"The Petrology and Sulfide Mineralization of the Partridge...

Conadian MineralogistYot.22, pp. 23-38 (1984)

ABSTRACT

The Partridge fuver troctolite, a basal intrusive body ofthe Duluth Complex, is located in the Ely-Hoyt Lakesregion of northeastern Minnesota. The upper part of thisbody is a sequence of troctolites and augite troctolites. Thebasal zone is 100-150 m thick, contains numerous xenolithsand is quite variable in composition; it contains most ofthe sulfides (pyrrhotite, cubanite, chalcopyrite andpentlandite). Rock typc present are augite troctolite, olivinegabbro, gabbro and gabbro-norite. In the basal zone. theiron content of olivine, the sodium content of plagioclaseand the silica and potassium contents of the bulk rock in-crease with depth as do the sulfur content and the copper-nickel and iron-nickel ratios of the rocks. The chemicaltrends in the rocks of the base are a result of contamina-tion from the underlying Virginia Formation. Minor con-tamination was caused by the mixing (near xenoliths andwithin 30 m of the base) of a partial melt derived from theVirginia Formation. The dominant contaminant from thismelt was silica. Most of the contaminants were added tothe magma by a hydrous S-bearing fluid derived from theVirginia Formation by the heat of the intrusion. This fluidtransported copper, iron and alkalis. It mixed with themagma at a fairly early stage in the crystallization of thetroctolite to cause the changes seen in plagioclase andolivine. Values of the copper-nickel ratio of the sulfidesin the region suggest that localization of copper-rich sulfideswas due to the availability of Vtginia Formation in the foor-wall rocks.

Keywords: Duluth Complex, copper-nickel sulfides, con-tamination, Partridge River troctolite, Minnesota.

Sounaarnn

La troctolite de Partridge River, massif intrusif d la basedu complexe de Duluth, est situ6e dans la r6gion des lacsEly - Holt, dans le Nord-Est du Minnesota. La partie sup€-rieure de ce massif est une s6quence de troctolites et de troc-tolites A augite. La zone infdrieure, d,une dpaisseur de 100d 150 m, a une composition assez variable et contient ungrand nombre de x6nolites. Cette zone porte 6galement laplupart des sulfures (pyrrhotine, cubanite, chalcopyrite etpentlandite). Troctolite i augite, gabbro d olivine, gabbroet gabbro noritique constituent les roches observ6es. Daqs

23

THE PETROLOGY AND SULFIDE MINERALIZATION OF THEPARTRIDGE RIVER TROCTOLITE, DULUTH COMPLEX, MINNESOTA

R. MICHAEL TYSON

Department of Geology, Northern Arizona University, Flagstaff, Arizona A60II, U.S.A.

LUKE L.Y. CHANG

Deportment of Geology, University of Marylond, College park, Moryland 20742, U.S.A.

cette zone, les teneurs en fer de I'olivine, en sodium du pla-gioclase et en silice et potassium de la roche globale aug-mentent avec la profondeur, d l'instar de la teneur en sou-fre et des rapports cuivre-nickel et fer-nickel des roches.La zonation chimique de ces roches s'expliquerait par leurcontamination au contact de la formation Virginia sous-jacente. Autour des xdnolites et d moins de 30 m de la base,le produit d'une fusion partielle de la formation Virginias'est m6l6 au magma, le contaminant l6gdrement. Le con-taminant principal fut la silice. La plupart des 6l6mentsmobilis6s furent ajoutds au magma par un fluide aqueuxsulfur6, lib6r6 de la formation Virginia par la chaleur deI'intrusion. Ce fluide a transport6 le cuiwe, le fer et les alca-lins. I1 r6agit avec le magma d un stade de la cristallisationde la troctolite assez pr€coce pour causer les modificationsrelev6es dans le plagioclase et I'olivine. D'aprds le rapportcuiwe-nickel des sulfures de la r6gion, on dirait que la loca-lisation des sulfures riches en cuiwe n'a pu se produire quegrdce i la prdsence de la formation Virginia i la paroi inf6-rieure du massif intrusif.

(Traduit par la Rddaction)

Mots-clds: complexe de Duluth, sulfures de cuivre etnickel, contamination, troctolite de Partridge River,Minnesota.

INTRoDUCTIoN

The ll00 million-year-old Duluth Complex @aureet al. 1969),located in northeastern Minnesota, cropsout as an arcuate belt extending from Duluth north-ward to Hovland on the north shore of LakeSuperior near the Canadian border. It was not in-truded in one pulse but rather as a succession of in-trusions of mafic magma, leading to rocks rangingfrom troctolite to anorthosite in composition(Weiblen & Morey 1980). Copper-nickel sulfideswere emplaced along the base of the Duluth Com-plex in the Ely-Hoyt Lakes region (Fig. l).

This study was undertaken to investigate one ofthe basal intrusive bodies in the Ely-Hoyt Lakesregion, the Partridge River troctolite. The silicate andsulfide mineralogy and petrology are described, and

u THE CANADIAN MINERALOGIST

I DDH'297

2 D D H - 3 0 4

3 DDH-364

4 D D H . 9 I

B BOUCHER I975

c cooPER r978

H HARDYMAN 1969

M M O L L I N 6 I 9 7 9

R R E N N E R I 9 6 9

R i R I P L E Y I 9 7 8

EXPLANATIONDULUTH COMPLEX

T R O C T O L I T I C S E R I E ST^--Tu]ts- o o I

A N O R I H O S I T I C S E R I E S

O L D E R R O C K SW

KEWEENAWAN EASALT

S E C T I O N S

) 5 l (ff

S C A L E I N M I L E S

5 l o 1 5S C A L E I N K I L O M E T E R S

B.

a $tratigraphic subdivision of the Partridge Rivertroctolite and the mode of sulfide emplacement areproposed. Together, these may allow for the iden-tification of additional copper-nickel deposits alongthe base of the Duluth Complex in other areas.

