by John W. Lydon

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The Mesoproterozoic Belt-Purcell Basin outcrops over an area of about 200,000 km² in Idaho, Montana, and southeastern British Columbia, and preserves an 18 to 20 km thickness of sediments that were deposited in an intracontinental rift. The Prichard (U.S.A.) and Aldridge (Canada) Formations consist of 10 to12 km of marine turbidites with intercalated tholeiitic sills and reflect a combination of high accumulation rates (>500 m/my) and magmatic activity along the axial part of the rift. Relatively small strata bound Cu-Co massive sulphide deposits in central Idaho arguably represent Besshi-type mineralization in the axial zone of the rift, but the Proterozoic metallogenic events with the greatest metal endowment are associated with basinal dewatering during the rift fill stage of basin evolution. These events produced the world-class Sullivan Zn-Pb-Ag SEDEX deposit of British Columbia, hosted by the Aldridge Formation, and the world-class Coeur d’Alene Ag-Pb vein district of Idaho, hosted by Prichard- to Revette-aged rocks. The Revette Formation, corresponding to the earliest part of the rift sag stage, also hosts the Spar Lake, Rock Creek, and Montanore Red Bed Cu deposits of Montana, which together have a proven resource of more than 330 million tonnes of approximately 0.7% Cu and 50 g/t Ag. Reports of other Proterozoic mineral deposit types, including IOCG, Cu-Ni-PGE, and rift-sag stage SEDEX deposits, suggest that the mineral potential of the Belt-Purcell Basin has not yet been fully realized.

The Belt-Purcell Basin is a Mesoproterozoic intracontinental rift filled by marine and fluviatile sediments. Rocks of the basin comprise the Belt-Purcell Supergroup, which is up to 20 km thick (Winston and Link, 1993) and outcrops over an area of approximately 200,000 km². Only about 10% of this area is in Canada, the greater part being in the states of Montana and Idaho of the U.S.A. (Fig. 1). The portion of the basin and its sedimentary succession that occurs in the U.S.A is called the Belt Basin and Belt Supergroup, respectively, whereas their northward extensions in Canada are known as the Purcell Basin and Purcell Supergroup, respectively. The term Belt-Purcell refers to the combined American and Canadian segments and is used here when referring to the basin as a single entity.

Mining interest in the Belt-Purcell Basin started in the 1860s with the development of placer gold deposits, especially in the Bannack and Virginia City areas of Montana and the Boise basin area of Idaho. During the 1880s, prospecting away from the diminishing resources of the placer gold mining camps led to the discovery of the world-class Coeur d’Alene silver-lead district and the world-class copper-silver- gold-molybdenum porphyry-epithermal system at Butte (e.g. Klein, 2004). A similar history of mining development took place in Canada. Mining interest in southeastern British Columbia started with the Kootenay gold rush of 1864 to the placer deposits of Wild Horse Creek at the eastern edge of the Purcell Basin. Prospecting west of the placer gold fields led to the discovery in 1892 of the outcropping lead-silver ores of the North Star and the nearby Sullivan deposits. Although mining and smelting of lead-silver ores of the Sullivan deposit had started in 1900, its immense size was not fully realized until 1912, following its purchase in 1910 by the Consolidated Mining and Smelting Company of Canada (subsequently renamed Cominco) (Hamilton et al., 2000).

Up to 2001, the Coeur d’Alene district had produced about 7,400,000 tonnes of Pb, 2,900,000 tonnes of Zn, and 35,600 tonnes of Ag (data from Long, 1998; Larsen et al., 2004) and the Butte camp about 9,700,000 tonnes of Cu, 2,500,000 tonnes of Zn, 1,700,000 tonnes Mn, and 22,600 tonnes of Ag, as well as significant amounts of Mo, Pb, and Au (Robin McCulloch, Montana Bureau of Mines, pers. comm., 2005). Mining from the Sullivan deposit continued until the end of 2001, producing a total of 8,412,077 tonnes of Pb, 7,944,446 tonnes of Zn, and 9,264 tonnes of Ag plus significant quantities of Sn, Cd, Sb, Cu, Bi, and Au from 150 million tonnes of ore (British Columbia Geological Survey, Minfile 082FNE052).

The early establishment of this phenomenal metal endowment of the area has supported an almost permanent mineral exploration interest, and subsequent years have seen the discovery of a variety of mineral deposit types, including seafloor copper-cobalt deposits of the Blackbird mining district (Nash and Hahn, 1989; Bending and Scales, 2001) and the Sheep Creek deposit near White Sulphur Springs, Montana (Himes and Petersen, 1990), Red-Bed Cu-Ag deposits of the Montana Copper Belt, including the Spar Lake (Troy) mine (Harrison, 1972; Garlick, 1987; Hayes et al., 1989), and lode-Au deposits (e.g. Perry and Childs, 2003). More recently, the potential of the area for iron oxide copper-gold (IOCG) deposits (Stinson and Brown, 1994; Gillerman et al., 2003) and PGE mineralization in mafic intrusions (Buckley et al., 1994) has become apparent.

This range of mineral deposit types hosted by rocks of the Belt-Purcell Supergroup is the result of two main metallogenic intervals. The first was during the Mesoproterozoic accumulation of the Belt-Purcell Supergroup and consisted mainly of base metal seafloor deposits (SEDEX and Besshi types) and diagenetic veins and replacements. The ore-forming processes were driven by interplay of extensional tectonics with sediment compaction, and the emplacement of riftrelated mafic magmas, its related hydrothermal convection. The second was during the Cretaceous-Tertiary Laramide Orogeny and consisted of porphyry and epithermal styles of mineralization related to the emplacement of felsic magmas, and mesothermal veins associated with compressional and transtensional tectonics. This synopsis focuses on the first metallogenic interval and on the Canadian portion of the Belt-Purcell Basin, highlighting the one world-class deposit so far discovered in this area – the Sullivan SEDEX-type deposit.

Global Setting

Continental reconstruction puts the Belt-Purcell Basin near the continental margin of Laurentia, one of the continents formed by the Neoproterozoic break-up of the Rodinia supercontinent. The best constrained reconstruction (Sears and Price, 1978, 2000, 2003a,b) fits Laurentia adjacent to Siberia, and is supported by the matching of structural trends, zircon ages, and the alignment of the Mesoproterozoic Taimyr and Uzdha troughs with the Belt- Purcell Basin (Fig. 2). Other reconstructions fit Laurentia adjacent to Australia and Antarctica (Dalziel, 1991; Hoffman, 1991; Moores, 1991). It is interesting to note that in both reconstructed continental configurations, the world’s most productive Mesoproterozoic sedimentary basins for SEDEX deposits (the Australian Broken Hill, Mt. Isa, and McArthur basins and the Laurentian Belt-Purcell), were all at low latitudes and over the same segment of the earth’s mantle during sedimentation.

Tectonic Setting

The Belt-Purcell Supergroup fills part of an interconnected system of Mesoproterozoic rifts that extended from western North America to Labrador and were linked to the opening of the Grenville ocean (Gower and Tucker, 1994). The Belt-Purcell rift is the passive type caused by lithospheric tension, as opposed to the active type caused by the ascent and lateral spreading of a mantle plume (Chandler, 2000). The evidence for this is the preservation of basal quartzites (Fort Steele and Neihart formations) and the absence of pre-rift volcanism, which indicates a lack of thermal doming associated with active-type rifts. The REE chemistry of rift-related tholeiitic sills suggests magma derivation by partial melting of the mantle beneath a highly attenuated crust 35 to 40 km thick (Anderson and Goodfellow, 2000), consistent with a passive-type intracontinental rift.

Synsedimentary Faulting

Extensional faulting and sporadic tholeiitic to alkaline magmatism continued over most of the approximately 180 million years of sediment accumulation in the Belt- Purcell Basin, from about 1500 Ma to about 1320 Ma. Synsedimentary faults associated with rifting had their most profound effect in the lower part of the Belt-Purcell stratigraphic succession. This faulting is responsible for the contrast in thickness between the turbidites and intercalated sills of the Aldridge/Prichard rift-fill sequence and the stratigraphically equivalent shelf facies of the Greyson, Newland, Chamberlain, and Neihart formations (Fig. 3). The rift-fill sequence is about 12 km thick in the Purcell Mountains (Höy et al., 2000), in contrast to the cumulative thickness of 2.5 km for the shelf sequence at the eastern end of the Helena embayment (Chandler, 2000). Two directions of synsedimentary faults have been recognized: approximately northto northwest-trending rift-parallel faults, and east to northeast- trending transfer faults (Höy et al., 2000; Chandler, 2000; Turner et al., 2000a).

A prime example of a rift-parallel synsedimentary fault is the structure that separates the shallow-water facies of the Fort Steele Formation and the overlying condensed lower Middle Aldridge in the Hughes Range, from the deep-water thick turbidite sequence of the of the Lower and Middle Aldridge in the Purcell Mountains (see Figs. 3, 4). However, this structure has little or no affect on the thickness of the upper part of the Middle Aldridge and younger Mesoproterozoic rocks. Other examples include faults that control the north-trending Sullivan Corridor, various faults northeast of Moyie Lake, the Iron Range Fault northeast of Creston, and the north-trending elongation of many mudvolcano vents (“discordant fragmentals”) (Höy et al., 2000; Turner et al., 2000b). Examples of east- to northeast-trending synsedimentary faults include antecedents to the Moyie- Dibble Creek and St. Mary-Boulder Creek fault systems (see Fig. 4), across which facies and thickness of Aldridge rocks change, and in proximity to which there is an anomalous thickness of Moyie sills (Höy et al., 2000).

The Belt-Purcell Rift consists of two branches (Fig. 1). The main or Purcell branch, which contains the Sullivan deposit, trends northwest through the Purcell Mountains of southeastern British Columbia and is characterized by a basal, 12 km thick, turbidite-sill complex – the Aldridge Formation in Canada and the Prichard Formation in the U.S.A. To the northwest, rocks of the Purcell branch are covered by Neoproterozoic and Phanerozoic strata and to the southeast they are truncated against an east-northeast-trending transfer fault. The east-northeast-trending or Helena branch extends along the northern side of this transfer fault to form the Helena embayment (Fig. 1).

Stratigraphic relationships of the Belt-Purcell are shown in Figure 5. The lower part of the Supergroup consists of marine turbidites, which infilled the rift grabens, and stratigraphically equivalent shallow marine to fluvial sandstones, mudstones, and carbonates that were deposited on the surrounding rift platform. These synrift sequences are overlain by shallow marine to lacustrine and fluviatile mudstones, carbonates, and sandstones that extend over the rifted and platformal areas alike, forming a rift-cover or rift-sag sequence. Strata of the Belt-Purcell Basin can thus be divided into three main facies groups (Fig. 3):

1. Basinal facies of rift-fill sequence consisting mainly of deep-water turbidites in the Purcell Branch (Aldridge and Prichard formations) and deep-water calcareous argillite and turbidites that shoal upwards to midshelf carbonates and siliciclastics (Newland Formation) in the Helena Branch.
2. Shallow-water platformal and fan-delta facies deposited at the margins of the rift and surrounding shelf, and approximately synchronous with turbidite deposition within the rift. Lithologies include fluviatile and deltaic quartz-rich arenites (Fort Steele and Neihart formations), fan-delta complexes containing coarse-grained debris flows shed from fault scarps (Lahood Formation), distal deep-water argillite-siltite debris flows (Greyson Formation), and shallow-water platformal carbonates (Waterton and Altyn formations).
3. Shallow-water, mud flat, fluvial, lagoonal, alluvial, and playa facies of rift-cover or rift-sag sequence that covers both the rift and its adjacent platforms, and forms the upper part of the Belt-Purcell Supergroup. Rock types include red, purple, and green argillites and siltites of the Ravalli Group (Creston Formation in Canada), a transgressive carbonate-rich sequence of the Middle Belt Carbonate (Kitchener Formation in Canada), and northward and eastward deepening fine-grained clastics of the Missoula Group (Sheppard, Gateway, Phillips, Roosville, and Mount Nelson formations in Canada) that range from a large sandy alluvial apron in the southwest through marginal marine sand and mud flats, to shallow-marine sediments composed of siliceous and carbonate mud in the north and east.

