Review of Reducing Mechanisms Potentially Involved in the Formation of Unconformity-type Uranium Deposits and their Relevance to Exploration
G.M. Yeo 1 and E.G. Potter 2
Yeo, G.M. and Potter, E.G. (2010): Review of reducing mechanisms potentially involved in the formation of unconformity-type uranium deposits and their relevance to exploration; in Summary of Investigations 2010, Volume 2, Saskatchewan Geological Survey, Sask. Ministry of Energy and Resources, Misc. Rep. 2010-4.2, Paper A-12, 13p.
Abstract The essential ingredients for ‘basin-related’ uranium deposits are oxidizing, U 6+-bearing fluids and focussed reduction of mobile U 6+ to immobile U 4+. The more critical problem is the reducing mechanism, since it constrains exploration targeting (i.e., is graphitic metapelite essential in the basement of the Athabasca Basin, or is a broader range of basement rocks favourable?). Potential reducing systems in sedimentary basins may be carbon based (e.g., particulate organic material, fluid 2+ hydrocarbons, graphite or graphite-derived compounds) or inorganic (e.g., mineral surfaces, Fe or H2S from oxidation of sulphides, or Fe2+ from chloritization or illitization of ferromagnesian minerals). Microbial activity may be involved in both and more than one U 6+ reduction mechanism may have been responsible for development of the various Athabasca deposits.
Plant fragments reduced many Phanerozoic sandstone-hosted uranium deposits. Oncoids and biolaminites in the basal Athabasca sandstones suggest widespread primary particulate organic material. Early diagenetic hematitization of the sandstone, however, would have oxidized this prior to formation of the uranium deposits >1590 Ma.
Fluid hydrocarbons or related humates are also thought to have reduced some stratiform sandstone-hosted uranium deposits elsewhere. Athabasca Basin hydrocarbons derived from both the 1.54 Ga Douglas Point black shale and Devono-Mississippian strata are common, but texturally post-date primary uranium phases, except at Dufferin Lake.
4+ Graphite or CH4 and CO2 derived from basement graphitic metapelite have been proposed as reductants for U in the Athabasca Basin. Although graphite is chemically inert at diagenetic temperatures, it has been argued that radiolysis of graphite generated CO2. Most hydrocarbons from which the graphite was derived would have been lost during upper amphibolite facies metamorphism (ca. 750°C). If structurally trapped, CH4 can survive above 800°C, but it would not then be readily available to reduce U 6+. Experimental interaction of graphite and tritium has generated CH4, but it is doubtful that this could produce sufficient CH4 or CO2 to form an economic deposit. Uranyl ions can be incorporated in Fe-oxide or reduced on Fe-bearing mineral surfaces. Alternatively, U 6+ can be 2+ 2+ reduced by Fe or H2S generated during oxidation of pyrite, common in metapelite, or by Fe released by chloritization of ferromagnesian minerals. In either case, Fe3+ would precipitate as a hydroxide and dehydrate to “hydrothermal hematite”, a distinctive alteration at most Athabasca deposits. Aluminium phosphate-sulphate minerals are a potential synchronous sink for sulphate released during sulphide oxidation. Although commonly mentioned in the literature, mechanisms for reduction/precipitation of uranium in unconformity-related deposits remain contentious. This review supports arguments by Kyser, Alexandre, and others that the most likely reduction mechanism for primary uranium in Athabasca Basin is by Fe2+ released during 2+ chloritization of ferromagnesian minerals, and possibly also by Fe or H2S from oxidation of pyrite. Variation in paragenetic sequences at individual deposits suggests local variation in the dominant reduction mechanism. The basement rock type most important geochemically to formation of unconformity uranium deposits, however, is broadly metapelite – not just graphitic metapelite. Graphitic metapelite remains important for controlling reactivated faults which were conduits for fluids.
1 Denison Mines Corp., 230 - 22nd Street East, Suite 200, Saskatoon, SK S7K 0E9. 2 Natural Resources Canada, Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8.
