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Gem deposits and pegmatites in northern Canada.
Strong field and analytical skills.
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Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
This dissertation is a multipart study of the crystal-chemistry and structure of the mineral beryl. The H₂O and Na content of beryl are quantified and related for emerald and each major beryl variety using single-crystal X-ray diffraction methods. The crystal-chemistry of beryl is shown to have differences across beryl varieties. A model that accurately determines the permutations in the beryl crystal structure using measured chemistry is created and is used to explore the limitations of the mineral. Beryl from the Prof Pegmatite near Revelstoke, British Columbia, is examined, using knowledge gained during this research. Beryl (Be₃Al₂Si₆O₁₈) is a well-known mineral, most famously in its vivid green form of emerald, but also in a range of colors. Prominent varieties of beryl aside from emerald include aquamarine, red beryl, heliodor, goshenite, and morganite. Emerald is studied first to remedy problems in prior research and to prove that this methodology is reliable. A relation between Na and H₂O content in beryl is determined that is consistent for beryl with significant Na content. Natural red beryl is regularly anhydrous, and heliodor has too low a Na content to reliably determine H₂O content from measured Na. A complete crystal structure of beryl can be calculated using chemical composition by utilizing the average ionic radii of the cations within the Al- and Be-sites. The structure is predictable using the Al-site average ionic radius for octahedrally trending beryl or the Be-site average ionic radius for tetrahedrally trending beryl. This model makes it possible to explore limitations on the beryl structure and the possibility of unusual cation substitutions. It is robust for true beryl up to a high limit of cation substitutions. The beryl studied from the Prof Pegmatite demonstrates an application of the crystal-chemical models to a newly described locality.This research will help guide future beryl studies in classifying beryl variety by chemistry and structure and allow for the calculation of H₂O content in a range of beryl varieties from easily measured Na content. Future beryl studies will be able to determine crystal structures during standard chemical analyses using the predictive structural model.
This dissertation presents a two-component study: (1) the effect of contamination on the mineralogy of the Rau pegmatite group and implications for pegmatite contamination and (2) a statistically validated concept inventory that can assess understanding of mineralogy concepts.The Rau pegmatite group, which is made up of 10 F-rich, barren to mixed NYF + LCT pegmatite dikes, is located in the Yukon Territory, Canada. The dikes are hosted in dolostone of the Bouvette Formation and were derived from the nearby Rackla pluton: a ~63 Ma, weakly peraluminous biotite-muscovite granite. The most evolved pegmatites of the group contain abundant rare element phases such as columbite group minerals, fluorcalciomicrolite, and REE-bearing fluoro-carbonate minerals. Elevated F activity during pegmatite crystallization is evident from F enrichment in multiple pegmatite zones.The Rau pegmatite dikes show clear evidence of being contaminated by their dolostone host rocks. Primary Ca-bearing minerals as well as carbonate pockets are present throughout the pegmatite dikes. Stable C and O isotopic signatures of carbonate minerals indicate that the pegmatite dikes were subjected to pre-emplacement contamination, whereas the presence of endo-contact skarns at the borders of some pegmatite dikes is evidence for post-emplacement contamination.An extensive meta-analysis of the pegmatite literature suggests that pegmatite contamination that can modify the mineralogy of a pegmatite is a much more common mechanism than is currently recognized. Future studies of pegmatites should include a more thorough investigation of the host rocks, and results should be interpreted while considering the potential influence of the host rock composition on pegmatite mineralogy.The Mineralogy Concept Inventory (MCI) is a statistically validated assessment that can be used to measure learning gains in introductory mineralogy courses. Development of the MCI was an iterative process involving expert consultation, student interviews, and statistical analysis. The MCI was implemented in undergraduate mineralogy courses at two different Canadian universities that employ different pedagogies: student-centred and instructor-centred. Although average pre-assessment scores were approximately the same, normalized learning gains were significantly higher in the course that uses a student-centred pedagogy. These results suggest that the use of a student-centred pedagogy can significantly increase learning of mineralogy concepts.
