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Geology, Petrology, and Controls on PGE Mineralization of the Southern Roby and Twilight Zones, Lac des Iles Mine, Canada

J. G. Hinchey and K. H. Hattori

Department of Earth Sciences, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

M. J. Lavigne^*

North American Palladium Ltd., Thunder Bay, Ontario, Canada P7B 6T9

View larger version (81K): [in a new window]   FIG. 1. Regional geology of the western Superior province, illustrating the boundaries of subprovinces, the locations of the Nipigon plate and the Lac des Iles mine (modified from Ontario Geological Survey, 1991). The subprovince names are shown in italics. The Nipigon plate is the northern extension of the igneous province related to the Midcontinental rift. The inset displays a simplified map of the Superior province. The square outlines the area shown in the figure.

View larger version (65K): [in a new window]   FIG. 2. Simplified regional geologic map of the study area, showing the occurrence of mafic and/or ultramafic intrusions with PGE mineralization (modified after Sutcliffe and Smith, 1988). Note the circular distribution of mafic-ultramafic intrusions. The diabase sills belong to the igneous rocks of the Nipigon plate.

View larger version (45K): [in a new window]   FIG. 3. Simplified geologic map of the Lac des Iles intrusive complex, illustrating the distribution of the North Lac des Iles, Mine Block, and Camp Lake intrusions (modified after Sutcliffe et al., 1989).

View larger version (71K): [in a new window]   FIG. 4. Simplified geologic map of the Mine Block intrusion of the Lac des Iles intrusive complex (modified after Sutcliffe and Sweeney, 1986), showing the distribution of East Gabbro, the locations of the Roby, Twilight, and High-Grade zones, and the outline of the phase 3 open pit.

View larger version (73K): [in a new window]   FIG. 5. Illustration of the complicated distribution of various rock types in the southern Roby zone in the Mine Block intrusion after 1:60 scale mapping by Hinchey et al. (2003). Rocks are divided into early leucocratic, mineralized melanocratic, and late barren melanocratic rocks. The mineralized melanocratic rocks are commonly accompanied by pegmatitic veins and pods of various sizes from several centimeters to 50 cm. Only large pegmatites are shown in the map. Felsic and diabase dikes cut all rock types of the Lac des Iles intrusive complex. Numbers on the sides correspond to the mine grid.

View larger version (105K): [in a new window]   FIG. 6. Representative photographs of the ore zone and the concentrations of Pd in different rock types. Solid squares show the locations of samples used for Pd analysis after the photographs were taken. The Pd value of the melanonorite in the center of (C) is for a sample outside the field of view. The numbers in parentheses correspond to the rock types described in the text. Rock hammer for scale. A. Medium-grained melanogabbro breccia (2a) containing fragments of earlier leucocratic rocks (1b-c) in the northern section of the southern Roby zone. Note low concentrations of Pd in leucocratic rocks and a dike of late barren clinopyroxenite (3a) cutting the melanogabbro breccia (2a). B. Magma mingling between early leucogabbro (1d) and late melanogabbro (2c) from the southern portion of the southern Roby zone. C. Magmatic brecciation of late melanonorite (6) and earlier norite (5) in the Twilight zone.

View larger version (113K): [in a new window]   FIG. 7. Photomicrographs showing the textures of sulfide minerals. A. Chalcopyrite (Ccp) along cleavage planes of actinolite (Act). The sample is from the medium-grained clinopyroxenite (2b) of the southern Roby zone. B. Primary magmatic sulfide bleb with apparent exsolution of chalcopyrite (Ccp) and pentlandite (Pn) in late fractures within pyrrhotite (Po). The sample is from the medium-grained melanocratic gabbro (2c) of the southern Roby zone. Note the thin lamellae of chalcopyrite along cleavage planes of actinolite (lower left).

