Canadian Mineralogist Yol.22, pp. 39-53(1984) FURTHERINTERPRETATTON OF THE Gu-Fe-Ni SULFTDE MINERALIZATIONIN THE DULUTHCOMPLEX, NORTHEASTERNMINNESOTA JILL DILL PASTERIS Department of Earth and Planetary sciences, McDonnell center for the space sciences, Ilashington University, Box j169, St. Loui.s, Missouri 6SnA, U.S.e. ABSTRACT Par contre, la pr6sencede trofite et de graphite, d6pos6 en phasevapeur, et d'ilmdnite sansmagndtite, dansla suite Approximately lQ sample5mineralized wilh Cu, Fe and Minnamax, indiqueraientun milieu plus r6ducteurque celui Ni sulfidesfrom the South Kawishiwi (Inco deposit)and du glte Inco. Les observationssuivantes montrent qu'une Partridge River (Minnamax deposit) intrusive complexes phase fluide mobile r6active fut active au stade tardif du werestudied by meansof the optical microscopeand the magmatisme:graphite d6pos6par phasevapeur dans les electron microprobe. Cathodoluminescence,microther- fractures de silicates non alt6r6s, inclusions fluides secon- mometry and liaser-Ramanspectroscopy were used on some daires chlorif&res dans l'olivine, remplacementd'agrdgats samples.In the two suites,both dominatedby troctolite, grossiersde pyrrhotine par de la cubanite massive,et pr6- the sulfides are mineralogically similar (predominantly pyr- cipit6s de sulfure de fer et nickel i grain fin le long de frac- rhotite, pentlandite, chalcopyrite, cubanite) and also tex- tures dansl'olivine serpentinis6e.La grandevariation de turally similar. In the Inco suite, however, both magnetite composition de la pentlandite indiquerait aussi un r6dqui- and ilmenite are present,whereas troilite is rare. In con- libre post-magmatiquepartiel. Un bain sulfur6immiscible trast, the occurrence of troilite and vapor-deposited pr6coce(d rapport Fe:Cu 6lev6)et un fluide magmatique graphite, as also the presen.sel ilmenite without magnetite, ultdrieur enrichi en cuivre sont responsablesde la min6ra- in the suite from Minnamax, suggeststhat the environment lisation dansles troctolites:le liquide sulfur6, par pr6cipi there was more reducing than at Inco. The following obser- tation primaire et le fluide cuprifbre, surtout par rempla- vations show that a mobile, reactivefluid phasewas ac- cement. Le mouvement et la composition du fluide durent tive during late-stagemagmatism: vapor-deposited graphite avoir 6t6 gouvern6spar un procepsusintercumulus normal in fractures in unaltered silicates, Cl-bearing secondary dansle complexeintrusif, hypothbsequi du moins concorde fluid-inclusions in olivine, replacementof coarsepyrrhotite avecles donn€essur isotopesdu soufre(Ripley l98l) pour segregationsby massivecubanite, and precipitatesof fine- le glte de Dunka Road, situd dansle pluton de Partridge grainediron-nickel sulfide along fracturesin serpentiniz- River. ed olivine. Widely variable compositionsof pentlanditealso suggestpartial postmagmaticre-equilibration. Both an early Traduit par la Rddaction) immiscible sulfide melt (with a high Fe:Cu ratio) and a later Cu-enrichedmagmatic fluid producedmineralization in the Mots-clds:sulfures de Cu, Fe et Ni, complexede Duluth, troctolites, the sulfide melt by primary precipitation, the pluton de Partridge River, gite Minnamax, pluton de Cu-bearingfluid mainly by replacement.The movemenr South Kawishiwi, glte Inco, Minnesota. and composition of the fluid were controlled by normal intercumulusprocesses in the complex.This hypothesisis in accordwith the sulfur-isotopedata of Ripley (1981)for INtnorucrroN the Dunka Road depositin the Partridge River pluton. Sample locqtion Keywords:Cu, Fe and Ni sulfides,Duluth complex,Par- tridge River pluton, Minnamax deposit, South Approximately 20 specimen$of mineralized drill- Kawishiwi pluton, Inco deposit, Minnesota. core and hand samples were studied from the Inco and Minnamax mines sites in the Duluth Complex SOMMAIRE (Fig. l). The Inco material (provided by M. Boucher) consists predominantly of troctolites and augite- On a 6tudi6au microscopeet A la microsondeenviron bearing troctolites from the South Kawishiwi in- 20 6chantillons min6ralisdsen sulfures de Cu, Fe et Ni des trusive body, near the site ofthe Inco shaft (Cooper complexesintrusifs de South Kawishiwi (gite Inco) et par- et al. 1978t Fig. I ofthis paper). The exact localities tridge River Glte Minnamax). Certains dchantillons furenr of the individual samples are not 6tudi6s par cathodoluminescence,microthermom€trie et documented. spectroscopieRaman au laser.Dans les deux suites. domi- The Minnamax samples, representing a section of n6espar la troctolite, les sulfures(surtout: pyrrhotine, pent- the adjacent Partridge River troctolite body, are landite, chalcopyrite, cubanite) sont min6ralogiquementet macroscopically similar to the Inco suite. These texturellement semblables.Dans la suite Inco, la magndtire documented drill-core samples are from hole 146 in et I'ilmdnite sont presentes,alors que la troilite y est rare. the southern end ofthe Tiger Boy deposit, and repre- 39 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/22/1/39/3435397/39.pdf by guest on 02 October 2021 40 THE CANADIAN MINERALOGIST and fine-grainedgraphite was done by F. Adar (In- struments SA) and F. Purcell (Spex Corp.). The author performed later analyseswith an Instruments SA RamanorU-l 000. Quantitativewavelength-dis- persion analysis(WDS) was carried out on selected ..6 automated "so)Iir!"'