The Lake Harbour Group (LHG) metacarbonates on Baffin Island contain occurrences of the gemstones sapphire (corundum), spinel (including cobalt-blue), and lapis lazuli (blue haüyne-rich rock). This dissertation uncovers the regional geologic processes (e.g., metamorphic history, metasomatism, protolith geochemistry) that influence gemstone potential by developing genetic models for the LHG gem mineral occurrences. Both barren and gem-bearing metacarbonates were studied using field examination and sampling, petrological (optical) and scanning electron microscope petrography, and whole rock geochemistry. Boron isotope geochemistry, thermodynamic modelling, and age-dating (zircon U-Pb and mica ⁴⁰Ar-³⁹Ar) were employed for selected occurrences.Corundum formation was made possible by three equally important sequential metamorphic reactions: (1) formation of nepheline, diopside, and K-feldspar (inferred) at granulite facies peak metamorphic conditions; (2) partial retrograde replacement of the peak assemblage by phlogopite, oligoclase, calcite, and scapolite; and (3) retrograde break-down of scapolite + nepheline to form albite, muscovite, corundum, and calcite. The corundum-forming reaction only occurs in a
The Aappaluttoq, Siggartartulik, and Kigutilik gem corundum deposits are hosted by the Fiskenæsset Anorthosite Complex in SW Greenland. These deposits are some of the largest ruby and sapphire deposits in the world yet have received little scientific attention. During regional granulite metamorphism (2817 Ma; U-Pb EMPA monazite), leucogabbroic rocks alter towards more Na-enriched compositions causing Ca-feldspar to recrystallize. This liberates the Al required for metamorphic opaque corundum formation in an orthogneissic host. Subsequent retrogression through the amphibolite facies (2747-2555 Ma, U-Pb EMPA monazite) and local scale hydrous metasomatism (2660 Ma, U-Pb EMPA monazite) causes recrystallization of the mineral assemblage and the formation of three distinct gem corundum bearing lithologies: 1) phlogopitite, 2) altered leucogabbro, 3) altered ultramafic rock containing sapphirine and gedrite-anthophyllite. Intrusion of local granitoids (2747-2714 Ma, U-Pb TIMS zircon) provided a hydrous fluid which infiltrated and metasomatised the regional assemblage and was channeled along preexisting lithological boundaries (leucogabbro-ultramafic, leucogabbro-melanogabbro). The primary host for gem corundum is the 2-8 meter wide phlogopitite unit formed at 700 to 815°C and 5 to 8 kbar. This unit contains up to 20% corundum, of which at least 5% is gem quality. The phlogopite represents the quenched granitic-metasomatic fluid; EMPA and LA-ICP-MS analysis confirm a fluid composition depleted in S, Cu, Ca, Pr, Nd, Sm, Te, Cs, Ba, Se and transition metals, and enriched in Rb, Sr, Cr, and the HREEs. This is consistent with a granitic source rich in Si-K-Mg-Rb-Ba-Cs-Nb-Ta-Ga-H₂O±Cr±U±CO₂. The other two corundum-bearing units show similar variations except are typically more depleted in Cu, Te, S, Rb, and Sr. Corundum δ18O analyses define a tight range between 1.49‰ and 5.3‰, with a mean of 3.2 ± 0.9‰. This suggests the interaction or mixing of meteoric water with the metasomatic fluid at some point during gem recrystallization. Worldwide ultramafic-amphibolitic gem corundum deposits show similar low δ18O, mineralogy, mineral chemistry, and local geology. This study proposes the use of amphibolite-type as an all-encompassing gem deposit model to explain several enigmatic deposits elsewhere in the world.
