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The Crater Island Assemblage, Amisk Lake (Part of NTS 63L-9} 1

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The Crater Island Assemblage, Amisk Lake (Part of NTS 63L-9} 1 The Crater Island Assemblage, Amisk Lake (Part of NTS 63L-9} 1 B.A. Reilly Reilly, B.A. (1994): The Crater Island Assemblage, Amisk Lake (part of NTS 63L-9); in Summary of Investigations 1994, Sask­ atchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4. The main objective this summer was to complete revi­ the past few years coupled with geochemical and iso­ sion mapping of the south-central Amisk Lake area, and tope studies (Watters et al., in press; Stern et al., in improve the understanding of the relationships between press a and b) has led to the recognition of several dis­ the West Amisk and Muskeg Bay assemblages to the tinct lithotectonic assemblages (Figure 1) (Reilly et al., west and the Sandy Bay Assemblage to the east. Ap­ in press) in the region. proximately 200 km2 were mapped at 1:50 000 scale during the month of August in an area extending from Greenstone assemblages on the west side of Amisk the south shore of Missi Island to the edge of the Pre­ Lake are dominated by 1882 to 1888 Ma (Heaman et cambrian Shield at the south end of Amisk Lake (most al., 1993; Stern and Lucas, in press) felsic to intermedi­ of this area is covered by Amisk Lake), thus bridging ate calc-alkaline island arc volcanic rocks underlain by the gap in revisional geological mapping which existed largely tholeiitic island arc basalts2 (Fox, 1976a and b; between the east and west shores of Amisk Lake, ex­ Walker and Watters, 1982; Ashton, 1990, 1992; Watters cluding central and eastern Missi Island. and Ashton, 1991 ; Reilly 1992, 1993; Stern et al., in press a). These have been termed the West Amlsk As­ Detailed revisional bedrock mapping by the Saskatche­ semblage (Reilly, 1993) and the Muskeg Bay Assem­ wan Geological Survey in the Amisk Lake area during blage (Reilly et al., in press), respectively. r~.i ·-.s~ (rcup ~ SE->d .,•·c·r·"ts ,~,_ r-.V ljl"J,)..);J 1--:::J ,~ypohy ·S:,), R,.,,. kr, ~/J \,/~l s•·, L,.JI~(-· i!i:.] ,..,.ie:,~ i .C:niSk E3) M,.sl--"'g ll,1y l£i~J ::---o. t,•r- Js :~nd Ll Sa ndy Bey l..!_J2i2:4b.J "" Figure 1 • Lithotectonic assemblages of the Amisk Lake area. (1) Saskatchewan Project A.112 was funded in 1994 under the Canada-Saskatchewan Partnership Agreement on Mineral Development 1990-95. (2) All of the Precambrian rocks in the map area have been metamorphosed and the prefix meta will be omitted when referring to these rocks. Saskatchewan Geological Survey 11 On the east side of Amisk Lake, the Sandy Bay As· Typical ab(cd) turbidite sequences and soft-sediment semblage (Slimmon, 1991 b), consists predominantly of slump folds are described by Van Wagoner (1982). tholeiitic MOAB-like basalts (Parslow and Gaskarth, 1984, 1988; Gaskarth and Parslow, 1987; Slimmon, A lower and an upper subaerial sequence dominated by 1991a, 1993; Watters et al., in press, this volume; Slim­ tuff and lapilti tuft have been described by Ayres et al. mon, this volume; Stern et al., in press b). ( 1991) in rocks now included within the Crater Island As­ semblage (Figure 2). In the lower sequence, bedding is This report briefly describes the characteristics, contact generally poorly developed and flows form about 50 per­ relationships, and economic potential of a newly defined cent of the sequence. In the upper sequence, bedding lithotectonic assemblage, namely the Crater Island As­ is well developed and flows form approximately 15 per­ semblage. This assemblage is excellently displayed cent of the sequence. Individual beds, which range in due to lower greenschist facies metamorphism, rela­ thickness from 5 mm to 8 m, in places display graded tively low deformation, paucity of plutons, and the pres­ bedding, although most are ungraded. Accretionary ence of wave-washed, flat-lying, lichen-free shoreline lapilli and armored lapilli occur in many of the beds, and exposures. basal scours and cross-beds are found locally. A summary of the main features of the lithotectonic vol­ Amygdaloidal flows up to about 1.5 m thick containing canic assemblages of the Amisk Lake area are given in up to 30 percent amygdales of variable size, shape, Table 1. and distribution have been interpreted as subaerial de­ posits by Ayres (1978). Features in support of this inter­ pretation include the presence of basal pipe amygdales, 1 . Crater Island Assemblage thinly chilled upper flow surfaces, and the absence of pillows. The amygdales are generally concentrated in The Crater Island Assemblage comprises a thick mafic the upper part of the flows, average 5 to 1 O mm in size volcanic sequence which outcrops in the south-central but are locally as much as 5 cm long, and range in part of Amisk Lake and extends from lskwasoo Island shape from spherical to irregular. Plagioclase-phyric in the east to the southwest corner of Missi Island in and pyroxene-phyric textures occur locally. Ayres the west (Figure 2). The assemblage forms essentially (1978) recognized