PLATINUM-PALLADIUM GROUP MINERALS, GOLD, SILVER, AND COBALT IN THE MINNAMAX COPPER-NICKEL SULFIDE DEPOSIT, DULUTH COMPLEX, NORTHEASTERN MINNESOTA

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1 Contents ABSTRACT... 3 ANALYTICAL PROCEDURES... 4 PLATINUM-GROUP MINERALS... 5 SILVER AND GOLD... 8 NICKEL COBALT ZINC AND CADMIUM COPPER LEAD VERY LATE STAGE ELEMENTAL MIGRATION CONCLUSIONS REFERENCES APPENDIX Appendix Table 1. Sample descriptions and gold, platinum, and palladium concentrations Appendix Table 2. Summary of sample mineralogy Appendix Table 3. Average electron microprobe analyses of selected silicate minerals in sample

2 MINNESOTA GEOLOGICAL SURVEY D.L. Southwick, Director PLATINUM-PALLADIUM GROUP MINERALS, GOLD, SILVER, AND COBALT IN THE MINNAMAX COPPER-NICKEL SULFIDE DEPOSIT, DULUTH COMPLEX, NORTHEASTERN MINNESOTA Peter L. McSwiggen Report of Investigations 54 ISSN Saint Paul 1999

3 PLATINUM PALLADIUM GROUP MINERALS, GOLD, SILVER, AND COBALT IN THE MINNAMAX COPPER NICKEL SULFIDE DEPOSIT, DULUTH COMPLEX, NORTHEASTERN MINNESOTA Peter L. McSwiggen ABSTRACT Electron microprobe studies show that rocks from the Minnamax copper-nickel sulfide deposit in the Mesoproterozoic Duluth Complex in northeastern Minnesota contain at least two platinum-group minerals (PGMs); froodite (PdBi 2 ) and cabriite (Pd 2 SnCu). These PGMs are commonly associated with massive sulfide mineralization. Thirteen samples were investigated in detail, and five of them contain either froodite, or cabriite, or both. Although individual PGM grains are rare, just a few 1 µm grains would account for a reported whole rock concentration of 10 ppm. The platinum group element (PGE) concentrations reported for these rocks do not require solid-solution of PGEs in sulfide minerals. Evidence for silver mineralization is also documented. Silver is present either in maucherite (Ni 11 As 8 ), or as discreet grains of native silver. Gold is present in a few of the native silver grains. Concentrations as high as 16 wt. percent gold were documented for 5 10-µm diameter grains, but gold concentrations are more typically 1 wt. percent. Cobalt is present in concentrations ranging from 0.5 to 6.0 wt. percent in maucherite, and from 1.0 to1.4 wt. percent in pentlandite ([Fe,Ni] 9 ). Detailed microprobe inspections and analyses of samples from a core drilled in the Minnamax deposit show that they contain a range of rare minerals, including dienerite (Ni 3 As), shadlunite [(Pb,Cd) (Fe,Cu) 8 ], altaite (PbTe), laurionite [PbCl(OH)], and cotunnite (PbCl 2 ). INTRODUCTION This investigation is part of a project undertaken by the Natural Resources Research Institute (NRRI) and funded by Minnesota Technology, Inc. and Arimetco International, Inc. The purpose of the project was to evaluate the mineral potential of the Minnamax coppernickel sulfide deposit (also known as the Babbitt deposit), a prospect located a few miles to the south of Babbitt, Minnesota. The Minnamax deposit is one of several copper-nickel sulfide deposits present at the base of the Duluth Complex (Fig. 1). The Duluth Complex is a series of Mesoproterozoic mafic to felsic intrusions that represent a major portion of the exposed Mesoproterozoic Keweenawan (1.1 Ga) Midcontinent Rift system (Paces and Miller, 1993). At its western margin the Duluth Complex is in intrusive contact with Paleoproterozoic ( Ga) metasedimentary rocks of the Virginia and Thomson Formations, and the Biwabik Iron Formation. To the north, the Duluth Complex is in intrusive contact with Archean granite and greenstone of the Wawa Province. To the south, the Duluth Complex is overlain by the volcanic and sedimentary rocks of the Mesoproterozoic North Shore Volcanic Group. Details of the geology of the Duluth Complex can be found in Miller and Weiblen (1990), Weiblen (1982), and Weiblen and Morey (1980). The Minnamax deposit is primarily a copper-nickel sulfide deposit that consists of several mineralized zones. The highest-grade zone delineated thus far contains an estimated 5 million tons of ore, that has an average copper concentration of 1.89 wt. percent (Severson, 1991). The ore body consists of troctolitic and ultramafic rocks that contain hornfels inclusions. The inclusions are derived from the underlying Paleoproterozoic metasedimentary rock, and the overlying Mesoproterozoic North Shore Volcanic Group. The copper and nickel are predominantly contained in disseminated sulfide minerals that make up between one and five percent of the rock. The dominant sulfide minerals are chalcopyrite, cubanite, pyrrhotite and pentlandite (Severson, 1991; Severson and Barnes, 1991). In addition to the copper and nickel, the deposit also contains significant concentrations of platinum, palladium, gold, silver, and cobalt (Miller, 1999; Hauck and others, 1997). 3

