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- 89 Uranium Metallogenic Studies: Maurice Bay Deposit M. Mellinger Previous work in the Maurice Bay area and on the Maurice Bay uranium deposit has been surrmarized by Harper (1979) and ~llinger (1979); the reader is referred to these articles for a description of the regional geology, and the i.nnediate geologic setting of the uranium mineralizatico. Investigations during the 1980 field season carrprised a detailed study of the stratigraphy of the Athabasca Group in this locality and detailed sarcpling for lithogeochemi.cal, petrographic, and mineralogical studies. Stratigraphy of the Athabasca Group A detailed stu::iy of the stratigraphy of the Athabasca Group in the vicinity of the Maurice Bay deposit was carried out through the logging of drill holes along three main secticos (102 + OOE, 015 and 014 + OON, see Fig. 1); correlations ....:ere verified using intenrediate sectioos. The stratigraphy oonsists of a ~ r Sequence , characterised by abundant mud.stone, and an Upper Sequence of conglcrrerates and sandstones. J:.o..Jer Sequence a) b) Fanglarerate, discontinuous and occurring in lows at the sub-Athabasca unoonformity, is the basal unit. I t is a m.rldy conglomerate**, corrprising angular quartz pebbles and cobbles, angular to sub-rounded clay (yell<:M) granules and pebbl es, and hematitic (black) granules (rarely pebbles) , in a dark red muistone to sandy mudstone matrix. i'bere bleaching occurs, the matrix is a light orange . The fanglarerate is interpreted as a wadi de!X)si t of surficial llt\ld and fragrrents. Interlarninated sandy mudstane, muddy sandstooe and nudst<Xle overlie the fanglorcerate. The mud/sand ratio is usually between 1: 1 and 1: 3 and several cycles, fining upward fran sandy mudstone to mudstone, and 40 an to 1500 an thick are carm::mly present. Mudclasts and mudcracks are camon, as are thin * Saskatchewan Research Council. Project not funded by the Saskatchewan Geological Society ** petrogr aphic names are given according to Folk ' s c l assificatioo (Folk, 1974). conglareratic beds, oontaining angular quartz granules and pebbles and clay granules. Colour is dark red, except where bleaching has occurred. This tmi t is interpreted as representing a paralic environrrent wherein sedinentation was rrore persistent than the rrore erratic depositional episodes of the fanglorrerates. Upper Sequence 'lhe upper s~oe carprises muddy sandy conglarerates and muddy sandstones with a mud/ sand ratio usually l&er than 1: 3. The depositional envirannent is interpreted as fluvial. Four sucressive units can be distinguished: a) Unit 1: rrostly muddy sandy oonglarerates, and congloneratic muddy sandstones, with local muddy sandstones, ccmronl y as gradatiooal , f ining up.,,ard seqtena:s (cycles). Pebbles are nostly quartz, but include lithic fragmants of baserrent derivatiai;argillated granules are abundant. b) Unit 2: essentially muddy sandstme, locally granular; rare pebbles (mainly quartz); sandstones range fran very ooarse to very fine; sane mudstales. c ) Unit 3: muddy sandy conglarerate arrl cooglorreratic muddy sandstcne, with minor muddy sandstore. Includes fe....:er lithic and argillaa:ous fragrrents than Unit 1. d} Unit 4: characterised by muddy sandstone with dispersed well-rounded ("rare") quartz pebbles every f~ netres in core. The oontacts between the units are erosional. 'lhe gereral pattern of transportation and sedinentation seerrs t o be strcngly influenced by pree xisting tqx:,graphy . No evidence of widespread faulting was f otmd; bedding is generally undisturbed. local disturbana: is indicated; for exarrple, by vertical dips over a 1.5 m interval in drill hole MB-724, and can be rel ated to nearby clayey alteration and mireralizaticn. Harper (1979) suggests that post-Athabasca block faulting is responsible for the present relief at the sl.b-Athabasca unconfonnity. Stratigraphic - 90 - "015 " ,::· 104 + DON (112+50E) Cmodlfled of1er LEHNERT-THIEl et "015'' 19 79 l al. , MAP OF THE HELI KIAN UNCONFORMITY AND OUTLI NE OF Minerl)tizctlon T HE MAURI CE BAY URANIUM DEPOSIT WITH SATELLITE OCCU RENCES . J C > cu loff } Miriuollzollon ( < cutoll ) 2:J Heliklon untonfonnlt y { dt pth In meir u j SC~LE 0 100 20 0 300 M1 tr u / FIGURE 1 ( S ect ions ,rudit d I n de tcu l } .. 