Petrogenesis of pelitic xenoliths at the Babbitt CuNi deposit, Duluth Complex, Minnesota, U.S.A.

Lithos, 21 (1988) 143-159 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

143

Petrogenesis of pelitic xenoliths at the Babbitt Cu-Ni deposit, Duluth Complex, Minnesota, U.S.A. EDWARD M. RIPLEY and JOMAAH A. ALAWI Department ~![Geolo~y, Indiana University, Bloomington, IN 47405 (U.S.A.)

LITHOS

Ripley, E.M. and Alawi, J.A., 1988. Petrogenesis of pelitic xenoliths at the Babbitt Cu-Ni Deposit, Duluth Complex, Minnesota, U.S.A. Lithos, 21 : 143-159. The Babbitt deposit consists of disseminated Cu-Fe-Ni sulfdes found within mafic rocks of the Duluth Complex, generally near contacts with underlying metasedimentary rock types. Host rocks for the deposit include troctolites, olivine gabbros, gabbronorites, norites, and occasionally country, rock hornfels. Xenoliths of country rocks are abundant in the deposit, and suggest a relationship between sulfide mineralization and country rock contamination. Country rocks in the Babbitt area include those of the middle Precambrian Biwabik Iron Formation, and both calcareous and non-calcareous pelites of the Virginia Formation. Xenoliths contain the assemblage cordierite-plagioclase-biotite-orthopyroxene,and are thought to have been derived from Virginia Formation protoliths. Comparison of protoliths and xenoliths using composition-volume, element ratio and mass-balance techniques suggests that xenoliths have been strongly depleted in volatiles, alkalis and Si. Footwall rocks show only a depletion in volatiles. Neither fluid-phase transport nor diffusion through an intergranular fluid can account for the mass of material transferred. Extensive partial melting ofxenoliths, with residual enrichment of FeO, MgO and AI20~, is the most viable transfer process. The lack of SiO2 concentration gradients around xenoliths and anomalous igneous rock compositions suggest that desilicification occurred at a time when physical mixing of extracted partial melt and host magma was possible, and prior to final emplacement. Sulfide saturation may have been initiated due to Si assimilation with an auxiliary magma chamber. However, the composition of ores in the Babbitt deposit is consistent with saturation being achieved by addition of sediment-derived volatile sulfur, independent of majorelement assimilation. (Received September 10, 1986; accepted June 9, 1987)

Introduction

T h e D u l u t h C o m p l e x (Fig. 1 ) is a series o f multiple, p r e d o m i n a n t l y mafic i n t r u s i o n s o c c u r r i n g in n o r t h e a s t e r n M i n n e s o t a , U.S.A. O v e r the p a s t ten to fifteen years the C o m p l e x has been the focus o f several i n v e s t i g a t i o n s c o n c e r n e d with its petrogenesis or the origin o f a s s o c i a t e d C u - N i sulfides (e.g., Boucher, 1975; M a i n w a r i n g a n d N a l d r e t t , 1977; B o n n i c h s e n et al., 1980; W e i b l e n a n d M o r e y , 1980; N.K. G r a n t a n d Molling, 1981; Ripley, 1981; Foose, 1982; C h a l o k w u a n d G r a n t , 1983; R a o a n d Ripley, 1983; Pasteris, 1984, 1985; T y s o n a n d Chang, 1984). M o s t o f the studies d e a l i n g with sulfide generation have e m p h a s i z e d the i m p o r t a n c e o f the der i v a t i o n o f sulfur a n d volatiles from s e d i m e n t a r y

0024-4937/88/$03.50

© 1988 Elsevier Science Publishers B.V.

c o u n t r y rocks. Q u e s t i o n s still r e m a i n as to the t i m ing o f sulfur i n t r o d u c t i o n , w h e t h e r in situ o r p r i o r to e m p l a c e m e n t . M o s t recent e v i d e n c e favors the latter ( R i p l e y a n d A1-Jassar, 1987; Chalokwu, 1985; Ripley a n d Andrews, 1985). W h a t has not been well d o c u m e n t e d is the possible role o f n o n - v o l a t i l e elem e n t s in localizing sulfide m i n e r a l i z a t i o n . Sulfide s a t u r a t i o n m a y be i n i t i a t e d by c o m p o s i t i o n a l changes in a m e l t i n d u c e d by processes such as silica a d d i t i o n (e.g., Irvine, 1975), or m a g m a m i x i n g ( C a m p b e l l et al., 1983). T h i s c o m m u n i c a t i o n concerns a s t u d y o f the B a b b i t t C u - N i d e p o s i t (Fig. 2), where sulfide m i n e r a l i z a t i o n is f o u n d in areas cont a i n i n g a b u n d a n t c o u n t r y rock inclusions. T h e Babbitt d e p o s i t p r o v i d e s p e r h a p s the best locality within the entire D u l u t h C o m p l e x for e v a l u a t i n g the roles

144

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Fig. 1, Generalized bed-rock geologyof northeastern Minnesota ( after Cooper et al., 1978 ) showing location and regional geology oflhe Hoyt Lakes-Kawishiwiarea. of partial melting of country rock and selective contamination in the genesis of sulfide mineralization. Oxygen isotope studies of rocks from the Babbitt deposit by Ripley and AI-Jassar (1987) unequivocally show that xenoliths have interacted extensively with surrounding igneous material. Ripley and Al-Jassar (1987) model a process of diffusive exchange of oxygen isotopes at xenolith contacts• However, the oxygen isotopic exchange profiles preserved at margins of xenoliths and adjacent igneous rocks may have been generated relatively late in the petrogenetic history of the xenoliths. Our study is essentially aimed at evaluating transfer of material from xenoliths and footwall rocks during magma emplacement and contact metamorphism. Methods utilized include petrographic examination of polished thin sections, microprobe analysis of selective minerals, and chemical analyses of meta-

morphosed sedimentary country rocks and igneous host rocks.

Geological setting Rock types in the Duluth Complex are varied, but are in general characterized by either the presence of plagioclase-olivine or plagioclase cumulates. The Complex has been divided into an older Anorthositic Series (Davidson, 1972) and a younger Troctolitic Series (Bonnichsen, 1972). Contacts between individual units of the two series are obscure, and much conjecture remains in evaluating the genetic relationships between the major divisions of

145

EXPLANATION R12W

Duluth Complex

g T 60 N 47°40

Viro, ~

t,on

LBiwabik Iron Formation

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T 59 N

Giants Range massif

Metasedimentary rocks f f

Fault

0 [

0

5 Miles I

[I

i I !

ii

I

d

5 Km

Drill lines

Fig. 2. Geologic setting of the Babbitt and Dunka Road Cu-Ni deposits, showing location of Babbitt drill lines and projection of the 1700 ft. level.

