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Ludwigite-group minerals and szaibelyite: rare borate minerals from Vysoká – Zlatno skarn, Štiavnica stratovolcano, Slovakia

Bilohuscin V., Uher P.

Geology, Geophysics and Environment

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2016

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Vol. 42, no. 1

59--60

EN

Beside of sedimentary evaporitic rocks, borate minerals occur also in some high temperature contact-metamorphic rocks, especially in skarns, locally in association with Fe and Sn ore minerals (e.g., Anovitz & Grew 1996). The borate minerals are generally associated with the post-magmatic processes which occur in the contact aureoles of intrusive, acid to intermediary, calc-alkaline rocks (Pertsev 1991). Borate minerals of the ludwigite group and szaibelyite were identified from the Mg-skarn in the R-20 drilling core in depth of 1172 m during geological exploration for Cu- Au porphyry-skarn ores in the Vysoká – Zlatno area near Banská Štiavnica, in the Štiavnica Neogene stratovolcano, central Slovakia (Koděra et a l. 2010). Ludwigite-group minerals (LGM) form massive black aggregates (>5 cm large) of numerous acicular, euhedral to subhedral prismatic crystals (usually 0.2–3 mm long). Ludwigite associates with clinohumite, szaibelyite, clinochlore, serpentine-group mineral, magnesite, dolomite, hematite, rarely valeriite, chalcopyrite, and sphalerite. Under transmitted light, LGM crystals are mostly opaque; locally they are translucent with strong pleochroism in sections parallel to Z-axis (deep green – dark reddish brown). In BSE, LGM crystals show regular concentric, rarely oscillatory or irregular zoning caused by distinct element variations during their growth or partial alteration: the dark zones show enrichment in Mg, Al and Ti, in contrast to the pale zones which reveal larger amounts of Fe. The electron-microprobe analyses reveal growth evolution of LGM crystals from Al- rich azoproite with ≤ 79 mol.% of Mg 2 (Mg 0.5 Ti 0.5 ) (BO 3 )O 2 end-member] to Al ± Ti-rich ludwigite and Al-dominant LGM phase [“aluminoludwigite” with ≤ 53 mol.% of Mg 2 Al(BO 3 )O 2 end-member] in central zones, whereas rim zones of the crystals and secondary veinlets attain nearly pure ludwigite composition [87–99 mol.% of Mg 2 Fe 3+ (BO 3 )O 2 end-member]. Consequently, LGM from the Vysoká – Zlatno skarn show the largest composition al variations ever known from one occurrence and they reach the highest contents of Ti ( ≤ 17.4 wt.% TiO 2 , 0.39 apfu ) and Al ( ≤ 14.4 wt.% Al 2 O 3 , 0.53 apfu ) ever reported in LGM (Schaller & Vlisidis 1961, Marincea 2000, Pertsev et al. 2004, Aleksandrov & Troneva 2008, 2011). The compositional variations indicate the following substitution mechanisms in the studied LGM: Mg 2+ = Fe 2+ for the all compositions, Fe 3+ = Al 3+ for samples without higher amount of Ti, and 2Al = Mg 2+ + Ti 4+ or 2Fe 3+ = Mg 2+ + Ti 4+ for analyses including high Ti content. Szaibelyite MgBO 2 (OH) occurs as aggregates of fibrous crystals, up to 0.5 mm in size, replacing LGM. Zoning in szaibelyite was not observed. The amounts of Mg are uniform (0.98 to 0.99 apfu ), content of Fe 2+ oscillates from 0.2 to 1.2 wt.% FeO (0.002–0.014 apfu ) and indicates the Mg 2+ = Fe 2+ substitution. Szaibelyite also contains small ad mixtures of Mn (0.1–0.4 wt.% MnO), Al and Cr ( ≤ 0.3 wt.% Al 2 O 3 or Cr 2 O 3 ). The skarn mineralization originated as a result of contact thermal metamorphism of Miocene calc-alkaline granodiorite intrusion on host Middle to Upper Triassic limestones, dolomites, shales and evaporitic anhydrite beds (the Veľký Bok Group, Veporicum Unit). The evaporites were most likely the primary source of boron, where as Ti was probably derived from the granodiorite. Clinohumite and LGM (azoproite to Al ± Ti-rich ludwigite and “aluminoludwigite”) precipitated during the high-temperature contact metamorphic event at ~ 700°C and ≤ 100 MPa, whereas the youngest Al,Ti-poor ludwigite veinlets, szaibelyite, serpentine-group mineral, clinochlore, magnesite, dolomite, hematite and probably also sulfide minerals were formed during younger, lower-temperature hydrothermal-metasomatic event.