A significant knowledge gap between the fields of reflectance spectroscopy and rare earth element (REE) mineralogy prompted this research effort. It narrows the knowledge gap through detailed study of thirty three samples representing three important mineral classes: REE fluorocarbonates (bastnaesite, synchysite, and parisite), REE phosphates (monazite, xenotime, and britholite), and REE-bearing silicates (cerite, mosandrite, kainosite, zircon and eudialyte). Reflectance spectroscopy was carried out in the visible to short wave infrared regions (500 nm to 2500 nm) and each sample was characterized using scanning electron microscopy and electron microprobe analysis.Spectral features of these minerals are primarily related to numerous 4f-4f intraconfigurational electronic transitions of trivalent lanthanides (Ln³+), as well as 5f-5f electronic transitions of uranium and vibrational overtones and combinations of CO₃²-, H₂O, PO₄³- and OH- where applicable. In general, the respective spectra of these REE minerals are sufficiently distinct for spectral classifications, and compilation diagrams with representative spectra are given. Broadly speaking, the light REE-enriched minerals are dominated by sharp absorptions related to Nd³+, Sm³+, and Pr³+ with lesser input from Eu³+, whereas heavy REE-enriched minerals are dominated by sharp spectral features related to Er³+, Dy³+ and Yb³+ with lesser input from Nd³+, Tb³+, Ho³+ and Tm³+ depending on their specific concentrations. For those minerals that do not show strong preference for light or heavy REE, a mixed set of absorption patterns is seen. Spectral variability of specific 4f-4f absorptions were substantial between different minerals and these variations are interpreted to be driven by the specific anion coordination at the Ln³+site across various crystal structures. Shifts in wavelength position and relative strengths of related absorptions can be significant enough to be highly relevant for hyperspectral remote sensing. This is especially applicable for REE mineral identification in field-based settings and high spatial resolution imaging spectroscopy.Three themes of ongoing and potential research are discussed: additional REE mineral spectroscopy, exploitation of diagnostic features for REE mineral detection and identification, and REE ore grade estimation. Overall, the research presented in this dissertation sets the foundation for future interpretation and exploitation of reflectance spectra for the REE minerals.
This dissertation investigates the petrogenesis of the Fir carbonatite system (Monashee Mountainsof British Columbia), which is particularly interesting because of its high degree of deformation, the relatively minor presence of associated silicate rocks and its comparably high content of Ta.A detailed examination of the rock textures and microstructures shows that the two main fabrics,a primary gneissic and a secondary fine-grained, foliated fabric, are the results of plasticflow and dynamic recrystallization during deformation. The primary fabric developed under peakmetamorphic conditions and was overprinted by retrograde mylonitic shear zones.The microtextural record and the equilibration temperatures are compared to regional marblesthat share the same tectonometamorphic history. Both lithologies reveal a very similar petrologicalrecord indicative of metamorphic equilibration, however, some calculated temperatures in the carbonatites (∼700 °C) exceed the peak-metamorphic conditions (620 - 650 °C), which indicates thatthe magmatic crystallization temperatures are preserved despite amphibolite-facies metamorphism.Apart from minor calcium and sodium amphiboles the Fir system contains predominantly the sodium-calcium amphiboles winchite and katophorite which define two major mineralogical facies. The amphiboles have high F contents (
This detailed scientific study of the carbonate-hosted gem corundum occurrences near Revelstoke, British Columbia and Kimmirut, Nunavut, Canada was completed in order to: (1) characterize the gem corundum mineralization; (2) develop genetic models for gem corundum mineralization; and (3) develop exploration strategies for gem corundum in carbonate-hosted deposits. These unique localities were chosen to help develop exploration strategies for gem corundum deposits in Canada since existing models of gem corundum genesis are unable to explain their origin.The Revelstoke occurrence is located in the Monashee Complex of the Omineca belt of the Canadian Cordillera. Pink (locally red or purple) corundum crystals occur in thin, folded and stretched layers containing the assemblage of green muscovite + Ba-bearing K-feldspar + anorthite ± phlogopite ± Na-poor scapolite. Mineral assemblages and textures in these silicate layers and thermodynamic modeling suggest that corundum formed from muscovite dehydration at the peak of metamorphism (~650-700 °C at 8.5-9 kbar). Observed trends in whole rock geochemical data indicate that the corundum-bearing silicate (mica-feldspar) layers formed by mechanical mixing of carbonate with the host gneiss protolith; the bulk composition of the silicate layers was modified by Si and Fe depletion during prograde metamorphism. High element mobility is supported by homogenization of δ¹⁸O and δ¹³C values in carbonates and silicates for the marble and silicate layers. The Kimmirut Sapphire Occurrence is located in the Lake Harbour Marble of the Baffin Island segment of the Trans Hudson Orogen. Blue and colourless zoned gem corundum crystals occur in coarse-grained calc-silicate pods with albite + calcite + muscovite ± K-feldspar. Corundum-bearing zones are separated from a phlogopite + plagioclase symplectite around violet diopside crystals by scapolite which fluoresces in UV light. Corundum likely formed during retrograde metamorphism at P-T