the Complex. Rocks of the Complex are overlain and underlain by the Keeweenawan lavas of the North Shore Volcanic Group (Green, 1977). Magmatic activity is centered on what is known as the Mid-Continent Gravity High, which is interpreted as a fossil continental rift zone. The Duluth Complex is also intrusive into a series of early and middle Precambrian rock types, including the Biwabik Iron Formation, predominantly pelitic metasediments of the Virginia Formation, and igneous rocks of the Giants Range Massif. Sulfide mineralization in the Duluth Complex is almost exclusively confined to the basal portions of the Troctolitic Series, where footwall material consists of abundant metasedimentary material. The Troctolitic Series has been divided into a series of separate intrusives (Weiblen and Morey, 1980; Weiblen, 1982). Contacts between individual intrusions are obscure, and based largely on mineral textures, fabric and geophysical indicators. Although sulfide mineralization is found in many of

the intrusives, most exploration has centered along the basal contacts of the South Kawishiwi and Partridge River intrusives (Fig. 2). The Babbitt deposit occurs in the Partridge River intrusion, where immediate country rocks are those of the Virginia Formation. Sulfide mineralization at the Babbitt deposit generally occurs near the basal contact with the Virginia Formation. However, some mineralization (referred to as cloud zones) occurs as much as 300-400 m above the basal contact (Fig. 3). Igneous rock types are variable (see below), and include troctolite, gabbronorite, olivine gabbro, norite and anorthosite. Several igneous rock types may host sulfide mineralization; a unifying feature appears to be the presence of country rock inclusions, even within cloud zones• Principal sulfide minerals in the deposit include pyrrhotite, cubanite, chalcopyrite, and minor pentlandite. Details of sulfide mineralization are given in Ripley and Alawi (1986). Although xenoliths are found at all levels

146 NW

SE 329

333

379

203

239

189

156

138

136

135

137

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COMPLEX

d3Z~ [T~T~ Vir i

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333

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60o Ft

Fig. 3. Cross-section along drill line 3600 E showing general geologic features, Babbitt deposit.

throughout the Partridge River Intrusion, they are most highly concentrated near the present footwall. In general, xenoliths are more abundant below a depth of ~ 400 m, where the contact with the Virginia Formation tends to flatten out (Fig. 3). In the Babbitt area, the volume proportion of xenoliths may reach as high as 25% within the deeper portions of the igneous sequence. We have examined over 300 samples ofxenoliths and footwall material from the Dunka Road and Babbitt areas. Our sampling has concentrated on xenoliths occurring in drill cores from lines 3600 and 4000 E and the 1700 ft. (518 m) level of the Babbitt deposit (Figs. 2 and 3).

Metamorphic rock types in the Babbitt area In the vicinity of the Babbitt high-grade sulfide mineralization, the mineralogy of metamorphic rock types is similar in both xenoliths and footwall. This situation is quite different from that at the Dunka Road deposit, located ~ 9 km to the southwest (Fig. 2), where variations in mineral assemblages and metamorphic grade are discernible from the contact outward (Ripley and Andrews, 1985 ). Lapotka et al. ( 1981 ) have also distinguished various metamorphic grades over comparable thicknesses of the

Rove Formation (equivalent to the Virginia Formation) in the extreme northeastern section of the Complex. One indicator of the intensity of metamorphism in the Babbitt area is the occurrence of orthopyroxene in all pelitic units examined, regardless of their distance from the apparent contact. In contrast, orthopyroxene is found only very near the intrusive contact or within xenoliths in both the Dunka Road deposit (Ripley and Andrews, 1985) and the Gunflint Lake area (Lapotka et al., 1981 ). Metamorphic rocks in the Babbitt area are variable in bulk rock composition, ranging from lowCaO pelitic rocks to calc-silicate units. Based on mineralogy and composition, three major divisions are evident. Calcium content is the most useful elemental indicator for classification purposes (Table 1 ). Pelitic rock types are characterized by CaO values less than ~ 2.5%. Calcareous pelitic rocks are defined as having CaO values between 2.5% and 12.0%, whereas calc-silicate units are those with CaO values in excess of 12.0%, and commonly greater than 20%. Calc-silicate units tend to be less than ~ 25 cm in thickness and are easily recognized by their pale color and coarse grain size. These units correspond to either carbonate-rich layers present in unmetamorphosed Virginia Formation or to calcareous pods found within pelitic units. Intermediate CaO values correspond to calcareous shale and

147 T~BLE I Summaq, of chemical analyses of metasedimenlary xenoliths from the Babbilt deposiL footwall hornfels from the Dunka Road deposit, and calcareous argillHe t'rom drill hole MDI)2 (values in wt.%) Rock q, pe

Specific densib

Ix)w-Ca() xenoliths

Inter.-CaO xenoliths

High-CaO xenoliths

Dunka Road footwall *~

a~ e. I;z= 32 )

ave. (,'1= 171

ave. ( n = 12)

ave. (,,1= 12)

range

range

range

2.86

2.72- 3.10

2.91

2.82- 3.15

3.08

2.85- 3.15

2.71

SK) 1i() &l.(), Fc() Mn() Mg() ('a~) Nat) K() P() S ( It()

48.l 1.24 20.5 15.3 I).13 8.25 1.12 1.02 1.44 0.21 0.77 061 123

43.6 -51.6 0.36- 1.68 12.9 -22.9 13.7 -22.8 0.09- 0.83 5.50 13.3 0.40- 2.63 0.05- 2.50 0.34- 3.01 0.14- 0.32 0.23- 1.48 0.03- 1.06 ~).45- 2.77

51.5 1.01 18.1 12.0 0.11 5.51 5.17 2.95 1.04 0.23 1.05 0.31 0.89

44.6 -58.4 0.22- 1.46 13.2 -23.9 8.3 -19.4 0.03- 0.22 3.81- 7.96 3.17-12.0 1.50- 4.25 0.33- 1.79 0.13- 0.66 0.06- 4.45 0.12- 0.98 0.27- 2.32

47.8 0.53 11.9 7.57 0.35 2.55 24.0 0.69 0.55 0.25 1.37 0.96 0.79

38.9 -52.4 0.13-0.93 3.85-21.4 4.30- 9.84 0.11- 0.59 0.63- 4.78 12.5 -39.5 0.02- 1.52 0.04- 2.15 0.07- 0.58 0 0 1 - 3.29 0.14- 3.22 0.62 1.70

59.8 1.25 16.1 8.07 n.a. 3.52 1.11 2.11 3.74 n.a. 0.14 2.14 1.98

final

99.92

99.87

99.91

Argillite .2 MDD2

range

2.63

2.75

2.78

55.1) - 6 2 . 4 0.94- 1.55 15.7 18.6 5.54- 9.50 n.a. 2.47- 5.10 0.54- 2.12 1.20 2.75 1.92- 5.05 n.a. 0.01- 1.00 1.69 2.51 1.16- 3.53

65.1 0.62 12.1 4.81 n.a. 2.53 5.10 3.80 1.04 n.a. 1.15 n.a. 2.0

99.92

98.29

n a . : n o t ana]~zed. * RaoandRiplc~ t l 9 8 3 t . *'Bonnichsen (1972).

graywacke beds. Compositionally distinct units may be interlayered on scales ranging from a few centimeters to tens of meters. It is this type of compositional variability within the Virginia Formation that hinders an accurate analysis o f element mobility, Figure 4 is a cross-section showing a generalized distribution of metamorphic units in the Babbitt deposit. In the area o f drill cores 156 and 136 (drill line 3600 E), the contact between the Duluth Complex and the Virginia Formation is extremely irregular. The lowest package o f Virginia Formation identified is designated footwall, although greater thickness of Virginia Formation may be present as either large xenoliths or portions of footwall cut by apophyses of Duluth Complex (Fig. 4). Footwall material in drill core 156 consists predominantly of rock types with intermediate CaO contents (Table 2). A sample from 559 m is a low-CaO pelite, whereas samples at 547, 549 and 500 m are characterized by anomalously high MgO contents, possibly indicative of the presence o f dolomite in the protolith. Low-CaO pelites and banded CaO-poor CaO-rich rock types predominant in overlying xenoliths. The high CaO content o f footwall rocks at Babbitt contrasts sharply with those present at the

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High-CaO

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C3 500

550 -

Fig. 4. Distribution of country rock xenoliths along drill cores 136 and 156. Also based on underground drift sampling and fan drilling from the 518 m (1700 ft.) level.

