THE SR-87 SR-86 VALUES OF CANADIAN SHIELD BRINES AND FRACTURE MINERALS WITH APPLICATIONS TO GROUNDWATER MIXING, FRACTURE HISTORY, AND GEOCHRONOLOGY

Analyses of saline waters, fracture minerals, and host rocks from seven localities on the Canadian Shield demonstrate the utility of the 87Sr 86Sr ratio in the study of groundwater systems in crystalline rocks. The ratios range from 0.704 to 0.753 and have obtained their signatures by mineral/rock interactions, primarily involving the feldspars. We have identified brines from isolated pockets in the same mines where extensive flow regimes exist. There is mixing of different brines as well as mixing with meteoric waters. The isotopic results on calcites from fractures and shear zones show more than one generation of mineral growth in a given fracture. The 87Sr 86Sr ratios of the calcites vary from values identical to the present-day brine in the fracture zone to ratios with Archean signatures. This implies that activity may occur in fault zones over a very long time. The brines are very rich in Sr (up to 2400 mg/1), very low in Rb, and have relatively radiogenic 87Sr 86Sr ratios. They are ubiquitous in Shield rocks and, if they were present throughout geological time, they may be one reason why Rb Sr ages of felsic plutons are commonly younger than associated U Pb ages. © 1990.