©2010 Society of Economic Geologists, Inc.
Economic Geology, v. 105, pp. 669–688
Secular Variation of Magmatic Sulfide Deposits and Their Source Magmas
A. J. NALDRETT,†
Economic Geology Research Unit, School of Geosciences, The University of Witwatersrand, P.O. Box 150, Wits,
Johannesburg, South Africa
Abstract
Magmatic sulfide deposits are divisible into two major groups, those that are valued primarily for their Ni
and Cu and that are mostly sulfide rich (>10% sulfide), and those that are valued primarily for their PGE and
tend to be sulfide poor (<5% sulfide). Most members of the Ni-Cu group formed as a result of an interaction
of mantle-derived magma with the crust that gave rise to the early onset of sulfide immiscibility. Of the different classes of deposit in this group, the komatiite-related class ranges from 2.7 to 1.9 Ga in age, the Flood
basalt-related class from 1.1 to 0.25 Ga, and the Mg basalt- and basalt-related group from the Archean to the
present. There is only one example each of anorthosite complex- and impact-related deposits, so that one cannot generalize about their secular distribution, except to say that anorthosite complexes are Proterozoic.
Ural-Alaskan intrusions are dominantly Phanerozoic (some Archean deposits have been included with this
group), but as yet no examples have been found with economic sulfide bodies.
Seventy-five percent of known PGE resources occur in three intrusions—the Bushveld, Great Dyke, and
Stillwater, the rocks all of which have crystallized from two magma types, an unusual, high SiO2, MgO, and Cr
and low Al2O3 type (U-type) that was emplaced at an early stage and a later, normal tholeiitic-type magma
(T-type); the PGE are concentrated in layers close to the level at which the predominant crystallization
switches from one magma type to the other. The U-type magma is interpreted as a PGE-rich, komatiitic magma
(possibly the product of two-stage mantle melting) that has interacted to varying degrees with the crust, becoming SiO2 enriched in this way. These three intrusions are Neoarchean to Paleoproterozoic in age.
All known examples of komatiites, with one exception, are Paleoproterozoic or older and their secular
distribution is thought to be due to cooling of the Earth. Known deposits do not occur in the oldest (>3.0 Ga)
komatiites but appear at around 2.7Ga in continental (Kambalda, Western Australia) or island-arc (Perseverance-Mount Keith, Western Australia) environments, possibly because it was these environments that offered
the opportunity for interaction with felsic rocks. It is suggested that the development of these environments in
the Archean was an additional control on the age distribution of these deposits. It is postulated that the
restricted secular distribution of PGE-enhanced intrusions is also due to the need for a hot mantle to give rise
to U-type magmas.
Introduction
MAGMATIC nickel-copper-PGE sulfide deposits form as the
result of the segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these droplets from the
silicate magma. While a wide range of base, precious, and semimetals are recovered from the deposits, the most important products are Ni and Pt. The size of the deposits, their
grades, and ratios of economic metals are very variable. This
is illustrated in Table 1, which is a summary of available information on tons of resources + production; grades of Ni,
Cu, Co, and PGE; tons of contained metal; and value of the
ore and of the individual metals at current (February 14,
2009) metal prices.
The deposits fall naturally into two major groupings, those
that are of value primarily because of their Ni and Cu content
and that tend to be rich in sulfide, with the ore containing 10
to 90 percent sulfide, and those of value primarily because of
their PGE that tend to be sulfide poor with the ore containing 0.5 to 5 percent sulfide (Fig. 1). With certain exceptions,
sulfide-rich types cluster at the nickel apex of the diagram;
many of the smaller deposits and camps, which are not included in this compilation, also fall in this area. Sulfide-poor
PGE-rich deposits cluster near the PGE apex. Exceptions to
this grouping are the Platreef, which consists of a cloud of
† E-mail:
[email protected]
0361-0128/10/3893/669-20
stratigraphically controlled, weakly disseminated sulfides in
the Bushveld Complex, and the deposits of the Noril’sk,
Siberia, and Duluth, Minnesota, regions.
Considering first Ni-Cu deposits, Naldrett’s (2004) classification is summarized in Figure 2, along with a brief description of each class and a sketch to assist with the description.
The classification is in terms of their associated magma type.
Class NC-1 comprises those related to komatiitic magmatism.
Deposits fall into two subclasses, those related to Archean komatiites (e.g., the deposits of Western Australia, Zimbabwe,
and the Abitibi belt of Canada, and, to a lesser extent, Finland) and those related to Proterozoic komatiites (e.g., those
of the Raglan and Thompson belts, which are both in
Canada). Class NC-2 consists of deposits that are associated
with flood basaltic magmatism. The principal examples considered here are those of the Noril’sk region in Siberia and
those of the Lake Superior area in North America. Class NC3 comprises a relatively uncommon magmatic association—
that of ferropicrite—for which the only significant example is
the Pechenga camp of the Kola peninsular of Russia. Class
NC-4 covers those deposits that are related to anorthositegranite-troctolite complexes, such as the Nain Plutonic Complex of Labrador, Canada. For many years this association was
not thought to be important as a source of magmatic sulfide
deposits, but the 1994 discovery of the Voisey’s Bay deposit
changed this prevailing viewpoint. Class NC-5 comprises a
miscellaneous grouping of deposits that are all associated with
669
Submitted: March 16, 2009
Accepted: June 30, 2009
670
A. J. NALDRETT
TABLE 1. Resources and Value of Deposits
Ore resource Ni
(106 t)
(wt %)
Deposit/camp
Yilgarn WA, type 11
Yilgarn WA, type 21
Zimbabwe, type 12
Zimbabwe, type 23
Ontario, type 14
Thompson5
Thompson low grade6
Raglan7
Pechenga8
Noril’sk9
Duluth10
Jinchuan11
Selibi-Phikwe, Botswana12
Tati (Phoenix + Selkirk),
Botswana13
Kabanga, Tanzania14
Monchegorsk (NKT orebody)15
Voisey’s Bay16
Montcalm17
Aguablanca18
Sudbury19
Great Dyke20
Total Merensky21
Total UG-222
Platreef23
Total Bushveld
Stillwater24
Portimo Area25
Lac des Iles26
Yilgarn WA, type 11
Yilgarn WA, type 21
Zimbabwe, type 12
Zimbabwe, type 23
Ontario, type 14
Thompson5
Thompson low grade6
Raglan7
Pechenga8
Noril’sk9
Duluth10
Jinchuan11
Selibi-Phikwe, Botswana12
Tati (Phoenix + Selkirk),
Botswana13
Kabanga, Tanzania14
Monchegorsk (NKT orebody)15
Voisey’s Bay16
Montcalm17
Aguablanca18
Sudbury19
Great Dyke20
Total Merensky21
Total UG-222
Platreef23
Total Bushveld
Stillwater24
Portimo Area25
Lac des Iles26
Ni (106
$US)
Yilgarn WA, type 11
Yilgarn WA, type 21
Zimbabwe, type 12
Zimbabwe, type 23
Ontario, type 14
35,142
64,088
3,093
2,625
2,087
0361-0128/98/000/000-00 $6.00
Cu
(wt %)
Co
(wt %)
Pt
(g/t)
Pd
(g/t)
Rh
(g/t)
Ru
(g/t)
Ir
(g/t)
Os
(g/t)
Total PGE
(g/t)
0.148
0.020
0.214
0.016
0.067
0.300
0.039
0.155
0.046
0.006
0.024
0.140
0.021
0.056
0.034
0.012
0.118
0.069
0.015
0.132
0.614
0.100
2.179
0.535
0.046
0.072
0.033
0.041
0.150
0.005
0.228
0.007
0.005
0.003
0.015
0.374
0.007
0.124
0.007
0.010
0.007
0.004
0.065
0.004
0.032
0.003
0.010
0.002
0.001
0.080
0.006
0.047
0.003
0.011
0.002
0.001
0.803
0.110
0.551
0.000
2.793
0.827
0.000
3.759
0.317
10.030
0.655
0.262
0.149
0.351
161.8
1,021.9
42.4
50.0
10.1
154.0
380.4
32.8
339.0
1,257.0
4,000.0
515.0
49.4
336.8
2.07
0.60
0.70
0.50
1.98
2.32
0.64
2.87
1.18
1.84
0.20
1.06
1.04
0.26
0.15
0.02
0.05
0.81
0.63
3.75
0.60
0.75
1.67
0.21
0.057
0.045
0.092
0.019
0.019
0.825
0.121
1.900
0.146
0.127
0.088
0.046
2.266
0.173
7.700
0.490
0.098
0.047
0.284
48.2
3.1
136.7
3.6
19.7
1,648.0
2,574.0
4,210.0
5,742.6
1,597.3
11,549.9
323.2
218.6
94.1
2.71
5.10
1.59
1.56
0.66
1.20
0.21
0.15
0.04
0.41
0.13
0.04
0.08
0.05
0.37
2.90
0.85
0.75
0.48
1.08
0.14
0.06
0.02
0.20
0.06
0.02
0.18
0.06
0.189
0.230
0.090
0.540
1.460
0.075
0.048
0.246
0.463
2.770
3.566
2.661
1.765
2.870
4.340
0.380
0.180
0.810
6.920
0.097
0.014
0.236
0.583
2.130
1.850
1.708
2.006
1.800
15.850
1.540
1.660
Ni
(103 t)
Cu
(103 t)
Co
(103 t)
Pt (t)
Pd (t)
Rh (t)
Ru (t)
Ir (t)
3,347.2
6,104.2
294.6
250.0
198.7
3,573.9
2,437.2
942.3
4,000.0
23,128.8
8,000.0
5,459.0
514.2
872.2
242.4
210.8
23.0
23.9
20.4
0.0
34.6
16.7
2.8
48.48
40.32
6.58
7.50
5.93
1.00
22.74
21.91
2.37
87.7
249.5
0.0
7.1
6.2
15.4
21.92
82.43
7.07
265.5
2,150.2
47,137.5
24,000.0
3,888.0
827.8
718.0
1.9
15.2
115.6
75.7
9.9
0.0
0.0
27.1
41.0
2,388.3
585.0
65.5
4.4
15.4
74.32
58.65
9,678.90
1,959.00
50.57
2.30
95.70
4.92
1.85
286.00
27.26
2.53
0.14
5.05
1,306.2
158.1
2,173.5
55.5
129.0
19,776.0
5,405.4
6,315.0
2,411.9
6,549.1
15,014.9
129.3
174.9
49.9
178.3
89.9
1,162.0
26.7
94.6
17,798.4
3,603.6
2,652.3
1,033.7
3,194.7
6,929.9
64.6
393.5
58.3
9.1
0.7
12.3
0.0
0.3
62.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
26.0
4.5
10.2
0.2
4.8
762.7
7130.0
15,012.0
15,279.0
2,820.0
33,148.2
1,402.7
83.1
16.9
39.04
21.45
13.21
0.05
4.65
961.19
5,482.62
7,790.00
9,809.00
3,204.00
20,789.82
5,122.72
336.64
156.22
Cu (106 Co (106
$US)
$US)
826
718
78
299
948
808
0.87
0.16
0.046
0.017
0.038
Pt (106
$US)
Pd (106
$US)
1,225
591
100
354
294
48
219
160
670
0.007
0.013
0.002
0.002
0.002
0.056
0.130
0.216
0.428
0.114
0.307
0.270
0.044
0.289
0.449
0.710
0.165
0.539
0.100
0.019
0.052
0.082
0.131
0.038
0.100
0.096
0.008
0.047
0.051
0.062
0.033
0.054
0.043
0.008
0.000
0.000
0.000
0.65
0.64
0.01
PGE (t)
5.42
12.04
4.99
11.20
15.01
5.59
129.96
111.92
23.35
11.03
5.01
6.39
28.11
127.33
12.26
2.48
155.77
27.04
5.14
0.37
1.42
2.13
1.36
39.66
10.64
5.26
0.08
0.38
2.62
2.07
58.75
11.75
5.76
0.10
0.29
123.30
107.39
1,2607.38
2,620.72
134.75
7.36
118.29
0.22
0.01
0.32
91.78
72.54
335.32
743.76
909.00 1,891.00
2,457.00 4,077.00
182.00
263.00
3,545.82 6,225.40
87.26
32.32
31.36
134.88
345.00
752.00
60.00
1,154.99
31.03
13.13
119.76
214.00
356.00
53.00
623.69
13.90
326
314
34
1.350
8.590
0.188
0.080
0.482
1.173
5.418
6.214
5.700
4.121
5.670
20.699
1.920
1.848
Os (t)
1.01
0.05
Rh (106 Ru (106
$US)
$US)
2,183
1,726
292
0.210
0.005
0.002
0.78
0.00
Ir (106
$US)
Value
Os (106
Total
US$/t of PGE
$US) (106 $US) of ore (106 $US)
81
181
75
142
190
71
0.01
65.07
26.63
25.65
0.28
9.50
1,932.70
13,946.32
26,161.00
32,730.00
6,582.00
65,487.93
6,689.92
419.71
173.95
41,227
68,911
3,790
2,625
2,764
255
67
89
52
275
% as
PGE
4,311
3,297
619
10.5
4.8
16.3
379
13.7
671
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
TABLE 1 (Cont.)
