Soil geochemical survey over concealed kimberlites in the Attawapiskat area in northern Canada

A soil geochemical survey was conducted over kimberlites in a discontinuous permafrost zone in the James Bay Lowlands, southeastern Hudson Bay Lowlands. The kimberlites are concealed by 10 to 30 m of tills and Tyrell Sea clay sediments. Samples of humus and B-horizon soil were collected at 25-50-m intervals over traverses with the total lengths of 562 m over the Whiskey kimberlite and 740 m over the Yankee kimberlite pipe. B-horizon soil samples, sieved to <80 mesh and dried at 60 degrees C, have high carbonate contents above the margins of the kimberlite pipes. Ammonium acetate leach at pH 5 (AA5) dissolves most of these carbonates, and shows anomalies of REE, Y and Ni above the margins instead of the centres of kimberlites. Since the leach dissolves not only carbonates but also amorphous Al-O-OH originating from the clastic component, the ratios of metals to Al are effective to display geochemical anomalies associated with kimberlites. The concentrations of high field strength elements, such as Nb and Ta, are low in AA5 relative to their detection limits, whereas these elements show anomalies in the Enzyme leach. The ratios of these metals to Mn in the Enzyme leach are useful to display the anomalies associated with kimberlites because soil samples over kimberlites contain high contents of these metals and low contents of Mn. Our data suggest that a soil geochemical survey may be used to assist in discriminating kimberlites from other targets during diamond exploration in sub-Arctic settings.