Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the...

PII S0016-7037(99)00083-6

Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneaththe Duluth Complex, Minnesota

YOUNG-ROK PARK,1 EDWARD M. RIPLEY,*,1 MARK SEVERSON,2 and STEVEN HAUCK2

1Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA2Minnesota Natural Resources Research Institute, University of Minnesota, Duluth, Duluth, MN 55811, USA

(Received July6, 1998;accepted in revised form February11, 1999)

Abstract—Thin (3–20 m), picritic to noritic sills are found in the Proterozoic metasedimentary footwall rocksof the Duluth Complex, Minnesota. The sills were emplaced prior to the major intrusions that comprise theDuluth Complex, and underwent contact metamorphism along with the country rocks. Oxygen, hydrogen, andsulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals, melts,and hydrothermal fluids in the high temperature environment below the major plutonic bodies of theMidcontinent Rift system. The pre-Duluth Complex sills exhibit a range ind18O values from 4.9‰ to 14.8‰,with values between 6‰ and 7‰ generally found in sill interiors. Highd18O values near sill contacts withhigh 18O metasedimentary rocks of the pelitic Virginia Formation or Biwabik Iron Formation, coupled witha smooth sigmoidal isotopic profile centered at the contact, suggest that oxygen diffusion was an importantexchange mechanism. The elevatedd18O values near the center of the thickest sills are thought to reflect theemplacement of isotopically contaminated basaltic magma. Dehydration reactions in the pelitic rocks of thecontact aureole liberated high18O fluids that enhanced subsolidus diffusive exchange. Advective displacementof the diffusion profiles toward the sill interior is less than 40 cm, and suggests that layer parallel flowdominated in the dehydration of the contact aureole. Elevated, but uniformd18 O values (9.7‰ to 10.5‰) inthinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity,perhaps related to extensive development of microcracks. AlthoughdD values of the pelitic country rocksrecord a history of dehydration, systematic variations ofdD (264‰ to2143‰) and H2O (0.15 to 5.40 wt.%)content are not found in the sills.d18O values of coexisting plagioclase and pyroxene from the sills indicatea close approach to isotopic equilibrium, and are consistent with a diffusion-dominant exchange process attemperatures near 500°C. Results of diffusion modelling suggest a duration of isotopic exchange that mayhave extended from tens of thousands of years to 1.4 Ma, depending on local controls of porosity andpermeability, as well as rates of fluid production in the contact aureole.

Localized areas of18O and D depletion in the sills (values as low as 4.9‰ and2143‰, respectively) denoteexchange with meteoric water after interaction with the high18O metamorphic fluids in the contact aureole.Although all of the elevated18O samples in the contact environment may have suffered18O depletion, mostexchange with meteoric water appears to be spatially localized, and is thought to reflect highly channelized,fracture-controlled fluid flow. Sulfur isotopic values of the sills are variable (22.7‰ to 11.2‰), and indicativeof an evolution involving pre-emplacement contamination of basaltic magma, and sub-solidus exchange withan H2S-bearing metamorphic fluid. Sulfur contents exceed 3.0 wt.% only within troctolitic to melatroctoliticsills, andd34S values of 7.8‰ to 8.3‰ are strongly suggestive of pre-emplacement contamination by sulfurderived from a Proterozoic sedimentary unit. Highd18O and d34S rocks, particularly at sill margins, areconsistent with either hydrothermal precipitation of fine-grained sulfide minerals, or isotopic exchangebetween magmatic sulfides and an H2S-bearing metamorphic fluid.Copyright © 1999 Elsevier Science Ltd

1. INTRODUCTION

Voluminous flood basalts and intrusive rocks of the LakeSuperior region are thought to have been produced by thecoincidence of intracontinental rifting and the presence of amantle plume (Green, 1983; Hutchinson et al., 1990; Nicholsonand Shirey, 1990; Cannon, 1992). Thermal anomalies associ-ated with the emplacement of large volumes of mafic magmainto the crust are expected to initiate fluid convection in thecountry rocks. Several stable isotopic studies have focused onthe development of flow systems in oceanic rift environments(e.g. Gregory and Taylor, 1981; Bowers and Taylor, 1985; Altet al., 1986), and continental-oceanic rift transitions, mainly of

the North Atlantic Igneous Province (e.g. Sveinbjornsdottir etal., 1986; Forrester and Taylor, 1976, 1977; Taylor and For-rester, 1979; Fehlhaber and Bird, 1991; Manning and Bird,1991; Brandiss et al., 1995). Stable isotopic studies of intra-continental rift systems where an ocean basin has not devel-oped are few. Although in the intracontinental setting of the 1.1Ga Midcontinent Rift system meteoric water and “basinal”brines are expected to have been the primary fluid types,seawater may have been of local importance where restricted“arms” of the sea may have developed during protracted ex-tension. In addition, hydrothermal fluids may have also beenderived during metamorphic reactions and crystallization ofmagmatic bodies. Previous studies dealing with the origin ofbasalt-hosted native copper mineralization in the BerglandGroup in Michigan (south shore of western Lake Superior) byLivnat (1983), Jolly (1974), and Bornhorst and Woodruff

*Author to whom correspondence should be addressed ([email protected]).

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Geochimica et Cosmochimica Acta, Vol. 63, No. 5, pp. 657–674, 1999Copyright © 1999 Elsevier Science LtdPrinted in the USA. All rights reserved

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657

(1997) have emphasized the role of hydrothermal fluids in thealteration and mineralization of the volcanic rocks. Althoughnative copper deposition in the North Shore Volcanic Group ofMinnesota (north shore of western Lake Superior) is consider-ably less extensive, the occurrence of abundant amygdules atflow tops and bottoms testifies to the presence of hydrothermalfluids. Schmidt (1990, 1993), Schmidt and Green (1992), andSchmidt and Robinson (1997) have detailed the mineralogicalvariation and burial metamorphic zones present in the NorthShore Volcanic Group. The emplacement of the multiple,sheet-like, mafic intrusions that comprise the Duluth Complexnear the contact of the North Shore Volcanic Group withunderlying Proterozoic and Archean sedimentary and igneousrocks has produced an opportunity to examine fossil hydrother-mal systems that may have developed within contrasting rocktypes above and below the plutonic rocks.

Park and Ripley (1996, 1997) have previously reported onreconnaissance isotopic studies of the hydrothermal systemdeveloped in the North Shore Volcanic Group above the DuluthComplex. A more detailed treatment of this work is the subjectof a companion paper. We here report on the oxygen, hydrogen,

and sulfur isotopic compositions of Proterozoic metasedimen-tary rocks and sills located in the footwall of the Complex. Thesills pre-date the emplacement of the major portions of theDuluth Complex, and show isotopic effects that reflect igneous,metamorphic, and hydrothermal processes. The isotopic dataare used to evaluate the relative importance of melt countryrock and hydrothermal fluid-country rock interaction that oc-curred in the high temperature environment beneath a majorplutonic complex.

2. REGIONAL AND LOCAL GEOLOGY

The Midcontinent Rift system is a large extensional featurethat extends from Kansas northward to beyond Lake Superiorand southeast beneath the Michigan Basin (Van Schmus, 1992;Van Schmus and Hinze, 1985; Van Schmus et al., 1987).Recent seismic studies have shown that over 20 km of volcanicrocks and 10 km of sedimentary rocks occur in the deepestportion of the rift (Behrendt et al., 1988; Cannon et al., 1989;Allen et al., 1997). Volcanic rocks are preserved primarilyalong the margins of the rift. In the Lake Superior area, volca-

Fig. 1. Geologic map of the Babbitt area showing location of drill cores sampled in this study.

658 Y. R. Park et al.

nic rocks are exposed on the northwest and south shores. Thevolcanic rocks are composed primarily of tholeiitic basalts,with up to 25% rhyolite flows in the North Shore VolcanicGroup, and lesser amounts of rhyolite in the Keweenawanrocks along the southern shore (Vervoort and Green, 1997).Troctolitic, gabbroic, and anorthositic rocks of the DuluthComplex constitute the major plutonic portion of the rift sys-tem. Several discrete layered intrusions have been identified,many of which intrude anorthositic rocks, as well as oneanother (Miller and Ripley, 1996). Many of the layered intru-sions are sheet like in form, with a resulting “stacked” igneousstratigraphy. The growth of the plutonic sequence due to mul-tiple magma inputs could also have led to the development ofdiscrete, and possibly overlapping, hydrothermal circulationsystems.

