- © 2007 Canadian Institute of Mining, Metallurgy and Petroleum
The Contact Lake Belt forms the NW-trending flank of a collapsed andesite stratovolcano complex adjacent to a subvolcanic intrusion within the northern, 1.87 to 1.85 Ga, Great Bear magmatic zone, Northwest Territories, Canada. It belongs to the Port Radium-Echo Bay historical district that hosts past producers of U, Ag, Cu (± Ra, Ni, Co, Bi) from polymetallic sulfide and arsenide veins. A re-examination of the belt has revealed widespread IOCG-type polymetallic mineralization exposed in numerous veins, stockworks, breccias, and replacement zones within extensive areas of polyphase hydrothermal alteration. The effects are most visible and intense in andesite, but also affect associated synvolcanic intrusions. A weak, pervasive chlorite-epidote-carbonate-sericite alteration is observed in the least-altered volcanic rocks. Subsequent hydrothermal alteration that is progressively superimposed on earlier facies includes: sericitic (sericite, quartz); phyllic (sericite, quartz, pyrite); potassic (K feldspar); earthy and specular hematite; K feldspar-scapolite-albite-magnetite-actinolite-apatite as veins, stockwork, and pegmatitic recrystallization; K feldspar-tourmaline-Fe-oxide-silica-sulfides; and massive albitites locally laced with amphibole. Hydrothermal breccias and diatremes occur locally throughout the belt. The style, size, overprinting relationship, mineralogy, and chemical composition of alteration zones and mineralization support the current IOCG exploration model for polymetallic mineralization in the Contact Lake Belt, as well as for mineralization elsewhere in the Port Radium-Echo Bay district and the Great Bear magmatic zone overall.
- Contact Lake-Echo Bay mining district
- Great Bear magmatic zone
- polyphase hydrothermal alteration
- polymetallic mineralization
The Contact Lake Belt refers to the northwest-trending zone of volcanic and intrusive rocks in the Contact Lake area of the Great Bear magmatic zone (GBMZ), south of Echo Bay and east of Great Bear Lake in the District of Mackenzie, Northwest Territories (Figs. 1⇓, 2⇓). This belt is located about 10 km southeast of the Port Radium-Echo Bay mining district, one of the historically (pre-1960) important sources of uranium in Canada, and a significant producer of silver plus lesser copper, nickel, cobalt, bismuth, radium, lead, and polonium between 1931 and 1982. The Contact Lake Belt includes the Contact Lake mine, a minor producer of Ag and U between 1934 and 1979. Fifty kilometers to the south is the Camsell River district that includes the former Terra and Norex Ag-Cu-polymetallic mines (Fig. 1⇓; Hildebrand, 1986; Gandhi et al., 1996). At the southern extremity of the GBMZ are the two best-known Canadian iron oxide copper-gold (IOCG) deposits, the NICO Co-Au-Bi and the Sue-Dianne Cu-Ag-Au deposits (Goad et al., 2000a,b; Fig. 1⇓). In contrast, the northern GBMZ is known for its past-producing vein-type U-Ag-Co-Cu mines, spectacular but weakly mineralized pyrite gossans, and albite and magnetite-amphibole-apatite alteration zones (Robinson and Badham, 1974; Hildebrand, 1986; Reardon, 1992a,b).
The Port Radium-Echo Bay district comprises folded volcanic and sedimentary rocks of the LaBine Group of the McTavish Supergroup, plutons of the coeval, prefolding, ca. 1.87 Ga, Mystery Island intrusive suite, and also postfolding 1.86 to 1.84 Ga magmatism (Figs. 2⇑, 3⇓; Hildebrand, 1983; Bowring, 1984; Bowring et al., 1984). The Contact Lake Belt is underlain primarily by andesitic volcanic rocks of the Echo Bay Formation of the LaBine Group, and roughly concordant, sheet-like Mystery Island intermediate to felsic plutons (Fig. 3⇓). The southern boundary is roughly coincident with the intrusive contact of the Contact Lake Belt supracrustal rocks and the younger, larger, and more alkalic plutons that predominate in the GBMZ (formerly Great Bear Batholith).
The Great Bear magmatic zone constitutes a 1.88 to 1.84 Ga Andean-type calc-alkaline volcano-plutonic arc complex, developed over the western half of the Wopmay orogen at the margin of the Archean Slave craton (Fig. 1⇑; Hildebrand et al., 1987; Gandhi et al., 2001). The GBMZ was not recognized to be prospective for IOCG styles of mineralization until long after closure of the last producing mine in 1982 (Hildebrand, 1986; Gandhi and Bell, 1993). Goad et al. (2000a, b) described the geology of the NICO Co-Bi-Au deposit discovered in the mid-1990s, and demonstrated that it is hosted in iron- and potassium-altered, brecciated basement sedimentary rocks at and beneath a volcanic unconformity, showing similarities with the Salobo IOCG deposit in Brazil (Requia et al., 2003). Camier (2002) reported on the geology and origin of the Sue-Dianne Cu-Ag-Au iron oxide-rich breccia complex, and demonstrated its many similarities to the Olympic Dam breccias. These and other exploration and geoscientific studies resulted in recognition that the Great Bear magmatic zone is host to several variants of the IOCG class of mineralization (Gandhi et al., 1996; Goad et al., 2000a,b; Camier, 2002; Gandhi, 2004; Mumin, 2006).
Economic polymetallic IOCG deposits include diagnostic alteration zones with regional-scale sodic-calcic (e.g., Kiruna-type albitization, scapolite, and amphibole-magnetite-apatite assemblages) or potassic alteration overprinted by focused iron-rich alteration. This commonly includes magnetite ± biotite and a main polymetallic ore zone associated with K-Fe enrichment in which either sericite or K feldspar prevail as the potassic phase. Striking alteration zones are known to occur in the northern and central parts of the GBMZ, but remain under-explored (Hildebrand, 1986; Reardon, 1992a, 1992b; Webb, 2001). Although the exploration potential is high, geological knowledge has not yet been updated in the light of new information about IOCG deposits and their complex settings (Hitzman et al., 1992; Kerrich et al., 2000; Porter, 2002; Sillitoe, 2003; Barton and Johnson, 2004; Williams et al., 2005). Furthermore, the IOCG model has only recently been applied to Canadian Precambrian terranes (Goad et al., 2000b; Mumin and Perrin, 2005). Consequently, many key regional and local indicators of IOCG deposits (Nisbet et al., 2000; Mumin and Perrin, 2005) were not systematically recognized during early regional work of the northern and central GBMZ, as pointed out for the K2 showing of the Contact Lake Belt by Webb (2001).
Iron oxide copper-gold deposits are economically and strategically attractive exploration targets for Canada (Goad et al., 2000b; Mumin, 2002; Corriveau, 2007). They tend to be polymetallic in nature (Cu, Au, Ag, U, Co, Bi, rare earth elements, etc.), have very large size potential, and are located in geological terranes that are non-classical exploration targets in the Canadian Shield. Hence, even highly prospective IOCG settings such as the Great Bear magmatic zone in the Northwest Territories remain under explored (Fig. 1⇑).
