the origins of anomalously graphitic rocks and quartzite

Saskatchewan Geological Survey 1 Summary of Investigations 2012, Volume 2

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.


Figure 1 – Current subdivision of lithostructural domains in northern Saskatchewan and northeastern Alberta. The dashed box represents the outline of NTS area 74H and the extent of Figure 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 the Mudjatik and Wollaston domains, which is thought to be favourable for uranium exploration in the Athabasca Basin, runs northeastward across the 74H map area from the southwest corner (Figure 2) 1. The map area is ideal for compilation as there is good bedrock mapping along the basin’s flanks and recognisable aeromagnetic features of various orientations that can potentially be quantified with drillhole information (Figure 2). In addition, depth to the unconformity is relatively shallow for most of the 74H area and therefore the magnetic signal from the basement rocks is less muted 2. The mapping process will include compilation of drillhole data from assessment information, selected field studies to help with the identification of rock units, and interpretation of geophysical information.

2. Field Studies Drillcores were selected for examination to provide insight into various aeromagnetic features in NTS area 74H. Cores 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).

Mudjatik

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Figure 2 – Total magnetic intensity for NTS area 74H. Diamond drillholes mapped in 2012 are denoted by white dots. The black dotted line represents the interpreted transitional boundary between the Wollaston (to the southeast) and Mudjatik domains and the white line denotes the southeastern extent of the Athabasca Basin. Note the location of the deposits mined at Key Lake and McArthur River (grey squares), and of provincial Highway 914 (solid black line).

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 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 information will be incorporated into subsequent remote predictive mapping. This report details the mesoscopic textures observed in very common graphitic basement rocks in NTS area 74H and in basement sections mapped as metamorphosed orthoquartzites by previous workers.

a) Basement-driven Exploration Criteria

Both graphitic basement rocks, commonly interpreted as having pelitic protoliths, and long core intervals of interpreted ‘orthoquartzite’, commonly referred to as quartzite 3 ridges, are key uranium exploration

features in the eastern Athabasca Basin. Graphitic pelitic rocks are common, but not ubiquitous (Yeo and Savage, 1999) in the Karin Lake formation of the lower Daly Lake group (Yeo and Delaney, 2007), and rare in the rest of the Wollaston supergroup. The Karin Lake formation commonly sits unconformably on Archean granitoid basement throughout much of the western Wollaston Domain and it has been proposed that the competency contrast between the relatively rigid rocks, in this case Archean orthogneisses and granites, and less-rigid Wollaston metasedimentary rocks might control the location of structures that were later important in focussing uranium mineralisation (e.g., Jefferson et al., 2007). As a result, anomalously graphitic units are commonly targeted in the search for unconformity-related uranium deposits, and patterns indicate that drilling commonly follows regional-scale graphitic horizons that are imaged through a variety of electromagnetic techniques. At McArthur River (Figure 2), conductive graphitic pelite is situated adjacent to a ridge of quartzite, which theoretically provides the competency contrast necessary to localise strain along the P2 fault system (McGill et al., 1993). As a result of the anomalous size and grade of the McArthur River deposit (e.g., Saskatchewan Ministry of Energy and Resources, 2011), exploration targets adjacent to quartz-rich units are considered desirable and have led to other discoveries, e.g., Phoenix deposit (Gamelin et al., 2010).

Two subjects related to exploration targets in the basement to the Athabasca Group will be detailed and discussed below: 1) the origin of anomalously graphitic units near Key Lake and McArthur River; and 2) the origin of the quartzite units. Evidence will be based on mesoscopic investigation of drillcore and will focus on mineral textures in a variety of basement units.

3 Quartzite refers to a “well crystallized quartz dominated rock” (Shelley, 1983, p77). The term orthoquartzite implies a metamorphosed quartz arenite and quartzolite, a quartz-dominated igneous rock.

McArthur River

Key Lake

914

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Table 1 – Location and orientation information for basement cores mapped in the summer of 2012. Inclination is calculated by subtracting the plunge of the drillhole from horizontal (i.e., 0°-90° = -90°, which is a vertical drillhole). Depth to unconformity (UC) refers to metres of core drilled to reach the unconformity and has not been corrected for true depth. The depth to the unconformity has been taken from the author’s log and is rounded to the nearest metre. Note all UTM coordinates in this document are in NAD 83, zone 13.

