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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Department of Earth Sciences, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5
North American Palladium Ltd., Thunder Bay, Ontario, Canada P7B 6T9
e-mail, john.hinchey{at}science.uottawa.ca
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Abstract |
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 Top Abstract IntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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Bulk chemical compositions suggest that all mafic igneous rocks in the mineralized zones, except for late clinopyroxenite, are cogenetic. The hypothetical parental magmas have high MgO and low (~15x chondrite), unfractionated rare earth elements (REE) with (Ce/Yb)chondrite <3, suggesting that the magmas formed through high degrees of partial melting in a moderately depleted mantle. Fractional crystallization of olivine, chromite, and high-temperature platinum group minerals (PGM) resulted in high concentrations of Pt, Pd, and Rh relative to Ir, Os, and Ru in the parental magmas. Extreme enrichment of Pd in the late melanocratic magmas is interpreted to have been attained through the incorporation of earlier formed sulfide melt. This interpretation is supported by high Cu/Pd in early barren leucocratic rocks and low Cu/Pd in fertile melanocratic rocks. Rocks in the volumetrically minor but economically important High-Grade zone (>35% of Pd in the deposit) on the eastern margin of the Roby zone have much higher concentrations of Pd than any other rocks and do not show correlations between sulfur and precious and base metals. Furthermore, the rocks are intensely and pervasively altered to actinolite, talc, anthophyllite, hornblende, chlorite, sericite, calcite, and quartz. These observations suggest subsolidus enrichment of Pd and mobility of S. The lack of apparent fluid pathways within the High-Grade zone and the distribution of the zone are consistent with the enrichment of Pd at high temperatures by fluids that originated from the mafic magmas.
The textures of the Lac des Iles deposit are similar to those of contact-type PGE deposits, but there are fundamental differences between the two. The Lac des Iles deposit is not localized near the contact between the host intrusion and the country rocks and evidence of the assimilation of the host rocks is lacking. Instead, the mineralization at Lac des Iles has many features in common with layered intrusion-hosted deposits, in which pulses of primitive magma introduced the PGE. Unlike the quiescent magma chambers of most layered deposits, the magmas at Lac des Iles were intruded energetically, forming breccias and magma-mingling textures.
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Introduction |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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Previous work and exploration history
Geologic investigations in the area began with reconnaissance mapping by
Jolliffe (1934), followed by more detailed mapping of the area by Pye (1968).
Economic interest in the area was sparked by the discovery of aeromagnetic
anomalies in the late 1950s. Significant Pd mineralization was first discovered
in the Roby zone in 1963 by prospectors and was subsequently investigated by
Gunnex Ltd. and Anaconda Ltd. The Ontario Geological Survey conducted several
mapping projects in the area (Sutcliffe and Sweeny, 1986; Macdonald, 1988;
Sutcliffe et al., 1989), and M.Sc. thesis projects on the deposit were completed
by Dunning (1979), Sweeny (1989), and Michaud (1998).
Madeleine Mines Ltd. commenced mining in 1990 but this lasted only several months. Lac des Iles Mines Ltd. began production in 1993 at a rate of 2,000 t/d. Today the Lac des Iles mine is an open-pit operation conducted by North American Palladium Ltd. Current proven and probable reserves consist of 88 million metric tons (Mt) grading 1.51 g/t Pd, 0.17 g/t Pt, 0.12 g/t Au, 0.06 percent Cu, and 0.05 percent Ni, with additional measured and indicated resources of 65 Mt grading 1.58 g/t Pd, 0.17 g/t Pt, 0.11 g/t Au, 0.05 percent Cu, and 0.05 percent Ni (North American Palladium Ltd. Annual Report, 2002).
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Geology |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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Geology of the Lac des Iles complex
The Lac des Iles intrusive complex (2692+4–2 Ma; Blackburn et
al., 1992) is subdivided into three main intrusive bodies (Lavigne and Michaud,
2001; Fig. 3): (1) the North Lac des Iles ultramafic intrusion, centered on the
lake, (2) the Mine Block intrusion, consisting of lithologically and texturally
complex gabbroic rocks, and (3) the Camp Lake intrusion, which is a homogeneous
hornblende gabbro southwest of Camp Lake. The three intrusions are separated by
tonalitic country rocks (Lavigne and Michaud, 2001). All rocks in the area have
been intruded by diabase dikes and sills, which range from ~2120 to ~1140 Ma (Buchan
and Ernst, 2004).
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Mineralized Zones |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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Most of the Roby zone, including the High-Grade zone, is being mined in the phase 3 open pit, but the southwest part of the Roby zone was still exposed on the surface in 2003 and was mapped in detail in this study (approx 65 to 70 m north-south and 35 to 40 m east-west: Fig. 5). The Twilight zone, with a surface exposure of 175 to 200 m north-south and 200 to 225 m east-west, lies southeast of the Roby zone (Fig. 4), with 50 to 70 m of East Gabbro between the two zones. The Baker zone is located approximately 1 km northeast from the Roby and Twilight zones (Fig. 4) and contains rocks and textures similar to those in the Roby and Twilight zones.
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Petrology of the Southern Roby Zone |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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Most rocks of the southern Roby zone show cumulate textures, consisting of subhedral to euhedral cumulus crystals of clinopyroxene, plagioclase, and minor orthopyroxene with intercumulus material consisting of the same assemblage plus minor biotite, magnetite, ilmenite, and sulfides. Breccias are common and are named on the basis of matrix composition. For example, a breccia with melanogabbro matrix is referred to as a melanogabbro breccia (Fig. 6A). Gabbroic rocks with varying grain size from fine to coarse and local pods and veins of pegmatite are termed "varitextured" gabbro.
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Mineralized melanocratic rocks (group 2)
The mineralized melanocratic rocks range from gabbro to clinopyroxenite,
contain sulfide, and have irregularly shaped pods and veins of pegmatite.
