Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Štefan Ferenc, Pavel Uher, Ján Spišiak, Viera Šimonová

Chromium- and nickel-rich micas and associated minerals in listvenite from the Muránska Zdychava, Slovakia: products of hydrothermal metasomatic transformation of ultrabasic rock

Journal of Geosciences, volume 61 (2016), issue 3, 239 - 254

DOI: http://doi.org/10.3190/jgeosci.217

The Cr-Ni-rich micas, Ni-Co sulphide phases and associated minerals occur in a small body of listvenite, an extensively altered serpentinite, in Lower Palaeozoic paragneisses near Muránska Zdychava village in Slovenské Rudohorie Mts. (Veporic Superunit, central Slovakia). The main rock-forming minerals of the listvenite are magnesite, dolomite and a serpentine-group mineral, less frequently calcite, quartz and talc. Accessory minerals of the listvenite include Cr-Ni-rich micas, chromite, and Ni-Co-Fe-(Cu-Pb) sulphide minerals (pyrite, pyrrhotite, pentlandite, millerite, polydymite, violarite, siegenite, gersdorffite, cobaltite, chalcopyrite and galena). The micas from the Muránska Zdychava listvenite (Cr-Ni-rich illite to muscovite and Ni-dominant trioctahedral mica) contain the highest Ni concentrations ever reported in the mica-group minerals (up to 22.8 wt. % NiO or 1.46 apfu Ni). The Cr concentrations are also relatively high (up to 11.0 wt. % Cr₂O₃ or 0.64 apfu). Compositional variations in both Cr-Ni-rich mica minerals are characterized by the negative Cr vs. Ni correlation that indicates a dominant role of 2^OM³⁺ + ^O□ = 3^OM²⁺ and ^OM³⁺ + ^TAl = ^OM²⁺ + ^TSi substitution mechanisms. Chromite is dominated by Fe²⁺ (0.82-0.90 apfu) and Cr (1.38-1.79 apfu).
The listvenite contains 0.3 wt. % Cr₂O₃ and 0.2 wt. % NiO. It represents a good example of multistage transformation of an ultrabasic protolith, reflecting variable alteration. The incipient alteration leads to relatively high SiO₂, MgO, and Cr₂O₃ contents, the latter two typical of ultrabasic rocks. The more advanced alteration stage shows lower SiO₂, but higher content of volatiles (c. 35 wt. % of LOI) bound in carbonates and hydrated silicate minerals. Based on geochemical and mineralogical characteristics, the studied listvenite body originated during three principal evolutionary stages: (1) peridotite stage, (2) serpentinization stage, and (3) hydrothermal-metasomatic stage (listvenitization). The listvenite origin was probably connected with Alpine (Late Cretaceous) late-orogenic uplift of the Veporic Superunit crystalline basement and retrograde metamorphism; we assume P-T conditions of the final listvenite stage at 200 MPa and up to 350 °C. The NE-SW and NW-SE trending fault structures played a key role during the process of listvenitization as they channelized the CO₂-rich fluids that transformed the serpentinized peridotite into the carbonate-quartz listvenite.