THE METER READER

Coordinated by John Peirce

Geophysical methods for kimberlite exploration in northern Canada

MIKE POWER and GEORGES BELCOURT, Aurora Geosciences, Whitehorse, Canada
ED ROCKEL, Diamondex Resources, Vancouver, British Columbia, Canada

In 1991, the discovery of diamondiferous kimberlite in

Canada's Northwest Territories (NWT) precipitated the
largest mineral staking rush in North America and sparked
a remarkable exploration effort which extended across north-
ern Canada. Thirteen years later, two diamond mines with
estimated annual productions of $1 billion (Can) are in oper-
ation, a third is in permitting, and several other projects are
approaching development. Ekati mine, operated by BHP
Billiton since 1998, produced 4.8 million carats from 4.2 mil-
lion tons of ore in 2003 while the nearby Diavik diamond
mine yielded 2.7 million carats from 1.0 million tons of ore
during the same period. When Diavik has achieved full pro-
duction in 2004, Canada will account for 15% of global dia-
mond output by value. The exploration programs which
created this new industry relied heavily upon the efficient
application of complementary geophysical techniques. This
paper overviews geophysical exploration methods currently
used in diamond exploration in northern Canada.

Slave Craton kimberlites. Diamond deposits in northern

Canada are hosted in kimberlite intrusions in and adjacent
to the Archean Slave Craton in the NWT and Nunavut
Territory (north of 60°N). Kimberlite is a volatile-rich ultra- Figure 1. Principal kimberlite pipes and dykes in the NWT and Nunavut
basic igneous rock composed of macrocrysts of mantle- located since 1991.
derived material enclosed in a matrix of olivine,
clinopyroxene, micas, carbonates and serpentine. Kimberlite
magmas are generated at depths in excess of 100 km and
reach the surface by means of a very rapid ascent through
the crust, frequently terminating in explosive eruptions. The
violent stopping associated with magmatic ascent results in
the inclusion of significant quantities of mantle peridotite and
eclogite, commonly in centimeter-sized nodules. When dia-
monds are present in either the magma source region or in
an area through which the magma ascends, they are incor-
porated into the kimberlite intrusion.
In the earth, diamond is present as a stable mineral only
at depths greater than 150 km and temperatures less than
1200°C. These unusual P-T conditions are largely confined
to mantle roots situated beneath stable Archean cratons.
Beneath these tectonic elements, the depth of the 1200°C
isotherm is deflected from 75 km to as much as 200 km, cre-
ating a diamond stability field at depths of 150-200 km.
Kimberlite magmas originating within and beneath these
regions can tap this material and are potentially diamon-
diferous. The rapid ascent of kimberlite magma at veloci-
ties in the order of 100 km/hr, preserves diamonds entrained
in the magma during transport through the upper asthenos-
phere. Only about 1% of kimberlite intrusions contain dia-
monds in economic concentrations.
Figure 1 shows the location of most kimberlite intrusions
in the NWT and Nunavut. The majority intrude granitoid
and lesser supracrustal rocks of the Slave Craton but they
are also found in adjacent areas where thin Proterozoic and
Phanerozoic sequences overlie Archean basement rocks.
Intrusion emplacement dates from a limited number of Figure 2. Block diagram of a kimberlite pipe. Pipes are eroded to different
pipes in the central Slave Craton vary from Permian to levels and some facies may be missing in many cases.

