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

Anastasia E Starikova,Sergei V Kanakin,Elena A Khromova,Eugene V Sklyarov,Ekaterina P Bazarova,Valentina B Savel’Eva

doi:10.3390/min9080490

Abstract

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.

Highlights

IntroductionPyrochlore-group minerals (part of the pyrochlore supergroup of minerals) are broadly occurring in nature and are of important economic interest due to their ability to host Ta, Nb, U, and REE
Pyrochlore-group minerals are broadly occurring in nature and are of important economic interest due to their ability to host Ta, Nb, U, and REE.They are common accessory phases in carbonatite and are often found in such lithologies as nepheline syenite, alkaline gabbro and granitoids, pegmatite, and in albite and greisen granites [1,2,3,4,5,6,7,8]
There are three main types of pyrochlore identified according to crystallization sequence and chemistry: Primary pyrochlore found as isolated crystals or aggregates; secondary-I pyrochlore looking darker-shaded in BSE images, which follows cracks, replaces earlier pyrochlore in grain rims, and is occasionally enclosed in zircon from Aeg-Arf granites; and the latest secondary-II in most strongly altered and deformed grain parts (Figure 2)

Summary

Introduction

Pyrochlore-group minerals (part of the pyrochlore supergroup of minerals) are broadly occurring in nature and are of important economic interest due to their ability to host Ta, Nb, U, and REE. They are common accessory phases in carbonatite and are often found in such lithologies as nepheline syenite, alkaline gabbro and granitoids, pegmatite, and in albite and greisen granites [1,2,3,4,5,6,7,8].

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