Abstract
The Vuoriyarvi Devonian carbonatite–ijolite–pyroxenite–olivinite complex comprises several carbonatite fields: Neske Vara, Tukhta-Vara, and Petyayan-Vara. The most common carbonatites in the Tukhta-Vara and Neske-Vara fields are calciocarbonatites, which host several P, Fe, Nb, and Ta deposits. This paper focuses on the Petyayan-Vara field, in which the primary magmatic carbonatites are magnesian. The least altered magnesiocarbonatites are composed of dolomite with burbankite and are rich in REE (up to 2.0 wt. %), Sr (up to 1.2 wt. %), and Ba (up to 0.8 wt. %). These carbonatites underwent several stages of metasomatism. Each metasomatic event produced a new rock type with specific mineralization. The introduction of K, Si, Al, Fe, Ti, and Nb by a F-rich fluid (or fluid-saturated melt) resulted in the formation of high-Ti magnesiocarbonatites and silicocarbonatites, composed of dolomite, microcline, Ti-rich phlogopite, and Fe–Ti oxides. Alteration by a phosphate–fluoride fluid caused the crystallization of apatite in the carbonatites. A sulfate-rich Ba–Sr–rare-earth elements (REE) fluid (probably brine-melt) promoted the massive precipitation of ancylite and baryte and, to a lesser extent, strontianite, bastnäsite, and synchysite. Varieties of carbonatite that contain the highest concentrations of REE are ancylite-dominant. The influence of sulfate-rich Ba-Sr-REE fluid on the apatite-bearing rocks resulted in the dissolution and reprecipitation of apatite in situ. The newly formed apatite generation is rich in HREE, Sr, and S. During late-stage transformations, breccias of magnesiocarbonatites with quartz-bastnäsite matrixes were formed. Simultaneously, strontianite, quartz, calcite, monazite, HREE-rich thorite, and Fe-hydroxides were deposited. Breccias with quartz-bastnäsite matrix are poorer in REE (up to 4.5 wt. % total REE) than the ancylite-dominant rocks (up to 11 wt. % total REE).
Highlights
Rare-earth elements (REEs) are widely viewed as critical metals due to their extensive use in modern and “green” technologies [1,2]
We suggest that the mechanism for HREE precipitation was the interaction of orthomagmatic carbonatitic Ba–Sr–REE–S-rich fluid with phosphate-bearing rocks formed simultaneously with high-Ti carbonatites or immediately after them
We suggest that the Ba–Sr–REE-rich sulfate (+Cl?) fluid played an important role in the transport of REE within the Petyayan-Vara field
Summary
Rare-earth elements (REEs) are widely viewed as critical metals due to their extensive use in modern and “green” technologies [1,2]. Minerals 2020, 10, 73 account for more than 50% of the global resources of REE [4]. Rare earth carbonatites are a Minerals 2020, 10, x FOR PEER REVIEW major focus of critical metal research. Over the past five years, several dozens of papers have been published on than the subject of rare earth carbonatites (RETherefore, carbonatites) in Angola [5,6], Australia for more. Rare earth carbonatites are a major[7,8,9], of critical metal research. Over[18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37], the past Finland five years,[38], several dozens of papers been published
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