Rare element pegmatites represent some of the last stages of igneous differentiation and are influential in element redistribution in the upper crust, leading to significant enrichment/depletion of various trace elements. Research into the processes that form these intrusions increases our understanding of the geochemical evolution of silicate earth and improves the potential for successful pegmatite exploration. This study focussed on the dikes comprising the rare element Little Nahanni Pegmatite Group (LNPG), Mackenzie Mountains, northern Canadian Cordillera. These peraluminous dikes have high concentrations of several rare elements, e.g., Li (up to 14,000 ppm), Cs (up to 500 ppm), Ta (up to 190 ppm), and Rb (up to 7,500 ppm). Orientation of the dikes was influenced during emplacement (2-3 kbar, ~400-500 °C) at ~90 Ma (apatite, U-Pb) by pre-existing foliation in the strongly deformed, stratified host rock of the Fork anticlinorium (axial planar cleavage and bedding). Differences in ⁴⁰Ar/³⁹Ar dates on pegmatite minerals (muscovite 77.1±3.6 and ~80 Ma and lepidolite 65.8±0.8 Ma) indicate the presence of an elevated paleogeothermal gradient (~60°C/km). Structural and contact metamorphic evidence identify a local heat source within the anticlinorium that may have been the source chamber for the dikes. Whole rock trace element concentrations and ratios, mineralogical and textural variations, and fractionation of Li isotopic ratios (δ7Li = -0.94‰ to +11.36‰) record a range of magmatic fractionation. Approximately 85% of the dikes are spodumene-rich, with discontinuous REEN patterns and low degrees of Li isotope fractionation, the remaining ~15% show greater magmatic fractionation, with little spodumene, and have flat or listric REEN patterns and strongly fractionated Li isotopic ratios. The replication of the REEN patterns by P and F saturation (mineral precipitation and fluid separation), illustrates the influence of flux components on the composition of late stage melts.The Li isotope composition of rapidly crystallized, co-precipitated mineral assemblages appears to show the retention of a kinetic isotopic fractionation signature; providing a potential method to assess the chemical equilibrium of the system. This integrated study advances our understanding of rare element pegmatite formation in several aspects, in particular the role of fluxes in their geochemical evolution.
Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
The unusual black shale hosted emerald deposits of Colombia are an important source of the finest quality emeralds. The majority of world emerald deposits are related to granite pegmatites and metamorphic environments, however Colombian-type emerald is amagmatic and is associated with sedimentary-hydrothermal brines of largely evaporitic origin where the key emerald-forming elements are remobilized from black shale during regional compressional tectonism. The valuable gem quality mineralization of Colombian emerald deposits is so far unique, however the basic geological conditions that led to mineralization may not be as unusual. Similar black shale basins hosting evaporites are found worldwide, and could be prospective for Colombian-type emerald mineralization. Canada’s Selwyn Basin and periphery is one similar setting with the distinction of hosting two interesting beryl showings: the Mountain River Beryl (a variation of the Colombian beryl model) and the Lened emerald showing (in which black shale provided vanadium as an emerald chromophore). This thesis presents the results of applying Colombian-type emerald exploration criteria to Selwyn Basin stream sediment geochemistry data, in addition to clarifying the origin of emerald at the Lened showing located within the Selwyn Basin.The unusual black shale hosted emerald deposits of Colombia are an important source of the finest quality emeralds. The majority of world emerald deposits are related to granite pegmatites and metamorphic environments, however Colombian-type emerald is amagmatic and is associated with sedimentary-hydrothermal brines of largely evaporitic origin where the key emerald-forming elements are remobilized from black shale during regional compressional tectonism. The valuable gem quality mineralization of Colombian emerald deposits is so far unique, however the basic geological conditions that led to mineralization may not be as unusual. Similar black shale basins hosting evaporites are found worldwide, and could be prospective for Colombian-type emerald mineralization. Canada’s Selwyn Basin and periphery is one similar setting with the distinction of hosting two interesting beryl showings: the Mountain River Beryl (a variation of the Colombian beryl model) and the Lened emerald showing (in which black shale provided vanadium as an emerald chromophore). This thesis presents the results of applying Colombian-type emerald exploration criteria to Selwyn Basin stream sediment geochemistry data, in addition to clarifying the origin of emerald at the Lened showing located within the Selwyn Basin.We identified several prospective areas in Selwyn basin for further Colombian-type emerald exploration based on Na, K, Be, Cr/V, and rare earth element values, however there are important caveats regarding analytical techniques and the regional nature of silt sampling. We found that Lened is a unique skarn-hosted hydrothermal emerald occurrence (~100 Ma) in which Be was provided by a nearby granite pluton of similar age. The chromophore V was provided by local black shale/mudstone.
