The Eagle and East Eagle sulfide ore-bearing mafic-ultramafic intrusions in the Midcontinent Rift System, upper Michigan: Geochronology and petrologic evolution
Abstract
[1] The Eagle and East Eagle intrusions are small, subvertical dike-like mafic-ultramafic bodies that cut Proterozoic sedimentary strata in the Baraga Basin in northern Michigan. The Eagle intrusion hosts a newly discovered magmatic Ni-Cu-PGE deposit. The nearby East Eagle intrusion also contains sulfide mineralization, but the extent of this mineralization has yet to be determined by further drilling. Both intrusions contain olivine-bearing rocks such as feldspathic peridotite, melatroctolite, and olivine melagabbro. Sulfide accumulations range from disseminated at both Eagle and East Eagle to semimassive and massive at Eagle. U-Pb baddeleyite dating gives a crystallization age of 1107.2 ± 5.7 Ma for the Eagle intrusion, coeval with eruption of picritic basalts at the base of the volcanic succession in the Midcontinent Rift System (MRS). The Fo contents of olivine cores in the Eagle and East Eagle intrusions vary between 75 and 85 mol %, higher than those of olivine in larger layered intrusions in the MRS such as the Duluth Complex. The FeO/MgO ratios and Al2O3 contents of the parental magmas for the Eagle and East Eagle intrusions inferred from olivine and spinel compositions are similar to those of picritic basalts in the base of the MRS volcanic succession. These petrochemical data suggest that the Eagle and East Eagle intrusions are the intrusive equivalents of high-MgO basalts that erupted in the early stages of continental magmatism associated with the development of the rift. Variations in mineral compositions and incompatible trace element ratios suggest that at least three major pulses of magmas were involved in the formation of low-sulfide rocks in the Eagle intrusion. Lower Fo contents of olivine associated with semimassive sulfides as compared to that of olivine in low-sulfide rocks suggest that the magma associated with the semimassive sulfide was more fractionated than the parental magmas of the low-sulfide rocks in the Eagle intrusion. Accumulation of suspended olivine crystals and sulfide droplets from ascending magmas as they passed through wide parts of the conduits at Eagle and East Eagle played a critical role in the genesis of olivine-rich rocks and sulfide ores in the intrusions. The Eagle Ni-Cu-PGE deposit typifies the conduit-style of magmatic sulfide deposition that is associated with continental basaltic magmatism.
1. Introduction
[2] The recent discovery of the Eagle Ni-Cu-PGE deposit in Upper Michigan represents a major breakthrough in the exploration for magmatic sulfide deposits in the Midcontinent Rift System (MRS) (Figure 1a). Prior to the Eagle discovery, sulfide mineralization in the MRS was known to occur mainly in the basal zones of large layered mafic intrusions within the Duluth and Mellen complexes [Klewin, 1990; Seifert et al., 1992; Miller and Ripley, 1996; Ripley et al., 2007]. The sulfide deposits in these large sheet-like mafic intrusions generally have low grades of Ni (<0.2 wt %) and Cu (<0.6 wt %) and were considered economically unattractive until recent technological advancements. Inspired by the genetic model developed for the world's second largest Ni-Cu-PGE deposit located in the subvolcanic intrusions associated with the Siberian flood basalts [Naldrett, 1992], the focus of exploration for magmatic sulfide deposits in the MRS has shifted in recent years from large layered intrusions to smaller sill-like or dike-like intrusions that may have served as magma conduits for flood basalts. This strategy has proven successful as geologists from Kennecott Exploration Ltd. in 2002 discovered the Eagle deposit hosted by a small, dike-like mafic-ultramafic body. The grades of Ni (up to 6.11 wt %), Cu (up to 4.15 wt %), PGE (up to 25 ppm Pt in Cu-rich stringers), and Au (up to 0.32 ppm) are much higher than other previously discovered magmatic sulfide deposits in the MRS. The host rocks of the Eagle deposit are significantly more olivine-rich than mineralized sheet-like intrusions in the MRS. The geometry, internal lithological structure and style of sulfide mineralization based on the results from drilling are similar to other conduit-type magmatic sulfide deposits such as the Voisey's Bay deposit in Labrador [Li and Naldrett, 1999]. Thus, the Eagle deposit may serve as a good example for regional mineral exploration. For this reason we initiated an integrated research program to study the Eagle deposit and host rocks. The purpose of this paper is to report the results of our geochronological study and to utilize these results in conjunction with geochemical data to constrain the temporal position of the Eagle intrusion in terms of rift evolution.
2. Regional Geological Background
[3] The Eagle Ni-Cu sulfide deposit is hosted by the Eagle mafic-ultramafic intrusion which includes Paleoproterozoic rocks of the Marquette Range Supergroup deposited during the Penokean orogeny (Figure 1b). The Proterozoic rocks fill a structural trough that is informally known as the Baraga Basin (Figure 1c). The Baraga Basin is located north of the southern boundary of the Great Lakes Tectonic Zone (GLTZ (Figure 1b)), a 2.7 Ga, ∼40 km wide, oblique collision zone [Sims et al., 1980]. To the south of the GLTZ the Archean basement comprises gneiss, migmatite and amphibolite of predominantly Middle Archean age, intruded in places by Late Archean granite [Peterman et al., 1986]. To the north of the GLTZ the Archean basement is composed of gneiss, granite and low-grade metamorphosed sedimentary and felsic volcanic rocks of predominantly Late Archean age [Sims et al., 1993]. The Archean assemblage to the south of the Baraga Basin includes volcanic and metasedimentary rocks that contain variable amounts of pyrite. Rocks of the Marquette Range Supergroup are subdivided into the Chocolay, Menominee and Baraga groups [Van Schmus et al., 1987]. Paleoproterozoic sedimentary rocks that host the Eagle intrusion are all thought to be part of the Baraga group which have been informally subdivided by Kennecott geologists into units traceable by mapping and geophysics. From oldest to youngest these rocks consist of the Goodrich quartzite, a chert carbonate member, the Lower Slate, the Upper Greywacke, and the Lower and Upper Fossum Creek Slates (Figure 1c). All of the Archean and Paleoproterozoic sedimentary rocks in the Baraga Basin have been metamorphosed to the greenschist facies.
[4] Tectonic and thermal events that may have affected the Baraga Basin are the ∼1.85 Ga Penokean Orogeny corresponding with the suturing of the Wisconsin Magmatic terrane to the southern margin of the Superior craton ∼75 km to the south [Schulz and Cannon, 2007], the ∼1.1 Ga continental rifting event and associated basaltic magmatism [Nicholson et al., 1997], and the ∼1.08 Ga Grenville Orogeny that occurred ∼500 km to the east [Cannon, 1994]. Voluminous basaltic magmatism is associated with the development of the Midcontinent Rift System (MRS), including numerous mafic intrusions in the Lake Superior region (Figure 1b). In Michigan and Wisconsin the early volcanic phases associated with rift development are the Siemens Creek and Kallander Creek picritic basalts [Nicholson et al., 1997]. Early stage picritic volcanic rocks are also found in the southwestern portion of the North Shore Volcanic Group (Ely's Peak Basalts) and the Group 1 volcanic rocks at Mamainse Point [e.g., Berg and Klewin, 1988; Nicholson and Schulz, 2009]. The overlying lavas are predominantly tholeiitic basalts and basaltic andesites [Green, 1972].