Saskatchewan Geological Survey 1 Summary of Investigations 2010, Volume 2 Keywords: Athabasca, unconformity, uranium, graphite, reductant.
1. Introduction The essential requirements for any uranium-mineralizing system are suitable uranium source rocks, suitable fluids and fluid pathways, and effective reducing mechanisms (Skirrow et al., 2009). The reducing mechanisms are particularly important, since they are generally tightly focussed, resulting in small exploration targets compared to the volume of potential host rock. In the Athabasca Basin, the source of uranium remains somewhat contentious with a weak consensus suggestive of derivation from the basin-fill sandstones (Hoeve and Sibbald, 1978; Fayek and Kyser, 1997; Alexandre et al., 2009a), whereas a minority view holds that uranium was derived from underlying crystalline rocks (Hecht and Cuney, 2000; Madore et al., 2000; Richard et al., 2010). Regardless of the source, there is a consensus that dissolved uranium was likely transported in oxidizing basin fluids in response to hydrodynamic gradients. Mineralogy and fluid inclusion analyses show that the diagenetic basin fluids were slightly acidic (kaolinite-illite equilibrium), hot (160° to 220°C), oxidized (f O2 in the hematite stability field) Na-Ca chloride brines (Derome et al., 2005; Cuney, 2009a). Fluid movement was constrained broadly by the unconformity between relatively permeable sandstones and impermeable crystalline rocks and more narrowly by the topography of the unconformity surface and by fault zones, particularly reverse faults. Although a range of reducing mechanisms have been proposed, the spatial association of many Athabasca deposits with basement graphitic pelites led Hoeve and Sibbald (1978) to suggest a diagenetic-hydrothermal reduction mechanism involving graphite for the formation of the deposits. According to this model,
“... at elevated temperatures and under a thick sedimentary cover, oxidizing diagenetic solutions of the Athabasca Formation penetrated the metamorphic basement along breccia and fault zones and reacted with graphitic rocks to yield reducing solutions containing carbon dioxide and methane. The newly generated reducing solutions, which formed part of a percolating system, may have flowed upwards along other portions of the same fault or breccia zone, eventually reaching the unconformity.
Mineralization resulted from interaction of flows of methane-bearing reducing solutions and of oxidizing diagenetic solutions carrying ore constituents and hence was subject to hydrodynamic controls.”
Alternative mechanisms suggested include reduction by: fluid hydrocarbons (Alexandre and Kyser, 2006); direct 6+ reduction of U by radiolysis of graphite (Alexandre et al., 2005); H2S from the breakdown of pyrite (Cheney, 1985; Ruzicka, 1993); Fe2+ from pyrite oxidation, chloritization of biotite or illitization of hornblende (Wallis et al., 1985; Alexandre et al., 2005); or mixing of geochemically distinct brines (Richard et al., 2010).
Recent discussions of unconformity-type uranium deposits are commonly not specific about potential reduction mechanisms (e.g., Jefferson et al., 2007; Kyser and Cuney, 2008a; Skirrow et al., 2009; Burrows, 2010), typically referring to them only as basement-derived reducing fluids. Graphite, or CH4- and CO2-bearing fluids generated from oxidation of graphite, however, are still widely considered the reductants for U6+ in the oxidizing basinal fluids (e.g., Cuney, 2009a, 2009b; Belyck, 2010; International Atomic Energy Agency, 2010). If, as inferred by Hoeve and Sibbald’s (1978) diagenetic-hydrothermal model, graphitic pelites are essential to the formation of Athabasca Basin unconformity deposits, then exploration should be focussed on areas underlain by graphitic pelites, such as the western Wollaston Domain, and electromagnetic conductors should continue to be the exploration targets of choice. On the other hand, if graphitic pelites are not essential to the formation of Athabasca Basin deposits, then a much broader area of the basin is relatively prospective and other methods may be effective in defining exploration targets. To resolve the question of whether graphitic pelites are essential to the formation of Athabasca unconformity uranium deposits, various reducing mechanisms for U6+ and the evidence for them in the Athabasca Basin are reviewed below.