DrillholeNTS Area

(50k) UTM-E UTM-NInclination

(°)Azimuth

(°)Depth to UC

(m)Storage Location Assessment File

4557-1-82 74H-06 490012 6358685 -90 - 185 Regina 74H06-00604560-1-82 74H-06 501112 6369952 -90 - 195 Regina 74H06-0060

69-1 (P1-H1) 74H-16 532104 6427685 -90 - 330 Regina 74H15-000269-3 (P1-H3) 74H-16 530116 6417084 -90 - 318 Regina 74H15-0002

AH-004 74H-05 469490 6345632 -60 138 112 Kapesin Lake 74H-0045AH-007 74H-06 471570 6346731 -90 - 128 Kapesin Lake 74H-0047AH-009 74H-04 469273 6345234 -60 318 96.6 Kapesin Lake 74H-0047AL-015 74H-04 460977 6343744 -90 - 89 Kapesin Lake 74H04-NE-0077AL-017 74H-04 462791 6344416 -90 - 99 Kapesin Lake 74H04-NE-0077AL-018 74H-04 461024 6342451 -60 138 72 Kapesin Lake 74H-04-NE-0097AL-020 74H-04 462807 6342847 -57 138 97.4 Kapesin Lake 74H-04-NE-0097BD-005 74G-08 420412 6354081 -88 - 201 Kapesin Lake 74H04-NE-0041BD-006 74G-08 420403 6354938 -90 - 191 Kapesin Lake 74H04-NE-0041BD-007 74G-08 421356 6355268 -90 - 206 Kapesin Lake 74H04-NE-0041

CB95-057A 74H-15 506109 6411063 -90 - - Regina 74H15-SW-0049EL-071 74H-05 458639 6345609 -60 318 103 Kapesin Lake 74H-04-NE-0093EL-074 74H-05 459517 6346842 -60 318 174 Kapesin Lake 74H04-NE-0087EL-078 74H-05 460642 6346865 -70 318 132 Kapesin Lake 74H04-NE-0087

GRL-131 74H-05 466581 6357978 -80 - 327 Kapesin Lake 74H05-173GRL-133A 74H-05 466638 6357908 -79 - 219 Kapesin Lake 74H05-173GTB-82-33 74H-05 462092 6353171 -90 - 258 La Ronge 74H05-0093KAP-005 74H-04 465496 6338411 -60 318 78 Kapesin Lake 74H-04-NE-0094KLI-006 74H-04 462801 6341759 -90 - 73 Kapesin Lake 74H-04-NE-0092LE-061 74G-07 393381 6358906 -58 275 139 Kapesin Lake 74G07-0044LE-064 74G-07 394164 6360389 -90 - 152 Kapesin Lake 74G07-0044LE-065 74G-07 394488 6361604 -90 - 168 Kapesin Lake 74G07-0044

MAC-059 74H-10 508285 6382629 -60 320 202 La Ronge 74H10-0052MAC-167 74H-10 500075 6395193 -90 - 420 La Ronge 74H-0040MAC-177 74H-11 493201 6398103 -90 - 584 La Ronge 74H-0040MK-020 74H-04 460610 6341520 -90 - 73 Kapesin Lake 74H04-NE-0070MK-028 74H-04 460477 6340764 -90 - 61 Kapesin Lake 74H04-NE-0070MK-032 74H-04 460255 6341009 -90 - 55 Kapesin Lake 74H04-NE-0070MK-038 74H-04 461827 6341561 -90 - 48 Kapesin Lake 74H04-NE-0070ML-008 74H-04 465591 6345328 -90 - 96 Kapesin Lake 74H04-NE-0074ML-014 74H-04 463521 6345075 -90 - 119 Kapesin Lake 74H05-SE-0075ML-015 74H-04 463012 6345019 -90 - 106 Kapesin Lake 74H05-SE-0075ML-018 74H-04 463004 6345103 -90 - 114 Kapesin Lake 74H05-SE-0075ML-059 74H-05 468841 6346817 -62 138 123 Kapesin Lake 74H-04-NE-0091ML-063 74H-06 471178 6348064 -62 318 214 Kapesin Lake 74H-04-NE-0091MW-007 74H-04 461992 6340398 -90 - 65 Kapesin Lake 74H-04-NE-0091MW-010 74H-05 461124 6346283 -58 165 113 Kapesin Lake 74H-04-NE-0092P-051 74H-04 465199 6340544 -65 21 92 Kapesin Lake 74H04-NE-0100P-052 74H-04 465146 6341105 -90 - 112 Kapesin Lake 74H04-NE-0100

PP-001 74H-04 467401 6336658 -90 - 48 Kapesin Lake 74H04-NE-0072RL-042 74H-14 491667 6401844 -60 133 636 Regina 74H14-0019RL-047 74H-11 488386 6393624 -60 313 708 Regina 74H14-0019

