Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike, Copper Cliff North Mine, Sudbury, Ontario: Evidence for Multiple Stages of Emplacement

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1 Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike, Copper Cliff North Mine, Sudbury, Ontario: Evidence for Multiple Stages of Emplacement J.H. RICKARD North American Palladium Ltd., Lac des Iles Mines Ltd. Thunder Bay, Ontario, Canada, P7B 6T9 D.H. WATKINSON Department of Earth Sciences, Carleton University Ottawa, Ontario, Canada, K1S 5B6 Received April 15, 2002; accepted September 27, Explor. Mining Geol., Vol. 10, Nos. 1 and 2, pp , Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved. Printed in Canada /00 $ Abstract Detailed petrographic and quantitative (electron microprobe, SEM-EDS) mineralchemical analyses of sulfide ore from the 100 orebody at the Copper Cliff North mine, Copper Cliff, Ontario, reveal numerous stages of ore development. Sharp contacts between weakly mineralized quartz diorite and blebby to massive sulfides indicate multiple intrusive events within the Copper Cliff offset dike. A spatial association of sulfides and hydrous minerals, such as amphibole, biotite, chlorite and epidote, suggests that most primary ore has been affected by later hydrothermal fluids. Sulfides are often enclosed or bordered by these secondary silicate phases. The ore consists primarily of pyrrhotite, pentlandite, chalcopyrite, with trace michenerite, cobaltite/gersdorffite, irarsite, hessite, tsumoite, and bismuth tellurides. Geological and petrological comparison of the 100 orebody and the adjacent 900 and 890 orebodies has revealed distinct similarities in ore distribution. Data show that the 100 orebody is depleted in platinum-group elements (PGE) relative to the 900 orebody. Platinum-group elements from the 100 orebody may have been hydrothermally remobilized and partly emplaced in the 900 orebody, in adjacent veins, or in magmatic breccia. Analysis of Cu, Ni, and PGE assay data of ore samples has characterized their distribution among the eight different ore textures in the orebody. There is a strong association between Pt and interstitial sulfide, and to a lesser extent massive sulfide, but little correlation between chalcopyrite-rich ore and PGE in the 100 orebody, unlike many other deposits in the Sudbury region Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved. Introduction The Copper Cliff North (CCN) mine is located approximately 4 km west of Sudbury, Ontario, in the town of Copper Cliff. It is one of two mines currently in production that is located along the Copper Cliff offset dike. Operations at the CCN mine had produced 44.8 million tons of ore (grading 0.83% Cu, and 0.83% Ni) from the initiation of mining in 1960 to the end of The seven main orebodies have variable sulfide contents and metal ratios, with an average copper to nickel ratio of 1.10:1, and a pyrrhotite to nickel ratio ranging from 9.7:1 to 20.5:1 (Inco Ltd., internal report). Sulfides in the Copper Cliff offset form large pod-like orebodies that may be found at the center or near the edges of the dike. Each of the orebodies at the CCN mine has a designated number that roughly refers to its position on one of several grid coordinate systems. The 100 orebody is the focus of this study, with additional information being included from the adjacent 900 and 890 orebodies. This paper addresses the controls on the location of sulfides with respect to the quartz diorite host, and the extent of hydrothermal remobilization of Cu and platinum-group elements (PGE) within the orebodies and surrounding quartz diorite. In so doing, it follows the work by Farrow and Watkinson (1996, 1997), Molnár et al. (2001), and Everest (1999) in the North Range of the Sudbury Structure, which suggests that the interaction of magmatic sulfides with saline fluids resulted in the stripping and remobilization of metals into footwall rocks. The close spatial association of Cl-bearing hydrous minerals to sulfides, occurrence of fluid inclusions, and the distinct texture of the sulfides is used as evidence for this interpretation. Geological Setting The Sudbury Structure includes all rocks affected by the Sudbury event, a catastrophic episode that occurred at approximately 1850 Ma. Emplaced into the structure is the 111

2 112 Explor. Mining Geol., Vol. 10, Nos. 1 and 2, 2001 Sudbury Igneous Complex (SIC), which outcrops as an oval-shaped feature (Fig. 1) approximately 58 km long and 26 km wide, and which occupies the contact area between Archean rocks of the Superior province and Proterozoic rocks of the Southern province (Card, 1964, 1978; Fleet et al., 1987). The SIC may be subdivided into several units from bottom to top: footwall breccia, contact sublayer, norite, quartz gabbro, and granophyre. Sulfide ore is typically found within the contact sublayer, footwall breccia, Sudbury breccia, and in quartz diorite offset dikes that extend radially and concentrically into the footwall rocks from the main mass norite. Rocks within the basin comprise the Whitewater Group, and consist of three formations: Onaping (oldest), Onwatin, and Chelmsford (youngest) (Dressler, 1984a). Most researchers of the Sudbury region accept the hypothesis that the Sudbury Structure was the site of a meteorite impact that resulted in the formation of a large crater and a unit of brecciated and volcanic material, the Onaping Formation (Peredery and Morrison, 1984). Sudbury breccia (pseudotachylite), observed throughout the perimeter of the structure, is interpreted as a shock feature and is proposed as evidence for an impact, as are shatter cones and shock-metamorphic (planar) features in quartz (Dressler, 1984b). Peredery and Morrison (1984) proposed that subsequent sedimentation formed the rest of the Whitewater Group; unstable conditions in the lower part of the crust and mantle allowed basaltic magma to rise and intrude the perimeter of the basin, generating the SIC at around 1.85 Ga. Marsh and Zieg (1999) hypothesize that the entire SIC is an impact melt sheet derived mostly from the crust. They propose two possible scenarios: either, (1) the granophyre was produced by melting of the granitic upper crust, while the underlying norite was formed from the lower crust; or Fig. 1. Regional geological map of the Sudbury Basin, with the location of the Copper Cliff North mine. (2) both the granophyre and norite are differentiates of the same melted body. Marsh and Zieg (1999) prefer