Abstract
Lanthanoids or ‘Rare Earths’ are critical to a range of advanced technologies; however, the formation processes that result in concentrated economic deposits of lanthanoids remain enigmatic. This research has focused on the evolution of the Norra Kärr syenite which has measured ore grades up to 0.61% TREO (of which 52.6% are HREO) and an indicated resource of 31.1 Mt at 0.4% TREO cut-off grade (Saxon et al. 2015) making it mainland Europe's largest lanthanoid resource. Core samples and analytical data kindly supplied by Tasman Metals Ltd. has provided an unrivalled opportunity to further investigate the evolution of this complex and to build on the previous studies of Sjöqvist et al. (2013) and Atanasova et al. (2017).
Mössbauer analysis of the Fe2+/Fe3+ ratios of minerals within bulk-rock powders from different lithologies within Norra Kärr shows that the complex is comprised of two distinct intrusive phases with differing redox properties. The first intrusive phase was oxidised, is rich in Fe3+ and contains the ore-bearing eudialyte-group minerals. The second intrusive phase contains non-ore bearing minerals and is more reduced than the first phase with Fe-bearing minerals from enclaves being richer in Fe2+ than anywhere else in the intrusion.
A detailed petrographic study has shown that both phases of intrusions display an overall crystallisation sequence of aegirines followed by zirconosilicate phases and finally feldspars. Electron probe microanalysis (EPMA) of amphiboles, biotites and zirconosilicates has provided data on the volatile content of each phase. These analyses have been used to model the relationships between volatile bearing phases within the magmas at Norra Kärr using the method of Andersen et al. (2010) to show the activity of a given volatile (F−, Cl−, OH−) within each lithology. When this modelling is combined with the petrogenetic sequences observed the two intrusive phases are shown to display two differing trends of volatile evolution: the first phase displays a trend in decreasing OH− activity with fractionation, while the second phase displays a trend in decreasing F− activity with fractionation.
To demonstrate how the magmatic mineral fractionation sequence and volatile fractionation pattern affect the distribution of lanthanoids throughout the magmatic system laser ablation-inductovely coupled plasma-mass spectrometry (LA-ICP-MS) analyses from all the minerals within each of the lithologies at Norra Kärr were collected. These results form the basis of a comprehensive model for understanding the enrichment and depletion of lanthanoids at different stages within the system. Preliminary results show that enrichment of lanthanoids within a magma may be due to the prolonged crystallisation of non-[Ln] bearing zirconosilicate phases due to the relative volatile activities within the magma. The point at which the volatile conditions alter to favour eudialyte crystallisation will therefore dictate whether a concentrated or diffuse ore-body will form. Further work will finalise this model and synthesise the data from Norra Kärr into a holistic ore-deposit model for future exploration of alkaline-igneous ore deposits.