1124 THE LEADING EDGE NOVEMBER 2004

 shallow and deep-seated structures. On the whole, the kim-
berlites are intruded on a NNW-trend, extending from
Victoria Island to the eastern end of Great Slave Lake.
Individual kimberlite clusters consisting of 3-10 pipes fol-
low subsidiary NNE- and ENE-trends.
Figure 2 illustrates the shape and setting of a typical kim-
berlite intrusion. The location of a kimberlite intrusion is con-
trolled by deep-seated structures at a regional scale but at
kilometer scale, the intrusions tend to be emplaced along
faults or precursor diabase dykes. A completely preserved
intrusion generally consists of a pipe- or carrot-shaped body,
containing three distinct facies. Hypabyssal kimberlite at the
base of the pipe is confined to dikes and knot-like root zones
(blows). Above this level, the main diatreme consists of a
mixture of predominantly kimberlite and lesser fragments
of the intruded country rock. The walls of the diatreme are
steeply dipping—typically in the order of 75-85°. Crater
facies kimberlite consists of a mixture of pyroclastic kim-
berlite, clay and country rock fragments. Large blocks of
country rock (xenoliths) may be found as discrete rafts in
the crater. Slave Craton kimberlites have surface areas of 10-
900 hectares with the majority being less than 100 hectares.
Most Slave Craton pipes contain crater facies kimberlite but
some can contain only diatreme or mainly hypabyssal facies
material. As a consequence of differential glacial erosion, the
majority of Slave Craton kimberlites are found beneath or
immediately adjacent to lakes. The surface expression of
pipes can vary depending upon the depth of erosion. While
kimberlite pipes are currently the primary exploration tar-
Figure 3. Contrast in physical properties between kimberlites (green) and get, kimberlite dykes are nonetheless very attractive. The
granitic rocks (red). Most kimberlites located to date intrude into granitic third diamond mine to be put into production in the NWT
and high-grade metamorphic rocks with similar physical properties. will likely be a high-grade diamondiferous kimberlite dyke
at Snap Lake.

Kimberlite exploration. Successful kimberlite exploration

strategies in the Slave Craton normally consist of a staged
program of indicator mineral sampling followed by geo-
physical surveys and drilling. Geochemical sampling is crit-
ical for identifying a potentially diamondiferous kimberlite
and in defining an area in which pipes and dikes are likely
situated. Geophysical surveys can directly locate kimberlite
intrusions and define optimal locations to test these targets.
Pleistocene glaciation eroded and dispersed kimberlite
material for extensive distances down ice from the sources.
The distinctive suite of minerals associated with kimber-
lites—pyrope garnet, chrome diopside, and Mg-ilmenite
(picroilmenite)—can be found in basal till near the pipes and
in glaciofluvial sediments at much greater distances from
the pipes. Multiple glaciation and interglacial fluvial
processes greatly complicate the process of tracing these indi-
cator mineral trains up-ice to their sources. Geochemical
exploration for kimberlite consists of sampling both glacial
till and glaciofluvial deposits, concentrating the heavy min-
erals and recording indicator mineral occurrences. The cal-
cium and chrome concentrations in pyrope garnets can be
used to assess whether the source kimberlite is potentially
diamondiferous; consequently garnet geochemistry is often
examined by electron microprobe analysis. Unfortunately,
Figure 4. Representative magnetic field responses of kimberlite pipes indicator mineral sampling can only identify a large prospec-
include (top left) dipole response, (top right) kimberlite as magnetic high, tive area and rarely will terminate immediately at a kim-
(bottom left) kimberlite as a magnetic low breaking diabase dyke, and berlite pipe.
(bottom right) magnetic low kimberlite adjacent to diabase dyke.
Geophysical methods have proven to be essential in suc-
cessful Slave Craton kimberlite exploration programs.
Eocene but the majority of the intrusions are either Kimberlite intrusions have distinct geophysical signatures
Cretaceous or Eocene. As a consequence, the intrusions cut controlled by the physical property contrast between the
all known rock units and subcrop beneath a variable depth kimberlite and surrounding rock and by the shape and size
of glacial overburden. Emplacement is controlled by both of the intrusions and their associated anomalies. In addi-