pahoehoe toes in one subaerial sec­ a folded sequence about three kilometres thick and tion. comprises largely tholeiitic basalt and basaltic andesite (Fox, 1976a and b; Ferreira, 1984; Gaskarth and Pars­ Coarser volcaniclastic rocks occur as massive to low, 1987). crudely bedded, clast-supported to matrix-supported tuft breccia, lapilli tuff, and minor coarse tuff. Fragments are Physical volcanological studies by Lorne Ayres and rounded to angular with chilled margins, amygdaloidal graduate students at the University of Manitoba have to non-amygdaloidal, monolithic in composition, and been conducted at Amisk Lake since the late 1970s. De­ have an observed maximum size of about 75 cm. The tailed stratigraphic sections concentrating on the Crater matrix consists of coarse tuff and small lapilli of similar Island Assemblage can be found in Ayres (1978, 1980a composition to the fragments. Ayres et al. (1991) de­ and b, 1981), Ferreira (1981), Van Wagoner (1982), scribes these coarser volcaniclastic rocks in terms of Van Wagoner et al. (1982), Ferreira (1984), Van two emergent and one intervening submergent zones of Wagoner and Van Wagoner (1987), and Ayres et al. flow foot breccia produced when lava entered the ocean. (1981 , 1991). Gabbro is a common component of the Crater Island The Crater Island Assemblage is composed of approxi­ Assemblage. Fine- to medium-grained dykes and sills mately equal amounts of lava flows and intercalated vol­ range from less than one metre to 500 m in width. canic\astic rocks of basaltic composition. Subaqueous Chilled margins are found in places and rhythmic igne­ massive to pillowed lava flows are the dominant rock ous layering is typically associated with the larger_ i~tru­ type and range in thickness from 3 m to greater than sions. The similarities in appearance and compos1t1on, 50 m. Plagioclase-phyric flows are more abundant than and intimacy of association of the gabbro and basalt aphyric flows. Phenocrysts generally comprise no more country rock suggests that the gabbro is synvolcanic. than 5 percent of the flow and are typically less than 5 mm in size. Microphenocrysts are also characteristic The Crater Island Assemblage is interpreted by Ayres of the plagioclase-phyric flows. Amygdaloidal flows are et al. (1991} as a composite basaltic shield volcano. He commonly intercalated with the massive and pillowed considers that the volcaniclastic rocks were erupted flows. Spherical to ovoid amygdales average less than largely by phreatomagmatic explosions on the coastal 5 mm in size and comprise less than 5 percent of the plain of a rapidly subsiding volcano with low slope an­ flow. Rare olivine-phyric flows are present. gles, and that some tephra w~s subaerially dE:posited as distal surge-and-fall deposits, but the remainder fell lnterflow tuff and lapitli tuff comprise about 40 percent on the upper submarine slopes where it was interca­ of the subaqueous sequences. These volcaniclastic lated with lava flows. units range from massive to bedded, ungraded to nor­ mal graded, and are less than one metre to several hun­ dred metres thick. Ayres et al. (1991) documented 2. Structural Geology and Metamorphism predominantly ungraded units on the west limb of the Winterton Island Anticline and graded units on the east Five main deformation events have been documented limb. Basal scours and flame structures are common. in the Amisk Lake area (Reilly, 1993; Slimmon, 1993): 12 Summary of Investigations 1994 ~ Table 1 - Characteristics of lithotectonic volcanic assemblages of the Amisk Lake area. ~ () ;? Assemblage Sandy Bay Crater Island Muskeg Bay West Amisk ::,i Dominant Lithology Mafic volcanics Mafic volcanics Malic volcanics Felsic to intermediate volcanics G) ~ o" Volcanic Rock Type Lava flows Volcaniclastics lava flows Lava flows > volcaniclastics? Volcaniclastics > lava flows 10 = [ gi Characteristics: Subaqueous aphyric, pillowed Subaqueous massive to pillowed Subaqueous massive to pillowed Subaerial volcaniclastics dominate. ~ flows dominate. Amygdaloidal feldspar-phyric flows dominate. aphyric flows dominate. Feldspar­ Tuff breccia, lapilli tuff, and luff are '<: flows are subordinate; amygdales Aphyric and amygdaloidal flows phyric and amygdaloidal flows are present. Fragments are volcanic, < 5 mm in size and comprise < 5% of are common. Amygdales com­ found locally. Amygdales are < 5 mm heterolithic. and poorly soned. flows. Massive flows and syn­ prise < 5% of flows and are < 5 mm in size and comprise < 5% of flows. volcanic gabbro intrusions are also in size. lnterflow tuff and lapilli tuff Olivine-normative base and quartz· Rare subaqueous volcaniclastics common. Flow breccias occur and synvolcanic gabbro intrusions normative top. Synvolcanic gabbro are chiefly monolithic tuff breccias. locally. are also common. Olivine- and intrusions are common. pyroxene-rich flows are rare. Poorly exposed. Minor subaqueous flows and flow Volcaniclastics are rare. breccias are typically feldspar± Subaerial volcaniclastics of tuff Volcaniclasti cs are rare and found hornblende-phyric.
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