INDICATIONS FOR FLUID MIXING IN ARCHEAN EPIDOSITE – FLUID INCLUSION DATA FROM THE ~2.7 GA FLAVRIAN PLUTON, NORANDA, CANADA

The ~2.7 Ga Archean Blake River Group (BRG) comprises part of the Abitibi Greenstone Belt of Ontario and Quebec, Canada. In the Noranda area of Quebec the BRG is intruded by the Flavrian Pluton. It has a surface expression of ~14 km by ~9 km, and it has been interpreted as a multy-phase sill-like intrusion. The rocks belong to the tonalite-trondhjemite-granodiorite suite typical of intrusions in Archean greenstones. The pluton is interpreted to be the intrusive equivalent to its volcanic host rock and is thought to have intruded its own ~8 km thick volcanic pile during cauldron generation of a large volcanic edifice in the seafloor environment. The formation of 18 known VMS deposits in the hanging wall of the pluton are believed to be due to a large hydrothermal system driven by the pluton. Fluid inclusions from magmatic quartz, patchy epidosite, and pipe vesicle fill have been analyzed microthermometrically. The results show two distinct fluids; the first is Na-dominated whereas the second is Ca-dominated. The former is found in two- and three- phase halite-bearing inclusions, believed to have formed due to exsolution of fluids from the fractionated magma. Chlorinities in this fluid range from ~5000 mmol/kg to ~9500 mmol/kg. The Ca-dominated fluid is interpreted to be Ca-enriched, evolved Archean seawater where high Ca values are due to widespread albitization of the intrusion and Na (fluid) – Ca (intrusion) exchange as well as a higher initial Ca value. The chlorinities of this fluid range between ~500 mmol/kg and ~2200 mmol/kg. Uncorrected low homogenization temperatures of 100°C to 150°C and the lack of separate vapor-rich inclusions suggest that phase separation did not occur. A pressure correction of ~100°C must be applied according to a pressure of ~1 - 1.5 kbar during pluton emplacement. Homogenization temperatures and chlorinities are consistent with those found in younger ophiolites and modern arc-related epidosite. However, in younger systems all fluids are Na-dominated.