At the time of deposition of the lower rift-fill sequence, the Belt-Purcell rift was a deep, elongate but relatively narrow water-filled trough into which one or more very large rivers drained at its southern head forming a fluviatiledeltaic complex. Turbidites derived from the delta complex fill moved northwestward during most of the rift-fill stage (Chandler, 2000; Höy et al., 2000). This transport direction, deduced from current direction indicators and sedimentary facies distributions, is consistent with the age profiles of detrital zircons that match the ages of basement terranes of the North American craton to the south and to the east of the Belt-Purcell Basin (Ross et al., 1992; Ross and Villeneuve, 1999). During the latter part of the rift-fill stage, and continuing throughout the rift-sag phase, sediment supply to the Belt Basin was mainly from the southwest (Chandler, 2000). Zircons in this sediment have age characteristics consistent with an Australian (Ross and Villeneuve, 1999) or Siberian (Sears and Price, 2000) source. Although the rift-cover sequence was deposited in a dominantly shallow-water environment, it contains deep-water facies deposited in local areas of greater differential subsidence. This local differential subsidence reflects a continuation of rift activity, albeit on a diminished scale, to at least the stratigraphic level of the Sheppard and Gateway formations (Höy, 1993).

The combination of various lines of evidence suggests that the Belt-Purcell Basin was marine rather than lacustrine:

1. Salt casts, evaporitic nodules, oolites, and syneresis cracks in the shallow-water facies of the lower Belt- Purcell and in strata of the rift-cover sequence indicate that the permanent water column was saline (Chandler, 2000).
2. Chamosite pellets, most common in shallow marine waters (Johnson, 1978), occur in the Fort Steele and Newland formations (Schieber, 1993; Chandler, 2000), and pelletal phosphorite, unknown as a Precambrian lacustrine mineral, occurs in the Spokane Formation (Gulbrandsen, 1966)
3. Strong tidal action, which is generally indicative of a marine environment, is suggested by herringbone crossbeds and ripples in the Fort Steele, Prichard, Altyn, Yellowjacket, Newland, and Creston formations, and by extremely elongated stromatolites in the Altyn Formation (Chandler, 2000).
4. Strontium concentrations of the Newland Formation and boron content of the Middle Belt Carbonate (Schieber, 1993) are typical for marine sediments.

From the time of the Creston Formation onwards, there is evidence for both marine and lacustrine environments (see discussion in Lydon, 2000a). During this period of time, the rift had been filled by fluviatile, deltaic, turbidite, and hemipelagic sediments, and its surrounding platform had become covered with fluviatile to shallow-water sandstones, fine-grained-clastics, and carbonates.

During deposition of the Aldridge-Pritchard turbidites, the deepest part of the water column may have been anoxic. Perhaps the strongest evidence for this is that the hemipelagic component of the sedimentary succession, which is preserved as inter-turbidite silty argillite, contains up to 3 wt.% organic carbon (Goodfellow, 2000). However, sulphur/carbon ratios of Aldridge sedimentary rocks with >0.3% organic carbon span the range for normal marine sediments, with only 30% plotting in the field for sediments deposited in an anoxic marine water column (see Fig. 12-19, Goodfellow, 2000). Intermittent stratigraphic excursions of sulphur isotope ratios of disseminated iron sulphides to heavy values in the Aldridge Formation (Goodfellow, 2000) and the Newland Formation (Lyons, 1993; Lyons and Luepke, 1995; Lyons et al., 2000) are interpreted by the authors to indicate periodic restriction of marine circulation and the development of a stratified water column. The great majority of Aldridge sedimentary rocks have MnO contents of <0.1%, similar to anoxic black sediments (Goodfellow, 2000), but this evidence is somewhat ambiguous because fine-grained Aldridge sedimentary rocks (argillites) have low concentrations of all elements (except Si, Al, and K) compared to average shales, due to dilution by an unusually high quartzo-feldspathic content (Lydon et al., 2000b).

Stratigraphically controlled U-Pb ages of zircons from volcanic rocks (Evans et al., 2000) include 1454 Ma for a bentonite near the top of the Helena Formation, 1443 Ma for felsic tuffs at the stratigraphic level of the Purcell lavas and equivalent Nicol Creek volcanics, and 1401 Ma for airfall tuff at the contact between the Bonner and Libby formations. These are plotted at their Purcell stratigraphic equivalents to the formations of the Belt Supergroup in Figure 6. Moyie sills in the Aldridge (Anderson and Davis, 1995; Schandl and Davis, 2000) and the Pritchard (Sears et al., 1998) formations have the same age of 1468+3/-2 Ma, and are plotted in Figure 6 midway between the base of the Middle Aldridge and the top of the MA2 sedimentary cycle (Fig. 5) of the Middle Aldridge Formation (Höy et al., 2000), which is the stratigraphic interval over which the 1468 Ma suite of sills were emplaced (see discussion in the section on “Stratigraphic interval corresponding to the 1468 Ma Moyie sill event” below). Metamorphic migmatites of the Salmon River Arch (Fig. 1), dated at 1370 Ma, are plotted at their calculated thermobarometric depth (Doughty and Chamberlain, 1996), and metamorphic titanites from the Sullivan deposit dated at 1325 to 1330 Ma (Schandl and Davis, 2000) are plotted at the 11 km depth of its peak Mesoproterozoic metamorphic pressure (De Paoli and Pattison, 2000). A smoothed curve through these seven points extrapolates to the seismically-imaged base of the Aldridge Formation at 1485 Ma, and gives average accumulation rates of 570 m/my from the beginning of Purcell sedimentation up to the top of the Kitchener Formation (Lydon, 2000a). These accumulation rates are lower than those calculated by Evans et al. (2000). However, Evans et al. (2000) appear to have assumed that only about 500 m of Prichard- Aldridge Formation had accumulated by 1468 Ma, the time at which the Moyie sills were being emplaced.ASm/Nd date of 1470 Ma for the footwall tourmalinite (Jiang et al., 2000), which predates the Sullivan deposit, is only slightly older than the 1468 Ma U/Pb zircon age for the mine sill that postdates the Sullivan deposit, and supports the 1470 Ma age for the Sullivan deposit deduced from the accumulation rates.

Rocks of the Purcell Supergroup have been affected by at least three distinct episodes of regional metamorphism and deformation, which have been termed the East Kootenay Orogeny (1350-1300 Ma), the Goat River Orogeny (900- 800 Ma), and Jurassic-Cretaceous (160-60 Ma) metamorphism, deformation, and plutonism (McMechan and Price, 1982). In addition, the rocks have been affected by Paleozoic faulting.

East Kootenay Orogeny

The regional metamorphic grade of rocks in the Purcell anticlinorium (Reesor, 1973; McMechan and Price, 1982) and stratigraphically equivalent rocks of the Belt Supergroup in the U.S.A. (Maxwell and Hower, 1967; Eslinger and Sellars, 1981) generally increases with stratigraphic depth. This progression has been interpreted to reflect burial diagenesis and metamorphism (Maxwell and Hower, 1967; Eslinger and Sellars, 1981; Doughty and Chamberlain; 1996; Lydon et al., 2000b). Peak metamorphic conditions of 1370 Ma migmatites of the Salmon arch yields estimates of 680°C at 450 MPa (14 km burial) and 690°C at 650 MPa (20 km burial) (Doughty and Chamberlain, 1996). These thermobarometric estimates are consistent with the maximum thickness of the Belt Supergroup of 15 to 20 km (Harrison, 1972) or at least 18 km (Winston and Link, 1993) estimated from field mapping. The peak metamorphic conditions at Sullivan calculated to be 450°C and 380 MPa (De Paoli and Pattison, 1995, 2000), equivalent to 11 km of burial, are consistent with a geothermal gradient of 38 to 45°C/km during accumulation of the Belt-Purcell Supergroup.

A Mesoproterozoic age for this regional metamorphism has been well established by radiometric dating of metamorphic titanite (1330-1360 Ma) from a Moyie sill in the U.S.A. (Ross et al., 1992), hydrothermally altered rocks at Sullivan, which give Pb-Pb ages of 1325 to 1329 Ma (Schandl et al., 1993; Schandl and Davis, 2000), and zircon U-Pb ages of 1370 Ma from migmatitic leucosome at the base of the Belt Supergroup (Doughty and Chamberlain, 1996). A foliation accompanying burial metamorphism, commonly axial planar to large folds, is associated with most Belt-Purcell rocks and tends to increase in intensity with depth (Höy, 1993). Mesoproterozoic tectonics was dominated by extensional (rift-parallel) and transfer (transverse to the rift trend) faults associated with the development of the Belt-Purcell rift. The East Kootenay Orogeny reflects the burial metamorphism of a thick sedimentary pile in the high geothermal gradient of an actively rifting environment.

The Goat River Orogeny

The Goat River Orogeny was proposed by McMechan and Price (1982) to describe an 800 to 900 Ma episode of uplift, block faulting, and metamorphism that marked the onset of deposition of the Windermere Supergroup. Evidence for a regional metamorphic event at this time is restricted to resetting of K-Ar and palaeomagnetic signatures, indicating that associated regional metamorphism was a low-grade event (McMechan and Price, 1982). On the basis of U-Pb dating of titanites and zircons, Anderson and Davis (1995) suggested a thermal disturbance at 1030 to 1120 Ma that Anderson and Parrish (2000) correlate with Grenville-age metamorphism.

Paleozoic Tectonics

The major impact of Paleozoic tectonics on the area of the Purcell anticlinorium was a 5 to 10 km downthrow to the northwest along the antecedent to the Moyie-Dibble Creek Fault between Early Cambrian and Late Devonian time. The vertical fault movement that uplifted the area to the south (“Montania”) resulted in a 240 km right-hand deflection of the boundary between the Cordilleran miogeocline and the North American cratonic platform (Price and Sears, 2000).

Jurassic-Cretaceous and Cretaceous-Paleogene Orogenies

Most of the major structural features of the Purcell anticlinorium are of Jurassic-Tertiary age and are associated with the development of the Rocky Mountain fold and thrust belt. However, many of the major faults and other deformational features can be related to antecedent structures that date back to Proterozoic and Archean structures (Höy et al., 2000; Price and Sears, 2000). Mesozoic-Cenozoic deformation and metamorphism of Purcell rocks are related to a Late Jurassic-Early Cretaceous episode of left-lateral transpression and a Late Cretaceous-Paleocene episode of right-lateral transpression, both being due to the oblique collision of the tectonic collage of Cordilleran accreted terranes (“Quesnellia”) with the North American craton (Price and Sears, 2000). These two periods of transpression were separated by the intrusion of mid-Cretaceous (90-115 Ma) granite plutons. The movement of the different Jurassic- Paleocene thrust panels has been modeled as a rotation of approximately 30° about an Euler pole situated near Helena, Montana by Price and Sears (2000). Much of the relative lateral movement between the different thrust panels was along reactivated transfer faults of the Mesoproterozoic rift, notably the Moyie-Dibble Creek, St. Mary, and Hall Lake faults, and divides the Purcell anticlinorium into structural blocks. Palinspastic reconstruction, based on compensating for the different relative movements of the thrust panels, restores rocks of the Belt-Purcell Basin to their configuration prior to the Jurassic collisional event. Although this reconstruction changes the orientation of the Purcell Branch of the rift to a more westerly trend, at less of an angle to the Helena Branch, geological relationships within each structural block are not affected.

Most mineral exploration of the Belt-Purcell Supergroup in Canada has focused on the Aldridge Formation, the target being SEDEX deposits similar to the Sullivan deposit. The Aldridge Formation of southeastern British Columbia and the Prichard Formation in the contiguous U.S.A. (Cressman, 1989) form the basal part of the Belt-Purcell Supergroup and, as shown in Figure 6, are essentially a sediment-sill complex that infilled an actively spreading intracontinental rift (Höy et al., 2000). Aldridge sedimentary rocks are dominantly turbidites, which most commonly consist of an arenite base and a siltite or argillite top, representing the A and E parts, respectively, of the Bouma turbidite model, with the B, C, and D parts commonly being indistinct (Hamilton et al., 1982). The Aldridge Formation has been subdivided by Cominco into three informal, but widely used and accepted members (Reesor, 1958, 1973).

The Lower Aldridge Formation is a sequence of thin- to medium-bedded, pyrrhotite-rich, distal argillaceous turbidites, whereas to the south, near Creston, more proximal, thick-bedded quartzites of the Ramparts Facies are dominant (Höy et al., 2000). Only about 2 km of Lower Aldridge turbidites are exposed at the surface, but seismic imaging suggests that the sediment-sill complex extends into the subsurface for an additional 6 km (Cook and van der Velden, 1995). East of the Rocky Mountain Trench, the deltaic to fluviatile Fort Steele Formation is considered to be the shallow-water equivalent of the upper part of the Lower Aldridge Formation (Höy et al., 2000).

