Geochemical analysis of thermal fluids from southern Mount Meager, British Columbia, Canada

Abstract

Geothermal exploration has been ongoing intermittently in the Mount Meager geothermal area, British Columbia, Canada, since the 1970s. However, a geochemical interpretation of the many fluid samples collected during this time period has not been carried out for several decades, leaving gaps in understanding of the system which modern techniques can help fill. Thermal fluids from springs and wells in the southern Mount Meager geothermal field were analyzed to understand the fluid origin, controls on chemistry from effects of mixing and water-rock interaction, and to determine the reservoir temperature and composition. Chloride and boron concentrations ranged from 100 to 3300 ppm and 0.3 to 28 ppm, respectively. These very high conservative element concentrations cannot alone be explained by rock dissolution and are instead likely supplied by a single magmatic source. Calculated reservoir compositions suggest that select deep wells have experienced significant CO2 degassing from reservoir to the surface, and high SO4 content present from surface samples is traced down to the reservoir. These data corroborate the hypothesis that a magmatic component exists and contributes to B, Cl, CO2 and SO4 fluid composition, although CO2 and SO4 may have alternate sources. Select geothermometers calculated reservoir temperatures of up to 283 °C for central deep wells and as low as 30 °C and 5 °C for hot springs and cold springs, respectively. Like many high-temperature geothermal systems, the compositions of thermal fluids appeared to be controlled by the equilibrium between the fluid and observed secondary minerals. Hot springs and wells on the eastern and northern sides of the reservoir are of low temperature and likely define the boundaries of peripheral waters. Wells to the southeast contained anomalously high Cl and SO4, suggesting a possible magmatic input of these components which may be controlled by the east-west running Meager Creek Fault Zone. There is a significant source of hot, Cl- and CO2-rich thermal waters supplying deep wells MC-1, MC-2, MC-6 and MC-8, and possibly MC-3. These NaCl waters likely define the high temperature, central location of the geothermal reservoir.