2. Reducing Mechanisms Reducing mechanisms for U6+ in oxidizing solutions can be grouped into two broad categories, those involving carbon-based reductants and those involving inorganic reductions. In addition, U6+ may be adsorbed directly onto
Saskatchewan Geological Survey 2 Summary of Investigations 2010, Volume 2 metal surfaces, and anaerobic bacteria may be involved in the reducing process. Discussion of the latter, however, is outside the scope of this review. a) Carbon-based Reducing Mechanisms In addition to fluid hydrocarbons, basement graphite or graphite-derived hydrocarbons, another carbon-based reducing mechanism to consider is early diagenetic reduction by particulate organic matter.
Detrital Organic Material Fossil plant fragments (or diagenetic sulphides associated with them) are widely considered the principal reductants for dissolved U6+ in Silurian and younger sandstone basins due to the development of vascular plants (Kyser and Cuney, 2008b). Sandstone roll-front uranium deposits occur at the redox boundary between oxidized hematitic red beds and reduced organic- or pyrite-rich grey beds (De Voto, 1978; Kyser and Cuney, 2008b). Although there were no vascular plants in the Proterozoic to provide organic detritus to the Athabasca Basin, oncoids and other biolaminites in the basal Athabasca Smart and Read formations (Yeo et al., 2007) indicate that bacteria were widespread. Such biolaminites were the plant fragment analogues in the Proterozoic. The bacteria that produced them may also have been a source of diagenetically mobile humate material. Early diagenetic hematization of the Athabasca sandstones prior to the earliest ca. 1.6 Ga uranium mineralizing event (Alexandre et al., 2009b), however, likely destroyed all such organic material.
Fluid Hydrocarbons
Fluid hydrocarbons, or light hydrocarbons such as CH4 derived from fluid hydrocarbons, are considered to have been reductants for uranium in several sandstone basins, most notably the late Cretaceous-Cenozoic Chu-Sarya and Syr-Darya basins of Kazakhstan (Jaireth et al., 2008), as well as South Texas (Adams and Smith, 1981; Nicot et al., 2010) and the Tarim (Qin et al., 2005) and Ordos basins of China (Cai et al., 2007; Xue et al., 2010).
Proterozoic hydrocarbons, thought to be derived from the 1.54 Ga Douglas Formation, are common in the Athabasca Basin (Wilson et al., 2007). However, such hydrocarbons post-date the earliest uranium mineralization ca. 1.59 Ga, although they are interpreted to have co-precipitated with 1.54 Ga uranium at Dufferin Lake (Alexandre and Kyser, 2006).
Methane or CO2 Associated with Graphitic Pelite
According to Price (1997), CH4 in a C-CH4-H2O-CO2 system can remain stable above 800°C. Survival of pre- metamorphic CH4 at upper amphibolite faces metamorphic temperatures should be favoured by the presence of water, high fluid pressure, and trapping in a closed system. It is uncertain whether sufficient CH4 could survive metamorphism to be a significant reductant and, if it did survive (e.g., as fluid inclusions), whether sufficient quantities could be released to act as a reductant.