4 ;;;;; yyyyy ;;; yyy ;;; yyy yyy ;;; ;; yy ;;; yyy ;; yy yyybabbitt Minnamax deposit drill hole T57N T58N T59N T60N T61N T62N Mesoproterozoic Paleoproterozoic Archean MAP AREA EXPLANATION Duluth Complex Cu-Ni deposits Virginia Formation Biwabik Iron Formation Giants Range Granite Metasedimentary and metavolcanic rocks Faults 0 5 mi N R15W R14W R13W R12W R11W 0 5 km Figure 1. Location of the Minnamax copper-nickel sulfide deposit and Duluth Complex in northeast Minnesota. Geology after Sims and others (1970) and Green (1982). Locations of copper-nickel sulfide deposits from Severson (1991), For sample descriptions see Appendix. Location of drill hole shown by white dot. Core samples from the Minnamax deposit (see Appendix) contain as much as 3.1 ppm platinum, 7.0 ppm palladium, and 13.1 ppm gold. The high concentrations of platinum group elements (PGEs), other precious metals, and cobalt have been shown by others to be associated with the high-grade copper zones (Severson, 1991; Morton and Hauck, 1987; Hauck and Barnes, 1989; Geerts and others, 1990; and Kuhns and others, 1990). Therefore the precious metals may add significantly to the value of the deposit, if they can be extracted economically. The presence of PGEs and other precious metals in these rocks leads to a number of questions. How are the precious metals distributed in the ore deposit? What are their concentrations? What form are they present in? The identity of minerals that host the PGEs, and the sites that the PGEs occupy within the host minerals influence the ease with which these metals might be extracted. In this investigation the mineralogical setting of the platinum group elements (PGEs), as well as that of other precious metals (gold, silver, and cobalt) is reported. Thirteen samples were selected from a 300-ft core (underground drill hole 10198; see Appendix) for which conventional assay methods had already indicated high concentrations of copper-nickel sulfide minerals, PGEs, and other precious metals (see Appendix). In this study, the samples were analyzed with an electron microprobe to determine the location of the PGEs and other precious metals, as well as their mineralogical context and paragenetic histories. ANALYTICAL PROCEDURES A polished thin section of each sample was supplied by NRRI. The thin sections were repolished prior to carbon coating for electron microprobe analysis in order to remove any tarnish that had accumulated. Analyses were conducted by P. L. McSwiggen on a JEOL 8900 electron microprobe with wavelength dispersive spectrometers in the Department of Geology and 4

5 Table 1. Platinum-group minerals previously reported from the Duluth Complex Mineral Formula Reference sperrylite PtAs 2 Ryan and Weiblen (1984) Kuhns and others (1990) Ripley (1990) Mogessie and others (1991) froodite PdBi 2 Morton and Hauck (1987) Mogessie and others (1991) michenerite (Pd,Pt)BiTe Morton and Hauck (1987) Mogessie and others (1991) laurite RuS 2 Sabelin and others (1986) irarsite (Ir,Ru,Rh,Pt)AsS " moncheite (Pt,Pd)(Te,Bi) 2 Mogessie and others (1991) taimyrite (Pd,Cu,Pt) 3 Sn " unknown Pd 7 (Sb,Bi) 8 Morton and Hauck (1987) Pt Fe alloys Sabelin and others (1986) Pd alloys " Pt S arsenides " Pd arsenides " Geophysics of the University of Minnesota. An accelerating voltage of 20 kv was used for the sulfide minerals, and 15 kv for the silicate minerals. In all cases a probe current of 20 namps was used. Beam diameter varied with the size of the area being analyzed, and ranged from maximum focused to 20 mm in diameter. The standards used for the sulfide minerals were mostly pure metals, with the exception of pyrite (which was used for iron and sulfur), and galena (which was used for lead). Several approaches were used to determine the location of the PGEs and other precious metals. Initially, X-ray maps were used to show the distribution of different element concentrations for entire thin sections at scales that should show grains as small as a few microns. This approach required large amounts of electron microprobe time, and the results were not satisfactory, because only a very few grains were located and identified. Greater success was achieved by using the backscattered electron detector to search systematically for bright grains. At a magnification of about 250x, a one-micron diameter PGM grain could be recognized, provided that the brightness and the contrast of the image were adjusted appropriately. When a bright grain was located, its elemental constituents were rapidly identified by using an energy dispersive spectrometer. This method permitted a thorough evaluation of an entire polished section in only a few hours. This procedure worked well for most samples, except those that contained an abundance of galena grains (which can be difficult to distinguish from the PGMs in the backscattered electron image), because each bright grain had to be evaluated individually with the energy dispersive spectrometer. In some samples, this continuous checking was very time consuming, and not all bright grains were analyzed; some PGMs could have slipped past inspection. Out of the thirteen samples that were analyzed, five contained PGMs. A quick calculation shows that only two grains of pure palladium (each 1 µm in diameter) are required per thin section to produce a measured whole rock palladium concentration of 10 ppm. It would be easy to overlook these PGM grains, even in samples with high (~10 ppm) whole-rock PGE concentrations. PLATINUM-GROUP MINERALS In copper-nickel sulfide deposits, PGEs are most likely to be present either as discrete PGMs or in solidsolution in the accompanying sulfide minerals. Platinumgroup minerals have been documented from the Duluth Complex by various researchers (Table 1). Ripley (1990) and Ripley and Chryssoulis (1994) investigated the sulfide and arsenide minerals from the basal zone of the Duluth Complex to assess whether they carry the PGEs in solid solution. Ripley and Chryssoulis (1994) used an ion microprobe, with detection limits of: Pt 25 ppb, Pd 250 ppb, Rh 15 ppb, and Ir 22 ppb. They determined that palladium is the most abundant PGE, with concentrations of as much as 750 ppm in arsenide minerals [maucherite, (Ni 11 As 8 )], and as much as 15 ppm in sulfide minerals. However, they determined the average palladium concentration in maucherite to be about 280 ppm, and the average palladium concentrations for the sulfide minerals to be: pyrrhotite 1.4 ppm; pentlandite 4.0 ppm; chalcopyrite 0.79 ppm; cubanite 0.77 ppm, and mackinawite 5.4 ppm. It was difficult for Ripley and Chryssoulis (1994) to establish the background level of PGEs in solid solution in the sulfide and arsenide minerals because submicroscopic PGE inclusions were present. They concluded that PGE solid solution in the sulfide minerals was sufficient to account for normal background PGE concentrations in samples from the Minnamax deposit. Maucherite carries significantly higher concentrations of platinum, palladium, iridium and rhodium than recorded for the sulfides, but typically it is only present in minor proportions in the rocks. It is therefore likely that 5