Cores from four nearly vertical drill-holes (pro-vided by AMAX Exploration, Inc., Babbitt, Min-nesota), located in St. Louis County in thesouthwestern corner of T.60N, R.12W (Fig. l), wereselected for detailed study of the Partridge Rivertroctolite. They were chosen from nearly 400available cores; the four cores provide closely spac-ed penetrations of the intrusive body, with a trendnearly parallel to the strike of the base of the com-plex in this area. Each core was describedmegascopically, and preliminary stratigraphic unitswere determined (Tyson 1979). Representativesamples from each stratigraphic unit in each corewere chosen for detailed study. The spacing ofsamples ranges from 0.3 m to 6 m and averages 3m. Forty-five thin sections from samples represen-tative of the stratigraphic units in each drill core werestudied. Modal compositions of 75 samples from two

V I R G I N I A F O R M A T I O N

r---lB I W A B I K I R O N F O R M A T I O N

--;;-'tl < - ' ' r

G I A N T S R A N G E G R A N I T E

of the holes have been determined; the tabulated dataare available, at nominal cost, from the Depositoryof Unpublished Data, CISTI, National ResearchCounCil of Canada, Ottawa, Ontario KIA 0S2. Thecomposition of olivine in most samples collected wasdetermined using X-ray diffraction by measuring the131 peak (Agterberg 1964). Plagioclase was analyz-ed optically for anorthite content using the disper-sionln the index of refraction (Morse 1968) and theMichel-L6vy method. Whole-rock analyses for ton,strontium and rubidium were carried out using X-ray fluorescence.

GEOLOGY OF THE ETY - HOTT LAKES I{EGION

The Duluth Complex lies in the Superior Provinceofthe Canadian Shield. The oldest rocks present inthe Ely - Hoyt Lakes region are an Archean granite-greenstone terrane. Unconformably overlying the

eranite is the Animikie Group (1.7 Ga). The gOger-most unit in the Animikie Group is the Virginia For-mation, which forms the footwall of the basal con-tact of the Duluth Complex in much of the Ely -

Frc. l. Geologic map of the Ely-Hoyt Lakes region, northeastern Minnesota, showing the location of the differentintrusive cimpleies, of previous study-at.as and of the present study-area. The cross sections are hypothetical but

were derived from relationships observed in drill core. This diagram was adapted from Bonnichsen (1974).

si'qt tnrnustot't

{is";*

:i?.dL 6r\ /._::lt- .>

* YPARTRT-DGE<""1;qiik

\ J / O o a

61.:::i7\

7,/vtd*;'S:'"

o o - o - o ^ o - o

Y; : .o "

THE PARTRIDGE RIVER TROCTOLITE. MINNESOTA 25

Hoyt Lakes region (Fig. l). The Virginia Formationis characterized by argillite, although argillaceoussiltstone and fine-grained greywacke are also present.Small amounts of limestone, dolomite, chert andsideritic iron-formation are present locally (Morey1972a). The argillite is black to dark grey and com-monly contains carbonaceous material. Quartz andplagioclase are the dominant minerals. Muscovite,chlorite and pyrite are commonly present (Moreyr972d.

The intrusive bodies in the Ely - Hoyt Lakes regionare made up of the lower stratigraphic portion ofthe Duluth Complex. These include the basal Troc-tolitic Series and the lowermost part of the Anor-thositic Series, as defined by Weiblen & Morey(1980). Two major troctottic intrusive bodies, SouthKawishiwi and Partridge River, are present along thebase.

The South Kawishiwi body is the basal troctolitein the northeastern part of the Ely - Hoyt Lakesregion (Fig. l). Green et al. (1966), Phinney (1969),Bonnichsen (1972), Bonnichsen & Tyson (1975) andBonnichsen et al. (1980) recognized a series ofstratigraphic units. A Basal Zone, 60 to 400 m thick,is composed of fine- to medium-grained troctolites,gabbros and norites and medium- to coarse-grainedtroctolites, melatroctolites and dunites. An AugiteTroctolite unit, approximately a thousand metresthick, overlies the Basal Zone. A sharp contact, local-ly complicated by xenoliths of anorthosite or horn-felsed basalt, separates the two units. Overlying thisis a Troctolite unit. A gradational contact exists be-tween these two units. The top of the Troctolite unitis not exposed in the region.

Exposures of the Partridge River troctolite, to thesouthwest (Fig. 1), are extremely limited owing toextensive glacial deposits covering the pluton. As aresult, it has not been studied in as much detail asthe South Kawishiwi pluton. Previous studies, mostlyfrom single drill-cores, have shown that the PartridgeRiver troctolite does not have the same stratigraphyas that exhibited by the South Kawishiwi pluton, nordoes its plagioclase exhibit as strong a foliation (Har-dyman 1969, Bonnichsen & Tyson 1975, Boucher1975, Molling 1979, Tyson 1979).

PETRoGRAPHY oF THEPARTRIDGE RTvBn TnocToIlrg

The basal 550 m of the Pafiridge River troctolite,as revealed by the four drill-cores, consist dominantlyof a mixture of troctolite, anorthositic troctolite,augite troctolite and melatroctolite. The most abun-dant minerals are plagioclase, olivine and augite.Biotite, hypersthene, ilmenite and sulfides occur inaccessory amounts.

Ftc. 2. Textures typical of unit M. Plagioclase is tabular(l) and irregular (I). Augite, at extinction, is poikilitic.Cross-polarized light, Scale bar is I mm.

Ftc. 3. Symplectitic intergrowth of hypersthene andplagioclase on the edge of an olivine grain. A selvageof pyroxene separates the intergrowth from the olivine.Note the ilmenite rods in the plagioclase. Plane-polarized light. Scale bar is 0.3 mm.

Plagioclase

Plagioclase exhibits two habits in the PartridgeRiver troctolite: the earlier plagioclase is tabular, andthe later generation has an inegular (or interstitial)habit @ig. 2). Both types of plagioclase arelabradoritic. The tabular plagioclase occurs as sub-hedral to euhedral crystals 0.5 to l0 mm in length,with size decreasing as the basal contact is ap-proached. These crystals exhibit normal zoning;however, the transition from core to mantle is notgradational. If, as suggested by Bonnichsen (1972),the magma was intruded as a crystal mush, the corescould represent the plagioclase crystals developedprior to intrusion and the mantles would representthat portion solidified in situ.

The irregular plagioclase embays the tabularplagioclase and locally exhibits an amoeboid shapeas if it had partly assimilated the earlier-formed

""ffi

26 THE CANADIAN MINERALOGIST

plagioclase. The zoning in the irregular plagioclaseis patchy and exhibits no regular pattern.