3. Geology of the Eagle and East Eagle Intrusions
[5] The Eagle intrusion and the nearby East Eagle intrusion (Figures 2a and 2b) together were historically known as the Yellow Dog Peridotites [Morris, 1977; Klasner et al., 1979]. Geologists from Kennecott Exploration Ltd. began using the new name after the identification of potentially economic Ni mineralization in 2002. Outcrops of the intrusions are scarce and virtually all knowledge of the size and extent of the bodies is a result of extensive drilling operations. A portion of the East Eagle intrusion forms a small hill rising up to 10 m above a flat, glaciated landscape known as the Yellow Dog Plains (Figure 2a). The Eagle intrusion is only exposed in a small outcrop on the north side of the Salmon Trout River (Figure 2a). The Eagle and East Eagle intrusions occur ∼600 m apart on the surface. Their long axes are parallel to numerous E–W trending mafic dikes in the region that belong to the Mesoproterozoic Baraga-Marquette dike swarm formed during the early stages of basaltic magmatism in the MRS [Wilband and Wasuwanich, 1981; Green et al., 1987]. Like the nearby mafic dikes, the Eagle and East Eagle intrusions are characterized by very prominent magnetic highs relative to the surrounding sedimentary rocks. Drilling to 2500 m in the area between these two intrusions indicates that they are not connected at depth but occur as separate subvertical, dike-like bodies that intrude the gently dipping strata of the Upper Fossum Creek Slate. The Eagle intrusion has a length of ∼480 m and a width of ∼100–200 m (Figure 2b). The vertical downward extension of the Eagle intrusion exceeds 300 m (Figures 2c and 2d). The East Eagle intrusion is larger than the Eagle intrusion. Its maximum length and width are ∼600 m and ∼150 m, respectively (Figure 2b). The vertical downward extension of the East Eagle intrusion exceeds 500 m (Figure 2e).
[6] To make direct comparison with other mafic intrusions of the MRS easier, we have adopted the rock classification system used by the Minnesota Geological Survey [Miller et al., 2002]. This system differs from others in that it raises the maximum threshold for plagioclase content for peridotites from the traditional 10%–20% to 25% (Figure 3). Based on this classification, 3 major rock types have been observed in the drill cores of the Eagle intrusion (Figures 2c and 2d). They are, in the order of decreasing olivine content, feldspathic peridotite, melatroctolite and olivine melagabbro. In addition, minor feldspathic pyroxenite is present locally in thin (<20 m) intercepts that may represent dikes (Figure 2c). The mineral modal compositions change abruptly (<1 cm) between feldspathic pyroxenite and the olivine-bearing rock units. Based on the core-logging data provided by the company and our own observations, feldspathic peridotite, the most olivine-rich unit in the intrusion, occurs as a sheet at the top of the intrusion (Figures 2c and 2d), which is not consistent with the structure of a typical layered intrusion. The melatroctolite and olivine melagabbro units form a sandwich structure with an olivine melagabbro core wrapped by melatroctolites. No visible igneous layering is present in any of the rock units.
[7] One drill core in the east end of the East Eagle intrusion (Figure 2e) is in melatroctolites only. This drill core encountered an interval of ∼450 m of melatroctolites that show no visible igneous layering or foliation. Based on drill core data provided by Kennecott geologists, other drill cores in the East Eagle intrusion except those close to the basal contact of the intrusion are also melatroctolites. The basal zone of the East Eagle intrusion contains quartz and carbonate inclusions and is highly altered.
[8] Sulfide mineralization is present in both intrusions, but economic ore bodies in the East Eagle intrusion have not been delineated by drilling. According to a company report released in 2009 the geologic resource of economic sulfide ores in the Eagle intrusion was estimated at 4.05 million tons with an average grade of 3.57% Ni, 2.9% Cu, 0.10% Co, 0.28 ppm Au, 0.73 ppm Pt, and 0.47 ppm Pd. More than 90% of the sulfide ores of the Eagle deposit occur in a single ore body resembling a 90° bent tube (Figure 4) above the keel of the intrusion (Figure 2c). The outlines of the ore body are discordant with the boundaries of different rock units (Figure 2c). Most massive sulfides are concentrated in the bent part of the tube. In places massive sulfide ores extend outward beyond the contacts of the intrusion into the metasedimentary rocks (Figure 2d). Within the sulfide ore body the boundaries between semimassive (11–19 wt % sulfur) and massive sulfide ores are marked by sudden changes (<1 cm) in silicate mineral contents from 50% to 75% to almost none. The silicate portion of semimassive sulfide ores is dominantly olivine (Figure 3). The boundaries between semimassive and disseminated sulfide ores (0.2 to 6 wt % sulfur; present in feldspathic peridotite, melatroctolite and olivine melagabbro) commonly transition over a few centimeters but sometimes transition gradationally over 10 cm. Principle sulfide minerals in the ores are pyrrhotite, chalcopyrite, pentlandite and cubanite. Up to 5 vol % of disseminated magnetite is present within the sulfide assemblages.
4. Petrography
[9] Most of the samples used in this study were collected from 3 drill cores in the Eagle intrusion (03EA034, YD0106 and 04EA047 (Figure 2c)) and one drill core in the East Eagle intrusion (04EA044 (Figure 2e)). 150 polished thin sections were examined using transmitted and reflected light microscopy. Twenty-six representative samples from the Eagle intrusion and 10 representative samples from the East Eagle intrusion using area measurement software. The results are shown in Figure 3. The modal mineral compositions of other samples were estimated by visual observation.
[10] Feldspathic peridotite, melatroctolite and olivine melagabbro differ in both olivine grain size and mode. Coarse-grained (>5 mm diameter) olivine is dominant in the feldspathic peridotite (Figure 5a) and medium-grained (3–5 mm diameter) olivine is dominant in melatroctolite (Figure 5b) and olivine melagabbro (Figure 5c). Feldspathic peridotite contains 30–60% olivine, 5–15% clinopyroxene, 15–40% orthopyroxene and 15–25% plagioclase. Melatroctolite contains 30–45% olivine, 5–10% clinopyroxene, 20–25% orthopyroxene and 25–35% plagioclase. Olivine melagabbro contains 10–35% olivine, 5–15% clinopyroxene, 20–45% orthopyroxene and 25–40% plagioclase. Olivine in the above three types of rocks occurs as subhedral to anhedral granular grains and are randomly orientated (Figures 5a–5c). Some olivine crystals contain small euhedral Cr-spinel inclusions. Although some relatively small olivine grains are enclosed in large augite oikocrysts, more than 80% of clinopyroxene occurs as subhedral prisms or anhedral grains in all of the units (Figures 5d and 5e). Orthopyroxene occurs predominantly (90%) as anhedral grains, and minor orthopyroxene occurs rims (100 to 300 μm) surrounding olivine (Figures 5d and 5e). Plagioclase laths are present in melatroctolite and olivine melagabbro (Figure 5f), with interstitial plagioclase more common in the feldspathic peridotite (Figure 5g). Lesser amounts of interstitial plagioclase occur in the melatroctolite and olivine melagabbro; hornblende, biotite, titaniferous magnetite, and sulfide occur as interstitial minerals in all units (∼10%). In the olivine melagabbro (Figure 5h), plagioclase may also occur as needles intergrown with clinopyroxene. Feldspathic pyroxenite (Figure 5i) is free of olivine and the general grain size is finer (pyroxene < 2.5 mm in diameter) than olivine-bearing rocks in the Eagle intrusion. In addition the intensity of alteration is less than in other rock types in the Eagle intrusion. Feldspathic pyroxenite contains 30%–45% orthopyroxene, 25%–30% clinopyroxene, and 15%–25% plagioclase. Minor amounts of sulfides (up to 5%) are present in the interstitial spaces along with hornblende, biotite and magnetite. All of the units in the Eagle and East Eagle intrusions show mutual grain boundary textures involving olivine, orthopyroxene, and clinopyroxene (Figures 5c–5e and 5i) suggesting textural maturation after expulsion of interstitial liquid.
[11] The silicate portion of semimassive sulfide ores (Figure 5j) is composed of medium-grained (3–5 mm diameter) olivine and smaller (<2.5 mm diameter) pyroxene and plagioclase. It contains 85%–90% olivine, 5%–10% clinopyroxene, 1%–2% orthopyroxene and 1%–2% plagioclase. Semimassive sulfides form a net textured matrix enclosing olivine, pyroxene and feldspar grains.
[12] Most olivine grains in the samples from the Eagle and East Eagle intrusions are partially altered to serpentine plus secondary magnetite. Some pyroxene and plagioclase in the rocks are partially altered to talc or tremolite, and sericite or chlorite, respectively. In contrast, pyroxene and plagioclase in the feldspathic pyroxenite of the Eagle intrusion show little alteration.
