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Saskatchewan Geological Survey 1 Summary of Investigations 2004, Volume 2

A Field Description of the Myo Rhyolite, Flin Flon and Creighton, Saskatchewan

K.A. Bailey 1 and H.L. Gibson 1

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, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.2, CD-ROM, Paper A-1, 11p.

Abstract The Flin Flon Mining Camp, in the Flin Flon Belt of the Paleoproterozoic Trans-Hudson Orogen, is host to three world-class volcanogenic massive sulphide ore bodies: the Flin Flon, Callinan, and 777 deposits. The deposits are hosted in the felsic rocks of the Millrock Member of the Flin Flon Formation, in the eastern limb of the Beaver Road anticline. The Myo Member, in the west limb of the Beaver Road anticline, is thought to be the time-stratigraphic equivalent of the Millrock Member and, thus, is of particular interest for its volcanogenic massive sulphide potential. The purpose of this project is to construct a stratigraphic and lithogeochemical comparison between the ore-hosting Millrock Member and presently barren Myo Member.

Initial findings for this study, based on the mapping of two 1:2000 scale stratigraphic sections are that the Myo Member comprises four types of rhyolite. The rhyolite types have a variety of facies including massive, flow banded, flow brecciated, and volcaniclastic. Locally, there appears to be a duplication of stratigraphy that suggests fault repetition.

Keywords: Paleoproterozoic, Flin Flon Mining Camp, volcanogenic massive sulfide deposit, Myo Member, rhyolite, rhyolite facies.

1. Introduction The Paleoproterozoic Flin Flon Mining Camp hosts three volcanogenic massive sulfide (VMS) deposits (Flin Flon, Callinan, and 777) that collectively total approximately 85.5 million tonnes of massive sulfide, making it one of the largest VMS camps in the world (Gibson et al., 2003). The VMS deposits are located within the Millrock Member, a north-northwest–trending, east-dipping unit of heterolithologic breccias and rhyolites (Devine et al., 2002) in the east limb of the Beaver Road anticline.

The Myo Member is a 4 km long, up to 300 m thick, northwest-trending felsic volcanic unit in the western limb of the Beaver Road anticline. Thomas (1989) and MacLachlan et al. (2002) recognized that the Myo Member ranges in composition from rhyolite to dacite and interpreted it as a time-stratigraphic equivalent of the Millrock Member. This interpretation is tentative as the Myo Member has not been examined in detail. The objective of this project, which forms the basis for an M.Sc. thesis at Laurentian University, is to determine whether the Myo Member does represent the stratigraphic and paleoenvironmental equivalent of the Millrock Member. The study involves detailed mapping and geochemistry aimed at establishing the litho- and chemo-stratigraphy of this unit as well as its mechanism(s) and environment of formation. It is anticipated this investigation will provide important insights into the exploration potential of the Myo Member.

This paper, which provides a preliminary report on the first year of field work, presents a comprehensive description of the felsic volcanic rocks that comprise the Myo Member and describes their contact relationships with enclosing strata. Hereafter, the felsic volcanic rocks of the Myo Member will be referred to as rhyolite, although they are in part dacitic in composition.

2. Regional Geology The study area is in the Flin Flon Belt, part of the 1.9 to 1.8 Ga Trans-Hudson Orogen (Figure 1). This Paleoproterozoic orogen is interpreted to have formed as a result of collisions between older Archean continental blocks, the Superior and Rae-Hearne cratons, and the intra-oceanic terranes between them (Lewry and Collerson,

1 Mineral Exploration Research Center, Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, ON P3E 2C6.


Figure 1 - Tectonic assemblage map of the Flin Flon Belt (modified from Syme et al., 1996). Location of Figure 2 indicated by the black box. THO, Trans-Hudson Orogen; MORB, mid-ocean ridge basalt.

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1990). Four tectono-stratigraphic assemblages have been recognized within the Flin Flon Belt: juvenile oceanic arc (1.9 to 1.88 Ga), oceanic floor (1.9 Ga), oceanic plateau/ocean island (undated), and evolved arc (1.92 to 1.9 Ga) (Lucas et al., 1996). These assemblages were formerly known collectively as the Amisk Group (1920 to 1870 Ma; Syme et al., 1999).