6 B C Fr D Fr Fr A B Fr Fr Fr Ccp Opx Ccp Cb Ccp C D Figure 2. Backscattered electron images of PGMs in sample 73. (A) Low magnification of area containing PGE minerals; shows PGM and chalcopyrite veins, and individual PGM grains; scale bar is 100 µm. (B) Froodite vein (Fr, white) in biotite; scale bar is 10 µm. (C) Froodite (Fr, white) and chalcopyrite (Ccp, light gray) surrounded by orthopyroxene (Opx). (D) Froodite (Fr, white), cabriite (Cb, light gray), and chalcopyrite (Ccp, medium gray) in vein cross cutting biotite; scale bar is 100 µm. anomalously high PGE concentrations (mean 2,516 ppb platinum, and 2,583 ppb palladium) require the presence of maucherite, or the presence of discrete platinum-group minerals (Table 1), or both. As part of this study, sulfide and arsenide minerals were analyzed for platinum and palladium, even though the detection limits for the electron microprobe are much higher then those of the ion microprobe used by Ripley and Chryssoulis (1994). Randomly selected grains of maucherite (Ni 11 As 8 ), chalcopyrite (CuFeS 2 ), bornite (Cu 5 FeS 4 ), pentlandite [(Fe,Ni) 9 ], sphalerite [(Zn,Fe)S], pyrrhotite (Fe 1-x S), cubanite (CuFe 2 S 3 ), and galena (PbS), as well as the newly discovered minerals shadlunite [(Pb,Cd)(Fe,Cu) 8 ], and dienerite (Ni 3 As) were analyzed using count times that ranged in length from 40 seconds to 10 minutes. The minimum detection limit for platinum and palladium, using a count time of 40 seconds, is about 0.05 wt. percent, whereas a count time of 10 minutes extends the detection limit to about 0.01 wt. percent. No trace of platinum or palladium was found in any of these sulfide or arsenide minerals. Two PGE-bearing minerals (froodite, PdBi 2, and cabriite, Pd 2 SnCu) are documented from the Duluth Complex (Figs. 2 and 3) in this study. Froodite was first described by Hawley and Berry (1958) from nickeliferous ores of the Flood Mine, Sudbury, Ontario. It is monoclinic, gray, and has a density of Carbri and Harris (1973) describe froodite as creamy white in reflected light, with a pale brown tinge in air. Cabriite was first described from the Oktyabr sk deposit, Noril sk district, USSR by Evstigneeva and Genkin (1983). It is white with a slight grayish tinge and has a density of It is typically present in massive copper-nickel sulfide ores composed mainly of chalcopyrite-group minerals. 6

