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Courageous Lake: GEOLOGY
The Slave Structural Province, bounded to the north by Coronation Gulf and to the south by Great Slave Lake and wedged between the Bear Province to the west and the Churchill Province to the east, covers approximately 310,000 sq km in northeastern Northwest Territories and Nunavut. The predominantly Archean rock types of the Province are granite plutons, broad platforms of sedimentary rock, and narrow volcanic belts.
The Archean sedimentary and volcanic supracrustal rocks that crop out in the Province have been termed Yellowknife Supergroup by Henderson (1943). Rocks of this terrane are a product of diverse tectonic origin including: rift and shelf assemblages, breakup-type tholeiitic basalt sequences, arc sequences, turbidites, and late-tectonic clastic basins. Postulated to underlie the Yellowknife Supergroup, possibly separated by a regional décollement, is significantly older sialic basement, termed the Central Slave Basement Complex (Bleeker et al., 1999) composed of a diorite to tonalite gneiss.
Courageous-MacKay Lake Greenstone Belt
The Courageous-MacKay Lake Greenstone Belt (CLGB) is a steeply east dipping, north to northwest trending, homoclinal sequence. These sequences of metavolcanic and metasedimentary rocks of the Yellowknife Supergroup form a composite pile 3 to 7 km wide and about 70 km long. Regional mapping demonstrates that the sequences are not overturned and stratigraphic tops are to the east. The CLGB is bounded to the west by a sodic granite pluton referred to as the Courageous Lake Batholith, and to the east by conformably overlying turbidite metasedimentary rocks (Moore, 1956). Dynamothermal regional metamorphism within the CLGB has created mineral assemblages indicative of mid-greenschist facies metamorphic grade. Lower-amphibolite facies metamorphic grade have been identified at the north and south limits of the CLGB. Four discrete deformational events are recorded in these rocks.
Based on lithogeochemical analysis and mapping done by Wells (1998) on the CLGB, the depositional environment is envisioned as one or more evolving island arcs. Early widespread tholeiitic volcanism was followed by more restricted calc-alkaline eruptive centers, or islands. Bimodal tholeiitic/calc-alkaline volcanic sequences of this type are common in Archean greenstone belts (Wells, 1998). Yellowknife Group sedimentary rocks, mainly greywackes and siltstones, are interpreted to represent later stage proximal basin filling.
The CLGB extends north-south for about 70 km, reaching a maximum thickness near Matthews Lake, and narrowing to a few tens of meters at either end. It has been described as consisting of two mafic to felsic cycles of volcanism. The basal cycle comprises mainly mafic to intermediate flow and pyroclastic rocks, which have been largely assimilated by the Courageous Lake Batholith to the west. The basal cycle of basalt and andesite is capped by a narrow band of rhyolite flow and tuff, which locally reaches a thickness of 60 meters. The interpreted lower cycle contains numerous base metal showings, including the Deb deposit (Dillon-Leitch, 1981; Ransom and Robb, 1983).
The second cycle contains mafic to felsic flow and pyroclastic rocks. The felsic rocks are much more extensive in cycle two, attaining a thickness of 1800 meters near Matthews Lake, and a lateral extent of 25-30 km. They are composed of massive to porphyritic flow, tuff and coarse pyroclastic units. Felsic volcanic rocks of cycle two are conformably overlain by the Yellowknife Group sedimentary rocks. The upper part of cycle two interfingers with the overlying sedimentary rocks and is host to nearly all known gold occurrences in the belt (Ransom and Robb, 1983).
An alternative interpretation of the volcanic stratigraphy is proposed based on work conducted by Seabridge. This interpretation calls for a single evolving volcanic succession rather than two distinct cycles. This modification is based on the observations that the "second cycle" is preserved only in the central part of the greenstone belt and is intimately associated with dome-like rhyolite intrusions. On the margins of the CLGB only a single cycle of mafic to felsic eruptive rocks are present. Seabridge's alternate interpretation would have the original basin filling with basaltic and andesitic lavas. As the basin evolved, rhyolitic volcanic rocks began to be deposited with the mafic rocks. There are indications of bimodal volcanism throughout the volcanic succession. In the central part of the basin, the volume of rhyolitic volcanic rocks swamp out the contribution of mafic volcanic rocks, leading to a thick sequence of "cycle 2" volcanic rocks. Only locally in the central part of the basin are mafic rocks preserved once the rhyolitic volcanism began. As the rhyolitic volcanism waned up-section, sedimentation became more common until it completely overtook volcanism, giving way to the turbidite sequence.
