Project 8811

Mudjatik Domain, Geology and Gold Studies: Porter Lake Area 1

C.T. Harper

Harper, C.T. (1988): Mudjatik Domain, geology and gold studies: Porter Lake area; in Summary of Investigations 1988; Saskat· chewan Geological Survey, Saskatchewan Energy and Mines, Miscellaneous Report 88-4.

Porter Lake, 85 km northeast of Buffalo Narrows, is sur­rounded by rugged rocky terrain and provides excellent transects through the felsic gneisses of the Mudjatik Domain as well as access to a number of supracrustal belts. In view of the knowledge gained at lthingo Lake (Harper, this volume), field investigations in this area were more of a fact.finding nature. Once major lithologi­cal differences had been identified in the felsic gneisses, selected areas were chosen for closer scrutiny. The widest sections of supracrustal belts were examined be· cause of the greater potential for primary sedimentary and/or volcanic features being preserved.

1. General Geology

The Mudjatik Domain is composed mainly of granitoid felsic gneisses of probable Archean age (hereafter referred to as the Mudjatik felsic gneisses), which are considered to be basement to narrow, arcuate to closed belts of supracrustal rocks of sedimentary and volcanic origins. Polyphase deformation has produced dome­and-basin structures, and accompanying metamorphism attained granulite facies conditions over large parts of the domain (Pearson, 1977; Pearson and Lewry, 1974; Lewry and Sibbald, 1977). In the Porter Lake area, Al­cock (1935) and Pearson (1977) described the Mudjatik falsie gneisses as a highly variable group of rocks, and left them as undifferentiated on their maps. Sibbald (1975), during detailed geological mapping around areas of anomalous radioactivity at the north end of Blackstone Lake and at Porter Lake, identified four sub­divisions of the falsie gneisses, including pegmatite, granite, granite gneiss and augen granite gneiss. The present author has subdivided the Mudjatik felsic gneis­ses around Porter Lake into three units: 1) tonalitic gneiss, 2) generally layered falsie gneiss of probable supracrustal origin, and 3) granite and granite peg­matite. These subdivisions are essentially the same as those identified at lthingo Lake (Harper, this volume).

Supracrustal rocks at Porter Lake also include the same types as at lthingo Lake: psammitic to pelitic gneiss, iron formation, hypersthene-bearing mafic granulites (mafic metavolcanics), and minor quartzofeldspathic gneiss (interpreted as falsie metavolcanics). Ultramafic intrusions are more common than at lthingo Lake.

2. Mudjatik Felsic Gneisses

a) Tonalitic Gneiss

At Porter Lake, the tonalitic gneiss is typically grey, coarse grained and has a well-developed swirly foliation, much the same as tonalitic gneiss at lthingo Lake. In the core of the Barker Bay Dome (Figure 1) and in other areas at the north end of Porter Lake, the tonalitic gneiss weathers yellowish-brown and locally contains hy· persthene as well as resinous brownish or greyish-green plagioclase, features which are indicative of granulite facies metamorphism.

Along the west shore of the central part of Porter Lake, the tonalitic gneisses are white to grey weathering, lack hypersthene and display a transition from strongly foliated to unfoliated. Also in this area, several different phases are distinguishable based upon grain size and mineralogical variations as well as crosscutting relation­ships. An early fine-grained dioritic phase occurs as scattered rafts and blocks in the typical coarse-grained well-foliated tonalite. Both types are intruded by a weak­ly foliated to massive anatectic leucogranodiorite, which creates a large scale-agmatite. The leucograno­diorite also intrudes large enclaves of amphibolitic gneiss engulfed in the tonalitic gneiss.

The tonalitic gneiss complex is intruded in a few places by mafic to intermediate dykes, the latter being composi­tionally similar to the tonalite. Younger aplitic to peg· matitic dykes cut all of the above. All the dykes have un­dergone some deformation, but the mafic dykes are the most deformed.

On several islands and the mainland in south-central Porter Lake is a body of weakly foliated biotite­hornblende tonalite-quartz diorite that appears to be less deformed and of lower metamorphic grade than the rest of the tonalitic gneisses. The relatively un­deformed state of this body suggests that it may be a younger phase of the tonalitic rocks.

The mineralogical differences observed in tonalitic gneis· ses at Porter Lake are probably a function of the present erosion surface exposing different structural levels and crossing metamorphic isograds such that higher metamorphic grades are progressively exposed in the cores of stuctural domes. The compositional and textural features of the tonalitic gneisses suggests that they are mainly of plutonic origin.

