Investigation of the Wollaston-Mudjatik Transition, Charcoal Lake and Cochrane River (parts of NTS 64L-9, -10, -11, -14, -15, and -16) C.D. Card, C.T. Harper, N. Barsi 1, J. Lesperance 1, and J.S. Smith Card, C.D., Harper, C.T., Barsi, N., Lesperance, J., and Smith, J.S. (2006): Investigation of the Wollaston-Mudjatik transition, Charcoal Lake and Cochrane River (parts of NTS 64L-9, -10, -11, -14, -15, and -16): in Summary of Investigations 2006, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2006-4.2, CD-ROM, Paper A-5, 16p. Abstract In 2006, a second mapping transect was completed across the northwest Wollaston Domain and the Wollaston- Mudjatik boundary. Basement rocks to the Wollaston Supergroup core elongated and sometimes overturned domes that are mantled by the younger supracrustal rocks. Basement supracrustal rocks include intermediate to mafic volcanic rocks, ferruginous metasedimentary rocks, and banded iron formation, which were intruded by quartz diorite, granodiorite, tonalite, and granite. As in 2005, two distinct successions were identified in the Paleoproterozoic Wollaston Supergroup. The lower succession contains graphitic pelite, psammopelite to pelite, calcareous psammopelite to pelite, and psammite that is spatially associated with basement rocks. The extreme southeast part of the transect is dominated by an upper succession of arkose, calcareous arkose, and calc-silicate rocks. The transition between the two successions is abrupt. Trans-Hudson–age anatectic granites and rare, Mackenzie suite diabase dykes intruded both the Wollaston Supergroup and its basement rocks. All of the rocks in the region, with the exception of the youngest intrusive rocks contain a well-developed, composite S1 foliation. Early mylonitic foliations are parallel to the S1 foliation. Regional-scale, northwest-trending F2 folds are in a low-strain panel west of Charcoal Lake. Orthogonal, high-amplitude, northeast-trending F3 folds interfered with F2 to form elongate basins and domes, except in the low-strain panel where the structures are symmetrical. The F3 folds were overturned and commonly had their limbs sheared off. D3 structures postdate the peak metamorphic assemblages and all lithologies except for the Mackenzie diabase dykes. Open northwest-trending F4 folds enhanced the depressions and culminations in the F3 axes. North-trending brittle-ductile shear zones were the youngest structures identified. Metamorphic assemblages in both the basement and Wollaston Supergroup rocks indicate moderate-pressure, granulite-facies conditions. Rare kyanite-bearing, cordierite restites suggest that these rocks passed through the kyanite stability field, either early on a prograde path or at extreme pressure and temperature, and that kyanite then remained metastable. The sharp boundaries separating the upper and lower Wollaston Supergroup successions are interpreted as syn- to post-D3 shear zones focussed on the limbs of F3 folds. The McRae Bay and Esker Lake faults accommodated west- side-up displacement that exposed a lower level of Wollaston stratigraphy. A quartzite and amphibolite succession exposed at the north end of Wollaston Lake is restricted to the west block of the McRae Bay fault and is interpreted to lie beneath the rest of the Wollaston Supergroup. There is no recognisable discontinuity between the Mudjatik and Wollaston domains. Basement to the Wollaston Supergroup comprises a volcano-plutonic succession that was intruded by tonalite and granite plutons. Both the lower and upper successions in the Wollaston Supergroup are thought to be part of a <1.88 Ga foreland basin succession. These rocks were subjected to high-grade metamorphism, and D1 and D2 deformation prior to 1.83 Ga. A second metamorphic episode occurred at ca. 1.82 to 1.81 Ga. D3 deformation post- dated this metamorphic episode and was characterised by tight northeast-trending folds with shear zones developed on their limbs. Northwest-trending F4 folds and north-trending, brittle-ductile faults were the final structures to develop. Keywords: Charcoal Lake, Cochrane River, basement, Wollaston Supergroup, Wollaston Domain, Mudjatik Domain, Hearne Province, fold interference, sheared F3 limbs, McRae Bay–Esker Lake faults, quartzite- amphibolite succession, granulite facies metamorphism. 