Brecciation and magma-mingling structures with the earlier leucocratic rocks of
group 1 are common. From north to south, the melanocratic rocks are (2a)
medium-grained meso- to melanogabbro breccia, (2b) medium- to relatively
coarse-grained clinopyroxenite, and (2c) medium-grained, dark gabbro (Fig. 5).
Pegmatite (2d) is common within and in contact with these melanocratic rocks.
Most rocks are altered with very rare relict pyroxenes and plagioclase.
Clinopyroxene is commonly replaced by a mixture of hornblende, actinolite, and
chlorite, whereas interstitial clinopyroxene is replaced by chlorite.
Plagioclase is mostly sericitized.
Medium-grained mesogabbro to melanogabbro breccia (2a): This breccia contains fragments of leucocratic rocks of group 1 (Fig. 6A). The matrix of the breccia is dominated by subhedral, equigranular clinopyroxene (1–6 mm, 50–80 vol %) that commonly forms aggregates, with lesser amounts of plagioclase (1–4 mm, 30–50 vol %) and minor (<5 vol %) orthopyroxene and biotite. Interstitial minerals are plagioclase (1–4 mm with minor aggregates of small ~0.5-mm crystals), minor clinopyroxene and biotite, and sulfide and oxide minerals. A mixture of sulfide and oxide minerals occur as blebs (1–6 mm diam) and fine-grained disseminations (1–3 vol % of the matrix). The sulfide minerals are mostly pyrrhotite with exsolved chalcopyrite and pentlandite. Minor pyrite and magnetite are present, and minor amounts of calcite occur in the blebs of sulfide minerals. Chalcopyrite also commonly occurs along cleavage planes of actinolite.
Medium- to coarse-grained clinopyroxenite (2b): These rocks range from massive, medium-grained clinopyroxene-rich melanogabbro to clinopyroxenite and consist of equigranular clinopyroxene (1–7 mm, 85–100 vol %) with interstitial clinopyroxene, plagioclase, orthopyroxene, biotite, sulfides and oxides, and rare gahnite. The rocks have an orthocumulate texture, with a minor adcumulate texture. Sulfides (1–5 vol %) are disseminated, blebby (0.5–2 mm), and net textured, consisting of pyrrhotite with intergrown chalcopyrite and pentlandite. The texture suggests that the sulfides were once monosulfide solid solution. The clinopyroxenite also contains minor chalcopyrite along cleavage planes of secondary actinolite (Fig. 7A).
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Pegmatite (2d): Pegmatite occurs as dikes and pods and is composed of coarse-grained plagioclase and clinopyroxene. Isolated pods of pegmatite on the surface are commonly connected to large pods of pegmatite through veinlets. The cores of the pods are commonly quartz rich and contain minor biotite, magnetite, ilmenite, pyrite, and traces of chalcopyrite. The sulfide aggregates commonly enclose fine-grained (<0.5-mm) calcite.
Late-stage intrusions (group 3)
Postmineralization intrusions include (3a) medium-grained, sulfide-free
clinopyroxenite and (3b) medium-grained, salt-and-pepper-textured gabbro. The
clinopyroxenite cuts earlier leucocratic rocks (group 1) and commonly displays
magma mingling with the earlier clinopyroxenite (2b). The clinopyroxenite (3a)
and gabbro (3b) display a cumulate texture and are altered with uralization of
clinopyroxene and sericitization of plagioclase.
Intrusions unrelated to the Lac des Iles complex
Late intrusions that cut rocks of the southern Roby zone are felsic and
diabase dikes. The felsic dikes are several centimeters to 1 m in width with
random orientations and range in composition from tonalite to granodiorite. The
dikes are commonly accompanied by narrow (<5-cm) symmetrical alteration halos
with reddish K-feldspar. Very fine grained diabase dikes, ranging from a few
centimeters to 1.5 m in width, cut all rock types.
Contact relationships between different rock types
The contacts between the early leucocratic rocks of group 1 and the
melanocratic rocks of group 2 are well exposed on the outcrop. The latter
contains fragments of leucocratic gabbro of group 1 (Fig. 6A) and also intruded
the partially solidified leucocratic rocks, developing magma-mingling structures
(Fig. 6B). This suggests that mafic magmas intruded prior to the solidification
of the earlier magmas, which produced the leucocratic rocks. Varitextured and
pegmatitic phases (rock type 2d) are commonly developed near the contact between
groups 1 and 2, suggesting the release of aqueous fluids from the magmas that
produced the melanocratic rocks.
Hydrothermal alteration
All rocks show some degree of alteration, especially melanocratic rocks which
are pervasively altered, but rocks adjacent to the Roby zone, including those of
the Twilight zone and East Gabbro, are not altered. Although minor faults occur
in the southern Roby zone, as observed by Michaud (1998), alteration is not
controlled by these faults. The alteration most likely took place at high
temperatures and was possibly related to aqueous fluids released from the magmas
for the melanocratic rocks. This interpretation is supported by abundant pods
and veins of pegmatite associated with the melanocratic rocks.
The High-Grade zone within the Roby zone is composed of intensely altered medium-grained melanogabbro and pyroxenite, similar to the melanocratic rocks of group 2 in the southern Roby zone. Primary minerals are entirely replaced by secondary amphiboles, talc, anthophyllite, chlorite, sericite, and calcite. The occurrence of blue quartz is also common. Sulfide minerals are dominated by pyrite (up to 10 vol %), with lesser amounts of pyrrhotite, chalcopyrite, and pentlandite.
