Regional Seismic Images Beneath the McArthur River Ore Bodies, Saskatchewan

Transcription

1 Regional Seismic Images Beneath the McArthur River Ore Bodies, Saskatchewan Z. Hajnal 1, E. Takacs 1, D. White 2, B. Reilkoff 1, B. Powell 3, and R. Koch 4 Hajnal, Z., Takacs, E., White, D., Reilkoff, B., Powell, B., and Koch, R. (2002): Regional seismic images beneath the McArthur River ore bodies, Saskatchewan; in Summary of Investigations 2002, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep , CD-ROM, Paper D-4, 5p. Abstract Two subparallel, regional seismic reflection profiles of the multidisciplinary and multi-institutional EXTECH IV project outline fundamental relationships between the local structures of the McArthur River uranium mine of the Mesoproterozoic Athabasca Basin and the underlying deep crustal tectonic framework. This deep reflection sounding investigation crosses over the P2 ore body, which is located in the region of a magnetic low. Tomographic inversion of the first-break arrival times indicated lateral changes in P-wave velocities of the sandstone along the survey lines. Distinct high-velocity zones mark alteration of the sandstone over known mineralized zones in the P2 ore body. The migrated time sections are characterized by robust reflection signals from near surface to past-moho depths. The sandstone-basement unconformity is clearly imaged, however, its correlation is more difficult along the central and southern part of the profiles due to influences of major structural disturbances. The differing patterns of reflections observed along the profiles will potentially allow distinction of the Wollaston/Mudjatik Transition Zone from the Wollaston Domain. South dipping reflections are traceable to a 4 km depth in the crust and are correlated with the P2 reverse fault that hosts the P2 ore bodies of the McArthur mine. The P2 structure is over 2.5 km thick and its seismic signature suggests multi-phase deformation. Strong, arcuate, and in places subparallel, zones of reflectivity dominate the middle and lower crust. The origin of these prominent seismic events appears to be located at the southern margin of the seismic investigation. Highly prominent, bright and sub-horizontal reflections, at 2.3 s two-way time (TWT), are the most distinct seismic signatures on the sections. These events are directly comparable to reflections observed along the northern half of the 1994 LITHOPROBE regional line S2B. The origin of this enigmatic zone of reflectivity is still under considerable discussion; this zone in 1994 data has been associated with the post-hudsonian Mackenzie Igneous Event (1.265 Ga). The Moho is well defined at about 10.5 s TWT and is characterized by a number of laterally recognizable gently south-dipping reflections. Below the Moho, a zone of strong reflectivity delineates the upper mantle, revealing considerable tectonic involvement of the lithospheric mantle within the depth range of 35 to 45 km. A gentle southeastern dip in the tectonic development of the region symbolizes the overall structural attitude of this highly reflective zone. Keywords: Seismic, reflection, data processing, crust, lithosphere, structure, convergence, sandstone, unconformity, seismic images, uranium, Athabasca Basin, Mesoproterozoic. 1. Introduction This regional seismic reflection survey was conducted at the McArthur River mine site in northern Saskatchewan (Figure 1) in the eastern part of the Athabasca Basin, as part of the EXTECH IV Athabasca Uranium Multidisciplinary Study (Jefferson et al., this volume; White et al., this volume). The setting of the basin, in relation to the major geological provinces, is discussed by Hoffman (1990). In the McArthur River area, the sedimentary sandstone fill (approximately 550 m) of the basin is limited to the a, b, c, d members of the Manitou Falls formation (Matthews et al., 1997). The ore bodies in the basin are recognized as characteristic unconformity-type uranium deposits, the characteristics of which are well documented in the literature (Sibbald, 1986; Hoeve and Quirt, 1984). The uranium deposits are in close proximity to the Sub-Athabasca unconformity and the paleo-weathered zone developed is basement rocks. The ore is structurally controlled by steeply dipping faults, which offset the 1 Dept. of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2. 2 Continental Geoscience Division, Geological Survey of Canada, 615 Booth Street, Ottawa, ON K1A 0E9. 3 Cameco Corp., th Street West, Saskatoon, SK S7M 1J3. 4 COGEMA Resources Inc., th Street West, Saskatoon, SK S7L 5X2. Saskatchewan Geological Survey 1 Summary of Investigations 2002, Volume 2