The tabular and, less commonly, the irregularcrystals of plagioclase contain exsolved rods of il-menite (Fig. 3) and plates of nearly opaque, reddishbrown rutile. Both minerals were identified optical-ly, the ilmenite in reflected light and the rutile intransmitted light. These two oxide minerals, wherepresent in the irregular plagioclase, are uniformlydistributed throughout the grain. In the tabularplagioclase, ilmenite and rutile tend to be concen-trated in the core, leaving the mantle relatively freeof the Ti-rich species. If, as has been postulatedabove, the core and mantle of the tabular plagioclaserepresent two periods of crystallization, then thesetextural data suggest that the titanium content of themagma fluctuated with time as crystallization of themagma progressed.

Olivine

Olivine occurs in three textural forms in the rocks:irregular, equant and poikilitic to interstitial. Thetiming of the cryslallization of olivine indicated bythese textures is one characteristic used to identifythe stratigraphic units (Table l). Olivine in differentunits formed prior to, at the same time as, and afterplagioclase. Petrographic evidence indicates it was

always earlier than augite, hypersthene, biotite, il-menite and the sulfides.

Augite

Augite was the last major silicate phase to form.It exhibits two textural forms, which are mutuallyexclusive in any particular sample. Poikilitic augitemost commonly encloses plagioclase crystals. It mayalso incompletely to completely enclose olivine,depending upon the unit in which it is observed.Augite may also be interstitial to the other silicates.In most occurrences, augite will exhibit exsolution.The most common products of exsolution are opa-que rods of ilmenite. These are accompanied by lesscommon reddish brown anisotropic plates, tentative-ly identified as rutile. The exsolved minerals can oc-cur as a light dusting in the pyroxene or be so con-centrated that the augite appears opaque. Locally,the exsolved phases are arranged en 6chelon. The oc-currence of these titanium-rich exsolved phases inthe augite suggests that titanium was abupdant in thelater stages of the differentiating magma.

Hypersthene

Hypersthene is present as interstitial crystals andas symplectitic intergrowths with plagioclase.

TABI,E 1. CHARACTERISTICS OF THE STRATIGRAPHIC UNITS

character is t ic

P lag ioc l aseTexture

composi t ion

Mode

Ol iv ineTexture

Composi t ion

Mode

Augi teTexture

Mode

Biot i teTexture

Mode

HyperstheneMode

Unit M

Tabular &Irregular

A h - ^ nf b t u

60 -1 42

Po i k i l i t i c &T r r a d t r l - r

) ) 0 6

L 3 - 2 3 6

Interst i t iaL-Po i k i l i t i c

2 - L 2 Z

Un i t P

Anhedral

r .0-78s

Equant &Euhedral

) q I z

1 0 - 7 3 I

Po i k i l i t i c

q - L z 6

un i t c

Tabular

An6 1-An6 4

5 0 - 6 0 8

Po i k i l i t i c

F o 4 0 - F o 6 0

8 - 1 9 8

Po i k i l i t i c

2 0 - 3 0 ?

Epitaclic to

4 -72

0 -42

Uni t B

Tabular

A n - A n) f , f , b

5 5 - 7 5 8

In te r s t i t i a l -Subpoik.

J f , ] O

L O - 2 2 2

Po i k i l i t i c

z - L v 6

Un i t A

Tabular

An- " -An- -

f , r ) f

5 Z - ) d 6

I n t e r s t i t i a l -Subpoik.

J f , ) U

0 . 5 - 8 8

In te r s t i t i a l -Po i k i l i t i c

t - 0 - 17 I

Interst i t ia l^ r h i + i ^ + i ^

0 -54 T race

Interst i t ia l Interst i t ia lor EPitacLic

2 - 4 2 7 - L 0 Z

u - 2 6 0-22 l - -3 ? 7 - 2 L Z

Hypersthene typically makes up less than 2t/o of therocks except at the base, where it exhibits a muchhigher abundance (up to 20t/o).

A symplectitic intergrowth of plagioclase andhypersthene (Frg. 3) is more common than the in-terstitial crystals of hypersthene. It can compose upto 990 of the rock. The symplectite most commonlyoccurs at the contact between olivine and plagioclase,embaying the latter. It may also be present along thecontact between two plagioclase crystals or associatedwith biotite. The intergrowth is separated fromolivine by a selvage of augite or orthopyroxene.Other minerals, such as amphibole and serpentine,may also form the selvage.

Biotite

Biotite is an accessory mineral in nearly all samplesstudied. It occurs in two ways: as irregularly shapedcrystals commonly associated with the sulfides andthe oxides, and as epitactic overgrowths on augiteand, locally, olivine.

STRATIGRAPHY oF THEPentnrocs Rtven Tnocrot-trs

Five correlatable units (informally labeled M, P,C, B and A) have been identified in all four cores(Iable l). Furthermore, Molling (1979; pers. comm.)found all but unit B in the core he studied (nearlyI km to the southeast), suggesting that these unitsare widespread in the Partridge River troctolite.

Unit M

Unit M forms the upper part of this section. It isquite variable laterally and vertically with respect tomineral textures and rock types present. Neither ofthese parameters could be correlated between holes.The rock types and their textures may be continuousvertically for hundreds of metres or for less thanthree metres. Their contacts may be sharp or grada-tional for over three or more metres. Part of the com-plexity in this unit may be due to the presence ofmore than one intrusive pulse, as suggested by Mol-ling et al. (1979) and Grant & Molling (1981).

Unit P

Unit P, a group of xenoliths with similar textures,is completely enclosed by unit M. The thickness ofthese rocks varies from 10 to 150 metres, and theirheight above the basal contact varies from core tocore. The contacts between this unit and unit M aresharp. Dykes of unit M commonly cut unit P. Local-

THE PARTRIDGE RIVER TROCTOLITE, MINNESOTA

Frc. 4. Texture typical of unit P. Olivine (O) is equant oreuhedral. Augite (A) is poikilitic to olivine, forming aglomerophyric mass. This example shows less extensiveserpentinization than most samples of unit P. Plane-polarized light. Scale bar is I mm,

Frc. 5. A texture characteristic of unit C. Biotite (B) isepitactic to poikilitic augite. The biotite is parallel tothe schiller in the augite. The darker patches in the coresof plagioclase crystals are concentrations of ilmeniterods. Plane-polarized light, Scale bar is lmm.

