Boron and strontium isotope study of fluids situated in fractured and unfractured rock of the Lac du Bonnet Batholith, eastern Manitoba

Abstract

<p>Groundwater from fractured and unfractured rock in the Lac du Bonnet Batholith, Manitoba, has been analyzed for B and Sr isotopes, and ion chemistry. Fluids from unfractured rock (pore fluids) have been sampled by several methods 1) direct sampling of undiluted pore fluids flowing into boreholes; 2) leaching of pore fluids into boreholes filled with distilled deionized water; 3) extraction of intercrystalline fluids from drill cores. Pore fluids can be very saline, up to 100g/L total dissolved solids, and are chemically distinct from fracture waters. They are 1) enriched in radiogenic ⁸⁷Sr, exhibiting ⁸⁷Sr/⁸⁶Sr ratios as high as 0.801; 2) depleted in Sr with respect to Ca having Ca/Sr molar ratio > 918; 3) depleted in Na and Sr with respect to Cl (Na/Cl < 0.18; Sr/Cl < 0.00085); and 4) have low δ¹¹B-values (<13.5%) and low boron concentrations (<0.27 μg/g). In contrast, fracture waters exhibit ⁸⁷Sr/⁸⁶Sr ratios between 0.713-0.734, have an average Ca/Sr molar ratio of 236, Na/Cl ratio of 0.55, and a Sr/Cl ratio of 0.0014. They are also enriched in ¹¹B up to a δ¹¹B-value of 52.7% and have relatively high boron concentrations compared to the pore fluids; as high as 1.57 μg/g. The dissimilarity in compositions between the two groundwater types indicates that two distinct chemical environments are present in the Lac du Bonnet Batholith. The fracture water chemistry is strongly controlled by the dissolution and alteration of minerals, in particular plagioclase. The dominance of plagioclasle dissolution is seen in the Na/Ca molar ratios (5.60), Ca/Sr molar ratios, and Sr isotopic compositions similar to the granite's oligoclase. The fracture waters are interpreted as an open system where chemical equilibrium between groundwaters and rock has not been attained. Boron isotopes in fracture waters exhibit marine-like compositions, however, due to boron's low concentration in the fracture waters relative to seawater and it's relationship to bromide and chloride concentrations in solution, the source of the boton is not considered to be marine in origin. A model is proposed where the exchange of boron between host rock and solution has resulted in boron concentrations and isotopic compositions of the fracture waters reaching a steady state. In contrast, pore fluid chemistry is not controlled by plagioclase dissolution as evidenced by the absence of a plagioclase ⁸⁷Sr/⁸⁶Sr and Ca/Sr molar ratio signature, and low Na/Cr molar ratio. The high Ca/Sr and low Na/Ca ratios are attributed to the formation of albite. The similarity between the isotopic composition of pore fluids and granite indicate that these waters have reached isotopic and chemical equilibrium with the host rock. The pore fluids can, therefore, be considered a closed system. The similarity in salinities of the most saline fluid inclusions (≈200g/L) and the predicted salinities of pore fluids below 1000m depth in the Lac du Bonnet pluton may indicate a genetic link between a pore fluids and fluid inclusions.</p>