Hydrothermal alteration of a chevkinite-group mineral to a bastnäsite-(Ce)-ilmenite- columbite-(Fe) assemblage: interaction with a F-, CO2-rich fluid

Abstract

The results are presented of a textural and mineral chemical study of a previously undescribed type of hydrothermal alteration of chevkinite-(Ce) which occurs in a syenitic pegmatite from the Vishnevye Mountains, Urals Region, Russia. The progressive alteration of the chevkinite to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage through a series of texturally complex intermediate stages is described and electron microprobe analyses are given of all the major phases. Unusual Nb±Th-rich phases formed late in the alteration sequence provide evidence of local Nb mobility. The main compositional fluxes are traced, especially of the REE, HFSE, Th and U. It appears that almost all elements, with the exception of La, released from the chevkinite-(Ce) were reincorporated into later phases, such that they did not leave the alteration crust in significant amounts. The hydrothermal fluids are inferred to have been F-and CO 2 -rich, with variable levels of Ca activity, and with fO 2 mainly between the nickelnickel oxide and magnetite-hematite buffers. This occurrence represents a new paragenesis for a columbite-group mineral.

Chemical and textural relations of britholite- and apatite-group minerals from hydrothermal REE mineralization at the Rodeo de los Molles deposit, Central Argentina

“…In this study, the low oxide totals polygonal cracks in strong alteration zones (Fig. 8) show strong similarities with the "mosaic textures" of ThSiO4 phases described by Seydoux-Guillaume et al (2007) and Macdonald et al (2015). Those authors relate these textures to the formation of a gel-like amorphous, highly porous, and probably hydrated substance, initially caused by radiation damage.…”

Extreme Alteration of Chevkinite-(Ce) by Pb-CO2-Rich Fluids: Evidence from the White Tundra Pegmatite, Keivy Massif, Kola Peninsula

“…Such a fluid is consistent with theoretical studies that the REE form the most stable complexes with F and moderately strong complexes with CO 2 (Haas et al [17]; Samson and Wood [18]). In comparison, Macdonald et al [19] described, in a syenitic pegmatite from the Vishnevye Mountains, Russia, chevkinite-(Ce) being altered, via a series of texturally complex intermediate steps, to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage. The hydrothermal fluids were inferred to have been F-and CO 2 -rich, with variable levels of Ca activity.…”

Behaviour of ThSiO4 during hydrothermal alteration of raremetal rich lithologies from peralkaline rocks

“…Sample K4 is from an aegirine-microcline syenite pegmatite from the Vishnevye (Cherry) Mountains, southern Urals, Russia. Chevkinite-(Ce) was progressively altered by hydrothermal fluids to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage through a series of texturally complex intermediate stages (Macdonald et al, 2015b). The focus of this study is ThSiO 4 occurring in association with cerite-(Ce) and ilmenite in veinlets formed during the very latest stages of the alteration of chevkinite-(Ce).…”

“…The strings of ThSiO 4 in the Vishnevye pegmatite ( Fig. 5b) are associated with cerite-(Ce) and ilmenite and were formed during the very latest stages of the alteration of chevkinite-(Ce) to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage, promoted by CO 2 -F-rich hydrothermal fluids (Macdonald et al, 2015b). Oxide totals are low, 82.60-97.24 wt.% (average 90.77 wt.%), and the sums of cations high (2.06-2.19 apfu), clear indicators that the mineral has been altered significantly.…”