1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2. Saskatchewan Geological Survey 1 Summary of Investigations 2006, Volume 2 1. Introduction The ‘Wollaston Northeast’ mapping project, initiated in 2005, was designed to increase the understanding of the nature and context of the Wollaston Supergroup (Yeo and Delaney, in press) northeast of Wollaston Lake. A new high-resolution airborne gamma-ray spectrometric and total field magnetic survey at 400 m line spacing (Ford et al., 2005), flown as part of the Government of Saskatchewan’s Mineral Exploration Incentive Program, provided framework geophysical data for the mapping program. Bedrock 1:50 000-scale geological mapping of the central part of the survey began in 2005 (Harper et al., 2005a, 2005b) and continued in the northern part in 2006 (Figure 1). Bedrock exposure is limited throughout much of the region and outcrop is commonly restricted to the edges of till- and glaciofluvial-draped ridges; major esker systems are common. Much of the 2006 map area has been subject to forest fires within the last 20 years, with some 2006 burns o located near the elbow in the 60 Cochrane River. These fires Dodge served to better expose the bedrock, and outcrops not included in previous RAE compilations (Scott, 1972, 1980) PROVINCE were documented. Mudjatik H P E R A Tantato O R a) Summary of 2005 V N IN E Mapping C E A transect across the north- Black Lake “Cochrane central part of Wollaston Lake Elbow” Charcoal (Harper et al., 2005a, 2005b, Lake 2005c), including the south part Cochrane 2006 of the Cochrane River, identified River two distinct lithologic Esker Lake fault successions (e.g., Ray, 1978; ATHABASCA BASIN Yeo and Delaney, in press). The 2005 west part of the transect is McRae Bay dominated by basement rocks Pasfield fault n McRae with intervening psammopelitic Lake to as Bay to pelitic rocks and a quartzite- oll A amphibolite succession. East of B W H McRae Bay, the stratigraphy is ke dominated by calcareous and La arkosic rocks interpreted to correlate with units higher in the Wollaston Supergroup stratigraphy. The quartzite- Cree amphibolite succession at the e Lake k north end of Wollaston Lake was a r L e r de correlated with the Hidden Bay n te ein o e R Assemblage (Wallis, 1971), as st P was originally suggested by Ray lla o e (1978). Given that the quartzite- W k La amphibolite succession is spatially associated with, and SON unconformably overlies, NS-HUD basement lithologies, it was TRA GEN N ORO interpreted to lie stratigraphically E n below the foreland basin– W a e n e dominated (Tran, 2001) S m to g a s n 0 10 30 50 th n o Wollaston Supergroup a e R W tt a stratigraphy (Harper et al., km o L R o 2005a). 102 Figure 1 - Major geological subdivisions and domains of northeast Saskatchewan. The location of the 2005 map area is outlined in red, 2006 in blue, and the extent of b) 2006 Mapping the Peter Lake and Wollaston Lake multi-parameter airborne survey is outlined in In 2006, a bedrock and surficial heavy black. Note the location of the elbow in the Cochrane River. The location of known outcrops of the Hidden Bay Assemblage (HBA) on the west shore of mapping transect was carried out Wollaston Lake is outlined in green. from Charcoal Lake, west to the Saskatchewan Geological Survey 2 Summary of Investigations 2006, Volume 2 elbow in the Cochrane River, and then south on the Cochrane River to the limit of 2005 mapping. The mapping was undertaken from two bush camps: one on Charcoal Lake and the second just south of the elbow in the Cochrane River. The camps were serviced by float-equipped aircraft based at Points North Landing. Bedrock was accessed via inflatable boats, foot traversing and, in two cases, by set-outs with a small, float-equipped aircraft. Mapping (Figure 2) was aided by the new aeromagnetic and radiometric data (Ford et al., 2005), which were used to extrapolate outcropping units and to help in interpreting the complex structure in places where no outcrop control was available. The bedrock study was complemented by a study of the Quaternary geology, the results of which are detailed by Smith (this volume). 2. Bedrock Geology a) Regional Geology The Wollaston and Mudjatik lithotectonic domains are in the Hearne Province (Figure 1). The Mudjatik Domain is dominated by Archean granitoid gneiss (e.g., Lewry and Sibbald, 1980), the oldest of which was emplaced at ca. 3.0 Ga (Orrell et al., 1999). Migmatitic tonalite ranges in age from 2.73 to 2.70 Ga (Annesley et al., 1997; Harper and van Breemen, 2004) and may locally be as old as 2.78 Ga (Annesley et al., 1999). Younger intrusions are generally granitic and vary from 2.64 to 2.58 Ga in age (Annesley et al., 1997; Annesley et al., 1999). Supracrustal septa are not common and generally not well understood, with the exception of an outlier of the 2.71 to 2.68 Ga Rankin-Ennadai greenstone belt (Delaney et al., 1990; Harper et al., 2001; Harper and van Breemen, 2004) and outliers of the Paleoproterozoic Hurwitz Group (Harper et al., 2001; Harper and Davis, this volume), both in the northeast corner of Saskatchewan. South of the Athabasca Basin, the age and origin of supracrustal rocks is not well known, with most workers suggesting that there are both Archean and Paleoproterozoic units (e.g., Lewry and Sibbald, 1977). The Wollaston Domain contains Archean rocks similar in appearance and age to those of the Mudjatik Domain, as well as the unconformably overlying Paleoproterozoic supracrustal rocks (e.g., Lewry and Sibbald, 1977) now referred to as the Wollaston Supergroup (Yeo and Delaney, in press).