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Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 © 2007 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved. Printed in Canada. 0964-1823/00 $17.00 + .00 Structural Controls on Massive Sulfide Deposition and Hydrothermal Alteration in the South Sturgeon Lake Caldera, Northwestern Ontario A.H. Mumin1,†, S.D. Scott2, A.K. Somarin1, and K.S. Oran3 (Received July 7, 2000; accepted January 30, 2007) Abstract — Synvolcanic structures played a fundamental role in the genesis, morphology, and siting of volcanogenic massive sulfide ores and associated hydrothermal alteration in the Archean South Sturgeon Lake caldera complex. The most voluminous and persistent hydrothermal venting and massive sulfide deposition occurred along synvolcanic rifts and grabens associated with faults and tectonic fissures that created permeable fracture zones deep enough to access the underlying hydrothermal reservoir. The type of fracturing is highly variable and changes with the composition, competency, degree of consolidation, and alteration of host rocks. Synvolcanic structures and fracture styles also vary according to the amount and type of tectonic movement, including extension-related collapse, shearing and faulting perpendicular to the principal direction of extension, and orthogonal faulting and shearing. Permeable conduits were created by tension fracturing along fault zones, brittle deformation at the intersections of orthogonal faults, and by extensional fractures in stockworks. In texturally uniform footwall rocks, the distribution of alteration zones was controlled by the morphology of the structural conduit. In rocks with vertical and/or lateral facies, permeability, and competency changes (e.g., Lyon Lake graben), there was an additional stratigraphic control over fluid migration. Some crosscutting synvolcanic structures, alteration zones, and intrusions appear as stratiform units at the present erosion surface due to regional deformation and the present attitude of the volcanic stratigraphy. Hydrothermal mineral assemblages (e.g., quartz, carbonates, chlorite, pyrite, chalcopyrite) infilling structurally induced fractures provide good evidence of fluid migration pathways. However, mineralogy can vary significantly according to the fluid characteristics, host rock geochemistry, and subsequent metamorphic history of the area. Clearly, one of the best methods for locating volcanogenic massive sulfide deposits is to delineate the attitudes of synvolcanic structures, and explore those that show evidence of associated high-temperature hydrothermal mineral assemblages. Excellent exploration targets occur where synvolcanic structures with hydrothermal alteration intersect paleo-seafloor horizons. © 2007 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved. Key Words: Volcanogenic massive sulfide deposits, Sturgeon Lake, hydrothermal alteration, structural geology. Sommaire — Les structures synvolcaniques ont joué un rôle important dans la genèse, la morphologie et la localisation des minerais de sulfures massifs volcanogènes et de l’altération hydrothermale qui leur est associée dans la caldeira complexe archéenne de South Sturgeon Lake. Les exhalaisons hydrothermales les plus considérables et persistantes ainsi que l’accumulation de sulfures massifs ont eu lieu le long de rifts et de grabens synvolcaniques associés à des failles et des fractures tectoniques qui ont généré une zone fracturée perméable suffisamment profonde pour atteindre le réservoir hydrothermal sous-jacent. Ce type de fracturation est très variable et change en fonction de la composition, de la compétence, du degré de consolidation et d’altération des roches hôtes. Les structures et les styles de fracturation synvolcanique varient aussi en fonction de l’importance et du type de mouvement tectonique, incluant l’effondrement en contexte de distension, le cisaillement et les failles perpendiculaires à la direction principale d’extension ainsi que les failles et cisaillement orthogonaux. Des conduits perméables ont été créés par rupture de tension le long de zones de faille, par déformation cassante à l’intersection de failles orthogonales et des fractures en extension dans des stockworks. Quand les roches du mur sont texturalement homogènes, la distribution des zones d’altération est contrôlée par la morphologie du conduit structural. Là où les roches présentent des variations verticales et latérales de faciès, de perméabilité et de compétence (e.g. Graben de Lyon Lake), on note que la stratigraphie exerce un degré de contrôle additionnel sur la circulation des fluides. Certaines structures synvolcaniques, zones d’altération et intrusions sécantes doivent leur aspect stratiforme le long de la surface d’érosion actuelle à la déformation régionale et à l’attitude présente de la stratigraphie. Les assemblages de minéraux hydrothermaux (e.g. quartz, carbonates, chlorite, pyrite, chalcopyrite) en remplissage dans les fractures résultant de l’activité structurale témoignent bien de la trajectoire empruntée par les fluides. Leur minéralogie peut toutefois varier considérablement selon les caractéristiques du fluide, la géochimie de la roche hôte et l’histoire métamorphique subséquente du secteur. On peut constater que l’une des meilleures méthodes pour trouver des gisements de sulfures massifs volcanogènes est clairement de définir l’attitude des structures synvolcaniques, puis d’explorer celles qui présentent des assemblages de minéraux hydrothermaux de haute température. D’excellentes cibles d’exploration peuvent être trouvées à l’intersection d’horizons correspondant à la paléo-surface du plancher océanique avec des structures synvolcaniques associées à une altération hydrothermale. © 2007 Canadian Institute of Mining, Metallurgy and Petroleum. All rights reserved. 1 Department of Geology, Brandon University, Brandon, Manitoba, R7A 6A9. Department of Geology, University of Toronto, Toronto, Ontario, M5S 3B1. 3 Connor, Clark & Lunn, 925 West Georgia Street, Vancouver, British Columbia, V6L 3L2. † Corresponding Author: [email protected] 2 84 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Introduction Modern seafloor studies show that volcanogenic massive sulfide (VMS) deposits form in areas of active extension. During rifting, subsidence, and thinning of the crust, hot asthenospheric mantle rises to the base of the crust causing bimodal mantle-derived mafic and crustal-derived felsic volcanism. Water–rock reactions result in metal leaching and formation of hydrothermal convection systems, which may ultimately form VMS deposits (Franklin et al., 2005). The majority of deposits are found along fault-bounded axial rifts, or within seamount calderas adjacent to extensional structures along or near spreading ridges, submerged island arcs, and back-arc basins (Scott, 1992; Fouquet, 1997; Gibson et al., 1999). Ancient deposits now preserved on land include the type locality, the Hokuroku district of Japan. Here, Miocene VMS deposits are preserved within a volcanic complex along a failed rift of the Japanese island arc, with individual deposits localized around the intersections of orthogonal faults (e.g., Scott, 1978, 1979; Cathles, 1983; Guber and Green, 1983; Cas, 1992). Identification of similar structures around Archean deposits has been more difficult due to the camouflaging effects of their later deformation and metamorphism (Scott, 1979). Nevertheless, considerable progress has been made in recent years in documenting the structural setting and controls for many ancient deposits (Barrie and Hannington, 1999; Stix et al., 2003), particularly in Australia (e.g., Cas, 1992; Large, 1992; Corbett, 2001; Sharpe and Gemmell, 2002) and in the Canadian Abitibi Belt (e.g., Kerr and Gibson, 1993; Larson and Hutchinson, 1993; Bleeker, 1999; Gibson et al., 2000; Yang and Scott, 2003). In the south Sturgeon Lake area of northwestern Ontario (Fig. 1), six VMS deposits have been mined (F Group, Mattabi, Lyon Lake, Creek Zone, Sub-Creek Zone, and Sturgeon Lake) and several sub-economic sulfide lenses remain unexploited (Table 1). The deposits are located within an Archean volcanic caldera complex and occur at several paleo-seafloor horizons (Groves et al., 1988; Morton et al., 1990, 1991, 1996; Mumin and Scott, 1991, 1994; Hudak and Morton, 1999; Hudak et al., 2003). This paper examines the evidence for synvolcanic structural controls on the site-specific location and morphology of these massive sulfide deposits and their associated hydrothermal alteration. It also documents and discusses Table 1. Mineral Deposits of the Sturgeon Lake Mining Camp* Grade Deposit Zn Cu Pb (wt.%) (wt.%) (wt.