Deformation-induced Rare Earth Elements (REE) redistribution in apatite from the Fen Carbonatite Complex (Norway)

The Fen Carbonatite Complex in Norway contains the largest deposit of Rare Earths Elements (REE) in Europe, with estimated resources in the range of 30 – 50 Mt of total Rare Earth Oxides. If Fen will be targeted as an exploitable mineral resource, the geological processes that formed it must be understood, with specific emphasis on what controls the location and composition of the REE resources.

The Fen complex formed at 580 Ma through different stages of carbonatite melt intrusions followed by hydrothermal alteration. Fluid- and melt-assisted deformation accompanied the intrusive and hydrothermal evolution of the Fen Complex extensively and resulted in the formation of shear zones and breccias. However, the mechanism and significance of carbonatite deformation are poorly understood, and so are the effects of post-crystallization deformation processes on the remobilization of trace elements in carbonatites.

This study investigates deformation processes and REE remobilization in shear zones in dolomite-carbonatites from Fen. The shear zones display a compositional banding defined by alternating dolomite- and apatite-rich layers, where apatite grains are variably elongated with aspect ratio ranging from 2 to 11 and grain length from 50 to 500 µm. SEM images reveal the presence of carbonatitic melt pseudomorphs in the form of intergranular beads, cusps, films, and pools, which are particularly evident in the apatite layers, where individual grains are locally entirely rimmed by melt films. The apatite grains appear zoned in cathodoluminescence (CL) images, with dark cores and bright rims that are thicker parallel to the foliation. In the most elongated grains, the dark core forms less than 20% of the grain area, which is otherwise dominated by the bright rim. On the contrary, more equidimensional grains are dominated by the dark core. Hyperspectral analysis of CL images indicates that the elongated rims of apatite are enriched in REE (particularly in Nd) compared to their core. Electron backscatter diffraction (EBSD) analysis demonstrates that (1) the elongated apatite grains are internally strain free, and (2) grain elongation occurs parallel to apatite c-axis.

Our data show that deformation of apatite occurred by melt-assisted dissolution-precipitation creep, which was responsible for grain elongation and remobilization of REE. Thus, post-crystallization deformation and melt-rock interaction played an important role in redistributing REE within the Fen Complex.