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Petrology of the Twilight Zone |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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The mineralized melanocratic norite and/or gabbronorite (6) have a texture similar to other norite and/or gabbronorite although it is rich in sulfides. This rock consists of subhedral to euhedral cumulus orthopyroxene (3–10 mm, 70–95 vol %), intercumulus plagioclase (5–30 vol %), intercumulus clinopyroxene (<10 vol %), minor biotite, and interstitial blebby sulfides and oxides (2–8 vol %). The sulfides consist of intergrown pyrrhotite, chalcopyrite, and pentlandite and oxides are magnetite and ilmenite. As in the other noritic and/or gabbronoritic rocks, clinopyroxene occurs as an exsolved phase in orthopyroxene (inverted pigeonite). Minor amounts of chalcopyrite and magnetite occur along the cleavage of secondary amphiboles.
Contact relationships of different rocks
The three noritic and/or gabbronoritic rocks display breccia and
magma-mingling structures (Fig. 6C). Matrix and clast relationships indicate
that the leucocratic norite and/or gabbronorite (4) is the earliest and that the
melanocratic norite and/or gabbronorite orthopyroxene (6) is the youngest. The
magma-mingling structures suggest that successive intrusions occurred while
earlier intrusions were not completely solidified.
Anorthosite (7), gabbro (8), and melanogabbro and/or clinopyroxenite (9) were contemporaneous with the norite and/or gabbronorite (5). These rocks commonly occur as fragments in a matrix of norite and/or gabbronorite. Fragments of gabbro and anorthosite are especially common in the northern part of the Twilight zone. These rocks are medium grained, moderately to intensely altered, and free of sulfide.
The south-central portion of the Twilight zone is dominated by late, medium-grained melanogabbro and/or clinopyroxenite (9), which produces breccias containing fragments of norite, gabbronorite, and gabbro. The breccias and intrusions contain irregularly shaped pods and veins of pegmatite, which consist of plagioclase, pyroxene, quartz, magnetite, and sulfide dominated by pyrite. The rocks are more intensely altered than the volumetrically dominant norite and/or gabbronorite. Alteration has produced actinolite after clinopyroxene, and chlorite and sericite after intercumulus clinopyroxene and plagioclase. These rocks commonly contain blebs (up to 5 vol %) that consist of pyrrhotite, chalcopyrite, pentlandite, and minor magnetite. The textures and alteration are very similar to those of the melanogabbro and clinopyroxenite of the southern Roby zone.
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Geochemistry of Mineralized Zones |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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The concentrations of Ir, Os, and Ru were determined at the University of Ottawa by an isotope dilution technique using a solution enriched in 191Ir, 190Os, and 99Ru. The analytical procedure was essentially identical to that described in Guillot et al. (2000). Precious metals in 3-g samples mixed with 6 g of Na2B4O7 were concentrated into a Ni sulfide bead at 1,050°C. The bead was dissolved in HCl, and the insoluble residue was dissolved in HNO3 before isotope ratio measurements using an inductively coupled plasma-mass spectrometry (ICP-MS; model HP-4500). Blank contributions of Ir, Ru, and Os, mostly from the Ni powder, were 0.005, 0.011, and 0.008 ng/g of flux, respectively, and less than 1 percent of the concentrations of the metals in the samples. Gold, Pt, Pd, and Rh were determined by a Pb collection fire assay followed by an ICP-MS analysis on 30-g sample splits at Acme Analytical Laboratories Ltd. in Vancouver, with detection limits of 1, 0.1, 0.5, and 0.05 ppb, respectively. Precision and accuracy of the analysis based on nine replicate analyses of a reference were 4.50 and 9.2 percent for Au, 0.61 and 2.03 percent for Pt, 0.49 and 4.68 percent for Pd, and 16.4 percent and an undetermined accuracy for Rh. The concentrations of REE and other trace elements were determined at Acme Analytical Laboratories Ltd., using ICP-MS after digesting samples with HNO3-HClO4-HF-HCl. Acid digestion technique rather than fusion technique was selected because of lower detection limits for many elements and because of the lack of refractory minerals in the samples. The precision of the REE analyses based on four replicates was mostly better than 10 percent, but the analyses of samples with concentrations close to the detection limits had precisions of only 28 percent. The precision and accuracy of REE analyses of reference materials with high concentrations of REE were better than 10 percent. Concentrations of Cu and other chalcophile elements were determined at Acme Analytical Laboratories Ltd. after aqua regia digestion followed by an ICP-MS analysis. The precision and accuracy of the Cu determinations based on replicate analyses were 0.85 and 0.87 percent, respectively.
Compositional variation
In all of the studied rocks, FeO(total) (total Fe expressed as
FeO) and MnO concentrations increase with MgO, whereas Al2O3,
Na2O, K2O, and Sr decrease (Fig. 9). The higher
concentrations of Al2O3, Na2O, and Sr
correspond to greater abundances of plagioclase in the rocks, whereas higher FeO(total)
and MgO correspond to greater abundances of pyroxene and oxides in the rocks.
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Primitive mantle-normalized plots of Ni, Cu, and PGE in mineralized rocks show low Ni, Os, Ir, and Ru compared to Cu, Rh, Pt, Pd, and Au (Fig. 14). The High-Grade zone samples have slightly higher ratios of platinum-group PGE (Pt-PGE) to iridium-group PGE (Ir-PGE) than other samples (Fig. 14). The Pt-PGE include Pt, Pd, and Rh and the Ir-PGE include refractory Ir, Os, and Ru. This is also illustrated by the low Ir/Pd, with an average value of 1.5 x 10–4 (Fig. 15A).
Element mobility during alteration and metamorphism
The total concentrations of REE and Zr show a positive correlation (Fig.
16A), suggesting that REE acted as immobile elements during alteration. In
contrast, considerable scatter is evident in the plot of Rb versus Zr (Fig.
16B), as expected for a mobile alkali element. The immobile behavior of the REE
is further supported by the similar, relatively flat patterns of
chondrite-normalized REE for all rocks, regardless of the intensity of
alteration (Fig. 11). As light REE are more soluble in fluids than heavy REE,
this pattern would have been modified during alteration.
