AQUEOUS GEOCHEMICAL ANOMALIES IN KIMBERLITES IN NORTHEASTERN ONTARIO ? EXPLORATION AND ENVIRONMENTAL IMPLICATIONS

Groundwaters found in kimberlite rocks are as anomalous as the rocks themselves and display characteristics unusual for waters in crystalline rock. This has lead to the on going development of a new method of kimberlite exploration using the aqueous geochemical characteristics of groundwaters. Environmental implications have also arisen out of this project due to the threat that alkali mine drainage from open-pit diamond mining may pose to the environment. Sixty groundwater samples were taken from 5 kimberlites in the Lake Timiskaming and Kirkland Lake kimberlite fields of northeastern Ontario. Almost one half of the waters exhibit very high pH (10 to 12.4) and low Eh (-200 to -538 mV), with OH-type alkalinity. An increase of pH is associated with an increase in K, and a decrease in the concentrations of Mg, Ca and Rb. The relationships that K/Mg, K/Ca and K/Rb have with pH is interpreted to be related to low temperature serpentinization of the kimberlite minerals, during which Mg and Ca are consumed in the formation of brucite and serpentine. Such water characteristics are consistent with the alteration of ultramafic rock and low-temperature serpentinization. The waters are all of modern recharge origin, however one kimberlite (highest pH, lowest Eh and most gas) hosts waters that do not fall along a local meteoric water line because a possible 2H fractionation during alteration processes and the liberation of H from the system. 87Sr/86Sr ratios in kimberlite groundwaters are low and are consistent with the influence of ultramafic mineral weathering, However, differences between the 87Sr/86Sr ratios of the groundwaters are different from the world-wide average for kimberlite whole-rock samples and suggest preferential weathering of high Sr, low 87Sr/86Sr minerals, and partial mixing with groundwaters from surrounding Archean felsic rock. Geochemical and isotopic comparisons with groundwaters in surrounding host country rock and the kimberlite-water reactions will aid in determining vectors towards kimberlite targets, and determine if there is a threat to the environment from alkali mine drainage.