(1) Prolect funded under the Saskatchewan component of the canada - Saskatchewan Subsidiary Agreement oo Mlneral Development 1984 - 89

Saskatchewan Geological Survey 49

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Figure 1 - Geological sketch map of the Porter Lake ama (modified from Pearson, 1977). 1, undifferentiated Mudjatik fe/sic gneiss. Units 2 to 4 are the authors' subdivisions: 2, tonalitic gneiss; 3, layered felsic gneiss; 4, granne, leucogranne. 5, metasedimentaty gneiss; 6, mafic granu/Hes and ultramafic intrusions; 7, banded iron formation.

b) Layered Felslc Gneiss

Layered falsie gneisses commonly border and apparent­ly underlie the supracrustal belts. As at lthingo Lake, the layered falsie gneisses have two main components, a supracrustal paleosome and a granitic neosome. The paleosome is a pink to grey, colour banded and layered, generally fine-grained, equigranular, granoblas­tic biotitic quartz-feldspar rock of probable psammitic to psammopelitic origin. The paleosome has undergone

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local as well as extensive anatexis, yielding a coarser grained equigranular massive to weakly foliated leucogranite neosome, which occurs as lit-par-lit veins and ill-defined masses. Larger masses commonly con­tain relict inclusions of the paleosome. Where hyper­sthene accompanies biotite (as at Jim Lake), the gneis­ses weather yellowish brown and have a rather greasy appearance.

Summary of Investigations 1988

c) Granite and Granite Pegmatite

Granite and granite pegmatite are the youngest mem· bers of the Mudjatik felsic gneisses. The pink to red weathering granite varies from fine to coarse to por­phyritic, is typically massive to weakly foliated, but has. augen texture developed locally and is co~posed ~a1n· ly of distinctive grey to black quartz and pink potassium feldspar with only minor biotite and plag1ocla~e. In places, these granites ~ave sharp c?ntact~ with .t~e tonalitic and felsic gneisses, appearing to overlie th~m and also to have been involved in the dome·and·basm forming events. This relationship might be explai~ed if the granites were emplaced as sheeHike bod,es 1r:1to the migmatite nappe lobes, before the dome-and·bas1n fold· ing. Elsewhere, the granites have gradational contacts . with both tonalitic and layered felsic gneisses. The transl· tion of tonalitic gneiss into granite is marked by a progressive increase of potassium feldspar, possibly by metasomatic replacement of plagioclase and loss ~f the gneissic textures. Transition from layered felsic gneiss into a homogeneous, coarse·grained to porp~yr~ic leucogranite is characterized by the progressive m·. crease of neosomal granite to the complete exclusion of supracrustal relicts.

Porphyritic granite intrusions were found in s~veral areas during the present investigations. These include: 1) an eye·shaped body, approximately 2 km long by 0.5 km wide, that occurs in the centre of the ovo1d­shaped area of mafic granulites east of the north end of Porter Lake; 2) a 1 km long body that intrudes the weak­ly deformed tonalite found in south-central Porter Lake; and 3) an elongate body, at least 3 km long and 0.5 km wide, that occurs along the northern margin of the supracrustal belt between Splash and Porter Lakes (Figure 1 ).

Pink granite pegmatite intrudes all rocks at Porter Lake. It also occurs as poorly defined veins in the granite, suggesting that it is the last phase related to the formation of the granite. On the west shore of Porter Lake and parallel to the northern margin of the east­west supracrustal belt (Figure 1) is a zone, up to 100 m wide, in which a swarm of pegmatite dykes intrudes both supracrustals and adjacent 'felsic' gneisses. Some of the dykes are folded, indicating that they were emplaced prior to at least the last deformation. Farther west near Splash Lake, several large pegmatites, which intrude mafic granulites and iron formation, are parallel to a north-northeast-trending axial planar fracture cleavage related to the last phase of dome-and-basin folding.