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Discussion |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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The REE in the calculated parental melt for each rock type showed relatively flat normalized patterns, about 10 to 15x chondrite values (Fig. 11). The primary melt would have contained less than the calculated values because the magmas were enriched in REE through fractional crystallization of chromite and olivine that do not incorporate REE. The average (Ce/Yb)chondrite for the parent magmas of the melanocratic and leucocratic rocks in the southern Roby zone are 2.4 and 2.5, respectively, whereas the average values for the melanonorite and the norite and/or leuconorite of the Twilight zone are 1.7 and 2.6, respectively.
Light REE are preferentially incorporated into a melt during partial melting. The relatively unfractionated REE patterns of our samples suggest that the source was a moderately refractory mantle where previous partial melting resulted in lower light REE than heavy REE. This interpretation is further supported by the similarity between the calculated concentrations and patterns of REE in the parental melts and enriched midoceanic ridge basalts. The melt for typical oceanic ridge basalts forms through relatively high degrees of partial melting in the moderately depleted mantle (e.g., Saunders, 1984).
The MgO content of the parental melt of the melanocratic rocks is calculated
to be 8.9 wt percent, using the partition coefficient for MgO between
clinopyroxene and melt of Hart and Dunn (1993) and an MgO content of 15.7 wt
percent for clinopyroxene in group 2 rocks. The primary melt probably had much
higher concentrations of MgO due to fractional crystallization of olivine. For
example, 20 percent fractional crystallization of olivine (Fo92) from
the melt yields 17.3 wt percent MgO in the original parental melt. Fo92
is the composition of olivine in equilibrium with mantle peridotites (e.g.,
Arai, 1992), but the choice of olivine composition does not significantly affect
the result. Therefore, the MgO content of the primary melt was similar to that
of picritic or komatiitic basalts which form through high degrees of partial
melting in the mantle. We suggest that the primary magmas for the Lac des Iles
intrusive complex were most likely products of relatively high degrees of
partial melting of a moderately depleted, refractory mantle source. This
interpretation is further supported by Nd isotope compositions of the
mineralized rocks. Recalculation of 
Nd values at
2690 Ma from data obtained by Brügmann et al. (1997) yields values ranging from
1.0 to 1.5 for most rocks in the Mine Block intrusion. The values are similar to
the Nd value of 2 for Late Archean igneous rocks
derived from a depleted mantle (e.g., Stern et al., 1989; Hattori et al., 1996).
Sulfur saturation in the parental magma
The ratio of Cu/Pd changes in a magma during sulfide separation because Pd
has a higher partition coefficient between sulfide and silicate melt than Cu at
a given temperature and fO2 (i.e., ~34,000 vs. 1,400; Peach
and Mathez, 1996, Crocket, 2002). Therefore, the ratio of Cu/Pd reflects the
timing of sulfur saturation in the silicate magmas (e.g., Barnes et al., 1993).
A lower Cu/Pd ratio than that of the primitive mantle implies that there was no
early removal of sulfide from the magma. Higher ratios of Cu/Pd imply early
removal of sulfide from the magma or the retention of sulfide at the source.
Samples from the Lac des Iles complex have a large range of Cu/Pd ratios (Fig.
17). The majority of the melanocratic rocks have lower Cu/Pd than the primitive
mantle (Fig. 17), suggesting that there was no early separation of sulfide melt.
In contrast, the leucocratic phases have high Cu/Pd, suggesting early removal of
sulfide from the magma or that residual sulfide remained in the mantle source.
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There are two possible processes for mafic magmas to acquire fractionated PGE with high Pt-PGE: low degrees of partial melting and fractional crystallization. Ir-PGE are preferentially incorporated into olivine, chromite, and high-temperature PGM, such as laurite and iridosmine (Puchtel and Humayun, 2001; Andrews and Brenan, 2002; Sattari et al., 2002; Righter et al., 2004). Retention of these minerals in the mantle during low degrees of partial melting results in low Ir-PGE in the melt. This is consistent with low Ir/Pd in gabbro-hosted deposits compared to komatiite-hosted deposits (Fig. 15). Evolved magmas also have fractionated PGE because olivine, chromite, and high-temperature PGE are removed during the early stages of fractional crystallization. We discount low degrees of partial melting of the parental magmas as the cause of high Pt-PGE because this is not consistent with the flat normalized REE patterns and the high MgO contents of the calculated parental magmas. Therefore, high Pt-PGE in our samples is best explained by the removal of Ir-PGE during early crystallization of the magmas.
High palladium in the southern Roby and Twilight zones
The average concentration of Pd in the melanocratic rocks, excluding the
High-Grade zone, is estimated to be ~4 ppm, on the basis of average grade,
tonnage, and abundance (~20 vol %) of mineralized melanocratic rocks in the Roby
zone. This is very high compared to average Pd concentrations of basalts (0.46
ppb) and komatiites (~11 ppb; Crocket, 2002). Various processes were suggested
for such enrichment of Pd. Watkinson and Dunning (1979) proposed that sulfur,
Ni, Cu, and PGE were enriched during evolution of the parental magmas, followed
by immiscible separation of Pd-bearing aqueous fluid from the magmas. Talkington
and Watkinson (1984) argued for an important role of hydrothermal activity in
the mineralization, based on the occurrence of PGM containing Te, As, and Bi and
the spatial association of PGM with secondary hydrous minerals and pyrite.