27

Frc. 6. A texture characteristic of unit B. Biotite (b) oc-curs along the plagioclase - poikilitic augite boundaries.Note that the biotite is not oriented relative to anycrystallographic direction ofthe augite. The darker pat-ches in the core zone of the plagioclase are concentra-tions of ilmenite rods. Plane-polarized light. Scale baris I mm.


Frc. 7. Apatite inclusions associated with plagioclase andaugite. This occurrence is typical of unit C. Plane-polarized light. Scale bar is 0.1 mm.

ly associated with unit P is a short interval of horn-fels composed of Virginia Formation or, less com-monly, Keweenawan basalt.

Unit P is the only unil in the Partridge River troc-tolite to exhibit rhythmic layering. A melatroctoliteor, locally, a dunite occurs at the base of eachrhythmic unit. It grades upward into a troctolite con-taining sporadic layers of melatroctolite a few cen-timetres thick. Above this, there is a sharp contactwith the overlying melatroctolite or dunite of the nextunit, and the sequence is repeated. The thickness ofthese rhythmic units varies from 2 to 15 metres. Therhythmic layering is approximately parallel to adistinct foliation exhibited by the rocks in this unit.The dip of this foliation varies within a single coreand between cores. The dip is commonly at a highangle to the core axis.

The textures seen in unit P (Fig. 4) are quite dif-ferent from those seen in the other units (Table l).Furthermore, unlike samples from the re$t of theunits, olivine and augite in unit P have.commonlybeen serpentinized. Plagioclase, in most places, isfresh, although locally it is saussuritized. As the rocksin unit P become more felsic. the effects of altera-tion decrease.

Units A, B, and C

Units A, B and C together form a zone of con-taminated rocks in the basal 60-90 m of the PartridgeRiver troctolite. Petrographically, these three unitsare distinguished from each other and from unit Mby their modal mineralogy and the textural featuresexhibited by olivine, augite and the accessoryminerals (Table l). Modal data show that, except forlocal variation, the abundance of olivine decreaseswith depth. Over the same interval, augite, biotite,plagioclase - orthopyroxene symplectite andhypersthene increase in abundance.

Unit C, the uppermost of the contaminated units,has gradational contacts with unit M above and unitB below. Short intervals of unit C can be foundwithin unit M, several metres above the contact. Twofeatures are diagnostic of unit C. Olivine is poikiliticthroughout and forms oikocrysts up to 15 mm indiameter. Biotite commonly occurs as subhedralepitactic crystals on augite and, Iocally, olivine (Fig.5).

Unit B has no distinguishing textural features. Itcan be recognized by its placement between units Aand C, the lack of textures characteristic of theenveloping units and the continuation of the chemicaltrends exhibited by units A and C, Olivine is in-terstitial to subpoikilitic. Biotite occurs along theboundary between augite and plagioclase crystals,but it lies parallel to this contact, and its growth isnot oriented relative to the augite (Fig. 6).

Unit A, the lowermost of the contaminated units,lies directly above the contact of the Partridge Rivertroctolite with the Virginia Formation and commonlycontains xenoliths of the slates. The xenoliths aretypically fine grained and composed of plagioclase,pyroxene, biotite and, locally, cordierite. Olivine,where present in unit A, occurs as interstitial to sub-poikilitic crystals. With depth, olivine decreases inabundance and disappears. In the same intervai,hypersthene increases in abundance. The commonoccurrence of hypersthene is diagnostic of unit A.A second characteristic of unit A is the oscurrenceof apatite as inclusions in plagioclase (Fig. 7) orassociated with biotite or augite.

Bosal contoct

The contact between unit A and the Virginia For-mation is the basal contact of the Partridge Rivertroctolite. In the four cores studied, the contact maybe sharp or gradational over one metre. Elsewherein the region, the contact zone may be tens of metresthick (Kirstein 1979, Weiblen & Morey 1980).Recognition of the basal contact is hampered by thefine-grained nature of the intrusive and metasedi-mentary rocks in this zone. It is further complicatedin drill core by the presence of xenoliths of theVirginia Formation and Keweenawan basalt and oftroctolitic dykes in the footwall rocks.

Chemistry

Several chemical trends are apparent in the con-taminated units, as indicated by whole-rock composi-tions (Fig. 8), modal data and mineral compositions(Fig. 9).Data from DDH-297 (Fig. 8) aretypical ofthe trends present in the four cores. The anorthitecontent of plagioclase decreases from Ary6 to An'

THE PARTRIDGE RIVER TROCTOLITE, MINNESOTA 29

? R n

375

400

425

450

475

500

525 l 2 t 4 1 6 t 8 0

"/"Fe

ro 20 30 40200 225 ?50 27s 3000 50 roo t50 200

ppm Rb ppm sr ppm zn

Ftc. 8. Whole-rock concentration of strontium, rubidium, iron and zinc in DDH-297 determined by X-ray fluorescence.The petrographic units (M, P, C, B, A, H and BIF) and depth in metres are shown to the left. H hornfels derivedfrom the Virginia Formation; BIF Biwabik iron-formation. The curves are interpreted in the text,

within the bottom 200 m of DDH-297. Over thesame interval, the forsterite content of olivinedecreases from Fo6a to Fo3r. This trend is parallel-ed by an increase in iron in the rock (Fig. 8). Theolivine curve does not exhibit a smooth transition,perhaps reflecting inhomogeneities in the magma orincomplete mixing of an iron-bearing componentfrom a source external to the magma. The initiationof sodium enrichment in plagioclase and iron enrich-ment of olivine coincides approximately with the topof unit C (defined by petrography). In the same in-terval, whole-rock rubidium increases and strontiumdecreases.

Unit A shows some definite chemical trends thatdo not occur higher up in the section. Biotite in-creases from 2-3t/o of the rock in unit M to l09o inunit A (Fig. 9), which reflects an increase inpotassium content with depth. An increase in theamount of Si in unit A is inferred by the decreasein olivine and the increase in hypersthene. As willbe discussed below, these compositional variationsat the base of the section are due to contaminationfrom the underlying metasedimentary rocks,especially the Virginia Formation.

