sturgeon

Exploration and Mining Geology, Vol. 16, Nos. 12, p. 83107, 2007 2007 Canadian Institute of Mining, Metallurgy and Petroleum.

All rights reserved. Printed in Canada.0964-1823/00 $17.00 + .00

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 meta-morphic 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. 2007CanadianInstituteofMining,Metal-lurgyandPetroleum.Allrightsreserved.

Key Words: Volcanogenic massive sulfide deposits, Sturgeon Lake, hydrothermal alteration, structural geology.

Sommaire Les structures synvolcaniques ont jou un rle important dans la gense, la morphologie et la localisation des minerais de sulfures massifs volcanognes et de laltration hydrothermale qui leur est associe dans la caldeira complexe archenne de South Sturgeon Lake. Les exhalaisons hydrothermales les plus considrables et persistantes ainsi que laccumulation de sulfures massifs ont eu lieu le long de rifts et de grabens synvolcaniques associs des failles et des fractures tectoniques qui ont gnr une zone frac-ture permable suffisamment profonde pour atteindre le rservoir hydrothermal sous-jacent. Ce type de fracturation est trs variable et change en fonction de la composition, de la comptence, du degr de con-solidation et daltration des roches htes. Les structures et les styles de fracturation synvolcanique varient aussi en fonction de limportance et du type de mouvement tectonique, incluant leffondrement en con-texte de distension, le cisaillement et les failles perpendiculaires la direction principale dextension ainsi que les failles et cisaillement orthogonaux. Des conduits permables ont t crs par rupture de tension le long de zones de faille, par dformation cassante lintersection de failles orthogonales et des fractures en extension dans des stockworks. Quand les roches du mur sont texturalement homognes, la distribution des zones daltration est contrle par la morphologie du conduit structural. L o les roches prsentent des variations verticales et latrales de facis, de permabilit et de comptence (e.g. Graben de Lyon Lake), on note que la stratigraphie exerce un degr de contrle additionnel sur la circulation des fluides. Certaines structures synvolcaniques, zones daltration et intrusions scantes doivent leur aspect stratiforme le long de la surface drosion actuelle la dformation rgionale et lattitude prsente de la stratigraphie.

Les assemblages de minraux hydrothermaux (e.g. quartz, carbonates, chlorite, pyrite, chalcopyrite) en remplissage dans les fractures rsultant de lactivit structurale tmoignent bien de la trajectoire emprun-te par les fluides. Leur minralogie peut toutefois varier considrablement selon les caractristiques du fluide, la gochimie de la roche hte et lhistoire mtamorphique subsquente du secteur.

On peut constater que lune des meilleures mthodes pour trouver des gisements de sulfures massifs volcanognes est clairement de dfinir lattitude des structures synvolcaniques, puis dexplorer celles qui prsentent des assemblages de minraux hydrothermaux de haute temprature. Dexcellentes cibles dex-ploration peuvent tre trouves lintersection dhorizons correspondant la palo-surface du plancher ocanique avec des structures synvolcaniques associes une altration hydrothermale. 2007Cana-dianInstituteofMining,MetallurgyandPetroleum.Allrightsreserved.

1Department of Geology, Brandon University, Brandon, Manitoba, R7A 6A9.

2 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]

StructuralControlsonMassiveSulfideDepositionandHydrothermalAlterationintheSouthSturgeonLakeCaldera,NorthwesternOntario

A.H. MuMin1,, S.D. Scott2, A.K. SoMArin1, AnD K.S. orAn3(Received July 7, 2000; accepted January 30, 2007)

84 Exploration and Mining Geology, Vol. 16, Nos. 12, p. 83107, 2007

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.