148 TABLE 2 Chemical analyses of metasedimentary footwall rocks from the Babbitt deposit, drill hole 156 (values in wt.%) Sample No.

1796

1800

1803

1811

1818

1826

1833

1843

1.8

3.0

4.0

6.4

8.5

11.0

] 2.5

16.2

3.05

3. I 1

3.12

3.08

2.99

2.79

3.06

3.02

Si() YiO AI:O, FeO MnO MgO ('aO Na~O K,O Pc(), S (" H:O

49.8 1.07 14.0 10.1 0.15 10.1 11.3 1.26 0.75 0.25 0.08 0.05 0.36

49.0 0.93 12.7 10.9 0.16 12.8 9.58 0.86 1.51 0.22 0.06 0.05 0.54

46.5 0.88 10.1 11.8 0.18 19.5 7.50 1.29 0.56 0.21 0.06 0.04 1.26

49.1 1.82 15.5 12.6 0.18 7.76 8.33 2.26 0.34 0.36 0.31 0.03 0.45

49.6 1.00 15.0 9.6 0.14 9.04 I 1.2 1.72 1.29 O. 15 0.26 0.03 0.45

50.8 1.21 16.3 11.3 0.15 8.65 6.84 2.30 0.97 0.29 0.02 0.03 0.36

51.8 1.33 19.1) 11.7 0.08 5.60 2.59 2.39 2.84 O. 19 0.41 1.20 0.65

49.4 1.30 14.6 9.70 0.15 7.46 I 1.2 1.89 2.75 0.26 0.23 0.05 0.63

Total

99.27

99.31

99.88

99.04

99.48

99.22

99.78

99.62

Distance below

contact ( cm ) Specific densit}

Dunka Road deposit (Rao and Ripley, 1983), where only low-CaO pelitic rocks are found. Variations of this type are to be expected, as the base of the Duluth Complex transects various stratigraphic heights within the Virginia Formation. Mineralogy within each of the three compositional classes is similar. Low-CaO rocks contain cordierite, orthopyroxene, plagioclase, biotite, Kspar, and locally quartz and sillimanite, with minor amounts of ilmenite, graphite, apatite and sulfides (pyrrhotite+chalcopyrite). Orthopyroxene may occur as discrete grains or as large aggregates enclosing other phases. Plagioclase may also occur as granular discrete grains, or as large poikiloblasts enclosing other minerals. Intergrowths of biotite, quartz and orthopyroxene are common, and are of particular importance in evaluating dehydration and melting reactions. Intermediate-CaO rock types are characterized by a varied mineralogy, containing in addition to the minerals found in CaO-poor rocks, clinopyroxene, amphibole and calcite. CaO content appears to be dependent on the presence of clinopyroxene and/or plagioclase composition. Calc-silicate units are composed of plagioclase, Ca-rich clinopyroxene, calcite, wollastonite, Ca-amphibole and vesuvianite. Ilmenite, sphene, graphite and sulfides are minor phases. Within calc-silicate units a distinct

/

38 I o~

°o

•

28f 18 80

e°° 5i

•

110

I 1~

2tO

J 25

Wt. % AI20~ Fig. 5. P l o t o f C a O vs. A I , O ~ c o n t e n l f o r h i g h - C a O h o r n f e l s .

relationship exists between CaO and A1203 abundance (Fig. 5). This relationship is thought to represent varying proportions of A1 minerals (principally clays) to carbonate in the protolith sediments. It is apparent that a wide range of compositions existed in calcareous units (lenses, pods, or beds), which limits their usefulness in the evaluation of element mobility. The restricted mineralogy exhibited by compositionally similar untis prevents the tracing of progressive metamorphic reactions. Muscovite is not present in metamorphic rocks of the Babbitt area, but is abundant in lower-grade pelitic rocks of the Virginia Formation. Reactions such as: muscovite + biotite + quartz cordierite + K-spar + H ~0 and

149

muscovite + quartz~sillimanite + K-spar + H 2 0 are presumed to have occurred. Muscovite has been eliminated in all rock types through dehydration reactions. Textures present in the pelitic rocks suggests that further dehydration and possible melting reactions have involved biotite and the production of orthopyroxene. Two possible reactions are: biotite + quartz Al-orthopyroxene + K-spar + H 2 0 and biotite + quartz ~- cordierite + orthopyroxene + K-spar+ H 2 0 Compositions of coexisting minerals in xenolitbs and footwall rocks from Babbitt (e.g., Fig. 11 ) show no well-defined trends. Variations in mineral compositions are largely a function of differences in protolith composition.

Element mobility in the metamorphic rocks Representative chemical compositions of Virginia Formation argillites occurring outside the contact aureole of the Duluth Complex have been given by Bonnichsen (1972) and Rao and Ripley (1983). Compositions are similar to those of footwall rocks from the Dunka Road deposit (Rao and Ripley, 1983), and rocks from U.S. Steel Corp. drill hole 24981, located at the outermost margin of the aureole (see Tables 1 and 3). Footwall rocks from the Dunka Road deposit are cordierite-plagioclasebiotite hornfels, with orthopyroxene present only within a few meters of the contact. Rocks from drill hole 24981 contain the assemblage quartz-plagioclase-chlorite-muscovite-biotite-graphite, and show little effects of contact metamorphism. Rao and Ripley (1983) have shown that Dunka Road footwall rocks have been subjected to devolatilization (loss of C and H20), but non-volatile elements have been immobile. Similar conclusions have been reached with respect to footwall rocks in the South Kawishiwi area (Bonnichsen, 1972) and the Gunflint Lake region in the northern section of the Complex (kapotka et al., 1984). A simple comparison of low-Ca xenolith compositions versus those of weakly metamorphosed Virginia Formation or

