Application of Airborne Magnetics, EM and Gravity to the Ring of Fire Intrusive Complex,
Ontario
Ken Witherly, Condor Consulting, Inc.
Peter Diorio, GeophysicsOne,
Summary
The Ring of Fire is an intrusive complex composed of
mafic and ultramafic rocks hosted in the Archean age
McFauld’s greenstone belt located in James Bay lowlands
of northern Ontario. Due to low topographic relief and an
extensive cover of Paleozoic platform carbonate rocks, the
area remained largely under explored until kimberlites were
found in 1988. This lead to the development of the Victor
diamond mine in 2006. Subsequent exploration for
kimberlites resulted in the serendipitous discovery of the
McFaulds VMS deposits in 2002 (ref Mugall 2010). With
this discovery came the recognition that there was a
greenstone belt present in the area and it could host
economic deposits. A semi-regional Geotem survey was
flown in 2003. Exploration in the area was complicated as a
number of junior companies had positions in the area and
while they would share the costs of expensive surveys, they
were competing for what was deemed the best land
positions. Ground surveys were conducted based on the
Geotem results, with the targeting model being either
kimberlite or VMS. This work eventuated in the discovery
of a major deposit of chromite and a number of significant
nickel sulfide deposits in 2007-2008. During this time,
numerous airborne and ground surveys were carried out
including a regional Falcon AGG and mag survey in early
2011. While various technical and commercial
presentations have been made on the Ring of Fire
geophysical work, due to the complicated claim ownership
most of these have tended to focus on the results controlled
by one group. This review is intended to look at the overall
area which hosts at three significant deposit styles; VMS,
magmatic nickel and chromite.
Fig. 1: TMI with McFauld’s greenstone belt highlighted and
known deposit locations.
Introduction
The aeromagnetic coverage (Fig. 1) shows the overall
structure of the McFauld’s greenstone belt which is located
in the eastern side of the Oxford-Stull terrain, which in turn
is part of the Archean age Superior province. Fig. 2 shows
the basic geology for the same area as shown in Fig 1 along
with the major deposit locations. The Ring of Fire intrusive
rocks have been dated at 2,734.5 ± 1.0 Ma. (Mugall et al.
2010).
While there are a number of greenstone belts present in the
Superior province, many of which host VMS style deposits,
the presence of a major mafic-ultramafic intrusive system
geologically interwoven with the normal greenstone belt
Fig. 2: Basic geology of McFauld’s greenstone belt and Ring of
Fire intrusive system (from Mugall et al. 2010).
sequence is quite unusual. The Ni-Cu and chromite
mineralization are believed to be genetically related and the
presence of additional resources is considered possible.
While the deposits are now sub-vertical, at the time of
formation it is believed that deposition occurred in a
shallowly dipping setting, likely similar to the sill-like
character of the Bushveld Complex in South Africa.
Subsequent tectonic activity has rotated the rocks into their
Airborne Geophysical Responses over Ring of Fire
now sub-vertical orientation. The published resources for
the area are outlined in Table 1.
Table 1
Deposit
Eagle’s Nest
Major
Ni
Minor
Cu
Resource
11.1 Mt @ 1.7% Ni, 0.9% Cu.
0.9 gpt Pt & 3.1 gpt Pd
Blackbird
Cr
NA
25 Mt @ 35.8%
Black Creek
Cr
NA
10 Mt @ 38%
Big Daddy
Cr
NA
50 Mt @ 38%
Black Thor
Cr
NA
70 Mt @32%
McFaulds
VMS
Cu
Zn
0.8 Mt 3.8% Cu, 1.1% Zn
(McFauld’s #3)
Early Discoveries to Present
The exploration work which lead to the discovery of the
resources in Table 1 was driven largely by the earlier
discovery of diamondiferous kimberlites in area. The
McFauld’s VMS zone (a number of discrete deposits with
the largest to date being #3) was discovered in 2002 and
then there was a hiatus until 2006-2007 when the Eagle’s
Nest, Eagle 2 and the chromite deposits were found in rapid
succession. The commercial situation was complex as the
primary players were all junior explorers with limited funds
and often interlocking agreements and shared personnel
that lead to both a considerable degree of cooperation but
as well, conflicts. The history of the exploration is largely
captured in the numerous NI 43-101 documents the various
explorers produced. Two of the most useful are Armstrong
et al. 2008 and Lahti 2008. The way the program in the area
unfolded changed in a major way when Cliffs Natural
Resources acquired much of the chromite assets from
several juniors in late 2009-early 2010. Currently,
development is on hold as companies and governments
work through major issues related to infrastructure and how
the development in this very remote area will impact the
indigenous people in the area.