RegionalGeology

The South Sturgeon Lake volcanic pile is a northward-younging 10 km-thick homoclinal sequence of Archean felsic through mafic volcanic rocks, with intercalated vol-caniclastic and chemical sediments. The volcanic pile is capped by a 300 to 1500 m-thick sequence of sediment-ary rocks dominated by graywacke, argillite, and con-glomerate, 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 pyrite- pyrrhotite-bearing horizons in each cycle (Shegelski, 1978). Morton et al. (1990, 1991, 1996) and Hudak and Morton (1999) have demonstrated that the lower two vol-canic cycles comprise progressive subaerial to subaqueous caldera fill. They subdivided the caldera fill into: (1) a pre-caldera 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 eastwest hinge with a superimposed broad warping about a northsouth 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

IntroductionModern 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. Waterrock 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 exten-sional 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 intersec-tions 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). Neverthe-less, 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; Mor-ton 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 struc-tural controls on the site-specific location and morphol-ogy of these massive sulfide deposits and their associated hydrothermal alteration. It also documents and discusses

Grade

DepositZn

(wt.%)Cu

(wt.%)Pb

(wt.%)Ag(g/t)

Au(g/t)

MetricTonnes

F Group 9.51 0.64 0.64 60.4 340 000

Mattabi 8.28 0.74 0.85 104.0 11 400 000

Lyon Lake and SubCreek Zone 6.53 1.24 0.63 141.5 0.5 3 945 000

Creek Zone 8.80 1.66 0.76 141.5 0.5 908 000

Sturgeon Lake 9.17 2.55 1.21 164.2 0.5 2 070 000

Table 1. Mineral Deposits of the Sturgeon Lake Mining Camp*

* After Franklin (1995).

margins (Franklin et al., 1977; Trow-ell, 1983; Groves et al., 1988; Mumin, 1988; Mumin and Scott, 1991, 1994; Mumin et al., 1991). Detailed accounts of the regional geology and volcanol-ogy are given by Trowell (1974, 1983), Franklin et al. (1977), Friske (1983), Hinzer (1981), Severin (1982), Groves (1984), Morton et al. (1985, 1990, 1991, 1996, 1999), and Hudak and Morton (1999).

Sulfide mineralization is associated with episodic eruption of felsic quartz-crystal 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

zer, 1981; Severin, 1982; Morton et al., 1991). The mas-sive 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-pyrite-rich 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).

FGroupDeposit

The most westerly zone of economic mineralization in the South Sturgeon Lake volcanic pile is the F Group de-posit, 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 megabrec-cias 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 suc-cession is composed of similar quartz-phyric pyroclastic flow deposits (Morton et al., 1990, 1999; Hudak and Mor-ton, 1999).

Regional deformation of the volcanic pile has resulted in eastwest-striking stratigraphy in the F Group area, with an average dip near the deposit of about 50N. 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 eastwest foliation that dips approximately 80N. One of the north- to northeast-

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.


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 sys-tematically zoned within the deposit. Both stratabound and crosscutting stringers with chalcopyrite and pyrite min-eralization 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 chal-copyrite-rich zones, massive sulfides grade into sphalerite-silver-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. 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.

Fig. 3. Photograph of footwall mafic mesobreccia deposit from ~300 m stratigraphically below the F Group pit. Pencil is ~14 cm long.


trace of the east-dipping F Group faults, and set c fractures are parallel to the regional foliation associated with late deformation and metamorphism of the volcanic pile.

Most tension veins (set a) strike 030 to 050, and are in the plane of the east-dipping synvolcanic faults. They are typically 1 to 3 cm in width (locally up to 20 cm) and have lengths of several meters. Tension gashes formed as a result of faulting parallel to set a veins, have an average strike of ~135 and intersect the veins at angles of 60 to 80. They range from 0.5 cm to 20 cm in width and up to 30 cm in length. Both types of tension fractures are rela-tively undeformed (although locally sigmoidal), suggest-ing minimal amounts of shearing during deformation that was synchronous with hydrothermal alteration. Rocks of the Copper Mountain outcrop are part of the structural con-duit that focused fluid discharge for the F Group deposit.

