Evolution of the fluid in the REE-rich pegmatites of the Strange Lake pluton

Abstract:

Five types of aqueous inclusions are observed in the Mid-proterozoic peralkaline granites and pegmatites of the Strange Lake pluton (Québec-Labrador, Canada). From earliest to latest they are: primary aqueous inclusions associated with melt inclusions (type 1), with CH₄ inclusions (types 2 a and b enriched in H₂ and higher hydrocarbons respectively), and CO₂ inclusions (type 3), primary aqueous inclusions, which show no apparent association with any gas-rich fluid (type 4) and secondary aqueous inclusions associated with mineral pseudomorphs (type 5).

The salinity of the fluid decreases from the earliest type 1 (24.1 wt.% NaCl eq.) to type 2 a and b (21.2 and 14.2 wt.% NaCl eq. respectively), to type 3 (10.1 wt.% NaCl eq.) and type 4 inclusions (8.2 wt.% NaCl eq.). The salinity of type 5 inclusions is high, i.e., 16.6 wt.% NaCl eq.. The gas composition changes systematically from CH₄ ± H₂ in type 1 and 2a to CH₄ + higher hydrocarbons in type 2b and to CO₂ dominant type 3 inclusions. Most of the inclusions show re-equilibration texures (‘implosion’ halos). Another typical feature is a tendency to decrepitate at temperatures higher than 140-150 °C.

The fluid evolution started with the exsolution of a saline aqueous liquid (~25 wt.% NaCl eq.) from the pegmatitic melt at a temperature of about 400 °C and a pressure around 1100 bars. Further evolution occurred due to cooling (isobaric) and oxidation of the fluid. Early initial oxidation led to formation of higher hydrocarbons by oxidative coupling of methane, and later, full-blown oxidation led to the production of CO₂. The CO₂ reacted with Na-rich fluid to form nahcolite, which caused a significant decrease in the apparent salinity down to a critical level, below which further oxidation led to an increase in the CO₂/CH₄ ratio and eventual disappearance of CH₄. Isobaric cooling continued down to at least 150-170 °C, at which temperature the CO₂ component was consumed to form REE-rich flurocarbonates. Nahcolite decomposed to release additional CO₂ and Na. The CO₂ was consumed by further fluid-rock interaction, whereas the Na contributed to an increase in fluid salinity.