3. Supracrustal Rocks Supracrustal rocks at Porter Lake are consistently o: higher metamorphic grade than supracrustals at lth1ngo Lake. At Porter Lake, hypersthene is a common con­stituent, but apart from gross lithological layering and quartz.magnetite banding in iron formation being con· sidered as original bedding, primary sedimentary and volcanic features have not been positively identified. Be· cause the supracrustal rocks at Porter Lake are essential-

Saskatchewan Geological Surw,y

ly the same as at lthingo Lake, only some significant ob­servations will be included here.

a) Quartzite

On the west shore of Centipede Lake. about 500 m south of the creek that flows into Porter Lake (Figure 1), is a small synformal keel, about 70 m long by 15 m wide, of massive to weakly foliated, b lue-grey quartzite that appears to rest unconformably on layere~ (psam­mitic) falsie gneiss. At the northwest _end of _this st~uc: ture, layering/foliation in the underlymg fels,c gneiss 1s perpendicular to the foliation evident in the quartzite .. Along the east side of the quartzite keel, the respective foliations are divergent. Farther along on the north shore of Centipede Lake is another area of quartzite which contains thin interlayers of meta·arkose. Layer· ing/ foliation in the quartzite is again divergent from t~at of the layered (psammitic- psammopelitic) felsic gneiss which crops out a little further inland.

b) Iron Formation and Associated Rocks

An area designated as psammitic gneiss (unit O of Pear­son, 1977) on the north shore of the large island near the middle of Fuller Lake is interpreted by the author to be felsic metavolcanic rock. Associated with these rocks and separating them from mafic granulites are strongly iron-stained siliceous sulphide·facies iron for­mation. Similar rocks occur along strike both west and north of the island, but they are not indicated on Pearson's map. Oxide facies iron formation is also present in this zone.

Iron formation is generally much more widespread than indicated on Pearson's (1977) map. One horizon can be traced intermittently for nearly 30 km from near the Mud­jatik River east to Dot Lake. Traverses across the supra· crustal belt between Splash and Porter Lakes revealed a continuous layer of iron formation, which occurs w ithin a sequence of mafic granulites (mafic metavolcanics) and is closely associated with ultramafic intrusions (Figure 1). Banded quartz ~agnetite is the .~omin~nt facies, but in places magnetite-poor quartzit1c fac1es (metachert) dominates. Most magnetite-rich b~nds are less than 3 cm thick, but a spectacular 3 m wide band of massive magnetite was found on the west side of a small lake midway between Splash and Porter Lakes. In several places along this belt, folding has thickened the iron formation unit to as much as 300 m in width.

4. Ultramafic Intrusions As with the iron formations. ultramafic intrusions were found to be more abundant than indicated on the pre· vious geological maps, occurring almost exclusively in the mafic granulite belts and very commonly intimately associated with iron formation. A series of pods and len­ses, perhaps the remnants of a once-continuous band of ultramafic rocks, can be traced for at least 16 km along the southern margin of the supracrustal belt from west of Splash Lake to a point about 2 km east of Porter Lake. Similar rocks are known to occur farther east along this same belt. Most of the ultramafics ap-

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pear to be metaperidotites, but metapyroxenites also exist as individual pods or more commonly as layers up to 30 cm thick in the metaperidotite. Magnetite is normal­ly abundant and sulphides are present locally.

5. Economic Geology

Al~ck (1935) reported that a number of small groups of claims had been staked in the Porter Lake region in the early thirties, mainly covering quartzitic material and ap­parently as gold prospects. Grab samples collected by Alcock gave negative results for both gold and silver. Very limited mineral exploration was carried out in the Porter Lake area until the period betvveen 1967 and 1977 when large areas were prospected for uranium, base and precious metals and iron. The discovery of sig­nificant gold values at lthingo Lake has refocussed ex­ploration activty on gold.

a) Gold and Silver

Trace amounts of gold (generally less than 0.04 oz./ton) have been reported from grab samples of gossanous mafic granulites and ultramafic rocks that contain disseminated sulphides (mostly pyrite, pyrrhotite and chalcopyrite). Silver values in these samples were typically ten to tvventy times the gold concentration and similar values have also been reported from banded quartz-magnetite iron formation (Saskatchewan Mineral Deposit Index for 748-6).

Prospective targets for gold mineralization in the Porter Lake area are still to be found in the mafic granulites, ultramafic rocks and iron formation, particularly where disseminated sulphides are present. Associated silicifica­tion and/or quartz veining would enhance the potential for finding elevated gold concentrations.

b) Base Metals

Disseminated to semimassive sulphide mineralization is normally found in the mafic granulites and ultramafic rocks throughout the region, and locally may contain up to several percent copper with lesser nickel and typically low concentrations of zinc, lead, gold and silver (Sas­katchewan Mineral Deposit Index for NTS area 748-6). An example of this type of mineralization is a grab sample of metapyroxenite containing disseminated pyrite, pyrrhotite and chalcopyrite, collected by the author from an island near the east shore of south central Porter Lake. This sample contains 1703 ppm Cu, 434 ppm Ni, 2.t ppm Ag, 119 ppb Au, 18 ppm Zn and <10 ppm Pb.