emulsion mixing and crystal compaction over magmatic differentiation to explain the layered nature of the igneous complex at Sudbury. The cross-cutting nature of the quartz diorite offset dikes through Sudbury breccia indicates that they were injected into the surrounding country rocks after the impact event. Lightfoot et al. (1997) showed that the quartz diorite fills the compositional gap between the felsic norite and the granophyre. Therefore, the quartz diorite may correspond to either the original magma that formed the SIC, or a later phase of intermediate composition produced from the main mass of the SIC. Metamorphic grade is of lower amphibolite facies in most of the Southern province rocks and gradually decreases northward from upper greenschist facies in the South Range to lower- or sub-greenschist facies in the North Range (Card, 1978; Fleet et al., 1987; Magyarosi, 1998). The timing of the major metamorphic event that has affected the Sudbury Basin is still a matter of debate. Card (1978) argued that the metamorphism was related to the Penokean orogeny and occurred before intrusion of the SIC, citing geochronological evidence and some field relationships. Fleet et al. (1987) and Magyarosi (1998) suggested that metamorphism (Penokean) occurred after intrusion of the SIC. All agree that the isograd between greenschist facies and amphibolite facies rocks crosses the SIC into the Southern province. Metamorphic zones within the Sudbury Basin are therefore believed to be a mappable continuation of the nodal pattern observed in the rocks south of the Basin. Riller and Schwerdtner (1997) presented evidence that suggests amphibolite facies metamorphism and deformation was caused by early Blezardian tectonism (2.4 Ga to 2.2 Ga), and greenschist metamorphism and deformation of the SIC was produced during later Penokean thrusting from the southeast. The intrusion of the SIC produced a contact metamorphic aureole that extends up to 1.5 km into the surrounding rocks. Riller et al. (1996) presented detailed magnetic fabric and microstructural data that indicates that amphibolitegrade metamorphism of the Murray pluton adjacent to the SIC was a contact metamorphic effect. In the NW portion of the Murray pluton, adjacent to the SIC, a structural overprint is recorded by a magnetic fabric parallel to the contact with the SIC. In the SE portion of the pluton away from the SIC, the rocks are not recrystallized, are of greenschist facies, and the magnetic fabric is sub-parallel to the mineral foliation in the granite. Most ore within the sublayer, footwall breccia, and offset dikes has magmatic textures and resulted from the segregation of sulfide liquid from a silicate melt (Ebel and Naldrett, 1996). In many cases, the density contrast between the sulfide and silicate liquids has resulted in the formation of deposits at or near the base of the sublayer, especially in large embayments (Binney et al., 1994; Morrison et al., 1994). However, recent studies of the Ni-Cu-PGE deposits

3 Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike J.H. RICKARD AND D.H. WATKINSON 113 in the Sudbury Structure indicate that contact metamorphism and later adjustments to the Sudbury Structure may have played a role in the redistribution of base and precious metals and other elements as chloride complexes in hydrothermal fluids. Chlorine-bearing hydrous minerals and calcic minerals occur together with ore minerals, and fluid inclusion studies reveal that saline fluids were in contact with sulfides. The presence of these assemblages in both the North and South ranges indicates that their formation cannot be exclusively related to regional metamorphic processes in the South Range (Molnár et al., 1997; Molnár and Watkinson, this volume). Many researchers have presented evidence suggesting that Cl-rich brines, originating in the footwall rocks of the North and South ranges, stripped metals from deposits in the sublayer and footwall breccia and redeposited them in the footwall country rocks. These secondary deposits are generally enriched in Cu and PGE (Farrow and Watkinson, 1996, 1997; Molnár et al., 1997, 2001; Everest, 1999; Watkinson, 1999; Molnár and Watkinson, this volume). Sudbury breccia and fracture systems provided suitable zones for fluid circulation (Molnár et al., 1997, 2001). These saline fluids produced varied alteration assemblages that consist mainly of epidote, chlorite, ± magnetite, ± quartz, ± K-feldspar, ± titanite, ± garnet (Farrow and Watkinson, 1996, 1997). The relative timing of the intrusive phases in the SIC is controversial, but considering the fact that essentially no sulfides are present in the overlying noritic rocks, it may be assumed that the sublayer was intruded first and the subsequent heat engine for circulation of hydrothermal fluids was provided by the later norite (Molnár et al., 1997; Watkinson, 1999; Molnár and Watkinson, this volume). This is also consistent with Riller et al. (1996) who suggest an intrusive origin for the norite. The Copper Cliff offset dike extends south from the SIC into rocks of the Southern province. This quartz diorite dike begins as a 2 km wide funnel-shaped embayment that gradually thins to approximately 40 m, and continues as a linear feature for a further 6 km to the south (Fig. 2). The dike dips steeply to the west, with a strike varying from southeast to south. The area mapped in this study begins at the south end of the Clarabelle open pit, adjacent to the CCN mine headframe, and extends 400 m to the south (Fig. 3). The sulfide assemblages exposed within this area are of low grade but are considered to be the surface expression of the 100 orebody (Fig. 4). The western edge of the dike is in contact with the Creighton granite throughout the field area, but contacts both metavolcanic and metasedimentary rocks of the Stobie Formation and the Creighton granite on the eastern side. The footwall and hangingwall rocks to the 100 orebody at depth are typically those of the Stobie Formation. Cochrane (1984) pointed out that the sulfides occur within elongate, pipe-like orebodies located either near the center of the dike or along the eastern edge. Sulfides tend to be associated with inclusion-bearing quartz diorite, so named for the abundant mafic and ultramafic rock inclu- Fig. 2. Simplified geological map of the Copper Cliff offset, showing the location of the detailed mapping area, mines and deposits (after Inco Ltd.). sions. The inclusions are derived from the surrounding