Saskatchewan Geological Society Special Publication Number 14: MINEXPO'96 SYMPOSIUM - Advances in Saskatchewan Geology and Mineral Exploration, Proceedings of a Symposium, Saskatoon, Saskatchewan 21 - 22 November, 1996. Editors: K.E. Ashton and C.T. Harper, 1999

New Ideas on the Classification, Age, Interpretation, and Tectonic History of the Precambrian Shield in Saskatchewan, Pages 27-43.

Recognition of Felsic Volcanic Rocks and Hydrothermal Alteration in Moderately to Highly Metamorphosed Parts of the Flin Flon Volcanic Belt

K.E. Ashton
Saskatchewan Geological Survey, Saskatchewan Energy and Mines, 2101 Scarth Street, Regina, SK S4P 3V7

E. Froese
Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8

A. Legault
Department of Geology, University of Regina, Regina, SK S4S 0A2

In the amphibolite fades terrains north of Flin Flon, the Flin Flon volcanic belt is represented both as continuous extensions (e.g. the Wildnest-Mirond and Kisseynew lakes areas) and more isolated occurrences (e.g. Sherridon and Scimitar Lake areas). In the absence of primary features, it has generally been assumed that the felsic gneisses in these terrains were derived from plutonic or sedimentary precursors. However, the composition of many of these felsic gneisses could also indicate volcanic protoliths, and the spatial association of intermediate to mafic gneisses, amphibolite, VMS-type sulphide mineralization (e.g. Sherridon, FON, Teejay, andDRS deposits), and metamorphosed hydrothermal alteration supports a volcanic origin.

Four alteration types are recognized: calcic (clinopyroxene, epidote, calcite), ferromagnesian (cordierite, garnet, orthoamphibole, cummingtonite), potassic (muscovite, biotite), and aluminous (aluminum silicates). Several of these commonly occur together (e.g. at Sherridon and Wildnest Lake), which allows easy recognition of potentially mineralized areas. The association of altered rocks with felsic gneisses and amphibolites is particularly interesting and should act as an incentive for more detailed exploration of these high-grade terrains.

The Flin Flon Domain has long been recognized as one of the major volcanogenic massive sulphide (VMS) camps in Canada (Figure 1). Flin Flon and Snow Lake have historically been the main mining centres, but the Hanson Lake area has also seen significant activity in the past and continues to be a focus for exploration. All three of these areas escaped the high grades of metamorphism associated with the Trans-Hudson Orogen by remaining at relatively shallow depths of the crust. Metamorphic grades in these southern locales are generally greenschist to lower amphibolite, which generally facilitates the determination of protoliths and stratigraphic relationships, while allowing relatively easy recognition of VMS mineralization and its associated host rocks and alteration. Since VMS deposits form during or shortly after volcanism, however, there is no reason that this type of mineralization should not be found in similar volcanic environments that have been metamorphosed to higher grades. The difficulty is in identifying these moderately to highly metamorphosed volcanic environments and then "seeing through" the metamorphic overprint to recognize favourable VMS settings.

Figure 1 - Lithotectonic domainal classification in Saskatchewan as of about 1990 showing the extent of the Flin Flon Domain. The previously distinct Attitti Block had been correlated, but relationships with (and between) the former Scimitar Complex and Glennie Domain were largely unknown. Black zones represent >2450 Ma rocks exposed in tectonic windows.

Extent of the Flin Flon Volcanic Belt

Relationships between the weakly metamorphosed rocks hosting the VMS deposits in the Snow Lake-Flin Flon-Hanson Lake corridor and the adjacent moderately to highly metamorphosed gneisses to the north have been hotly debated virtually since they were first mapped by Bruce (1918). However, a wealth of new knowledge gained by detailed mapping and follow-up studies of both weakly and highly metamorphosed areas have gone a long way towards clarifying these issues. It is known that the rocks comprising the low-grade areas do not stop at the historic domain boundaries (Figure 1). At the western end of the traditional Flin Flon Domain, they have been traced continuously into a gneissic terrain exhibiting the same rock types and in similar proportions, constituting what had been formerly called the Attitti Block (Ashton et al., 1987; Ashton and Leclair, 1991). A better understanding of the tectonic history of the Trans-Hudson Orogen (Lucas et al., 1994; Lewry et al., 1994), and of the relationship between the ≥2450 Ma rocks of the Pelican Window and the adjacent Paleoproterozoic hornblendic gneisses in particular (Ashton et al., 1999), has led to reinterpretation of the Sturgeon-weir Shear Zone (Ashton et al., 1987) as a splay off the Pelican Decollement Zone (formerly Pelican Slide), rather than a major, continuous, domain-bounding fault (Lewry, 1994). Therefore, the major regional structural discontinuity is between the ≥2450 Ma rocks of the Pelican Window and the adjacent Paleoproterozoic volcano-plutonic rocks, not between similar volcano- plutonic packages on either side of the Sturgeon-weir Shear Zone (Figure 2). This has led to reinterpretation of the historic Hanson Lake Block as comprising a core of ≥2450 Ma rocks in the Pelican Window surrounded by Paleoproterozoic volcano-plutonic rocks representing a western extension of the Flin Flon Domain.

