Nb mineralization in the nepheline syenite in the Saima area of the North China Craton, China

Abstract

The Triassic Saima alkaline complex on the northern margin of the North China Craton (NCC) is composed mainly of nepheline syenite, along with some nepheline syenite pegmatite and mineralized nepheline syenite, and it contains REEs (bastnäsite) + Nb (Nb-rich rutile and pyrochlore) and Zr (zircon) mineralization. However, the effects of magmatic and hydrothermal processes on Nb enrichment and mineralization remain unclear. Thus, we conducted a comprehensive study of the Nb mineralization in the Saima alkaline complex. Biotite in the altered nepheline syenite and pegmatite has inverse isochron 40Ar/39Ar ages of 234.30 ± 6.30 and 230.08 ± 2.75 Ma, respectively. Together with the close spatial relationship and the reported ages of the aegirine nepheline syenite associated with Nb mineralization, we propose that the altered and mineralized nepheline syenite and pegmatite were formed at the same time. In addition, clinopyroxene and biotite compositional trends, an increase in the proportion of hydrous minerals including natrolite, hydromuscovite, and arfvedsonite, and the growth of K-feldspar suggest that the magma evolved from nepheline syenite to mineralized nepheline syenite and nepheline syenite pegmatite. The altered nepheline syenites yield low initial 87Sr/86Sr ratios (0.709625–0.712132) and negative εNd(t) values (−15.32 to −14.15), consistent with previous studies of fresh samples, suggesting that the Saima alkaline rocks were produced by the partial melting of metasomatized lithospheric mantle. The early-crystallized magmatic Nb-rich titanite and rutile in the nepheline syenite and pegmatite indicates an initial Nb-rich parent magma, and fractional crystallization of Nb-poor minerals increased the Nb content of the residual melts. The crystallization of Nb minerals (pyrochlore) and Nb-rich minerals (altered rutile, titanite, and ilmenite) may have been the result of metasomatism by post-magmatic Na- and F-rich fluids, indicating the redistribution and precipitation of Nb. Thus, we conclude that Nb was enriched through magmatic processes, and the Nb mineralization was related mainly to hydrothermal activity.