View larger version (40K): [in a new window]   FIG. 8. Illustration of the complicated distribution of different rock types in the Twilight zone based on the 1:120 scale mapping by Hinchey et al. (2003). Dashed lines show the mine grid. The numbers in parentheses after rock names correspond to the rock types in the text. The mineralized melanonorite (6) is the latest rock type of the Lac des Iles intrusive complex in the outcrop. It shows the intrusive contacts with fine-grained gabbro (10), leuconorite (4), norite (5), and medium-grained gabbro (8).

View larger version (24K): [in a new window]   FIG. 9. MgO (wt %) vs. Al2O3 (wt %) and Sr (ppm) for whole rocks from the southern Roby, Twilight, and High-Grade zones. Note the well displayed correlations suggesting a common parental origin of all rock types. Numbers in parentheses are rock types in the text.

View larger version (11K): [in a new window]   FIG. 10. Hf vs. Zr concentrations for the rocks from the southern Roby, Twilight, and High-Grade zones. Note the well displayed correlation suggesting a cogenetic origin of all rock types. See legend in Figure 9. Correlation coefficient (r) is shown in the lower right and error bars in 2 are given in the upper left of the diagram.

View larger version (31K): [in a new window]   FIG. 11. Chondrite-normalized rare earth element (REE) plots for a variety of rocks: (A) the melanocratic rocks and (B) the leucocratic rocks from the southern Roby zone, (C) the melanonorite and (D) norite and/or leuconorite rocks from the Twilight zone, and (E) normal and enriched midoceanic ridge basalts (N- and E-MORB, respectively) compared to the hypothetical parental melt for the rocks in the southern Roby and Twilight zones. The composition of the hypothetical melt was calculated following the method of Bédard (1994). Note the low degrees of REE fractionation for both the bulk rocks and calculated melt. The calculation of the composition of the hypothetical melt used CIPW normative minerals of plagioclase, clinopyroxene, orthopyroxene, and olivine. The remaining is attributed to a trapped melt fraction. Using the REE concentrations of bulk rocks, the modal abundance of minerals, and partition coefficients of the minerals and melt, the concentrations of REE in different phases were calculated using mass balance. We used the concentration of REE in clinopyroxene and the partition coefficients between clinopyroxene and melt listed in Bédard (1994) to calculate the REE concentrations of the parental melt. Chondrite and MORB values are from McDonough and Sun (1995), and Sun and McDonough (1989), respectively.

View larger version (19K): [in a new window]   FIG. 12. Bivariate plots of MgO (wt %) vs. Au, Pd, and Pt (ppm). Note the high concentrations of precious metals in the melanocratic rocks in the southern Roby and Twilight zones. The dashed line broadly separates the melanocratic from the leucocratic rocks.

View larger version (14K): [in a new window]   FIG. 13. Bivariate plots of sulfur (wt %) vs. Ni, Pt, and Pd (ppm). Note the positive correlations, suggesting a sulfide control on mineralization. Elevated Ni concentrations in samples without sulfides suggest the presence of minor Ni in silicate minerals. Preliminary electron microprobe data confirm that clinopyroxene contains significant Ni. Correlation coefficients (r) are calculated for samples, excluding the High-Grade zone.

View larger version (23K): [in a new window]   FIG. 14. Primitive mantle-normalized plot for Ni, Cu, and PGE of the southern Roby, Twilight, and High-Grade zones. The data have been recalculated to 100 percent sulfide following Naldrett (1981). Note similar patterns for all rocks with low concentrations of Ni, Ir, Os, and Ru and high concentrations of Cu, Rh, Pt, and Pd. Primitive mantle values for Ni and Cu are from McDonough and Sun (1995) and PGE values from Guillot et al. (2000).