At the east end of Segment Lake, a sediment-hosted base metal occurrence, consisting of disseminated galena, pyrite and pyrrhotite in feldspathic metapsam­m ite, contains up to 2.4 percent Pb, 0.08 percent Zn and 0.4 oz./ton Ag (Pearson, 1977).

c) Iron

As mentioned above, banded quartz-magnetite iron for­mation can be traced intermittently for about 30 km

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from the Mudjatik River east to Dot Lake. Several high magnetic anomalies associated with this band have been investigated in the past to determine their economic potential (e.g., Mawdsley, 1960; Harper, 1983). The magnetic anomalies occur in areas where the iron formation has been tightly folded to produce greater thickness, but the size and grade have not war­ranted exploitation under existing economic conditions. In view of the world-wide association of gold deposits hosted in iron formation, iron formations of the Mudjatik Domain represent a virtually untested exploration target.

d) Uranium

Two types of uranium mineralization have been recog­nized in the area; 1) occurrences in remobilized base­ment and 2) occurrences in supracrustals (Sibbald, 1975; Sibbald et al., 1977). Radioactive occurrences in the remobilized basement are caused by uraninite and secondary uranium bloom in the weakly foliated pink granites, leucogranites and granite pegmatites, all of which are believed to be the anatectic fraction derived from the Mudjatik falsie gneisses. Radioactive occurren­ces in supracrustal rocks in the Porter Lake area belong primarily to a pelite-pegmatite type (Sibbald et al., 1977). Uraninite and secondary uranium bloom occur in white anatectic segregation pegmatites derived from the host graphitic pelitic gneisses. Sulphide mineralization, includ­ing pyrite, molybdenite and chalcopyrite, is commonly associated with this type. To date, no economic con­centrations of uranium have been found.

6. Discussion

Sibbald et al. (1977) proposed that the supracrustal rock sequences of the Cree Lake Zone are most consistent with deposition on a relatively stable, shallow marine to subaerial continental platform. Harper (1983) reiterated this idea and described a general stratigraphic se­quence involving an early period of elastic sedimenta­tion followed by widespread extrusion of mafic volcanics accompanied by local sedimentation, intrusion of ultramafic rocks and precipitation of iron formation. This was in turn followed by further deposition of elastic sedi­ments, carbonates and more iron formation. Harper indi­cated that this succession was similar to other well­known Precambrian iron-bearing areas, for example the Limpopo mobile belt of southern Africa, the Krivoy Rog area of U.S.S.R. and the Hammersley Group of Western Australia.

The present investigations confirm this general model, but have also shown that volcanism was locally bimodal (i.e., basalt-rhyolite) and, in some areas such as lthin­go Lake, intermediate volcanics were also extruded. Across the Mudjatik Domain, the mafic granulites have great lateral extent, but are commonly of limited thick­ness (generally several hundred metres) compared to typical greenstone belts. It is suggested here that in­cipient continental rifting may have been responsible for localization of the volcanic sequences and could also have provided the linear loci for intrusion of ultramafic rocks, such as seen in the Porter Lake area.

Summary of Investigations 1988

A striking similarity to the Mudjatik Domain is found in the association of iron formation with both metabasaltic and elastic metasediments in the Southern Margin Zone of the Damara Orogen in Namibia (Breitkopf, 1988). Breitkopf states that the mafic rocks are equivalent to continental within-plate tholeiitic basalts, probably derived from lithospheric material during crustal exten­sion and thinning. On the basis of their great lateral ex­tent and limited thickness (generally < 30 to 40 m) Breit­kopf suggested the metabasalts were erupted along nor­mal faults related to crustal extension. Lithogeochemical studies currently underway on the Mudjatik mafic granulites and amphibolites should help clarify the presumed continental setting of the Mudjatik Domain.

Iron-rich rocks in the Mudjatik Domain occur in both vol­canic and sedimentary environments. It is possible and most likely that iron formation was deposited during a number of episodes, as shown by the numerous thin horizons in the mafic granulites and amphibolites, but it is equally plausible that the major iron formation units are diachronous, being deposited contemporaneously with both volcanics and sediments. The author con­siders the iron formations to have formed by exhalative processes related to volcanism in the Mudjatik Domain.