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Composition of Nb-Ta-Ti-Sn-W oxide minerals: indicators of magmatic to hydrothermal evolution of the Cínovec granite intrusion and Sn-W deposit (Czech Republic)

Chladek S., Breiter K., Uher P.

Geology, Geophysics and Environment

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2016

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Vol. 42, no. 1

61--62

EN

The Cínovec (Zinnwald) Sn-W ore deposit is genetically linked to intrusion of late Variscan, highly fractionated granite which expresses the latest evolutionary stage of a volcano-plutonic system of the Teplice caldera. Whole intrusion is relatively highly fractionated and from bottom ( ~ 1500 m) to top part of cupola-shaped deposit is obviously following succession from biotite (annite) granodiorite-granite-zinnwaldite granite, with the partly greisenized uppermost part at 300–500 m depth (Štemprok 1965, 1971). In 1961−63 the Czechoslovakian Geological Survey (CGS) drilled a 1596 m deep borehole in the Sn-W-mineralized Cínovec granite cupola (Štemprok 1965, Štemprok & Šulcek 1969). All studied rock types include W- and Sn-bearing minerals (wolframite series, scheelite and cassiterite) and disseminated accessory Nb-Ta-Ti-W- Sn minerals (Štemprok & Šulcek 1969, Štemprok 1989, Johan and Johan 1994) which were obtained from the collection of CGS in Prague and studied by BSE and electron microprobe. They crystallized in following succession: rutile + columbite + cassiterite (biotite granodiorite) → rutile + columbite + W-rich ixiolite + cassiterite + scheelite in zinnwaldite granite. Textural relationships of these Nb- Ta-Ti-Sn-W minerals indicate predominantly their magmatic origin and part of them (e.g., cassiterite and columbite) show minor post-magmatic alteration phenomena like distinctly inhomogeneous mixtures of secondary pyrochlore-group minerals (“oxykenopyrochlore” and oxycalciopyrochlore). Nb/Ta and Fe/Mn fractionation trends led to characteristic Mn and Ta enrichment from bottom (biotite granite) to uppermost zinnwaldite granite, especially in columbite-group minerals. While Nb/ Ta fractionation is limitedly applied, effective Fe/ Mn fractionation led to significant Mn – enrichment of late-magmatic phases [columbite-(Mn) and W-rich ixiolite]. Post-magmatic to hydrothermal metasomatic fluids caused partial greisenization of the granites and this stage is represented by latest columbite + scheelite + cassiterite + wolframite assemblage. The last two minerals were objects of extensive mining in the past. Although the hydrothermal system was enriched in F and Li (presence of topaz and zinnwaldite), there are only relatively limited Nb/Ta and Fe/Mn fractionations in post-magmatic columbite. Similarly to primary fractionation, both Nb/Ta and Fe/Mn ones take place and overlap characteristic primary Mn-enrichment. Effective Mn-redistribution is predominantly controlled by crystallization of Mn-dominant wolframite like hübnerite in the hydrothermal stage. Scandium is typical rare element in primary (magmatic) and secondary (hydrothermal) mineral assemblage. While primary Sc-fractionation continues the ongoing Sc-enrichment mostly in columbite to uppermost parts of intrusion, the hydrothermal Sc-redistribution is controlled by crystallization of main ore mineral – wolframite, which consumed a major part of scandium. Main substitution mechanisms in rutile-cassiterite-wolframite-columbite assemblage include following heterovalent substitutions: (i) Ti 3 (Fe,Mn) 2+ −1 (Nb,Ta) −2 , (ii) Ti 2 Fe 3+ −1 (Nb,Ta) −1 , (iii) (Nb,Ta) 4 Fe 2+ −1 W −3 . Moreover, a part of minor cations can enter via: (iv) (Fe,Mn) 2+ 1 W 1 (Fe,Sc) 3+ −1 (Nb,Ta) −1 into wolframite lattice, (v) W 1 (Ti,Sn) 1 (Nb,Ta) −2 , (vi) (Sc,Fe) 3+ 3 (Fe,Mn) 2+ −2 (Nb,Ta) −1 , and (vii) W 2 Sc 3+ 1 (Nb,Ta) −3 into columbite lattice. Calculated Fe 3+ can be introduced into rutile lattice predominantly via mechanism (ii), while via (iv) into wolframite lattice and together with Sc 3+ via (vi) into columbite lattice. The last mechanism results in charge imbalance of A and B positions of columbite lattice entering R 3+ cations to. The distinctly varying calculated Fe 3+ values can refer to changing f O 2 during columbite, rutile, W-rich ixiolite and wolframite crystallization. Therefore, the textural and crystallo-chemical features of studied Nb-Ta-Ti-Sn-W oxide minerals in the Cínovec granite cupola reflect a complex geochemical development of this granite system and ore mineralization from primary magmatic stage, through late-magmatic to subsolidus conditions, and ending in distinct hydrothermally – metasomatic overprint of pre-existing phases.

