Stratigraphy, Structure, and Silicification: New Results From Mapping in the Flin Flon Mining Camp, Creighton, Saskatchewan
Kate MacLachlan
MacLachlan (2006): Stratigraphy, structure, and silicification: new results from mapping in the Flin Flon Mining Camp, Creighton, Saskatchewan; in Summary of Investigations Volume 2, 2006, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2006-4.2, CD-ROM, Paper A-9, 25p.
Abstract Two areas in the Flin Flon mining camp were mapped at a scale of 1:5000, one area between Douglas Lake and West Arm Road on the west limb of the Beaver Road Anticline, and the other on the peninsula between Green Lake and Phantom Lake. In the Douglas Lake area, mapping was undertaken in the hanging wall of the west-younging, felsic-dominated Myo member of the Flin Flon formation. Significant findings include, an extrusive felsic unit above the upper contact of the Myo member, abundant peperite at several stratigraphic levels, an up to 100 m wide zone of semi-conformable silicification, and pervasive sinistral shearing that post-dates the Phantom Intrusive Suite.
In the Green Lake Peninsula area, three fault-bounded successions with opposing younging directions were recognized. The easternmost succession, east of the newly defined Burley Lake Fault (a modified version of the Phantom Lake Fault), is part of the west-younging Louis formation on the east limb of the Burley Lake Syncline. The east-younging succession between the Burley Lake and Green Lake faults comprises a large component of fine- grained mafic volcaniclastic rocks intercalated with aphyric and plagioclase ± pyroxene porphyritic flows, and is tentatively correlated with the Hidden formation. The west-younging succession west of the Green Lake Fault is tentatively correlated with the west limb of the Beaver Road Anticline. The Green Lake Fault is a fairly late structure that overprinted a multiply reactivated fault zone that runs down the centre of the peninsula and may be a continuation of the Flin Flon Lake Fault. The boundary between the east- and west-facing successions occurs within this complex fault zone and the timing and nature of the original boundary in not known.
Keywords: Flin Flon Mining Camp, stratigraphy, structure, silicification, peperite, sinistral shear zones, faulting.
1. Introduction The Flin Flon Mining Camp is in the Paleoproterozoic Flin Flon Belt of the Trans-Hudson Orogen in Manitoba and Saskatchewan (Figure 1). Because it is a world-class volcanogenic massive sulphide (VMS) mining district, the Flin Flon–Creighton area has been the focus of many detailed geological studies over the years (e.g., Stockwell, 1960; Bailes and Syme, 1989; Thomas, 1992; 1994; Fedorowich et al., 1995; Price, 1997; Gale et al., 1999). As a result of the first “Targeted Geoscience Initiative” (TGI) in the Flin Flon Belt, undertaken from 2000-2002, significant progress was made in understanding the volcanological framework of the VMS hosting succession (Gibson et al., 2003b; 2005; Devine et al., 2002; Ames et al., 2002; MacLachlan and Bailey, 2002; Bailey and Gibson, 2004; 2005), and the previous informal stratigraphic nomenclature (e.g., Thomas, 1994) was revised (Devine et al., 2002; Figure 2). A consistent stratigraphic and volcanological framework for the entire camp, however, remains incomplete and is one of the objectives of the present Flin Flon TGI 3 project. The VMS deposits are hosted by a steeply northeast-dipping succession (Figure 2) on the upright, shared limb of the Beaver Road Anticline (BRA; Stockwell, 1960) and Hidden Lake Syncline (HLS; Stockwell, 1960). These regional, inclined, D2 folds are disrupted by several phases of superposed faulting (e.g., Stockwell, 1960; Thomas, 1994; Gale et al., 1999; Fedorowich et al., 1995). The Flin Flon Lake Fault is a northerly-trending regional structure that truncates the shared limb of the anticline-syncline pair and precludes tracing specific units around the BRA to the west-younging, western limb (Figure 2). Breccias of the ‘Blue Lagoon member’1 of the ‘Flin Flon formation’, however, are interpreted to occur on both sides of the Flin Flon Lake Fault (H. Gibson, pers. comm., 2006), thus providing a stratigraphic datum for regional correlations. There also is general agreement that sequences on either limb of the BRA are broadly correlative (K. Gilmore, pers comm., 2002), but lithologically different, and detailed stratigraphic and genetic relationships remain unclear. An M.Sc. thesis (Bailey, 2006) study of the “Myo Rhyolites”
1 Informal nomenclature first appears in single quotation marks; subsequently, the quotation marks are dropped.
Saskatchewan Geological Survey 1 Summary of Investigations 2006, Volume 2
E NAIN V A RAE L S NE AR Hudson HE Bay HO T 50 N 50 N 60 W 130 W SUPERIOR G
Flin Flon TGI study area N I M Atlantic O Y Ocean W
<1.8 Ga Orogens Archean Cratons Cu-Zn MINES Paleoproterozoic Orogens B - Birch Lake C - Callinan Ce - Centennial T NE Arm shear zone Co - Coronation Cu - Cuprus C Bear Lake block TR D - Don Jon Flin Flon N F - Flin Flon F D Fl - Flexar Cu K - Konuto M WL M - Mandy S N - North Star Ce S - Schist Lake T - Trout Lake Scotty Lake block Fl TR - Triple 7 K W - Westarm B W WL - White Lake
Amisk Lake Co ake w L ko us ap Phanerozoic cover p o a 102 15' W th Phanerozoic cover o 10 km A 54 30' N Sask. Man.
PRE-ACCRETION ASSEMBLAGES (1.92 to 1.88 Ga) POST-ACCRETION ROCKS (1.87 to 1.84 Ga) Missi Group Major mafic-felsic tholeiitic SUCCESSOR BASINS sandstone, conglomerate JUVENILE ARC intrusive rocks Schist-Wekusko suite Calc-alkaline volcanic rocks 1.90 to greywacke, mafic sills 1.88 Ga Arc rift basalt Tholeiitic, mafic-felsic SUCCESSOR ARC Felsic volcanic rocks INTRUSIVE ROCKS Intermediate-mafic OCEAN FLOOR Layered mafic-ultramafic complex N-type MORB/E-type MORB (BACK-ARC) 1.90 Ga OCEANIC PLATEAU Tholeiitic basalt (age unknown) Accretion-related Younger shear zones shear zones (D >1.87 Ga) and/or faults (D -D ) Felsic plutonic rocks 1 25 'EVOLVED ARC' (~1.920 to 1.903 Ga) F VMS deposit (see inset key)
ARCHEAN SLICES Granitic rocks (~2.5 Ga)
Figure 1 - Tectonic assemblage map of the Flin Flon Belt. Inset shows location of the Flin Flon Belt within the Trans- Hudson Orogen (modified from Syme et al., 1998).
Saskatchewan Geological Survey 2 Summary of Investigations 2006, Volume 2
0 km 1 Legend
C Intrusive Rocks FF VMS deposit C FF=Flin Flon F F Phantom Intrusive Suite C=Callinan L Tr=Triple 7 C C Boundary Intrusions RF M=Mandy C S=Schist Lake 10 Bootleg Pluton Tr Mine shaft Tr H Annabel Pluton 106 Tailings N=North Main Pond L S=South Main S Cliff Lake Pluton Tr= Triple 7 Tr F Missi Group Highway L 167 F Ross F Lake 2007 Provincial Flin Flon Assemblage Border N Louis formation 10A Fault
FF Manitoba 10 Hidden formation Saskatchewan F1 Syncline/ Anticline B R Douglas formation A X S F2 Syncline/ Anticline 2007 X X Flin Flon formation
P Younging L Millrock member R direction
167 W Myo member (felsic/mafic)
A Study Areas R Creighton member
Blue Lagoon member MacLachlan CL 02 Club member Bailey 20 R Schist B M Simard L Lake Douglas L F
F Lake S Unassigned volcanic rocks X Lewis D L 2006 R Potter Bay PLF M 2007 Phantom Lake F 2006 R S
Burley 2006 Lake Figure 2 - Simplified geological map and informal stratigraphic subdivisions of the Flin Flon-Creighton area, showing locations of previous G and ongoing Flin Flon Targeted Geoscience Initiative 3 studies; BLS, Burley Lake Syncline; BRA, Beaver Road Anticline; CCF, Creighton LF Creek Fault; CL, Carlisle Lake; CLF, Club Lake Fault; DLR, Douglas Lake Road; FFLF, Flin Flon Lake Fault; GLF, Green Lake Fault; HLS, Hidden Lake Syncline; MF, Mandy Fault; PLF, Phantom Lake Fault; PLR, Phantom Lake Road; RF, Railway Fault; RLF, Ross Green Lake Fault; and WAR, West Arm Road. Lake
Saskatchewan Geological Survey 3 Summary of Investigations 2006, Volume 2 (Thomas, 1994), the potential time-stratigraphic equivalent of the VMS-hosting rhyolites, resulted in redefinition of the rhyolite-bearing succession as the ‘Myo member’ of the Flin Flon formation (Bailey, 2006). More work remains to be done on the successions above and below the Myo member. The first part of this report describes the stratigraphy and geology of part of the western limb of the BRA, in the hanging wall of the Myo member. The second part focuses on a structurally and stratigraphically complex area south of the Phantom Lake Fault (Stockwell, 1960; Figure 2), where a rhyolite on the western shore of the Green Lake Peninsula, is thought to be a lateral continuation of the Myo member (K. Bailey, pers. comm., 2006). This potential correlation infers that younging in the western part of the peninsula is toward the west, whereas the succession youngs to the east in the eastern part of the peninsula (Stockwell, 1960). There is currently no interpretation of how or where the younging change occurs. During the summer of 2006, a joint mapping project by Saskatchewan Industry and Resources (SIR) and the Manitoba Geological Survey (MGS) was initiated in the Green Lake Peninsula and adjacent Burley Lake–Carlisle Lake areas (Figure 2). This paper reports on the results of the SIR component of 1:5000 scale mapping and how it integrates with the MGS mapping to resolve some of the stratigraphic and structural issues in the area.