Methane or CO2 Derived from Graphitic Pelite
Historically, destruction of graphite to form reduced phases such as CH4 or CO2 has been proposed through the following reactions with basin-derived fluids (Hoeve and Sibbald, 1978; Kyser et al., 1989):
2C + 2H2O = CH4 + CO2 and
2H2O +CO2 = CH4 + 2O2. In an examination of the relationship between graphite and uranium mineralization, Kyser et al. (1989) noted uniform δ13C values in graphite both distal and proximal to the uranium ore deposits. This led the authors to propose that destruction of basement graphite did occur via oxidation of basinal fluids from the sedimentary rocks, but that 12 13 significant quantities of C-rich CH4 were not produced near the deposits. Furthermore, the δ C composition of bitumen samples from ore zones were isotopically indistinguishable from graphite, indicating that if the bitumen formed from degradation of graphite, it did so with no significant isotopic fractionation. The authors proposed the mechanism for this lack of isotopic fractionation was radiolysis. During radiolysis, graphite would have been ionized under the actions of α-, β-, and γ-rays emitted from radioactive elements and would have reacted with H2O. This would require significant concentrations of radioactive elements, which could not be met through the circulation of uranium-bearing fluids, since the radiolytic products such as CO and CH4 are powerful inhibitors of graphite oxidation via radiolysis (Minshall et al., 1995). Therefore, formation of a uranium deposit by radiolysis of
Saskatchewan Geological Survey 3 Summary of Investigations 2010, Volume 2 graphite is unlikely, although this process imparts post-depositions effects on uranium ore deposits. These observations are supported by the work of Derome et al. (2003), who identified CH4 in sandstone about 1 m from ore and C2H6 and CO2 about 10 m from ore, which they interpreted to have been generated by post-ore hydrolysis of graphite. Maozhong et al. (1994) further proposed that radiolysis of groundwater may give rise to several other alteration products observed in unconformity-related uranium deposits – mainly hemitization, argillitization, and decolouration.
Direct Reduction of U6+ by Graphite Alexandre et al. (2005) suggested that U6+ might be directly reduced from basinal fluids by graphite:
6+ + U + 3H2O + ½C → UO2 + ½CO2 + 6H . If graphite acts directly as a reductant, however, we should invariably see an intimate association between uranium and graphitic pelite. At the Eagle Point, Raven-Horseshoe, Cluff Lake, and Centennial deposits, however, graphitic pelite is absent. At other deposits (e.g., Gartner (Key Lake) and Shea Creek), uranium is more strongly associated with other lithologies. This suggests that direct reduction of U6+ by graphite in the Athabasca Basin is not likely. b) Inorganic Reducing Mechanisms 6+ As noted above, inorganic mechanisms by which U could be reduced include: H2S released during alteration of pyrite, Fe2+ released from pyrite, chloritization of biotite, and illitization of hornblende.
Hydrogen Sulphide from Pyrite
Cheney (1985) was the first to note that the Wollaston graphitic schists are sulphide-rich and proposed that H2S, generated from the breakdown of pyrite (e.g., to pyrrhotite), may have acted as a reductant for U6+:
FeS2 + H2 = FeS +H2S.
6+ Recently, Beyer et al. (2010) have suggested that H2S from pyrite was the primary reductant for U at the Boomerang Lake unconformity-related uranium prospect in the Thelon Basin, NWT.
Ferrous Iron from Fe-sulphides
Alternatively, Fe2+ liberated during oxidation of pyrite, is a potential reductant:
2+ 2- + FeS2 +7/2O2 +H2O = Fe + 2SO4 +2H and
6+ 2+ + U + 5H2O + 2Fe = UO2 + Fe2O3 + 10H .
2+ 3+ 2- On reduction of Fe to Fe , the latter could produce hematite, but where would the sulphate (SO4 ) go? Ore zone sulphide δ34S values at McClean Lake are comparable to those of basement pyrite, suggesting derivation of ore zone sulphur from the basement sulphides (Bray et al., 1982), but the ore zone sulphides are typically late in the paragenetic sequences of most Athabasca uranium deposits (Table 1; Figure 1). Aluminium phosphate-sulphate (APS) minerals are a possible sink for sulphate, yet Gaboreau et al. (2007) documented sulphate contents in the APS minerals which contradict such a relationship. They noted an increase in phosphorus/sulphur ratios within the APS minerals proximal to the Athabasca ore deposits, which likely reflects the different physiochemical natures of the fluids, with more oxidizing conditions distal to ore zones and more reduced fluids (phosphorus- and light rare earth element–bearing APS minerals) occurring proximal to ore. The authors suggest early remobilization of diagenetic sulphides as the source of sulphur for the distal sulphur-rich APS minerals.