The tension fractures occur within a broad zone (up to 1500 m wide) of hydrothermal alteration (Fig. 2). The most intensely altered rocks in the center of the zone are severely leached of Na, Ca, Mg, Fe, and Mn, but are sig-nificantly enriched in SiO2 and K, typical of some types of VMS feeder zones (Sangster and Scott, 1976; Franklin et al., 1981; Franklin, 1995; Gibson et al., 1999). Due to leaching of most alkalis and base metals, and the relative

Footwall Structures and Hydrothermal AlterationExtensive normal and strike-slip orthogonal faulting and

brecciation occur in footwall rocks of the F Group deposit. A near-horizontal set of normal faults (a in Fig. 5A) form a 15E-dipping set of fractures in the south wall of the pit, the same attitude as the plunge of the massive sulfide lens-es. These east-dipping fractures are disrupted and offset by a north- to northeast-trending orthogonal set of strike-slip faults (b in Fig. 5A) that dip ~40 to the west. Associated with this orthogonal fault system are zones of brecciation that occur around their intersections, and in linear arrays following the fault planes. The formation of these types of linear breccias is believed to result from repeated, alternat-ing, small-scale movements along two intersecting faults, as illustrated schematically in Figure 6. The fractures in the F-Group footwall are filled with brecciated and altered host-rock fragments and variable hydrothermal mineralogy dominated by chlorite, Fe-rich carbonate, sericite, pyro-phyllite, and quartz. Locally, the brecciated rocks are min-eralized with chalcopyrite and pyrite (locally exceeding 90% in some individual fractures), Fe-rich carbonate, and Mg-rich chlorite (Figs. 5 and 7). Parallel, closely spaced fractures and tension gashes filled with Fe-rich carbonate and chlorite sulfide are common. Some fractures have

Fig. 4. Simplified section through the F Group structure, rotated approximately 45 to its ori-ginal attitude at the time of ore deposition. The deposit formed within a small graben, and has both crosscutting and stratabound hydrothermal alteration. Massive sulfide ore is hosted within Mattabi succession felsic pyroclastic rocks, which are separated from footwall pyroclastic flows, breccia, and debris by the thick line.

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, silici-fied, and subsequently metamorph-osed to quartz-sericite-pyrophyllite- andalusite-chloritoid-rich rocks.

Footwall rocks near the center of the F Group pit contain en echelon ten-sion 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 synvol-canic faulting, and the regional eastwest foliation. The average strikes of two distinct fracture sets are 040 and 355 (Fig. 9), a and b, respectively. A third fracture set, c, is conformable to the regional foliation, with an average trend of 080. Set b fractures correlate with the projected surface trace of the west-dipping faults, set a is the surface


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 45W. They are disrupted by west-plunging faults b dipping 15E. 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-Mg-rich 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, pyro-phyllite, 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-pyro-phyllite 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 (Hold-away, 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 Fe-carbonate 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 chalcopyr-ite and/or pyrite. Fibrous Mg-rich chlorite forms comb-textured selvages along some fractures (Fig. 8E). Chlorite commonly has deformation and kink-banding that locally occurs oblique or parallel to vein walls, indicating minor


surfaces (Fig. 11A); (2) faulted offsets (Fig. 11B); and (3) a combination of the two caused by reactivation of the same structure (Fig. 11C). In the latter type, the lower stratigraphic contact and paleo-seafloor horizons are offset by reactivated faulting, and the upper part of the structure is overlain by drape folded rocks. Drape folds over paleo-topography could be distinguished from faulted offsets and regional deformation by the following criteria: the footwall contact of a drape-folded unit conforms to paleo-topog-raphy such as a seafloor fault scarp or rift; however, the upper contact of the same unit tends toward a subparal-lel alignment with the regional paleo-surface, essentially seeking a paleo-horizontal layering due to gravitational settling (Fig. 11A,C).

The principal ore-controlling structures, called the F Group structure and the Jackpot structure (Fig. 12), were traced for considerable distances down plunge below sur-face as systematic offsets in stratigraphy, including: (1) the upper and lower contacts of massive sulfide lenses with host rocks; and (2) the contacts of distinct geological units that could be traced with confidence, either locally or re-gionally. (The Jackpot structure was previously called the Darkwater structure by Mumin, 1986, 1988, and Mumin and Scott, 1991, 1994, but is changed here to Jackpot to avoid confusion with a different structure called Darkwater by Morton et al., 1996, 1999.)