Ni (106
$US)
Thompson5
37,523
Thompson low grade6
25,589
7
Raglan
9,893
Pechenga8
41,996
Noril’sk9
242,829
Duluth10
83,992
Jinchuan11
57,314
Selibi-Phikwe,
5,398
Botswana12
Tati (Phoenix + Selkirk),
9,157
Botswana13
Kabanga, Tanzania14
13,714
Monchegorsk
1,660
(NKT orebody)15
Voisey’s Bay16
22,820
Montcalm17
583
Aguablanca18
1,355
19
Sudbury
207,628
Great Dyke20
56,751
Total Merensky21
66,301
25,322
Total UG-222
Platreef23
68,759
Total Bushveld
157,641
Stillwater24
1,357
Portimo area25
1,836
26
Lac des Iles
524
Cu (106 Co (106
$US)
$US)
Pt (106
$US)
Pd (106
$US)
Rh (106 Ru (106
$US)
$US)
10,499
Value
Os (106
Total
US$/t of PGE
$US) (106 $US) of ore (106 $US)
850
281
545
602
2,057
158
75
81
905
7,326
160,597
81,768
13,247
2,820
74
603
4,589
3,004
395
957
1,450
84,503
20,700
2,317
155
543
428
70,687
14,307
369
17
1,431
539
83,216
7,931
736
41
176
36
2,234
388
74
5
32
20
595
160
79
1
547
699
1,469
20
6
921
160
285
157
189
2,446
608
306
361
28
3,959
91
322
60,639
12,277
9,036
3,522
10,884
23,610
220
1,341
199
488
363
6
13
171
2,464
26,986
252,274
531,157
540,604
99,778
1,172,855
49,630
2,939
599
96
186
0
2
34
7,020
26,704
40,041
97,565
56,892 264,483
71,637 714,890
23,399
52,955
151,832 1,031,693
37,412
25,390
2,459
1,141
227
US$/t
Metal prices as of
February 14, 2009
Ir (106
$US)
3,407
14
1
% as
PGE
274
67
428
155
517
53
145
171
3,518
8.3
33
26
745
149
73
1
42,172
25,589
14,043
52,425
649,995
212,398
74,603
8,439
3,172
2,500
241,979
43,634
3,648
220
22.6
4.8
37.2
20.5
4.9
2.6
4
14,347
43
2,744
19.1
15,889
2,501
330
807
1,206
506
7.6
20.2
27,934
683
1,896
333,118
473,115
962,874
1,430,234
26,1118
2,652,137
115,115
8,574
2,690
204
192
96
202
184
229
249
163
230
356
39
29
667
9
205
62,387
404,087
887,537
1,401,390
181,475
2,470,886
113,538
5,398
1,968
2.4
1.4
10.8
18.7
85.4
92.2
98.0
69.5
93.2
98.6
63.0
73.1
3
0
4
1,040
471
10,665
2,024
27,116
5,177
58,463 11,285
3,771
900
89,270 17,332
463
466
166
1,518
2,712
4,511
672
7,904
176
0
US$/troy oz
39,682
1,097
226
9,020
445
465
393
1 Tonnage and Ni grades from Barnes (2006); Cu/Ni, Co/Ni, PGE/Ni from Naldrett (2004); supplementary data on Mt. Keith from Reid Keays, pers. commun. (2006)
2 Tons of ore and Ni grade, Harry Mason, pers. commun.; data for PGE obtained from Ni/PGE ratios of 21 personally collected samples, analyzed by
AJN in 1980
3 Martin Prendergast, pers. commun. (March 5, 2009)
4 Shebandowan press release, Mengold resources citing production and Ni and Cu grades of Shebandowan from 1971–1998, along with Pd/Ni and Pt/Ni ratios for the adjacent Shebandowan West deposit, which have been extended to Shebandowan; Langmuir 1 and 2: Tons mined and Ni grade from Inspiration
Mining website (March 5, 2009) citing production to 1991; Cu, Pt, and Pd from metal/Ni ratios from 31 personally collected samples analyzed by AJN in 1980
5 Reserves of Ni from INCO Ltd., Cu/Ni ratio from Theyer (1980), Co/Ni and PGE/Ni ratios from data of Bleeker (1990)
6 Data from Layton-Mathews et al. (2005)
7 Data on tonnage and Ni and Cu grade from Lesher (2005); Co and PGE grades from Lesher et al. (1999)
8 Ni data from Green and Dupras (1999); Cu/Ni and PGE/Ni ratios from Brugmann et al. (2000)
9 Estimate, Naldrett and Searcy (unpub. data), based on analytical data of Naldrett et al. (1996), Distler and Kunilov (1994)
10 Tonnage, Ni, and Cu grade from Listerud and Meinecke (1977); Co/Ni and PGE/Ni ratios from Naldrett (1989)
11 Tonnage and Ni grade from Chai and Naldrett (1992); Cu/Ni, Co/Ni , PGE/Ni ratios from Tang (1993, table 4)
12Tons of ore, Ni and Cu grades from Maier et al. (2002); data for PGE obtained from Ni/PGE ratios of 21 personally collected samples, analyzed by
AJN in 1980
13 Figure for tons of resources and Ni and Cu grade as of December 31, 2006, obtained from Noril’sk Nickel website March 5, 2009; Pt/Ni and Pd/Ni
obtained from production figures given as salable tons of Ni and oz of Pt and Pd produced in 2007; figures for Rh, Ru, Ir, and Os obtained from metal/Ni
ratios for disseminated ore from Barnes and Maier (2002)
14 Reserves from Barrick website (February 16, 2009), quoting data for end of 2007 (Barrick has 50% share); Cu and Co calculated from average of
grades published on Sutton Resources website with ratios related to Ni; PGE/Ni ratios as presented by Evans et al. (1999); note with respect to Evans’
data, precedence was given to net and disseminated ores, since massive ore can easily lose Pt and Pd to the surroundings
15 Data from a report by Oxus Resources (2001), kindly provided by Tatiana Grohovskaya
16 Tonnage and Ni, Cu, and Co grades from Lightfoot and Naldrett (1999); PGE data obtained from PGE/Ni ratios of 190 samples with >20% sulfide
from Naldrett et al. (2000)
17 Tonnage and Ni and Cu grades from Falconbridge website December 2004, Pt/Ni, Pd/Ni, Rh/Ni, Ru/Ni, and Ir/Ni ratios from samples personally collected and analysed by AJN in 1980
18 Data from Lundin Mining Corporation technical report (website March 5, 2009)
19 Reserves and Ni grade after Naldrett (2004); Cu/Ni, Co/Ni, and PGE/Ni ratios after data of Naldrett et al. (1999)
20 PGE data from Vermaak (1995); Ni/(Pt + Pd) and Cu/(Pt + Pd) ratios from study of Naldrett and Wilson (1990)
21 Tons of PGE from Vermaak (1995, table 2.7); grade of total PGE from Vermaak (1995, table 2.1); Ni and Cu grades using Ni/(Pt + Pd) and Cu/Ni ratios of Naldrett (1989)
22 Tons of PGE from Vermaak (1995, table 2.7); grade of total PGE from Vermaak (1995, p. 17); Ni and Cu grades from Naldrett (1989)
23 Data from Vermaak (1995); no reliable data are available for Ni and Co contents
24 Tons and grade of (Pt + Pd) of proven and probable reserves for the Stillwater and East Boulder mines from Zientek et al. (2002); Pt/Pd ratio from
Vermaak (1995); Ni and Cu calculated using data for Pt and Pd from Zientek et al. (2002) and ratios for Ni/(Pt + Pd) and Cu/(Pt + Pd) from Naldrett (1989)
25 Information from release on internet by Arctic Platinum Partnership (Goldfields and Outokompu), July 2002
26 Data for tonnage and Pt and Pd grades as given at Cordilleran Round-up, Vancouver, January 2000; metal/Pd ratios from Naldrett (unpub. data)
0361-0128/98/000/000-00 $6.00
671
672
A. J. NALDRETT
PGE
0 .2
0 .4
of
P
L
PR
Po
0 .6
N
0 .8
ive
lat
Re
0.4
R Mo
Ta S
O1
A
Y1
TK P
J
Y2
S-P
MtV
0.2
1
0.2
0.4
Ni-Cu Rich
Y1 - Yilgarn Type 1 (Australia)
Y2 - Yilgarn Type 2 (Australia)
Z - Zimbabwe Hi grade
O1 - Ontario Type 1
T - Thompson Hi grade (Canada)
R - Raglan (Canada)
N - Noril’sk (Russia)
D - Duluth (USA)
J - Jinchuan (China)
S-P- Selibe-Phikwe (Botswana)
Ta - Tati (Botswana)
K - Kabanga (Tanzania)
Mo - Monchegorsk (Russia)
D
0.6
0.8
1
Ni
u
fC
eo
va
0.6
lu
va
lue
ive
0.8
la t
U
M
B
GD
Re
GE
1
Cu
P - Pechenga (Russia)
V - Voisey’s Bay (Labrador)
Mt - Montcalm (Ontario)
A - Aguablanca (Spain)
S - Sudbury (Ontario)
PGE Rich
B
M
U
PR
GD
-Total Bushveld (South Africa)
- Merensky Reef (Bushveld)
- UG-2 (Bushveld)
- Platreef (Bushveld)
- Great Dyke of Zimbabwe
- Stillwater (USA)
Po - Portimo Complex (Finland)
L - Lac des Iles (Canada)
FIG. 1. Relative value of the contributions of Ni, Cu, and PGE to magmatic sulfide deposits. Data from Table 1.
