Illuminating Kimberlites: Kimberlite detection, delineation and area selection 
from electromagnetic studies

ABSTRACT:
Kimberlites are vehicles that bring diamonds to the surface. However,
they also are barren when rising through lithosphere that is too thin or
is otherwise devoid of diamonds. Accordingly, there are three aspects to
efficient and effective diamond exploration, and these are first of all
to explore in areas that are likely to host diamondiferous kimberlites,
second to find the kimberlites, and third to determine the subsurface
geometry of the kimberlite pipes (or dykes). Electromagnetic methods can
contribute to all three of these aspects.

For area selection, clearly it is imperative to search for kimberlites
in regions where the lithosphere thickness exceeds the graphite/diamond
stability field. Thus the depth to the lithosphere-asthenosphere
boundary (LAB) is an important parameter to know. The asthenosphere is
the region where partial melt initiates, and laboratory studies have
shown that electrical conductivity rises by two or more orders of
magnitude when the rock melt fraction is as low as 0.1% - melt connects
in an efficient network thus facilitating flow of ions. This step-change
in conductivity, from resistive lithosphere to conductive asthenosphere
over at most 25 km, can be determined to within 10 kilometres using
deep-probing magnetotelluric (MT) methods. In addition, deep-MT studies
on the Slave craton have shown a remarkable spatial correlation between
the mapped Eocene kimberlites in the Corridor of Hope and an upper
mantle region (~80 km depth) of anomalously high electrical
conductivity. The Central Slave Mantle Conductor (CSMC) is currently
interpreted in terms of interconnected graphite, and is therefore
indicative of a region of the mantle with higher concentrations of
carbon. A similar anomaly exists in the North Caribou terrane in the
western part of the Superior craton, suggesting that if there are
kimberlite pipes there, they are likely also the carriers of diamonds
from depth.

For detection and delineation studies of kimberlite pipes by Em methods,
it is important that the kimberlite has an EM signature, i.e., that its
electrical properties be different from those of the host rock.
Laboratory studies and borehole logs show that kimberlite can have a
wide range of electrical resistivity, from tens of ohm.metres to tens of
thousands of ohm.metres. Crater and diatreme facies rocks are generally
more porous and susceptible to weathering, which tends to result in
lower resistivities. The hypabyssal phase rocks can also weather, but
are less likely to do so and, due to their generally lower porosities,
result in higher resistivities. In the Slave and Superior cratons, the
host rock is typically tens of thousands of ohm.metres resistivity, and
the kimberlite pipe is a conductive target. In contrast, in the
Prairies, the host rocks are often very low resistivity, resulting in
the kimberlite pipe being a resistive target.

Examples will be shown of target detection using airborne
controlled-source EM methods from both the craton regions and the
Prairies. Target delineation using ground-based high frequency
audio-magnetotelluric (AMT) will be shown through numerical simulations.
Kimberlite pipes of varying diameter and properties will be imaged, and
also the resolution capabilities for imaging a kimberlite dyke at depth
will be presented.