2. Regional Geology The Flin Flon Belt is a collage of 1.92 to 1.87 Ga, predominantly juvenile oceanic rocks and 1.87 to 1.83 Ga successor arcs, basins and plutonic rocks (Lucas et al., 1996; 1999; Syme et al., 1998; Stern et al., 1999). The Flin Flon–Creighton area is underlain by the 1.90 to 1.88 Ga Flin Flon Arc Assemblage (Syme and Bailes, 1993; Stern et al., 1995), which in Manitoba has been subdivided into several structural blocks (Bailes and Syme, 1989). The present study area is in a continuation of the Flin Flon block in Saskatchewan, which comprises predominantly juvenile, tholeiitic, mafic volcanic and volcaniclastic rocks, with a volumetrically minor, but economically significant component of rhyolite. The unconformably overlying 1.83 to 1.85 Ga Missi Group comprises predominantly continental sandstone and conglomerate. The ca. 1842 Ma Boundary Intrusions (Heaman et al., 1992), a distinctive suite of ultramafic to monzonitic intrusions, cut rocks of the Missi Group.
Two phases of folding and several phases of faulting have been recognized in volcanic rocks of the Flin Flon– Creighton area (e.g., Stockwell, 1960; Stauffer and Mukerjee, 1971; Thomas, 1994; Fedorowich et al., 1995; Gale et al., 1999; Figure 2). The first phase of folding (F1) is represented by the Burley Lake Syncline, which predated deposition of the Missi Group (Stockwell, 1960). The fault-modified, second generation, north-northwest–trending BRA–HLS fold pair is responsible for the regional-scale disposition of map units (Figure 2). The F2 HLS clearly refolds the F1 Burley Lake Syncline (Figure 2). The northern limit of the volcanic rocks is the south-dipping Club Lake thrust fault, which places the volcanic rocks on top of the Missi Group. The Club Lake Fault and a series of related thrust faults imbricate the ‘Hidden formation’ and are folded by the Hidden Lake Syncline (Stockwell, 1960; Fedorowich et al., 1995; Gale et al., 1999). Evidence for early thrust faults has also been documented in the hinge of the BRA, where they are interpreted to post-date folding related to the HLS-BRA and pre-date subsequent tightening of those structures (Lewis et al., this volume). A similar timing relationship was suggested for the Railway Fault and HLS by D. Price (pers. comm., 2002). Regional sinistral and dextral shearing and late brittle dextral faulting post-dated folding and thrust faulting (Thomas 1992; Fedorowich et al., 1995; Gale et al., 1999; Lewis et al., this volume).
Stratigraphy of the Flin Flon Block A detailed informal stratigraphy has been developed for the volcanic and volcaniclastic rocks of the immediate mine succession (Figure 2, Devine et al., 2002; Ames et al., 2003). The VMS-bearing unit and its footwall have been defined as the Flin Flon formation, which has been subdivided into: the ‘Club’, Blue Lagoon, ‘Creighton’ and ‘Millrock’ members from oldest to youngest (Devine et al., 2002). The Club member comprises aphyric mafic flows, rhyolite flows and rhyolite-bearing volcaniclastic rocks. The Blue Lagoon member is characterized by plagioclase porphyritic volcaniclastic rocks and minor flows. The Creighton member consists predominantly of aphyric mafic flows and the Myo member comprises intrusive and extrusive rhyolite and rhyodacite and intercalated mafic volcanic and volcaniclastic rocks on the west limb of the BRA. The Millrock member, which hosts the Flin Flon, Callinan and Triple 7 deposits, comprises heterolithic mafic breccia, mafic tuff and rhyolite flows and autoclastic and resedimented rhyolite breccias. The Flin Flon formation is overlain by the Hidden formation a sequence of aphyric and plagioclase porphyritic basaltic andesite flows and sills with abundant fine-grained, thinly- bedded interflow sediments (DeWolfe and Gibson, 2006). The overlying ‘Louis formation’, is a succession of predominantly plagioclase and pyroxene 2 porphyritic basaltic andesite flows (DeWolfe and Gibson, 2006).
2 Mafic mineral inferred to have been pyroxene, but now pseudomorphed by greenschist facies mafic minerals throughout study area.
Saskatchewan Geological Survey 4 Summary of Investigations 2006, Volume 2 3. Geology of the Douglas Lake Area An area from the Bootleg Pluton to Highway 167, and between Douglas Lake and West Arm Road (Figure 2) was mapped at 1:5000 scale (Figure 3). This succession corresponds roughly to the ‘Bomber Lake’ and ‘Newcor’ members of Thomas (1992). Bailey (2006), however, included parts of the Bomber Lake and Newcor members that occur between the “Myo Rhyolites” as part of the Myo member. To avoid confusions the names Newcor and Bomber will be dropped for the time being and rocks not included in the Myo member will be referred to as the hanging-wall succession. a) Terminology In the following descriptions of lithologic units, the non-genetic terminology of Fisher (1966) will be used for volcaniclastic rocks. The terms tuff (<2 mm), lapilli (2 to 64 mm) and block/breccia (>64 mm) will be used strictly to distinguish clast size and do not imply a mechanism of formation by pyroclastic fall. The term “cryptodome” is used for high-level felsic to intermediate intrusions that may locally break the surface to become extrusive (McPhie et al., 1993). In the case of less viscous mafic lavas the term “cryptoflow” or “invasive flow” (Gibson et al., 2003a) is used to describe flows that are erupted just below the surface. These types of synvolcanic intrusions are especially favoured in subaqueous, mixed volcanic-sedimentary successions. The water column contributes to the confining pressure (McPhie et al., 1993), which allows magma to rise to very high levels where it is likely to encounter intervals of weak, poorly consolidated, less dense sediments. Some of the rock units described below as flows, may in fact be more informatively termed “invasive flows” (H. Gibson, pers. comm., 2002), as they have contact relationships that are similar to both flows and synvolcanic sills. “Flows” that have peperitic upper contacts may form in one of several ways: 1) they may have flowed down into less dense, unconsolidated wet sediments; 2) they may have been fed sideways from a sill or lava tube into an adjacent synvolcanic basin filled with unconsolidated sediment; or 3) they may have been fed from below by a dyke and inflated up into the overlying unconsolidated sediments. Thus, although some of the flows erupted below the sea floor, they erupted into unconsolidated wet sediment and thus have flow features (e.g., pillows) similar to flows erupted underwater. In this way, a pillowed flow may actually engulf wet sediments and/or peperite 3 within the flow and massive flows may have peperitic tops as well as bottoms. Gibson et al. (2003a) have used these types of features to indicate a local hiatus in volcanism at the mine horizon in the Flin Flon camp, which allowed the deposition of a significant thickness of waterlain tuff prior to eruption of the hanging wall stratigraphy. This tuffaceous succession is inferred to have played a roll in confining mineralizing fluids to the mine horizon, contributing to the large size of the VMS deposits in the Millrock member (H. Gibson, pers. comm., 2002). b) Stratigraphy The base of the succession mapped in the Douglas Lake area is the uppermost felsic unit of the Myo member, an aphyric to sparsely plagioclase porphyritic, massive rhyodacite (unit 1, Figure 3; compiled from Bailey 2006), which was interpreted to be predominantly intrusive (Bailey, 2006). A small lens of quartz porphyritic rhyolite at the same stratigraphic interval (unit 2, Figure 3) is also part of the Myo member. The Myo member is overlain by a succession of aphyric, weakly amygdaloidal, massive mafic flows with lobate to weakly pillowed tops, and pillowed flows that commonly have interpillow hyaloclastite or tuff (unit 3, Figure 3; Figure 4A). The flows range from approximately 4 to 8 m in thickness and commonly have a greenish brown weathered surface. Locally intercalated with these flows are discontinuous, medium bedded lapilli tuff (Figure 4B) and thin bedded to laminated mafic tuff that are too thin to map at this scale. Overlying the weakly amygdaloidal flows in the southeastern part of the map area is a heterolithic breccia (unit 5, Figure 3) consisting predominantly of aphyric mafic volcanic clasts, with up to 1% felsic volcanic fragments, in a tuffaceous mafic matrix. Within the breccia is a lens of massive aphyric rhyolite/rhyodacite (unit 1, Figure 3). Based on the work of Bailey (2006) on the Myo member rhyolites, the lack of flow features and associated autoclastic or resedimented breccias, suggests that this rhyolite is likely intrusive. Above the heterolithic breccia in the southeast and directly above the massive and pillowed flows in the north, is a succession of brownish-weathering, massive to pillowed mafic flows (unit 6, Figure 3) that are irregularly intercalated with areas of peperite (unit 7, Figure 5). The peperite is comprised of a mixture of light greenish- yellow-weathering, massive aphanitic mafic tuff and brownish-weathering, weakly amygdaloidal mafic volcanic rock. A predominantly massive, aphyric rhyolite (unit 1) intruded by abundant mafic dykes and sills (Figure 6a), is within this unit. At its south end, the massive rhyolite locally grades upward into a clast-supported, in situ, autoclastic breccia. The autoclastic breccia grades upwards and outwards into a matrix-supported breccia with a
3 Peperite is a rock generated by mixing of coherent lava or magma with unconsolidated wet sediment (Fisher, 1960), and is characterized by a clastic texture in which either component may form the matrix. Peperite occurs at the contact between intrusions and wet sediments, and along the basal contacts of lava flows that override or burrow into unconsolidated sediments (McPhie et al., 1993 and refs. therein).