magmas ranging from picritic to tholeiitic in composition; in
discussing the secular distribution of members of this class
below, a distinction is made between those for which the
source magma likely had >10 wt percent MgO, which are
grouped with class NC-3, and those with <10 wt percent
MgO that are considered on their own. This has been done
because the objective was to see if there is a relationship between MgO-rich magmatism and time. Sudbury (class NC-6)
is unique and comprises a class of its own, that is, mineralization that has developed from the melt produced by extraterrestrial impact (see summary by Naldrett, 2003). Class NC-7
covers a class of ultramafic complex that is usually referred to
as Ural-Alaskan or Alaskan. Normally this class does not contain significant accumulations of Ni-rich sulfide, although
some are enriched in PGE that are not related to sulfide concentrations and have given rise to placer PGE deposits. However, certain examples from Alaska and British Columbia that
contain Ni sulfides include Duke Island, Alaska, Turnagain
River, British Columbia, and Salt Chuck, Alaska (Thakurta et
al., 2008).
Considering now sulfide-poor, PGE-rich deposits, the initial division is on the basis of petrologic association, as has
been done above for Ni-Cu deposits. It has become apparent
(Helz, 1985; Irvine and Sharpe, 1986; Iljina, 1994; Alapieti
and Lahtinen; 2002; Miller and Andersen, 2002) that the
largest PGE deposits of the world occur in intrusions
(Bushveld Igneous Complex; Stillwater Igneous Complex;
Great Dyke of Zimbabwe) that are characterized by a high
proportion of an early magma with a distinctive Al2O3-poor
0361-0128/98/000/000-00 $6.00
and MgO-, Cr-, and yet SiO2-rich (U-type, sometimes referred to as magnesian andesite) composition, which was followed in the same intrusion by one with a more typical tholeiitic (T-type) composition. Many of the PGE concentrations
occur at levels in the intrusions at which there is trace element and isotopic evidence of variable degrees of mixing of
these two magma types. This association was grouped by Naldrett (2004) as a distinctive class, PGE-1. Deposits in intrusions that show evidence of both U-type and tholeiitic magmas, but in which the tholeiitic component is dominant, are
grouped as class PGE-2. Class PGE-3 comprises intrusions
for which there is no evidence of an early U-type magma but
for which the magma is clearly tholeiitic. Some Keweenawan
intrusions of the Lake Superior area, including the Sonju
Lake intrusion within the Duluth Complex and the Coldwell
intrusion (Barrie et al., 2002), are members of this class, along
with the Cap Edvard Holm and Skaergaard intrusions of East
Greenland. Calc-alkaline magmatism (class PGE-4) is known
to host PGE concentrations, although none of these have yet
proved to be economic. Examples include intrusions of the
Platinum belt in the Ural Mountains of Russia, where at both
the Volkovsky deposit and the Baron prospect PGE are concentrated in zones rich in titaniferous magnetite, apatite, and
Cu sulfides and the Lac des Iles deposit in Canada. The
Longwoods Intrusive Complex at the southern tip of the
southern end of the South Island of New Zealand (Cowden et
al., 1990) is clearly calc-alkaline and forms part of an accreted
volcanic terrane. PGE-bearing gold placers have been derived from this intrusion, but the nature and origin of the primary PGE mineralization is not understood at the present
time. In addition to this primary division based on magma
types, PGE-rich deposits can be viewed further on the basis
of morphology and predominant mineralogical association of
the orebodies, including whether they are stratiform, strata
bound, or discordant, and whether the PGE show a sulfide,
chromite, or magnetite association. Figure 3 shows a hypothetical layered intrusion with different classes and styles of
mineralization located within it where they are most likely to
be found (Naldrett, 2004). It must be emphasized that a single intrusion will never have all types associated with it; Figure 3 is merely an attempt to summarize a wide variety of
styles of mineralization in one diagram.
Figure 4, also based on Table 1, summarizes the contributions that each of the forgoing classes of Ni-Cu– and PGEdominant deposit types contribute to the overall Ni and Pt resources of magmatic sulfide deposits. Between them, the
Flood basalt class (NC-2) and Sudbury class (NC-6) account
for 50 percent of all known Ni resources, with a sizeable contribution coming from type PGE-1 layered intrusions (those
with a high proportion of U-type magma) for which Ni is a byproduct. PGE resources are totally dominated by PGE-1 layered intrusions with a small contribution coming from Noril’sk class NC-2 (note that if one considers Pd, about 25% of
the world’s resources are present in the Noril’sk deposits).
The Origin of Ni-Cu– and PGE-rich Sulfide Deposits
As emphasized above, magmatic sulfide deposits are the result of the development of sulfide immiscibility in a mafic
and/or ultramafic magma, the partitioning of chalcophile metals into the immiscible sulfides, and then their subsequent
672
NC-2
Type example Noril’sk-Talnakh, Siberia Very Ni-, Cu- and PGErich zones of massive and disseminated sulfide within elongate (up
to 20 km long, 1-2 km wide, 100-300 m thick) feeders to overlying
flood basalt lavas. The feeders show internal layering ranging from
picrite (50% olivine) to gabbro, and are connected to 10 to 30 mthick sills of unknown extent. Certain units within the volcanics
show marked depletion in chalcophile metals that are thought (Li et
al., 2009) to be the source of the metals in the ores. The intrusions
occur within or have ascended through a thick evaporite sequence
that sulfur isotopes indicate was the source of S in the ores.
Type example Pechenga The Pechenga graben is filled with 4
volcanic sequences separated by sedimentary horizons, amounting
to 10,000 m in total thickness.Disseminated, massive and
brecciated sulfides occur within and below the base of
differentiated wherlite to gabbroic bodies that have intruded
sandstones, black, sulfidic shales and tuffs comprising the upper
sedimentary horizon. They are interpreted as feeders to the
overlying, partly ferropicritic volcanic sequence. Re-Os isotope
data indicate that sulfur in the ores has come from the shales.
There are 226 wherlitic intrusions of which 25 are mineralised.
2
1
Type example, Kambalda, WA Lenses of massive
sulfide, overlain by net-textured sulfide at the base of
relatively MgO-rich, dominantly mesocumulus peridotite.
The principal ore zones occur at the base of lava tubes
that fed inflating flows (Hill et al., 1995, 2004). The high
temperatures beneath the tubes led to thermal erosion
of the substrate (Huppert & Sparkes, 1985a,b, Barnes,
2006), formation of elongate troughs that contain the
mineralisation and incorporation of sedimentary sulfide.
Size mostly < 5 mt.
Type example Mt Keith, WA Accumulations of lowgrade, disseminated sulfide within lenses of typically
adcumulus dunite. The sulfides may be globular (2a) or
interstitial (2b). Both are thought to be due to cotectic
precipitation of olivine+sulfide, modified by some sorting
during transportation. (Barnes, 2006). Some adcumulate
dunites are interpreted in part as intrusive, shallow,
subvolcanic feeders to komatiitic volcanism (Naldrett &
Turner, 1977; Grguric et al. 2006),while others were
probably extrusive,or transitional. Size 400 mt
Type example, Raglan, Quebec Accumulations of sulfide within
a komatiitic volcanic environment (maximum MgO of magma 1719wt%) comprising conduits (variably interpreted as intrusive or
extrusive) and less MgO-rich sheet facies. The principal ore
deposits comprise “pools” of massive sulfide passing up into nettextured sulfide both at the base and at internal contacts within the
conduit (similar to type 1 above) and also zones of disseminated
sulfide (similar to type 2).. Numerous small pools occur within a
single conduit, each rarely exceeding 1 mt .
Brief Description
Olivine-sulfide
adcumulate
Layered
olivine orthocumulate
Sulfides
Olivine adcumulate
(or olivine-sulfide adcumulate),
formed in erosion channel
Cooled magma
Olivine
Hot flowing lava
Detail
Hot lava
Spinifex-textured lava flows
Pyroxene-phyric basalt
Tholeiitic basalt
Sulfidic, graphitic semi-pelite
Olivine-phyric
basalt
Main Body
C.Gr Gabbro
Cross-section through Pilgujarvi Body
Basalt
Wherlite
Ophitic Gabbro sills SE
Copper Ore
Stringer Ore
in contact gd
600
400
200
0m
Contact and
Lower Olivine gd
Residual Series
rocks
Upper
Copper Ore
Shales, Tuffs
Clinopyroxenite
M.Gr Gabbro
Massive Ore
Picritic gd
Olivine gd
Olivine-bearing
gabbrodolerite (gd)
Leucogabbro and
upper taxitic gd
Lower Taxitic gd
Sparse sulfides
in Olivine gd
Disseminated Ore
Weak sulfide
mineralization
Schematic section showing facies at Raglan
Chanennelised sheet
Facies
Gabbro
Deeply erosive conduit facies
Magnetite-rich
Layer
Disseminated sulfide
NW
Sulfide liquid
Sulfide liquid
Cross-section through mineralised lens
Sulfides in
substratum rocks
Olivine
Hot contaminated lava
Sketch
Cross-section through mineralised intrusion
Vertical scale is
exaggerated
FIG. 2. The different classes of Ni-Cu–dominant sulfide deposits, along with their age ranges, brief descriptions and a sketch to augment the description. Credits for
the sketches are as follows: NC-1, 2.7 Ga: type 1 (modified from Hill et al., 1990); type 2 (modified from Hill et al., 1990, after the observations of Grguric et al., 2006);
1.9 Ga (from Lesher, 2007): NC-2, after Naldrett (2004); NC-3, simplified after Smolkin (1977); NC-4, modified after Lightfoot and Naldrett (1999); NC-5, after Chai
and Naldrett (1992); NC-6, after Coats and Snajdr (1984); NC-7, downloaded from the “Hard Creek Nickel” Turnagain project website, March 13, 2009.