2 unconformity by 20 to 50 m and obliquely intersect the traces of conductive graphite pelite gneisses in the basement. Beneath the eastern part of the basin, the basement comprises the Wollaston and Mudjatik domains of the Archean Hearne Craton. Portella and Annesley (2000) suggested the basement includes the western Wollaston Domain and transition zone between the Wollaston-Mudjatik domains; they recognized six major stages of deformation and metamorphism. The southeastern margin of the Hearne Craton was deformed and metamorphosed during the circa 1.8 Ga continentcontinent collision of the Trans- Hudson Orogen (THO). Figure 1 - The area of interest on the Magnetic Vertical Derivative Map. White marks the regional lines, as well as the ore bodies. 2. Seismic Survey The deep reflection survey consisted of two north-northwest trending profiles Line-A which is approximately 10 km in length and Line-B, approximately 30 km. The profiles transect the P2 and P2 North ore bodies, in the region of a magnetic low. The seismic source consisted of three, kg IVI-2400 Vibroseis units with a total peak force of kg. The data were collected with a 960 channel recording system. Vibration point (VP) intervals were 25 m for 3 km at the ends of the line and 50 m through rest of the profiles. The upsweep frequencies were 10 to 84 Hz, with 10 sweeps at each VP site, sweep length of 28 s and recording length (correlated) of 18 s. Due to the short length of the profiles, the nominal stack fold reached only 120%. 3. Data Processing and Interpretation Tomographic inversion of the first-break (FB) data provided an intriguing insight into the acoustic properties of the basin fill. A distinct high velocity zone is coincidental with the location of the P2 North ore body. More specifically the seismic data defined a significant halo zone within the sandstone section and vertically above the ore zone. This seismic halo is interpreted to correspond to a zone of hydrothermal silification of the sandstone that has been recognized in lithologic logs of boreholes (McGill et al., 1993) and is detectable by other geophysical techniques (e.g., Craven et al., 2001; Mwenifumbo et al., 2001). The reflection data were subjected to a first phase of editing, geometry assignment, and refraction static corrections. Surface consistent deconvolution and spectral whitening assisted in reduction of the amplitudes of the coherent noise events. Suppression of highly contaminating coherent ground roll was attempted by implementation of surface wave noise elimination routines and by following a multi-step F-K filtering operation. Residual static, as well as standard NMO corrections and a CDP stack followed these signal enhancement procedures. A number of random noise suppression operations (F-X deconvolution, trace mixing, and coherency filter) were also applied to the data prior to the finite difference migration. The migrated time sections are characterized by robust reflection signals from near surface to past-moho depths (Figure 2). Strong reflections mark the sandstone-basement unconformity (UC). These events are clearly Saskatchewan Geological Survey 2 Summary of Investigations 2002, Volume 2

3 Figure 2 - The upper 6 s of the Line B migrated time section. UC=unconformity; O=ore body; P2=shear/fault zone; BR=bright reflector; DR=dipping reflections in the middle and lower crust; and M=Moho. Saskatchewan Geological Survey 3 Summary of Investigations 2002, Volume 2

4 distinguishable along the northern half of the profile. Correlation is more difficult within the central and southern region of the line due to influences of major structural disturbances. The P2 basement structure dominates the section from approximately 0.25 to 2.3 s (TWT). The south dipping P2 shear/fault zone is first imaged below the main ore body of the McArthur mine (see Gyorfi et al., this volume for detail) and its interpreted seismic signature continues beyond 4000 m depth to the south-southeast, probably extending outside the southern limit of the seismic line. Seismic signatures related to this prominent structure are over 2500 m thick and the multiple nature of the discontinuities suggests multi-phase deformation. The P2 fault image transects patterns representing gently folded structures of the basement rocks, indicating that it is a relatively young tectonic feature. The seismic data confirm the northwesterly increase in thickness of the Athabasca Group sandstones. The weak amplitude of the seismic signatures along the northern third of the profile is interpreted to represents Archean basement. The highly prominent, bright, subhorizontal reflectivity at 2.3 s TWS (BR) predates the P2 structure. This series of sub-horizontal reflections is the most distinctive seismic signature of the section. This zone is at least 0.25 s (TWT) wide and is offset in several places by several steeply dipping discontinuities. The horizontal reflections image heterogeneous internal structures with multi-cycle reflected arrivals and numerous diffractions. The origin of this sheet-like complex is speculative. Mandler and Clowes (1997) suggested that comparable exceptionally strong signal images of the region are parent bodies which fed ca Ma (post Hudsonian) Mackenzie diabase intrusions. The Mackenzie diabase dikes and sill-like bodies crop out in the Athabasca Basin. Madore and Annesley (1994) also described older but similar intrusive bodies in the same area, such as the Sandy Islands gabbro complex. The latter is 1826 Ma (Hudsonian) in age and its parent thus could also be a potential source for the seismic reflections. On the other hand, the latter is difficult to explain as a uniform horizontal structure, considering the post 1828 Ma tectonic events that affected Trans-Hudson Orogen. Below the zone of bright reflectors, a north dipping set of sub-parallel reflections (DR) dominate on the section down to 10.5 s TWT. Given consistent trends, these highly visible reflection patterns would project to the surface to the south of the end of the seismic profile. This consistent reflectivity pattern dominates the entire crust down to Moho level (M). Based on projection of regional crustal tectonics (Hajnal et al., 1996), these images might mark the westernmost extent of subduction zone related to the convergence of the Paleoproterozoic Trans-Hudson Orogen. The Moho is well defined around 10.5 s TWT, although there are indications that it is a result of a number of gently south-dipping reflectors. Zones of strong reflectivities also characterize the apparent lithospheric root of the sub- Moho lithosphere. 4. Conclusions 1) The highly successful regional reflection survey of the EXTECH IV project mapped in detail the unconformity surface, an important framework element for uranium exploration in the Athabasca Basin. 2) Beneath the P2 ore zone, this survey imaged the P2 fault system, which offsets the unconformity by only a small amount in regional terms (20 to 50 m), but is a leading target for exploration because it is thought to have played a significant role in the ore-forming process by serving as a conduit for hydrothermal fluids. 3) The very bright horizontal reflection zone at 2.3 s TWS predates the P2 structure and is interpreted as a very large, young intrusive body that extends regionally beneath a significant segment of the Athabasca Basin. Such a large intrusive body may represent a suitable heat source for the hydrothermal system that generated the world-class P2 ore bodies. 4) The crustal and Moho reflection signatures are consistent with the mineralized district of the McArthur River Mine Camp being located at the western flank of the convergent margin of a major collisional orogen. 5. Acknowledgments This sub-project of the EXTECH IV Athabasca Uranium Multidisciplinary Study is funded by Natural Sciences and Engineering Research Council, the Geological Survey of Canada, Cameco Corp., and COGEMA Resources Inc. as part of a partnership with Saskatchewan Industry and Resources and the Alberta Geological Survey. This paper was critically read by Drs. Bhaskar Pandit and Charles Jefferson. This is Geological Survey of Canada Contribution No Saskatchewan Geological Survey 4 Summary of Investigations 2002, Volume 2