Much like pyrite, pyrrhotite is also susceptible to reduction under acidic conditions, leading to the generation of H2S and Fe2+ (Belzile et al., 2004):
2+ FeS + H2 = Fe + H2S.
Saskatchewan Geological Survey 4 Summary of Investigations 2010, Volume 2 Table 1 - Summary of predominant host rocks, structural controls, alteration features, and inferred reducing mechanisms for selected Athabasca Basin uranium deposits. See Figure 1 for locations of the deposits listed in this table. Ore Host-rock Underlying Structural Controls Alteration Proposed Deposit Ore Settings Lithology Basement Lithology on Ore Pre-ore Syn-ore Post-ore Other Reductant(s) References chlorite, dravite, euhedral quartz, Basement-hosted pelitic to graphitic pelitic gneiss, carbonates, Deposit Model basement hosted pelitic gneisses granitoids, pegmatites yes illite, chlorite illite pyrite, hematite various Alexandre et al. (2009a) dravite, kaolinite, Athabasca-hosted coarse-grained pelitic to graphitic pelitic gneiss, kaolinite, illite, dravite, rutile, Deposit Model sandstone hosted sandstones granitoids, pegmatites yes chlorite, chlorite pyrite various Alexandre et al. (2009a)
conglomeratic sandstone (Bird Member drusy quartz, (A) sandstone of the Manitou Falls graphitic pelitic gneiss, calc-silicates, basement "ridge" cut by illite, quartz illite, chlorite, pyrite, Thomas et al. (2000); Thomas and Wood 1. Cigar Lake hosted Formation (MFb)) granitic pegmatites minor faults dissolution hematite hydrocarbons (2007) sandstone (Read minor faults in footwall of silicification, reduced fluids (A) sandstone and Formation); sericite- mylonitic psammite, quartzite, dextral-oblique Dufferin hematite, hematite, desilicification, from graphitic 2. Centennial basement hosted chlorite schist granite Lake Fault illite, chlorite chlorite chlorite dravite, pyrite fault Jiricka (pers. comm., 2009) Peter River paragneiss at least three fault sets; (A) basement and and Smart Formation (D breccia (zone à boules) sulphides, 3. Cluff Lake sandstone hosted pit) Earl River orthogneiss, granodiorite common hematite chlorite, illite Koning and Robbins (2006) illite, quartz, (A) basement and sandstone (Read dolomite, Hoeve and Sibbald (1978); Hoeve et al. 4. Collins Bay sandstone hosted Formation) paragneiss, granite gneiss Collins Bay Fault (reverse) siderite, dravite ( 1981) dravite, chlorite, metapelites, calc-silicates, biotite steep northeast-trending kaolinite, Sask. Ministry of Energy and Resources (A, B) basement gneiss graphitic and pyritic pelite, fault; no offset in euhedral quartz, graphite and Fe- Assessment File 64L05-SW-0149; 5. Dawn Lake hosted Wollaston Supergroup pegmatite unconformity illite, chlorite illite pyrite, hematite chlorite Alexandre et al. (2005) dravite, rutile, 6. Dufferin Lake clay alteration, chalcopyrite, (Southwest (A) sandstone sandstone (Read rutile, pyrite, chlorite, pentlandite, Occurrence) hosted Formation) Dufferin Lake Fault chlorite bitumen calcite bitumen Alexandre and Kyser (2006) graphite-filled structures hematite, in Wollaston semi-pelitic to pelitic hosted in Riedel-style dravite, (B) basement Supergroup and gneiss+graphite, feldspar porphyry, faults in the hanging wall carbonate, 7. Eagle Point hosted orthogneiss pegmatite of the Collins Bay Fault muscovite, APS hematite chlorite, pyrite Cloutier et al. (2010) sandstone and 8. Key Lake (A) sandstone and conglomerate (MFb) and psammopelite and pelitic gneiss with hematite, silica, silica, illite, kaolinite, illite, (Deilmann) basement hosted Wollaston Supergroup up to 25% graphite Key Lake Fault dickite chlorite, dravite silica, chlorite Harvey and Bethune (2007) hematite, conglomeratic hematite, bornite, (A) sandstone sandstone (Fair Point kaolinite, illite, chalcopyrite, 9. Maurice Bay hosted Formation) migmatitic gneisses, granodiorite horst structure chlorite hematite carbonate Fe2+ Alexandre et al. (2009a) conglomeratic chlorite, (A) sandstone sandstone (Fair Point pelitic to graphitic pelitic gneiss, south-southeast–trending desilicification, hematite, pyrite, drusy 10. Maybelle River hosted Formation) mylonitic leucogranite fault clay carbonate quartz Kupsch and Catuneanu (2007)