The Deer Horn property is located 150 km south of Smithers in west-central British Columbia and covers 51 km². The deposit is an intrusion-related polymetallic system enriched in Au-Ag-Te-W-Cu with lesser amounts of Bi-Pb-Zn-Mo; the Au and Ag are hosted in telluride minerals. The quartz-sulfide vein system containing the main zones of Au-Ag-Te mineralization and sericite alteration is found in the hanging wall of a local, spatially related thrust fault. The age of the sericite alteration is 56 ± 2 Ma. Biotite K-Ar ages of 57–48 Ma for the nearby Nanika granodiorite intrusive suite indicates that it is likely genetically responsible for the Au-Ag-Te mineralizing event. The telluride minerals are 0.1–525 μm and commonly form whole euhedral to subhedral grains or composite grains of Ag-, Bi-, Pb-, and Au-rich telluride minerals (e.g., hessite, tellurobismuthite, volynskite, altaite, and petzite). Panchromatic cathodoluminescence imaging revealed four generations of quartz. Locally, oscillatory zoning observed in quartz II suggests the participation of hydrothermal fluids. Fine-grained veinlets of quartz III and IV intersect quartz I and II, which is evidence of at least two shearing events; veinlets of calcite intersect all generations of quartz. Three types of fluid inclusions were observed: (1) aqueous liquid and vapour inclusions (L-V); (2) aqueous carbonic inclusions (L-L-V); and (3) carbonic inclusions (V-rich). Fluid inclusions that are thought to be primary or pseudosecondary and related to the telluride mineralization were tested with microthermometry. Homogenization temperatures are 130.0–240.5 °C for L-V inclusions and 268.0–336.4 °C for L-L-V inclusions. Four of eight aqueous carbonic inclusions had solid CO2 melting temperatures from –56.8 to –62.1 °C, indicating the presence of 0.5–13.2% dissolved methane in these inclusions. Sulfur isotope analysis of ³⁴S/³²S using 20 samples of pyrite was conducted. δ³⁴S readings are close to 0 (from –1.6 to 1.6 per mil) and confirm that the sulfur is very likely magmatic/igneous in origin.
Rare earth element- and Nb-bearing NYF-type pegmatites are located on the KIN property, approximately 95 km northeast of Revelstoke, British Columbia. They intrude amphibolite grade rocks of the Neoproterozoic Horsethief Creek Group in the Omineca Belt of the Canadian Cordillera. The Cordillera has traditionally been associated with LCT-type pegmatites, making the presence of NYF-type pegmatites on the KIN property particularly unusual. These pegmatites are found in-situ in four localities and contain significant amounts of allanite-(Ce), monazite-(Ce), chevkinite-(Ce), aeschynite-(Ce), euxenite-(Y), Nb-rich rutile, ilmenite, amphibole, and fluorapatite within plagioclase and Ba-rich feldspar and quartz. Additionally, the pegmatites contain textures and minerals, such as epidote-rimmed allanite and the breakdown of monazite into apatite and allanite in a corona texture, which can be attributed to Ca, F, and Si-rich fluids having been introduced during metamorphism. These pegmatites were dated by U-Pb zircon methods at approximately 79 Ma, and likely formed from an A-type source. Along with the NYF-type pegmatites, A-type REE-bearing syenites, coarse grained I-/S-type granites, and tourmaline bearing granitic pegmatite float samples are located on the property. The granite and syenite were identified as potential parental rocks for the mineralized pegmatites and this hypothesis was tested using geochemistry and geochronology. The granite is undeformed and has been dated by U-Pb zircon methods at approximately 76 Ma; this evidence, along with its geochemical signature suggests that the granite cannot be the parent for the pegmatites. Geochemical and elemental characteristics within the syenites suggest possible linkage to the pegmatites; however, syenite in the immediate area has been dated to 378 Ma, nearly 300 million years older than the pegmatites. In view of this the geochemical match and age discrepancy, it is possible that the pegmatites formed from partial melting of these older syenites at approximately 79 Ma.