..$_..n' sulfides,oxides and silicateswith a fully ..a. t\i ... - trr JEOL 733 electron microprobe at Washington University. The standards are natural minerals and synthetic materials. Correction proceduresfor the analysesare based on Bence& Albee (1968) and Albee & Ray (1970).The operatingconditions were 15 kV acceleratingvoltage with currents of 50 nA for oxide, 30 nA for feldsparand 30 nA for sulfide phases.The sulfide dataarethe primary concernof this paper. l:;cli'.'".0* Previous studies Two previous detailed studies addressedthe Yi."'"' mineralogy,textures, relative abundances and com- .oap"ff positions of sulfide phasesin troctolitic sequences at the Inco site and in a drill hole south of Babbitt [Weiblen& Morey (196) and Boucher(1975), respec- tivelyl. Other reports describe the more general geologicalrelationships of the sulfide depositsto the from Coopgr, potential 6t a|.1978 Duluth Complexas well astheir economic (e.g., Bonnichsen 1972, I 974, Tyson & Chang I 984). More recently, oxygen-and sulfur-isotopeanalyses have been periormed on the Dunka Road deposit (Ripley& Rao 1980'Ripley 1981)and rubidium and FIG. l. Generalizedgeologic map of a portion of the strontium-isotopeanalyses on a Minnamax drill core Duluth Complex,showing sampling areas and other (Grant & Molling 1981),b.oth in the PartridgeRiver sulfidedeposits. 1 Inco site,2 Minnamaxsite, 3 U.S. studies, together with the (map troctolite. The above Steelsite, 4 WaterHen intrusivecomplex after & NaldretN(1977) Cooperet al. 1978and Ripley1981). sulfur-isotopework of Mainwaring on the Cu-Ni mineralizationin the Water-Hen in' trusive complexto the south, suggestthat reaction of melt and country rocks may havestrongly affected sentdepth levelsof 173to 586metres (samples pro- the sulfide precipitation in the Duluth Complex; they vided by S. Watowich, AMAX). The mine site is concludedthat much of the sulfur may have been pres(:ntlyowned by Kennecott,but the name Min- derived from the intruded sediments.The chemistry nam€x is retained here because AMAX was the and pfunicaltransport aspects of the mechanismremain operator during this program of drilling and sam- to be explained.It thus appears,from petrographicand pling. The silicaterock-types are predominantly troc- strontium-isotopeanalysis (Grant & Molling 1981), tolitic, but a hornfels zone that may represent that vast amountsof the underlying carbonaceous, xenoliths of underlying metasedimentary and sulfide-bearing Virginia Formation were not metavolcanicrocks also was s€urpled.The Cu-Ni physically assimilated by the Duluth magmas. sulfide mineralization consists of both the However, H2O, CO2 and other volatiles may have disseminated and massivg (Local Boy) type been selectivelyassimilated by the melt (c/ Weiblen (watowich 1978). & Morey 1976,Hollister 1980,Ripley 1981). The immediateaims of the presentstudy are (l) Techniques of analysis to compare and contrast sulfide relationshipsob- servedin the two depositschosen (Iable l) with those This overview involved a study by optical previously reported and (2) to re-interpret thesedata microscopyof about 2.0doubly polishedthin sections in light of more recentinformation on the composi- jn transmitted and reflected light. Microther- tion of late magmatic fluids and on intercrrmulus mometry, Raman spectroscopyand cathodolumines- processes.Of particular coniern are the following cencewere performed on severalsamples. The in- points: the timing of sulfide immiscibility in the itial Raman-microprobeanalysis of fluid inclusions magmatic sequence, the factors causing sulfide Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/22/1/39/3435397/39.pdf by guest on 02 October 2021 SULFIDE MINERALIZATION IN THE DULUTH COMPLEX 41 'I. TABLE MINEMLOGYOF 1}IOOCCURRENCES OFSULFIDES IN THEDUTUTH COMPLEX Flne spinel Coarse Graphlte Coarse Coarse & llm. lamellae Sample Po Tr Cp Cb graphlte in veinlets ilmenite rngnetite in sllicates ARAA A in plag & Cpx MINNAI'IAX ASAA A in p'lag & Cpx Key'lJte' to abbrevlations: A abundant, S some, R rare, N not observed, po pyrrhotlte, Tr trol- Cp chalcopyrite, Cb cubanite. precipitation,the numberof episodesof mineraliza-