HIGHLY ALKALINE GROUNDWATERS IN KIMBERLITES – GEOCHEMICAL AND ISOTOPIC EVIDENCE FOR LOW-TEMPERATURE SERPENTINIZATION

A groundwater study of the Lake Timiskaming and Kirkland Lake kimberlite fields was undertaken to understand the hydrology and hydrogeochemistry of groundwater-kimberlite interaction and to develop groundwater as a kimberlite exploration tool. Groundwater from exploration drill holes was sampled at five kimberlites: B30, C14, A4, Diamond Lake (DL), and 95-2. The pH, oxidation-reduction potential (Eh), dissolved oxygen, electrical conductivity and temperature were measured in-situ. Field results indicate that many groundwaters have unusually high pH, with values at B30, C14 and A4 in the range of 11.00 to 12.45. Groundwaters have been analyzed for major and trace elements (ICP-OES, ICP-MS) and stable isotopes (C, O, H, S). d²H and d¹⁸O isotopes and plot along a local meteoric water line (LMWL). The A4 kimberlite is the exception, with waters up to 10 per mil more enriched in d²H compared to the LMWL. Reasons for this enrichment may include fractionation due to high amounts of OH^-, longer reaction times with rocks, or the waters may be older than the others collected and possibly have been recharged at the end of glaciation. The d¹³C analyses show variation consistent with dissolution of Paleozoic carbonate rocks with an additional source of carbon such as organic acids, for DL waters. For all waters, the amount of K increases with increasing pH to a maximum of 39600 mg/L. As K and pH increase, Mg decreases to a minimum value of less than the detection limit. The relationship between K and Mg likely indicates that the Mg is consumed in the formation of brucite and serpentine and their supersaturation in the waters. The same is true for Ca where the same trend observed with pH vs K/Mg is also seen in pH vs K/Ca.