Figure 2 - Lithotectonic domainal classification in Saskatchewan today. Black zones represent >2450 Ma rocks exposed in tectonic windows; PDZ (and cross-hatched areas)=Pelican Decollement Zone, and SWSZ=Sturgeon-weir Shear Zone.

Evidence from farther north and west suggests that the amalgamated package of dominantly volcanic and plutonic rocks making up the Flin Flon Domain does not stop at the western boundary of the former Hanson Lake Block either (Figures 1 and 2). Detailed mapping has shown that the former Scimitar Complex, a large area of hornblende and biotite gneisses centred 70 km north of Pelican Narrows between the Glennie and Kisseynew domains (Pearson, 1973; Kirkland, 1976; Fuh, 1979), consists of the same rock types, and in roughly the same proportions, as the northern Glennie Domain (Ashton et al., 1996a, 1996b, 1997). Conventional U-Pb age dating of a felsic volcanic rock in the southern Scimitar Lake area yielded a crystallization age of 1873 +8/-5 Ma (Hartlaub et al., 1997a), well within the age range for volcanism in both the Glennie Domain (Heaman et al., 1992) and Hanson Lake area (Heaman et al., 1993). Although most of the volcanism in the Amisk Collage of the low-grade Flin Flon Domain occurred in the 1.91 to 1.88 Ga range (Gordon et al., 1990; Heaman et al., 1992; David et al., 1993; Stern et al., 1993; Stern and Lucas, 1994), an 1864 ±3 Ma shoshonitic boulder from the Flin Flon area (David et al., 1993) suggests that at least minor volcanism lasted until then. Given the absence of any obvious structural discontinuities in the Reindeer River area, rocks of the Scimitar Lake area have been re­interpreted as an eastern extension of the Glennie Domain volcano-plutonic complex (Ashton et al., 1997).

The rock types and proportions constituting the northern Glennie Domain (including those of the Scimitar Lake area) are also very similar to those of the northern Flin Flon Domain. Since mapping along the Kisseynew-Flin Flon (Hartlaub et al., 1996) and Kisseynew-Glennie (Ashton et al., 1996a; Hartlaub et al., 1997b) domain boundaries showed that both exhibited similar relationships between the volcano- sedimentary-plutonic package and the Burntwood Group, there is no obvious reason to suspect that the Glennie and Flin Flon domain packages are different. In fact, the simplest interpretation is that the volcano- sedimentary-plutonic package is continuous beneath a regional synform cored by rocks of the Kisseynew Domain (Figure 2). A wide, layer-parallel, mylonite zone spanning the southern boundary between the Scimitar Lake area of the Glennie Domain and the Kisseynew Domain has been interpreted as a northern extension of the Pelican Decollement Zone, which has also re-emerged on the northern limb of this synform (Ashton et al., 1996a; Hartlaub et al., 1997b). Tran et al. (1996) have similarly suggested that variably mylonitized and migmatitic orthogneisses of the northwestern Flin Flon Domain extend continuously underneath rocks of the Kisseynew Domain into the Glennie Domain in the Wood-Pelican lakes area, and Maxeiner et al. (1999) have traced the Northern Lights volcanic package across the Tabbernor Fault from west of Hanson Lake into the Glennie Domain.

North of Flin Flon, on the southern flank of the Kisseynew Domain, volcano-plutonic rocks of the Flin Flon Domain are structurally intercalated with Burntwood and Missi rocks and extend for at least tens of kilometres north of the domain boundary (e.g. Zwanzig, 1990; Lucas et al., 1994). Numerous VMS deposits are known in these rocks, including the past- producing Sherridon ore body (Froese and Goetz, 1981; Ashton and Froese, 1988).

The term Flin Flon-Glennie Complex has been assigned to this volcano-sedimentary-plutonic package extending continuously from Flin Flon to the Lac La Ronge-Reindeer Lake area in the west and occurring discontinuously to the north due to structural imbrication and fold interference. Having made this correlation, it is also important to point out that the potential for VMS deposits is not restricted to rocks of the Flin Flon-Glennie Complex. The ability to locate favourable environments is equally applicable to other terrains dominated by volcanic arc rocks (e.g. La Ronge-Lynn Lake belt).