5. Analytical Methods
[13] U-Pb dating was carried out at the Jack Satterly Geochronology Laboratory, University of Toronto, Canada. A large (∼3 kg), composite (splits of 6 drill core samples DX 13–18) feldspathic peridotite sample was crushed and pulverized and heavy minerals were concentrated using standard methods. Feldspathic peridotite was chosen because it is the most abundant rock type in the Eagle intrusion. Baddeleyite and zircon grains were examined using a binocular microscope and clear crystals lacking visible imperfections and inclusions were selected for analysis. These were dissolved using ∼0.10 ml of concentrated hydrofluoric acid (HF) and ∼0.02 ml of 7N HNO3 in Teflon dissolution bombs at 200°C [Krogh, 1973] for 5 days, and then dried to a precipitate, followed by redissolution in ∼0.08 ml of 3.1N HCl overnight. U and Pb were isolated by using anion exchange column separation and loaded together onto outgassed rhenium filaments with silica gel [Gerstenberger and Haase, 1997]. Pb and U were analyzed with a VG354 mass spectrometer using a Daly collector in pulse counting mode. All common Pb in zircon (0.5–0.7 pg) was assigned to the isotopic composition of the laboratory Pb blank. Dead time of the measuring system for Pb is 22.8 ns and 20.8 ns for U. The mass discrimination correction for the Daly detector is constant at 0.07% per atomic mass unit. Amplifier gains and Daly characteristics were monitored using the SRM 982 Pb standard. Thermal mass discrimination corrections are 0.10% per atomic mass unit. Decay constants are those of Jaffey et al. [1971]. All age errors quoted and error ellipses in the concordia diagram are given at the 95% confidence level. Plotting and concordia age calculation are from Isoplot 3.00 [Ludwig, 2003].
[14] Mineral compositions were determined by wavelength dispersive analysis using a CAMECA SX50 electron microprobe at Indiana University. An accelerating voltage of 15 kV was used. Beam current and peak counting time for major elements were 20 nA and 20 s, respectively. Nickel was analyzed at a beam current of 100 nA and a peak counting time of 100 s. The detection limit for Ni under these conditions was about 60 ppm. The accuracy of analyses was monitored using reference material of similar compositions. Sample reproducibility varied by less than 2%.
[15] Major element analyses were conducted by X-ray fluorescence spectrometry at the University of Cincinnati. Trace elements were analyzed by ICP-MS in the Geoscience Laboratories, Sudbury, Ontario, Canada. Samples were digested in a closed vessel using a combination of up to four acids (hydrofluoric, hydrochloric, nitric and perchloric). Sulfur was analyzed using an Eltra CS 2000 at Indiana University. Multiple analyses of standards gave an absolute standard deviation of ±0.07 wt %.
6. Analytical Results
6.1. Baddeleyite U-Pb Age
[16] Results of U-Pb isotopic analysis of four baddeleyite crystals and one abraded zircon crystal from the Eagle feldspathic peridotite are given in Table 1. The baddleyite data are plotted in Figure 6. Two of the baddeleyite data are concordant and the other two are 4–5% discordant. The weighted mean 207Pb/206Pb age of the four baddeleyite crystals is 1107.3 ± 3.7 Ma (MSWD = 1.1). A regression line through all the data yields an identical age of 1107.2 ± 5.7 Ma (MSWD = 1.6) and has a lower intercept at −8 ± 320 Ma, indicating that minor Pb loss occurred recently.
| Fraction | Weight (mg) | U (ppm) | PbComa (pg) | 207Pb/204Pb Measuredb | Th/Uc | 206Pb/238U | 207Pb/235U | 207Pb/206Pb Age (Ma) | % Discd |
|---|---|---|---|---|---|---|---|---|---|
| Baddeleyite fragment | 0.0002 | 1036 | 0.58 | 341.6 | 0.04 | 0.18576 | 1.9583 | 1107.1 | 0.9 |
| Baddeleyite fragment | 0.0001 | 664 | 0.47 | 140.2 | 0.59 | 0.17973 | 1.9018 | 1114.6 | 4.8 |
| Zircon | 0.0011 | 434 | 0.73 | 3667.4 | 0.17 | 0.49434 | 12.0522 | 2623.3 | 1.6 |
| Baddeleyite fragment | 0.0002 | 396 | 0.47 | 163.8 | 0.25 | 0.17894 | 1.8823 | 1102.8 | 4.1 |
| Baddeleyite fragment | 0.0002 | 335 | 0.51 | 135.4 | 0.04 | 0.18687 | 1.9703 | 1107.4 | 0.3 |
- a Pbcom is common Pb assuming the isotopic composition of laboratory blank: 206/204, 18.221; 207/204, 15.612; 208/204, 39.360 (2σ errors of 2%).
- b Values of 207Pb/204Pb corrected for fractionation and common Pb in the spike; Pb/U ratios also corrected for blank (no initial Pb in these analyses).
- c Th/U calculated from radiogenic 208Pb/206Pb ratio and 207Pb/206Pb age assuming concordance.
- d Disc is percent discordance for the given 207Pb/206Pb age.
[17] The baddeleyite age for the Eagle feldspathic peridotite indicates that the emplacement of the Eagle intrusion is coeval with the first phase of regional magmatism associated with the development of the MRS, including the Siemens Creek volcanic suite (Figure 7a) and the Group 1 volcanic suite at Mamainse Point (Figure 7c). As summarized by Nicholson et al. [1997], the first phase of magmatism took place from 1108 to 1105 Ma and is characterized by the presence of picritic basalts.
[18] The 207Pb/206Pb age of the zircon analysis is 2623.3 ± 1.4 Ma, which is similar to the age of other Archean rocks in the area of the Eagle deposit [Bickford et al., 2006; Schmitz et al., 2006]. The zircon grain analyzed is interpreted to be a xenocryst because it has an age much older than the MRS magmatism. This interpretation is consistent with the nature of SiO2 undersaturation of the parental magma for the peridotite as discussed below.
6.2. Mineral Compositions
[19] There are two types of olivine in the Eagle and East Eagle intrusion. One is present in low-sulfide rocks, including feldspathic peridotite, melatroctolite and olivine melagabbro units with Fo contents between 75 and 85 mol % (Data Set S1). The second type of olivine occurs disseminated in semimassive sulfides and has Fo content between from 70 to 79 mol % (Data Set S1). The olivine in the Eagle and East Eagle intrusions are significantly more magnesian than in coeval large layered intrusions in the MRS [Miller and Ripley, 1996; Ripley et al., 2007], suggesting crystallization from a more primitive magma. Significant compositional zoning is observed in large olivine crystals in the olivine melagabbro and some melatroctolite samples from the Eagle intrusion. The zoned olivine crystals (Figures 5k and 5l) are characterized by a large core with almost constant Fo and Ni contents and a thin rim with Fo and Ni content decreasing rapidly outward (Figures 8a–8d). The core to rim Fo content differences are up to 8 mol %. Olivine crystals in the feldspathic peridotite unit are only slightly zoned; the core to rim Fo variations are <2 mol %. The most primitive olivine (highest Fo content) is the core of large, zoned crystals in the olivine melagabbro (Figures 9a and 9b). Olivine with core compositions ranging between Fo82 and Fo86 may occur in the olivine melagabbro samples (Figure 9b). The Fo contents of olivine in the melatroctolite samples from the Eagle and East Eagle intrusions are similar (Figure 9c). However, olivine crystals from the East Eagle intrusion have smaller core-rim Fo variations (<3 mol %). Olivine crystals in the low-sulfide samples have a positive Fo-Ni relationship. In contrast, olivine associated with semimassive sulfides has a negative correlation between Fo and Ni contents (Figure 9d). The reasons for these different variations will be evaluated in the discussion below.
[20] The compositions of Cr-spinels inclusions in olivine from the Eagle and East Eagle intrusions are similar (see Data Set S2). Within the Eagle intrusion both the Cr # and Mg # of Cr-spinel inclusions in olivine crystals from olivine melagabbro are generally higher than those from feldspathic peridotite and melatroctolite.
[21] The core to rim differences of clinopyroxene Mg # in olivine melagabbro and melatroctolite in the Eagle intrusion are up to 9 (see Data Set S3). The Mg # of cores vary between 82 and 84, and the rim values vary between 74 and 78. In contrast, the core to rim differences of clinopyroxene Mg # in feldspathic peridotite are comparatively small, less than 3. The Mg # of clinopyroxene cores of the East Eagle intrusion are similar to those from the Eagle intrusion, varying between 82 and 83. Variations of clinopyroxene Mg # from core to rim in the East Eagle intrusion are around 4. Clinopyroxenes from the Eagle and East Eagle intrusions are characterized by much higher Mg # than those in intrusions of the Duluth Complex [Ripley et al., 2007; Miller and Ripley, 1996], again consistent with derivation from a more primitive magma.