McFauld’s Area VMS
The initial geophysical anomalies of interest were derived
from aeromagnetic surveys flown as part of the kimberlite
exploration program. Prior to drilling, ground surveys were
carried out, including EM which were used to guide the
drilling. Subsequently, both Geotem (2003) and VTEM
(2004 & 2005) were flown over the area (Condor 2005).
The comparison between the Geotem and VTEM time
constant response is shown in Fig. 3.
Fig. 3: Geotem and VTEM AdTau over McFauld’s VMS deposits
#1-4. (Condor 2005)
The conductive features were all clearly mapped with both
systems. Profile analysis for the two systems showed that
the VTEM had considerably less lateral detection than the
Geotem. This outcome was expected due to the differences
in the spatial foot-prints of the two systems. The slight
increase in conductivity moving SE in the VTEM image is
interpreted to be caused by an increase in the regolith
conductivity or thickness present in the area.
Eagle’s Nest Nickel and Blackbird Chromite
After the McFauld’s VMS discoveries in 2003, it was not
until 2007 when the Eagle’s Nest was discovered. The only
airborne data available prior to this discovery was the 2003
Geotem. Fig. 4 shows an assessment of the Geotem
(Condor 2005) and the later VTEM EM picks (Bournas and
Kumar 2009) on a regional geology image. Both outcomes
show the major conductive features and it is understood
that the Eagle deposit was discovered as the result of drill
testing ground geophysical surveys over a discrete airborne
response (Lahti per com 2014). The Eagle’s nest deposit is
a interpreted to be a mineralized conduit (chonolith)
associated with near-by intrusive rocks which host the
chromite deposits. Drilling has shown the deposit extends
to over 1.6 km depth and maintains the rod-like shape
apparent at surface.
While the conductive response associated with deposit can
be attributed to pentlandite and pyrrhotite, there is also a
weaker conductivity associated with the chromite deposits.
This was attributed magnetite and talc within serpentinized
ultramafic rocks adjacent to the chromite ore.
The potential field results in Fig. 5 show a complex pattern
in the magnetic results but a simpler response in the
gravity. A dashed white line circles a zone of high density
which is suggestive of a buried high density mafic body
Airborne Geophysical Responses over Ring of Fire
body that underlies the area. The magnetic results show an
array of complex events near surface, including the discrete
magnetic high associated with Eagle 1. The main airborne
and ground results over Eagle’s Nest have been reviewed in
detail by Balch et al. 2010.
Big Daddy and Black Thor Chromite Deposits
After the initial chromite discoveries near Eagle’s Nest,
ground gravity soon became the geophysical technique of
choice to map these high density features. While there was
an associated EM response noted above, it was not deemed
as directly associated with the chromite mineralization as a
mapping the density provided. Fig. 6 shows a composite
over several deposits; the Big Daddy and Black Thor as
well as several smaller zones. The amplitude and linearity
of these features would have made them distinctive targets
for drilling.
In Fig, 7, the Falcon TMI and gravity (Gd) results are
presented along with the VTEM picks. At this scale there is
clearly a close relationship between the major geophysical
responses and the deposits.
Fig. 4: Geotem Interpretation (top) and VTEM EM picks (bottom)
over Eagle-AT 5 deposits. (Condor 2005, Bournas and Kumar
2009 and OGS-GSC 2011)
Fig. 5: Falcon Gd (top) and TMI (bottom) over Eagle’s Nest and
Blackbird 1 and 2. (OGS-GCS-2011)
To better understand the petrophysical character of the
mineralized systems, 2.5D modeling was undertaken of the
magnetic and gravity results. Fig. 8 shows the result along a
Fig. 6: Ground gravity over the Big Daddy-Black Thor trend.
(Franklin 2013).
Airborne Geophysical Responses over Ring of Fire
Fig. 9: 3D inversion model of Falcon TMI and Gdd (gravity).