7 Cb Bi Pl and Chl Fr A B Figure 3. Backscattered electron images of area containing PGE minerals in sample (A) Froodite and cabriite (pale gray, arrowed) fills veins and is surrounded by plagioclase and chlorite (Pl and Chl) and biotite (Bi, upper left third of image); scale bar is 10 µm. (B) Enlargement of (A) shows textural relations between the froodite (Fr, light gray, lower half) and cabriite (Cb, medium gray, upper half); scale bar 1 µm. In the samples examined, PGM grains are typically a few microns in diameter, consequently they are difficult to analyze even with a focused electron beam (Fig. 3). With such small grains, there is always the risk that the surrounding minerals are contributing to the results. Therefore the compositions provided in Table 2 are from the larger grains, where it was possible to avoid elemental contributions from surrounding minerals. Cabriite is only present as very small grains (a few microns in diameter), therefore it is possible that some of the iron, sulfur and nickel (Table 3) are from the surrounding phases. However, the amount of sulfur (analysis 2, Table 3) is so small that it excludes the possibility of interference from the surrounding sulfide grains. Therefore most of the iron reported in that analysis is likely to be in solid solution in the cabriite. No significant platinum was recorded in the cabriite grains analyzed in this study. The froodite in these samples contains about 1.5 wt. percent platinum and very minor amounts of copper and iron in addition to palladium and bismuth (Table 2). Platinum group minerals are most abundant in a sample of hornfels (sample 73, see Appendix). In this sample most of the PGM grains are concentrated in a 1mm area (Fig. 2A), with froodite being the dominant PGM. It is present within veins that cross-cut the sample, and as individual subhedral grains. The veins that contain froodite also contain chalcopyrite (Figs. 2C and 2D), and cut across biotite, cordierite, plagioclase and orthopyroxene grains. In some veins cabriite grains are in contact with, or are in the same vein as froodite (Figs. 2D and 3). The longest and widest vein segment containing PGMs is about 300 µm long and 10 µm wide. Table 2. Electron microprobe analyses of froodite (PdBi 2 ) in sample 73 Sample Analysis # Weight percent of element Cu Fe S Ni Co Pt Pd Sn Bi As Total Normalized atomic percent of element Cu Fe S Ni Co Pt Pd Sn Bi As

8 Table 3. Electron microprobe analyses of cabriite (Pd 2 SnCu) in sample 134 Sample Analysis Weight percent of element Cu Fe S Ni Co Pt Pd Sn Bi As Zn Pb Total Normalized atomic percent of element Cu Fe S Ni Co Pt Pd Sn Bi As Zn Pb Individual PGM grains are typically about 10 x 5 µm, but grains as large as 30 x 15 µm are present (Fig. 2C). They are almost always in contact with chalcopyrite grains, which may form inclusions in orthopyroxene, biotite, or chalcopyrite. The second most PGM-enriched sample is a massive sulfide (sample 134, see Appendix). A total of twelve evenly distributed PGM grains (five froodite and seven cabriite) were identified in this sample. The PGM grains are typically very small (1 2 µm diameter), the largest grains are about 1 x 10 µm. The froodite grains are all contained within intergrown pentlandite and chalcopyrite (Fig. 4A) that form an exsolution texture. Cabriite is present within the intergrown pentlandite and chalcopyrite, as well as in areas of cubanite and chalcopyrite, although the latter two minerals do not exhibit any exsolution features (Fig. 4B). In summary, froodite was documented in five samples (samples 24.7, 73, 74.5, 100, and 134) and cabriite was documented in three samples (73, 74.5 and 134). PGMs present in these samples each have textural characteristics that fall into one of the groups discussed above. SILVER AND GOLD In the samples analyzed, silver is present either within maucherite (Ni 11 As 8 ), or as discrete grains of silver alloy that include small amounts of copper, nickel, and iron. Most maucherite grains are free of silver, but where it is present, it is typically segregated into zones (Fig. 5) that have silver concentrations as high as 30 wt. percent (Table 4). In some cases the silver appears to be Pn and Ccp Fr Cb Pn Cub Pn A B Figure 4. Backscattered electron images of PGMs in sample 134. (A) Froodite (Fr, white) surrounded by pentlandite (Pn, medium gray), and pentlandite intergrown with chalcopyrite (Pn and Ccp); scale bar is 10 µm. (B) Cabriite (Cb, white) in cubanite (Cub, medium gray); scale bar is 10 µm. 8

9 homogeneously distributed within the maucherite (Fig. 6A), but higher magnification shows it to be exsolved on a submicron scale ( Fig. 6B). Due to the very small scale of the intergrowths shown in Figure 6B (<0.1 µm wide ), the composition of the exsolved phases could not be determined. In other cases, however, exsolved silver forms lenses of native silver as wide as 2 3 µm. Native silver, or silver alloys unrelated to maucherite are also present in the Minnamax samples. They typically occur as small grains (approximately 10 µm) within larger sulfide grains or along sulfide grain boundaries (Fig. 7), implying that some of the silver had an early paragenesis. However, the presence of silver in veins (Fig. 8) and vugs (Fig. 9) implies that native silver or its alloys also formed late in the paragenetic history of these rocks. Sulfide grains were analyzed for gold following the same approach used for the PGEs, although count times ranged from 40 seconds to 30 minutes. Longer count times result in detection limits that range to as low as wt. percent. No gold at or above that level was found in any of the analyzed sulfide minerals. Gold is only present in native silver and silver-alloy grains. The most goldenriched grain found to date contains approximately 12 wt. percent gold, but gold concentration within the grain Ag-enriched Table 4. Electron microprobe analyses of silver-rich and silver-poor parts of maucherite grain in sample 48.5 Ma Ma Figure 5. Backscattered electron image of maucherite grain (Ma, gray) in sample Silver-enriched zone (arrowed) is white. Composition of silver-rich and silverpoor zones is shown in Table 4. Scale bar is 10 µm. Sample Analysis # 1 2 Weight percent of element Cu Ni Fe Zn Co Ag Pb Cd S As Au Total Ma Ccp Ag A B Figure 6. Secondary electron images of maucherite in sample 103. (A) Maucherite (Ma, light gray lath) in chalcopyrite (Ccp, medium gray), surrounded by silicate minerals; scale bar is 100 µm. (B) Close-up of the maucherite (scale bar is 100 µm) shows submicron-scale exsolution lamellae of silver (light gray) in maucherite. 9