more strongly metamorphosed li)otwall rocks suggests a major difference in SiO~ content. However, the principal difficulty encountered in assessing gains or losses of elements from xenoliths in the Babbitt area is an accurate determination of protolith compositions. Because of the original compositional variability in sedimenta~' rocks of" the Virginia Formation such a determination is difficult. However, analyses by G.B. Morey (unpublished data, 1984) ofunmetamorphosed Proterozoic shales and graywackes from Minnesota indicate that SiO2 contents vary, between 55.27% and 68.08% and between 68.12% and 87.99%, respectively. Compositions ofargillitic rocks from the footwall at Dunka Road and drill hole 24981 fall in the same range as those determined by Morey. A total of sixty samples of Proterozoic sedimentary rocks from northeastern Minnesota have been analyzed to assess possible protolith compositions. A Iow-SiO: protolith similar to the xenoliths has not been detected. Although the raw data strongly suggest that xenoliths have been desilicified, a more quantitative procedure was pursued in order to better define element mobility in the Babbitt deposit. Several authors (e.g., Wood et al., 1976: Coish, 1977: Schultz, 1977) have utilized xanation plots of presumed mobile or immobile elements in assessing mass transfer during metamorphism. Plots of two immobile elements show regular relationships, and frequently define a constant ratio. Diagrams of immobile versus mobile elements may show linear relationships, but with a negative slope and a changing ratio. Plots of mobile versus mobile elements are expected to show a scattering of points and no regular relationship, although distinct compositional groupings may be observed. The constancy of element ratios computed versus a presumed immobile element (typically AI) has been used by Ferry (1982) and Lapotka el al. (1984) in assessing immobility in both contact and regional metamorphism. A series of variation diagrams was prepared in order to elucidate compositional differences between possible protoliths for xenoliths in the Babbitt area. Diagrams were constructed to illustrate compositional variations of presumed refractory elements, as well as mobile species. Rock types plotted include weakly metamorphosed rocks occurring outside the contact aureole (Bonnichsen. 1972: Rao

15O TABLE 3 ('heroical a n a b s e s of low-grade Virginia Formation argillites, drill hole 24981 (values in wt.%) Sample No.

136

156

167

176

186

196

197

206

212

223

Average

Specific 2.72

2.72

2.75

2.72

2.64

2.68

2.67

2.78

2.69

2.68

2.71

57.7 0.85 18.6 6.64 0.06 3.30 0.72

II {)

4.16 0.23 (),56 1.23 3.59

70.6 0,56 12.0 6,29 0.06 2.79 1.89 2.49 1.04 0.21 0.41 0.53 2.10

62.4 0.77 15.7 6.82 0.04 3.23 0.65 2.23 3.08 0.23 0.62 0.85 2.74

57.9 0.84 18.1 7.29 0.05 3.61 0.76 1.60 3.95 0.23 0.47 1.14 3.39

65.3 0.67 14.3 6.17 0.04 2.95 1.08 2.32 2.68 0.23 0.50 1.04 2.16

57.0 0.93 18.8 7.42 0.06 3.63 0.62 1.66 4.41 0.24 0.24 0.84 3.38

59.3 0.79 17.4 6.92 0.04 3.54 0.85 1.60 3.88 0.24 0.55 1.13 3.14

61.5 0.78 15.6 5.91 0.03 2.62 1.05 2.16 3.36 0.21 1.40 2.61 2.49

60.7 0.77 15.4 6.90 0.03 2.59 0.96 2.06 3.39 0.2 1 2.25 2.36 2.34

59.6 0.73 19.3 3.65 0.02 2.51 0.41 1.79 4.98 0.22 0.32 2.44 3.11

01.2 0.77 16.5 6.42 0.04 3.09 0.90 1.96 3.48 023 0.73 1.42 2.84

1 oral

t)9.50

100.96

99.35

99.33

99.44

99.23

99.38

99.72

09.96

99.1)7

99.57

dellSlI\

Si( ) li(), kl,() Fc( Mn( ) Mg( (a() Na ,( ) K () P (),

1.77

S (

AI,,O~

/

/

/

/

/

/

/

/

Cordierite

\

/

,/ / // /' f /

/

/

/

/

/

Biotite ' ~

"\

Orthopyroxene

~

MgO

FeO

Fig. 6. Representanve compositions of coexisting cordierite, biotite and orthopyroxene in xenoliths from the Babbitt deposit.

and Ripley, 1983: G.B. Morey, unpublished data, 1984), footwall rocks from the Dunka Road area ( Rao and Ripley, 1983: this study), and Babbitt area (this study), as well as xenoliths from Babbitt (this study). Variation diagrams show positive slopes for Fe-Mg. Fe-Ti and Mg-Ti plots, and negative slopes for Fe-Si, Fe-K and Fe-Na plots (Fig. 7). Relatively unmetamorphosed or weakly metamorphosed footwall rocks plot closer to the origin in refractory element plots than do metamorphosed rocks. This

relation is consistent with the premise that metamorphosed rocks were derived from protoliths similar in composition to the weakly metamorphosed rocks occurring both within and outside the contact aureole. Although the Si-Fe plot is essentially linear, the Si/Fe ratio varies from ~ 13.5 in weakly metamorphosed rocks, to 2.4 in xenoliths. A plot of Na versus K, both presumed to be more mobile elements, does not show a linear relation (Fig. 7). Instead three distinct fields are discernible. Two are characterized by low K values ( < ~2%), and Na values between 0 and 2% and from 2% to 4%. Lower Na values correspond to low-Ca pelites, whereas higher Na values are representative of high-Ca pelites, Weakly metamorphosed rock types have higher K values and intermediate Na contents. This relationship suggests that K has been depleted from metamorphosed samples, and that high-Ca pelites contained greater quantities of Na than most of the analyzed weakly metamorphosed samples (Table 1). Samples from the Babbitt footwall rocks plot off the linear trends displayed in most of these diagrams (see Fig. 7). As mentioned above, these rocks are enriched in Mg, and probably represent a protolith rich in dolomite. A summary of element ratios relative to Mg for various rock types is presented in Fig. 8. The ratios confirm the conclusions reached from the variation diagrams, principally that ratios of refractory ele-

151

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7.

ments are similar for weakly metamorphosed material, and differ greatly from those of xenoliths. Pelitic xenoliths show particularly pronounced depletions in Si, Ca, Na, K and P. A more quantitative treatment of comparing probable protoliths to metamorphosed equivalents was undertaken following a computational technique developed by Gresens (1967). Gains and losses of elements from a sample are related to a parent through mass-balance and volume considerations. The following equation applies:

where: 3 X , = l o s s or gain of an element i (in g or wt.%) in producing rock fl from rock ~: m = the initial quantity of each element (g), c o m m o n l y i n = 100 g;/; = t h e volume factor, the ratio of the final volume to the initial volume of the rock mass: p~L p/~= specific gravity of the parent rock and product rock, respectively; and W'L W/¢=weight fraction of component i in rocks ~ and/~, respectively.

152 12 I

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2O I N,a/Mg

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Babbitt Xenoliths • Low CaO pellte • Int CaO pelite

Fig. 8. Elemen! ratios for xenoliths and mildly metamorphosed Virginia Formation. The successful application of this technique is highly dependent on a precise knowledge of protolith composition and volume changes during metamorphism. Use of the entire range of compositions for both xenoliths and possible protoliths leads to the same conclusions as does utilization of average compositions. Figure 9 was constructed using average bulk-rock compositions with possible volume factors estimated using results from variation diagrams which suggest that Fe, Mg and Ti have behaved as refractory elements. Pelitic rocks from the contact aureole and xenoliths at Babbitt fall within rather narrow compositional ranges (Table 1 ). These rocks are compared to a series of pelitic rocks located at the outer margin of the contact aureole (U.S. Steel Corp. drill hole

