Intrusive and Extrusive Rocks of the Western Part of Missi Island, Amisk Lake1

C.T. Harper

Harper, C.T. (1993): Intrusive and extrusive rocks of the western part of Missi Island, Amisk Lake; in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93-4.

During 1993 an area, of approximately 15 km2 along the western shore of Missi Island, was geologically mapped at 1: 1 O 000 scale as part of the regional remap­ping program, Project Seagull, in the Flin Flon-Hanson Lake area. The area extends from about 0.5 km north of Laural Lake2 to the south end of Hannay Bay (Figure 1}, and about 1.5 km in from the shores of Amisk Lake.

The west shore of Missi Island is about 25 km west of Creighton and 13 km west of the community of Denare Beach, from which boat access to the area follows the East, North, and West Channels of Amisk Lake around the north end of Missi Island.

The topography of the western part of Missi Island is generally undulating, with the greatest vertical relief of 50 to 75 m being adjacent to the shoreline of Amisk Lake. The highest elevation on Missi Island is about 110 m above Amisk Lake. Bedrock is exceptionally well­exposed along the shoreline, but is poor inland, except for areas underlain by some of the intrusive stocks. Ma­ture and very mature forests cover large areas of the is­land, which having extensive understory growth and thick mats of roots, moss and forest litter, effectively mask the bedrock. Travel on foot is exceptionally diffi­cult in areas of very mature forest, because of fallen spruce, poplar, and birch trees up to one metre in di­ameter, and by a very dense secondary growth of bal­sam fir.

Although this area of Missi Island contains numerous gold and base metal prospects (Figure 1 ), the main em­phasis of the project was to examine and determine the intrusive-extrusive and age relationships of the numer­ous porphyritic rocks exposed in the area. Previous mapping (Bruce, 1918; Wright, 1933; Byers and Dahlstrom, 1954) had identified an assortment of inter­mediate to felsic, possibly subvolcanic dykes, sills, and stocks emplaced into a sequence of mafic to felsic vol­canic rocks. Many of these rocks were lumped into unit 6 of Byers and Dahlstrom, therefore, subdividing this unit was a key aspect of the project.

Earlier geological investigations by the Geological Sur­vey of Canada in the Amisk Lake area include work by: Mcinnes (1910, 1913), Bruce (1915, 1916, 1918), Wright (1933), Wright and Stockwell (1934a and b, 1935), and Mawdsley (1931) for the Saskatchewan De-

partment of Natural Resources. Subsequent to Byers and Dahlstrom's (1954) mapping for the Saskatchewan Department of Mineral Resources, Fox (1976a and b), Ayres and Findlay (1976), Ayres (1977, 1980, 1981), Chute and Ayres (1977), and Ayres et al. (1981) carried out additional independent mapping in the area. In 1990 Saskatchewan Energy and Mines began 1 :20 000 scale revision mapping in the west Amisk area, and includes work by Ashton (1990, 1992), Reilly (1992, this volume) and Slimmon (1992, this volume).

Exploration activity in the west Amisk Lake area has been on-going since the early 1900s. Some of the more recent work related to gold mineralization in the West Channel area by Saskatchewan Energy and Mines staff and colleagues includes: Pearson (1980, 1983}, Coombe (1984), Pearson and Galley (1985), and Pear­son, McDougall and Galley (1986).

1 . Regional Geology

Missi Island is situated in the western part of the Early Proterozoic Flin Flon-Snow lake greenstone belt (Flin Flon Domain), which lies along the southern edge of the Precambrian Shield. The belt forms the southern­most part of the exposed Trans-Hudson Orogen (Lewry and Collerson, 1990). The lower grade greenstone belt is transitional northwards into the gneisses of the Kis­seynew Domain, and to the south is overlain unconfor­mably by Ordovician dolomite and sandstone.

The greenstone belt comprises: Amisk Group mafic to felsic volcanics, volcanogenic sediments, and mafic to felsic syn- to post-volcanic dykes and sills; Missi Group fluvial to alluvial sandstones and conglomerates which unconformably overlie the Amisk Group; and a variety of intrusive rocks that range in age from synvolcanic to post-Missi. Both Amisk and Missi groups have been af­fected by several periods of deformation (Stauffer and Mukherjee, 1971 ; Thomas et al., 1993). Metamorphic grade ranges from sub-greenschist to amphibolite fa­cies.

Amisk volcanism occurred about 1905 to 1885 Ma based on U-Pb zircon dating of felsic volcanics and por­phyries (Syme et al., 1987; Bickford et al., 1986; David et al., 1993; Heaman et al., this volume); however, rhy­olites at Hanson Lake have given ages as young as

( 1) Saskatchewan Project A. 117 was funded in 1993 under the Canada-Saskatchewan Partnership Agreement on Mineral Development 1990·95. (2) Laural Lake is correct, but it is commonly misspelled as 'laurel' (Saskatchewan, Gazetteer of Canada, Energy, Mines and Resources Canada).

30 Summary of Investigations 1993

1875 Ma (Heaman et al., this volume). Interestingly, quartz phyric rhyolite from Laural Lake on Missi Island contain an inherited zircon population which yield 207Pb/206pb ages of 1923 to 1934 Ma (Heam an et al., this volume). In the Snow Lake area, at the east end of the greenstone belt, felsic volcanics contain inherited zir· cons with Pb/Pb ages between 2650 and 2824 Ma indi· eating that interaction with Archean crustal material had taken place (David et al., 1993). Amisk greywacke sedi· ments west of Amisk Lake contain detrital zircons which have ages ranging between 1913 and 1884 Ma, indicat­ing that sedimentation occurred shortly after volcanism (Heaman et al., this volume). Similar greywacke tur­bidites in the File Lake area of Manitoba contain detrital zircons ranging from 2345 to 1850 Ma indicating that in

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that part of the belt sedimentation occurred after much of the tectonic jostling of the arc assemblages and em· placement of numerous felsic plutons had taken place (David et al., 1993).