10 Ccp Ag Ag-alloy Fr Ccp Figure 7. Backscattered electron image of sample 100, showing native silver or silver-alloy grain (Ag, white grain) and froodite (Fr, white grain) in chalcopyrite (Ccp, medium gray). Scale bar is 10 µm. Figure 10. Backscattered electron image of sample 16.4 showing an gold-rich, silver-alloy grain (white) contained within a chalcopyrite grain (Ccp, light gray). The chalcopyrite is surrounded by olivine, chlorite and serpentine. Scale bar is 10 µm. The composition of the gold-rich, silver-alloy grain is given in Table 5. Ag Table 5. Electron microprobe analyses of goldrich silver alloy grain in sample 16.4 [Element concentration given in wt. percent] Sample Analysis # Figure 8. Backscattered electron image of sample 74.5 showing a native silver vein (Ag, white) cutting across a veined and altered greenalite-like sheet silicate that shows many generations of veining and alteration. Scale bar is 10 µm. Cu Ni Fe Zn Co Ag Pb Cd S As Au Total Ccp Figure 9. Backscattered electron image of sample 73 showing silver-filled vugs (Ag, white) in chalcopyrite (Ccp, dark gray). Some of the vugs are unfilled or partially filled. Scale bar is 10 µm. Ag 10 ranges from 7.5 to wt. percent (Table 5, analyses 1 3). The grain is approximately 120 x 10 µm (Fig. 10), and is an Ag-Au-Cu alloy. It is an inclusion within a chalcopyrite grain, which is surrounded by remnant olivine, chlorite, and serpentine. Other Ag-Au-Cu alloy grains contain gold in much lower concentrations, typically about 1 wt. percent (Table 5, Analysis 4); this grain in sample 16.4 is 40 x 20 µm, and forms an inclusion within chalcopyrite. Gold is also present in silver-alloy lenses in maucherite grains, such as in sample 98, which is a hornfels (Fig. 11). The maucherite grain is about 300 x 40 µm, and is situated between chalcopyrite, plagioclase and cordierite grains. Three distinct zones are documented

11 Table 6. Electron microprobe analyses of a silver- and gold-enriched maucherite (Ni 11 As 8 ) grain in Sample 98 [Element concentration given in wt. percent] normal maucherite Ag-enriched maucherite Ag-Ni-Au alloy Sample Analysis # Cu Ni Fe Zn Co Ag Pb Cd S As Au Total Ag-Ni-Au alloy Ma Ma 10 µ A 10 µ B 10 µ C Figure 11. Backscattered electron image (A) and element concentration maps (B, C, and D) for brecciated maucherite grain in sample 98. Scale bars are all 10 µm. (A) Grain is brecciated and contains zones and lenses of a silver-nickel alloy that contains about 1 wt. percent gold; white area is silver-nickel alloy, light gray is maucherite. (B) Element concentration map for silver; white area is silver-nickel-gold alloy; gray is silverenriched maucherite. (C) Element concentration map for nickel; pale gray area includes as much as 55 wt. percent nickel. (D) Element concentration map for arsenic; pale gray area includes as much as 42 wt. percent arsenic. The composition of normal maucherite, silver-enriched maucherite, and Ag-Ni-Au alloy are given in Table µ D

