Uranium unconformity

Uranium unconformity

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  Abstract. Unconformity relateduranium deposits from the Atha- basca basin (Saskatchewan, Canada) represent the world largest high-grade U ore-bodies. They are located in the vicinity of an unconformity between an Archean to Paleoproterozoic basement and a Paleo- to Meso-proterozoic intracontinental clastic basin. Their exceptional grade and size results from the combined effi- ciency of a series of U-fractionation mechanisms, which will be reviewed here. U-enrichment of the basement is exceptional. U- extraction efficiency from source rocks is shown by the alteration of highly refractory minerals such as monazite and zircon in the sandstone, the regolith, and the basement by slightly acidic, hot (160 to 220°C, about 1kbar), oxidized and Na-Ca-Mg rich (up to 6 molar Cl) basinal diagenetic brines generated within continental, oxidized, organic free, clastic formations. Exceptional trapping conditions resulted from (i) the strong redox gradient developed between the oxidized Paleo- to Meso-Proterozoic sediments and the epicontinental Paleoproterozoic organic-rich metasediments of the basement and (ii) the creation of large openings by the combined effects of reverse faulting and intensive quartz disso- lution in the sandstone. U-Pb geochronology and REE patterns of the U-oxides provide evidence for a succession of mineralization events rather than one unique ore-forming event. Keywords. Uranium, unconformity related, Proterozoic, metallo- genesis 1 Introduction Unconformity related uranium deposits from the Atha- basca basin (Saskatchewan,Canada) represent more than half a million tons U @ more than 10 % in average. With the development of the Sue, Midwest and the high grade - large tonnage McArthur (225,000t @15.34 %U) and Ci- gar Lake (142,000t @ 15 % U) deposits, the production of Canadian unconformity-type deposits is expected to reach about 18,000 metric tons U by 2007. Most of the results presented here derive from recent studies of the Athabasca Uranium Province (AUP) with offers more than 30,000 continuously cored drill holes through the basin, generally down to the basement, and to a lesser extend from the East Alligator Rivers Uranium Province (EARUP) with large but lower grade deposits. The un- derstanding of the mechanisms leading to the forma- tion of such metal accumulations at that time represents a major scientific challenge. The efficiency of the suc- cessive fractionation processes occurring during metal extraction from the source, its transport and its deposi- tion will be evaluated here. 2 Relative importance of the uranium sources A first parameter to consider is the level of the metal con- centration in the potential source rocks and the stability of each metal hosting phase. Possible uranium sources of unconformity related deposits were the basement and the sandstone cover. 2.1 Potential U sources in the basement Archean in the AUP is mainly composed of U-poor mag- netite bearing tonalities and cannot have represented an uranium source, but in the EARUP high K-Th-U Archean granites does exist. Paleoproterozoic sediments in both provinces consist of epicontinental clastic to chemical de- posits.DuringPaleoproterozoic,thestrongoxygenincrease in the earth atmosphere,led to a worldwide storage of con- siderable amounts of uranium and carbon matter in these sediments. Graphitic schist, meta-arkose and calc-silicate showsyngeneticU-enrichmentsinbothprovinces.Graphitic schist, cannot have been a major U-source because the re- ducing conditions created by graphite and associated sul- fides, prevent uranium leaching. But, uraninite bearing calcsilicatesandmeta-arkosesmayhavebeeneasilyleached by oxidizing fluids.Leucogranites and pegmatoids derived by partial melting of the Paleoproterozoic metasediments (Sibbald et al.1976; Annesley et al.2003) are frequently en- riched in uraninite and monazite. Paleoproterozoic high- K calc-alkaline granitoids, rnriched in Th and U represent up to 50% of the basement rocks in the Western AUP (Brouand et al. 2003) and also exist in the Eastern AUP (Annesley & Madore, 1991) and in the EARUP (Ferguson et al 1980).They are 1800±30 Ma old,in EAUP and EARUP, but of Taltson age (1930±30 Ma) in the WAUP (Brouand et al., 2003). Late Hudsonian vein type U-deposits (e.g. Beaverlodge, Gunnar) also demonstrate the ability of the PaleoproterozoicbasementtorepresentafavorableU-source and these deposits may also themselves have contributed as U-sources for the unconformity related deposits. 2.2 Potential U sources in the cover The clastic sediments are considered as the major U- source by many authors because they hosted oxidized flu- ids able to transport uranium. However, average U-con- Chapter 3-7 World-class unconformity-related uranium deposits: Key factors for their genesis M.L. Cuney Unité Mixte: Géologie et Gestion des Ressources Minérales et Energétiques, and UMR G2R 7566 - CREGU - UHP, BP 239, F-54 501 Vandoeuvre les NANCY Cedex, France 3-7