Saskatchewan Geological Survey 5 Summary of Investigations 2006, Volume 2
E E Chance Legend
LOM 0 0 0 0 0 0 167 Intrusive Rocks 3 2 1 1 2 3 3 Strongly amygdaloidal, greenish, pillowed and 3 18 Bootleg Pluton: diorite-tonalite-granodiorite 9 massive mafic flows; > 6 m thick
R Broadly synvolcanic mafic intrusive: includes aphyric L 17 8 Silicified scoriaceous breccia D R plag-phyric and plag+px-phyric. A W Hanging wall Succession Heterogeneous, brownish, mafic flows with Plagioclase crystal-rich heterolithic mafic breccia 7 abundant peperite; peperite > in tact flows 16 6069000 N and tuff-breccia, locally with felsic fragments Weakly to moderately amygdaloidal, brownish, LOM 6 massive and pillowed mafic flows; local peperite, 15 Plagioclase porphyritic pillowed mafic flow 7 3 10 7 17 5 Heterolithic, mafic breccia 14 Strongly amygdaloidal, aphyric greenish pillowed 6 5 9 mafic flows, in part with abundant flow top breccia 4 Thin bedded to laminated mafic tuff
12 3 13 Mafic tuff and lapilli-tuff, thin to medium bedded 6 1 Cor Weakly amygdaloidal, aphyric, pillowed L 10 O 3 and massive mafic flows, 4-10 m thick M Moderately amygdaloidal, aphyric, greenish, 12 massive and pillowed mafic flows, < 4 m thick Myo Member 4 Heterogeneous, brown, mafic flows with 11 abundant peperite 2 Massive quartz-porphyritic rhyolite 8 6 11 7 W Massive and pillowed, mafic to intermediate flows 1 Massive, aphyric rhyolite; 1a, redeposited 7 10 13 with abundant amoeboid flow top; silicified 1a rhyolite breccia 1 5 Faults Bedding (top known): Newcor upright, overturned 10 6 Unity Main foliation (flattening fabric): 1 12 6068000 N Au and Cu-Au showings and mines inclined, subvertical Spaced cleavage; inclined, subvertical 13 Post-main foliation Douglas 18 5 inclined, subvertical N Lake 11 Bedding (top unknown): 9 inclined, subvertical 6
lt 13 u a M F 15 O 14 io L R 15 15 0 250 18 L O 16 M Metres Bootleg/Rio
Figure 3 - Simplified geology of the Douglas Lake area.
Saskatchewan Geological Survey 6 Summary of Investigations 2006, Volume 2 A A Pp
LT
P
B B
Flow
Pp P Dyke
Figure 4 - Field photographs of representative lithologies of Figure 5 - Field photographs of unit 7 in the Douglas Lake unit 3 in the Douglas Lake area. A) Pillowed mafic flow area, Pp, peperite. A) Pillowed mafic volcanic flow with overlain by mafic lapilli tuff (312717E, 6069349N). intra-pillow (within pillow) peperite (312556E, 6068574N). B) Pillowed mafic volcanic with epidotized rims and 4 Pillows are outlined with dotted line. B) Peperitic interpillow hyaloclastite (313016E, 6068678N ). Hammer termination of massive mafic volcanic flow (312556E, for scale in each photo is about 40 cm long. 6068574N). Contact shown with dotted line. more chloritic matrix and variations in the size and concentration of rhyolite fragments (unit 1a, Figure 6b), suggesting that the fragments have been redeposited.
Above the succession of peperitic flows is a distinctive breccia comprised predominantly of highly silicified scoriaceous mafic fragments that weather light green to white (unit 8, Figure 3). At one locality a weakly altered scoriaceous flow top breccia grades upward into a strongly silicified scoriaceous flow top breccia and then into a scoriaceous breccia with silicified fragments in a chloritic matrix (Figure 7). Along strike within this unit there are variations in clast size and abundance, which suggest bedding and, thus, redeposition of the scoriaceous fragments. This breccia is commonly overlain by a thin bedded to laminated mafic tuff, less than a meter thick and not distinguishable as a separate unit on the map. The timing of silicification will be discussed in a subsequent section. The scoriaceous breccia is overlain by a succession of light greenish, thick bedded (>6 m), aphyric, variably amygdaloidal massive and pillowed flows (unit 9, Figure 3). The massive flows are commonly weakly amygdaloidal5 (1 to 2%, 1 to 4 mm) throughout, with ubiquitous quartz-epidote alteration patches and flow banding. The massive bases grade up into lobate, weakly pillowed and/or flow brecciated tops that are more strongly amygdaloidal (5 to 8%, 1 to 6 mm) and commonly characterized by large, quartz-filled gas cavities. The pillowed flows typically have well-defined selvedges, and strongly amygdaloidal (25 to 35%, 2 to 10 mm in diameter) cores that are intensely quartz-epidote altered.
4 All UTMs are in NAD 83, Zone 14. 5 Unless otherwise noted, amygdules are quartz ± feldspar.
Saskatchewan Geological Survey 7 Summary of Investigations 2006, Volume 2
A
Figure 7 - Field photograph of unit 8 in the Douglas Lake area, showing silicified scoriaceous clasts in a reddish- weathering chloritic matrix (312445E, 6068669N). Unit 9 is overlain by a succession of aphyric, weakly amygdaloidal (1 to 5%, 1 to 5 mm), predominantly massive, mafic to intermediate flows with amoeboid flow tops (unit 10, Figure 3) that are commonly as thick as the flow itself. Minor, moderately amygdaloidal (5 to 10%, 2 to 8 mm) pillowed flows are also present (Figure 8). All flows of this unit contain chlorite (±quartz) amygdules, rather than quartz-feldspar amygdules. Most distinctive is the highly silicified B nature of the amoeboid flow tops and pillows, as well as the margins of the massive flows (Figure 8). The silicified amoeboid flow unit is overlain by and locally intercalated with brownish, thin (1 to 2 m thick), weakly to moderately amygdaloidal, heterogeneous, massive and pillowed peperitic flows (unit 11, Figure 9A). This peperitic unit commonly has thin (<1 m), discontinuous, thin-bedded to laminated, mafic tuff interbedded within it (Figure 9B).
Overlying this peperitic unit is a succession of 1 to 4 m thick variably amygdaloidal, aphyric massive and pillowed flows (unit 12). The massive flows are weakly amygdaloidal (1 to 3%, 1 to 5 mm) throughout and become more strongly amygdaloidal at the top, commonly with lobate to weakly pillowed tops or flow Figure 6 - Field photographs of unit 1 in the Douglas Lake top breccia. The pillowed flows are more strongly area. A) Resedimented rhyolite breccia (313329E, amygdaloidal (20 to 30%, 2 to 8 mm) and commonly 6067957N). Hammer for scale is about 40 cm long. display strong quartz-epidote alteration in pillow cores. B) Massive aphyric rhyolite cut by mafic dyke/sills A 5 to 10 m thick, discontinuous, thin- to medium- (313221E, 6068134N). bedded mafic tuff and lapilli-tuff (unit 13, Figure 3, see Figure 10) is interbedded with and overlying unit 12. The tuffs are in turn overlain by strongly amygdaloidal (25 to 30%, 2 to 8 mm), predominantly pillowed flows with strong quartz-epidote alteration in the pillow cores (unit 14, Figure 3). Towards the north end of the map area there are also abundant flow breccias in this unit. The succession of aphyric flows east of Douglas Lake is cut by various aphanitic to fine-grained mafic dykes and sills (unit 17), comprising aphyric, plagioclase porphyritic and plagioclase-pyroxene porphyritic varieties. These broadly synvolcanic dykes and sills have been described in more detail by MacLachlan and Bailey (2002). South of Douglas Lake, overlying the aphyric succession of flows east of Douglas Lake, is a sequence of plagioclase porphyritic flows and volcaniclastic rocks, corresponding to the ‘Douglas formation’ of Thomas (1992). The Douglas formation is dominated by plagioclase crystal-rich, heterolithic breccia that ranges from clast-
Saskatchewan Geological Survey 8 Summary of Investigations 2006, Volume 2 A A
Flow
B B
Figure 9 - Field photographs of unit 11 in the Douglas Lake area. A) Peperitic mafic volcanic flow (312791E, 6067847N). C Lava is dark grey and tuff is buff coloured. B) Thin-bedded to laminated mafic tuff (312825E, 6067770N).