The Middle Aldridge Formation consists of up to 2400 m of medium-bedded quartzitic turbidites with inter-turbidite intervals of siltstone or argillite, which extend both over the central or axial part of the rift basin in the Purcell Mountains and the more marginal areas of the basin in the Northern Hughes Range east of the Rocky Mountain Trench. Within the Middle Aldridge Formation are intervals of laminated carbonaceous siltite consisting of alternating light and dark layers, 1 to 10 mm thick, whose relative thicknesses remain constant over distances in excess of 300 km (Huebschman, 1973). The barcode-like patterns formed by the alternating light and dark layers (see Lydon, 2000c for details) are diagnostic of a specific stratigraphical horizon, and have been used by Cominco to establish stratigraphical correlations within the otherwise monotonous sequence of turbidites. More than a dozen of these “Marker” intervals have been recognized (Hamilton et al., 2000). The Middle Aldridge Formation has been divided into three upward-fining cycles, the bottom of each representing a period of tectonic extension and basinal subsidence (Höy et al., 2000). These cycles are termed, from stratigraphically lowest to stratigraphically highest, MA1, MA2, and MA3, respectively (Fig. 5). The top of MA1, in particular, has a high incidence of sedimentary fragmental rocks, tourmalinites, and sulphide deposits, including the Kootenay King SEDEX deposit and the disseminated sulphide deposits of Canam and Star (Fig. 4).

The Upper Aldridge Formation is approximately 300 m thick and consists of thin-bedded to laminated, pyrrhotiterich argillite and siltite, and represents a shallowing basin (Höy et al., 2000). The laterally most extensive stratiform tourmalinization in the Purcell Basin occurs in the Upper Aldridge Formation in the Doctor Creek-Findlay Creek area (Fig. 4) (Greig et al., 2001).

Moyie Sills

The gabbroic Moyie sills occupy about 40 vol.% of the Aldridge stratigraphic sequence in the area immediately to the west of the Sullivan deposit, and between 5 and 30 vol.% elsewhere in the Aldridge Formation. Isoliths of sills within the exposed rocks indicate over 1 km of aggregate sill thickness along the axis of the Belt-Purcell rift (Fig. 7) (Höy et al., 2000), implying a minimum total volume of more than 50,000 km³ of magma emplacement (Anderson and Goodfellow, 2000). Inclusion of the deeper part of the Prichard-Aldridge sediment-sill complex, which has only been seismically imaged, would greatly increase this estimate.

U-Pb dates on zircons from mafic sills associated with the rifting have given ages of 1469 Ma (Sears et al., 1998), 1468 Ma (Anderson and Davis, 1995; Schandl and Davis, 2000), 1457 Ma (Sears et al., 1998), 1445 Ma (Höy, 1989), 1449 Ma (D.W. Davis cited in Brown and Woodfill, 1998), 1433 Ma (Zartman et al., 1982), and 1379 Ma (Doughty and Chamberlain, 1996), suggesting recurrent mafic magmatic upflow over the period 1470 to 1430 Ma, with perhaps major episodes at about 1470 and 1445 Ma, and a final upflow at about 1380 Ma (see Fig. 6). Additional U-Pb ages of felsic magmatic products associated with the mafic magmatism in the Purcell lavas (stratigraphic equivalent to the Nicol Creek volcanics) of 1443 Ma (Aleinikoff et al., 1996; Evans et al., 2000), of porphyritic granite in the Salmon River Arch of 1370 Ma (Evans and Zartman, 1990), and the Hell Roaring Creek stock of 1340/1360 Ma, indicates either association of felsic differentiates with mafic magmatic upflow (Doughty and Chamberlain, 1996) or anatectic melts generated by mafic magma emplacement at high metamorphic temperatures.

Hydrous autometamorphism of most sills, the formation of peperite and “granofels” along sill margins, and the local disruption, fluidization, and expulsion upwards of adjacent sediments as mud-volcano eruptions, indicates that the sills were emplaced at shallow depth into wet, semiconsolidated sediment (Höy, 1989; Buckley and Sears, 1998; Anderson and Höy, 2000; Höy et al., 2000) and were synchronous with Middle Aldridge Formation sedimentation. Not uncommonly, and especially near the top of the Lower Aldridge (Doug Anderson, pers. comm., 1995), the sills have partially melted or incorporated wet sediment forming “granophyre” (Cominco terminology), “granofels” (Turner et al., 2000b), or “granosediment” (Poage et al., 2000) at their margins.

Mud Volcanoes and Tourmalinites

The Aldridge and Prichard formations record a high incidence of mud-volcano activity that is manifest as bodies of sedimentary fragmental rocks (Figs. 4, 7). In the majority of cases, these discordant and concordant bodies respectively represent the vent breccias of the conduits and the eruption breccias (or wastage aprons of the extrusive mounds) of mud volcanoes. On a local scale, discordant sedimentary fragmental bodies are rooted in disrupted and fragmented sedimentary strata that have been fluidized along the upper contact of Moyie sills (Anderson and Höy, 2000; Höy et al., 2000). Individual bodies of sedimentary fragmental rocks vary in maximum dimension from a few tens to hundreds of metres and occur over a stratigraphic interval of at least 3000 m, with the greatest volume occurring at or just below the Lower to Middle Aldridge contact, and a second incidence near the top of the MA1 sedimentary cycle (Anderson and Höy, 2000). By far the largest mud-volcano field is that of the Sullivan-North Star corridor, which is at least 7 km long and up to 3 km wide (Ransom and Lydon, 2000; Turner et al., 2000a). Discordant fragmental, or vent-breccia bodies, are commonly elongated along rift-parallel synsedimentary faults. Some of the larger concordant sedimentary fragmental bodies may be debris flows related to the slumping of unstable submarine slopes.

Tourmalinite in the Belt-Purcell Basin is a rock consisting of >30 vol.% tourmaline that was formed by the replacement of the aluminosilicate fraction of a sedimentary rock by tourmaline (Slack et al., 2000). Most tourmalinite occurrences are in the Aldridge or Prichard formations, and most are within or immediately adjacent to sedimentary fragmental rocks, particularly discordant bodies (Anderson and Höy, 2000). About 30% of all bodies of sedimentary fragmental rocks in the Aldridge Formation are at least partially tourmalinized (Anderson and Höy, 2000). The genetic association of mud volcanoes, tourmalinization, and Moyie sill emplacement is reflected by the three being co-extensive on a regional scale (Fig. 7). The great majority of tourmalinite occurrences in the Prichard-Aldridge formations and the sedimentary fragmental bodies, with which they are associated, are restricted to those areas where there is a significant cumulative thickness of Moyie sills, especially along the presumed locus of gabbroic magma upflow at the rift axis (Lydon, 2000b).

Stratigraphic Interval of the 1468 Ma Moyie sill Event

As discussed by Anderson and Parrish (2000), the predominance of sills over dykes and lava flows, together with sediment fluidization and the formation of “granofels” at sill margins, indicate that the magmas of the Moyie sill event were emplaced into water-saturated, unconsolidated sediments at the level of neutral magma buoyancy. Under these conditions, sill emplacement tends to migrate to successively higher stratigraphic levels, because the compaction and thermal induration of unconsolidated sediments surrounding a newly emplaced sill reduces the capacity of the sediments to accommodate subsequent magma emplacement by lateral flow. As noted above, mud volcanos in the Aldridge are spatially and genetically associated with sill emplacement, and reflect the release of pressure created by compaction of sediments by sill emplacement. The stratigraphically lowest expressions of mud-volcano activity are the sedimentary fragmental rocks just below the Sullivan Horizon in the Sullivan- North Star corridor and near the top of the Lower Aldridge at other localities (Anderson and Höy, 2000). The stratigraphically highest expression is the St. Joe “tuff” (Fig. 5B) near the base of the MA3 fining-upward cycle of the Middle Aldridge (Anderson and Parrish, 2000; Höy et al., 2000).

The stratigraphic level of the lowest mud-volcano eruption is not conclusive evidence for the timing of initiation of the Moyie sill magmatic event, because sill emplacement into wet, unconsolidated sediment does not necessarily produce mud-volcano activity at the seafloor. The stratigraphic separation between the St.Joe “tuff” and the base of the Middle Aldridge is about 2.2 km in the Moyie area, which, assuming the accumulation rate of 570 m/my of Figure 5, represents a time interval of 3.85 my. The maximum duration of Moyie sill emplacements is the 5 my encompassed by the analytical uncertainty (1468 +3/-2 Ma) for the U/Pb dating of zircons from both Lower Aldridge and Middle Aldridge Moyie sills (Anderson and Davis, 1995). Therefore, Moyie sill emplacement must have commenced no more than 1.15 my prior to initiation of Middle Aldridge sedimentation. Because accumulation rates of the Lower Aldridge cannot have been higher than 570 m/my (Fig. 5), the lowest stratigraphical level corresponding to the time at which emplacement of Moyie sills commenced is therefore about 650 m below Lower-Middle Aldridge contact. However, because the Sullivan Horizon, just below the Lower-Middle Aldridge contact, represents a period of sedimentation starvation with very much lower average accumulation rates, it is likely that the 1.15 my time period is represented by a much smaller thickness than 650 m. Initiation of sill emplacement was therefore likely very close in time to the tectonic event that caused a deepening of the basin in the Purcell anticlinorium, which in turn caused the sediment starvation at the top of the Lower Aldridge (Höy et al., 2000). The metallogenetic significance of this timing is that it means that the Sullivan ore deposit formed during the early stages of sill emplacement into Lower Aldridge turbidites.

Overview of Deposit Types and Classification

The Belt-Purcell Basin contains a variety of base metal mineral deposits and occurrences (Figs. 1, 4). Höy et al. (2000) have classified Mesoproterozoic deposits occurring in the Purcell anticlinorium into four main types, which, with the addition of Besshi-type deposits of Idaho and Redbed Copper-type deposits of Montana, make up the variety of Proterozoic mineral deposit types that occur in Belt-Purcell rocks. These six deposit types can be classified into the three groups that are briefly described below. More detailed descriptions of some of these deposits are described in later sections.

Seafloor Sulphide Deposits

SEDEX deposits represent the accumulation of iron, zinc, and lead sulphides on the seafloor, or just below the sediment surface, around hydrothermal vents of sedimentary basins. The hydrothermal fluids, which in general cannot be directly linked to magmatism, are thought to generally have a temperature <250°C and represent the discharge of formational brines from a compacting sedimentary pile (Lydon, 1996). Sullivan is the prime example in the Belt-Purcell Basin and is described in detail below; other examples include the Sheep Creek (Fig. 1) and Kootenay King (Fig. 4) deposits.

Besshi-type deposits are stratiform Cu-bearing, massive sulphide deposits that typically have a very high length:thickness ratio and occur in sill-sediment complexes of oceanic extensional environments. They formed from high temperature (>300°C) seafloor hydrothermal systems of seawater convection cells driven by the heat of subsurface mafic magmatic intrusions. They are a variant of the volcanogenic massive sulphide (VMS) class of deposits. Deposits of the Idaho Cu-Co belt, including Blackbird (Fig. 1), Iron Creek, and Black Pine have been interpreted to be of this type (e.g. Nold, 1990).

Vent complex / feeder pipe deposits are interpreted to represent the upflow zone of seafloor hydrothermal vents that did not develop a substantial overlying concordant sulphide deposit, or alternatively the root zone of a SEDEX deposit from which the concordant stratiform part has been eroded. The Stemwinder and Fors (Fig. 4) deposits are examples.

Stratabound Disseminated Sulphide Deposits

Redbed Copper deposits form by deposition of copper sulphides from oxic groundwaters at a reducing interface, such as black shales, usually during the early burial and compaction history of a sedimentary succession. Redbed Copper deposits of the Revette Formation in Montana contain three major deposits of this type (Fig. 1): Troy (Spar Lake), which has already been mined, Rock Creek, which is in its final stages of permitting, and Montanore.

Disseminated sphalerite and galena occupy large volumes in arenaceous beds over stratigraphic intervals of tens of metres in the Middle Aldridge. The Star and Canam deposits (Fig. 4) are examples.

Veins

Mesozoic veins. Vein deposits in Aldridge rocks have been divided into Cu types, Pb-Zn-Ag types, and Au types (Höy, 1993). Many of these deposits are associated with Mesozoic intrusions and are not considered further here.

Mesoproterozoic veins. A Mesoproterozoic age for the galena-sphalerite and galena-tetrahedrite veins of the Coeur d’Alene district of Idaho (Fig. 1), and the St. Eugene and Vine (Cominco data) sphalerite-galena veins of the Purcell anticlinorium (Fig. 4) is indicated by the Pb isotope compositions of galena (Beaudoin, 1977).