Clues from the Low-grade Flin Flon Volcanic Belt

Much of the work in the weakly metamorphosed areas has dealt with the environment of VMS mineralization and the nature of its attendant alteration.

1. It has been suggested that the Flin Flon Domain comprises a collage of volcanic rocks which were deposited in different tectonic settings and then tectonically amalgamated prior to widespread intrusion by dominantly granodioritic to tonalitic plutons (Lucas et al., 1996). Virtually all of the economic VMS deposits are hosted by tholeiitic, basalt-dominated sequences which are thought to have been emplaced in one or more island arc environments.
2. Although some of the more Cu-rich deposits are hosted solely by mafic volcanic rocks (e.g. Coronation, Birch, Flexar, Konuto Lake), the really large Cu-Zn deposits (e.g. Flin Flon, Trout Lake, Callinan, Western Nuclear) contain significant rhyolitic horizons (Syme and Bailes, 1993; Reilly, 1995), and the Zn±Pb-Cu deposits (e.g. Western Nuclear, McIlvenna Bay, Chisel, Lost, Ghost) occur in dominantly calc-alkaline felsic volcanic sequences. Thus, there is a strong tendency for mineralization in all three mining centres to occur either within calc-alkaline felsic volcanic or pyroclastic rocks sitting atop thick mafic volcanic piles, or at the contact between these mafic and felsic end members.
3. There are several types of locally extensive, hydrothermal alteration associated with these mineralized settings (i.e. ferromagnesian, aluminous, potassic, silicic, and calcic) which can be useful in identifying areas of high VMS potential (Thomas, 1993; Syme and Bailes, 1993; Bailes and Galley, 1996). Therefore, explorationists searching for VMS deposits in moderately to highly metamorphosed terrains can focus their attention on calc-alkaline felsic volcanic rocks and on rocks displaying evidence of intense hydrothermal alteration.

Recognition of Moderately to Highly Metamorphosed Felsic Volcanic Rocks

Since the effects of metamorphism and deformation obliterate almost all primary features, the term "felsic volcanic rocks" in these terrains must be expanded to include pyroclastic rocks, fine-grained syn-volcanic intrusions, and rare, recycled, syn-volcanic felsic sediments, as well as flows. While less precise, this broader definition is still very useful in delineating felsic volcanic centres. Besides, many of the VMS deposits in felsic rocks occur in volcaniclastics rather than flows (e.g. Callinan; Thomas, 1994), and the heat provided by syn-volcanic intrusions is considered by many to be critical in forming VMS deposits (Lydon, 1988). Silicified (and/or feldspathized) mafic volcanic rocks may be mistaken for felsic volcanic rocks in the field, although this does not necessarily lessen their VMS potential since the silicification process itself is a form of intense hydrothermal alteration.

In spite of this expanded view, geological maps of moderately to highly metamorphosed areas released prior to about 1985 are virtually devoid of felsic volcanic units. There are three main reasons for this.

1. During the last generation of mapping, workers tended to use descriptive, mineral-based names (e.g. garnet-biotite gneiss). Protolith determination was considered largely guess work and was relegated to a line or two of text in largely descriptive reports.
2. Quartzofeldspathic metasedimentary rocks of the Missi Group were well known in the low-grade terrains, so there was a natural tendency to interpret rocks of broadly similar composition in the high-grade terrains as correlative paragneisses.
3. In spite of their importance in the VMS camps, it seems that workers weren't asking themselves what a felsic volcanic rock would look like after being metamorphosed to upper amphibolite facies conditions.

One of the most obvious changes to take place in felsic volcanic rocks during medium- to high-grade metamorphism is their coarsening by recrystallization to a medium-grained, granoblastic texture. Their composition is equally problematic because it generally produces quartzofeldspathic biotite, hornblende and/or garnet gneisses, similar to psammitic sedimentary protoliths. In order to aid in the distinction between Missi-type psammitic metasedimentary rocks and felsic metavolcanic rocks, a set of criteria has been established (Table 1).

Table 1 - Criteria for the distinction of metamorphosed psammitic sedimentary rocks and felsic volcanic rocks in the Flin Flon-Glennie Complex (including rocks in the South Flank of the Kisseynew Domain).