country rocks, an unknown ultramafic source, and the quartz diorite itself. The most common sulfide textural type at CCN mine is that of inclusion massive sulfide, which is inclusion-bearing quartz diorite that contains abundant blebs of pyrrhotite with less abundant pentlandite and chalcopyrite. Massive sulfide is generally composed of 90% to 95% sulfide + magnetite, consisting of massive pyrrhotite with small blebs of pentlandite, magnetite, and rare chalcopyrite (Fig. 5a). Blebby sulfide displays 1 cm to 2 cm, sub-round blebs of pyrrhotite and pentlandite in quartz diorite with some chalcopyrite occurring as grains in the blebs or as veinlets joining the blebs (Fig. 5b). Disseminated sulfide rarely comprises more than 5% of the total sulfide and is typically chalcopyrite-rich (Fig. 5c). Interstitial sulfide is distinguished by the presence of sulfides that occupy the interstices between euhedral plagioclase, amphibole, quartz, and pyroxene grains. A rare feature in the orebody is the occurrence of ragged disseminated sulfides, characterized by jagged, cusp-like edges on all blebs (Fig. 5d). This is dissimilar to the ragged disseminated sulfide or net-textured ore of the contact sublayer. All

4 114 Explor. Mining Geol., Vol. 10, Nos. 1 and 2, 2001 of these textures indicate that the sulfide minerals were derived from a primary sulfide magma. Within the study area, the quartz diorite contacts with the Creighton granite are commonly barren of sulfides and marked by a thin chilled margin. The quartz diorite is finegrained for at least 5 cm to 10 cm from the contact, and then locally develops into a zone of sub-spherical textured quartz diorite. This sub-spherical textured zone (approximately 30 cm wide) consists of long, randomly oriented blades of amphibole (up to 2 cm long) set in a medium-grained plagioclase/quartz groundmass. The margins of the dike are typically composed of sulfide-poor, medium-grained quartz diorite with local sub-angular to sub-rounded granitic and quartzo-feldspathic inclusions (less than 2 cm in diameter), and some small, fine-grained greenstone inclusions. Greater concentrations of sulfide typically coincide with the occur- Fig. 3. Detailed surface geology of the sulfide-bearing quartz diorite dike, just south of the Copper Cliff North mine. Numbers refer to sample locations. Fig. 4. Longitudinal section of the Copper Cliff North mine looking west. The surface mapping area and the 100, 900, and 890 orebodies are shown (after Inco Ltd. vertical section).

5 Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike J.H. RICKARD AND D.H. WATKINSON 115 center of the dike, although some parts of the orebody are located at the edge of the dike and may extend for a short distance into the country rocks. Where the orebody is centrally located, the outer edge contains weakly disseminated sulfide that gradually changes into blebby disseminated or ragged disseminated ore toward the center (from the footwall toward the hangingwall). The blebby disseminated ore contains more inclusions and the blebs are slightly more abundant away from the footwall. The contact between the disseminated sulfide and the massive sulfide is typically knife-sharp (Fig. 6a), showing a transition from approximately 5%, to 80% or 90% total sulfide. This grades into inclusion massive sulfide that may contain 5% to 15% quartz diorite, with a variable amount of mafic and ultramafic inclusions (Fig. 6b). The central part of the orebody may also have sections of massive sulfide, which contain only a few small inclusions. In one exposure at the 3500 ft level, the orebody is in contact with the metasedimentary rocks. Here, the orebody is dominantly composed of inclusion massive sulfide that grades into massive sulfide toward the footwall. This is in sharp contact with a narrow section of interstitial sulfidebearing quartz diorite. Closer to the footwall, massive sulfide veins (up to 0.5 m wide) extend into the surrounding amphibolite (Fig. 6c). A few chalcopyrite-rich quartz/carbonate veins continue into the footwall rocks and represent the limit of sulfide mineralization. The quartz diorite is host to several felsic dikes (Szentpéteri et al., 2001) that strike sub-parallel to the quartz diorite and appear to pinch and swell along strike. Their width varies from less than 0.1 m to 1 m and they may be traced for as much as 5 m to 15 m, at which point they become discontinuous. Most of the dikes may be correlated along strike, an indication that they are a late magmatic differentiate and were intruded into a zone of weakness in the partly crystalrence of larger and more abundant inclusions. These inclusions may be up to 7 cm long and comprise amphibolite or metasedimentary rocks. Inclusions are commonly rimmed by medium-grained biotite, probably formed by reaction with magma. The contact between the blebby disseminated sulfides (abundant inclusions) and the barren or sulfide-poor quartz diorite (few inclusions) is obscured in most places, but where visible, it is relatively sharp. The sulfide assemblage exposed at surface is that of finely disseminated to blebby disseminated and of low grade (less than 1% to 5% sulfide). Sulfide blebs are 1 cm to 2 cm in diameter and composed mostly of pyrrhotite with less abundant chalcopyrite and pentlandite. The outcrop surface is gossanous with many spherical void spaces that remain after weathering and dissolution of sulfides. Underground workings provided an excellent opportunity to observe sulfide distribution within the dike and fresh sulfide textures. The 100 orebody generally occupies the Fig. 5. a) Massive sulfide ore showing megacrysts of pentlandite. b) Blebby sulfide ore that displays 1 cm to 2 cm, sub-round blebs of pyrrhotite and pentlandite set in quartz diorite with some chalcopyrite occurring as grains in the blebs or as veinlets joining many of the blebs. c) Interstitial sulfide in quartz diorite, where the sulfide is found interstitial to silicate grain boundaries. d) Ragged disseminated sulfide, characterized by angular and drawn-out blebs joined by veins. Fig. 6. a) A sharp contact between blebby sulfide and massive sulfide. b) Typical INMS that may contain 5% to 15% quartz diorite, with a variable amount of mafic and ultramafic inclusions. Note the accumulation of chalcopyrite around the ultramafic clast. c) Pyrrhotite- and pentlandite-rich massive sulfide stringers (up to 0.5 m wide) that extend into the surrounding amphibolites on the 3500 level of the 100 orebody.