supported to matrix-supported. The breccia is locally interbedded with minor tuff breccia and lapilli-tuff (unit 16). The matrix of the volcaniclastic rocks comprised 1 to 4 mm long plagioclase crystals (15 to 20%), mafic tuff and locally 10 to 15% mafic lapilli. The blocks and lapilli in all units are predominantly mafic and are both aphyric and plagioclase (±pyroxene) porphyritic and commonly amygdaloidal (Figure 11). There are rare clasts of aphyric massive rhyolite, as well as fragments of silicified amygdaloidal basalt (Figure 11B). Intercalated with the crystal-rich volcaniclastic rocks are variably plagioclase (±pyroxene) porphyritic pillowed flows (unit 15). The lowermost succession of flows is sparsely pyroxene and plagioclase porphyritic Figure 8 - Field photographs of unit 10 in the Douglas Lake (1 to 2 % plagioclase, 1 to 2 mm long and less than 1% area. A) Silicified margin and amoeboid flow top of a massive flow (312844E, 6067860N). Flow margin shown pyroxene, 1 to 2 mm long) with amoeboid flow tops. with a dotted line. Arrow indicates tops. B) Strongly silicified They are light green to buff on the weathered surface amoeboid flow top (312844E, 6067860N). Hammer for scale and sparsely amygdaloidal (Figure 12). Both, pillows is about 40 cm long. C) Silicified pillowed flow (312616E, and amoeboid flow tops are heterogeneously silicified 6068139N). Arrow indicates tops. and commonly have quartz-epidote alteration. The other pillowed flow and flow breccia units in this succession are more strongly plagioclase porphyritic (8 to 10%) with 1 to 4 mm long plagioclase crystals, minor amygdales (~1%, 1 to 3 mm in diameter) and heterogeneous silicification.
Saskatchewan Geological Survey 9 Summary of Investigations 2006, Volume 2 A A
Tops
B B
S main
Figure 11 - Field photographs of unit 16 in the Douglas Lake area. A) Plagioclase crystal-rich mafic lapilli-tuff. B) Slightly rusty weathering, silicified, amygdaloidal fragment in mafic, plagioclase crystal-rich heterolithic tuff C breccia (312521E, 6067447N). c) Silicification The most distinctive and intense alteration is the semi- conformable silicification of the massive flows and amoeboid flow tops of unit 10 and the scoriaceous breccia of unit 8. In the amoeboid flow tops, the amygdaloidal to scoriaceous fragments are strongly silicified and weather white (Figure 8), whereas the matrix is weakly to moderately chloritized and weathers greenish-brown. A possible explanation for this comes from the Noranda area in the Abitibi Greenstone Belt Tops (Gibson et al., 1983). An amoeboid flow top forms when the flow breaks out of its chilled rind and flows Figure 10 - Field photographs of unit 13 in the Douglas into its own hyaloclastite carapace. The mode of Lake area. A) Bedded mafic lapilli-tuff with angular formation of hyaloclastite by quench fragmentation unconformity giving younging to top of photo (312526E, (McPhie et al., 1993) implies that the hyaloclastite is 6068032N). Hammer for scale is about 40 cm long. B) Mafic cold, whereas the magma flowing out into it is hot. If lapilli-tuff with elongate aphyric mafic volcanic fragments the sequence is buried quickly and the fragments retain (indicated by arrows; 312556E, 6068574N). C) Graded tuff enough of their heat, then that temperature difference giving younging to the right-hand side of the photo (southwest; 312217E, 6068467N). would be the key to causing silicification of the fragments, but not the matrix. This is due to the fact that at low pressure (>700 bars) hydrous fluids have maximum silica saturation at about 350°C, after which the amount of silica that can be in solution drops dramatically (Kennedy, 1950). When silica-rich fluids below 350°C pass through this flow sequence and interact with the hot scoriaceous (porous and permeable) fragments, which raise the fluid temperature above the silica saturation point, then excess silica is precipitated into the fragments leaving the matrix unsilicified. A similar process may have resulted in the silicification of fragments, but not the matrix in unit 8.
Saskatchewan Geological Survey 10 Summary of Investigations 2006, Volume 2 d) Structure A The Douglas Lake area is dominated by three main structural features: 1) an early, steeply southwest-dipping, approximately bedding-parallel flattening fabric; 2) an anastomosing network of northwest-striking ductile sinistral shear zones; and 3) a west-northwest–striking, steeply- dipping spaced cleavage that post-dates the flattening fabric. There are also several sets of fractures presumably related to brittle faulting. The early flattening fabric is defined primarily by flattening of pillows and fragments in various breccia types, but also locally by a preferred mineral orientation in tuff, interpillow hyaloclastite and pillow selvedges.
The sinistral shear zones commonly are at lithological B contacts, but also locally cut across them. Sinistral movement is defined by C-S fabrics, shear bands, and counter-clockwise rotation of other features into the shear zone margins. Most lithological contacts have at least minor shearing along them. The shear zones generally trend northwest and dip steeply west. The west-northwest–striking spaced cleavage occurs sporadically and locally shows slight sinistral offset of units it is developed in. The late cleavage is in Phantom porphyry dykes, one of which is strongly deformed in the sinistral shear zone that hosts the Newcor Deposit, indicating that both sinistral shearing and the west- northwest cleavage post-dated intrusion of the ca. 1838 Ma Figure 12 - Field photographs of unit 15 in the Douglas Lake area showing. A) Light (Heaman et al., 1992) Phantom green to white weathering, flattened, sparsely plagioclase+pyroxene-porphyritic Intrusive Suite. There are also pillowed flow (312383E, 6067657N). Hammer for scale is about 40 cm long. B) Close late north-south–striking, up of sparsely plagioclase+pyroxene porphyritic flow (312383E, 6067657N). subvertical brittle dextral faults that post-dated all other deformation. e) Mineral Showings and Deposits There are numerous historic Au, Cu-Au, and Cu showings and mines in the Douglas Lake area, including the past producers Newcor and Bootleg/Rio (Figure 3). The following descriptions are taken from the Saskatchewan Mineral Deposit Index (2006) unless otherwise specified. The Cor (SMDI# 0014), Unity (SMDI# 0013), and Chance (SMDI# 0026) Au/Cu-Au showings are associated with quartz veins in northwest-striking sinistral shear zones (Figure 13) and are characterized by disseminated sulphide (pyrite, pyrrhotite, chalcopyrite±sphalerite, galena,
Saskatchewan Geological Survey 11 Summary of Investigations 2006, Volume 2 arsenopyrite) in the wall rock and sulphide veinlets associated with the quartz veins. The Newcor Deposit (SMDI# 0005) is also within one of the sinistral shear zones, but is characterized predominantly by massive, gold-bearing arsenopyrite, pyrite and sphalerite, although Au-bearing arsenopyrite also occurs as well formed crystals in massive white quartz veins and in chlorite schist (Sabina, 1987). The Bomber Lake showing (SMDI# 0038) is also in one of the northwest- striking shear zones and is characterized by massive pyrite and chalcopyrite. The Bootleg/Rio Gold Mine (SMDI# 0011) is in the northeast-striking Rio Fault, along the northern margin of the Bootleg Pluton. It is characterized by two different types of mineralization. The first is comprised of up to 15% disseminated pyrite in bedding parallel bands in silicified and carbonate- altered rock in the hanging wall of the fault. The second type of mineralization is pyrite-chalcopyrite-sphalerite- galena stringers and quartz veinlets within the fault zone.
4. Geology of the Green Lake Peninsula The Green Lake Peninsula in Phantom Lake is mostly south of the Phantom Lake Fault of Stockwell (1960), which separates this part of the stratigraphy from the mine succession. It is largely because of the Phantom Lake Fault that the stratigraphic position of the rocks in this area has remained uncertain. Recent mapping of the Myo member by Bailey (2006) suggested that there Figure 13 - Field photograph of a previously prospected is no significant offset along the Phantom Lake Fault shear zone and quartz vein, which is along strike from the (Figure 2), and detailed Mag and VLF maps (HBED, main Cor showing (312767E, 6068410N). pers. comm., 2006) of the area do not show any evidence of a major structure coincident with the Phantom Lake Fault. a) Stratigraphy Based on previous work (e.g., Stockwell, 1960; Bailey, 2006), the rocks in the eastern part of the peninsula were reported as younging east and stratigraphy in the western part of the peninsula to be west facing. One of the primary objectives of the mapping was to confirm the difference in younging directions, and to determine where and how it occurs. In the following section the stratigraphy is subdivided into three different successions separated by either observed or inferred faults (Figure 14).
East of the Burley Lake Fault
Within the eastern side of the peninsula is a succession of thick (>6 m), aphyric, locally spherulitic, massive mafic flows with pillowed and flow breccia tops (unit 1; Figure 14). Based on the transition from massive base to pillowed or lobate tops, this unit faces southwest. A few hundred metres to the southeast is a unit of thick, massive, plagioclase porphyritic (12 to 15%, 2 to 5 mm) mafic flows/sills (unit 2) that are inferred to be part of the same west-facing succession. Both units 1 and 2 are separated from units to the southwest by the newly named Burley Lake Fault.