As pointed out earlier, one of the most important steps in protolith recognition in moderately to highly metamorphosed terrains is to determine the make-up of the same belt at lower metamorphic grades, where primary features allow a greater degree of certainty. By knowing the options (i.e. which rock types might be confused), a set of criteria can be established to use in the higher-grade rocks. For high-grade gneisses derived from the Flin Flon Domain, there are only two rock types known to have similar compositions: the granodiorite-tonalite plutonic suite and the conglomerate-arkose sequence of the Missi Group (and Ourom Group of the Glennie Domain). In general, the plutons can be distinguished on the basis of their coarser grain size, K-feldspar-bearing character (felsic volcanic rocks are generally sodic rather than potassic), large size, and elliptical to tabular shapes. In addition, partial melting of granodiorite produces distinctive hornblendic K-feldspar-rich veins and leucosomal layers in gneissic to migmatitic rocks by the reaction:

biotite+plagioclase+quartz = K-feldspar-rich melt+hornblende±sphene±magnetite,

in which the presence of sphene and magnetite is dependent on the relative Ti and Fe concentrations in the reactant biotite (Winkler, 1974, p298).

It is much more difficult to distinguish moderately to highly metamorphosed felsic volcanic rocks from psammitic gneisses of the Missi and Ourom groups, which were deposited in extensive alluvial fans and shallow marine settings (Stauffer, 1990). With deformation, these deposits are attenuated and tend to form long, continuous units. By contrast, felsic volcanic rocks have a much more patchy distribution. Rhyolitic compositions are highly viscous and therefore tend not to travel far. Dacitic endmembers are less viscous but are also spatially restricted. Both can be quite voluminous at felsic volcanic centres but completely absent from the volcanic record a few kilometres away.

The Missi and Ourom groups were unconformably deposited on an older volcano-plutonic basement which included the felsic volcanic rocks. The presence of basal polymictic conglomerates indicate that they also unconformably overlie Burntwood Group pelites in the vicinity of the Flin Flon-Kisseynew domainal boundary (Ashton and Wheatley, 1986; Ashton, 1989), but the presence of transitional compositions at the same stratigraphic position farther north suggests that the contact there represents a facies change (Ashton, 1992). Volcanic rocks, including some of felsic composition, are known in the Missi and Ourom groups, but are volumetrically minor. The older Amisk-type felsic volcanic suite is an integral part of the volcanic succession and stratigraphically underlies virtually all of the sedimentary rocks.

Only the coarsest of primary features are preserved in the moderately to highly metamorphosed terrains. In the Missi and Ourom groups, this includes things like cobbles and pebbles which are both competent and compositionally distinct (usually granitoid) from the matrix, and structures such as crossbedding which are defined by mineral concentrations. Even fewer primary features are preserved in the felsic volcanic suite. Fragments are commonly similar in composition to the matrix of volcaniclastic rocks and tend to become indistinguishable upon recrystallization. Phenocrysts lose their euhedral shape and generally become recrystallized to a finer-grained aggregate, although relicts are known, particularly from competent syn- volcanic intrusions.

Oxidation state has long been used to distinguish magnetiferous arkoses of the Missi Group from graphitic pelites of the Burntwood Group. The near- ubiquitous presence of magnetite, together with the absence of graphite, in rocks of the Missi Group is consistent with deposition in a highly oxidized alluvial/fluvial setting. It is unclear how much of this magnetite is actually derived from the older rocks and survives the erosional process since much of the iron is tied up in hematite in weakly metamorphosed Missi and Ourom rocks. The hematite is only converted to magnetite upon metamorphism under amphibolite facies conditions. The deep-water equivalent of the Missi Group is represented by the upper Burntwood pelites (Machado and Zwanzig, 1995), whose graphitic nature indicates a more reducing environment. Graphite is also common in broadly syn-volcanic, first- cycle pelites delineated in the weakly metamorphosed Flin Flon Domain as the Welsh Lake Assemblage (Reilly, 1993). The presence of pillows in most volcanic assemblages further points to a marine setting, so it is not surprising that many felsic volcaniclastic rocks contain graphite. However, magnetite, which has been attributed to pre-metamorphic alteration by oxidizing fluids, is also known from both volcanic and syn-volcanic pelitic rocks northwest of Flin Flon. A better criterion to distinguish felsic volcanic rocks is the near-ubiquitous presence of trace amounts of sulphides. This usually takes the form of pyrite in weakly metamorphosed rocks, and pyrrhotite at higher metamorphic grades. It is possible to find minor amounts of sulphide in arkosic rocks, but this seems extremely rare in the Missi and Ourom groups.

Alteration

Altered rocks of the Missi Group are rare and generally restricted to fault zones which provide conduits for late fluids. Felsic volcanism, however, is commonly accompanied by hydrothermal fluids, which can result in both intense and widespread alteration. Five major types of alteration occur in volcanic settings: ferromagnesian, aluminous, silicic, calcic, and potassic (Table 2). Since potassic alteration only locally accompanies VMS deposits, it will not be dealt with in this paper. The other four types are widespread but were not generally recognized prior to the 1970s, and even now are commonly misinterpreted.