6 116 Explor. Mining Geol., Vol. 10, Nos. 1 and 2, 2001 lized magma. Contacts with the quartz diorite are sharp and most evident on exposed surfaces where the more biotite-rich quartz diorite displays a lower weathering profile. Sulfide Mineralogy and Textures Pyrrhotite (Fe 1-X S) is the most abundant sulfide mineral. The two major polymorphs of pyrrhotite are hexagonal pyrrhotite and monoclinic pyrrhotite, both of which are found in the CCN deposit. High-temperature pyrrhotite has hexagonal symmetry while lower temperatures yield both hexagonal and monoclinic types (Klein and Hurlbut, 1985). Seventeen samples were analyzed with an automated Philips PW 3710 powder X-ray diffractometer at the University of Ottawa (Rickard, 2000). Qualitative analyses confirm that hexagonal pyrrhotite is the dominant polymorph. Hexagonal pyrrhotite was predominant in 14 samples, and the remaining three samples were mostly monoclinic pyrrhotite. The monoclinic samples were distributed throughout the deposit. Pentlandite [(Fe,Ni) 9 S 8 ]is the most important Ni-bearing mineral in the South Range of the Sudbury Basin. By comparison, Ni-bearing sulfides such as millerite are volumetrically very limited and are much more common in the North Range. Pentlandite is present in all sulfide textural types at the CCN mine, but is rare within chalcopyrite-rich quartz veins. Unlike pyrrhotite and chalcopyrite, it does not occur in veinlets. Argentopentlandite [Ag(Fe,Ni) 8 S 8 ] is a trace phase within the ore, and is typically hosted by chalcopyrite, an association also noted by Cabri and Laflamme (1976). It commonly forms a branching texture with pentlandite along presumed chalcopyrite parting planes. In blebby to massive sulfides, it may also form in chalcopyrite along magnetite grain boundaries. Representative electron microprobe data of some of the common sulfide minerals are presented in Table 1. Many textures associated with chalcopyrite (CuFeS 2 ) suggest that it was either the latest sulfide mineral to have crystallized and/or has been remobilized by secondary processes. The spatial zonation of chalcopyrite relative to the other major sulfides is obvious on any well-exposed rock face. Most sulfide blebs are joined by thin veinlets of chalcopyrite (±pyrrhotite), an indication that the chalcopyrite has been remobilized. Fractures in rock inclusions, within the quartz diorite, likely provided excellent pathways for later fluids and are commonly filled by chalcopyrite, less abundant pyrrhotite and locally magnetite. Chalcopyriterich veins proximal to the orebody often have abundant pyrrhotite (±pentlandite) at the center, whereas, the more distal chalcopyrite-rich quartz/carbonate veins have very little pyrrhotite. Two of these veins were examined in detail. The margin of one is dominated by chalcopyrite with small, irregularly shaped masses of partially replaced pyrrhotite. The dominant sulfides in the other vein are pyrrhotite and pentlandite, with minor chalcopyrite restricted to the marginal zones. Pyrrhotite and pentlandite at the core of the veins are coarse-grained and blocky in texture. The grain boundary between chalcopyrite and the inner pyrrhotite is irregular and commonly marked by flames or skeletal grains of pentlandite. Even the partially replaced pyrrhotite at the margins is rimmed by pentlandite flames. This texture suggests that the pentlandite flames were exsolved after a Curich fluid had infiltrated and partially replaced sulfides at the vein margins. Electron-microprobe analyses show that there is a distinct difference in Co content between subhedral pentlandite grains at the center of veins and pentlandite exsolution flames in contact with marginal chalcopyrite (Fig. 7a). Twelve analyses of the inner subhedral pentlandite grains in two polished thin-sections reveal Co abundances of 0.21 atoms per formula unit (a.p.f.u.), S.D. = 0.01 a.p.f.u. (or 1.6 wt%, S.D. = 0.1 wt%), whereas, eleven analyses of marginal pentlandite flames give a Co range of 0.13 a.p.f.u., S.D.= 0.01 a.p.f.u. (or 1.0 wt%, S.D. = 0.4 wt%). The Ni and Co content of pentlandite from different ore types throughout the 100 orebody are presented in Figures 7b and 7c, Table 1. Representative electron microprobe analyses of sulfides from CCN mine Analysis CCN3 3A CCN07-2B CCN09-6D CCN46-1B CCN06 1b CCN07-2A CCN09-5A CCN2B 2C CCN2B 1A CCN46-1A CCN3 3B CCN19-D2 Mineral Pn Pn Pn Pn Pn Po Po Po Po Po AgPn AgPn Texture bleb masu vein vein diss masu vein bleb bleb vein bleb vein Ag Fe Co Ni Cu As S Total Ag Fe Co Ni Cu As S Subtotal Pentlandite (Pn), pyrrhotite (Po), and argentopentlandite (AgPn). Data presented in weight percent and atomic proportion.