24981 ), which have suffered only mild regional and contact metamorphism. Pelitic rocks from the Babbitt area are also compared to footwall rocks from the Dunka Road deposit. Although the latter show a well-defined temperature gradient, Rao and Ripley (1983) have demonstrated that contact metamorphism in the Dunka area was essentially isochemical, except for the loss of volatile phases such as H20 and C. Average compositions of the mildly metamorphosed Virginia Formation and footwall rocks from the Dunka Road deposit are listed in Tables 1 and 3. These compositions are very similar to the "average pelite" of Shaw (1956). Analyses of likely protoliths for samples containing higher CaO contents are rare. Although a probable protolith could not be as accurately defined as for low-CaO pelites, an analysis of a calcareous siltstone from a Virginia Formation unit occurring outside of the contact aureole (MDD2; Bonnichsen, 1972) was chosen as a reference. This sample contains 5.1% CaO and was compared to the average of footwall and xenolith samples containing between 2.5% and 8.0% CaO. Petrographic and chemical analyses of pelitic rocks from the Babbitt deposit leave little doubt that devolatilization has occurred. Figure 9A illustrates a composition-volume diagram for the average Babbitt low-CaO pelite relative to the average Virginia Formation pelite found outside the contact aureole. Results of the computations suggest that the Babbitt rocks are indeed depleted in H~O, S and C, as well as SiO2, A1203 Na20 and K~O. Comparison of Babbitt pelites relative to those in the Dunka Road area also indicates a loss of SiO2, A120~, Na20 and K20 (Fig. 9B). Results of computations comparing the high-CaO pelites suggest that the xenoliths are strongly depleted in silica, and to a lesser extent in A1203, alkalis and H20 relative to mildly metamorphosed material ( Fig. 9C ).

M e c h a n i s m s of mass transfer

Analyses of both composition-volume relations and element ratios suggest that relative to weakly metamorphosed protoliths, xenoliths in the Babbitt area are depleted in SiO2, AI_,O3, K20 and Na20, as well as volatile phases such as H20, C and S. Local depletions in CaO are also detected. Analyses of over 30 contact zones indicate that there are no well-de-

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fined gradients in SiO~ content either within xenoliths or in surrounding igneous rock (Fig. 10). In the majority of cases examined, the proportion of SiO~ in xenoliths is only slightly higher than that of surrounding igneous material. There is no doubt that if the xenoliths were compositionally similar to unmetamorphosed shales and graywackes, a strong initial SiO_~ concentration gradient between xenoliths and melt would have existed. In rare instances

where S i O 2 gradients could be defined (e.g., Fig. 10, upper xenolith), mass-balance computations indicate that the volume o f the possible SiO,-enriched area around the xenolith is insufficient to account for the mass of SiO2 lost from the xenolith. Available evidence strongly suggests that element transfer must have occurred early enough that homogenization in the melt was possible (perhaps when the xenoliths were still part of the roof rocks,

154

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Fig. 10. Silica content of representative xenoliths and surrounding igneous rocks from the Babbitt deposit: (A) and (B) drill core 156; (C) drill core 146; and (D) drill core 136. Numbers in parentheses are orthopyroxene percentage. or soon after their emplacement in the magma), Evidence for loss of Na, K, and locally Ca from xenoliths, as well as Si and A1, suggests that vapor-phase transport may have been important in element transfer. Many workers (e.g., Maury and Bizouard, 1974', Ferry, 1982) have noted that Na, K and Ca may be transported within a high-temperature fluid phase. During dehydration ofxenoliths the amount of H20 liberated could transport the amounts of Na and K thought to have been released from the xenoliths. However, calculations of the amount of Si which could be transported via a fluid phase suggest that sufficient Si could not be transported. A magma column of dimensions 100 m X 3 0 m X 3 0 m containing 20% xenoliths was utilized for the mass-balance computations. A total of 4.5.106 kg of SiO2 and 3.37.106 kg of H,O were removed from the xenoliths based on analyses of unmetamorphosed sediments. According to solubility data of Walther and

Helgeson (1977), only 0.7% of the transferred Si could be accounted for by vapor-phase transfer. A second possible transport mechanism involves transfer through an intergranular fluid phase. Such a mechanism circumvents the material-balance problems associated with direct transfer of a fluid. Water of dehydration has been liberated by the xenoliths, but material could also have been transported through such a fluid, provided that diffusion was rapid enough. SiO2 diffusivities in aqueous solutions at 550°C and 1 kbar water pressure measured by Ildefonse and Gabis (1976) and those estimated by Brady (1983) are less than 10 - 4 c m 2 S - i . In order to transport the amount of SiO: lost from the xenoliths in reasonable time periods, D-values would have to have been considerably larger. The combination of high temperature and large differences in/ZSlO~-values may have caused a rapid transfer of SiO~ through a standing pore fluid. However, the required diffusivity is much larger than any experimentally determined values would suggest, and renders diffusion through an intergranular fluid an unlikely transport mechanism. The mechanism most frequently considered for element exchange between pelitic inclusions and marie magma is partial melting accompanied by diffusive transport, or extraction of the melt followed by magma mixing (e.g., Leake and Skirrow, 1960; Barker, 1964; Evans, 1964: Gribble and O'Hara, 1967). We have evaluated the likelihood of partial melting of country rock xenoliths using the mass-balance approach of MacRae and Nesbitt (1980). The average mildly metamorphosed Virginia Formation argillite from drill hole 24981 was used as a starting composition. Two possible minimum melt compositions were utilized, one based on the experimental results of Hoffer and Grant (1980), and the other that of MacRae and Nesbitt (1980) (S-type granite of White and Chappell, 1977). Subtraction calculations (Table 4) suggest that approximately 55% partial melting is required to achieve a restite composition similar in SiO2 content to that of the xenoliths. Partial melting of this magnitude results in complete elimination of the Na20 component using the melt of Hoffer and Grant (1980). Results indicate enrichments in FeD, MgO and A1203, and are consistent with the relative mobility estimates derived from Gresens' (1967) approach. Our calculations do not take into account the possibility of compositional changes

155 T~BLE 4 S u m m a r y of partial melting mass balance c o m p u t a t i o n s (values in wt.%: S a n d C free basis) Ave. Iow-CaO pelite *~

SiC) 3-i(), AI ,()~ FeO MgO ('a() Na4) K x) H ,()

62.1 0.78 16.8 7.24 3.13 0.92 1.99 3.53 2.88

Mineral melt

Restite 55% melting

,4 * 2

B*

.4 *~

B **

Babbitt lowCaO xenolith *~,

72.85 0.27 12.47 1.47 0.30 0.98 2.99 4.66 4.00

71.42 -13.70 1.20 0.28 -5.40 4.03 4.00

48.95 1.40 22.09 14.29 6.58 0.84 0.77 2.15 1.51

50.71 1.73 20.58 14.62 6.62 2.04 0.1)0 2.91 1.51

48.9 1.26 20.86 15.60 8.39 I. 14 1.03 1.50 1.25

*~l)rill Hole 24981. * 'White and Chappell (1977) S-type m i n i m u m melt, normalized to 4.00 wl.% H20. * ~Experimentally produced partial melt of Hoffer and G r a n t ( 1980), normalized to 4.00 wt.% H.,O. *~Subtraction of 55% partial melt of composition A. *~Subtraclion of 55% partial melt of composition B. *"A~erage of 32 analyses.