NOVEMBER 2004 THE LEADING EDGE 1125

 tion, kimberlites with very weak
geophysical signatures can be
discriminated from spurious
sources by the location of anom-
alies in relation to structural fea-
tures and known kimberlite
intrusions. Figure 3 illustrates the
physical property contrast
between the various kimberlite
facies, granitic rocks common in
the central Slave Craton and the
overlying overburden, common
in the central Slave Craton.
Contrasts in magnetic suscepti-
bility, electrical resistivity, den-
sity, dielectric permittivity and
seismic velocity between kim-
berlites and their host rocks gen-
erate anomalies which, in the
case of kimberlite pipes, are fur-
ther enhanced by their distinc-
tive geometry. Crater facies
kimberlite generally displays the
greatest contrast in physical
properties and kimberlite pipes
or dykes containing this facies
are most readily detectible with Figure 5. Representative horizontal loop electromagnetic responses of kimberlite pipes. In-phase profiles
geophysical methods. Unfor- solid, quadrature dashed. Shown are (top left) crater facies kimberlite, (top right) diatreme facies kimberlite,
tunately, spurious anomalies can (bottom left) offset response, and (bottom right) near surface weathered kimberlite.
be generated by a wide variety
of bedrock and surficial features. In recent years, geophys- lows in this area which, when combined with coincident
ical programs tend to incorporate a suite of surveys designed resistivity lows creates numerous false anomalies.
to exploit several different physical property contrasts. This Figure 4 illustrates the range of total magnetic field
approach is particularly necessary in the more heavily responses associated with kimberlite pipes in the Slave
explored portions of the Slave Craton. Craton. We can see the ability of remanent magnetisim to
In general, exploration programs begin with an airborne cause a wide range of values in kimberlites. The magnetic
total magnetic field and electromagnetic (EM) survey of a moments within magnetic materials, if present in the kim-
large area defined by indicator mineral sampling. Follow- berlite body, align to the local field after the intrusion occurs
up ground surveys are then conducted over a suite of anom- and remain in that orientation once the kimberlite body
alies screened from the airborne survey results. The primary cools. The size and shape of the body in addition to these
objective of the ground surveys is to fix the location of the orientated magnetic materials may produce a wide variety
airborne magnetic field and/or EM anomalies and to deter- of magnetic results. Many kimberlites have been discovered
mine if a density, ground radar or seismic velocity anom- in association with diabase dykes, regional faults and geo-
aly is also present. The most favorable of the ground logical contacts and thus regional scale geological struc-
follow-up targets are then tested by drilling. If diamondif- tures play an important part in the selection of magnetic
erous kimberlite is intersected, additional detailed geo- targets for investigation. These structures and their inter-
physical surveys may be conducted to delineate the intrusion sections produce local areas of weakness in the crust, pro-
and assist in the definition of phases in the pipe prior to com- viding a path of lower resistance for kimberlite intrusions.
mencing detailed drilling or bulk sampling. In all phases of Intersections of multiple structures are given more weight
this work, geophysical interpretation incorporating knowl- in the target selection process. It is helpful that in many
edge of local kimberlite responses and geology is essential. instances these regional structures can be readily identified
using magnetics surveys.
Magnetics. Kimberlites are ultramafic rocks with reported
susceptibilities in the range of 1 to 80 ǂ 10-3 SI units. In most Electromagnetics. There is generally a strong contrast in elec-
intrusive settings, there is a positive susceptibility contrast trical resistivity between kimberlites and surrounding
between the kimberlite and the surrounding country rock. granitic rocks in the Slave Craton. This contrast is most pro-
In addition, the magnetic responses of many kimberlite nounced for crater facies kimberlite and is largely due to
intrusions are greatly enhanced by strong remnant mag- serpentinization and clay alteration. Pipes with crater facies
netism. Kimberlite pipes in a cluster tend to have the same kimberlite are generally evident as resistivity lows with
intensity and direction of remnant magnetism but it is also apparent resistivities in the range of 1-100 ohm-m. Spurious
possible to find pipes in close proximity with different mag- EM/resistivity anomalies are also generated by a range of
netic signatures. Spurious anomalies are generated princi- surficial and bedrock sources. Lacustrine clays and some
pally in situations where the kimberlites are expressed as glacial deposits have electrical resistivities in the same range
weak lows. This occurs immediately north of the Ekati Mine as kimberlite; consequently, these sediments are major
Property where pipes tend to be expressed as smooth 50- sources of false anomalies. Similarly, localized zones of
150 nT lows. Lakes filled with anomalously thick sections argillaceous alteration, particularly at the intersection of
of nonmagnetic overburden generate similar magnetic field faults and along diabase dikes, can generate bedrock resis-