Table 2 - Diagnostic minerals of various alteration types at low and medium/high grades of metamorphism.

Ferromagnesian and aluminous alteration commonly occur together and result from a combined addition of Mg and/or Fe, together with alkali depletion, which causes a relative increase in the concentration of aluminum. In weakly metamorphosed rocks, ferromagnesian alteration commonly takes the form of chloritization, but at conditions roughly corresponding to the transition from greenschist to amphibolite facies, chlorite breaks down to almandine garnet and/or cordierite, plus an orthoamphibole of the anthophyllite- gedrite series. Such rocks are generally distinctive; the garnet and cordierite are commonly in much greater concentration and grain size in these altered rocks than in other rocks in which they might occur in the same area (i.e. Mg-Al-rich pelites). This is because the altered volcanic rock has a composition that more closely approximates the compositions of these ferrromagnesian minerals. These compositional constraints may stabilize a mineral not observed in unaltered rocks. Such is the case in the northern Flin Flon volcanic belt of Saskatchewan where the Mg- enriched character of the altered volcanic rocks stabilizes cordierite, which is not present in pelitic or other rocks of that area due to the relatively high metamorphic pressures. The orthoamphiboles are also easily recognized by their unusually coarse size, bladed nature, and by their distinctive brown colour, which contrasts markedly with the more common green-black hornblende.

The depletion of alkalis and calcium facilitates development of garnet-anthophyllite±cordierite rocks by destabilizing minerals such as hornblende. It also results in a relative increase in the concentration of aluminum, which in extreme cases, results in the growth of alumino-silicate minerals. In weakly metamorphosed rocks, and particularly in felsic volcanic rocks, sericite and/or andalusite may develop. At higher grades, the same compositions will produce kyanite or sillimanite. As with ferromagnesian alteration, the compositional constraints of rocks affected by intense aluminous alteration may produce either very abundant, and/or very coarse grains, which persist beyond the normal stability limit of the mineral in unaltered rocks (e.g. kyanite in the alteration zones at the Anderson Mine in Snow Lake and in altered rocks at Hanson Lake). Even textures such as quartz- sillimanite faserkiesel, typically associated with sedimentary rocks, occur in altered felsic volcanic rocks (e.g. Sherridon). Many altered rocks containing these peraluminous phases have been historically misinterpreted due to their similarity to aluminous metasedimentary rocks.

Silicification is widespread in the weakly metamorphosed part of the Flin Flon volcanic belt. Intense zones of silicified basalts have been documented at Snow Lake (Skirrow and Franklin, 1994), and workers have drawn attention to the problem of silicification of rhyolitic rocks at Flin Flon (E.C. Syme, pers. comm., 1989). There is no reason why this syn-volcanic process would not have taken place in the more strongly metamorphosed component of the Flin Flon-Glennie Complex.

Carbonatization, like silicification, may result from several processes, including sea-water alteration, hydrothermal alteration, and metasomatism. Ankerite and other carbonate minerals are common in weakly metamorphosed volcanic terrains, but most workers during the last generation of mapping were not looking for their high-grade equivalents in the gneissic terrains. As a consequence, calc-silicate rocks and impure marbles in the moderately to highly metamorphosed terrains were historically considered metamorphosed marls and mixed chemical and clastic sediments. While such sedimentary carbonates may exist, many of these carbonate occurrences in the northern Flin Flon volcanic belt are considered to reflect alteration for the following reasons:

1. They are spatially associated with volcanic rocks.
2. They have a patchy distribution (chemical sedimentary rocks tend to form more continuous units).
3. Contacts marking the extent of the carbonate and/or calc-silicate mineral phases locally crosscut map units defined on the basis of colour index (i.e. they resulted from metasomatic fluids which flowed across stratigraphic boundaries).
4. Such units commonly pass into carbonate- and calc-silicate-free rocks with an otherwise similar composition along strike.
5. There must be some high-grade equivalents to the carbonate-rich rocks of the weakly metamorphosed terrains.

Many calc-silicate occurrences are found on old geological maps spatially associated with amphibolite, meta-arkose or psammite, sulphide occurrences, and in some instances, cordierite-anthophyllite rocks (e.g. Scimitar Lake area, Pearson, 1973; Ashton et al., 1996b). If the meta-arkose/psammite is re-interpreted as felsic or silicified volcanic rock, this package is readily seen as having a high potential for volcanogenic massive sulphide mineralization since it includes most of the necessary ingredients: 1) a thick mafic volcanic pile; 2) intercalated felsic volcanic horizons, which host many of the VMS deposits of the Flin Flon and Snow Lake areas; 3) alteration; and 4) the sulphides themselves. Re-mapping of such areas has predictably revealed more garnet/cordierite- anthophyllite ferromagnesian alteration and geochemical and/or other justification for re­interpreting the meta-arkose/psammites (see below). This type of calc-silicate/impure marble setting may indicate that such rocks form by the addition of Ca removed from the nearby Ca-depleted ferromagnesian and aluminous alteration zones.

