Rare-Earth Mineralization in Rift Formations of the Ishlya Graben (Western Slope of the Southern Urals)

Rift formations of the Ishlya graben, widespread on the western slope of the Southern Urals (Bashkir meganticlinorium), are represented by alternating terrigenous rocks (carbonaceous shales, siltstones, and siltstones) of the volcanoplutonic association (gabbrodolerites and basic effusives with a small amount of pyroclastic material). Terrigenous rocks of the Ishlya graben contain rare-earth mineralization: allanite-(Ce), REE-bearing epidote, monazite-(Ce), xenotime-(Y), chevkinite-(Ce), fergusonite-(Nb), rare-earth fluorocarbonates (bastnaesite-(Ce), hydroxyl bastnaesite-(Ce), parisite-(Ce), and synchysite-(Ce) characterized by a wide variety of morphological types and the presence of complex associations. The chemical composition of metamorphic minerals was used to determine the P–T parameters of rock metamorphism (T = 250–600°C, P = 2–10 kbar), the chemical composition and temperature of the fluid phase (CaCl2 + NaCl, T = 180–408°C for primary inclusions and FeCl2, T = 121–248°C for secondary ones), as well as the hematization temperature (465–593°C) and re-equilibration of the ilmenite–titanomagnetite association (T = 501–576°C at oxygen fugacity from –23.15 to –21.25). It is shown that the processes of rare-earth mineralization in the natural environment are diverse and multifactorial, with the chemistry of the local-scale mineralization environment being of great importance. The comparative analysis of the chemical composition of monazite and xenotime revealed that the primary source of rare-earth phosphates in channel and alluvial deposits were represented by the Riphean–Vendian metamorphosed rocks in the eastern subzone of the Bashkir meganticlinorium.