Eudialyte Geochronology

Abstract

The Proterozoic Kipawa Syenite Complex and Red Wine Intrusive Suite have both been explored as potential REE ore bodies and are a heretofore unexploited REE resource. This study improves upon the internal-isochron eudialyte geochronology method developed by Sjöqvist et al. (2020) through the addition of Electron Microprobe mapping prior to precise MicroMill sampling to build Sm/Nd internal mineral isochrons to directly date this potential rare earth element ore mineral. We show that Nb and Ta concentrations correlate well with Sm/Nd ratios in zoned eudialyte crystals, providing a qualitative map to guide microsampling. At the Kipawa Syenite Complex two internal eudialyte isochrons yield ages of 1066±56 Ma (MSWD=1.7) and 1109±53 Ma (MSWD=1.2) while a multi-sample eudialyte bulk isochron produces an age of 1092±53 Ma (MWSD= 1.5). The weighted average of the three isochrons is 1090±31 Ma, and gives the age of eudialyte formation across the Kipawa Syenite Complex. Nd model ages confirm derivation from older continental crust with TDM=2.28. At the Red Wine Intrusive Suite single internal eudialyte isochron yields an age of 765±240 Ma (MSWD=3.7) while the high-Nb sector of this crystal yields an age of 704±120 Ma (MSWD=1.6). A multi-sample eudialyte and mosandrite bulk isochron produces an age of 989± 150 Ma (MSWD=15). The latter age reflects original Grenvillian crystallization of REE ore-minerals, while the age of the high-Nb zone reflects a younger, heretofore unrecognized recrystallization event. Nd model ages suggest derivation from the Proterozoic crust with TDM=1.80. Examination of Nd model ages and geochemical data from five agpaitic deposits (Red Wine, Kipawa, Ilímaussaq, Norra Kärr, Lovozero) reveals three distinct deposit types identified as the Lovozero type, the Grenville type, and the Kipawa type.