7 Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike J.H. RICKARD AND D.H. WATKINSON 117 sulfide, interstitial sulfide, veins, and chalcopyrite-rich quartz/carbonate veins. The magnetite in massive sulfide forms sub-round blebs with or without oxyexsolution lamellae of ilmenite, and may account for 7% to 10% of the rock. Locally, magnetite grains are mantled by ilmenite. Petrographic examination of the inclusion massive sulfide, proximal to a major fault (No. 2 cross-fault) that crosscuts the ore, reveals extensive brecciation of the rock and development of parallel chains of pyrite in pyrrhotite. The formation of pyrite may indicate oxidation of sulfides by fluids that moved along the fault. Sphalerite (ZnS) is a trace mineral in most Sudbury ores. Sphalerite stars in chalcopyrite occur in some ragged disseminated sulfide samples from the 100 orebody. Tiny grains of sphalerite are also in blebby sulfides and disseminated sulfides. Electron-microprobe analyses reveal that sphalerite is often associated with sulfarsenides, arsenides, bismuth tellurides, and hessite. The sphalerite either partially or completely envelops the sulfarsenides. An unmistakable feature is the spatial association of hydrous minerals such as amphibole, biotite, chlorite, and locally epidote with all of the sulfide minerals. The former typically occur as dark green to black haloes in quartz diorite or country rocks, around blebs of sulfide, or along sulfide vein margins. The modal proportion of amphibole increases significantly proximal to sulfide minerals. Chlorite forms long, bladed crystals around, and is often intergrown with sulfide. It may be the most abundant alteration mineral proximal to the sulfides but is not nearly as common in sulfide-poor quartz diorite. Thin chloritefilled fractures locally cross-cut the sulfides. Platinum-Group Mineralogy Fig. 7. a) Covariation between pentlandite flames (solid circles) and subhedral grains (open circles) in two stringers from the 100 orebody. b) Histogram of the Ni contents in pentlandites throughout the 100 orebody. c) Histogram of the Co contents in pentlandites throughout the 100 orebody. All analyses recalculated to 17 atoms based on the pentlandite structure of (Fe,Ni,Co) 9S 8. respectively. The average Ni content of pentlandite determined by electron-microprobe analyses of 41 grains is 4.54 a.p.f.u., S.D. = 0.14 a.p.f.u (or wt%, S.D. = 1.09 wt%) and the average Co content is 0.21 a.p.f.u., S.D. = 0.10 a.p.f.u. (or 1.57 wt%, S.D. = 0.79 wt%). Magnetite (Fe 3 O 4 ) is a minor mineral in massive sulfide and inclusion massive sulfide, with lesser amounts in blebby The major platinum-group metals (PGM) associated with the ore in the 100 orebody are iridium and to a lesser extent rhodium. This is reflected in assay data provided by Inco Ltd. that shows low overall total precious metal (TPM, typically Pt, Pd, Rh, and Au) contents in the orebody. Cobaltite contains the only Ir-bearing phase in the ores. This Ir occurs in crystals of the sulfarsenide irarsite [(Ir,Ru,Rh,Pt)AsS] at the core of cobaltite grains (Fig. 8a). Those with no appreciable Ir may have a very small amount of Rh at the core and, in one crystal, the center was host to five tiny bismuth telluride grains. Many of the cobaltite crystals are strongly zoned, with Co-rich rims and more Ni-rich cores (cobaltian gersdorffite), with or without irarsite. This zonation is not commonly seen in reflected light, but is very distinct with sharp contacts in backscattered electron images. Figure 9a is based on three microprobe analyses taken respectively within each zone of a cobaltite crystal from core to rim. One unusual cobaltite has a barren Co-rich core, enveloped by a Rh-rich phase, and an outer Co-poor rim (Fig. 9b). The associated ternary diagram is based on three separate analyses within the single crystal.

8 118 Explor. Mining Geol., Vol. 10, Nos. 1 and 2, 2001 Fig. 9. a) Zoned cobaltite in pyrrhotite from the 100 orebody, which has a Co-rich rim and a Ni-rich core. b) Zoned cobaltite in pyrrhotite from the 100 orebody, which has a Co-poor rim and a Co-rich core. A small amount of Rh is present in the inner rim. Note: Pyrrhotite (Po). Fig. 8. a) Crystals of the sulfarsenide irarsite (Ir), at the core of a cobaltite grain. b) Cobaltite (Cob) partially enveloped by sphalerite (Sph), with small hessite (Hess) (Ag 2Te) crystals at the grain margins. c) Gersdorffite (Ger) along a silicate/sulfide/oxide grain boundary adjacent to bismuth-tellurides. d) Altaite (Alt) (PbTe) as anhedral grains in pyrrhotite (Po) and magnetite (Mt). e) An anhedral sperrylite (Sper) adjacent to a sulfide grain in weakly disseminated quartz diorite. f) Pyrrhotite partially replaced by irregular blebs of bismuth-telluride (BiTe), and adjacent to froodite (Fr) (PdBi 2). Chalcopyrite Cp) has replaced pyrrhotite along the margins of a stringer from the 890 orebody. Note: Quartz (Qtz), Amphibolite (Amph), Pentlandite (Pn). The majority of cobaltite crystals (>35) observed in polished thin section occur as discrete grains in pyrrhotite, or less commonly in chalcopyrite (approximately 12) and pentlandite (1). Some are completely or partially enveloped by sphalerite, with small hessite crystals (Ag 2 Te) at or near the grain margins (Fig. 8b). Others occur along silicate/sulfide grain boundaries adjacent to bismuth tellurides (Fig. 8c). Most cobaltite grains occur in massive sulfide and inclusion massive sulfide, and less commonly in sulfide veins. This may be related to a genetic process or simply to the volume of sulfide found in the different textural types of ore. Michenerite (PdBiTe), although rare, is the main Pdbearing phase in the orebody. Small michenerite grains occur Fig. 10. a) Michenerite (Mich) grain with tsumoite (Ts) along a silicate/pyrrhotite grain boundary, from a sulfide stringer in the 100 orebody. b) Michenerite with tsumoite from a different stringer in the 100 orebody. c) Michenerite compositions from the 100 orebody reveal two groups based on the Sb content (Sb appears to substitute for Bi in the michenerite structure). Note: Amphibole (Amph), Pyrrhotite (Po), Pentlandite (Pn).