OxroE aro Sur.r'rne PETRocRApHy

The opaque oxides occur as grains interstitial tothe silicates. They are commonly associated withbiotite, which typically mantles them. The predomi-nant opaque oxide is ilmenite, which exhibits no ex-solution. Magnetite, the other oxide present, occurslocally. Its presence shows no relationship to thepreviously described stratigraphy or chemical trends.The magnetite is titaniferous, as evidenced by thepresence of ulv6spinel and ilmenite exsolutionlamellae.

Sulfides occur sporadically throughout the drillcores examined. The "cloud zones" and the basalzone are two horizons of zulfide concentration. The"cloud zones" are 15- to 30-metre-thick intervalsnormally containing less than I modal 9o sulfides(e.9., note 355 m depth on Fig. 13). They occur hun-dreds of metres above the other major area of sulfideconcentration, in the basal zone. The "cloud zones"may be sulfides that were original components of themagma and not the result of contamination by theVirginia Formation.

The basal zone has a higher abundance (0.590 to

M

Y_

M

B

H

A

H

3 l F

30

< q n

? 7 R

400

425

450

475

q o n

s25

>3Vo) of sulfides and tends to be thicker (100-200m) than the "cloud zones". The major sulfide

Frc. 10. Textures typical of the sulfides in the PartridgeRiver troctolite, The sulfides (opaque) are interstitialto the silicates, locally replace plagioclase, form veinletswithin plagioclase cleavages and exhibit eutectoid rela-tionships with pyroxene. All these textures are illustratedin this photogaph. Note the biotite O) mantling thesulfides. Plane-polarized transmitted light, Scale bar isI mm.

THE CANADIAN MINERALOGIST

506070 0 1o2030 0 lo o lo% A n m o d o l 7 " m o d o l o / o m o d o l % b i o l i t e

o I i v tne hypers thene

minerals in both horizons are pyrrhotite, cubanite,chalcopyrite and penllandite. Minor amounts oftroilite, bornite and valeriite are also present.

Chalcopyrite is the predominant sulfide in the"cloud zone" and in the upper part of the basal zone.However, with increasing depth in the basal zone,cubanite and pyrrhotite become more importantmodally. At the bottom of the basal zone, pyrrhotiteand cubanite predominate, and locally chalcopyriteis absent. Pentlandite is present in variable amounts,exhibiting no particular pattern throughout eithersulfide zone.

The sulfides exhibit three textural relationshipswith the silicates. The bulk of the sulfides occurs asgrains interstitial to the silicates (Fig. 10). In the basalzone, this texture locally grades into a net textureof sulfide around silicate crystals and, rarely, tomassive sulfide.

Sulfides also occur as fine veinlets connecting in-terstitial grains. These veinlets generally occur withinplagioclase crystals, occupying cleavages (Fig. 1l)within the silicate. The veinlets are typically enrichedin copper relative to the associated interstitial sul-fides. A third textural occurrence of the sulfides is

35 45 55 65"h Fo

Frc. 9. Proportion of forsterite component in olivine, of anorthite component in plagioclase and modal abundancesof olivine, hypersthene and biotite versas depth in DDH-297. The petrographic units (defined in Fig. 8) and depthin metres are shown at the left. These curves are interpreted in the text.

?

THE PARTRIDGE RIVER TROCTOLITE. MINNESOTA 3 l

FIc. ll. A sulfide veinlet (S) following cleavages inplagioclase. The sulfide is dominantly chalcopyrite.Plane-polarized reflected light. Scale bar is 0.1 mm.

as eutectoid intergrofihs with either ortho- orclinopyroxene (Frg. 10). One (rarely, more) of thesulfides chalcopyrite, cubanite and pyrrhotite occurin these intergrowths.

Each of the major sulfides can make up an entiregrain, but commonly, two or more sulfides will com-prise a grain. In this case, a variety of textures canbe seen between the sulfides. Where cubanite andchalcopyrite occur together, they commonly exhibitmutual exsolution textures (Fig. l2).

The textural relationships between pyrrthotite andchalcopyrite are varied. Some chalcopyrite (andcubanite) is present as lamellae within pyrrhotite.More commonly, blebs of the copper-iron sulfidesare present on the edges of pyrrhotite grains (Fig.l2). This latter texture may be interpreted in one ofthree ways. Chalcopyrite and cubanite may havereplaced already crystallized plrrhotite, as has beensuggested by Boucher (1975); the sulfides may havebeen annealed, with the copper-iron phasesmigrating toward the boundary of the pyrrhotite, orthe texture may be explained by fractionation of asulfide liquid, enriching it in copper. In their discus-sion of the phase relationships in the system Cu-Fe-S, Kullerud et al. (1969) indicated that a sulfideliquid will crystallize Ms,r more iron-rich than theoriginal liquid; the resulting liquid would be pro-gressively enriched in copper.

Pentlandite, the other major sulfide phase, occursas rounded blebs within pyrrhotite. Pentlandite iscommonly cut by fractures that may be filled withchalcopyrite or cubanite. Troilite occurs as exsolu-tion lamellae in pyrrhotite in the copper-poor zones.Conversely, bornite has exsolved from chalcopyritein the copper-rich zones.

Chemical trends of the sulfides are presented inFigure 13. Sulfur exhibits a sharp increase that coin-cides with the contact between units B and C. Below

Frc. 12. Typical texture of sulfides in the basal zone of thePartridge River troctolite. Copper-iron sulfide is pre-sent as a bleb at the edge of a pyrrhotite grain (mediumgrey). Note tle exsolution relationship betlveen cubanite(ight grey) and chalcopyrite (white). Plane-polarizedreflected light. Scale bar is I mm.

the level of this increase, the sulfur content remainshigh (generally greater than 0.5Y0). This same trendis exhibited by the other three cores. The copper-nickel ratio of the sulfides shows an increase withdepth that starts at the top of unit C. The increasein this ratio suggests an influx of copper relative tonickel into the magma or, conversely, an increasingpartition of nickel into olivine with depth. The iron-nickel and iron-copper ratios are quite variable.However, both curves exhibit a marked increase im-mediately at the base of the intrusive body.