In the Amisk Lake area the oldest intrusive rocks identi­fied thus far are the Mystic Lake tonalite-granodiorite­diorite suite, which intrude highly strained metavol­canics (Reilly, 1990, 1991 ). This intrusive suite has given a U-Pb zircon age of 1906 ±2 Ma (Heaman et al., 1992), and indicates the presence of an older supracrus­tal sequence. A number of synvolcanic plutons have been identified in the Manitoba portion of the belt with ages ranging from 1890 to 1870 Ma (Bailes et al., 1988; Gordon et al., 1990). Pre-Missi intrusions, such

, • •• Mis~r Con9 lomerate ~or/e"o

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Gron odiorite

Plag1oclase Porphyry

Micro d1or i te

ln~ermed1ole to fe !sic vole on i cs (B&D I/hit 2)

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.'!JJ./ Hanna y Boy r e ls 1c Volc a nic Comp lex

~i:J Mixed Mafi c Volcan ics - vo lca n1 c lasl1cs

~.~'i ~ Lauro ! LaKe Rhyo lite Dome Comp lex ~ •,\ //

~ Hyslin Bay Fel s1c Complex

~ Mafic Volcan ic s

~J Maj or Gold Deposit s

L:!.J G old occurr e nce s

Figure 1 - Geological sketch of the western part of Missi Island, Amisk Lake. Geology beyond the limits of the current mapping is from Byers and Dahlstrom (1954). Granodiorite intrusions are: Mll=Missi Island Intrusion, HL=Hambly Lake Intrusion, ML=Meaney Lake Intrusion, and HP=Hannay Peninsula intrusions. Major gold deposits are: 1. Laural Lake, 2. Monarch-Prince Albert, and 3. Amisk Gold Syndicate.

Saskatchewan Geological Su,vey 31

as the Annabel lake, Mari lake, and Reynard lake plu­tons, in the Amisk Lake area have ages of 1860 to 1853 Ma (Ansdell and Kyser, 1990; Heaman et al., 1992). A group of post-Missi intrusions include the mafic to ultramafic Boundary Intrusions (1842 ±3 Ma} which intrude the Missi, and a variety of quartz mon­zodiorites and feldspar porphyritic granodiorites (1838 ±2 Ma) (Heaman et al., 1992). Detrital zircons from the Missi Group yield ages ranging from 2529 to 1847 Ma (Ansdell et al., 1991; Ansdell reported by Syme et al. , 1993). Thus deposition of the Missi Group occurred be­tween 1847 and 1842 Ma and had a diversity of source terranes.

The age of the Missi Island stock was determined to be 1848 ±11 Ma by the lead evaporation technique on zir­cons (Ansdell et al. , 1991 ). Based on this age, it is not clear whether this intrusion and others like ii on Missi Is­land predate or are contemporaneous with Missi Group sedimentation. Only further refinement of the ages of these intrusive rocks would clarify the problem, because there are no direct contact relationships. According to Byers and Dahlstrom (1954) a small outlier of Missi con­glomerate occurs in fault contact with one of the Missi Island intrusions on the west shore of Meaney lake (Figure 1 ). Unfortunately lime did not allow examination of that locale.

2. Missi Island

The major rock types in the western part of Missi Island are: mafic volcanic flow and pyroclaslic rocks, felsic vol­canic (extrusive) and subvolcanic (intrusive) assem­blages, and the Missi Island granodiorite-leucograno­diorite intrusive suite. Minor rock types include interme· diate volcanics and intrusions, metasedimentary rocks, mafic dykes, breccia dykes, and a group of distinctive feldspar-porphyritic quartz diorite dykes and sills.

The following rock descriptions represent the relative stratigraphic sequence based on a rather limited num­ber of top determinations and other relevant sequence determining observations.

a) Mafic Volcanics (M)

Mafic volcanics occur mainly east and south of Laural lake as far as Hambly lake; wrap around the Hambly Lake Intrusion and outcrop along the eastern margin of the Meaney lake Intrusion (Figure 1). Several narrow bands of mafic volcanics occur east of Hannay Bay, and probably represent higher stratigraphic levels.

Thematic volcanic rocks are predominantly flows (Mf), with minor flow breccias, interflow ash tuff (Mt) and/or sediment. They are typically dark green, greenish black to black. very fine grained, and weakly to strongly schis· lose. The tuffaceous/sedimentary sequences tend to show more colour variation from pale to dark green. The mafic volcanics are typically composed of blue

green hornblende, biotite, plagioclase, epidote±quartz with accessory opaque minerals, sphene and carbon­ate; chlorite is commonly present as amygdales. but shows overgrowth by amphibole. Some rocks still pos­sess a greenschist tacies assemblage of chlorite, bi­otite, epidote, plagioclase±actinolite±quartz. Sheared mafic volcanics are schistose and consist mainly of chlo· rite and biotite with carbonate. epidote and opaque grains. Patchy epidote and carbonate alteration is spo­radically developed in all types.

The flows, generally 1 to 3 m thick, are massive (Mfm) and pillowed (Mfp), and can be aphyric, plagioclase phyric and/or amygdaloidal. Well-defined pillows are not common, because of alteration and deformation , but where observed, they range from a few tens ot centime­tres to 2 min length and up to 50 cm wide in plan view. Three dimensional shapes are rarely observed. Pillow selvages range from a few millimetres to 4 cm wide, in­dicating different rates of pillow development. Amygdales are typically less than 5 mm in diameter, but in places are up to 10 cm in diameter. They are composed of chlorite-epidote-quartz±carbonate. quartz±teldspar, feldspar, and carbonate. Plagioclase phenocrysts range from 2 to 6 mm and compose up to 15 percent of some flows. Most show extensive replace­ment by epidote-clinozoisite, calcite, chlorite, and biotite.

Flow breccia and pillow fragment breccia are apparently a minor component of the flow sequences. Fragments are monolithologic, enclosed in fine-grained mafic ma­trix, and generally 1 O to 30 cm across.

lnterflow tufts and sediments are very fine grained, finely layered (from 1 to 50 mm thick), and in a few places have filled in the underlying flow surface irregu­larities. Channel scours, graded bedding, and small scale slump structures are also preserved.