According to Dillon-Leitch (1981), CLGB supracrustal rocks have undergone three stages of metamorphism analogous to the history of the Yellowknife area reported by Ramsey and Kamineni (1977). These events include:
- early regional greenschist facies metamorphism associated with northwest-southeast compression.
- laterally discontinuous contact thermal metamorphic aureoles of greenschist and amphibolite facies associated with granite intrusions.
- late hydrothermally induced retrograde metamorphism in amphibolite facies rocks.
All metamorphic events took place under moderate confining pressures between 2 Kb and 4 Kb (Dillon-Leitch, 1981). The metasedimentary rocks exhibit the greatest variety and continuity of metamorphic assemblages.
Prograde mineral assemblages in and adjacent to the FAT deposit are the product of the early mid-greenschist facies grade metamorphic event. The constituent minerals are chlorite+muscovite+biotite with minor almandine garnet porphyroblasts. The discontinuous and scattered distribution of the garnets probably indicates compositional control rather than increasing pressure conditions. No mineralogical indication of a retrograde metamorphic event has been noted in the FAT deposit area. The high quality of preservation of the original rock textures in the FAT deposit and the lack of a mineralogically distinct post-metamorphic hydrothermal event demonstrates that no regional metamorphic or hydrothermal events have affected these rocks since the initial greenschist facies metamorphism.
Chlorite group minerals are common pro-grade mineral assemblages associated with mid-greenschist regional metamorphism in the CLGB. In areas where garnet zone metamorphic grade and higher was achieved, chlorite is rare and probably retrogressive pseudomorphs after almandine+corderite+biotite. Chlorite alteration is noted in drilling as dark green, scaly masses on fractures and indistinct halos around zones of abundant garnets and biotite. Chlorite is commonly associated with sericite+biotite alteration. Chlorite alteration in the FAT deposit is not associated with gold mineralization and is mentioned here as a probable artifact of the metamorphic events that affected the mineral system.
Folds and cleavages formed during four deformation phases have been identified during various generations of work in the CLGB. The distinctions between various phases are based on field relationships and oriented thin sections by Dillon-Leitch (1981). As with metamorphic phases, the metasedimentary rocks have more clearly preserved the structural history of the area.
The earliest phase of ductile deformation is manifest as an east facing homocline. The structure is open, has an axial trace that trends roughly north-south, and is flat lying to shallowly south plunging. The interlimb distance of the structure is approximately 2 km. It is postulated by Dillon-Leitch (1981) that the homocline formed in response to diapiric upwelling of sodic granitoids along a preexisting north-south oriented, deep seated fault on the western margin of the CLGB. Synclines, along the eastern margin of the CLGB, were formed in Yellowknife Group sedimentary rocks. These features formed against a static granite body during the diapiric rise of granite on the west side of the CLGB. Continued tilting of the homocline and east-west compression developed major isoclinal folds in the sedimentary rocks. The axial traces of these folds parallel the trend of the CLGB, except where folds deflect around the static granite plutons. Successive periods of regional, subhorizontal compression created S1 and S2 foliation fabrics and cleavages. The maximum strain over the belt is believed to coincide with peak metamorphism and thermal doming during granite emplacement (Dillon-Leitch, 1981).
Late stage, brittle faulting in the CLGB is generally restricted to 2 repeated orientations, north-northwest and east-northeast. In the deposit area north-northwest faults are dextral and have a right-lateral sense of movement. The east-northeast faults exhibit sinistral rotation with left-lateral displacement. Orientations and sense of movement of the faults indicate they are probably coeval with emplacement of gabbro dikes, and Proterozoic in age (Zhang, 1998).
Petrography and Lithogeochemistry
Various generations of petrographic and lithogeochemical analyses have been done on the rock units of the FAT deposit and the various igneous lithologies of the CLGB. The objectives of these investigations were:
- characterize the volcanic and intrusive rock lithologies.
- explain the genesis of source magmas.
- determine the controls on gold mineralization.
Geochemical signatures from sample suites of the CLGB metavolcanic rocks indicate a typical tholeiitic to calc-alkaline, Archean greenstone volcanic succession. Mafic volcanic rocks are metaluminous, sub-alkaline, of tholeiitic affinity and basalt to basaltic-andesite in composition. Felsic volcanic rocks are dacite to rhyolite in composition with chemical affinity to subduction related magmas (Wells, 1998).