1126 THE LEADING EDGE NOVEMBER 2004

tivity lows with signatures similar to those exhibited by
kimberlite pipes.
Multicoil helicopter-borne EM systems and, less fre-
quently, fixed-wing time-domain EM systems are used in
first-pass surveys for most exploration programs. Ground
follow-up surveys usually include horizontal loop EM
(HLEM) surveys operating from 220 Hz to 28 KHz at a coil
spacing of 100 m. Ground time-domain EM surveys have
been used less frequently, principally because of the cost.
Coincident loop TDEM data, however, can be inverted to
yield some information on the anomaly source geometry
using stacked 1D inversions and this information can be use-
ful in discriminating surficial targets from bedrock con- Figure 6. Capacitively-coupled resistivity response of kimberlite pipes at
ductors. In contrast, kimberlite HLEM responses do not 100 m dipole spacing. Pipe outlines are shown in green. (a) Kimberlite
pipe beneath a lake (b) Kimberlite pipe on land. In both cases, conductive
lend themselves to unassisted inversion. lake bottom sediments or overburden overprint the response.
Figure 5 illustrates typical HLEM responses associated
with kimberlite pipes in the Slave Craton. A wide range of
responses can occur and can be attributed to the facies that
are preserved in the kimberlite. Generally, the crater facies
and the upper portions of the diatreme facies are more
porous and weathered. These portions of the kimberlite
may contain conductive clay particles as well. These facies,
if preserved, are relatively electrically conductive and may
be visualized with the horizontal loop EM survey.
Kimberlites are generally much softer than the sur-
rounding rocks and are thus preferentially scoured by glacia-
tion events. After such events, it is observed that the greatest
amount of material is scoured and deposited directly down
ice of the kimberlite body. These depressions once filled
with sediments and clays will produce an EM response
which can be offset down ice. Many ground geophysical sur-
veys over kimberlites have produced magnetic and EM
responses which are offset from one another due to these
glacial scouring events. For these reasons it is important not
to rely solely on one geophysical method. The horizontal
loop EM does provide information that, when added to the
larger exploration picture, aides in the identification and pri-
oritization of kimberlite targets.
A recent innovation has been the introduction of the
capacitive coupled resistivity (CCR) method to kimberlite
exploration. Originally developed in Russia, and further Figure 7. Gravity response of a kimberlite pipe beneath a lake. (a)
refined in Canada, this method relies upon capacitive as Bouguer gravity after all conventional corrections. (b) Bouguer gravity
after lake water depth correction. (c) Total magnetic field response indicat-
opposed to galvanic coupling to introduce a relatively high ing location of granitic xenolith in southern section of the pipe. (d) 3D
frequency (8 KHz) electric field into the earth. Twenty-meter gravity model cross-section through small magnetic field high. Inversion
receiving and transmitting dipole antennas consisting of of water-depth corrected gravity identifies this feature clearly.
braided wire are separated at a fixed spacing of 100 m or
more and the entire array is dragged along the ground in a sity range which overlaps that of the surrounding granitic
manner similar to an HLEM survey. The recorded voltage and metasedimentary rocks. Kimberlite pipes containing
and fixed current are converted to an apparent resistivity crater facies will commonly generate obvious Bouguer lows
and plotted at the midpoint between the antennas. The of 0.5 to 1.0 mGal. There are numerous sources of false
depth of investigation is in the order of the antenna sepa- anomalies including anomalously thick sections of lake-bot-
ration and the survey results are a useful complement to tom overburden, alteration zones at the intersections of
airborne resistivity results. Figure 6 illustrates the responses structures and small stocks of low-density felsic igneous
of kimberlite pipes both beneath a lake and on land. In both rocks within larger granitic intrusions. Gravity surveys are
cases, responses from lake bottom sediments are apparent considered a subsidiary complement to magnetic field and
but the kimberlite pipes are nonetheless apparent as clear EM surveys, given the relatively low-amplitude contrast
resistivity lows. It is worth noting that the response over and the presence of spurious sources. The introduction and
the land-based pipe is significantly stronger than the air- successful application of the Falcon airborne gravity gra-
borne resistivity response, a consequence of the antennas diometer system at the Ekati Mine property in the late 1990s
on surface providing stronger signal and better coupling spurred the application of this technique elsewhere in the
with the ground. Slave Craton.
Improvements over the past decade in gravity survey
Gravity. In the Slave Craton, there is a range of density con- techniques have greatly increased field efficiencies, reduced
trasts between kimberlites and surrounding country rocks. costs and reduced sources of error. These improvements have
The density contrast is most pronounced for both crater also enhanced the utility of gravity data in identifying kim-
and diatreme facies kimberlite where serpentinization and berlite intrusions. The principal innovations have been the
clay alteration are present. Hypabyssal kimberlite has a den- introduction of kinematic carrier phase GPS surveys for