%) Ag (g/t) F Group 9.51 0.64 0.64 60.4 Mattabi 8.28 0.74 0.85 104.0 Lyon Lake and SubCreek Zone 6.53 1.24 0.63 141.5 Creek Zone 8.80 1.66 0.76 141.5 Sturgeon Lake 9.17 2.55 1.21 164.2 * After Franklin (1995). some of the stratigraphic, deformational, and alteration features that distinguish ore-related from late structures, and how these structures may be identified in an ancient deformed and metamorphosed terrain. Regional Geology The South Sturgeon Lake volcanic pile is a northwardyounging 10 km-thick homoclinal sequence of Archean felsic through mafic volcanic rocks, with intercalated volcaniclastic and chemical sediments. The volcanic pile is capped by a 300 to 1500 m-thick sequence of sedimentary rocks dominated by graywacke, argillite, and conglomerate, with magnetic iron formation (Fig. 1). Early investigators divided the rocks into four mafic to felsic cycles (Trowell, 1974, 1983; Franklin et al., 1977; Hinzer, 1981), with several laterally extensive, graphitic ± pyritepyrrhotite-bearing horizons in each cycle (Shegelski, 1978). Morton et al. (1990, 1991, 1996) and Hudak and Morton (1999) have demonstrated that the lower two volcanic cycles comprise progressive subaerial to subaqueous caldera fill. They subdivided the caldera fill into: (1) a precaldera sequence dominated by amygdaloidal to massive basalt flows with scoria and tuff cone deposits; (2) an early caldera sequence comprised of felsic pyroclastic rocks, megabreccia, mesobreccia, and debris flow deposits, with lesser amounts of dacite and andesite; and (3) a late caldera sequence comprised of felsic pyroclastic rocks, rhyodacite, dacite, and andesite flows, and volcaniclastic sedimentary rocks. Known economic mineralization occurs in the upper felsic portions of the early caldera sequence (Mattabi and F Group deposits), and the late caldera sequence (Lyon Lake deposits). Caldera rocks were dated at 2735.5 ± 1.5 Ma, and overlying post-caldera rocks at 2717.9 +2.7/-1.5 Ma (U-Pb zircon; Davis and Trowell, 1982; Davis et al., 1985). In addition to some late faults, much of the stratigraphy has been offset by synvolcanic faulting with abundant horst and graben structures across the caldera complex (Mumin, 1988; Mumin and Scott, 1991, 1994; Morton et al., 1996, 1999). Subsequent deformation folded the Sturgeon Lake volcanic pile about an east–west hinge with a superimposed broad warping about a north–south axis (Trowell, 1970; Franklin et al., 1977). Most of the volcanic pile has been subjected to greenschist facies metamorphism; however, amphibolite facies rocks occur in the eastern and southern margins (Franklin et al., 1977; Trowell, 1983; Groves et al., 1988; Mumin, 1988; Mumin and Scott, 1991, 1994; Mumin et al., 1991). Detailed accounts Au Metric of the regional geology and volcanol(g/t) Tonnes ogy are given by Trowell (1974, 1983), Franklin et al. (1977), Friske (1983), – 340 000 Hinzer (1981), Severin (1982), Groves – 11 400 000 (1984), Morton et al. (1985, 1990, 0.5 3 945 000 1991, 1996, 1999), and Hudak and 0.5 908 000 Morton (1999). Sulfide mineralization is associated 0.5 2 070 000 with episodic eruption of felsic quartzcrystal ash-flow tuffs (Harvey and Hin- Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 85 Fig. 1. Volcanic stratigraphy of the Archean South Sturgeon Lake volcanic pile (after Franklin et al., 1975; Morton et al., 1996, 1999). The Sturgeon Lake caldera-fill complex extends stratigraphically upwards (north) from the base of the mesobreccia, debris, and pyroclastic deposits. The eastern, western, and upper limits of the caldera complex are not defined. zer, 1981; Severin, 1982; Morton et al., 1991). The massive sulfide deposits are typical Archean Zn-Cu-Ag-rich volcanogenic massive sulfide lenses, with anomalously high lead values of about 1 wt.%, and minor gold. They are compositionally zoned with a Cu-rich footwall near the region of hydrothermal discharge, and a sphalerite-pyriterich upper and distal portion. Individual ore deposits may be single lenses up to 70 m thick or multiple stacked lenses with intercalated host rock (cf. Sangster and Scott, 1976; Franklin et al., 1981; Large, 1992; Franklin, 1995; Poulsen and Hannington, 1995). F Group Deposit The most westerly zone of economic mineralization in the South Sturgeon Lake volcanic pile is the F Group deposit, located 5 km west of the Mattabi deposit (Fig. 1). The F Group district is underlain by up to 750 m of mafic, carbonated, and chloritic heterolithic meso- and megabreccias intercalated with pyroclastic and debris-flow deposits (Groves et al., 1988; Morton et al., 1991, 1999; Fig. 2). The breccias form most of the footwall rocks beneath the F Group deposit (Fig. 3) and comprise quartz, chlorite, calcite, dolomite, plagioclase, biotite, epidote, apatite, sphene, ilmenite, magnetite, and pyrite. Bedded, felsic, quartz-phyric pyroclastic flow and ash deposits overlie the breccia and debris deposits, and host the massive sulfides at a horizon approximately 90 to 150 m stratigraphically below the Mattabi ore horizon. The overlying Mattabi succession is composed of similar quartz-phyric pyroclastic flow deposits (Morton et al., 1990, 1999; Hudak and Morton, 1999). Regional deformation of the volcanic pile has resulted in east–west-striking stratigraphy in the F Group area, with an average dip near the deposit of about 50ºN. However, variable dips are common in the vicinity of mineralization and have been observed to reverse to a southerly direction in the immediate hanging wall and footwall of the F Group mineralized zone, particularly on the Darkwater property (Fig. 2). This localized reversal is due to the influence of primary synvolcanic structures, as well as perturbations caused by the regional deformation. Rocks in the F Group deposit are also characterized by an east–west foliation that dips approximately 80ºN. One of the north- to northeast- 86 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 2. Surface geology of the F Group and Darkwater properties showing footwall alteration and important syn-volcanic structures associated with mineralization. The original nature of the structural features is distorted by deformation and tilting of the volcanic pile. a = surface strike of east-dipping faults, b = surface strike of west-dipping faults. See Figure 5A for orientation of set a and b faults within the F Group pit. trending faults (b in Fig. 2) has segmented and moderately displaced the western part of the mineralized horizon to the north. Economic mineralization of the F Group deposit is concentrated in a linear wedge of massive sulfides nestled against a synvolcanic fault scarp (Fig. 4). Metals are systematically zoned within the deposit. Both stratabound and crosscutting stringers with chalcopyrite and pyrite mineralization occur in the footwall below the paleo-seafloor, and the base of the massive sulfide lens is copper enriched. Stratigraphically upward and laterally away from the chalcopyrite-rich zones, massive sulfides grade into sphaleritesilver-rich ore with a central portion enriched in galena. High-grade mineralization (minimum 10 wt.% Zn over 2 m or equivalent) extends eastward for at least 300 m beyond the F Group pit (Fig. 2). In the distal portion, the sulfides are contained within three separate stacked lenses, each averaging 5 m in thickness. In both the hanging wall and laterally distal portions, economic mineralization grades into massive and finally disseminated pyrite. Fig. 3. Photograph of footwall mafic mesobreccia deposit from ~300 m stratigraphically below the F Group pit. Pencil is ~14 cm long. Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 87 minor amounts of tourmaline along vein margins (Fig. 7D), and many have narrow to coalescing selvages of cryptocrystalline quartz alteration (Fig. 5C). Host rocks are generally leached of some alkali metals, silicified, and subsequently metamorphosed to quartz-sericite-pyrophylliteandalusite-chloritoid-rich rocks. Footwall rocks near the center of the F Group pit contain en echelon tension gashes filled with typical footwall stockwork mineralization including quartz, Fe-carbonates, pyrophyllite, and kyanite. The gashes are related to shearing of the east-dipping faults, and occur in rocks that were leached and silicified. Rocks that outcrop approximately 300 m southwest of the center of the F Group pit (Copper Mountain, Fig. 2) are severely altered and fractured with abundant shear- and alteration-related tension veins and gashes (Fig. 8A). The attitude of fractures is variable due to the interaction of orthogonal synvolcanic faulting, and the regional east– west foliation. The average strikes of two distinct fracture sets are 040º and Fig. 4. Simplified section through the F Group structure, rotated approximately 45° to its ori355º (Fig. 9), a and b, respectively. A ginal attitude at the time of ore deposition. The deposit formed within a small graben, and has third fracture set, c, is conformable to both crosscutting and stratabound hydrothermal alteration. Massive sulfide ore is hosted within the regional foliation, with an average Mattabi succession felsic pyroclastic rocks, which are separated from footwall pyroclastic flows, trend of 080º. Set b fractures correlate breccia, and debris by the thick line. with the projected surface trace of the west-dipping faults, set a is the surface Footwall Structures and Hydrothermal Alteration trace of the east-dipping F Group faults, and set c fractures Extensive normal and strike-slip orthogonal faulting and are parallel to the regional foliation associated with late brecciation occur in footwall rocks of the F Group deposit. deformation and metamorphism of the volcanic pile. A near-horizontal set of normal faults (a in Fig. 5A) form Most tension veins (set a) strike 030º to 050º, and are a 15ºE-dipping set of fractures in the south wall of the pit, in the plane of the east-dipping synvolcanic faults. They the same attitude as the plunge of the massive sulfide lensare typically 1 to 3 cm in width (locally up to 20 cm) and es. These east-dipping fractures are disrupted and offset by have lengths of several meters. Tension gashes formed as a north- to northeast-trending orthogonal set of strike-slip a result of faulting parallel to set a veins, have an average faults (b in Fig. 5A) that dip ~40º to the west. Associated strike of ~135º and intersect the veins at angles of 60º to with this orthogonal fault system are zones of brecciation 80º. They range from 0.5 cm to 20 cm in width and up to that occur around their intersections, and in linear arrays 30 cm in length. Both types of tension fractures are relafollowing the fault planes. The formation of these types of tively undeformed (although locally sigmoidal), suggestlinear breccias is believed to result from repeated, alternating minimal amounts of shearing during deformation that ing, small-scale movements along two intersecting faults, was synchronous with hydrothermal alteration. Rocks of as illustrated schematically in Figure 6. The fractures in the Copper Mountain outcrop are part of the structural conthe F-Group footwall are filled with brecciated and altered duit that focused fluid discharge for the F Group