The Origins of Anomalously Graphitic Rocks and Quartzite Ridges in the Basement to the Southeastern Athabasca Basin C.D. Card Card, C.D. (2012): The origins of anomalously graphitic rocks and quartzite ridges in the basement to the southeastern Athabasca Basin; in Summary of Investigations 2012, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2012-4.2, Paper A-6, 15p. Abstract Remote-predictive mapping for the basement to the Athabasca Basin in NTS area 74H was augmented by a core mapping program using drillcores stored at Kapesin Lake, near the Key Lake mine, and in La Ronge and Regina. The unaltered basement includes: pre-Wollaston supergroup, the Karin Lake formation of the Wollaston supergroup, and younger intrusions of pegmatitic granite. Pre-Wollaston rocks include granite, leucotonalite, and gneissic tonalite. Rocks of the Karin Lake formation include psammopelite, the most common unit, pelite and rare psammite, all of which contain traces of graphite. Younger intrusions are dominantly pegmatitic granites, which in some cases contain more than 50% normative quartz. The majority of the rocks present in the basement sections contain hydrothermal alteration assemblages. Although graphite in concentrations of <5% can be expected in psammopelitic rocks of the type found in the Karin Lake formation, concentrations of >5% and locally up to 25% in altered rocks are clearly anomalous and require some form of concentration. Many such rocks contain no ferromagnesian minerals (e.g., biotite), as might be expected if they had been derived from psammopelitic precursors, and have undergone pervasive alteration. In addition, pre- Wollaston supergroup orthogneisses and pegmatitic granites locally contain metasomatic graphite. Contacts between anomalously graphitic units and non-graphitic rocks are gradational. In some instances, the graphite is foliation parallel and appears to have pseudomorphed biotite. Elsewhere, the graphite is late and randomly oriented. Graphite is also present in late fractures, particularly those developed in pegmatitic granites. Anhedral pyrite in veins and as amorphous replacements of various rock-forming minerals is typically associated with the graphite-rich rocks. Rocks containing 80 to 100% quartz are locally encountered in basement cores. Some of these may well be Wollaston supergroup quartzite, but others lack evidence of sedimentary features common in orthoquartzites and range from massive, where they resemble quartz veins, to well foliated. Such quartzite rocks encountered in the studied core are most common in gradational into pegmatitic granites, suggesting they are quartzolites. Layered quartz-rich rocks associated with the quartzolites contain relict textures such as gneissosities and likely represent silicified country rock. Anomalous concentrations of graphite are thought to have been precipitated from fluids generated during the prograde metamorphic cycle. Dehydration reactions generate free H2O that can consume some of the graphite present in originally carbonaceous metasedimentary rocks, such as black shales. Reactions associated with biotite melting under granulite-facies conditions can also consume graphite. Graphite likely precipitated locally from carbon-rich fluids following peak metamorphic conditions. Multiple graphitisation events are probable. The quartz- rich intersections are speculated to have formed due to immiscibility in H2O-rich, low-viscosity pegmatitic granite melts. Fluid inclusion studies in the quartz cores of pegmatitic granites indicate that H2O-rich melt fractions enriched with carbonate have the potential for increased silica solubility. This interpretation better fits the characteristics of the quartzites and provides a more attractive explanation for several quartzite features that form prominent topographic features in the basement to the Athabasca Basin. Hydrothermal solutions likely enhanced basement permeability. Enhanced permeability has the potential to lead to more efficient mixing of fluids derived from the Athabasca Group and the basement rocks, a process essential to creating the redox reactions necessary to precipitate uranium. Quartzite ridges may have provided a competency contrast that focussed fault systems and, if magmatic, might have also been a local source of uranium. High-grade deposits such as McArthur River and Phoenix are directly adjacent to ‘quartzite ridges’. Keywords: Wollaston supergroup, Karin Lake formation, uranium system, graphite, hydrothermal alteration, C-O- H fluid, massive quartz, quartz ridge, pegmatitic granite, silicification. Saskatchewan Geological Survey 1 Summary of Investigations 2012, Volume 2 1. Introduction Although there is modern bedrock mapping of the basement rocks around much of the