Fluids associated with an Archean regional hydrothermal system, Abitibi subprovince, Canada; Fluid inclusion characteristics and hot cathode cathodoluminescence characteristics of host minerals.

摘要

The central part of the Archean Abitibi greenstone belt underwent prehnite-pumpellyite to lower greenschist facies regional metamorphism. These conditions are ideal to constrain characteristics of fluids involved in a regional submarine hydrothermal system. Hot cathode cathodoluminescence shows that quartz associated with this submarine hydrothermal system is of primary hydrothermal origin. Quartz textures, including growth zoning, primary depositional textures, and a lack of triple junctions typical of recrystallization, suggest that the quartz is of primary Archean origin.; Fluids from the Ben Nevis area, interpreted to be a hydrothermal recharge zone, have low eutectic temperatures between -32°C and -52°C, indicating Ca to be an important fluid constituent. Homogenization temperatures between ∼100°C and ∼160°C as well as high salinities (4--12 CaCl2 wt% equiv.) are consistent with data from other Precambrian seafloor related rocks.; The majority of fluid inclusion microthermometric data from the Ansil volcanogenic massive sulfide deposit shows temperatures between 200°C and 250°C, consistent with temperatures during precipitation of sphalerite. A small amount of data suggest maximum fluid temperatures of 390°C and boiling during chalcopyrite precipitation, consistent with a water depth of ∼900 m. Salinities are highly variable; fluids are Ca-dominated. Fluid inclusion microthermometry fails to identify all fluids associated with the formation of the Ansil deposit proposed by previous workers.; Fluid inclusion microthermometry on magmatic quartz and patchy epidosite from the plagiogranitic Flavrian pluton has identified two distinct fluids. A Na-dominated fluid is found in halite bearing inclusions, which homogenize by halite disappearance at temperatures between 150°C and 450°C. Chlorinities range between 5000 and 10,000 mmol/kg. This fluid is interpreted to be of magmatic origin. A Ca-dominated fluid is found in liquid dominated two phase inclusions; pressure corrected homogenization temperatures range from ∼200°C to ∼300°C, chlorinities range from ∼500--∼2000 mmol/kg. This fluid is interpreted to be modified seawater. The data is consistent with Mesozoic and modern equivalents.; Distinct temperatures and chlorinities show that the investigated fluids were trapped at the time of deposition of the host minerals and hence are of Archean origin. The data suggest that the Archean seawater was likely more saline and Ca-rich.