The most important IOCG deposit worldwide is Olympic Dam in South Australia (Roberts and Hudson, 1983; Oreskes and Einaudi, 1992; Skirrow et al., 2002). With quoted total resources of 3810 Mt at 1.1% Cu, 0.4 kg/t U3O8, and 0.5 g/t Au, the Olympic Dam deposit comprises the world’s largest individual uranium resource (1.4 Mt) and the fourth largest copper and gold resources with 42.7 Mt and 55.1 Moz, respectively (Western Mining Corporation, press release, 2004). This iron oxide breccia-hosted deposit formed at shallow levels in a granitic intrusion, which forms part of an A- to I-type calc-alkaline plutonic suite within a mature orogen at the margin of an Archean craton (Reeve et al., 1990; Ferris et al., 2002; Giles et al., 2002). Although Olympic Dam is the best known example of an IOCG-type system, there is a wide range of seemingly disparate deposits that form a number of subgroups within the IOCG family. This has led to the origin of several competing models (Barton and Johnson, 2004; Haynes, 2000; Pollard, 2000; Hitzman and Valenta, 2005; Williams et al., 2005), but they all have in common some form of igneous-hydrothermal origin, and widespread and abundant alkali-iron (alkali feldspar-iron oxide) metasomatism.
The similarities between the ages of host rocks, style of mineralization/alteration, and composition of known mineralization in the Contact Lake area with those of the southern GBMZ and other IOCG districts worldwide warrant re-appraisal of the Port Radium-Echo Bay district for its IOCG potential. This paper discusses the geological and geochemical results of early exploration-stage work and government geoscience initiatives. Current work consists of satellite and air photo image acquisition and interpretation, a helicopter-flown magnetometer-VTEM survey, reconnaissance and detailed geological mapping (lithologies, type and extent of hydrothermal alteration zones, structure, and mineralization), resampling of historic showings, and ground magnetometer, spectrometer, and scintillometer surveys. With its extensive exposures of intensely altered and mineralized rocks, the Contact Lake Belt provides classic and well exposed examples of polymetallic IOCG alteration and mineralization superimposed on host rocks initially devoid of significant amounts of iron oxides.
Regional Setting of the Great Bear Magmatic Zone
The Great Bear magmatic zone refers to the large (100 × 450 km exposed extent) volcano-plutonic complex formed during eastward subduction-related, Early Proterozoic arc magmatism (ca. 1.88–1.84 Ga) that dominates the western portion of the Early Proterozoic north-trending Wopmay orogen (Fig. 1⇑; Hildebrand et al., 1987). Other main tectonic elements include the ca. 2.10 to 1.90 Ga arc-related Hottah Terrane to the west, and the pre-Great Bear Calderian accretionary wedge to the east of the GBMZ. The Calderian accretionary wedge consists of lowermost volcanic rocks overlain by continental margin clastic and carbonate sedimentary sequences. A compositionally diverse suite of peraluminous to metaluminous granitoid plutons (Hepburn intrusive suite) intruded this sequence during compression and regional shortening between 1.90 and 1.88 Ga. Deformation culminated in folding, imbrication, and eastward thrusting of the accretionary wedge (Calderian supracrustal rocks plus Hepburn intrusions) onto the Slave craton, forming the internal metamorphic zone of the Wopmay orogen (Fig. 1⇑; Hildebrand et al., 1987).
The GBMZ consists of mainly calc-alkaline felsic to mafic volcanic and associated sedimentary rocks, intruded by voluminous calc-alkaline to alkaline plutons that occupy the contact zone between the Hottah Terrane and the Calderian accretionary wedge (Gandhi et al., 2001). The substrate to this 3 to 4 km-thick magmatic zone is interpreted to be the Hottah Terrane and its amphibolite facies sedimentary and intermediate volcanic rocks (Gandhi et al., 2001; Fernández et al., 2005), although alternative interpretations exist (e.g., Cook et al., 1999; Wu et al., 2005). Among the supracrustal rocks preserved within the GBMZ are ca. 1.88 Ga Treasure Lake sedimentary rocks interpreted to overlie Hottah Terrane metamorphic rocks (e.g., DeVries Lake and NICO deposit areas in Fig. 1⇑; Gandhi and van Breemen, 2005). The GBMZ supracrustal rocks and early plutons were folded during northeast–southwest compression between ca. 1.86 and 1.85 Ga. Great Bear magmatism resumed with late syn to postfolding emplacement of alkaline syenogranitic batholiths that ended by 1.84 Ga. The GBMZ is cut by an extensive system of northeast-trending fault zones formed during regional transcurrent (strike-slip) faulting between ca. 1.84 and 1.81 Ga (Hildebrand et al., 1987), as well as by diverse swarms of diabase dikes.
The GBMZ is buried to the north under gently north-dipping sedimentary sequences of the unconformable Proterozoic Coppermine homocline (Hoffman and Hall, 1993). Geophysical data and deep drilling indicate that the GBMZ extends to the south and west under unconformable Paleozoic sedimentary cover of the Mackenzie platform (Hildebrand et al., 1987; Goad et al., 2000a,b; Cook and Erdmer, 2005).
Supracrustal Stratigraphy of the Great Bear Magmatic Zone
Hoffman and McGlynn (1977) and Hoffman (1978) established the regional stratigraphy of the Great Bear Lake region, and pointed out similarities between the Great Bear Batholith and arc-related plutons in the Andes. Comprehensive 1:50 000-scale mapping in the eastern Great Bear Lake region, including the Port Radium-Echo Bay district (Hildebrand, 1981, 1983) and the Camsell River district (Hildebrand, 1984, 1985), refined stratigraphic relationships in the region, especially within the supracrustal rocks hosting the mineralization.
Volcanic and sedimentary units occur as (semi-) isolated bodies within the granitoid batholiths that dominate the GBMZ. The supracrustal sequences are generally interpreted as roof pendants to the larger and in part coeval intrusions (Hildebrand, 1984). The discontinuous nature of the supracrustal exposures hinders regional stratigraphic correlations. As a result, the stratigraphic terminology for the magmatic zone is often specific to geographic regions of supracrustal exposures.
Supracrustal rocks in the GBMZ that are younger than and unconformably deposited on those of the Hottah Terrane and overlying Treasure Lake sediments are assigned to the McTavish Supergroup (Fig. 3⇑; Hoffman, 1978; Hildebrand, 1984). In the Great Bear Lake region, the supracrustal rocks have an estimated total thickness of over 10 km in three volcano-sedimentary sequences separated by unconformities. From approximately west to east across the zone, and from oldest to youngest, they are the LaBine, Sloan, and Dumas groups (Fig. 3⇑; Hoffman, 1978). Felsic volcanic rocks in the southern GBMZ that unconformably overlie the Treasure Lake Group are assigned to the Faber Group of the McTavish Supergroup (Goad et al., 2000a,b; Gandhi and van Breemen, 2005). The Faber Group consists of an approximately 5 km-thick sequence of predominantly felsic ignimbrites that, based on Nd isotope data, originated from crustal material (Johnson and Hattori, 1994). The Faber Group is lithologically similar to felsic ignimbrites of the Sloan Group, occupies a similar central position within the GBMZ, and is interpreted to be broadly correlative with the Sloan Group.
Cannuli (1989) discussed the stratigraphy proposed by Hoffman (1978), and suggested the possibility that the Dumas and LaBine groups are age-equivalent sequences on opposite sides of the GBMZ. The Sloan Group locally overlies LaBine Group units. From these observations, the entire GBMZ volcano-plutonic complex may be viewed as a large synform with Sloan-Faber Group felsic volcanic rocks occupying the core of a regional-scale syncline, and the LaBine and Dumas groups occupying equivalent positions on the west and east limbs, respectively.