during fractional melting. Comparison of the composition of the computed restite with that of the average low-CaO pelite suggests that subtraction of the estimated minimum melt compositions do not accurately predict FeO, MgO and K20 values. It appears that the bulk composition of an extracted melt would have been slightly lower in FeO, MgO and Na20, and higher in K20. Part of the compositional discrepancies associated with the mass-balance computations may be related to vapor-phase transfer of elements such as potassium. Substantial quantities of K20 may have been liberated from xenoliths during dehydration, although a decrease in K,O is not detected in footwall rocks that have also been dehydrated (e.g., Bonnichsen, 1972; Rao and Ripley, 1983). Watson (1982) has discussed selective contamination of mafic melts, especially by K20. He attributes this feature to a high diffusivity of K relative to other components, and an initial deviation from a two-liquid equilibrium. It is interesting to note that according to Watson (1982), a basaltic melt which shows little evidence of K20 contamination will most likely not demonstrate evidence of major-element contamination. Such is apparently the case in the Babbitt area, where K20 values of igneous material vary between 0.4 and 1.4 wt.%. White and Chappell (1977) have used SiO2 variation diagrams similar to that of Fig. 7B to illustrate mixing lines between a derived minimum melt and restite. The composition of the source material plots

on the line, with other compositions representing progressive separation of melt and restite. White and Chappell (1977) estimate that a minimum melt derived from a pelitic source rock should have a SiO2 content of nearly 75%, with low FeO and MgO values. Extrapolation of the trend shown in Fig. 7B to less than 1% FeO gives a composition of ~73% SiO2. A potential mixing line is thus generated which passes through the restite (xenoliths) and source (low-grade Virginia Formation) compositions, and renders a reasonable SiO2 value for a minimum melt. Although precise estimates of restite xenolith compositions are difficult to obtain, the computations are strongly suggestive that xenoliths in the Babbitt area have undergone extensive partial melting. Textural evidence for partial melting is observed in those xenoliths where quartz still remains (e.g., drill hole 136, 519-525 m). Figure 11A illustrates the occurrence of interstitial quartz at triple junctions. Adjoining phases are cordierite, plagioclase and K-spar. Extensive microprobe studies confirm that the interstitial phase is composed of only quartz. This feature is intriguing, as it suggests that the composition of the residual liquid remaining after major melt extraction was essentially pure SiO2. Cooling of the intrusive at depth apparently prevented the preservation of glassy material. A second possible textural indicator of partial melting is the corroded nature of biotite ( Fig. 11B), and the occurrence of quartz and orthopyroxene inter-

15~

modified by Powell and Powell (1977). Estimates based on the composition of coexisting potassium feldspar and plagioclase typically yield temperatures in the range of 500-700°C (see Alawi, 1985). These temperatures are low if partial melting occurred, and possibly represent temperatures of reequilibration achieved after melt extraction. Temperature estimates may also be low due to uncertainties in computing the activity of albite, and failure to consider order-disorder relationships. In contrast, simplified heat-transfer calculations based on models outlined by Carslaw and Jaeger (1959) suggest much higher temperatures. For example, using a shale heat diffusivity of 4 . 1 0 - ~ cm 2 s- ~, a xenolith of 6 m radius would achieve the temperature of the surrounding melt ( 1000-1100 ° C) within five years. Only the largest xenoliths in the Babbitt deposit would resist complete heating prior to crystallization of the enclosing magma. However, heattransfer computations are far more complex than this example, due especially to the locally large proportion of xenoliths in the melt, and uncertainties in the effective magma/xenolith ratio.

Summary of chemical studies

Fig. 11. ~,. Photomicrograph showing interstitial quartz (Q) in a lo~-('aO pelitic xenolith t'rom the Babbitt area. Adjoining minerals arc cordierite, plagioclase and K-spar. Sample ~idlh is 0.85 ram. Reflected, plane-polarized light, carboncoated section. Sample 136-1708. B. Photomicrograph shov, ing highly corroded, cavernous nature ofpartialb melted biotite. Sample width is 0.85 ram. Reflected, plane-polarized light, carbon-coated section. Sample 136-I 708.

growths within biotite. The assemblage may be indicative of dehydration melting of biotite via reactions similar to (Thompson, 1982: J.A. Grant, 1985): qtz + biot + v a p o r ~ o p x + liq With respect to possible partial melting, estimates of temperatures achieved by the xenoliths provide disparate results. Temperatures reached by the xenoliths were estimated using the two-feldspar thermometer developed by Stormer (1975) and

Bulk-rock chemical studies of count ry rocks, xenoliths and igneous rocks of the Babbitt deposit indicate that there has been extensive interaction between magma and country rocks. Xenoliths have experienced an appreciable loss of volatiles, silica, alumina and alkalis. Vapor-phase transport and diffusion through an intergranular fluid cannot account for the mass of material transferred. Extensive partial melting of xenoliths and efficient melt extraction provides the most viable mechanism for element transfer. The lack of concentration gradients around xenoliths, or the presence of compositionally anomalous igneous material suggests that the partial melt was extracted at a time when physical mixing and homogenization were possible in the melt. Addition of the SiO~ lost from xenoliths would cause only a 2-3% increase in overall SiO~ abundance within a representative magma volume. Compositional data from this study suggest that partial melting and magma contamination must have occurred during the early stages of magma emplacement or prior to final intrusion. Partial melting and devolatilization of roof rocks may have

157 commenced before blocks began to sink in the chamber. Convection in the chamber could have lead to mixing and homogenization of the contaminated melt. In this model chemical interaction between the xenoliths and m a g m a would have ceased long before final accumulation near the base of the intrusive pulse. Upward rafting of country rock fragments from the floor of the shallow m a g m a chamber is considered unlikely. Specific density differences between country rock hornfels ( p = 2 . 7 - 3 . 1 ) and mafic melts ( p = 2 . 7 - 3 . 0 ; e.g., Huppert and Sparks, 1980; Chalokwu, 1985) are not sufficient to permit floating of footwall material. It is possible that xenoliths may have been desilicified at depth when in contact with an auxiliary magma chamber, or perhaps when as part of conduit walls. Although chemical homogenization of the melt could be established by such processes, it is unlikely that magma emplacement was turbulent enough to carry from depth the larger xenoliths in the Babbitt area.

Country rock contamination and sulfide saturation Irvine ( 1975 ) has demonstrated that addition of Si to a mafic m a g m a may cause sulfide saturation. Assimilation of felsic country rock has also been cited as the primary cause of sulfide saturation of the host m a g m a for the Sudbury deposits (Naldrett, 1981, 1984). Results of this study suggest that assimilation of Si from xenoliths has occurred in the Babbitt area, and that desilicification was initiated early in the magmatic crystallization history, at a time when magma mixing promoted widespread chemical homogeneity. For this reason it is difficult to discount the premise that saturation in sulfur was achieved principally due to the introduction of a siliceous country-rock-derived partial melt. In fact, Ripley and Alawi (1986) propose, based on sulfide distribution and composition, olivine composition and isotopic data, that sulfide saturation must have been achieved either prior to or during m a g m a emplacement. Hence, relatively early Si assimilation is consistent with the host melt being saturated in sulfide early in its evolution. The copper-rich nature of the sulfide assemblages at Babbitt (Cu/Ni from ~ 4 to 10) indicates that given sufficient metal content in the magma, the timing of sulfide liquation is not of major impor-

tance in controlling ore composition. This is a function of copper behaving as an incompatible element, with strong preference for the sulfide phase (e.g., Rajamani and Naldrett, 1978). In other words, even if sulfide saturation were achieved very late in the crystallization history of the melt, sulfides enriched in copper would still be produced. Methods of enriching a melt in copper due to fractional crystallization and magma blending, and required magma/sulfide ratios are discussed in Rao and Ripley (1983) and Ripley (1986). Perhaps the most obvious, and crucial, feature related to ore genesis in the Duluth Complex is sulfur introduction. Assimilation of sulfur derived from country rock devolatilization is a much more important process than assimilation of major elements such as silica. Although the incorporation of a granitic partial melt derived from xenoliths could lead to a decrease in sulfide solubility, there is no evidence which suggests that silica introduction was in any way required to promote sulfide liquation. The erratic distribution of sulfide mineralization at both the Dunka Road (Rao and Ripley, 1983) and Babbitt (Ripley and Alawi, 1986) deposits is more consistent with the attainment of sulfide saturation in the magmas being caused by mass action effects associated with sulfur assimilation and/or temperature decreases.