[22] The Mg # of orthopyroxene cores vary between 78 and 85 (see Data Set S4), a larger range than exhibited by coexisting clinopyroxene crystals in the intrusions. Orthopyroxene crystals from olivine melagabbro and melatroctolite units in the Eagle intrusion tend to have higher Mg # than those from feldspathic peridotite in the intrusion. The Mg # of orthopyroxene from melatroctolite in the East Eagle intrusion is lower than that from melatroctolite in the Eagle intrusion. Mg # for orthopyroxene in both the Eagle and East Eagle intrusions are higher than those of orthopyroxene in the Duluth Complex [Ripley et al., 2007].
[23] The An contents of plagioclase cores in the Eagle intrusion vary between 59 and 65 mol % (see Data Set S5). Rim compositions range from An44 to An48, with most core to rim variations in the range of 12 to 17 mol %. The An contents of plagioclase cores in the East Eagle melatroctolites vary between 61 and 62 mol % and rim compositions range from An47 to An53. Plagioclase in both intrusions contains <0.2 wt % K2O. Plagioclase core An values are similar to the most calcic cores found in troctolitic rocks of the Duluth Complex [Ripley et al., 2007; Miller and Ripley, 1996].
[24] Figure 10 illustrates the stratigraphic variations of mineral modes and compositions in drill core 03EA034 that penetrated the central part of the Eagle intrusion (see Figure 2c). The Fo content of olivine cores decreases upward in the olivine melagabbro zone, continues decreasing in the overlying melatroctolite zone, but increases upward in the feldspathic peridotite zone. The rim Fo content of olivine remains constant in the olivine melagabbro and melatroctolite zones, and parallels the increases in the core compositions in the feldspathic peridotite zone. The Mg # of clinopyroxene cores is generally constant in the entire drill core. The rim Mg # of clinopyroxene remains constant in the olivine melagabbro zone, and increases upward from the melatroctolite zone to the feldspathic peridotite zone. The An content of plagioclase slightly increases upward. As shown in Figure 2c, the lower part of the olivine melagabbro zone which contains the most primitive olivine is close to the center of the intrusion, not to the basal contact of the intrusion with country rocks.
[25] Variations of mineral modes and compositions with depth in drill core 04EA044 (see Figure 2e) in the East Eagle intrusion are illustrated in Figure 11. The core and rim Fo contents of olivine are constant for the entire interval. The core and rim Mg # of clinopyroxene also remain constant in the drill core as do plagioclase An contents.
6.3. Whole-Rock Major Element Compositions
[26] Major element compositions of the anhydrous silicate portion of the bulk rocks are listed in Table 2. The amount of total sulfide in a sulfide-bearing sample was estimated using the content of S multiplied by FeS/S atomic ratio of 2.8 [Naldrett, 1981]. The content of FeOt (total iron calculated as FeO) was corrected for Fe in the sulfide with a composition assumed to be FeS. The corrected FeOt was then split between FeO and Fe2O3 using a calculated FeO/(FeO + Fe2O3) ratio of 0.9 [Kilinc et al., 1983]. The normalized values are used in the following discussion.
| Sample | ||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| DX2 | DX4 | DX6 | DX08 | DX11 | DX15 | DX17 | DX19 | DX21 | DX23 | DX25 | DX28 | DX31 | DX32 | DX33 | DX35 | DX36 | DX38 | DX75 | DX79 | DX81 | DX85 | DX87 | DX89 | DX91 | DX93 | DX95 | DX97 | DX99 | DX101 | DX103 | DX105 | DX107 | DX109 | DX111 | DX113 | DX115 | DSC-7B | |
| Drill core | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | |
| Depth (m) | 32.09 | 40.48 | 50.16 | 59.50 | 72.15 | 94.00 | 106.25 | 117.65 | 128.30 | 137.45 | 148.55 | 165.15 | 183.15 | 191.80 | 199.81 | 207.81 | 212.70 | 222.15 | 54.10 | 94.35 | 114.00 | 155.15 | 174.90 | 194.05 | 214.30 | 234.00 | 254.00 | 274.50 | 294.00 | 314.50 | 334.45 | 354.70 | 373.75 | 393.22 | 414.30 | 434.40 | 454.95 | |
| Rock type | F-PED | F-PED | F-PED | F-PED | F-PED | F-PED | F-PED | GBN | MTR | MTR | MTR | MTR | MGB | MGB | MGB | MGB | MGB | MGB | MGB | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MAM |
| SiO2 | 47.13 | 46.44 | 48.01 | 44.89 | 47.15 | 49.61 | 51.56 | 55.19 | 47.25 | 48.07 | 47.55 | 48.46 | 53.01 | 51.78 | 49.57 | 50.52 | 49.52 | 49.86 | 47.39 | 47.04 | 46.53 | 47.00 | 46.70 | 46.01 | 46.08 | 46.54 | 46.56 | 46.92 | 47.31 | 46.42 | 47.69 | 46.74 | 45.60 | 47.42 | 48.26 | 48.87 | 46.92 | 47.89 |
| TiO2 | 0.92 | 0.98 | 0.89 | 0.86 | 0.87 | 1.02 | 0.86 | 1.29 | 1.25 | 1.32 | 0.97 | 1.35 | 0.97 | 0.92 | 1.26 | 1.10 | 0.97 | 1.18 | 0.86 | 1.00 | 0.91 | 0.96 | 0.88 | 0.94 | 0.96 | 0.92 | 0.91 | 0.94 | 0.95 | 0.89 | 0.86 | 0.86 | 0.97 | 0.98 | 0.98 | 0.98 | 0.96 | 1.81 |
| Al2O3 | 5.33 | 4.69 | 4.80 | 4.65 | 4.43 | 5.30 | 4.65 | 7.76 | 6.35 | 6.42 | 6.12 | 7.39 | 6.01 | 6.89 | 7.03 | 6.73 | 6.63 | 8.07 | 6.36 | 5.84 | 5.63 | 6.74 | 5.86 | 5.58 | 5.41 | 5.44 | 6.57 | 5.90 | 6.09 | 6.79 | 5.96 | 5.78 | 4.25 | 6.83 | 6.76 | 7.19 | 6.20 | 11.66 |
| FeOT | 15.78 | 16.14 | 15.68 | 14.61 | 12.83 | 10.47 | 9.92 | 8.37 | 13.30 | 12.88 | 12.79 | 13.16 | 9.95 | 9.07 | 10.94 | 9.13 | 11.28 | 12.61 | 13.91 | 14.06 | 14.38 | 14.13 | 14.37 | 14.13 | 13.93 | 13.95 | 14.22 | 14.10 | 14.02 | 14.22 | 13.73 | 14.16 | 14.47 | 13.92 | 13.54 | 12.61 | 14.06 | 12.54 |
| MnO | 0.22 | 0.23 | 0.22 | 0.20 | 0.19 | 0.20 | 0.20 | 0.19 | 0.18 | 0.18 | 0.18 | 0.17 | 0.19 | 0.20 | 0.17 | 0.18 | 0.20 | 0.18 | 0.21 | 0.20 | 0.21 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.20 | 0.22 | 0.20 | 0.19 | 0.20 | 0.20 | 0.21 |