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  246 tent of thesandstones, away from the mineralized areas, is below 1 ppm, and 50 to 80% of U is presently bound to zircon.But,original U-contents may have been higher and leached during the diagenetic events (Kotzer and Kyser 1995). In clastic sediments, U can be distributed among 4 major sites: (i) in detrital accessories, (ii) adsorbed on clay minerals and Ti-Fe oxides, (iii) precipitated in or- ganic matter bearing layers, (iv) bound to acidic volcanic ash. However, the amount of U, originally present as U- oxide, must have been very low, owing to the oxidizing conditions during deposition of these Proterozoic conti- nental sandstone, free of organic matter. Only monazite may have represented a significant U-source. 3 Efficiency of uranium extraction Incongruent dissolution of monazite with new formation of a Th-U silicate with lower Th/U ratio and Ca-REE-Sr Al-phosphates-sulfates(APS),bothintheclasticsediments and in the altered sections of the basement allowed the extraction of about 75% of its uranium (Hecht and Cuney 2000a, b). Zircon was also altered with enrichment in U, Ca, LREE, Al, P, elements typical of the monazite alter- ation products (Hecht andCuney 2000a).Such alterations, first observed in the Franceville basin,Gabon (Cuney and Mathieu 2001) also occur basin wide in the Athabasca and Kombolgie clastic sediments. Average Th content of the sandstone being below 10 ppm (except in the lower Manitou Falls Formations of the Eastern Athabasca), the amount of U deriving from monazite alteration was lim- ited. Moreover, a significant part of the liberated U was trapped in altered zircon (U increase from a few hun- dreds to several thousand ppm), in Fe-Ti oxides deriving from detrital Fe-Ti oxide alteration. 4 Efficiency of uranium transport 4.1 Fluid characteristics Most authors agree that U was transported by the basinal brines. U solubility was favored by their high fO2 (well within the hematite field) and chlorinity (up to 6 molal). High fO2 resulted from the lack of organic matter in the sandstone. The pH, controlled by the kaolinite-illite paragenesis, was slightly acidic (ca. 4.5 at 200°C) because of the lack of feldspar in highly mature quartzose sand- stones or from its alteration during diagenesis. Early di- agenetic brines from detrital quartz overgrowths are Na- rich and derive from evaporitic layers in the upper part of the basin, whereas the brines trapped later in perva- sively silicified zones and drusy quartz, close to the min- eralized zones, were enriched in Ca through their inter- action with Ca-rich basement lithologies (Derome et al. 2003, 2005). One of the key factors for the genesis of high grade U-ore is the high-Ca content in the mineralizing fluid which led to: (i) dissolution of U, even from mona- zite, (ii) new formation of U-poor APS, (iii) Ca, REE, U, Al,P enrichment of zircon altered zones (Hecht and Cuney 2000a, b; Cuney et al. 2000), (iv) strong change in quartz solubility, which may have contributed to its dissolution in the mineralized zones. No experimental data exist to quantify the effect of such fluid compositions on U-solu- bility, but laser ablation ICP-MS analyses (ongoing stud- ies, coll. C. Heinrich, ETH, Zürich) reveal that the Ca-rich brines are the richest in uranium.U solubilities of 30 ppm were calculated by Raffensperger & Garven (1995) for 5 molal Na-Ca chloride solutions at 200°C for a fO2 of –20. The concentrations of U-ligands other than chlorine (F, P, CO2, S) was severely limited by the low solubility prod- uct of fluorite, apatite, calcite and gypsum imposed by high Ca-activity in the brines. 4.2 Temperature regime The temperature at the base of the basins during the min- eralization event was assumed to be slightly higher than during early diagenesis and to be constant everywhere at about 200-250°C (Kotzer and Kyser 1995).Such high tem- peratures would mean anomalously high thermal gradi- ents: 40-50°C/km for a 5 km thick basin. New fluid inclu- sion studies (Derome 2003, 2005) show that T and P at the unconformity decrease from the early diagenetic (160- 220°C,1-1.25 kbar) tothe mineralization stage (140-160°C, 0.6 kbar). A late, low saline, CH4 bearing fluid, deriving from the basement which mixed with the basinal brines is common in the EARUF but rare in the AUF. 4.3 Duration and age of ore deposition The duration of the alteration and mineralization pro- cesses cannot be quantified by geochronology. U-Pb ages on U-oxides have an error of several tens of Ma.The scat- ter of the data can be explained by Pb diffusion out of U- oxides because of its larger ionic radius. Duration of the processes obtained by numerical modeling require nu- merous parameters which are not all well constrained. The formation of an ore body, such as Cigar Lake, re- quires a few million years and several tens of km3 of fluid, using 5 to 10 ppm U in the solution and a 0.lm/year fluid percolation rate. Mass balance calculations using the amount of dissolved silica in a volume derived from GOCAD 3D modeling of chlorite breccias associated with U-ore deposition,gives a minimum fluid volume of about 1 km3 in the EAUP, and a fluid/rock ratio of ~10,000 (Lorilleux et al., 2003). Recent U-Pb dating by ion probe have given ages up to 1570 Ma (Alexandre and Kyser 2003) much older than previous estimations. Combined U-Pb isotopic dating and REE analyses by ion probe (Bonhoure et al. 2005) show that different U-oxide generations in the same deposit may have different ages and distinct REE M.L. Cuney