9 Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike J.H. RICKARD AND D.H. WATKINSON 119 in two sulfide veins and in one sample of massive sulfide ore. The overall chemical composition of all of the michenerite grains is virtually identical, except for their antimony (Sb) contents (Figs. 10a, 10b, and 10c). Six michenerite grains from the sample of massive sulfide ore and one from the veins have an average Sb content of 0.18 a.p.f.u. (S.D. = 0.02 a.p.f.u.), whereas, five michenerites from the other vein have an average Sb content of 0.08 a.p.f.u. (S.D. = 0.01 a.p.f.u.). Cabri and Harris (1973) found that Sb may substitute for Bi in the michenerite structure. Rare minerals in the ore are altaite (PbTe) and sperrylite (PtAs 2 ), both of which were observed by backscattered electron imaging. The altaite occurs as anhedral grains in pyrrhotite and magnetite (Fig 8d). Only one anhedral sperrylite grain was identified and it occurs as an isolated grain in weakly mineralized quartz diorite (Fig. 8e). Some representative electron microprobe data of PGM, arsenides, and tellurides are presented in Table 2. The 890 and 900 orebodies are mineralogically and texturally quite similar, but there are some subtle differences in the nature of the accessory minerals and the distribution of sulfides. Both the 890 and 900 orebodies contain the same ore textures as the 100 orebody and the distribution of sulfides within the quartz diorite dike is also comparable. The 900 orebody was studied only on the 2400 ft level of the CCN mine. Mapping completed by Inco geologists, together with observations made by the authors, indicate that the ore occurs at the margin of the dike and also partially intrudes the footwall metasedimentary rocks. The inclusion massive sulfide, toward the center of the quartz diorite, contains abundant large inclusions locally surrounded by chalcopyrite. The contact with the metasedimentary rocks is marked by many veins (up to 1 m wide) of pyrrhotite and pentlandite. The presence of inclusions (up to 2 cm) in some of the large veins implies that they may have formed by intrusion of sulfide magma. However, the extensive silicification of the footwall rocks and abundance of quartz/carbonate veins suggest that at least some of the sulfide veins are the product of sulfide remobilization. Table 2. Representative electron microprobe analyses of PGM, arsenides, and tellurides from CCN mine Analysis CCN2A-2 CCN12-2A CCN3A 4 CN64B-E2 CCO8-2 CCN46-A4 CN64B-E3 CN64B-A2 CN64A-F2 CCN1-4B CCN46-A8 CCN46-A5 CCN19-K1 CN64B-E1 Mineral Cob Cob Cob Ger Mich Mich Pk Fd Fd Ir Ts Ts Alt Nic Ore Type bleb vein bleb vein masu vein vein vein vein cob pn, silic pn, silic vein vein Pd Ru Rh Pt Os Ir Au Ag Sb Te Bi Fe Co Ni Cu Pb As S Total Pd Ru Rh Pt Os Ir Au Ag Sb Te Bi Fe Co Ni Cu Pb As S Subtotal Cobaltite (Cob), gersdorffite (Ger), michenerite (Mich), froodite (Fd), irarsite (Ir), tsumoite (Ts), altaite (Alt), nickeline (Nic). Data presented in weight percent and atomic proportion.

10 120 Explor. Mining Geol., Vol. 10, Nos. 1 and 2, 2001 Platinum-group minerals and telluride mineral assemblages in samples taken from the 890 and 900 orebodies have some distinct differences as compared to those from the adjacent 100 orebody. For example, one sample of chalcopyrite-rich vein from the 890 orebody contained abundant argentopentlandite along the vein margin adjacent to silicate minerals (dominantly amphibole). Most PGM grains border bismuth tellurides and lead tellurides, an association which has also been recognized in the North Range (Watkinson, 1999). The tellurides also occur as elongate anhedral grains in pyrrhotite. As occurs in veins from the 100 orebody, it appears that chalcopyrite has replaced pyrrhotite at the vein margins in the 890 and 900 orebodies. Textural observations indicate that the tellurides and argentopentlandite were precipitated as pyrrhotite was being resorbed. Figure 8f shows partially replaced pyrrhotite with disseminated, irregular blebs of bismuth telluride, adjacent to froodite (PdBi 2 ). Electron-microprobe analyses of zoned cobaltite grains from a sample of inclusion massive sulfide reveal Co-rich rims and Ni-rich cores, similar to those in samples from the 100 orebody. Most of these are barren of PGE, but two did contain trace Ir and Rh at the core. Cobaltite is a trace phase in an inclusion massive sulfide sample from the 900 orebody. Most of the cobaltite grains occur as discrete, euhedral crystals in pyrrhotite, but two were observed to occur in pentlandite and two others have tiny chalcopyrite and pentlandite grains attached to them. The cobaltite grains typically have Co-rich rims, and the cores may contain up to 5 wt% Rh. A carbonate/quartz vein in the 900 orebody has an assemblage of chalcopyrite, pyrrhotite, nickeline (NiAs), Au-bearing froodite (PdBi 2 ), parkerite [Ni 3 (Bi,Pb) 2 S 2 ], bismuth telluride, hessite, galena (PbS), argentopentlandite, and native bismuth in 2 mm to 3 mm wide masses of gersdorffite (NiAsS). Unlike the gersdorffite found at the core of most cobaltite grains in the studied orebodies, this gersdorffite is Co-poor. Nickeline occurs as symplectic intergrowths or as subhedral masses with pyrrhotite in the gersdorffite. A carbonate mineral (opaque in reflected and transmitted light) locally forms a symplectic intergrowth with nickeline proximal to the gersdorffite/carbonate margin. The spatial association of this feature indicates that it may have been precipitated with the carbonate during precipitation of the metals by a hydrothermal fluid. The only PGM, froodite, is observed as small euhedral grains in gersdorffite (Fig. 11a) and may be solitary or intergrown with parkerite [Ni 3 (Bi,Pb) 2 S 2 ] (Fig. 11b), galena, bismuth telluride, or native bismuth. Small concentrations of these minerals, with or without hessite, occur throughout the carbonate/quartz vein. Froodite has been observed to occur with parkerite at the Levack West (McCreedy West) and Frood mines by Cabri and Laflamme (1976). Chemical data for froodite from Sudbury mines were presented by Farrow and Watkinson (1997). Electron-microprobe analyses reveal that most froodite in the 900 