| MgO | 25.73 | 26.53 | 25.41 | 29.42 | 28.50 | 26.98 | 26.85 | 18.93 | 25.34 | 24.34 | 25.24 | 22.81 | 22.32 | 25.34 | 23.45 | 24.05 | 24.53 | 21.27 | 24.84 | 25.21 | 26.60 | 24.83 | 25.95 | 27.14 | 27.57 | 26.48 | 25.46 | 25.88 | 25.23 | 25.39 | 24.76 | 25.93 | 27.85 | 23.46 | 23.19 | 23.13 | 24.91 | 13.64 |
| CaO | 3.85 | 4.19 | 4.29 | 3.28 | 3.82 | 3.94 | 3.79 | 5.67 | 4.03 | 4.48 | 5.03 | 4.20 | 4.64 | 4.58 | 5.19 | 5.86 | 5.58 | 5.31 | 5.42 | 5.49 | 4.92 | 4.88 | 4.83 | 4.94 | 4.70 | 5.34 | 4.78 | 4.79 | 4.77 | 4.71 | 5.65 | 4.96 | 5.50 | 5.56 | 5.54 | 5.27 | 5.31 | 9.81 |
| Na2O | 0.74 | 0.50 | 0.44 | 1.82 | 1.97 | 2.18 | 1.91 | 2.29 | 2.10 | 2.06 | 1.93 | 2.19 | 2.53 | 0.92 | 2.19 | 2.24 | 0.95 | 1.12 | 0.76 | 0.80 | 0.53 | 0.90 | 0.88 | 0.75 | 0.76 | 0.82 | 0.98 | 0.93 | 1.03 | 1.03 | 0.84 | 1.01 | 0.72 | 1.27 | 1.18 | 1.33 | 1.07 | 1.75 |
| K2O | 0.23 | 0.20 | 0.18 | 0.22 | 0.17 | 0.22 | 0.22 | 0.21 | 0.08 | 0.14 | 0.11 | 0.16 | 0.31 | 0.24 | 0.09 | 0.10 | 0.27 | 0.31 | 0.18 | 0.27 | 0.21 | 0.28 | 0.24 | 0.23 | 0.30 | 0.24 | 0.25 | 0.26 | 0.31 | 0.27 | 0.23 | 0.28 | 0.34 | 0.30 | 0.29 | 0.31 | 0.30 | 0.48 |
| P2O5 | 0.07 | 0.10 | 0.08 | 0.07 | 0.07 | 0.08 | 0.06 | 0.10 | 0.11 | 0.11 | 0.08 | 0.12 | 0.07 | 0.07 | 0.10 | 0.09 | 0.08 | 0.09 | 0.07 | 0.09 | 0.08 | 0.10 | 0.08 | 0.08 | 0.08 | 0.07 | 0.07 | 0.08 | 0.09 | 0.08 | 0.07 | 0.08 | 0.08 | 0.08 | 0.08 | 0.09 | 0.09 | 0.21 |
| Total | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 | 100.00 |
- a F-PED, feldspathic peridotite; GBN, feldspathic pyroxenite; MTR, melatroctolite; MGB, olivine melagabbro; MAM, sample from Group 1 of the Mamainse Point volcanics. Unit is wt %.
[27] Variations of whole-rock compositions in the Eagle and East Eagle intrusions are illustrated in Figure 12. The MgO content of the Eagle feldspathic pyroxenite is close to 18 wt %. It is characterized by high SiO2 (Figure 12a) and low FeO (Figure 12b) in comparison with other types of rocks in the intrusions. The MgO contents of all olivine-bearing rocks in the Eagle and East Eagle intrusions vary between 23 and 29 wt %. Feldspathic peridotites have the highest MgO contents between 25 and 29 wt %. Melatroctolites and olivine melagabbros have lower MgO contents, varying between 21 and 25 wt %. In the olivine-rich rocks the contents of SiO2, Al2O3, and CaO increase with decreasing MgO contents (Figures 12a, 12c, and 12d). No significant correlation between MgO and FeO contents is observed in these rocks (Figure 12b). Melatroctolites in the Eagle and East Eagle intrusions have similar MgO, FeO, SiO2, Al2O3 and CaO contents but differ slightly in TiO2 and K2O contents. Figure 12 illustrates that most of the major element variations shown by rocks of the Eagle and East Eagle intrusions are accounted for by mixtures of olivine, clinopyroxene, orthopyroxene and plagioclase. Relative enrichments in K2O are related to the presence of biotite, and TiO2 enrichment is a function of biotite, titaniferous magnetite and Cr-spinel.
6.4. Whole-Rock Trace Element Compositions
[28] Whole-rock trace element compositions of the Eagle and East Eagle intrusions are listed in Table 3. Variations of Zr concentrations, Zr/Y and La/Yb with depth are plotted together with mineral compositions in Figures 10 and 11. The whole-rock Zr concentrations decrease in the olivine melagabbro from 75 to 38 ppm, remain relatively high in the melatroctolite between 55 and 85 ppm, and sharply decrease in the feldspathic peridotite to values between 40 and 54 ppm (Figure 10e). Another important feature in this drill core is that the whole-rock Zr/Y and La/Yb ratios, which are not sensitive to fractional crystallization of olivine and clinopyroxene because of their incompatible nature, vary from 4.75 to 6.01, and from 4.84 to 6.76, respectively (Figures 10f and 10g). Using the partition coefficients for Zr, La, Y and Yb from Bedard [2001], our calculations indicate that 50% of olivine±clinopyroxene fractional crystallization from a basaltic magma results in less than an 0.1 change in Zr/Y ratio and less than an 0.5 change in La/Yb ratio of the residual liquid. These values are insignificant in comparison with the variations in the rocks. Variations in whole-rock Zr/Y and La/Yb ratios in the melatroctolite of the East Eagle intrusion (Figures 11f and 11g) are similar to those observed in the melatroctolite of the Eagle intrusion.
| Sample | |||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| DX2 | DX4 | DX6 | DX08 | DX11 | DX15 | DX17 | DX19 | DX21 | DX23 | DX25 | DX28 | DX31 | DX32 | DX33 | DX35 | DX36 | DX38 | DX75 | DX79 | DX81 | DX85 | DX87 | DX89 | DX91 | DX93 | DX95 | DX97 | DX99 | DX101 | DX103 | DX105 | DX107 | DX109 | DX111 | DX113 | DX115 | |
| Drill core | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 03EA034 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 | 04EA044 |
| Depth (m) | 32.09 | 40.48 | 50.16 | 59.50 | 72.15 | 94.00 | 106.25 | 117.65 | 128.30 | 137.45 | 148.55 | 165.15 | 183.15 | 191.80 | 199.81 | 207.81 | 212.70 | 222.15 | 54.10 | 94.35 | 114.00 | 155.15 | 174.90 | 194.05 | 214.30 | 234.00 | 254.00 | 274.50 | 294.00 | 314.50 | 334.45 | 354.70 | 373.75 | 393.22 | 414.30 | 434.40 | 454.95 |
| Rock type | F-PED | F-PED | F-PED | F-PED | F-PED | F-PED | F-PED | GBN | MTR | MTR | MTR | MTR | MGB | MGB | MGB | MGB | MGB | MGB | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR | MTR |