The Duluth Complex was emplaced along the unconformitybetween the North Shore Volcanic Group and the underlyingProterozoic and Archean rocks. In the area of this study (Bab-bitt Cu-Ni deposit to just south of Birch Lake, Fig. 1), theimmediate footwall country rocks are those of the early Pro-terozoic (;1.8 Ga) Virginia Formation, underlain by the Bi-wabik Iron Formation. The Virginia Formation consists ofinterbedded argillites and graywackes, and minor limestonesand chert (Lucente and Morey, 1983). The known thickness ofthe Virginia Formation exceeds 500 m to the southwest of thestudy area, but the sequence thins northward. In the study area,large xenoliths of the Virginia Formation occur in the lower-most troctolitic units of the Duluth Complex, and render evi-dence pertaining to the mechanism of intrusive emplacement.In the Babbitt area the Virginia Formation ranges in thicknessfrom 0 to 180 m. Contact metamorphism has converted thepelitic rock types into cordierite-biotite-plagioclase6 orthopy-roxene hornfels, and carbonate layers to calc-silicates (Ripleyand Alawi, 1988). The Biwabik Iron Formation consists ofalternating layers of iron bearing chert, iron oxides, and slatesthat have been divided into four members referred to as UpperSlaty, Upper Cherty, Lower Slaty and Lower Cherty. The IronFormation varies in thickness, but is less than 120 m in the area

of the Babbitt Cu-Ni deposit. Contact metamorphism of theBiwabik Iron formation has resulted in the formation of horn-fels containing iron rich pyroxenes and amphiboles, as well asiron oxides (Bonnichsen, 1975).

Pre-Duluth Complex sills occur in both the Virginia Forma-tion and the Biwabik Iron Formation (Severson, 1994). Hauckand coauthors. (1996) have divided these sills into two types.One is a Cr-poor (,600 ppm) tholeiite that bears geochemicalsimilarities to the Logan sills which intrude rocks of the RoveFormation (Virginia Formation equivalent) and Gunflint IronFormation (Weiblen et al., 1972) at the northern margin of theComplex (Fig. 1). Hauck and coauthors (1997) refer to thesesills as “Logan-type”, with no implication with respect to agesimilarities. The other is a Cr-rich (.600 ppm) tholeiite thatHauck and coauthors (1996) suggest is part of an early feedersystem for the overlying basaltic lavas. The Cr-rich sills occurprimarily in the Virginia Formation and are found in type twotextural varieties. One type is a fine grained, massive, graycolored unit (“massive gray” units of Severson et al., 1996) thatgenerally appears to be the outer marginal zone of a com-posite sill (Fig. 2). The central or interior zones of these sillsare coarser grained, green in color, and rich in olivine andlocally hornblende. Thicknesses of the high-Cr sills in theVirginia Formation range from 0.15 to 60 m (Severson et al.,1996). The Cr-poor Logan type sill occurs near the top of theBiwabik Iron Formation where it varies in thickness from;0.6to 5.5 m (Fig. 3).

Each of these sill types may contain phenocrysts of bothplagioclase and pyroxene, and show a granoblastic textureproduced during contact metamorphism by the Duluth Com-plex. Hornblende (up to 20 volume %) locally characterizes thefine grained zones of the high-Cr sills. Similiar abundances ofhornblende may occur in the coarser grained areas of thehigh-Cr sills, which are also olivine-rich (locally up to 80%)and usually strongly serpentinized. The olivine-rich zones aresimilar to the melatrotolite layers found in the South Kawishiwiand Partridge River intrusions, and are locally sulfide rich (3.4to 3.6 wt.% s (sulfur)). Orthopyroxene is the dominant ferro-

Fig. 2. Schematic diagram showing the relationship between Duluth Complex, metasedimentary footwall rocks (VirginiaFormation, Biwabik Iron Formation), and pre-Complex sills (Cr-rich and Cr-poor Logan-type). Modified from Severson etal. (1996)

659Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks

magnesian mineral in the fine grained zones of the high Cr sillsand in the Cr-poor sill, both of which would be classified asnorite (the orthopyroxene is not a metamorphic reaction prod-uct). Orthopyroxene occurs in both ophitic and granular form.Optically clear plagioclase in both sill types is normally in therange of An 50 to 60, but turbid varieties range from An 20 to6. Biotite is a common accessory mineral in both sill varieties;secondary alteration minerals are found locally and includeserpentine, chlorite, actinolite, epidote, prehnite, calcite, andalbite. Mineralogical alteration varies in intensity within bothsill types, but together with isotopic evidence strongly point toan interaction with a hydrothermal fluid after, or perhaps duringemplacement of the overlying intrusions of the Duluth Com-plex. Representative modal analyses of both sill types are givenin Table 1.

3. PREVIOUS STUDIES

Both of the Duluth Complex intrusions that occur in contactwith the Virginia and Biwabik Iron Formations (the PartridgeRiver Intrusion in the Babbitt area and the South KawishiwiIntrusion to the north) show mineralogical and isotopic evi-dence for limited hydrothermal alteration. Alteration mineralspresent in the troctolitic to gabbroic rocks include chlorite,epidote, albite, amphibole, prehnite, serpentine, and calcite(e.g. Tyson, 1979; Chalokwu, 1985; Sassani and Pasteris, 1988;Ripley and Alawi, 1988; Ripley et al., 1993). Mineralogic andstable isotopic studies (Ripley and Al-Jassar, 1986; Ripley andAlawi, 1988; Ripley et al., 1992; Ripley et al., 1993) haveindicated that devolatilization reactions in Virginia Formationxenoliths and footwall rocks have liberated large volumes of a

Fig. 3. Stratigraphic columns showing Duluth Complex, metasedimentary footwall rocks, and pre-Complex sills in drillcores B1-360, B1-357, B1-255, B1-331, B1-140, B1-131, B1-148, B1-119, B1-249, B1-171, B1-179, and B2-5. Depth tothe contact between Duluth Complex and metasedimentary footwall rock (Virginia Formation) from surface is shown foreach drill core.

Table 1. Average modal abundances for various types of pre-Duluth Complex sills.

Olivine Plagioclase Clinopyroxene Orthopyroxene AmphiboleBiotite 1

oxides1 sulfides

Cr-rich, coarse-grained 25–50 30–60 5 0–30 10Cr-rich, fine-grained 50 20 20 10Cr-poor 55 30 15


CO2-CH4-H2S-H2O fluid. In addition, evidence exists for theinvolvement of late stage magmatic fluids in pervasive chlorite-actinolite-serpentine alteration zones, and meteoric H2O inlocalized highly serpentinized melatroctolites. Fluid inclusionstudies by Pasteris and coworkers, (1995) also suggest thatfluids of distinct origins interacted with the Duluth Complexover an extended period of magmatic crystallization, cooling,and subsolidus re-equilibration.

Oxygen isotopic studies by Ripley and Al-Jassar (1987)indicated that zones of elevatedd18O values occur in closeproximity to Virginia Formation xenoliths. Depletions in18O atxenolith margins and enrichment of18O in the surroundinggabbroic material was modelled as a diffusion process at sub-solidus temperatures. Lee and Ripley (1996) have identifiedmagmatic pulses in the South Kawishiwi intrusion that arecharacterized by elevatedd18O values, but lack country rockxenoliths. They have attributed the enrichment in18O to con-tamination by country rock in sub-volcanic magma chambers.