All previous work has reiterated the strong associations between alkali depletion, sericite (K-metasomatic replacement of Ca, Na) alteration and silica alteration, with gold concentrations. Strataform quartz introduction and secondary silica alteration are shown to have good correlation with As, Au, Ag and W enrichment. TiO2 vs. Zr scatterplots done by Wells (1998) and Madeisky (1999) on full suites of FAT host lithologies suggest that gold is not concentrated in any unique lithology.
The FAT deposit is a strataform series of near-vertical, elongate lenticular ore domains hosted in Archean tuffaceous clastic rocks and ash-flow tuff. Gold mineralization is interpreted to be a product of an episodic, epithermal-like, submarine and subaerial, hydrothermal system. Regional deformation has imparted minor metamorphic mineralogical and geometric modifications to the deposit. The hydrothermal system is interpreted to have formed within an emerging, peraluminous, calc-alkaline rhyolite to rhyodacite volcanic edifice. Although there is no strict lithological control to the gold distribution in the FAT deposit, each of the identified ore domains has a consistent stratigraphic architecture that distinguishes them. Gold concentrations are associated with:
- K-metasomatism manifested as sericitic alteration
- strataform quartz zones accompanied by broad, variably intense silicic alteration
- concentrations of acicular arsenopyrite crystals
FAT Deposit Description
Since its discovery, several common points have been used to describe the FAT deposit, including:
- A series of gold zones concentrated in long and narrow bands.
- The deposit is hosted by metasedimentary and metavolcanic rocks.
- Gold concentrations are associated with the introduction of silica, muscovite and sulfide minerals.
Several early operators (e.g. Giant Yellowknife Mines and Noranda Mining Inc.) in the CLGB utilized gold deposit models based on metamorphogenic lode gold concepts developed in the Superior Province to describe the FAT deposit (Ransom and Robb, 1985, Kemp, 1987). Placer Dome demonstrated that the FAT deposit did not form through metamorphic processes (Lau, 1990, Wells, 1998). Seabridge now favors an epithermal-like genesis for the FAT deposit.
The FAT deposit is located between the north shore of Matthews Lake and the south shore of Courageous Lake. It is made up of at least 13 discrete, steep east-dipping, elongate lenticular zones that vary in thickness from 20 to 125 meters wide. The continuity of these 13 zones has been demonstrated to be at least 1900 meters long (between UTM 7,108,700N and 7,110,600N), 800 meters wide (between UTM 486,000E and 486,800E) and, although open at depth, at least 1200 meters deep. The thirteen mineral zones are shown in a perspective view in the following figure which is looking northwesterly.
The rocks of the FAT deposit have been metamorphosed, however in the following discussion the prefix "meta" has been omitted from their lithologic description for convenience of the reader.
The volcanic rocks of the CLGB represent a tholeiitic to calc-alkaline suite of volcanic rocks, common to many Archean greenstone belts of the world. U-Pb and Rb-Sr age determinations of a general suite of CLGB volcanic lithologies give an age of 2.66 Ga (Dillon-Leitch, 1981).
Mafic volcanic rocks are classified as basalt and basaltic-andesite (Moore, 1956). This unit crops out along the western margin of the greenstone belt as low relief ridges. The flows are holocrystalline, massive, fine-grained and medium-to-dark green in color. They are commonly amygdaloidal and pillowed indicating a shallow, subaqueous depositional environment. No mafic volcanic rocks are known in the FAT deposit.
Felsic volcanic rocks and their intrusive equivalents in the CLGB were derived from peraluminous, sub-alkaline magmas of calc-alkaline affinity (Wells, 1998). The rocks are dacite to rhyolite and range in color from pale grey to light green. Lithic-bearing tuff, ash and agglomerate are the principal rock textures. These felsic volcanic lithologies are the predominant host to the FAT deposit.
The felsic volcanic rocks are the best described units in the area because of their association in the FAT gold deposit. In general, these rocks are a package of fine-grained pyroclastic units that regionally form a relatively thick but laterally restrictive pile. Compositionally these rocks seem to vary little, but textural variation is diverse. The most common variety of felsic volcanic rock is lapilli-tuff, generally composed of about 30% juvenile phyric fragments in a phyric groundmass as shown in the following figure. Note that the scale bar in this figure is approximately 7 centimeters long. This rock shows ubiquitous welding and compaction layering. Lithic-tuff is less common and generally contains 10% cognate lithic clasts of porphyritic rhyolite in a phyric groundmass. Crystal-tuff units are uncommon and seem to be limited in lateral and vertical extent. These rocks are typically very fine-grained with a trace to 20% ß-quartz crystal inclusions and rare accidental lithic inclusions. The second most common pyroclastic lithology is ash tuff. This unit is composed of 60-80% very fine phyric fragments with minor amounts of pumice fragments of lapilli size.