| Nb | 2.90 | 3.30 | 2.90 | 2.50 | 2.50 | 2.60 | 2.20 | 4.00 | 3.80 | 4.40 | 3.20 | 4.70 | 2.10 | 2.60 | 3.50 | 3.10 | 2.90 | 3.90 | 2.80 | 3.90 | 3.20 | 3.50 | 3.30 | 3.40 | 3.80 | 3.20 | 3.30 | 3.50 | 3.70 | 3.20 | 3.00 | 2.90 | 3.40 | 3.70 | 3.70 | 3.80 | 3.60 |
| Zr | 44.10 | 54.20 | 45.90 | 43.20 | 43.30 | 44.10 | 38.80 | 74.30 | 65.00 | 75.80 | 55.00 | 84.60 | 38.20 | 48.50 | 62.80 | 60.80 | 57.90 | 74.60 | 46.80 | 67.90 | 50.00 | 57.90 | 55.00 | 57.10 | 64.30 | 55.80 | 55.30 | 59.90 | 62.00 | 53.70 | 52.20 | 48.60 | 60.40 | 60.60 | 63.70 | 63.90 | 61.30 |
| Y | 7.34 | 9.18 | 8.20 | 7.28 | 7.57 | 7.80 | 7.10 | 13.38 | 11.18 | 13.03 | 10.01 | 15.17 | 8.05 | 9.89 | 12.58 | 11.90 | 11.68 | 14.64 | 8.69 | 11.71 | 9.33 | 9.98 | 9.55 | 9.81 | 10.65 | 9.42 | 9.35 | 10.26 | 10.58 | 9.29 | 9.89 | 9.18 | 10.26 | 10.94 | 11.49 | 11.06 | 10.81 |
| La | 4.13 | 5.27 | 4.38 | 4.04 | 4.12 | 4.34 | 3.94 | 7.00 | 6.72 | 7.30 | 5.45 | 7.60 | 3.58 | 4.63 | 5.98 | 5.22 | 5.22 | 6.65 | 4.62 | 6.82 | 5.46 | 5.83 | 5.54 | 5.83 | 6.34 | 5.28 | 5.37 | 6.12 | 6.26 | 5.44 | 5.23 | 5.18 | 6.04 | 6.22 | 6.48 | 6.34 | 6.14 |
| Ce | 10.56 | 13.06 | 10.65 | 10.06 | 10.26 | 10.71 | 9.58 | 16.85 | 16.07 | 17.61 | 12.99 | 18.29 | 8.72 | 11.01 | 14.10 | 12.51 | 12.49 | 15.85 | 10.88 | 16.11 | 12.80 | 14.03 | 13.08 | 13.83 | 14.88 | 12.59 | 12.66 | 14.46 | 14.74 | 12.98 | 12.67 | 12.27 | 14.11 | 14.73 | 15.51 | 15.07 | 14.63 |
| Pr | 1.40 | 1.77 | 1.48 | 1.40 | 1.40 | 1.45 | 1.27 | 2.25 | 2.17 | 2.40 | 1.77 | 2.48 | 1.18 | 1.50 | 1.90 | 1.72 | 1.71 | 2.19 | 1.51 | 2.18 | 1.74 | 1.89 | 1.75 | 1.83 | 2.00 | 1.67 | 1.67 | 1.95 | 1.99 | 1.75 | 1.72 | 1.66 | 1.90 | 1.98 | 2.11 | 2.04 | 1.97 |
| Nd | 6.60 | 8.21 | 6.90 | 6.46 | 6.49 | 6.67 | 5.74 | 10.34 | 10.07 | 11.05 | 8.18 | 11.28 | 5.48 | 6.87 | 8.86 | 8.04 | 7.91 | 10.15 | 6.93 | 9.89 | 7.90 | 8.58 | 7.96 | 8.38 | 9.04 | 7.64 | 7.64 | 8.92 | 9.03 | 7.96 | 7.83 | 7.70 | 8.47 | 8.96 | 9.66 | 9.26 | 8.97 |
| Sm | 1.68 | 2.10 | 1.81 | 1.62 | 1.66 | 1.71 | 1.49 | 2.69 | 2.51 | 2.81 | 2.13 | 2.91 | 1.49 | 1.84 | 2.34 | 2.20 | 2.18 | 2.74 | 1.83 | 2.50 | 1.96 | 2.18 | 2.04 | 2.06 | 2.29 | 1.96 | 1.92 | 2.19 | 2.23 | 1.99 | 2.01 | 1.93 | 2.16 | 2.27 | 2.41 | 2.30 | 2.23 |
| Eu | 0.58 | 0.69 | 0.61 | 0.56 | 0.56 | 0.58 | 0.49 | 0.94 | 0.84 | 0.91 | 0.72 | 0.97 | 0.52 | 0.62 | 0.81 | 0.77 | 0.73 | 0.93 | 0.62 | 0.80 | 0.68 | 0.72 | 0.67 | 0.69 | 0.76 | 0.66 | 0.66 | 0.72 | 0.74 | 0.66 | 0.69 | 0.66 | 0.71 | 0.76 | 0.80 | 0.76 | 0.75 |
| Gd | 1.79 | 2.22 | 1.94 | 1.80 | 1.80 | 1.83 | 1.58 | 2.93 | 2.75 | 3.01 | 2.29 | 3.17 | 1.74 | 2.08 | 2.63 | 2.55 | 2.51 | 3.06 | 1.91 | 2.63 | 2.11 | 2.31 | 2.18 | 2.19 | 2.43 | 2.09 | 2.02 | 2.34 | 2.35 | 2.11 | 2.17 | 2.10 | 2.28 | 2.44 | 2.59 | 2.43 | 2.38 |
| Tb | 0.27 | 0.33 | 0.30 | 0.27 | 0.28 | 0.28 | 0.25 | 0.46 | 0.42 | 0.46 | 0.35 | 0.50 | 0.28 | 0.34 | 0.43 | 0.42 | 0.40 | 0.49 | 0.30 | 0.41 | 0.33 | 0.35 | 0.33 | 0.35 | 0.37 | 0.33 | 0.32 | 0.36 | 0.37 | 0.33 | 0.34 | 0.33 | 0.36 | 0.38 | 0.40 | 0.37 | 0.37 |
| Dy | 1.63 | 1.97 | 1.79 | 1.61 | 1.63 | 1.66 | 1.51 | 2.79 | 2.50 | 2.79 | 2.15 | 3.02 | 1.69 | 2.03 | 2.61 | 2.51 | 2.45 | 2.99 | 1.85 | 2.46 | 1.95 | 2.15 | 2.00 | 2.05 | 2.23 | 1.99 | 1.91 | 2.13 | 2.17 | 1.96 | 2.08 | 1.99 | 2.15 | 2.27 | 2.42 | 2.28 | 2.26 |
| Ho | 0.30 | 0.37 | 0.34 | 0.31 | 0.31 | 0.32 | 0.29 | 0.53 | 0.48 | 0.52 | 0.40 | 0.59 | 0.32 | 0.40 | 0.51 | 0.48 | 0.47 | 0.58 | 0.35 | 0.47 | 0.38 | 0.40 | 0.38 | 0.39 | 0.43 | 0.38 | 0.37 | 0.41 | 0.41 | 0.38 | 0.40 | 0.37 | 0.41 | 0.44 | 0.46 | 0.43 | 0.43 |
| Er | 0.82 | 0.97 | 0.90 | 0.81 | 0.81 | 0.84 | 0.79 | 1.44 | 1.27 | 1.39 | 1.08 | 1.60 | 0.88 | 1.08 | 1.34 | 1.29 | 1.25 | 1.54 | 0.95 | 1.26 | 1.01 | 1.08 | 1.03 | 1.03 | 1.16 | 1.04 | 0.99 | 1.09 | 1.12 | 1.00 | 1.07 | 1.02 | 1.11 | 1.15 | 1.24 | 1.17 | 1.15 |
| Tm | 0.11 | 0.13 | 0.12 | 0.11 | 0.11 | 0.11 | 0.11 | 0.20 | 0.17 | 0.19 | 0.15 | 0.22 | 0.12 | 0.15 | 0.19 | 0.17 | 0.17 | 0.21 | 0.13 | 0.17 | 0.14 | 0.15 | 0.14 | 0.14 | 0.16 | 0.14 | 0.14 | 0.15 | 0.15 | 0.14 | 0.14 | 0.14 | 0.15 | 0.16 | 0.17 | 0.16 | 0.16 |
| Yb | 0.67 | 0.78 | 0.74 | 0.66 | 0.68 | 0.71 | 0.67 | 1.20 | 1.05 | 1.15 | 0.90 | 1.35 | 0.74 | 0.89 | 1.13 | 1.06 | 1.07 | 1.27 | 0.80 | 1.04 | 0.87 | 0.90 | 0.87 | 0.87 | 0.98 | 0.87 | 0.85 | 0.91 | 0.94 | 0.84 | 0.90 | 0.84 | 0.95 | 0.97 | 1.03 | 0.98 | 0.98 |
| Lu | 0.10 | 0.11 | 0.11 | 0.09 | 0.10 | 0.10 | 0.10 | 0.17 | 0.15 | 0.16 | 0.13 | 0.20 | 0.11 | 0.13 | 0.16 | 0.15 | 0.15 | 0.18 | 0.12 | 0.15 | 0.13 | 0.13 | 0.13 | 0.12 | 0.14 | 0.12 | 0.12 | 0.13 | 0.13 | 0.12 | 0.13 | 0.12 | 0.13 | 0.14 | 0.15 | 0.14 | 0.14 |
| Hf | 1.20 | 1.50 | 1.30 | 1.20 | 1.20 | 1.20 | 1.10 | 2.10 | 1.90 | 2.10 | 1.60 | 2.40 | 1.10 | 1.40 | 1.80 | 1.70 | 1.60 | 2.10 | 1.30 | 1.90 | 1.40 | 1.60 | 1.50 | 1.60 | 1.80 | 1.50 | 1.50 | 1.60 | 1.70 | 1.50 | 1.50 | 1.40 | 1.70 | 1.70 | 1.80 | 1.80 | 1.70 |
| Ta | 0.18 | 0.21 | 0.18 | N.D. | N.D. | 0.17 | N.D. | 0.25 | 0.26 | 0.29 | 0.20 | 0.30 | N.D. | N.D. | 0.23 | 0.20 | 0.19 | 0.25 | 0.17 | 0.25 | 0.21 | 0.22 | 0.21 | 0.22 | 0.24 | 0.21 | 0.21 | 0.23 | 0.23 | 0.21 | 0.20 | 0.19 | 0.22 | 0.23 | 0.24 | 0.25 | 0.23 |
| Th | 0.58 | 0.73 | 0.62 | 0.60 | 0.60 | 0.65 | 0.66 | 1.19 | 1.11 | 1.20 | 0.88 | 1.35 | 0.62 | 0.85 | 1.07 | 0.88 | 0.93 | 1.15 | 0.68 | 1.07 | 0.81 | 0.93 | 0.90 | 0.92 | 1.04 | 0.87 | 0.84 | 0.95 | 0.98 | 0.86 | 0.80 | 0.77 | 1.00 | 0.95 | 0.99 | 1.00 | 0.95 |
- a F-PED, feldspathic peridotite; GBN, feldspathic pyroxenite; MTR, melatroctolite; MGB, olivine melagabbro. Unit is ppm.