Between Burley Lake and Green Lake Faults The lowest unit in the succession, in the southwest, comprises massive, aphyric flows/sills (>90%) intercalated with minor thin bedded to laminated mafic tuff (unit 4, Figure 15). The massive flows/sills are weakly amygdaloidal (~2%, 2 to 6 mm) with an increasing proportion of amygdules toward the top indicating that younging is to the east. Along strike to the northwest of unit 4 is a thin unit of thin bedded to laminated mafic tuff (unit 3, Figure 14). Both units 3 and 4 grade upward into a volcaniclastic succession comprising mafic tuff and lapilli-tuff with minor amounts of intercalated massive, aphyric flows (<20%; unit 5, Figure 15). The volcaniclastic rocks are thin to medium bedded (Figure 15), commonly with up to 20% pyroxene and plagioclase crystals and are greenish
Saskatchewan Geological Survey 12 Summary of Investigations 2006, Volume 2
E E
Legend 0 0 0 0 0
0 Intrusive Rocks 6 7 1 BRA? 1 14 Massive and pillowed aphyric mafic volcanic flows 3 3
n N 56 ? M P a 25 Phantom Porphyry Intrusion
Phantom w Mafic lapilli-tuff and tuff, e P 70 Lake a 13 in part with abundant plagioclase crystals h
b
51 c
(Potter Bay) o
t Boundary Intrusion ? t 24 i
a M Thin aphyric mafic flows, commonly peperitic, intercalated n ? k 11 a s with thin bedded to laminated mafic tuff a
? 64 M Fine- to medium-grained gabbro, aphyric and M S 23 ? plagioclase porphyritic M 10 Thin bedded to laminated mafic tuff ?
B 22 Aphyric aphanitic to fine-grained mafic intrusive Thin to medium bedded mafic tuff and lapilli-tuff, with M L 9 M ? 84 F minor intercalated massive mafic flows P G L F West of Green Lake Fault M 8 Thin bedded to laminated mafic and felsic tuff and lapilli-tuff P ? P Aphyric massive rhyolite Thin to medium bedded mafic tuff and lapilli-tuff, 21 7 B intercalated with minor aphyric and porphyritic massive flows P ? L 76 F Sparsely plagioclase porphyritic massive rhyolite 74 20 Scoriaceous breccia interbedded, with minor tuff, lapilli-tuff 6067000 N 79 6 P ? tuff-breccia and plagioclase-porphyritic pillowed mafic flows P P 19 Quartz and plagioclase porphyritic massive rhyolite P 5 Thin to medium bedded mafic tuff and lapilli-tuff, intercalated pep ? P with minor aphyric to sparsely porphyritic massive mafic flows Phantom P P P 18 Thin bedded to laminated mafic tuff ? 4 Aphyric massive mafic flows (>90%) intercalated with Lake thin bedded to laminated mafic tuff (<10%) (west arm) P P M/P Plagioclase +/- pyroxene porphyritic pillowed and P Thin bedded to laminated mafic tuff P pep 17 massive flows 3 ? P Burley East of Burley Lake Fault ? M/P 75 16 Aphyric amygdaloidal massive and pillowed flows 89 Lake P 85 P P 2 Thick massive plagioclase prophyritic mafic flows P Between Burley Lake and Green Lake Fault P M ? Aphyric pillowed mafic flows with concentric 1 Thick massive aphyric mafic flows 70 M 15 O cooling rings pep L P ? P pep P Structures ? ? Bedding (top known): Polyphase deformation zone 6066000 N Brittle Fault upright, overturned pep P Main foliation (flattening fabric):
P ? Shear zone inclined, subvertical P 82 P - Pillowed Spaced cleavage; M Bedding (top unknown): Pep - Peperitic M GLF inclined, subvertical inclined, subvertical M - Massive M Post-main foliation ? inclined, subvertical M M P pep pep M Green M Lake
P M
P Figure 14 - Simplified geology of the Green Lake Peninsula; BLF, Burley Lake Fault; BRA, Beaver Road Anticline; GLF, Green Lake LOM 0 Metres 250 Fault; and LOM, limit of mapping.
Saskatchewan Geological Survey 13 Summary of Investigations 2006, Volume 2 A A
B B
Tops
Figure 15 - Field photographs of unit 5 on Green Lake Peninsula. A) Plagioclase and pyroxene crystal-rich mafic lapilli-tuff (316235E, 6066153N). B) Thin bedded mafic tuff Figure 16 - Field photographs of unit 6 on Green Lake (316113E, 6066592N). Peninsula. A) Scoriaceous mafic tuff-breccia (316105E, 6066697N). B) Graded bedding in thin bedded mafic tuff to weathering. This unit grades upwards into a coarser lapilli-tuff (316105E, 6066697N). volcaniclastic succession, which is characterized by a distinctive scoriaceous breccia (unit 6, Figure 14; Figure 16) and minor mafic tuff, lapilli tuff, and tuff breccia. The volcaniclastic rocks are intercalated with minor discontinuous, plagioclase (15 to 20%, 2 to 8 mm long) ±pyroxene (up to 2%, <1 to 2 mm long) porphyritic, pillowed mafic flows. The breccia unit is massive to thick bedded, varies from matrix to clast-supported and comprises aphyric and plagioclase-porphyritic, subangular to subrounded scoriaceous fragments in a tuffaceous matrix. The tuff, lapilli tuff and tuff breccia units are thin to medium bedded and matrix supported, with a mafic tuffaceous matrix and aphyric and plagioclase-phyric scoriaceous lapilli and blocks. The tuff and lapilli-tuff units locally contain up to 15% pyroxene crystals. The breccia unit thickens toward the south and is sandwiched by tuff and lapilli tuff with similar scoriaceous fragments. The sudden increase in thickness of units 4, 5, and 6 northwest of Burley Lake suggests the presence of synvolcanic faults. Unit 6 is overlain by aphyric and sparsely plagioclase (1 to 3%, 1 to 4 mm) and pyroxene (~1%, 1 to 2 mm) porphyritic massive flows or subvolcanic sills, intercalated with ?minor thin bedded to laminated mafic tuff (unit 7). Scattered amygdules (1 to 2%, 2 to 5 mm) within the massive flows/sills increase in abundance toward the top or the northeast. This unit is overlain by a 10 to 15 m thick succession of thinly laminated mafic tuff, thin to medium- bedded felsic lapilli tuff, and thin bedded to laminated felsic tuff (unit 8, Figure 17). A massive to flow banded, aphyric, spherulitic rhyolite (Figures 17A and 17C) also occurs within this succession, but not always at the same stratigraphic horizon, and is therefore interpreted to be intrusive. Above the bedded felsic tuff is a unit comprising medium- to thin-bedded mafic tuff and lapilli tuff, locally with up to 10% pyroxene crystals, intercalated with minor massive and pillowed aphyric and sparsely pyroxene (1 to 2%, 1 to 3 mm) and plagioclase (3 to 5%, 1 to 4 mm) porphyritic flows (<10%; unit 9). Some of the tuff beds are graded; however, facing directions are not consistent, suggesting both normal and reverse grading.
Saskatchewan Geological Survey 14 Summary of Investigations 2006, Volume 2 Unit 9 is overlain by a succession of weakly A amygdaloidal, sparsely pyroxene (1 to 2%, 1 to 2 mm) and plagioclase (1 to 2%, 1 to 2 mm) porphyritic, massive to pillowed flows (unit 11), which are intercalated with thin bedded to laminated mafic tuff (unit 10). Some of the massive mafic volcanic ‘flows’ have well-developed chilled margins against the intervening tuff unit (Figures 18A to 18C) and are likely subvolcanic sills. Two individual flows/sills are separated from each other and adjacent units by laminated mafic tuff (Figure 18A), ranging from a few tens of centimetres to a few metres thick. Because the flows/sills and intervening tuff units can be traced along strike for some distance, the thickness of the tuff units have been somewhat exaggerated so as to represent them on the map. Toward the southeast, the succession thickens, the two massive flows/sills become pillowed (Figures 18D and 18E) and the tuff units are less continuous. Further along strike, B discontinuous lenses of medium-bedded mafic tuff and lapilli tuff appear (Figure 18E) and give facing directions to the northeast. There are also abundant, strongly plagioclase porphyritic (10 to 15%, 1 to 6 mm), pillowed flows and minor aphyric, massive and pillowed flows.
Units 10, 11, and 12 also occur near the northern tip of the peninsula in an isolated fault block, where bedding trends roughly east-west, faces south and is transected by numerous sinistral shear zones. The massive flows there here have amygdaloidal and commonly lobate or pillowed tops and faint compositional layering parallel to bedding (Figure 19). One of these fine- to medium- grained flows has slightly coarser leucocratic patches and compositional layering in its centre (Figure 19) and was sampled for U-Pb geochronology to date the time C of eruption. Unit 11 passes upward into a succession of thin, aphyric, weakly amygdaloidal, massive and pillowed flows commonly with peperitic tops, intercalated with thin bedded to laminated mafic tuff (unit 13) and poorly-bedded lapilli tuff (unit 12). The flows range from 1 to 4 m thick and have more strongly amygdaloidal tops and/or pillowed or peperitic tops (Figure 20C), indicating younging to the east. The interflow laminated mafic tuff (Figure 20D) ranges from tens of centimetres up to two metres and is not distinguished as a separate map unit at this scale. The lapilli tuff is generally not well bedded, but more than one bed is evident by differences in grain-size, clast type and size. Most of the lapilli tuff beds are dominated by a mafic tuff component, with 15 to 20% Figure 17 - Field photographs of unit 8 on Green Lake aphyric mafic lapilli (Figure 20A) and rare aphyric Peninsula. A) Laminated felsic tuff broken up by numerous amygdaloidal blocks. There is one distinctive lapilli- small fracture sets in a variety of orientations (315995E, tuff unit that contain abundant plagioclase crystals (25 6067034N). B) Thin-bedded felsic tuff and lapilli-tuff to 30%, 2 to 4 mm in size) and dark grey, flattened (315995E, 6067034N). C) Spherulitic massive aphyric aphyric mafic clasts (Figure 20B). rhyolite (315626E, 6067551N). The uppermost unit of this fault bound succession comprises weakly to non-amygdaloidal, aphyric pillowed flows with multiple concentric cooling cracks, large flattened pillows (up to 2 m long) and abundant quartz veining in the pillow margins and interstices (unit 14). The facing direction of this unit is not evident. It is separated from the next unit to the east by the Burley Lake Fault.