NC-3
1.9Ga
2.7Ga
Class Age Type
NC-1
Komatiite
Flood Basalt
Ferropicrite
673
1.1-0.25Ga
Vertical scale is
exaggerated
Peripheral Sills
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1.9Ga
A
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
673
Class Age
NC-4
NC-5
NC-6
Impact
NC-7
B
Anorthosite
complex
Hi-Mg basalt
Basalt
1.3Ga
2.7-0.1Ga
Ural-Alaskan
674
1.85Ga
0361-0128/98/000/000-00 $6.00
2.7-recent
most<0.5Ga
Sketch
0
1 km
FIG. 2. (Cont.)
Type example Turnagain BC Ural-Alaskan intrusions are
composed of dunite, clinopyroxenite+/-hornblendite, often zoned
outward from a dunite core. They occur in linear belts, usually at
accreting margins, although some examples (Konder; Inagli) occur
in stable shields. Their origin is debated, although some may be
feeder pipes for andesitic volcanoes.The dunites often contain
chromite segregations and disseminations with associated Pt that
has given rise to important placer deposits. Magmatic sulfides are
uncommon. Turnagain contains a large zone of low Ni-tenor sulfide
possibly due to assimilation of S from country rocks.
Only example Sudbury Ore bodies occur around the base of an
impact melt, about 2 km thick, that fills the bottom of a 1.85Ga
impact structure. Unlike most impact melts, the Sudbury melt is
strongly layered, ranging from granophyre at the top to quartz
norite at the base. Most ore bodies are associated with a
distinctive igneous rock ranging from quartz gabbro to norite
from which the sulfides settled to permeate what is interpreted as
a basal impact breccia, with some setlling farther into footwall
fractures. As the sulfides settled from the overlying impact melt this
became progressively more chalcophile depleted.
Type example Jinchuan Sulfides occur within a 6 km long,
>1 km deep,150 m wide disjointed body of ultramafic rock that
tapers with depth. The margins are lherzolitic, the core dunitic.
Sulfides occur interstitial to the olivine of the dunite, most of which
constitutes ore. The average MgO content of the body is >30 wt%,
but Chai et al. (1992) have shown that the magma contained 12
wt% MgO (Ni/Cu=1.76). It is interpreted as the ultramafic root of a
largely gabbroic, crustally contaminated intrusion. Zircon dating
has shown the age to be about 0.831Ga, and it is proposed that
emplacement was related to the South China plume (Li et al. 2009)
West-central subchamber
( Ore body No 1)
700
500 m
900
1100
1300
1500
1700
Lherzolite
Lherzolite
Dunite
Plagio
clase
East subchamber
( Ore body No 2)
0
un
iss
Gn
e
400 m
try
af
ic
cN
or
ite
Fe
lsi
Strathcona
Deep Zone
or
ite
N
twa
ll
Foo
Lower Coleman
Bx
N
ic
an
1 km
Vo
lc
wa
ck
di
c
ouosed
tlin p
e it
e Prop
Su
lfi
ph
yl
e
lit
Diorite-Tonalite
Metavolcanic
Hornblendite
Clinopyroxenite
Wherlite
Dunite
Turnagain Intrusion BC
Cross section through Strathcona Mine
0
Co
Copper
Zone
M
Longitudinal profile through ore bodies at Jinchuan
Net-textured
sulfide
Disseminated
sulfide
West subchamber
(Ore body No 24)
Type example Voisey’s Bay The Voisey’s Bay deposit is
DISCOVERY
W
E
OVOID
hosted by troctolite that is part of the Nain anorthosite suite. Mafic REID BROOK ZONE HILL ZONE
EASTERN
DEEPS
CHAMBER
magma rose into the crust and ponded to form a lower intrusion
(Upper chamber))
within a psamitic to pelitic sediment containing graphite and sulfide.
Interaction between sediments and magma developed immiscible
sulfides. The magma+sulfides rose 1.5 km higher up a vertical
REID BROOK CHAMBER
dyke to form an upper intrusion. Sulfides occur within thickened
(Lower chamber)
MAGMA CONDUIT (FEEDER DIKE)
zones in the dyke and at the entry line of the dyke to the upper
Sulfide mineralisation
intrusion. Subsequent passage of magma through the system is
believed to have upgraded the early sulfides.
Longitudinal profile through the Voisey’s Bay ore zones
Brief Description
674
A. J. NALDRETT
675
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
[PGE-1] Waterberg type mineralization
Hybrid and
exocontact rocks
PGM
Lac des Iles
Baron type mineralization
Ultramafic rocks
Mainly
mafic rocks
Sul
[PGE-3] Skaergaard and
Sonju Lake mineralization
Sul
Sul+ Chr
PGE-1 Merensky and J-M reefs
[PGE-4] Volkovsky type
mineralization
Marginal mineralization
in Bushveld(Platreef,
Sheba’s Ridge) [PGE-1]
and intrusions of Finland
and Ontario [PGE-2]
PGE-1 UG-2 sulfide-bearing chromitite
Chr+Sul
PGM
Sul
TiMt+Ap
+Sul
[PGE-1] Dunite pipes
Appearance
of cumulus
plagioclase
Alternating ultramafic,mafic
and plagioclase cumulates
Chr
Ultramafic cumulates
[PGE-1] Sulfide zones
of the Great Dyke
[PGE-1] Sulfide-free
chromitites rich in
Os, Ir, and Ru
FIG. 3. A hypothetical layered intrusion showing the locations and styles of PGE mineralization that could be present. It
should be stressed that it is highly unlikely that all styles would be present in the same intrusion. The classes of mineralization are labeled to conform with the discussion in the text. Figure modified after Naldrett (2004).
concentration into economic concentrations. There are a number of key steps in this process that are summarized in Figure 5.
The first step is partial melting in the mantle. As a volume
within the mantle melts, the chalcophile elements enter the
melt at different rates, depending on the degree of melting.
This is illustrated in Figure 6A, which is based on the melting
A
of a spinel lherzolite mantle at 20 kbars (~65 km; Hart and
Zindler, 1986). The modeling was undertaken using the program MELTS (Ghiorso et al., 2002) using currently accepted
mantle concentrations of Ni, Cu, PGE, and S, sulfur solubilities, and partition coefficients (Table 2). The concentration of
Ni rises during melting as olivine melts progressively and its
B
Ni
PGE-2 intrusions
Pt+Pd
Other
Archean
Komatiite
Anorthositerelated
Paleoproterozoic
Komatiite
Flood Basaltrelated
Impact meltrelated
Mg-Basalt
Ferropicrite
Miscellaneous
Gabbro-related
PGE-dominant
PGE-1 Intrusions
PGE-dominant
PGE-2 Intrusions
FIG. 4. A summary of the contributions that the different classes of Ni-Cu and PGE deposits contribute to the known resources of (A) Ni and (B) Pt + Pd. Data from Table 1.
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676
A. J. NALDRETT
STAGES IN THE LIFE OF A MAGMATIC SULFIDE DEPOSIT
CRUST
7. FULL MATURITY
Magma extrudes at surface or sills out
in the crust, sulfide liquid
concentrates and crystallises
6. NOURISHMENT
Enrichment of sulfide by continued
magma flow
5. GROWTH
Concentration of sulfide,
as a result of magma emplacement
4. DELIVERY
Magma (now with immiscible sulfide)
rises higher through crust
MANTLE
3.
FERTILISATION OF SOURCE
Magma sills out within crust and
undergoes a period of cooling,
crystallisation and interaction
with crust - sulfide liquid forms
2. DEVELOPMENT OF SOURCE
Magma ascends through mantle
and into crust
1. BIRTH OF SOURCE
Partial Melting in mantle
FIG. 5. Stages in the conception, delivery, and development of a magmatic Ni-Cu sulfide deposit from partial melting in
the mantle to solidification in/on the crust.
contained Ni is released into the magma. Pt, Pd, and Cu are
held back in the mantle while sulfide is present, but, once
there is sufficient magma to dissolve all of the sulfide, they
reach their maximum concentrations, subsequently to be diluted with further melting as the mass of magma increases
without the addition of PGE and Cu. Pt and Pd, being much
more chalcophile, are held back to a greater extent than Cu.
A
Wt% MgO in melt
10 15
20
25
30
35
50
250
No sulfide remaining in mantle
350
40
100
20
10
50
0
0
0.00
0.10
0.20
Ni
0.30
0.40
0.50
0.60
fraction of mantle melted
Cu
0.70
0.80
0.90
Sulfide saturated magma
50
200
40
150
30
100
20
50
10
0
0
10
20
30
40
50
60
70
Wt% of magma fractionated
(Pt+Pd)
FIG. 6. A. Variation in the Ni, Cu, and (Pt + Pd) contents of partial melts derived through mantle melting at 20 kbar (≈
60 km depth) as calculated using the program pMELTS (Ghiorso et al., 2002). Cu and PGE increase with increased degree
of melting until all sulfide is dissolved in the melt, after which they decrease. Note that because it is less chalcophile than Pt
and Pd, the Cu/(Pt + Pd) ratio decreases with progressive melting until no sulfide remains in the mantle. Parameters used
in the modeling are given in Table 3. B. Fractional crystallization of the magma produced by 18 percent partial melting that
is illustrated in (A). It is assumed that the partial melt produced at 20 kbar rises toward the surface and sills out under 3 kbar
pressure (≈ 9 km depth), at which stage it cools and fractionally crystallizes. In the 18 percent melt chosen for this example,
all of the sulfide in the mantle had just been dissolved at the stage that the partial melt left its source, so that Cu, Pt, and Pd
are at their maxima. Note how rapidly Pt and Pd concentrations decrease once sulfide saturation is achieved and sulfides are
removed from the magma.
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676
60
250
0
1.00
70
Sulfide unsaturated magma
80
90
ppb (Pt+Pd) in magma
30
ppb (Pt+Pd) in melt
150
B
300
ppm Ni/2, Cu in magma
Sulfide
in
mantle
200
ppm Cu, ppm Ni/10 in m elt
The result is that if a magma is released from its source after
only a modest degree of melting, it will be very poor in PGE
and have rather modest amounts of Ni and Cu. Magmas released when no sulfide remains in the mantle will be rich in
PGE and Cu, whereas those that are not released until advanced stages of melting will have high Ni contents and
Ni/Cu ratios.