NOVEMBER 2004 THE LEADING EDGE 1127

 Figure 8. GPR responses of kimberlite pipes and dykes. (a) Response of a kimberlite pipe beneath a lake at 25 MHz. (b) Response of a kimberlite pipe at
12.5 MHz beneath 40 m of overburden. (c) GPR response of a hypabyssal kimberlite dyke intruding bedded limestone at 25 MHz. (d) Response of a
shallow dipping dyke at 25 MHz; kimberlite occurs beneath a diabase dyke, the top of which is the strong reflector.

topographic leveling; the use of accurate digital terrain mod- the presence of electrolytes in the bound water. Con-
els in performing terrain corrections; and the employment sequently, kimberlites have a higher relative dielectric per-
of improved water depth correction algorithms. The latter mittivity than host rocks and the contact between the two
is a significant factor given that most kimberlite targets rock types is a radar reflector.
occur beneath or adjacent to small lakes. The application of GPR surveys are most efficiently run during winter
water corrections based on algorithms developed for use on months where optimal access maximizes survey efficiency.
sea ice or large lakes is completely inappropriate in these Helicopter-supported reconnaissance surveys are normally
situations. Instead, survey crews take soundings at gravity carried out on foot by a two-man crew while detailed sur-
stations on the ice and merge this data with other sound- veys can be run with a snow machine to tow the equipment.
ings and the mapped edge of the lakes to create digital GPR instruments are not designed for cold weather opera-
bathymetry models. Water depth corrections are then tion. Consequently, the instruments and operating com-
applied by performing a terrain correction using the bathym- puter must be shielded in a hot-box and the fiber-optic
etry model as input. cables shrouded to prevent shattering.
Figure 7 illustrates the gravity response of a kimberlite Figure 8 illustrates the GPR response of kimberlite pipes
pipe beneath a lake showing the Bouguer anomaly before and dykes. Figure 8a displays the response of a kimberlite
and after water column corrections, (a) and (b). Water depth pipe beneath a lake. The top of the pipe is clearly evident
contours are superimposed on both anomalies. The final cor- as a smooth planar reflector with evident attenuation. The
rected Bouguer anomaly data indicates that the source cen- fractured surrounding granite displays multiple diffraction
ter of mass is located north of the deepest point in the lake. hyperbolas and trails, generating a contrast in texture. The
At this location, drilling intersected a granite xenolith with steep granitic walls of the pipe and the lake bottom sedi-
surface dimensions of 50 ǂ 50 m which evidently slid into ments are clearly visible in the radargram. The response of
the pipe crater following intrusion. This feature is evident a kimberlite pipe on land is shown in Figure 8b. Crater
as a subsidiary total magnetic field high (c) immersed within facies kimberlite subcropping at a depth of 38 m beneath
the larger low generated by the kimberlite pipe. Three- boulder till produces a clear strong reflection at a center fre-
dimensional inversion of the gravity data, shown in (d), accu- quency of 12.5 MHz.
rately illustrates both the location and dimensions of the While GPR surveys can image kimberlite pipes to depths
granitic xenolith. in the order of 50 m, the results cannot be used to conclu-
sively identify kimberlite without supporting magnetic,
Ground-penetrating radar. Kimberlite intrusions can be resistivity, or gravity data. There are numerous surficial fea-
detected by ground-penetrating radar (GPR) surveys as a tures including lacustrine clay deposits and rafts of metased-
consequence of the contrast in both dielectric permittivity imentary rocks which can produce spurious GPR anomalies.
and electrical resistivity between kimberlites and sur- Instead, GPR surveys have proven to be most useful in
rounding rocks. Both serpentinization and clay alteration delineating the extent of a kimberlite pipe following an ini-
cause kimberlites to have higher liquid bound water con- tial confirmatory drill hole. Radar reflections known to be
centrations than the relatively dry surrounding host rocks. associated with the pipe can then be mapped to determine
This contrast persists in permafrost conditions because of the areal extent of the pipe and the depth of overburden.