Deposit Distribution

The geographical distribution of major Mesoproterozoic deposits is shown in Figure 1. Smaller deposits of the Canadian portion of the Belt-Purcell Basin are shown in Figure 4. All the major Mesoproterozoic mineral deposits occur in the lower part of Belt-Purcell stratigraphy (Fig. 5). Seafloor sulphide deposits appear to be restricted to Aldridge (Pritchard) host rocks and their stratigraphic equivalents, and fall into the time interval of rift fill sedimentation. Epigenetic deposits span the range from Aldridge to Creston Formation time (see age-stratigraphic relationships in Fig. 6 and Canada-U.S.A stratigraphic correlations in Fig. 5). It is interesting to note that vein deposits hosted by Aldridge-age rocks dominantly have a Pb-Zn-Ag metal association, whereas epigenetic deposits in Creston age (Ravalli Group) rocks, whether they be veins (e.g. tetrahedrite-bearing veins of the Coeur d’Alene district) or stratabound disseminated sulphides (e.g. Spar Lake, Rock Creek) dominantly have a Cu-Ag metal association.

Grade and Tonnage Statistics

A selection of grade and tonnage data for Mesoproterozoic mineral deposits of the Belt-Purcell Basin are shown in Table 1.

Production

The greatest production of metals has come from the Sullivan deposit, amounting to over 16,000,000 tonnes of Pb+Zn and 9,000 tonnes of Ag. The aggregate base metal production from the Coeur d’Alene district is a little over half that from Sullivan, but its aggregate silver production is about three times that of Sullivan. As reflected in the statistics of Table 1, the bulk of silver production from the Coeur d’Alene district is from the tetrahedrite-bearing set of veins rather than the sphalerite-galena set of veins. During 2004, about 5.5 million ounces (approximately 171 tonnes) of Ag was produced from the Coeur d’Alene district, mainly from the Galena, Coeur, and Lucky Friday mines. Production of Pb, Zn, and associated Ag from other deposits, such as the St. Eugene vein, North Star SEDEX, and Stemwinder Vent Complex deposits has been very small (Table 1) in comparison to Sullivan or the Coeur d’Alene district. Production from the Spar Lake Cu-Ag Red Bed Copper type of deposit from 1981 until suspension of mining in 1993 was 33,742,281 tons (approximately. 30.6 million tonnes) of ore at an average grade of 0.66% Cu and 1.50 opt Ag (approximately. 42 g/t) (Reipas and Couture, 2005). Production from the Blackbird Cu-Co mine for the period 1951 to 1959 is reported as 6350 tons of Co from an average ore grade of 0.6% Co and 1.5% Cu by Nash and Hahn (1989) (i.e. 960,099 tonnes ore treated if 100% Co recovery), but Bending and Scales (2001) state that the mine produced >5,000,000 tons of ore during the period 1949 to 1965.

Resources

Mining at Sullivan ceased in 2001 with depletion of its reserves. Exploration for the continuation of the Sullivan deposit on the north side of the Kimberley fault (Fig. 8) has so far met with some, though limited, success. At the end of 2004, proven and probable reserves at the Galena, Coeur, and Lucky Friday mines of the Coeur d’Alene district amounted to about 1,337,000 tonnes with an average grade of approximately 472 g/t Ag (data from 2005 annual reports of the Coeur d’Alene Mines Corporation and the Hecla Mining Company). Proven and probable reserves remaining at the Troy deposit at resumption of production in 2004 were estimated as 8.7 million tons (approximately. 7.9 million tonnes) at 0.65% Cu and 1.57 opt (approximately 44 g/t) Ag (Reipas and Couture, 2005). The Rock Creek deposit, which is at an advanced stage of production feasibility study, is estimated to contain a total inferred mineral resource of about 124,000,000 tonnes at 0.72% Cu and 45 g/t Ag.Amarginally greater resource is inferred for the nearby Montanore deposit, which is currently undergoing a permitting procedure in anticipation of a future mining operation. Between 1978 and 1982, exploration by Noranda Ltd. defined 5 million tons of “mining reserves” and 7.3 million tons of “indicated reserves” of Cu-Co ores in the Blackbird mining district (Nash and Hahn, 1989), but it is uncertain whether these resources include the approximately 2 million tonnes listed in Table 1 for the Ram and Sunshine deposits. Other Cu-Co resources have been delineated at the Sheep Creek deposit (Table 1).

Within the Canadian portion of the Belt-Purcell Basin, the Sullivan deposit is by far the most important mineral deposit. At the time of its closure at the end of 2001 it was Canada’s longest-lived continuous mining operation and had produced metals worth over $20 billion in terms of 2005 metal prices. Only the Sullivan deposit will be described in detail here. Detailed descriptions of the Sullivan deposit have previously been given by Freeze (1966), Ethier et al. (1976), Ransom (1977), Campbell et al. (1980), Hamilton et al. (1982, 1983), Campbell and Ethier (1983), McClay (1983), Conly et al. (2000), Lydon et al. (2000a), Owens, (2000), Ransom and Lydon (2000), and Lydon (2004).

Geological Setting

The Sullivan deposit occurs just below the contact between the Lower and Middle Aldridge Formation. On a regional scale, the top of the Lower Aldridge is marked by up to 20 m of a black laminated mudstone that is termed “CWL” (carbonaceous wacke laminite) by Cominco (Ransom and Lydon, 2000). This interval, called the Sullivan Horizon, records a period of sediment starvation following a sudden tectonic downdrop of the basin (Höy et al., 2000), which in effect caused a regression of the delta complex to the south and a removal of this part of the basin beyond the northern limit of its turbidite fan. The subsequent northward progradation of the turbidite fan marks the beginning of the Middle Aldridge. In the Kimberley area, the stratigraphic interval occupied by the CWL is up to 200 m thick due to the intercalation of products from contemporaneous mud-volcano and hydrothermal activity (Ransom and Lydon, 2000). The area of stratigraphic thickening and mud-volcano activity is called the Sullivan Subbasin and measures at least 13 km in a north-south direction and 3 to 5 km in an eastwest direction (Fig. 8). In cross-section, the Subbasin is asymmetric and is thickest in the west where it is marked by a 1 km wide, north-south trending zone of mud volcanoes, synsedimentary faults, disrupted sediments, and hydrothermal alteration called the Sullivan-North Star Corridor (Turner et al., 2000a, b). The Subbasin gradually thins to the east and terminates against an interpreted, west-facing synsedimentary fault that separates the Subbasin proper from the stratigraphically equivalent plume deposits of the Concentrator Hill Horizon (see Fig. 12J in Ransom and Lydon, 2000).

Rocks of the Sullivan-North Star corridor have been affected by a variety of hydrothermal alteration types that are similar to those occurring in the footwall and hanging wall of the Sullivan deposit itself (Leitch et al., 2000; Turner et al., 2000c). The most extensive is sericite alteration, which is strongly controlled by the distribution of coarse sedimentary fragmental rocks stratigraphically lower than the Sullivan Horizon. Sericite alteration is only marginally enriched in K₂O (as measured by K₂O/Al₂O₃ ratios) compared to unaltered Aldridge sedimentary rocks, and its typical pale yellowish grey-green colour is due to the absence of biotite that gives the brown colour to unaltered Aldridge sedimentary rocks. Other alteration types include tourmalinization, chloritization, and albitization (Turner et al., 2000c).

The Sullivan Subbasin is close to the axis of maximum Aldridge turbidite thickness (Fig. 1) and maximum aggregate thickness of Moyie sills (Fig. 7) (Price and Sears, 2000), and is interpreted to be a local structurally controlled halfgraben or graben related to the spreading of the Belt-Purcell rift. The Sullivan deposit is situated at the northern end of the Sullivan-North Star Corridor (Fig. 8). It has been suggested (Höy et al., 2000; Turner et al., 2000b) that the Sullivan deposit was localized by the intersection of the Sullivan Corridor and an ancestral Kimberley Fault, which is parallel to synsedimentary rift transfer faults identified elsewhere in the Aldridge Basin (Fig. 4) (Höy et al., 2000; Price and Sears, 2000). However, if this ancestral Kimberley Fault existed as a synsedimentary fault, it seems to have had little effect on subbasin lithofacies, because they can be traced in drill core across the present Kimberley Fault without any major change in thicknesses or lithologies (Ransom and Lydon, 2000). On the other hand, at the time of Moyie sill emplacement, there was an east-west structural grain to subbasin rocks, because dykes rising from the Mine sill follow this orientation. Furthermore, pyrite-chlorite-carbonate alteration, which appears to be genetically related to emplacement of the mine sill (Turner and Leitch, 1992), extensively overprints the Sullivan orebody along the present Kimberley Fault (see Fig. 9).

Morphology and Architecture of the Sullivan Deposit

The Sullivan deposit is a semiconcordant sulphide body whose economic portion is approximately 2,000 m by 1,600 m in area (Figs. 9, 10), and up to 100 m thick (Fig. 10) (Hamilton et al., 1982) that accumulated in the crater or on the flanks of a mud volcano (Ransom and Lydon, 2000; Turner et al., 2000b). The orebody can be divided into a thicker, more massive, western “Vent Complex” and a thinner, bedded eastern part, the two being separated by a structurally complex Transition Zone (Fig. 9, 10). The western part overlies a subvertical, hydrothermal upflow zone and alteration pipe, and is termed the Vent Complex. The main sulphide minerals are pyrrhotite, pyrite, sphalerite, galena, and minor boulangerite, with associated magnetite, cassiterite, manganiferous carbonate, and garnet. The ore horizon has been traced about 7 km east and 12 km south of Sullivan (Fig. 8) as the Concentrator Hill Horizon, a 3 to 7 m thick interval that consists of thin pyrrhotite layers and disseminations in laminated quartz-muscovite siltites and argillites containing geochemically elevated values of Pb and Zn (Fig. 13E) (Ransom and Lydon, 2000).

Footwall

The immediate footwall to the Sullivan deposit consists of vent breccias, extrusion breccias and mudflows of a mud volcano (“Footwall Conglomerate”), which is more than 80 m thick under the Vent Complex but tapers eastwards and northwards to a few metres of bedded sandstones and siltstones. Under the Vent Complex, the Footwall Conglomerate and footwall turbidites have been tourmalinized (Fig. 11A) to form an impressive alteration pipe about 1 km in diameter that can traced to a depth of 450 m below the orebody where it is cut by the gabbro and granofels of the Mine Sill (Fig. 10). Over an area that is more extensive than the tourmalinite pipe, footwall rocks are cut by a variety of sulphide-bearing veins and are impregnated with sulphide disseminations that locally almost entirely replace the host rock (Fig. 11A). Veins containing pyrrhotite, with minor sphalerite, galena, arsenopyrite, chalcopyrite, and quartz and/or calcite as gangue minerals are locally abundant beneath the massive sulphides of the Vent Complex (Hamilton et al., 1982).

Sullivan Ore Zone

Vent Complex. Massive sulphides of the Vent Complex comprise about 70% of Sullivan ore tonnage. The main sulphide body is up to 100 m thick and is coarsely layered, consisting of a lower uneconomic pyrrhotite-rich sulphide zone (Fig. 11B), a middle zone of tectonically foliated pyrrhotitegalena- sphalerite (Fig. 11C), and an upper zone of galenasphalerite- pyrrhotite massive sulphides that are intercalated with lithic units.

Bedded ores. The Bedded Ores consist of five concordant layers of interlaminated sulphides and argillite (Fig. 11D, E, F), termed the “Ore Bands” by Cominco, that are separated by beds of arenaceous to argillaceous mud flows, termed the “Waste Bands” (Fig. 12). From base to top, the principal sulphide layers have been termed the “Main”, “A”, “B”, “C”, and “D” Ore Bands, and the intervening siliciclastic layers, the “A”, “B”, “C”, and “D” Waste Bands (Fig. 12). The Bedded Ores are more than 30 m thick near the Transition Zone and gradually taper to <10 m at the economic limit of the orebody. The bottom part of the Main Band is fragmental in texture, consisting of subangular to rounded clasts of wall rock, pyrite, coarsely crystalline calcite, sphalerite, and quartz in a matrix dominated by pyrrhotite (Fig. 13A). This fragmental sulphide layer is a zone of durchbewegt sulphides, formed along a tectonic décollement at the base of the sulphide body during Cretaceous deformation.