RL-79-15 74H-06 471918 6355296 -90 - 173 La Ronge 74H06-0047SP-001 74H-07 520399 6372999 -60 220 58 Kapesin Lake 74H10-0019SP-004 74H-10 519513 6373729 -62 220 48 Kapesin Lake 74H10-0019

TUE-06-06 74K-05 232295 6469491 -90 - 1212 Regina 74K03-0019WC-79-1 73 I-5 466838 6484374 -90 - 939 Regina 74I-0012

WL-001-Dejour-Wapata 74 I-13 457736 6525629 -90 - 768 Regina 74I12-0002WL-002 74H-06 469960 6347141 -90 - 131 Kapesin Lake 74H05-SE-0027WL-004 74H-06 469893 6347205 -90 - 161 Kapesin Lake 74H05-SE-0027WL-021 74H-05 466878 6349333 -60 318 217 Kapesin Lake 74H04-NE-0088WL-022 74H-05 465084 6347545 -60 138 158 Kapesin Lake 74H04-NE-0088WR-008 74H-09 530941 6382055 -74 90 58 Kapesin Lake 74H10-0019ZQ-010 74H-06 472889 6370510 -90 - 256 La Ronge 74H06-NW-0080


Figure 3 – A) Well-foliated leucotonalite (MW-007, 105.8 m). B) Gneissic leucotonalite. Layering is due to both injected leucosome and in situ neosome (WL-022, 252 m). C) Hydrothermally altered gneissic leucotonalite. Banding in this rock was imparted by injected leucosome, in situ partial melting, and alteration. Pegmatitic granite is also present (P-052, 193 to 195 m).

3. Lithostratigraphy The rocks described below, with the exception of the younger intrusions, are well foliated to gneissic and have been metamorphosed beyond the limit of minimum melting, implying metamorphic conditions of at least upper amphibolite facies. Younger pegmatitic granitoid rocks (<1.815 Ga; Annesley et al., 2005) are typically massive, but well foliated examples are not uncommon, indicating a late foliation-forming deformational event in the region.

a) Pre-Wollaston Basement

Granite, leucotonalite, and gneissic tonalite are uncommon constituents in the drillcores observed in 2012 and represent the only pre-Wollaston supergroup rocks that were encountered. Fresh granite is pink, has a 1 to 3 mm grain size and is well foliated, homogeneous and contains 5 to 10% biotite. It is typically associated with 20% younger pegmatitic granites. Fresh leucotonalite is grey, medium grained (1 to 2 mm), homogeneous, well foliated and contains 15% biotite (Figure 3A). Pink and white altered varieties were also present in drillcore MW-007 (Table 1). Gneissic leucotonalite (Figure 3B) is the most common of the pre-Wollaston units. It typically comprises a 1 to 2 mm paleosome containing 5 to 10% biotite, 30% or less quartz, plagioclase and local magnetite. Layers of injected leucosome and in situ neosome development have imparted a migmatitic gneissic banding (Figure 3B). Some of the gneissic tonalite displays compositional layering in addition to that defined by neosome (Figure 3C). Such rocks may have metasedimentary protoliths; however, the difference in the alteration of feldspars in adjacent layers suggested that it is a product of metasomatism.

b) Karin Lake Formation (Wollaston supergroup)

The most common protoliths for unaltered rock types in drillcore from NTS area 74H are metasedimentary rocks of the Karin Lake formation, lower Daly Lake group (Yeo and Delaney, 2007). Psammopelite is the most common rock type. It contains 1 to 2% garnet that is commonly elongate (Figure 4A), implying later deformation, as well as 10 to 20% biotite and trace to 1% graphite. Sillimanite is rare. Although cordierite is common elsewhere in the Karin Lake formation (e.g., Highrock Lake; Yeo and Savage, 1999), no porphyroblasts were observed in any of the

B

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Figure 4 – A) Psammopelite with elongate garnet (Grt) porphyroblasts. White leucosome bands make up less than 10% of this example, with thin bands of melanosome (M) at the margins of the leucosome (RL-047, 769.4 m). B) Pelite with about 30% white, dismembered melt leucosome. A ragged garnet (Grt) porphyroblast is preserved near the bottom right of the image (AL-017, 140.7 m).

cores examined. The psammopelite is commonly migmatitic, containing millimetre- to centimetre-scale layers of tonalitic leucosome and narrow bands of biotite-rich melanosome (Figure 4A). Pelite is less common. It contains 2 to 3% elongate garnet porphyroblasts and 25% biotite (Figure 4B). No cordierite was documented. Graphite is absent or present in trace amounts. The pelite is also migmatitic but, relative to the psammopelite, contains a higher proportion of tonalitic melt leucosome, which forms centimetre-scale bands that are rarely dismembered (Figure 4B). Psammite is uncommon and typically interlayered with the other metasedimentary gneisses. It contains 1 to 2% garnet grains, typically no larger than 1 mm in size, in a fine-grained quartzofeldspathic groundmass that contains 5 to 10% biotite.

