The partition coefficients of Nd and Sm for the sulphides: first data from Palaeoproterozoic layered Cu-Ni-PGE complexes of Fennoscandian Shield

The Sm-Nd systematics is one of the most demanded isotope-geochronological tools to study ancient geological complexes. With the accumulation of knowledge about the REE in various geological processes, the question arises of extending the capabilities of the Sm-Nd method by using new mineral geochronometers. The research focused on defining the time of the ore process and its position in the general geochronological scale of formation of the geological site become particularly important. There is a pressing need for defining possible forms of REE occurrence in a lattice of geochronometer minerals in the Sm-Nd study of accessory minerals (e.g. fluorite, burbankite, eudialite, ruthile, etc.) and ore minerals (ilmenite, chrome-spinellid, sulfide minerals). The Sm-Nd method of dating ore processes using sulphide minerals, successfully used on several geological objects, made it possible to determine the main stages of ore formation and confirm geochronologically the conclusions about the syngenetic or epigenetic nature of the ore process.

Pyrite, pentlandite, chalcopyrite and pyrrhotite from the main industrial fields of the Fennoscandinavian shield were studied: Monchegorsk pluton, Fedorovo-Pansky intrusion, Pechenga, Penicat intrusion and Ahmavaara (Finland). Using a mass-spectrometric method 35 sulphide monofractions were analyzed. The partition coefficients for Nd and Sm were established: for pyrite - 0.229 (Nd) and 0.169 (Sm); for pyrrhotite - 0.265 (Nd) and 0.160 (Sm); for chalcopyrite - 0.229 (Nd) and 0.161 (Sm); for pentlandite – 0.158 (Nd) and 0.082 (Sm). The mean values for D_Nd are 0.201, for D_Sm=0.145 and resulting D_Nd/D_Sm about 1.4.

Probably, the distribution of REE in sulfide minerals is inherited from fluids during sulfide formation. REE concentrations in sulphide may reflect the composition of the fluid.

Thus, for the first time data on Sm and Nd concentrations have been obtained by mass spectrometry. Coefficients of neodymium and samarium distribution in sulfides have been calculated for major Cu-Ni-PGE complexes of Fennoscandia.

 

This study performed under the theme of scientific research 0226-2019-0053 and were supported by the RFBR  18-05-70082.