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.
Igneous Rock Associations 23. The Bushveld Complex, South Africa: New Insights and Paradigms
SUMMARYThe Bushveld Complex has continued to serve as the basis for study into the fundamental nature of petrological processes for layered intrusion formation and for oxide and sulphide hosted Platinum Group Element (PGE)–Cu–Ni ore deposits. These studies have included discoveries in terms of the physical extent of Bushveld magmatism, both laterally and internally. Lateral variations in the mafic to ultramafic Rustenburg Layered Suite of the Northern Lobe of the complex have also revealed petrologically distinctive Upper Critical Zone equivalent rocks (the so-called Flatreef) with enhanced contamination and mineralization traits that reflect a transition between Eastern and Western Lobe equivalent stratigraphy and Platreef-style complexity. Traditional magma mixing models have been re-examined in light of radiogenic isotopic evidence for crustal involvement early in the chromite precipitation or formation process, combined with evidence for associated heterogeneous fluid contents, cryptic layering profiles, and textural evidence. A wide variety of alternative ore-genesis models have been proposed as a consequence. The fundamental mechanics of magma chamber processes and the existence of the magma chamber as an entity have been called into question through various lines of evidence which have promoted the concept of progressive emplacement of the complex as a stack of not-necessarily-quite-sequentially intruded sills (with or without significant quantities of transported phenocrysts), emplaced into variably crystallized and compacted crystal-liquid mush mixtures, modified by compaction-driven late magmatic fluid (silicate and aqueous) activity. Alternatively, petrological and geochemical observations have been used to discount these interpretations in favour of more conventional cooling and gravity-driven accumulation of silicate and ore minerals in a large, liquid-dominated system.RÉSUMÉLe complexe de Bushveld a demeuré à la base d’études sur la nature fondamentale des processus pétrologiques de formation d’intrusions litées et des gîtes des éléments du groupe platine (ÉGP)-Cu-Ni hébergés dans les oxydes et les sulfures. Ces études ont comporté des découvertes sur l’étendue physique, à la fois latérale et interne, du magmatisme de Bushveld. Les variations latérales de la suite stratifiée et mafique à ultramafique Rustenburg du lobe nord du complexe ont également révélé des roches équivalentes pétrologiquement distinctes de la zone critique supérieure (le communément désigné Flatreef) avec des traits de contamination et de minéralisation accrus qui reflètent une transition entre la stratigraphie équivalente des lobes est et ouest et la complexité de type Platreef. Les modèles traditionnels de mélanges magmatiques ont été réexaminés à la lumière de preuves isotopiques radiogéniques indiquant une implication de la croûte au début du processus de précipitation ou de formation de la chromite, combinées à des preuves de contenu fluide hétérogène associé, de profils de litage cryptique et de preuves texturales. Ainsi, une grande variété de modèles alternatifs de genèse de minerai a été proposée. La mécanique fondamentale des processus de la chambre magmatique et l'existence de la chambre magmatique en tant qu'entité ont été remises en question au moyen de divers éléments de preuve qui ont mis en avant le concept de mise en place progressive du complexe sous forme d'un empilement non-nécessairement séquentiel de sills injectés (avec ou sans quantités significatives de phénocristaux transportés) mis en place dans des mélanges de bouillie cristaux/liquide à cristallisation et compaction variable, modifiés par une activité tardive de fluide magmatique (silicaté et aqueux) induite par la compaction. Alternativement, des observations pétrologiques et géochimiques ont été utilisées pour écarter ces interprétations en faveur d'un processus plus conventionnel de refroidissement et d’accumulation de minérais silicatés et minéralisés induite par la gravité dans un vaste système à dominance liquide.