The correlation of stratigraphic offsets associated with the F Group deposit indicates that the ore lies nestled against one wall of a 125 m-wide graben (Figs. 4 and 12). The central down-dropped portion of the graben and frac-turing associated with the south boundary faults of the gra-ben is photographed in Figure 10A. The structure is 25 to 50 m deep and partially filled by quartz-phyric felsic pyro-clastic rocks. The remainder of the graben is filled with massive and disseminated sulfides, bedded quartz-phyric pyroclastic flow and ash deposits, and in particular, their hydrothermally altered variants. Parts of the boundary faults to this graben are the east-dipping faults observed in the footwall of the pit (Fig. 5A). The entire structure

Fig. 6. Plan view of development of fracture breccias as a result of repeated, alternating, small-scale orthogonal faulting. Arrows indicate direction of repeated movement of the mobile block.

late or synchronous shearing. In the distal portions of the outer zone, the frequency of gashes and veins decreases. Here, the fractures are mainly filled by quartz with spor-adic pyrophyllite and Fe-carbonate, and wall-rock altera-tion is dominated by Mg-rich chlorite plus lesser amounts of sericite and (Fe, Mg)-carbonates.

Hanging-Wall Structures and Hydrothermal AlterationStructural deformation extends stratigraphically up-

wards into hanging-wall rocks of the F Group deposit, although it did not significantly affect the massive sulfide lenses. Multiple fracturing and shearing is evident on the east side of the deposit. Preserved in the east wall of the pit (Fig. 10A) and stratigraphically above the chalcopyrite-filled fracture breccia zone are areas with closely spaced (310 cm apart) parallel fracture veins in leached and sili-cified hanging-wall felsic rocks. Vein mineralogy is similar to that in footwall fractures (Fig. 5C) and comprises (Fe, Mg)-carbonates and chlorite with minor pyrite chalco-pyrite, and bleached wall rock of cryptocrystalline quartz. The veins in parts of these banded zones are boudinaged and/or fragmented by orthogonal shearing. In places, this produced numerous centimeter-sized cryptocrystalline quartz spheres with cores of Fe-carbonate, chlorite, pyrite chalcopyrite, sericite, chloritoid, and pyrophyllite, within a matrix of fine-grained quartz, pyrophyllite, sericite, anda-lusite, and chloritoid (Figs. 5D and 10B). This phenomenon is also observed on the weathered surface of some rocks. Disaggregated hydrothermal veins are visible as rounded and lensoid domains made visible by the brown weathering of a small amount of Fe-sulfide, chlorite, and/or carbon-ate (Fig. 10C). The breakup and rounding of hydrothermal veins is preserved in every stage of formation, from paral-lel veins with incipient orthogonal shearing, disaggrega-tion, and/or boudinage (Fig. 7A), to near-spherical clasts of hydrothermal minerals encased in cryptocrystalline quartz (Fig. 5D). Occasional snowball textures are observed, as is often seen with metamorphic porphyroblasts that grew dur-ing deformation strain. This phenomenon is quite distinct

from the altered and silicified pumice fragments described by other investi-gators (Hudak, 1989; Walker, 1993) and is one of the unique features of F Group rocks that preserve evidence of orthogonal shearing that was syn-chronous with hydrothermal altera-tion.

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 max-imum depth of about 900 m. Three types of stratigraphic offsets were used to delineate synvolcanic structures: (1) drape folds over paleo-topographic


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 ~85 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.


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 host-ing the F Group mineralization; field of view is 1.7 mm. D. Photomicrograph taken in cross-polarized transmitted light showing chloritoid and anda-lusite 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. Transi-tion 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.


fracture veins in silicified felsic footwall rocks contain Fe-carbonate and chlorite with minor sulfides, very similar to those at F Group.

One of the structural features still visible in the Mattabi footwall is a west-dipping (~60) fracture zone with abun-dant en echelon tension gashes flanking both sides of the fracture zone (Fig. 13). The tension gashes are up to 5 cm wide and 0.5 m to 3 m long. They are relatively undeformed and sub-perpendicular to the plane of shearing. The gashes are primarily filled with quartz and coarsely crystalline Fe-carbonate indicative of open-space filling. Many of these gashes contain minor amounts of disseminated chalcopyr-ite; however, it can be abundant locally. The brittle shear zone is composed of fragmented blocks of host rock with weathered copper minerals within and around fractures. The steep westward plunge of this shear zone is consistent with a documented westward plunge of massive sulfide ore in the Mattabi mine (Mattabi mine sections and level plans; pers. commun., S. Kerr, Senior Geologist).

