au-pge-u and ree-th-u mineralization in altered tantato domain … · normand, c. (2016): au-pge-u...

Saskatchewan Geological Survey 1 Summary of Investigations 2016, Volume 2

Au-PGE-U and REE-Th-U Mineralization in Altered Tantato Domain Basement Gneiss, Stony Rapids Area, Saskatchewan

Charles Normand 1 Information from this publication may be used if credit is given. It is recommended that reference to this publication be made in the following form:

Normand, C. (2016): Au-PGE-U and REE-Th-U mineralization in altered Tantato Domain basement gneiss, Stony Rapids area, Saskatchewan; in Summary of Investigations 2016, Volume 2, Saskatchewan Geological Survey, Saskatchewan Ministry of the Economy, Miscellaneous Report 2016-4.2, Paper A-5, 25p.

Abstract Bedrock geological examination of Tantato Domain granulite facies metamorphic rocks that crop out along the south shore of the Fond du Lac River two kilometres west of Stony Rapids was undertaken to characterize known Au-PGE-U and REE-Th-U mineralization in the area. The high-grade metamorphic rocks are exposed in a series of low-lying outcrops that have been affected by regolithic alteration below the unconformity with unmetamorphosed Athabasca Group siliciclastic sedimentary rocks.

Au-PGE-U and REE-Th-U mineralization occur in close spatial association with the paleoregolith. The Au-PGE-U mineralization is part of a more extensive, narrow belt of similar Au-PGE-U–mineralized occurrences located in basement rocks along the northern edge of the Athabasca Basin between the Black Bay fault and the Cora Lake shear zone. One occurrence of Au-PGE-U mineralization was visited in 2016 and is hosted by an altered, bleached and oxidized zone developed in metagabbro/norite. Contact relationships between this mineralization (the mineralized basement rocks) and paleoregolith or Athabasca Group sediments were not observed. Similar alteration zones in the larger Athabasca Basin region have been assigned a hydrothermal origin postdating deposition of Athabasca Group siliciclastic rocks. The presence of a pre-existing thick overlying accumulation of non redox- and non pH-buffering sandstone on top of the basement, now apparently eroded at all reported occurrences, is believed to have been a requirement for the formation of Au-PGE-U mineralization.

The REE-Th-U mineralization occurs in paleoregolith, a few tens of metres north of the overburden-covered unconformity between basement and the Manitou Falls Formation. It represents the only known occurrence of this style of mineralization in the province. The mineralization consists of a small, gently dipping, quartz- and monazite-rich layer of an as yet undetermined nature in the paleoweathered basement. The paleoregolith exhibits a diagenetic foliation cut by numerous generations of fractures that extend into unaltered basement rocks. Alteration haloes observed in garnetiferous diatexite at the margins of some of these fractures near paleoregolith show an increase in eTh concentrations of up to 8.5 times that in the host rock, suggesting mobility of thorium during circulation of hydrothermal fluids of probable basinal origin. Calculated eU/eTh ratios of monazite mineralization and Th-rich alteration haloes associated with fractures are similar, suggesting a possible genetic link between the two.

Widely differing types of alteration and radiometric eU and eTh signatures between Au-PGE-U and REE-Th-U mineralization suggest that the two types of mineralization are unrelated.

Keywords: Tantato Domain, granulite, norite, tonalite, diatexite, paleoregolith, unconformity, gold, platinum group elements, uranium, rare earth elements, thorium, monazite

1. Introduction This report represents the third contribution from a multi-year project aimed at providing further understanding of the Tantato Domain metallogeny. It contains the results of fieldwork undertaken to document gold–platinum group element–uranium (Au-PGE-U) and rare earth element–thorium–uranium (REE-Th-U) mineralization close to a paleoregolithic zone developed in high-grade metamorphic rocks exposed along the northern margin of the Athabasca Basin.

1 Saskatchewan Ministry of the Economy, Saskatchewan Geological Survey, 1000-2103 11th Avenue, Regina, SK S4P 3Z8 Although the Saskatchewan Ministry of the Economy has exercised all reasonable care in the compilation, interpretation and production of this product, it is not possible to ensure total accuracy, and all persons who rely on the information contained herein do so at their own risk. The Saskatchewan Ministry of the Economy and the Government of Saskatchewan do not accept liability for any errors, omissions or inaccuracies that may be included in, or derived from, this product.


The area investigated is immediately south of the Fond du Lac River, approximately two kilometres west of Stony Rapids (Figure 1), and is easily accessed by boat. Bedrock exposure of >50% is present in two zones. The first and largest zone (outcrop zone 1, Figure 1) is oriented north-northwest and covers a surface area of ~280 000 m2 between Universal Transverse Mercator (UTM) coordinates2 449750E and 450490E (NAD83 datum, Zone 13). The second zone (outcrop zone 2, Figure 1) is exposed close to the shore of the river between UTM coordinates 449330E and 449485E.

Figure 1 – Location of the study area along the Fond du Lac River near Stony Rapids. Areas delimited by dotted lines in the box outlined in red indicate the locations of interest in this study, in which >50% of the bedrock is exposed. The white diamond symbol shows the location of the MacAskill monazite occurrence. The yellow square symbol shows the location of the Au-PGE-U occurrence examined as part of this study. Topographic contours are spaced 10 metres apart. An active rock quarry held by the Saskatchewan Ministry of Highways and Infrastructure is at the southern end of outcrop zone 1. The area outlined in red refers to the more detailed map in Figure 2. The inset map in the upper right corner of the figure shows the location of Stony Rapids near the boundary between the Tantato Domain (green) and the Athabasca Basin (light yellow) in the Precambrian Shield of northern Saskatchewan.

Work was focused on 1) description of bedrock lithology; 2) documentation of structures and associated alterations; 3) characterization of Au-PGE-U and REE-Th-U mineralization; and 4) verification of whether any relationship (association) exists between the two types of mineralization. Because the various types of mineralization investigated are radioactive, finding them in the field was greatly aided by using a handheld spectrometer3.

2 All UTM coordinates in this paper are in Nad83, Zone 13. 3 Model RS-230; Radiation Solution Inc.


2. Previous Work a) Au and PGE Mineralization Associated with Uranium Gold mineralization associated with anomalous Pd, Pt and U was recently identified in the Stony Rapids area by Navis Resources Corp. (named Star Minerals Group Ltd. at the time) and ALX Uranium Corp. (named Lakeland Resources Inc. at the time). According to company news releases and website project descriptions, the mineralization is associated with fracture zones characterized by strong hematite-chlorite alteration within an approximately 50-metre-wide northwest-trending envelope. Mineralized grab samples containing up to 5.7 ppm Au, 0.39 ppm Pd and 0.36 ppm Pt (Navis Resources Corp., 2014; ALX Uranium Corp., 2014a) were collected from outcrop zone 1.

b) REE-Th-U Mineralization The discovery of light rare earth element (LREE)–dominant mineralization in outcrop zone 1 (Figure 1) was first reported by Eldorado Nuclear Ltd./SMDC in 1976-1978 (their Gibbons Creek property; Olsson, 1976). Chemical analyses of bedrock samples collected during this time period returned a maximum of 4659 ppm total LREE (TLREE), including 2337 ppm Ce and 149 ppm Y (close to an equivalent 1% monazite), at the MacAskill occurrence4 (Figure 1), and three other samples containing between 554 and 664 ppm Ce (along line 11+00W; Ellerington, 1977). Possibly similar mineralization has recently been reported from the same area by ALX Uranium Corp. (2014a) and Navis Resources Corp. (2014). To date, this area remains the sole surface location where significant LREE mineralization5 has been identified in the Tantato Domain. The exact mode of occurrence, location and nature of the REE occurrences reported by ALX Uranium Corp. and Navis Resources Corp. have not been communicated, except for the mention of a sample reported to contain 23.6% monazite, with 16.75% kaolinite/illite clay (sample #79446; ALX Uranium Corp., 2014a; Navis Resources Corp., 2014). Such elevated kaolinite/illite clay proportions have not previously been reported from monazite-rich mineralization elsewhere in the province (Normand, 2014b, and references therein).