PBrnocsNesrs oF UNrrs M am P

Of the five silicate units described, unit Mrepresents the main intrusive body of the PartridgeRiver troctolite. It exhibits no foliation, rhythmiclayering or chemical trends indicative of accumula-tion of crystals. Unit M in this part of the Ely-HoytLakes region appears to have crystallized in place.Units A, B and C are a result of contamination ofunit M as described below.

Unit P, as de,scribed above, is composed of olivineorthocumulates and, in its more felsic portions,olivine-plagioclase orthocumulates. It is quite dif-ferent texturally and compositionally from the otherstratigraphic units in the Partridge fuver lroctolite.It is enveloped by rocks of the Main Unit and ex-hibits rhythmic layering typical of that seen in otherlarge mafic intrusive complexes (Wager & Brown1967). Unit'P was not intruded into unit M as in-dicated by its characteristics:

l. Unit P is mafic to ultramafis. The magma fromwhich such rocks crystallized would have been veryhot (ca. 1300'C) and would have metamorphosed

M

P

M

c

B

H

A

H

BIF


- .o 05 l .o t5 2 .o o lo o 1o2030 0 ro20

%S CulN i Fel5 I Fel6u

Frc. 13. Copper-nickel, iron-nickel and iron-copper ratios in the sulfides and the sulfur content versus depth inDDH-297. The petographic units (defined in Fig. 8) and the depth in metres are shown at left. The raw data fromwhich these curves were derived are confidential and property of the company. These curves are interpreted in the text.

l q n

3 76 ,

400

425

450

475

5 n n

3 2 3

the surrounding rocks in unit M. Unit M near thecontact lacks high-temperature metamorphism oreven metamorphic textures.

2. No chilled margins are evident in Unit P.3. Unit P is highly serpentinized, whereas the

plivine in the surrounding rocks is fresh. The serpen-tinizing fluids affected only unit P and not the en-closing troctolites, suggesting that unit P is older thanunit M and was altered prior to the emplacement ofunit M.

4. Unit P is cut by dykes of unserpentinized rocksimilar to those found in the enclosing troctolites.

5. The foliation of the rocks in unit P and theirheight above the basal contact vary from core tocore. Locally, they can be found on the base of thecomplex (Hardyman 1969).

6. Unit P is commonly associated with hornfelsderived from the Virginia Formation and the Ke-weenawan basalts.

7. Unit P is not found throughout the PartridgeRiver troctolite.

8. Pieces of unit P may be found at more thanone depth within unit M in any one core.

The rocks making up unit P are probably xenotthsfrom an ealier intrusive phase of the Duluth Com-plex in the Ely-Hoyt Lakes region. Unit P was ini-tially intruded along the contact between rocks ofthe earlier Anorthositic Series and the older Precam-brian rocks (Fig. 14). In this position, the rocks ofunit P crystallized, forming the rhythmic layers seen.They were then serpentinized. With the intrusion ofthe Partridge River troctolite, pieces of unit P formedas a result of this earlier intrusion were ripped offthe floor and rafted up into the magma chamber.The present height of unit P above the base is a func-tion of its rate of sinking and the number of intrusivepulses pushing it up.

PrtnoceNEsrs oF UNITS, A, B eNl C

The mineralogical and chemical trends exhibitedby the silicates and the sulfides in the basal zone(units A, B anb C) are probably a result of the addi-tion of material to the Partridge River troctolite fromthe underlying Virginia Formation. The addition wasaccomplished in two ways. A partial melt, formed

THE PARTRIDGE RIYER TROCTOLITE. MINNESOTA 33

3*"n"';g'fiffi

ffifu**o o o o . - ;

ffik:;Ho o o o o o - o o

G,ANrs ,iAffi

{tk,.,.

Ftc. 14. The proposed model for the emplacement of unit P. This is a hypothetical cross-section drawn along A-A'in Figure l. Part A shows the situation prior to the emplacement of unit P. Part B is a hypothetical view of unitP in place after its emplace4ent. Part C shows the present occurrence of unit P. Details of this model are presentedin the text.

due to heating during the intrusion of basic magma,was derived from the Virginia Formation, mixed withand contaminated the magma. Secondly, sulfur inthe Virginia Formation *u, srslilized by the heatofthe intrusion and contaminated the basal zone ofthe Partridge River troctolite; Both mechanisms wereimportant, but to varying degrees in the core samplesstudied.,

Psrtial melting

The enrichment of silicon, sodium and potassiumexhibited by the troctolites in the basal zone (these

elements have been added, as indicated by thecrystallization of hypersthene instead of olivine, theenrichment in Na and Si of plagioclase and the in-crease in the abundance of biotite with depth) arewhat would be expected should a granitic melt bemixed with the magma. Experimental work on themelting of shale in the presence of water indicatesthat melting commences at about 7@oC at a pressureof 2.5 kbar (Wyllie & Tuttle l96l)..The inirial liquidhas a granitic composition, rich in Na, K and Si. Therefractory material is composed of quartz, mullite,cordierite, hypersthene and, if sufficient calcium ispresent, Ca-plagioclase.


Evidence for such a process is also available fromnatural occurrences. In his study of the basal zoneof the Stillwater Complex, Barker (1975) noted thatmetasediments in the contact aureole consist of cor-dierile, orthopyroxene, plagioclase and quartz. Heproposed that water, K, Rb, Na, Si and Zr were lostfrom the metasediments and migrated into themagma via a partial melt. Similar effects were notedin the metamorphic aureole of the Cashel - LoughWheelaun intrusive complex in Eire (Leake & Skir-row 1960, Evans 1964). In both cases, the partialmelts caused contamination of the basal part of thepluton. Is there evidence supporting the occurrenceof this process in the basal Partridge River troctolite?

The Virginia Formation rocks, which have beenunaffected by heat from the Duluth Complex, arecomposed of chlorite, plagioclase, muscovite, quar1iz,,carbonates and pyrite. Xenoliths of these rockswithin the Duluth Complex commonly containplagioclase, cordierite, quartz and biotite, similar toWyllie's & Tuttle's (1961) refractory as$emblage.Renner (1969) described the basal igneous rocks ofthe South Kawishiwi intrusive body and the footwallVirginia Formation in the Erie Mining Company'sDunka River mine (Frg. l). The contact-zone in-trusive body has been enriched in sodium and siliconrelative to the overlying igneous rocks @enner 1969).Renner estimated that some of the Virginia Forma-tion must have been melted to produce the con-tamination noted. The partial melt, he proposed,then moved into the magma and contaminated it,forming the contact zone.