c) Younger Intrusions

Pegmatitic granite is present in most drillholes, regardless of host-rock type. The granites are typically two-feldspar syenogranites (Figure 5A) containing up to 5% biotite and rare pyrite, garnet and tourmaline. Plagioclase is typically metasomatised and replaced by epidote (Figure 5A). Prismatic, radiating epidote crystals (Figure 5B) are also presumed to be the product of plagioclase alteration. Many of the granites are silica rich with over 50% normative quartz (Figure 5C). The silica-rich varieties are commonly gradational into nearly massive quartz (Figure 5D), which typically cores the pegmatitic intersection.

d) Graphitic Rocks

The general interpretation for graphite-bearing units of the dominantly psammopelitic to pelitic Karin Lake formation (Yeo and Delaney, 2007) in the basement to the Athabasca Basin is that the protoliths were carbon bearing prior to prograde metamorphism. Organic-rich mudstones and black shales (e.g., Madore and Annesley, 1997) are the most commonly proposed protoliths. Other metasedimentary units that would not classify as pelites, i.e., containing >25% aluminosilicate minerals (e.g., biotite, garnet, sillimanite, cordierite in the classification of Maxeiner et al., 1999), are present and contain 5% or less graphite. Extremely graphite-rich units (25% or greater) are sporadically present. In many cases concentrations of graphite are thought to have formed via pressure solution, with the inferred removal of quartz and feldspar occurring during deformational events (e.g., Thomas et al., 2000). This mode of graphite concentration would be favoured where the rocks show signs of intense deformation.

Rocks with greater than 5% (volume) graphite are common in drillholes along the major uranium exploration trend (Figure 2) in NTS area 74H and many of these are correctly interpreted as metamorphosed carbonaceous sediments. However, rocks containing metasomatic graphite are also present. Rocks from pre-Wollaston basement, the Karin Lake formation and younger intrusions are either locally or commonly graphitic. For example, a wide intersection of metasomatised gneissic leucotonalite in drillcore P-052, with a similar texture and appearance to unaltered gneissic tonalite in drillcore WL-022 (Figure 3B), is graphitic between 119.5 and 129.5 m. In contrast to the weakly graphitic, weakly altered psammopelite and pelite described above, strongly graphitic varieties are pervasively metasomatised, and minerals such as garnet (replaced by chlorite), and fresh biotite are absent (Figure 6A). Up to 25% graphite is present and it appears to take the place of matrix biotite. In both the tonalite and the metasedimentary rocks, contacts between anomalously graphitic and non-graphitic rocks are diffuse or gradational. In most cases this transition is from rocks with preserved, albeit commonly altered, mica (likely originally biotite, but typically colourless), into rocks with a mix of matrix graphite and mica, and ultimately into graphite-rich rocks

GrtGrt

M

A B


Figure 5 – A) Metasomatised pegmatitic granite with plagioclase replaced by epidote. Vein quartz (Qtz) is also present in this section of core (GRL-133A, 308.6 m). B) Radiating, prismatic epidote crystals in pegmatitic granite (GRL-133A, 365 m). C) Metasomatised, silica-rich pegmatitic granite containing over 50% normative quartz (MW-007, 195.2 m). D) Massive quartz from a pegmatitic granite intersection with a centimetre-scale, green, crystal of altered plagioclase(?) and a smaller crystal of less-altered K-feldspar(?) (GRL-133A, 285 m).

without preserved matrix mica implying that mica provides a nucleus for graphite growth. Biotite and/or chlorite are only preserved in pseudomorphed porphyroblasts, presumed to have originally been garnet. Altered mica that appears dusted by fine-grained graphite is commonly noted. In drillholes GRL-133A and AL-015, late randomly oriented or radiating graphite is preserved in rocks of unknown protolith (Figures 6B, 6C, and 6D). Graphite is also common in late, pegmatitic segregations, particularly along crystal boundaries and in other planes of weakness. Graphite also replaces biotite in some metasomatised pegmatitic samples. In drillhole Al-015, an entire domain of original biotite at least 2 cm in long dimension appears pseudomorphed by graphite and the accompanying feldspars are strongly altered (Figure 6D). In other cases, graphite fills or partly fills late fracture sets or forms radiating patterns in the pegmatitic rocks (Figure 6E). The graphite in fractures, in particular, implies mobilisation of carbon well after the emplacement of the pegmatitic granites, although more than one emplacement event remains a possibility (Annesley and Millar, 2011).