4. SAMPLING AND ANALYTICAL METHODS

Samples for this study were collected from both outcrops and drillcores provided from the Natural Resources Research Institute at theUniversity of Minnesota-Duluth. In the Babbitt to Birch Lake area drillcores B1-119, B1-131, B1-148, B1-171, B1-179, B1-249, B1-255,B1-331, B1-357, B1-360, and B2-5 were sampled (Fig. 1). Sampleswere slabbed for thin sections, and aliquots prepared for oxygen,hydrogen, and sulfur isotopic analysis. Whole rock powders werecrushed to;100 mesh, and stored in a vacuum oven prior to analysis.Mineral separates of plagioclase and pyroxene were prepared by mag-netic separation using a Frantz separator, followed by hand picking.Purity was in excess of 98% for plagioclase and 95% for pyroxene.Plagioclase phenocrysts from the sawed slabs were also microdrilledusing a 0.75 mm diamond drill bit. Samples for oxygen isotopicanalyses were prepared using the BrF5 method of Clayton and Mayeda(1963). Hydrogen for isotopic analysis was liberated by melting wholerock powders in vacuum, followed by zinc reduction of water to H2 insealed quartz tubes using a method slightly modified from that ofVennemann and O’Neil (1993). Water yields were measured using theion gauge of the mass spectrometer following calibration of the inletsystem with standard water samples. Whole rocks were analyzed fol-lowing vacuum combustion with CuO (Fritz et al., 1974; Park andRipley, 1998). Sulfur content was determined by manometer measure-ment of liberated SO2. Oxygen and hydrogen isotopic measurementswere made using either a Finnigan MAT 252 or Delta E stable isotoperatio mass spectrometer. Sulfur isotopic determinations were madeusing a Nuclide 6“ 60° sector instrument. Results are presented instandardd notation, relative to VSMOW for O and H, and CDT for S.NBS-28 quartz and NCSU quartz haved18O values of 9.66 0.2 and11.45 6 0.2‰, respectively, in our laboratory. Hydrogen isotopiccompositions of BA66 (serpentinized komatiite) and KGA-1 (wellcrystallized kaolinite) havedD of 266 6 2 and261 6 2‰, respec-tively.

5. RESULTS

Results of stable isotopic analyses are given in Table 2 andFig. 4. d18O values of rocks from the Virginia Formation andBiwabik Iron Formation range from 8.9‰ to 13.6‰, and10.2‰ to 16.4‰, respectively. The values of the VirginiaFormation are similar to those of unmetamorphosed samplesrecorded by Ripley and Al-Jassar (1987) and Arcuri et al.(1998).d18O values of the pre- Duluth Complex sills show awide range from 4.9‰ to 14.8‰. Coexisting pyroxene andplagioclase from the sills range from 6.5 to 9.6 and 7.4 to11.7‰, respectively. Two samples of microdrilled plagioclasephenocrysts show a range ind18O values from 11.1‰ to 12.7‰

(Table 3). In a given sample,d18O variation with An content issmall. In sample Block 20, fresh plagioclase (An 48-53) rangesfrom 11.5‰ to 11.6‰, whereas altered plagioclase (An 7-18)ranges from 11.2‰ to 11.9‰. A second altered plagioclase (An8-20) phenocryst from the same sample shows ad18O rangebetween 12.1‰ and 12.7‰.

dD values of the pre-Complex sills show a wide range of2143‰ to261‰, with rocks lacking obvious signs of hydro-thermal alteration falling in the more restricted range of266‰to 294‰. Water contents range from 0.15 to 5.48 wt.%. Nosystematic variation between water contents anddD values wasobserved, although all samples withdD less than298‰ arecharacterized by water contents that exceed 1 wt.%.

Spatial variations involvingd18O, dD, and H2O content areillustrated in Fig. 5. Profiles were constructed perpendicular tocountry rock sill contacts in three drill cores. Each profileillustrates a different relationship betweend18O, dD, and H2Ocontent. A Cr-rich sill in the Virginia Formation in drill coreB1-255 (Fig. 5 A) has a thickness of;14.9 m, and is charac-terized byd18O values that progressively increase from valuesof 6.6‰ to 7.7‰ in the interior of the sill to values of 8‰ to9‰ at its margin. In contrast to thed18O values, no spatialtrend is shown by thedD or H2O values. In drill core B2-5,another Cr-rich sill sampled in the Virginia Formation (Fig. 5B) has a thickness of only 2.68 m, and bothdD andd18O valuesshow low values throughout the lower two meters of the silland much higher values near the sill top. H2O content showsthe reverse trend, with values decreasing from;1.2–1.4 wt.%in the lower two meters to 0.6 wt.% at the sill top. In a Cr-poorsill (; 4 m) in the Biwabik Iron Formation from the same drillcore (Fig. 5 C),d18O values show relatively constant elevatedvalues in the range of 9.7‰ to 10.5‰. Water contents areuniformly low throughout the sill, andd18O values are clearlyelevated relative to those considered normal (5.5‰ to 7‰) formafic magmas.

Sulfur isotopic compositions of whole rocks from the Vir-ginia Formation and pre- Duluth Complex sills range from 0.12to 6.1 and22.7 to 11.2‰, respectively (Figs. 6 and 7). Onlythe troctolitic to picritic sills have sulfur abundances that ex-ceed 1 wt.%, and theird34S values are high (7.8‰ to 8.3‰).Low sulfide sills have a wide range ind34S values (see above).Two features of thed34S data are particularly noteworthy. First,virtually all low-sulfide rocks in the Duluth Complex (PartridgeRiver Intrusion, South Kawishiwi Intrusion, Anorthositic Se-ries rocks) haved34S values near 0‰ (Ripley, 1981; Ripley andAl-Jassar, 1987; Lee and Ripley, 1996; Arcuri et al. (1998). Incontrast, many of the low sulfide rocks of the sills are charac-terized byd34S enrichment. Second, low sulfide sill rocks thathaved34S values in excess of;5‰ also show enrichment in18O (d18O 5 8.8‰ to 14.8‰).

6. DISCUSSION

It is clear from the results of previous studies referencedabove that isotopic values of the igneous rocks in the DuluthComplex, and by extension the pre-Duluth Complex sills, mustbe interpreted in terms of both magma country rock, and laterigneous rock hydrothermal fluid interaction. The sills examinedas part of this study exhibit variable isotopic systematics thatcan be related to both of these endmember time frames. Sill


Table 2. Analytical data for pre-Duluth Complex sills and country rocks.

Drill hole/outcrop

Depth(cm)

OverlyingDuluth1

Complexintrusion

Rocktype

Host2

rockThickness3

(m)Distance4

(m)H2O

(wt.%)S

(wt.%)d18O(‰)

dD(‰)

d34S(‰)

d18Opx

(‰)d18Opl

(‰)