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
TABLE 2. Assumptions Made in Modeling
Mantle is from Hart and Zindler (1986),
adjusted to QFM-1, with added Cr2O3:
SiO2 46.36; TiO2 0.182; Al2O3 4.10; Fe2O3 0.59; Cr2O3 0.40; FeO 6.67;
MgO 38.1; CaO 3.24; Na2O 0.33; K2O 0.32; H2O
Derived melts at 20 kbars:
18% melt: SiO2 44.82; TiO2 0.72; Al2O3 12.79; Fe2O3 1.74; Cr2O3 0.064;
FeO 7.70; MgO 17.90; CaO 9.58; Na2O 1.73; K2O 1.81; H2O 1.13;
(in ppm) Ni 675; Cu 125; (in ppb) Pt 18; Pd 18; Ir 0.51
10% melt: (in wt %) SiO2 46.49; TiO2 0.82; Al2O3 14.99; Fe2O3 1.55;
Cr2O3 0.0375; FeO 6.36; MgO 13.94; CaO 7.72; Na2O 2.79;
K2O 3.26; H2O 2.04; (in ppm) Ni 619; Cu 69; (in ppb) Pt 1.11;
Pd 0.63; Ir 0.20
Mantle contents of Ni, Cu, Pt, Pd, and Ir are from Crockett (2002):
Ni 2,500 ppm; Cu 30 ppm; Pt 4 ppb; Pd 4 ppb; Ir 3 ppb
Partition coefficients
Ol
Cpx
Opx
Spinel
Garnet
Sulfide
D (Ni)
D (Cu)
D (Pt)
D (Pd)
D (Ir)
6.5
2
1
8
0.24
500
0
0
0
4.2
0
1,000
0.05
0.05
0.05
0.3
0
14,000
0.05
0.05
0.05
0.1
0
25,000
10
2
2
28
0
36,000
Sulfur solubility:
In partial melt at mantle depth = 1,200 ppm
In magma at crustal depth = 2,200 ppm
Wendlandt (1982) and Mavrogenes and O’Neill (1999) have
shown that during ascent from the mantle (stage 2, Fig. 5),
the decrease in pressure will cause an increase in the ability
of a magma to dissolve sulfur. The increase is appreciable; ascent through 55 km will more than double the ability of a typical basaltic magma to dissolve sulfur from about 1,000 ppm
S at 64-km depth to 2,300 ppm S at ~9-km depth. The result
is that even a magma that was saturated in sulfur when it left
its source will be far from saturated on nearing the surface.
On cooling, the magma will crystallize silicates and the sulfur content will build up in the remaining magma, eventually
reaching the saturation level. Figure 6B shows the variation in
Ni, Cu, and (Pt + Pd) resulting from fractional crystallization
at 3-kbars pressure (~9-km depth) of an 18 percent melt of
the mantle used as the illustration for Figure 6A; this has
been calculated using the program MELTS (Ghiorso and
Sack, 1995). Sulfide immiscibility is not achieved until 45 to
50 percent of the magma has crystallized, by which stage most
of the Ni has been removed in early-forming silicates.
It is now appreciated that in the case of most Ni-Cu deposits, sulfide immiscibility has been induced as a result of interaction of a magma with the crust, either due to the addition of crustal sulfur or to the assimilation of crustal material
in a way that has changed the magma’s ability to dissolve sulfur (stage 3, Fig. 5). This conclusion is based on evidence
from S, Sm-Nd, and Re-Os isotope measurements, and/or
trace element analyses of many deposits, including those at
Noril’sk, Voisey’s Bay, Pechenga, and Kambalda. In the case of
PGE-rich deposits interaction with the crust may not be the
immediate cause of sulfide immiscibility, although it may
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677
have occurred during magma emplacement (Barnes and
Maier, 2002).
In some cases, only a portion of the magma involved has reacted with crustal rocks in a manner causing sulfide immiscibility, and the ratio of sulfide formed to magma affected is relatively high. This situation is referred to by students of
magmatic sulfides as one characterized by a low R factor,
where the R factor is the ratio of magma to sulfide involved in
the interaction (Campbell and Naldrett, 1979). The consequence is that the Ni, Cu, and PGE contents of a restricted
amount of magma are contained within a relatively large
amount of sulfide, so that the tenors of these metals in the
sulfide are low and the ore is often uneconomic. Study of
some key ore deposits, notably those at Noril’sk (Naldrett et
al., 1996, Li et al., 2009), Voisey’s Bay (Li and Naldrett, 1999),
and Kambalda (Lesher and Barnes, 2008), have shown that
the development of deposits of reasonable metal tenor requires an additional factor, commonly the interaction of earlyforming low tenor sulfides with fresh, sulfide-unsaturated
magma (stage 6, Fig. 5). The new magma displaces the earlydepleted magma, adds additional chalcophile elements to the
sulfides, and, because it is sulfide unsaturated, dissolves some
of the sulfide liquid, principally FeS, leaving the remaining
sulfide liquid enriched in its more chalcophile constituents
(see Kerr and Leitch, 2005). Enrichment of this kind may
occur during ascent (stage 4) of the mixture of magma and
immiscible sulfide resulting from stage 3 or once the sulfides
have reached their final resting place (stage 5). Many deposits
(Kambalda, Voisey’s Bay, Noril’sk) occur in magmatic feeder
systems, in which flow has resulted in the sulfides becoming
concentrated in hydrodynamic traps, and later surges of
magma have followed the same channels, thus giving the new
magma access to and the opportunity to enrich early sulfides.
The effect of interaction of fresh magma with early sulfide is
illustrated in Figure 7.
Once a sulfide liquid cools and starts to crystallize (stage 7),
it tends to fractionate, with the early-forming monosulfide
solid solution (mss) concentrating Ru, Ir, and Os, and the remaining liquid becoming enriched in Cu, Pt, Pd, and Au. Ni
and Rh range from being compatible to incompatible in mss,
depending on the prevailing fS2 (Mungall et al., 2005). The
fractionated Cu-, Pt- and Pd-rich liquid may migrate away
from the main body of mss to give rise to Cu- and PGE-rich
portions of the body or footwall veins.
Models for the formation of PGE-enriched horizons or
zones in layered intrusions tend to be more complex and subject to more disagreement than those for Ni-Cu deposits (see
Naldrett et al., 2008, and references within). One feature that
the horizons have in common is that they occur in intrusions
for which isotopic and major and trace element data indicate
that more than one magma was involved, typically the magmas described as U- and T-type in this paper. Figure 8 illustrates the differences between the two magmas on a normative quartz-feldspar (plagioclase + K-feldspar)-pyroxene
(augite + hypersthene)-olivine plot. The T-type magmas are
all richer in feldspar and olivine than the U-type and are characterized by much lower Cr contents.
Profiles through the three major igneous complexes that
dominate in terms of PGE production (Bushveld, Stillwater,
and Great Dyke) along with a profile representative of a series
677
678
A. J. NALDRETT
24
320
Fra ction s ulfide remaining
1.0
280
0.1
20
240
0.01
Cu (wt%)
6
5
4
12
3
2
Log R
1
200
0
160
Pt ( ppm )
16
120
8
80
4
40
0
0
6
5
4
3
2
1
0
Logarithm R
YCu (R-modeling)
YPt (R-modeling)
YCu (upgrading
with dissolution)
YPt (upgrading
with dissolution)
FIG. 7. The effect of variations in silicate magma-sulfide liquid mass ratio
on the concentrations of Cu and Pt of sulfides in equilibrium with basaltic
magma containing typical concentrations of these elements (100 ppm Cu, 15
ppb Pt; DCu = 1,000, DPt = 10,000). One set of curves illustrates simple variations in R factor. The other set illustrates the variation in metals if a series
of pulses (or a continuous stream) of magma interact with the sulfides, exchanging chalcophile metals and dissolving away some of the FeS component
of the sulfide liquid. In the example shown, it is assumed that each successive pulse of magma has 100 times the mass of the remaining sulfide, and that
the magma dissolves an amount of sulfide equal to 0.05 percent of its own
mass. R in this case is the sum of all pulses of magma that have passed through the system. The inset shows how much sulfide remains as Rcum increases
in the case of the model involving sulfide dissolution.
of intrusive complexes in Finland (Pennikat-Portimo-Koillismaa) that are probably the current most promising prospects
for future production are compared in Figure 9. The areas of
the stratigraphic columns with a gray-shaded background indicate the parts of the profiles where the rocks formed predominantly from a U-type magma, while the unshaded areas
indicate rocks where the dominant magma was T-type. It
should be noted that the example of the Naukas block shown
in Figure 9 as representing the Portimo complex gives a biased view of the relative amounts of U- and T-type magmas
involved. Other parts of this complex consist almost entirely
of T-type magma, so that U-type magma was the lesser component of the complex as a whole. The major PGE-rich horizons occur close to the level at which rocks from the U-type
magma give way upward to those from the T-type magmas.
The changeover from one type to the other is not simple in
the Bushveld and Stillwater complexes, and there is evidence
for considerable intermingling between the two in intervening transition zones (Helz, 1985; Seabrook et al., 2005).
Compositions of Magmatic Sulfide Deposits
Figure 10A-C shows the relationship between the estimated MgO content of the magma involved and the Ni content in 100 percent sulfides, the Ni/Cu, and the Pd/Ir for a
0361-0128/98/000/000-00 $6.00
number of deposits for which there is adequate information
(the data are from Naldrett, 2004). Both the Ni in 100 percent sulfides and Ni/Cu ratio increase with increasing MgO
content of the source magma. This is consistent with the
modeled changes for these elements with the increasing of
degree of partial melting required to increase the MgO content of the partial melt, coupled with fractional crystallization
of the resultant magma up to the point at which sulfides separate. The Pd/Ir ratio decreases with increasing MgO. This is
because Ir, along with Ru and Os, behave as though they are
compatible in the mantle. The reason for this is not known
with certainty, although it may be that they are present as submicroscopic alloy inclusions in olivine and/or other mantle
phases and are not released until the host phase is taken into
the melt.
The wide spread in Ni contents of 100 percent sulfide of
deposits of a given type, associated with magmas of a similar
MgO content that is shown in Figure10A, is largely due to
varying R factors or different degrees of enrichment by the
passage of subsequent magma as discussed above. The
spreads in Ni/Cu and Pd/Ir ratio between different deposits
of the same type are due to the cooling and fractionation of
the sulfide liquid as discussed above.
Distribution of Magmatic Sulfide Deposits through Time
The secular distribution of magmatic sulfide deposits is
shown in Figure 11A-B. In assessing the significance of this diagram and comparing it with similar diagrams that could be
drawn for porphyry Cu, VMS, or SEDEX deposits, one must
appreciate that the population base on which this is drawn is
much smaller, with a few camps totally dominating the picture.