deposit. host-rock fragments and variable hydrothermal mineralogy The tension fractures occur within a broad zone (up dominated by chlorite, Fe-rich carbonate, sericite, pyroto 1500 m wide) of hydrothermal alteration (Fig. 2). The phyllite, and quartz. Locally, the brecciated rocks are minmost intensely altered rocks in the center of the zone are eralized with chalcopyrite and pyrite (locally exceeding severely leached of Na, Ca, Mg, Fe, and Mn, but are sig90% in some individual fractures), Fe-rich carbonate, and nificantly enriched in SiO2 and K, typical of some types Mg-rich chlorite (Figs. 5 and 7). Parallel, closely spaced of VMS feeder zones (Sangster and Scott, 1976; Franklin fractures and tension gashes filled with Fe-rich carbonate et al., 1981; Franklin, 1995; Gibson et al., 1999). Due to and chlorite ± sulfide are common. Some fractures have leaching of most alkalis and base metals, and the relative 88 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 5. A. Footwall of the F Group pit (looking south) showing the fracture brecciation caused by intersecting orthogonal faults, shearing, and associated tension veins and gashes. The regional eastward plunge of linear structures associated with extension is visible as fault set a dipping 45ºW. They are disrupted by west-plunging faults b dipping 15ºE. B. Close-up of a fracture breccia in the F Group footwall. Most of the fractures in this photograph are filled with Fe-Mg-rich carbonate, chlorite, quartz, pyrophyllite, pyrite, and chalcopyrite. Field of view is 3 m. C. Brittle fracturing of silicified felsic tuffs provided pathways for fluid discharge at the F Group deposit. The fractures are filled with chlorite, Fe-Mg-rich carbonates, quartz ± accessory minerals including pyrite, chalcopyrite, arsenopyrite, chloritoid, and tourmaline. Field of view is 30 cm. D. Cryptocrystalline quartz spheres with cores of Fe-Mgrich carbonate and chlorite ± accessory minerals including pyrite, pyrophyllite, or chloritoid in an altered felsic tuff with abundant sericite, pyrophyllite, quartz, and chloritoid. This unique feature of the F Group deposit is the result of orthogonal shearing synchronous with hydrothermal fluid alteration. Two sets of orthogonal fractures (orientation indicated by dashed lines) are visible in the photo. immobility of Al2O3, the most intensely altered rocks are residually enriched in aluminum. The mineral assemblage consists of, in order of abundance, quartz, sericite, pyrophyllite, and andalusite, with accessory apatite, epidote, and zoisite (Fig. 8C,D). Tension gashes normally contain quartz and 0% to 85% pyrophyllite as selvages along the fracture veins or as radiating crystals in cavities. Euhedral quartz may also line these cavities. Tension veins are filled with quartz ± kyanite and pyrophyllite. Locally, some veins contain as much as 90% light blue kyanite (Fig. 8A,B). Prismatic crystals and mats of sillimanite that penetrate and/or replace quartz are also present in some quartz-pyrophyllite veins (Fig. 8F). The presence of the four aluminum silicates, andalusite (in host rocks), pyrophyllite (in tension gashes and veins), kyanite (in some tension veins), and sillimanite (in the occasional quartz-pyrophyllite vein), remains an interesting but unresolved enigma. It appears that andalusite formed in country rocks within its normal low-pressure stability field (Holdaway, 1971), whereas pyrophyllite is a likely consequence of hydrous alteration of the siliceous and aluminous rocks. However, kyanite and sillimanite appear quite out of place with respect to their normal pressure-temperature stability range (Holdaway, 1971). Peripheral to the central zone, hydrothermal alteration of the footwall debris and pyroclastic breccia (megabreccia and mesobreccia of Morton et al., 1999) is characterized by whole-rock Na and Ca depletion with Fe plus Mg enrichment. Mg-rich chlorite, sericite, and Fecarbonate dominate the mineral assemblage of this outer zone (Fig. 2). Tension fractures are filled with abundant Fe-carbonate (siderite-ankerite), pyrophyllite in radiating rosettes, quartz, Mg-rich chlorite, and traces of chalcopyrite and/or pyrite. Fibrous Mg-rich chlorite forms combtextured selvages along some fractures (Fig. 8E). Chlorite commonly has deformation and kink-banding that locally occurs oblique or parallel to vein walls, indicating minor Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 89 from the altered and silicified pumice fragments described by other investigators (Hudak, 1989; Walker, 1993) and is one of the unique features of F Group rocks that preserve evidence of orthogonal shearing that was synchronous with hydrothermal alteration. Regional Setting of the F Group and Jackpot Structures Approximately 200 diamond-drill holes from the F Group and adjoining properties were replotted on 30 m and 120 m-spaced sections over a strike length of about 2 km and to a maximum depth of about 900 m. Three Fig. 6. Plan view of development of fracture breccias as a result of repeated, alternating, smalltypes of stratigraphic offsets were used scale orthogonal faulting. Arrows indicate direction of repeated movement of the mobile block. to delineate synvolcanic structures: (1) drape folds over paleo-topographic late or synchronous shearing. In the distal portions of the surfaces (Fig. 11A); (2) faulted offsets (Fig. 11B); and outer zone, the frequency of gashes and veins decreases. (3) a combination of the two caused by reactivation of Here, the fractures are mainly filled by quartz with sporthe same structure (Fig. 11C). In the latter type, the lower adic pyrophyllite and Fe-carbonate, and wall-rock alterastratigraphic contact and paleo-seafloor horizons are offset tion is dominated by Mg-rich chlorite plus lesser amounts by reactivated faulting, and the upper part of the structure of sericite and (Fe, Mg)-carbonates. is overlain by drape folded rocks. Drape folds over paleotopography could be distinguished from faulted offsets and Hanging-Wall Structures and Hydrothermal Alteration regional deformation by the following criteria: the footwall Structural deformation extends stratigraphically upcontact of a drape-folded unit conforms to paleo-topogwards into hanging-wall rocks of the F Group deposit, raphy such as a seafloor fault scarp or rift; however, the although it did not significantly affect the massive sulfide upper contact of the same unit tends toward a subparallenses. Multiple fracturing and shearing is evident on the lel alignment with the regional paleo-surface, essentially east side of the deposit. Preserved in the east wall of the seeking a paleo-horizontal layering due to gravitational pit (Fig. 10A) and stratigraphically above the chalcopyritesettling (Fig. 11A,C). filled fracture breccia zone are areas with closely spaced The principal ore-controlling structures, called the (3–10 cm apart) parallel fracture veins in leached and siliF Group structure and the Jackpot structure (Fig. 12), were cified hanging-wall felsic rocks. Vein mineralogy is similar traced for considerable distances down plunge below surto that in footwall fractures (Fig. 5C) and comprises (Fe, face as systematic offsets in stratigraphy, including: (1) the Mg)-carbonates and chlorite with minor pyrite ± chalcoupper and lower contacts of massive sulfide lenses with pyrite, and bleached wall rock of cryptocrystalline quartz. host rocks; and (2) the contacts of distinct geological units The veins in parts of these banded zones are boudinaged that could be traced with confidence, either locally or reand/or fragmented by orthogonal shearing. In places, this gionally. (The Jackpot structure was previously called the produced numerous centimeter-sized cryptocrystalline Darkwater structure by Mumin, 1986, 1988, and Mumin quartz spheres with cores of Fe-carbonate, chlorite, pyrite and Scott, 1991, 1994, but is changed here to Jackpot to ± chalcopyrite, sericite, chloritoid, and pyrophyllite, within avoid confusion with a different structure called Darkwater a matrix of fine-grained quartz, pyrophyllite, sericite, andaby Morton et al., 1996, 1999.) lusite, and chloritoid (Figs. 5D and 10B). This phenomenon The correlation of stratigraphic offsets associated with is also observed on the weathered surface of some rocks. the F Group deposit indicates that the ore lies nestled Disaggregated hydrothermal veins are visible as rounded against one wall of a 125 m-wide graben (Figs. 4 and 12). and lensoid domains made visible by the brown weathering The central down-dropped portion of the graben and fracof a small amount of Fe-sulfide, chlorite, and/or carbonturing associated with the south boundary faults of the graate (Fig. 10C). The breakup and rounding of hydrothermal ben is photographed in Figure 10A. The structure is 25 to veins is preserved in every stage of formation, from paral50 m deep and partially filled by quartz-phyric felsic pyrolel veins with incipient orthogonal shearing, disaggregaclastic rocks. The remainder of the graben is filled with tion, and/or boudinage (Fig. 7A), to near-spherical clasts of massive and disseminated sulfides, bedded quartz-phyric hydrothermal minerals encased in cryptocrystalline quartz pyroclastic flow and ash deposits, and in particular, their (Fig. 5D). Occasional snowball textures are observed, as is hydrothermally altered variants. Parts of the boundary often seen with metamorphic porphyroblasts that grew durfaults to this graben are the east-dipping faults observed ing deformation strain. This phenomenon is quite distinct in the footwall of the pit (Fig. 5A). The entire structure 90 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 7. Fracture mineral assemblages from the footwall of the F Group deposit. A. Multiple close-spaced parallel fractures intersect in an orthogonal pattern at ~85º to each other. The fractures are filled with Fe-Mg-rich chlorite and carbonate ± accessory amounts of sulfides, chloritoid, and pyrophyllite. The host rock is silicified felsic pyroclastic tuff from the graben boundary fracture zone (see Fig. 10A) adjacent to the F Group deposit. This type of fracturing and alteration is a precursor to formation of the cryptocrystalline quartz spheres shown in Figure 5D. B. Fe-Mg rich chlorite-carbonate-sulfide-quartz vein from the F Group footwall fracture zone; field of view is 3.6 cm. C. Close-up of B; field of view is 9 mm. D. Photomicrograph taken in cross-polarized transmitted light showing the vein margin of B: Fe-Mg-rich carbonate, quartz, and sulfides abut the cryptocrystalline quartz matrix of silicified felsic pyroclastic tuff; tourmaline (shown here within quartz) is commonly found along vein selvages, and is sometimes abundantly intergrown with stringer sulfides; field of view is 1.7 mm. E. Photomicrograph taken in plane-polarized transmitted light showing the carbonate, chlorite, and quartz gangue in footwall mineralization at F Group; field of view is 1.7 mm. F. Photomicrograph taken in plane-polarized reflected light of chalcopyrite, arsenopyrite, pyrite, and sphalerite stringer mineralization within chlorite-carbonate-quartz fracture veins, F Group footwall; field of view is 1.7 mm. Abbreviations: Asp = arsenopyrite, Carb = carbonate, Chl = chlorite, Cpy = chalcopyrite, Py = pyrite, Qtz = quartz, Sph = sphalerite, Sulf = sulfides, Tm = tourmaline. Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 91 Fig. 8. Aluminum silicate-rich mineral assemblages in fractures from the footwall of the F Group deposit. A. Photograph of the Copper Mountain outcrop in the F Group footwall hydrothermal alteration. Mafic mesobreccia deposits of this outcrop are thoroughly leached and silicified producing a quartz-sericite-andalusite-rich host rock. Tension fractures and gashes are variably filled with quartz, pyrophyllite, and kyanite. Some white patches (e.g., under the lens cap) are chatter marks from heavy machinery. B. Photomicrograph taken in plane-polarized transmitted light showing kyanite blades in pyrophyllite from the footwall fracture zone, F Group pit; black opaque minerals are sulfides; field of view is 1.7 mm. C. Photomicrograph taken in cross-polarized transmitted light showing andalusite porphyroblasts in the quartz-pyrophyllite matrix of altered felsic pyroclastic rocks hosting the F Group mineralization; field of view is 1.7 mm. D. Photomicrograph taken in cross-polarized transmitted light showing chloritoid and andalusite in a matrix of very fine-grained felted pyrophyllite; fracture vein fill from the F Group footwall; field of view is 1.7 mm. E. Photomicrograph taken in cross-polarized transmitted light showing comb-textured chlorite growing perpendicular to vein walls, intergrown with pyrophyllite. Transition zone between the quartz-sericite-aluminum silicate hydrothermal core to peripheral Fe-Mg-rich chlorite-carbonate alteration (Oran, 1987); field of view is 2.2 mm. F. Photomicrograph taken in cross-polarized transmitted light showing very fine-grained felted needles of sillimanite intergrown with quartz and pyrophyllite. Fracture vein fill from the F Group footwall (Oran, 1987); field of view is 2.3 mm. Abbreviations: And = andalusite, Chl = chlorite, Chld = chloritoid, Ky = kyanite, Pyroph = pyrophyllite, Qtz = quartz, Sil = sillimanite. 92 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 much more extensive than at F Group (Franklin et al., 1975, 1977; Groves, 1984; Morton et al., 1985, 1990, 1991, 1996; Morton and Franklin, 1987; Groves et al., 1988; Walker, 1993; Franklin, 1995; Hudak and Morton, 1999). A detailed structural analysis has not been carried out within the Mattabi ore deposit, and we recognize that minimal structural information is available due to the limited access and present exposure of mine workings. However, examination of some features that are known in Mattabi country rocks demonstrate the regional influence of the same tectonic stresses that affected the F Group area (cf. Morton et al., 1999). Fracture breccias (described below) are preserved in silicified rocks of the Mattabi deposit. Fractures containing quartz, chlorite, Fe-carbonate, pyrite, chalcopyrite, and locally minor Fig. 9. Fracture orientation and mineral assemblages from the Copper Mountain outcrop, F sphalerite are common, and were comGroup footwall alteration zone (after Oran, 1987). General orientation of the two main fracture monly found in subconcordant footsets (a and b), and the metamorphic foliation (c) are indicated. Map location shown on Fig. 2. wall copper-rich horizons (comparable to F Group). In some areas, parallel plunges approximately 15º to the east. fracture veins in silicified felsic footwall rocks contain FeThe parallel Jackpot synvolcanic fault was delineated carbonate and chlorite with minor sulfides, very similar to at approximately 350 m downdip from the center of the those at F Group. F Group graben (Fig. 12). This structure forms a signifiOne of the structural features still visible in the Mattabi cant paleo-fault scarp and hosts persistent hydrothermal footwall is a west-dipping (~60º) fracture zone with abunalteration and sulfide mineralization. It has been traced for dant en echelon tension gashes flanking both sides of the 2.5 km down plunge to the east, beyond which there is no fracture zone (Fig. 13). The tension gashes are up to 5 cm information. The lower stratigraphic contact that defines wide and 0.5 m to 3 m long. They are relatively undeformed displacement along the Jackpot structure has been offset by and sub-perpendicular to the plane of shearing. The gashes 45 to 60 m. However, above the hanging-wall dacitic rocks, are primarily filled with quartz and coarsely crystalline Festratigraphic offsets are minimal. The systematic structural carbonate indicative of open-space filling. Many of these offsets correlated between drill sections make it possible to gashes contain minor amounts of disseminated chalcopyrillustrate schematically the relationships among structure, ite; however, it can be abundant locally. The brittle shear hydrothermal alteration, and sulfide deposition. In Figure zone is composed of fragmented blocks of host rock with 12, we project the stratigraphic location of known mineralweathered copper minerals within and around fractures. ization from 1800 m along the plunge of the Jackpot StrucThe steep westward plunge of this shear zone is consistent ture onto one section, and also rotate the section by ~45º on with a documented westward plunge of massive sulfide ore a horizontal axis to show its original, approximately flatin the Mattabi mine (Mattabi mine sections and level plans; lying orientation. The current erosional surface is indicated pers. commun., S. Kerr, Senior Geologist). for reference. Sulfide mineralization similar to the F Group East- and southeast-dipping faults are also present in the deposit occurs as stacked lenses over a stratigraphic interMattabi footwall, although they have not been thoroughly val of at least 240 m, and straddles both sides of the fault investigated (no access). They appear to correlate with scarp. To date, only sub-economic bodies consisting of east-plunging trends that have been documented for some either narrow high-grade Zn or Zn-Cu-Ag sulfide lenses ore lenses (Mattabi mine sections and level plans) and are (up to 25 wt.% Zn), or wide low-grade pyrite-dominated inferred to be northeast-trending faults (Fig. 1). sulfide lenses have been found along this structure. Stratigraphically below the Mattabi deposit, northeasttrending alteration zones are documented at surface by Mattabi Deposit Morton et al. (1990, 1991), and at depth by Walker (1993). They are associated with aluminum silicate and/or Fe-rich Mattabi is the largest deposit discovered to date in the alteration of host rocks, and contain fractures with hydrosouth Sturgeon Lake area (Table 1). Alteration mineral asthermal mineral assemblages similar to those at F Group. semblages and mineralization at Mattabi are similar but Morton et al. (1990) concluded that these alteration zones Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 93 Fig. 10. A. East wall of the F Group pit (looking east) showing the south boundary fracture zone of the F Group graben structure. The interior downdropped block of the graben and direction of movement is indicated by the arrow. The orientation of the boundary fractures gives the false impression of reverse faulting, which is a consequence of the north tilted deformation of the volcanic pile. Field of view ~40 m. B. Close-up showing development of a cryptocrystalline quartz sphere (ovoid in this photo) with a darker core containing carbonate, chlorite, sulfide, sericite, and chloritoid. The sheared and altered host rock is comprised of quartz-sericite-pyrophyllite-chloritoid-altered felsic pyroclastic tuff. C. Rounded boudins from F-Group pit. The larger spheres appear darker due to weathering of Fe-rich minerals, and encase smaller spheres of cryptocrystalline (Crypto) quartz. The matrix is sheared and siliceous felsic pyroclastic tuff. Abbreviations as in caption to Figures 7 and 8, plus: ser = sericite. represent synvolcanic conduits for hydrothermal fluids that fed the Mattabi deposit. West- and east-dipping structures at Mattabi, associated fractures and mineral assemblages, and northeast trending alteration and fracture zones are similar to the F Group area and are thought to be related to the same synvolcanic stresses. Lyon Lake Deposits The Lyon Lake deposits (Figs. 14 and 15) have been classified as Mattabi-type Archean Cu-Zn volcanogenic massive sulfide deposits by Morton and Franklin (1987) and Franklin (1995). However, they vary considerably in their geological setting and alteration mineralogy from the nearby Mattabi and F Group deposits. The Lyon Lake deposits occur about 1000 m stratigraphically above the Mattabi-F Group horizon near the top of the late caldera sequence (Morton et al., 1996, 1999). Their immediate host rocks are predominantly dacitic to rhyolitic pyroclastic flows and tuffs. The deposits lie stratigraphically above the currently-recognized eastern extremity of the Beidelman Bay intrusion, the synvolcanic heat source believed to have driven hydrothermal activity at Sturgeon Lake (Fig. 1; Franklin et al., 1977; Campbell et al., 1981; Davis et al., 94 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 deposit (Mumin, 1988; Mumin et al., 1991). The area also coincides with a regional change in strike of the volcanic rocks from east–west at Mattabi to a southeasterly direction east of Lyon Lake. A total of 6.9 Mt of Zn-Cu-Ag-rich massive sulfide ore with minor Pb and Au values is distributed amongst four main deposits (Lyon Lake, Creek Zone, Sturgeon Lake, and Sub-Creek) and several smaller lenses in the Lyon Lake area (Figs. 14 and 15; Table 1). The deposits range in size from small lenses of 20 000 t to the 2.1 Mt Sturgeon Lake deposit. They differ from most other Archean deposits only in that they are relatively rich in lead, averaging about 1.0 wt.