PMM-Block 12 Outcrop SKI Sill BIF 1.14 0.16 9.2 280 9.9PMM-Block 20 Outcrop SKI Sill BIF 1.52 0.03 10.1 283 6.7PMM-Crusher #2 Outcrop SKI Sill BIF 1.75 0.23 14.4 267 10.8Dunka Pit Sill Outcrop SKI Sill BIF 1.8 0.21 6.1 2113 21.7Wentworth sill Outcrop PRI Sill BIF 3.84 0.03 14.8 264 5.0B1-119 609.75 PRI Sill VF 19.2 6.25 B 5.48 3.60 5.22111 8.2B1-131 581.62 PRI Sill VF 12.2 2.07 B 0.84 0.07 7.5 280 22.1 7.2 8.3B1-140 591.92 PRI Sill VF 17.07 5.18 T 0.49 0.15 283 22.0B1-148 604.60 PRI Sill VF 17.37 7.80 T 4.36 0.04 4.92143 20.5B1-171 194.31 SKI Sill BIF 2.14 0.46 T 1.5 0.18 10.3 277 11.2B1-179 239.42 SKI Sill BIF 3.05 1.37 B 0.18 0.03 9.4 288 5.6 9.2 11.1B1-249 635.51 PRI Sill VF 14.32 2.74 B 3.68 3.37 5.72111 7.7B1-255 499.81 PRI Norite PRI 0.17 10.1 287B1-255 514.87 PRI VF 0.34 0.06 11.1 283 6.1B1-255 521.91 PRI Sill VF 4.75 0.09 B 0.23 0.19 10.1 273 2.3B1-255 522.98 PRI VF 0.42 0.33 11.2 274 4.2B1-255 523.86 PRI VF 0.37 4.6B1-255 524.62 PRI VF 0.94 11.0 283B1-255 525.23 PRI VF 10.7B1-255 525.48 PRI VF 0.64 10.2 272B1-255 526,.18 PRI Sill VF 14.94 0.35 T 0.55 0.02 9.0 277 22.6B1-255 526.79 PRI Sill VF 14.94 0.96 T 0.69 9.1 261B1-255 527.00 PRI Sill VF 14.94 1.17 T 1.6 8.5 270B1-255 527.91 PRI Sill VF 14.94 2.09 T 0.74 0.07 7.5 282 0.6B1-255 529.07 PRI Sill VF 14.94 3.25 T 0.08 7.7 22.6B1-255 530.02 PRI Sill VF 14.94 4.19 T 0.63 7.6 281B1-255 533.13 PRI Sill VF 14.94 7.30 T 0.17 7.6 20.0B1-255 536.33 PRI Sill VF 14.94 4.44 B 0.5 0.13 6.6 278 21.7B1-255 539.44 PRI Sill VF 14.94 1.33 B 1.45 0.07 6.9 –71 0.1 6.6 7.9B1-255 540.32 PRI Sill VF 14.94 0.45 B 0.27 8.2 270B1-255 540.50 PRI Sill VF 14.94 0.27 B 1.44 8.0 284B1-255 541.05 PRI VF 0.42 10.9 280B1-255 541.23 PRI VF 0.77 1.23 10.3 288 4.5B1-255 542.33 PRI VF 0.2 0.07 8.9 269 0.1B1-255 544.16 PRI BIF-A 15.6B1-255 546.60 PRI BIF-B 14.9B1-331 270.72 PRI Sill VF 19.81 5.24 T 1.49 0.23 6.6 285 21.0B1-331 276.97 PRI Sill VF 19.81 8.32 B 0.07 20.6B1-357 497.68 PRI Sill VF 17.07 0.25 T 0.23 9.1 0.2B1-357 500.12 PRI Sill VF 17.07 2.69 T 0.07 9.2 21.6B1-357 501.70 PRI Sill VF 17.07 4.27 T 0.88 0.17 6.2 268 21.8 6.3 7.3B1-357 505.66 PRI Sill VF 17.07 8.23 T 0.33 0.2 6.8 266 21.3 6.5 8.0B1-360 519.07 PRI Sill VF 7.92 3.05 T 0.65 0.1 7.5 285 0.5B1-360 520.051 PRI Sill VF 7.92 1.61 B 1.43 7.0 279B2-5 47.27 SKI VF 0.71 0.01 13.6 273 4.8B2-5 47.49 SKI Sill VF 2.68 0.10 T 0.66 10.0 274B2-5 48.19 SKI Sill VF 2.68 0.80 T 1.41 0.06 7.1 291 0.8B2-5 49.68 SKI Sill VF 2.68 0.39 B 1.44 0.09 6.6 298 1.9B2-5 49.99 SKI Sill VF 2.68 0.10 B 1.23 6.8 299B2-5 50.17 SKI VF 0.29 8.9 265B2-5 74.98 SKI BIF-C 16.4B2-5 75.59 SKI BIF-C 15.1B2-5 76.60 SKI BIF-C 11.5B2-5 77.42 SKI BIF-C 10.6B2-5 77.82 SKI BIF-C 0.49 10.4 285B2-5 78.00 SKI Sill BIF 4.27 0.09 T 0.29 10.3 294B2-5 78.24 SKI Sill BIF 4.27 0.33 T 10.0B2-5 78.52 SKI Sill BIF 4.27 0.61 T 10.3B2-5 78.88 SKI Sill BIF 4.27 0.97 T 10.2B2-5 79.16 SKI Sill BIF 4.27 1.25 T 0.15 0.07 10.0 293 11.0 8.35 10.0B2-5 79.58 SKI Sill BIF 4.27 1.67 T 10.0B2-5 80.16 SKI Sill BIF 4.27 2.01 B 0.16 9.7 288B2-5 80.77 SKI Sill BIF 4.27 1.40 B 10.0B2-5 81.26 SKI Sill BIF 4.27 0.91 B 0.26 0.03 10.2 286 20.8 9.6 11.7B2-5 81.93 SKI Sill BIF 4.27 0.24 B 10.2


country rock interaction without the influence of the largerintrusions of the Duluth Complex may have produced contam-inated, high18O magmas, and/or diffusive exchange zones atsill contacts with high18O country rocks (e.g. Virginia Forma-tion with d18O ' 8–14‰). Sulfur may have also been derivedfrom country rocks and incorporated into the parent magmaprior to, during, or after emplacement. The sills evaluated in thestudy area are located within the contact aureoles of the Par-tridge River and South Kawishiwi Intrusions (Miller and Rip-ley, 1996). Dehydration reactions that occurred in the peliticcountry rocks produced metamorphic fluids that may haveenhanced isotopic exchange between the sills and adjacentmetasedimentary rocks. Larger convective flow systems relatedto the continued emplacement of mafic intrusions of the over-lying Duluth Complex may have led to progressive alteration ofthe sills. Sulfide minerals may also have precipitated fromhydrothermal fluids during the alteration process. The contactmetamorphic aureole of the Duluth Complex is an environmentwhere fluids of diverse origin (magmatic, metamorphic, mete-oric) may have been involved in isotopic exchange reactions atdifferent times, with the result of possible overlapping, andvariable, isotopic distributions.

The sills in the Virginia Formation and Biwabik Iron For-mation showd18O values that range from those considered”normal“ for basaltic rocks (5.5‰ to 7.0‰, Taylor, 1968), tovalues that are strongly elevated. RepresentativedD values formafic rocks are more difficult to constrain because of isotopicfractionation that may accompany the liberation of a volatilephase (e.g. Taylor, 1988). Accordingly, we denote a field for”normal“ sills in Fig. 4 A that ranges from260‰ to 290‰.The oxygen and hydrogen isotopic compositions of the sillsdisplayed in Fig. 4 suggest that exchange processes have oc-curred and that in some cases acted to increased18O values,whereas in other casesdD, and possiblyd18O, values werestrongly lowered. The high water contents (and hence abun-dance of hydrous secondary alteration minerals) of the lowdDsamples suggests that interaction with a fluid phase may haveoccurred at lower temperatures and/or higher water/rock ratiosthan samples that show little mineralogical evidence of hydro-thermal alteration.

The lowestd18O values of plagioclase and pyroxene pairsare 7.4‰ and 6.3‰ (Table 2), respectively, and are higher thanthose expected for unaltered basaltic rocks (;6.0‰ and 5.5‰).

These values are found at the center of sills that show little orno evidence for alteration, and suggest that sills may havecrystallized from18O-rich, contaminated magma. This conclu-sion is consistent with the abundance of orthopyroxene in thesills (Table 1), which may have formed rather than olivine dueto SiO2 contamination from metasediments. Proper interpreta-tion of the isotopic systematics must then consider country rockcontamination, followed by isotopic exchange during contactmetamorphism and hydrothermal alteration.

Although many of the oxygen isotopic values of the sills areelevated relative to those of normal basaltic rocks, the anom-alous oxygen isotopic values of the sills are not mirrored in theenclosing sediments. The isotopic variability present in theunmetamorphosed sedimentary rocks (8‰–14‰) is alsopresent in the metamorphosed, hornfels equivalents. In thecontact aureole, partial melting, and loss of a SiO2-rich liquid,occurred only locally within;1 meter of the contact with theoverlying intrusive rocks. Compositional changes throughoutthe rest of the aureole are related to devolatilization reactions(Andrews and Ripley, 1989). Unmetamorphosed pelitic rocktypes in the Virginia Formation and Biwabik Iron Formationcontain from 3.6 to 4.4 wt.% H2O, whereas hornfels typicallycontain less than 1 wt.% H2O. Iterative calculations using aporosity of 1%, T5 500°C, and bulk rock waterD values of2‰ indicate that dehydration causes a change of less than 0.1‰in thed18O value of the residual rock. The fluid released duringdehydration is characterized by ad18O value in the range of10‰ to 15‰, depending on the isotopic composition andmineralogy of the source sedimentary layer. No systematicvariations ind18O values are present in the Virginia Formation,except near the contacts with sills. Perry and coworkers (1973)report variations inD values between quartz and magnetite inthe Biwabik Iron Formation in the nearby Dunka River areathat give a temperature gradient of from 400°C to 650°C acrossthe contact aureole, but layer to layer isotopic heterogeneity ismaintained. It appears that pervasive interaction with a fluidphase that may have homogenized isotopic values in the con-tact aureole did not occur. Hydrogen isotopic studies by Ripleyand coworkers (1992) have also shown that large, systematicvariations indD values between unmetamorphosed and meta-morphosed Virginia Formation that result from dehydrationhave also been preserved.