View larger version (29K): [in a new window]   FIG. 15. A. Plot of Pd (ppm) vs. Ir/Pd for samples from the southern Roby, Twilight, and High-Grade zones compared to other PGE deposits. Note that the Lac des Iles data plots along the general trend of other orthomagmatic deposits. B. Plot of Pd (ppm) vs. Pt/Pd for rocks from the Lac des Iles deposit. Note that the majority of the melanocratic rocks show comparable Pt/Pd ratios, but samples from the High-Grade zone have higher Pd and therefore low Pt/Pd. The scatter is most likely due to a nugget effect of coarse-grained platinum group minerals. Data sources: Komatite-hosted deposits (Picard et al., 1995); Marathon deposit in the Coldwell Complex, Ontario (Good and Crocket, 1994); Duluth Complex (Theriault et al., 1997); J-M reef (J-M: Naldrett, 1981); Merensky reef, Bushveld Complex (M: Naldrett, 1981).

View larger version (12K): [in a new window]   FIG. 16. A. Plot of total REE vs. Zr for all rock types. Zirconium is an incompatible and immobile element. The positive correlation suggests that the REE were also immobile. B. Plot of Rb vs. Zr, illustrating that Rb acted as a mobile element during alteration. C. Plot of H2O (wt %) vs. Pd (ppm) for variably altered melanocratic rocks of the southern Roby zone. The lack of any positive correlation suggests that aqueous fluid likely was not important in the concentration of Pd.

View larger version (29K): [in a new window]   FIG. 17. Pd vs. Cu/Pd plot for samples from the southern Roby and Twilight zones compared to other PGE deposits. Note that the rocks from Lac des Iles show a wide variation in Cu/Pd ratios. The mineralized melanocratic rocks show low Cu/Pd ratios, similar to those for other primary PGE deposits, whereas the barren leucocratic rocks display high Cu/Pd ratios. Abbreviations: JM = JM reef, Stillwater, United States (Barnes and Naldrett, 1985); MR = Merensky reef, Bushveld (Maier et al., 1996) and UG = UG2 reef, Bushveld (Maier and Bowen, 1996), South Africa; N = Noril’sk (Smirnov, 1966), FP = Federov Pansky (Schissel et al., 2002), Russia; Sud = Sudbury, Canada (Naldrett, 1981).

View larger version (8K): [in a new window]   FIG. 18. Sc vs. Sc/Y for samples from the southern Roby zone. Note that the Sc/Y ratios of the melanocratic rocks are similar to or lower than those of the leucocratic rocks.

View larger version (91K): [in a new window]   FIG. 19. Schematic model showing the history of mineralization at the southern Roby zone. Magmas formed by a high degree of partial melting in a depleted mantle source (A1) became enriched in Cu, Pt, and Pd through fractional crystallization of olivine, chromite, and high-temperature PGM (A2), segregated sulfide melt that had low Cu/Pd ratios along the conduit and the base of the magma chamber (A3), and solidified as the early leucocratic gabbros. A second episode of partial melting in the mantle source produced another batch of fertile magma. As with the early magma, this magma was enriched in Cu, Pt, and Pd through fractional crystallization (A2). This magma incorporated the earlier sulfide melt and intruded forcefully into the partially crystallized leucocratic rocks (B1), causing brecciation and magma mingling, and solidified as fertile melanocratic gabbro. Aqueous fluids that separated from the melanocratic magma percolated through the cumulates, partially dissolving Pd and concentrating it in the High-Grade ore zone adjacent to barren East Gabbro (B2).

View larger version (29K): [in a new window]   FIG. 20. Temporal evolution of palladium mineralization at the southern Roby and Twilight zones, Lac des Iles mine. The concentrations of MgO (wt %) and Pd (ppb) are shown by box-whisker plots where a box is defined by the 1st and 3rd quartile values. The total ranges of values are shown as lines and the median values of rock types are connected by the thick line. Note the increased MgO and Pd concentrations associated with the sulfide-bearing melanocratic rocks. The numbers on the left side of the diagram correspond to the rock types: 1a = leucogabbro and/or anorthosite, 1b = varitextured gabbro, 1c = medium-grained light gray gabbro, 1d = medium-grained gabbro, 2a = matrix of melanogabbro breccia, 2b = clinopyroxenite, 2c = dark gabbro, 3a = late clinopyroxenite, and 3b = late gabbro.