Mineralogical and geochemical zoning and mass transfer between the Khudolaz complex water-saturated gabbro and the Zilair Formation sandstones (Vostochnyi Buskun, Southern Urals)

   Research subject. The contact aureole rocks of the Vostochnyi Buskun multiphase gabbroic intrusion of the Khudołaz differentiated complex (Southern Urals).

   Aim. To study geochemical interactions between the magmatic body and host volcanogenic-sedimentary rocks, to determine the physicochemical conditions of metamorphism and metasomatism, to identify mineralogical and geochemical zoning of the contact aureole, and to assess the metallogenic potential of the rocks.

   Materials and methods. Minerals were studied by optical and scanning electron microscopy; the bulk composition of rocks was determined by XRF and ICP–MS. Contact metamorphism temperatures were determined using biotite and chlorite geothermometers.

   Results. Several types of gabbroids in the Vostochnyi Buskun massif have been identified. The contact aureole of the massif is composed of three main types of rocks, corresponding to the interaction between taxitic leucogabbro and host volcanogenic-terrigenous rocks: hypersthene and clinopyroxene hornfelses in direct contact with the intrusion rocks, and chl ± ms ± bt hornfelses, forming the greater part of the exocontact. Clinopyroxene metasomatites, in comparison with more distant from the contact hornfelses, are characterized by an addition of Ca and Sr, and removal of Ti, Fe, Mg, Li, Rb, and Ba. The mineral and chemical composition of hypersthene hornfelses correspond to mafic igneous rocks and show similarities with beerbachites. Poor ore mineralization in the contact aureole rocks is represented by ilmenite, copper, and iron sulfides, including platinum-bearing pyrrhotite in chlorite-mica hornfelses.

   Conclusion. The Vostochnyi Buskun massif was formed via at least three petrogenetic phases, which resulted in the multi-stage metamorphic and metasomatic alteration of host rocks. The contact aureole formation is associated with injection of second-phase fluid-saturated taxitic olivine-hornblende leucogabbro, which were in direct contact with the host volcanogenic-sedimentary strata. The contact aureole formed in two stages: (1) a high-temperature stage (526–616 °C) accompanied by pegmatoid segregations in gabbro, hypersthene, clinopyroxene, and biotite-bearing hornfelses; and (2) a low-temperature stage (253–458 °C), which is associated with dolerite dikes and propylitic quartz-chlorite-albite veins, as well as chlorite replacing biotite. The presence of chlorine-containing apatite and amphibole indicate the participation of chlorine-bearing fluid in metasomatism, while the features of the mass transfer of ore elements indicate the absence of sulfide ore bodies under the Vostochnyi Buskun massif contact zone.

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