Pyrochlore-Group Minerals in the Granite-Hosted Katugin Rare-Metal Deposit, Transbaikalia, Russia

Pyrochlore group minerals are the main raw phases in granitic rocks of the Katugin complex-ore deposit that stores Nb, Ta, Y, REE, U, Th, Zr, and cryolite. There are three main types: Primary magmatic, early postmagmatic (secondary-I), and late hydrothermal (secondary-II) pyrochlores. The primary ma...

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Bibliographic Details
Published in:Minerals (Basel) 2019-08, Vol.9 (8), p.490
Main Authors: Starikova, Anastasia E., Bazarova, Ekaterina P., Savel’eva, Valentina B., Sklyarov, Eugene V., Khromova, Elena A., Kanakin, Sergei V.
Format: Article
Language:English
Subjects:
alkaline granites
Biotite
Cryolite
Crystallization
East Transbaikalia
Electron microscopes
Evolution
Fluorides
fluornatropyrochlore
Geology
Heavy metals
Hydrothermal alteration
Iron
Isotopes
Katugin rare-metal deposit
Mineral deposits
Mineralization
Mineralogy
Minerals
Nepheline
Occupancy
Petrology
pyrochlore-group minerals
Pyrochlores
Syenite
Thorium oxides
Uranium dioxide
Zirconium
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Summary:Pyrochlore group minerals are the main raw phases in granitic rocks of the Katugin complex-ore deposit that stores Nb, Ta, Y, REE, U, Th, Zr, and cryolite. There are three main types: Primary magmatic, early postmagmatic (secondary-I), and late hydrothermal (secondary-II) pyrochlores. The primary magmatic phase is fluornatropyrochlore, which has high concentrations of Na2O (to 10.5 wt.%), F (to 5.4 wt.%), and REE2O3 (to 17.3 wt.%) but also low CaO (0.6–4.3 wt.%), UO2 (to 2.6 wt.%), ThO2 (to 1.8 wt.%), and PbO (to 1.4 wt.%). Pyrochlore of this type is very rare in nature and is limited to a few occurrences: Rare-metal deposits of Nechalacho in syenite and nepheline syenite (Canada) and Mariupol in nepheline syenite (Ukraine). It may have crystallized synchronously with or slightly later than melanocratic minerals (aegirine, biotite, and arfvedsonite) at the late magmatic stage when Fe from the melt became bound, which hindered the crystallization of columbite. Secondary-I pyrochlore follows cracks or replaces primary pyrochlore in grain rims and is compositionally similar to the early phase, except for lower Na2O concentrations (2.8 wt.%), relatively low F (4 wt.%), and less complete A- and Y-sites occupancy. Secondary-II pyrochlore is a product of late hydrothermal alteration, which postdated the formation of the Katugin deposit. It differs in large ranges of elements and contains minor K, Ba, Pb, Fe, and significant Si concentrations but also low Na and F. Its composition mostly falls within the field of hydro- and keno-pyrochlore.
ISSN:2075-163X
2075-163X
DOI:10.3390/min9080490