Volcanic strata of the Amisk Group that host the Flin Flon, Callinan, and 777 VMS deposits are part of the juvenile oceanic arc assemblage and consist of tholeiitic basalts, mafic volcaniclastic rocks, and a volumetrically minor, but economically important, component of rhyolitic volcanic and volcaniclastic rocks (Figure 2). Unconformably overlying the Amisk Group are fluvial and alluvial sandstones and conglomerates of the Missi Group dated at 1832 Ma (Heaman et al., 1992). Late tectonic intrusions include the Boundary and Phantom intrusions dated at

Figure 2 - Simplified geology of the Flin Flon–Creighton area showing location of 1:2000 scale stratigraphic sections of this study. FFLF, Flin Flon Lake Fault; CCF, Creighton Creek Fault; RLF, Ross Lake Fault; CLF, Club Lake Fault; BRA, Beaver Road anticline; HLS, Hidden Lake syncline; C, Callinan; F, Flin Flon; RF, Railway Fault; and BLS, Burley Lake Syncline (after Bailes and Syme, 1989; modified from MacLachlan et al., 2002).

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1842 ±3 Ma (Heaman et al., 1992) and 1820 Ma (MacQuarrie, 1980) respectively. Compositions of the Boundary Intrusions range from pyroxenite to quartz diorite. The Phantom Intrusions include porphyritic granites, granodiorite, and quartz diorite (Thomas, 1989). Rocks in the Flin Flon–Creighton area are affected by multiple deformation events including several phases of faulting and two phases of folding (F1 and F2; Thomas, 1994). Two regional scale F2 folds, the Beaver Road anticline and Hidden Lake syncline, trend northwest and plunge southeast (Figure 2).

The stratigraphy hosting the ore deposits in the Flin Flon Mining Camp is subdivided into three formations (Figure 2). The Flin Flon Formation at the base is overlain by the Hidden Formation. The Creighton Formation is separated from the Flin Flon Formation by a fault and its stratigraphic position is currently uncertain (Devine, 2003). The Flin Flon Formation comprises, from oldest to youngest, the Club, Blue Lagoon, Millrock, and Myo members, with the latter two possibly being correlative. The base of the Hidden Formation is marked by the Bomber Member, which is overlain by rocks of the Newcor Member (Figure 2).

Rhyolite flow and flow breccia, heterolithic breccia, and minor mafic flows make up the lowermost Club Member (Devine et al., 2002). The Blue Lagoon Member in the east limb of the Beaver Road anticline consists of heterolithic mafic breccia with variable amounts and sizes of feldspar crystals and minor feldspar porphyritic and lesser aphyric basalt flows (Devine et al., 2002). The Blue Lagoon Member in the west limb of the Beaver Road anticline consists predominantly of aphyric and feldspar porphyritic pillowed basalt flows with subordinate interflow volcaniclastic rocks (MacLachlan et al., 2002).

The Flin Flon, Callinan, and 777 VMS deposits occur in the Millrock Member, located in the east limb of the Beaver Road anticline. The Millrock Member consists of heterolithic and monolithic breccias, mafic and felsic volcaniclastic rocks, aphyric to quartz- and feldspar-phyric rhyolite flows, domes and cryptodomes, and associated autoclastic volcaniclastic rocks (Devine et al., 2002). Correlation across to the Myo Member in the west limb of the Beaver Road anticline is complicated by faulting that has removed most of the hinge zone of the fold. The Myo Member was described by Thomas (1989) as consisting of quartz ± feldspar porphyritic rhyolite to dacite flows, flow breccias, porphyritic intrusions, and felsic fragmentals.

The Bomber Member of the Hidden Formation immediately overlies the felsic volcanic rocks and is characterized by abundant synvolcanic basalt sills and interflow/sill volcanic tuff. The basaltic tuffs are fine-grained (typically <2 mm), plane-bedded, and locally cross bedded (Tardif, 2003; MacLachlan, pers. comm., 2004). The Bomber Member is overlain by pillowed basalt flows and flow breccias of the Newcor Member (MacLachlan, pers. comm., 2004).

3. Stratigraphic Sections During the 2004 mapping program, two stratigraphic sections through the Myo Member and immediate footwall and hanging wall strata, were mapped at a scale of 1:2000 (Figure 2). One section is northwest of Myo Lake and the other is southeast of West Arm Road. In addition, both the external and internal contacts between different felsic units within the Myo Member were traced between the two sections and correlated with an intervening section mapped by MacLachlan et al. (2002). Fifty-five samples were taken for major, trace, and rare earth element analyses: 18 from the rhyolites, 25 from the immediate mafic footwall and hanging wall strata, as well as 12 samples representative of the different intrusive rocks. In this paper the terms tuff, lapilli tuff, and block are used in a non-genetic sense and in accord with Fisher’s classification scheme for volcaniclastic rocks (Fisher, 1966).