The mafic volcanics are intruded by synvolcanic dykes and sills of basaltic and microdioritic composition.

b) Hyslin Bay Felsic Complex (H)

The 'Hyslin Bay•3 Felsic Complex is used here to desig­nate the predominantly plagioclase-porphyritic dacitic rocks (Hd) that underlie the area east of Hyslin Bay {Figure 1 ). The dacites typically occur as frost-heaved, blocky to rubbly outcrop ridges caused by a prominent network of fractures. These rocks are pale greenish grey to light grey weathering and darker grey on fresh broken surfaces. They contain up to 20 percent plagio­clase phenocrysts which are 2 to 4 mm in length, en­closed in an aphanitic to very fine-grained matrix con­sisting of quartz. plagioclase, calcite, sericite with minor biotite, chlorite, apatite and opaques. Plagioclase phe· nocrysts are moderately to strongly replaced by calcite and sericite. Pale green biotite flakes. up to 3 mm long and matrix biotite compose up to 15 percent of the rocks. Numerous zircon inclusions produce dark pleo-

(3) Unofficial place names appear in single quotes when first referenced; subsequently the quotes are dropped.

32 Summary of Investigations 1993

chroic haloes in the biotite. A few percent quartz phe­nocrysts, 1 to 2 mm in diameter, occur locally in this unit. The uniform and rather massive character of the rocks suggests an intrusive rather than extrusive origin.

Locally the plagioclase-porphyritic dacite is capped by a pinkish weathering, quartz phyrlc rhyodaclte-rhyolite unit (Hqr) of 50 to 100 m thickness. The quartz 'eyes' are 2 to 4 mm in diameter and compose 5 to 10 per­cent of the rock. This may be the stratigraphic equiva­lent of the Laural Lake Rhyolite Dome Complex.

c} Laural Lake Rhyolite Dome Complex (L)

The Laural Lake Rhyolite Dome Complex is exposed mainly west of Laural Lake and on Gull Island (Figure 1 ). The complex probably underlies most of Laural Lake and is exposed along the east margin of the lake. The presumed lower-most part of the dome complex is a feldspar-porphyritic daclte (Ldp), exposed east of Laural Lake. It is very similar to the Hyslin Bay dacite.

The main part of the complex consists of: a white weath­ering, quartz-feldspar phyric rhyolite (Lwr), a pink weathering, pyritlc, muscovite-bearing quartz-phyric rhyolite (Lpr), and pink weathering, quartz-phyric rhy­olite fragmental (Lrtb). A few small dacitic lenses and mafic dykes are also part of the dome complex.

The white weathering quartz-feldspar phyric rhyolite (Lwr) occupies the southern half of the dome. Quartz phenocrysts compose up to 10 percent, average 1 to 2 mm diameter, but are up to 4 mm diameter near the 'upper' contact. Plagioclase and minor potash feldspar phenocrysts are 2 to 3 mm long and compose up to 1 O percent of the rock. Plagioclase phenocrysts are partly replaced by sericite and calcite, and also show re­placement by potash feldspar. Phenocrysts are en­closed in a very fine-grained matrix of quartz, feldspar, muscovite, and calcite. The subunit is mainly massive, suggesting an intrusive origin; however, near the strati­graphic 'top' thin , faint monolithologic, matrix-supported felsic lapilli layers indicate a pyroclastic origin.

The pink weathering, pyritic, muscovite-bearing quartz-phyric rhyollte (Lpr) occupies the northern part of the dome complex and is the principal host for the Laural Lake gold-silver deposit. The rocks are coarse grained, have a well-developed schistosity, and have 15 to 25 percent, clear to grey quartz phenocrysts, that av­erage less than 2 mm adjacent to the white rhyolile, but are typically 3 to 5 mm in diameter in the 'middle' and 'upper' parts of the dome. Abundant potassium feldspar may reflect early potassic alteration (e.g. adularia­sericite) related to the formation of the epithermal gold­silver mineralization. The matrix consists of fine-grained quartz, potash feldspar, muscovite and calcite, and mi­nor tourmaline. Disseminated pyrite is ubiquitous, aver­aging 1 to 2 percent, and increasing dramatically in the mineralized zones. Fuchsitic-looking, emerald green mi­caceous lozenges from 2 to 1 O mm long and up to 3 mm thick are a common accessory. The homogene­ous character of the rocks suggests that this subunit is an intrusive component of the dome.

Saskatchewan Geological Survey

The pink to buff weathering, quartz-phyrlc rhyolite fragmental subunit (Lrtb) occurs both east and west of the 'massive' pink rhyolite and similar rocks occur on the southern third of Gull Island and the adjacent islets. The predominant quartz-phyric rhyolite fragments are generally strongly flattened, range from a few millime­tres to 50 cm long and up to 40 cm wide, and occur in a fine.grained, quartz·phyric matrix composed of quartz, feldspar, muscovite, and calcite with minor tourmaline and disseminated pyrite. Some fragments may have been pumiceous. Fuchsitic micaceous lozenges (frag­ments ?) up to 10 mm long are also present in these rocks.

Fragmental rocks represent the pyroclastic and/or epi­clastic carapace of the dome complex. The coarseness (up to 40 cm size) and angularity of the lragmental rocks on Gull Island suggests that there is either an ex­tension of the Laural Lake dome or another quartz­phyric rhyolite dome at the south end of Gull Island. Evi­dence for this can be seen on 'Little Gull Island', east of the south end of Gull Island where a small intrusive apo­physis, 2 m long by 40 cm wide, of quartz-feldspar rhy­olite porphyry with chilled margins was emplaced into the rhyolite tuft breccia. The porphyry is composed of up to 40 percent quartz and feldspar phenocrysts 3 to 5 mm in diameter.

d) Mixed Mafic Volcanic- Volcaniclastic Assem-blage (V)

Overlying the Hyslin Bay and Laural Lake complexes is a mixed assemblage of mafic pyroclastics, flows, and fine (greywacke-argillite) to coarse (conglomerate) epi­clastic rocks. Throughout this assemblage veins and stringers of quartz and buff to red brown weathering ankerite are a common feature.

The pyroclastics consist mainly of mafic tuft breccia (Vtb) which grades laterally into ash tuft. These rocks commonly overlie the rhyolite dome complexes and con­tain abundant fragments of the underlying rhyolites. North of the Laural Lake dome, the mafic tuft breccia im­mediately overlying the pink quartz-phyric rhyolite con­tains rhyolite fragments up to 1 m long in a fine-grained mafic (chlorite+biotite) matrix. The size and abundance of fragments decreases to the northeast and southwest indicating that these rocks reflect a proximal eruptive fa­cies. About 500 m southwest, on the shore of Amisk Lake the rock is a fine mafic tuft (Vt) with only a few small rhyolite fragments (<1 cm). The tufts consist of chlorite, biotite, muscovite, plagioclase, quartz, and car­bonate.

