Mineralogical characterisation, fluid inclusions and formation conditions of aplite, pegmatite, and quartz-tourmaline rich veins, Elba (Italy)

Sebastian Schilli

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Aplitic-pegmatitic veins, usually present as dykes, sills and irregular shaped bodies were investigated around the Monte Capanne Pluton and the Calamita peninsula (Elba, Italy). These veins consist of aplitic-pegmatitic bodies in the eastern part and a porphyric dyke (Orano porphyry) in the western part of the Monte Capanne Pluton. Samples were collected in the field to determine the local geological context of these veins by describing in detail the relationship between host rock and veins. Thin sections were prepared from these samples to investigate the mineralogy and textures in detail. In addition, several thick sections (about 100 µm thickness) were prepared for fluid inclusion research. Both minerals and fluid inclusions were analyzed by optical microscopy, Raman spectroscopy, electron microprobe analyses and microthermometry. The aplitic/pegmatitic veins consist mainly of quartz, plagioclase, K-feldspar with abundant immiscibility features, and locally mica, tourmaline and andalusite. The porphyry contains phenocrysts (up to 8 mm in size) of plagioclase, biotite and anhedric rounded quartz grains within a fine grained matrix. Accessory minerals in the aplitic/pegmatitic veins are zircon, xenotime, monazite, rutile/brookite/anatase, apatite, garnet, cordierite, allanite, titanite, columbite, cassiterite, hematite, magnetite, biotite, hedenbergite, sillimanite, and pyrite. Tourmaline and andalusite crystals are brittly deformed (boudins) due to magmatic flow. Tourmaline is strongly zoned which is visualized by green, brown, blue and colourless pleochroic colours under the optical microscope with transmitted light. The composition ranges from schorlitic to foitic tourmaline, and dravitic tourmaline in pegmatites around the Monte Capanne Pluton. The colourless tourmaline of the schorlitic group is a Mn-rich elbaitic tourmaline. At the Calamita peninsula, the tourmalines display two compositional groups of schorlitic and dravitic tourmaline. The pink colour of andalusite is directly related to the Fe-content (up to 2.9 mass% FeO). The Mg/Fe ratio of coexisting garnet and biotite (sample 57A) were used to estimate a temperature of approximately 600 ˚C. Rutile contains up to 7.0 mass% Nb2O5, 4.3 mass% SnO2, 3.9 mass% Ta2O5 and 11.5 mass % FeO. Columbite contains 48.0-56.1 mass% Nb2O5, 9.3-15.7 mass% Ta2O5, 11.9-16.3 mass% FeO, 7.9-11.7 mass% WO3 and 0.4-5.0 mass% Sc2O3. Some columbites are extremely enriched in HREE (heavy rare earth elements), and contain additionally up to 22 mass% Y2O3 and 11.8 mass% UO2. The aplitic/pegmatitic veins contain abundant fluid inclusions of primary origin. The inclusions occur mainly in quartz, but tourmaline, andalusite, plagioclase, allanite, and garnet also contain large fluid inclusions. Quartz in pegmatites in the ophiolitic unit at the eastern border of the Monte Capanne Pluton contains large inclusions (up to 20 µm in diameter) with approximately 42 vol% vapour phase. This phase is a mixture of CO2 and CH4 with minor amounts of N2 and H2. The liquid phase contains sassolite (H3BO3) daughter crystals, occasionally arsenolite (As2O3) and mica. Total homogenization occurs around 365 ˚C. A second type occurs in trails and contains CH4-H2 gas mixture in a relative small bubble (15 vol.%, TH ≈ 200 ˚C). Tourmaline contains two types of fluid inclusions: 1) CH4-rich vapour phase (± CO2); 2) no gas in vapour bubble, and sassolite (±arsenolite) in liquid phase. Both types have relative large vapour bubbles (40-60 vol%). Homogenization temperatures are about 370 ˚C.
Translated title of the contributionMineralogische Charakterisierung, Flüssigkeitseinschlüsse und Bildungsbedingungen von Apliten, Pegmatiten und Quarz-Turmalin-reichen Ganggesteinen, Elba (Italien)
Original languageEnglish
Awarding Institution
  • Montanuniversität
  • Bakker, Ronald Jack, Supervisor (internal)
Award date27 Mar 2015
Publication statusPublished - 2015

Bibliographical note

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  • Elba
  • Italy
  • pegmatite
  • tourmaline
  • andalusite
  • fluid inclusions
  • borate

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