Original paper

Ferroan magnesioaxinite from hydrothermal veins at Lazany, Brno Batholith, Czech Republic

Nová k, Milan; Filip, Jan

Abstract

Hydrothermal veins, up to 2cm thick, consisting of dominant ferroan magnesioaxinite and minor magnesian ferroaxinite, less abundant clinozoisite and very rare actinolite, cut hydrothermally altered metadiorite at Lažany, the Brno Batholith, Czech Republic. Light grey to pinkish grey axinite is commonly massive and sporadically forms tabular crystals, up to 5mm in size, in small open vugs. Axinite is commonly homogeneous in the BSE image, a weak sectorial zoning was locally observed. Chemical composition is close to the general formula, it varies in the range: Mg = 1.02-0.71, Fe2+tot = 0.98-0.62, Mn = 0.41-0.19 (all in apfu); ≈2% of Fe is trivalent from Mössbauer spectroscopy. The substitutions derived from the end-member magnesioaxinite include dominant FeMg-1 less significant MnMg-1 and negligible Fe3+Al-1. The mineral association of axinite-bearing veins suggests temperatures of the greenschist facies at ≈300-400 °C and low XCO2. Significant influx of B from an external source, potentially leucocratic granites adjacent to the metabazite zone, was necessary. The other elements (Ca, Mg, Fe, Al and Si) were very likely derived from the host rock. Formation of compositionally similar tourmaline-bearing veins in metabasite zone of the Brno Batholith instead axinite-bearing veins rather reflects higher activity of CO2 then lower activity of Ca. Scarcity of magnesioaxinite in contrast to abundance of rocks with Mg > Fe is controlled chiefly by crystal chemical constraints. Axinite behaves as apparent sink for Mn.

Keywords

magnesioaxiniteelectron microprobemossbauer spectroscopyhydrothermal veinslazanybrno batholithczech republic