Fluid evolution in the Strange Lake granitic pluton, Canada: Implications for HFSE mobilisation

Abstract

10Strange Lake is a mid-Proterozoic peralkaline granite pluton (Québec-Labrador, Canada) that 11 underwent extreme enrichment in high field strength elements (HFSE), including the rare earth 12 elements (REE). The HFSE mineralisation is confined to highly altered pegmatites and the most 13 altered parts of the granites, implying a genetic association between hydrothermal fluids and 14 HFSE enrichment. This study uses analyses of fluid inclusions to investigate the hydrothermal 15 evolution of the Strange Lake pluton and the role of hydrothermal processes in concentrating the 16 HFSE to potentially exploitable levels. 17Five groups of inclusions were distinguished. From earliest to latest, these groups are: primary 18 aqueous inclusions (~25 wt.% NaCl eq.) associated with melt inclusions (Group 1); primary 19 Fluid evolution commenced with the exsolution of a saline aqueous liquid (~25 wt.% NaCl eq.) 28 and an immiscible CH 4 +H 2 gas from the pegmatitic melt at temperatures of ~450-500 °C and a 29 pressure of ~1100 bars. During isobaric cooling, the gas component of the fluid was gradually 30 oxidised, evolving from being CH 4 -dominant to a CH 4 fluid with a significant proportion of 31 higher order hydrocarbons (due to oxidative coupling of methane induced by the consumption of 32 O 2 through the alteration of arfvedsonite to aegirine; ~325-360°C), and finally to a CO 2 -33 dominated fluid at ~300 o C. The apparent salinity of the aqueous fluid decreased from ~25 to 34 ~4.5 wt.% NaCl eq. due to fluid-rock interaction. The latter also caused precipitation of nahcolite 35 (as a result of the reaction of newly formed CO 2 with sodium from decomposing minerals), 36 formation of pseudomorphs after primary Na-zirconosilicates and Na-titanosilicates and 37 replacement of primary REE-silicates by bastnäsite-(Ce). Owing to this interaction, the carbonic 38 component of the fluid was consumed which, together with the consumption of H 2 O to form Al-, 39 K-and Fe-phyllosilicates, contributed to an increase in the fluid salinity (up to ~19 wt.% NaCl 40 eq.). 41 3 Light rare earth elements (LREE) were remobilised over 10s to 100s of metres by the high 42 temperature high salinity fluid (preserved as Group 1 and 2a inclusions), whereas heavy rare 43 earth elements (HREE) were remobilised on a much smaller scale by a late, low temperature 44 high salinity fluid (trapped as inclusion Group 5). Fluid preserved as inclusion Group 3 may have 45 been responsible for remobilisation of Zr and Ti. 46

Rare Earth Elements in Mineral Deposits: Speciation in Hydrothermal Fluids and Partitioning in Calcite

“…Igneous processes such as partial melting, crustal assimilation, melt immiscibility, and fractional crystallization, along with crustal metasomatism, can lead to the formation of REE-enriched carbonatite, alkaline, and peralkaline melts [2,[10][11][12][13][14][15]. Numerous studies have also indicated the importance of hydrothermal processes for the mobilization and concentration of the REE in the late magmatic evolution stages of carbonatite and alkaline/peralkaline systems [4,5,[16][17][18][19].…”

Rare metal indicator minerals in bedrock and till at the Strange Lake peralkaline complex, Quebec and Labrador, Canada

“…2). Mineral paragensis and alteration assemblages at Strange Lake have been described by several researchers, including Boily and Williams-Jones (1994), Miller (1986), Salvi and Williams-Jones (1990, 1996, 2006, Gysi and Williams-Jones (2013), Williams-Jones (2014, 2018), and Vasyukova et al (2016).…”

Fractionation of Rare Earth Elements in Greisen and Hydrothermal Veins Related to A-Type Magmatism

“…This is explained mainly by the abundant incorporation of these metals into the accessory minerals fluorite, monazite, and apatite (Figures 4(a) and 4(b)). The high concentrations in monazite and apatite indicate that the retention was primarily a consequence of the high uptake by phosphates [6], whereas the high concentrations in fluorite also show that this mineral constituted a significant trap at the magmatic stage, in line with observations elsewhere that fluorite can incorporate significant quantities of REE from a melt [57]. Unlike the phosphates, fluorite was also abundant in greisen and veins, with an overall pattern of decreasing REE concentrations in the sequence granite>greisen>veins (Figure 8(a)).…”

“…The growing demand for rare earth elements (REE) in modern techniques for a sustainable environment has been associated with extensive prospecting for these metals worldwide [1]. Numerous laboratory experiments and genetic models have incorporated fluid-inclusion and isotopic data and/or trace element partitioning calculations to understand how REE are transported, fractionated, and ultimately deposited in ores typically associated to the formation or alteration of carbonatitic and alkaline to peralkaline rocks [2][3][4][5][6]. REE enrichment in these systems is often ascribed to magmatic processes that can concentrate REE in fluoride liquids over silicate melt through liquid immiscibility [7] or in primary REE-rich minerals such as phosphates and zircon crystallizing from magma [8].…”