sandstone and hematite, conglomerate (Read chlorite, (A) sandstone and Formation) and pelitic to graphitic pelitic gneisses, silica, hematite, dolomite, pyrite, Derome et al. (2005); Thomas and Wood, 11. McArthur River basement hosted Wollaston Supergroup quartzite P2 Fault (reverse) dravite dravite, chlorite bitumen mixing of brines (2007); Richard et al. (2010)
12. McClean Lake (A) basement Wollaston Supergroup graphitic pelitic gneiss, psammitic hematite, (Sue C, D, and E) hosted graphitic paragneiss gneiss, granitoids, pegmatite graphitic fault zone illite, chlorite chlorite Tourigny et al. (2007)
Saskatchewan Geological Survey 5 Summary of Investigations 2010, Volume 2 Table 1 (continued) - Summary of predominant host rocks, structural controls, alteration features, and inferred reducing mechanisms for selected Athabasca Basin uranium deposits. See Figure 1 for locations of the deposits listed in this table.
Ore Host-rock Underlying Structural Controls Alteration Proposed Deposit Ore Settings Lithology Basement Lithology on Ore Pre-ore Syn-ore Post-ore Other Reductant(s) References sandstone, conglomerate, and sericite, sericite, chlorite, mudstone (MFb); chlorite, sericite, carbonates, (A) sandstone and Wollaston Supergroup garnet pelite, locally graphitic and kaolinite, chlorite, sulphides, desilicification, CH4 from 13. Midwest Lake basement hosted pelite sillimanite or cordierite bearing northeast-trending fault hematite hematite brecciation hematite graphite Wray et al. (1985) illite, chlorite, (B) basement Wollaston Supergroup graphitic pelitic gneiss and schists, stratabound between muscovite, muscovite, dravite, quartz, graphite, Fe- 14. Millennium hosted pelitic gneiss calc-pelites, granitic pegmatites reverse faults rutile hematite, APS pyrite, chlorite chlorite Cloutier et al. (2009)
sandstone (Read Formation) and desilicification, (A) sandstone and Wollaston Supergroup graphitic pelite, garnet pelite, pelite, illite, chlorite, euhedral quartz, reductant 15. Phoenix Zone basement hosted graphitic pelite quartzite WS shear (reverse) dravite hematite, pyrite unknown Kerr (2010)
east-northeast–trending (B) basement Wollaston Supergroup pelitic to graphitic pelitic gneiss, fault; dravite-quartz chlorite, illite, chlorite, wallrock 16. P-Patch hosted pelitic gneisses pegmatite breccia hematite, clay dravite reactions Mercadier et al. (2010) "red alteration"- hematite and chlorite and breccia, silica, dravite, chlorite, "green carbonatization, CO2 and CH4 (A) basement meta-arkose and Rabbit Lake Fault dravite, alteration"- silicification, from graphitic 17a. Rabbit Lake hosted plagioclasite plagioclasite (reverse) chlorite carbonate sulphides hematite pelite Hoeve and Sibbald (1978) meta-arkoses, calc-silicate gneiss, dravite, chlorite, graphitic plagioclasite, calc-silicates kaolinite, (A) basement and dolomitic marbles, granitic euhedral quartz, graphite and Fe- 17b. Rabbit Lake hosted meta-arkose pegmatite Rabbit Lake Fault illite, chlorite illite pyrite, hematite chlorite Alexandre et al. (2005)
2+ Fe associated with chlorite, possible northeast chlorite, clay illite and 18. Raven and (B) basement quartzite and calc– trending; southeast- (illite-sudoite), drusy quartz, hematite Horseshoe hosted meta-arkose pelite, quartzite, meta-arkose dipping fault hematite sulphides formation Rhys et al. (2008)