3. Local Geology The two outcrop areas examined in this study include granulite-facies metamorphosed igneous rocks altered to regolith below their unconformity-bound contact with unmetamorphosed Athabasca Group siliciclastic sedimentary rocks of the Manitou Falls Formation (Gilboy and Ramaekers, 1981). A description of the rock types, structures, alteration, radioactive anomalies and mineralization observed in the map area (Figure 2) follows.

a) Rock Types The bedrock in the study area is composed of rock types typically observed elsewhere in the upper deck (Hanmer et al., 1991) of the Tantato Domain along the north shore of the Fond du Lac River, with the exception of psammopelitic and granitic gneiss (Hanmer, 1997; Knox and Lamming, 2014; Normand, 2014a). Moderately to strongly foliated, homogeneous, coarse-grained, pinkish- to white-weathering garnetiferous diatexite that contains, on average, 15% ribboned quartz and 7% garnet (5 mm in diameter on average) is intimately associated with strongly foliated and ribboned quartzofeldspathic, tonalitic gneiss that weathers light grey. The tonalitic gneiss locally contains up to 25% medium-grained, retrograde biotite. The most common rock types are fine- to coarse-grained, foliated and lineated, homogeneous to layered, 2.62 to 2.60 Ga (Williams and Hanmer, 2006) mafic granulites of gabbroic/noritic to anorthositic composition (Hanmer et al., 1995). The gabbroic/noritic granulites contain 25 to 45% crystals of originally mafic composition that are on average 3 mm in diameter and are now recrystallized to a fine-grained mineral assemblage (Kranck, 1955; Kopf, 1999), with less than 5% quartz of probable metamorphic origin. Melanocratic

4 This sample was collected from an old trench (designated #2; Ellerington, 1977) at the “MacAskill showing” (MacAskill, 1959), as designated by Ellerington (1977). Following Rogers and Hart (1995) procedural guidelines for qualitative mineral potential evaluations, the mineralized location should be termed an occurrence. The name MacAskill monazite occurrence has been adopted in this paper to designate the location where monazite was identified as the bearer of the REE during the field work presented here. 5 Where reported, concentrations exceed minimum grade requirement to qualify as an occurrence of ≥0.5 wt. %, or 1% equivalent monazite; Rogers and Hart (1995).


layers and patches of coarse-grained, pyroxene-garnet granulite up to 60 cm thick are common in this unit. Crosscutting relationships indicate that the gabbroic/noritic to anorthositic and melanocratic granulites are younger than the garnetiferous diatexite and tonalitic gneiss, consistent with relationships reported by Hanmer (1997), Knox and Lamming (2014) and Ashton (personal communication, 2016). Granitic pegmatites less than 60 cm thick occur sporadically. They are locally zoned, with portions composed almost exclusively of quartz.

Measurements of magnetic susceptibility6 indicate that the melanocratic layers and patches within the gabbroic/noritic to anorthositic granulites are the most magnetic rocks, followed by the gabbroic/noritic rocks, the garnetiferous diatexite, and finally the tonalitic gneiss (Table 1). Data obtained from handheld gamma ray spectrometer measurements on outcrops of the gabbroic/noritic and melanocratic granulites, and garnetiferous diatexite, are presented in Table 2. The spectrometric data suggests that the mafic granulites contain half the amount of potassium (K) and two times the amount of equivalent thorium (eTh) held in garnetiferous diatexite.

Table 1 – Descriptive statistics of magnetic susceptibility measurements (10-3 SI units) of melanocratic granulites, gabbroic/noritic to anorthositic granulites, garnetiferous diatexite and tonalitic gneiss.

Melanocratic

Granulite Gabbroic/Noritic and

Anorthositic Granulite Garnetiferous

Diatexite Tonalitic Gneiss Paleoregolith

Mean 2.54 0.48 0.29 0.10 0.06 Standard Error 0.41 0.02 0.05 0.02 0.01 Median 1.84 0.48 0.19 0.08 0.06 Mode 1.85 0.31 0.16 0.08 0.04 Standard Deviation 2.38 0.16 0.36 0.06 0.04

6 Measurement of magnetic susceptibility was carried out using a handheld magnetometer (Exploranium® Kappameter KT-9 instrument).

Figure 2 – Location of the outcrop areas examined in this study (surrounded by dotted lines), including bedrock geology, approximate location of paleoregolith, structures and radioactive occurrences (see text). The area outlined in red represents the detailed map area covered in Figure 8.


Melanocratic

Granulite Gabbroic/Noritic and

Anorthositic Granulite Garnetiferous

Diatexite Tonalitic Gneiss Paleoregolith Sample Variance 5.69 0.03 0.13 0.00 0.00 Kurtosis 8.24 0.30 20.05 1.12 5.77 Skewness 2.56 0.40 4.14 1.43 1.71 Range 12.07 0.81 2.26 0.18 0.20 Minimum 0.33 0.17 0.03 0.04 0.00 Maximum 12.40 0.98 2.29 0.22 0.20 Sum 86.32 27.68 14.19 1.03 1.91 Count 34 58 49 10 32 Table 2 – Median values of handheld gamma ray spectrometer data from mafic granulites (gabbroic/noritic and melanocratic granulites), garnetiferous diatexite and paleoregolith/saprolith. N = number of readings; eU = equivalent uranium; eTh = equivalent thorium.

Rock Type Dose Rate (nSv/hr) K (%) eU (ppm) eTh (ppm) eU/eTh

Garnetiferous diatexite (N=11) 26.5 (20.9, 69.9)* 1.2 (0.9, 2.3) 0.5 (0.4, 1.5) 1.4 (0.6, 12.1) 0.36 (0.05, 1.33) Mafic granulites (N=16) 23.2 (16.0, 36.8) 0.7 (0.5, 1.2) 0.45 (0.2, 1.1) 2.6 (0.9, 5.1) 0.21 (0.08, 0.44) Paleoregolith (N=7) 32.1 (20.3, 87.7) 0.5 (0.4, 1.9) 3.5 (2.0, 7.5) 0.3 (0.1, 2.8) 9.00 (2.68, 24.00) * Values in brackets represent minimum and maximum values, respectively.

b) Paleoregolith Relatively large exposures (~20 to 40 m2) of thickly developed paleoregolith only occur near the base of the southwestern edge of outcrop zone 1. Very few small, thin patchy remnants (<1 m2) of paleoregolith have been preserved from Quaternary erosion and weathering higher up on outcrop zone 1, and occur widely scattered. No regolith was observed in outcrop zone 2.

