Paleomagnetic Directions as Indicators of Fluid Movement in the Athabasca Basin 1

J.C. Dobrohoczki 2, T.K. Kyser 2, and J. Baker 3

Dobrohoczki, J.C., Kyser, T.K., and Baker J. (1993): Paleomagnetic directions as indicators of fluid movement in the Athabasca Basin; in Summary of Investigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93-4.

The Athabasca Basin has been extensively explored for uranium following the discovery of high-grade unconfor­mity-type uranium deposits in the late 1960s. A wide range of techniques has been applied in the search for new deposits and in developing a model for deposit for­mation. Of great importance to any model is documenta­tion of the fluid history of the basin. Minerals deposited coevally from a mineralizing fluid may provide a more widespread indication of uranium deposit formation than the metal itself. One such mineral is hematite, which is pervasive throughout the sandstones of the Athabasca Basin as well as the underlying regolithic basement (Ho­eve and Sibbald, 1978). Hematite has been deposited by various fluid events including diagenesis, uranium mineralization, and later events (Kotzer et al. , 1992).

Hematite can record the paleomagnetic direction due to its high coercivity which allows it to acquire the declina­tion (horizontal component) and inclination (vertical com­ponent) of the paleomagnetic field at the time of crystal­lization (Tarling, 1971; Irving, 1964). This property serves to characterize the hematite formed in a particu­lar fluid event.

This report presents some of the preliminary paleomag­netic data obtained from unoriented drill core (no decli­nation) from various sites in the eastern half of the Athabasca Basin and a general interpretation and dis­cussion of the results.

1 . Regional Geology

The Athabasca Basin, located in the Churchill Province of northern Saskatchewan, is the youngest of a series of intracratonic basins that formed after the Huclsonian Orogeny (Aamaekers, 1981). In the study area, the Helikian Athabasca Group comprises the Manitou Falls, Wolverine Point, and Locker lake formations. The ba­sal Manitou Falls Formation is an eastward thickening wedge of fluvial and marine mature quartz sandstones and alluvial fan conglomerates (Aamaekers, 1979a; 1980a). Conformably overlying the Manitou Falls Forma­tion are the sandstones, siltstones, and mudstones of the Wolverine Point Formation (Ramaekers, 1981 ). The Locker Lake Formation overlies the Wolverine Point For­mation and consists of pebbly sandstones interbedded with a few thin siltstones (Ramaekers, 1981). The

Athabasca Group unconformably overlies Archean and Early Proterozoic gneisses and metasedimentary rocks within which there is a well developed paleoregolith sev­eral metres thick (Hoeve and Sibbald, 1978).

2. Previous Work

The first investigation of paleomagnetism in the Athabasca Basin was by Fahrig et al. (1978) who exam­ined numerous samples of essentially vertical, unori­ented (no declination) drill core samples from through­out the basin. Both reverse and normal inclinations were found distributed throughout numerous drill holes. Fahrig et al. (1978) indicated the possibility of using pa­leomagnetism to correlate lithologies, assuming the po­larity of reversals occurred during deposition of the for­mation and had not been altered since. Later investiga­tions showed that the hematite carrying the magnetiza­tion had been extensively remobilized by later post-de­positional fluid events (Ramaekers 1979b, 1980b; Lar· son and Walker, 1975).

Additional paleomagnetic studies did not occur until ex­tensive work had been done on determining the fluid history of the Athabasca Basin through petrology, fluid inclusions, and stable and radiogenic isotope analyses (Hoeve and Quirt, 1984; Wilson and Kyser, 1987; Kotzer and Kyser, 1990a and b, 1991 , 1992). Only af­ter, was it possible to relate fluid events, hematite par­agenesis, and paleomagnetism (Kotzer et al., 1992) (Figure 1).

Kotzer et al. (1992) determined that, Early Proterozoic (1600 to 1700 Ma) magnetization (A-magnetization) has a normal polarity and is associated with local fluid migra­tion during early diagenesis. Middle Proterozoic (1450 to 1600 Ma) magnetization (B-magnetization) has a re­verse polarity, and is associated with regional fluid mi­gration during peak burial diagenesis, and is coeval with formation of unconformity-type uranium mineraliza­tion that resulted from the mixing of saline, metal-bear­ing oxidizing basin fluids with reducing basement fluids. Late Precambrian (- 900 Ma) magnetization (C-magneti· zation) has a high-angle normal polarity and is associ­ated with a fluid pulse probably initiated by early uplift­ing and faulting of the basin. Extensive remobilization and deposition of uranium in some areas, such as at

(1) Funded by a joint NSERC University-Industry (Cameco and Uranerz) CAD grant. (2) Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, S7N OWO. (3) Pacific Geoscience Centre, Sidney, British Columbia, V8L 482.