(A) basement garnet-cordierite pelitic gneiss, reductant 19. Roughrider hosted pelitic gneiss graphitic pelitic gneiss, pegmatite at least three faults illite, chlorite hematite unknown McCready et al. (2009) desilicification, (A, P, B) sandstone kaolinite, illite, hosted plus graphitic metasedimentary rocks, chlorite, dravite, 20a. Shea Creek basement veins Manitou Falls sandstone pelitic gneisses, orthogneiss graphite-rich reverse fault APS illite mixing of brines Kister et al. (2006); Laverret et al. (2006) (A, P, B) basement and sandstone felsic gneiss and Smart thrust faults (footwall), chlorite, illite, 20b. Shea Creek hosted Formation sandstone hanging-wall garnetite, pelite widespread breccia hematite Robbins and Koning (2006) 21. Tamarack (A) sandstone sandstone and graphitic biotite gneiss, graphitic reductant (Collins Creek) hosted conglomerate (MFb) pelite, calc-silicates multiple fault strands hematite, pyrite unknown Hirsekorn (2008)
Notes on abbreviations: APS, aluminium phosphate-sulphate minerals. Ore Settings: A, deposit at or near unconformity; B, deposit below unconformity in basement; and P, deposit above unconformity perched in Athabasca sandstone.
Saskatchewan Geological Survey 6 Summary of Investigations 2010, Volume 2
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Table 1.
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4 REINDEER REINDEER NCE
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FORMATIONS OF THE ATHABASCA GROUP ATHABASCA THE OF FORMATIONS GR Undivided
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Major faults and reactivated shear zones: BB, Black Bay fault; BLSZ, Black Lake shear zone; CB, Cable Bay fault; GR, Grease River H, Harrison fault; RO, Robillard fault; STZ, Snowbird tectonic zone; River shear zone and VRSZ, Virgin
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Saskatchewan Geological Survey 7 Summary of Investigations 2010, Volume 2 Ferrous Iron from Alteration of Silicates In addition to suggesting direct reduction of U6+ by graphite, Alexandre et al. (2005) proposed that Fe2+, liberated through chloritization of biotite or illitization of hornblende could act as a reductant. These reactions (not stoichiometrically balanced) can be generalized as:
+ 2+ K(Mg,Fe)3AlSi3O10(OH,F)2 (biotite) + H + H2O + Mg => Mg2(Al,Fe)3Si3AlO10(OH)8 (chlorite) + + 2+ K + SiO2 + Fe ,
+ + Ca2(Fe, Al, Fe)5(Si7AlO22)(OH)2 (hornblende) + K + H => K(Al,Mg,Fe)2(Si,Al)4O10(OH)2 (illite) + + 2+ 2+ 2+ Na + Ca + Fe + Mg + SiO2 +H2O, and
6+ 2+ + U + 5H2O + 2Fe = UO2 + Fe2O3 + 10H . Both chloritization of biotite and illitization of feldspar, biotite, and amphibole result in a significant mineral volume reduction (Kogure and Banfield, 2000; Alexandre et al., 2005; Kyser and Cuney, 2008a). Chloritization of biotite typically involves transformation of two biotite layers into one chlorite layer, losing two K interlayer sheets and two tetrahedral sheets, although less commonly, a potassium interlayer sheet is replaced by a brucite-like sheet (Kogure and Banfield, 2000). These void spaces increase permeability and facilitate subsequent fluid movement. However, as noted by Alexandre et al. (2005), the timing of chloritization is important, as chlorite can fill voids resulting in alteration without uranium mineralization. Although up to five different chlorite compositions have been documented in the basement (Quirt and Wasyliuk, 1997), chlorite paragenetically and spatially associated with Athabasca deposits becomes increasingly magnesium-rich proximal to the ore zones.