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  247 abundance, but allgenerations having the same REE pat- tern. Bell shaped REE patterns centered on the middle HREE seems to be typical of the uranium oxides from unconformity related uranium deposits.However,it is still uncertain to certify if accretion of multiple stages of U deposition is required to explain the high grade-large ton- nage of these deposits. 5 Efficiency of trapping conditions Accumulation of such high-grade ore requires the cre- ation of open space and an efficient reaction to destabi- lize the U-complexes and to reduce UVI+ . 5.1 Tectonic reactivation Space can be first created by tectonic reactivation of base- ment faults.Most unconformity-type deposits are related to tectonic contraction leading to reverse faulting of the basement and creation of preferential opening near the strong mechanical anisotropy represented by the unconformity. Such faults were already active during the retrograde orogenic events affecting the basement with graphite-sulfide deposition and during sedimentation. If faulting is important to create or rejuvenate the per- meability of the structures, the magnitude of the dis- placement along the fault does not seem to control the size of the ore-body. For the two largest deposits, the offset of the reverse fault reach 85 m at McArthur River, but just a small bump exists at Cigar Lake. 5.2 Quartz dissolution Another major way of creating space is the huge quartz dissolution associated with U-mineralization and asso- ciated breccia bodies (Lorilleux et al. 2003). A volume loss up to 90% was estimated from mass balance calcu- lations in altered sandstone. The upward decrease of al- teration intensity from the unconformity and the input of basement-derived elements (Mg for sudoïte and dravite, K for illite) indicate that the fluids derived from the basement (Kister et al. 2005). As no temperature in- crease, no boiling or unmixing is observed during this event, quartz dissolution may be related to the sharp de- crease of silica solubility in the basement derived Ca- rich aqueous fluid with respect to basinal Na-rich or to less saline aqueous fluid as shown by experimental data (Marshall and Chen 1982). 5.3 Basement derived reduced fluid A basement derived reduced fluid was proposed to ex- plain U deposition,the source of Mg (dravite and sudoite alteration), B (dravite) in the sandstones, and Ni, Co, Cu, Zn, Au in the polymetallic deposits. However, direct evidences for such a reduced fluid are still very weak. Current studies in the AUP and EARUP have identi- fied CH4 in the gas phase of some highly saline inclu- sions and in gas-rich inclusion in the basement. Hydro- carbons reported in the Rabbit Lake (Pagel et Jaffrezic 1977) and Nabarlek and Jabiluka (Wilde et al. 1989) de- posits have only been detected by global analyses of crushed quartz. Leaching of iron in the sandstone in the alteration envelope of the U-deposits represents a fur- ther indication of the percolation of a reducing fluid (Kister et al. 2005). 5.4 Role and origin of organic compounds A genetic link between basement graphite and bitumens associated with the U-deposits has been rejected by sev- eral authors.But,the similar isotopic composition of bar- ren bitumens and basement graphite suggests a deriva- tion of the bitumens by hydrogenation of carbon (Landais 1996), as well as the intimate relations between graphite and hydrocarbons determined by synchrotron (Annesley et al. 2001). Sangely et al. (2005) from in situ analyses of C-isotopic composition by ion probe and aliphaciticy by Fourier Transform Infrared Microspectroscopy propose an abiogenic origin for bitumens. Thus, the genetic link between organic matter and U-deposition is still contro- versial. 6 Conclusions Advanced analytical techniques, and geochemical and 3D modeling have permitted to reassess the various hypotheses proposed for the genesis of the exception- ally large tonnage high-grade unconfor-mity-related U deposits. The degree of U and graphite enrichment of the Paleoproterozoic basement was a key factor, but, the extreme reactivity of the oxidized, highly saline, Ca- Na brines generated at the base of thick, organic matter and K-feldspar free, Proterozoic sandstone basins, able to scavenge U even from very refractory minerals such as monazite, seems to be the most important key factor. The intensive quartz dissolution produced by large vol- umes of basement derived reduced and silica undersatu- rated fluids was also a key factor for creating the space necessary for high-grade ore deposition. The role of a reducing fluid remains not well understood.At the world scale, several Proterozoic basins (e.g. Thélon, Canada; Riphean of Siberia, Jotnian of the Baltic shield with the Karku deposit) share similarities with the Athabasca and Kombolgie basins, but need to be better characterized to evaluate their potentialities for hosting unconformity- related deposits. More recent continental clastic basins contain interstratified formations rich in organic mat- ter, preventing the genesis of large volumes of highly oxidized fluids. Chapter 3-7 · World-class unconformity-related uranium deposits: Key factors for their genesis
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