[29] Figures 13a and 13b are the primitive mantle-normalized alteration-resistant trace element patterns for the Eagle and East Eagle intrusions and coeval volcanic rocks. The trace element patterns of the Eagle and East Eagle intrusions are similar. They generally match the pattern of average picritic basalts in the Group 1 volcanic suite at Mamainse Point (data from Shirey et al. [1994]) but are significantly different from the pattern of average picritic basalts in the Lower Siemens Creek volcanics (data from Nicholson et al. [1997]). With the exception of Th all elemental concentrations in the intrusions are lower than the average of the Mamainse Point Group 1 volcanics. This is expected if the intrusions represent higher crystal/liquid ratios than do the volcanics. In detail, the negative anomalies of Nb and Ta are more pronounced in the intrusive rocks than in the average picritic basalts in the Group 1 volcanic suite at Mamainse Point.
7. Discussion
7.1. Composition of Parental Magma, Mineral Accumulation, and Olivine Fo-Ni Relations
[30] No chilled margin that may represent parental magma is found in the Eagle and East Eagle intrusions. Textures, mineral modes and mineral compositions suggest that the rock types represent mixtures of crystals and trapped liquid (Figure 12). The baddeleyite U-Pb age indicates that the Eagle, and by association the East Eagle intrusion, are the oldest tholeiitic intrusions of the MRS. Compared to other larger tholeiitic intrusions, such as those within the Duluth and Mellen complexes, the Eagle and East Eagle intrusions have higher whole-rock Mg # as well as higher olivine Fo contents. The age and stratigraphic correlations suggest that the Eagle and East Eagle intrusions were temporally related to the eruption of picritic basalts that occur near the base of the oldest volcanic sequences of the MRS (Figure 7).
[31] The FeO/MgO ratio of the parental magma for the most primitive olivine in the Eagle intrusion (olivine melagabbro) is estimated to be 1.04 using the KD (FeO/MgO)olivine/(FeO/MgO)liquid of 0.3 given by Roeder and Emslie [1970]. The estimated value is within the range of picritic basalts (0.7–1.4) in the Group 1 volcanic suite at Mamainse Point [Shirey et al., 1994] and the Lower Siemens Creek volcanics [Nicholson et al., 1997]. The Al2O3 contents of parental magmas in equilibrium with spinels from all units in the Eagle intrusion estimated using the relation (Al2O3)spinel = 0.035(Al2O3)liquid2.42 (wt.%) by Maurel and Maurel [1982] are from 8.41 to 10.87 wt %. These values are also generally within the range of picritic basalts (9.53–13.62 wt %) in the Group 1 volcanic suite at Mamainse Point and the Lower Siemens Creek volcanics. In addition, the picritic basalts of the Group 1 volcanic suite at Mamainse Point and the Eagle intrusive rocks have similar trace element patterns (Figure 13a).
[32] As described by Berg and Klewin [1988], the picritic basalts of the Group 1 volcanic suite at Mamainse Point contain up to ∼20% olivine phenocrysts that crystallized before eruption. This suggests that the picritic basalts are mixtures of a liquid with variable amounts of olivine phenocrysts. Among the picritic basalt samples analyzed by Berg and Klewin [1988], sample DSC-7B has low olivine phenocryst content and high MgO and Zr concentrations. This sample is therefore a reasonable choice for a primitive liquid composition (Table 2). The MgO and Zr contents in this sample normalized to anhydrous composition are 13.64 wt % and 112 ppm, respectively. Using the bulk composition of this sample to represent an initial liquid, we have simulated its fractional crystallization and equilibrium crystallization using reasonable values for shallow rift-related tholeiites of 1 kb and fO2 of FMQ-2 [Righter et al., 2008] using the MELTS program of Ghiorso and Sack [1995]. The very low grade of metamorphism recorded in the Proterozoic country rocks (only partially recrystallized pelitic rocks composed of chlorite, muscovite and quartz; see also Vallini et al. [2007] suggest that emplacement of the Eagle intrusion occurred at pressures less than 2 kb. The initial content of H2O in the liquid is assumed to be 0.5 wt %, again a reasonable value for rift-related intrusions that have not undergone degassing [Ripley et al., 2007]. Results for both equilibrium and fractional crystallization indicate a crystallization sequence of olivine (first-formed = Fo86), clinopyroxene and plagioclase. The results are at odds with the clear presence of abundant primocryst orthopyroxene in all of the rock types in the Eagle intrusion. Orthopyroxene crystallization characterizes more siliceous compositions such as that of the feldspathic pyroxenite (e.g., DX19, Table 4). The zoning pattern observed in olivine from the olivine melagabbro (Figure 8) suggests that the primitive olivine was not in equilibrium with surrounding liquid. Rapid cooling, as evidenced by the plagioclase crystal form in the olivine melagabbro (Figure 5h), was required to preserve this relation. It is therefore likely that material around at least the cores of olivine in the olivine melagabbro does not represent the liquid from which the olivine grew. We estimated the composition of the rock which encloses the olivine by subtracting the weighted olivine core composition from the bulk rock composition. Results for samples DX35 to DX38 are shown in Table 4. The compositions are very similar to that of the feldspathic pyroxenite, DX19. Because clinopyroxene zoning is similar to that of olivine we also subtracted clinopyroxene from the olivine melagabbro with similar results. Such MgO- and SiO2-rich compositions cannot be produced by fractional crystallization of the DSC-7B composition from Mamainse Point, nor can they be produced by contamination of such a parental melt by assimilation of siliceous country rocks. For both of these processes the concentration of MgO is too low for the appropriate SiO2 concentration. If we consider DX19 to represent a liquid composition then assimilation of siliceous country rocks would have required the parent melt to have been more MgO-rich than that represented by DSC-7B, or any other picritic lava in the MRS. We conclude that the MgO- and SiO2-rich compositions of DX19 and those computed from the subtraction of olivine from olivine melagabbro are not liquid compositions and must represent the accumulation of orthopyroxene. As a corollary we also conclude that the high-MgO rock types in the Eagle and East Eagle intrusions represent the accumulation of not only olivine, but also orthopyroxene, in the conduit system. Our analysis strongly suggests that picritic liquids similar in composition to DSC-7B at Mamainse Point were the parent liquids from which the most primitive olivine in the Eagle intrusion crystallized. However, combined fractional crystallization and, potentially, assimilation processes lead to the abundant crystallization of orthopyroxene in deeper chambers. Both olivine and orthopyroxene were entrained in ascending magmas and accumulated in favorable locations in the conduit now represented by the Eagle intrusion.