% Pb. The ore occurs as a single massive sulfide lens filling a small graben (Sturgeon Lake deposit; Fig. 16), a series of stacked lenses separated by barren waste rock (Lyon Lake and Sub-Creek Zone deposits), or a combination of the above where a single massive lens splits into several stacked lenses (Lyon Lake A Zone; Fig. 17). Both the Lyon Lake and Sub-Creek deposits are the aggregate result of coalescence of individual sulfide lenses. Sulfide mineral zoning of the deposits is both vertical and lateral. The ores are chalcopyritepyrrhotite-rich at their base near areas of hydrothermal discharge, and sphalerite-pyrite-rich in the more distal portions (Figs. 16 and 17). Geophysical conductors, a thick sequence (~250m) of mixed volcanic (rhyolite, dacite, andesite, and basalt), volcaniclastic, and chemical sedimentary rocks (silicate- and oxide-facies iron formation), and extensive fracturing define a graben structure beneath and hosting the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits Fig. 11. Three principal types of stratigraphic offsets can be used to distinguish syn-volcanic (Fig. 14; Mumin, 1988; Mumin and from late structures in a typical VMS setting: A. Drape folds; the lowermost units follow paleoScott, 1991,1994; Morton et al., 1999). seafloor topography whereas upper contacts tend toward subparallel alignment with the regional In contrast, the Sturgeon Lake deposit topography and paleo-horizontal layering; B. Simple offsets indicate post-depositional faulting, but only at the horizon that is offset, and may provide conduits for hydrothermal fluid discharge is hosted by quartz ± feldspar-phyric at a stratigraphically higher horizon; C. Drape-folds over faulted stratigraphy are excellent indipyroclastic deposits that overlie mafic cators of syn-volcanic faults that may have provided hydrothermal conduits. Sulfide lenses can intrusions (Figs. 14 and 16). It has appear as both offset and conformable to the paleo-seafloor horizon. been suggested by some investigators (Hudak and Morton, 1999; Morton et 1985; Franklin, 1995). Greenschist facies metamorphism al., 1999) that these mafic intrusions are post-caldera, and persists from F Group and Mattabi to just west of the Lyon therefore post-date mineralization. However, we find strucLake deposits. Here, an increase in metamorphic grade octural and alteration features (described below) that suggest curs from lower-greenschist facies west of the Lyon Lake a pre- or syn-mineralization presence, and believe they are deposit to lower-amphibolite facies east of the Creek Zone part of a complex of intermediate to mafic intrusions that Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 95 Fig. 12. Simplified cross section through the F Group and Jackpot graben structures, rotated 45º about a horizontal axis, to approximate their original attitude at the time of ore deposition. Mineralization and alteration from a 1.8 km strike length along the structure (i.e., perpendicular to the section) have been projected onto this section to illustrate the relationships among structure, hydrothermal alteration, and massive sulfide deposition. Footwall alteration is not shown on this section. The south wall of the F Group structure graben is shown in photograph Figure 5A, and the south graben boundary faults in photograph Figure 10A. both pre- and post-date the evolution of the Lyon Lake deposits and immediate host rocks. Hydrothermal alteration associated with the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits is characterized by Fe-enrichment, most evident as Fe-rich chlorite, carbonates, grunerite, biotite, pyrrhotite, and magnetite in both stratabound and crosscutting zones within the footwall (Mumin, 1988; Mumin and Scott, 1994). However, at the Sturgeon Lake deposit, hydrothermal alteration includes a more typical pipe-like feeder with a footwall chalcopyrite stringer zone. Here, the felsic volcanic host rocks are leached and silicified, with (Fe-Mg)-rich and aluminous alteration mineral assemblages (Severin, 1982; Mumin, 1988; Jongewaard, 1989; Mumin et al., 1991; Mumin and Scott, 1994; Hudak, 1996). Some investigators have suggested that the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits are structurally displaced from their footwall feeder and alteration zones, and are contained within a displaced thrust sheet (Koopman, 1993; Hudak, 1996; Morton et al., 1999). Their evidence is based on the presence of sheared rocks in both the footwall and hanging wall of the deposits, as well as the lack of alteration comparable to the Mattabi, F Group, or Sturgeon Lake deposits. We recognize the presence of sheared rocks at the contact of the Lyon Lake andesite with mine sequence pyroclastic rocks, but the shearing occurs sporadically in pockets of variable thickness across the mine area, and commonly consist of heterolithic breccia (Fig. 17). For these reasons, and the lack of evidence for any displaced or terminated footwall alteration or mineralized zones, we suggest that the contact breccias may be a paleo-regolith of mixed debris, talus, and flow breccia accumulated at the base of the Lyon Lake andesite flows. This contact is a zone of competency contrast and probable weakness, and will likely have accommodated some shearing during regional deformation of the Sturgeon Lake volcanic pile. The basal shear of the proposed thrust sheet also occurs as sporadically distributed stratiform zones of variable thickness in footwall rocks of the Lyon Lake deposits. These stratiform zones are intensely altered pyroclastic tuff and breccia deposits (mostly altered rhyolite agglomerates 96 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 13. A. Aerial photograph looking southwest toward the footwall of the Mattabi pit, location of the fracture zone in B is indicated by the white square. B. Parallel tension gashes visible in the footwall of the Mattabi pit are similar to those found in the footwall of the F-Group deposit. These tension fractures are filled with Fe-carbonate (partially weathered out), minor pyrite and chalcopyrite, and rare kyanite. These gashes are orthogonal to, and were caused by, the steeply west-dipping shear visible as a small scarp in the upper right side of the photograph. Field of view is ~10 m. in mine terminology; Fig. 17) with abundant Fe-rich chlorite, carbonates, amphibole, biotite, pyrrhotite, magnetite, and quartz, which is the dominant hydrothermal alteration assemblage associated with the Lyon Lake, Creek, and Sub-Creek deposits (Mumin 1984, 1988; Mumin et al., 1986, 1991, Mumin and Scott, 1994). Due to their high degree of alteration and relative weakness, shearing of these rocks is expected as a normal consequence of the regional deformation at Sturgeon Lake. However, due to lack of evidence for any significant displacement of any unit along this horizon, we interpret them to be intermittent stratabound lenses of footwall alteration, intimately associated with the ore lenses and synvolcanic structural zones (as faults and fracture zones in Fig. 1). Geological mapping, alteration, and geochemical studies (Mumin et al., 1986, 1991; Mumin and Scott, 1994) document variations in hydrothermal mineral assemblages, whole rock geochemical patterns, and in the evolutionary patterns of the hydrothermal system over time and between districts and deposits. Distinct variations in alteration assemblage and morphology occur between the Lyon Lake group of deposits and those at Mattabi and F Group. In particular, Fe-rich alteration assemblages at the Lyon Lake, Creek, and Sub-Creek deposits occur as fine disseminations and fracture filling in discordant feeder zones beneath chalcopyrite-pyrrhotite zones of the ore lenses (Figs. 16 and 17). These discordant zones tend to dissipate quickly into altered footwall rocks, but link with the intensely altered stratabound breccias described above. The regional foliation at Lyon Lake is subparallel to stratigraphy and dips steeply to the north at about 80º. This contrasts with an average 60º northward dip of the volcanic and sedimentary rocks at Lyon Lake. As a result, foliation-parallel features are sometimes confused with primary bedding, and narrow lens-like mafic sills along late shear planes can be confused with primary stratigraphy. Throughout most of the mine, only minor offsets and bends in stratigraphy are related to these late structures. One possible exception is the banded iron formations in the footwall, which tend to be highly folded (Dubé et al., 1989; Koopman, 1993), although these contortions appear to be internally restricted to the relatively narrow and intermittent iron formation units. These contortions are not observed in the overlying or underlying units. Between the 425 m and 550 m levels (vertical depth), the host rocks to the Sub-Creek orebody, and the orebody itself, are folded, sheared, and offset about 100 m