e) Silica-rich Rocks

Thick intersections of quartz-rich rock in the basement to the Athabasca Basin are commonly referred to as quartzite in publications (e.g., Jefferson et al., 2007) and company reports and are typically interpreted as orthoquartzites. Orthoquartzites are bedded and commonly contain minor amounts of metamorphic minerals indicative of their original composition, for example muscovite and/or sillimanite in the case of aluminous varieties, and diopside in calcareous varieties. Feldspar may be recrystallized, but is still represented in metamorphosed feldspathic quartz arenites and heavy mineral laminae may be preserved as transposed layers rich in magnetite and/or other accessory minerals (Figure 7; Card et al., 2008). Recrystallisation accompanying metamorphism generally modifies the original clastic texture and grain size, but both the quartz and the associated mineral impurities tend to have grain sizes ranging from <1 to 2 mm (Figure 7). Although quartzite is present in the Wollaston supergroup, it is rare and

Qtz

A B

C D


Figure 6 – A) Metasomatised graphitic pelite. No ferromagnesian minerals (e.g., biotite, garnet) were identified in this specimen. The grey, metallic mineral is graphite, which is estimated to form 15 to 20% of the rock volume. The yellow mineral is altered feldspar (AL-018, 83.4 m). B) Randomly oriented graphite (silver) and 0.5 mm scale spots of fine-grained pyrite (Py) developed in a metasomatised granitoid(?) rock (AL-015, 130.3 m). C) Randomly oriented, millimetre-scale graphite (Gr) flakes (dark grey) in a metasomatised migmatitic rock of unknown protolith (GRL-133A, 318.9 m). D) Top core is an unaltered section of pegmatitic granite with centimetre-scale domains of biotite (Bt); the bottom is a metasomatised pegmatitic granite (Gr) where books of biotite were at least partly replaced by a centimetre-scale mass of near-monomineralic graphite and plagioclase is epidotised and chloritised(?). Core diameter is about 4.75 cm (top core is from AL-017, depth unknown; bottom core from AL-015, 130.8 m). E) Radiating graphite (Gr) associated with pyrite (Py) overgrowing pegmatitic granite. The country rock is also anomalously graphitic (GRL-133A, 326 m).

Py

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Figure 7 – Well-bedded sedimentary-derived quartzite crosscut by pegmatitic granite. Note the relatively small grain size. The pen magnet indicates the presence of magnetite-rich beds that likely originated as heavy mineral laminae on foresets (image from central Black Birch Lake, UTM 470194 m E, 6366982 m N).

the stratigraphic units likely to contain it, the Hidden Bay assemblage and Souter Lake group (Yeo and Delaney, 2007), are preserved only sporadically in the region. Orthoquartzite is also locally preserved to the west in the Mudjatik Domain (Figure 1; Card and Bosman, 2007).

Quartzite intersections in many drillcores are essentially massive quartz that appears coarse grained (Figure 8A) and are typically associated with quartz-rich pegmatitic granites of varying thickness (Figure 8B and 8C). Intrusive contacts are not developed between the quartzite intervals and the pegmatitic granites (Figure 8D), implying consanguinity. In other cases, quartzite contains a planar fabric that ranges from a spaced cleavage (Figure 8B) to a well-developed (shear?) foliation (Figure 8C). The massive quartzite locally grades into siliceous, layered rocks (Figures 8E and 8F). These represent silicified country rocks that have maintained their originally gneissic banding. In drillhole WR-185 (Figure 8E), these layered rocks can be traced into their relatively unaltered, pelitic equivalents. In other cases, such as in MAC-167, unaltered examples were not intersected and, therefore,

the layered rocks are more difficult to interpret. Given the massive nature of the quartz in this drillcore, however, the layered rocks likely represent inclusions of silicified country rock.

4. Discussion

a) Origin of Graphitic Rocks

Given that unaltered metasedimentary rocks of the region typically contain only 0 to 2% graphite, whereas rocks containing up to 25% graphite typically display evidence of intense post-peak metamorphic hydrothermal alteration, particularly those of metasedimentary origin, it seems logical that the extra graphite in the altered rocks is also hydrothermal. The carbon-bearing aqueous solutions necessary to facilitate this process are likely the product of metamorphic processes occurring at depth (Pattison, 2006). Most prograde metamorphic reactions are dehydration reactions, such as:

Ms + Qtz Als + Kfs + H2O 4,

which typically take place under upper amphibolite facies conditions at moderate pressures (Yardley, 1989). The aqueous fluid generated during the prograde process has the potential to consume graphite via the following reaction:

2C + 2H2O CO2 + CH4,

although it is clear that some metamorphic graphite is conserved in pelitic rocks throughout the regional metamorphic cycle if there was originally greater than 0.2% volume (Pattison, 2006). Further graphite is consumed at granulite facies via biotite melting by reactions such as:

Bt + Sil + Qtz ± Pl melt + Crd + Kfs.