12 Table 7. Electron microprobe analyses of pentlandite ([Fe,Ni] 9 ), sample 134 [element concentration given in wt. percent] Sample Analysis Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Total Number of atoms calculated on basis of 8 sulfur atoms Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As Bi Pk Fr Figure 12. Backscattered electron image of sample 73 shows vein containing froodite (Fr, light gray) and parkerite (Pk, medium gray) in biotite (Bi, black). Scale bar is 10 µm. Pk Fr for this maucherite grain: 1. Normal maucherite with little to no gold or silver; 2. zones of maucherite enriched in silver; and 3. Silver-alloy lenses containing gold. The grain appears to have been brecciated, with silver filling the resultant interstices. There is no evidence for brecciation in the surrounding mineral grains, so the brecciation and infilling may represent a very early process. The native silver lenses of zone 3 are only 1 2 x 3 5 µm. They are a silver-nickel alloy that contains approximately 1 wt. percent gold (Table 6). It is possible that the silver-enriched zone (zone 2) formed at the same time as the silver-nickel alloy lens of zone 3, but differs only in scale. The silver-rich maucherite in zone 2 does not contain measurable gold as it does in zone 3, however, which implies some genetic differences. Native silver grains examined in this study only rarely contain gold. However, it can be shown numerically that it would only require three grains, each 5 µm in diameter, and each containing 1 wt. percent gold to produce the whole rock gold concentrations of 10 ppm gold documented for these samples. NICKEL In the samples analyzed, four minerals contain nickel as a major constituent. They are pentlandite (Fe, Ni) 9, maucherite (Ni 11 As 8 ), parkerite (Ni 3 (Bi,Pb) 2 S 2 ), and dienerite (Ni 3 As). Mackinawite (Fe 9 ), which contains nickel as a minor component, is also present. Pentlandite is the volumetrically dominant nickel-bearing mineral in these samples, even though it does not contain the most nickel. It contains wt. percent nickel (Table 7). This compares with about 50 wt. percent nickel in the maucherite (Table 8), and 70 wt. percent in the dienerite (Table 9). The small grain size of the parkerite (< 10 µm) precluded quantitative analysis. Pentlandite constitutes as much as 25 percent by volume of some samples, and exhibits a wide variety of textural relations. The wide range of textures indicates that it represents several paragenetic stages. Severson and Barnes (1991) recognized four textural types of pentlandite, that reflect the time at which each type formed. The earliest pentlandite formed during the crystallization of sulfide minerals such as pyrrhotite. The latest pentlandite formed as a secondary mineral in fractures in cubanite and chalcopyrite. Maucherite is a minor nickel arsenide phase in most samples, but it is significant here because of its silver and cobalt contents, as well as its nickel content. PGM inclusions in maucherite grains have already been described for the Duluth Complex (Ryan and Weiblen, 1984; Iwasaki and others, 1986). Textural evidence suggests that maucherite formed early in the paragenetic 12

13 Table 8. Electron microprobe analyses of maucherite (Ni 11 As 8 ) in samples 73, 98, 100, 103, 134 and 259 [element concentration given in wt. percent; blank indicates not analyzed for] Sample Analysis Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Total Normalized atomic percent of element Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S sequence (Severson and Barnes, 1991). As an earlycrystallizing phase, it may have played an important role in scavenging elements from still molten magma that ultimately crystallized to form the Duluth Complex. Although parkerite is an accessory mineral, it may be important in this system because it is a bismuth-bearing phase, like froodite. In sample 73, parkerite and froodite are intergrown in a vein that cross cuts biotite (Fig. 12). Dienerite is a very minor accessory phase that forms exsolution lamellae in a few maucherite grains (Fig. 13). Dienerite is interesting because it is rare. It was first described in 1921, from a single loose crystal collected near Radstadt, Salzburg, Austria, that has since been lost (Anthony and others, 1990). Here, it is documented from the Duluth Complex for the first time. The presence of dienerite as exsolution lamellae in maucherite of the Duluth Complex provides constraints on the subsolidus phase relationships in the nickel-arsenic system, and on the paragenesis of dienerite. The element maps in Fig. 13 and the electron microprobe analyses (Table 9) show that dienerite has a much lower ability to take in cobalt into its structure than the host maucherite (Table 8). The maucherite grain contains almost 2 wt. percent cobalt, whereas the exsolved dienerite contains virtually none. Mackinawite is a rare accessory mineral. It is typically intergrown with phases such as pentlandite and chalcopyrite. It contains between about 3 and 5 wt. percent nickel (Table 10), which falls in the range reported by others (Ostwald, 1978). However, the copper concentrations for the samples analyzed are as high as 17 wt. percent, whereas previously reported copper concentrations reported for mackinawite range from near zero (Anthony and others, 1990) to 7 8 wt. percent (Ostwald, 1978; Severson and Barnes, 1991). The mackinawite reported in Table 10 is an inclusion in pentlandite; therefore it is unlikely that the high copper concentration was the result of contamination from the surrounding phase. It is possible, though, that a copperbearing phase may have intergrown with the mackinawite on a submicron scale. 13

14 COBALT Only two phases contain significant cobalt; these are maucherite ( wt. percent; Tables 4 and 8), and pentlandite ( wt. percent; Table 7). The concentration of cobalt in maucherite varies both between and within grains. Within a single maucherite grain cobalt content ranges from 3.8 to 6.0 wt. percent (Table 4, sample 48.5). The lower cobalt concentration is from a silverenriched part of the grain, but this inverse relationship between silver and cobalt is not consistent for all the samples. For example, both the silver-enriched and normal parts of a single maucherite grain (Table 6) contain about 1.0 wt. percent cobalt, although the silver-enriched maucherite may contain slightly more cobalt than the nonsilver-enriched parts. Cobalt concentrations in pentlandite range from approximately 1.0 to 1.5 wt. percent (Table 7). Cobalt concentrations show very little variation between or within samples. Very minor amounts of cobalt (about 0.1 wt. percent) can also occur in mackinawite (Table 10). Di Di Ma Ma 20 µ A 20 µ B Di Di Ma Ma 20 µ C 20 µ D Figure. 13. Backscattered electron image (A) and element concentration maps (B, C, and D) for maucherite grain in sample 134. Scale bars are all 20 µm. Exsolved dienerite (Di) is in upper right of grain. (B) Element concentration map for cobalt; light gray area contains as much as 2 wt. percent cobalt. (C) Element concentration map for nickel; light gray area contains approximately 70 wt. percent nickel; medium gray area contains approximately 55 wt. percent nickel. (D) Element concentration map for arsenic; darker gray area contains approximately 30 wt. percent arsenic; lighter gray area contains approximately 40 wt. percent arsenic. 14

