Hydrothermal alteration of chevkinite-group minerals: products and mechanisms. Part 1. Hydration of chevkinite-(Ce)

Abstract

Samples from Russia and Scotland are used to examine the interaction of the REE-Ti silicate chevkinite-(Ce) with hydrothermal fluids. Altered zones in crystals are distinguished by using areas of low intensity on backscattered-electron images, low analytical totals, increasingly large departures from stoichiometry and, in some cases, the presence of micropores. Initial alteration of the chevkinite results in strong Ca enrichment. With increasing degrees of alteration, Ca abundances drop sharply, as do those of the REE, Fe and Si. In contrast, Ti levels increase strongly, usually accompanied by higher Nb ± Th levels. The most altered zones contain up to 36 wt.% TiO2 and the formula cannot be expressed in the standard chevkinite formula. In detail, samples follow different alteration trends, presumably reflecting different P, T, fO2 and fluid composition. The Ti enrichment may have been related to a reaction front of dissolution-reprecipitation passing through the outer zones of the original chevkinite, leaving behind a reprecipitated Ti-enriched phase which may or may not be chevkinite.

Hydrothermal metasomatism of a peralkaline granite pegmatite, Khaldzan Buragtag massif, Mongolian Altai; complex evolution of REE-Nb minerals

“…It should be stressed that the sequence is seen as essentially continuous, as the mineral assemblages changed in response to changing fluid composition and temperature decrease. Jiang (2006), Vlach and Gualda (2007) and Bagiński et al (2015) to hydrothermal alteration. During that process, however, Nb tends to increase and the La/Ce ratio decreases, features not seen in the study samples.…”

“…Recent studies of the hydrothermal alteration of the chevkinite group of minerals (CGM) have revealed a diversity of alteration products, dependent, inter alia, on the composition of the host rock, the composition of the hydrothermal fluids and the pressure and temperature at which the alteration proceeded (Jiang 2006;Vlach and Gualda 2007;Bagiński et al 2015;Macdonald et al 2015Macdonald et al a, b, 2016. The diversity has consequences for the mobilization or retention of rare-metals and the significance for the formation of rare-metal deposits.…”

Hydrothermal alteration of chevkinite-group minerals. Part 2. Metasomatite from the Keivy massif, Kola Peninsula, Russia

“…Full details of the analytical conditions used are given in Baginśki et al (2015) and will not be repeated here.…”

“…In our studies of the low-temperature alteration of the CGM, we have divided alteration schemes into two types: (1) those which started with compositional modification of the CGM and the production of a zone of 'altered chevkinite'; and (2) those where formation of this zone was followed by the formation of a new mineral assemblage. In a companion paper, Baginśki et al (2015) described examples of type (1) alteration. Here we describe an example of type (2), where the CGM was initially altered by interaction with hydrothermal fluids, followed by crystallization of assemblages dominated by epidote-supergroup minerals.…”

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

“…The distribution of allanite and chevkinite in the studied carbonatites dikes suggests that the composition of the magmas is an important control on the stability fields of these minerals, a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing magmatic fluids and a later hydrothermal fluid. Bagiński et al [76] and Macdonald et al [77] documented the alteration of chevkinite-(Ce) to a phase strongly enriched in Ti and depleted in Si, REE and Fe compared to the unaltered phase. In contrast to studies chevkinite did not alter to a member of the epidote-group, possibly because REE + Y abundance in chevkinite (43-50 wt %) is almost twice that observed in allanite.…”