Acknowledgements Appreciation is expressed to Mr. Stan Watowich, now of Gold Fields Exploration and formerly of AMAX, for his valuable assistance throughout much of our research at the Babbitt deposit. Thanks also go to Mr. Jack Malcolm for his logistical support during our stays in the Babbitt area. We thank Mr. Cedric Iverson for supplying samples of the lowgrade Virginia Formation from the United States Steel Corp. drill core, and for his encouragement of our research efforts over the years. Permission to publish this work has been granted by Kennecott Copper Company. Drs. D.G. Towell, E. Merino and P. Ortoleva reviewed an earlier draft of this manuscript; their comments are appreciated. Research on the Babbitt Cu-Ni deposit has been supported through NSF Grants EAR-8108536 and EAR8312744.

158

References

Alawi, J.A., 1985. Petrography, sulfide mineralogy and distribution, mass transfer, and chemical evolution of the Babbitt Cu-Ni deposit, Duluth Complex, Minnesota, Ph.D. Thesis, Indiana University, Bloomington, Ind., 350 pp. Barker, F., 1964. Reaction between basic magmas and pelitic schists, Cortland, New York. Am. J. Sci., 262:615-634. Bonnichsen, B., 1972. Southern p a n of the Duluth Complex. In: P.K. Sims and G.B. Morey (Editors), Geology of Minnesota, A Centennial Volume. Minn. Geol. Surv., St. Paul, Minn., pp. 361-388. Bonnichsen, B., Fukui, L.M. and Chang, L.L.Y., 1980. Geologic setting, mineralogy, and geochemistry of magmatic sulfides, South Kawishiwi intrusion, Duluth Complex, Minnesota. In: J.D. Ridge (Editor), Proceedings Fifth Quadrennial IAGOD Symposium 1. Schweizerbart, Stuttgart, pp. 545-565. Boucher, M.L., 1975. Copper-nickel mineralization in a drill core from the Duluth Complex of northern Minnesota. U.S. Bur. Min., Rep. Invest. No. 8084. Brady, J.B., 1983. Intergranular diffusion in metamorphic rocks. Am. J. Sci., 283(A): 181-200. Campbell, I.H., Naldrett, A.J. and Barnes, S.J., 1983. A model for the origin of the platinum-rich sulfide horizons in the Bushveld and Stillwater Complexes. J. Petrol., 24: 133-165. Carslaw, H.S. and Jaeger, J.C., 1959. Conduction of Heat in Solids. Oxford University Press, London, 2nd ed., 510 pp. Chalokwu, C.I., 1985. Chemical, petrochemical, and compositional study of the Partridge River intrusion, Duluth Complex, Minnesota. Ph.D. Thesis, Miami University, Oxford, Ohio. Chalokwu, C.I. and Grant, N.K., 1983. The importance of trapped liquid abundance to the re-equilibration of primary mineral compositions in the Partridge River troctolite, Duluth Complex, Minnesota. Geol. Soc. Am., Abstr., 15: 541. Coish, R.A.. 1977. Ocean floor metamorphism in the Betls Cove Ophiolite, Newfoundland. Contrib. Mineral. Petrol., 60: 255-270. Cooper, R.W., Morey, G.B. and Weiblen, P.W., 1978. Topographic and aeoromagnetic lineaments and their relationship to bedrock geology in glaciated Preambrian terrane, northeastern Minnesota. In: D.W. O'Leary and J.L. Earle (Editors), Proc. 3rd Int. Conf. on Basement Tectonics, Denver, Colo., pp. 137-148. Crank, J., 1975. The Mathematics of Diffusion. Oxford University Press, London, 2nd ed., 414 pp. Davidson, Jr., D.M., 1972. Eastern part of the Duluth Complex. In: P.K. Sims and G.B. Morey (Editors), Geology of Minnesota: A Centennial Volume. Minn. Geol. Surv., Spec. Vol., pp. 354-360. Duke, J.M. and Naldrett, A.J., 1978. A numerical model of the fractionation of olivine and molten sulfide from komatiite magma. Earth Planet. Sci. Lett., 39: 255-266. Eggler, D.H., 1972. Water-saturated and undersaturated melting relations in a paracutin andesite and an estimate

of water content in natural magma. Contrib. Mineral. Petrol., 34: 261-271. Evans, B.W., 1964. Fractionation of elements in the pelitic hornfelses of the Cashel-kough Wheelaun intrusion, Connemara, Eire. Geochim. Cosmochim. Acta, 28:127-156. Ferry, J.M., 1982. A comparative geochemical study of petitic schists and metamorphosed carbonate rocks from South-Central Maine. Contrib. Mineral. Petrol., 80: 59-72. Foose, M.P., 1982. Structural, stratigraphic, and geochemical features of the South Kawishiwi intrusion, Duluth Complex, Minnesota. Geol. Soc. Am., Abstr. Prog., 14: 490. Grant, J.A., 1985. Phase equilibria in low-pressure partial melting of pelitic rocks. Am. J. Sci., 285: 409-435. Grant, N.K. and Moiling, P.A., 1981. A strontium isotope and trace-element profile through the Partridge River troctolite, Duluth Complex, Minnesota. Contrib. Mineral. Petrol., 77: 296-305. Green, J.C., 1977. Keweenawan plateau volcanism in the Lake Superior region. In: W.R.A. Baragar, L.C. Coleman and J.M. Hall (Editors), Volcanic Regimes in Canada. Geol. Assoc. Can., Spec. Pap., 16: 407-422. Gresens, R.L.. 1967. Composition-volume relationships of metasomatism. Chem. Geol., 2: 47-65. Gribble, C.D. and O'Hara, M.J., 1967. Interaction of basic magma with pelitic materials. Nature (London), 314: 1198-1201. Hoffer, E. and Grant, J.A., 1980. Experimental investigation of the formation of cordierite-orthopyroxene paragenesis in pelitic rocks. Contrib. Mineral. Petrol., 73:15-22. Huppert, H.H. and Sparks, R.S.J., 1980. The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense, ultrabasic magma. Contrib. Mineral. Petrol., 75: 279-289. lldefonse, J.P. and Gabis, V., 1976. Experimental study of silica diffusion during metasomatic reactions in the presence of water at 550°C and 1000 bars. Geochim. Cosmochim. Acta, 40: 297-303. Irvine, T.N., 1975. Origin ofchromitite layers and similar deposits of other magmatic ores. Geochim. Cosmochim. Acta, 39: 991-1020. Irvine, T.N., 1980. Magmatic infiltration metasomatism, double diffusive fractional cyrstallization, and adcumulus growth in the Muskox intrusion and other layered intrusions. In: R.B. Hargraves (Editor), Physics of Magmatic Processes. Princeton University Press, Princeton, N.J., pp. 325-383. Lapotka, T.C., Papike, J.J., Vaniman, D.T. and Morey, G.B., 1981. Petrology of contact metamorphosed argillite from the Rove Formation, Gunflint Trail, Minnesota. Am. Mineral., 66: 70-80. Lapotka, T.C., White, C.E. and Papike, J.J., 1984. The evolution of water in the contact-metamorphic aureole of the Duluth Complex, northeastern Minnesota. Gol. Soc. Am. Bull., 95:788-804 Leake, B.E. and Skirrow, G., 1960. The pelitic hornfelses of the Cashel-Lough Wheelaun intrusion, Co. Galway, Eire. J. Geol., 68: 23-40. MacRae, N.D. and Nesbitt, H.W., 1980. Partial melting of common metasedimentary rocks: A mass balance approach. Contrib. Mineral. Petrol., 75: 21-26.