East- and southeast-dipping faults are also present in the Mattabi footwall, although they have not been thoroughly investigated (no access). They appear to correlate with east-plunging trends that have been documented for some ore lenses (Mattabi mine sections and level plans) and are inferred to be northeast-trending faults (Fig. 1).

Stratigraphically below the Mattabi deposit, northeast-trending alteration zones are documented at surface by Morton et al. (1990, 1991), and at depth by Walker (1993). They are associated with aluminum silicate and/or Fe-rich alteration of host rocks, and contain fractures with hydro-thermal mineral assemblages similar to those at F Group. Morton et al. (1990) concluded that these alteration zones

plunges approximately 15 to the east.The parallel Jackpot synvolcanic fault was delineated

at approximately 350 m downdip from the center of the F Group graben (Fig. 12). This structure forms a signifi-cant paleo-fault scarp and hosts persistent hydrothermal alteration and sulfide mineralization. It has been traced for 2.5 km down plunge to the east, beyond which there is no information. The lower stratigraphic contact that defines displacement along the Jackpot structure has been offset by 45 to 60 m. However, above the hanging-wall dacitic rocks, stratigraphic offsets are minimal. The systematic structural offsets correlated between drill sections make it possible to illustrate schematically the relationships among structure, hydrothermal alteration, and sulfide deposition. In Figure 12, we project the stratigraphic location of known mineral-ization from 1800 m along the plunge of the Jackpot Struc-ture onto one section, and also rotate the section by ~45 on a horizontal axis to show its original, approximately flat-lying orientation. The current erosional surface is indicated for reference. Sulfide mineralization similar to the F Group deposit occurs as stacked lenses over a stratigraphic inter-val of at least 240 m, and straddles both sides of the fault scarp. To date, only sub-economic bodies consisting of either narrow high-grade Zn or Zn-Cu-Ag sulfide lenses (up to 25 wt.% Zn), or wide low-grade pyrite-dominated sulfide lenses have been found along this structure.

MattabiDeposit

Mattabi is the largest deposit discovered to date in the south Sturgeon Lake area (Table 1). Alteration mineral as-semblages and mineralization at Mattabi are similar but

Fig. 9. Fracture orientation and mineral assemblages from the Copper Mountain outcrop, F Group footwall alteration zone (after Oran, 1987). General orientation of the two main fracture sets (a and b), and the metamorphic foliation (c) are indicated. Map location shown on Fig. 2.

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 fea-tures that are known in Mattabi country rocks demonstrate the regional influ-ence 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, pyr-ite, chalcopyrite, and locally minor sphalerite are common, and were com-monly found in subconcordant foot-wall copper-rich horizons (comparable to F Group). In some areas, parallel


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 pyroclas-tic flows and tuffs. The deposits lie stratigraphically above the currently-recognized eastern extremity of the Beidel-man 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.,

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.

LyonLakeDeposits

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)

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 down-dropped 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.


Fig. 11. Three principal types of stratigraphic offsets can be used to distinguish syn-volcanic from late structures in a typical VMS setting: A. Drape folds; the lowermost units follow paleo-seafloor topography whereas upper contacts tend toward subparallel alignment with the regional 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 at a stratigraphically higher horizon; C. Drape-folds over faulted stratigraphy are excellent indi-cators of syn-volcanic faults that may have provided hydrothermal conduits. Sulfide lenses can appear as both offset and conformable to the paleo-seafloor horizon.