a) The Myo Lake Section The Myo Lake section (Figure 3), along the west side of Myo Lake, is approximately 800 m thick. The footwall succession (Blue Lagoon Member) consists of an aphyric, amygdaloidal pillowed basalt flow and auto-breccia, overlain by a feldspar porphyritic massive to pillowed basalt flow which, in turn, is conformably overlain by mafic lapilli tuff. A series of aphanitic to fine-grained, aphyric to feldspar ± pyroxene-phyric sills intruded the upper part of the feldspar porphyritic flows and tuffs and the basal part of the overlying Myo Member. These intrusions dilated stratigraphy, increasing the thickness of the section by about 150 m. The overlying Myo Member consists of a 300 m thick section of quartz and feldspar porphyritic rhyolite. The Myo Member is overlain by a succession of aphanitic, aphyric, variably amygdaloidal pillowed to massive basalt flows that range from 2.5 to 40 m in thickness. The hanging wall flows occupy the stratigraphic position of the Bomber Member; however, they are not characterized by the abundant sills and interflow tuff which characterize this unit elsewhere.

b) The West Arm Road Section The West Arm Road section (Figure 4), located 2.5 km southeast of the Myo Section, is approximately 1300 m thick. The Myo rhyolite exposed in this section has a more complex stratigraphy. The footwall Blue Lagoon


Member consists of about 200 m of mafic intrusions with thin screens of aphanitic, sparsely feldspar porphyritic, amygdaloidal, massive basalt flows. The overlying Myo Member consists of 450 m of quartz-feldspar porphyritic rhyolite, which is overlain by aphanitic, aphyric, amygdaloidal massive to pillowed basaltic flows of unknown stratigraphic position. These mafic rocks are overlain by a 200 m thick unit of finely bedded to laminated mafic tuff intercalated with basalt flows, mafic sills, and aphyric to feldspar-phyric felsic volcanic rock (Myo Member). This succession is overlain by a second quartz-feldspar porphyritic rhyolite unit, which in turn is overlain by a second aphyric to feldspar-phyric rhyolite, with minor basalt flows. The upper aphyric rhyolite defines the top of the Myo Member and only differs from the lower aphyric felsic unit by the absence of mafic tuffs. The upper contacts of the upper quartz and feldspar porphyritic and aphyric rhyolite are obscured by gabbro intrusions. A thin, discontinuous bedded tuff at the upper contact of the Myo Member is overlain by aphanitic, aphyric, amygdaloidal mafic pillowed flows (Bomber Member). Numerous aphyric to feldspar ± pyroxene porphyritic mafic sills and dikes intrude all of the above rock types. The youngest intrusions in the section are the Boundary and Phantom intrusions.

The two units of quartz-feldspar porphyritic and aphyric to feldspar porphyritic rhyolites of the Myo Member, and the abundance of mafic flows and

tuff may reflect near contemporaneous or alternating mafic and felsic volcanism. However, the similarity in the stratigraphic successions suggest that the upper rhyolite may be a fault repetition of the lower rhyolite. The location of the fault, however, is unknown and it is unclear if the mafic flows between the upper and lower rhyolites are Bomber Member or Blue Lagoon Member rocks.

4. Myo Member Rhyolites Detailed mapping has facilitated subdivision of the felsic volcanic rocks of the Myo Member into four types of rhyolites based on phenocryst type and abundance; within these four types of rhyolite, four facies have been distinguished based on textures and structures. The four facies are massive, flow banded, flow breccia, and volcaniclastic.

Figure 3 - Simplified stratigraphic section of the Myo Member and flanking units, West Myo Lake area. Units without patterns or symbols are massive.

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Figure 4 - Simplified stratigraphic section of the Myo Member and flanking units, West Arm Road area.

a) Type 1 Rhyolite Type 1 rhyolites are aphyric to feldspar phyric, with 2 to 7%, less than 1 mm feldspar phenocrysts, and are locally amygdaloidal, with amygdules generally less than 1 mm but reaching 1 cm in size (Figure 5A). Type 1 occurs solely as massive facies, which consists of massive coherent rhyolite that commonly has a recognizable spherulitic texture indicating that it was originally glass rich. In general, the massive facies of rhyolite is volumetrically the most significant throughout both sections. Type 1 rhyolite occurs in two stratigraphic positions in the West Arm Road section with upper and lower contacts that are generally conformable. Locally, the upper contact truncates bedding on a centimetre-scale, which combined with the entirely massive nature and lack of flow features, suggests that Type 1 is intrusive.

