOROGUMMITE AND XENOTIME-(Y) OCCURRENCE IN DITRAU ALKALINE INTRUSIVE MASSIF, EAST CARPATHIANS, ROMANIA PAULINA HIRTOPANU 1* , GYULA JAKAB 2, C. JENS ANDERSEN 3 and J. ROBERT FAIRHURST 3 1 Department of Mineralogy, University of Bucharest, Romania 2 IG Mineral Gheorgheni, Romania 3 University of Exeter, Camborne School of Mine, UK Corresponding author : *[email protected] Received July 10, 2013 The thorite and thorogummite, closely associated with the xenotime-(Y) represent the characteristic paragenessis of the Belcina type mineralization of the Ditrau alkaline intrusive massif. The Belcina vein mineralization is localized in the South-East of the massif, within the Cambrian metamorphic Tulghes Group. Genetically the Belcina mineralization is linked to the red alkaline syenites, the fluid responsible for the mineralisation belonging to the late-stage hydrothermal event. Late stage calcite, siderite, ferrodolomite veins and development of the Fe-oxides and hydroxides (hematite, goethite, lepidocrocite), minor sulphides of Fe, Pb, Zn, Mo and Hg, may be present. Also, REE-apatite, monazite-(Nd), monazite-(Ce), brabantite, ferrocolumbite, manganocolumbite, Nb-rutile, zircon, magnetite, thorianite, Y-fluorite are present as primary mineralization sequence. The gangue associated to the ore is represented by feldspars (microcline, albite, orthoclase), quartz, different common carbonates of at least two generations, natrolite, pectolite and different chlorites. Electron microprobe analyses of many individual grains of thorite gave mainly ThO 2=70-80.00, SiO 2=15- 17.00, ZrO=0-6, FeO=0-4% and very little Ce 2O3, UO 2, Y 2O3, PbO and P 2O5 contents. Thorogummite has less ThO 2, more SiO 2, ZrO, FeO, CaO, PbO and P2O5, than thorite. Thorite and especially thorogummite have an oscillatory chemical composition. The xenotime-(Y) has a constant composition, with of about Y2O3=60-62 and P 2O5=38-40% (wt). Because of so many common carbonates, as mineral gangue, the Belcina Th-Y mineralization could be attributed to a carbothermal process more than to a hydrothermal one. Key words: thorite, thorogummite, xenotime-(Y), Nb-rutile, monazite-(Ce), brabantite, apatite, ferrocolumbite, zircon, natrolite, pectolite, Fe oxides/hydroxides, hydrothermal/carbothermal activity. INTRODUCTION/GEOLOGICAL affiliation to anorogenic alkaline magmas explains SETTING 1 their general feature Nb>>Ta and the predominance of LREE in the first mineralization The Ditrau alkaline intrusive complex is stages and the Th-Y one in the last hydrothermal situated within the Cambrian metamorphic Tulghes /carbothermal stages. The Ditrau alkaline intrusive Group of the inner part of the East Carpathians. It complex represents the end of a long lasting has a distinct ring structure (Streckeisen, 1960), fractionation processes in the crust and deep upper having a succession of magmatic events from mantle, respectively. This explains the modest gabbroic and dioritic magma to syenitic and dimensions of Ditrau intrusion and its enrichment various postmagmatic events as well, developed with incompatible elements. The high petrographic between a Triassic extensional stage and a Jurassic complexity of Ditrau massif is doubled by a great rifting stage (Krautner & Bindea, 1996). The diversity of mineral occurrences. In addition to the succession of the magmatic events in Ditrau, could 85 minerals, previously mentioned (Jakab, 1998, be completed with carbonatite intrusion that Constantinescu, 1999), about 200 were recently followed the alkaline intrusion and used the same identified, many of them being first occurrences in pathways as the previous alkaline silicate melt. Its Romania (Hirtopanu et al. , 2003, Hirtopanu et al. , Proc. Rom. Acad., Series B, 2013 , 15( 2), p. 111–131 112 Paulina Hirtopanu et al. 2010a, 2010b). The rare element minerals determined Ditrau massif (SE), in the surrounding metamorphic in Ditrau complex intrusion, belong to the following rocks of the Tulghes Group, at 2.5 km distance of 6 classes (in the predominant order): REE(Y) – massif (Jakab, 1998) (Fig. 1). The Belcina occurrence Carbonates; Nb, Ta, REE(Y), Ti, Zr, Th, U –Oxides; comprises a complex mineralization, different from REE(Y) –Phosphates; REE(Y), Th, Zr, Pb –Silicates; that reported in Jolotca. It is mostly constituted of Th Halides and Tellurides (Hirtopanu et al., 2010b). silicates and Y-phosphates, less zircon, niobian rutile, They occur in a small mineralization areas (Lazarea, ferrocolumbite, Y-fluorite, less sulphides, and almost Hereb-Cianod, Ditrau Valley/ Pricske, Aurora, Putna, no Nb-Ta pyrochlores (which occur frequently in Creanga Halasag) and in two big vein mineralization Jolotca area), small/almost no REE(Y) – silicates areas, Jolotca and Belcina (Fig. 