Geochemistry of Felsic Gneisses Derived from Volcanic and Psammitic Precursors

The distinction of igneous and sedimentary protoliths in medium- to high-grade gneiss terrains using geochemical criteria is extremely difficult. Due to the large compositional range, the greatest success has resulted from situations in which the two sets of rocks to be distinguished were derived from different tectonic settings (e.g. marine pelites vs. calc-alkaline tonalites), but such rocks are usually so compositionally different as to make their geochemical distinction redundant. The most difficult situation is one in which both rock types are derived from the same tectonic setting. Such is the case for the felsic volcanic suite and the Missi/Ourom arkoses of the Flin Flon-Glennie Complex. The felsic volcanic rocks are arc-derived, calc-alkaline rocks, whereas the arkoses are derived from a provenance, which includes both this volcanic suite and intrusive, calc-alkaline, granodiorite-tonalite plutons, which are thought to have been derived in a subsequent island arc setting (Lucas et al., 1996). Thus, it isn't surprising that the arkoses are nearly identical compositionally to the felsic volcanic rocks. Nevertheless, the mineralogical differences noted above indicate that some elemental redistribution must have taken place during the erosional and sedimentary processes.

Data Sets

Most of the research dealing with the distinction of para- and orthogneisses is valid only for restricted compositional ranges and has been tested solely on rocks from a specific area. In order to test the viability of using geochemical criteria to distinguish moderately to highly metamorphosed felsic volcanic rocks from psammitic sedimentary rocks in the Reindeer Zone, weakly metamorphosed rocks of known origin were included as standards. A total of six data sets were compiled:

1. granodioritic to tonalitic plutonic rocks from the moderately to highly metamorphosed parts of the Flin Flon and southern Kisseynew domains;
2. low-grade felsic volcanic rocks from the immediate Flin Flon area;
3. low-grade felsic volcanic rocks from Hanson Lake;
4. moderately to highly metamorphosed felsic gneisses interpreted as having volcanic protoliths from the Attitti Lake, Scimitar Lake, and Sherridon areas;
5. Missi psammitic paragneisses from the southern Kisseynew Domain; and
6. pelitic paragneisses, including: a) weakly metamorphosed syn-volcanic rocks from the Amisk Lake area, b) moderately to highly metamorphosed syn-volcanic rocks from the Attitti Lake area, and c) moderately to highly metamorphosed rocks of the younger Burntwood Group from the southern Kisseynew Domain.

The analytical work was carried out in commercial laboratories, with several exceptions. The major and trace element data for some of the moderately to highly metamorphosed felsic volcanic rocks from Attitti Lake and some of the Missi and Burntwood rocks from the southern Kisseynew Domain were analyzed at the University of Regina (G. Parslow, pers. comm., 1996). The rocks from Hanson Lake were also analyzed at the University of Regina as part of an M.Sc. thesis by R. Maxeiner (many of the analyses can be found in Maxeiner et al., 1995). The Sherridon rocks were analyzed at Carleton University as part of an unpublished Ph.D. thesis (Goetz, 1980). Analyses from the six data sets were plotted on a series of discrimination diagrams, some of which were gleaned from the literature, to find ways to geochemically distinguish them.

The protoliths of medium- to high-grade felsic gneisses in the Attitti and Scimitar Lake areas were either not considered or were thought to be sedimentary prior to about 1985 (e.g. Byers and Dahlstrom, 1954; Pyke, 1961, 1966; Pearson, 1973; Kirkland, 1976; Fuh, 1979). Detailed mapping has allowed distinction of gneisses having felsic volcanic precursors in several areas (Ashton and Leclair, 1991; Ashton, 1992; Ashton et al., 1993, 1995, 1996a, b, 1997), some of which are known to have VMS-type deposits nearby (e.g. Schotts Lake, Pearson, 1986; Knife Lake).