11 1! H<.ilo~ t ll[ATllERfO I GREEN ZONE (A) UNWEATHERED RED ZONE\ GREEN ZONE (8) Rock Color: "feldspar s " pi nkish , whi ti sh "biot it e" dark brown Mi ne r alogy cordie rit e feldspa r s -- - yellow greenish - ...... - -- --- green -- red il 1 it e --- garnet biot it e i 11 ite --- c hl ori te --- c hl or ite --- hemH 1te quartz (una ~fected) gra ph it e (unaf fected ) Figure 2 • Changes in rock color a nd mineralogy i n a typical weathering profi l e (pel it ic gneiss) - 91 - evidence , as well as the absence of widespread faulting in the Athabasca Group near the edges of the basenent horst, suggests that the pr esent basen-ent topography is essentially pre-Athabasca; however, l ocal reactivation of pre-Athabasca fault zones i s considered as a possibility, and may even be a prerequisite to uranium mineralization . clayey alteratim facies, can be traced fran cne· drill hole to the next. The alteration patterns are not related to specific stratigraphic units, and as a whole, are far rrore extensive on the southern side of the basetrent horst, where northward mi.grating fluids seem to have been stopped against the southern f lank of the structure. Paleo-weather ing i n Basement Rocks Sandstme Secondary Facies Seccodary facies in the Athabasca Gr oup ("sandstone" here afte r) are related to diagenetic and hydrothermal processes. Three facies in the sandstone are considered as resulting fran general diagenetic processes not directly related to mireralization. Purple sandstone aoguired its rolour through the deposition of a thin hematite coating on quartz grains. Primary heavy mineral cmcentrations (hematite grains) occur, although thin section examination shows these hematite grains t o have been overgr0vn by a new generaticn of hematite. '!he purple facies could correspond to an earl y pervasive oxidizi ng diagenet i c stage, as other facies always appear to be younger than the purple facies where interacticn occurs . Thus , redbed characteristics might have been predcmi.nant at an earl y stage . The orange facies in sandstone correspcnds to a general bleadring (reduction) of pre-existing hematitic (purpl e) sandstone . Where diss olution of hemati te i s not a:mplete , ghost heavy mireral streaks are still distinguishable along bedding pl anes. Light green sandstone i s associated with orange sandstone, and is referred to as the "regular green" facies, in ocntrast to another green facies which is related to alterati01 processes (see bel ow) • A well developed pal.ea-weathering profile can be d::>served in baseirent rocks fran the unconformi. ty domwards. The rocks show a specific up..rard zonation in col our, due to successive mineral weathering reactions (Fig. 2). In hand specirren first a "green" zooe is d::>served, in which the rocks have a :rrediurn green to yellc:kl col our, depending on original mineralogy, ~ a "red" zme marked by hematite on iron-bearing minerals (micas). 'Ihe transition between the green and red zones camonly occurs as hematitizaticn of the green alcng fractures. Similar observations are cormonly made elsewhere in the Athabasca Basin. In thin section , cordierite is rare l y fresh and was prd::>ably altered to hydromuscovite during retrograde rretanorphism. Both potassic feldspar .and plagioclase are altered to illite (argillation) , apparently bef ore garnet and biotite are affected; this justifies the s ubdivisirn of the green zcne i nto two sub-zones (A and B, see figure 2) . Biotite is the l ast netanorphic mineral to be altered, arxl i t is progressivel y chloritized (sOTietines partly sericitized) with a parallel exsolutioo of Fe-Ti oxides in cl eavage pl anes . In the red zone , the majn change in mineralogy is the develcprrent of hematite m chlori te, which is respcnsible for the strcng red col ouration. Zirocn, apatite, graphite and quartz are unaffected by weathering reactions. I t is ilrportant to note that the ori ginal texSeveral a l teration facies are recognized, ture of the rock i s well preserved throughout which are related i n varying degrees to the weathering profile . Thus, variations in uranium mi.reralization . 'nle green alteratim the aspect of a rock for the sane weathering f acies occurs near strongl y altered zcnes, stage is dependent upcn both original mineralogy and is recognized by a rredium to dark green and texture. Grain size is an inportant variacolouring of the rock. Where alteration bility factor; a roarse- grained mineral r eactions i nvol ve consurrptim of quartz grai ns , (especially feldspars and garnet) will resist a clayey al teration facies (usually of yellow weathering longer than its fine- grained equivato whi te colour) i s developed; this facies i s lent, thus introducing an apparent ananaly in indicative of very strong chemical activity . the weathering sequenoe. Two other alteration facies seem to grade into each other: a ~ and a dark red, both Al ter ation in Baserrent Rocks related to a strong hernatitizatian of the rock matrix. All the alteratim facies appear t o be re l at ed t o uraniun mi.reralization; the In plares where the uranium mineralizat im is dark r ed (hemati tizatim) facies is the nost cl ose t o or straddl es the unoonformity , a strong closely rel ated as it i s frequantly accaipanied argilli c alteraticn smri.lar to that ooserved in the Athabasca Group affects baserrent rocks by pitchblende concentrations. (Mellinger, 1979). Quartz solution is well Sare of the alteratioo facies , particularl y the defined, and inte rrrediate stages of quartz - 92 - digesticn can be observed in thin secticn, Clay minerals in the altered rocks are illite (white mica) and chlorite. In one case, anphibolite is affected (Section 106 + 25E), and extensive carbanatization accacpanies the develq:ment of clay minerals suggesting co2 as a prominent conponent of the alteration and prdlably also mir.eralizing fluids. 