Chlorite, illite, and hematite alteration are typically closely associated with primary uranium mineralization at Athabasca deposits, both spatially (Thomas et al., 2006) and paragenetically (Hiatt and Kyser, 2007; Jefferson et al., 2007; Table 1; Figure 1).
3. Discussion The corridor between Key Lake and Eagle Point in the eastern Athabasca Basin, coincident with the western Wollaston Domain is notable as the locus of a high proportion of large, high-grade uranium deposits, including McArthur River and Cigar Lake (Figure 1). Graphitic pelites are common in the basement rocks underlying this uranium-rich corridor (Tran, 2001; Annesley et al., 2005; Jefferson et al., 2007; Yeo and Delaney, 2007, and others). Most of the Wollaston Supergroup is a classic foreland basin succession, in which basal quartzites, graphitic pelites, iron formation, carbonates, and calc-silicates represent an early starved-basin phase, overlain by upward-coarsening, compositionally immature clastics (flysch and molasse successions) shed from the central part of the Trans-Hudson Orogen towards the craton (Yeo and Delaney, 2007). Structurally, the Wollaston Domain is a broad synclinorium (Tran, 2001); hence the basal Wollaston starved-basin strata, including abundant graphitic pelite, are at a high structural level toward its western margin.
Although the case for reduction of U6+ by graphite or graphite-derived hydrocarbons is not convincing, graphitic pelites are important as potential structural controls. They are typically associated with underlying Archean granitic gneisses, interbedded quartzites and Trans-Hudson granites and pegmatites generated by partial melting of lower Wollaston pelites. The extreme rheological contrast between such rock types and graphitic pelites resulted in localized Trans-Hudson ductile deformation as well as syn- and post-Athabasca brittle faulting. As a result, in many of the Athabasca deposits, ore-controlling deformation zones are rooted in graphite-bearing basement units (Table 1; Figure 1).
4. Conclusions Reducing mechanisms involving Fe2+ are more likely than those involving graphite. The close spatial and paragenetic association of chlorite, illite, and hematite alteration with ore suggests Fe2+ was more likely derived from silicates than basement pyrite. This is reinforced by the lack of syn-genetic sulphides or sulphates produced during alteration. Although hornblende-bearing basement rocks are common, biotite-rich ones predominate; hence chloritization of biotite is likely the main source of Fe2+. Note that excess Fe2+ can also be taken up through illitization of plagioclase and chloritization of illite (Alexandre et al., 2005). If biotite-rich rocks (i.e., pelites) are the key favourable rock type for localization of unconformity-type deposits, much more of the Athabasca Basin is prospective than just the Key Lake–Rabbit Lake corridor.
Saskatchewan Geological Survey 8 Summary of Investigations 2010, Volume 2 Although graphitic pelites are not essential to create a redox trap for U6+, they are still important as loci of reactivated faults which acted as conduits for mineralizing fluids.
5. Acknowledgments This review was initially presented orally at the 2010 Saskatchewan Geological Survey Open House in response to encouragement from Colin Card. Ken Ashton, Roger Wallis, and others suggested that we write it up, and we thank Ken and Colin for their constructive comments on the manuscript. The idea that Fe2+ from alteration of silicates is certainly not new. The earliest suggestion that it might be involved in U6+ reduction appears to be in a paper by Wallis et al. (1985), written during the first main phase of Athabasca Basin exploration.
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