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Uranium-rich monazite-(Ce) from the Krivá type granitic boulders in conglomerates of the Pieniny Klippen Belt, Western Carpathians, Slovakia : composition, age determination and possible source areas

Uher P., Plašienka D., Ondrejka M., Hraško L., Konečný P.

Geological Quarterly

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2013

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Vol. 57, No. 2

343–-352

EN

Monazite-(Ce) is a widespread accessory mineral in granitic cobbles of the Krivá type (Zástranie and Krivá localities) in polymict conglomerates of Cretaceous flysch sequences, the Pieniny Klippen Belt, Western Carpathians, NW Slovakia. The granites show leucocratic muscovite-biotite granodiorite composition and peraluminous calc-alkaline, S-type character. The monazite contains unusually high U, commonly 1 to 3, and in some places up to 6.6 wt.% UO2, together with 5 to 7.7 wt.% ThO2. A cheralite-type substitution [Ca(U,Th)REE–2 is the dominant mechanism of U4+ + Th4+ incorporation into the monazite structure in the Zástranie sample, whereas both cheralite- and huttonite-type substitution [(Th,U)SiREE–1P–1] are evident in the Krivá granitic cobble. Uranium alone prefers the CaU4+(REE)–2 mechanism, whereas Th favours the huttonite substitution. The chemical U-Th-Pb dating of monazite from both granitic cobbles show an Early Carboniferous age (346 ± 2 Ma), which is consistent with the main meso-Variscan, orogen-related plutonic activity in the Central Carpathian area (Tatric and Veporic superunits). Analogous U-rich monazites were detected in some Variscan S-type leucogranites of the Rimavica massif (South Veporic Unit) and the Bojná and Bratislava massifs (northern part of the Tatric Unit). On the basis of structural and palaeogeographic data, the North Tatric Zone is the most plausible source of the monazite-bearing granitic boulders in the Pieniny Klippen Belt. However, the source granitic body was most likely hidden by ensuing tectonic shortening along the northern Tatric edge after deposition of the Coniacian–Santonian Upohlav type conglomerates.

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New approach to garnet redistribution during aeolian transport

Lisá L., Buriánek D., Uher P.

Geological Quarterly

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2009

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Vol. 53, No 3

333-340

EN

Garnet composition within Late Pleistocene (Weichselian) loess and loess-like deposits was studied in 13 samples of sediment heavy mineral fractions from Moravia and Silesia (Czech Republic). Four areas differing in garnet chemistry were identified, and some regional trends in garnet composition changes were documented. The data obtained support the generally accepted conclu ion of prevailing westerly winds during Weichselian loess deposition. Metamorphic rocks of the Bohemian Massif together with contributions fromig neous (mainly granitic) and sedimentary rocks were indicated as a source for the Weichselian loess and loess-like deposits studied. Local differences in garnet composition depend on the basement source rocks, on prevailing wind direction, on regional geomorphology and on transport distance.