Saskatchewan Geological Survey 15 Summary of Investigations 2006, Volume 2
A D
E
B
Tf Tf Flow Ltf
C
Figure 18 - Field photographs of unit 11 on Green Lake Peninsula. A) Thin-bedded to laminated mafic tuff (315943E, 6067376N). B) Chilled margin of sparsely pyroxene and plagioclase porphyritic massive flow/synvolcanic sill (bottom) against mafic tuff (top). Contact shown by dotted line (315943E, 6067376N). C) Fine- to medium-grained centre of pyroxene and plagioclase porphyritic massive flow/synvolcanic sill (315943E, 6067376N). D) Sparsely pyroxene and plagioclase porphyritic, amygdaloidal, pillowed mafic flow (316146E, 6067136N). E) Pillowed mafic volcanic overlain by a wedge of lapilli tuff (Ltf) and tuff (Tf; 316335E, 6066868N).
Saskatchewan Geological Survey 16 Summary of Investigations 2006, Volume 2 West of the Green Lake Fault A A northwest-trending line through the middle of the peninsula, separates thinner, well-bedded, continuous units in the northeast, from thicker, poorly bedded units that are cut by large massive intrusive bodies. This change is inferred to coincide with a loosely constrained reversal in facing directions (Figure 14). It also coincides with a broad, multiply reactivated deformation zone (stippled pattern in Figure 14) that overprinted the inferred contact zone. Therefore, the contact is drawn as an inferred fault, although the timing and nature of this fault are uncertain. At the south end of the map area, the change in facing directions coincides with the Green Lake Fault of Stockwell (1960), which is marked by a pronounced airphoto lineament. The Green Lake Fault is interpreted as a late structure that overprinted the cryptic contact between the northeast and southwest facing successions. B Current mapping has indicated the Green Lake Fault is not as laterally extensive as indicated by Stockwell (1960). The base of the southwest-facing sequence comprises aphyric (unit 15) and plagioclase (±pyroxene) porphyritic pillowed flows and massive flows/sills (unit 16) intercalated with thin bedded to laminated mafic tuff and minor mafic lapilli tuff (unit 17). In the northwest, the flows are predominantly pillowed, whereas in the southeast, the flows are mainly massive and could be subvolcanic sills. All of these flows commonly weather dark green to black (Figure 21) and lack the bedding parallel flattening fabric and foliation typical of the northeastern part of the peninsula. The porphyritic flows range from strongly plagioclase porphyritic (12 to 20%, 2 to 8 mm) with only minor pyroxene (1 to 2%, 1 to 4 mm), to moderately C plagioclase and pyroxene porphyritic (5 to 7% plagioclase, 1 to 4 mm; 2 to 4% pyroxene, 1 to 4 mm). The massive flows are commonly fairly coarse-grained in the centre (Figure 22). All of the above units occur as in situ screens between broadly synvolcanic aphyric and plagioclase porphyritic mafic intrusive rocks (units 21 and 22). The massive flows/subvolcanic sills are weakly amygdaloidal with more strongly amygdaloidal tops, indicating younging to the west. To the northwest, this succession is overlain by a thick succession of aphyric to sparsely plagioclase porphyritic massive and pillowed flows that are in part peperitic (unit 15, Figure 21C). An abrupt change from the thick succession of aphyric flows, to the sheeted tuff-flow/sill complex in the southeast suggests a faulted contact (as shown in Figure 14). The aphyric flows are overlain by a west- facing succession of plagioclase±pyroxene porphyritic Figure 19 - Field photographs of unit 11 on Green Lake (plagioclase 5 to 7%, 1 to 5 mm; pyroxene 1 to 2%, Peninsula. A) Fine-grained, massive, mafic flow with faint ~1 mm) pillowed flows (Figure 22). Southeast of the compositional layering in part defined by coarser leucocratic patches (315744E 6067693N). B) Coarser-grained, leucocratic sheeted tuff-flow/sill succession is an abrupt change patches in massive mafic flow (315744E, 6067693N). C) Close into a thick sequence of plagioclase±pyroxene up of coarser-grained leucocratic patches in massive mafic porphyritic flows, suggesting the possibility of another flow (315744E, 6067693N). faulted contact. This potential fault contact is presently occupied by a thin northerly trending branch off a larger mafic intrusion.
Saskatchewan Geological Survey 17 Summary of Investigations 2006, Volume 2 A C M
Tops
P
B D
Figure 20 - Field photographs of unit 12 on Green Lake Peninsula. A) Mafic lapilli-tuff (316141E, 6067248N). B) Plagioclase crystal-rich mafic lapilli tuff with flattened aphyric mafic lapilli (parallel to horizontal pencil), with the late spaced cleavage parallel to the other pencil (316345E, 6067044N). C) Massive, aphyric flow with pillowed top and minor interpillow tuff (316345E, 6067044N). Hammer for scale is about 40 cm long. D) Thin-bedded to laminated mafic tuff (316300E, 6067129N) with kink folds of uncertain generation. Fractures have developed along the hinges of the kink folds (dashed lines). The pillowed plagioclase and pyroxene porphyritic flows are overlain by quartz and plagioclase porphyritic massive rhyolite (unit 18), which is in turn overlain by sparsely plagioclase porphyritic massive rhyolite and then aphyric massive rhyolite. Bailey (2006) correlated all these rhyolites with the Myo member and based on their massive nature interpreted them to be most likely intrusive in origin. b) Structure A broad, multiply reactivated, northwest-trending fault zones extends down the centre of Green Lake Peninsula and separates it into two halves. The rocks on the east side of this fault zone are characterized by: 1) a strong, northwest- striking, steeply west-dipping, bedding-parallel flattening fabric (e.g., Figures 19B to 19D) and locally a parallel foliation defined by preferred mineral alignment of chlorite; 2) north-northwest–striking and northwest-striking (bedding-parallel), steeply-dipping, sinistral, ductile shear zones, in part defined by C-S fabrics and shear bands; and 3) a roughly west-northwest–striking, steeply dipping spaced cleavage (Figures 19B and 23A). Near the northern tip of the peninsula, in two isolated fault blocks, bedding strikes almost perpendicular to that elsewhere and dips moderately to the southeast (Figure 14). Small scale folds of bedding, bedding cleavage relationships and a strong stretching lineation indicate that this is the hinge zone of a map-scale fold (Figure 24). The axial planar cleavage strikes about 140° and dips steeply west. Small scale folds and a parallel stretching lineation trend about 135° to 145° and plunge about 35º to 45º southeast. The fold is dissected by numerous northwest-trending sinistral, ductile shear zones with minor offset.
Saskatchewan Geological Survey 18 Summary of Investigations 2006, Volume 2
A A
Tops
Tops
B B
Ep
Tf
C C
Figure 21 - Field photographs of unit 15 on Green Lake Figure 22 - Field photographs of unit 16 on Green Lake Peninsula. A) Sparsely quartz-plagioclase amygdaloidal peninsula. A) Weakly amygdaloidal, sparsely pyroxene and aphyric pillowed mafic flow, tops to top of photo (315757E, plagioclase porphyritic mafic volcanic rock with well- 6066555N). Hammer head for scale is about 10 cm long. formed pillows, tops to top of photo (315687E, 6067441N). B) Close-up of sparsely quartz-plagioclase amygdaloidal Hammer for scale is about 40 cm long. B)Chilled margin of aphyric pillowed mafic flow. C) Interpillow blocky peperite sparsely plagioclase and pyroxene porphyritic massive in pillowed aphyric mafic flow (315924E, 6066214N). mafic flow/synvolcanic sill (above) against mafic tuff (Tf). There is a weakly amygdaloidal zone near the margin of the flow, which is epidotized (Ep; 316331E, 6065833N). C) Fine- to medium-grained centre of massive, sparsely porphyritic flow (316331E, 6065833N).