Supracrustal rocks in the Port Radium-Echo Bay district are assigned to the LaBine Group (Hoffman, 1978; Hildebrand, 1981, 1983). Only formations of the LaBine Group will be described in the following section.
Geological Setting of the Echo Bay-Contact Lake Area
The Port Radium Formation consists of a lower sequence of siltstone, argillite, and cherty metasedimentary rocks with minor carbonate horizons, and is overlain to the northeast by a thick sequence of andesitic volcanic rocks of the Echo Bay Formation (Fig. 3⇑). The lower part of this sequence (Mile Lake Member) consists of fine-grained to microporphyritic andesitic and trachytic flows intercalated with siltstone, greywacke, and tuffaceous beds. The upper part of the sequence (Surprise Lake Member) consists of massive porphyritic trachyandesitic flows. The Port Radium and Echo Bay formations represent a single caldera fill sequence (Hildebrand et al., 1987). To the north and northeast, the Echo Bay Formation is overlain by the Cameron Bay Formation, a succession of rhyodacitic ash-flow and ash-fall tuff intercalated with coarse clastic sedimentary rocks, conglomerate, and andesitic lava. The basal member of the Cameron Bay Formation is a crumbly-weathering, ferruginous conglomerate that is overlain by poorly cemented sediments. These features suggest that there was a short time span between the deposition of the Echo Bay Formation, uplift and erosion, deposition of the Cameron Bay Formation, and intrusion of granitoids.
A number of intrusive rocks have been emplaced into the volcanic succession (Fig. 4⇓) including: hornblende-plagioclase porphyry (or flow?) of the Cobalt porphyry; monzodiorite to quartz monzonite of the Mystery Island intrusive suite; biotite-hornblende granite/quartz monzonite and diabase of the younger phase of the GBMZ; and diabase dikes. The Cobalt porphyry is exposed in the Port Radium area, where it was emplaced as subvolcanic dikes and sills into the Echo Bay Formation. The Cobalt porphyry is commonly associated with strongly iron-metasomatized wall rocks (magnetite-bearing) and polymetallic sulfide veins. The Mystery Island intrusive suite of monzodiorite and quartz monzonite was intruded into the volcanic successions both as subconcordant subvolcanic sills and laccoliths, and clearly discordant dikes and stocks. When approximately 5 km of postemplacement movement along the NE-trending Cameron Bay Fault is taken into account, they occur as three discrete plutons within the Port Radium (Bertrand Lake pluton), Echo Bay (Tut, Glacier Lake and Contact Lake plutons), and Cameron Bay formations (Fig. 2⇑).
The plutons are flanked by a metasomatic aureole in the wall rock that appears most intense within about 1 to 2 km of the upper contacts (Fig. 4⇑). The alteration halo shows a general outward zonation from albite, to magnetite-actinolite-apatite, to hematite, to potassic, to phyllic ± chalcopyrite ± siliceous gossans, and finally to propylitic assemblages. In most areas, early alteration has been obliterated and overprinted by younger alteration assemblages. Additional alteration zones occur locally and are described in detail below. Pebbles of quartz monzonite are found within conglomerates of the Cameron Bay Formation, and a related monzonitic plug cuts the sequence to the east of Cameron Bay. Such relationships suggest that magmatism related to the Mystery Lake intrusive complex may have been contemporaneous with volcanism related to the Cameron Bay Formation. Syenogranitic intrusions of the Great Bear magmatic zone batholith flank the supracrustal units of the Echo Bay region to the southwest, the northeast, and the northwest. Intrusive contacts are often well defined and discordant, in sharp contrast to those of the monzonitic Mystery Island suite intrusions.
The Hogarth and Gilleran plutons, located to the northwest and northeast of the Contact Lake belt respectively, are predominantly biotite-hornblende quartz monzonite. These plutons are generally fine grained, and have wide contact metamorphic aureoles. The upper granodiorite and aplite margin of the Hogarth pluton has shed boulders into the overlying volcanic pile of the Feniak Formation, suggesting this magmatism was coeval with volcanism (Hoffman and McGlynn, 1977).
Emplacement of diabase dikes and sills is the latest intrusive event in the area. Mapping indicates that both series of dikes and sills cut rocks of the supracrustal succession, as well as the Mystery Island intrusive suite and the younger granitic batholiths (Hildebrand, 1981, 1983; Hildebrand et al., 1987). The dikes commonly follow the major structural trends (Fig. 4⇑). Field relationships suggest that the EW- and NW-trending dikes may be older than the NE-trending dikes. The dikes collectively appear to be earlier than some of the faults but later than others, and have been reported as both pre- and post-mineralization in age (Mursky, 1973). A series of NE-trending dikes of fine- to medium-grained leucocratic monzonite and quartz monzonite cut the Cameron Bay Formation. It is uncertain if these are related to the Mystery Island intrusive suite.
Structure of the Echo Bay-Contact Lake Area
The volcanic stratigraphy in the Echo Bay-Contact Lake area defines a broad syncline between Echo Bay and Lindsley Bay (Fig. 2⇑). The sequence has been folded about a gently plunging, NW-trending axis. The Contact Lake area is centered on one limb of the homocline, with locally more obvious folding in the Cameron Bay area to the north. Some investigators (e.g., Hildebrand, 1981; Hildebrand et al., 1987; Mumin et al., 2005) attribute at least some of this folding to collapse of the Echo Bay stratovolcano complex. The volcanic center appears on air photographs and satellite images as a prominent circular structure ∼5 km in diameter, which occupies the east central part of the district. Within the Echo Bay-Contact Lake area, widespread shear and fault zones are recognized as prominent lineaments marked by elongate depressions with narrow bays, lakes, or rivers, and nearly vertical cliffs. The Southwest and southeast arms of Echo Bay (Fig. 2⇑) appear to be expressions of such structures.
All of the volcanic and plutonic rocks of the GBMZ have been cut by a series of near-vertical, N- to NE-striking, late dextral faults. These structures are the most widespread features, and are considered to be related to a regional compressional event and the Wopmay fault zone ∼75 km to the east of the study area (Fig. 1⇑). Five prominent, subparallel fracture systems of this type are up to 25 km long, and are spaced ∼5 km apart throughout the area. One of these systems is referred to as the Cameron Bay Fault, which passes through Echo Bay, Cameron Bay, and the southeast end of Mackenzie Island. The fault displays an apparent dextral displacement recognized as over 5 km of offset of various sills and laccoliths of the Mystery Island intrusive suite, and their associated gossan zones. Some earlier dip-slip faults that developed during caldera formation and collapse also have traces that are parallel to the younger northeasterly faults. These synvolcanic structures have less displacement and shorter strike lengths, and are truncated by the flanking batholiths. Apparent displacements on these faults are both dextral and sinistral.
The Contact Lake Belt is characterized by a strong northwest- to southeast-trending series of structural lineaments along the southern flank of the stratovolcano center. merge with S-trending structures near its southeast margin, and are segmented by orthogonal northeast structures sub-parallel to those mentioned above. Similarly, north–south lineaments are cut by east–west faults, which are particularly noticeable near the center of the belt, and also accommodate the former Contact Lake mine. These two principal orthogonal structures and a myriad of less prominent fractures and faults of all orientations, are preferential sites for hydrothermal veining and mineralization throughout the belt as veins, stockworks, and breccias.