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Figure 5 - Photographs of rhyolites: A) aphyric to feldspar-phyric amygdaloidal, massive Type 1 rhyolite; B) flow banded facies of Type 2 rhyolite with flow banding parallel to strike; C) flow banded facies of Type 2 rhyolite with contorted flow banding; D) volcaniclastic facies of Type2 rhyolite showing crude layering of light coloured felsic clasts (arrows); E) volcaniclastic facies of Type 2 rhyolite near upper contact showing massive facies clasts in a fine-grained, hyaloclastite matrix; and F) massive, coherent facies of Type 3.

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b) Type 2 Rhyolite The remaining three rhyolite types are quartz and feldspar phyric. Type 2 is sparsely quartz phyric (1 to 2%) with quartz eyes generally less than 1 mm in diameter. Feldspar phenocrysts compose 5 to 7% of this unit and most are typically less than 1 mm in diameter. Type 2 occurs predominantly in the Myo Lake section and to a lesser extent in the West Arm Road section (Figures 3 and 4). In the Myo Lake section, it forms the lowermost felsic volcanic unit of three that comprise the Myo Member. It exhibits all four facies, but the massive facies predominates and occurs as small (<2 m thick), massive lobes separated by weak to strongly flow banded and flow brecciated facies.

The flow banded facies weathers greenish grey to greenish brown and consists of distinct lenticular bands that range from centimetres to millimetres thick and are differentiated by subtle variations in colour and/or spherulite development. Flow banding ranges from faint to pronounced and is typically oriented parallel to strike (Figure 5B); however, areas of contorted flow banding (Figures 5C) are common at or near the contacts with flow breccia or massive lobes. Flow banding is a record of the movement of the lava mass (McPhie et al., 1993).

The flow breccia facies weathers greenish grey to greenish brown and contains clasts of both flow banded and massive rhyolite that range from 1 to 30 cm long in an aphyric to porphyritic spherulitic groundmass. The close spatial association of the flow breccia and flow banded facies and their gradational contacts where contorted flow bands break-up to form flow breccia, indicate they are related to each other. Bonnichsen and Kauffman (1987) and McPhie et al. (1993) interpret this relationship to be a product of viscous laminar flow.

The volcaniclastic facies was only observed in the Type 2 rhyolite at one locality in the West Myo Lake section. The volcaniclastic facies ranges from matrix to clast supported and is composed of angular to subangular, lenticular massive clasts from 2 to 40 cm long that decrease in size up section. Local variations in the abundance of lapilli- to block-sized clasts (5 to 75%) define a crude layering (Figure 5D). The matrix is a fine-grained, granular hyaloclastite that has a brown-weathered, spherulitic texture. The rock becomes chlorite altered near the upper contact of the facies (Figure 5E). Closer to the upper contact, the felsic clasts also show colour changes that indicate chlorite alteration. Both the clasts and the matrix have the same phenocryst abundance and size.

Although the contacts of the Type 2 rhyolite are obscured by overburden in the Myo Lake section, the occurrence of the volcaniclastic facies suggests that it is a flow or dome. Crude layering and clast-supported beds of massive fragments in a fine hyaloclastite matrix are interpreted by Gibson et al. (1999) as characteristics of a flank breccia formed by the slumping and redeposition of autobreccia and hyaloclastite from subaqueous mass flows or domes. In the West Arm Road section, Type 2 has an upper contact marked by a brown-weathered, chlorite-altered zone approximately 6 cm wide along the contact with Type 3 rhyolite.

c) Type 3 Rhyolite Type 3 rhyolite contains quartz phenocrysts that are less than 1.5 mm in diameter and comprise 5 to 8% of the rock. Feldspar phenocrysts constitute 5 to 10% of this type and do not exceed 1.5 mm in diameter. Type 3 occurs in both the Myo Lake and West Arm Road sections (Figures 3 and 4) as massive (Figure 5F), flow banded (Figures 6A and 6B), and flow brecciated facies and stratigraphically overlies Type 2. The flow banded facies is volumetrically dominant (~65 to 70%). The massive facies constitutes 20 to 25% and the remaining 5 to 10% is flow breccia. This rhyolite type is characterized by large massive lobes (up to 6 m thick) that are traceable along strike for at least 175 m, surrounded by flow banded and flow breccia facies.