upwards and to the north by a low-angle, sheared drag-fold (Mumin, 1988; Dubé et al., 1989; Koopman et al., 1990; see fig. 9 of Koopman, 1993). This structure is also associated with small Zn-Cu-Ag-rich massive sulfide lenses up to 600 m east of the Sub-Creek deposit (Fig. 15). Its intersection with the ore horizon plunges shallowly to the east, and it appears to be part of a significant fault system characterized by an annealed, white-quartz breccia that has been traced in drill core for several kilometers. In many areas the structure contains abundant veinlets and disseminations of pyrrhotite, magnetite, and Fe-rich silicates (mostly chlorite, biotite, and amphibole) and carbonates. This hydrothermal mineral assemblage is similar to those documented in fracture zones beneath the other Lyon Lake deposits (Mumin and Scott, 1994). Considering its mineralogy and intimate spatial association with several sulfide lenses (Fig. 15), this structure may represent a reactivated synvolcanic fault related to mineralization, or late deformation along a pre-existing structural weakness. Also, as documented by Koopman (1993), it has the characteristics of a parasitic fold on the south limb of the east plunging syncline associated with regional deformation of the south Sturgeon Lake volcanic pile. At present, we believe there is insufficient evidence to conclusively constrain its origin, timing, and relationship to mineralization. Synvolcanic faulting is evident in the Lyon Lake region. Ore related structures form a near-orthogonal grid with one set plunging shallowly to the east, and the complementary structures plunging steeply to the west. The most significant structure associated with mineralization delineated to date in the Lyon Lake region is a drape fold and offset Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 97 Fig. 14. Geological map of the Lyon Lake area showing the projected surface strike of some important syn-volcanic faults and fracture zones. The fractured fissure beneath the Sturgeon Lake deposit appears to be subparallel to stratigraphy because of the shallow plunge of the structure (20ºE) and steep dip of the volcanic stratigraphy (65ºN). in geology depicted in correlations of diamond-drill holes extending east of the Sturgeon Lake deposit (Noranda Exploration Company Limited data, ca.1980–1986). West of the deposit, it correlates with a fracture zone evident in drill holes, has a sub-crop bearing of about 285º, and strikes into the footwall of the Sturgeon Lake deposit (Fig. 14). This structure (Sturgeon Lake graben) was developed in the felsic footwall host rocks and was partially filled by massive sulfide ore (Fig. 16). The structure plunges eastward at approximately 20º (Fig. 15). In mafic rocks below the deposit, it forms a fracture stockwork at least 60 m wide (e.g., DDH 23-72, from ~30 m to the end of the hole at 205 m, with ~175 m of anastomosing fractures filled with hydrothermal minerals; Fig. 18). The fracture density varies from isolated veinlets, typically 2 to 10 mm thick, to an anastomosing or irregular stockwork with floating wall rock fragments (Fig. 18B). Six hundred to 900 m below the ore horizon, the stockwork veins are comprised of quartz, calcite, Ferich carbonates ± magnetite, sulfides, and chlorite, and correlate with fracture breccias beneath the Lyon Lake graben. Host rocks to the fractures commonly have a selvage leached of Na and dusted with abundant very fine-grained 98 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 15. Vertical longitudinal projection of the Lyon Lake ore horizon. The distribution of massive sulfide lenses and some of the structural trends are evident in this section. The dip of the stratigraphy produces an ~10% shortening of the vertical scale. pyrite and/or pyrrhotite. Many of the fracture selvages contain minor to locally abundant tourmaline (Fig. 19A), and minor porphyroblastic chloritoid is present locally in the immediate host rock (Fig. 19C). Stratigraphically upwards, vein mineralogy changes with an increase in the amount of iron-rich minerals. Calcite is replaced by increasingly Fe-rich carbonates intergrown with quartz, and some of the most Fe-rich carbonate veins are spotted with magnetite (Fig. 19C). Nearer to the paleo-seafloor in footwall mafic rocks beneath the Sturgeon Lake deposit, fracture veins contain Fe-rich carbonates, pyrrhotite, chlorite, magnetite, minor to trace pyrite and chalcopyrite, and decreasing amounts of quartz. Host rocks are depleted in Na and Ca, and typically enriched in Fe and Mn ± K (Mumin, 1988; Friske, 1983). The style of fracturing changes in the immediate footwall of the Sturgeon Lake deposit where the fracture zone disrupts quartz-phyric felsic pyroclastic tuffs. The stockwork in the felsic rocks is comprised of fine veinlets, in contrast to the open style of stockwork veining in deeper footwall rocks. It appears that the felsic rocks must have been unconsolidated to semi-consolidated at the time of ore formation, and could not support open stockwork veining to the same extent as the deeper underlying rocks. The fractures in the felsic tuffs contain chalcopyrite, pyrrhotite, and pyrite that form a typical Cu-rich footwall stringer zone beneath the Sturgeon Lake deposit. They also contain Mg-rich chlorite and (Mg, Fe)-rich carbonate in abundance, and aluminum silicates are reported by Friske (1983), Jongewaard (1989), and Hudak (1996). Even though the Sturgeon Lake structure is believed to be a continuous fracture zone extending into the footwall, there is a strong contrast in the alteration mineralogy and fracturing patterns between the felsic tuffs that immediately host the deposit and the underlying mafic intrusions and other footwall rocks. Part of the difference can be attributed to host rock composition. More importantly though, Mg-enrichment and the Mg-rich alteration mineral assemblage in the immediate footwall indicate that greater permeability of felsic pyroclastic rocks permitted an influx of cool seawater, which mixed with the hydrothermal fluid and caused precipitation of the footwall stringer zone. The deeper-seated Fe-rich alteration assemblages are attributed to evolved, near-neutral hydrothermal fluids, whereas the overlying Mg ± Fe-rich and aluminous assemblages are attributed to regeneration of acidic fluids resulting from seawater entrainment in footwall rocks, and mixing with the evolved hydrothermal fluid (Mumin, 1988; Mumin et al., 1991; Mumin and Scott, 1994). Based on the morphology of the Sturgeon Lake deposit (Fig. 16), correlation of linear sulfide-associated structures, and widespread open-space fracturing in footwall rocks, we interpret the Sturgeon Lake structure to have been a fissure within a graben that was oriented perpendicular to the main direction of extensional deformation (cf. F Group). The presence of mafic intrusions and smaller felsic dikes indicate that the fissure also provided a conduit for both mafic and felsic magmas, some of which post-dated mineralization. Repeated activation of the structure created the deep fracture zone that disrupted the country rocks, and Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. Fig. 16. Mine section 9900E through the Sturgeon Lake ore deposit. Massive sulfides fill a small graben at the top of a fissure zone. An extensive fracture stockwork underlying the deposit has been traced for over 1 km below the deposit. A footwall Cu-stringer zone occurs in the upper 60 m where the fracture stockwork disrupts felsic pyroclastic rocks that were permeable to seawater infiltration. Circles with dashed lines represent drill holes. Abbreviations: Carb = carbonates, Chl = chlorite, Cpy = chalcopyrite, Gn = galena, Mt = magnetite, Po = pyrrhotite, Py = pyrite, Qtz = quartz, Sph = sphalerite. provided access to the hydrothermal reservoir. Brecciation and open fractures provided an excellent conduit for rapid and voluminous hydrothermal discharge leading to formation of the Sturgeon Lake orebody. An estimate of extension can be derived from the density of open-space fracturing. Six hundred meters stratigraphically beneath the deposit, diamond drilling indicates that the zone is at least 60 m wide with a minimum average fracture density of at least 12%, suggesting the possibility of 7 m of combined extensional fracturing plus any volume loss that may have occurred during alteration. However, multiple intersecting fractures and disruption of the veining suggest that this was not a single event, but occurred as a series of tectonic disruptions over time. Fracture zones orthogonal to the Sturgeon Lake structure dip steeply to the west and occur repeatedly throughout the district. Some of these faults are currently believed to be related to the paleo-graben that lies stratigraphically below the Lyon Lake, Creek, and Sub-Creek Zone ore bodies (Fig. 14). Graben filling with mixed volcanic and sedimentary rocks has resulted in abundant lateral and vertical facies changes, which complicate delineation of structures and hydrothermal feeder zones related to the Lyon Lake ore deposits. However, fracturing in and below the graben is similar to the open-space fracture stockworks described above for the Sturgeon Lake deposit (Fig. 18). The lower 99 part of the graben contains about 60 m of felsic pyroclastic rocks, locally averaging up to 25 vol.