The Nature and Origin of the REE Mineralization in the Wicheeda Carbonatite, British Columbia, Canada

TLDR: The Wicheeda carbonatite is a deformed plug or sill that hosts relatively high grade light rare earth elements (LREE) mineralization in the British Columbia alkaline province.

Abstract: The Wicheeda carbonatite is a deformed plug or sill that hosts relatively high grade light rare earth elements (LREE) mineralization in the British Columbia alkaline province. It was emplaced within metasedimentary rocks belonging to the Kechika Group, which have been altered to potassic fenite near the intrusion and sodic fenite at greater distances from it. The intrusion comprises a ferroan dolomite carbonatite core, which passes gradationally outward into calcite carbonatite. The potentially economic REE mineralization is hosted by the dolomite carbonatite. Three types of dolomite have been recognized. Dolomite 1 constitutes the bulk of the dolomite carbonatite, dolomite 2 replaced dolomite 1 near veins and vugs, and dolomite 3 occurs in veins and vugs together with the REE mineralization. Carbon and oxygen isotope ratios indicate that the calcite carbonatite crystallized from a magma of mantle origin, that dolomite 1 is of primary igneous origin, that dolomite 2 has a largely igneous signature with a small hydrothermal component, and that dolomite 3 is of hydrothermal origin. The REE minerals comprise REE fluorocarbonates, ancylite-(Ce), and monazite-(Ce). In addition to dolomite 3, they occur with barite, molybdenite, pyrite, and thorite. Minor concentrations of niobium are present as magmatic pyrochlore in the calcite carbonatite. A model is proposed in which crystallization of calcite carbonatite preceded that of dolomite carbonatite. During crystallization of the latter, an aqueous-carbonic fluid was exsolved, which mobilized the REE as chloride complexes into vugs and fractures in the dolomite carbonatite, where they precipitated mainly in response to the increase in pH that accompanied fluid-rock interaction and, in the case of the REE fluorocarbonates, decreasing temperature. These fluids altered the host metasedimentary rock to potassic fenite adjacent to the carbonatite and, distal to it, they mixed with formational waters to produce sodic fenite.

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The nature and origin of the REE mineralization in the Wicheeda Carbonatite,
British Columbia, Canada
By
Joel Trofanenko
A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment
of the requirements for the degree of Master of Science
Department of Earth and Planetary Sciences
McGill University, Montreal, QC, Canada
December 2014
© Joel Trofanenko 2014

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Abstract
In response to rising demand of the rare earth elements (REE), recent exploration of the
British Columbia alkaline province has identified the Wicheeda Carbonatite, which
contains an estimated 11.3 million tons of light REE-enriched ore grading 1.95 wt.%
TREO, to be the highest-grade prospect known in British Columbia. However, research
of the deposit is restricted to one paper describing mineralization in carbonatite dikes
adjacent to the main plug. This study describes the nature and origin of REE-
mineralization in the Wicheeda plug.
The carbonatite was emplaced in metasedimentary limestone and argillaceous limestone
belonging to the Kechika Group, which has been altered to potassic fenite immediately
adjacent to the carbonatite and to sodic fenite at greater distances from it. The
carbonatite comprises a ferroan dolomite core, which passes outwards gradationally into
calcite carbonatite. Three texturally distinct varieties of dolomite have been recognized.
Dolomite 1 constitutes most of the carbonatite; Dolomite 2 replaced Dolomite 1 near
veins and vugs; Dolomite 3 occurs as a fracture and vug-lining phase with the REE
mineralization. Stable carbon and oxygen isotopic ratios indicate that the calcite
carbonatite is of mantle origin, that Dolomite 1 is of primary igneous origin, that
Dolomite 2 is largely primary igneous with minor hydrothermal signature contamination,
and that Dolomite 3 is of hydrothermal origin. Rare-metal mineralization in the deposit
is, with the exception of pyrochlore, which occurs in the calcite carbonatite, restricted to
veins and vugs in the dolomite carbonatite. There it occurs as hydrothermal veins and in

