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

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.% Cr 2 O 3 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 O M 3+ + O □ = 3 O M 2+ and O M 3+ + T Al = O M 2+ + T Si substitution mechanisms.Chromite is dominated by Fe 2+ (0.82-0.90 apfu) and Cr (1.38-1.79apfu).The listvenite contains ~0.3 wt.% Cr 2 O 3 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 SiO 2 , MgO, and Cr 2 O 3 contents, the latter two typical of ultrabasic rocks.The more advanced alteration stage shows lower SiO 2 , 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 CO 2 -rich fluids that transformed the serpentinized peridotite into the carbonate-quartz listvenite.