Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

A large number of assessment file reports concerning the geology and mineral deposits of small areas
within the Sioux Lookout orogenic belt are available via either the Sioux Lookout Resident Geologist
Office or the ERLIS system.
Areas surrounding the Sioux Lookout orogenic belt are: 1) to the NW and N, the eastern Winnipeg
River subprovince (Breaks and Bond 1977, Breaks 1988, Sanborn--Barrie 1988, Beakhouse 1991, Corfu et
al. 1995, Ciceri et al. 1996, Cruden et al. 1997); 2) to the NE and E, the Zarn Lake area (Page 1978, Page
and Moller (1979a,b) and other areas which bridge the gaps between the Sioux Lookout orogenic belt and
the Sturgeon Lake and Savant Lake belts (e.g., see Trowell references in Blackburn et al. 1991); 3) to the
SE, the Basket Lake batholith (Sage et al. 1974, Szweczyk and West 1976), which is part of the western
margin of the central Wabigoon region (Blackburn et al. 1991); and 4) to the S and SW, the Dinorwic and
Dryden areas (Pettijohn 1939, Satterly 1943; Blackburn et al. 1985, 1991, 1998; Berger 1989, 1990;
Chorlton 1990, 1991). At a regional scale, supracrustal rocks in the Sturgeon Lake and Savant Lake areas
extend to the east and northeast of the Sioux Lookout orogenic belt (Blackburn et al. 1991, Figures 9.3,
9.40), and the recent interpretation of the evolution of the “Sturgeon–Savant belt” by Sanborn--Barrie and
Skulski (1999) includes important similarities and differences with the proposed evolution of the Sioux
Lookout orogenic belt (Devaney 1999a, and herein).
Geology of Stratigraphic Units
STRATIGRAPHIC OVERVIEW
The Sioux Lookout orogenic belt is composed of the following tectonized stratigraphic units, from north to
south: 1) the Northern Volcanic belt (or Botham Bay Group); 2) the Patara Formation; 3) the “Big
Vermilion–Daredevil unit”; 4) the Ament Bay Formation; 5) the Daredevil Formation stratotype; 6) the
Central Volcanic belt (or Neepawa Group); 7) the Minnitaki Group; and 8) the Southern Volvanic belt
(Figures 1, 2). The Sioux Lookout orogenic belt is bounded to the north by the Winnipeg River
subprovince (Beakhouse 1991) and to the south by the poorly known Basket Lake batholith (Szewczyk and
West 1976, Berger 1989, Blackburn et al. 1991). Previous stratigraphic and regional geological studies of
the Sioux Lookout orogenic belt were summarized by Johnston (1969, 1972) and Blackburn et al. (1985,
1991). Most of the major stratigraphic units of the Sioux Lookout orogenic belt continue to the southwest,
an area described in recent reports by Berger (1989, 1990), Chorlton (1990, 1991), and Page (1984), and
some units continue to the east (Page and Moller 1979a,b; Trowell et al. 1982, 1983). Studies by Davis and
Trowell (1982), Davis et al. (1988), Davis (1990) and Devaney (1999a) have provided a small number of
radiometric ages for the rocks in and around the Sioux Lookout orogenic belt (Figure 2).
The distribution of the linear, belt--like lithological units which form the various stratigraphic units of
the Sioux Lookout orogenic belt is shown in Figure 1, on the accompanying map (Devaney et al. 1995b),
and on the older maps of Johnston (1969, 1972) and Blackburn et al. (1985; 1991, Figures 9.12, 9.40). The
map width of the typically very steeply dipping units approximates their tectonized “stratigraphic”
thickness. Geographic localities referred to below are shown on the accompanying map (Devaney et al.
1995b), the maps of Johnston (1969, 1972), and other maps cited herein.
Owing to: 1) a lack of detailed structural and geochronological studies; 2) the low amount of
continuous exposure (including large covered intervals); and 3) the near total absence of exposed
boundaries (contacts) between the major stratigraphic units, the degree of structural disturbance, both
between and within the various stratigraphic units of the Sioux Lookout orogenic belt, is mostly unknown
(or not reliably known), and in most cases lateral and vertical (upward) trends within the formation--like
units cannot be reliably interpreted as true stratigraphic trends. As is common in the other Archean
greenstone belts of northern Ontario, the nature of the exposure and potential structural complexity severely
limit attempts at conventional stratigraphic descriptions and interpretations, and a more generalized,
lithofacies--oriented approach must be taken. References regarding more general aspects of interpretation
common to the supracrustal rocks of most Archean greenstone belts (e.g., the subaqueous origin of
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