Stratigraphic correlation between the Bedded Ores and the Vent Complex is lost in the Transition Zone, which is characterized by spectacular, convolute folding of the Bedded Ores (Fig. 13B). This deformation developed during the eastward movement of the Vent Complex over the Bedded Ores along reverse faults, which depressed the nose of the Bedded Ore sequence downward to the base of the Vent Complex massive sulphides. Galena-rich sulphides have been injected upward into the stratigraphic level of the “HU” hanging-wall ores (Paakki et al., 1995).

Hanging-wall ores. The hanging wall sequence to the main Sullivan orebody consists of three graded beds, the “I”, “H”, and “HU”, each up to 10 m thick and marked by a basal, quartz-rich arenite and an upper interval of laminated, pyrrhotitic and carbonaceous argillite up to about 3 m thick. Above the Transition Zone and arcing around the northern part of the hanging wall albitite zone, the laminated, pyrrhotitic, carbonaceous argillite intervals of the “I”, “H”, and “HU” graded beds contain bodies of laminated sulphides, very similar in appearance to those of the “A” through “D” Ore Bands (Fig. 11E). Parts of these sulphides have been mined. Conly et al. (2000) interpret these ores to reflect synsedimentary deposition but Paakki et al. (1995) point out that at least some of the Hanging Wall Ores represent galena-rich tectonic mobilization from the main orebody into the hanging wall.

Hydrothermal Alteration

There are at least five “stages” and more than seven types of hydrothermal alteration associated with the Sullivan deposit (Leitch et al., 2000):

1. Tourmaline alteration of the large footwall “tourmalinite pipe” (Fig. 10; 11A) and also small discontinuous bodies that are preserved around the periphery of albite-chlorite in the hanging wall.
2. Chlorite-pyrrhotite alteration occurs principally as a semiconformable body at the contact of the massive sulphides and footwall tourmalinite (Fig. 10) and also in discontinuous bodies at the hanging wall contact of the massive sulphides in the western part of the deposit (Shaw et al., 1993a). Argillite laminae within the Bedded Ores are almost invariably chloritized (Lydon et al., 2000a).
3. Chlorite-albite-pyrite alteration is most abundant in the immediate hanging wall of the Sullivan deposit (Fig. 10, 13C), vertically above the tourmalinite pipe (Shaw et al., 1993b), where there is a general zonation from an albite-rich core to a chloritepyrite- rich periphery. Chloritealbite- pyrite alteration also occurs along the western side of the Sullivan-North Star corridor and along contacts of gabbro sills (Turner et al., 2000c).
4. Pyrite-carbonate alteration. Immediately below the body of hanging wall albite-chloritepyrite alteration the main sulphide body has been replaced by a pyrite-chlorite-carbonate assemblage (Fig. 13D) (Shaw et al., 1993a), which is most intense in the “iron-core” (Fig. 9) (Hamilton et al., 1982). Along the Kimberley Fault, sulphides of the Ore Bands have been replaced by a pyrite-chlorite-calcite assemblage (Fig. 9) (Hamilton et al., 1982; Owens, 2000).
5. Carbonate alteration. Especially near the southern part of the Transition Zone, Waste Bands and to a lesser extent the Ore Bands, are heavily impregnated with calcitic carbonate. In some cases the waste bands are so heavily carbonated that they may be classified as an impure limestone.
6. Muscovite (sericitic) alteration forms an envelope around the entire deposit (Fig. 10) (Shaw and Hodgson, 1980; Leitch et al., 2000; Turner et al., 2000c), and also occurs along the extent of the Sullivan Corridor (Turner et al., 2000a). The envelope of sericite alteration around the deposit but the lack of significant potassium enrichment of sericitised rocks, suggests that it may be a metamorphic effect, in which biotite is sulphidized to iron sulphide and muscovite.
7. Other alteration types are of only local or sporadic extent, including chlorite-biotite-garnet and carbonatemuscovite near the eastern margin of the tourmalinite zone (Shaw et al., 1993a) and silicification near zones of muscovite-altered tourmalinite (Leitch et al., 2000).

These various types of alteration span the history of hydrothermal events at Sullivan, but most are not directly related to the main ore-forming event. The tourmalinite alteration pre-dates ore formation and may represent the venting of “vapour” from the phase separation of formational waters heated to high temperatures by sill emplacement at depth in the Lower Aldridge (Lydon, 2000b). The main role of the tourmalinite pipe in the genesis of the deposit may therefore have been to prepare a rigid hydrothermal conduit that channeled the upflow of later hydrothermal systems. Only the chlorite-pyrrhotite alteration has been linked to the main oreforming event (Shaw and Hodgson, 1986). The chloritealbite- pyrite, pyrite-carbonate, and carbonate alteration are all probably linked to the emplacement of the Mine Sill (Turner and Leitch, 1992).

Deformation

Although the Sullivan deposit appears to have preserved its overall original architecture in remarkably pristine fashion, there is intense deformation within the sulphide body. During Jurassic-Cretaceous deformation, horizontal compressive stresses produced horizontal movement of the hanging wall with respect to the footwall along décollements in the sulphide body (Campbell et al., 1980; McClay, 1983). The major décollement was along the base of the Main Band, which is marked by a zone of durchbewegung structure up to several metres thick (Fig. 13A). Movement along low-angle thrusts and bedding-parallel surfaces are expressed in the Vent Complex as gneissic banding and schlieren textures (Fig. 11C) and in the Bedded Ores as chloritic, slickensided, sharp contacts of sulphide layers with Waste Bands and bedding-parallel zones of durchbewegt sulphides (e.g. Fig. 11E). The Bedded Ores commonly contain zones of bedding-parallel compression (e.g. tightly folded layers) juxtaposed against zones of bedding-parallel extension (e.g. boudinaged or dismembered layers).

Zonation

Thickness

The thickness of the Sullivan deposit generally decreases in all directions away from the central part of the Vent Complex. The main interruption to this trend is due to tectonic thinning along the Transition Zone where sulphides of the main body have been squeezed out laterally by the downward movement of the Bedded Ore sequence along a reverse fault system towards the base of the main sulphide body. This squeezing produced the convolute folding typical of the eastern margin of the Transition Zone (Fig. 13B). Generally, the eastward decrease in thickness of the “A” through “D” Ore Bands is very gradual, although tectonic effects locally influence the isopachs of individual layers of the Bedded Ore sequence (Ransom and Merber, 2000). Thickness of the Waste Bands tends to mimic the thickness of the overlying Ore Band.

Grade and Composition

The Sullivan Deposit as a whole is strongly concentrically zoned with respect to Pb/(Zn+Pb) ratios, which gradually decrease outwards (Fig. 9) (Hamilton et al., 1982). Contours for Pb/(Zn+Pb) crosscut stratigraphy of the Bedded Ores and are steepest near the Transition Zone. In three dimensions, these contours form a concentric shell that is convex upwards about the Vent Complex. Grades of both Pb and Zn decrease outwards, the grade of Pb dropping off at a greater rate than that of zinc, so that the Pb/(Zn+Pb) ratios of the ore decrease both outwards and stratigraphically upwards. Tin, occurring mainly as cassiterite (Hamilton et al., 1982), and Cu are concentrated along zones marked by the highest Pb/(Zn+Pb) ratios. Copper minerals include tetrahedrite and chalcopyrite (Hamilton et al., 1982). Arsenic, which occurs in arsenopyrite and tetrahedrite, is highest in a zone surrounding the Iron Core (Freeze, 1966), whereas Sb is concentrated in a zone outside the high tin zone. The zone of high Sb content corresponds to high boulangerite content in the Bedded Ores. Manganese has more than twice the concentration in the Bedded Ores than it does in the Vent Complex ores but does not show a simple concentric zonation like Zn or Pb (Lydon et al., 2000a).

Mineralogy

There is a pronounced zonation of the main ore-forming sulphide minerals outwards from the base of the Vent Complex. In general, this zonation is primarily a decrease in the proportion of pyrrhotite relative to other sulphides of the ores away from the “barren” basal part of the Vent Complex where it is by far the dominant sulphide. The proportions of sphalerite and galena in the ores are greatest above and lateral to the barren pyrrhotite zone in the Vent Complex. Laterally away from the Vent Complex, as the grade of sphalerite and galena decrease, the ratio of fine-grained pyrite to pyrrhotite in the ores increases. Magnetite is intimately associated with this fine-grained pyrite both as layers and as porphyroblasts (Fig. 11E), the latter suggesting that some of the magnetite is of a retrograde metamorphic origin (Lydon and Reardon, 2000). Superimposed on this concentric zonation is a replacement of pyrrhotite-bearing assemblages by a late coarse-grained pyrite-carbonate-chlorite assemblage in both the core of the Vent Complex and along the Kimberley Fault (Fig. 9).

SEDEX Deposits

Sheep Creek

The Volcano Valley Fault is close to the northern limit of Belt- Purcell rocks of the Helena embayment and marks the locus of a Mesoproterozoic synsedimentary fault system. Along the southern side of the fault, laminated calcareous shales of the Newland Formation contain sedimentary debris flows and massive to laminated stratiform pyritic layers and lenses. The most extensive zone of pyritic beds occur near the upper contact of the lower Newland Formation , and is up to 100 m thick, has a strike length of about 15 miles (24 km), and extends up to 5 miles (8 km) south of the Volcano Valley Fault (Zieg, 1993). Gossans formed by weathering of these pyritic bodies were mined for iron in the 19th century. Locally, sphalerite, galena, and pyrrhotite replace the pyritic layers to form ore textures similar to the bedded ores of the Sullivan deposit. Although the pyritic Newland Formation at the Sheep Creek property (Fig. 13F ) was originally explored for its Zn-Pb potential, exploration attention became focused on its Cu-Co content. The Sheep Creek deposit consists of a stratigraphically lower pyrite zone associated with debris flows containing 4.0 million tonnes at 4% Cu and a stratigraphically upper baritic pyrite zone containing 4.5 million tonnes at 2.5% Cu and 0.1% Co (Fig. 14) (Himes and Petersen, 1990; Rankin and Zieg, 1990; Zieg, 1993; Zieg and Leitch, 1993). The Cu occurs as chalcopyrite in thin layers along microfractures and interstitial to pyrite; the Co occurs as cobaltian pyrite, which also has an anomalously high nickel content.

Other SEDEX Deposits

Within the Canadian portion of the Belt-Purcell Basin, the North Star deposit occurs at the same stratigraphic level and about 4 km south of Sullivan. It is an erosional remnant from which about 60,000 tons of Pb ore was mined from 1895 to 1929. Kootenay King is a small stratiform massive sulphide lens occurring in the Middle Aldridge Formation on the eastern side of the Rocky Mountain Trench, and from which about 13,000 tons of ore was mined from 1952 to 1953. Barite-base metal sulphide mineralization at theWilds Creek (Leg) and Dave deposits along the western edge of the Purcell anticlinorium have been mapped as occurring in the Dutch Creek or Kitchener Formation (Fig. 5), and may also be of the SEDEX type (Brown and Klewchuck, 1995). The Mineral King deposit, from which about 2.1 million tonnes of baritic Zn-Pb ores were mined from 1954 to 1967, was also suggested to be a SEDEX deposit in the rift-sag sequence of the Purcell Basin (Lydon, 1996), but it has since been shown to occur in a thrust slice of Devonian rocks (Lydon and Graf, 2000).

Within the American portion of the Belt-Purcell Basin, the Soap Gulch deposit near Melrose, Montana, may, like Sheep Creek, represent Mesoproterozoic SEDEX mineralization.

Besshi-Type Deposits

Blackbird District

The Cu-Co deposits of the Blackbird District (Fig. 15) occur in the middle unit of the Yellowjacket Formation, as a 5,200 m thick coarsening upward sequence of argillites, siltites, and quartzites (Nash and Hahn, 1989). Mafic dykes and sills are also common in the area (Nash and Hahn, 1989). The Cu-Co mineralization occurs as stratiform layers of cobaltite, chalcopyrite, and pyrite associated with biotite-garnet ±chloritoid layers (interpreted to be mafic tuffs or volcaniclastics) and tourmaline-bearing chemical sediments in the middle part of the sequence (Nash and Hahn, 1989; Bending and Scales, 2001). Some of the biotite-rich layers have a high Cl content (Nash and Connor, 1993). Cobaltitebearing, tourmaline-cemented breccias are common in the lower part of the sequence, but are generally of low Co grade (Nash and Hahn, 1989). Based on the association of the sulphide deposits with rocks interpreted to be mafic volcanics, the occurrence of both stratiform and discordant (vent or feeder zones) sulphide mineralization and hydrothermal alteration, and metal associations similar to Besshi-type deposits, the sulphide deposits are interpreted to be of seafloor origin (Nash and Hahn, 1989; Nold, 1990; Bending and Scales, 2001).At the Iron Creek deposit, 28 km southeast of the Blackbird deposit, the mineralization consists mainly of cobaltiferous pyrite associated with bedded magnetite.