Fig. 7: Falcon TMI (top) and Gd (bottom) over Big Daddy-Black
Thor; VTEM picks displayed (refer to Fig. 4 for scale). (OGS-GSC
2011)
line across the Big Daddy deposit. Petrographic and
chemical evidence from the Big Daddy property (Mungall
2010) indicate that the McFaulds Lake Sill is a well
fractionated, body comprising lower (to the northwest)
olivine‐rich units overlain by olivine‐poor units. The
principal chromite bodies lie at the top of the olivine‐rich
unit. (Greenough and Palmer 2010)
A close examination of the drilling and modeling shows
that the strongest gravity response coincides with the
pyroxenite zone which lies stratigraphically directly above
the chromite bearing peridotite zone. An intense magnetic
anomaly coincides with the upper part of the dunite and
provides an excellent marker horizon. This relationship
appears consistent for all the deposits in the trend including
Big Daddy, Black Creek, Black Thor South and Black Thor
North.
The dense chromite ore certainly contributes to the
response but this is too thin, at least on this section, to be
resolved as a separate unit. Perhaps surprisingly the dunite
appears to have only slightly elevated density (2.73 to 2.75
g/cc) which is at odds with textbook density for dunite of
3.28g/cc. This suggests that the dunite has been heavily
serpentinized resulting in much lower density.
Fig. 8: Detailed interpretation of magnetic and gravity data over
the Big Daddy deposit (refer to Fig. 7 for location).
Fig. 9 shows a 3D inversion of the Falcon TMI and Gdd
results. The aspect of the intrusive being one large system
is quite apparent. A more systematic assessment of these
data along with ground information should be able to
provide a much better understanding of the overall extent
of the Ring of Fire intrusive system and provide guidance
as to where additional resources might be.
Airborne Geophysical Responses over Ring of Fire
REFERENCES
Armstrong, T., Puritch, E., Yassa, A., Pearson, J. L.,
Hayden, A., and Partsch, A., 2008, Technical Report and
Preliminary Economic Assessment On The Eagle One
Deposit Double Eagle Property McFaulds Lake Area James
Bay Lowlands, Ontario P & E Mining Consultants Inc.
October 20, 2008
Balch, S. J., Mungall J.E. and Niemi, J., 2010, Present and
Future Geophysical Methods for Ni-Cu-PGE Exploration:
Lessons from McFaulds Lake, Northern Ontario; 2010
Society of Economic Geologists, Inc. Special Publication
15, pp. 559–572
Bournas, N. and Kumar, H., 2009, Thin-Plate Modeling of
VTEM Anomalies over VMS Deposits-A Few Case
Studies; presented at KEGS-PDAC Symposium;
Geophysical Applications to VMS Deposits, Toronto
February 28, 2009
Condor Consulting, Inc., 2005, McFauld’s Trend Airborne
EM and Magnetic Surveys; report for Billiken Management
Services, Ltd., November 2005.
Franklin, J. M., 2013, Ring of Fire and Beyond: The
Exceptional Mineral Potential of Ontario's Far North;
presented at PDAC conference Toronto Canada, March 7,
2013
Greenough ,G. and Palmer, P., 2010, Technical Report and
Resource Estimate, McFaulds Lake Project, James Bay
Lowlands, Ontario, Canada, for Noront Resources Ltd.,
April 2010
Lahti, H., 2008, Updated Technical Report on the
McFaulds Lake Project Porcupine Mining Division James
Bay Lowland, Ontario, Canada 84°45 – 86°20’ W 52°20’ –
53°30’ N Situated in parts of NTS 43CNW, 43DNE,
43ESE & 43FSW; for UC Resources Limited and Spider
Resources Inc.; Deep Search Exploration Technologies
Aug 30, 2008
Mungall, J.E., Harvey, J.D, Balch S.J., Azar, B., Atkinson,,
J. and Hamilton, M.A.,2010, Eagle’s Nest: 2010, A
Magmatic Ni-Sulfide Deposit in the James Bay Lowlands,
Ontario, Canada 2010 Society of Economic Geologists, Inc.
Special Publication 15, pp. 539–557
Ontario Geological Survey and Geological Survey of
Canada, 2011, Ontario airborne geophysical surveys,
gravity gradiometer and magnetic data, grid and profile
data (ASCII and Geosoft® formats) and vector data,
McFaulds Lake area; Ontario Geological Survey,
Geophysical Data Set 1068.
Buck, M., 2011, NI 43‐101 Technical Report on the
Preliminary Economic Assessment of the Big Daddy
Chromite Project, McFaulds Lake Area, for KWG
Resources Inc., May 2011
GEOPHYSICAL ASSESSMENT
OF THE
RING OF FIRE
Condor Consulting Inc.
Bedrock in the deposit area belongs to the
Oxford-Stull domain of the North Caribou
superterrane of the Archean Superior province
and is locally overlain by Paleozoic sandstone
and platform carbonate of the James Bay
lowlands.