The pegmatite field on Mount Begbie, near Revelstoke, B.C., is a Li-rich pegmatite field in the southern Canadian Cordillera that is relatively well-known among prospectors, but until now, under-studied by academics. Mapping the pegmatite field on the northeastern slope of the mountain revealed a dense population of S-type granitic pegmatites belonging to the LCT family with diverse ranges of mineralogy corresponding to barren, beryl-columbite, beryl-columbite-phosphate, and lepidolite-subtype compositions. Typically, the pegmatites are not strongly metamorphosed and show only rare foliation, meaning they primarily postdate the exhumation of the Thor-Odin Culmination that occurred during the Late Paleocene to Early Eocene. They are likely related to other granitic bodies (i.e., the Ladybird granite suite) resulting from the exhumation event rather than any other known intrusions in the Monashee complex. The dikes in the study area are elongate, with most at least 10 m long and are usually not more than one meter wide; the largest is approximately 520 m long and 10 m wide. Orientations of the pegmatites are dictated in part by conjugate shear planes developed in the host rock prior to the emplacement of the dikes; nearly all strike between 295° and 330° and have a subvertical dip. Fractionation is variable within the most primitive of the pegmatites and it may be controlled by localized fluid accumulation or the composition of the original source rock. Despite variability, fractionation within the pegmatite field increases from the southeast to the northwest, suggesting the source pluton may be located to the southeast of the study area at depth. Detailed examination of the minerals tourmaline, sekaninaite, beryl, rare-element oxides, and zircon as well as the mostly qualitative study of phosphates and other minerals provides insight into the geochemistry and mineralogy of the individual dikes. The results suggest that some of the primitive dikes are more fractionated than they appear and highlight other pegmatites as having unusual compositions for typical beryl-columbite and beryl-columbite-phosphate-subtype pegmatites. Elevated contents of Be and Li in sekaninaite and the presence of qitianlingite in the Mount Begbie pegmatite field are somewhat unique compared to other cordierite and rare-element oxide localities worldwide.
This thesis explores in detail the petrology and petrogenesis of the Ren carbonatite sill andassociated fenites, located in the Monashee mountain range of British Columbia. Thecarbonatite body and fenites have been significantly deformed and metamorphosed, whichhas provided a unique petrological research opportunity, since only a few other carbonatiteoccurrences worldwide have been described from highly metamorphosed orogenic settings.This work aims to address knowledge gaps pertaining to the petrology, petrogenesis andeconomic exploration of comparable carbonatites in similar geotectonic settings.The effects of amphibolite facies regional metamorphism and progressive deformationare apparent throughout the carbonatite body and adjacent fenites. Many of the rocks displayremobilized, passively mixed components, boudinaged structures, and rheomorphic bands, aswell as foliation and porphyroblastic textures. Recrystallization of minerals at peakmetamorphic conditions (580–730 °C) is indicated primarily in undifferentiated calcite forwhich metamorphic solvus temperatures (~690 °C) were derived. Other minerals related tometamorphic recrystallization include rims of monazite-(Ce) around earlier fluorapatite,interstitial REE-silicates, and Ca- and Mg-amphibole forming late in the parageneticsequence, after primary silicates.Despite the metamorphic overprint and alteration, many petrological featurespertaining to the igneous paragenetic record have been preserved, such as textures of primaryminerals, compositional trends in phlogopite, clinopyroxene and amphibole, and whole-rockgeochemistry of rock units. Solvus temperatures (~760 °C) of calcite, higher than peakmetamorphic conditions, highlight preservation of the igneous component. The carbonatite isinferred to be a product of primitive mantle melts(s) that did not undergo significantfractionation processes, and intruded the crustal environment relatively undifferentiated. Thewhole-rock compositional trend of the fenites and partially fenitized host rocks suggestssodic-potassic alteration of the country rock during emplacement of the carbonatite sill.The Nb-Ta and REE mineralizations of the Ren occurrence are both insufficient foreconomic extraction. Nevertheless, rare and new mineral candidates, (Fe,OH)-analogue tovästmanlandite-(Ce) and (Mg)-analogue to biraite-(Ce), discovered in the deposit by theauthor, emphasize its petrological and mineralogical significance.