Most authors correlate the Yellowjacket Formation with the Prichard Formation (Chandler, 2000). However, based on a comparison of detrital zircon ages, Link and Fanning (2003) suggested the Yellowjacket Formation is comparable to the Missoula Group (see stratigraphic correlations in Fig. 5) of Montana. This might imply that the volcanism may be associated with the episode of magmatism that also produced the Purcell lavas of the Snowslip Formation (see Figs. 3, 5) and Nichol Creek volcanics (see Fig. 5) of the Purcell anticlinorium, and not the magmatic event that produced the Moyie sills.

Vent Complex / Feeder Pipe Deposits

Fors

The Fors prospect (Britton and Pighin, 1995) is a blind deposit discovered by drill-testing low-grade surface mineralization. Fors is a pipe-shaped body of discordant sedimentary fragmentals that extends from below the Lower-Middle Aldridge contact to approximately 380 m above this horizon with a small overlying concordant cap (Fig. 16). The discordant sedimentary fragmental is variably altered to tourmalinite, and the concordant cap consists of actinolite-, talc-, and muscovite-alteration assemblages with disseminated to bedded sulphides. A 2 m thick semi-massive sulphide vein of pyrrhotite with variable amounts of sphalerite and galena, with accessory scheelite, chalcopyrite, and bismuthinite, cuts most of the actinolite-rich alteration zone and underlying albitized sediments. Although similar in many respects to the feeder pipe at Sullivan, Fors is at a higher stratigraphic level, contains abundant magnesium-rich calcsilicate and biotite alteration, and has elevated values of Ag, Au, W, As, and Bi. No resource has been calculated for this small deposit. The highest grade drill intercept was 1 m of massive sulphides with 9.35% Pb, 16.4% Zn, 0.09% Cd, and 98 g/t Ag, and 42 m of disseminated Pb-Zn mineralization, including 7 m of pyrrhotite that resembles the Concentrator Hill Horizon near Sullivan.

Other Vent Complex / Feeder Pipe Deposits

The Stemwinder deposit, about 2 km south of Sullivan, consists of discordant pyrrhotite, sphalerite, and galena associated with tourmalinized fragmental rocks. Production and reserves are about 150,000 tonnes at 16.6% Zn, 3.1% Pb, and 82 g/t Ag.

Stratabound Disseminated Sulphide Deposits

Redbed Copper Deposits

The Spar Lake, Rock Creek, and Montanore deposits of northwest Montana are hosted by the Revette Formation, a sequence of sandstones in the middle part of the Ravalli Group, which are the stratigraphic equivalent of the Creston Formation in Canada (Fig. 5). The Spar Lake deposit is laterally zoned, with the dominantly bornite- chalcocite-calcite assemblage of ore-grade mineralization fringed by lower grade zones of chalcopyrite, galena, and pyrite (Hayes et al., 1989). The ores are thought to have formed by the migration of warm (120ºC) fluids along bedding or along vertical fluid-escape structures with deposition of metal sulphides in quartzite pore spaces at a pre-existing pyrite-leucoxene interface before any major compaction of the rocks (Hayes and Einaudi, 1986; Hayes et al., 1989).

Disseminated Sphalerite and Galena

The Canam and Star mineral claims in the southern part of the Purcell anticlinorium are of this type. Höy et al. (2000) state that published Cominco lead isotope data of Canam galena indicates that it is of similar lead isotopic composition as Aldridge SEDEX and vein deposits (Beaudoin, 2000). Höy et al., (2000) suggest that the mineralization represents diagenetic sulphides deposited from hydrothermal fluids rising along active growth faults and spread laterally along permeable arenaceous layers

Veins

Coeur d’Alene District

The quartz-Fe carbonate-galena-sphalerite-tetrahedrite veins of the Coeur d’Alene district form one of the world’s most productive silver camps, having produced over 34,000 metric tonnes of silver up until the late 1990s (Long, 1998). The veins of the district can be divided into of two types based on their ore mineralogy:a low-silver galena-sphaleritepyrrhotite- pyrite type and a high-silver galena-tetrahedrite type (Leach et al., 1998). Veins hosted by the Prichard and Burke formations are typically of the low-silver type, whereas veins hosted by the Revette and St. Regis formations are typically of the high-silver type (Leach et al., 1988). Most veins occur within two kilometres of a 40 km long segment of the Osburn fault, a major west-northwesttrending structure with a 26 km right-lateral displacement within the Lewis and Clark structural zone (Fryklund, 1964; Bennett and Venkatakrishnan, 1982; Leach et al., 1998). The Lewis and Clark structural zone marks a major crustal structure that has been intermittently active since the Proterozoic (Harrison, 1972; Winston, 1986; White, 2000). The age of the mineralization is controversial: Pb/Pb isotope model ages for galena give ages of 1400 to 1500 Ma, (Zartman and Stacey, 1971) suggesting genesis during burial diagenesis, but Ar/Ar and Rb/Sr dating of gangue minerals give ages as young as Cretaceous suggesting a second metallogenetic event associated with emplacement of the Idaho and Kaniksu batholiths (Leach et al., 1998; Fleck et al., 2002). Leach et al. (1998) suggest that the galena-sphalerite-pyrrhotite-pyrite type of veins formed during the Proterozoic, and the galenatetrahedrite type formed during the Late Cretaceous to early Tertiary.

St. Eugene

The St. Eugene deposit (Fig. 4), the largest vein deposit in the Purcell Supergroup, consists of galena, sphalerite, pyrite, pyrrhotite, and minor chalcopyrite and tetrahedrite with a gangue of quartz and calcite that infilled a fracture system over a vertical extent of about 1300 m and over a strike length of about 3,000 m. About 1.5 million tonnes of ore were mined from the St. Eugene vein prior to 1916 from which about 113,000 tonnes of Pb, 15,000 tonnes of Zn, 180,000 kg of Ag, and 80 kg of Au were recovered (Höy et al., 2000). The St. Eugene deposit shares many similarities with veins of the Coeur d’Alene district, including Pb isotope ratios, (Beaudoin, 1997, 2000), mineralogy and oxygen isotope values of quartz (Joncas and Beaudoin, 2002). Joncas and Beaudoin (2002) suggest that the St. Eugene vein formed from metamorphic fluids with a heavy oxygen isotope signature prior to the Mesoproterozoic regional metamorphism.

Vine

The Vine deposit (Höy and Pighin, 1995) consists of massive pyrrhotite, sphalerite, and galena, with minor chalcopyrite and arsenopyrite and an associated quartz-calcite-chlorite gangue that follows a Moyie dyke over a vertical extent of about 800 m (Fig. 17). Lead isotope compositions of galena are similar to other Mesoproterozoic vein deposits of the Belt-Purcell Basin (Beaudoin, 2000), and Höy et. al. (2000) suggest that the fluid flow that precipitated the mineralization is associated with Aldridge extensional faulting and magmatism.

The different types of Proterozoic mineral deposits of the Belt-Purcell Basin can all be related to different stages in the sediment filling of an intracontinental rift (Fig. 18).

Early Rift – Besshi Deposits During initial rifting, when the sediment cover is thin (Fig. 18A), high-temperature (>300°C) seawater convection cells are able to develop within the competent rock of the crystalline basement, driven by the heat of mafic magma chambers. If discharged at the seafloor, they may form Besshi-type volcanogenic massive sulphide deposits. The stratiform Cu-Co deposits of the Blackbird mining district are in this category (Nash and Hahn, 1989; Bending and Scales, 2001). Their occurrence in the Yellowjacket Formation, which might correlate with the mid-Belt Missoula Group (Link and Fanning, 2003) and hence the 1443 Ma mafic magmatism, may only reflect the southward propagation of the Belt Purcell rift (Burtis et al., 2003) so that the onlapping Belt-Purcell sediments were close to basement at this southern extremity of the preserved parts of the basin.

Rift Fill - SEDEX Deposits

The most favourable environment for the formation of SEDEX deposits has long been recognized to be intracratonic rifts that have been filled by marine sediments, with the deposits being spatially associated with synsedimentary faults (Large, 1983; Lydon, 1996). Climatic conditions that produce evaporitic environments, and consequently highly saline formational waters, are an integral part of the classic genetic model for SEDEX deposits (Lydon, 1983), and therefore intracontinental rifts of low latitudes are the optimal geological setting. The ore fluids for SEDEX deposits are thought to be these saline formational waters that leached metals from argillaceous sediments during the smectite-illite transformation during burial diagenesis (Lydon, 1983) and were expelled to the seafloor as a consequence of basinal dewatering, either by the breaching of geopressured reservoirs by synsedimentary faulting (Lydon, 1983; 1986), by compaction-driven stratal migration (Cathles and Smith, 1983; Cathles, 1986) or by thermally-driven convection (Russell, 1983; Yang et al., 2004).

Typically, SEDEX deposits occur in the rift cover sedimentary sequence deposited during the rift-sag stage (Lydon, 1996), such as the SEDEX deposits of the Mt. Isa and McArthur basins of Australia, the Paleozoic deposits of the Selwyn Basin in Canada, and those of the Central Irish Basin. Sullivan, like Broken Hill of Australia, however, occurs in the rift-fill sequence, and is associated with the tectonically and magmatically most active stage of rift development (Fig. 18B). However, in all other aspects, Sullivan conforms to the classic SEDEX model: the Belt-Purcell rift developed in a low-latitude environment (Chandler, 2000), there is direct fluid inclusion evidence for highly saline formational brines (Leitch and Lydon, 2000); and it is associated with the synsedimentary faulting and the sedimentary fragmental rocks of the Sullivan-North Star Corridor (Turner et al., 2000a,b).

To explain 1) the spatial association of one of the world’s largest Zn-Pb deposits and one of the world’s largest tourmaline deposits (the alteration pipe below Sullivan); 2) the spatial association between tourmalinization and mud volcanos at Sullivan and elsewhere in the Belt-Purcell Basin; 3) the co-extent of mud volcanos and Moyie sills in the Belt- Purcell Basin; and 4) the formation of Sullivan during the early stages of the Moyie sill magmatic event, Lydon (2004) proposed a segregated hydrothermal diapir genetic model (Fig. 19). The essence of this model is that initiation of sill emplacement prematurely both compacted and superheated the Lower Aldridge turbidite sequence. The superheating caused sill emplacement into a low-density low-salinity fluid and a high-density hypersaline fluid, which, because of their large density contrasts, physically segregated into different hydrothermal reservoirs. Ascent of hydrothermal fluids from this reservoir system is envisaged as a slowly rising diapir. The initial release of formational fluid overpressure created by sill-induced compaction resulted in mud-volcano activity (Fig. 19, Stage 1). The overpressure created by phase separation of saline formational fluids was relieved by the ascent and venting of the low-density fluid, which utilized the conduits afforded by the mud-volcano vents (Fig. 19, Stage 2). This low-density low-salinity fluid tourmalinized the substrate along its ascent path, indurating the unconsolidated sediments and forming a mechanically competent crossstratal fluid conduit that served to channel the subsequent upflow of the high-salinity fluid. This high-salinity fluid, or brine (Fig. 19), was presumably a mixture of connate saline formational waters and the hypersaline fluid formed near sill contacts by high-temperature phase separation of superheated formational fluids. Ascent of the brine diapir took place only after the hydrothermal reservoirs had been evacuated of their low-density fluid, and only after the brine had been heated to buoyancy with respect to shallow formational fluids (Fig. 19, Stage 3). Venting of these brines at the seafloor produced the sulphide deposit. The hanging wall albitite is related to the local heating of ambient saline groundwaters surrounding the deposit by the Mine Sill, after the Sullivan deposit had been buried to at least 200 m, and the level of neutral buoyancy for sill emplacement had progressed upwards into the upper kilometre of Lower Aldridge sediments. The sulphide of the ores was likely both of hydrothermal origin and locally produced by the bacteriogenic reduction of the water column and/or hydrothermal sulphate (Campbell et al., 1978; Goodfellow, 2000; Taylor and Beaudoin, 2000).

