X-ray Microanalysis Phase Map on Rare Earth Minerals with a Conventional and an Annular Silicon Drift Detector
Rare earth elements (REEs) have become important components in over 200 products across an extensive range of applications, causing the demand for these elements keeping rising and the rare earth industry keeping flourishing [1]. The Nechalacho rare earth deposit, located in the Northwest Territories in Canada, is one of the most important REE sources in the world [2]. The aim of this work is to use one high spatial resolution and high count rate scanning electron microscope/x-ray energy dispersive spectrometry (SEM/EDS) system to characterize rare earth minerals (REMs) in this deposit using phase maps. The Xray microanalysis was achieved by a conventional silicon drift detector (SDD) with 60 mm collecting area located on the side of specimen and an annular SDD with a same collecting area inserted below the objective lens. The position of the annular detector provides the shortest working distance and the largest solid angle, allowing the X-ray microanalysis with a count rate as high as 2,000,000 cps and short acquisition time. Because of the high count rate, the relatively low accelerating voltage can be used, providing a high spatial resolution, making the characterisation of small features in micron-scale possible. The traditional EDS elemental map shows distribution of elements, but it cannot provide enough information for phase identification. The problem is more obvious in geological materials, where the samples have numerous micro phases with unknown composition. In order to overcome that, we develop a phase map method using the traditional EDS map and the f-ratio method [3]. The method was applied to acquire the compositional information of REM and to show their distribution.
Controls on the Concentration of Zirconium, Niobium, and the Rare Earth Elements in the Thor Lake Rare Metal Deposit, Northwest Territories, Canada
The Thor Lake rare metal (Zr, Nb, REE, Ta, Be, Ga) deposits in Canada’s Northwest Territories represent one of the largest resources of zirconium, niobium, and the heavy rare earth elements (HREE) in the world. Much of the potentially economic mineralization was concentrated by magmatic processes. However, there is also evidence of autometasomatic processes and remobilization of Zr and REE by hydrothermal fluids.

The deposits are situated at the southern edge of the Slave province of the Canadian Shield, within the 2094 to 2185 Ma alkaline to peralkaline Blachford Lake Intrusive Complex. A layered alkaline suite dominated by aegirine nepheline syenite occurs in the center of this suite of rocks and is considered to represent the youngest phase of the complex.

Much of the rare metal mineralization occurs in two subhorizontal tabular layers, which form upper and lower zones of the Nechalacho deposit (formerly the Lake zone), and in which Zr is hosted primarily by zircon, Nb primarily by ferrocolumbite and fergusonite-(Y), and HREE by fergusonite-(Y) and zircon. The LREE are present mainly in monazite-(Ce), allanite-(Ce), bastnasite-(Ce), parisite-(Ce), and synchysite-(Ce). Much of the HREE mineralization in the lower mineralized zone occurs in secondary zircon, which forms small (10–30 μm) anhedral grains in pseudomorphs after probable eudialyte. In the upper zone, zircon is a magmatic cumulate mineral, which was replaced locally by secondary REE-bearing minerals. Element distribution maps of zircon crystals in the upper zone indicate that the HREE were mobilized from the cores and locally precipitated as fergusonite-(Y) along microfractures. The light rare earth elements (LREE) were also mobilized locally from both primary zircon and inferred primary eudialyte. The occurrence of zircon in fractures, wrapped around brecciated K-feldspar fragments, and as a secondary phase in pseudomorphs are evidence of its hydrothermal origin and/or of remobilization of primary zirconium.

A model is proposed in which injection of separate pulses of miaskitic and agpaitic magma resulted in the crystallization of an upper zone rich in zircon and a lower zone rich in eudialyte. Primary eudialyte was later altered in situ to zircon-fergusonite-(Y)-bastnasite-(Ce)-parisite-(Ce)-synchysite-(Ce)-allanite-(Ce)-albite-quartz-biotite-fluorite-kutnahorite-hematite–bearing pseudomorphs by an inferred fluorine-enriched magmatic hydrothermal fluid. Zirconium, niobium, and REE in both the upper and lower zones were subsequently mobilized during multiple metasomatic events, which, for the most part, served to further enrich the primary layers in REE (albitization generally dispersed REE and high field strength elements (HFSE)) and created new secondary REE-bearing phases.
The origin and mode of emplacement of lujavrites in the Ilímaussaq alkaline complex, South Greenland
Abstract Lujavrites are rare meso- to melanocratic agpaitic nepheline syenites that are characterized by elevated contents of elements such as Li, Be, Zr, REE, Nb, Th and U. They are the most evolved members of the three large composite agpaitic complexes – Lovozero, Kola Peninsula, Russia; Pilansberg, South Africa; and Ilimaussaq, South Greenland – and are inferred to stem from the same deep fractionating magma sources that fed the earlier members of the complexes. The composition of the melts that evolved into lujavrites is, however, not well known. The agpaitic part of the Ilimaussaq complex is divided into a roof series, a floor series of cumulates and an intermediate series of lujavrites sandwiched between the two. In the traditional view, the lujavrites formed from residual melts left between the downward crystallizing roof series and the floor cumulates. New field observations and geochemical data suggest that the floor cumulates and the main mass of lujavrites constituted a separate intrusive phase which was emplaced into the already consolidated roof series rocks largely by piecemeal stoping. Studies of the contact facies of the floor cumulates indicate that the initial magma of the floor cumulate–lujavrite sequence was peralkaline nepheline syenitic with enhanced contents of Zr, Hf, HREE, Y, Nb, Ta, F, Ba and Sr. Subsequent crystallization in a closed system resulted in the formation of the floor cumulates and lujavrites. Chemical analyses of dykes within and outside the complex represent stages in the magmatic evolution of the agpaitic rocks.