Project

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REE-mineralization in pegmatites of the South Platte district in the Pikes Peak Batholith, Colorado (USA)

Applicant Allaz Julien
Number 185009
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.10.2019 - 30.09.2023
Approved amount 279'114.00
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All Disciplines (4)

Discipline
Geochemistry
Mineralogy
Geochronology
Geology

Keywords (7)

Fluid Inclusions; Mineralization; Syenogranite-monzogranite; Pegmatite; Rare-earth-elements; Colorado; REE-speciation

Lay Summary (French)

Lead
Les nouvelles technologies requièrent de plus en plus d’éléments de terres rares (TR: cérium, néodyme, yttrium, etc.). Ils sont notamment utilisés dans la fabrication de batteries, d’ampoules LED, d’aimants permanents pour éoliennes, de catalyseurs pour les voitures, ou de verres spéciaux (p.ex. écrans tactiles). De nos jours, la grande majorité de ces éléments proviennent essentiellement de Chine, et il est important de trouver d’autres sources potentielles en Europe et ailleurs dans le monde afin de limiter les risques d’approvisionnement d’un marché en pleine expansion.
Lay summary

Malgré leur nom de terres « rares », ces éléments sont aussi abondants dans la croûte terrestre que le cuivre et bien plus courant que l’or ou l’argent. Toutefois, il est souvent difficile de les extraire car, au contraire de certains métaux tels que l’or, ils sont très rarement concentrés dans les roches. Il y a toutefois quelques exceptions à la règle, mais les conditions qui entrainent un tel enrichissement restent actuellement mal connues.

Une de ces exceptions à l’enrichissement des éléments de TR concerne les roches granitiques riches en alcalins (sodium, potassium), et plus particulièrement les pegmatites qui représentent le liquide final de cristallisation d’un granite. La présente étude vise à étudier le granite de Pikes Peak au Colorado, où une quantité significative de pegmatites présentent des teneurs très élevées en TR qui induisent la formation d’une large variété de minéraux de TR. Cette étude va permettre de comprendre comment le granite et les pegmatites se sont enrichis, et va notamment questionner le rôle du magma et des fluides notamment riches en éléments ligands tels que le chlore, le fluor, et les fluides carboniques. Ces ligands semblent jouer un rôle prépondérant soit dans le transport, la concentration, ou la déposition des éléments de TR. Au final cette étude devrait permettre de recentrer les recherches de tels éléments dans des environnements nouveaux et prometteurs pour de futures exploitations minières en Europe et dans le monde.

Direct link to Lay Summary Last update: 10.04.2019

Lay Summary (English)

Lead
New technologies require more and more rare earth elements (REE: cerium, neodymium, yttrium, etc.). They are used for the fabrication of batteries, LED lights, permanent magnets for windmill, catalysts for cars, or special glasses (e.g., touch screen). Nowadays a vast majority of these elements are mined in China, and it is important to find potential alternate sources in Europe and in the world in order to limit the risk of supply in a market in full expansion.
Lay summary
Despite their name of « rare » earth, these elements are as abundant as copper in the Earth crust, and more common than gold or silver. However, it is commonly difficult to extract them as, unlike other metals such as gold, they are rarely concentrated in the rocks. There are exceptions to the rule, and the conditions that lead to such an enrichment remains currently badly constrained.

One of the exceptions to the enrichment of REE concerns magmatic rocks rich in alkaline elements (sodium, potassium), and in particular the pegmatites that represent the final liquid of crystallisation of a granite. The present study aims at investigating the Pikes Peak granite in Colorado, where a significant quantity of pegmatites displays a high REE content that induce the crystallisation of a large variety of REE minerals. This study will allow to understand how the granite and the pegmatites got enriched, and will notably question the role of magma and fluids notably enriched in ligands such as chlorine, fluorine, and carbonic fluids. The latter are suspected to play an important role either in the transportation, the concentration, or the deposition of these elements. Finally, this study should allow to re-centre the search for such elements in novel and promising environments for future mining in Europe and elsewhere in the world.
Direct link to Lay Summary Last update: 10.04.2019

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Abstract

New technologies require more and more rare earth elements (REE) to be used in electronics, optical fibers, magnets, etc. Yet, our current economy relies heavily on several major sources found in China; these elements are therefore considered as critical due to their growing demand by the industry and their high risk of supply. There are, however, other potential sources, notably REE-enriched alkali-rich intrusions. This project aims to understand the REE mineralization processes of several pegmatites hosted in the northern part of the Pikes Peak Batholith (PPB), a typical anorogenic “A-type” subaluminous to peralkaline intrusion, where over 50 known REE-bearing pegmatites of large size (pluridecametric) crop out. These pegmatites show a concentric zonation with a border facies against the granite, a microcline-rich intermediate zone, and a quartz or quartz + microcline core with a REE-rich mineralization occurring as veins and pockets near the interface between the intermediate zone and the core of the pegmatite. Previous works suggested an REE-enrichment, trending toward more heavy-REE + Y (HREE) or more light-REE (LREE) enrichment between different pegmatite. However, these limited data remain overall not conclusive. Several studies, notably in the 1980s, have highlighted the importance of F-rich liquid in the mineralization process, yet only one study on the aqueous mineralizing fluid in one pegmatite exists. Prior efforts on South Platte have been sporadic, on pegmatites actively mined at times, and not systematic.I propose to investigate and compare these pegmatites and their host rocks in greater petrological and geochemical detail and with state-of-the-art analytical methods in order to answer one fundamental questions: what are the processes, melt- or aqueous/carbonic fluid-related, that lead to the REE-enrichment in the batholith, and the concentration, transport, and precipitation of REE-rich phases in the pegmatite. Several pegmatite bodies and their associated host granites will be investigated, and their mineralogy and geochemistry will be compared. It is hypothesized that the mineralization stems from an initial REE-enrichment in the granite, and that the mineralization is primarily of magmatic origin and results from a fractionation process or melt immiscibility, with a possible late hydrothermal remobilization. Detailed petrological work on the REE-mineralization and the associated pegmatite and granite will represent the core of this study, along with whole rock analysis and mineral geochemistry, and will clarify whether there is a true distinction to be made between LREE- versus HREE-rich pegmatites. Our preliminary study reveals a complex mineralogy with numerous finely intergrown REE mineral species in a single pegmatite body, variable mineral chemistry from LREE- to HREE-rich compositions, co-existence of LREE- and HREE-dominant phases, and an apparent variation in the mineral assemblages from one pegmatite to the other. Geochronology on zircon and other actinide-bearing phases (e.g., monazite and thorite), fluid inclusions, and isotopic studies will also be necessary to answer these questions. To gain a better understanding at the nature of the granite and possible source, I propose to obtain U-Th-Pb ages and Hf-isotopes in zircon. Oxygen isotopes will be measured in zircon, quartz, and microcline from both the granite and the different pegmatite zones in order to shed some light on the fractionation or melt immiscibility process and possible late hydrothermal alteration that lead to the REE-mineralization in the pegmatite. This study will also investigate the fluid inclusions found in minerals from the granite and the pegmatite, notably quartz and fluorite, in order to gain a better understanding of the aqueous or carbonic fluid(s) that are involved in the mineralization process. These results will be compared with existing experiments on REE-speciation with notably F-rich melt or Cl-rich aqueous or carbonic fluids. This 4-year project will be done in collaboration with colleagues from ETH Zürich, University of Colorado at Boulder, and Colorado School of Mines, and will involve one PhD student, two Master’s students, and a few Bachelor’s students.
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