5. The source for uranium may be within the Athabasca Group, as a background concentration, or the underlying unweathered or weathered baserrent carplex, as a background or pre-existing ore ooncentration (similar to the A zone or to other known baserrent occurrences) • Acknowledgerren ts Guidelir.es to Ore Genesis The foll<Ming are ccnsidered guidelines to ore genesis at Maurice Bay. 1. 2. 3. 4. The baserrent horst structure to which mineralizaticn is spatially related is of pre-Athabasca age. This is inferred from the paleo--athering profiles developed along faults (M=llinger, 1979), as -n as from stratigraphic relationships within the Athabasca Group; Uranium mineralization in the A zone may have been enplaced before the deposition of the Athabasca Group, at a tine of major Hudsonian fault develq:,nent in baserrent rocks. It is thus older than that in the Main and B zones; 'lhe Athabasca Group suffered late anchizone (transitional to netarrorphism) diagenesis (Kubler, 1968; Dt.moyer de Segonzac, 1970). ValU=s for the Kubler (crystallinity) index of illite in mudstones usually fall between 5.0 and 6.0. In this type of environrrent, primary porosity is destroyed and pore fluids (rrost likely brines) can only travel in rocks where secondary f()rosity has developed through mineral dissolution and/or local fracturing (Hayes, 1979; Sdunidt and M:::Donald, 1979); Uranium deposited in the Main and B zones is thought to have been transf()rted and precipitated under late diagenetic physi(X)chemical conditions. Uranium was carried in solution by brines flooing along paths determined by secondary porosity reveloprrent. Potassilnn and magnesium were present in solution, resulting in the developnent of white mica and chlorite, and co may have played a significant role 2 in ronplexing u6+ (or rather uoi+). The simultaneous def()sition of pitchblende and hematite (a cornrcri association at Maurice Bay) is possible under specific physicochemical =ditions if a sufficiently high terrperature is attained (Langmuir, 1978). reposition of the Main Zone along the southern flank of the basenent uplift could have resulted from significant changes in hydrogeological conditions d\E to baserrent topography, as -11 as fran discharge of solutions of contrasting chemistry through reactivated baserrent faults; Uranerz Exploration and Mining Ltd., Eldorado Nuclear Ltd., and the Saskatchewan Mining r::evelopnent Corporation are gratefully acknowledged for their support of the present research project and for their permission to publish this contribution. Thanks are extended to Uranerz Exploration and Mining Ltd. for their co-q:,eration and hospitality during field v.0rk. References Dunoyer de Segonzac, G. (1970): 'lh.e transformation of clay minerals during diagenesis and low-grade rretarrorphism: a review. Sedinentology, 15, 281-346. FoJk, R.L. (1974) : Petrology of Sedirrentary Rocks. Henphill Publishing Co. , Austin, Texas, 182 pages. Harper, C.T. (1979): Uranium !!€tallogenic studies: Maurice Bay area, geology and mineralization. In: Surrrnary of Investigations, 19 79 , Sask-:-Geol. Survey Misc. Report 79-10, 96-106. Hayes, J.B. (1979): Sandstone diagenesis, the hole truth. In: As~cts of Diagenesis, P.A. Schollce and P.R.S. Schulger ed., SEPM S~cial Publ. #26, 127-139. Kubler, B. (1968): Evaluation quantitative du rretarrorphisne par la crystallinite de l'illite. Etat des progres realises ces dernieres annees. Bull. Centre Rech. Pau S.N.P.A., I, 385-397. Langmuir, D. (1978) : Uranit.nn solution - mineral equilibria at low temperatures with applications to sedirrentary ore deposits. Geochirn. Cosrrochirn. Acta, 42, 547-569. ~llinger, M. (1979): Uranium metallogenic studies: Maurice Bay deposit, geology and petrography. In: Surrmary of Investigations 1979, Sask. Geol. Survey Misc. Report 79-10, 107-llO. Schmidt, V., McD::mald, D.A. (1979): 'lhe role of secondary porosity in the o:,urse of sandstone diagenesis. In: As~cts of Diagenesis, P. A. Scho]ke and P.R,S. Schluger ed., SEPM Special Publ. #26, 175-207.