Medium- to high-grade felsic gneisses at Sherridon, Manitoba, which were originally interpreted as having sedimentary precursors, contain several Cu-Zn deposits, including the past-producing Sherritt Gordon mine (Goetz, 1980; Froese and Goetz, 1981). Cordierite-anthophyllite rocks reflecting Fe-Mg metasomatism occur along a stratigraphic horizon hosting the deposits and form intense, near- monomineralic alteration at the mine site. Calc-silicate rocks and impure marbles are also common, and aluminous alteration in the form of quartz-sillimanite faserkiesel occurs within tens of metres of both the east and west orebodies. The felsic gneisses were re­interpreted about ten years ago as having felsic volcanic precursors (Ashton and Froese, 1988). In the Meat Lake area, centred 17 km east of Sherridon, Zwanzig (1992) has re-interpreted another large area of felsic gneiss as having volcanic precursors. These had also been previously considered sedimentary, but new exposures revealed rare primary volcanic and volcaniclastic textures, along with associated calc- silicate rocks and cordierite-anthophyllite alteration.

Results

Figure 3 illustrates the variation in major element composition, as illustrated on AFM diagrams for the igneous and Missi psammitic paragneiss data sets. As expected, the felsic volcanic rocks and younger plutonic suite define calc-alkaline trends, which are mimicked by the derived Missi sedimentary rocks. However, the felsic volcanic trends display significantly more scatter than that of the plutonic rocks, and when their high-grade equivalents are plotted, the trend becomes near indistinguishable. This is partly due to alteration, which, as mentioned above, is commonly associated with volcanism. For instance, alkali depletion which is commonly associated with Fe-Mg metasomatism, would have the effect of moving a composition away from the Na₂O + K₂O apex, whereas spilitization (i.e. albitization of a basalt by addition of Na) would move a point in the opposite direction. Since the plutonic rocks were deposited well after the volcanic episode, they escaped the associated alteration. The highly scattered results from the more strongly metamorphosed felsic volcanic rocks reflect the added effects of metamorphic processes, which include partial melting and significant elemental redistribution by near-ubiquitous metamorphic fluids. Although the plutonic rocks have undergone similar metamorphic conditions, they occur as large, less permeable bodies, which tend to inhibit the passage of metamorphic fluids. Scatter into the tholeiitic field displayed by the Missi data may reflect a component of Missi volcanism, or mafic volcanic detritus from the older Amisk Collage.

Figure 3 - Major element geochemical data from the Missi Group, plutons, and felsic volcanic rocks plotted on AFM diagrams (fields from Irvine and Baragar, 1971). Note similarity in trend of Missi data to that of felsic volcanic rocks.

Similarly, plots designed to chemically classify volcanic rocks do not provide a unique way of distinguishing between felsic rocks of volcanic and sedimentary precursors either. Figure 4 illustrates a common classification scheme based on SiO₂, Zr, and TiO₂, the latter two of which are generally considered relatively immobile elements. Although the Missi data exhibit a near constant Zr/TiO2 ratio relative to the positive correlation shown between SiO₂ and the Zr/TiO₂ ratio in the plutonic and felsic volcanic rocks, there is significant overlap and this test alone is not sufficient for confident distinction between volcanic and sedimentary protoliths. Both the volcanic and sedimentary data groups span the rhyolite-rhyodacite- dacite fields, whereas the plutonic rocks display more intermediate compositions, plotting in the rhyodacite- dacite-andesite fields.

Figure 4 - Geochemical data for the Missi Group, plutonic, and felsic volcanic data sets plotted on an SiO2 vs. Zr/TiO2 diagram designed to help classify volcanic rocks (diagram from Winchester and Floyd, 1977). Note the near-constant Zr/TiO2 ratio displayed by the Missi data relative to the positive correlation shown between SiO2 and Zr/TiO2 by the igneous rocks.

The haphazard use of trace elements is similarly incapable of discriminating between various origins for moderately to highly metamorphosed felsic rocks (Figure 5). The Missi data is more tightly clustered than that of the igneous rocks but plot in essentially the same place.

Figure 5 - Geochemical data for the Missi Group, plutonic, and felsic volcanic data sets plotted on a diagram designed to determine the tectonic environment of granitoid rocks (diagram from Pearce et al., 1984). Note general similarity of Missi and igneous data. WPG=within-plate granites; ORG=ocean ridge granites; VAG=volcanic arc ridges; and syn-COLG=syn-collision granites.