1985; Franklin, 1995). Greenschist facies metamorphism persists from F Group and Mattabi to just west of the Lyon Lake deposits. Here, an increase in metamorphic grade oc-curs from lower-greenschist facies west of the Lyon Lake deposit to lower-amphibolite facies east of the Creek Zone

deposit (Mumin, 1988; Mumin et al., 1991). The area also coincides with a regional change in strike of the volcan-ic rocks from eastwest 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 Stur-geon 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 de-posits), 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 ag-gregate result of coalescence of indi-vidual sulfide lenses. Sulfide mineral zoning of the deposits is both vertical and lateral. The ores are chalcopyrite- pyrrhotite-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 sediment-ary rocks (silicate- and oxide-facies iron formation), and extensive fractur-ing define a graben structure beneath and hosting the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits (Fig. 14; Mumin, 1988; Mumin and Scott, 1991,1994; Morton et al., 1999). In contrast, the Sturgeon Lake deposit is hosted by quartz feldspar-phyric pyroclastic deposits that overlie mafic intrusions (Figs. 14 and 16). It has been suggested by some investigators (Hudak and Morton, 1999; Morton et

al., 1999) that these mafic intrusions are post-caldera, and therefore post-date mineralization. However, we find struc-tural and alteration features (described below) that suggest a pre- or syn-mineralization presence, and believe they are part of a complex of intermediate to mafic intrusions that


both pre- and post-date the evolution of the Lyon Lake de-posits and immediate host rocks.

Hydrothermal alteration associated with the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits is character-ized 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 chalcopyr-ite 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 (Koop-man, 1993; Hudak, 1996; Morton et al., 1999). Their evi-dence 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 mineral-ized 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 prob-able 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 oc-curs as sporadically distributed stratiform zones of vari-able thickness in footwall rocks of the Lyon Lake deposits. These stratiform zones are intensely altered pyroclastic tuff and breccia deposits (mostly altered rhyolite agglomerates

Fig. 12. Simplified cross section through the F Group and Jackpot graben structures, rotated 45 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.


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 frac-tures 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 chlor-ite, carbonates, amphibole, biotite, pyrrhotite, magnetite, and quartz, which is the dominant hydrothermal altera-tion 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 associ-ated with the ore lenses and synvolcanic structural zones (as faults and fracture zones in Fig. 1).

Geological mapping, alteration, and geochemical stud-ies (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 as-semblage 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 chal-copyrite-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 80. This contrasts with an average 60 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 stratig-raphy. Throughout most of the mine, only minor offsets and bends in stratigraphy are related to these late struc-tures. One possible exception is the banded iron formations

in the footwall, which tend to be highly folded (Dub et al., 1989; Koopman, 1993), although these contortions ap-pear 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; Dub 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 struc-ture contains abundant veinlets and disseminations of pyr-rhotite, magnetite, and Fe-rich silicates (mostly chlorite, biotite, and amphibole) and carbonates. This hydrothermal mineral assemblage is similar to those documented in frac-ture zones beneath the other Lyon Lake deposits (Mumin and Scott, 1994). Considering its mineralogy and intim-ate 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 associ-ated 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 signifi-cant structure associated with mineralization delineated to date in the Lyon Lake region is a drape fold and offset


in geology depicted in correlations of diamond-drill holes extending east of the Sturgeon Lake deposit (Noranda Ex-ploration Company Limited data, ca.19801986). West of the deposit, it correlates with a fracture zone evident in drill holes, has a sub-crop bearing of about 285, and strikes into the footwall of the Sturgeon Lake deposit (Fig. 14). This structure (Sturgeon Lake graben) was developed in the fel-sic footwall host rocks and was partially filled by massive sulfide ore (Fig. 16). The structure plunges eastward at ap-proximately 20 (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 isolat-ed 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, Fe-rich carbonates magnetite, sulfides, and chlorite, and correlate with fracture breccias beneath the Lyon Lake gra-ben. Host rocks to the fractures commonly have a selvage leached of Na and dusted with abundant very fine-grained

Fig. 14. Geological map of the Lyon Lake area showing the projected surface strike of some important syn-volcanic faults and fracture zones. The frac-tured fissure beneath the Sturgeon Lake deposit appears to be subparallel to stratigraphy because of the shallow plunge of the structure (20E) and steep dip of the volcanic stratigraphy (65N).


pyrite and/or pyrrhotite. Many of the fracture selvages con-tain 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, magnet-ite, 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 dis-rupts 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 uncon-solidated 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 foot-wall, there is a strong contrast in the alteration mineralogy and fracturing patterns between the felsic tuffs that im-mediately host the deposit and the underlying mafic intru-sions 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 at-tributed to regeneration of acidic fluids resulting from sea-water 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 fis-sure 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 min-eralization. Repeated activation of the structure created the deep fracture zone that disrupted the country rocks, and

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.


provided access to the hydrothermal reservoir. Brecciation and open fractures provided an excellent conduit for rapid and voluminous hydrothermal discharge leading to forma-tion of the Sturgeon Lake orebody.