orebody contains small amounts (up to 6wt%) of Au in solid solution. Given the balanced stoichiometry of the froodite, and that Au has a similar ionic radius to Pd, it is reasonable to assume that the Au is substituting for Pd in the froodite structure. Cabri and Harris (1973) identified froodite associated with gold and michenerite in a sample from the Vermilion mine, but the gold was not contained in the froodite structure. Figure 11c reveals that the froodite may also be divided into two compositional groups: those that are Au-rich, and those relatively barren of Au. The five Au-rich grains that were analyzed have an average Au content of 0.14 a.p.f.u. (S.D. = 0.03 a.p.f.u.), and the eight Au-poor grains have an average Au content of 0.04 a.p.f.u. (S.D. = 0.02 a.p.f.u.). Geochemistry Although platinum and palladium are typically the most economically important of the PGE, they are not found in great abundance within the 100 orebody. Base- and precious-metal distributions within the 100 orebody, and within different sulfide textures, were investigated using 685 assay samples supplied by Inco Ltd. There is no distinct correlation between PGE and Cu (Table 3), which is commonly observed in North Range deposits (Farrow and Watkinson, Fig. 11. a) Au-bearing froodite (Fd), bismuth telluride (BiTe), argentopentlandite (AgPn), and chalcopyrite (Cp) in gersdorffite (Ger), hosted by a carbonate/quartz vein within footwall metasedimentary rocks. b) Au-bearing froodite and parkerite (Pk) in gersdorffite hosted by the same carbonate/quartz vein. c) Ternary plot of Au-bearing froodite compositions from the 900 orebody, CCN mine. Au substitutes for Pd in the more Au-rich grains. Analyses are in atomic percent.

11 Cu-Ni-PGE Mineralization within the Copper Cliff Offset Dike J.H. RICKARD AND D.H. WATKINSON 121 Table 3. Pearson correlation coefficients for 684 assay samples from the 100 orebody, CCN mine (includes all textural types) Fe Cu Ni Co Pt Pd Au Fe Cu Ni Co Pt Pd Au Table 4. Pearson correlation coefficients for massive sulfide (MASU) and interstitial sulfide (INSU) samples from the 100 orebody, CCN mine MASU Fe Cu Ni Co Pt Pd Au Fe Cu Ni Co Pt Pd Au INSU Fe Cu Ni Co Pt Pd Au Fe Cu Ni Co Pt Pd Au Data based on 23 MASU and 5 INSU samples (subset of the larger 684 sample dataset). 1997). However, the average Pt and Pd contents relative to sulfide texture reveal a stronger association between Pt and interstitial sulfide, and to a lesser extent, massive sulfide (Table 4). The high average Pt content in massive sulfide is due to a nugget effect in a few samples. The average Pt and Pd contents in 23 massive sulfide samples is 0.03 oz/ton (S.D. = 0.06 oz/ton) and 0.01 oz/ton (S.D. = 0.02 oz/ton), respectively. In five interstitial sulfide samples, the average is 0.03 oz/ton (S.D. = 0.02 oz/ton) Pt and 0.01 oz/ton (S.D. = 0.01 oz/ton) Pd. The lower limit of analytical detection for these Pt and Pd data is reported as oz/ton (Brian Huston, senior staff geologist, CCN mine, pers. comm.). Gold content in the 100 orebody is also low, Au being most commonly associated with interstitial sulfide and also present in veins. No Au-bearing minerals were observed in polished thin section. The average Au content in five interstitial sulfide samples is oz/ton (S.D. = oz/ton), and in 83 vein sulfide samples is oz/ton (S.D. = oz/ton). Data from underground workings and diamond-drill logs have been examined using a Datamine 3D model constructed by Inco Ltd. The existing Datamine model includes the adjacent 120, 100, 900, and 890 orebodies within the quartz diorite dike. The orebodies may or may not be connected by zones of weakly disseminated sulfides. The Datamine model shows that the total precious metal (TPM) content in parts of the 900 orebody is clearly higher than that in the 100 orebody. TPM values in the 100 orebody are typically less than oz/ton, with a maximum value of between 0.05 and oz/ton, whereas, the 900 orebody contains a wide range of TPM values from less than 0.02 oz/ton to greater than 0.2 oz/ton (Rickard, 2000). Discussion The orebodies in the Copper Cliff offset are similar to many of the deposits found within the offset dikes that are located throughout the Sudbury Structure. However, there are also many important differences in the nature of the sulfide distribution, intrusive contacts, and inclusion mineralogy that make them quite unique. An understanding of these differences is useful in the application of genetic models to such deposits. A detailed study of the Worthington offset by Lightfoot et al. (1997) revealed that formation of the dike involved multiple, closely spaced intrusive phases. Marginal phases consist of quartz diorite barren of inclusions, while rocks toward the center contain progressively larger sub-round inclusions derived from the adjacent country rocks. The largest concentrations of sulfide in the offset dike are associated with the largest inclusions of country rock. Unlike those in the Worthington offset, sharp contacts between intrusive phases in the Copper Cliff offset are not readily identifiable at surface. Where they do exist, they tend to be sharp contacts between barren and sulfide-bearing inclusion quartz diorite, or blebby sulfide and massive sulfide. Zoning within the Copper Cliff offset dike has long been thought to be the result of flow differentiation (Cochrane, 1984) with the association of sulfides and country rock inclusions, at the center of the dike, being explained by the flow dynamics of the dike. Thus, the occurrence of larger inclusions has been related with magma moving at a greater velocity. However, this hypothesis does not account for inclusions found near dike margins, and observations made in this study suggest a much more complex intrusive history for the dike. Separate bands within the Copper Cliff offset dike likely resulted from several closely spaced intrusive events that took advantage of planes of weakness in the previously intruded magmas. This resulted in unevenly distributed intrusive phases. For example, the quartz diorite along the margins of the dike that exhibits a distinctive sub-spherical texture may have been the first magma to intrude those areas. The conditions necessary for growth of this sub-spherical texture were only optimal near the edges of the dike. In other cases, the marginal quartz diorite may be a later phase with or without sulfides. Evidence for this is the development of elongate clinopyroxene crystals perpendicular to the contact between quartz diorite and massive sulfide (Fig. 12). The occurrence of these oriented clinopyroxenes may be likened to the skeletal or dendritic olivine crystals found in comb-layered harrisitic rocks. Donaldson (1977)

