Abstract
This article is devoted to the geology of titanium-rich varieties of the Petyayan-Vara rare-earth dolomitic carbonatites in Vuoriyarvi, Northwest Russia. Analogues of these varieties are present in many carbonatite complexes. The aim of this study was to investigate the behavior of high field strength elements during the late stages of carbonatite formation. We conducted a multilateral study of titanium- and niobium-bearing minerals, including a petrographic study, Raman spectroscopy, microprobe determination of chemical composition, and electron backscatter diffraction. Three TiO2-polymorphs (anatase, brookite and rutile) and three pyrochlore group members (hydroxycalcio-, fluorcalcio-, and kenoplumbopyrochlore) were found to coexist in the studied rocks. The formation of these minerals occurred in several stages. First, Nb-poor Ti-oxides were formed in the fluid-permeable zones. The overprinting of this assemblage by residual fluids led to the generation of Nb-rich brookite (the main niobium concentrator in the Petyayan-Vara) and minerals of the pyrochlore group. This process also caused niobium enrichment with of early generations of Ti oxides. Our results indicate abrupt changes in the physicochemical parameters at the late hydro (carbo) thermal stage of the carbonatite formation and high migration capacity of Ti and Nb under these conditions. The metasomatism was accompanied by the separation of these elements.
Highlights
Carbonatites are defined in the International Union of Geological Sciences (IUGS) system of classification as igneous rocks containing more than 50 modal percent carbonate [1]
Raman spectroscopy indicated Ti oxides occur as three polymorph modifications: anatase, rutile, and brookite
Five morphotypes of titanium oxide isolations were detected in titaniferous carbonatites: segregations of elongated crystals (I type), xenomorphic rims of carbonatite grains overgrowths (II type), amoeboid isolations (III type), specific “cementing” intergrowths with sulfides (IV type), and small idiomorphic individuals of brookite (V type) (Figure 4a–e, respectively)
Summary
Carbonatites are defined in the International Union of Geological Sciences (IUGS) system of classification as igneous rocks containing more than 50 modal percent carbonate [1]. Varieties of carbonatite are named based on the dominant carbonate mineral [2]. They have been chemically classified according to the percent by mass of CaO, MgO, and FeOT and MnO [1], which has an evolutionary aspect as well. Most of the known carbonatite complexes have a common scheme of carbonatite genesis, whereby early calciocarbonatites were replaced by magnesiocarbonatites, and ferrocarbonatites, as the most recent [3]. The latter are considerably affected by carbo(hydro)thermal-metasomatic processes as they develop [4]. Carbonatites account for more than 50% of global rare-earth elements (REE)
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