Geochemical Distinction

The use of selected immobile trace elements has shown positive results in previous studies aimed at chemically distinguishing medium- to high-grade gneisses of differing origins. Tarney (1976) studied the partitioning of SiO₂ and TiO₂ in calcic pelites and calc- alkaline tonalitic orthogneisses from a variety of sources and found that the TiO₂ tended to concentrate in the sedimentary gneisses. Although not all of the rocks in the present study fit into these lithological classes, it was decided to test the SiO₂-TiO₂ discriminant as a way of distinguishing igneous from sedimentary rocks (Figure 6). The results are remarkable given the mobile nature of SiO₂ and draw attention to the effects of silicification on the volcanic rocks. As expected, most rocks of the Missi Group plot within Tarney's paragneiss field, whereas the vast majority of plutonic rocks plot within the orthogneiss field and display a negative correlation trend between TiO₂ and SiO₂. The two weakly metamorphosed felsic volcanic data sets exhibit a similar trend but show a displacement into more siliceous compositions of the sedimentary field due to silicification. Typical rhyolites worldwide contain about 71 to 75 percent SiO₂ (Nockolds et al., 1978, p49), but many of the known felsic volcanic rocks in this study contain in excess of 80 percent SiO₂. Therefore, it can be assumed that these rocks have undergone a significant degree of silicification, which has shifted their positions to the right and into the paragneiss field. The more highly metamorphosed felsic volcanic rocks exhibit the same negative correlation trend and displacement to more silica-rich compositions, suggesting that they too have been effected by silicification.

Figure 6 - Missi Group, pelite, pluton, and felsic volcanic data plotted on a TiO2 vs. SiO2 diagram used by Tarney (1976) to distinguish calcic pelites from calc-alkaline orthogneisses. Although the aluminous pelites in this study show a wide scatter, the Missi data plot dominantly in Tarney's sedimentary field. By contrast, the igneous rocks mainly plot in the igneous field and display a negative correlation trend. Deviation at the high- silica end is caused by silicification of the volcanic rocks.

Cr and Ni, together with Ti, have previously been used to distinguish ortho- and para-amphibolites by comparing them to known pelitic compositions (Leake, 1964). A data set comprising Archean pelitic migmatites from the Pelican Window (Ashton and Shi, 1994), syn-volcanic pelitic paragneisses from the low- grade part of the Flin Flon Domain, and high-grade Burntwood pelites from the Kisseynew Domain was compiled to test the validity of this discriminant on rocks of the Reindeer Zone. The majority of pelitic rocks from all three sources plot within Leake's designated compositional field for pelites (Figure 7). The majority of Missi rocks exhibit similarly high Cr values, whereas the igneous rocks average about an order of magnitude less, suggesting that Cr has been concentrated during the sedimentation process (due to a Cr contamination problem, the Hanson Lake felsic volcanic data set has not been plotted). In spite of the dearth of Cr analyses for the plutonic and Flin Flon volcanic rocks, and minor scatter in the results, the sedimentary and igneous data groups are easily distinguished. Similar results are obtained by plotting Ni versus TiO₂.

Figure 7 - Missi Group, pelite, pluton, and felsic volcanic data plotted on a Cr vs. TiO2 diagram previously used by Leake (1964) to distinguish para- and ortho-amphibolites. Dashed lines mark Leake's pelite field; solid line added during this study to distinguish fields for sedimentary and igneous precursors.

The concentration of V also appears to successfully distinguish sedimentary and igneous protoliths (Figure 8). V analyses were not obtained for the Hanson Lake rocks or for the majority of plutonic or Flin Flon felsic volcanic rocks, but the available data shows that V, like Cr, tends to concentrate in the sedimentary rocks. An arbitrary line has been added to distinguish sedimentary and igneous fields.

Figure 8 - Missi Group, pelite, pluton, and felsic volcanic geochemical data plotted on a V vs. TiO2 diagram, showing distinction of sedimentary and igneous protoliths.

Discussion of Geochemical Discriminants

The elements that successfully discriminate between felsic gneisses derived from volcanic rocks and those derived from psammitic paragneisses have two things in common: 1) they are relatively immobile and thus, their concentrations are least susceptible to modifications by alteration and metamorphism, and 2) they are elements which tend to concentrate in mafic volcanic rocks relative to their felsic counterparts. Since the Missi and Ourom psammitic paragneisses are derived from a dominantly mafic volcanic belt, they tap an enormous reservoir of these elements. This probably explains the generally higher Cr, Ni, and V concentrations in the psammitic rocks relative to the felsic igneous rocks.

Regardless of the partitioning process, it appears that moderately to highly metamorphosed felsic volcanic and Missi/Ourom psammitic rocks can be geochemically distinguished. Although none of the discriminant functions work perfectly for individual samples, the use of several discriminants together on a set of samples should yield good results.

The possibility of a felsic volcanic origin for quartzofeldspathic supracrustal rocks in moderately to highly metamorphosed terrains containing a significant abundance of amphibolite should be considered. Packages comprising amphibolite, felsic volcanic rocks (including those misinterpreted as meta- arkose/psammite), calc-silicate/impure marble, sulphide occurrences, and, in particular, those with cordierite/garnet-anthophyllite rocks, are highly prospective VMS targets and should be considered focal points in any exploration program in moderately to highly metamorphosed terrains.

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