Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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The abundance of very coarse and angular clasts of volcanic composition, including mega--blocks, suggests a proximal to medial setting. The proportion of lava flow facies present is minor, suggesting that the depositional sites were more commonly in medial, rather than proximal, zones. Most of the section does not appear to be distal in nature; a greater proprtion of finer, more thinly bedded facies with more abundant evidence of sedimentary reworking would be expected in a distal setting. The variety of facies and sedimentological and volcanological features described above suggest deposition in a stratovolcanic cone setting subdivisible into proximal and distal zones (e.g., models of: Easton and Johns 1986, Cas and Wright 1987, Hackett and Houghton 1989). Page and Clifford (1977) suggested that the source vent for the pyroclastic deposits was only a few kilometres away, with the Northeast Bay pluton representing the subvolcanic feeder pluton to the pyroclastic succession. However, for the present study, it has not been assumed that only one vent area was present, and the potential presence of multiple satellite vents would complicate the generalizations about proximal--distal trends. Although the physical volcanological and sedimentological features present seem to indicate a stratovolcano succession, the geochemical compositional variability typical of stratovolcanoes (Cas and Wright 1987) is not present. Page and Clifford (1977) described this upper pyroclastic part of the succession as a mix of basaltic, andesitic, dacitic and felsic rocks. The variable colour of the rocks in field exposures does suggest such a range in composition, but the geochemical analyses of the present study (see “Geochemistry”) show that these rocks are mostly of basaltic andesite composition. It is postulated that transgression--regression cycles (Figure 4) may be preserved in the upper pyroclastic part of the succession, an apparently homoclinal, continuous and orderly section of well preserved subaerial to subaqueous facies. In this scenario, major episodes of volcanic aggradation and progradation would have caused subaerial emergence and a regressional shift of paleoenvironmental facies, versus major episodes of submergence, and possibly also subsidence (e.g., caused by the loading effect of the volcanic pile, and/or evacuation of magma chambers?), which would account for transgressive shifts to subaqueous facies (Devaney 1998). Three large--scale stratovolcanic transgression--regression cycles (or sequences), about 1 km thick each, may be present, and would represent the cyclic alternation and accumulation of subaerial and subaqueous facies belts (stratigraphic units), somewhat similar to the cyclic transgression--regression cycles of clastic marine successions (e.g., Galloway 1989). The transgressive units need not be marine; volcanic deposits can dam fluvial drainage to form lakes. At the scale of the Sioux Lookout orogenic belt, the volcanic succession at Northeast Bay represents a distinctive, relatively small, local volcanic centre within the Central Volcanic belt. Similar volcanic strata are thought to be present over extents of about 10 km along strike to the west and northeast (Page and Clifford 1977; Page, unpublished map; Page and Moller 1979a,b), but the local nature contrasts strongly with other much more extensive volcanic units in the Central Volcanic belt. The fault--bounded nature of the volcanic succession at Northeast Bay (an ovoid area suggested to be a crustal flake rotated during late tectonism; see the introduction to this Northeast Bay section, above, and discussion of site 15 in Figure 23 under “Analysis of a transect across the Sioux Lookout orogenic belt,” below) suggests structural reactivation of an originally fault--bounded basin such as a volcanic graben (see below and Devaney 1999a). In western Wabigoon subprovince, the sporadic distribution of andesitic and dioritic units of ca. 2732 Ma age (Blackburn et al. 1991), the geochemical composition of older, coeval and younger arc--like units, and the presence of the ca. 2734 Ma VMS deposits at Sturgeon Lake (Davis et al. 1985, Morton et al. 1991) collectively suggest the interpretation of partially rifted arc settings for the various ca. 2732–2734 Ma rocks, including the Northeast Bay succession of the Sioux Lookout orogenic belt (see below and Devaney 1999a). CENTRAL VOLCANIC BELT: NORTHEAST BAY PLUTON (DIORITIC COMPLEX), NORTHEAST BAY OF MINNITAKI LAKE AREA The Northeast Bay pluton was previously described by Johnston (1972), Page and Clifford (1977) and Trowell et al. (1983). As noted above, Page and Clifford (1977) considered this pluton to represent a volcanic neck or core (feeder pluton) below a vent area located at or near the west end of the pluton, and 26
geochemical data (see “Geochemistry”) confirm the co--magmatic link between this pluton and the nearby andesitic volcanic rocks. In one relatively small part of this western area, multiple dikes and lithological variability suggest the presence of a marginal dike swarm (Devaney and Babin 1996; see below). Based on lithological and geochemical similarities, the author correlated the Northeast Bay volcanic centre and subvolcanic pluton with the Sturgeon Lake volcanic centre (caldera succession of Morton et al. 1991) and subvolcanic Biedelman Bay pluton (Poulsen and Franklin 1981), located about 60 km to the east along strike. (Devaney 1997). D.W. Davis (Royal Ontario Museum) dated a rock from this dioritic plutonic complex at 2730–2734 Ma (Davis, written communication 1998; Devaney 1999a). Therefore the Northeast Bay pluton is the same age (within experimental error) as the Biedelman Bay subvolcanic pluton and the South Sturgeon volcanics (dates of Davis et al. 1985, Blackburn et al. 1991), and the ages confirm the previous lithological correlation. Lithologies Rocks of intermediate composition, identified in the field as diorite, tonalite, and trondjhemite (leuco--tonalite), are fine-- to medium--grained (minor coarse--grained areas too), with a good crystalline igneous texture of interlocking crystals. (One northeastern exposure appears to be more granitic.) Dike features include chill margins, porphyritic textures, and late quartz veins. Both felsic dikes and small fractures have the same “X” pattern (conjugate fractures?), suggesting that various types of fluids (felsic magmas, sulphidic and siliceous fluids) followed the same fracture pathways. Some marginal areas of the pluton contain small mafic bands or brecciated blocks with sharp contacts which may represent inclusions, xenoliths or dikes; poor exposure limits resolution. Sulphide minerals are concentrated along rusty fracture planes and 1 m wide pyritic gossans along vertical fractures in the diorite; malachite stain is rare. It is not known to what degree the sulphides may have been “remobilized” during recent weathering of the outcrops. Note that Cu--Mo showing on Johnston’s (1972) map is actually in the mafic volcanic country rock at the border area and not in the dioritic intrusion. The presence of very minor amounts of Cu and Mo in a dioritic pluton suggest a possible porphyry copper type of mineralization. Note the similarities with mineralization identified farther east in the coeval and lithologically similar Beidelman Bay intrusion (Poulsen and Franklin 1981). Heavily Diked Zone (Potential Pluton--margin Dike Swarm) The shoreline exposures along the islands exposing the southwestern end of dioritic complex display complex and unusual features. Numerous thin (
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 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 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
Page 82 and 83: structurally--controlled Archean me
Page 84 and 85: This andesitic sequence and its com
Page 86 and 87: young as 2701 Ma. However, a few mi
Page 88 and 89: volcanism (interpreted extensional
Page 90 and 91: eyond the resolution of surface bed
Page 92 and 93: asin would be adjacent to an orogen
Page 94 and 95: 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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