1). The biggest (allanite, which has a big development in Jolotca Jolotca occurrence from the NV of the massif is of area, chevkinite, etc), and small/no REE(Y)- vein type and comprises a complex sulphides carbonates (which also have a big development in mineralization. The rare elements mineralizations Jolotca area). Also, there are present as accessory/ followed after sulphides, almost all the rare element constituent primary minerals such as: Th-apatite, Fe- minerals being developed as veins in sulphides, as a oxides and hydroxides (magnetite, hematite, goethite, late hydrothermal phase. They are represented by lepidocrocite), some sulphides (pyrite, arsenopyrite, LREE-carbonates; Nb,Ta, REE(Y), Ti, Zr, Th, U galena, sphalerite, chalcopyrite, tetrahedrite, oxides; REE(Y), Th phosphates; REE(Y), Zr, Th, Ti, molybdenite, cinnabar) and native mercury Hg, in the REE silicates; Bi and Pb-Bi tellurides; and halides calcite-ferrodolomite-siderite and apatite gangue. The (Hirtopanu 2006, Hirtopanu et al ., 2010a, 2010b). samples studied here (around 50) were collected from The Jolotca rare elements mineralization has a the vein situated at the confluence of Belcina Valley gangue of ferrodolomite, ankerite, siderite and with Kisgyor Rivulet and were abbreviated with calcite. The other occurrence of vein type is the “Bel”. The samples collected from Jolotca were Belcina, less big than Jolotca, situated outside of the abbreviated with “Dt”. Fig. 1. The schematic geological map of Ditrau alkaline massif with mineralization areas : hornblendite-diorite complex (blue), red syenite (red), white nepheline syenite (yellow), Tulghes Group (grey), F = fault, G8 =crustal fracture (hachure interrupted line), (acc. to Jakab, Gyula). Thorite, thorogummite and xenotime-(y) occurrence in Ditrau alkaline intrusive massif, East Carpathians, Romania 113 MINERALOGY complete series. Also, thorite is related to a mineral called huttonite (ThSiO 4). Thorite and huttonite are The first and complete mineralogical description dimorphs. The two minerals have the same of Belcina mineralization belongs to Jakab (1982, chemistry, they just have different structures. 1998) who determined representative minerals of Huttonite is monoclinic and belongs to the following mineral classes: a. silicates: K feldspar, Monazite Group, being related structurally to its albite, chlorite, biotite, sericite, thorite, zircon, members. Also, there is a close related mineral epidote; b. oxides: ilmenorutile, rutile, anatas, thorogummite, that was once considered a variety columbite, pyrochlore, fersmite, magnetite, loparite, of thorite but it is a product of its hydrothermal ilmenite, hematite; c. phosphates: xenotime, apatite, alteration. Because thorite is highly radioactive, Y-fluorapatite; d. carbonates: calcite, ferrodolomite; specimens are often metamictized. This is a state sulphides: pyrite, arsenopyrite, galena, sphalerite, found in radioactive minerals and results from the chalcopyrite, tetrahedrite, molybdenite. The mineral destructive effects of their own radiation on their crystallization order (Jakab, 1998) is: Paragenesis I: crystal lattice. The effect can lead to destroying pegmatitic quartz-feldspar gangue; ferrodolomite+ completely its own crystal lattice while leaving the quartz; niobotantalates+xenotime+ thorite+fluorapa- external appearance unchanged. In the Belcina tite+arsenopyrite+sphalerite+molybdenite+pyrite+ chlo- area, thorite occurs as a constituent mineral, while rite; ferrodolomite; calcite+galena; Paragenesis the Jolotca thorite is an accessory mineral. The II:pyrite+sphalerite+galena+calcite+quartz. The K/Ar Belcina thorite occurs as irregular grains (Fig. 3) of microcline ages, shows for the Belcina mineralization a few mm and much longer up to 1cm in diameter. the same age as the Ditrau rocks (Jakab, 1998). Also, It could be massive and compact. The Jolotca the same author showed that the Jolotca thorite has idiomorph crystals with habits as square mineralization, having the predominance of Y in short prisms (Fig. 2), pseudooctahedral crystals. respect to Ce, and high Th content, belongs Macroscopically it is nearly opaque being genetically to the magmatic red syenites. The first transparent in thin fragments. In transmitted light it modern study of the Belcina occurrence was made in is yellow-orange, red, brownish yellow, brownish laboratories of CSM, Exeter University (2007–2009). black, black (Figs. 2 and 3). It has a distinct, poor After a minutely optical study, the samples were (110) cleavage (Fig. 2) and is uniaxial (+). The analyzed chemically by microprobe and SEM. The refringence (n ≈1.85), greater than that of the electron microprobe data, which
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