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

• Autorzy: Bilohuscin V. , Uher P.

Identyfikatory

DOI

10.7494/geol.2016.42.1.59

• Konferencja: XVIIth International Conference of Young Geologists Herl'any 2016
• Języki publikacji: EN

Abstrakty

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.

Słowa kluczowe

EN

sedimentary rock; evaporitic rock; temperature

• Wydawca: Wydawnictwa AGH
• Czasopismo: Geology, Geophysics and Environment
• Rocznik: 2016
• Tom: Vol. 42, no. 1
• Strony: 59--60
• Opis fizyczny: Bibliogr. 8 poz.

Twórcy

autor

Bilohuscin V.

• Comenius University, Department of Mineralogy and Petrology; Ilkovičova 6, 842 15 Bratislava, Slovakia

autor

Uher P.

• Comenius University, Department of Mineralogy and Petrology; Ilkovičova 6, 842 15 Bratislava, Slovakia

Bibliografia

• Aleksandrov S.M. & Troneva M.A., 2008. Heterovalent isomorphism in the Magnesium-Iron Borates. Geochemistry International , 46, 800–813.
• Aleksandrov S.M. & Troneva M.A., 2011. Genesis and Composition of Endogenous Borates in the Skarns of the Eastern and Central Pyrenees. Geochemistry International , 49, 802–814.
• Anovitz LM. & Grew E.S. (eds), 1996. Boron: Mineralogy, Petrology, and Geochemistry . Reviews in Mineralogy 33, Mineralogical Society of America.
• Koděra P., Lexa J. & Fallick A.E., 2010. Formation of the Vysoká-Zlatno Cu-Au skarn-porphyry deposit, Slovakia. Mineralium Deposita , 45, 817–843.
• Marincea S., 2000. The influence of Al on the physical and crystallographic properties of ludwigite in three Romanian occurrences. European Journal of Mineralogy , 12, 809–823.
• Persev N.N., 1991. Magnesian skarns. [in:] Aksiuk A.M., Skarns: Their Genesis and Metallogeny , Theophrastus Publications, Athens, 299–324.
• Pertsev N.N., Schreyer W., Armbruster T., Bernhard H.J. & Medenbach O., 2004. Alumino-magnesiohulsite, a new member of the hulsite group, in kotoite marble from east of Verkhoyansk, Sakha-Yakutia, Russia. European Journal of Mineralogy , 16, 151–161.
• Schaller W.T., & Vlisidis A.C., 1961. Composition of aluminian ludwigite from Crestmore, California. American Mineralogist , 46, 335–339.

Uwagi

PL

Opracowanie ze środków MNiSW w ramach umowy 812/P-DUN/2016 na działalność upowszechniającą naukę.