Saskatchewan Geological Survey 19 Summary of Investigations 2006, Volume 2
A C S1 S0
S2
B Flow D
Tf
Frac
Flow
Figure 23 - Field photographs showing some of the fault-related structural elements of the Green Lake Peninsula. A) Relationship between bedding and two foliations, an early one that is nearly bedding parallel and the later, approximately east-west striking spaced cleavage (316201E, 6067128N). B) An early chlorite-hematite fracture along the contacts between a massive flow and mafic tuff, offset by numerous small, brittle, north-trending dextral faults (315626E, 6067551N). Hammer for scale is about 40 cm long. C) Fold of a hematized early brittle fault (315562E, 6067569N). D) Subvertical, northwest- trending brittle-ductile fabric adjacent to the Green Lake Fault (316636E, 6067551N). Hammer for scale is about 40 cm long. The rocks west of the fault zone do not have a well-developed bedding-parallel flattening fabric or foliation. This is interpreted to be the result of early hornfels metamorphism suggested by the dark green to back colour of the volcanic rocks. This metamorphism is interpreted to have been caused by the large Boundary and Phantom intrusions along the western arm of Phantom Lake (Figure 2). There are, however, numerous roughly bedding- parallel, northwest-striking, steeply dipping shear zones, both dextral and sinistral, commonly along contacts between intrusive and volcanic/sedimentary rocks. There is also a set of north-northeast–striking, steeply-dipping shear zones that post-dated the northwest-striking ones and predated the late north-south brittle dextral faults. The earliest structures within the Green Lake Peninsula fault zone are northwest-trending chlorite-, hematite- and iron sulphide-filled fractures that are locally folded and have been transposed into parallelism with the overall northwest- trending fabric within the fault zone. The original orientation of these fractures is not known. They were overprinted by northwest-trending sinistral and dextral shear zones. These are in turn overprinted by ubiquitous subvertical north-south striking brittle faults with dextral offset, observable at centimetre to map scale (Figure 23). The pervasive late brittle faulting precludes detailed interpretation of the earlier faulting history. c) Intrusive Rocks There is a wide range of intrusive rocks including: 1) aphyric, aphanitic mafic dykes and sills; 2) aphyric, fine- to medium-grained gabbro; 3) plagioclase porphyritic, aphanitic mafic dykes and sills; 4) plagioclase porphyritic, fine- to medium-grained gabbro; 5) aphyric rhyolite; and 6) coarse-grained gabbro with acicular amphibole, interpreted to be part of the Boundary Intrusion Suite. The first five intrusive types occur predominantly as sills and are interpreted to be broadly synvolcanic. They have been sampled for geochemistry to test this hypothesis.
Saskatchewan Geological Survey 20 Summary of Investigations 2006, Volume 2 A 5. Conclusions and Discussion Some of the key findings in the Douglas Lake area are recognition of: 1) an extrusive, aphyric felsic volcanic unit, stratigraphically above the uppermost rhyodacite of the Myo member; 2) abundant peperite at several stratigraphic levels; 3) a semi-conformable silicification zone and evidence for other early silicification; and 4) pervasive sinistral shearing. Clvg The recognition of abundant intrusive felsic rocks S above the Myo member, suggests that using it as a 0 criteria to define the Myo member may be problematic. Abundant peperite at the base of this succession indicates that a significant amount of water-lain mafic tuff was deposited prior to eruption of the flows. This suggests a hiatus in volcanism similar to that represented by the tuffs of the Millrock member (Devine et al., 2002), which hosts the VMS deposits. B More mapping is required along strike and down section from the present map area to determine the best way to subdivide the stratigraphy on the west limb of the BRA.
The nature and intensity of the silicified amoeboid flow tops of unit 10 is similar to that in the footwall of major VMS deposits in both Snow Lake, Manitoba and Noranda, Quebec (H. Gibson, pers. comm., 2006). Several of the Au/Cu-Au showings and past producers in the Douglas Lake area are associated with massive sulphides, bedding-parallel disseminated sulphides, and Cu- and Zn-bearing sulphides, suggesting a link to VMS mineralization. Along the east shore of Douglas Lake, there is an abrupt change from predominantly aphyric flows to plagioclase porphyritic volcaniclastic rocks, which suggests the presence of a subsidence structure coincident with a change in magma type. Two C of the past producing mines in this area (Newcor and Bootleg/Rio) occur at or near this horizon (Figure 3). Clvg These observations suggest a favourable environment for the formation of VMS mineralization (Gibson et al., 1999) stratigraphically above the Myo member, and more specifically at the base of the Douglas formation of Thomas (1992). S0 Some of the key points for the Green Lake Peninsula area are (Figure 14): 1) the stratigraphy is separated into three fault-bounded successions with different facing directions; 2) the central fault block is composed of about 50% volcaniclastic material; 3) recognition of a broad, multiply-reactivated, northwest-trending structural zone down the centre of the peninsula; 4) The Phantom Lake Fault, as previously mapped, does not exist; and 5) presence of a map-scale fold closure at the Figure 24 - Field photographs of fold-related structural north end of the peninsula. elements on the Green Lake Peninsula. A) Bedding-cleavage relationship in the hinge of the fold. The axial planar The change in facing direction identified by previous foliation is defined by flattened lapilli (315760E, 6067663N). workers (Stockwell, 1960; Bailey, 2006) was B) Small-scale close folds of interpillow tuff with a mineral confirmed and a third was added, all occupying stretching lineation parallel to the fold hinge (315657E, 6067798N). C) Bedding in the hinge zone of the fold, separate fault blocks. The eastern block is a showing the steeply dipping axial planar cleavage (315744E, continuation of the west facing Louis formation on the 6067693N). east limb of the Burley Lake Syncline (Simard, 2006; Figure 25). The Burley Lake Syncline does not continue south of Potter Bay as previously suggested,
Saskatchewan Geological Survey 21 Summary of Investigations 2006, Volume 2
0 km 1 Legend
C Intrusive Rocks C VMS deposit F FF F Phantom Intrusive Suite FF=Flin Flon L C C Boundary Intrusions C=Callinan C RWF Tr=Triple 7 Bootleg Pluton M=Mandy 10 S=Schist Lake Annabel Pluton Tr H 106 Tailings Pond L S Cliff Lake Pluton Tr Mine shaft N=North Main Tr Missi Group S=South Main F
L Tr= Triple 7
F Ross
F Lake Flin Flon Assemblage 167 Highway
N Louis formation 10A Provincial Border FF 10 Hidden formation Fault
B R A Douglas formation S F1 Syncline/ Anticline Flin Flon formation F2 Syncline/
P Millrock member Anticline L
R Myo member (felsic/mafic)
167 W Younging
A Creighton member
R direction Blue Lagoon member CL Club member
Unassigned volcanic rocks R Schist B
M L Lake Douglas L F
F Lake B S R D A L F R F ? Potter L F Bay ? M Phantom Lake
F R S
Burley B L Lake F G L F Figure 25 - Revised regional map showing new interpretation of the regional structure and stratigraphy of the Green Lake–Burley Lake– Green Carlisle Lake area. Abbreviations same as in Figure 2. Lake
Saskatchewan Geological Survey 22 Summary of Investigations 2006, Volume 2 but is cut by the northwest-trending Burley Lake Fault (Simard, 2006; Figure 25), a subvertical sinistral shear zone that in part coincides with the Phantom Lake Fault of Stockwell (1960). The central block, between the Burley Lake and Green Lake faults, is an east-facing succession of fine-grained mafic volcaniclastic rocks (~ 50%) intercalated with massive and pillowed flows. The proportion, thickness, and grain-size of the volcaniclastic component increases toward the southeast, suggesting the presence of a depositional centre. This succession is tentatively correlated with the lithologically similar Hidden formation north of Potter Bay; this hypothesis will be tested geochemically. Although the hinge zone of the Burley Lake Syncline has been cut out by later strike-slip faulting along the Burley Lake Fault, the regional change in facing direction of the Louis and Hidden formations was likely originally caused by this fold. Thus, the finer volcaniclastic rocks of the Hidden formation on the Green Lake Peninsula are tentatively interpreted to be a lateral facies equivalent of the west facing Hidden formation in the Carlisle Lake area (Simard, 2006; Figure 25), which comprises a large proportion of breccia (Simard, 2006). The west-facing sequence in the third block, west of the Green Lake Fault is tentatively correlated with the west limb of the Beaver Road Anticline. More work, including geochemical analyses, is required to determine its exact stratigraphic position. The complex, multiply reactivated fault zone down the centre of the Green Lake Peninsula is dominated by late brittle dextral faulting, but there is evidence for several earlier phases of faulting. One of the earliest faulting events is inferred to have juxtaposed the east and west facing successions, although the timing and nature of this inferred fault are unconstrained. This fault zone may be a continuation of the Flin Flon Lake Fault, which is also a broad zone with evidence for multiple phases of faulting, including abundant late brittle dextral faults (H. Gibson, pers. comm., 2006). This interpretation implies that the Phantom Lake Fault does not continue across Phantom Lake as proposed by Stockwell (1960; Figure 2). Thus, the Phantom Lake Fault of Stockwell (1960) is herein separated into two different structures: 1) the earlier Burley Lake fault, a north-northwest–trending sinistral shear zone with significant offset, which separates the west facing Louis Formation and the east facing Hidden formation along the east side of the Green Lake Peninsula; and 2) a series of anastomosing, northwest-trending, approximately bedding- parallel, sinistral shear zones, which characterize the whole western limb of the Beaver Road anticline and which Bailey (2006) showed to have very little offset in the vicinity of the northwestern end of the former Phantom Lake Fault. The bedding-parallel sinistral shear zones are also common along the eastern side of the Green Lake Peninsula where they show only minor offset of the Burley Lake Fault (Figure 14).
The fold hinge that occurs near the northern tip of the Green Lake Peninsula is similar in orientation, shape and plunge to the hinge zone of the Beaver Road anticline, which is cut out by the Flin Flon Lake fault north of the boundary intrusion on Phantom Lake (Figure 25). If, as proposed here, the Flin Flon Lake Fault continues onto the Green Lake Peninsula then this fold could be the continuation of the Beaver Road Anticline on the east side of the Flin Flon Lake Fault. More mapping is required to investigate this possibility.
