Selective leach geochemistry of soils overlying the 95-2, B30 and A4 kimberlites, northeastern Ontario

Studies were carried out by the Geological Survey of Canada and the Ontario Geological Survey at the B30, A4 and 95-2 kimberlites in the glaciated terrain of northeastern Ontario, Canada to evaluate the effectiveness of selective leaches for detecting soilgeochemical signatures over deeply (>30 m) buried kimberlites. In this region, Late Jurassic kimberlites were covered by up to 60 m of glacial sediments during the Late Wisconsin glaciation and since the region was deglaciated approximately 9000 years ago, soil-forming processes have beenactive. Two selective leaches, ammonium acetate pH 5 (AA5) and Mobile Metal Ions (MMI-D-commercial leach), were used along with aqua regia to detect and evaluate the geochemical signatures in mineral and organic soils over the kimberlites. Sampling was carried out using a depth-based protocol of 10to 20 cm, regardless of mineral soil horizon sampled. The strongest "kimberlitic" responses were detected over the 95-2 kimberlite, where mineral soils were most consistent and free of organic matter. Kimberlite pathfinder elements for selective leaches applied to mineral soils will depend on thegeochemical contrast between the kimberlite and its host rock. In this study over three kimberlites, pathfinder elements include: elevated concentrations of Ba, Co, Mg, Mn, Pb, REE, Sr, Ti, Ca, Ga, Cd, I Na, and U and depleted concentrations of Al, Cr, Fe, Rb, Ti and Th.