The Sheep Creek deposit has all of the geological characteristics of a SEDEX deposit, except for its high Cu and Co content. Based on textural and fluid inclusion evidence, Himes and Petersen (1990) suggested that the deposit originated as a seafloor to early diagenetic pyritic body, that was invaded by hot (>250°C), moderately saline fluids before lithification of adjacent sediments. The expulsion of hot fluids was likely triggered by synsedimentary fault movement (Rankin and Zieg, 1990). The position of the Sheep Creek deposit at the edge of the basin suggests a stratal migration of fluids from the thicker and higher temperature part of the axial zone of the rift.

Rift Cover – Redbed Copper, Veins, SEDEX (?)

The exact ages of emplacement of the Redbed Copper deposits of the Revette Formation in Montana or the veins of the Coeur d’Alene district and the Purcell anticlinorium are not known. Both deposit types are epigenetic and both occur in rocks of the Ravalli Group (Fig. 5). Sulphides of the Spar Lake Cu-Ag deposit were deposited prior to any significant compaction (Hayes and Einaudi, 1986; Hayes et al., 1989), which suggests that they formed during or before deposition of the overlying Wallace Formation carbonates (Fig. 5). Major galena-bearing veins with Proterozoic Pb isotope signatures do not occur in rocks younger than Ravalli age (Creston age in Canada), which suggests that veins of this type also formed no later thanWallace-Kitchener time. If so, both Redbed Copper deposits and Pb-Zn-Ag vein deposits are related to migrations of saline basinal fluids during the earliest stages of the rift-cover phase (Fig. 3; Fig. 18C) following the very high accumulation rates of Creston and older sedimentary rocks (Fig. 6).

If direct comparison is made to the most productive part of the stratigraphy of other highly productive intracratonic rifts, the rift-cover sequence of the Sheppard Formation and younger strata should be the most prospective part of the Belt-Purcell Supergroup for SEDEX deposits. As noted above, the Mineral King SEDEX deposit, which was previously thought to be an example of this SEDEX potential in the upper part of Belt-Purcell stratigraphy (Lydon, 1996) actually occurs in a thrust slice of Devonian age rocks (Lydon and Graf, 2000). Other showings of stratiform baritebase metal mineralization on the western limb of the Purcell anticlinorium that have been mapped as occurring in Dutch Creek or Kitchener Formation (Brown and Klewchuck, 1995), including the Wild Creek (Leg) and Dave deposits, may be indicative of a SEDEX metallogenetic event in the upper part of Belt-Purcell stratigraphy. However, barite from the Dave deposit has sulphur isotope ratios similar to the Mineral King deposit, and likewise may also be in a Devonian thrust slice (Lydon and Graf, 2000).

SEDEX Deposits

There has been no research on what is the maximum amount of base metal that can be mobilized in an intracratonic sedimented rift and concentrated as SEDEX deposits. The fact that no second major deposit has been discovered despite decades of exploration in both the Broken Hill, Australia, and Sullivan areas, both of which are very large Pb-rich deposits of a rift-fill sequence, does not bode well for the discovery of another Sullivan in the axial part of the rift within the Aldridge-Pritchard formations. However, the occurrence of the Sheep Creek deposit at the margin of the Belt-Purcell Basin shows that the basin margins, as for example in the case of the Central Irish Basin, have potential that is deserving of investigation. Pb-Zn-Ag veins and Cu- Ag Redbed deposits formed in Ravalli Group (Creston equivalent) rocks shows that metalliferous fluids circulated in at least some parts of the basin during the lower part of the rift cover sequence. Höy (1993) documented synsedimentary faulting in the Sheppard and Gateway formations, subsequent to the magmatic event that formed the Nichol Creek volcanics and Purcell lavas. This combination of evidence for metalliferous basinal fluids, tectonic activity, and high heat flow during accumulation of the rift-cover sequence are positive indicators for SEDEX potential in the upper part of the Belt-Purcell stratigraphy.

Redbed Cu Deposits

With two recently discovered large deposits waiting to come into production, the potential for the discovery of more Redbed Copper deposits in the Revett Formation of Montana is obvious. The quartzite facies, which has trapped the Cu- Ag mineralization, does not appear to continue northwards as part of the Creston Formation in Canada. However, the green and purple colours of the Creston Formation do indicate intraformational redox fronts, so there is potential for at least small deposits of this type.

Iron Oxide Copper-Gold (Olympic Dam type) Deposits

If the fundamental conditions for the genesis the Olympic Dam and Cloncurry subtypes of iron oxide-copper-gold (IOCG) deposits (e.g. Hitzman et al., 1992; Kerrich et al., 2000; Barton and Johnson, 2002) is met by the emplacement of a felsic pluton into a sedimentary basin containing saline formational waters, then there is potential for this deposit type in the Belt-Purcell Basin. Attention to this possibility has already been drawn with respect to the magnetitehematite deposits of the Iron Range Mountains in southeastern British Columbia (see Fig. 4) (Stinson and Brown, 1994) and the iron oxide-copper-thorium-REE veins of the Lemhi Pass area in Idaho, which have given a 1055 Ma age (Gillerman et al., 2003). The apparent occurrence of large 1370 Ma granites in the Salmon River area (Evans and Zartman, 1990) support potential for granite emplacement into Belt-Purcell rocks while they still possessed saline formational waters.

Ni-Cu-PGE Deposits

The Mesoproterozoic tholeiitic and alkaline sills represent an upflow of a large volume of mafic magma that has been partly contaminated by assimilation of crustal rocks (Anderson and Goodfellow, 2000). The assimilation of sulphide in crustal rocks is a key component of genetic models for such Cu-Ni deposits as Noril’sk (Eckstrand, 1996). The upflow zones of Belt-Purcell magmas, and perhaps the thickest sills (up to 500 m), are therefore good candidates for magmatic sulphide segregations containing copper, nickel, and PGE. The hydrous autometamorphism of the sills by interaction with wet sediments adds the possibility of Pd-rich hydrothermal Cu-PGE deposits. That the process has taken place, at least on a small scale, is illustrated by the occurrence near Dixon, Montana of a 3 metres thick deposit in a gabbro that post-dates the Ravalli group, which grades about 3 ppm PGE over a strike length of 8 km (Buckley et al., 1994; Lauer 1994), and by samples geochemically enriched in PGE from the Yahk area of British Columbia (Höy, 1989).

In order to more completely understand the timing and location of ore-forming processes and hence realize the full mineral potential of the Belt-Purcell Basin, particularly the Canadian portion, it is necessary to address the following knowledge gaps:

1. The need for direct radiometric dating of ore-forming events in order to relate the causes of ore formation to other geological events. Very few deposits have been radiometrically dated.

   This can be carried out by direct dating of the ores by Nd/Sm of carbonate and sulphides; Re/Os on pyrite and other sulphides; Rb/Sr of carbonates, sulphides, and fluid inclusions; U/Pb on xenotime; etc. or by bracketing the age of the ore horizon by U/Pb dating of host rocks, e.g. intercalated volcanic ash horizons. Important among these are the assumed Mesoproterozoic vein deposits and the assumed SEDEX deposits on the western limb of the Purcell anticlinorium.

2. The need to develop prognostic criteria as to the favourability of a sedimentary basin to contain SEDEX deposits.

   This is a problem that is applicable to SEDEX deposits in general. The investigation of the Belt-Purcell Basin, and its successor rift-controlled Widermere Basin (note the recent discovery of Windermere sills and dykes in Montana (Burtis et al., 2003)), would involve comparative studies of basinal evolution on productive and non-productive basins. Essential information would include

   1. Sediment accumulation rates (needs radiometric dating control).
   2. Evolutionary paths of mineralogical and lithogeochemical changes during diagenesis and burial metamorphism to understand the timing and controls of metalleaching windows. This would require stratigraphically controlled lithogeochemical and mineralogical profiles supported by radiometric dating of diagenetic and burial metamorphic minerals.
   3. Evolutionary paths of pore fluids in the basin, with emphasis on determining the stage or circumstances under which pore fluids become metalliferous. This would have to rely on measuring the chemical composition of fluid inclusions in cements and diagenetic minerals.
   4. History of tectonic and magmatic events within the basin. The recent discovery of rift-related magmatic events high in the stratigraphy of the Belt-Purcell Basin, and the continuation of these events in the succeeding Windermere Basin (e.g. Burtis et al., 2003; Chamberlain et al., 2003), shows that the history of the Belt-Purcell Basin is still incompletely understood. These knowledge gaps hinder the recognition of the full potential of the basin for the occurrence of mineral deposits.
   5. Paleoclimatic and water column history of the basin. This would require various geochemical and isotopic (e.g. sulphur, carbon, and strontium) profiles through the basin and systematic observations on lithological and sedimentological environmental indicators.

3. Need to develop prognostic criteria as to the most favourable stratigraphic interval(s) and the most favourable spatial position(s) for SEDEX deposits in a sedimentary basin.

   This requires coming to a fundamental understanding as to the circumstances that lead to a focused expulsion of formational waters in a sedimentary basin. This would require developing expertise on the hydrology of modern sedimentary basins and the factors that control the migration of large volumes of basinal fluids. Much of this could be accomplished by compilation (especially of data generated by the oil exploration industry) and mathematical modeling of fluid flow, using rock property parameters for the Belt-Purcell Basin.

Most of the author’s awareness of the Belt-Purcell Basin was developed in conjunction with the 1991-96 Sullivan Project carried out by the Geological Survey of Canada in collaboration with Cominco Ltd. (now Teck Cominco), the British Columbia Geological Survey Department, the United States Geological Survey, and various universities. In particular Trygve Höy, Doug Anderson, Paul Ransom, Ray Price, and Jerry Zieg are thanked for sharing their knowledge of the Belt-Purcell Basin, both during the project and subsequently. Cominco personnel who had worked at the Sullivan mine, especially Owen Owens, John Hamilton, Paul Ransom, and Norris Del Bel Belluz, are thanked for sharing their knowledge and experience, and for supplying some of the photographs used in this paper. Karen Kelly and Wayne Goodfellow are thanked for their careful reviews of an earlier version of this paper and for making suggestions to improve its clarity and quality.

Table 1: A selection of grade and tonnage data for Mesoproterozoic deposits of the Belt-Purcell Basin

Deposit type	Deposit name	Cu %	Co %	Zn %	Pb %	Ag g/t	Au g/t	Tonnes	Comments	Reference
Seafloor Sulphide deposits
SEDEX
	Kootenay King	0.00		15.60	5.35	67	0.00	13,000	Mineral resource	BC Minfile Number: 082FGNW009
	North Star	0.00		6.12	35.50	673	0.00	61,000	Production	BC MINFILE Number: 082FNE053
	Sullivan	0.00		5.86	6.08	67	0.00	161,970,000	Original mineral resource	Hamilton eta al., 1983
	Sheep Creek Lower Zone	4.00						4,000,000	Mineral resource	Zieg, 1993
	Sheep Creek Upper Zone	2.50	0.10					4,500,000	Mineral resource	Zieg, 1993
Besshi
	Blackbird	1.50	0.60					960,099	Production 1951-1959	Nash and Hahn, 1989
	Ram & Sunshine	0.54	0.68				0.56	1,922,568	Proven, probable and inferred resources	Bending and Scales, 2001
Vent Complex deposits
	Stemwinder	0.00		15.60	3.70	76	0.00	25,000	Production	BC Minfile 082FNE116
Stratabound disseminated sulphide deposits
Redbed Copper Deposits
	Spar Lake	0.70				43		80,000,000	Pre-mining mineral resource	Calculated from data in Reipas and Couture, 2005
	Rock Creek	0.72				45		124,200,000	Mineral resource	Calculated from data in Couture and Tanaka, 2005
	Montanore	0.78				60		136,000,000	Mineral resource	Long et al., 1998
Veins
Coeur d'Alene district
	Bunker Hill	0.01	0.00	3.72	0.28	45	0.01	1,279,729	Production 1981-90.	Bennett and Singer, 1992
	Sunshine	0.36	0.00	0.00	0.01	706	0.01	1,499,832	Production 1981-90.	Bennett and Singer, 1992
	Coeur	0.75	0.00	0.00	0.00	559	0.08	1,357,406	Production 1981-90.	Bennett and Singer, 1992
	Galena	0.58	0.00	0.00	0.00	621	0.08	1,797,867	Production 1981-90.	Bennett and Singer, 1992
	Lucky Friday	0.22	0.00	1.42	11.29	582	0.30	1,520,362	Production 1981-90.	Bennett and Singer, 1992
	Star-Morning	0.00	0.00	5.90	4.24	95	0.00	497,068	Production 1981-90.	Bennett and Singer, 1992
	Coeur d'Alene district 1884-1997	0.00	0.00	2.62	6.66	313	0.00	109,496,626	Production 1884-1997. Grades estimated	Long, 1998
Purcell anticlinorium
	St. Eugene			0.98	7.66	124	0.05	1,475,266	Production	BC Minfile 082GSW025
	Vine			2.40	4.60	52	1.80	547,000	Mineral resource	Höy and Pighin, 1995

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Anderson, H.E., and Davis, D.W., 1995, U-Pb geochronology of the Moyie sills, Purcell Supergroup, southeastern British Columbia: Implications for the Mesoproterozoic geological history of the Purcell (Belt) Basin: Canadian Journal of Earth Sciences, v. 32, p. 1180-1193.