6. Acknowledgments Many of the ideas in this report evolved in part from field trips and discussions with various colleagues on the Flin Flon TGI 3 project, all of whom are thanked for their input and encouragement. In particular, I would like to thank Renee-Luce Simard with whom I worked closely to develop ideas about the structure, stratigraphy and volcanology of the Green Lake Peninsula area. I would also like to thank Harold Gibson for sharing freely his extensive knowledge of volcanology and VMS deposits. The SIR crew is very appreciative of the generosity and hospitality of the Manitoba Geological Survey for sharing their wonderful trailer camp at the Centennial Mine Site. I am also very grateful for the time, expertise and information provided by the geologists of Hudson Bay Exploration and Development. Last, but not least I would like to thank my field crew, Cassia Johnson, Ron LeRay and David Lewis for their cheerful cooperation and assistance. Ralf Maxeiner and Charlie Harper are thanked for thorough and thoughtful reviews, which significantly improved this report.
7. References Ames, D.E., Tardif, N., MacLachlan, K., and Gibson, H.L. (2002): Geology and hydrothermal alteration of the Flin Flon–Triple 7–Callinan VMS horizon hanging wall stratigraphy: Flin Flon Targeted Geoscience Initiative; in Summary of Investigations, 2002, Volume 2, Sask. Industry Resources, Rep. 2002-4.2, CD-ROM, Paper B-3, 12p. Bailes, A.H. and Syme, E.C. (1989): Geology of the Flin Flon–White Lake Area; Manit. Energy Mines, Geol. Rep. GR 87-1, 313p. Bailey, K.A. (2006): Emplacement, petrogenesis and volcanic reconstruction of the intrusive and extrusive Myo Rhyolite complex, Flin Flon and Creighton, Saskatchewan; unpubl. M.Sc. thesis, Laurentian Univ. Sudbury, 124p.
Saskatchewan Geological Survey 23 Summary of Investigations 2006, Volume 2 Bailey, K.A. and Gibson, H.L. (2004): A field description of the Myo Rhyolite Flin Flon and Creighton Saskatchewan; in Summary of Investigations, 2004 Volume 2, Sask. Industry Resources, Misc. Rep. 2004-4.2, CD-ROM, paper A-1, 11p. ______(2005): Distribution and geochemistry of the Myo Rhyolite Flin Flon and Creighton Saskatchewan; in Summary of Investigations, 2005 Volume 2, Sask. Industry Resources, Misc. Rep. 2005-4.2, CD-ROM, Paper A-8, 10p. Devine, C.A., Gibson, H.L., Bailes, A.H., MacLachlan, K., Gilmore, K., and Galley, A.G., (2002): Stratigraphy of VMS-hosting volcanic and volcaniclastic rocks of the Flin Flon Formation, Flin Flon–Creighton area, Saskatchewan and Manitoba; in Summary of Investigations, 2002 Volume 2, Sask. Industry Resources, Rep. 2002-4.2, CD-ROM, Paper B-4, 11p. DeWolfe, Y.M. and Gibson, H.L. (2005): Physical description of the Bomber, 1920 and Newcor members of the Hidden formation, Flin Flon, Manitoba (NTS 63K16SW); in Report of Activities 2005, Manit. Ind. Econ. Dev. Mines, Manit. Geol. Surv., p7-19. Fedorowich, J.S., Kerrich, R., and Stauffer, M.R. (1995): Geodynamic evolution and thermal history of the central Flin Flon Domain Trans-Hudson Orogen: constraints from structural development, 40Ar/39Ar, and stable isotope geothermometry; Tecton., v14, p472-503. Fisher, R.V. (1966): Rocks composed of volcanic fragments; Earth Sci. Rev., v1, p287-298.
Gale, D.F., Lucas, S.B., and Dixon, J.M. (1999): Structural relations between the polydeformed Flin Flon arc assemblage and Missi Group sedimentary rocks, Flin Flon area, Manitoba and Saskatchewan; Can. J. Earth Sci., v36, p1901-1915.
Gibson, H.L., Ames, D., Bailes, A., Tardif, N., Devine, C., Galley, A., MacLachlan, K., and Gilmore, G. (2003a): Invasive flows, peperite and Paleoproterozoic massive sulphide, Flin Flon Manitoba and Saskatchewan; Geol. Assoc. Can., Abstr. Vol. 28, CD-ROM, abstr. #357.
Gibson, H., Devine, C.A., Galley, A., Bailes, A., Gilmore, K., MacLachlan, K., and Ames, D. (2003b): Structural control on the location and formation of Paleoproterozoic massive sulphide deposits as indicated by synvolcanic dike swarms and peperite, Flin Flon, Manitoba and Saskatchewan; in Geol. Assoc. Can./Mineral. Assoc. Can., Soc. Econ. Geol., Jt. Annu. Meet. 2003, Abstr. Vol. 28, p35.
Gibson, H., DeWolfe, M., Bailey, K., Devine, C., Lafrance, B., Gilmore, K., Simms, D., and Bailes, A. (2005): The Flin Flon caldera: ore localization within a Paleoproterozoic synvolcanic subsidence structure defined through mapping; Manitoba Mining and Minerals Convention 2005, Prog. Abstr., p49.
Gibson, H.L., Morton, R.L., and Hudak, G.J. (1999): Submarine volcanic processes, deposits and environments favourable for location of volcanic-associated massive sulphide deposits; in Barrie C.T. and Hannington M.D. (eds.), Volcanic-associated Massive Sulphide Deposits: Processes and Examples in Modern and Ancient Settings, Soc. Econ. Geol., Reviews in Economic Geology, v8, p13-51.
Gibson, H.L., Watkinson, D.H., and Comba, C.D.A. (1983): Silicification: hydrothermal alteration in an Archean geothermal system within the Amulet Rhyolite Formation, Noranda, Quebec; Econ. Geol., v78, p954-971. Heaman, L.M., Kamo, S.L., Ashton, K.E., Reilly, B.A., Slimmon, W.L., and Thomas, D.J. (1992): U-Pb geochronological investigations in the Trans-Hudson Orogen, Saskatchewan; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4, p96-99. Kennedy, G.C. (1950): A portion of the system silica-water; Econ. Geol., v25, p629-653. Lucas, S.B., Stern, R,A., Syme, E.C, Reilly, B.A., and Thomas, D.J. (1996): Intraoceanic tectonics and the development of continental crust: 1.92 - 1.84 Ga evolution of the Flin Flon Belt; GSA Bull., v108, p602-629. Lucas , S.B., Syme, E.C., and Ashton, K.E. (1999): New perspectives on the Flin Flon Belt, Trans-Hudson orogen, Manitoba and Saskatchewan: an introduction to the special issue on the NATMAP Shield Margin Project, Part 1; Can. J. Earth Sci., v36, p135-140. MacLachlan, K. and Bailey, K. (2002): Bedrock geology, Myo Lake section, Creighton, Saskatchewan (part of NTS 63K-12); with Summary of Investigations 2002, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.2, 1:3000 scale map.
Saskatchewan Geological Survey 24 Summary of Investigations 2006, Volume 2 McPhie, J., Doyle, M., and Allen, R. (1993): Volcanic Textures: A Guide to the Interpretations in Volcanic Rocks; CODES, University of Tasmania, Hobart, 198p. Price, D. (1997): Flin Flon–Creighton generalized geological map; internal map prepared for Hudson Bay Exploration and Development Company Ltd., 1:10 000 scale. Sabina, A.P. (1987): Rocks and Minerals for the Collector, La Ronge–Creighton, Saskatchewan; Flin Flon- Thompson, Manitoba; Geol. Surv. Can., Misc. Rep. 42, p19-20. Simard, R.-L. (2006): Geology of the Schist Lake–Mandy mines area, Flin Flon, Manitoba (part of NTS 63K12); in Report of Activities 2006, Manit. Sci. Tech, Energy Mines, Manit. Geol. Surv., p9-21. Saskatchewan Mineral Deposit Index (2006): URL
Stockwell, C.H. (1960): Flin Flon–Mandy, parts of 63K/12 and 13; Geol. Surv. Can., A-series Map 1078, 1:12 000 scale.
Syme, E.C. and Bailes, A.H. (1993): Stratigraphic and tectonic setting of volcanogenic massive sulphide deposits, Manitoba; Econ. Geol., v88, p566-589.
Syme, E.C., Lucas, S.B., Zwanzig, H.V., Bailes, A.H., Ashton, K.E., and Haidl, F.M. (1998): Geology, NATMAP shield margin project area Flin Flon Belt, Manitoba/Saskatchewan accompanying notes; Geol. Surv. Can. Map 1968A/Manit. Energy Mines Map A-98-2/Sask. Energy Mines Map 258A.
Thomas, D. (1992): Highlights of investigations around the Flin Flon Mine: reassessment of the structural history; in Summary of Investigations 1992, Saskatchewan Geological Survey; Sask. Energy Mines, Misc. Rep. 92-4, p3-15.
______(1994): Stratigraphic and structural complexities of the Flin Flon Mine Sequence; in Summary of Investigations, 1994, Saskatchewan Geological Survey, Sask. Energy and Mines, Misc. Rep. 94-4, p3-10.
Saskatchewan Geological Survey 25 Summary of Investigations 2006, Volume 2