Secular distribution of Ni-Cu deposits
Looking first at Ni-dominant deposits, the high MgO komatiite-related deposits (class NC-1a) are restricted to the
Archean, while the lower MgO komatiite-related deposits
formed at or close to 1.9 Ga (class NC-1b). The secular distribution of komatiites and the reasons for it are discussed
below. The Mg-basalt-ferropicrite class (NC-3) is restricted to
the Proterozoic, although this could be the consequence of
the small population base that is available. Although most are
too small in size to appear in Figure 11A, reference to Table
1 will show that there are some deposits associated with miscellaneous gabbroic intrusions that occur throughout the time
period for which data are available.
There is only one known example of the anorthosite complex (NC-4) class, Voisey’s Bay. Anorthosite complexes themselves are Proterozoic phenomena ranging in age from Amanuat (2120 Ga) to Rogaland (925 Ga; Scoates and Mitchell,
2000), which places Voisey’s Bay at 1.33 Ga in among the
lower third of ages. The reason why anorthosite complexes
occur only in the Proterozoic is not understood.
Flood basalt-related deposits (class NC-2) appear to be restricted to the younger Proterozoic and Phanerozoic. This is
probably because the large flood basalt provinces themselves
are relatively young in age; it is possible that some of the volcanism classed here as komatiitic is an early equivalent of
more recent flood basalt provinces.
Regarding impact-related deposits (class NC-6), here again
the existence of only one prime example (Sudbury) restricts
678
679
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
Feldspar
358
132
314
952
Quartz
Olivine
638
1870
Pyroxene
Bushveld
U
T
Stillwater
U
T
Portimo
U
T
The number beside each point is the estimated ppm Cr in the magma
Estimates of Magma Composition: Bushveld, Barnes and Maier (2002)
Stillwater, Helz (1985)
Portimo, ilijna (1995)
FIG. 8. Normative plot showing the differences between the estimated compositions of U-type and T-type magmas involved in the Bushveld, Stillwater, and Portimo intrusions. The number adjacent to the plot of each liquid is the ppm Cr.
Note the less feldspathic. More quartz rich and higher Cr contents of the U-type in comparison with the T-type from the
same intrusion. Estimated liquid compositions are from Barnes and Maier (2002) for the Bushveld, Helz (1985) for the Stillwater, and Iljina (1995) for Portimo.
generalization in so far as age is concerned. The 144 Maaged Morokweng crater in South Africa has some resemblances to Sudbury (Andreoli et al., 2008). It is the only other
impact site that has a major associated Ni anomaly (fragments of meteorite recovered around small impact sites are
excluded from this generalization). Similarities include a
thick melt sheet, relatively high concentrations of Ni and
PGE in the impact melt, presence of sulfides in some parts
of the impact melt, and evidence that some of the dikes of
impact melt injected into the footwall were sulfide saturated.
On the other hand, the PGE have chondritic rather than the
terrestrial profiles characteristic of Sudbury and much of the
Ni occurs as trevorite, the latter observation making it unlikely that the impact melt was ever sulfide saturated
throughout. In this author’s opinion, it cannot be grouped in
class NC-6, although it serves to show that impacts resulting
in Ni enrichment are not restricted to any period in the
Earth’s history.
Intrusions that are generally accepted as belonging to class
NC-7 (Ural-Alaskan intrusions) are found in convergent margin environments in Alaska, northern British Columbia, Columbia, New South Wales, Kamchatka, and the Ural platinum
belt, and in stable shield environments at Konder and Inagli
in eastern Siberia. They are Phanerozoic in age (Urals ~450
Ma; Salt Chuck, Alaska ~420 Ma; Fifield, NSW ~400 Ma;
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Inagli ~355 Ma; Konder ~340 Ma; British Columbia ~175
Ma; other Alaskan bodies ~100 Ma, Alto Condoto, Columbia
~20 Ma; Johan, 2002). As emphasized above, only a small
number of these bodies have primary magmatic sulfides, and
these have not yet proved to be economic in modern-day
terms, although 300,000 t grading 0.77 wt percent Ni and
0.33 wt percent Cu were mined at Salt Chuck (Nixon, 1998).
Ural-Alaskan intrusions are known primarily for their
chromite-associated Pt contents that have not proved to be
economic in situ but have given rise to important placer deposits. A small group of zoned, equant intrusive 2.7 Ga mafic
and/or ultramafic pipes containing PGE-rich magmatic sulfides occur in the metasedimentary Quetico subprovince of
northwestern Ontario. They closely resemble the Phanerozoic Ural-Alaskan bodies (Pettigrew and Hattori, 2006) and
probably formed in a similar, albeit much older setting. Because of their, as yet, unproven economic viability, deposits of
this class are not considered further in this paper.
The secular distribution of PGE-rich deposits
Turning to the distribution of PGE deposits in time, this is
illustrated in Figure 11B. Note that the vertical scale in this
figure is logarithmic. This was necessitated because the contribution of layered intrusions, particularly those of class
PGE-1, is so overwhelming that other types would not have
679
680
A. J. NALDRETT
Bushveld
Zone
Zone
GNIII
2000
OBV
Main & Lower
Sulfide Zones
1
0
Ultramafic
1
Critical
Lower Lower Upper
2
UG-2
800
1500
MCU III
2
3
Height in m
4
1000
J-M Reef
600
6
7
8
500
400
9
0
10
11
200
12
13
14
Marginal
Cumulus
olivine
Cumulus
pyroxene
SK Reef
5
MCU II
GNII
NII
GNI
NI
Bron
zitite
Merensky
Reef
3
OBIV
OBIII
ANI
OBII
Peridotite
Banded
Middle
4
Lower
Main
Height (km)
ANII
5
Narkaus
Kilvenjarvi Block
MCU I
6
Cyclic Unit
Upper
Series
Upper
7
Great Dyke
Southern part of
Darwendale subchamber
Stillwater
8
0
Cumulus
plagioclase
Rocks crystallised from dominantly U-type magma
Areas with no shading are from dominantly T-type magma
FIG. 9. Typical profiles for the Bushveld, Stillwater, Great Dyke, and Narkaus block of the Portimo intrusions. The gray
shading indicates portions of the profile for which the magma was dominantly U type. The proportion of U type shown for the
Great Dyke is exagerated, because much of the profile composed of T-type rocks has been eroded. The proportion of U-type
for Portimo is also exagerated because of the choice of the Narkhaus block for the illustration. Other blocks are composed totally, or dominantly of T type, but Narkaus is shown because it contains the best example of Merensky-style mineralisation (the
SK reef) and illustrates clearly the position of this with respect to the changeover in magma type. Note that in all of the profiles, the principal PGE reef lies close to this changeover. The Bushveld and Stillwater profiles are from Naldrett (1997), that
for the Great Dyke is from Wilson and Prendergast (2001) and that for Narkaus is from Alapieti and Lahtinen (2002).
been visible if the scale had been linear. It could be said that
high PGE concentrations in magmatic sulfides are restricted
to the Archean and Paleoproterozoic, if it were not for the
Permo-Trassic Noril’sk Ni-Cu deposits being so rich in PGE
and the overall restricted number of examples that makes
generalizations like this so risky. What can be said is that mineralized intrusions with a high proportion of rocks that
formed from the U-type magma referred to above appear to
be restricted to Paleoproterozoic and older times, and, as illustrated in Figure 11B, these account for the vast preponderance of PGE associated with layered intrusions. Other deposits contributing to our PGE resources are of relatively
minor importance and occur throughout the geologic time
scale considered here.
Komatiites and Their Genesis
Before one can assess why komatiite-related ore deposition
is so time dependent, it is necessary to understand current
ideas on what komatiites are and how they formed. The presently accepted definition is perhaps best expressed by Arndt
0361-0128/98/000/000-00 $6.00
(2008, p. 11) who stated “Komatiite can therefore be defined
as a rock whose field relations or textures provide evidence of
a volcanic or subvolcanic origin and whose mineral assemblages or major element contents indicate an ultramafic composition.” Arndt (2008) goes on to set a minimum MgO content
of 18 wt percent for komatiite magma, with volcanic rocks of
the same affiliation but for which the magma contained <18
percent MgO being referred to as komatiitic basalts.
The high MgO contents of komatiite samples with textures
indicating that they are representative of liquid compositions
means that they had high extrusion temperatures in the range
of 1,400° to 1,600°C. Komatiites fall into two major types:
Munro-type and Barberton-type.1 Some chemical differences
between the two types are illustrated in Figure 12. Munrotype komatiites, sometimes referred to as Al undepleted, are
1 Other variants exist, e.g., Gorgona-type, Commondale-type, Krasjokitype, but these are of lesser importance and in order not to confuse the
reader of this volume with excessive petrologic data are not considered further here. Those interested will find a detailed discussion in Arndt (2008).
680
681
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
100
A
10000
B
C
1000
20
10
10
Pd/Ir
100
Ni/Cu
Average Wt% Ni in 100% sulfide
30
1
10
0
0
0
5
10
15
20
25
30
Approximate wt% MgO of Magma
35
1
0
5
10
15
20
25
30
Approximate wt% MgO of Magma
Pechenga NC-3 1.9Ga
Jinchuan NC-5 0.825Ga
Archean Komatiites NC-1a 2.7Ga
Proterozoic Komatiites NC-1b 1.9Ga
0
35
5
10
15
20
25
30
Approximate wt% MgO of Magma
35
Voisey’s Bay NC-4 1.33Ga
Noril’sk 0.25Ga
Flood Basalt NC-2 Duluth 1.1Ga
FIG. 10. Plots of (A) Ni content in 100 percent sulfide, (B) Ni/Cu ratio, and (C) Pd/Ir ratio against the estimated MgO
content of the magma for deposits for which data are available.
characterized by Al2O3/TiO2 >15 (commonly with the chondritic ratio of 20), depleted light REE, and Gd/YbN close to
unity. Barbeton-type komatiites, sometimes referred to as Al
depleted, are characterized by Al2O3/TiO2 <15, relatively flat
100,000
Paleoproterozoic
Komatiite
Noril’sk-Talnakh
Jinchuan
Aguablanca
1.0-0.5
0.5-0.0
Sudbury
Duluth
Kabanga
1.5-1.0
3.0-2.5
Age in Ga
Age in Ga
Archean
Komatiite
Thompson
Raglan
1
2.0-1.5
Noril’sk-Talnakh
Jinchuan
1.0-0.5
10
0.5-0.0
Voisey
Bay
1.5-1.0
Duluth
Pechenga
Thompson
Raglan
2.0-1.5
2.5-2.0
3.0-2.5
Largely
W. Australia
0
Largely Bushveld
Stillwater
Great Dyke
5
100
Largely Bushveld
10
1000
Monchegorsk
Sudbury
15
10,000
2.5-2.0
20
B
Largely
W. Australia
25
Tonnes of (Pt+Pd) (NOTE logarithmic scale)
A
30
Stillwater
Great Dyke
35
103 tonnes of Ni
light REE profiles, and high Gd/Yb ratios. The difference
between the two is ascribed to whether or not garnet was present in the source at the time that the magma separated from
it (see discussion on partial melting below).