An excellent analog to the Laural Lake setting is Mount Tarawera, North Island, New Zealand, where in 1886 several coalescing rhyolite domes were covered by an explosive eruption of basaltic scoria and ash deposits containing blocks and fragments of the rhyolite making up the domes. The basaltic material erupted through an 8 km long fissure which cut across the domes (Houghton, 1982; personal observation).

Mafic tuft breccias with angular quartz-phyric rhyolite and dacite fragments, from 1 to 40 cm long, overlie the

33

Hyslin Bay Felsic Complex. The fragments compose up to 40 percent of the rocks, and are contained in a ma­trix of chlorite and biotite.

Matlc flow rocks (Vf) in this sequence include: amygdaloidal, plagioclase-phyric massive flows (Vfmk,a), amygdaloidal pillowed flows (Vfp), with minor flow breccias and intertlow sediments.

Sedimentary rocks are interlayered with fine tuffaceous rocks and occur as localized lenses, perhaps as cal­dera fill. At the north end of Gull Island pelltlc schist (Vps) consisting of muscovite, biotite, quartz and feld­spar is interlayered with mafic tuff. Buff to brown weath­ering carbonate (calcite and ankerite) occurs in layer parallel seams up to 6 cm thick. The continuity of some ot these layers suggests they may in part represent pri­mary carbonate sediments.

Polymictlc conglomerates (Ve) occur on the headland east of Gull Island, and on the small peninsula on the east side of the entrance to Hyslin Bay (Figure 1). At the first locality, the matrix-supported, flattened clasts consist of mafic and felsic volcanic and sedimentary rocks, and are typically less than 1 O cm long, but locally are up to 40 cm long.

At the Hyslin Bay locality, the matrix-supported conglom­erate overlies a thinly bedded sequence of light to dark grey greywack~arglllite (Vga) which exhibits normal grading and slump structures. The conglomerate has a biotite and chlorite matrix containing flattened clasts, up to 30 cm in length, of mafic and felsic extrusive and in­trusive volcanic as well as sedimentary origins. Several large boulders (30 to 40 cm across) of the thinly bed­ded argillite are also present. Rounded masses of ank­erite are suggestive of a elastic origin; alternatively they could be shaped tectonically, as rotated boudins. The greywacke-argillite and conglomerate beds are intruded by intermediate to felsic breccia dykes, which incorpo­rate numerous wall-rock fragments as well as exotic fragments. These rocks are also intruded by several feldspar-porphyritic rhyodacitic dykes, which appear to be the feeder dykes to a dome exposed on several small islands immediately north of the peninsula.

e) Hannay Bay Felsic Volcanic Complex (HB)

The Hannay Bay felsic volcanic complex underlies the area around Hannay Bay and extends to the southern end of the large peninsula between West Channel and Missi Bay (Figure 1 ). The unit comprises a complex se­quence of: high-level, subvolcanic intrusions of plagio­clase-porphyritic rhyodacites and dacites; quartz±feld­spar phyric rhyolites; dacitic and rhyolitic tuff breccias; minor lapilli to ash tuffs; and minor andesitic and basal­tic rocks. Other minor intrusions include microdiorite and granodiorite.

The plagioclase-porphyritic rhyodacltes (HBrd) are similar to the Hyslin Bay plag!oclase-porphyritic dacites (Hd), but are more siliceous, contain less than 10 per­cent mafic minerals and have a lighter colour index than dacites of the Hyslin Bay and Hannay Bay com­plexes. The rhyodacites are generally buff to light pink-

34

ish grey weathering and light grey to light pinkish grey on fresh surfaces. A few percent quartz phenocrysts of 1 to 2 mm diameter are commonly present. Finely dis­seminated pyrite Is nearly ubiquitous. The Hannay Bay porphyritic dacites and rhyolites of this unit are essen­tially similar to the equivalent rocks of the Hyslin Bay Complex. The intrusive character of the porphyritic rhyo­dacites and dacites is well-demonstrated by cross-cut­ting relationships, but the subtle colour variations and changes in feldspar grain size and content, that are vis­ible in shoreline outcrops, are rarely observable inland.

Extrusive components of the complex are mainly distin­guished by fragmental textures, and include: rhyollte tuft breccla (HBrb), daclte tuft breccla (HBdb), and crystal llthic tufts (HBt), which are best developed on the west shore of Hannay Bay. The most extensively ex­posed deposit is about 500 m long and 200 m thick. Fragments are angular, generally less than 30 cm long, monolithologic and similar to the adjacent felsic rocks. These pyroclastic deposits probably represent the rem­nants of pyroclastic aprons built up around felsic domes.

Andesltlc (HBa) rocks, a minor component of the com­plex, are green grey, plagioclase-phyric, with a very fine­grained groundmass of plagioclase, biotite, and amphi­bole. Plagioclase phenocrysts are 2 to 4 mm long and compose up to 25 percent of the rock. The rocks could be flows or high-level subvolcanic intrusions.

Sheared rocks of the Hannay Bay complex are repre­sented by rusty pyritic, biotite-chlorite schists, which are commonly silicified and cut by carbonate veins. Associ­ated intense fracturing between narrow high-strain zones, which produces tectonic breccias, also results in further chloritization of the rocks. Increasing mafic con­tent accompanying shearing has resulted in some of the rocks being mapped as andesites in the past.

f) Greywacke-Argillite (Sg)

Greywacke-argillite occurs in several areas on 'Hannay Peninsula' (Figure 1 ). These sediments are extremely fine grained to fine grained, finely laminated to thinly bedded, and have distinct to gradational colour variation from black to very light grey. Although tops can be de­termined, the overall stratigraphic facing directions are complicated by complex isoclinal folding. Locally, very siliceous layers possibly represent chert or fine rhyolite ash tuft beds. Quartz-phyric rhyolite flows are interiay­ered with these rocks.

g) Minor Intrusive Rocks

A variety of dykes and sills intrude the volcanic se­quence. The most common types are microdiorite, quartz microdiorite, with minor gabbro, and mafic to fal­sie breccia dykes. Feldspar-porphyritic quartz diorite is the youngest of the minor intrusions.

