The influence of magmatic and volatile evolution upon lanthanoid fractionation within peralkaline magmas (Norra Kärr, Sweden)

Abstract

The lanthanoid elements, and in particular the heavier lanthanoids, are essential in the manufacture of advanced technologies due to their unique magnetic, spectral and luminescence properties. However, despite advances in the understanding the behaviour of lanthanoids within chemistry, physics and materials science, the behaviour of these elements within geological systems and in particular the processes by which they become concentrated to form economic ore deposits is still poorly understood.The southern Swedish Norra Kärr lanthanoid-deposit represents a significant European lanthanoid resource of 0.19 Mt T-LnO, with the principal ore domains exhibiting ore-grades between 0.48 -0.69 % T-LnO. Despite its economic significance, the formation of the Norra Kärr intrusion and its economic ore bodies is still not fully understood. However, the intrusion represents an ideal locality to study the concentration of lanthanoids within a peralkaline magma due to the presence of an economically important ore resource; a particular bias towards the economically important heavy- [Ln]; and zonation between light and heavy-[Ln] within separate ore domains.Previous work on the intrusion has primarily focused on analysing the main ore-domains at the expense of other lithologies with the data collected used to develop models for the whole intrusion. This study has analysed the petrology, minerology and whole rock geochemical characteristics of all the lithologies of Norra Kärr and placed the intrusion into the wider structural context of deformation within the Baltic shield at the time of emplacement. These new data and interpretations have been used to test the existing models and develop a new model for the emplacement of Norra Kärr; to better contextualise the formation of the ore domains; and to offer a new model for the light-[Ln] vs heavy-[Ln] zonation observed within the ore-body as a whole.This study has demonstrated that Norra Kärr was emplaced in two magmatic phases, possibly as a series of dyke intrusions; with the first phase magmas more enriched in [Ln] than the second phase magmas. The ore domains developed as a series of cryptic units due to fractionation of the Grennaite-portion of the first magmatic phase. The [Ln]-concentrations were shown to initially increase from the Contact Grennaite to the Pegmatite-Host Grennaite, before enriching in the light- [Ln] and depleting in the heavy- [Ln] as observed in the Crenulated Grennaite. The second phase magmas (Pulaskite-type) display a different minerology to the first phase partially due to due to higher aHF and a more reduced magmatic chemistry. These second phase magmas may have been emplaced due to the effects of ongoing shearing of the intrusion, with the Lakarpite developing its complex chemistry and petrology due to interaction with the still warm Pegmatite-Host Grennaite.This study has demonstrated that a progressive increase in [Ln] within a melt is possible due to aegirine crystallisation, and that the observed variation in light-[Ln] vs heavy-[Ln] within th...