Borosilicates are oxygen-bearing boron minerals in which SiO₄ tetrahedra form an important structural component. They represent widespread constituents of rocks originating in the Earth’s crust and commonly provide insight into the rock-forming processes. Important borosilicates include tourmaline, axinite, werdingite, boralsilite, dumortierite, holtite, howlite, and grandidierite. Tourmaline is the most widespread borosilicate mineral and it can be used to study a wide variety of geological processes based on its high compositional variability. Dumortierite [(Al,☐)Al₆(BO₃)Si₃O₁₃(O,OH)₂] is second only to tourmaline as the most abundant borosilicate mineral, but remains relatively understudied in comparison. Isostructural holtite [(Al,Ta,Nb,☐)Al₆(BO₃)(Si,Sb,As)₃O₁₂(O,OH,☐)₃] is poorly constrained chemically. The ability to incorporate elements such as As, Sb, Bi, Nb and Ta make dumortierite and holtite unusual for silicate minerals. Synthesis experiments designed to better determine the relationship between holtite and dumortierite by synthesizing dumortierite and gradually replacing Si with As and Sb were carried out at the GeoForschungZentrum (GFZ) in Potsdam, Germany. Synthesis conditions were designed based on the previous work by Werding and Schreyer (1990). Experiments ranged from 3-5 kbar at 550-650 °C and 15-20 kbar at 600-700 °C. The less common borosilicate boralsilite [Al₁₆B₆Si₂O₃O₃₇] was the dominant phase produced in the attempts at synthesizing dumortierite. The results showed an increased stability range for boralsilite than had been previously studied and gave insight to simpler methods of synthesis. A detailed case study of the mineralogy of the Uvil’dy Lake pegmatite revealed a potentially unique locality for dumortierite and tourmaline evolution. The tourmaline shows a high Mn/(Fe + Mg) ratio within primary phases and increased Li, Fe and Mn-enrichment in later phases. Apart from being unusually yellow, the dumortierite from Uvil’dy was also anomalously high in Bi (~0.03 apfu). Eight additional samples of dumortierite from different global localities were analysed and were compared to approximately 1100 dumortierite and holtite compositions assembled from both published and unpublished data from worldwide localities. Including the analysed Uvil’dy samples, this extensive dataset gives a detailed look at the chemical relationship between dumortierite and holtite and allows them to be better constrained in terms of their chemical constituents.
Carbonatites are rare magmatic rocks composed of greater than 50% carbonate minerals. They are generally associated with continental rift-related tectonic settings and are commonly enriched in rare earth elements (REE), Nb, and P. The Wicheeda carbonatite complex, located 80 km northeast of Prince George in British Columbia, Canada, has been historically explored for its REE potential, but until recently there has been very little scientific study. The purpose of this study was to explore the geology, mineralogy, geochemistry, and geochronology of the Wicheeda Carbonatite Complex. The complex consists of a carbonatite plug with a number of carbonatite and potassic-syenite dykes and sills emplaced into the sedimentary rocks of the upper Cambrian and lower Ordovician Kechika Group. Sodic-fenitization is common around the carbonatites and the degree of alteration and abundance of syenite outcrops increases away from the carbonatite plug. The complex was mapped over an area of 1.45 km². The REE mineralogy of the Wicheeda carbonatite was defined through optical petrography (86 thin sections), scanning electron microscopy, electron probe microanalysis (371 points on 14 mineral species), and single-crystal X-ray diffraction (7 samples). The REE mineralogy is complex, with multiple stages of primary, late-stage, rapidly cooling crystallization. It consists of Ca-REE-fluorocarbonates, Ba-REE-fluorocarbonates, ancylite-(Ce), monazite-(Ce), euxenite-(Y), and allanite-(Ce); the majority of these minerals are LREE rich. Whole rock isotopic analysis was completed for the Rb-Sr and Sm-Nd systems. An isochron age of 316 ± 36 Ma was determined using the Sm-Nd system, giving values for εNdT and ⁸⁷Sr/⁸⁶SrT that range from -0.5 to 0.5 and 0.70526 to 0.70659, respectively. Evidence from the Wicheeda Carbonatite Complex along with comparisons with other worldwide carbonatites, suggests that the complex formed from a dominantly silicate, parental, mantle melt emplaced into the continental lithosphere. The lithosphere underwent metasomatism and, potentially, low degrees of partial melting and/or the incorporation of previously subducted sediments for carbonatite generation.