The consumption of biotite generates elemental Fe that must be accommodated in the crystal lattices of the product minerals – cordierite in the example above. Biotite contains a much larger proportion of ferric iron (Fe3+) than cordierite and therefore reduction to the ferrous species (Fe2+) is necessary for the bulk of the iron to be consumed (Cesare et al., 2005). Graphite can help to facilitate this process during the redox reaction:

2Fe2O3 + C 4FeO + CO2 (ibid.).

4 Abbreviations in the equations: Ms = muscovite; Qtz = quartz; Als = one of the three aluminosilicate polymorphs (sillimanite, kyanite or andalusite); Kfs = K-feldspar; H2O = water; C = carbon; CO2 = carbon dioxide; CH4 = methane; Bt = biotite; Sil = sillimanite; Pl = plagioclase; Crd = cordierite; Fe2O3 = iron (3+) oxide; and FeO = iron (2+) oxide.


Figure 8 – A) Background image: section of massive to foliated quartz (Qtz) that grades into a narrow interval of metasomatised pegmatitic (Peg) granite; Inset: close-up of quartz-rich and metasomatised pegmatitic granite. The feldspar-rich part of the intersection is locally interrupted by domains of massive quartz (WR-185, UTM 334554 m E, 6307584 m N, no depth information available, image courtesy of Sean Bosman). B) Metasomatised pegmatitic granite cored by massive quartz. Note that contacts are gradational rather than intrusive and that within the black oval outline the quartz core is continuous with quartz in the granite. A spaced fracture cleavage (Cl) is developed in the upper row of core parallel to the dashed black line, (MAC-167, 427.8 m). C) Well-foliated and metasomatised, siliceous pegmatitic granite. The cross-cutting pink vein is quartz dominated and contains fragments of the host rock (MW-007, 94.5 m). D) A metasomatised, feldspar-rich section of pegmatitic granite that changes abruptly into massive quartz, although quartz is continuous across the boundary (MAC-167, 480.2 m). E) Massive quartz (Qtz) that grades into siliceous, layered rock (arrow) near the bottom of the image. These layered rocks likely represent silicified country rock of unknown protolith (WR-185, UTM 334554 m E, 6307584 m N, no depth information available, image courtesy of Sean Bosman). F) Silicified gneiss(?) in a quartz-rich intersection (MAC-167, 433.8 m).

Qtz

Qtz

Peg

ClB

C D

E

A

F


The foregoing reactions indicate that, as a result of the prograde metamorphism 5 of carbonaceous pelitic rocks, carbon-bearing aqueous fluids (C-O-H fluid) are likely products, in addition to graphite-bearing pelitic gneisses. The Karin Lake formation is apparently a suitable source environment to derive carbon, as graphitic pelites are a common constituent (Yeo and Delaney, 2007). In order to re-precipitate this carbon, the fluid would later need to become supersaturated during cooling. There are two scenarios under which this could occur: 1) a closed system, where there is a high fluid-to-rock ratio; or 2) an open system with a low fluid-to-rock ratio (Huizenga, 2011). In the first scenario, a high fluid-to-rock ratio is achieved by transporting fluid in fault or shear systems that can accommodate high volumes, and in which oxygen fugacity (fO2) is moderated by changes to the fluid rather than being influenced by the wall rocks. In an open fluid system, the fO2 of the fluid is controlled by a mineral buffering system, i.e., fluid chemistry is influenced by reactions with the rock it is invading, and consequently changes in fO2 cause variability in the ratio O/(O+H) (Huizenga, 2011). The typical hydrothermal graphite observed in this study is relatively coarse and crystalline. This type of graphite is typically formed at high temperatures, e.g., upper amphibolite or granulite-facies conditions (Pasteris, 1999; Foustoukos, 2012); however, highly crystalline graphite can also grow from lower temperature fluids, such as at the Borrowdale graphite deposit in the United Kingdom (Luque et al., 2009). In the latter case, the hydrothermal graphite must nucleate on a pre-existing mineral phase, such as metamorphic graphite (Pasteris, 1999), poorly ordered hydrothermal graphite or organic material (Foustoukos, 2012), or silicate minerals (Luque et al., 2009). It is likely that graphite was deposited over a range of fluid temperatures during the unroofing of the Wollaston supergroup after the peak of orogenesis in the Trans-Hudson orogeny (Rantitsch et al., 2004). Annesley and Millar (2011) proposed four to five major carbon-bearing fluid events over a span of ca. 50 million years in the region. Graphitised exploration drillcore indicate that these hydrothermal fluids were focussed near the Karin Lake formation/Archean unconformity in the southeast Athabasca Basin and therefore graphitisation was not restricted to rocks of the Wollaston supergroup.