Macdonald (1988) also suggested hydrothermal enrichment of PGE by fluids that
originated from the parental magmas, also based on the abundant hydrothermal
minerals in the mineralized zones. Brügmann et al. (1989) proposed
"constitutional zone refining" for the enrichment of Pd and other
Pt-PGE relative to Ir-PGE. Constitutional zone refining (McBirney, 1987)
involves the formation of volatile-rich magmas through fractional
crystallization and partial melting of earlier formed cumulates. Brügmann et
al. (1989) suggested that a volatile-rich silicate magma remelted gabbro
cumulates and selectively incorporated the Pt-PGE from sulfides in the
cumulates, and that the melanocratic and leucocratic rocks in the Roby zone
represent the residue and partial melt, respectively. In another model, Lavigne
and Michaud (2001) suggested that the mineralization involved the forceful
intrusion of a PGE-, Ni- and Cu-rich immiscible sulfide liquid into the
partially crystallized overlying magma chamber. Exsolution of aqueous fluids
from magmas resulted in the redistribution of precious metals.
Our data, showing positive correlations between sulfur and base and precious metals, are not consistent with hydrothermal concentration of Pd in the southern Roby and Twilight zones. Furthermore, our detailed mapping shows that the bulk of PGE are in late melanocratic rocks. Hydrothermal processes cannot explain the preferential enrichment of Pd in the mafic rocks. Therefore, we discount any significant contributions of hydrothermal fluids to the Pd mineralization in the southern Roby and Twilight zones. However, aqueous fluids may have been responsible for mineralization in the High-Grade ore samples as indicated by the scatter in the plots of sulfur and base and precious metals (see below).
The zone-refining process is also not consistent with our data. First, the zone-refining process produces leucocratic melt and melanocratic and ultramafic rocks as the residue (Brügman et al., 1989). Our detailed mapping shows that melanocratic rocks carry most of the Pd, and leucocratic rocks are essentially barren (Fig. 12, Table 2). Second, the zone-refining process involves partial melting of a cumulate (Brügmann et al., 1989) containing clinopyroxene, orthopyroxene, and a solidified, trapped melt. Partial melting would dissolve the solidified melt first, then clinopyroxene. The solidified melt contains high concentrations of incompatible elements, such as REE, compared to any other phases. Clinopyroxene also contains high concentrations of incompatible elements compared to olivine and orthopyroxene. Therefore, any partial melt should contain elevated incompatible elements compared to earlier rocks. The late clinopyroxenites (3a) are the only rock type with high REE, but these rocks are free of sulfides and barren of Pd. Similar concentrations of REE in all rocks in the southern Roby and Twilight zones (Fig. 11) argue against the zone-refining process. Dissolution of significant amounts of clinopyroxene would have increased the Sc content of the melt, as Sc is preferentially included in clinopyroxene, but this is inconsistent with the similar Sc/Y ratios of all rocks (Fig. 18).
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Enrichment of palladium in the High-Grade zone of the Roby zone
Although the High-Grade zone was not the focus of this investigation, it is
important because it contains high Pd, ~8 ppm in most samples, and hosts
approximately 35 percent of the Pd in the mine. The samples from the High-Grade
zone do not plot on the correlation trends of sulfur versus base metals and
sulfur versus precious metals (Fig. 13). The data suggest that Pd enrichment in
this zone may have been caused by aqueous fluids. This interpretation is
supported by abundant quartz aggregates, the lack of exsolution textures in
sulfides, and the common occurrence of pyrite.
The High-Grade zone is located on the eastern margin of the Roby zone, adjacent to East Gabbro that is older than the rest of the Mine Block intrusions. The High-Grade zone is adjacent to high-grade breccia ore in the Roby zone (Lavigne and Michaud, 2001), suggesting a genetic link between the two. We propose that aqueous fluid exsolved from late fertile melanocratic magmas migrated into the area of the High-Grade zone. The barren East Gabbro would have acted as a physical barrier for such an aqueous fluid, resulting in precipitation of Pd along the boundary with the East Gabbro. The mineralized zone is subvertical at present, but this steepening probably occurred during the docking of the Quetico accretionary prism and Wawa arc to the Wabigoon subprovince to the north (Percival, 1989).
A model of igneous activity and PGE mineralization at Lac des Iles
This section summarizes the evolution of the southern Roby and Twilight
zones, which is illustrated schematically in Figure 19. First, relatively high
degrees of partial melting in a moderately depleted mantle formed the parental
magmas of early gabbroic rocks. These magmas became enriched in Cu and Pt-PGE
during the fractional crystallization of olivine, chromite, and high-temperature
PGM. The magmas eventually reached sulfur saturation, forming an immiscible
sulfide melt with low Cu/Pd in the conduit, which resulted in high Cu/Pd ratios
in the evolving magma. The magmas reached the site of the deposit and partly
solidified as leucocratic gabbros. A new batch of magmas, also formed by
relatively high degrees of partial melting of a similar source, passed through
the same conduit as the earlier, leucocratic magmas and incorporated the
preexisting sulfide melt formed by the earlier magmas. The melanocratic magmas
reached sulfur saturation, forming a second sulfide melt and inheriting the low
Cu/Pd ratios of the earlier sulfide melt. In this model the late mafic magmas
became rich in water and other volatiles, which caused the brecciation and
pegmatite formation observed in the surface outcrops. The hydrothermal fluids
released from melanocratic magmas migrated upward along the boundary between the
East Gabbro and mineralized gabbroic rocks, resulting in intense hydrothermal
alteration at the margin of the Roby zone and forming the High-Grade zone.
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Comparison with contact-type PGE mineralization
The southern Roby and Twilight zones of the Lac des Iles intrusive complex
share many characteristics with contact-type deposits. They include low sulfide
abundance in the mineralized rocks, high Pd compared to other PGE, and the
common occurrence of breccias and pegmatite. However, the PGE mineralization at
the southern Roby and Twilight zones is not localized near the contacts with
country rocks. Instead, the mineralization is in the center of the Mine Block
intrusion. In addition, there is no evidence that enrichment of SiO2 resulted
in sulfur saturation in the ore deposit. In contrast, the most primitive,
melanocratic rocks are the most PGE and sulfide rich. Thirdly, there is no
evidence suggesting the interaction between mineralized mafic magmas and felsic
magmas and/or rocks. Sutcliffe (1989) and Sutcliffe et al. (1989) described
contemporaneous emplacement of granitoid magmas with gabbroic rocks, but later
studies (e.g., Michaud, 1998; Lavigne and Michaud, 2001), including our study,
failed to identify any xenoliths of granitic rocks in the area of
mineralization. Minor inclusions of tonalitic rocks are present, but they are
restricted to the barren margin of the Mine Block intrusion adjacent to the
surrounding tonalite. These observations suggest that the origin of Pd
mineralization at Lac des Iles is fundamentally different from that of the
contact-type deposits.
