Special Report 117

Author(s) Date 2024-03-13

Geophysical studies on several continents have shown that diamondiferous kimberlites are sometimes spatially associated with geophysical anomalies in the lithospheric mantle. This includes magnetotelluric (MT) studies that have shown that diamondiferous kimberlites are sometimes underlain by zones of low electrical resistivity (conductors) in the lithosphere. A number of explanations have been proposed for the low resistivity of these regions and include (1) lithosphere enrichment, (2) the formation of hydrous minerals by metasomatism, or (3) the presence of conductive mineral phases such as graphite and sulphides. The relationship between these low resistivity anomalies and kimberlite diamond potential has been suggested as having value in exploration. However, this link has not been definitively established. Northern Alberta is an ideal location to further evaluate this relationship because the region hosts two distinct kimberlite fields, the Buffalo Head Hills and Birch Mountains fields. These kimberlite fields are underlain by regions of the mantle with different values of electrical resistivity. The Buffalo Head Hills kimberlite field has good diamond potential and is underlain by a resistive lithospheric mantle. The Birch Mountains kimberlite field has poor diamond potential and is underlain by a major lithospheric mantle conductor. The study of the kimberlites and the surrounding regions has produced xenolith analytical, mantle petrological, and regional geophysical data that can be integrated with the MT data in order to provide a more rigorous interpretation of the properties of the lithosphere. Ultimately, the spatial relationship between lithospheric mantle conductors and diamondiferous kimberlites may be a useful tool in area selection during diamond exploration. 
The number of MT stations previously deployed in northern Alberta was quite limited, and parts of the resulting resistivity model were poorly resolved. To improve model resolution, the University of Alberta and the Alberta Energy Regulator undertook a long-period MT survey in northern Alberta in the summer and fall of 2022. In total 15 stations were deployed and spaced approximately every 50 km. The survey was aimed at filling in a gap in MT station coverage between the two kimberlite fields as well as placing additional MT stations above the conductor beneath the Birch Mountains kimberlite field. Time-series data were acquired and used to compute apparent resistivity and phase data in the period range of 1–10 000 seconds. Three-dimensional (3D) MT inversion was used to convert these MT data into an electrical resistivity model of the region that imaged the entire lithosphere. The 3D resistivity model shows that the conductor beneath the Birch Mountains kimberlite field does not extend west towards the Buffalo Head Hills kimberlite field, and that it is narrower than previously imaged. The new MT data also help to remove uncertainty about the spatial position and geometry of this conductor, which was previously constrained by data from only a few MT stations. Initial modelling results suggest that the prior interpretations of the conductor being caused by lithosphere enrichment or hydrous minerals is valid. A more extensive mantle resistivity modelling approach will be undertaken in the future to fully evaluate which conductivity-enhancing mechanism can best explain the low resistivity observed beneath the Birch Mountains kimberlite field. These results will be integrated with regional xenolith analytical, mantle petrological, and geophysical data to determine if there is a genetic relationship between the low resistivity anomaly and kimberlite diamond potential.
This work was completed under the Mineral Grant provided by the Government of Alberta dated June 22,

Chase, B.F.W., Unsworth, M.J., Pană, D.I. and Wang, E. (2024): Deep electrical structure of the Buffalo Head Hills and Birch Mountains, northern Alberta: implications for diamond exploration; Alberta Energy Regulator / Alberta Geological Survey, AER/AGS Special Report 117, 40 p.