1128 THE LEADING EDGE NOVEMBER 2004

Figure 9. Seismic reflection survey over Snap Lake kimberlite dyke.

GPR surveys are proving to be a primary tool in the loca- northwest and downdip from the surface outcrop of the Snap
tion of kimberlite dikes however. Moderate-to-steeply dip- Lake dyke. The 1-2 m wide dyke is clearly visible as a dis-
ping kimberlite dikes have very clear GPR responses and crete feature to depths of 1800 m.
GPR surveys have been used to successfully locate dia-
mondiferous kimberlite dikes near Kenady Lake, Great Slave Conclusion. The discovery and rapid development of the
Lake, and Victoria Island. In Figure 8c, the 25 MHz response diamond mining industry in northern Canada is a direct
is shown over a 25-m wide vertical dyke intruding Paleozoic result of the coordinated application of geophysical and
limestone. Clear diffractions from the margins of the dyke geochemical methods. Magnetic and electromagnetic meth-
assist in defining the walls of the intrusion. Figure 8c illus- ods commonly locate the majority of kimberlite intrusions
trates the GPR response over a shallow-dipping kimberlite in a new area. Complementary techniques are more com-
dyke. The dominant reflection is produced by a selvage monly used in heavily explored areas to identify kimberlite
overlying a precursor diabase dyke; kimberlite up to 1-m intrusions with weak magnetic or resistivity responses.
thick occurs beneath the diabase. Many areal extensive indi- Geophysical techniques are also being used to define the
cator mineral trains are thought to originate from kimber- geometry of diamondiferous kimberlite intrusions during
lite dykes or sills. In these areas, GPR surveys can be used development drilling. With the development of several long-
to identify prospective dykes or sills for drill testing. life diamond mines and the discovery of numerous other
potentially economic deposits, there will be a continuing
Seismic. Despite the clear contrast in seismic velocity requirement to apply, adapt and improve geophysical meth-
between kimberlites and surrounding country rocks, seis- ods for kimberlite exploration in northern Canada.
mic surveys have not been used extensively in Slave Craton.
Seismic refraction surveys have been conducted over sev- Suggested reading. “Geotectonic controls of primary diamond
eral kimberlite pipes and results suggest that seismic veloc- deposits: implications for area selection” by Helmstaedt and
ity measurements from reversed refraction profiles might Gurney (Journal of Geochemical Exploration, 1995). “Slave Province
be useful screening targets in granitic rocks. Two orthogo- Kimberlites, NWT” by Kjarsaard (in Searching for Diamonds
nal reversed profiles are recommended over a target to mit- in Canada, Geological Survey of Canada Open File 3228, 1996.)
igate against velocity anisotropy. In metasedimentary rocks,
the possibility of velocity overlap suggests that seismic Acknowledgments: The authors thank Aber Resources, Darnley Bay
refraction surveys may not provide conclusive results. Resources, Diamondex Resources, Diamonds North Resources, Diavik
High-resolution 2D seismic reflection surveys have been Diamond Mines, Intertech Minerals, and Snowfield Development
conducted at Snap Lake to delineate a diamondiferous kim- Corporation and Southern Era Resources for permission to present the
berlite dike hosting reserves of 22.8 million tons at 4.0 ct/t. data in this paper. The authors gratefully acknowledge help from Anne
The survey employed a 1024-channel ARAM system and Hall and Karen Jane Weir (nee Wright) to keep us on the geological
used vibroseis and dynamite as energy sources. Receivers straight and narrow and patient assistance in image preparation from Jim
consisted of single phones at a 4-m spacing and source Robinson and Holly Stirling.
points were spaced 8 m apart along two lines. Figure 9 illus-
trates the reflection section recorded over a line, extending Corresponding author: aurora@klondiker.com

NOVEMBER 2004 THE LEADING EDGE 1129