Rare-Earth Minerals in Ultrabasites of the Ildeus Massif (Stanovoy Cratonic Superterrane): Influence of Post-Collision Processes on Deep Ore-Magmatic Systems of Convergent Plate Boundaries

The Ildeus ore-magmatic basite–ultrabasite system in the central part of the Stanovoy superterrane formed in the Triassic as a result of subduction of oceanic lithosphere of the Mongol–Okhotsk basin. In the Jurassic and Early Cretaceous, it underwent hydrothermal–metasomatic alterations during collision and postcollision processes within the southern margin of the Siberian continent. Hydrothermally altered ultrabasites and felsic–alkaline albite–quartz–biotite–K-feldspar–apatite (with calcite and barite) metasomatites of the Ildeus contain rare earth element (REE) minerals, represented by monazite in association with apatite, xenotime, oxides, REE carbonates, and REE silicates. Native metals and alloys of Ag, Au, Cu, Ni, Zn, Pt and sulfides of Fe, Co, Ni, Cu, Pb, and Zn are associated with REE minerals in rocks of the massif. The total REE content in these rocks varies from 10 to 1938 ppm, correlating positively with the concentrations of barium (up to 5876 ppm) and thorium (up to 74.5 ppm). Alkaline-silicate metasomatites with REE minerals are spatially associated with adakite dikes and veins (Sr/Y from 27 to 1716) and were formed in the presence of aqueous–carbonate–sulfate–phosphorus–potassium fluids with hybrid properties characteristic of the fluid phase of mantle alkaline–ultrabasic–carbonatite complexes, on the one hand, and lower crustal adakitic melts, on the other. It is suggested that the enrichment Ildeus ultrabasites and metasomatites with rare earth minerals, metals, and sulfides is associated with the melting of the sulfidized thickened lower crust under the influence of deep mantle fluids and intrusion of Early Cretaceous metal-bearing adakites into Triassic suprasubduction igneous complexes.