Geology of the Contact Lake Belt
The Contact Lake Belt is underlain almost entirely by andesitic volcanic rocks and associated monzonite to monzodiorite intrusions (Fig. 4⇑). Fine-grained andesite varies from massive tuff to plagioclase porphyritic and amygdaloidal tuffs, lavas, and breccia. Amygdules are filled with quartz ± chlorite, carbonate, epidote, and hematite or pyrite, depending on the alteration affecting the immediate host rock (Fig. 5a⇓). Rounded and irregular blocks of amygdaloidal andesite in homogenous fine-grained tuffs are a common feature. Exposures of collapse and pyroclastic debris flow deposits are located in the north-central part of the belt along the shoreline of Echo Bay, and immediately southeast of the southeastern tip of Echo Bay.
The large Contact Lake pluton is a sill-like body of fine- to medium-grained monzonite/monzodiorite. It has a hypidiomorphic granular texture, is quite homogeneous, and is comprised of plagioclase, K feldspar, and 10% to 20% amphibole and pyroxene. Accessory minerals include quartz, chlorite, epidote, and carbonate.
Diorite and diabase dikes have intruded along SE- and E-trending structural zones. These dikes are of limited width and length, although larger gabbroic dikes are exposed in the southwest corner of the belt. The dikes are moderately magnetic and generally barren, and display only minimal to moderate hydrothermal alteration effects. However, they locally host vein-type mineralization such as the high-grade Thompson polymetallic showing. Some diorite dikes appear to be syn-volcanic, whereas gabbroic dikes that have intruded the syenogranite are clearly substantially younger than its immediate host. The syenogranite may be genetically associated with the volcanic rocks.
Granitic and syenogranitic batholiths of the Great Bear plutonic suite define the southern, southeastern, and south-western boundaries of the Contact Lake Belt, and these batholiths are presumed to underlie at least part of the belt (Hildebrand, 1983). An early phase of albite and magnetite-actinolite-apatite alteration has a demonstrated special and probable genetic association with the Mystery Island suite of monzodioritic intrusions (Reardon, 1992b), whereas late-stage polymetallic sulfide and arsenide mineralization may be genetically linked to the granitic intrusions. Also, the alkaline nature and close spatial association of these granitic intrusions with altered and mineralized rocks in the belt suggest a possible genetic relationship to various types of hydrothermal alteration and metal enrichment.
Arkose and intercalated conglomerate outcrop as a small They body south of the southeastern tip of Echo Bay, and form a zone that straddles both sides of the east arm of Echo Bay about 2 to 4 km northwest of the southeast termination. These polymictic conglomerates contain abundant clasts of andesite, as well as monzodiorite, granite, and mafic rocks. Variably altered andesite clasts are also common in the conglomerate. Fragments of altered vein material, including quartz and jasper, commonly occur. Some conglomerate beds are strongly and pervasively hematite altered. Further north, on Bay Island, conglomerate and arkose beds are intercalated with very fine-grained, less than 1 m-thick, cream-colored to iron-stained ash beds. This sequence does not display the same degree of hematization, nor the abundance of altered clasts common in the sedimentary rocks along the southern shore. Local hematization of some conglomerate and arkose beds, and their intercalation with volcanic rocks may suggest a synvolcanic formation of at least some of the sedimentary rocks.
Extensive polyphase hydrothermal alteration affects all rocks of the belt (Figs. 4⇑, 6⇓), although the effects are most visible and intense in andesite. Alteration styles include large zones of pervasive hydrothermal alteration, hydrothermal veining, and hydrothermal stockwork, breccia, and diatreme. The different types of pervasive alteration are characterized by their dominant hydrothermal mineral assemblages including: propylitic (chlorite-epidote-carbonate ± albite ± sericite); phyllic (quartz-sericite-pyrite; Fig. 5b⇑); potassic (proximal K feldspar or distal sericite; Fig. 5c⇑); hematite (Fig. 5d⇑); actinolite; magnetite (magnetite-actinolite-scapolite-alkali feldspar ± apatite; Fig. 5e–f⇑); sodic (albite; Fig. 7a⇓); silicification (quartz ± pyrite); and sulfide (pyrite ± chalcopyrite ± bornite ± arsenopyrite ± sulfarsenide minerals; Fig. 7b–d⇓).
Two of the large pyrite-dominated sulfide-rich alteration halos are located at Gossan Island and K2 at the northwest end of the Contact Lake Belt (Figs. 2⇑, 7b–e⇑). Although dominated by pyrite, both contain varying amounts of Cu. The pervasive zones are generally kilometer-scale features that crosscut geological boundaries (Fig. 4⇑). In particular, propylitic, potassic, and albitic zones straddle the andesite-monzodiorite contact at several locations, but effects are markedly more intense in andesite. Sharp contact relationships with albite-altered volcanic rocks are observed at two localities. At the southeastern end of the belt, the monzodiorite contains sharply bounded xenoliths of albitite. To the east of the Contact Lake Mine and at various other locations, the monzodiorite is crosscut by amphibole ± magnetite and potassic veins, indicating that alteration is cogenetic with, or overprints, the monzodiorite (Fig. 4⇑). Magnetite, hematite, actinolite, silicification, and sulfide zones are best expressed in andesite north of, and to a much lesser degree south of, the monzodiorite contact.
Pervasive zones are generally superimposed on each other in order of increasing intensity of alteration. The alteration, in most cases, is ubiquitous throughout the zone, but in some places potassic and sodic alterations are highly gradational from incipient to strong. Least-altered rocks have well-preserved volcanic textures, but are themselves altered to a propylitic assemblage. Other types of alteration are superimposed on propylitic rocks in a progressive sequence with considerable overlap in mineral assemblages (Fig. 6⇑). Some alteration enhances textures (Fig. 7f⇑), whereas others are texturally destructive (Fig. 5d,f⇑). Alteration progressively intensifies toward several distinct hydrothermal centers that are characterized by one or other of the following assemblages: massive albitite ± amphibole ± magnetite; Cu-Au-Co sulfarsenides in tourmaline + K feldspar + chlorite + quartz ± magnetite and hematite alteration; magnetite-actinolite-apatite zones; hematite + K feldspar ± Cu-Au-Co sulfarsenides; or diatreme and stock-work (Fig. 6⇑). Based on field relationships delineated, hydrothermal alteration mineral assemblages progress outwards from proximal albite, magnetite, to distal potassic, phyllic, and propylitic zones, as illustrated in Figures 4⇑ and 5⇑. Not all mineral assemblages are present at any one locality.
Hydrothermal vein infilling and common throughout the belt. Veins occur in all rock types, are widely distributed, most common in andesite (but also found in monzodiorite), and comprise mineral assemblages similar to those characterizing the pervasive alteration (quartz, hematite, K feldspar, magnetite, actinolite, and epidote). Monomineralic and multiphase veining are common. Alteration selvages can be homogeneous or zoned, and may reach a few tens of centimeters in width. Manganese oxide, tourmaline, carbonate, and jasper-bearing veins also occur and are locally abundant, but most important is the localized but widespread occurrence of polymetallic sulfide veins with varying amounts of Cu, Ag, Au, U, Bi, and Co.