The paleoregolith is a fine-grained, greenish grey, red, brown or beige, shale-like rock essentially composed of phyllosilicate minerals that display a penetrative, subhorizontal foliation that is absent in the gneiss. The subhorizontal foliation is interpreted to have developed in the altered, regolithic basement during diagenesis. It is attributed to the vertical stress imposed by the thick column of Athabasca Group sediments that had accumulated on top of the unconformity. The terminology employed by Passchier and Trouw (1996) to designate this type of foliation, i.e., diagenetic foliation, is used in the rest of this paper. At one location, a shallow-dipping (N272/30°), medium- to coarse-grained, quartz-rich layering is also observed in the regolith. The nature of this layering could not be ascertained in the field. It may represent a preserved, relict gneissosity in the precursor granulites, in which case a significant proportion of the altered rocks could represent a paleosaprolith assemblage (saprolite + saprock, following the terminology of Eggleton, 2001; see also Macdonald, 1980), or a coarse-grained sedimentary deposit, in which case the rock would represent a product of mechanically reworked, weathered, regolithic granulite. In the study area, these rocks were interpreted in part by Clark (1978) to represent mudstone deposited in a low energy and moderately oxidizing environment at the same time as the regolith was forming.

Based on diamond-drill logs presented in Currie (1980), the Athabasca Group sedimentary rocks south of outcrop zone 1 overlie the gneiss and paleoregolith along an unconformity that dips southward at an estimated 3°. Such low-relief topography is known to favour development and preservation of regolithic alteration profiles (Butt et al., 1997). Paleoregolith has been recognized in the area in zones up to 50 m thick at the base of the Manitou Falls Formation (Gilboy and Ramaekers, 1981). Macdonald (1980) and Wilson (1986) proposed that the material contained in the basal sedimentary sequences of the Athabasca Basin was derived from erosion of the paleoregolith. Yeo et al. (2001) suggested that the paleoregolith below the Athabasca Basin formed by the development of a lateritic profile in an arid climate. By contrast, Bray et al. (1987, 1988) suggested that, based on Ar isotopic data from illites in


basement, sandstone and “regolith”, the alteration of the bedrock below the unconformity of the Athabasca Basin may have been caused, in part or wholly, by diagenetic processes after deposition of the Athabasca Group sediments.

As would be expected from such oxidized rocks, the magnetic susceptibility of the paleoregolith assemblage is the lowest of all rock types encountered in the study area (Table 1). The paleoregolith assemblage is enriched in uranium with respect to the underlying gneiss (Table 2) by a factor of ~7 to 8. Spectrometer measurements also show that the paleoregolith assemblage is depleted in Th relative to the underlying gneiss: 0.3 ppm eTh compared to 1.9 ppm eTh (median value of all 27 values of granulite facies metamorphosed rocks; Table 2), respectively (Figure 3A). In addition, the paleoregolith assemblage shows much higher calculated eU/eTh ratios (9.0 vs. 0.2). Radiometrically measured potassium concentrations in the paleoregolith are significantly lower than those measured in the underlying gneiss (Table 2). Based on the information available, the nature of this uranium enrichment (whether the result of regolith development, later diagenetic processes accompanying the evolution of overlying Athabasca Group sediments, or subsequent hydrothermal activity) remains uncertain (see also Macdonald, 1980; Tremblay, 1982, 1983). In recent years, it has been proposed that the uranium contained in the unconformity-related uranium deposits of the Athabasca Basin has been derived from such diversified sources as Athabasca Group sediments and basement rocks, including the paleoregolith underlying the unconformity (Cuney, 2005).

Figure 3 – Relationships between radiometrically measured U and Th concentrations in A) mafic granulites, garnetiferous diatexite and granitic pegmatite, Th-rich patchy anomalies in granulite-facies rock types, weathered granulite-facies rock types (paleoregolithic assemblages), Th-rich fracture zones and monazite-rich paleoregolith; and B) granulite-facies rock types, weathered granulite-facies rock types (paleoregolithic assemblages), and U-rich fracture zones and U anomalies in paleoregolith. The area in green shows the eU and eTh signature of monazite-rich occurrences in the Beaverlodge and Zemlak domains (Normand, 2010, 2011, 2012, 2014b). The black square symbol in A) represents average chemical analysis of nodular monazite-rich mudstones in south-central Wales (Smith et al., 1994). Black vertical and horizontal lines represent the one standard deviation errors on Th and U, respectively, of the data presented in Smith et al. (1994).


c) Structures The garnetiferous diatexite and tonalitic gneiss exhibit an early penetrative foliation (S1) expressed by extreme ribboning of quartz that also parallels contacts and layering in gabbroic/noritic to anorthositic and melanocratic granulites. The S1 foliation dips predominantly north-northeast at a shallow angle of ~36° (Figure 4). Steeply dipping S1 foliation planes are present only very locally. A second penetrative foliation (S2) is also present and is best developed in the gabbroic/noritic granulites, where it is manifest by the alignment of flattened and lineated, recrystallized mafic crystals. The second foliation is represented by local recrystallization of quartz ribbons parallel to S2 in garnetiferous diatexite and tonalitic gneiss.

A third, mild deformation event, D3, that affected both the S1 and S2 foliations has resulted in the formation of metre- to decametre-scale, open F3 folds. The best fit great circle calculated through the poles of S1 and S2 foliations shown in Figure 4 suggests that D3 deformation of the foliations produced F3 folds with axes that plunge shallowly to the east.

Brittle-ductile and brittle structures that formed following a rapid uplift of the high-grade gneisses of the Tantato Domain after 1.85 Ga (Mahan et al., 2006) include shear zones, faults, fractures and veins. The shear zones vary in style from mylonitic to cataclastic through zones of intense, submillimetre-spaced fractures to mildly deformed zones with centimetre-spaced fractures developed over strike lengths of 30 m or more. All shear zones dip steeply (69° to 89°), with one exception (39°), to the south-southwest or the north-northeast. They strike N287° to N299° or N097° to N111° and may be associated with the D3 event. Breccias were produced locally where dense sets of anastomosing shears and fractures developed.

A wide array of fracture generations, along which displacement is unrecognizable, cut both granulite facies metamorphic rocks and paleoregolith derived from weathering of the granulites. They are shown in the rose diagrams in Figure 5. The presence of fractures that cut the diagenetic foliation in the paleoregolith indicates that they developed after a thick accumulation of Athabasca Group sediments had taken place on top of the unconformity.

