LOW – T SERPENTINIZATION AND THE PRODUCTION OF HYDROGEN AND METHANE GAS IN KIMBERLITES IN NORTHEASTERN ONTARIO, CANADA

Groundwater interaction with kimberlite rocks produces waters characteristic of low temperature serpentinization reactions. Groundwaters were sampled from drill holes in kimberlites in the Kirkland Lake and Lake Timiskaming kimberlite fields of northeastern Ontario. A4 and B30 kimberlitic groundwaters contain little Mg, but high K and Ca, and high pH (up to 12.45) due to the hydration of unaltered kimberlitic minerals. C14 and Diamond Lake kimberlitic waters show higher Mg, but lower K, Ca and pH values (9 – 10.5) due to water interaction with completely altered kimberlite rock. The groundwaters may be classified as being in one of two groups: 1) A Mg–HCO₃ type water and, 2) a more alkaline K (Ca) – OH water, which is void of virtually any Mg. The production of H₂ gas is common in kimberlitic waters, which have high pH values and are of the K (Ca) – OH type. These waters also contain little Dissolved Inorganic Carbon (DIC) and may be classified as waters originating in a closed system. Hydrogen gas comprises up to 10 % of the total gas concentration found dissolved in the waters. The H₂ gas also has extremely depleted dD_(H2) ratios (-771 to –801 ‰), which is some of the most depleted deuterium ratios ever seen in a natural environment. The dD_(H2) ratios indicate that H₂ is in equilibrium with the waters and may account for the slight enrichment in dD_(H2O) from the Kimberlite Water Line (KWL) for waters from kimberlites A4 and B30. There is one gas sample that contains CH₄. The d¹³C_(CH4) and the dD_(CH4) isotopic ratios suggest that the CH₄ formation was due to a bacterial source rather than through low temperature serpentinization. Reaction - modeling of water - kimberlite interactions produces similar results to what is observed through analysis.