Zones with strongest hydrothermal alteration include: massive albitite in the extreme southeast (Fig. 7a⇑); recrystallized feldspar-magnetite-actinolite-apatite-scapolite at the Mag Hill (Fig. 5e–f⇑); diatreme and the stockwork at the northwest of Mag Hill; and feldspar-tourmaline-quartz-chlorite-Cu-Au-Co-Ag sulfarsenide mineralization at K2 (Fig. 7b,c⇑). All major hydrothermal alteration types have been observed locally in association with potential economic sulfide mineralization.
Progression of polyphase pervasive alteration and discrete veining is well exposed from the shore of Echo Bay to the Gazelle (Gz in Fig. 4⇑) and Mag Hill showings, and highlights the spatial relationships of key alteration zones and mineralization (Fig. 4⇑). The shore exposure, northwest of Gazelle, is dominated by least-altered porphyritic andesite laden with faint patches of weakly magnetite-altered andesite. Magnetite content and intensity of the gray tone of the andesite in weathered surface vary across the outcrop, presenting evidence that the extent of alteration is highly variable and patchy. This alteration is non-destructive in terms of texture, and in fact enhances texture because the plagioclase phenocrysts are preferentially albitized and the matrix is preferentially replaced by actinolite and magnetite. The shape of phenocrysts is well preserved and outlines are sharp.
Elsewhere in the area, a similar process led to a dark gray magnetite-rich rock with strikingly white plagioclase phenocrysts (Fig. 7f⇑). The subsequent faint, but pervasive albite alteration consists of anastomosing, fine-grained, metasomatic veins, with sporadically distributed greenish patches of fine-grained amphibole up to a few to tens of centimeters in size. The porphyritic texture is partially to completely obliterated. A few discrete millimeter- to centimeter-wide veins of magnetite, amphibole, or pyrite crosscut this alteration. Those filled with amphibole are flanked by an incipient pink alkali feldspar alteration envelope a few centimeters in width.
Anastomosing pegmatitic rosette-textured metasomatic fronts form preferentially at the expense of the early albitization zones (Fig. 5f⇑) and extend into least-altered volcanic rocks, destroying textures, crosscutting early alteration veins, and forming a faint arborescent network of interlocking heterogranular alkali-feldspar crystals. This pervasive pegmatite-textured alteration consists of alkali feldspar grains with prismatic shapes, square cross sections, and ragged and fuzzy outlines. Feldspar crystal cores are albitic with hydrothermal K feldspar overgrowth. Distribution of crystals is random or forms well-defined rosettes (Fig. 5f⇑). Grains can reach tens of centimeters in length. Where this alteration is incipient, feldspar prisms are whitish to pinkish and very strongly poikilitic, and the observed interstitial magnetite and amphibole is fine grained. Boundaries of such alteration fronts are sharper than those of the early albite alteration, and characterized by feldspar crystals interwoven with their wall rock.
The extent and the intensity of this alteration vary significantly at the outcrop scale, and increase significantly toward the center of Mag Hill (Fig. 4⇑). Towards Mag Hill, the pink color of the alkali feldspar and the sharpness of the crystals increase, and interstitial amphibole and magnetite become coarse-grained (Fig. 5e⇑). Also, K feldspar rims form around magmatic feldspar grains indicating a transformation to potassic alteration. Apatite also becomes apparent megascopically, and is intergrown with magnetite and amphibole.
At central Mag Hill, the rosette-textured alteration prevails, but discrete, straight-walled alkali feldspar veining becomes common, leading to veins of a few millimeters to centimeters width and several meters long, with spectacular comb-textured selvages developed over widths up to tens of centimeters (Fig. 5f⇑). Mineral assemblage and textural relationships remain the same as the well-formed, rosette-textured alteration. The mineralogy of precursor andesite is totally obliterated.
Discrete monomineralic to polymineralic veining with apatite, magnetite, and/or amphibole is common in zones where the rosette-textured alteration is both incipient and pervasive. In the former case, feldspar alteration selvages occur, and early, silicified crackle breccias are locally preserved.
Toward Gazelle, the rosette-textured alteration is only sporadically developed and overprints a fine-grained, texture-preserving, bright pink, pervasive siliceous and potassic alteration. Late-stage, open space-filling carbonate veins with cockade textures are locally mineralized with chalcopyrite. At the Gazelle Cu showing, the outcrop consists of pervasively altered porphyritic andesite, bright pink in weathered and fresh surface, with medium-grained chalcopyrite disseminated in zones at least a few meters in length. Volcanic textures are largely preserved except where the andesite is replaced by rosette-textured alkali feldspar alteration along metasomatic fronts. Sodium dominates over potassium (Table 1⇓, sample CQA-05-226F, 226J). The outcrop displays no rusty zones, except over a few centimeter-wide clots of pyrite, and malachite staining is limited to a few cracks, easily missed during regional mapping or exploration. The cryptic nature of mineralization in this outcrop is indicative of many “hidden” IOCG-associated altered and mineralized zones occurring throughout the Contact Lake and Echo Bay districts, and indeed elsewhere in the GBMZ. This is in marked contrast to many spectacular but weakly mineralized gossans, such as Pyrite Snake and the Echo Bay gossan looming across the bay and to the southeast, which readily attract attention away from some of the important, but cryptic, zones of alteration and mineralization.
Hydrothermal diatremes and breccias are important diagnostic features of some IOCG-related hydrothermal systems. They may take many forms, ranging from structural breccias with a strong hydrothermal overprint to well developed clast- and matrix-supported polymictic diatremes. Several variants are exposed along the Contact Lake belt, including quartz-sulfide breccias with tourmaline matrix at K2 (Fig. 8a⇓), and hematite-cemented breccia with phyllic and altered andesite clasts at the Hematite breccia showing ∼300 m northwest of K1 (Fig. 5d⇑). Another unusual breccia is located at the Mile Lake area west of the southwest arm of Echo Bay (Fig. 2⇑), approximately 5.5 km west of Contact Lake. In this breccia, potassic-altered clasts have been cemented by skarn assemblages including clinopyroxene, garnet, vesuvianite, and remnant carbonate. It appears that brecciation and skarnification occurred after an early potassic alteration, but hydrothermal alteration continued after this stage as well. The breccia contains minor to significant amounts of bornite, chalcocite, and chalcopyrite as disseminated grains and clots. The origin of this breccia is presently not known.
Previous mining in the Port Radium-Echo Bay district, and in the geologically similar Camsell River district 40 km to the south, exploited high-grade U-Ag ± Cu mineralization that occurred locally as complex polymetallic arsenide and sulfide assemblages in relatively narrow quartz-carbonate veins (Mursky, 1973; Kissin 1993a,b; Fig. 8b,c⇑). The arsenide-sulfide veins are also enriched typically in one or more of the elements As, Ni, Co, Bi, Au, Zn, Pb, Sb, W, REE, and Ra, and many of these were episodically recovered, mostly as by-products of the Ag, U, and/or Cu mining. Economic veins are hosted mainly by roughly ENE-striking structures that exhibit evidence for multiple hydrothermal events during episodic ductile shearing to primarily brittle fracturing. Cumulative silver production from the Great Bear magmatic zone mining districts is greater than 50 Moz, of which 32 Moz are from the Port Radium-Echo Bay district (Normin, 2006). The Port Radium-Echo Bay district also reportedly yielded greater than 5100 t Cu and greater than 15 million lb U3O8. Copper production from the Camsell district is unknown.