Within the felsic volcanic rocks of the FAT deposit are abundant lense shaped, epiclastic intercalations. Whole-rock analyses by Wells (1998), and others, have shown many of the sedimentary rocks are derived from a tuffaceous source. They are generally light brown to grey-black in color. Flame structures, graded bedding, load casts and slump features are common in these rock units which correspond to an inter-volcanic low-energy depositional environment. Metatamorphic grade is low and primary sedimentary structures are preserved. The lithologies are tuffaceous greywacke, thinly laminated siltstone and fine-grained arkosic sandstone. The coarser clastic units vary from thickly bedded to massive and generally show graded bedding. The coarser clastic rocks can form massive beds up to 15 meters thick. Fine grained siltstone is thinly laminated and seldom exceeds 7 meters in thickness.
Intruding and post dating all rocks groups are Proterozoic gabbro dikes. Selected dike samples in the province have yielded ages of approximately 2.0 Ga (Dillon-Leitch, 1981). In the FAT deposit vicinity, a prominent east-northeast dike offsets mineral domains of the FAT deposit by about 25 meters. Narrow gabbro dikes are encountered elsewhere in the FAT resource as north-northwest trending features; most are not exposed at the surface. These intrusive rocks have no economic importance.
The mineral domains of the FAT deposit are defined by a discrete suite of hydrothermal alteration assemblages. The lateral continuity and stratigraphic thickness of the hydrothermal system indicates that the FAT deposit was robust in volume and duration. Alteration styles are of varying intensity and can exist independently and in combination.
The predominant hydrothermal alteration minerals in the FAT deposit are illite group sheet silicates, referred to as a single mineral, 'sericite'. Sericite alteration is identified wherever fine-grained, white-grey, aligned mica (muscovite/paragonite) is encountered. This alteration style is best developed in the felsic volcanic rock units, probably due to the original glassy nature of these rocks. It is recognized that original devitrification of the felsic volcanic rock followed by greenschist metamorphism is likely to create a mineral assemblage that mimics sericite alteration associated with hydrothermal fluids. In the absence of a definitive feature to discriminate the source of sericite, Seabridge has followed the convention of previous workers and cataloged all sericite occurrences as alteration. The following photograph is a typical example of sericitic alteration at Courageous Lake. Note that the yellow tape measure is graduated in centimeters. This is a sample of drill core from the FAT deposit.
Intensity of sericite alteration varies widely in the FAT deposit. In most cases it is associated with other alteration styles but can be found independently. All the recognized sulfide minerals in the FAT deposit can be found in sericite alteration. The presence of sericite alteration is a necessary component of gold occurrences but the intensity of sericite alteration alone is not diagnostic of gold mineralization. This association of sericite alteration and gold suggests that ore forming fluids in the FAT mineral deposit are in part K-bearing and capable of leaching Ca and Na.
Silicic alteration of varying intensity is ubiquitous throughout the defined ore domains and much less common between ore domains in felsic volcanic rock of the FAT deposit. It is texturally retentive in volcanic rocks, only rarely overprinting and destroying primary pyroclastic textures. Silica flooding of groundmass material in volcanic rock is microcrystalline, blue-grey in color and closely related to strataform quartz zones. The most intense zones of silicic alteration are not generally indicative of higher gold concentrations. This gold-poor silica alteration may be another expression of the original devitrification of the glassy volcanic pile that hosts the FAT deposit.
Two distinct varieties of quartz have been identified in the FAT deposit ore domains (see figure below). The predominant variety is blue-grey, cryptocrystalline quartz as thin, sulfide-bearing strataform anastomosing veinlets and strataform lenses. Less common is medium-grain crystalline, white-grey quartz zones, often containing abundant iron carbonate minerals (predominantly ankerite) and calcite at the margins of these quartz zones. The white-grey quartz zones and veins typically crosscut the blue-grey quartz in the ore domains. White-grey, quartz-ankerite-calcite veins up to 0.5 meters wide are common between ore domains. These veins cut roughly perpendicular to the original depositional layering preserved in the volcanic rocks. The two types of quartz alteration are shown in the photograph of drill core in the following figure.