Table 2. (Continued)

Drill hole/outcrop

Depth(cm)

OverlyingDuluth1

Complexintrusion

Rocktype

Host2

rockThickness3

(m)Distance4

(m)H2O

(wt.%)S

(wt.%)d18O(‰)

dD(‰)

d34S(‰)

d18Opx

(‰)d18Opl

(‰)

B2-5 82.14 SKI Sill BIF 4.27 0.03 B 10.4B2-5 82.20 SKI BIF-C 0.77 10.1 296B2-5 82.30 SKI BIF-C 10.3B2-5 82.39 SKI BIF-C 10.5B2-5 82.91 SKI BIF-C 11.6B2-5 83.82 SKI BIF-C 12.7

1: Duluth Complex Intrusions: SKI-South Kawishiwi Intrusion, PRI-Partridge River Intrusion2: VF-Virginia Formation, BIF-Biwabik Iron Formation, with submember (A–D)4: Distance from top (T) or basal (B) contact with country rock


6.1. Spatial Variations of d18O, dD, and H2O and TheirRelationship to Isotopic Exchange Mechanismsand Duration

Oxygen isotopic variations in the sills appear to be stronglycontrolled by their proximity to high18O metasedimentaryrocks. An evaluation of the spatial variations ind18O, dD, andH2O content for the three profiles across sill country rock

contacts described above (Fig. 5) is instructive with respect tothe processes that caused the isotopic distributions in the sills.The simplest case is that shown in Fig. 5A where symmetricald18O exchange zones are developed at the contacts of the sillwithin the Virginia Formation. There is a clear progressionfrom low d18O values in the central portion of the sill to highervalues found at sill margins. No systematic trends are observedfor dD values or H2O content, with values ranging from261‰to 284‰, and 0.23 to 1.6 wt%, respectively. The sill has athickness of 14.9 m in this locality, whereas the other two sillsshown in Fig. 5 are located in the Virginia Formation and theBiwabik Iron Formation and are only 2.7 and 4.3 m thick,respectively. Despite the relatively thin nature of these sills,variations ind18O of up to 3‰ are observed. The Cr-rich sill indrill core B2-5 from 47 to 50 m (Fig. 5 B) shows an asymmetricvariation in bothd18O anddD values, with the relative mag-nitude of oxygen isotopic exchange at the margins of the sillrelated to the isotopic composition of the immediate countryrock. Water contents of this sill are highest whered18O valuesare lowest. The Cr-poor sill encountered in drill core B2-5 (Fig.5C) shows relatively constant and highly elevatedd18O valuesthroughout the sill. The values at both upper and lower contactsapproach those of the enclosing iron formation.dD values ofthis sill vary only between283‰ and293‰, whereas H2Ocontent is low and relatively homogeneous, varying from 0.18to 0.25 wt.%.

The low H2O contents of samples from the Cr-poor sill indrill core B2-5 (Fig. 5C), coupled withd18O values in excess of9.7‰, contrasts sharply with the H2O contents andd18O valuesof the Cr-rich sill in the same core. These features indicate thatelevation ofd18O values may proceed without the crystalliza-tion of secondary hydrous minerals, and offers support for thelocal importance of diffusive exchange mechanisms. In thestratigraphically highest sill encountered in drill core B2-5 (Fig.5B), d18O values of samples that contain in excess of 1.2 wt.%H2O are less than 7‰. Such relationships suggest that hydra-tion reactions involving a second fluid may have acted to lowerpreviously elevatedd18O values.

The shape of the profiles that are developed at sill countryrock contacts could in part be related to kinetic effects duringisotopic re-equilibration. Plagioclase phenocrysts in the pre-Duluth Complex sills show only a small variation ind18Ovalues relative to that shown in phenocrysts from extrusiverocks in the North Shore Volcanic Group (NSVG) that overliethe Complex. For example, in sample Block 20 from the Peter

Fig. 4. (A) d18O–dD variations in pre-Complex sills. The hatchedarea represents primary, unexchanged mafic igneous rocks. Area Irepresents rocks with elevatedd18O values due to diffusive exchangewith high 18O country rocks and exchange with dehydration waters.Area II represents highly altered rocks thought to have been producedby localized accumulation of fluid produced by devolatilization reac-tions. Area III shows the effects of late-stage interaction with a low D,low 18O meteoric water. (B)dD–H2O variations in pre-Complex sills.Areas I, II, and III encompass the same samples as in (A). (C) H2Ocontent–d18O variations in pre-Complex sills. Areas I, II, and IIIencompass the same samples as in (A) and (B).

Table 3. Variation ofd18O values with An content in plagioclasephenocryst from the pre-Complex sill.

Sample No. Grain d18O (‰) An content Comment

Block 20 a 11.15 7 Altered11.8 14 Altered11.95 16 Altered11.67 18 Altered11.47 52 Fresh11.6 53 Fresh

b 12.12 8 Altered12.5 14 Altered12.67 16 Altered12.26 20 Altered12.11 20 Altered


Mitchell mine, thed18O variation of a single phenocryst is0.8‰ (11.1‰–11.9‰), but with no obvious zonation trend. Incontrast, plagioclase in lavas from the NSVG may show avariation ind18O values of up to 5‰ within a single grain, withevidence of isotopic exchange proceeding from grain marginsinward, or along fracture zones (Park and Ripley, 1996). Al-though Na-rich areas are locally found in the plagioclase phe-nocrysts from the sills, there is no systematic variation ind18Ovalues with plagioclase An content or with turbidity index. Asecond plagioclase phenocryst from sample Block 20 shows asimilar range ind18O values to the first, but larger absolutevalues (12.1‰ to 12.7‰). Such distributions attest to smallscale variability in the establishment of isotopic exchangeequilibria in the contact aureole, but sharply contrast with thatshown by plagioclase in the NSVG where low temperature,kinetically dominated exchange has occurred in a fluid bufferedenvironment.

Inspection of thed18O values of plagioclase-pyroxene min-eral pairs provides further evidence for only a small kineticeffect in sill samples from the contact aureole. Even consider-ing the;1‰ variation between plagioclase phenocrysts in thesame sample, plagioclase and pyroxened18O values indicate anexchange history that is very different from that of plagioclaseand pyroxene in hypabyssal sills and plutonic rocks that intrudethe NSVG. The relatively shallow slope exhibited by samplesof pre-Duluth Complex sills ind18Oplag–d18Opyr space (Fig. 8)suggests a near approach to isotopic equilibrium, and elevationof both plagioclase and pyroxene values relative to presumedinitial values near 6.0‰ and 5.5‰, respectively. In contrast,samples from the hypabyssal intrusions associated with theNSVG show a very steep slope on thed18Oplag–d18Opyr dia-gram, characteristic of kinetically-dominated exchange with afluid phase (Gregory and Criss, 1986; Gregory et al., 1989)where pyroxene exchange has been limited. Such kinetically

Fig. 5. Profiles ofd18O, dD, and H2O contents across sill country rock contacts encountered in drill cores B1-255 (A) andB2-5 (B and C). Filled diamond5 sill, open diamond5 Virginia Formation, open square5 Biwabik Iron Formation.


dominated systems are often linked to large hydrothermal sys-tems at relatively low temperature (,350°C). Although thehigher temperatures of the contact metamorphic environmentpromoted isotopic exchange, the data shown on Figure 8 do notindicate an equilibrium geotherm. Plagioclase-pyroxene pairswith the higherd18O values record a lower temperature than dosamples with lowerd18O values. High18O samples occur nearsill margins, and have experienced greater exchange with meta-sedimentary country rocks. Temperatures computed using thefractionation factors of Bottinga and Javoy (1973) suggest atemperature near 500°C, which is below the peak temperatureof 650°C estimated by Andrews and Ripley (1989) using min-eralogical indicators and Perry and coauthors (1973) usingquartz-magnetite oxygen isotopic fractionation in the iron for-mation. The data points along the lowd18O portion of the arrayin Figure 8 are found in sill interiors, and plagioclase-pyroxenepairs may in part retain their high temperature, magmaticrelatedd18O fractionation.