Mlcrodiorite (mDi) occurs throughout the volcanic se­quence as generally narrow sills, 3 to 10 m thick, and up to 100 m thick and 500 m long. They are grey green to dark green, fine grained (<2 mm grain size), and well foliated. They are composed essentially of plagioclase,

Summary of Investigations 1993

chlorite, biotite±amphibole (blue green), quartz, carbon­ate, epidote, with minor muscovite, opaque grains (mag­netite, pyrite), apatite and zircon. The microdiorite sills are interpreted to be synvolcanic, and interestingly are associated with a number of the gold occurrences.

Quartz Mlcrodiorite (qmOi) sills, up to 10 m thick and 300 m long are a minor component intruding greywacke­argillite and rhyolites on the northern-most headland of Hannay Peninsula. They are dark green to green black, very fine grained and contain about 15 percent millime­tre--size, knobbly weathering quartz crystals, which are typically clear, and locally bluish. In sheared varieties the quartz crystals show evidence of rotation and stretching. In one moderately deformed sill, millimetre­size plagioclase phenocrysts compose about 15 per­cent of the rock. The principal minerals are chlorite, bi­otite, epidote, calcite, and a very fine mosaic of quartz and plagioclase. Similar quartz diorites are found in the Comeback Bay area of Amisk Lake (see Slimmon, this volume) .

Minor gabbro {Gb) occurs as narrow sills up to 6 m thick. One sill on the east shore of Hyslin Bay consists of a lower massive, greenish black, coarsely crystalline (1 to 2 cm) melanogabbro (95 percent hornblende, 5 percent plagioclase), which grades up into a layered mesocratic (up to 50 percent plagioclase) gabbro. The upper contact with overlying mafic flows, features 2 cm long hornblendes that have grown perpendicular to the contact.

An unusual matic intrusion of possible lamprophyric af­finity {GL) occurs on the headland at the south end of Hyslin Bay. It forms an irregular-shaped, xenolithic body that averages 10 m wide, is exposed for 50 m across the headland, and has a number of apophyses extend­ing into the surrounding rocks. The rock is dark grey, generally fine grained, and contains 10 to 15 percent black, lath-like aggregates up to 10 cm long and 5 cm wide. Intersecting laths exhibit star and tomahawk shapes. A single thin section shows the aggregates to be completely recrystallized to a tine aggregate of bi· otite and chlorite with minor to abundant magnetite. Dis­seminated magnetite forms 2 to 3 percent of the ground· mass, which consists mainly of fine plagioclase laths, carbonate, quartz, biotite, and chlorite.

Maflc to felslc breccia dykes do not form mappable units, but are indicated on the accompanying map as a subordinate rock type {e.g. fbrd). Braccia dykes, best ex­posed in shoreline outcrops, are generally less than a metre wide, and are rarely traceable for more than a few metres. They are sharp walled, generally highly dis­cordant, and contain abundant wall-rock xenoliths as well as exotic xenoliths. Intrusion breccias composed of angular wall-rock xenoliths forming a jig-saw mosaic, are developed locally.

Several episodes of breccia dyke emplacement are rec­ognized. They are mostly late features as they cut de­formed rocks, although some dykes are folded about north-south trending axial planes and have a well-devel· oped foliation. Most dykes have no noticeable fabric, but have been offset sinistrally by late north-south trend-

Saskatchewan Geological Survey

ing brittle fractures. Possibly related to this late brittle faulting event are some straight-walled, dark grey, very fine-grained breccia veins, up to 6 cm wide that cut across all fabrics and some of the early folded breccia dykes.

Plagloclase-Porphyrltic Quartz Dlorlte (pQD) intrudes all rocks in the west Missi Island area, occurring as nar· row dykes (<1 m wide) to larger, sill·like bodies (150 m wide and at least 800 m long), of which the largest oc· cur at the north end of Hannay Peninsula. The dykes are most commonly highly discordant to the 'layering' and range from shallow to vertical dipping. These por­phyries are the youngest of the minor intrusions. They were emplaced after an early period of deformation, (01) as indicated by previously fractured, altered, veined, and folded xenoliths, but were emplaced prior to a later event (02) which produced the prominent north-south fabric typical of the region. They may be closely related to the major granodiorite intrusions, be­cause they are found both intruding them and as xeno· liths in them.

The typical plagioclase·porphyritic quartz diorites are grey to dark grey, and contain up to 25 percent, white plagioclase phenocrysts 2 to 12 mm long and up to 5 mm wide, which occur in a fight to dark grey schis­tose matrix of biotite, chlorite, quartz and feldspar, with minor muscovite, epidote and carbonate. The phe­nocrysts have been rotated, pulled apart and stretched, and have developed pressure shadows, which are com· manly filled with carbonate±chlorite. The carbonate is commonly leached on the weathered surfaces which ac­centuates the shadows. The tails and alignment of the pull-apart fractures indicate sinistral style movement. A lighter grey, slighter more felsic variety intrudes the darker variety, and in at least one locality the dark grey variety has a fine-grained, phenocryst-poor marginal phase up to 50 cm wide.

Quartz eyes, 1 to 5 mm in diameter, compose 10 to 15 percent of the rocks. They are less obvious on weathered surfaces because of their grey to deep blue colour; they are best observed on wet, fresh-broken sur­faces.

h) Mlssi Island Granodiorite, Leucograno­diorlte Intrusions (Gd)

The Missi Island granodiorite intrusions include the Missi Island, Hambly Lake, Meaney Lake, and Hannay Peninsula intrusions {Figure 1 ). The Missi Island Intru­sion, the main body underlying the central part of Missi Island has yielded a single zircon Pb-evaporation tech· nique age of 1848 ±11 Ma (Ansdell and Kyser, 1990). This year's mapping coverage of these intrusions is lim­ited, however, they appear very similar in appearance, mineralogy, fabric development, and chemistry, and as such are all considered to be coeval.