Much of the technical literature for the FAT deposit uses "vein" to describe the silica alteration in the ore domains. Textures in these zones of strataform silica alteration are more consistent with pervasive inundation of silica-bearing fluids into the rock, rather than a fracture-filling. The nature of the fluid responsible for the silica alteration is a silica saturated and metal-bearing hydrothermal fluid. Textural evidence indicates the fluid invaded the rock at pressures that for the most part did not exceed the lithostatic lode in the volcanic pile.
Carbonate alteration is a ubiquitous constituent affecting the rocks hosting the FAT deposit. Whether the source of abundant carbonate is from seawater or as a product of cation liberation during alkali leaching is unknown. Carbonate as calcite, ankerite and siderite are common accessory minerals with white, medium crystalline quartz zones and veins. Calcite is common as infill of late stage fractures and small shear zones and is found in minor amounts in the matrix of most rocks in the FAT deposit. Carbonate alteration is a major and widespread component of the assemblages of the FAT deposit but is not believed to be an important process in introduction of gold mineralization in the rocks of the FAT deposit.
The most pervasive zones of carbonate alteration are found in the stratigraphically older ore domains of the FAT deposit. Stratigraphically below and as part of Zone 8 are breccias derived from clastic debris flows with a matrix of calcite. In addition, sucrossic calcite is found replacing lapilli clasts in volcanic rocks of Zone 8. Younger ore domains (Zones 1 to 5) contain approximately similar abundances of calcite fracture infill, quartz/carbonate veins and strataform masses, but fine crystalline calcite impregnating matrix material is much reduced and spottier in nature. Throughout the deposit ankerite is a common mineral associated with white-grey quartz zones as subhedral, irregular masses embayed at the boundaries of these quartz zones.
Potassic alteration is restricted to the external margins of gabbro intrusions within the FAT deposit and is not associated with gold concentrations. This alteration type is manifested in microvein filling and vein salvages or patchy zones as distinctive pink orthoclase, biotite, quartz and pyrrhotite. Potassic alteration is intense and distinct within a few meters of a gabbro intrusion but not recognized elsewhere in the FAT deposit.
Sulfide mineralogy in the FAT deposit is relatively simple and consists of pyrite, pyrrhotite, arsenopyrite, sphalerite and chalcopyrite in decreasing order of abundance. All of these minerals can be found in the ore domains but only arsenopyrite has a consistent correlative relationship to gold concentrations.
Pyrite is present throughout the volcanic pile ranging in abundance from a trace to about 5%. It is disseminated in the rock or along fractures and microveinlets. The pyrite habit is euhedral to subhedral, ranging in size from 0.5 millimeters to about 4.0 millimeters.
Pyrrhotite has a more restrictive distribution but can be very abundant in sedimentary rocks intercalated with the volcanic pile. Concentrations of pyrrhotite range from a trace to 8%. It is found in fractures and microveins in the volcanic rocks and as semi-massive lenses in sedimentary rocks. An anhedral habit or clotty composite crystals are typical in the size range of 0.5 to 3.0 millimeters.
Arsenopyrite is recognized in 3 distinct habits and it is the best guide to the occurrence of gold. Concentrations of arsenopyrite can range from a trace to about 10%, but typically where present it is less than 1% of the rock. Arsenopyrite is found as:
- Acicular crystals disseminated or as partial to full replacement of lapilli fragments, the discrete needle-like crystals are 0.5 to 2.5 millimeters long.
- Anhedral disseminated clots of arsenopyrite is in the size range of 0.5 to 2.0 millimeters.
- Euhedral arsenopyrite in fractures, from 1.0 to 5.0 millimeters across.
Both the fine anhedral and acicular arsenopyrite are associated with gold concentrations but the acicular variety seems to have the clearest association in much of the deposit. Coarse arsenopyrite is not common and seems to have little association with gold.
Sphalerite and chalcopyrite are trace components of the mineral system. They are typically euhedral to subhedral discrete minerals ranging in size from 0.3 to 1.5 millimeters. There most common occurrence is at the margin of and within the chill margin of the younger gabbro intrusions.
All disclosure of a scientific or technical nature was prepared by, or under the supervision of, William E. Threlkeld (Licensed Registered Geologist #790 in the State of Washington), a Vice President of Seabridge. Mr. Threlkeld is a "Qualified Person" under National Instrument 43-101.