Composition and origin of rare-metal (Nb–Ta, REE) and sulfide mineralization in magnesiocarbonatites from the Yenisei Ridge, Central Siberia

Abstract

The Penchenga Neoproterozoic linear fenite-carbonatite complex in the Yenisei Ridge consists of several magnesiocarbonatite sheet-like bodies which lie nearly concordantly with the metamorphic country rocks and are surrounded with alkaline metasomatic fenite aureoles. Their mineralogy includes primary high-temperature phases (ferrodolomite, calcite, phlogopite, sodic amphibole of the eckermannite – magnesio-arfvedsonite series, fluorapatite, magnetite, and ilmenite), late-magmatic accessories (fluorocalciopyrochlore, pyrrhotite, pyrite, Cu-Pb-Zn-Ag sulfide and hessite, monazite-(Ce) and REE-carbonates of synchysite, cordylite, cebaite, and ancylite), as well as secondary phases (Sr-Ba pyrochlore, ferrocolumbite, fersmite, and Fe and Sr carbonates) of hydrothermal and supergene origin that replace primary magmatic minerals. According to their main features of mineralogy and chemistry, the carbonatites correspond to magnesian derivates of a high-pressure alkali-dolomite melt that forms by melting of carbonated peridotite, as obtained in experiments. The Nd, Sr, and Pb isotope compositions of the Penchenga rocks and minerals (εNd ≈ +6; εSr ≈ −19; ∼0.81 207Pb/206Pb; ∼1.99 208Pb/206Pb) suggest a mantle PREMA + EM 2 source of carbonatite magma and mineralization (including Nb and REE). The presence of carbonates with relatively high δ18O and pyrrhotite enriched in δ34S and 207Pb may be due to continental crust inputs to the source of metals. As estimated approximately from the composition of the pyrrhotite-sphalerite assemblage, sulfide minerals crystallize at a depth of ∼30 km in the lower crust, under a pressure of ∼8 kbar. Degassing of the juvenile alkali-dolomite melt and its interaction with meteoric waters leads to fenitization and formation of late magmatic CO2–H2O fluids. These “carbohydrothermal” fluids form from salt melts, and increase in H2O upon cooling from 600 to 200 °C. At lower temperatures of 480–240 °C, the O–H isotope systems of magmatic minerals undergo inversion and ensuing closure. REE-bearing mineral phases precipitate from REE-carrier fluids.