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

Abstract

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°M3+ + °□ = 3°M2+ and °M3+ + TAl = °M2+ + 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.