12 122 Explor. Mining Geol., Vol. 10, Nos. 1 and 2, 2001 Fig. 12. Clinopyroxene perpendicular to an intrusive contact of weakly mineralized quartz diorite and massive sulfide, 100 orebody, 3500 ft level, Copper Cliff North mine. Microphotograph at 2.5x. showed that the formation of comb layers requires continuous, gradual changes in the degree of supercooling and supersaturation of a melt. It was also suggested (Donaldson, 1977) that formation of comb-layered harrisite may be dependent on the creation of a thermal gradient within the host magma, close to the chamber wall. The absence of a chilled boundary at the marginal contact of the quartz diorite may reflect the absence of a large contrast in temperature between the quartz diorite and massive sulfide. Textures such as blebby sulfides and disseminated sulfides indicate that sulfide liquid crystallized first while the surrounding quartz diorite was incompletely solidified. For spherical sulfide blebs to retain their shape, they must be solid before the crystallization and growth of adjacent silicate crystals in the quartz diorite, because otherwise the latter would impinge on their borders. Interstitial sulfide is the result of the opposite relationship, in which the sulfides form after the silicate has crystallized. These sulfide textural differences may be related to varying volatile contents of the different magma pulses. Interstitial sulfide is typically associated with marginal phases of quartz diorite and may have formed in an anhydrous magma that had a high solidus temperature. The blebby sulfides may have formed in a hydrous magma, in which the silicate solidus temperature was lower than that of the sulfide liquid. The felsic dikes within the quartz diorite represent the latest phase of the quartz dioritic magma. The dikes all have the same orientation and pinch and swell through the quartz diorite indicating that late magma infilled small linearly oriented fissures or dilation zones in the crystallizing quartz diorite magma. Hydrothermal remobilization of sulfides within the 100, 900, and 890 orebodies is spatially limited, but is nevertheless a significant feature in most of the sulfides. The consistent link between sulfides and hydrous minerals such as amphibole, chlorite, biotite, and epidote throughout the orebodies can only be explained by the occurrence of a hydrothermal or metamorphic event. Such hydrous minerals typically occur at the margins of sulfide veins and veinlets, a texture that can only develop by remobilization of sulfides into fractures after solidification of the quartz diorite. Similar features have also been identified by Magyarosi et al. (1999) at Copper Cliff South mine, Stewart et al. (1999) in deposits near Kelly Lake, and Carter et al. (this volume). The hydrothermal events affecting the Sudbury Structure have been outlined by Farrow and Watkinson (1992, 1997) and Molnár et al. (1997, 2001). Fluid inclusion, petrographic, and microthermometric studies indicate that five stages of hydrothermal activity are associated with the formation of the Sudbury deposits. These episodes are represented, for example, by primary and secondary fluid inclusions in K-feldspar from the Murray granite and quartz intergrown with sulfides at the Lindsley mine (Molnar et al., 1997). Textures observed in the rocks from the 100 orebody are compatible with these aforementioned hydrothermal events. The first hydrothermal event is characterized by the remobilization of chalcopyrite that often replaces pyrrhotite in veins, magnetite in inclusion massive sulfide, and also infills fractures. Precipitation of cobaltite/gersdorffite grains around PGM may also be associated with this event. Evidence for a second hydrothermal episode is the development of sphalerite, hessite, and bismuth tellurides around or at the margins of cobaltite grains. The zonation of cobaltite/gersdorffite grains suggests that the core was formed first with successive growth zones of sulfarsenide proceeding outward. Zonation of sulfarsenides is common throughout the Copper Cliff offset (Magyarosi et al., 1999; Stewart et al., 1999; Szentpéteri et al., 2001; Carter et al., this volume). The cobaltite/gersdorffite likely nucleated on irarsite or bismuth telluride observed to occur at the center of the crystals. Diffuse, iridium-rich zones are found in the cobaltite around the irarsite, an indication that iridium has diffused out of the irarsite into the surrounding cobaltite, and has been incorporated into the cobaltite structure. Similar iridium-bearing cores in sulfarsenides have been reported by Cabri (1981). Zoning in sulfarsenides has also been documented by Ohnenstetter et al. (1991) in hollingworthite (RhAsS) from the Two Duck Lake intrusion. A common relationship within the inclusion massive sulfide ore is that of chalcopyrite that forms amalgamated blebs around inclusions of country rock. This could be a primary feature associated with the Cu-rich phase (Naldrett, 1989; Ebel and Naldrett, 1996) of the original sulfide liquid, in which the liquid moved into areas of low pressure around inclusions. Conversely, these textures could be the result of hydrothermal replacement of pyrrhotite and pentlandite by chalcopyrite. Chalcopyrite-filled veinlets join many of the sulfide blebs in inclusion massive sulfide, and each bleb is surrounded by hydrous minerals. Therefore, weak tectonic stresses acting on the quartz diorite may have allowed fluid flow into low-pressure zones around the country rock inclusions, much like the formation of pressure shadows associated with porphyroblasts in strongly deformed rocks (see Fig. 6b).