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vugs infilled by REE-fluorocarbonates, i.e., bastnäsite-(Ce), ancylite-(Ce), and monazite-
(Ce) together with accessory pyrite, barite, molybdenite, and thorite.
A model is proposed in which calcite carbonatite was the earliest magmatic phase to
crystallize. The calcite carbonatite magma saturated with niobium relatively early,
precipitating pyrochlore. The magma later evolved to a dolomite carbonatite composition
which, upon cooling exsolved an aqueous carbonic fluid, which altered the Kechika
metasediments to potassic fenite and mixed with formational waters further from the
carbonatite to produce sodic fenite. This fluid mobilized the REE as chloride complexes
into vugs and fractures in the dolomite carbonatite. Upon progressive fluid-rock
interaction, the REE precipitated largely in response to cooling and pH. Hydrothermal
concentration led to remarkable grade consistency, with virtually all of the dolomite
carbonatite containing >1 wt.% TREO, making the Wicheeda Carbonatite a very
attractive exploration target.

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Sommaire
En réponse à la demande industrielle croissante pour les éléments du groupe des terres
rares (ETR), l’exploration de la province alcaline de Colombie Britannique a récemment
permis d’identifier la carbonatite de Wicheeda (11.3 millions de tonnes de minerai enrichi
en terres rares légères, affichant une teneur de 1.95 % TREO (l’oxide des terres rares
total) comme l’un des prospects de terres rares les plus prometteurs. Les précédentes
investigations menées sur ce gisement se limitent aux rapports des compagnies ayant
exploré la zone pour ses ressources potentielles en terres rares. La présente étude décrit la
minéralisation en éléments des terres rares de la carbonatite de Wicheeda, et combine
l’étude de données pétrographiques et géochimiques dans le but de développer un modèle
génétique pour ce gisement.
La carbonatite de Wicheeda s’est mise en place dans les formations de calcaires
métasédimentaires et calcaires argileux du Groupe de Kechika, qui ont été altérées en
fénite potassique au contact de la carbonatite et en fénite sodique de manière plus distale.
La composition de la carbonatite montre une évolution graduelle depuis son cœur
(dolomite ferrifère) vers sa périphérie (carbonatite calcitique). Trois variétés de dolomites
ont été distinguées sur la base d’évidences texturales. La dolomite 1 est le composant
principal de la carbonatite, la dolomite 2 remplace la dolomite 1 à proximité des veines et
des pores. La dolomite 3 se met en place en fracture et en remplissage de la porosité, et
accompagne la minéralisation en terres rares. Les ratios des isotopes stables du carbone
et de l’oxygène indiquent que la carbonatite calcitique est d’origine mantellique et la
dolomite 1 d’origine ignée. La dolomite 2 montre une signature ignée faiblement

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modifiée par l’altération hydrothermale subséquente et la dolomite 3 est d’origine
hydrothermale.
Un modèle est proposé dans lequel la carbonatite calcitique est la phase magmatique la
plus précoce. Cette phase a atteint la saturation en niobium très tôt, entrainant la
précipitation du pyrochlore. Le magma a évolué plus tardivement vers une composition
de carbonatite dolomitique qui, lors de son refroidissement, a exsolvé un fluide aquo-
carbonique altérant les métasédiments de Kechika en fénite potassique et se mélangeant
avec les eaux formationelles, plus distales, pour produire les fénites sodiques.
Le fluide aquo-carbonique a mobilisé et transporté les ETR sous la forme de complexes
chlorurés, au sein de la porosité et des fractures de la carbonatite dolomitique. A la suite
des interactions fluide-roche prolongées, les ETR ont précipité du fait du refroidissement
et de l’augmentation du pH ; la présence d’apatite a entrainé la formation de monazite-
(Ce). La concentration hydrothermale a permis une répartition homogène des teneurs,
avec la carbonatite dolomitique contenant dans son ensemble plus de 1 % TREO, faisant
de Wicheeda un gisement très attrayant.
Contributions of Authors
This thesis has been written in manuscript format in accordance with the regulations put
forth by the Faculty of Graduate Studies and Research at McGill University. It contains
one manuscript submitted to Economic Geology. In addition, there are two supporting
chapters and eleven appendices.