Saskatchewan Geological Survey 161

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Hematite is altered to goethite (G1), which looses its magnetic susceptibility giving an incoherent D-magnetization. Uplift is likely to have occurred periodically be-tween - 800 Ma and the present.

3. New Work

a) Sampling and Analysis

The 130 samples used in this study are from vertically drilled, un­oriented (no declination) drill core from: McArthur River P2 North·

Figure 1 - Proposed mineral paragenesis-fluid history in the Athabasca Basin, devel­oped from petrographi~, fluid inclusion, and stable and radiogenic isotope systematics. Pa:agenes1s of hematite has been established using field and petrographic relation­ships and Pf:!leo~gnet~sm (mo:1ified from Kotzer et al., 1992). Abbreviations are: altn.=alterafton; d1ag.=d1agenet1c; mag.=magnetization; drav.=dravite· hem.=hematite· kaol.=kaolinite; sst.=sandstone; and qtz.=quartz. ' '

Sue Zones A, B, C, D, and E; Tele· phone Lake (located a few km southwest of the Sue Zone); and Rumple Lake (located in the mid· die of the Athabasca Basin). The Rumple Lake drill core was used as a control hole to study the deeper part of the basin because it was located far from any known mineralization and major faulting.

t~e Eagle Point deposit, is associated with C-magnetiza· t1on. ln~oh~rent magnetization of Phanerozoic age (D· magnetization) was caused by the incursion of low tem­perature meteoric waters which altered hematite to goethite.

In the initial stages of diagenesis, heavy-mineral suites within the Manitou Falls Formation were strongly altered to hematite (H1), and early quartz (01) overgrowths were formed (Kotzer et al., 1992). The H1 -hematite car­ries the A-magnetization. Coeval apatite gives a U-Pb age of 1650 to 1700 Ma (Cummings et al., 1987).

Prograde peak burial diagenesis at -200°C, involving the mixing of saline, oxidizing basin fluids with reducing basement fluids resulted in the remobilization of hema­tite (H2) and formation of diagenetic clays (K1 , 11 , C1 ). euhedral quartz (02), dravite (T1), and polymetallic ura­nium mineralization (S1 . U1) (Kotzer and Kyser, 1992). H2-hematite from this fluid mixing event carries the B· magnetization. lllite intergrown with H2-hematite has a Rb-Sr age of 14n ±57 Ma (Kotzer and Kyser, 1990b).

A late, high-temperature fluid pulse, probably initiated by early uplifting and fracturing of the basin, formed the ~oung~st hematite (H3) which carries the C-magnetiza­tton. llhte, formed together with H3-hematite, gives a Rb­Sr and K-Ar age of about 900 Ma (Kotzer and Kyser, 1990b).

Uplift and fracturing of the basin resulted in incursion of low temperature oxidizing meteoric fluids, formation of r~trograde mineral assemblages, and destruction of pre· v1ously formed unconformity-type uranium deposits. T~is proc_ess is ~anifested by pervasive kaolinite (K2) with drav1te (T2) in fractures, followed by remobilization of uranium (U2) and pyrite ($2), which were redepo­sited in late fault fractures, and late kaolinite (K3) forma· tion in reactivated fractures (Kotzer and Kyser, 1991 ).

162

As the drill core samples had to be re-cored for paleomagnetic analysis, samples were se­lected that had no fractures. All the samples were from the Manitou Falls Formation or the underlying regolith and basement, except for two samples from Rumple Lake which are from the upper Wolverine Point Forma­tion. Two oriented surface hand specimens were also collected from the Maw zone.

Paleomagnetic analysis of the drill core was done at the Pacific Geoscience Centre (PGC) in Sidney, B.C. Where possible, drill core samples were re-cored to ob· tai~ two one-inch diameter by one-inch high paleomag­~ellc cores. Analyses of the natural remnant magnetiza­tion and thermal demagnetization were made on the PGC's automated Schoenstadt SSM1 fluxgate spinner magnetometer.

b) Results

Of the 130 drill core samples that were analyzed, 58 had thermal demagnetizations that were reasonably strong and coherent enough to be classified as A, B or C. The remainder of the samples gave incoherent direc­tions and are tentatively classified as D-incoherent mag­netization. The two oriented surface hand specimens from the Maw zone gave D-incoherent magnetizations. The majority of the D-incoherent samples are from the upper units of the drill holes; in the Maw zone hand samples, hematite has been partially or entirely altered to low temperature goethite.