Satellite images and air photo interpretation confirm the strong structural control of vein-related sulfarsenide mineralization and identify several structures that warrant further investigation. Vein type mineralization is common in the Great Bear magmatic zone where at least ten past-producing mines are situated. They include the Rayrock U mine at its southern extent, the Terra and Norex Ag mines in the Camsell River district, and the Eldorado, Echo Bay, Contact Lake, Bonanza, and El Bonanza U-Ag-Cu ± CoNi-Bi mines in the Echo Bay district. These deposits are structurally controlled, high-level, quartz or quartz-carbonate vein systems associated with varying degrees of iron oxide (hematite dominant) and alkali metasomatism. The presence of low-temperature mineral assemblages, vuggy nature, and open space-filling textures suggest an epithermal environment for formation of these deposits. They occur within regionally extensive IOCG mineralizing systems and are situated peripheral to the centers (source) of hydrothermal activity.
Volcanic rocks of the Echo Bay district were dated by Robinson and Morton (1972) at 1770 ± 30 Ma (Rb-Sr). However, a crosscutting granite body gave a U-Pb date of 1820 ± 30 Ma (Jory, 1964). Clearly the volcanic rocks must be older than the granite. It has been suggested that formation of diabase sills and vein mineralization occurred between 1500 and 1400 Ma based on K-Ar dates for a diabase sill at Port Radium (1400 ± 75 Ma; Thorpe, 1971; Steiger and Jäger, 1978), K-Ar dates for actinolite-magnetite veins in the Echo Bay mine (1420 ± 60 Ma; Thorpe, 1971), and U-Pb dates for pitchblende at Port Radium (1450 Ma by Jory, 1964; 1500 ± 10 Ma to 1424 ± 29 Ma by Miller, 1982). However, these younger ages are suspect because they may have been reset by later metamorphic events.
More than forty different sulfide and arsenide minerals are reported from the Eldorado mine (Port Radium) alone (e.g., Mursky, 1973). Sulfide and arsenide minerals typically overprint quartz-hematite (± carbonate, chalcopyrite, pyrite, and pitchblende) veins (Fig. 8b,c⇑). In the Port Radium district, sulfide-arsenide-quartz-carbonate-hematite veins occur in northeast- and east-trending, steeply dipping fault and fracture zones. They are typically narrow veins averaging only 1.2 m wide (but locally up to ∼13 m) and form multiple subparallel veins within a ∼1.6 km-long and ∼600 m-wide northeast-trending structural corridor that extends eastward from the Eldorado mine (Fig. 2⇑). These veins occur within an extensive halo of alkali-iron -pyrite-chalcopyrite alteration that is similar to alteration throughout the district.
A second type of high-grade, but generally narrow (<2 m) and short (<100 m) vein, is found at several localities along the Contact Lake Belt. The best examples are the Contact Lake Mine, Thompson, Bornite Lake, South Contact, and Azurite showings. They are characterized by high-grade stringer to semi-massive chalcopyrite-bornite veins with quartz, carbonate, hematite ± arsenides, and are variably enriched in uranium, silver, cobalt, bismuth, zinc, and gold.
At Port Radium, the Eldorado U-Ag-Cu-Co sulfide and arsenide veins occur along brittle fracture zones in meta-sedimentary rocks constrained between granitoid intrusions (batholiths) and their overlying volcanic rocks. The veins are intimately associated with the margins and apices of feldspar porphyry intrusions, which are themselves spatially linked to granitoid bodies marginal to, and underlying, the Echo Bay supracrustal (volcanic and sedimentary) rocks.
Mineralization in the Contact Lake Belt
Many different sites examined along the ∼15 km × 5 km Contact Lake Belt during the present study returned anomalous to exceptionally high values of one or more of Cu, Au, Ag, U, Bi, Co, and Zn. These potentially economic metals are accompanied by enrichments in one or more of K, Na, Fe, S, Pb, As, Mn, Bi, F, P, and CO2, depending on style of mineralization and associated hydrothermal alteration. Most of the mineralization appears distal or peripheral to the cores of hydrothermal centers, in phyllic (quartz, sericite, pyrite), potassic, and/or hematite alteration zones, and in stratigraphically high-level epithermal-type vuggy quartz-carbonate-hematite veins (see below). However, some are deeper seated (without vuggy texture) and occur within or peripheral to magnetite-actinolite-apatite alteration. The most important mineralized sites are described below.
This is the site of the most intensive and extensive hydrothermal magnetite-actinolite-feldspar-apatite ± sulfide alteration along the Contact Lake Belt (Fig. 4⇑). Host rocks are alkali- (sodic or potassic) and/or actinolite ± epidote-altered andesites. The core zone comprises a pervasive pegmatite-textured alkali feldspar-scapolite-magnetite-actinolite-apatite hydrothermal assemblage (Fig. 5f⇑). Distal alteration includes magnetite-scapolite-actinolite-apatite±sulfides, actinolite, and alkali feldspar ± epidote assemblages. Predominantly pyritic mineralization is present intermittently throughout the Mag Hill region, with only traces of malachite locally visible. In spite of the barren appearance of mineralization, several samples from different areas returned significant assays of up to 0.45 wt.% Cu and 21.7 g/t Ag (Table 1⇑).
K2 is a prominent Cu-Au-Ag-Co-enriched gossan with patchy intermittent to continuous sulfide mineralization dominated by pyrite (1–20 vol.%). The K2 mineralization is hosted within potassic-altered, minor hematite-bearing amygdaloidal, porphyritic, and tuffaceous andesite. Hydrothermal alteration is characterized by intense potassic alteration and abundant tourmaline and quartz-tourmaline veining, breccias, and crackle breccias (Figs. 7b,c⇑, 8a,d⇑). Where not overprinted by the potassic alteration, the andesites are altered to a propylitic assemblage with abundant chlorite, carbonate, epidote ± albite, and sericite. The K2 zone is one of the important hydrothermal centers along the Contact Lake Belt (Fig. 4⇑). Locally, magnetite-actinolite and magnetite-jasper veins are exposed in small patches. The auriferous economic sulfide assemblage includes pyrite, chalcopyrite, bornite, arsenopyrite, cobaltian arsenopyrite, and glaucodot (Fig. 7c⇑). High-grade grab samples from mineralized veins exposed near the northeast shore of K-2 Lake (west K2 zone) returned values up to 5.85 wt.% Cu, 2.7 g/t Au, 12.0 g/t Ag, and 0.71 wt.% Co (Table 1⇑). Chip samples across the face of the K2 gossan (Fig. 7b⇑) include 14 m at 0.78 wt.% Cu, 0.20 g/t Au, 2.31 g/t Ag, and 1647 ppm Co.
The J1 showing is located approximately 500 m southeast of K2 (Fig. 4⇑). It is a SE-trending zone of steeply dipping disseminations and veins of pyrite exposed over several meters width. Alteration includes a small zone of magnetite-actinolite ± apatite stockwork and en echelon veining with potassic selvages, which grades into a zone with quartz-tourmaline veining and potassic- and hematite-altered andesite. In the general area, brecciated jasper veins with specular hematite are common. A sample from the J1 zone with 15% to 20 vol.% pyrite and trace malachite returned 3.09 wt.% Cu, 1.80 g/t Au, 15.5 g/t Ag, and 308 ppm Co (Table 1⇑).