Mg-Basalt
Ferropicrite
Miscellaneous
Gabbro
Anorthositerelated
Flood Basaltrelated
Impact meltrelated
PGE-dominant
Layered Intrusions
FIG. 11. Secular variation in (A) total Ni and (B) Pt+Pd contained in different classes of deposit. Note that the high-Mg
members of class NC-5 have been combined with those of NC-3, while the low-Mg members are classed as miscellaneous
gabbro related. Note also that the ordinate in (A) is normal while that in (B) is logarithmic. A logarithmic scale was necessary for (B) because the dominance of deposits related to layered intrusions is so overwhelming that little else would have
been visible on the diagram. Data are from Table 1.
0361-0128/98/000/000-00 $6.00
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682
Sample/prim itive mantle
A. J. NALDRETT
10
Most Barberton Komatiites (Al2O3 /TiO2 < 15, Zr
25 ppm)
1
Most Munro Komatiites (Al2O3 /TiO2 >15, Zr
0.1
Th
Nb
La
Ce
Nd
Sm
Gd
17 ppm)
Dy
Er
Yb
FIG. 12. Illustration of some trace and major element differences between Barberton- and Munro-type komatiites.
o
Temperature ( C)
1000
1200
1400
1600
100
6
Solid Peridotite
B
150
8
A
at
therm
e geo
Mantl
10
Depth (k m)
es
Pressur e (Gpa)
C
0
50
komatiit
matiites
4
2000
Liquid
Peridotite
Barberton
Munro ko
2
1800
Path of
40%
h of
Pat
50%
0
%
200
250
day
herm to
a
2.7 G
geot
Mantle
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by garnet which coexists with orthopyroxene and olivine. At
still higher pressure, orthopyroxene is no longer stable and
garnet and olivine are the stable minerals at and for some
temperature interval above the solidus. Contours in Figure 13
indicate the percentage of melting of mantle peridotite as a
function of temperature and pressure. The mantle geotherms
that are shown are purely for illustrative purposes—they have
30
Partial melting regimes
Originally it was supposed that komatiites originated in the
deep mantle and were the consequence of rapid, adiabatic
rise of volumes of this material to shallow depths (today these
volumes would be called mantle plumes); during the ascent
release of pressure caused significant melting, and on reaching a shallow depth the magma separated from its source.
Two important concepts have emerged that have cast doubt
on this simple model. The first is that mantle melting is likely
to be much more complicated, with melt leaving its source regime after only a few percent has been generated. In effect,
this gives rise to fractional melting, with the early melt having
a basaltic composition (depending on the pressure at which it
separated) and subsequent aliquots being progressively more
primitive. The compositions of lavas extruded on the surface
indicate that the different melt aliquots often pond and mix,
or become mixed during their ascent, so that while generated
by fractional melting, their compositions approximate the results expected from batch melting..
The second concept, that of hydrous melting, was first raised by de Wit and Stern (1980), and discussion continued in
a series of talks and/or papers by de Wit et al. (1983, 1987),
Grove et al. (1994, 1996, 1999), and Parman et al. (1997,
2001, 2004). The implications of this concept are that MgOrich melts will form at much lower temperatures in wet environments related to subduction (the effect of water enlarging
the field of olivine during partial melting has been well known
for many years—see Kushiro, 1969; Inoue, 1994), and therefore that the tectonic regime for komatiite genesis is that of
subduction and not that of an ascending mantle plume. Arndt
(2008) reviewed this hypothesis and concluded that it does
not apply in the case of komatiite genesis. In the subsequent
discussion in this paper the essentially dry melting, plume hypothesis is accepted.
Phase relationships of a typical mantle composition as a
function of temperature and pressure are shown in Figure 13,
which is based on the data of Herzberg (1995) and Herzberg
and O’Hara (2002). At pressures below 4 Gpa, clinopyroxene,
orthopyroxene, and olivine are the principal phases on the solidus (spinel, or at very low pressures, plagioclase, will also be
present), and, with rising temperature and melting, disappear
into the increasing volume of melt in this order. Above 4 Gpa,
spinel and clinopyroxene are absent and their place is taken
300
Principal minerals in equilibrium with liquid
Cpx + Opx + Ol
Gt + Ol
Opx + Ol
Gt + Opx + Ol
Ol
Path of ascent of partial melt before effective separation from its source
Path of ascent of partial melt after effective separation from its source
FIG. 13. P-T diagram showing the pase assemblages stable between liquidus and solidus for typical mantle peridotite (after Herzberg, 1995; Herzberg
and O’Hara, 2002). The mantle geotherms are after the relationship between
mantle geotherm and peridotite solidus given by Schubert et al. (2001) for
different stages in the Earth’s history, and are for illustrative purposes only.
The melting paths shown are from Arndt (2008).
682
683
SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
been drawn after the coupled core-mantle thermal evolution
model of Schubert et al. (2001) for whole mantle convection,
which indicates that the present mantle temperature (expressed in °K) will be 85 percent of the solidus temperature and
that it would have been 93 percent of the solidus temperature
at 2.7 Ga. The solidus used by Schubert et al. (2001) is not
that of Herzberg and O’Hara (2002), so the precise position
of the geotherms is not known; what is likely, from the steady
exhaustion of heat-producing radioactity in the Earth, is that
the relative positions shown in Figure 13 are realistic, i.e., that
the present geotherm is substantially lower than the Archean
geotherm.
Arndt (2008) has used the phase relationships shown in Figure 13 to explain the genesis of Barberton- and Munro-type
komatiites. As emphasized above, the low Al2O3/TiO2 ratio,
REE profiles, and other trace element characteristics of Barberton komatiites indicate that the melt segregated from its
source while garnet was still present; this implies that segregation occurred at high pressure. Path A shown in Figure 13
is close to that of Arndt (2008). Within the temperature-pressure regime shown for Barberton-type komatiite genesis, the
data of Stolper et al. (1981) and Rigden et al. (1984, 1988) indicate that a komatiitic liquid would have a density close to
that of olivine and, therefore, would not escape from a source
of dominantly olivine composition. This would mean that
melting would approximate to true batch melting, which accounts for the path for Barberton melting shown in the figure.
Arndt (2008) emphasized that Barberton komatiites have higher concentrations of highly incompatible trace elements,
such as zircon, than Munro komatiites, which indicates that
they are the result of lower degrees of partial melting—Arndt
(2008) suggests 30 percent. This melt then coalesced and rose
rapidly to the surface to extrude at about 1,600°C, as shown
in Figure 13. Arndt’s explanation for Al-undepleted Munrotype komatiites (path B) is that all garnet that might have been
present in the source was exhausted by the stage that fractional melting was complete and the resultant melts coalesced
and rose adiabatically to the surface to extrude again at about
1,600°C, in this case not influenced by the retention of elements compatible in garnet. As seen in the figure, the plume
giving rise to Barberton-type komatiites impacted the peridotite solidus at a greater pressure and higher temperature than
that for Munro-type komatiites, which means that either they
ascended from a greater depth or that the mantle geotherm
was somewhat higher in temperature than that responsible for
Munro komatiites. This latter possibility is consistent with the
generally greater age of Al-depleted komatiites.
It is apparent from Figure 13 that the hotter Archean geotherm promotes the formation of komatiites, and that they
will not form by adiabatic ascent of peridotite from the same
depths today. The present mantle geotherm is likely to produce melts that follow path C. It is also likely that an intermediate mantle geotherm existed in the Proterozoic, which is
why the Proterozoic komatiites are not as magnesian as those
of Archean age. While declining mantle geotherms may be
the explanation for the relative rarity of komatiites in younger
rocks, this is not to say that komatiites cannot form at all
today. Derivation from a deeper, hotter level in the present
mantle, or effective segregation of the melt from its source at
a higher degree of partial melting could produce a more magnesian melt than the 13 to 14 wt percent MgO picrites that
are thought to be the primary magmas of most present-day
MORB and oceanic island basalts. The 90 Ma komatiites on
Gorgona Island (Echevaria, 1980; Echevaria and Aitken,
1980) formed from a liquid containing 18 to 22 wt percent
MgO, less than the more magnesian Archean komatiites, but
higher than post-Archean picrites. Kerr (2005) concluded
that this liquid was the result of fractional melting at 3 to 4
Gpa, i.e., from a plume impacting the solidus between paths
B and C in Figure 13. Thus, in rare circumstances, very MgO
rich magmas can form today, although they have done so in
tectonic environments that are not conducive to magmatic sulfide formation (see next section).
Tectonic setting of mineralized and unmineralized komatiites
Arndt (2008) analyzed the occurrence of komatiites in
terms of the settings into which they were emplaced and a
modified version of his analysis appears in Table 3. It is clear
that they can erupt into a wide range of tectonic settings ranging from oceanic plateaus (Arndt refers to these as “mafic
plains”) to active island-arc volcanism. Proponents of hydrous
melting emphasized the association with subduction-related
phenomena, but, as stated above, Arndt (2008, p. ) dismisses
the few examples where the association exists as coincidental,
saying “a mantle plume does not know what it will meet at the
surface”! When one looks at the mineralized komatiites, the
tectonic settings are not so wide ranging. The oceanic plateau
setting has so far not been favorable. All of the productive settings have been where komatiitic magmatism has been closely
associated with continental or island-arc rocks. As discussed
TABLE 3. Tectonic Setting of Komatiitic Volcanism
Locality
Age
Setting
Ni mineralization
Gorgona Island
Thompson nickel belt, Canada
Gilmour Island and Raglan belt, Canada
Shaw Dome, ON, Canada
Munro Township, ON, Canada
Belingwe, Zimbabwe
Hunter mine group, Canada
Kambalda, W. Australia
Wiluna-Leonora belt, W. Australia
Prince Albert group, Canada
Barberton, South Africa
90 Ma
1.88 Ga
1.92 Ga
2.7 Ga
2.7 Ga
2.7 Ga
2.7 Ga
2.7 Ga
2.7 Ga
2.7 Ga
3.5 Ga
Uplifted oceanic plateau
Rifted continental margin
Rifted continental margin
Eruption into active island arc
Oceanic plateau
Oceanic plateau
Eruption onto island arc
Eruption onto submerged continental platform
Eruption into active island arc
Eruption into shallow marine-continental setting
Oceanic plateau
None
Well mineralized
Raglan is well mineralized
Modest mineralization
None
None
None
Well mineralized
Well mineralized
None
None
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above, all magmas are likely to arrive at crustal depths significantly undersaturated in S, and in most cases become sulfide
saturated through interaction with the crust, particularly sulfide-rich crust. Contamination of komatiitic magma with felsic rocks has been demonstrated for both the Kambalda ores
and those of the Raglan nickel belt (Lesher and Barnes,
2008). Bekker et al. (2009, p. 1087) have concluded, on the
basis of ∆33S data, that komatiite-related Ni sulfide deposits
of the Agnew Wiluna greenstone belt in Western Australia
and deposits in the Alexo-Dundonald area of Ontario show
mass-independent fractionation requiring “almost all of the
sulfur in these deposits to have come from the assimilation of
hydrothermal massive sulfides.” Thus, the paucity of deposits
in the oceanic plateau setting is understandable.