% disseminated to massive pyrrhotite, pyrite, and magnetite (Fig. 19D). Mixed volcanic and sedimentary rocks filled the rest of the graben prior to deposition of the Lyon Lake, Creek, and Sub-Creek Zone orebodies. Consequently, most of the ore lenses and adjacent host rocks for these deposits show no clear evidence of any pre-existing large structures, only subtle drape folds resulting from pre- and syn-depositional subsidence within the graben (Fig. 17). The Lyon Lake and Creek Zone deposits were examined by Roberts (1981) for evidence of structural deformation and displacement. Based on observations at that time, Roberts stated that no significant structural deformation of the deposits was evident, other than minor offsets of less than 1 m. Rather, existing structure of the deposit was interpreted to be the result of deposition over pre-existing topographic features and syn-depositional subsidence. Although we have mapped possible displacements of up to several meters, these observations remain consistent with our present findings, the only significant exception being the Lyon Lake structure that deforms the Sub-Creek ore deposit. Hydrothermal feeder zones have been located beneath the Lyon Lake, Creek, and Sub-Creek Zone deposits with the aid of geochemistry, alteration, and mineralogy (Mumin, 1988; Mumin et al., 1991; Mumin and Scott, 1994). The ore lenses are zoned, and several distinct chalcopyrite-pyrrhotite-rich proximal zones with distal pyrite-sphalerite envelopes have been mapped in detail (e.g., Fig. 17). The chalcopyrite-pyrrhotite-rich proximal sulfides grade, over short distances, into disseminated pyrrhotitemagnetite-chlorite, and locally into quartz-carbonate-amphibole-biotite-bearing discordant and stratiform footwall zones. Variable texture, competency, permeability, and degree of consolidation and geochemistry of the host rocks influenced the fracture and alteration patterns observed in these rocks. Consequently, some fracture zones appear to be discontinuous and exhibit some variation in mineralogy. Locally, fracturing is dominated by quartz veins and tension gashes. Some veins contain abundant pyrrhotite and Fe-carbonate, with trace magnetite and chlorite. Fine, anastomosing to irregular fracture stockworks with abundant pyrrhotite and magnetite, moderate amounts of (Fe ± Mg)-rich carbonates and chlorite, some tourmaline (e.g., Fig. 19B), occasional cryptocrystalline quartz selvages, and trace chalcopyrite occur beneath Cu-rich portions of some of the sulfide lenses. Their close relationship to ore, 100 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 17. Mine section 11 500E through the A-zone of the Lyon Lake ore deposit. The detailed section is rotated 70º about a horizontal axis to approximate its original attitude at the time of ore formation. The massive sulfides formed in a basin filled with mixed sedimentary and volcanic rocks, and drape over minor structures along the paleo-seafloor. Footwall stringer and disseminated mineralization consists of pyrrhotite, magnetite, chlorite, carbonate, quartz, and trace to minor tourmaline, pyrite, and chalcopyrite in fractured zones. hydrothermal mineral assemblage, and termination of most of the veining in the sulfide lenses suggest that they were part of a synvolcanic feeder system for the deposits. At greater depth (~300 to 700 m) below the deposits, extensive open-style fracturing is present in drill holes beneath and within the graben structure with the same mineralogy as described above for deep fracturing beneath the Sturgeon Lake deposit (Figs. 18 and 19). In addition to crosscutting structures, a significant amount of hydrothermal fluid was channeled into stratabound zones by variable permeability in the mixed volcanic and sedimentary rock fill of the graben. Geochemical investigations have delineated a footwall zone of Na depletion beneath the Lyon Lake group of deposits (Severin, 1982; Friske, 1983). More recently, Mumin (1988), Mumin et al. (1991), and Mumin and Scott (1994) used residual alteration indices to document regional alteration and fluid evolution patterns in the Lyon Lake area, and outlined an extensive region character- ized by Mg ± K enrichment, Na depletion, and high Rb/ Sr ratios. They interpreted this feature to be an extensive reservoir zone about 1 to 3 km stratigraphically below the Lyon Lake group of deposits, where evolution of the hydrothermal fluid and metal leaching took place. The reservoir is separated from the mineralized horizon by Late Caldera Sequence rocks (Fig. 14). Oxygen stable isotope analyses (e.g., high positive whole rock δ18O values: B.M. Smith, unpublished data; Moss, 1992; Holk and Taylor, 2000) and geochemical and mineralogical distribution patterns in country rocks indicate that only minor to moderate amounts of low-temperature fluids affected these intermediary rocks, causing albitization and carbonatization of feldspars (Mumin, 1988; Mumin et al., 1991; Mumin and Scott, 1994). Consequently, deep synvolcanic faulting was necessary to access the high-temperature hydrothermal reservoir deep beneath the Lyon Lake group of deposits. In contrast, the reservoir zone beneath Mattabi and F Group was much closer to the paleo-seafloor horizon (Franklin, Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 101 Fig. 18. A. Drill core from hole 23-14, which intersected the footwall fracture zone near the base of the Lyon Lake graben. Fractures form an open-type stockwork filled with Fe-rich carbonates, quartz, chlorite, minor to locally abundant magnetite and pyrrhotite, and trace to minor chalcopyrite and pyrite. Fractures are clearly visible due to weathering of Fe-rich carbonates and minor sulfides in an altered dacitic host rock. Field of view is 29 cm. B. Close-up from the fracture stockwork intersected in drill hole 23-71-72, showing brittle fragmentation of dacitic host rock (light colored) within an Fe-carbonatequartz-chlorite-biotite matrix with minor magnetite and traces of pyrite, pyrrhotite, and chalcopyrite (dark). Altered dacitic host rock is predominantly quartz-chlorite-biotite-grunerite-magnetite. Field of view is 4 cm. C. Polished core from the fracture stockwork zone that disrupts mafic rocks in the deep footwall of the Lyon Lake Group ore deposits. The open-space brittle fractures are variably filled with quartz, carbonates, and magnetite. Vein selvages are dusted with ultra-fine-grained pyrrhotite, which, along with chlorite, gives the altered mafic rock a very dark appearance. The quartz-carbonate veins have been subjected to orthogonal shearing and boudinage, resulting in the segmented appearance of some of the veins. 1976; Morton et al., 1985, 1990; Groves et al., 1988; Walker, 1993). In these districts, shallower faulting would have been sufficient to tap into the reservoir and focus hydrothermal discharge. An interpretation of the structural setting for the faulting, rifting, and mineralization in the Lyon Lake area is schematically illustrated in Figure 20. Although schematic, the illustration retains the true spatial distribution of the deposits and stratigraphy, but is rotated approximately 60° to the horizontal axis to return stratigraphy to its original, near-horizontal orientation. Linear structures form subparallel faults that repeat throughout the Lyon Lake area in an orthogonal pattern. Fault scarps, grabens, and halfgrabens caused by extension and collapse of the volcanic rocks are common. Both sets of orthogonal structures fractured footwall rocks deep enough to access the underlying hydrothermal reservoir, and formed conduits for focused discharge of hydrothermal fluids at the paleo-seafloor 102 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Fig. 19. A. Photograph taken with a stereomicroscope of a carbonate-tourmaline vein from the deep fracture stockwork zone of the Lyon Lake graben, drill hole 23-67 at 286’; field of view is 4.4 mm. B. Photomicrograph taken in plane polarized transmitted light of a pyrrhotite-tourmaline stringer with minor pyrite and trace chalcopyrite. Sample is from a footwall sulfide-rich zone ~150 m below the Lyon Lake deposit; drill hole 23-75 at 336’; field of view is 0.43 mm. C. Photograph taken with a stereomicroscope of a carbonate-chlorite-quartz vein from the deep fracture stockwork zone ~600 m below the Lyon Lake deposits. The fracture also contains abundant magnetite and pyrrhotite, with minor chalcopyrite and trace pyrite and arsenopyrite. The host rock is dominantly chlorite-quartz-sericite with chloritoid porphyroblasts. Sample is from drill hole 17-23 at 240’; field of view is 13.5 mm. D. Photograph taken with a stereomicroscope of a magnetite-pyrrhotite-rich (with minor chalcopyrite and trace pyrite and arsenopyrite intergrown with the pyrrhotite) formation near the base of the Lyon Lake graben. Fine carbonate-quartz-chlorite-grunerite veins are abundant. Abundant magnetite porphyroblasts overgrow and include pyrrhotite and chalcopyrite. The host rock is dominantly grunerite-actinolite-chlorite-quartz-carbonate-garnet. Sample is from drill hole 23-67 at 350’; field of view is 13.5 mm. Abbreviations as in captions to Figures 7 and 8, plus: Act = actinolite, Grun = grunerite, Gt = garnet, Mt = magnetite, Po = pyrrhotite, Ser = sercite. horizon. Figure 20 illustrates a further important finding of this investigation, that some intrusions, structures, and alteration zones which appear stratabound at surface, are in fact crosscutting in the 3rd dimension (depth; e.g., Sturgeon Lake structure, Fig. 14). These unusual structural and stratigraphic relationships are the result of late regional deformation. Conclusions and Implications for Exploration Within the Sturgeon Lake caldera, the most voluminous and persistent hydrothermal venting and massive sul- fide deposition occurred along synvolcanic rift and graben faults and fissures that created permeable fracture zones deep enough to access the underlying hydrothermal reservoir. These permeable structures are a normal consequence of the development of large massive sulfide-hosting caldera complexes (Poulsen and Hannington, 1995; Goodfellow et al., 1999; Franklin et al., 2005). They most commonly result from regional extensional tectonics, and/or caldera collapse as the underlying magma reservoir is depleted. In the Hokuroku district of Japan, massive sulfide deposits are localized at the intersections of orthogonal faults associated with the failed rift of the Japanese island-arc, and with Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 103 Fig. 20. Schematic block diagram