Mineral Assemblages of Chromitites of the Alapaevsk Dunite–Harzburgite Massif (Middle Urals)

A scheme of the sequence of mineral formation in chromite ores has been developed. This scheme includes primary and secondary early and late mineral assemblages. Primary assemblages of high-chromium (Cr2O3 > 50 wt %) and alumina-rich (Cr2O3 < 50 wt %) ores are represented by the same main minerals—chrome spinel, clinopyroxene, and olivine with characteristic differences in the compositions of these minerals for each type of ores. Accessory minerals of primary assemblage are synchronous with chrome spinel and are represented by pentlandite, Cu pentlandite, chalcopyrite, pyrrhotite, and bornite, as well as platinum-group minerals (laurite RuS2, erlichmanite OsS2, native osmium, and copper gold). There are minerals of an early secondary assemblage within polyphase inclusions in chrome spinel. These polyphase inclusions consist of Cr-chlorite, amphibo, garnet, sulfides (millerite, heazlewoodite), and noble metals (Ni,Cr-bearing copper, nickel-bearing copper, (Cu,Fe,Cr)-bearing nickel, awaruite). Minerals of noble metals of an early secondary assemblage represented by laurite, Pt and Pd arsenides and stibnides, Ru-pentlandite and high-grade native gold were only found in alumina-rich ores. The late secondary assemblage is represented by the intergrowths of native copper and awaruite with serpentine within high-chromium chrome spinel. The temperature formation conditions of secondary assemblages were estimated using a chlorite geothermometer. The formation temperatures of the chlorites studied from chromitites ranged from 250 to 284°С. Minerals of noble metals in secondary assemblages formed at temperatures below 350°C together with garnet, amphibole, chlorite, and nickel sulfides. Grains of primary Os–Ir–Ru alloys underwent sulfurization during epigenetic processes, forming a fine-grained porous mixture of native and sulfide phases, sometimes with As, as well as the replacement by Ru-pentlandite. The presence of awaruite and native Cu and Ni in both primary and secondary assemblages of chromitites indicates the reducing conditions under which noble-metal minerals formed. The limited development of high-chromium ores, along with the processes of sulfurization of the primary grains of Os–Ir–Ru alloys, has led to the scarcity of detrital platinum mineralization in the Alapaevsk massif area.