Mineralogy and Fluid Regime of Formation of the REE-Late-Stage Hydrothermal Mineralization of Petyayan-Vara Carbonatites (Vuoriyarvi, Kola Region, NW Russia)

Abstract

The Vuoriyarvi Devonian alkaline–ultramafic complex (northwest Russia) contains magnesiocarbonatites with rare earth mineralization localized in the Petyayan-Vara area. High concentrations of rare earth elements are found in two types of these rocks: (a) ancylite-dominant magnesiocarbonatites with ancylite–baryte–strontianite–calcite–quartz (±late Ca–Fe–Mg carbonates) ore assemblage, i.e., “ancylite ores”; (b) breccias of magnesiocarbonatites with a quartz–bastnäsite matrix (±late Ca–Fe–Mg carbonates), i.e., “bastnäsite ores.” We studied fluid inclusions in quartz and late-stage Ca–Fe–Mg carbonates from these ore assemblages. Fluid inclusion data show that ore-related mineralization was formed in several stages. We propose the following TX evolution scheme for ore-related processes: (1) the formation of ancylite ores began under the influence of highly concentrated (>50 wt.%) sulphate fluids (with thenardite and anhydrite predominant in the daughter phases of inclusions) at a temperature above300–350 °C; (2) the completion of the formation of ancylite ores and their auto-metasomatic alteration occurred under the influence of concentrated (40–45 wt.%) carbonate fluids (shortite and synchysite–Ce in fluid inclusions) at a temperature above 250–275 °C; (3) bastnäsite ores deposited from low-concentrated (20–30 wt.%) hydrocarbonate–chloride fluids (halite, nahcolite, and/or gaylussite in fluid inclusions) at a temperature of 190–250 °C or higher. Later hydrothermal mineralization was related to the low-concentration hydrocarbonate–chloride fluids (<15 wt.% NaCl-equ.) at 150–200 °C. The presented data show the specific features of the mineral and fluid evolution of ore-related late-stage hydrothermal rare earth element (REE) mineralization of the Vuoriyarvi alkaline–ultramafic complex.

Rare earth element mobility in and around carbonatites controlled by sodium, potassium, and silica

“…In run CbNa, REE occur in Na-rich burbankite-group phases. Natural burbankite is a rare and ephemeral mineral, often pseudomorphed by fine-grained assemblages of REE fluorcarbonates, monazite, baryte, strontianite, and rock-forming carbonates (6,27,(29)(30)(31)(32)(33)(34)(35), and occasionally described in fluid inclusions (15,22,36). Our experiments also contain extensive REE hosted in the Na-carbonates bonshtedtite and rouvilleite (table S1).…”

“…Carbonatites preserved in the geological record are mostly calcic, containing no alkali phases other than silicates. Fluid and melt inclusions in carbonatites or fenites are often highly alkali-rich ( 11 , 13 – 16 , 34 , 36 , 49 , 50 , 62 , 68 – 70 ), suggesting original liquid compositions equivalent to the combined composition of both carbonatites and fenites. Our experiments support this duality: An originally homogenous alkali-bearing carbonatite crystallized to an assemblage of solid calcic carbonates with a separate alkali-rich fluid, which in nature fenitizes the surrounding rock ( 10 , 14 , 66 ).…”

Enrichment of heavy REE and Th in carbonatite-derived fenite breccia

“…During crystallization of the carbonatitic melt, the HREE are incompatible in the crystallizing assemblage so are retained in the residual Na-and K-rich fluid phase (Anenburg et al 2020). Similar alkali-rich fluids have been proposed as the origin of fenite alteration aureoles adjacent to carbonatites, as supported by the presence of alkali carbonate daughter minerals in fluid inclusions (Bühn & Rankin, 1999;Williams-Jones & Palmer, 2002;Dowman et al 2017a;Prokopyev et al 2020).…”

Mass balance of complementary metasomatic processes using isocon analysis

“…However, a numerical experiment showed that the use of one (common) protolith has almost no effect on the final result. We emphasize strong evidence that the primary fluids altering the source rocks contained very few x and y components under consideration [ 12 , 15 , 16 ]. We note that the considered components (Sr, Ba, and REEs) are abundant in carbonatites and very few in the host rocks.…”

“…We note that the considered components (Sr, Ba, and REEs) are abundant in carbonatites and very few in the host rocks. Thus, their introduction from an external source is unlikely, while their redistribution between the indicated types of carbonatites is confirmed by geological methods [ 12 , 15 , 16 ]. The proposed methodology is applicable to the specified geological profile of the processes.…”