The geochemistry, age, and origin of groundwater in a mafic pluton, East Bull Lake, Ontario, Canada

Abstract

The geochemistry of groundwater in the East Bull Lake gabbro-anorthosite pluton near Elliot Lake, Ontario, Canada, has been investigated. Three chemical types of groundwater are present to depths of about 650 m, and are controlled by the nature of the groundwater flow systems and the water/rock interactions in the fractures. Ca HCO 3 water is present in the recharge area of the local flow system and evolves rapidly along the direction of groundwater flow to a high-pH, Na HCO 3 water primarily by cation exchange reactions. Saline Na Cl water is present below a depth of about 350 m. On the basis of its chemistry and hydraulic head measurements, the Na Cl water is believed to be part of a deep regional groundwater flow system. Carbon-14 measurements of the Na HCO 3 water suggest that this water was recharged shortly after the last Pleistocene deglaciation. This is supported by δ 18O values that are as much as 6%. lighter than shallow modern groundwater. Despite the high Mg content of the enclosing rocks, Mg 2+ concentrations in the groundwater of this unit are <1.0 mg L −1, and may be controlled by present-day serpentinization or formation of talc. Laumontite is thermodynamically stable in the Na HCO 3 and Na Cl waters and hence could still be forming under the present low-temperature conditions (<25°C). The stable isotope composition of the Na Cl water suggests that this water type is not a geochemically evolved product of the overlying waters, although present-day albitization of the anorthosite could be responsible for the increased Ca Na ratio in the Na Cl water compared to the other water types. The salinity of this water appears to have been derived outside the pluton, possibly as an ancient infiltration into crystalline basement rocks of saline formation waters present in Paleozoic sedimentary rocks, which formerly overlay the pluton, or from nearby Proterozoic formations. This hypothesis is supported by Br Cl ratios that are similar to sea water and many present-day formation waters, and the isotopic composition of the dissolved sulphate. The He “age” of the saline water is of the order of 10 8 years. Therefore both the saline water and many of its solutes appear to have ancient origins.