Steeply dipping mode II shear fractures (Pollard and Segall, 1987; Scholz, 2002) were observed where they cut a marker (usually metamorphic layering that defines S1), producing an apparent offset. Fractures along which slip was recognized are rare. Mode II fractures that strike north-northwest or south-southeast and cut layered basement rocks produce apparent sinistral displacements of mafic layers of up to 5 cm. Mode II fractures that strike west-northwest or east-southeast and cut layered basement rocks produce

Figure 4 – Equal area stereographic projection showing the distribution of the poles of measured S1 (black filled circles) and S2 foliations (white filled circles). The foliations were affected by a late folding event, F3, producing gentle waving of S1 and S2 with open fold axes that plunge shallowly to the east. The calculated pole of the best fit great circle (N173/83°) through 28 poles of S1 planes and 6 poles of S2 planes plunges 7° at N083 (πS1-S2; black square). Black x symbols represent measured intersection lineations between S1 and S2. The grey square with a white border represents the pole of a measured bedding plane or relict gneissosity in paleoregolith (see ‘Paleoregolith’). Contouring of S1 poles (only) is for 1% area at intervals of 3%. All stereonet calculations presented in this paper were produced using the program Stereonet, version 9.3.1 (Allmendinger et al., 2013; Cardozo and Allmendinger, 2013).


apparent dextral displacements of mafic layers of up to 7 cm. The two sets are possibly conjugate, resulting from northwest-southeast shortening. Slickenlined fracture surfaces were not observed.

Figure 5 – Rose diagrams showing the orientation of fractures measured in A) granulite facies metamorphic rocks, and B) paleoregolith.

Three groups of mode I extensional (Pollard and Segall, 1987; Scholz, 2002), and other possibly hybrid shear-extensional or reactivated, fractures were recognized. The first and oldest group is comprised of steeply northeast-dipping (75° on average) individual fractures, or fracture zones up to 10 cm wide, with a strike of N335° to N345°. These fractures and fracture zones are cored by dolomite, forming veins up to 2 cm wide. Weak to strong, red hematitic staining of the wall rock is commonly observed at the margins of these fractures and fracture zones. Fractures of the first group were only observed within a distance of approximately 20 m from the shore of the Fond du Lac River. No anomalous radioactivity was recorded from these fractures.

The second group of fractures is characterized by a dark, biotite-rich filling and are <5 mm wide. These fractures are intimately associated with the shear zones oriented at ~N290° to N300°, which they parallel or cut at highly variable angles. Narrow, <2 mm wide, white feldspathic alteration haloes are generally present, extending from both sides of the fractures into the wall rock. As for group 1 fractures, no anomalous radioactivity was recorded associated with these fractures.

The third group of fractures are spatially associated with patches of paleoregolith. They are typically quartz filled and have silicified margins with serrated edges (Figure 6). Their dip is moderate to steep (40° to 90°), and their orientation is variable: north-northeast to northeast, east-southeast and north-northwest. Red staining by very fine-grained hematite is common in the alteration haloes. These fractures are also usually not radioactive.

At one location (UTM 449946E, 6569747N), however, wide, reddish brown alteration haloes 3 to 10 cm wide have developed at the margins of crosscutting fractures exposed along the southwest-facing margin of an outcrop of garnetiferous diatexite. This set of fractures is illustrated by the photographs in Figure 7 (fractures are described in Table 3). Where the fractures are closely spaced, the alteration haloes have merged to form much wider alteration zones. These haloes disappear gradually northward along the fractures up the south-sloping face of the outcrop, and presumably away from fluid pathways connected to the paleoregolith (circle 1 in Figure 7) and, by inference, the unconformity. These haloes are radioactive and enriched in radiometrically measured equivalent thorium (eTh) and uranium (eU) concentrations by up to 8.5 and 6.7 times that in the garnetiferous diatexite host, respectively. The thorium-rich fractures numbered 7 and 8 in Figure 7 are oriented parallel to one of the most prominent sets of fractures present in both granulite facies metamorphic rocks and paleoregolith, striking between N300° and N320° (28% of all fractures). Where the fractures intersect rusty brown paleoregolith (circle 1 in Figure 7), uranium enrichment is much more pronounced: up to 26.5 times that in the garnetiferous diatexite host (26.5 vs. 1 ppm eU). The source of the elevated thorium in the haloes may be monazite.


Figure 6 – Outcrop photo showing the relationship between the fractures, granulite facies gneiss and a thin overlying veneer of paleoregolith (PR). Black arrows point to fractures with strong, serrated alteration haloes immediately adjacent to the paleoregolith. These alteration haloes are probably present below the paleoregolith patch, at the interface with granulite facies gneiss.

Figure 7 – A) Outcrop photo showing alteration haloes developed at the margins of narrow quartz-filled fractures (features numbered 2 to 8) in garnetiferous diatexite. All the alteration haloes show enrichment in thorium and uranium relative to the garnetiferous diatexite (Table 3). A thin cover of paleoregolith occurs on the southwest-facing side of the outcrop (feature numbered 1). B) A close-up view showing in more detail the relationships between garnetiferous diatexite, fractures and alteration haloes around features 3, 4, 5 and 6. Compass lid indicates north in both photographs.


Table 3 – Description and radiometric data from numbered features indicated in Figure 7.

Feature # ― 1 2 3 4 5 6 7 8 1―4 1―6 4―5

Description Garnetiferous diatexite Paleoregolith Fracture Fracture Fracture Fracture Fracture Fracture Fracture Intersection Intersection Intersection

Orientation ― ― 071/48 315/72 010/72 073/73 080/71 304/75 310/76 ― ― ―

Dose Rate (nSv/hr) 69.9 n.m. n.m. n.m. n.m. n.m. n.m. 386.1 327.2 307.3 317.9 192.9

K (%) 1.5 n.m. n.m. n.m. n.m. n.m. n.m. 2 1.8 3.2 3..4 2.3

eU (ppm) 1 n.m. n.m. n.m. n.m. n.m. n.m. 2.8 6.6 26.1 26.5 6.7

eTh (ppm) 12.1 n.m. n.m. n.m. n.m. n.m. n.m. 102.9 78.4 26.3 27.6 34.4

eU/eU°* 1 ― ― ― ― ― ― 2.8 6.6 26.1 26.5 6.7

eTh/eTh°* 1 ― ― ― ― ― ― 8.5 6.5 2.2 2.3 2.8

eU/eTh 0.08 ― ― ― ― ― ― 0.03 0.08 0.99 0.96 0.19

*: eU° and eTh° are background measurements from bedrock (garnetiferous diatexite). n.m.: not measured.


At all locations where such alteration haloes are present in basement rocks at the margins of narrow fractures, the latter are always located near paleoregolith, suggesting the possibility that downward-flowing basinal fluids were responsible for this alteration. The limited extent of the haloes along the fractures (probably less than one or two metres in a direction normal to the plane defined by the paleoregolith―basement rock interface) suggests that irreversible reactions were quickly neutralized by the gneiss, and fluids became unreactive along their sustained flow path.

d) Radioactive Anomalies Handheld spectrometer-measured background radioactivity of bedrock outcrops in the map area, expressed in terms of dose rate, varies between 16 and 88 nSv/hr. Locations where radioactivity exceeded a dose rate of 100 nSv/hr were considered anomalous. The radioactive anomalies were caused by geological features that are enriched in either thorium, uranium, or both.