For a few samples, the magnetization changes over a core length of only a few centimeters from a reverse 8-to ~ n~rmal A- or C-magnetization or from A- to C-mag­net1zat1on. These changes occur even in drill core sam­ples of the same lithology and hematite staining and in­dicate that the fluid movement is permeability controlled.

Summary of Investigations 1993

The inclinations of the reasonably stable magnetizations (at 500°C thermal demagnetization) were combined with inclinations of the stable magnetizations of Fahrig et al. (1978) (at 560°C) to give a histogram of paleo­magnetic inclinations (Figure 2). The inclinations nor­mally can be separated into the A-, 8-, and C-magneti­zation events.

The Maw zone drill hole ZQ-11 (Figure 3) contains D-in­coherent magnetization throughout the core, except for some A and C directions and very strong 8-magnetiza­tion within about 1 O m of the unconformity. Core sam­ples from Sue Zone A, B, C, D, and E yielded predomi­nantly 0-incoherent magnetization, except samples from Sue Zone A (A 114 at 52.0 m) and B (8118 at 30.2 m) that are near uranium mineralization and which give 8-magnetizations. The few samples collected from Telephone Lake drill hole SP-66 are of A-magnetization (52.7 m and 69.0 m) and C-magnetization (64.0 m). The Rumple Lake drill core (74-J) has a mixture of A-, 8-, and C-magnetizations throughout the drill core, with 8-magnetization occurring in the regolithic basement. Petrographic relations indicate that the two D-incoher­ent magnetizations may occur in sandstones leached of hematite during the early diagenesis of the basin before quartz overgrowth (Figure 1 ).

McArthur River P2 North samples (Figure 4) are from the drill holes along a northeast-trending reverse fault that has uranium mineralization at or near the unconfor­mity. At drill hole MAC204, which intersects the mineral­ized zone at about 500 m depth, the magnetization above the unconformity is predominantly D-incoherent magnetization with a few B-magnetizations. Well below the mineralization and the unconformity, in the fault frac­tured basement, are numerous B· and C-magnetiza­tions. MAC214 is predominantly 0 -incoherent magneti­zation with some 8-magnetizations throughout the drill hole along with some A-magnetizations near the base. MAC206 is a mixture of A-, 8-, and D-incoherent mag­netizations throughout the drill hole, with some C-mag­netizations. MAC138 is predominantly D-incoherent and

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C-magnetizations, with 8-magnetizations occurring near the unconformity.

4. Discussion

A-, 8-, C-, and incoherent D-magnetizations are gener­ally present throughout the entire basin. These magneti­zations are intermixed throughout the drill core with inco­herent D-magnetizations typically present in the upper sections of the drill core as a result of pervasive altera­

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tfon of hematite to goethite by late, low-temperature meteoric waters. In very late fault fracture zones, D­incoherent magnetization is the pre­dominant magnetization throughout drill core (Figure 5).

Fluid flow was permeability control­led as indicated by the change in magnetization of samples within a few centimeters of each other. This is the result of partial overprinting by later fluids constrained by the permeability of the rock altered by previous fluid and diagenetic events or fracture episodes.

Figure 2 - Histogram of paleomagnetic inclinations at soo•c thermal demagnetization, in­cluding the data from Fahrig et al. (1978) at 560°C thermal demagnetization.

The 8-magnetization appears to be present throughout the basin near th e unconformity. This indicates that this was a basin wide event. The 8-magnetization also occurs

Saskatchewan Geological Survey 163

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Figure 4 • Paleomagnetism data from drill holes at McArthur River P2 North. Magnetizations separated by a slash are sam­ples where the two paleomagnetic cores have two distinct mag­netic inclinations.

in the upper sections of the Rumple Lake drill core, indi­cating that fluids associated with B-magnetization oc­curred both near the unconformity and in horizons in the Athabasca Group sediments well above the uncon­formity. Since the fluids associated with 8-magnetiza­tion seem to be a basin wide event, major fluid move­ment and mixing occurred during tectonic activity that re­activated basin fault fractures. Uranium mineralization, generally located at the intersection of major basement fault zones with the basinal sediments, occurred be­cause these fault zones allowed horizontally controlled saline, metal-bearing oxidizing fluids to mix with reduc­ing basement fluids. Fracturing in the fault zone control­led the permeability and thereby the flow rates and mix­ing ratios between oxidizing and reducing fluids, thus determining whether mineralization occurred at, above or below the unconformity.

5. Acknowledgments J.C.D. thanks T.K. Kyser and M. Fayek for collecting the first batch of samples from the Athabasca Basin and the staff at the Pacific Geoscience Centre; E. Irving and J. Wyne for helpful advice. Also thanks are ex­tended to Cameco and Uranerz for their assistance and funding through a joint NSERC University-Industry grant.