The granodiorites are light grey to light pinkish grey weathering, coarse grained, ranging from 2 to 3 mm av­erage in the smaller Hannay Bay intrusions to 4 to 10 mm in the larger intrusions, and are moderately foli­ated. Mafic xenoliths are common near the margins and

35

are typically less than 10 cm in length. The grano­diorites are composed of 20 to 25 percent quartz, which is bluish in places, 40 to 65 percent plagioclase, 10 to 15 percent K-feldspar, and <5 to 25 percent biotite. Pale yellow green colouration of the plagioclase in places suggests saussuritic alteration. Byers and Dahlstrom (1954) did not indicate the presence of K­feldspar and thus called this group of intrusions quartz­eye diorites; however, microcline is ubiquitous as an in­terstitial mineral to the dominant plagioclase.

The Mlssl Island Intrusion appears to be the only body to have a narrow, up to 50 m wide, discontinuous, finer grained diorite margin which is intruded by grano­diorite dykes. The granodiorite becomes less mafic ( <5 percent biotite, i.e. leucogranodiorite), more quartz rich, and coarser grained from the margin inwards. In places two foliations can be distinguished.

The northwestern end of the Hambly Lake Intrusion is more mafic having 20 to 25 percent biotite, and con­tains large greyish to bluish quartz aggregates up to 1 O mm long. At the southern tip of the intrusion radiat­ing dykes intrude the bordering mafic volcanics.

The eastern margin of the Meaney Lake Intrusion is very strongly foliated to mylonitic along its contact with the mafic volcanics. The northward projecting arm of this intrusion (west shore of Missi Bay, Figure 1) is vari­ably tectonized, with numerous discontinuous en eche­lon narrow high-strain zones and breccia zones. Chlo­rite and muscovite are extensively developed in these zones. Pyritic shear zones, of 2 to 3 m width are heav­ily iron stained (gossanous). Xenolith-rich zones are also developed in which mafic to falsie intrusive and ex­trusive and sedimentary rock types are represented. Sheets of granodiorite are intruded into the surrounding volcanics for at least 100 m from the contact.

Two of the Hannay Peninsula Intrusions were exam­ined. The northern-most body was mapped as two sepa­rate bodies by Byers and Dahlstrom (1954), but addi­tional outcrops were found during the present mapping which links the two previously mapped parts. At the northeast end this granodiorlte is intruded by a metre­wide plagioclase-porphyritic quartz diorite dyke. Sheared mafic dykes containing granodiorite xenoliths occur near the west margin at the north end of this body.

At the northern end of the western-most intrusion there is a spectacular xenolith-rich zone and adjacent intru­sion breccia in the surrounding felsic rocks. Angular blocks up to 50 cm long of greywacke-argillite, mafic vol­canics, various felsic porphyries, and plagioclase por­phyritic quartz diorite are enclosed in a porphyritic gra­nodiorite matrix. This zone extends southward for about 100 m with a decrease in the size and abundance of xenoliths and a corresponding increase in granodiorite.

3. Structure

Although primary sedimentary, volcanic, and intrusive structures are commonly well preserved in low-grade metamorphic terranes, synvolcanic deformational tea-

36

tures, such as faults and folds, are difficult to distin­guish from later superimposed regional events. Several candidates for pre-regional deformational events are the mineralized structures (faults) which host the deformed and metamorphosed Laural Lake Au-Ag deposit and other similar epithermal-type mineralizations found in the west Missi Island area.

a) 01 Deformation

The earliest regional deformational event, D1, is respon­sible for the flattening of pyroclastic fragments, clasts in conglomerates, and the development of a generally layer-parallel foliation, S1, typically defined by the growth of chlorite, muscovite, and biotite. The 81 folia­tion is axial planar to F1 regional scale folds that had an east-west trend (see Reilly, this volume; Slimmon, this volume; Thomas, this volume). The mixed rnafic vol­caniclastic and volcanic assemblage extending east of Hyslin Bay occupies the core of an overturned, south­ward dipping, F1 syncline. The dacitic rocks to the north and south appear to represent adjacent anticlinat culmi­nations. Unfortunately the general lack of outcrop and top determinations farther east makes it difficult to deter­mine the extent of these structures.

Numerous east-west trending quartz, calcite­chlorite±quartz. quartz-carbonate and quartz-feldspar­carbonate veins and weakly pyritized (gossanous) nar­row high-strain zones are probably related to waning stages of the 01 event, because they are deformed by subsequent events.

b) 02 Deformation

01 structures are deformed by moderately tight, north­south to northwest-trending 02 folds that have subverti­cal to moderately west-dipping axial planes. Minor F2 folds typically plunge moderately northwest, but reversal to the south and southeast occur in several places. These folds are defined by lithological contacts, S1 and 82 foliations. A penetrative axial planar 82 fracture and crenulation cleavage is characteristic and can com­pletely obliterate S1 in fold hinges. The S2 fabric is rep­resented by chlorite, muscovite and biotite growth.

c) 03 Deformation

03 deformation is probably closely related to D2, and produces north- to northwest-trending, brittle-ductile high-strain zones. These structures are subvertical, range from small to large scale, and are represented by breccia zones, intensified foliation, related shear folding and further development of chlorite. The shear folds commonly indicate sinistral movement. Many of these shear zones are pyritic and with weathering become gossanous. They may also have associated silicification and carbonatization, and have elevated gold contents.

Major shear zones of this age include the West Chan­nel Fault (Byers and Dahlstrom, 1954; Ashton, 1990; 1992; Reilly, 1992) which occurs just west of Missi Is­land. The larger shear zones on west Missi Island may be subsidiary structures to the West Channel Fault.

Summary o f Inves tigations 1993

There is also an apparent westward arching of the shears from south to north along the west side of Missi Island, perhaps caused by a large scale sinistral rota­tional movement of Missi Island between the West Channel Fault and MacDonald Creek Shear Zone. Such movement might explain the large-scale 'rotated porphy­roblast-like' geometry of Missi Island.

d) 04 Deformation

Slimmon (this volume) and Reilly (this volume) describe the effects of 04 deformation in the Amisk Lake region as being related to the east-northeast-trending Embury Lake Antiform (Stauffer and Mukherjee, 1971). Minor structures or associated fabrics are not found; however. reorientation of earlier structures is indicated. Similarly in the west Missi Island area, no minor structures are developed; however, a broad-scale undulation is indi­cated by reversals of 01 and 02 fold plunges and min­eral lineations about generally easterly trending axes. This may reflect the effects of 04 deformation or they may result from superposition of 02 folds on 01 struc­tures.

