Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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Units of chert–magnetite (BIF), up to tens of metres thick, comprise a minor amount of the Northern Volcanic belt. The “bands” are tectonized laminae and beds; thicker bands are tens of centimetres thick. Pyritic BIF is less common (e.g., thin massive to internally layered pyrite bands). A pyrite deposit in the Northern Volcanic belt, the former North Pines mine, was mined during the 1920s (Johnston 1972). The BIF units form prominent stratigraphic (or lithotectonic) units, with the lateral continuity and strike length of these being best expressed on aeromagnetic maps (OGS 1982, 1997). GEOCHEMISTRY Trowell and Johns (1986) demonstrated the tholeiitic nature of the Northern Volcanic belt basalts and their strong similarity to the Jutten Group basalts. Data presented below (see “Geochemistry”) and analyses by D. Wyman (written communication, 1998) show that the Northern Volcanic belt basalts have a MORB (mid--ocean ridge basalt) composition. Using Wyman’s analyses, the Northern Volcanic belt can be subdivided into two geochemically defined units: a main northwestern unit, and a relatively thin unit along the southeast margin of the Northern Volcanic belt. (Importantly, the basaltic part of the northwestern Central Volcanic belt also displays the same two--part subdivision, suggesting correlation of the Northern Volcanic belt and Central Volcanic belt.) Interpretation The abundance of pillowed basalt flows, MORB--like composition, and the great extent of the Northern Volcanic belt appear to be diagnostic of oceanic crust, a deep marine mafic plain paleoenvironment. The chert--magnetite BIF units should represent exhalites precipitated from seawater to form interflow chemical sediments. Although undated, such mafic sequences in western Wabigoon subprovince are thought to be older than the “diverse intermediate to felsic volcanic sequences” (Blackburn et al. 1991). TOP INDICATIONS Tops indications in the Northern Volcanic belt to the south or southeast, based mostly on pillow shapes, are very consistent. Also, reports of this top direction have been consistent from study to study: for example, Pettijohn (1934, 1935), Johnston (1972), and (Devaney et al. (1995b). However, despite brief mentions of the overturned nature of the Northern Volcanic belt sequence by Trowell et al. (1977, 1978) and Page (1984), it has not been previously stressed that these widespread tops to the southeast occur in moderately to steeply northwest--dipping strata. (Note that it is the orientation of schistosity, rather than bedding, which is more commonly recorded on most maps of the Northern Volcanic belt.) Based on the maps of Johnston (1972) and Page (1984), and the results of the present study (Devaney et al. 1995b), overturned basaltic strata in the Northern Volcanic belt are widespread. This suggests the possibility that the entire Northern Volcanic belt has been overturned. PATARA FORMATION Introduction, Definition and Age Use of the term “Patara Formation” herein is consistent with Pettijohn’s (1934, 1935) original definition and mapping of the distinctive mafic--clast conglomerate unit present along the southeast margin of the 6
Northern Volcanic belt from central Vermilion Lake to south Pelican Lake. This 20 km long formation extends for a relatively short distance along strike compared with the other formations, lithological units or “belts” in the map area. Other more felsic rocks lumped together as “Patara sediments” by Johnston (1972) and Blackburn et al. (1985) are herein considered to be stratigraphically equivalent to the Daredevil Formation, and are herein informally termed the “Big Vermilion–Daredevil unit” (see below, and Devaney 1999a). This an important point because Davis et al. (1988), using the geological framework of Johnston (1972), reported a ca. 2712 Ma age from a sample of supposed “Patara sediment.” That sample is from the eastern part of the Big Vermilion–Daredevil unit, and is closely similar in age to other samples from the volcanic Daredevil Formation (type area) and the clastic Minnitaki Group dated by Davis et al. (1988). Fralick and Davis (1999) report an age of ca. 2700 Ma for detrital zircon grains from the sandy component of the mafic--clast conglomerate in the Patara Formation, which confirms the previous interpretation by the author (Devaney 1997, 1999a) of the synorogenic nature of the Patara Formation Johnston (1972) reported trachytic dikes from the roadside section of the Patara Formation described below. Given the interpretations proposed herein for the Patara Formation, more study of the dikes is warranted. Lithofacies CONGLOMERATIC FACIES Conglomerate--dominant outcrops or intra--outcrop horizons are typically crudely bedded, with vague coarser versus finer layers and rare well bedded lenses. Subtle compositional layering (e.g., clusters of clasts with recessively weathered rims) also defines bedding. Sandy interbeds are up to tens of centimetres thick, and commonly contain pebble bands. The conglomerate has a tightly clast--supported framework of pebbles and a coarse sandstone matrix. Clast composition is mostly oligomict, consisting of mafic volcanic clasts (commonly dark brown to bluish coloured; some clasts weather very light brown or grey), plus a minor component of clasts of gabbro, magnetite BIF (with one 41 by 26 cm magnetite boulder clast), chert--magnetite BIF, and chert. Rare clasts with recessively weathered rims could represent individual pillows transported as clasts, or intense weathering of some clast rims. Rare quartz pebbles are probably chert (recrytallized?). At some sites, gabbroic cobbles are the largest clasts; clast size may partly reflect the jointing and “toughness” of the source area lithologies. The rounded form of many of the clasts appears to be a primary feature. Regarding the preservation of such primary features, it is notable that superbly preserved pillowed basalts are close by in the adjacent Northern Volcanic belt, and well preserved sedimentary structures are found to the south in the sandier facies of the Patara Formation, illustrating that well preserved primary fabrics are common. Johnston (1972) considered the conglomerate along the line of inferred (unexposed) contact with the Northern Volcanic belt to be a “basal” horizon and described the Patara Formation as having a “basal breccia” unit. However, at several sites near the contact the clasts are round to sub--angular, and are thus do not constitute a breccia. Also, the view presented herein (see below) is that the conglomerates along the contact with the Northern Volcanic belt likely represent the top, not the base, of the Patara Formation Small rare exposures of mafic--clast conglomerate may be present along strike east and west from the known disribution of the Patara Formation (along potential fault zones). One such exposure was identified just east of the town of <strong>Sioux</strong> <strong>Lookout</strong> (next to a bypass bridge built after publication of the map of Devaney et al. 1995b). 7
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 28 and 29: SANDIER FACIES At central Vermilion
Page 30 and 31: wrench--induced verticalization of
Page 32 and 33: alteration). Low--intensity mafic a
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
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Note that similar felsic volcanic r
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22) At Southeast Bay of Minnitaki L
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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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