Monchegorsk Mafic–Ultramafic Layered PGE-Bearing Complex (2.5 Ga, Kola Region, Russia): On the Problem of Relationships between Magmatic Phases Based on the Study of Cr-Spinels

Abstract

The composition of Cr-spinels from rocks of the Monchegorsk layered complex (2.5 Ga) basically corresponds to the evolutionary trend that is typical for layered mafic–ultramafic intrusions (late magmatic phases contain Cr-spinels enriched in Fe and depleted in Mg, Cr, and Al). Cr-spinels within the Dunite Body of the Sopcha massif are almost identical to those within the Dunite Block rocks and are close to those from harzburgite of the NKT massif. Cr-spinels within the satellite bodies of the Ore Layer 330 are shown to have zonal structure, which confirms their origin from a new portion of melt, which may have been injected with several pulses. The composition of accessory Cr-spinels may indicate that the layered complex of rocks of the South Sopcha massif was formed from the most evolved portion of magmatic melt (linked with the Monchetundra intrusion), and its vein complex may be considered the one formed at the final stages of the magmatic system evolution. The composition of Cr-spinels from the Pentlandite Gorge mafic–ultramafic rocks may indicate that they are fragments of the NKT massif and not of the Monchetundra massif, as it was believed earlier.