5 6. References Craven, J.A., McNeice, G., Wood, G., Powell, B., Koch, R., Annesley, I.R., and Mwenifumbo, J. (2001): Magnetotelluric investigation at McArthur River A preliminary look at the data; in Summary of Investigations 2001, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep , CD B, p Hajnal, Z., Lucas, S.B., White, D.J., Lewry, J.F., Bezdan, S., Stauffer, M., and Thomas, M. (1996): Seismic reflection images of high-angle faults and linked detachment in the Trans-Orogen; Tect., v15, p Hoeve, J. and Quirt, D. (1984): Uranium mineralization and host rock alteration in relation to clay mineral diagenesis and evolution of the Mid-Proterozoic Athabasca Basin, Saskatchewan, Canada; Sask. Resear. Counc., Publ. R B, 190p. Hoffman, P.F. (1990): Subdivision of the Churchill province and extent of the Trans-Hudson Orogen; in Lewry, J.F. and Stauffer, M.R. (eds.), Early Proterozoic Trans-Hudson Orogen of North America, Geol. Assoc. Can., Spec. Publ. 37, p1-14. Madore, C. and Annesley, I.R. (1994): A field petrographic and geochemical study of gabbros and related rocks from the Sandy Islands gabbro complex; in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4, p Mandler, H.A.F. and Clowes, R.M. (1997): Evidence for extensive tabular intrusions in the Precambrian Shield, western Canada: A 160 km-long sequence of bright reflections; Geol., v25, p Matthews, R., Koch, R., and Leppin, M. (1997): Advances in integrated exploration for unconformity uranium deposits in western Canada; in A.G. Gubins (ed.),geophysics and Geochemistry at the Millennium, Prospectors and Developers Association of Canada, p McGill, B., Marlett, J., Matthews, R., Sopuck, V., Homeniuk, L., and Hubregtse, J. (1993): The P2 North uranium deposit Saskatchewan, Canada; Expl. Min. Geol. v2, no4, p Mwenifumbo, C.J., Pflug, K.A., Elliott, B.E., Jefferson, C.W., Koch, R., Robbins, J., and Powell, B. (2001): Multiparameter borehole geophysical logging at Cluff Lake and McArthur River Projects: New parameters for exploration, stratigraphy, and high-resolution seismic studies; in Summary of Investigations 2001, Volume 2, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep , CD B, p Portella, P. and Annesley, I.R. (2000): Paleoproterozoic tectonic evolution of the eastern sub-athabasca basement, northern Saskatchewan: Integrated magnetic, gravity and geologic data; in GeoCanada 2000, Calgary, Conference CD, ext. abstr. #647. Sibbald, T.I.I. (1986): Overview of the Precambrian geology and aspects of the metallogenesis of northern Saskatchewan; in Gilboy, C.F. and Vigrass, L.W. (eds.), Economic minerals of Saskatchewan, Sask. Geol. Soc., Spec. Publ. 8, p1-16. Saskatchewan Geological Survey 5 Summary of Investigations 2002, Volume 2