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 CO2-rich, with variable levels of Ca activity, and with fO2 mainly between the nickel-nickel oxide and magnetite-hematite buffers. This occurrence represents a new paragenesis for a columbite-group mineral.
Highlights
The two most abundant members of the chevkinite group of minerals (CGM) are chevkinite and perrierite, which have the general formula (REE,Ca)4Fe2+(Fe2+,Fe3+,Ti)2Ti2(Si2O7)2O8
With ΣREE2O3 contents up to 50 wt.%, members of the group have a high potential for REE mobilization during hydrothermal alteration and the mineralizing potential of the fluids
The CGM have the potential to be used for radiometric dating; Vazquez et al (2014), for example, presented the results of ion microprobe 238U-230Th dating of chevkinite from high-silica rhyolites of La Primavera caldera, Mexico, which were broadly consistent with corresponding K-Ar ages
Summary
The two most abundant members of the chevkinite group of minerals (CGM) are chevkinite and perrierite, which have the general formula (REE,Ca)4Fe2+(Fe2+,Fe3+,Ti)2Ti2(Si2O7)2O8. Bagiński et al (2015) and Macdonald et al (2015) documented the alteration of chevkinite-(Ce) to a phase strongly enriched in Ti (and to a lesser degree in Nb and Th) and depleted in Si, REE and Fe compared to the unaltered phase We complement these studies by documenting a previously unrecorded type of interaction between a CGM and hydrothermal fluids, where chevkinite-(Ce) was altered to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage via a complex series of intermediate steps which were arrested before equilibrium was achieved. The specimen was collected from a small trench or surface exposure, or from a small pegmatite vein accompanying pegmatite #35 at its northern end At present, this part of Dolgaya has been completely mined out by the quarry company and is buried under dumps. Along its boundary with the microcline matrix, the crystal has been replaced by a 5–12 mm b
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