Graphitisation via high-temperature metamorphic fluids likely began relatively close to the fluid source Karin Lake formation rocks (Henne and Craw, 2012; Huizenga and Touret, 2012) before the fluids began migrating through the crust and cooling during the late stages of metamorphism 6, likely aided by shear/fault systems. Closed fluid system graphitisation was possible in these conduits. Graphitisation in situ or in the wall rocks of faults or in situ during exhumation following the metamorphic peak occurred in an open fluid system with high initial O/(O+H). Graphite precipitation in an open fluid system is suggested as the origin for the Borrowdale graphite deposit in the United Kingdom (Ortega et al., 2010) and is the most likely process for forming apparent replacement textures such as those in Figure 6.

b) Origin of Silica-rich Rocks

None of the quartz-rich intersections observed in this study 7 are best interpreted as orthoquartzites. Most have one, but typically more, of the following features indicating late-stage quartz growth: 1) massive, non-foliated quartzite; 2) massive non-foliated quartzite that grades into quartz-rich pegmatitic granite; 3) foliated quartzite that grades into foliated, quartz-rich pegmatitic granite; and/or 4) silicified country rock adjacent to massive non-foliated or foliated quartzite (Figure 8). One option for the origin of the quartzite is a magmatic system related to the pegmatitic granites. In order to create the quartz-rich zones in igneous granites, low-viscosity, high-H2O melts must be generated during partial melting during regional metamorphism (Thomas and Davidson, 2012 and references therein). Such melts are not stable with declining temperature and pressure resulting in immiscibility and the production of two distinct melt fractions in addition to an aqueous liquid (ibid.). In the latter case, one of the resulting melt fractions is H2O-rich (>10%) and has a low viscosity (Thomas et al., 2000). Such melts tend to contain other volatiles, such as Li2O, B2O3 and P2O5, Fl, and Cl, and have the potential to act as solvents and consume trace elements as they migrate (ibid.) and may be elevated in carbonate (CO3

-2) (Thomas and Davidson, 2012). Thomas and Davidson (2012) state that fluid inclusions found in the quartz cores of pegmatitic granites suggest that carbonate-rich melts are a likely key in concentrating silica, with silica solubility increasing as a function of temperature. This is a potential source of the SiO2-rich melt necessary to produce the observed massive quartz cores in pegmatitic granites (Figure 8D).

An alternative fluid source might be the fluids responsible for the graphitisation described above. One of the consequences of the destruction of silicate minerals, such as biotite and feldspar during the deposition of metasomatic graphite, is a net loss of SiO2 in the altered rock (Galbreath et al., 1998). That SiO2 is presumably taken up by the hydrothermal fluids and could be re-precipitated due to changes in fluid properties leading to silicification of nearby rocks. One argument against this process is the relative immobility of Al2O3 in most geochemical systems, as it would be a necessary constituent to produce the feldspar components observed in most of the quartz-rich intersections (Figure 8).

5 Although it is acknowledged that most exposures of Wollaston supergroup contain upper amphibolite facies assemblages, granulite-facies conditions are likely to have persisted deeper in the crust. 6 The final significant metamorphic episode and associated pegmatitic granite emplacement in the eastern Hearne Province is ca. 1.815 Ga (e.g., Annesley et al., 2005). 7 Quartzite are found in drillcores 4560-1-82, GTB-82-33, MAC-167, MW-007, MW-010, P-052, and ZQ-010.


c) Synthesis

It appears that fluids and magmas produced during metamorphic processes in the hinterland to the Trans-Hudson Orogen had a profound influence on the basement rocks of the southeastern Athabasca Basin. Therefore, basement rocks associated with unconformity-related uranium deposits must be classified not only in terms of protolith, but also the alteration assemblages overprinting them. The relative timing between the various alteration events, which include epidotisation of plagioclase, graphite/sulphide precipitation, and the development of quartz-rich rocks remains unresolved. Although Harvey and Bethune (2007) mentioned the possibility of hydrothermal graphite in rocks in the Deilmann pit at the Key Lake mine, Annesley and Wheatley (2011) were the first to identify it in fault-zone rocks, although they suggested that the graphite postdated the Athabasca Group. Annesley and Millar (2011), however, hinted at the complexity of pre-Athabasca Group alteration in the eastern Athabasca Basin. They suggested that there were four to five episodes of carbon-sulphur-boron (CSB) geochemical cycling in the basement and that this cycling could be related to both pre- and post-Athabasca Group uranium deposition. In addition to the CSB fluid cycling necessary to produce the graphitisation and associated metasomatism observed in drill cores, it is suggested here that the quartz-rich rocks observed in the core are the products of relatively late H2O-enriched melts and/or SiO2-rich aqueous fluids.