Favourable host rocks for gold mineralization in the Mudjatik Domain include the widespread oxide-facies banded iron formation, mafic granulites and ultramafic intrusions. A similar host rock association for gold mineralization is found in the Archean Murchison and Southern Cross Provinces of the Yilgarn Block in the Western Australian Shield (Groves and Phillips, 1988). According to them, this type of stratigraphic association, comprising mainly basaltic volcanics, stable platform sediments and oxide-facies banded iron formation, is consistent with the tectonic setting for platform-phase greenstones. Extensional tectonics are minimal in this setting. The Mudjatik Domain appears to represent a transition between purely platform-phase greenstones and rift-phase greenstones, in that localized rifting of the Mudjatik platform helped control sites of deposition, vol­canism and intrusion.

Geochronological studies in the Mudjatik Domain are still at an early stage. The results to date indicate the presence of extensive Archean crust that underwent a major regional thermotectonic event about 2000 Ma (Bickford et al., 1986, 1987) and was strongly affected by the Hudsonian Orogeny about 1850 Ma ago. A variety of granulite facies rocks contain clear euhedral zircons, which yield the 2000 Ma age, and thus may indi­cate the timing of the granulite facies metamorphism. The June,'tion Granite (Sibbald, 1974), a late megacrystic granite, yields a Hudsonian age of 1820 Ma (Bickford et al., 1986), but also contains relict xenocrystic zircons which are believed to have been incorporated during anatexis of the Archean crust. The megacrystic and por-

Saskatchewan Geological Survey

phyritic granites at lthingo Lake and the massive to weakly foliated granites at Porter Lake may also be of Hudsonian age. A more thorough discussion of the geological history awaits the results of geochronological studies to be carried out following this field season.

7. References Alcock, F.J. (1935): Mudjatik-Haultain area, Saskatchewan;

Geol. Surv. Can., Mem. 180, 16p.

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

Bickford, M.E., Van Schmus, W.R., Collerson, K.D. and Mac­donald, R. (1987): U-Pb zircon geochronology project: new results and interpretations; in Summary of Investiga­tions 1987, Sask. Geol. Surv., Misc. Rep. 87-4, p76-79.

Breitkopf, J.H. (1988): Iron formations related to mafic vol­canism and ensialic rifting in the Southern Margin Zone of the Damara Orogen, Namibia; Precam. Research, v38, p111·130.

Groves, D.I. and Phillips, G.N. (1988): The genesis and tec­tonic control on Archaean gold deposits of the Western Australian Shield-a metamorphic replacement model; Ore Geol. Rev., v2, p287·322.

Harper, C.T.(1983): Iron ores of northern Saskatchewan; Sask. Energy Mines, Rep. 220, 102p.

Lewry, J.F. and Sibbald, T.1.1. (1977): Variation in lithology and tectonometamorphic relationships in the Precambrian basement of northern Saskatchewan; Can. J. Earth Sci., v14, p1453-1467.

Mawdsley, J.B. (1960): Iron ore occurrences northwestern Saskatchewan; Sask. Dep. Miner. Resour., Rep. 50, 19p.

Pearson, D.E. (1977): The geology of the Mudjatik area (south­west quarter), Saskatchewan; Sask. Dep. Miner. Resour., Rep. 166, 31p.

Pearson, D.E. and Lewry, J.F. (1974): Large-scale fold inter­ference structures in the Mudjatik River area of northern Saskatchewan; Can. J. Earth Sci., v11, p619-634.

Sibbald, T.1.1. (1974): La Loche (north) area; in Summary Report of Field Investigations by the Saskatchewan Geological Survey 1974; Sask. Dep. Miner. Resour., p38-45.

____ (1975): Investigation of certain radiometric anomalies in Crown Reserve 621; in Summary of Inves­tigations 1975 by the Saskatchewan Geological Survey; Sask. Dep. Miner. Resour., p123-129.

Sibbald, T.1.1., Munday, R.J.C. and Lewry, J.F. (1977): The geological setting of uranium mineralization in northern Saskatchewan; in Dunn, C.E. (ed.), Uranium in Sas­katchewan; Sask. Geol. Soc., Spec. Publ. 3, p51-99.

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