This showing is located approximately 3 km southeast of the K2 zone (Fig. 4⇑). It is a series of intermittent gossanous outcrops of limited extent, hosted in andesite with minor potassic ± hematite ± epidote alteration. Mineralization, as veins and breccias, comprises 1 to 20 vol.% sulfides (pyrite ± arsenopyrite ± chalcopyrite) and up to 20 vol.% hematite ± actinolite. Samples contain up to 0.53 wt.% Cu, 0.3 g/t Au, 51.7 g/t Ag, 161 ppm Co, and 646 ppm Bi (Table 1⇑). Northwest of K1, pervasive hematite-altered andesite grades into a zone of hematite-cemented breccia of unknown extent. The breccia comprises angular to sub-rounded fragments of variably altered andesite including phyllic clasts similar to the Echo Bay gossan, and have a matrix and fine veins of hematite (Fig. 5d⇑).
Echo Bay Gossan:
A large pyrite gossan is exposed in prominent cliffs immediately east-southeast of the southeast arm of Echo Bay (Fig. 4⇑). This gossan occurs in silicified and locally sericitized amygdaloidal andesite, and grades from potassic, to phyllic (Fig. 5b⇑), and siliceous alteration styles. The gossan is relatively barren of economic sulfides, although minor malachite appears locally in sheared zones with elevated pyrite content, and significant Ag (up to 118 g/t; Griep, 1996) has been reported in previous work.
Contact Lake Mine and East Trench:
The Contact Lake mine is located near the northeast-central shoreline of Contact Lake (Fig. 4⇑). It is comprised of several narrow high-grade veins within potassic-and/or albitic-altered monzodiorite of the Mystery Island intrusive suite. The Contact Lake mine has historical production of 625 035 oz Ag and 6933 lbs U3O8 from narrow quartz-carbonate-hematite veins that also contained high values of Cu, Zn, Ni, Co, and Bi (Normin, 2006). A single sample from the Contact Lake mine rubble containing quartz-hematite-ankerite-chalcopyrite-bornite ± pitchblende returned 3.07 wt.% Cu, 0.4 g/t Au, 950 g/t Ag, 0.29 wt.% Co, 0.82 wt.% U, and 3.42 wt.% Bi. Samples of rubble from the east trench returned up to 9.44 wt.% Cu and 31.3 g/t Ag (Table 1⇑).
This showing is the most spectacular of the high-grade veins previously prospected along the Contact Lake Belt (with the exception of the former Contact Lake Mine). It is located 5 km south-southwest of K2 (Fig. 4⇑). Thompson comprises a branching vein system hosted within and sub-parallel to a WNW-trending, approximately 25 m-wide, magnetite-bearing, fine-grained diabase dike. The Thompson vein comprises a series of veinlets, stringers, and semi-massive veins of chalcopyrite, bornite, and hematite along with quartz, carbonate, and pyrite (Fig. 7d⇑). Malachite is abundant and minor erythrite (Co3(AsO4)2·8H2O)is present in some samples. The vuggy nature and open space-filling textures, and mineral assemblages suggest a near-surface epithermal environment at the time of formation. The analyzed samples were consistently polymetallic with metal values up to 31.32 wt.% Cu, 6.2 g/ t Au, 493 g/t Ag, 0.63 wt.% Co, 0.37 wt.% U, 0.41 wt.% Bi, and 0.41 wt.% Zn in grab samples (Table 1⇑).
South Contact Showing:
The South Contact showing is exposed in a series of trenches near the north shore of Contact Lake, approximately 2.5 km southeast of the Contact Lake mine (Fig. 4⇑). The vein consists of quartz, Fe-Mn-carbonate, hematite, chalcopyrite, bornite, malachite, and minor erythrite and pitchblende. The vein occurs in potassic and sodic altered monzodiorite near its southern contact with syenogranite, and is associated with minor mafic rocks. Selected grab and chip samples taken from the vein returned up to 32.78 wt.% Cu, 0.20 g/t Au, 273 g/t Ag, 0.41 wt.% Co, 0.92 wt.% U, 0.31 wt.% Bi, and 0.20 wt.% Zn (Table 1⇑).
The Bornite Lake showing (Fig. 4⇑) is a series of up to three bifurcating veins of up to 1 m thick exposed over a ∼45 m length. The veins consist of chalcopyrite-bornite and accessory amounts of chalcocite, hematite, quartz, and carbonate with abundant malachite on surface exposures. The trench rubble samples yielded up to 2.29 wt.% Cu and 57.7 g/t Ag (Table 1⇑). Historical drilling returned values of 220 g/t Ag and 9.06 wt.% Cu over 0.73 m, and 100 g/t Ag and 6.42 wt.% Cu over 1.9 m (Fingler, 2005).
The Azurite vein system is located near the andesite/monzodiorite contact approximately 700 m south of the K1 showing, in an area of regionally pervasive hematite alteration (Fig. 4⇑). Copper-rich seams are predominantly quartz-carbonate-hematite veins with chalcopyrite and bornite. Surface weathering has produced abundant malachite staining along surface exposures and fractures, and some deep blue azurite. Two samples of narrow copper-rich seams from the showing yielded assays of up to 5.05 wt.% Cu, 0.30 g/t Au, and 11.5 g/t Ag (Table 1⇑).
Andesite Breccia (Avalanche):
An andesite debris flow (avalanche) and pyroclastic breccia deposit is well exposed in outcrop along the shore of Echo Bay approximately 2.5 km southeast of K2. The unit consists of angular fragments and blocks of porphyritic and amygdaloidal andesite up to 1 m in diameter, in an aphanitic andesitic matrix. The breccia contains approximately 5 vol.% of rounded, concentrically zoned clasts up to ∼40 cm in size that appear to be volcanic bombs (Fig. 8e⇑). The breccia is moderately hematite-altered, both pervasively and along bands. Quartz-hematite ± pyrite and chalcopyrite ± malachite occur in 100°- to 105°- trending veins. Two samples from a ∼10 cm-wide section of a quartz-hematite vein with pyrite and chalcopyrite yielded assays up to 1.47 wt.% Cu and 0.10 g/t Au (Table 1⇑).
Contact Lake Southeast:
A series of sub-parallel linear magnetic high anomalies extend southeast from the Mag Hill region for an additional 3 km towards the extreme southeast limit of the Contact Lake property. Regional potassic alteration grades into actinolite ± magnetite in the southeast. Along the extreme southeast boundary of the property, a significant zone of massive albite alteration is present (Fig. 7a⇑). Intermittent sulfides, predominantly pyrite with chalcopyrite in places, occur in patches as disseminations and shear-hosted veins and stringers.
The current investigation confirms the presence of geo-chemically anomalous to potentially economic concentrations of Cu, Au, Ag, U, Co, Bi, and Zn at more than seventeen localities along the Contact Lake Belt. Extensive hydrothermal alteration zones (potassic, sodic, phyllic, sericite, hematite, magnetite, actinolite ± apatite, tourmaline, quartz, chlorite-calcite-epidote, and sulfide) with or without associated surface gossan, exposures of breccia and brecciation including possible diatremes, shear zones, and stockworks have been identified. These alteration types are spatially and genetically associated with vein, breccia, and replacement styles of mineralization.