illustrating the interpreted structural setting for the Lyon Lake ore deposits. Some features that appear stratabound in plan view (Fig. 14) are in fact crosscutting in the 3rd (depth) dimension. caldera formation within the failed rift (Hashimoto, 1977; Scott, 1978, 1979; Cathles et al., 1983; Guber and Green, 1983; Ohmoto and Takahashi, 1983; Finn et al., 1994; Yoshida, 2001). The regional association of synvolcanic structures and massive sulfide ores is also well documented in the main caldera of the Noranda massive sulfide district, Quebec, where the entire district forms a collapse structure with many internal synvolcanic faults (Gibson and Watkinson, 1990). Detailed studies at Millenbach (Knuckey et al., 1982) show the intimate relationship of a series of sulfide lenses distributed along synvolcanic structures and their intersections. Gibson et al. (2000) discussed the graben within which the giant Horne Cu-Au-rich VMS deposit formed. Similar structural features are also documented for the giant Kidd Creek Zn-Cu-Ag-rich VMS deposit near Timmins, Ontario, which also formed within a synvolcanic graben (Bleeker, 1999; Gibson et al., 2000). The style of synvolcanic fracturing is highly variable throughout the Sturgeon Lake district and changes with the composition, competency, degree of consolidation, and alteration of host rocks. Synvolcanic structures and fracture styles vary also according to the amount and method of tectonic movement including: (1) extensional rupturing, collapse, shearing, and faulting perpendicular to the principal direction of extension (e.g., F Group, Jackpot, and Sturgeon Lake structures); and (2) orthogonal faulting and shearing, and shear-induced tension fracturing (e.g., west plunging structures at F Group and Mattabi). Permeability was particularly enhanced at the intersections of synvolcanic faults, where repeated orthogonal movements resulted in significant rock brecciation and the formation of planar breccia zones. In texturally uniform footwall 104 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 rocks, the distribution of alteration zones was controlled by the morphology of the structural conduit. In rocks with rapid vertical and/or lateral facies, permeability, and competency changes (e.g., Lyon Lake graben), there was an additional stratigraphic control over fluid migration. Variations in competency and consolidation of individual units within the Lyon Lake graben appear to have varied from impermeable soft muds and chemical sediments, to loosely consolidated tuffs and breccias, to rigid dikes, flows, and hydrothermally cemented and sealed layers. Consequently, the nature of fracturing in these rocks varied considerably. Soft muds and loosely consolidated debris will not sustain open fractures unless under the influence of pressurized fluids. This intercalation of facies of variable permeability and competency interfered with the direct discharge of hydrothermal fluids, and inhibited seawater infiltration into the footwall zone. Consequently, the Lyon Lake deposits do not exhibit typical pipe-like footwall alteration and Cu-stringer zones such as those at Noranda (Sangster and Scott, 1976; Gibson and Watkinson, 1990; Poulsen and Hannington, 1995). Many structures in the vicinity of VMS ores are commonly dismissed as late, post-ore features unrelated to mineralization because they displace ore and host rocks. However, the evidence from Sturgeon Lake suggests that most of these are reactivated synvolcanic structures. They off-set and deform ore and host stratigraphy because they can be active before, during, and after massive sulfide deposition. Some form conduits for hydrothermal discharge at higher stratigraphic horizons, whereas reactivation of others merely offsets stratigraphy. Some of these structures form conduits for magmatic intrusions, and these intrusive bodies may obliterate the evidence of structure and previous hydrothermal activity. Investigations at Sturgeon Lake revealed several criteria for distinguishing synvolcanic from late structures. Synvolcanic structures form subparallel rift and graben fault scarps that repeat at irregular intervals varying from tens to hundreds of meters. They occur in a semi-orthogonal pattern with the main set of extensional ruptures forming planar features perpendicular to the principal direction of extension. They form as a consequence of caldera doming and collapse, as a result of episodic resurgence and depletion of the magma chamber. At Sturgeon Lake, these structures can be identified along paleo-seafloor horizons by offsets in stratigraphy or drape folds that occur only in the footwall of a particular unit. Alternatively, the lowermost of a series of units will drape over or fill structures, whereas overlying units conform to the regional paleo-topography, and tend toward paleo-horizontal layering. Footwall contacts of undeformed sulfide deposits can provide excellent definition of paleo-topography at the time of ore deposition, including many of those that may have formed as sub-seafloor replacements, provided that the replaced unit conformed to paleo-topography. Additionally, any undeformed geological marker unit can be used. The strike of structural conduits at the current erosional surface can vary from perpendicular to stratigraphy, to apparently stratabound but crosscutting in the third dimen- sion. However, it is not always possible unequivocally to identify those geological features that are stratabound at surface but discordant at depth, especially where only surface information is available and outcrop exposure is limited or minimal. In practice, it can be very difficult, and we suspect that in deformed terrains like Sturgeon Lake many of these apparently stratabound, but actually discordant, features remain unrecognized. Oblique, subconcordant structures are common, and variations in the strike of synvolcanic structures and associated hydrothermal alteration zones occur because of the interrelation of regional stratigraphic dip and the plunge of linear features. Hydrothermal conduits in faults can be recognized by their vein mineral assemblages, which vary according to fluid characteristics, host rock geochemistry and the subsequent metamorphic history of the area. Our observations at Sturgeon Lake are consistent with the structural aspects of the model for VMS-producing hydrothermal systems presented by Franklin (1995), whereby sulfide deposition is associated with the boundary faults of a graben structure. The most significant difference between our observations and Franklin’s (1995) model is in the variety and style of geochemical alteration and fluid migration, which can show differences between districts and between individual deposits within districts (Mumin, 1988; Mumin and Scott, 1994). As with many other VMS regions, we find that synvolcanic structures are the most important features controlling the locations of economic massive sulfide deposits. In particular, graben and halfgraben structures are excellent sites for the accumulation and preservation of thick sulfide lenses in a natural containment. Rapid infill and accumulation of volcanic, volcaniclastic, and chemical deposits further enhance the chances of preservation by burial or formation beneath a thin veneer of sediments. Grabens are a consequence of extension and collapse, and extension promotes the formation of permeable faults and fracture networks. A variety of structure and fracture patterns created permeable discharge zones for hydrothermal fluids at Sturgeon Lake. Hydrothermal mineral assemblages associated with structural conduits also vary significantly within the mining camp, between deposits of a single group, and even within a single structure as it transects stratigraphic units of varying composition, competency, and consolidation. Clearly, one of the best methods for locating VMS deposits is to delineate the attitudes of synvolcanic structures, and explore those with associated high-temperature hydrothermal minerals. Excellent exploration targets occur where synvolcanic structures with high-temperature hydrothermal alteration intersect paleo-seafloor horizons. Acknowledgments This research was made possible by Noranda Inc., Mattabi Mines Limited, and Noranda Exploration Company Limited, who generously funded the project and provided unrestricted access to geological data. We thank INMET (formerly Corporation Falconbridge Copper) for access to their data and the Sturgeon Lake mine site. We Structural Controls on Massive Sulfide Deposition and Hyrdothermal Alteration in the South Sturgeon Lake Caldera • A.H. Mumin et al. 105 have benefited from discussions with company geologists, with whom the senior author worked as a geologist in the Sturgeon Lake camp for 7 years. R.L. Morton from the University of Minnesota-Duluth and his research team provided many insights into the volcanology of the Sturgeon Lake caldera. We thank the reviewers for excellent suggestions improving this manuscript. 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Yoshida, T., 2001, The evolution of arc magmatism in the NE Honshu arc, Japan: Tohoku Geophysical Journal, Science Reports Tohoku University, Series 5, 36-2, p. 131–149. 108 Exploration and Mining Geology, Vol. 16, Nos. 1–2, p. 83–107, 2007 Excalibur Property Xstrata Excalibur Property Property Xstrata Property P-4 Rhy An 662000 661000 660000 659000 658000 657000 5527000 P-3 D D 5526000 P-2 CLAW LAKE 51-6 P-1 51-3 51-5 D 51-2 Sed 51-4 An D A-1 5525000 N-3 Rhy N-2 N-1A Sed Glitter Lake Compilation Map: A-5 Compilation of previous exploration data UTM: NAD 27 B. Jones and H. Mumin, 2008 D An An Andesite, Basalt D Gabbro/Diorite Intrusions Rhy Rhyolite, Rhyodacite Sed Metasediments, Tuff GLITTER LAKE VLF Conductors Diamond Drill Hole Massive, stringer, or disseminated sulphides N-4A Property Boundary 0 0.5 Kilometers 1 Excalibur Resources Sturgeon Lake Mining Camp District Northwestern Ontario, Canada