159 Mainwaring. P.R. and Naldrett, A.J., 1977. Country rock assimilation and genesis of Cu-Ni sulfides in the Water Hen intrusion, Duluth Complex, Minnesota. Econ. Geol., 72: 1269-1284. Maury, R.C. and Bizouard, H., 1974. Melting of acid xenoliths into a basanite: an approach to the possible mechanism of crustal contamination. Contrib. Mineral. Petrol., 48: 275-286. Morse, S.A., 1979a. Kiglapait geochemistry, I. Systematics. sampling, and density. J. Petrol., 20: 555-590. Morse, S.A., 1979b. Reaction constants for En-Fo-Sil equilibria: An adjustment and some applications. Am. J. Sci., 279: 1060-1069. Naldrett, A.J,, 1981. Nickel sulfide deposits: classification, composition, and genesis. Econ. Geol., 75th Anniv. Vol.. pp. 628-685. Naldrett. A.J.. 1984. Summary, discussion, and synthesis. In: E.G. Pye, A.J. Naldrett and P.E. Giblin (Editors), The Geology and Ore Deposits of the Sudbury Structure, Ontario. Can. Geol. Surv.. Spec. Vol., 1: 533-571. Pasteris, J.D., 1984. Further interpretation of the Cu-Fe-Ni sulfide mineralization in the Duluth Complex, northeastern Minnesota. Can. Mineral., 22: 39-53. Pasteris, J.D., 1985. Temperature-oxygen fugacity relationships among Fe-Ti oxides in the Duluth Complex and their petrologic implications. Can. Mineral., 22: 39-53. Powell, M. and Powell, R., 1977. Plagioclase-alkali-feldspar geothermometry revisited. Mineral. Mag., 41: 253-256. Raedeke. L.D. and McCallum, I.S., 1980. A comparison of fractionation trends in the lunar crust and the Stillwater Complex. In: J.J. Papike and R.B. Merill (Editors), Proc. Conf Lunar Highlands Crust. pp. 133-153. Rajamani, V. and Naldrett, A.J., 1978. Partitioning of Fe, Co, Ni, and Cu between sulfide liquid and basaltic melts and the composition of Ni-Cu sulfide deposits. Econ. Geol., 73: 32-93. Rao, B.V. and Ripley, E.M., 1983. Petrochemical studies of the Dunka Road Cu-Ni deposit, Duluth Complex, Minnesota. Econ. Geol., 78: 1222-1238. Ripley, E.M., 1981. Sulfur isotopic studies of the Dunka Road Cu-Ni deposit, Duluth Complex, Minnesota. Econ. Geol., 76: 610-620. Ripley, E.M., 1986. Origin and concentration mechanism of copper and nickel in the Duluth Complex sulfide zones - A dilemma. Econ. Geol., 81: 974-979. Ripley, E.M. and Alawi. J., 1986. Sulfide mineralogy and chemical evolution of the Babbitt Cu-Ni deposit, Duluth Complex, Minnesota. Can. Mineral., 24: 347-368. Ripley, E.M. and AI-Jassar, T.J., 1987. Sulfur and oxygen iso-

topic studies of melt-country rock interaction, Babbitt CuNi deposit, Duluth Complex, Minnesota. Econ. Geol., 82: 87-108. Ripley, E.M. and Andrews, M.S., 1985. Devolatilization equilibria in the genesis of the Dunka Road Cu-Ni deposit, Duluth Complex, Minnesota. Geol. Soc. Am.. Abstr., 17: 700. Roeder, P.L. and Emslie, R.F., 1970. Olivine-liquid equilibrium. Contrib. Mineral. Petrol., 29: 275-289. Schulz, K.J., 1977. The petrology and geochemistry of Archean volcanics, Western Vermilion district, northeastern Minnesota. Ph.D. Thesis, University of Minnesota, Minneapolis, Minn. Shaw, D.M., 1956. Geochemistry' of pelitic rocks, P a n III. Major elements and general geochemistry. Geol. Soc. Am. Bull., 67:919-934. Stormer, Jr., J.C., 1975. A practical two-feldspar geothermometer. Am. Mineral., 80: 667-674. Thompson, A.B., 1982. Dehydration melting of pelitic rocks and the generation of H20-undersaturated granitic liquids. Am. J. Sci., 282: 1567-1595. Tyson, R.M. and Chang, L.L.Y., 1984. The petrology of sulfide mineralization of the Partridge River troctolite, Duluth Complex, Minnesota. Can. Mineral., 22: 23-38. Walther, J.V. and Helgeson, H.C., 1977. Calculation of the thermodynamic properties of aqueous silica and solubility of quartz and its polymorphs at high pressures and temperatures. Am. J. Sci., 277: 1315-1351. Watson, E.B., 1982. Basalt contamination by continental crust: Some experiments and models. Contrib. Mineral. Petrol., 80: 73-87. Weiblen, P.W., 1982. Keweenawan intrusive igneous rocks. In: R.J. Wold and W.J. Hinze (Editors), Geology and Tectonics of the Lake Superior Basin. Geol. Soc. Am., Mem., 156: 57-82. Weiblen, P.W. and Morey, G.B., 1976. Textural and compositional characteristics of the sulfide ores from the basal contact zone of the South Kawishiwi intrusion, Duluth complex, northeastern Minnesota. Proc. 37th Annu. Mining Symp. (Minn. Geol. Surv., Repr. Ser. 32). Weiblen, P.W. and Morey, G.B., 1980. A summary of the stratigraphy, petrology, and structure of the Duluth Complex, Minnesota. Am. J. Sci., 280A: 88-133. White, A.J.R. and Chappell, B.W., 1977. Ultrametamorphism and granitoid genesis. Tectonophysics, 43: 7-22. Wood, D.A., Gibson, I.L. and Thompson, R.N., 1976. Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland. Contrib. Mineral. Petrol., 55: 241-254.