The changes ind18O values across sill country rock contactsresult from diffusive and/or advective exchange that may haveoccurred at different times in the history of rift development.After initial emplacement of the sills exchange may haveoccurred during cooling. However, cooling and crystallizationtimes for thin magma sheets (;4 to 15 m) emplaced intorelatively cool country rocks are much less than 1000 years(Irvine, 1970) and isotopic exchange zones produced via dif-fusion at the contact are predicted to have been narrow. A moreprobable time frame for isotopic exchange would have beenduring larger scale contact metamorphism produced by re-peated magma inputs of the major intrusions of the DuluthComplex. Dehydration reactions that occurred in the peliticsedimentary rocks produced a fluid phase that would have beenimportant for both diffusive and advective transport. The mag-

nitude of oxygen isotopic exchange that occurred within thesills and along contact zones is variable (Fig. 5), and was afunction of variables such as isotopic compositions of both thesills and surrounding country rocks, thickness of the sill, po-rosity–permeability contrasts, and density of fractures or mi-crocracks.

As suggested above, high18O values at the centers of thethicker sills (Fig. 5) may be reflective of contamination by high18O country rocks prior to or during magmatic emplacement.d18O values between 7‰ and 8‰ are common for many noriticrocks in the Duluth Complex (e.g. Ripley and Al-Jassar, 1987;Lee and Ripley, 1996). The values are thought to representpre-emplacement contamination of tholeiitic magma by incor-poration of silicious partial melts derived from the VirginiaFormation. The 4.3 m thick Cr-poor sill in core B2-5 is char-acterized by relatively constantd18O values of 9.7‰ to 10.5‰.These values are too high to be the result of contamination atthe magmatic stage. Figure 9 shows results of two componentmixing calculations that illustrate thatd18O values of 7‰ to8‰ are consistent with;20% to 25% contamination fromcountry rock, but thatd18O values near 10‰ are indicative ofunreasonable values of;70% contamination by either high18O Archean or Proterozoic country rocks. It is far more likelythat the elevatedd18O values of the 4.3 m thick sill are relatedto subsolidus isotopic exchange in the presence of a fluid phase.

The smooth oxygen isotopic profiles developed at the mar-gins of the sill in drill core B1-255 (Fig. 10) are suggestive ofdiffusive broadening of an initiald18O step function. Both thesigmoidal shape of the profiles and the fact that the inflectionpoints are located very close to the contacts suggest that dif-fusion was the dominant transport mechanism for isotopicexchange. Because of the low diffusivities of oxygen in min-erals at temperatures of 500°C to 800°C (e.g. Cole and

Fig. 6. Sulfur isotopic compositions of the Virginia Formation and pre-Complex sills.


Ohmoto, 1986), meter scale transport of oxygen at sill countryrock contacts is thought to have occurred by diffusion througha fluid phase present in microfractures or along grain bound-aries. Pore space would have been filled with fluid duringdevolatilization of the contact aureole, and it is reasonable toassume that diffusive exchange could have taken place duringthis time frame. Andrews and Ripley (1989), Ripley and co-workers (1992) and Ripley and coworkers (1993) have previ-ously proposed that the majority of fluid flow in the pelitichornfels during devolatilization was layer parallel. However,both isotopic (Ripley and coworkers, 1993) and fluid inclusion(Pasteris and coworkers, 1995) evidence indicate that fluidderived from the Virginia Formation did locally infiltrate thebasal troctolitic gabbroic rocks of the South Kawishiwi andPartridge River intrusions of the Duluth Complex. Inspection of

the profiles developed at the margin of the sill in core B1-255(Fig. 10) shows that the advective displacement of the inflec-tion points are less than;35 cm, consistent with a lack ofmajor fluid infiltration in the production of the profiles. Thesmooth transitions ind18O from the sill to the country rocks arealso consistent with the near attainment of equilibrium at alocal (grain) scale, as has been suggested based ond18O valuesof coexisting minerals, and mineral scale isotopic homogeneity.

d18O gradients observed at the contacts of sills and countryrocks may be evaluated using a one dimensional advective-diffusive transport equation. One form of the differential equa-tion, given by Bickle and Mckenzie (1987) is:

C

t Frskd

rf~1 1 f! 1 fG 1 vOf

C

x5 Deff

2C

x2 (1)

where C is the concentration of a chemical tracer or an isotoperatio, vo is the fluid velocity,f is porosity,rs andrf are solidand fluid densities,kd is the solid fluid partition coefficient forthe element, Deff is the effective bulk diffusion coefficient, t istime, and x is distance. Becausekd for oxygen is large com-pared to porosity, the termrskd/rf (1 1 f) 1 f can be reducedto rskd/rf. Bickle and Mckenzie (1987) suggest that this valueis ;1.6 for oxygen in metamorphic fluids. Since diffusive masstransport in this environment takes place primarily through afluid phase, Deff may be defined as:

Deff 5 Df f/t2 (2)

Fig. 7. Sulfur isotopic compositions of pre-Complex sills versus Sabundance. (A) Entire data set showing high-S sills. (B) Data setexcluding the high-S sills.

Fig. 8.d18Oplagioclasevsd18Opyroxenefor samples of pre-Complex sillsfound in metasedimentary rocks below the Duluth Complex, and forsamples from hypabyssal intrusives associated with the North ShoreVolcanic Group (Beaver Bay Complex, Silver Cliff Intrusion, KnifeRiver Diabase, and Endion Sill; Park (1998)).


where Df is the diffusion coefficient of the element in the fluidandt (t $ 1) is tortuosity (Richardson and McSween, 1989).For oxygen in fluids the diffusion coefficient at temperatures

between 450°C and 600°C is in the range of 1 to 5 x 1028

m2/sec (e.g. Bickle and Mckenzie, 1987). For the calculationsthat follow, we have takent as 1 (de Marsily, 1986). A solution

Fig. 9. d18O values of sills as a function of the mass fraction (f) of country rock assimilated. Computed from a mixingequation of the form:

d18Osill(‰) 5(CO, ini. melt)(1-f)d18Oini. melt

(CO, ini. melt)(1-f) 1 (CO, c.r.)(f)1

(CO, c.r.)(f) d18Oc.r.

(CO, ini. melt)(1-f) 1 (CO, c.r.)(f)(13)

where CO, ini. melt and CO, c.r. represent oxygen concentration in initial melt and country rock, respectively.

Fig. 10. d18O - distance plot for the upper contact of the sill in the Virginia Formation, drill core B1-255. Optimumparameters used to fit the advective-diffusive transport equation discussed in the text are Csill 5 7.6 ‰, Cc.r. 5 11.3 ‰,z9 5 0.35, and t9 5 0.485.


for diffusive broadening of a step function in an advectingframe of reference has been given by Bickle and Baker (1990)based on formulations by Crank (1975) and Carslaw and Jaeger(1959) as:

C 5 C1 1DC

2 U1 1 erf Sx9 2 z9

2Ît9 DU (3)

where

x9 5 x/hz9 5 z/h where z5 the advective displacement.

