Special Report 088
Geophysical studies in the Northwest Territories have shown that diamondiferous kimberlite pipes are spatially coincident with a zone of high electrical conductivity in the upper mantle. It has been suggested that the elevated conductivity is due to graphite, which may indicate a carbon-rich mantle, with carbon present as diamonds at greater depth. Magnetotelluric studies of the Trans-Hudson orogen detected a lithospheric mantle conductor close to the Fort a la Corne kimberlite field, which has similar geometrical and electrical properties to that in the Slave craton. The apparent spatial coincidence between diamondiferous kimberlites and an underlying mantle with peculiar geophysical properties may be a useful tool in area selection for diamond exploration. To test this hypothesis, the University of Alberta and the Alberta Geological Survey undertook long period magnetotelluric (MT) data collection across the Buffalo Head Hills kimberlite field of north-central Alberta, a region where a significant quantity of similar data had been previously acquired in the LITHOPROBE program.
MT data have been recorded at 13 locations spaced at approximately 24 km along Highway 88 between Lesser Lake Slave and the town of Vermilion, using University of Alberta instrumentation. Apparent resistivity and phase, as a function of signal period, were computed from the time series. Tensor decomposition showed a preferred geoelectric strike direction of N37 degrees east, which, as expected, is roughly parallel to the geometry of the basement terranes. Two-dimensional (2-D) MT inversion was used to convert signal period into depth for both the new MT data and the previously reported LITHOPROBE data. Resulting resistivity models of the Earth showed a decrease in electrical resistivity at a depth of approximately 200 km that is interpreted as the lithosphere-asthenosphere boundary (LAB). A low resistivity (high conductivity) zone was observed in the upper mantle at 50 to 80 km depth beneath the northern end of the Buffalo Head Hills. Three-dimensional (3-D) inversions produced similar resistivity models to those obtained by 2-D inversions of individual profiles.
Similarly to the diamondiferous fields in Northwest Territories and Saskatchewan, the north-central Alberta upper mantle conductor may be interpreted as a domain of graphite concentration, although elevated conductivity of the upper mantle can be also produced by partial melting and sulphide concentration. However, in contrast to the Slave craton, the northern Alberta upper mantle conductor is not directly below the diamondiferous kimberlites and may indicate that the originally inferred correlation of diamondiferous kimberlite fields and upper mantle conductivity anomalies is not universal. Additional MT data are required to better constrain a 3-D resistivity model of northern Alberta.
Turkoglu, E., Unsworth, M.J. and Pana, D.I. (2007): Deep electrical structure of Buffalo Head Hills, northern Alberta: implications for diamond exploration; Alberta Energy and Utilities Board, EUB/AGS Special Report 88, 34 p.