Four types of thorium-rich anomalies (eU/eTh <1; Figure 3A) were identified. A first type of thorium-rich radioactive anomaly, with eTh concentrations up to 302.1 ppm (asterisk symbols in Figure 3A), corresponds to thin, subhorizontal lenticular patches that are less than 10 cm in diameter and are associated with intimately interlayered gabbroic granulite and garnetiferous diatexite. The nature of the radioactive material at these anomalies has not been determined. They have an unusually low radiometric U signature that is significantly less than that of monazite-rich occurrences in the Beaverlodge and Zemlak domains (Normand, 2010, 2011, 2012, 2014b). They may represent altered monazite accumulations of unknown origin, from which uranium was stripped out during an alteration event, leaving a thorium-enriched residue. The eU/eTh ratios calculated at these anomalies are very low (<0.006).

Granitic pegmatites constitute a second type of thorium-rich radioactive anomaly, where eTh concentrations of up to 251.7 ppm were recorded (light green circle symbol in Figure 3A). The phase or phases that contain the thorium in these pegmatites has not been identified.

Fractures illustrated in Figure 7 (see also Table 3) constitute a third type of thorium anomaly (white square symbols in Figure 3A). The radiometric data suggests that the anomalous thorium was carried by the fluid(s) responsible for development of their associated alteration haloes, as mentioned in the previous section. A monazite-rich layer in paleoregolith (white diamond symbols in Figure 3A) constitutes the fourth and most interesting type of thorium-rich anomaly and is discussed in detail in ‘Monazite-rich Mineralization at the MacAskill Occurrence’. The eU/eTh ratios calculated at the monazite-rich paleoregolith occurrence (~0.1) are significantly greater than those calculated from monazite occurrences in the Beaverlodge and Zemlak domains (0.03 ±0.01; Figure 3A; Normand, 2010, 2011, 2012, 2014b).

Two types of uranium-rich anomalies (eU/eTh >1; Figure 3B) were identified. These anomalies were invariably encountered where strong hematitic alteration occurs and paleoregolith is present nearby. From one such location (UTM 450009E, 6569637N), a grab sample (# 98942) was reported by ALX Uranium Corp. (2014b) to contain 3.7 ppm Au, 0.22 ppm Pd+Pt and 177 ppm U. Observations made at this location are presented below in ‘U-rich Radioactive Anomalies and Au-PGE-U Mineralization’. Another location of interest corresponds to a highly altered zone at the interface between paleoregolith and basement gneiss (yellow diamond symbol in Figure 3B). At this location (UTM 449934E, 6569674N), concentrations of 2.9% K, 35.5 ppm eU and 0.8 ppm eTh were obtained (calculated eU/eTh ratio: 44.4) on heavily altered gneiss containing fine-grained red hematite, specular hematite and an emerald green mica. This assemblage is consistent with alteration at temperatures more consistent with diagenetic or hydrothermal processes in the Athabasca Basin than with near-surface weathering.

Where fractures intersect paleoregolith in Figure 7, spectrometer measurements reach 3.4% K, 26.5 ppm eU and 27.6 ppm eTh (calculated eU/eTh ratio: ~1; Table 3).


4. Au-PGE-U and REE-Th-U Mineralization a) U-rich Radioactive Anomalies and Au-PGE-U Mineralization A sample map by ALX Uranium Corp.7 was used to locate Au-PGE-U mineralization in the field. Some of the sampling sites were not found, likely due to recent westward expansion of the quarry located at the southern extremity of outcrop zone 1 (Figure 1).

Only one of the locations was found with certainty and verified in the field by the presence of marked flagging tape. This location was the source of Au-PGE-U–mineralized grab sample # 98942 reported by ALX Uranium Corp. in 2014 (ALX Uranium Corp., 2014b). The occurrence is hosted in a medium-grained, salt-and-pepper–textured, highly strained, gneissic metagabbro/norite containing 30 to 35% equant pyroxene and 15% foliation-parallel mafic bands 1 to 10 mm wide and 8 to 20 cm long. The strong, slightly undulating S2 foliation exhibited by the rock is oriented ~N308/51° to N310/41°. A handheld spectrometer assay of this rock returned the following: 2.3% K, 0.4 ppm eU and 0.9 ppm eTh. The mineralized area consists of a locally rusty and red hematite-stained, strongly bleached, poorly defined zone <3 m in length and 10 to 15 cm in width, and roughly oriented N309/24°. A handheld spectrometer assay collected from the strongly altered zone returned 2.4% K, 50.9 ppm eU and 1.6 ppm eTh. The calculated eU/eTh ratio of 31.8 is similar to that calculated from measured eU and eTh concentrations at the occurrence where the specular hematite + green mica assemblage was observed (see ‘Radioactive Anomalies’). The alteration zone at this location most likely represents a hydrothermal feature, not paleoweathering along a fracture. The formation of similar bleached zones that crosscut paleoregolith and basement along fracture zones elsewhere in the Athabasca Basin has been attributed to hydrothermal events that postdate deposition of Athabasca Group sandstones (Wilson, 1986).

The Au-PGE-U mineralization reported in the Stony Rapids area by ALX Uranium Corp. and Navis Resources Corp. is significant, as it adds further evidence for continuity of one or more Au-PGE-U mineralization event(s) that affected the area between the Black Bay fault to the west and the Cora Lake shear zone to the east. See ‘Discussion’, below, for more details.

b) Monazite-rich Mineralization at the MacAskill Occurrence REE mineralization reported by Ellerington (1977) from trench #2 (using the trench numbering scheme in Ellerington, 1977) at the MacAskill occurrence was inspected in detail and sampled (Figure 8). Mineralization consists of monazite occurring in a short, narrow zone exposed in paleoregolith on the south wall of trench #2. Mineralization of this type could not be located anywhere else in the study area. Preliminary field examination revealed that the monazite-bearing layer contains a high proportion of zone-parallel quartz stringers, is less than 10 cm thick, and dips westward at a low angle, from the top of the eastern end of the south wall of the trench. The layer can be followed westward down this wall towards the bottom of the trench for a short distance. Monazite crystals are most abundant at the eastern end of the mineralized layer, near the top part of the trench. The proportion of monazite in, and radioactivity of, the layer decreases abruptly to the west along the southern wall of the trench. A handheld spectrometer measurement on the most radioactive area returned 2.3% K, 93.2 ppm eU and 958.5 ppm eTh (white diamond symbol in Figure 8).

The monazite occurs as dark reddish brown individual grains generally <3 mm in diameter, and granular masses up to 2 cm in diameter. However, one large individual subhedral crystal 1 cm in diameter was observed. The diagenetic foliation in the paleoregolith at and near the contact with this monazite crystal presented a ‘fish-like’ texture, being deflected and partially wrapping around the monazite in a manner similar to what can be observed around pyrite nodules in shale and slate. Development of this texture, however, is not necessarily diagnostic of a unique process. The monazite may have been present in the rock before regolith development, or was deposited or grew in situ during or after formation of the regolith.