Komatiiticor nonkomatiitic origin of the Bushveld magmas?
It has been suggested that the U-type magmas involved in
the Bushveld (Hamlyn and Keays, 1985; Neilsen and Brooks,
1995) and the Portimo (Iljina, 1994) igneous complexes were
boninitic. Boninites are lavas with a high content of SiO2 and
MgO that develop over subduction zones as a result of a second stage of melting affecting mantle that has been melted
previously. They can contain high concentrations of PGE that
are attributed to the first melting having been insufficient to
dissolve all the sulfide present in the source. As discussed
above, sulfide remaining in the mantle will hold back PGE
that will then be available in some abundance to be released
into the second melt.
Momme et al. (2003) studied the Cu, S, and PGE content
of lavas belonging to the North Atlantic igneous province in
and offshore of East Greenland and in Iceland. They interpreted their results to indicate that the lavas were the result
of the mixing of magma from two sources, one picritic and
PGE rich, and the other tholeiitic and less enriched in PGE.
The picritic magma was the result of the continued melting of
mantle that, as discussed above, had undergone an earlier
partial melting event during which some sulfide had remained behind, holding back much of the PGE in the source.
Momme et al. (2003) noted that none of the North Atlantic
igneous province lavas are boninitic and argued that the special envionment that gives rise to boninites is not required to
produce PGE-rich magma. Barnes and Maier (2002) showed
that the trace element signatures of the rocks comprising the
lower layers of the Bushveld Complex indicate that the high
silica content is consistent with crustal contamination of a hot,
possibly komatiitic magma. This explanation is generally accepted today, although it should be noted that the source Utype magma for the Bushveld is estimated to have contained
13 to 14 wt percent MgO, while a typical komatiite magma
should contain >18 wt percent MgO (Arndt, 2008). Contamination is unlikely to have diluted the MgO content to such an
extent, so that the process of contamination probably involved
fractional crystallization in addition to assimilation. Thus, it is
a possibility, although by no means proven, that the magma
that seems to have been so important in the genesis of PGErich layered complexes developed from emplacement of a
primary magma that was hotter than those generally developed today. This hot magma was unusually enriched in PGE,
perhaps, as Momme et al. (2003) suggested, as a result of an
earlier melting episode and rose into the crust and underwent
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extensive contamination before ascending to form the magma
chamber. It is possible that the magmas responsible for Neoproterozoic and Phanerozoic mafic intrusions have not been
so hot, have not had the required composition, and have not
interacted with the crust to the same extent.
The rarity of intrusions greater in age than 2.9 Ga makes
any comment on the prevalence of deposits older than this
statistically highly unreliable. However, Maier et al. (2009)
have shown that the Pt contents of komatiites older than 2.9
decrease systematically with age (by a factor of 2.5 from 2.9 to
3.5 Ga). This is true for komatiites from both the Pilbarra
(Western Australia) and Kaapval cratons, and for both
Munro- and Barberton-type komatiites. After discussing and
rejecting a number of explanations for this, they concluded
that this is due to the progressive enrichment of an initially
depleted mantle (depleted due to core segregation) becoming
enriched by meteorite bombardment in the period 4.5 to 3.8
Ga (late veneer hypothesis). It took time for this late-stage,
PGE-enriched zone to become mixed throughout the mantle,
and so enrich the source region of most mantle plumes close
to the core-mantle boundary. Thus, although there is not a
statistically valid number of Paleoarchean intrusions on which
to demonstrate a lack of PGE deposits, the Maier et al. (2009)
hypothesis suggests that they are less likely to have formed
during this period.
Although a “red herring” in the context of the secular focus
of this volume, this author feels compelled to introduce a geographic note. Seventy-five percent of the world’s known
PGE resources occur in two complexes, the Bushveld and the
Great Dyke that now lie less than 600 km apart from each
other, and it is tempting to ask if the mantle in this part of the
world was particularly PGE enriched during the late
Neoarchean and Paleoproterozoic.
The 2.06-Ga Bushveld Complex intruded the Kaapvaal craton and the 2.58 Ga Great Dyke intruded the adjacent Zimbabwe craton. The latter craton grew as a result of a series of
south-southwest–directed collisions between 2.615 and 2.600
Ga and amalgamation with the Limpopo microcontinent by
about 2.60 Ga (Dirks and Jelsma, 2004). Sigma 3 switched
from a vertical to a horizontal orientation toward the end of
the initial accretionay process and the Great Dyke exploited
this stress regime (Paul Dirks, pers. commun., 2009). There
is some debate as to when this combined craton joined with
the Kaapvaal craton to form the Kalahari craton. Initial collision probably occurred at about 2.61 Ga (Griffin et al., 2003,
2004), with final welding at perhaps 2.0 Ga. Griffin et al.
(2008) and Begg et al. (2009) have coupled seismic tomography with petrologic profiles of the mantle obtained from
studies of inclusions from kimberlite pipes. Rather than interpreting the variable seismic velocities in terms of temperature alone, they interpret them in terms of refertilization of
depleted mantle by the subsequent intrusion of magma and
introduction of fluids. The Archean cratons are mostly underlain to depths of 300 km or more by steep-sided roots composed of highly depleted dunite and harzburgite. The Zimbabwe craton and the southern part of the Kaapvaal craton
are cases in point. However, the intervening northern Kaapvaal craton differs in having at depths of up to 175 km (see
Begg et al.’s figs. 5, 6) a belt of lower velocity extending eastward, beneath the Bushveld. Begg et al. (2009), partly on
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SECULAR VARIATION OF MAGMATIC SULFIDE DEPOSITS AND THEIR SOURCE MAGMAS
the evidence of strongly metasomatized mantle xenoliths and
xenocrysts from the 1.2-Ga Premier diamond mine, have interpreted the seismic tomography of this belt as being the result
of refertilization of this portion of the roots of the Kaapvall craton during the late Paleoproterozoic. As Begg et al. (2009)
point out, and as is shown in Figure 14 taken from their paper,
the coincidence of the belt with the overlying Bushveld intrusion is suggestive that the refertilization is related to Bushveld
emplacement. On the other hand, the Great Dyke cuts through
classic depleted Archean lithosphere. Thus, if there is a PGErich zone within the mantle in southern Africa, it is unlikely to
be in the roots of what is now the Kalahari craton but must be
a feature of the deep (>300 km) asthenosphere that survived
for the 500 m.y. intervening between emplacement of the two
bodies, or of a very long-lived, PGE-rich deep mantle plume
that cut through both asthenosphere and lithosphere
Conclusions
1. The composition of magmatic sulfide deposits is highly
dependent on the MgO content of the source magma involved, high MgO contents being associated with sulfides with
high Ni/Cu and low Pd/Ir ratios. These aspects can be modified as a consequence of the degree of enrichment of the sulfides; a high R factor or interaction with fresh, sulfide-unsaturated magma will lead to higher PGE/(Ni + Cu) ratios in
the sulfides.
2. Komatiites with >25 wt percent MgO in the liquid
magma are restricted to the Archean. Barberton-type (Al-undepleted) komatiites tend to be older and are not known to
host significant mineralization, perhaps because they extruded
and/or intruded in an oceanic setting that lacked the felsic
rocks or sulfur source that could have caused the magma to
become sulfide saturated.
3. Munro-type komatiites (Al-depleted) are generally younger and were extruded in a continental setting or during island arc volcanism. They are the host to all known Archean
komatiite-related sulfide deposits, probably because the opportunity for contamination was present.
4. Moderate MgO (18–22 wt % MgO) komatiitic magmas
are mostly Paleoproterozoic and the mineralized examples
were extruded and/or intruded into a rifted continental margin setting. A Phanerozoic (90 Ma) oceanic example of moderate MgO komatiitic volcanism occurs on Gorgona Island,
Colombia, and proves that in exceptional circumstances, komatiites have formed in very recent times.
5. The overall secular distribution of komatiites is attributed to the cooling of the Earth, coupled with, in the case of
associated ores, the development of more extensive areas of
continental-style crust.
6. Deposits associated with magmas of picritic to basaltic
composition have developed from the Neoarchean to recent
times.
7. There is only one example each of anorthosite complexand impact melt-related deposits, so one cannot generalize
about their secular distribution, except to say that anorthosite
complexes are a Proterozoic phenomena.
8. Ural-Alaskan intrusions are mostly Phanerozoic in age,
although Archean examples have been described. Significant
accumulations of magmatic sulfides occur in only a very few
of them, and none have so far proved to be economic.
9. PGE-rich deposits occur from the Neoarchean to the
recent. However, the major deposits, which are associated
Archean Lithosphere
Great
Dyke
Archean lithosphere
fertilised in Proterozoic
Proterozoic lithosphere
fertilized in Phanerozoic
Bushveld
N
Archean lithosphere
fertilized in Phanrozoic
Phanerzoic lithosphere
zones of uncertainty
1000 km
FIG. 14. Begg et al.’s (2009) interpretation of their 100- to 175-km-depth seismic tomgraphy model in terms of the tectonothermal age of the subcontinental lithosphereic mantle. Note the belt of Archean lithosphere refertilized in the Proterozoic that extends west-southwest beneath the northern part of South Africa and how the Bushveld complex relates to this.
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A. J. NALDRETT
with intrusions that crystallized from a high proportion of Utype magma, occur from the Neoarchean to Paleoproterozoic. It is possible that the conditions required to develop a
hot, PGE-enriched U-type magma, which would then interact with the crust, are time dependent and were much rarer
both prior to the Neoarchean and from the Mesoproterozoic
onward.
In summary, this author believes that a magmatic sulfide
deposit may form any time that a hot mafic and/or ultramafic
magma enters the crust, particularly if the area of the crust is
sulfide enriched. The style style and composition of the deposit will depend on the stage in the development of the
Earth and thus on the type of magmatism that could develop
at the stage in question.
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