164

Cummings, G., Krstic D., and Wilson J. (1987): Age of the Athabasca Group, northern Alberta; Geol. Assoc. Can., Prog. Abstr. 12, pA35.

Fahrig W.F., Christe K.W., and Freda G. (1978) : The paleolati­tude and paleomagnetic age of the Athabasca Formation, northern Saskatchewan; in Current Research, Part C, Geol. Surv. Can., Pap. 78-1C, p1-6.

_ _ _ _ (1979): The paleolatitude and paleomagnetic age of the Athabasca Formation, northern Saskatchewan·reply; Discussions and Communications in Current Research, Part C, Gaol. Surv. Can., Pap. 79-1C, p119-120.

Hoeve J . and Quirt D. (1984): Mineralization and host rock al­teration in relation to clay mineral diagenesis and evolution of the Middle-Proterozoic, Athabasca Basin, northern Sas­katchewan, Canada; Sask. Research Council, Tech. Rep . 187, 187p .

Hoeve J. and Sibbald T.1.1. (1978): Uranium metallogenesis and its significance to exploration in the Athabasca Basin; in G. R. Parslow (ed.), Uranium Exploration Techniques, Sask. Geol. Soc. , Spec. Publ. 4, p161-188.

Irving E. (1964): Paleomagnetism and its Application to Geo­logical and Geophysical Problems; John Wiley and Sons Inc.

Kotzer T. and Kyser T.K. (1990a): The use of stable and radio­genic isotopes in the identification of fluids and processes associated with the unconformity-type deposits; in Beck, L.S. and Harper, C.T. (eds.), Modern Exploration Tech­niques, Sask. Geol. Soc., Spec. Publ. 10, p115-131 .

___ _ (1990b): Fluid history of the Athabasca Basin and its relation to uranium deposits; in Summary of Investiga­tions 1990, Saskatchewan Geological Survey, Sask. En­ergy Mines, Misc. Rep. 90-4, p153-57.

_ _ __ (1991): Retrograde alteration of clay minerals in uranium deposits; Chem. Geo!. (Isotope Geosci. Sec.), v86, p307-321.

_ _ _ _ (1992}: Isotopic, mineralogical, and chemical evi­dence for multiple episodes of fluid movement during pro­grade and retrograde diagenesis in a Proterozoic basin; in Kharaka, Y.K. and Maest, A.S. (eds.), Water-Rock Interac­tion, p1177-1181 .

Kotzer T .. Kyser T.K., and Irving E. (1992): Paleomagnetism and evolution of fluids in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada; Can. J . Earth Sci. , v29, p1474-1491 .

Larson E.E. and Walker T.R. (1975): Development of chemical remnant magnetization during ear1y stages of red-bed for­mation in Late Cenozoic Sediment, Baja California; Geol. Soc. Amer. Bull., v86, p639-650.

Ramaekers, P. (1979a): Stratigraphy of the Athabasca Basin; In Summary of Investigations 1979, Saskatchewan Geologi­cal Survey, Sask. Miner. Resour., Misc. Rep. 79-10, p154-160.

___ _ (1979b): The paleolatitude and paleomagnetic age of the Athabasca Formation, northern Saskatchewan­Oiscussion; Discussions and Communications in Current Research, Part C, Geo!. Surv. Can., Pap. 79-1C, p117-119.

Summary of Investigations 1993

MAC206 MAC138

w Rurnple Lake MAC204 E MAC214

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Profile not to scale

Figure 5 - Profile of the eastern part of the Athabasca Basin showing the general movement and distribution of fluids over time as indicated by paleomagnetic evidence to date.

____ (1980a): Stratigraphy and tectonic history of the Athabasca Group (Helikian) of northern Saskatchewan; in Summary of Investigations 1980, Saskatchewan Geologi­cal Survey, Sask. Miner. Resour., Misc. Rep. 80-4, p99-106.

____ (1980b): The paleolatitude and paleomagnetic age of the Athabasca Formation, northern Saskatchewan­Further discussion; Discussions and Communications in Current Research, Part B, Geol. Surv. Can., Pap. 80-18, p297-299.

Saskatchewan Geological Survey

____ (1981 ): Hudsonian and Helikian basins of the Athabasca region, northern Saskatchewan; in Campbell, F.H.A. (ed.), Proterozoic Basins of Canada, Gaol. Surv. Can., Pap. 81-10, p219·233.

Tarling D.H. (1971): Principles and Applications of Paleomag­netism; Chapman and Hall Publishing.

Wilson M.A. and Kyser T.K. (1987): Stable Isotope geochemis­try of alteration associated with the Key Lake uranium de­posit, Canada; Econ. Geo!. v82, p1540·1557.

165