Mineralogy, petrogenesis, and SHRIMP U–Pb age of the “Ore Horizon 330” sulfide PGE-Cu-Ni deposit from the Paleoproterozoic layered Monchegorsk pluton, Kola Region, Russia

Abstract

The results of a comprehensive study of the Ore Horizon 330 (OH330) sulfide platinum group elements (PGE)-Cu-Ni deposit are presented. OH330 is located within the Sopcha intrusion orthopyroxenites of the layered Monchegorsk pluton (Monchepluton) in the Kola Region (Russia). It is a sill-like body, 3300 m long, 1200 m wide, and 4–6 m thick. The main rock-forming minerals of OH330 were olivine (Fo_87–84) and orthopyroxene (En_84–83). Light rare-earth element (LREE) fractionation is typical for all OH330 rocks, with the (Ce/Sm)_N (where the subscript N indicates normalization to the primitive mantle) values of 1.21 in dunite, 1.33–1.86 in harzburgite, and 1.86–2.15 in orthopyroxenite. The trace elements exhibited positive U and Ta anomalies in dunite and harzburgite and negative Nb and Ta anomalies in orthopyroxenite. The following crystallization parameters of the OH330 rocks were determined: liquidus temperature, 1220 °C; solidus temperature, 1190 °C; and pressure, 2.4 kbar. The SHRIMP (sensitive high-resolution ion microprobe) U–Pb age of magmatic zircon from the OH330 orthopyroxenite is 2492.5 ± 4.1 Ma. This indicates that OH330 was younger than the rocks of the Monchepluton ultramafic subchapter (about 2506 Ma). The calculated parental melt of OH330 has a boninite-like composition with an MgO content of 14 wt% and is contaminated with crustal material. The formation of OH330 occurred as a result of additional magma injection associated with the activation of a mantle diapir. Thus, the OH330 body is probably the ultramafic cumulate part of an intrusion associated with Paleoproterozoic continental rifting.