7 The sample location orthophoto can be viewed through Rockstone Research (2014).


Figure 8 – Bedrock geology in the area of the MacAskill monazite occurrence. The monazite occurrence (white diamond symbol), located in trench # 2 (T2) using the numbering scheme in Ellerington (1977), is hosted in paleoregolith (light grey) derived from weathering of granulite facies metamorphosed rocks of the Tantato Domain (medium grey). The unconformity with Athabasca Group siliciclastic sedimentary rocks (Manitou Falls Fm.) is located a few tens of metres south of the trench (trace of the unconformity is from the Geological Atlas of Saskatchewan; http://www.infomaps.gov.sk.ca/website/ sir_geological_atlas/viewer.htm).

The calculated radiometric eU/eTh ratios at the monazite-rich paleoregolith occurrence (0.09 to 0.21) compare reasonably well with those calculated from radiometric measurements obtained on the alteration haloes developed at the margins of the fractures shown in Figure 6 (0.03 to 0.19; Figure 3A). It is possible that the thorium enrichment associated with the fractures and the monazite occurrence is a product of diagenetic or hydrothermal activity that involved similar fluids that were active at the same time. The calculated radiometric eU/eTh ratios at the monazite-rich paleoregolith occurrence (~0.1) also compares well with the U/Th ratio calculated from average chemical analysis of nodular monazite-rich mudstones in south-central Wales (Figure 3A; ~0.1; Smith et al., 1994). Monazite of authigenic origin throughout the world, however, shows a very wide scatter in calculated U/Th ratios that can vary between 0.0003 and >60, making this criterion non diagnostic for the identification of authigenic monazite.

5. Discussion a) Regolithic Alteration The significance of the consistently low thorium concentrations in the paleoregolith relative to those in the basement gneiss protolith is difficult to determine, and interpretations necessarily depend on what the real nature of the material is, i.e., saprolithic or regolithic. As mentioned above, if the material represents a reworked product, (i.e., a regolith or possibly a mudstone), the low thorium concentration in the material could have resulted from mechanical separation


of the host minerals, which are usually dense and resistant to weathering (e.g., monazite) during transport. Assuming that the rocks represent paleosaprolith, that thorium remained immobile during saprolith development and that the volume did not change significantly (e.g., Shakotko, 2014), even after development of the diagenetic foliation, the density of the saprolith would have to have been decreased to roughly 16% that of the gneiss precursor. This is clearly impossible. Thorium would have to have been mobile in this case. For this element to achieve significant solubility in aqueous media (>1 ppb and up to ppm levels) in a weathering or hydrothermal environment, however, unusually acidic conditions (pH <4) are required and solubility is greatly enhanced by the presence of ligands, such as sulphate and phosphate (Langmuir and Herman, 1980; Neck et al., 2003; Ahmed et al., 2012). An indication that thorium was mobile during or after regolith development is clearly shown by the Th-enriched alteration haloes at the margins of the fractures shown in Figure 7.

b) Au and PGE in Hydrothermal Uranium Mineralization in Saskatchewan The association of gold and platinum group elements (essentially Pd and Pt) with hydrothermal uranium mineralization in Saskatchewan was first reported from the Nicholson Bay mine (Robinson, 1950), near the north shore of Lake Athabasca (Figure 9, Table 4), 16 km southeast of Uranium City. Hawley and Rimsaite (1953) reported phenomenal concentrations of up to 2242 ppm Au, 443 ppm Pd and 73 ppm Pt in uranium ore from the deposit. Exploratory work aimed at evaluating the Au-PGE economic potential of the deposit between 1987 and 1988 confirmed the presence of substantial amounts of the precious metals in the deposit. Significantly, Au and PGE were found to be closely associated with zones of intense fracturing, pitchblende-carbonate mineralization and hematitization (Mason, 1987; Hulbert, 1990). Remarkable Au and Pd+Pt grades were encountered in diamond-drill hole intersections during this time at the Nicholson Bay mine No. 2 deposit (e.g., 18.2 ppm Au, 8.67 ppm Pd and 2.05 ppm Pt in a 6.096-m-long intersection in diamond-drill hole MCD-5; Mason, 1987). Hulbert (1990) reported that strongly anomalous concentrations of Tl, Se, As, Bi and Cl accompany the Au-PGE-U mineralization. The latter also reported a very complex paragenetic mineral assemblage hosting the Au and PGE, including oosterboschite [(Pd,Cu)7Se5], froodite [α-PdBi2], stibiopalladinite [Pd5Sb2], native gold and electrum. According to Hulbert (1990), most of the platinum at the locality is present in solid solution in Bi selenides, cosalite [Pb2Bi2S5] and guanajuatite [Bi2Se3]. During 1987 and 1988, other Au-PGE-U mineralization was identified in the same general area, including the Quartzite Ridge8 and Missy9 occurrences (Figure 9, Table 4), which show the same spatial relationship between Au-PGE and intense fracturing and hematitization.

Gold and PGE associated with uranium are reported from 11 other occurrences in Saskatchewan (Figure 9, Table 4). In some uranium deposits, only Au is reported, locally in proportions that could potentially be economical as a secondary commodity: 8,090 ounces of gold was recovered from high-grade uranium leach residue (58 ppm gold) at the Cluff Lake D zone (SMDI 1150a) between 1987 and 1988 (AMEC Americas Limited, 2014). Gold at the Cluff Lake D zone was present in native form and in calaverite [AuTe2], occurring in uranium mineralization hosted mostly in pelitic basement rocks (AMOK Canada Limited, 1974). An additional 14 Au-U occurrences in, and on the periphery of, the Athabasca Basin are also presented in Figure 9 and Table 4.

In most of these occurrences, Au-U and Au-PGE-U mineralization is hosted predominantly in basement rocks. In the Shea Creek area, however, native gold, occasionally associated with Bi-tellurides, was identified in samples from Athabasca sandstone as well as basement rocks (UEX Uranium Corp., 2009). Perhaps significantly, all of the occurrences in which PGE are reported occur north of the present extent of the Athabasca Basin (Figure 9). Analyses for PGE in rocks from deposits located to the south of this zone are not reported (and possibly have not been analyzed). There are no obvious explanations as to why PGE should be absent from deposits south of the northern margin of the Athabasca Basin. Is it that fertile sources of fluid or rocks necessary for the PGE were not represented? Is it that all PGE-bearing mineralization is older, albeit only slightly, than unconformity-related uranium deposits in the Athabasca Basin? Some of these questions are addressed below.

8 A one metre chip sample, # 204022, is reported to have assayed 70 ppm Au, 14.8 ppm Pd and 14 ppm Pt from this location (Hilland, 1988). 9 A grab sample, # 7311, is reported to have assayed 55.9 ppm Au, 1.8 ppm Pd and 0.33 ppm Pt from this location (Bowe and Petrie, 1989).


Figure 9 – Distribution of Au-PGE-U (blue diamond symbols) and Au-U (orange square symbols) occurrences in and at the periphery of the Athabasca Basin, northern Saskatchewan, and tectonic domain names. The names of the occurrences and citations for their first published, or main, references are given in Table 4.

Table 4 – Au-PGE-U and Au-U occurrences in the province of Saskatchewan. The references cited are to the main or first published description of the occurrence/deposit and/or the relevant Saskatchewan Mineral Deposit Index (SMDI) number.