Comparison with stratiform-type mineralization in large layered intrusions
We propose that the bulk of Pd mineralization was introduced by pulses of
primitive magmas, as illustrated by the sequence of intrusions with various
compositions (Fig. 20). The proposed mode of the mineralization is analogous to
that suggested for typical stratiform deposits. However, the mode of emplacement
of magmas is different. Unlike the quiescent magma chambers necessary for the
formation of large, continuous ore horizons in stratiform deposits, the
intrusive environment at Lac des Iles was apparently dynamic, forming breccias
and magma mingling instead of layering. In addition, the Lac des Iles intrusive
complex shows highly fractionated PGE with high Pd compared to the ore in
layered intrusions, although the Lac des Iles ore lies in the general trend of
Ir/Pd versus Pd defined by many deposits (Fig. 15).
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Conclusions |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
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The mineralization at Lac des Iles is commonly compared to contact-type PGE ore because of similar textures in both types of deposits, but the Lac des Iles ore shows no evidence for host-rock assimilation. In addition, the ore is not localized near contacts with the country rocks. The Lac des Iles mineralization also shows similarities with PGE deposits in large layered intrusions, with the bulk of the PGE in both types being introduced by pulses of fertile primitive magmas. However, the intrusion of this fertile magma was much more energetic and dynamic at Lac des Iles than that associated with the quiescent magma chambers in large layered deposits.
|
Acknowledgments |
|---|
February 24, December 9, 2004
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Footnotes |
|---|
February 24, 2004; December 9, 2004
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References |
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TopAbstractIntroductionGeologyMineralized ZonesPetrology of the Southern...Petrology of the Twilight...Geochemistry of Mineralized...DiscussionConclusionsReferences |
|---|
Alapieti, T.T., Lahtinen, J., Iluhma, I.I., Ilanninen, E., Piirainen., T., and Sivonen, S., 1989, Platinum-group element bearing copper nickel sulfide mineralization in the marginal series of the Proterozoic Suhanko-Konttijarvi layered intrusion, northern Finland, in Prendergast, M.D., and Jones. M.J., eds., Magmatic sulfide—Zimbabwe volume: London, Institute of Mining and Metallurgy, 254 p.
Andrews, D.R.A., and Brenan, J.M., 2002, Phase-equilibrium constraints on the magmatic origin of laurite + Ru-Os-Ir alloy: Canadian Mineralogist, v. 40, p.1705 –1716.
Arai, S., 1992, Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry: Mineralogical Magazine, v. 56, p. 173–184.
Barnes, S.J., and Naldrett, A.J., 1985, Geochemistry of the J-M (Howland)
reef of the Stillwater Complex, Minneapolis adit area: I. Sulfide chemistry and
sulfide-olivine equilibrium: Economic Geology, v. 80, p. 627–647.
Barnes, S.J., Couture, J.F., Sawyer, E.W., and Bouchaib, C., 1993,
Nickel-copper occurrences in the Bellterre-Angliers belt of the Pontiac
subprovince and the use of Cu-Pd ratios in interpreting platinum-group element
distributions: Economic Geology, v. 88, p. 1402–1418.
Barrie, C.T., MacTavish, A.D., Walford, P.C., Chataway, R., and Middaugh, R.,2002 , Contact-type and magnetitite reef-type Pd-Cu mineralization in ferroan olivine gabbros of the Coldwell Complex, Ontario: CIM Special Volume 54, p.321 –337.
Bédard, J.H., 1994, A procedure for calculating the equilibrium distribution of trace elements among the minerals of cumulate rocks, and the concentration of trace elements in the coexisting liquids: Chemical Geology, v. 118, p.143 –153.[CrossRef][Web of Science][GeoRef]
Blackburn, C.E., Johns, G.W., Ayer, J., and Davis, D.W., 1992, Wabigoon subprovince: Ontario Geological Survey Special Volume 4, pt. 2, p. 303–381.
Brügmann, G.E., Naldrett, A.J., and Macdonald, J., 1989, Magma mixing and
constitutional zone refining in the Lac des lles Complex, Ontario: Genesis of
the platinum-group element mineralization: Economic Geology, v. 84, p.1557
–1573.
Brügmann, G.E., Reischmann, T., Naldrett, A.J., and Sutcliffe, R.H., 1997, Roots of an Archean volcanic arc complex: the Lac des Iles area in Ontario, Canada: Precambrian Research, v. 81, p. 223–239.[CrossRef][Web of Science][GeoRef]
Buchan, K.L., and Ernst, R.E., 2004, Diabase dyke swarms and related units in Canada and adjacent regions: Geological Survey of Canada A Series Map 2022A, 39 p.
Crocket, J.H., 2002, Platinum-group element geochemistry of mafic and ultramafic rocks: Canadian Institute of Mining and Metallurgy and Petroleum Special Volume 54, p. 177– 210.
Dunning, G.R., 1979, The geology and platinum-group mineralization of the Roby zone, Lac des Iles Complex, northwestern Ontario: Unpublished M.Sc. thesis, Ottawa, Ontario, Carleton University, 129 p.
