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

Abstract The results of a comprehensive research of the Ore Horizon 330 (OH330) deposit, which is located within the orthopyroxenite of the Sopcha intrusion of the layered Monchegorsk pluton (Monchepluton) in the Kola Region (Russia), are presented. The OH330 is a sill-like body 3300 m long, 1200 m wide, and 4–6 m thick. On its western flank, a regular alternation (from bottom to top) of interlayers of dunite, harzburgite and orthopyroxenite is observed. In term chemical compositions, olivine (Fo87–84) and orthopyroxene (En84–83) in the OH330 rocks have a similar compared to the Monchepluton rocks, differing only in lower Ni contents. The geochemical features of the OH330 rocks were defined by fractional crystallization and crustal contamination of the parental magma. The light rare earth elements (LREE) fractioning is typical of all OH330 rocks, with the (Ce/Sm)N values of 1.21 ppm in dunite, 1.69 ppm in harzburgite, and 1.81 ppm in orthopyroxenite. The trace element distribution in dunite and harzburgite is characterized by U, Ta, and Sr positive anomalies, whereas orthopyroxenite exhibits Nb and Ta negative anomalies. The following PT crystallization parameters were determined: liquidus temperature 1600–1470 °C, sodidus temperature 1290–1120 °C, and pressure 3.5 kbar. The SIMS SHRIMP U-Pb magmatic zircon age of the OH330 orthopyroxenite is 2492.5 ± 4.1 Ma, indicating that the OH330 is younger than the marginal zone rocks of the Monchepluton ultramafic subchamber. A concordant U-Pb age of 2818.0 ± 3.1 Ma was obtained for the xenocryst zircon population, which corresponds to the age of the Archean granitoid basement rocks.