Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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alteration). Low--intensity mafic alteration appears to have been developed, based on the presence of relatively high amounts of iron and magnesium oxides in some samples. Some exposures of lapillistone (or volcanic pebble conglomerate) are rusty weathering (pyritic gossan). Only low--grade disseminated sulphide minerals appear to be present in the outcrops. The weak alteration suggested by geochemical analyses and the calc--alkalic nature of the rocks (see Geochemistry section) do not suggest good potential for VMS (volcanogenic massive sulphide) deposits. The “Eimiller occurrence” is a gold showing on the south shore of Big Vermilion Lake (Johnston 1972, Trowell et al. 1978). Tuffs or sandy sedimentary rocks are in sharp, tectonized contact with quartz porphyry, with quartz–carbonate alteration and veins near the contact (especially within 1 m of it). This seems to be an example of veining at a site of competency contrast. HIGHWAY 664 AREA Along Highway 664 (the road to Hudson, which crosses the stratigraphy), the contact between the Patara Formation and the Big Vermilion–Daredevil unit lies beneath a large covered interval. Near this contact, tectonized sandstone and siltstone exposures that presumably are part of the Big Vermilion–Daredevil unit are coarser and more thickly bedded than an exposure of dark, fine grained, laminated, cross--laminated, and graded beds that is located to the northwest and is thought to be a southern portion of the Patara Formation Small folds may be soft--sediment (penecontemporaneous) features rather than tectonic, based on apparent partial erosion and truncation of some of the folds. A very rusty--weathered series of roadside outcrops contain clast--supported fine pebble conglomerate which is poorly sorted and crudely bedded, with sandy matrix and thin laminated sandstone interbeds (e.g., 5 cm thick). The distinctive white siliceous (felsic volcanic) clasts are small (few centimetres) and round to angular. The rounding of such a hard, physically and chemically resistant clast lithology indicates a significant degree of sedimentary reworking, hence the intense rusty weathering at this outcrop is probably not reflective of any volcanogenic sulphide mineralization. FROG RAPIDS AREA Near Frog Rapids (between the western parts of Pelican and Abram lakes), roadside outcrops consist mostly of highly sheared tuff or very fine--grained wacke to siltstone. Some beds are graded, with rare cross--laminae. More siliceous bands could represent felsic ash layers or alteration related to the local quartz veins. Other features noted in the vicinity include: graded clast--supported fine pebble conglomerate or lapillistone (fining to the south), pebbly sandstone or lapilli--tuff with light (“felsic”) clasts, and odd small (about 1 m long) dark alteration(?) lenses. Highly deformed bedded rocks, suggesting the presence of a fault zone, are prominent in one roadside outcrop. Distinguishing the fabrics of tectonic deformation from soft--sediment deformation can be difficult and uncertain, but in this case the following observed features are likely tectonic: thin lensoid layers (tectonic truncation via transposition of layering, rather than sedimentary erosion or slumping), veining, pinching and swelling of bed width/thickness (boudinage), cleavage, brecciation (with interlocking jigsaw--clast shapes and highly local (0.5 m scale) variation in breccia development), a “pebbly” layer of pseudo--clasts, a fault plane (obvious offset), kinks, Z--folds, rusty (pyritic) layers (probably the result of fault permeability along fractures rather than pyritic exhalite laminae), and sigmoidal fabrics (shear zones). PELICAN LAKE AREA Shoreline outcrops at Pelican Lake contain tectonized, laminated to thinly bedded tuff or wacke--siltstone, thinly layered magnetite (not mapped by Johnston 1972), and crudely bedded conglomerate (or lapillistone) 12
with small intermediate--felsic volcanic clasts and rare intraclasts. Rare very siliceous layers may be fine felsic tuff, similar to those at Frog Rapids (see above). Rare dikes may be the “trachyte” mapped by Johnston (1972). Conglomerates and coarse sandy beds interbedded with magnetite (lean BIF) and fine grained clastic facies suggest deposition of “resedimented” subaqueous conglomerates. In one outcrop area, polymict conglomerate contrasts with oligomict conglomerate located to the north; this may be the contact between the north margin of the Big Vermilion–Daredevil unit and the south margin of the Patara Formation. A western exposure of polymict volcanic pebble conglomerate is clast--supported, with small felsic pebbles common (e.g., felsic volcanic clasts; notable porphyry clasts; one quartz (chert?) cobble). This conglomerate is not as polymict as the Ament Bay Formation but is more polymict than the nearly oligomict Patara Formation. The contrasts in composition with other conglomeratic facies in the Big Vermilion–Daredevil unit, such as the white felsic clast conglomerate (along Highway 664; described above) and other conglomerate exposures nearby on Pelican Lake, indicate that a variety of different source areas provided clasts. CENTRAL ABRAM LAKE AREA Sample 7 (fraction 26) of Davis et al. (1988) is from this area, in which the rocks were classed as as “Patara sediments” by Johnston (1972). Davis et al. (1988) used Johnston’s terminology, so, in order to avoid confusion, the reader should note that the ca. 2712 Ma age of this “tuff” sample, herein considered to be representative of the Big Vermilion–Daredevil unit, should not be confused with a younger (ca.