Type 3 is traceable along strike over 1100 m from the Myo Lake section, to the lowermost quartz-feldspar phyric felsic volcanic unit of the West Arm Road section and beyond that section for 250 m to the southeast. The lower contact with the underlying Type 2 rhyolite is visible in the West Arm Road section and is sharp and conformable. The upper contact of Type 3 is exposed in the Myo Lake section where it is in conformable but sharp contact with Type 4 rhyolite (described below). The nature of the exposed contacts of Type 3 are such that the emplacement mechanism is uncertain.

d) Type 4 Rhyolite Type 4 rhyolite contains 8 to 10% light blue quartz eyes that range from 1 to 3 mm in diameter, and up to 10% white-weathering feldspar phenocrysts less than 1.5 mm in length (Figure 6C). Type 4 occurs as the uppermost quartz-feldspar felsic unit in both the Myo Lake and West Arm Road sections where it exhibits massive facies at Myo Lake and flow banded to flow breccia facies in the West Arm Road section (Figure 6D).

The lower contact of Type 4 with Type 3 in the Myo Lake section is sharp and marked by a narrow, 5 cm wide zone containing small, 1 to 4 cm aphanitic, aphyric mafic clasts. The clasts are more concentrated along the contact but can also be found internal to the unit and in one locality, at the upper contact with the overlying mafic volcanic flows. The upper contact of this unit in the Myo Lake section cross cuts overlying mafic volcanic flows and is


Figure 6 - Photographs of rhyolites: A) flow banded facies of Type 3 rhyolite with flow banding parallel to strike; B) flow banded facies of Type 3 rhyolite with contorted flow banding; C) massive facies of Type 4 rhyolite showing the large, abundant quartz phenocrysts; and D) flow banded and flow breccia facies of Type 4 rhyolite.

clearly intrusive at that locality. East of this section and on the east side of Myo Lake where MacLachlan et al. (2002) mapped, the massive facies of Type 4 intrudes massive to flow banded facies of Type 3.

The flow banded and flow brecciated facies of Type 4 constitute the uppermost quartz-feldspar phyric rhyolite in the West Arm Road section. The upper contact is not exposed in this section and the lower contact is intruded by mafic sills and sheared.

On the basis of the intrusive nature of Type 4 contacts with hanging wall basalt flows and locally with underlying Type 3 rhyolite, this unit is interpreted to be a high-level intrusion or cryptodome. The flow banded and brecciated facies of Type 4 in the West Arm Road section probably represent the extrusive equivalent of the massive facies exposed at Myo Lake.

5. Conclusions and Future Work Field mapping has shown that the rhyolite of the Myo Member can be subdivided into four types distinguished on phenocryst type and abundance. At least two of the rhyolites, Types 1 and 4, are intrusive, and only Type 2 is unequivocally extrusive. The stratigraphy of the Myo Member in the West Arm Road section may be complicated by thrust fault repetitions.

Future work will include detailed mapping of at least two more sections: one to the northwest near Hilary Lake and one to the southeast of the West Arm Road section, near Phantom Lake. In addition to these sections, the upper and lower contacts of the Myo Member will be walked out to examine all contact relationships with footwall and hanging wall strata. This will be done in conjunction with petrographic analysis of textures and structures and a

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comprehensive evaluation of all geochemical data, in order to define a chemostratigraphy. This work will aid in correlation of the various units that comprise the Myo Member and also help resolve potential fault repetitions. Furthermore, this approach will form the basis for determining the stratigraphic position of the Myo Member relative to the Millrock Member and aid in comparing and contrasting the mechanisms and environments of emplacement of these two units.

6. Acknowledgments Funding for this project is provided by an NSERC–Laurentian University–Hudson Bay Exploration and Development Co. Ltd. (HBED)–MIRARCO Collaborative Research and Development Grant with additional financial and logistic support provided by Saskatchewan Industry and Resources (SIR) and in part by a Laurentian University Summer Fellowship. Manitoba Geological Survey supplied housing for the summer. Thanks to Kate MacLachlan (SIR) for contributing invaluable time and knowledge all through the project. Also, Tom Lewis, Kelly Gilmore, Jim Pickell, Darren Simms and Christine Devine of HBED are thanked for their support both in the office and in the field. Data collection and sampling could not have been accomplished without the enthusiastic help of Natalie MacLean.

7. References Bailes, A.H. and Syme, E.C. (1989): Geology of the Flin Flon–White Lake Area; Manitoba Energy Mines, Geol.

Serv., Geol. Rep. 87-1, 313p.

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Devine, C.A. (2003): Origin and emplacement of volcanogenic massive sulphide-hosting, Paleoproterozoic volcaniclastic and effusive rocks within the Flin Flon subsidence structure, Manitoba and Saskatchewan, Canada; unpubl. M.Sc. thesis, Laurentian Univ., 93p.

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