Edgar, A.D., and Sweeny, J.M., 1991, The geochemistry, origin and economic potential of the platinum group element bearing rocks of the Lac des Iles Complex, northwestern Ontario, Ontario geoscience research grant program, grant no. 286: Ontario Geological Survey Open File Report 5746, 87 p.
Good, D.J., and Crocket, J.H., 1994, Genesis of the Marathon
Cu-platinum-group element deposit, Port Coldwell alkalic complex, Ontario: A
mid-continent rift-related magmatic sulfide deposit: Economic Geology, v. 89, p.131
–149.
Guillot, S., Hattori, K.H., and de Sigoyer, J., 2000, Mantle wedge
serpentinization and exhumation of eclogites: Insights from eastern Ladakh, NW
Himalaya: Geology, v. 28, p. 199–202.
Gupta, V.K., and Sutcliffe, R.H., 1990, Mafic-ultramafic intrusives and their
gravity field: Lac des Iles area, northern Ontario: Bulletin of the Geological
Society of America, v. 102, p. 1471–1483.
Hart, S.R., and Dunn, T., 1993, Experimental cpx/melt partitioning of 24 trace elements: Contributions to Mineralogy and Petrology, v. 113, p. 1–8.[CrossRef][Web of Science][GeoRef]
Hattori, K., Hart, S.R., and Shimizu, N., 1996, Melt and source mantle compositions in the Late Archean: A study of Sr-and Nd-isotope and trace elements in clinopyroxenes from shoshonitic alkaline rocks: Geochimica et Cosmochimica Acta, v. 60, p. 4551–4562.[GeoRef]
Hinchey, J.G., Hattori, K.H., and Lavigne, M.J., 2003, Preliminary report of field descriptions and contact relationships of different lithological units in the south Roby and Twilight zones, Lac Des Iles: Ontario Geological Survey Open File Report 6107, 16 p.
Jenner, G.A., 1996, Trace element geochemistry of igneous rocks: Geochemical nomenclature and analytical geochemistry: Geological Association of Canada Short Course Notes, v. 12, p. 1–77.
Jolliffe, F., 1934, Block Creek map-area, Thunder Bay district, Ontario:Geological Survey of Canada Summary Report 1933 , pt. D, p. 7–15.
Lavigne, M.J., and Michaud, M.J., 2001, Geology of North American Palladium
Ltd.’s Roby zone deposit, Lac des Iles: Exploration and Mining Geology, v. 10,
p. 1–17.
Lesher, C.M., Phillips, G.N., Groves, D.I., and Campbell, I.H., 1991, Immobility of REE and most high field-strength elements and first transition series metals during Archean gold-related hydrothermal alteration of metabasalts at the Hunt mine, Western Australia, in Ladeira, E.A., ed., The economics, geology, geochemistry and genesis of gold deposits: Rotterdam, Balkema, p. 327–334.
Lightfoot, P.C., and Naldrett, A.J., 1994, Proceedings of the Sudbury-Noril’sk symposium: Ontario Geological Survey Special Volume 5, 423 p.
Macdonald, A.J., 1988, Platinum-group element mineralization and the relative importance of magmatic and deuteric processes: Field evidence from the Lac Des Iles deposit, Ontario, Canada, in Prichard, H.M., Potts, P.J., Bowles, J.F.W., and Cribb, S.J., eds., GeoPlatinum ’87: Amsterdam, Elsevier, p.215 –236.
Maier, W.D., and Bowen, M.P., 1996, The UG2-Merensky reef interval of the Bushveld Complex northwest of Pretoria: Mineralium Deposita, v. 31, p.386 –393.[Web of Science][GeoRef]
Maier, W.D., Barnes, S.J., and Waal, S.A., 1998, Exploration for magmatic Ni-Cu-PGE sulphide deposits: A review of recent advances in the use of geochemical tools, and their application to some South African ores: South African Journal of Geology, v. 101, p. 237–253.[Abstract][Web of Science][GeoRef]
McBirney, A.R., 1987, Constitutional zone refining of layered intrusions:NATO Advanced Science Institutes Serial C , v. 196, p. 437–452.
McDonough, W.F., and Sun, S.S., 1995, The composition of the earth: Chemical Geology, v. 120, p. 223–253.[CrossRef][Web of Science][GeoRef]
Michaud, M.J., 1998, The geology, petrology, geochemistry and platinum-group element-gold-copper-nickel ore assemblage of the Roby zone, Lac des Iles mafic-ultramafic complex, northwestern Ontario: Unpublished M.Sc. thesis, Thunder Bay, Ontario, Lakehead University, 183 p.
Naldrett, A.J., 1981, Platinum-group element deposits: Canadian Institute of Mining and Metallurgy Special Volume 23, p. 197–232.
Ontario Geological Survey, 1991, Bedrock geology of Ontario, west-central sheet: Ontario Geological Survey Map 2542.
Peach, C.L., and Mathez, E.A., 1996, Constraints on the formation of
platinum-group element deposits in igneous rocks: Economic Geology, v. 91, p.439
–450.
Pearce, J.A., and Norry, M.J., 1979, Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks: Contributions to Mineralogy and Petrology, v. 69, p. 33–47.[CrossRef][Web of Science][GeoRef]
Peck, D.C., Keays, R.R., James, R.S., Chubb, P.T., and Reeves, S.J., 2001,
Controls on the formation of contact-type platinum-group element mineralization
in the East Bull Lake intrusion: Economic Geology, v. 96, p. 559–581.
Pettigrew, N.T., and Hattori, K.H., 2002, Palladium-copper-rich platinum-group element mineralization in Legris Lake mafic-ultramafic complex, western Superior province, Canada: Institution of Mining and Metallurgy Transactions, sec. B, v. 111, p. 46–57.
Percival, J.A., 1989, A regional perspective of the Quetico metasedimentary belt, Superior province, Canada: Canadian Journal of Earth Sciences, v. 26, p.677 –693.