Bonnichsen (1972) investigated the chemistry ofthe Virginia Formation in the contact zone and inxenoliths. The metamorphosed Virginia Formationcontained progressively more Ca, Al, Mg and Fe asthe metamorphic grade increased from the footwallup to the contact, and from large to small xenoliths.The enrichment could be explained by depleting themetasediments and enriching the magma in Na, K,P, Si and CO2 (Bonnichsetl9T2), Bonnichsen pro-posed that a partial melt of the Virginia Formation,originating at the contacts of the slates with the troc-tolites and from xenoliths, was responsible for themovement of these materials out of the metasediments. The abundance of xenoliths in a portion ofthe magma would dictate how much contaminationoccurred (Bonnichsen 1972).

One major problem with tle hypothesis is the hightemperature required for the formation of a graniticpartial melt (700"c: wyllie & Tunle 196l). However,such a temperature may have been achieved as aresult of three closely spaced events occurring in thefollowing sequence: (1) Initially, the intrusion of theAnorthositic Series would have raised the tempera-ture of the Virginia Formation near the contact. (2)The next intrusive went was the emplacement of unitP. This caused the temperature in the upper portion

of the Virginia Formation to increase to about600'C. At this temperature, some of the chlorite inthe metasediments at the contact would have beenconverted to biotite, which caused a release of water@eer et al, 96A, The water migrated into unit P,where serpentinization occurred during the laterstages of, or shortly after, crystallization. (3) Whenthe troctolites were intruded, the rise in temperatureof the immediate footwall rocks and xenoliths of theVirginia Formation only had to be on the order of100-200"C to allow partial melting to occur.

A second objection to the partial-meltinghypothesis is based on oxygen-isotope @ao & Ripley1980, Ripley & Rao 1980) and strontium-isotope data(Grant & Molling 1981); these data indicate thatlarge-scale assimilation of material from the VirginiaFormation by the intrusive complex is highly unlike-ly. However, contamination by a partial melt derivedfrom the counlry rocks on the basal contact andxenoliths of the Virginia Formation could take placeon a local scale (Rao & Ripley 1980, Ripley & Rao1980). Rare granitic veins and selvages, remnants ofthis melt, are present in the troctolites and aroundxenoliths (Fig. l5). Contamination did not occurthroughout the intrusive complex, but only in thebasal 100 m. Here the concentration of xenoliths ofVirginia Formation is greatest, and the texturalevidence of partial melting is strongest.

Two major contaminants that remain unexplainedby the partial-melting hypothesis are iron and sulfur.Grant & Molling (1981) suggested that the high con-tent of iron in olivine near the base is due to the reac-tion of cumulus olivine with an intercumulus liquidrich in iron. As noted in this study, the olivine inthe basal zone is nol a cumulus mineral but ratheran intercumulus phase. There is no evidence in-dicating that crystals accumulated in this portion ofthe Partridge River troctolite. No rhyhmic layeringis present, and there is no foliation of tJte plagioclase,the first phase to crystallize. The rocks appear tohave crystallized in place. A more reasonablehypothesis is that the iron enrichment is associatedwith the sulfur enrichment.

Sulfide contsminotion

The sulfides typically occur interstitially to thesilicates and locally show textures indicative ofreplacement of the silicates (Fig. l0). Layers ofmassive sulfide with graphite may have formed bygravity accumulation of an immiscible sulfide liquid.However, most of the sulfides in the basal zone ofthe Partridge River troctolite formed by crystalliza-tion from the magma. The sulfides solidified lete inthe sequence of crystallization of the magma, as in-dicated by the interstitial nature of the sulfides andtheir local replacement of the silicates.

Evidence from the present study and other in-


vestigations indicates that the bulk of the sulfur inthe Partridge River troctolite was derived from theVirginia Formation. The increase in abundance ofsulfur with depth in the basal zone supports thishypothesis. Sulfur-isotope studies (Mainwaring &Naldrett 1977, Ripley l98l) indicate thar rhe sulfuroriginated from a sedimentary source, inferred to bethe Virginia Formation. The Virginia Formation isa likely source, owing to the presence of pyrite-richbands and tabular bodies in it and its stratigraphicequivalents (Morey 1.972a, Matlack 1980).

Ripley (1981) proposed that sulfur was releasedas a result of the breakdown of pyrite to pyrrhotite.Other volatiles, liberated from the Virginia Forma-tion at the same time Oy the heat of the intrusion),mixed with the sulfur to yield a fluid composed ofCO2, CH4, H2S and H2O. Aided by the turbulentnature of the intrusion, this fluid migrated into themagma owing to lower partial pressure of water andsulfur in the silicate liquid. It then dissolved in tnemagma, contaminating it.

The increase of the copper-nickel and iron-nickelratios in the sulfides with depth (Fig. 13) lras not beenaddressed by previous workers. The trends of theratios could be explained by nickel entering olivinepreferentially with depth. Hence, there would be lessnickel to enter the sulfides. However, other studiesof the basal zone of the Troctolitic Series reveal thatthe amount of nickel entering olivine does not in-crease markedly with depth, and the amount of NiOpresent in olivine is rarely greater than 0.20 wt.go(Fukui 1976, Bonnichsen et ql.1980). Secondly, thewhole-rock analyses for iron show that this elemenris increasing in abundance with depth (Fie. 8) andis being introduced to the basal pafi of the magmacolumn. By analogy, the trend seen in the copper-nickel ratio could be due to the addition of copperto the basal zone.

The Virginia Formation, in the contact zone, son-tains thin stratiform bands composed of pyrrhotiteand graphite, which were interpreted to be syngeneticwith the sediments (Matlack 1980). The presence (orabsence) of copper within these bands was norestablished by chemical analysis. However, Morey(1972b) reported on pyrite- and pyrrhotite-richtabular bodies within the Thomson Formation (thestratigraphic equivalent of the Virginia Formationto the southwest), which had been investigated as apossible source of sulfur. Other sulfides present inthe bodies include minor amounts of chalcopyriteand sphalerite . Zinc avaages 0.029t/o of the rock andcopper about 0.0590 (Morey 1972b). As the Thom-son and Virginia Formations are stratigraphicallyequivalent and petrographically similar, the bandspresent in the Virginia Formation could be expectedto have a mineralogical composition similar to thebodies in the Thomson Formation.