Map number Occurrence or Deposit References

1 PITCHIE Uranium Zone 1 or the Uranium Ridge Uranium Mine (south extension of Lorado Deposit) SMDI 1229; Hulbert, 1987

2 Leonard Uranium Mine, Rix Athabasca No. 10 Adit SMDI 1414; Hulbert, 1987 3 Quartzite Ridge Au-Pt-Pd Occurrence SMDI 2319; Hilland, 1988 4 Missy Au Showings or Missy #1 Au Showing and Missy #2 Au Showing SMDI 2378; Bowe and Petrie, 1989 5 Nicholson Bay Uranium Mine (Nicholson Zones 1 to 6) SMDI 1264; Hulbert, 1987; Mason, 1987 6 Bearcat Au Showing or Bearcat No. 1, No. 2, and No. 3 (A and B) Zones SMDI 1258 7 Fish Hook Bay Uranium Mine B and C Zones SMDI 1294 8 EUNICE Claim No. 4 Uranium Occurrence SMDI 1326 9 MacIntosh Bay 1 (CANALASKA) SMDI 1278; Duff and Schimann, 2010 10 Felix Bay 1 (CANALASKA) SMDI 1279; Duff et al., 2009 11 Adair Bay 1 (CANALASKA) Duff et al., 2009 12 Gaitwin (Fall) (CANALASKA) SMDI 1567; Duff et al., 2009

13 Star Property ALX Uranium Corp., 2014a, 2014b; Navis Resources Corp., 2014


Map number Occurrence or Deposit References

14 BUTCH Claims U-Au-PGE Showing SMDI 1632; MacLachlan et al., 2009 15 Crackingstone Peninsula Uranium Occurrences KH-11 and KH-12 SMDI 2094 16 GIL Claims Au-U Veins 1 to 3 SMDI 2194

17 Goldfields Uranium Showing 50-TT-75 or Goldfields Uranium Zone No. 1 or Brenmac Mines East Zone SMDI 1480; Beck, 1969

18 Martin Lake Uranium Mine or Martin Lake No. 1 Adit or R.J. Harrison Mine SMDI 1361; Christie, 1953; Robinson, 1950

19 Bolger Uranium Deposit or Bolger Open Pit (surface expression of the VERNA Deposit) SMDI 1287; Robinson, 1950

20 Shea Creek Deposits SMDI 2670; UEX Uranium Corp., 2009 21 Cluff Lake D Uranium Deposit SMDI 1150a; Sibbald et al., 1991 22 Patterson Lake - Triple R Deposit SMDI 5340 23 Patterson Lake - Arrow Zone SMDI 5332 24 Key Lake Mine SMDI 1130; Sibbald et al., 1991 25 Millennium Uranium Zone SMDI 2742; Halaburda et al., 2008 26 Phoenix Deposit SMDI 5338; Liu et al., 2010 27 Cigar Lake Mine SMDI 1856; Chevalier and Schimann, 1982 28 Waterbury Lake J Zone SMDI 3463 29 Collins Bay A zone SMDI 0617; Sibbald et al., 1991 The most favourable conditions under which Au, Pd and Pt can be transported in hydrothermal fluids have been estimated, tested and confirmed by a large number of investigators over the last 30 years, and are too numerous to list exhaustively here (e.g., Mountain and Wood, 1988; Wilde et al., 1989; Jaireth, 1992; Wood et al., 1992; Mernagh et al., 1994; Gammons, 1995, 1996; Olivo and Gammons, 1996; Wood and Normand, 2008; Mei et al., 2015). Au, Pd and Pt are easily hydrolyzed and quite insoluble in aqueous surficial and hydrothermal fluids unless ligands, either inorganic or organic (e.g., humic and carboxylic acid moieties10 – Vlassopoulos et al., 1990; Wood et al., 1994; Wood, 1996; hydroxamate siderophore moieties – Hutchins et al., 1999; Normand and Wood, 2005), are present in significant concentrations. A common ligand in hydrothermal fluids, the Cl– ion, when present in large concentrations, can increase the solubility of these metals by several orders of magnitude under appropriate conditions (Gammons et al., 1992, 1997). Major controls on the solubility of these metals in concentrated aqueous chloride solutions are oxygen fugacity and pH. In order to obtain concentrations of upwards of tens of parts per billion or parts per million in concentrated hydrothermal brine solutions, very elevated oxygen fugacity and low pH conditions must be met. Very few hydrothermal environments meet these conditions, but one of them occurs in basinal brines. Transport of gold and PGE, in addition to uranium, by petroleum or its derivatives has also been proposed (Sugiyama et al., 2014; Fuchs et al., 2016), and control on solubility in such fluids might be less affected by variable redox conditions or fluid/rock reactions, but more by temperature and pressure variations. The presence of organic buttons, bitumen and thucholite has been reported from a number of uranium deposits in Saskatchewan (McCready et al., 1999; Chi et al., 2014, and references therein), including the Nicholson Bay mine (Robinson, 1955; Peiris and Parslow, 1988), as well as in uranium deposits in basins in other parts of the world, such as in South Africa (England et al., 2001; Drennan and Robb, 2006; Fuchs et al., 2016).

Fluid properties consistent with those for the uranium-mineralizing brines cited above were reported by Rees (1992) based on fluid inclusion data measured on samples obtained from the Nicholson Bay mine, the Fish Hook occurrences and the Quartzite Ridge occurrence: mineralization at temperatures between 100 and 200°C; the presence of vapour-rich fluid inclusions suggestive of phase separation, thus low pressure; and highly oxidizing,

10 In sedimentary basins, the largest volumes of carboxylic acids are generated during peak diagenetic conditions at temperatures between ~80 to 120°C (Klein, 1991).


complex chloride brines (30 to 40 wt. % eq. NaCl or ~7 to 11 m eq. NaCl) that contain significant proportions of divalent cations.

The physicochemical properties of the fluids involved in the formation of primary uranium mineralization throughout the Athabasca Basin were summarized by Richard et al. (2015, and references therein) as follows: comparatively low temperature (100 to 200°C), oxidizing, Na+Ca(±K±Mg)+Cl-rich with up to 6 m dissolved salts and deposited metals at low pressure. These properties match those reported by Rees (1992) remarkably well. Based on thermodynamic calculations, Richard et al. (2012) suggested that the oxygen fugacity of the uranium-mineralizing fluids that were responsible for the formation of major unconformity-related uranium deposits in the Athabasca Basin was very high (10-24 at 200°C to achieve concentrations of ~10-5 mol/1 total dissolved U, or more than 15 orders of magnitude above that fixed by the hematite-magnetite buffer). Richard et al. (2012) also estimated that the fluid was very acidic (pH ranging between 2.5 and 4.5) and probably evolved from the evaporation of seawater. Evaporation of water appears to be a critical factor in the complex process involved in achieving very low pH values, a process by which values below 1 may be attained (Mernagh et al., 2014).

