NEW PROJECTS AT SUDBURY
OFFERED BY THE IMPACT GEOLOGY GROUP AT
THE UNIVERSITY OF NEW BRUNSWICK, CANADA
Posting: 3 June 1999

Sudbury is one of the world’s largest known impact structures and is also
the site of a truly giant Cu-
Ni-PGE (platinum-group element) mining camp. It contains the world’s single
largest Cu-Ni-PGE
deposit: the Frood-Stobie ore body, from which 600 million tonnes of
sulphides have been extracted
over the last century. Many of the original ore discoveries have been mined
out, or are approaching
the end of their productive existence. Frood -Stobie is nearly extracted,
and, critically, this deposit
has been the mainstay of Inco operations over the last hundred years.
Consequently, there is a real
need to discover new giant ore bodies at Sudbury. Key to any exploration
program is the recognition
that Sudbury is a large impact structure. Furthermore, many, if not most, of
the ore is located in
tectonic settings that were created as a direct result of the impact
process. In order to identify
potential ore zones, the Impact Geology Group at the University of New
Brunswick has been
working in collaboration with industry and the Ontario Geology Survey (OGS)
to understand how
the generation of a large impact structure can create and control the siting
of large ore bodies. We
are seeking new graduate students at the MSc or PhD levels to undertake
multifaceted projects
involving detailed field mapping, followed by sample evaluation via
microprobe, major, trace and
REE geochemistry and geochronology. These projects provide excellent
all-round training for future
careers in industry or research. Three projects are currently available:

1. Geological setting of the Foy Offset
The Foy Offset is a radial dyke extending from the Sublayer of the Sudbury
Igneous Complex (SIC)
in the north range of the Sudbury impact structure. It is probably the
largest radial dyke of the
structure, extending for at least 25 km into Archean footwall rocks beyond
the SIC. The proximal
Foy probably feeds the concentric Hess Offset dyke, while the Foy’s distal
portion is itself fed via
Hess (Wood and Spray, 1998). Little work has been done on Foy, and virtually
none on its distal
segment. There are no publications dealing exclusively with Foy, though
there are internal reports
available produced by mining companies for parts of the dyke. Considerable
mineralization occurs
along the proximal 9 km length, where the dyke is widest, and there is now
renewed interest in
reassessing Foy’s mineral potential. The project will assess the geological
setting of Foy (i.e., the
nature of the host fracture/fault zone), the relationship of Sudbury Breccia
(pseudotachylyte) and
especially Footwall Breccia to the quartz diorite of the main Foy dyke and
establish the geochemical
signatures of these lithologies. Detailed mapping will form an important
part of the project via 3-4
month field seasons each year, followed by analytical work to be carried out
in Fredericton. The
project may involve camping, canoeing and helicopter support, as well as 4x4
vehicle usage. The
project will be financed in collaboration with Falconbridge and NSERC,
preferably at the PhD level.
Ideally, the project should start this summer (1999).

2. Geological setting of the Ministic Offset and related regional geology
The Ministic Offset is a radial dyke located in the northwest of the SIC in
the north range. Virtually
no work has been done on this dyke and its full radial extent has not been
established. Furthermore,
the geology of the northwest sector of the impact structure is poorly known,
with few regional maps
being available. Provisional work by the Impact Geology Group indicates that
the Ministic dyke may
be considerably more extensive than previously considered, and that it may
link with the Hess Offset
(Wood and Spray, 1998). The project will involve a field-based program of
mapping (3-4 months
per season), with emphasis on establishing dyke-wall rock relations and dyke
extent. Regional
structure will also be studied, particularly regarding impact-generated
radial and concentric fault
systems and their interrelations. Analytical studies will include microprobe
work and geochemical
analysis based in Fredericton. Field work will involve canoeing (with
excellent access via Ministic
Lake), camping and 4x4 usage, as well as helicopter support where necessary.
This project offers the
potential for realizing new insights into Sudbury geology for this part of
the impact structure. The
project will, in its initial phases, be NSERC supported at the MSc or PhD
level with assistance from
the OGS.

3. The metamorphic aureole of the Sudbury impact structure
As a 2.5 km thick, originally 100 km diameter subcircular melt body, the SIC
would have been
expected to metamorphose subjacent footwall rocks. Energy transformation
calculations indicate that
if the SIC was generated by impact melting, it was most probably superheated
to >2000°C. Such a
temperature would be expected to induce extreme contact effects beneath the
SIC. It is therefore
puzzling that the aureole is thin (<1.5 km) and poorly developed. In places
it is apparently non-
existent. Although some work has been done on the aureole (e.g., Dressler
1984; James et al. 1992),
there has been no detailed, systematic study made of contact effects at
Sudbury along the length of
the less-disturbed North Range contact. In the Levack Township area,
Dressler (1984) does
document an innermost pyroxene hornfels facies zone (~100 m), followed by a
hornblende hornfels
facies zone (~200 m), then merely recrystallization textures in plagioclase
(~1000 m). In the Windy
Lake area, provisional work by our group indicates that only static
recrystallization has occurred,
with no new mineral growth occurring, even adjacent to the SIC.
 There could be several reasons for the diminished nature of the aureole:
(1) the lower part
of the melt sheet was rapidly cooled in a buffer zone comprising melt mixed
with relatively cold rock
fragments (which together now comprise the Footwall Breccia); (2) the
effects of contact
metamorphism were overprinted and partly erased by Penokean
tectonometamorphism. Overprinting
is known to have affected the South Range (along with Grenvillian effects),
though these effects
have traditionally been considered minimal in the North Range; (3) the
present contact between the
SIC and footwall is locally tectonic (i.e., the contact is, in places, a
fault/thrust).
 In order to test the above and thoroughly characterize the aureole for the
first time, a graduate
student at the PhD level will be dedicated to determining the conditions of
formation and width of
the aureole. This will be done primarily in the less altered North Range via
the sampling and
mapping of several <3 km long swaths radiating out from the SIC-footwall
contact. Core material
from Falconbridge and Inco will also be made available. Different
lithologies will be sampled,
including Levack gneiss and migmatites, Cartier granitoids, basic-ultrabasic
rocks exposed in Wisner
Township, greenstones in Trill Township and Footwall Breccia. These
different rock types will be
used to establish PT conditions of metamorphism using established
thermobarometers via electron
microprobe analysis. For example, exchange (e.g., garnet-ilmenite) and
solvus (e.g., two-pyroxene)
thermometers, and net transfer equilibria (e.g.,
garnet-plagioclase-hornblende-quartz), as well as
hornblende barometry. Mapping the distribution of the aureole will also help
determine the degree
of post-impact disturbance and so complement the proposed structural studies
in the Western and
Eastern Sectors. This subproject will also address the origin of the Levack
gneisses which abut the
SIC in the North Range: were they exhumed from granulite facies conditions
prior to impact or
elevated by the impact event itself?

Reference

     Wood, C.R. and Spray, J.G. 1998. Origin and emplacement of Offset Dykes
in the Sudbury impact
     structure: constraints from Hess. Meteoritics and Planetary Science 33,
337-347.

All three projects provide funding at least $16,000 per year (probably more
for foreign students) for
two years for a Masters and four years for a doctorate. Interested
candidates should contact John
Spray for further details. Details of Dr Spray’s research program can be
found via website
http:\\www.unb.ca\geology\jgs\jgs.htm

Dr John Spray
Impact Geology Group
Department of Geology
University of New Brunswick
2 Bailey Drive
Fredericton                                         Tel: (506) 453-3550
New Brunswick, E3B 5A3                 Fax: (506) 453-5055
CANADA                                             e-mail: [log in to unmask]



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