An estimate of extension can be derived from the density of open-space fracturing. Six hundred meters stratigraphic-ally 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 struc-ture dip steeply to the west and occur repeatedly through-out 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 bod-ies (Fig. 14). Graben filling with mixed volcanic and sedi-mentary 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

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 chal-copyrite-pyrrhotite-rich proximal zones with distal pyr-ite-sphalerite envelopes have been mapped in detail (e.g., Fig. 17). The chalcopyrite-pyrrhotite-rich proximal sulfides grade, over short distances, into disseminated pyrrhotite-magnetite-chlorite, and locally into quartz-carbonate-am-phibole-biotite-bearing discordant and stratiform footwall zones. Variable texture, competency, permeability, and de-gree 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 mineral-ogy. 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 abun-dant 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,

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 up-per 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.

part of the graben contains about 60 m of felsic pyroclastic rocks, locally averaging up to 25 vol.% disseminated to massive pyrrhotite, pyrite, and mag-netite (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 evi-dence of structural deformation and displacement. Based on observations at that time, Roberts stated that no sig-nificant structural deformation of the deposits was evident, other than minor offsets of less than 1 m. Rather, exist-ing structure of the deposit was inter-preted to be the result of deposition over pre-existing topographic features and syn-depositional subsidence. Al-though we have mapped possible dis-placements of up to several meters, these observations remain consistent with our present findings, the only sig-nificant exception being the Lyon Lake structure that deforms the Sub-Creek


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 res-ervoir is separated from the mineralized horizon by Late Caldera Sequence rocks (Fig. 14). Oxygen stable isotope analyses (e.g., high positive whole rock 18O values: B.M. Smith, unpublished data; Moss, 1992; Holk and Taylor, 2000) and geochemical and mineralogical distribution pat-terns in country rocks indicate that only minor to moder-ate amounts of low-temperature fluids affected these inter-mediary 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,

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 permeabil-ity in the mixed volcanic and sedimentary rock fill of the graben.

Geochemical investigations have delineated a foot-wall 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 re-gional alteration and fluid evolution patterns in the Lyon Lake area, and outlined an extensive region character-

Fig. 17. Mine section 11 500E through the A-zone of the Lyon Lake ore deposit. The detailed section is rotated 70 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.


to the horizontal axis to return stratigraphy to its original, near-horizontal orientation. Linear structures form sub-parallel faults that repeat throughout the Lyon Lake area in an orthogonal pattern. Fault scarps, grabens, and half-grabens caused by extension and collapse of the volcanic rocks are common. Both sets of orthogonal structures frac-tured footwall rocks deep enough to access the underlying hydrothermal reservoir, and formed conduits for focused discharge of hydrothermal fluids at the paleo-seafloor

1976; Morton et al., 1985, 1990; Groves et al., 1988; Walk-er, 1993). In these districts, shallower faulting would have been sufficient to tap into the reservoir and focus hydro-thermal discharge.

An interpretation of the structural setting for the fault-ing, 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

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-carbonate-quartz-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.


fide deposition occurred along synvolcanic rift and graben faults and fissures that created permeable fracture zones deep enough to access the underlying hydrothermal reser-voir. 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 associ-ated with the failed rift of the Japanese island-arc, and with

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., Stur-geon Lake structure, Fig. 14). These unusual structural and stratigraphic relationships are the result of late regional de-formation.

ConclusionsandImplicationsforExploration

Within the Sturgeon Lake caldera, the most volumin-ous and persistent hydrothermal venting and massive sul-

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 por-phyroblasts 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.


