Differences in Sm/Nd ratios between early magmatic and late sulfides: The role of fluids and Nd mobility

Abstract

This paper presents the results of a SmNd study of sulfide minerals and whole-rock samples from Cu-Ni-PGE layered complexes of the Fennoscandian Shield. Syngenetic (early) and epigenetic (late) sulfides were analyzed in each complex using the SmNd method. Late sulfide minerals with low Sm/Nd values (the ¹⁴⁷Sm/¹⁴⁴Nd ratio is often in the range of 0.02–0.07) are associated with an increased mobility of Nd relative to Sm, resulting in a relative excess of Nd compared to Sm in these sulfides. Simultaneously, early sulfides, which are deposited during the magmatic stage of ore formation, typically exhibit higher Sm/Nd values (the ¹⁴⁷Sm/¹⁴⁴Nd ratio is frequently above 0.07). Additionally, SmNd isotope data for sulfide minerals were used to date ore-forming processes in two Cu-Ni-PGE complexes—Nyud-II (Monchegorsk area, Russia) and Ahmavaara (Finland). The SmNd ages of syngenetic and metamorphic ore from these complexes were determined. Syngenetic ores formed at 2496 ± 36 Ma (Nyud-II) and 2441 ± 93 Ma (Ahmavaara), while metamorphic ores formed at 1940 ± 32 Ma (Nyud-II) and 1904 ± 24 Ma (Ahmavaara). Thus, SmNd isochrons yield the timing of sulfide mineralization and its relationship with the ages of the rocks containing it, while Sm/Nd ratios in sulfides help understand the processes of ore formation. A comprehensive analysis of the full isotopic dataset (this study and other published data) showed the potential of using SmNd isotope data to trace the sequence of sulfide mineralization, which has been confirmed for some hydrothermal deposits. However, this sequence has not been confirmed for magmatic sulfides; this opens up the possibility for further research.