It is also proposed that multiple pulses of CSB aqueous fluids are likely responsible for generation of the graphitised/sulphidised rocks. Although tourmaline was not observed in association with the rocks described here, late-stage tourmaline is common across the region. The B necessary for the CSB fluids could be derived from recycling of tourmaline-bearing pelites common in the Karin Lake formation (Annesley et al., 2005). Alternatively, the B may be introduced through mixing with magmatic fluids. A mixing scenario is suggested by Galbreath et al. (1988) for an association of hydrothermal graphite and tourmaline in the Black Hills, although no mechanism was offered.

With respect to the origin of the quartz ridges, traditional models invoke a clastic sedimentary origin with subsequent transformation into ridges by pop-up structures relating to outwardly diverging faults (Kyser and Cuney, 2009). The textures observed during this study clearly preclude a sedimentary origin and instead support a relatively late-stage magmatic or aqueous fluid origin, which is not a mode of origin that has been specifically discussed in the region to the author’s knowledge.

The specific role played by pre-Athabasca Group hydrothermal graphite and massive quartz deposits in terms of contribution to the overall ore system is equivocal. Furthermore, the role played by graphite in unconformity-related uranium deposition is speculative, although all of the largest deposits are directly associated with concentrations of it (Thomas et al., 2000). The scenario presented above provides no insight into the direct influence of graphite on mineralisation; however, it implies altered basement rocks are likely in the direct vicinity of the uranium mineralisation and that graphitised/suphidised alteration zones are being targeted during exploration. This suggests that there are corridors of alteration that are loosely associated with the Karin Lake formation/Archean unconformity that have the potential for enhanced permeability as well as the narrower planar faults normally assumed. Such corridors could provide access for oxidising fluids originating in the Athabasca Group to infiltrate a much larger volume of the basement complex, perhaps increasing the prospectivity for basement-hosted deposits. Alternatively, the permeability corridors could provide a volumetrically more significant pathway for reduced, basement-derived fluid that is necessary for the redox-related precipitation of uranium.

Quartzite ridges are typically thought to provide a competency contrast that localises faulting. Although this may be true, it seems a coincidence that high-grade uranium deposits (e.g., McArthur River, Phoenix, and the BJ zone), are closely associated with what might be quartzolites generated during pegmatitic granite emplacement. The most obvious benefit of a pegmatitic granite precursor is the potential for a local source of uranium. Together, a permeable graphitic fluid corridor and quartzolite could potentially become predictive for mineral exploration.

5. Conclusions A hydrothermal origin for both anomalously graphitic rocks and quartz ridges of the southeastern basement to the Athabasca Basin has been implied after mesoscopic mapping of basement drillcores from NTS area 74H. Rocks that contain anomalous concentrations (>5%) of graphite and do not always exhibit clear evidence of having sedimentary precursors should be carefully scrutinised with respect to their degree of alteration and their ferromagnesian contents. Quartzite drillcores should be assessed for features that might indicate that they had quartz arenitic protoliths. Massive quartz associated with pegmatitic granite is better interpreted as having a hydrothermal/ magmatic origin.


In order to understand the true extent of pre-Athabasca alteration in the basement to the southeastern Athabasca Basin the following exercises are necessary.

1) Mapping graphitic alteration systems through the reassessment of historic drillcore in order to understand the extent of hydrothermal graphite in the eastern Athabasca Basin.

2) Reconsideration of the origin of quartzite ridges, including an assessment of the volume of actual quartzite versus the massive quartzolite suggested in this report.

A better understanding of these alteration systems will allow researchers to determine what role they might have played in formation of the younger, post-Athabasca Group unconformity-related deposits.

6. Acknowledgements Cameco Exploration is thanked for hosting us at the Key Lake mine and for providing us the logistical support necessary to access the core at Kapesin Lake. Scott MacKnight provided able and cheerful assistance in the field and Scott and Stephanie Boulanger are thanked for their assistance at the subsurface laboratory in Regina.

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