Some previous investigators classify polymetallic vein deposits at Port Radium as arsenide vein Ag and U (Ruzicka and Thorpe, 1995), and five-element veins (Kissin, 1993a,b). Detailed field mapping along the Contact Lake Belt shows that these veins are superimposed on larger, kilometer-scale hydrothermal alteration zones. This close spacial association, multiple alteration and mineralization centers, and available age dating (Reardon, 1992b), suggest a long history of episodic mineralization. Similar to early-stage magnetite-actinolite-apatite mineralization, late-stage polymetallic veins are also intimately associated with iron oxide and alkali metasomatism, both within and forming halos adjacent to the veins. This superimposed mineralization requires a genetic model that explains a large hydrothermal system(s), which evolved episodically over a long period of time, and affected an extensive area.
Geological mapping shows zoned hydrothermal alteration from several hydrothermal centers (Figs. 4⇑, 6⇑). Not all assemblages are present in any one region; however, the combined evidence shows a consistent outward progression of alteration zoning from subvolcanic intrusions as schematically illustrated in Figure 9⇓. There is a striking resemblance to porphyry copper type deposits; however, at Contact Lake, and for IOCG deposits in general, albite and iron oxide metasomatism is much more intense and pervasive. Core zones are proximal to the apex, margins, and periphery of subvolcanic stocks and are typified by albite and/or magnetite-rich assemblages. Intermediate zones are potassic (K feldspar) and/or hematite-rich, and more distal alteration consists of phyllic, sericitic, and/or propylitic assemblages.
Conventional interpretations suggest that the U-Ag-Cu-polymetallic sulfarsenide veins found throughout the district occurred long after host-rock deposition, whereas the alteration/mineralization in IOCG models is effectively synmagmatic, and typically associated with arc-related magmatism that characterizes the Great Bear magmatic zone. Their vuggy nature and low-temperature quartz-hematite-carbonate assemblages within extensive alkali-iron alteration zones suggest that the veins are peripheral, near surface, epithermal expressions of large IOCG systems, whereas their spatial distribution and setting show strong structural control. Veins of the former Eldorado and Echo Bay mines are situated in metasedimentary rocks, near the margins and apices of porphyry stocks, between underlying batholiths and overlying volcanic rocks (Fig. 8f⇑). The cores and intermediate zones of the hydrothermal systems that are responsible for these veins are possible loci for larger, more typical, bulk-mineable IOCG-type deposits. They all formed as a result of the volcano-plutonic event that led to episodic construction of the Echo Bay stratovolcano complex, with its associated episodic and multi-phase hydrothermal alteration and mineralizing events (cf. Fig. 9⇑). As a consequence, throughout the Echo Bay district, alteration zones are in direct spatial association with monzodioritic intrusions. This geological setting is similar to that of the NICO deposit in the southern Great Bear magmatic zone (Goad et al., 2000a,b; Camier, 2002). NICO ores occur in metasedimentary rocks situated between granitoid intrusions and their cogenetic overlying volcanic pile, and are marginal to bimodal feldspar porphyritic dikes and stocks that link the granitoids to their overlying volcanic rocks (Fig. 10⇓). In contrast, the Sue-Dianne (southern GBMZ) mineralization is more closely associated with the apex of a porphyry stock where it intrudes a large structure in rhyodacite ignimbrite (Fig. 10⇓).
There are many shared characteristics between porphyry copper and some felsic to andesitic volcanic-associated IOCG-type deposits, such as those found in the GBMZ. A schematic comparison of cordilleran-type porphyry copper systems with characteristics of the Echo Bay-Contact Lake district is illustrated in Figure 11⇓. The similarities are indicative of a continuum of deposit styles between porphyry copper and IOCG, and in essence, where iron-rich porphyry Cu-Au type ends, the IOCG deposit classification begins (cf. Kirkham and Sinclair, 1995). Characteristics of porphyry copper deposits that are shared with IOCG and present in the Echo Bay-Contact Lake district include widespread propylitic, phyllic, and potassic alteration zones, Fe-rich zones, large barren or low-grade pyrite zones, and close spatial and genetic association with the root zones of felsic to intermediate (andesitic) stratovolcanos. The mineralization in this district differs from classic porphyry-type systems in the abundance of hydrothermal iron oxides, widespread alkali-iron metasomatism, and unique metal assemblages including Cu, Ag, U, Au, Ni, Co, and Bi. The outward displacement of typical porphyry copper-type alteration halos in favor of core assemblages (sodic + magnetite + hematite + potassic) is, in essence, the transformation to the typical alkali-iron-dominated assemblage observed in IOCG systems (Figs. 9⇑, 11⇓).
Any exploration model for the Contact Lake Belt and Echo Bay district must consider the full continuum of possible IOCG mineralization styles that could form within an andesitic volcano-plutonic complex, from typical, very large core-type breccia and replacement deposits, to peripheral and distal high-grade epithermal vein mineralization. Continued geological studies of the Contact Lake Belt and Port Radium-Echo Bay district is highly recommended in order to better understand the nature of the mineralization, refine the geological and exploration models, and better understand the timing relationships between alteration, mineralization, and magmatic activity.
Current research and previous work indicate that the GBMZ satisfies many important geological criteria required for formation of IOCG deposits. Although there are limited geochronological data that specifically address the age of mineralization, it appears that all mineralization styles occurred episodically during the volcano-plutonic event at the margin of an Archean craton where arcs and successor arcs developed (cf. Groves and Vielreicher, 2001). This mineralization is associated with large-scale, continental, calc-alkaline, I-type granitic suites that include intermediate and mafic facies (cf. Creaser, 1996; Nisbet et al., 2000; Wyborn, 2002; Sillitoe, 2003). The GBMZ is disrupted by long-lived crustal-scale faults and splays. These faults provided channelways to magmas and associated hydrothermal fluids that were sourced from the upper mantle to lower crust (cf. Hitzman, 2000; Sillitoe, 2003), and exerted significant controls on magmatic centers and resultant mineralization. Finally, the GBMZ displays overprinting of Cu-Au ± polymetallic mineralization on earlier iron oxide and regional-scale potassic and calcic-sodic alteration zones (Goad et al., 2000a,b), analogous to other well-known IOCG deposits (cf. Skirrow et al., 2002; Barton and Johnson, 2004).
Collectively, the geological features discussed above for the Contact Lake-Echo Bay district and the GBMZ as a whole point to a high resource potential for the GBMZ in base, precious, strategic, and nuclear energy metals, which goes beyond the historical mining of vein-type mineralization.
The authors would like to thank all persons and organizations that contributed to the Contact Lake-Great Bear IOCG Project. J. Lehmann of Strasburg University, France, P.W. Stewart, and J. Perrin provided many excellent contributions to the field mapping and/or preparation of this paper. B. Elliott, B. Arbuckle, and V. Blanchet provided invaluable field support, and V. Antonoff kindly polished and stained rock slabs. Funding and logistical and technical support were provided by Alberta Star Development Corporation, Brandon University, the Northwest Territories Geoscience Office through the Strategic Initiatives in Northern Economic Development program, and the Geological Survey of Canada through the Targeted Geoscience Initiative program. The manuscript greatly benefited from review and comments by P. Weihed and N. Reardon. Geoscience Office through the Strategic Initiatives in Northern Economic Development program, and the Geological Survey of Canada through the Targeted Geoscience Initiative program. The manuscript greatly benefited from review and comments by P. Weihed and N. Reardon.
- Received April 2, 2006.
- Accepted April 14, 2007.