Mineralogy, Petrogenesis and SIMS SHRIMP U-Pb Age PGE-Cu-Ni Deposit of the “Ore Horizon 330” of the Sopcha Intrusion in the Paleoproterosoic Monchegorsk Pluton, Kola Region, Russia

Abstract

The “Ore Horizon 330” deposit (or OH330) is located among the orthopyroxenite of the Sopcha intrusion of the Paleoproterozoic layered Monchegorsk pluton (or Monchepluton) in the Kola Region. It is a sill-like body with a length of 3300 m, width of 1200 m, and thickness of 4‒6 m. OH330 was studied in two sections of its north-western part with a thickness of 5.2 and 5.5 m. There, it consists of regularly alternating interlayers (from bottom to top) of dunite, harzburgite, and orthopyroxenite. Olivine (Fo_87–84) and orthopyroxene (En_84–83) in the OH330 rocks were depleted in Ni compared to the Monchepluton rocks and were similar in Mn content. The total rare earth element (REE_tot) content did not exceed 1 ppm in dunite and harzburgite, with an average value of 3.15 ppm in orthopyroxenite. 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 on average. 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, in addition to positive U anomalies. The geochemical features of the OH330 rocks were determined by fractional crystallization and crustal contamination of the parental magma. The liquidus temperatures of dunite and harzburgite magmatic crystallization were determined using the olivine-melt thermometer, with values of 1600‒1470°C. This is due to the genetic link between the OH330 parental melt and the mantle diapir. The orthopyroxene-melt equilibrium temperatures the OH330 rocks were 1290‒1120°C. The calculated pressure values for the OH330 rocks varried from 1 to 6 kbar, with an average value of 3.5 ± 1.5 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 age of the marginal zone of the Monchepluton ultramafic subchamber. A concordant U-Pb age of 2818.0 ± 3.1 Ma was obtained from the xenocryst zircon population, which corresponds to the age of the Archean granitoid basement rocks.