REE -Sr-Ba minerals from the Khibina carbonatites, Kola Peninsula, Russia: their mineralogy, paragenesis and evolution

Abstract

ABSTR ACTCarbonatites from the Khibina Alkaline Massif (360 380 Ma), Kola Peninsula, Russia, contain one of the most diverse assemblages of REE minerals described thus far from carbonatites and provide an excellent opportunity to track the evolution of late-stage carbonatites and their sub-solidus (secondary) changes. Twelve rare earth minerals have been analysed in detail and compared with literature analyses. These minerals include some common to carbonatites (e. g. Ca-rare-earth fluocarbonates and ancylite-(Ce)) plus burbankite and carbocernaite and some very rare Ba,REE fluocarbonates .Overall the REE patterns change from light rare earth-enriched in the earliest carbonatites to heavy rare earth-enriched in the late carbonate-zeolite veins, an evolution which is thought to reflect the increasing 'carbohydrothermal' nature of the rock-forming fluid. Many of the carbonatites have been subject to sub-solidus metasomatic processes whose products include hexagonal prismatic pseudomorphs of ancylite-(Ce) or synchysite-(Ce), strontianite and baryte after burbankite and carbocernaite. The metasomatic processes cause little change in the rare earth patterns and it is thought that they took place soon after emplacement .

The origin of secondary heavy rare earth element enrichment in carbonatites: Constraints from the evolution of the Huanglongpu district, China

“…5G). Burbankite has not been directly observed, but this texture and assemblage is typical of burbankite replacement (Zaitsev et al, 1998(Zaitsev et al, , 2002. The subsequent development of the REE mineral paragenesis is recorded by reaction textures in phosphates and silicates, the colloform banded infill of niobate pseudomorphs, and the development of synto epitaxial alteration and overgrowth in the fluorcarbonates.…”

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

“…REE enrichment is "most commonly found only in the latest and most highly evolved parts of a carbonatite intrusion" [6]. The accumulation of REE in late carbonatites is assumed to be controlled by fluid activity [7][8][9][10][11][12][13][14][15][16]. Such a surplus in articles reflects a keen interest in REE occurring in late carbonatites.…”

Rare Earth Element Fluorocarbonate Minerals from the Olympic Dam Cu-U-Au-Ag Deposit, South Australia

“…Average La/Nd and La/Ce ratios in Olympic Dam synchysite are 1.147 and 0.419, respectively, within the field of published data for occurrences associated with felsic intrusive rocks ( Figure 10). Published compositional data for bastnäsite tend to be more varied than for synchysite, but overall, data for carbonatite-hosted bastnäsite [42], skarn bastnäsite [38], and from Bayan Obo [35] have higher La/Nd (4.103, 5.653, and 2.117, respectively) than bastnäsite associated with felsic intrusions: pegmatites [47]; alkaline igneous rocks [40]; or metamorphic rocks [48], which have lower La/Nd (1.372, 1.162, and 1.424, respectively). The distinction in terms of La/Ce ratios is less marked but ratios for intrusion-associated bastnäsite are nevertheless lower than for carbonatite-and skarnhosted types.…”

“…"Hydrothermal" synchysite, in which REE is sourced from fluids associated with igneous host rocks (syenite, granite) has average ratios La/Nd of 1.157 and La/Ce ratios of 0.470 [39][40][41]. In contrast, carbonatite-hosted synchysite has average La/Nd and La/Ce ratios of 2.471 and 0.617, respectively [42,43]. Pegmatite-hosted synchysite [44] has average La/Nd and La/Ce ratios of 1.692 of 0.524; synchysite precipitated in basalts has average ratios of La/Nd = 1.459 and La/Ce = 0.543 [45].…”