5

vOftrskd

rf(4)

t 5

h2rskd

rf

Deff~t9! (5)

h 5 scaling distance.DC 5 C2

2 C1

C1 5 initial isotopic composition of the sillC2 5 initial isotopic composition of the country rock

Fluid flow parameters such as time integrated fluid flux, fluidflux, porosity, permeability, and time can be estimated fromresults of the model calculations as outlined by Bickle andBaker (1990). The integrated fluid flux needed to satisfy theadvective displacement of the isotopic front is given byvo

ft 5 zrskd/rf. Porosity-permeability estimates require a par-ticular porosity structure. Bickle and Baker (1990) have de-scribed porosity-permeability relationships for flow along mi-crocracks and flow along an interconnected pore networkaround grain boundaries. In the case of flow within discretemicrocracks permeability (kf) is:

kf 5a2f3

A(6)

where

a 5 distance between the microcracks (in m)f 5 porosityA 5 12 (Walther and Orville, 1982)

For flow along an interconnected pore network:

kf 5a2fn

A (7)

where

a 5 grain diameter (in m)A 5 3000 and n5 2 for fluid-solid dihedral angles,50°

(Cheadle, 1989)

For a diffusive exchange zone advected a distance z,

f 5 3zhADft

a2P

zh2t94

1n21

(8)

wheren 5 3 for flow in microcracks and 2 for interconnectedgrain boundaries. The fluid pressure gradient (P/z) is taken as20 MPa/km andh, the fluid viscosity as 1024 pascalz seconds.Following the computation off, t may be computed from thedefinition of t9.

Figure 10 shows the results of a best fit profile for the topboundary of the sill in drill core B1-255. Optimum values forCsill, Ccountry rock, t9, and z9, with h 5 1 m, are 7.6‰, 11.3‰,0.485, and 0.35. The X2 statistic for the goodness of fit of thecomputed curve (Press et al., 1986), defined as

X2 1 Oi51

N Fd18Omi 2 d18Oc

s G 2

(9)

whered18Omi is the measuredd18O value at a distance x9, d18Oc

is the calculated value, ands is the uncertainty of thed18Omeasurement (here taken as 0.2), is 15 which indicates thatmodeling results are in a good agreement with the data. Thecalculated integrated fluid flux is low,; 0.56 m3/m2. Timecalculations vary greatly for flow toward the sill via micro-cracks or interconnected pore boundaries. Using a spacingbetween cracks of 0.5 to 1 mm, times between;7000 and15,000 years are computed for the development of the ex-change profiles. Utilizing flow through interconnected porestimes between 355,000 and 1.4 Ma are computed for graindiameters of 0.5 and 1 mm. These times are longer than thosecomputed for diffusive exchange equilibration to be attained inplagioclase, and suggest that grain scale equilibrium was ap-proached. Clear plagioclase grains in the sills show no evidenceof secondary alteration, and are thought to have exchanged viaa diffusion process. For a spherical geometry, the time requiredfor a mineral to isotopically equilibrate by diffusion (Td) isgiven by the following equation from Crank (1975):

Td 5 a2/4Ds (10)

where a is the grain radius in meters. Cole and Ohmoto (1986)give a diffusion coefficient for oxygen in plagioclase reactedwith a hydrothermal fluid at 550°C of 10218 m2/s. These datasuggest equilibration times for plagioclase grains of 0.5mm and1mm radius of about 2000 and 8000 years.

The 4.3 m thick sill in core B2-5 shows very differentoxygen isotopic systematics relative to the thicker sill in coreB1-255.d18O values in the sill are essentially constant, varyingonly between 9.7‰ and 10.5‰, whereas a modestd18O gra-dient is seen in the surrounding iron formation (Fig. 5). Thevery enriched nature ofd18O values in the sill cannot be relatedto a magmatic mixing process, as roughly a 70% contributionof oxygen from the immediate country rocks would be required(Fig. 9). The elevatedd18O values throughout the sill may haveoriginated in at least two ways. One is that the effectivediffusivity, Deff, was much higher in this sill relative to countryrocks. Figure 11 illustrates calculated profiles for both contactsof the sill utilizing slightly different country rockd18O values.Diffusion alone can account for the observedd18O distributionsif the Deff values for oxygen in the sill is;4 times larger thanthat in the country rock. Elevated Deff values would largely bea function of higher porosities in the sill, perhaps due toextensive development of microcracks. Chlorite filled fracturesare abundant in this sill, and do indeed distinguish it from the


thicker sill in core B1-255. Ripley and coworkers (1993) havenoted that picritic units in the larger intrusions of the DuluthComplex are often characterized by anomalously lowd18Ovalues, thought to be related to late stage flow of meteoricwaters through fractured, olivine-rich rocks. The thin sill hereis not picritic, but a similar physical mechanism for increasedporosity may have existed.

The elevatedd18O values in the 4.3 m thick sill may also beevaluated in terms of fluid flow being channeled in the sillrelative to surrounding country rocks. Consideration of apinned boundary solution to the advective diffusive transportequation was presented by Bickle and Baker (1990) as:

C 2 C1

C2 2 C15 2pePe~ x91z9!/ 2O

n51

`n

~Pe/ 2!2 1 n2p2 z @~21)nePe/ 2 2 1#

(11)

z exp{1 2 [(Pe/ 2)2 1 n2p2] t9}sin(np~ x9 1 z9!)(12)

where Pe is the Peclet number, defined asvofh/Deff. In thiscase, the;10‰ d18O values of the sill would represent a nearattainment of isotopic equilibrium with a fluid, with the gradi-ents observed in the country rocks resulting from diffusiveexchange at contacts. Although the fit to the observed data (Fig.12) is as good as that for the diffusion only solution describedabove (Fig. 11), the applicability of a model where flow ispreferentially channeled in the sill is uncertain. Fluid involvedin the isotopic exchange is thought to be derived primarily fromthe devolatilization of the metasedimentary rock types. It ispossible that due to the higher porosity of this sill relative tocountry rock, fluid derived from immediate country rocks was

concentrated within the sill. However, because thed18O valuesof the fluids liberated from the sedimentary layers duringcontact metamorphism were not lower than;10.5‰, thed18Ovalues of the sill could not record isotopic equilibration withsuch a fluid at temperatures below;600°C (bulk rock fluidDvalues are$2‰). For these reasons, the pinned boundarysolution is considered unlikely, and the diffusion model basedon higher Deff values in the sill is favored.

The estimated times required to generate the isotopic profilesat sill country rock contacts are critically dependent on porosityregimes. However, even a maximum time of;1.4 Ma esti-mated using a model of flow through an interconnected porefluid network is near the 0.5 to 1 Ma time period for emplace-ment of the major plutonic phases of the Duluth Complex(Paces and Miller, 1993). The intrusions increased in size as aresult of the emplacement of relatively thin magma sheets (e.g.Miller and Ripley, 1996). Temperatures at basal contact zonesmay have remained elevated due to continued magma injection,but must have waned as successive injections were emplaced atstratigraphically higher levels. Pasteris and coauthors (1995)have shown that fluids involved in late stage alteration oftroctolitic and gabbroic rocks near the base of the DuluthComplex were derived from the Virginia Formation. This sug-gests that either fluids were expelled from the Virginia Forma-tion over a prolonged interval, or that fluids present in theVirginia Formation did not gain access to the Duluth Complexuntil late stages of crystallization, essentially at the end of the0.5 to 1.0 Ma emplacement period. If fluid production wascontinuous over this time span, then diffusion through aninterconnected pore fluid network is viable. If fluid productionwas more episodic in nature, then time frames for advective-

Fig. 11.d18O– distance plot for the 4 m thick Cr-poor sill in drill core B2-5. Computed using a diffusion solution only,with Csill and Cc.r. at the upper contact of 7.3‰ and 13‰, and 7.6‰ and 12.4‰ at the lower contact, and enhanceddiffusivity ;4 times in the sill relative to the country rock.


diffusive transport and isotopic exchange are more consistentwith a porosity-permeability relationship governed by flowthrough microcracks. In either case the low computed timeintegrated fluid flux is in agreement with earlier suggestionsthat the majority of dehydration fluids lost from the AnimikieGroup contact rocks were expelled via layer parallel flow.

Exchange profiles also show localized perturbations that arethought to be the result of later influx of meteoric water. GroupIII samples are characterized by elevated H2O contents, highpercentages of serpentine and chlorite, and lowdD values.These features are all consistent with the involvement of me-teoric H2O at low temperatures. However, the occurrence ofD-depleted sill material appears to be highly localized. It is ofnote that Ripley and coworkers (1992) have reported a welldefined decrease indD values with metamorphic grade. Un-metamorphosed Virginia Formation is characterized bydDvalues in excess of;270‰, with values decreasing progres-sively to;2100‰ toward the contact. LowerdD rocks occurin the footwall hornfels, but are altered, with orthopyroxeneconverted to serpentine and chlorite, and cordierite and plagio-clase partially converted to chlorite. The presence of low D silland country rocks only in scattered localities suggests that fluidflow may have been strongly conduit or fracture controlled.

