Geology and Cu-Ni-PGE Deposits of
the Duluth Complex, NE Minnesota
Jim Miller
Department of Geological Sciences
Precambrian Research Center (NRRI)
University of Minnesota Duluth
A Looming
Stewardship
Question for
Minnesotans:
When should
we develop
this immense
copper-nickelprecious
metal mineral
resource?
Talk Outline
Geological origin of the Duluth Complex
Cause and character of the Cu-Ni-PGE
sulfide mineralization
World-class ore deposit by any measure
_______________
THE
MIDCONTINENT
RIFT
_______________
An attempt at
continental rifting 1.1
billion years ago
A. Rift Magmatism
Basalt
Flows
Gabbro
Crust
Mantle
Tectonic and Magmatic
Evolution of the
Midcontinent Rift
1,109-1,086 Ma
Mantle
Plume
B. Sediment Infilling
Sandstone
1,090-900 Ma
C. Compression
1,000-900 Ma
Midcontinent Rift
Geology in the
Lake Superior
Region
Duluth
Complex
Igneous Rocks of the
Midcontinent Rift
South
Kawishiwi
Intrusion
Partridge
River
Intrusion
Multiple
Emplacement
of Duluth
Complex
Intrusions
Sulfur Contamination creating the Cu-NiPGE Sulfide Deposits of the Duluth Complex
Cu
S
Ni
S
Co
Pd + Pt + Au
Copper-Nickel-Precious
Metal Deposits of the
Duluth Complex
Average Grades
Cu – 0.6%
Ni - 0.2%
Co – 0.005%
Pd+Pt+Au – 500-1000 ppb
Igneous Stratigraphy of the PRI and SKI
Based on Severson (1994)
History of Exploration
$/ton
From Severson,2008
1966 State lands open
for mineral leasing
Active exploration drilling
Polymet, Teck, Franconia,
Wallbridge (Duluth Metals)
1948 Cu-Ni
sulfide
discovered
at Spruce Rd
Cu-Ni Regional Study
State land closed to
new mineral leasing
1958 Bear
Creek drills
Babbitt
1954 INCO
drills Maturi
1999 LTV shuts down
1985 PGE
discovered
8,000
Cu Prices 1948-2006
7,000
Current $ - 7,060
6,000
5,000
4,000
3,000
2,000
1,000
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
1968
1966
1964
1962
1960
1958
1956
1954
1952
1950
1948
0
The Current Situation
Current Exploration Activity
Polymet – Northmet
Teck American – Mesaba
Duluth Metals – Nokomis, Maturi,
Dunka Pit, Birch Lake, Spruce Rd
Encampment– South Filson Cr.
Dominant Mineralogy
of Magmatic
Cu-Ni Sulfide Deposits
Pyrrohotite – FeS1-x
Chalcopyrite - CuFeS2
Pentlandite - (Fe,Ni)9S8
Bornite - Cu5FeS4
Talnakhite – Cu9(Fe,Ni)8S16
Chalcocite - Cu2S
Cobaltite - CoAsS
Why Sulfide Mining?
Most of the world’s metals comes from sulfide minerals
Chalcopyrite
CuFeS2
Bornite
Cu5FeS4
Chalcocite
Cu2S
Pentlandite
(Fe,Ni)9S8
Cinnabar
HgS
Sphalerite
ZnS
Molybdenite
MoS2
Galena
PbS
Cobaltite
CoAsS
The Environmental Challenge
Preventing Acid Rock Drainage (ARD)
2FeS2(s) + 7O2(g) + 2H2O(l) →
2Fe2+(aq) + 4SO42-(aq) + 4H+(aq)
Precious Metal Minerals
PMM Name
Formula
Total #
Sulfide Host Mineral
Total PMM
In Sulfide
cp
pn
cb
po
tn
bn
Kotulskite
Pd(Bi,Te)1-2
15
8
8
0
0
0
0
0
Froodite
PdBi2
11
10
3
6
1
0
0
0
Naldrettite
Pd2Sb
3
3
3
0
0
0
0
0
Paolovite
Pd2Sn
83
68
45
10
7
4
1
1
Sobolevskite
PdBi
2
2
1
0
0
0
1
0
Sperrylite
PtAs2
26
22
13
5
3
1
0
0
Stibiopalladinite
Pd5Sb2
11
5
4
1
0
0
0
0
Taimyrite
(Pd,Pt,Cu)3Sn
2
2
2
0
0
0
0
0
Telluropalladinite
Pd9Te4
1
1
1
0
0
0
0
0
93
68
31
24
3
4
3
3
Undetermined PGM
Silver sulfide
AgS
16
11
7
3
1
0
0
0
Empressite
AgTe
2
2
1
0
0
1
0
0
Hessite
Ag2Te
3
3
1
1
1
0
0
0
Electrum
AuAg
62
22
12
3
3
3
1
0
Gold
Au
17
7
3
1
0
3
0
0
347
234
135
54
19
16
6
4
67%
57%
23%
8%
7%
3%
2%
TOTALS
%
PMM study of NorthMet ore, Cervin, 2011
PMM-Sulfide Connection /Disconnection
PdSn
bt
sperrylite
bn
pn
cp
cpx
pn
cp
pn
cb
cp
cp
pn
Deuteric
alteration
seperates PMM
from host sulfide
– implications for
beneficiation
plagioclase
chl
PdBi
PdSbBi
cp
cp
cp
Cervin, 2011
Styles of
Mineralization
after Peterson (2001)
• Open (early)
– vertically extensive (> 450
meters)
– low - mod Cu-Ni grades
and very low Au+PGE
– Cu-Ni grades typically
increase towards the basal
contact
• Confined (late)
– vertically restricted (< 150
meters)
– mod - high Cu-Ni grades
and mod to high Au+PGE
– Cu-Ni grades typically are
the highest near the top of
the mineralized zone
Transition, Open - Confined Styles
P
Peterson, 2001
100% Sulfide Plots of SKI Open vs.
Confined Styles of Mineralization
Open Style
Open Style
N = 1,737
N = 1,737
Deposits
Spruce Rd
Dunka Pit
Serpentine
Confined Style
Confined Style
N = 13,868
N = 13,868
Deposits
Nokomis
Maturi
Birch Lake
Peterson, 2012
New Maturi Mineralization
Stratigraphy
Peterson, 201223
Emplacement/Mineralization Model
for the South Kawishiwi Intrusion
Peterson, 2012
World Class Ores of the Duluth Complex
Compared to other Magmatic Sulfide Deposits, the Duluth Complex is:
#1
or 2 in contained Copper
#4 in contained PGE
#3 in contained Nickel
Only the Bushveld, Great
Dyke, and Noril’sk contain
more PGE
From Peterson, 2010
World Class Scale of the
Maturi Deposit Alone
Nokomis
Maturi
Maturi
Maturi
From Peterson, 2010 and
DM press release 12/2012
The Largest UNDEVELOPED Cu-Ni-PGE
Deposit on Earth
Duluth deposits are perceived as low grade. But compared to the source
of most of the world’s copper - Porphyry Copper Deposits...
…the Duluth
Complex ores are
HIGHER GRADE
with MORE
CONTAINED
METAL than nearly
all porphyry systems.
The largest copper
ore deposits in the
USA are on this
diagram, and the
Duluth Complex ores
are much larger then
all of them.
From Peterson, 2010
CONCLUSION:
The base and
precious metal
deposits of the
Duluth Complex
are a World
Class Resource
that will be
mined ….
SOMEDAY
Stewardship question - If not now, when?