Picard, C., Amossé, J., Piboule, M., and Giovenazzo, D., Physical and chemical constraints on platinum-group element behavior during crystallization of a basaltic komatiite liquid: Example of the Proterozoic Delta sill, New Quebec, Canada: Economic Geology, v. 90, p. 2287–2302.
Puchtel, I., and Humayun, M., 2001, Platinum group element fractionation in a komatiitic basalt lava lake: Geochimica et Cosmochimica Acta, v. 65, p.2979 –2993.[CrossRef][Web of Science][GeoRef]
Pye, E.G., 1968, Geology of the Lac Des Iles area, District of Thunder Bay:Ontario Department of Mines Geological Report 64 , 47 p.
Righter, K., Campbell, A.J., Humayun, M., and Hervig, R.L., 2004, Partitioning of Ru, Rh, Pd, Re, Ir, and Au between Cr-bearing spinel, olivine, pyroxene and silicate melts: Geochimica et Cosmochimica Acta, v. 68, p.867 –880.[CrossRef][Web of Science][GeoRef]
Sattari, P., Brenan, J.M., Horn, I., and McDonough, W.F., 2002, Experimental
constraints on the sulfide- and chromite-silicate melt partitioning behavior of
rhenium and platinum-group elements: Economic Geology, v. 97, p. 385–398.
Saunders, A.D., 1984. The rare earth element characteristics of igneous rocks from the oceanic basins, in Henderson, P., ed., Rare earth element chemistry: Amsterdam, Elsevier, p. 205–231.
Schissel, D., Tsvetkov, A.A., Mitrofanov, F.P., and Korchagin, A.U., 2002,
Basal platinum-group element mineralization in the Federov Pansky layered mafic
intrusion, Kola Peninsula, Russia: Economic Geology, v. 97, p. 1657–1677.
Schoenberg, R., Kruger, F.J., Nägler, T.F., Meisel, T., and Kramers, J.D.,1999 , PGE enrichment in chromitite layers and the Merensky reef of the western Bushveld Complex: A Re-Os and Rb-Sr isotope study: Earth and Planetary Science Letters, v. 172, p 49–64.
Smirnov, M.E., 1966, The structure of Noril’sk nickel-bearing intrusions and the genetic types of their sulphide ores: Moscow, Nedra, 58 p.
Stern, R.A., Hanson, G.N., and Shirey, S.B., 1989, Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior province: Canadian Journal of Earth Sciences, v. 26, p. 1688–1712.[Web of Science]
Sun, S.S., and McDonough, W.F., 1989, Chemical and isotopic systematics of ocean basalts: Implications for mantle composition and processes: Geological Society of London Special Publication, v. 42, p. 313–345.[CrossRef]
Sutcliffe, R.H., 1986, Regional geology of the Lac des Iles area, District of Thunder Bay: Ontario Geological Survey Miscellaneous Paper 132, p. 70–75.
—— 1989, Magma mixing in late Archean tonalitic and mafic rocks of the Lac des Iles area, western Superior province: Precambrian Research, v. 44, p.81 –101.[GeoRef]
Sutcliffe, R.H., and Smith, A.R., 1988, Precambrian geology of the plutonic rocks in the Lac des Iles-Tib Lake area, District of Thunder Bay: Ontario Geological Survey Map 3098.
Sutcliffe, R.H., and Sweeny, J.M., 1986, Precambrian geology of the Lac des Iles Complex, District of Thunder Bay: Ontario Geological Survey Map 3047.
Sutcliffe, R.H., Sweeny, J.M., and Edgar, A.D., 1989, The Lac des Iles Complex, Ontario: Petrology and platinum-group element mineralization in an Archean mafic intrusion: Canadian Journal of Earth Sciences, v. 26, p.1408 –1427.
Sweeny, J.M., 1989, The geochemistry and origin of platinum group element mineralization of the hybrid zone, Lac des Iles Complex, northwestern Ontario:Unpublished M.Sc. thesis, London, Ontario, University of Western Ontario , 185 p.
Talkington, R.W., and Watkinson, D.H., 1984, Trends in the distribution of precious metals in the Lac des Iles Complex, northwestern Ontario: Canadian Mineralogist, v. 22, p. 125–136.
Theriault, R.D., Barnes, S.J., and Severson, M.J., 1997, The influence of country-rock assimilation and silicate to sulfide ratios (R factor) on the genesis of the Dunka Road Cu-Ni-platinum-group element deposit, Duluth Complex, Minnesota: Canadian Journal of Earth Sciences, v. 34, p. 375–389.
Todd, S.G., Keith, D.W., Schissel, D.J., Leroy, L.L., Mann, E.L., and Irvine,
T.N., 1982, The J-M platinum-palladium reef of the Stillwater Complex, Montana:
I. Stratigraphy and petrology: Economic Geology, v. 77, p. 1454–1480.
Von Gruenewaldt, G., Sharp, M.R., and Hatton, C.J., 1985, The Bushveld Complex: Introduction and review: Economic Geology, v. 80, p. 803–813.
Watkinson, D.H. and Dunning, G., 1979, Geology and platinum-group mineralization, Lac des Iles Complex, northwestern Ontario: Canadian Mineralogist, v. 17, p. 453–462.
Williams, H.R., 1992, Quetico subprovince: Ontario Geology Survey Special Volume 4, pt. 2, p. 383–403.
Wilson, A.H., and Prendergast, M.D., 2001, Platinum-group element
mineralization in the Great Dyke, Zimbabwe, and its relationship to magma
evolution and magma chamber structure: South African Journal of Geology, v. 104,
p. 319–342.
Wood, S.A., 2002, The aqueous geochemistry of the platinum-group elements
with applications to ore deposits: Canadian Institute of Mining, Metallurgy and
Petroleum Special Volume 54, p. 177–210.
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are given in the upper left of the diagram.