15 Table 9. Electron microprobe analyses of dienerite (Ni 3 As) in sample 134 [Element concentration given in wt. percent] Sample Analysis 1 2 Cu Fe Ni Co Pb Cd Ag As S Total Normalized atomic percent of element Cu Fe Ni Co Pb Cd Ag As S Table 10. Electron microprobe analyses of mackinawite (Fe 9 ) in sample 134 [Element concentration given in wt. percent] Sample Analyses Cu Fe Ni Zn Co Pb Ag Cd As S Total Number of atoms calculated on basis of 8 sulfur atoms Cu Fe Ni Zn Co Pb Ag Cd As ZINC AND CADMIUM The zinc- and cadmium-bearing minerals in the samples analyzed include sphalerite [(Zn,Fe)S], shadlunite [(Pb,Cd)(Fe,Cu) 8 ], and greenockite (CdS). Of these, sphalerite is the primary zinc-bearing phase. Sphalerite occurs in minor amounts, although some of the more mineralized portions of the Minnamax deposit contain as much as 5 modal percent sphalerite (Severson and Barnes, 1991). The sphalerite is typically intergrown with or included in chalcopyrite, cubanite or talnakhite, suggesting that it formed early in the paragenetic sequence. The sphalerite typically contains 1 2 wt. percent cadmium, 5 7 wt. percent iron, and typically less than 1 wt. percent copper (Table 11). Shadlunite [(Pb,Cd)(Fe,Cu) 8 ] contains approximately wt. percent cadmium (compared to wt. percent cadmium in sphalerite), but sphalerite is more abundant. Therefore, they probably contribute about equal amounts of cadmium to the whole rock compositions. Shadlunite in rocks from the Minnamax deposit forms exsolution features in chalcopyrite (Fig. 14). It also contains significant amounts of lead (Table 12). Ccp Φigure 14. Backscattered electron image of sample 103 showing exsolved shadlunite (Sh, white) in chalcopyrite (Ccp, gray). Scale bar 10 mm. Sh 15

16 The only mineral that contains cadmium as a primary component in these rocks is greenockite (CdS), which is present as an accessory mineral, typically as inclusions in chalcopyrite (Fig. 15) and less commonly as crystals along internal fractures. Greenockite has previously been reported (Anthony and others, 1990) as essentially pure CdS, although zincian greenockite (containing as much as 8.9 wt. percent zinc) has been recorded in stratiform Pb-Zn-Ag deposits in Queensland, Australia (Patterson,1985). Greenockite in the Duluth Complex contains as much as 9.2 wt. percent zinc, 10.8 wt. percent copper, and 3.3 wt. percent iron (Table 13). The elevated concentrations of zinc, copper, and iron in the greenockite are not the result of beam interaction with surrounding mineral, because the surrounding chalcopyrite lacks measureable quantities of zinc. Furthermore, if the beam was interacting with chalcopyrite, the sulfur concentration in the greenockite would have been higher. Chalcopyrite contains over 30 wt. percent sulfur, whereas stoichiometric greenockite contains only 22.2 wt. percent sulfur. In this study, the sulfur concentration in the greenockite ranges from 20.5 to 23.5 wt. percent. This suggests that the greenockite in rocks of the Minnamax deposit may be deficient in sulfur, and that the composition of greenockite varies significantly from the stoichiometric composition reported in the literature. COPPER The four main copper-bearing minerals documented from samples analyzed in this study are bornite (Cu 5 FeS 4 ) (Table 14), chalcopyrite (CuFeS 2 ) (Table 15), talnakhite (Cu 9 (Fe, Ni) 8 S 16 ) (Table 16) and cubanite (CuFe 2 S 3 ) (Table 17). Pyrrhotite (Fe 1-x S) was also analyzed to determine whether it contained minor concentrations of metals other than Fe. Table 11. Electron microprobe analyses of sphalerite [(Zn,Fe)S] in samples 103 and 49 [Element concentration given in wt. percent] Sample Analysis Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Total Numer of atoms calculated on basis of one S atom Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S

17 Ccp Ccp Gr Gr Gn A B Figure 15. Backscattered electron images of greenockite. (A) Greenockite (Gr) is the light gray mineral intergrown with chalcopyrite (Ccp, medium gray) and blades of galena (Gn, white); sample 24; scale bar is 10 µm. (B) Greenockite (Gr) is the light gray mineral intergrown with chalcopyrite (Ccp, medium gray); sample 24.7; scale bar is 10 µm. Table 12. Electron microprobe analyses of shadlunite [(Pb,Cd)(Fe,Cu) 8 ] in samples 103 and 259 [Element concentration given in wt. percent] Sample Analysis Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Total Number of atoms calculated on basis of 8 S atoms Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As