The physicochemical characteristics of the fluids reported by Rees (1992) for mineralization at the Nicholson Bay mine, the Fish Hook occurrences and the Quartzite Ridge occurrence (see also Sibbald, 1988) correspond with those believed responsible for allowing transport of the Au, Pd and Pt associated with unconformity-related types of uranium mineralization (Wilde et al., 1989; Jaireth, 1992; Mernagh et al., 1998). The geographic distribution of the Au-PGE-U occurrences (Figure 9) supports the idea that the fluids were derived from the Athabasca Basin. Upper intercept and maximum U/Pb and 207Pb/206Pb uraninite-pitchblende ages of 1549 and 1260 Ma, respectively, attributed to the Nicholson Bay and Fish Hook Bay deposits (Kotzer and Kyser, 1993) are also consistent with this, and possibly coincident with peak diagenesis in the Athabasca Basin around 1600 to 1450 Ma (Hiatt and Kyser, 2007).

It appears that all the elements favourable for transport and precipitation of Au, Pd and Pt, in addition to U, were assembled in the Mesoproterozoic environment of the Athabasca Basin. It is proposed that the ground was prepared first following evaporation of seawater (Richard et al., 2011) to produce an acidic, oxygenated and highly saline brine of complex composition. Constant recirculation of renewed brine through domains of flow systems (Klein, 1991) in Athabasca Group sediments would have ensured that the ƒO2- and pH-buffering capacity of the rock was obliterated (e.g., no feldspar left) after prolonged fluid/rock interaction (high water/rock ratio). This would in turn have ensured that dispersion of the precious metals and uranium from adsorption or precipitation in discrete phases was prevented in the portions (domains of flow systems) of the Athabasca Basin that served as ore fluid reservoirs11 (Mernagh et al., 1998; Wood and Normand, 2008). As a result of tectonic activity in the Athabasca Basin during the Mesoproterozoic Era, the oxygenated and acidic reactive brines contained in the reservoir could have been drawn from recharge areas into sections of fractured basement, and, through prolonged fluid/rock interation to neutralize the ƒO2- and pH-buffering capacity of the basement rocks, extract metals from appropriate, gold- and PGE-bearing source lithologies, such as ultramafic rocks, metalliferous pelites, iron formations, paleoplacers and regolith. The distance travelled by the fluids enriched in precious and other metals between the source and the site of mineral deposition could have been comparatively short. Hulbert (1990) suggested that the PGE in uranium mineralization hosted by metasediments at the Nicholson Bay mine was derived from adjacent ultramafic rocks. However, the source of the PGE in most hydrothermal deposits throughout the world, the mechanisms of transport and deposition of these metals, and the distance between the source where these metals are extracted and the site of metal deposition are not well constrained (Wilde, 2005). Despite all these uncertainties, it is reasonable to conceive that faulting or extensional fracturing during earthquake events (Sibson, 2001) could have allowed rapid flow of massive amounts of oxygenated and acidic ore fluids to penetrate deep into basement rocks and abruptly precipitate Au-PGE-U mineralization into the permeable structures by reduction and/or neutralization of acidity.

11 It is not suggested that the entire Athabasca Basin constituted the reservoir for any amount of fluid responsible for mineralization. The volume of the portions of the reservoir containing the fluid implicated in leaching the precious metals from the source and transporting them to the sites of mineralization is unknown.


c) REE-Th-U Mineralization in Paleoregolith Given the limited information presently available, only speculations can be made regarding the nature and origin of the MacAskill occurrence monazite-rich layer. The monazite-rich layer may represent a deposit formed in gneiss before development of the regolith, such as a paleoplacer, a pegmatite, a vein, etc. If the monazite-rich layer formed or was emplaced at or close to the surface during weathering and regolith development, it may represent an authigenic product, a product of transformation of clay-adsorbed REE (Bao and Zhao, 2008) by interaction with phosphorus (which could have been derived from phosphogenesis), a product of local mechanical transport of pre-existing monazite, such as in illuvium, eluvium, alluvium or colluvium, or a placer. If the monazite-rich layer formed after a considerable quantity of Athabasca Group sediments were laid on top of the regolith, it may have grown as an authigenic or hydrothermal product, possibly precipitated in a vein or shear zone as proposed by Ellerington (1977). The similarity in calculated eU/eTh ratios between alteration haloes developed at the margins of fractures spatially associated with paleoregolith and the monazite-rich occurrence hosted by paleoregolith suggest a close relationship and a possible origin from diagenetic or hydrothermal fluids derived from the Athabasca Basin. The composition of monazite can be an indicator of the environment in which it grew. Authigenic monazite, also called nodular or grey monazite in the literature, has a distinct chemical signature (e.g., low Th, high Eu) that characterizes it as a product of low-temperature processes (Donnot et al., 1973; Rosenblum and Mosier, 1983; Read et al., 1987; Burnotte et al., 1989; Milodowski and Zalasiewicz, 1991; Smith et al., 1994; Cabella et al., 2001; Kryza et al., 2004; Čopjaková et al., 2011). Hydrothermal monazite shares similar compositional characteristics (Gordon, 1939; Schandl and Gorton, 2004; Repina, 2008; Prsĕk et al., 2010; Grand’Homme et al., 2016). Magmatic or metamorphic monazite contains much higher thorium and lower europium concentrations in general (Overstreet, 1967; Schandl and Gorton, 2004).

6. Concluding Remarks Based on widely differing associated alteration and radiometric data, it appears that the monazite and Au-PGE-U mineralization in the study area are temporally and paragenetically unrelated.

Detailed petrographic, microchemical and fluid inclusion microthermometric analyses of fracture-bound, Th-rich alteration haloes and mineralization at the MacAskill occurrence will help to determine the nature of the monazite. Radiometric dating of this monazite may also provide critical information on the timing of growth and origin of the monazite, and its place in the development history of the Athabasca Basin.

The Au-PGE-U mineralization reported in this study is a small-scale example of unconformity-related, PGE-enriched uranium mineralization that may be more widespread than is presently recognized. There are possibly economic concentrations of PGE associated with gold and uranium mineralization south of the northern periphery of the Athabasca Basin. Whereas uranium occurrences and deposits elsewhere along the northern rim of the Athabasca Basin (e.g., the Nisto mine, SMDI 1621, and the Goldfields Uranium Showing 50-SB-1, SMDI 1578) should be evaluated for their Au and PGE potential, uranium deposits within the Athabasca Basin and elsewhere along its periphery should be analyzed routinely for Au and PGE. The information would greatly improve our understanding of the systems involved in precious metal enrichment of unconformity-related uranium mineralization.

7. Acknowledgments Fieldwork during the summer of 2016 benefitted greatly from the able assistance of Anastasia Comtois-Poissant, Dixon Dallas and John Kelley. Fruitful discussions were had with Thomas Ogilvie during a visit in the field. Julie and Adam Duff made the necessary effort to provide the team with all supplies required. We express our sincere gratitude to Steve Roberts, the executive director of the Wildfire Management Branch; Brent Zbaraschuk, the Fire Base Supervisor; helicopter pilot Digby Amess; Edwin Powder, Kelvin Whitedeer, and the rest of the Fire Crew in Stony Rapids who provided aid and support during the summer of 2016.

This report benefitted from constructive reviews by Ralf Maxeiner and Ken Ashton.


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