Athabasca Basin (Figure 1), very little information is available about the basement rocks directly beneath it. A preliminary remote predictive basement map for the western Athabasca Basin was prepared during the EXTECH IV multidisciplinary uranium study (Card, 2006), but the only published basement maps for the eastern part of the basin are those of Gilboy (1982a, 1982b). Since the time of Gilboy’s mapping, the database available for basement mapping in the Athabasca region has evolved considerably. In addition to the new mapping along the flanks of the basin, there has been 30 years of new drilling, information from which is stored in the Saskatchewan Geological Survey’s Mineral Assessment database (http://www.er.gov.sk.ca/smad), and new aeromagnetic surveys for the entire Athabasca Basin in Saskatchewan (e.g., Buckle et al., 2010). These new aeromagnetic data are the best means of interpreting the basement units beneath the magnetically transparent Athabasca Group in the absence of drillhole information. The Athabasca uranium ore systems project (Bosman et al., 2011) is a multidisciplinary geoscience project designed to achieve a four-dimensional understanding of the Athabasca region during the entire evolution of the multi- episode uranium system. Basement geology and later overprinting events, such as fault systems and alteration, are key components in understanding the uranium system. Basement geology in the Athabasca Basin will be compiled at 1:250 000 (250 k) scale and published as a series of 250 k NTS map sheets and geographic information system products. The first NTS map area chosen for compilation is 74H (Geikie River), which encompasses both the Key Lake and McArthur River uranium mines and numerous other uranium deposits and showings. Moreover, the transitional boundary between Nolan 60o N the Mudjatik and Wollaston Train Dodge 102 W domains, which is thought to be Zemlak BeaverlodgeRAE o favourable for uranium Basement Mudjatik exploration in the Athabasca Tantato Taltson Lake Athabasca Basin, runs northeastward across Black Lake the 74H map area from the Taltson zone southwest corner (Figure 2) 1. The map area is ideal for Athabasca Carswell compilation as there is good Basin Pasfield bedrock mapping along the Lake tectonic Wollaston Lake basin’s flanks and recognisable aeromagnetic features of various Lloyd orientations that can potentially NTS be quantified with drillhole Cree Lake 74H Snowbird Reindeer information (Figure 2). In Lloyd Peter Lake addition, depth to the Clearwater Wollaston Lake HEARNE unconformity is relatively shallow for most of the 74H area Virgin River Wathaman and therefore the magnetic signal from the basement rocks is less o muted 2. The mapping process 110 W Mudjatik La Ronge will include compilation of RottenstoneREINDEER ZONE drillhole data from assessment BBIW information, selected field Glennie studies to help with the KisseynewNW SASK IW Kisseynew identification of rock units, and Lac interpretation of geophysical La HW Ronge PW information. Alberta PHANEROZOIC Flin Flon Saskatchewan Archean Windows NW = Nistowiak Window Deschambault IW = Iskwatikan Window Sask craton Lake Amisk 2. Field Studies 0 10 30 50 km HW = Hunter Bay Window } Lake PW = Pelican Window BBIW = Black Bear Island Window } Hearne Drillcores were selected for 54o N examination to provide insight Figure 1 – Current subdivision of lithostructural domains in northern Saskatchewan into various aeromagnetic and northeastern Alberta. The dashed box represents the outline of NTS area 74H features in NTS area 74H. Cores and the extent of Figure 2. stored at the Kapesin Lake core 1 The Wollaston-Mudjatik transition zone (Annesley et al., 2005) is commonly referred to as favourable for uranium exploration. The zone has loosely defined dimensions, but it is thought be broadly coincident with the transitional boundary (Figure 2) between the Wollaston and Mudjatik domains. 2 Although it is generally non-magnetic and therefore magnetically transparent, anomalies sourced from below the thicker parts of the Athabasca Group tend to be less clear due to absolute distance from source to collector (e.g., Pilkington, 1989). Saskatchewan Geological Survey 2 Summary of Investigations 2012, Volume 2 cache near Key Lake, and the Saskatchewan Geological Survey’s facilities in La Ronge and Regina were remapped in the summer of 2012. These included 42 basement sections of cores McArthur River stored at Kapesin Lake, six sections from La Ronge, and 10 stored in Regina (Table 1). In addition, 17 full sections and one partial section of Athabasca Group were logged at Kapesin Lake and La Ronge, respectively (Bosman et al., this volume). The newly gathered basement 914 information will be incorporated into subsequent