18 Table 13. Electron microprobe analyses of greenockite (CdS) in samples 24 and 24.7 Table 14. Electron microprobe analyses of bornite (Cu 5 FeS 4 ) in sample 259 [Element concentration given in wt. percent; blank [Element concentration given in wt. percent] indicates element not analyzed for] Sample Analysis Sample Analyses Cu Ni Fe Zn Co Ag Pb Cd As Au 0.00 S Total Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Total Number of atoms calculated on basis of one S atom Cu Ni Fe Zn Co Ag Pb Cd As Au S Number of atoms calculated on basis of 4 S atoms Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Bornite typically ranges from trace amounts to as much as 5 modal percent of the samples investigated (Severson and Barnes, 1991). It is typically present as small grains (< 1 mm), and in some cases it is intergrown with chalcopyrite on a sub-micron scale, indicating an origin as an exsolution feature. The analyzed bornite grains have a near-stoichiometric composition, and contain little other than copper, iron and sulfur (Table 14). Chalcopyrite is the most abundant copper-bearing mineral. It exhibits a wide range of textural and mineralogical relations (Severson and Barnes, 1991). It deviates only very slightly from stoichiometric composition (Table 15), although some grains contain minor amounts (<0.3 wt. percent) of nickel. Talnakhite has a composition very close to that of the chalcopyrite (Table 16), but is considerably less abundant; it is present as exsolved lamellae in chalcopyrite and cubanite. Talnakhite contains <0.5 wt. percent nickel, which is slightly more than documented for the chalcopyrite in this study. Cubanite typically forms exsolution lamellae in the chalcopyrite. In some samples cubanite is the main copper-bearing mineral (Severson and Barnes, 1991). It has near-stoichiometric composition (Table 17), and contains negligible trace elements. Pyrrhotite contains little besides iron and sulfur (Table 18), and is typically the dominant sulfide mineral in the Minnamax deposit. LEAD Lead is present in several accessory minerals in the Minnamax samples, but galena is the only lead-bearing accessory mineral that is abundant. Galena (PbS) is typically present as minor blebs or blades in other sulfide minerals such as chalcopyrite or talnakhite (Fig. 15A). Although galena is an accessory mineral, it is typically 18

19 Table 15. Electron microprobe analyses of chalcopyrite (CuFeS 2 ) in samples 134 and 49 [Element concentration given in wt. percent] Sample Analysis Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Total number of atoms calculated on basis of 2 sulfur atoms Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S scattered throughout a sample. The Minnamax samples analyzed for this study also contain a lead-telluride mineral (altite, PbTe), and a lead-selenide phase. Both of these are present as 1-µm-diameter grains, which makes them difficult to analyze. Nonetheless, analyzed altite grains produced a consistent mean composition of: Pb: 58.2 wt. percent Te: 35.3 wt. percent Cu: 3.5 wt. percent Fe: 2.91 wt. percent S: 0.94 wt. percent Ideal altaite contains wt. percent lead and wt. percent tellurium). In contrast, the lead-selenide analyses show widely variable concentrations of copper, iron, and sulfur, implying that interference from surrounding grains was an analytical problem. The best analyses of the lead-selenide phase contained mean concentrations of: Se: 15.9 wt. percent Pb: 70.8 wt. percent S: 3.4 wt. percent Fe: 3.2 wt. percent Cu: 2.0 wt. percent These concentrations produce an atomic ratio Pb:Se of almost 2:1, which does not conform to any known mineral. Consequently, it is possible that even though these grains are only on the order of 1 µm across, they may be a composite of more than a single phase, such as intergrown galena (PbS) and clausthalite (PbSe). The mineralogical identity of the lead-selenide grains will not be resolved until larger grains are identified and analyzed. 19

20 Table 16. Electron microprobe analyses of talnakhite [Cu 9 (Fe,Ni) 8 S 16 ] in sample 103 [Element concentration given in wt. percent] Sample Analysis Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S A Total Number of atoms calculated on basis of 16 S atoms Cu Fe Ni Zn Co Pb Cd Pt Pd Ag Au As S Figure 17. Secondary electron images show the morphology of the silver-rich crystals growing on the surface of sample 134. (A) shows the twinned euhedral crystals typical of the early growth stage, and (B) shows the globular form associated with later stages of mineral growth. Scale bars are 1 µm. B Grl Ag Cub Figure 16. Secondary electron image showing silver crystals (Ag, light gray) growing on carbon-coated polished surface of sample 134. The underlying material is cubanite (Cub, medium gray) and greenalite (Grl, dark gray). Scale bar is 10 µm. 20 Figure 18. Secondary electron image of potassium chloride residue from a bubble on the carbon-coated polished surface of sample 134. The potassium chloride residue indicates that the sample contains significant chlorine-bearing phases that are volatile in a vacuum. Scale bar is 100 µm.