Lithium Isotope Geochemistry and Origin of Canadian Shield Brines

摘要

Hypersaline calcium/chloride shield brines are ubiquitous in Canada and areas of northern Europe. The major questions relating to these fluids are the origin of the solutes and the concentration mechanism that led to their extreme salinity. Many chemical and isotopic tracers are used to solve these questions. For example, lithium isotope systematics have been used recently to support a marine origin for the Yellowknife shield brine (Northwest Territories). While having important chemical similarities to the Yellowknife brine, shield brines from the Sudbury/Elliot Lake (Ontario) and Thompson/Snow Lake (Manitoba) regions, which are the focus of this study, exhibit contrasting lithium behavior. Brine from the Sudbury Victor mine has lithium concentrations that closely follow the sea water lithium-bromine concentration trajectory, as well as δ~6Li values of approximately -28 per thousand. This indicates that the lithium in this brine is predominantly marine in origin with a relatively minor component of crustal lithium leached from the host rocks. In contrast, the Thompson/Snow Lake brine has anomalously low lithium concentrations, indicating that it has largely been removed from solution by alteration minerals. Furthermore, brine and nonbrine mine waters at the Thompson mine have large δ~6Li variations of ～30 per thousand, which primarily reflects mixing between deep brine with δ~6Li of -35 +- 2 per thousand and near surface mine water that has derived higher δ~6Li values through interactions with their host rocks. The contrary behavior of lithium in these two brines shows that, in systems where it has behaved conservatively, lithium isotopes can distinguish brines derived from marine sources.