Anderson, H.E., and Goodfellow, W.D., 2000, Geochemistry and isotope chemistry of the Moyie sills: Implications for the early tectonic setting of the Mesoproterozoic Purcell Basin, Chapter 17 in Lydon, J.W., Höy, T., Slack, J.F., and Knapp, M., eds., The Geological Environment of the Sullivan Deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 1, p. 302-321.

Anderson, D., and Höy, T, 2000, Fragmental sedimentary rocks of the Aldridge Formation, Purcell Supergroup, British Columbia, Chapter 14 in Lydon, J.W., Höy, T., Slack, J.F., and Knapp, M., eds., The Geological Environment of the Sullivan Deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 1, p. 259-271.

Anderson, H.E., and Parrish, R.R., 2000, U-Pb geochronological evidence for the geological history of the Belt-Purcell Supergroup, southeastern British Columbia, Chapter 7 in Lydon, J.W., Höy, T., Slack, J.F., and Knapp, M., eds., The Geological Environment of the Sullivan Deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 1, p. 113-126.

Barton, M.D., and Johnson, D.A., 2002, Alternative brine sources for Feoxide (-Cu-Au) systems: Implications for hydrothermal alteration and metals, in Porter, T.M., ed., Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective: PGC Publishing, Adelaide, v. 1, p. 43-60.

Beaudoin, G., 1997, Proterozoic Pb isotope evolution in the Belt-Purcell Basin: Constraint from syngenetic and epigenetic sulfide deposits: Economic Geology 92, p. 343-350.

———2000, The lead isotope composition of the Sullivan deposit, Chapter 31 in Lydon, J.W., Höy, T., Slack, J.F., and Knapp, M., eds., The Geological Environment of the Sullivan Deposit, British Columbia: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 1, p. 574-581.

Bending, J.S., and Scales, W.G., 2001, New production in the Idaho Cobalt Belt: A unique metallogenetic province: Transactions of the Institute of Mining andMetallurgy, Section B,Applied Earth Sciences 110, p. 81-87.

Bennett, E.H., and Springer, D., 1991, Mine Production in the Coeur d'Alene District, 1981-1990: Idaho Geological Survey GeoNote 20, 1 p.

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Brown, D.A., and Klewchuck, P., 1995, The Wilds Creek (Leg) zinc-leadbarite deposit, southeastern British Columbia: Preliminary ideas: British Columbia Ministry of Energy, Mines, and Petroleum Resources, Geological Fieldwork 1994, Paper 1995-1, p. 157-164.

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Slack, J.F., Shaw, D.R., Leitch, C.H.B., and Turner, R.J.W., 2000, Tourmalinites and coticules from the Sullivan Pb-Zn-Ag Deposit and vicinity, British Columbia: Geology, geochemistry, and genesis, Chapter 39 in Lydon, J.W., Höy, T., Slack, J.F., and Knapp, M., eds., The Geological Environment of the Sullivan Pb-Zn-Ag Deposit, British Columbia, Geological Association of Canada, Mineral Deposits Division, Special Publication No. 1, p. 736-767.

Stinson, P., and Brown, D.A., 1994: Iron mineralization and ultramafic dikes on Iron Range Mountain, southeastern British Columbia: Northwest Geology, v. 23, Addendum, p. 99-101.

Swanson, C.O., and Gunning, H.C., 1945, Geology of the Sullivan Mine: Canadian Institute of Mining and Metallurgy Transactions, v. 48, p. 645-667.

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Turner, R.J.W., and Leitch, C.H.B., 1992, Relationship of albitic and chloritic alteration to gabbro dykes and sills in the Sullivan mine and nearby area, southeastern British Columbia, in Current Research, Part E; Geological Survey of Canada, Paper 92-1E, p. 95-105

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[Click on an image thumbnail to view a larger image, notice]

 

	Figure 1: Map showing the outcrop extent of the Belt-Purcell basin, the locations of major mineral deposits, and the simplified distribution of sedimentary facies of the lower part of the Belt-Purcell Supergroup (i.e. Aldridge-Pritchard Formations and stratigraphic equivalents).Numbers refer to profiles in Figure 3.  The locations of minor mineral deposits occurring in Canada are shown in Fig.
	Figure 2: Continental reconstruction for the Mesoproterozoic after Sears and Price (2000, 2003a, 2003b). Note that a different reconstruction (Hoffman, 1991) based on correlation of Grenville-age tectonic belts places Antarctica contiguous to the western edge of Laurentia, with Australia contiguous to the Hart River basin.
	Figure 3: Stratigraphic correlation and comparison of average thicknesses between formations of the rift fill, rift cover and platform sedimentary sequences of the Belt-Purcell Supergroup. Locations of profiles are shown in Fig. 1.
	Figure 4: Geological map of the southern part of the Purcell anticlinorium showing the locations of Mesoproterozoic sulphide deposits, tourmalinite occurrences, sedimentary fragmental bodies, and major synsedimentary faults. Thick blue lines indicate the approximate position of faults active during the Mesoproterozoic.  Lithologies of geological units are shown on Fig. 3. After Höy et al., 2000.
	Figure 5: Stratigraphic correlation of the formations of the Belt-Purcell Supergroup showing the stratigraphic positions of the major mineral deposits. Note that there is no vertical scale. Modified after Chandler, 2000.
	Figure 6: Average thickness of stratigraphic divisions of the Purcell Supergroup (Höy, 1993) and stratigraphic or depth-controlled events dated by U-Pb zircons or Pb-Pb titanite methods (see Lydon, 2000a for sources of data). The thick dark blue line is a smoothed joining of the geochronological data points, the slope of which gives averaged accumulation rates for the Belt-Purcell basin.  The data suggest an average accumulation rate of 570m/my for the Lower and Middle Aldridge Formation. The dotted blue line indicates the minimal accumulation rates for the Lower Aldridge, assuming a minimal erosion period of 25 my for exposure of the Priest River basement gneiss.  Modified after Lydon (2000a).
	Figure 7: Map of the outcrop area of the Belt-Purcell Supergroup showing that the occurrence of Aldridge-Pritchard age tourmalinite occurrences are largely coextensive with the tholeiitic Moyie sills. Note that the isoliths are for outcropping Moyies sills of the Middle Aldridge and the uppermost part of the Lower Aldridge Formation, and do not reflect the volume of sills seismically imaged in the lower part of the rift fill sequence (see Fig. 6). Data from Höy et al., 2000 and sources cited therein.
	Figure 8: Map of the major geological features of the Sullivan Sub-basin and the Sullivan-North Star Corridor.  CHH refers to the laminated muscovite and pyrrhotite-rich laminated facies of the Concentrator Hill Horizon.
	Figure 9: Map of Sullivan deposit showing the average Pb /(Zn+Pb) ratio for the entire ore thickness (after Hamilton et al., 1982) and the approximate distribution of pyrite-rich sulphides. Note that the grid coordinates are in feet.
	Figure 10: Semi-schematic west-east cross-section through the Sullivan deposit showing the distribution of major ore types and major alteration types (based on Hamilton et al., 1982)
	Figure 11: A. Tourmalinized Footwall Conglomerate about 30m below the bases of the Vent Complex of the Sullivan orebody. The clasts of the sedimentary fragmental rock are clearly preserved. The tourmalinite is cut by a network of pyrhhotite veinlets and disseminated grains which are concentrated in the matrix and highlight the sedimentary fabric of the rock. B. Contact relationships between a remnant block of galena-sphalerite-pyrrhotite banded ore in massive “barren” pyrrhotite which replaces the basal part of the orebody in the Vent Complex. 39-R-14 area. Width of photo about one metre. Photo by Cominco, from Leitch et al. (2000). C. Middle part of the Vent Complex massive sulphide body showing the strong tectonic foliation marked by galena-rich schlieren of high-strain zones. Vertical field of view about 1.5 metres. 34-R stope. Photo by Cominco. From Lydon et al. (2000a). D. Interlayered laminated spalerite-galena-pyrrhotite sulphides and argillite (dark layers) in the Bedded Ores “A” Band. Note limits of Ore Band are marked by white chalk, below and above which are the mud flows of the “A” and “B” Waste Beds respectively. Width of photo about two metres. Photo by Cominco. E. Polished slab from the “B” Band, 3200 SE Crosscut, Sullivan mine, showing the typically laminated nature of the Bedded Ores. Note the layer of durchbewegt pyrrhotite-rich sulphides which reflects bedding-parallel tectonic movement – a common feature of the Bedded Ores. From Lydon and Reardon (2000). F. Laminated pyrite interlayered with argillite, and thin layers of coarsely crystalline magnetite, in the Ramp Extension, Southeastern Fringe of the Sullivan orebody. Some of the pyrite layers have been partially replaced by pyrrhotite. The small “spots” in the laminated pyrite layers are porphyroblasts of manganifereous garnet. From Lydon et al. (2000a).
	Figure 12: Idealized mine stratigraphy in the Bedded Ores. Modified after Hamilton et al., (1982).
	Figure 13: A. Durchbewegt sulphide, basal part of Main Band, DDH 7246, 136 ft. Rounded clasts of pyrite, argillite and coarse-grained calcite in a pyrrhotite matrix. Durchbewegung structure is produced by autogenous milling of the more mechanically competent fragments during tectonic flow of the mechanically incompetent matrix. B. Convolute folding of galena-rich bedded ore in the Transition Zone. Dark layers are argillite. Note pyrrhotite-rich sulphides at bottom of photograph. Width of photo about two metres. 4250-P Scram 3. Photo by Cominco. From Lydon et al. (2000a). C. Interlayer albitised (white) areanceous part and chlorite-pyrite-albite altered (dark colured) siltite-argillite part of thin bedded turbidites from 45 Shaft in the hanging wall of the Sullivan deposit. Field of view about two metres. Photo by Cominco. D. Pyrite-carbonate alteration of the pyrrhotite-bearing massive sulphides of the main ore body in the “Iron Core” of the Vent Complex. Vertical extent of photograph is about one metre. Photo by Cominco. E. Polished slab of drill core from the distal hydrothermal sediments of the Concentrator Hill Horizon (CHH). The laminated rock consists mainly of a muscovite-rich argillite and siltite with thin pyrhhotite layers and is geochemically anomalously high in zinc and lead contents. F. Drill core intersection of the Upper Ore Zone, Sheep Creek, showing typically interbedded argillite layers (light grey), calcareous microturbidites (dark grey) and fine grained pyrite (brown). Evidence that the pyrite layers represent replacement of calcareous microturbidites is the absence of calcareous microturbidites in pyretic intervals and the incomplete replacement of calcareous microturbidite layers by pyrite from the top down at locations indicated by the white arrows.
	Figure 14: Cross section through the Sheep Creek deposit (modified after Himes and Peterson, 1990).
	Figure 15: Geological map of the Blackbird mine area, Idaho, showing locations og Cu-Co bodies. Note middle unit of Yellowjacket Foramtion is about 1000m thick. Cu-Co sulphides occur over its entire thickness. From Nash and Hahn (1985).
	Figure 16: Schematic cross-section of the Fors deposit. Abbreviations: po – pyrrhotite; sp – sphalerite; ga – galena; aspy – arsenopyrite; W – scheelite.  From Höy et al. (2000), after Höy and Pighin (1995).
	Figure 17: Schematic cross-section of the Vine vein deposit. From a diagram in Höy et al., 2000, after Britton and Pighin (1995).
	Figure 18: Schematic representation of the evolution of the Belt-Purcell basin and associated metallogenetic events.
	Figure 19: Model involving the segregation of a hydrothermal diapir driven by the emplacement of thick tholeiitic sills into a thick succession of unconsolidated marine turbidites to explain the three main sequential seafloor hydrothermal events at Sullivan: 1: mud volcano activity with minor deposition of manganese carbonates/oxides; 2: the intense tourmalinization of the mud volcano vent and surrounding sediments; and 3: the deposition of sulphides. From Lydon (2004).