4. Metamorphism

Mineral assemblages in volcanic and sedimentary rocks indicate that most of the west part of Missi Island is rep­resented by greenschist facies metamorphism, with only localized transition to lower-most amphibolite fa­des. Polymetamorphism is indicated by the preserva­tion of two foliation producing events.

Preliminary petrographic studies indicate that peak metamorphism, M1, accompanying 01 resulted in fab­rics consisting of chlorite, biotite and muscovite, and less commonly actinolite. Peak metamorphism, M2, that accompanied 02 resulted in further growth of chlorite, biotite and muscovite, and locally blue green horn­blende in mafic volcanics. Post peak M2 retrograde metamorphism, that accompanied 03 brittle-ductile shearing, is indicated by extensive chlorite development in faults and breccia zones.

5. Economic Geology

Numerous gold and a few base metal±gold occurrences exist in the west Missi Island area (Figure 1). The early exploration history and property descriptions for this area are contained in reports by Bruce (1918), Mawc:1-sley (1931), Wright (1935), Byers and Dahlstrom (1954), Beck (1959), Coombe (1984), and Pearson (1980, 1983), and therefore will not be discussed here.

The Laural Lake Au-Ag deposit is the most recent dis­covery in the area and was actively explored during the 1980s when a two level exploration decline was exca­vated. The property has been inactive for several years, but probably still warrants further investigation, and like­wise the surrounding area may still hold potential for economic gold mineralization based on an epithermal precious metal model. For example, White Island, off the north coast of the North Island, New Zealand is an active volcanic island with associated active hydrother-

Saskatchewan Geological Survey

mar systems which precipitate copper and gold minerali­zation (Hedenquist et al .. 1993). The island lies above a shallow magma body which is releasing vapour and in­teracting with local ground water and sea water. Metals are precipitated both below the ground surface and from metalliferous fluids discharging along the shore line in the flanking sedimentary environment. Perhaps gold deposits such as the Prince Albert/Monarch, hosted in greywacke-argillite sediments, are remobilized equivalents of deposits originally formed from subma­rine discharging metalliferous fluids.

6. References Ansdell, K.M. and Kyser, T.K. (1990): Age of granitoids from

the western Flin Flan Domain: An application of single zir· con Pb-evaporation technique; in Summary of lnvestiga· tions 1990. Saskatchewan Geological Survey, Sask. En­ergy Mines, Misc. Rep. 90·4, p136·142.

Ansdell, K.M., Stauffer, M.R., Kyser, T.K. , and Edwards, G. (1991 ): Age and source of detrital zircons from the Missi Group: A Proterozoic molasse deposit, Trans-Hudson Oro­gen; Geol. Assoc. Can./Miner. Assoc. Can., Jt. Annu. Meet., May 1991, Toronto, Prag. Abstr., vl6, pA3.

Ashton, K.E. (1990): Geology of the Snake Rapids area, Flin Flan Domain (parts of NTS 63L·9 and -10); in Summary of Investigations 1990, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 90·4, p4-12.

____ (1992): Geology of the Snake Rapids area: Up­date; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines , Misc. Rep. 92-4, p97-l 13.

Ayres, L.D. (1977): A transition from subaqueous to subaerial eruptive environments in the middle Precambrian Amisk Group at Amisk Lake, Saskatchewan; Univ. Man., Cent. Pree. Stud., Annu. Rep., p36·51.

____ (1980): Preliminary stratigraphic investigations of the upper falsie-intermediate component of the Early Pro­terozoic Amisk Group. Amisk Lake, Saskatchewan; Univ. Man., Cent. Pree. Stud., Annu. Rep., p14-46.

____ (1981): A subaqueous to subaerial transition zone in the Early Proterozoic metavolcanic sequence. Amlsk Lake, Saskatchewan; Univ. Man. , Cent. Pree. Stud., Annu. Rep. , p49·61 .

Ayres, L.D. and Findlay, D.F. (1976): Precambrian porphyry copper and molybdenum deposits in Ontario and Saskatch· ewan; in Report of Activities, Part B, Geo!. Surv. Can., Pap. 76-16, p39-41 .

Ayres, L.D .. van Wagoner, N., and van Wagoner, S. (1981): Physical volcanology of the Amisk Lake volcano; in Sum­mary of Investigations 1981, Saskatchewan Geological Sur­vey, Sask. Miner. Resour., Misc. Rep 81-4, p47-5 1.

Bailes, A.H., Syme, E.C .. Gordon, T.M .. and Hunt, P.A. (1 988): U-Pb zircon geochronology of the Richard Lake tonalite, a possible synvolcanic pluton in the Snow Lake area; in Manit. Energy Mines, Report of Field Activities 1988, p63-65.

Beck, L.S. (1959): Mineral occurrences in the Precambrian of northern Saskatchewan (excluding radioactive minerals); Sask. Dep. Miner. Resour., Rep. 36, 134p.

37

Bickford, M.E., Van Schmus, W.R., Macdonald, A., Lewry, J.F., and Pearson, J.G. (1986): U-Pb zircon geochronology project for the Trans-Hudson: Current sampling and recent results; in Summary of Investigations 1986, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 86-4, p101-107.

Bruce, E.L. (1915): Amisk Lake district, northern Saskatche­wan and Manitoba; Geol. Surv. Can., Summ. Rep. 1914, p67-69.

____ (1916): Amisk·Athapapuskow Lake area, northern Saskatchewan and northern Manitoba; Geol. Surv. Can., Summ. Rep. 1915, p126·130.

_ ___ (1918): Amisk·Athapapuskow Lake district; Geol. Surv. Can., Mem. 105, 91p.

Byers, A.A. and Dahlstrom, C.D.A. (1954): Geology and min· eral deposits of the Amisk-Wildnest takes area, Saskatche· wan; Sask. Dep. Miner. Resour .• Rep. 14, 1np.

Chute, M.E. and Ayres, L.D. (1977): Missi Island volcanic cen· tre, Saskatchewan; in Report of Activities, Part B, Geo!. Surv. Can., Pap. n-1B. p29·31.

Coombe, W. (1984): Gold in Saskatchewan; Sask. Energy Mines, Open File Rep. 84-1, 134p.