Much of the copper present would be removed by

Frc. 15. A portion of drill core showing a xenolith (X, inthe lower left portion) of Virginia Formation and augitetroctolite containing sulfide and biotite. The two rock-types are separated by a selvage (G) of light materialhaving a granitic composition. Note the accumulationof granitic material in the upper-left-hand portion ofthe photograph. Incident light. Scale bar is 5 mm.

the sulfide-bearing fluid formed from these bands,as would zinc. The hydrous fluid proposed by Ripley(1981) would have contained HS- and OH- owingto the dissociation of H2S and H2O. Iron and cop-per from the sulfide bands would complex with theseradicals and be transported by the fluid into themagma and mixed with it. Zinc would behave in asimilar manner. As a result, zinc should (and does)follow the trend displayed by copper (Fig. 8).

Nickel, on the other hand, is not reported as oc-curring in the Virginia Formation or its equivalents.It was probably derived from the magma and ap-pears in the sulfides, owing to its affinity for sulfur.Additional data indicating that copper and nickel hadtwo different sources are contained in a report onthe copper-nickel resources in the Ely-Hoyt Lakesregion (Fig. l) by Listerud & Meineke (1977).Although values of the copper-nickel ratio varywithin each area, the averages for each area aresignificant. To the northwest of Birch Lake, the ratiois 2.7:l.In that area, no Virginia Formation is ex-posed on the surface, and only short intervals of itwere encountered in drill cores. In the area of theDunka mine, southwest of Birch Lake but northeastof the present study-area (Fig. l) and near Renner's(1969) study-area, the ratio is 3.2:1. Here a limitedamount of Virginia Formation is present. In the areaof the present study, the ratio is 4:1, and extensiveamounts of Virginia Formation are present. Ripley(1981) reported that in the Dunka Road deposit tothe southwest (Fig. l), the copper-nickel ratio isgleater than 5: I . From these data, it can be inferredthat the abundance of copper relative to nickel isrelated not only to the presence but also the abun-dance of the Virginia Formation in xenoliths and thefootwall.


Could there be any other components of thehydrous sulfide-bearing fluid? Shamazaki & Clark(1973) and Burnham (1979) suggested that alkaliscould also be a component of this type of fluid. Ifsodium werr present in the fluid, it would be dis-solved in the magma with the other components. Itshould appear enriching some phase; as noted above,plagioclase does show an increasing content ofsodium with depth. Potassium may also have beenadded to the magma by the same mechanism.Potassium enrichment is indicated by the increasingabundance of biotits with depth. The common as-sociation of biotite with the sulfides could be ex-plained by their coprecipitation. However, the in-crease in the abundance of biotite roughly parallelsthe abundance of sulfur and $uggests that potas$iummay be a part of Ripley's (1981) sulfide-bearing fluid(Shamazaki & Clark 1973).. The increase in theabundance of rubidium with depth in the con-taminated zone @ig. 8; Grant & Molling 1981) andits decrease in xenoliths and Virginia Formation inthe contact zone (Grant & Moline 1981) may be ex-plained in t}te same way because Rb, an alkali, wouldbe expected to behave similarly to Na and K.

The hydrous S-bearing fluid may also provide themechanism for the addition of iron to olivine. Theabundance of rubidium in the contact zone parallelsthe iron enrichment in olivine (Grant & Mollingl98l), suggesting a similar source. We propose thatiion was also carried in from the Virginia Forma-tion by this fluid. When the fluid was dissolved inthe rnagma, iron was also. If the fluid was addedearly in the crystallization sequence of the magma,as suggested by the sodium enrichment of plagio-clase, iron would partition into the olivine when itcrystallized.

CouclusloNs

The Partridge River troctolite can be divided intofive stratigraphic units on the basis of mineralogy,textures and chemical data. Unit P is the onlycumulate present; it represents xenoliths from anearlier related intrusive body. Enveloping unit P isunit M. Its textures, mineralogy and chemical trends€ue very complex; it probably formed by more thanone intrusive pulse. The bottom three units (A, Band C) are correlatable along strike for a distanceof about I km. This basal zone is the result of con-tamination from the underlying Virginia Formationand its xenoliths within the intrusive body. Uponemplacement of the magma that gave the PartridgeRiver troctolite, a fluid rich in sulfur, water, alkalis,iron and copper was formed. It migxated into andwas dissolved into the magma. This dissolution prob-ably occurred during the crystallization of plagio-

clase,'the first phase to form. It enriched the plagio-clase in sodium. Later, as crystallization of themagma proceeded and olivine started to form, asmall amount of granitic partial melt was producedin the Virginia Formation along the troctolite con-tact and in xenoliths. This partial melt only affectedthe magma in the vicinity of the Virginia Formation,and thus remnants of melt have been preserved asveins or selvages around xenolilhs (Fig. l5). This par-tial melt added Si to the magma and caused the for-mation of orthopyroxene in place of olivine. Dur-ing the last stages of crystallization, the sulfidessolidified. They either formed an immiscible liquidor,solidified directly from the magma; evidence forboth processes is present. As most of the sulfide wasadded to the basal section and did not have the op-portunity to cdmpletely mix with the magma, it isin the basal zone that the sulfide is most concen-trated. The lack of complete mixing accounts for thechemical trends seen in the silicates and the sulfides.It indicates that not only was sulfur derived fromthe footwall rocks but alkalis, iron and copper wereas well. This is borne out by the regional values ofthe copper-nickel ratio, which correlate well with theproportion of Virginia Formation present in thefootwall.

AcrNowLnocslasNts

We especially thank Bill Bonnichsen for introduc-ing us.to the Duluth Complex and its many problems.Our appreciation is extended to AMAX Exploration,Inc., of Babbitt, Minnesota, and to Stan Watowich,formerly of that company, for providing us withsamples and assay data. Stan Watowich, GordonGumble, Bill Bonnichsen and Jill Pasteris are thank-ed for earlier discussions. The Bilby Research Centerat Northern Arizona University provided facilitiesduring part of this study.

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Received May 20, 1982, revised manuscript acceptedSeptember 7, 1983.

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