| Sample | ||||
|---|---|---|---|---|
| DX35 | DX36 | DX38 | DX19 | |
| Olivine mode (%) | 28 | 25 | 18 | |
| cpx mode (%) | 8.25 | 11.5 | 14.25 | |
| Computed Composition From the Subtraction of Olivine From Olivine Melagabbroa | ||||
| SiO2 | 54.71 | 52.96 | 52.08 | 55.19 |
| TiO2 | 1.52 | 1.29 | 1.44 | 1.29 |
| Al2O3 | 9.35 | 8.84 | 9.84 | 7.76 |
| FeOT | 6.76 | 9.99 | 12.28 | 8.37 |
| MnO | 0.17 | 0.20 | 0.19 | 0.19 |
| MgO | 16.23 | 17.73 | 15.97 | 18.93 |
| CaO | 8.07 | 7.37 | 6.43 | 5.67 |
| Na2O | 3.11 | 1.26 | 1.37 | 2.29 |
| K2O | 0.14 | 0.36 | 0.37 | 0.21 |
| P2O5 | 0.12 | 0.11 | 0.12 | 0.10 |
| Total | 100.18 | 100.12 | 100.09 | 100.00 |
| Computed Composition From the Subtraction of Olivine and cpx From Olivine Melagabbro | ||||
| SiO2 | 55.04 | 52.91 | 52.03 | |
| TiO2 | 1.82 | 1.46 | 1.56 | |
| Al2O3 | 11.33 | 10.11 | 10.78 | |
| FeOT | 6.68 | 10.64 | 12.90 | |
| MnO | 0.18 | 0.22 | 0.19 | |
| MgO | 15.25 | 17.61 | 15.54 | |
| CaO | 6.04 | 5.43 | 5.19 | |
| Na2O | 3.83 | 1.44 | 1.50 | |
| K2O | 0.17 | 0.42 | 0.41 | |
| P2O5 | 0.15 | 0.13 | 0.13 | |
| Total | 100.50 | 100.38 | 100.25 | |
- a Except for sample DX19, which is whole-rock composition.
[33] Variations of Fo and Ni contents in the cores of olivine in the Eagle intrusion can be modeled by fractional crystallization from a liquid with the composition of the Mamainse Point sample DSC-7B with ∼740 ppm Ni and an olivine-liquid DNi = 8.6 (Figure 9a). The DNi value is the average value for olivine crystallization from the Eagle magma using the equation of Wang and Gaetani [2008]. The variation in olivine Fo and Ni content recorded by core composition in the olivine melagabbro (Figure 9a) strongly suggest that olivine which crystallized from MgO-rich but variably fractionated liquid was ultimately incorporated in a more siliceous liquid capable of crystallizing orthopyroxene. This interpretation holds for the melatroctolite and feldspathic peridotite rock types where olivine core compositions follow an expected fractional crystallization trend in Ni versus Fo space (Figure 9a). Rim compositions of olivine in the olivine melagabbro and melatroctolite, however, represent reaction with, and minor olivine crystallization from, the more evolved transporting melt. Rim compositions of olivine in the feldspathic peridotite differ from core Fo contents by less than 2 mol % and may represent arrested “trapped liquid shifts” produced by reaction between early crystallizing olivine and evolved residual liquid [e.g., Barnes, 1986; Li et al., 2003]. The gentle olivine core to rim trend (Figure 9b) may be in part due to the slower diffusion of Ni relative to Mg-Fe in olivine [Ozawa, 1994; Nakamura, 1995].
[34] Olivine from melatroctolite in the East Eagle intrusion (Figure 9c) may have crystallized from a more Ni-rich magma, with rim compositions again representing reaction with trapped liquid. Alternatively, the core and rim compositions may represent stages of in situ crystallization preserved due to relatively rapid cooling in the conduit.
[36] As shown in Figure 9d, the Fo contents of olivine crystals associated with semimassive sulfides in the Eagle intrusion are significantly lower than those in the low-sulfide rocks in the intrusion. This suggests that the olivine crystals in the semimassive sulfide did not crystallize from the same magmas that formed the low-sulfide rocks, but from a more fractionated magma.
7.2. Multiple Magma Emplacements in the Conduit System
[37] The different degrees of olivine and clinopyroxene compositional zoning in the different rock units of the Eagle intrusion, along with textural differences, sharp contact relations, and distinct Fo values in olivine associated with semimassive sulfides suggest that at least four major pulses of magmas were involved in the formation of the sulfide-bearing rocks in the Eagle intrusion. Because Zr, Y, La and Yb are all incompatible during spinel, olivine, pyroxene and plagioclase crystallization [Bedard, 2001], the ratios Zr/Y and La/Yb should remain almost constant during crystallization of spinel and the other silicate minerals. Therefore, in situ crystallization cannot account for the incompatible element variations (Figures 10e–10g). The feldspathic peridotite and melatroctolite units are difficult to separate using Zr/Y and La/Yb ratios, but distinctly different Zr concentrations indicate that the ratio of primocrysts to trapped liquid is higher in the feldspathic peridotite. In addition the distinctly different olivine and clinopyroxene core-rim zoning characteristics indicate that the feldspathic peridotite and melatroctolite represent different cooling units. Much lower Zr/Y and La/Yb ratios in the olivine melagabbro are consistent with its origin from a distinct magma pulse.
[38] At Eagle and East Eagle the magmas within the conduit system carried variable amounts of olivine and pyroxene crystals that accumulated in the conduit system. We propose that the olivine and pyroxnene enrichments at Eagle and East Eagle were related to the changing geometry of a magma conduit. As shown in Figure 2, the Eagle intrusion may represent a wider part of a subvertical conduit. When an olivine/pyroxene- or sulfide-laden magma entered such a wide part, the velocity of the magma decreased, corresponding to a lower ability to carry dense materials such as olivine, pyroxene and immiscible sulfide droplets. As a result olivine and pyroxene crystals plus sulfide liquid droplets may have been concentrated, giving rise to massive sulfides and sulfide-silicate mixtures. As magma supply ceased, the liquids (some sulfide-bearing) trapped in the interstitial spaces of a framework consisting primarily of olivine, orthopyroxene, and clinopyroxene began to solidify. The feldspathic peridotite, melatroctolite, olivine melagabbro and olivine-rich semimassive sulfide units that comprise the Eagle intrusion represent a collection of crystals and variably fractionated, potentially contaminated, liquids that accumulated at different times in the magma conduit.
7.3. Conclusion and Implications for Regional Metallogeny
[39] Massive and semimassive sulfide ores are more common in the Eagle intrusion than in other much larger intrusions of the Duluth and Mellen complexes which formed by more evolved magmas during the later stages of the MRS [Ripley et al., 2007]. The Eagle intrusion is related to high-MgO basaltic magma, similar to magma that produced picritic basalts during the early stages of rift evolution (1107 ± 5.7 Ma). The other larger intrusions are primarily related to high-Al olivine tholeiitic magmas [Ripley et al., 2007]. High-MgO parental magmas also correlate with elevated Ni contents in the melt; the Ni enrichment observed in the sulfides at Eagle is in part related to the involvement of Ni- and MgO-rich magmas. In addition to the difference in parental magma composition and timing of magmatism recorded by intrusions of the Duluth and Mellen complexes, the Eagle intrusion represents a dynamic conduit whereas the larger sheet-like intrusions represent a more passive accumulation of magma. The Eagle and East Eagle intrusions are good examples of olivine, pyroxene and sulfide accumulation in a widened part of a dynamic conduit in response to decreasing magma velocity. Multiple pulses of magmas with different incompatible trace element ratios were involved in the development of the Eagle and East Eagle intrusions. The different pulses of magma were likely related to each other by assimilation/fractional crystallization processes in different staging chambers. The Eagle magmatic Ni-Cu-PGE deposit is an important conduit-type magmatic sulfide deposit that can be used as an exploration model for similar deposits in the MRS.
Acknowledgments
[40] This study was financially supported by Kennecott Exploration Ltd. and Kennecott Minerals Ltd., two subsidiaries of Rio Tinto Ltd. We thank the manager of the Eagle project, Andrew Ware, and his staff members for logistical support and useful discussions. This work was partially supported through NSF grant EAR 0710910 to C. Li and E. M. Ripley.