Page 1: Ontario Geological Survey Open File
Page 5: e Queen’s Printer for Ontario, 20
Page 11: Large--Scale Structures ...........
Page 15: 25. Stage 5 sedimentation and tecto
Page 19: A large volume of basaltic strata (
Page 22 and 23: Many of the Sioux Lookout orogenic
Page 24 and 25: pillowed basalt flows) are not give
Page 26 and 27: Units of chert-magnetite (BIF), up
Page 28 and 29: SANDIER FACIES At central Vermilion
Page 30 and 31: wrench--induced verticalization of
Page 34 and 35: few clues regarding the specific se
Page 36 and 37: seems to be the case here; see disc
Page 38 and 39: 1969, 1972; Trowell et al. 1977, 19
Page 40 and 41: Page and Clifford (1977) interprete
Page 42 and 43: upward. Examples of both westward--
Page 44 and 45: shapes, suggesting intrusion into s
Page 46 and 47: The abundance of very coarse and an
Page 48 and 49: One small island outcrop contains t
Page 50 and 51: AGE AND UNCERTAIN STRATIGRAPHIC CON
Page 52 and 53: and sedimentation). Other good exam
Page 54 and 55: (illustrating how an increased clas
Page 56 and 57: For the present study, attention wa
Page 58 and 59: Structural Geology Because the focu
Page 60 and 61: LARGE--SCALE STRUCTURES Description
Page 62 and 63: As, Pb, Zr, Y); inductively coupled
Page 64 and 65: DISCRIMINATION PLOTS Data from the
Page 66 and 67: It is notable that the Northeast In
Page 68 and 69: (ca. 2734 Ma) 60 km to the east at
Page 70 and 71: The Stage 5 cartoon (Figure 22) is
Page 72 and 73: also during later phases of Stage 5
Page 74 and 75: aquabasinal (lacustrine?) mudstones
Page 76 and 77: Note that similar felsic volcanic r
Page 78 and 79: 22) At Southeast Bay of Minnitaki L
Page 80 and 81: Given that the adjacent Sturgeon La
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structurally--controlled Archean me
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This andesitic sequence and its com
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young as 2701 Ma. However, a few mi
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volcanism (interpreted extensional
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eyond the resolution of surface bed
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asin would be adjacent to an orogen
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The moderately to steeply dipping,
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References Allen, R.L. 1988. False
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Ciceri, D.L., Cruden, A.R. and Robi
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Dupuy, C. and Dostal, J. 1984. Trac
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Horwood, H.C. 1938. Geology of the
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Nilsen, T.H. and McLaughlin, R.J. 1
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Sage, R.P., Breaks, F.W., Stott, G.
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Trowell, N.F., Blackburn, C.E. and
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Figure 1b. Location of Sioux Lookou
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Figure 3. Roadside cross--section o
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Figure 5a. Geochemical sample locat
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Figure 6. Discrimination plots show
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Figure 8. Plots of selected high fi
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Figure 9. Plots of selected oxides
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Figure 10. Plots of selected trace
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Figure 11. Continued. --77 to --81)
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Figure 12. N--MORB normalized exten
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Figure 12. Continued. m) Northeast
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Figure 14. For all Sioux Lookout or
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Figure 15. Zr/Yb versus Nb/Yb discr
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Figure 17. Discrimination diagrams
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Figure 19. REE plots for representa
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Figure 21. Plot of N--MORB normaliz
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Figure 22b. Corresponding stages of
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Figure 24. Schematic cross--section
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Figure 26. Stage 6 Sioux Lookout or
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Figure 28. Cartoon views of potenti
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Figure 30. Theory of small--scale k
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Figure 32. Riedel flake structure;
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Figure 34. Theoretical shear fractu
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Table 1. Continued. Sample 94JRD--0
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Table 1. Continued Sample 94JRD--00
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Table 1. Continued. Sample 94JRD--0
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Table 1. Continued. Sample 94JRD--B
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Table 1. Continued Sample 94JRD--B1
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Table 1. Continued. Sample 95JRD--B
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Table 2. Continued. Northeast Bay P
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Table 4. Altered Sioux Lookout orog
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Metric Conversion Table Conversion
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Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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