David, J., Machado, N., Bailes, A.H ., and Syme, E.C. (1993): U·Pb geochronology of the Flin Flon-Snow Lake Belt: New results; New Perspectives on the Flin Flon- Snow Lake­Hanson Lake Belt from the NATMAP Shield Margin Pro· ject, Flin Flon-Creighton CIM Branch, June 1993, abstr., 1p.

Fox, J.S. (1976a): Preliminary bedrock geology of the west Amisk Lake area (63L·7(W) and 63L-16(SW)); Sask. Re­search Council, Geol. Div., Misc. Rep.

____ (1976b): Some comments on the volcanic straligra· phy and economic potential of the west Amisk Lake area, Saskatchewan; Sask. Research Council, Circ. 9, 29p.

Gordon, T .M., Hunt, P.A., Bailes, A.H., and Syme, E.C. (1990): U-Pb zircon ages from the Flin Flon and Kisseynew belts, Manitoba: Chronology of crust formation at an Early Pro­terozoic accretlonary margin; in Lewry, J.F. and Stauffer. M.A. (eds.), The Early Proterozoic Trans-Hudson Orogen of North America, Geo!. Assoc. Can., Spec. Vol. 37, p177· 199.

Heaman, L.M., Kamo, S.L. , Ashton, K.E. , Reilly, B.A., Slim· mon, W.L., and Thomas, D.J. (1992): U-Pb geochronologi­cal investigations in the Trans-Hudson Orogen, Saskatche· wan; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4, p120-123.

Hedenquist, J.W., Simmons, S.F., Giggenbach, W.F., and Eldridge, C.S. (1993): White Island, New Zealand, vok:anic­hydrothermal system represents the geochemical environ· ment of high-sulphidation Cu and Au ore deposition; Geol. , v21 , p731-734.

Houghton, B.F. (1982): Geyserland: A guide to the volcanoes and geothermal areas of Rotorua; Geo!. Soc. N .Z., Guide· book 4, 48p.

38

Lewry, J.F. and Collerson, K.D. (1990): The Trans-Hudson Oro· gen: Extent, subdivision and problems; in Lewry, J .F. and Stauffer, M.A. (eds.), The Early Proterozoic Trans-Hudson Orogen of North America, Geot. Assoc. Can .. Spec. Pap. 37, p1-14.

Mawdsley, J.B. (1931): Mineral possibilities in northern Sas­katchewan with special reference to areas reconnais· sanced in 1931; Sask. Oep. Nat. Resour., unpubl. rep. 26p.

Mcinnes, W. (1910): Lac la Ronge district, Saskatchewan; Gaol. Surv. Can., Summ. Rep. 1909, p151 -157.

_ ___ (1913): The basins of the Nelson and Churchill riv­ers; Geol. Surv. Can., Mem. 30, 146p.

Pearson, J.G. (1980): Flin Flon gold project; in Summary of In· vestigations 1980, Saskatchewan Geological Survey, Sask. Miner. Resour., Misc. Rep. 80-4, p70-80.

____ (1983): Gold metallogenic studies: Flin Flon·Amisk Lake area; in Summary of lnvesUgations 1983, Saskatche· wan Geological Survey, Sask. Energy Mines, Misc. Rep. 83-4, p67-74.

Pearson, J.G. and Galley, A.G. (1985): Gold deposits of the Flin Flon- Amisk Lake area; in Summary of Investigations 1985, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 85·4, p83·89.

Pearson, J.G., MacDougall, F.H., and Galley, A.G . (1986): Ge­ology and evolution of gold occurrences in the Flin Flon­Amlsk lake area, Saskatchewan; in Clark, L.A. (ed.), Gold in the Western Shield, CIM Spec. Vol. 38, p399·411 .

Reilly, B.A. (1990): Bedrock geological mapping, Mystic- West Arm, Schist lakes area (NW part of NTS 63K·12); in Sum· mary of Investigations 1990, Saskatchewan Geological Sur· vey, Sask. Energy Mines, Misc. Rep. 90-4, p25-35.

____ (1991 ): Revisional bedrock mapping: Mystic lake­Kaminis Lake area (parts of NTS 63K-1 2 and 63l·9) in Summary of Investigations 1991 , Saskatchewan Geologi· cal Survey, Sask. Energy Mines, Misc. Rep. 91-4, p9·15.

____ (1992): Revision bedrock geological mapping, Neagle Lake-Errington Lake area (parts of NTS 63L-9 and -16); in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92·4, p16-22.

Stauffer. M.R. and Mukhe~ee, A.C. (1971): Superimposed de­formations in the Missi metasedimentary rocks near Flin Flon, Manitoba; Can. J. Earth Sci., v8, p217-242.

Syme, E.C., Bailes, A.H., Gordon, T.M. , and Hunt, P.A. (1987): U·Pb zircon geochronology in the Flin Flon Belt: Age of Amisk volcanism; Mani!. Energy Mines, Report of Field Ac· tivities 1987, p105·107.

Syme, E.C., Bailes, A.H., and Stem, R.A. (1993): Recognition of distinct lithogeochemical assemblages in the Flin Flon Belt; Part 1: lithology; New Perspectives on the Flln Flon­Snow Lake-Hanson Lake Belt from the NATMAP Shield Margin Project, Flin Aon- Creighton CIM Branch, June 1993, extend. abstr. 2p.

Thomas, O.J., Reilly, B.A., Slimmon, W.L., and Ashton, K.E. (1993): Structural synthesis of the Amisk Lake-Flin Flon area; New Perspectives on the Flin Flon- Snow lake-Han­son Lake Belt from the NATMAP Shield Margin Project, Flin Aon-Creighton CIM Branch, June 1993, unpubl. abstr.

Summary of Investigations 1993

Wright, J.F. (1933): Amisk Lake area, Saskatchewan; Geol. Surv. Can. Summ. Rep. 1932, ptC, p73-110.

Wright, J.F. and Stockwell, C.H. (1934a): Gold occurrences of Flin Flon district, Manitoba and Saskatchewan; Geol. Surv. Can., Summ. Rep. 1933, ptC, p1 ·11.

Saskatchewan Geological Survey

____ (1934b): West half of Amisk Lake area, Saskatche­wan; Geel. Surv. Can., Summ. Rep. 1933, ptC, p12-22.

____ (1935): Amisk Lake, Saskatchewan; Geol. Surv. Can., Map 314A.

39