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 frac-ture styles vary also according to the amount and method of tectonic movement including: (1) extensional rupturing, collapse, shearing, and faulting perpendicular to the prin-cipal 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). Permeabil-ity was particularly enhanced at the intersections of syn-volcanic faults, where repeated orthogonal movements resulted in significant rock brecciation and the formation of planar breccia zones. In texturally uniform footwall

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 Watkin-son, 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

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.


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 sur-face information is available and outcrop exposure is lim-ited 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 discord-ant, features remain unrecognized. Oblique, subconcordant structures are common, and variations in the strike of syn-volcanic structures and associated hydrothermal alteration zones occur because of the interrelation of regional strati-graphic 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 Franklins (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 half- graben structures are excellent sites for the accumula-tion 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 forma-tion of permeable faults and fracture networks.

A variety of structure and fracture patterns created per-meable discharge zones for hydrothermal fluids at Stur-geon 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 hydrother-mal minerals. Excellent exploration targets occur where synvolcanic structures with high-temperature hydrother-mal alteration intersect paleo-seafloor horizons.

Acknowledgments

This research was made possible by Noranda Inc., Mattabi Mines Limited, and Noranda Exploration Com-pany 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

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 com-petency changes (e.g., Lyon Lake graben), there was an additional stratigraphic control over fluid migration. Varia-tions 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 permeabil-ity and competency interfered with the direct discharge of hydrothermal fluids, and inhibited seawater infiltration into the footwall zone. Consequently, the Lyon Lake de-posits 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 com-monly dismissed as late, post-ore features unrelated to min-eralization because they displace ore and host rocks. How-ever, 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 high-er stratigraphic horizons, whereas reactivation of others merely offsets stratigraphy. Some of these structures form conduits for magmatic intrusions, and these intrusive bod-ies may obliterate the evidence of structure and previous hydrothermal activity.

Investigations at Sturgeon Lake revealed several criteria for distinguishing synvolcanic from late structures. Syn-volcanic 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 dom-ing 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 lower-most of a series of units will drape over or fill structures, whereas overlying units conform to the regional paleo-top-ography, and tend toward paleo-horizontal layering. Foot-wall 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 un-deformed geological marker unit can be used.

The strike of structural conduits at the current erosional surface can vary from perpendicular to stratigraphy, to ap-parently stratabound but crosscutting in the third dimen-


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Franklin, J.M., Kasarda, J., and Poulsen, K.H., 1975, Petrology and chemistry of the alteration zone of the Mattabi massive sulfide deposit: Economic Geology, v. 70, p. 6379.

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Franklin, J.M., Sangster, D.M., and Lydon, J.W., 1981, Volcanic associated massive sulfide deposits: Economic Geology, 75th Anniversary Volume, p. 485627.

Franklin, J.M., Gibson, H.L., Jonasson, I.R., and Galley, A.G., 2005, Volcanogenic massive sulfide deposits: Economic Geology, 100th Anniversary Volume, p. 523560.

Friske, P.W.B., 1983, Wall-rock alteration and ore gen-esis at the Lyon Lake deposits, Northwestern Ontario: Unpublished Ph.D. thesis, Halifax, University of New Brunswick.

Gibson, H.L., and Watkinson, D.H., 1990, Volcanic mas-sive sulphide deposits of the Noranda Cauldron and shield volcano, Quebec, in Rive, M., Verpaelst, P., Ga-gon, Y., Lulin, J.M., Riverin, G., and Simard, A., eds., The northwestern Quebec polymetallic belt: CIM, Spe-cial Volume 43, p. 119133.

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Groves, D.A., Morton, R.L., and Franklin, J.M., 1988, Phys-ical volcanology of the footwall rocks near the Mattabi massive sulphide deposit, Sturgeon Lake, Ontario: Can-adian Journal of Earth Sciences, v. 25, p. 280291.

Guber, A.L., and Green, G.R., 1983, Aspect of the sedi-mentologic and structural development of the Eastern

have benefited from discussions with company geolo-gists, 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 Stur-geon Lake caldera. We thank the reviewers for excellent suggestions improving this manuscript. This research was conducted with the support of NSERC.

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