In summary, thed18O anddD values of the sills in generalreflect isotopic exchange with a high18O, high D fluid, thoughtto be derived primarily from dehydration reactions occurring inthe contact aureole. Relatively fresh basaltic rocks in the centerof thicker sills are characterized by elevated18O contents,thought to be produced by contamination of magma by high18O country rock prior to or during emplacement. Isotopeexchange proceeded from the margin of sills inward, or alongfracture zones, by diffusive exchange through an intergranularfluid. This process has driven contaminated basaltic rocks toeven higherd18O values, but with little or no change indDvalues. Mafic rocks characterized by highd18O values and highH2O contents (e.g. Group II, Fig. 4) represent localized accu-mulations of metamorphic H2O that resulted in highly alteredzones in the sills. Elsewhere, incursion of meteoric H2O hasresulted in the lowering of bothd18O and dD values, (e.g.Group III, Fig. 4). Major influxes of meteoric H2O appear to

have been spatially restricted, and localized by conduit orfracture flow.

6.2. Sulfur Isotope Systematics

Sulfur isotopic values of the sills fall into a broad rangewhich mimics that found in the larger intrusions (Fig. 6).Uncontaminated, mantle-derived melts typically haved34S val-ues of 06 2‰, a range characteristic of low sulfide gabbroicand troctolitic rocks from the Partridge River and South Kaw-ishiwi intrusions (e.g. Taib and Ripley, 1993; Lee and Ripley,1996). Cu-Ni sulfide mineralization in the Duluth Complex ischaracterized by elevatedd34S, up to;16‰ (Ripley, 1981;Ripley and Al-Jassar, 1987; Lee and Ripley, 1996; Arcuri et al.,1998). These values are interpreted to be indicative of a largecomponent of sulfide derived from pyritic Proterozoic countryrocks. d34S values of the Virginia Formation are extremelyvariable, ranging from 0‰ to 30‰ (e.g. Ripley, 1981; Arcuri etal., 1998). The 30‰ range can be found over vertical distancesof less than 15 m. Analyses of Virginia Formation collected aspart of this study show a more restricted range of 0.1 to 6.1‰.Desulfidation of pyrite during metamorphic conversion to pyr-rhotite may release H2S that is isotopically similar to the parentminerals (e.g. Snyder, 1988). Because of the variability ind34Svalues of pyrite from the Virginia Formation it is difficult toaccurately estimate relative proportions of externally derivedsulfur found in the igneous rocks. However, because the ma-jority of sulfur found in the Virginia Formation and BiwabikIron Formation is characterized by positived34S values, thed34S values of 0‰ to23‰ found in the igneous rocks arestrongly indicative of a mantle sulfur source.

Samples of sill material with low sulfide content andd34Svalues of 06 3‰ (Fig. 7) are not thought to have beencontaminated with sedimentary sulfide. Most of these samplesare found near the center of sills, and are also characterized byrelatively lowd18O values. Samples with elevatedd34S valuesare contaminated, but the timing of sulfide introduction in theigneous material is difficult to accurately determine. Isotopi-cally anomalous sulfide may have been introduced prior to orduring magma emplacement, or as a part of a latter hydrother-

Fig. 12. Pinned boundary solutions for the upper (A) and lower (B) contacts of the 4 m thick Cr-poor sill in drill coreB2-5 using the variables shown on each plot.


mal fluid that reacted with the igneous rocks. The two samplesthat contain in excess of 3.0 wt.% S also haved34S values of7.8‰ and 8.3‰ (Fig. 7). The samples are melatroctolites fromthe Cr-rich sills described above that are extensively serpen-tinized. Sulfide minerals in these rocks are patch like to inter-stitial in form, and are texturally similar to sulfides that occurin the basal troctolitic units of the intrusions of the DuluthComplex. We interpret these sulfides to be related to a sulfidemineralizing event that deposited massive sulfides in the foot-wall rocks of the nearby Local Boy ore zones of the BabbittDeposit. Their proximity to a major N-S trending fault (Granofault of Severson, 1994) suggests a feeder vent in this vicinity.The sulfides in these two sills are not related to the fluids thatproduced the serpentinization.

Textures of low sulfide rocks are more difficult to assess, asthe very fine grained minerals could have formed during inter-action with highd18O, H2S-bearing metamorphic fluids. Isoto-pic exchange between high34S H2S in the fluids producedduring dehydration reactions in the contact aureole, and the sillscould also explain the presence of high18O, high34S sills. Thepresence of high34S and high18O rocks at the margins of sills(e.g. the 15 m sill in the drill core B1-255) suggests thatsubsolidus processes may be locally important in sulfide min-eral precipitation, and as a control ond34S values.

7. CONCLUSIONS

Oxygen isotopic values of pre-Duluth Complex sills locatedin Proterozoic metasedimentary country rocks beneath the Du-luth Complex are strongly elevated relative to values of 5.5‰to 7.0‰ considered normal for mafic melts.dD values arevariable, but generally much less anomalous thand18O values.Layer to layer oxygen isotopic variability is preserved in themetasedimentary rocks, and the range in whole rock values isthe same as that found in unmetamorphosed samples outside ofthe contact aureole. No evidence for variations in oxygenisotopic composition of the metasedimentary rocks is present,except near contacts with the sills. The elevatedd18O values ofthe sills is in part related to contamination by high18O countryrocks either prior to or during emplacement of the magmas.Further increases ind18O values progressed from contactzones, due primarily to diffusive exchange with the enclosingmetasedimentary rocks. Dehydration reactions in the peliticrocks within the aureole of the Duluth Complex caused littlevariation in oxygen isotopic composition of the mineral assem-blages, but provided fluid that enhanced the diffusive exchangeprocess. Modelling of a fluid assisted diffusion dominant ex-change process suggests exchange durations that may havevaried between tens of thousands of years and 1.4 Ma, depend-ing on local controls of porosity and permeability, as well asrates of fluid production in the contact aureole. Low computedtime integrated fluid fluxes are consistent with the premise thatthe majority of dehydration fluid lost from the hornfels wasexpelled via layer parallel flow.

Localized zones of high18O and high H2O contents repre-sent accumulation of metamorphic fluids released during dehy-dration reactions, and the production of hydrous, secondaryminerals. A later influx of meteoric water is locally detected bydecreases in silldD andd18O values, and is particularly man-ifest in highly serpentinized troctolite to melatroctolite zones of

Cr-rich sills. The distribution of low18O, low D alterationassemblages suggests that the flow of meteoric water waschannel or fracture controlled.

Sulfur isotopic values of the sills vary widely, between22.7‰ and 11.2‰. Sulfur content of sills other than themelatroctolite type are low at less than 0.3 wt.%. Sulfides in themelatroctolite sills are texturally similar to those found in theoverlying mineralized rocks of the Duluth Complex, andd34Svalues of 6‰ to 7‰ are consistent with contamination bycountry rock sulfur prior to emplacement of the sills. The sillswith high d34S values and less than 0.3 wt.% S are character-ized by elevatedd18O values (.8.0‰). This covariation sug-gests that either sulfur isotopic exchange occurred between H2Sproduced as part of the devolatilization fluid and igneous sul-fide minerals, or that sulfide minerals precipitated from the porefluid. Low sulfide sills withd34S values of 06 3‰ experiencedlittle, if any, pre-emplacement sulfur contamination or laterisotopic exchange with H2S bearing devolatilization fluids.

Acknowledgements—We would like to thank John Green, Jim Brophy,Dave Towell, and John Hayes for thoughtful reviews of earlier versionsof this paper. Discussions with Jim Miller concerning Lake Superiorgeology were stimulating and acknowledged with thanks. This researchwas supported through NSF Grants EAR 9417687 and EAR 9614165to E. M. Ripley.

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Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the...

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