EFFECTS OF THE GLACIAL-PERIGLACIAL CYCLE ON DEEP GROUNDWATER AT THE LUPIN MINE, NUNAVUT, CANADA

The periglacial-glacial cycle can impact groundwater conditions in the deep subsurface. Column studies in a laboratory have shown the impacts of freezing on groundwater chemical and isotopic compositions, but very little has been published regarding cryogenic processes and hydrogeology in crystalline rock currently under deep permafrost conditions. The study was conducted at the Lupin mine in Nunavut Territory, Canada, where permafrost extends to 540 meters below ground surface (mbgs). This site has experienced several glacial cycles. Striations were observed on rock outcrops, evidence of a warm glacial foot. Thermal and climate modeling suggests conditions under the last glacial advance were warm enough to melt pre-existing permafrost.

GPR measurements over several ice covered lakes in the area show taliks exist. Taliks are a potential conduit of modern fresh water through permafrost to deeper zones Many samples obtained from frozen levels of the mine were contaminated by Na-Cl brines used for drilling. However, many geochemical parameters such as rare earth element signatures of groundwaters from the permafrost are not likely from salt contamination and may be attributable to freeze-out processes.

Samples from deeper in the mine provide insight into chemistry of deep groundwaters below the permafrost. Waters range from brackish to saline, Na-Cl to Ca-Na-Cl dominant, and lack indication of contamination. Borehole waters have higher salinities than matrix fluids. Long horizontal boreholes intersected several fracture systems with waters of different salinity. Isotopic age tracers (¹⁴C, ³⁶Cl) collected at depth indicate a common recharge event around 24,000-26,000 ybp, regardless of the salinity of the water sampled. Occasional tritium in some samples at depth indicate a combination of gradients from mine drawdown and taliks allowing recharge of modern fresh water to depth. Composition of gases at depth suggest the freezing process has impacted isotopic and relative abundance of gases as well.