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1899-Exploration for diamonds in Ontario started.
1946-Kimberlite recognized in Michaud Twp.
1959-Selection Trust started diamond exploration program.
1962-Joined with De Beers (Monopros).
1988-Discovered Attawapiskat kimberlite cluster-Victor mine
opened 2008.
2002-Aeromagnetic programs initiated by De Beers & Spider/KWG;
results in discovery of McFauld’s VMS cluster.
2003-Geotem E/mag flown by Billiken for Noront/Spider/KWG.
2007/8-Eagle’s Nest, several chromite deposits located.
2009-Cliffs Resources takes control of Freewest-Black Thor chromite.
2010-Cliffs Resources takes control of Spider-Big Daddy chromite.
2014-april 28, Ontario gov. Offers $1B for infrastructure; hopes to
secure matching sum from federal government.
2014-summer-market speculates Cliff’s to sell their Ring of Fire
properties due to falling iron ore prices and removal of senior
management.
The complex, called the Ring of Fire, has been dated at 2734.5 ±
1.0 Ma and it was emplaced into 2773.37 ± 0.9 Ma felsic plutonic
rocks. The felsic rocks form a sill complex structurally beneath
metasedimentary and metavolcanic rocks considered to have
formed along a passive margin at ca. 2800 Ma within the OxfordStull domain of the North Caribou superterrane in the Archean
Superior province. Bedrock in the deposit area belongs to the
Oxford-Stull domain of the North Caribou superterrane of the
Archean Superior province and is locally overlain by Paleozoic
sandstone and platform carbonate of the James Bay lowlands.
The McFauld’s VMS deposits were discovered in the
course of drill testing several discrete magnetic
features thought to be potential kimberlites. While
several deposits have been located in this cluster, to
date drilling has failed to define any economic
resources. As the images show, both the Geotem
(first system flown over the Ring of Fire) and VTEM
produce coherent responses over McFauld’s #1 and
#3.
The McFauld’s VMS deposits have coincident EM
responses, and are considered similar to the Matagami
Lake style VMS deposits. There is a minor thickness of
saprolite above the Archean basement but this has only a
moderate conductivity.
The VTEM survey was carried out in 2008
over the area of the Eagle’s Nest and chromite
deposits. While the magmatic Ni-Cu Eagle’s
Nest deposit is the most conductive feature in
the survey, there is conductivity associated
with the chromite horizons as well.
This shows the detailed magnetic and EM responses
associated with Eagle’s Nest. The results of a ground
magnetic survey are stitched into the airborne image.
The EM response shows a coincident EM response but
the outline is larger. Note the discrete EM picks are
marked by light blue circles. The EM results appear to
suggest that the Eagle’s Nest has a larger conductive
body than is magnetic. This apparent zoning could be an
artifact of the ‘spill over’ of the EM response with respect
to the magnetics or could be showing a mineralogical
zoning; more detailed modeling is required.
Falcon airborne gravity gradiometry and
magnetics were carried out over the extent of the
Ring of Fire complex. Images of the magnetics and
vertical gravity (Gd) show there are strong direct
associations between the two geophysical surveys
and mineralization. The magnetics tends to show
direct association with the EM (picks from 2008
VTEM survey) and mineralized zones. The gravity
shows the mineralized bodies to have a proximal
association with a large body of high density
underling the area. This body could be the primary
magma chamber that the shallow mineralized sills
were derived.
Moving along strike to the NE, the magnetic
and gravity responses appear to be similar to
one and another. The chromite zones are still
however associated with the ridge of high
gravity, whereas some of the EM responses
are over high density rocks and some are
associated with lower density rocks.
The detailed ground gravity response over Big
Daddy and Black Thor shows that there is a close
association between high density and the chromite
horizon. The EM response is more complex and in
places appears to conform with the chromite
horizon, whereas in locations, it is off-set into the
hanging wall, appearing to be coming from the
peridotite-dunite zone. Additional modeling of the
gravity, magnetics and EM is required to properly
understand these relationships.
Modeling of the Falcon gravity data shows that
the chromite zone is accurately picked by the
magnetic results but slightly off-set from the
gravity response. This suggests that alteration
of the chromite host rock has reduced its SG to
a point where the presence of the high density
chromite is insufficient to raise the rock
packages SG to an anomalous level.
Reconciliation of the airborne and ground
gravity results is deemed warranted.