Front
I /T 7i9
Figure 1. Distribution of the Huronian Supergroup (in black) .
Table 1. Huronian Formations.
Nipissing Diabase (- 2.1 Ga old)
... intrusive contact...
COBALT GROUP
Bar River Fm.
Gordon Lake Fm.
Lorrain Fm.
H Gowganda Fm.
U unoonformable to conformable contact R QUIRKE LAKE GROUP
O Serpent Fm.
Espanola Fm.
N Bruce Fm.*
I local disconformable contact A HOUGH LAKE GROUP
N Mississagi Fm.
Pecors Fm.
Ramsay Lake Fm.
local disconformable contact ELLIOT LAKE GROUP
McKim Fm. A
Matinenda Fm. ?
"Mississagi Fm."
of
Collins, 1925.
. unconformity . Archean (- 2.5 Ga old)
* contains rocks of glaciogenic origin
Figure 2. Distribution of the "Mississagi Formation" at the surface (black) and in the subsurface (stipple) in the southern Cobalt Bnbayment.
This paper is an attempt to outline the stratigraphie and sedimentological framework of uraniferous strata in the Cobalt Plain, and provide an explanation for the differences in localization of uranium showings compared to the Blind River - Elliot Lake deposits. Sedimentological and mineralogical interpretations are based on recent observations in papers by Long (1981 and in press), Meyn and Matthews (1980 and in press) ; Mossman and Harron (1983) and Sauerbrei and Phipps (1983). Previous studies on the uranium potential of this area includes work by Thomson (1960) and summaries by Roscoe (1969) and Robertson (1981).
Most of the area (Fig. 2) has been mapped in detail by members of the Ontario Geological Survey (Card et al 1973; Dressler 1979, 1980, 1982; Grant 1964;
Meyn 1970, 1971, 1972, 1973a and b, 1977; Muir et al 1978; Thomson 1960; and Thomson and Card 1963).
GEOLOGICAL SETTING
Rocks of the Huronian Supergroup crop out in a broad belt between Sault Ste. Marie and Noranda (Fig. 1) occupying a structural position which straddles the Superior and Southern tectonic provinces (Card et al
1972) and is terminated to the southeast against rocks of the Grenville structural province. Metamorphic grade ranges from low or subgreenschist faciès in the north and northwest to almandine-anphibolite grade in the vicinity of the Grenville Front (Card et al 1972) . In the southern part of the Cobalt embayment most of the supergroup has been regionally meta-morphosed under lower greenschist faciès conditions (Meyn 1973a) with most
samples containing muscovite and biotite of metemorphic origin, and a few containing chlorite (Long, in press). Observations by Mossman and Harron
(1983) suggest that in some localities metamorphism may have reached lower anphibolite faciès conditions.
The Huronian Supergroup consists of an apparently cyclical association of conglomerates, siltstones and sandstones (Roscoe 1969; Fraray & Roscoe 1970)
and 2.2
deposited between 2.5/Ga ago (Van Schmus 1965). The conglomeratic parts of this sequence (Ramsay Lake, Bruce and Gowganda Formations) appear to have
been deposited in a glacial to paraglacial environment (Young 1966, 1970, 1981;
Parviainen 1973) in middle to high latitudes (Morris 1977; Syroons 1975, Nesbitt and Young 1982). Sandstones are predominantly fluvial (Long 1976) and mudstones were deposited in lacustrine and possibly restricted marine conditions. Details of individual formations and prior history of investigation of the area are summarized in papers by Collins (1925), Roscoe (1969) and Robertson (1973).
Further details of the tectonic, structural, stratigraphie and metamorphic setting can be found in papers by Card et al (1972), Sims et al (1981), Young (1982, 1983), Long and Lloyd (1983) and Long (1984).
The regional geology of the southern part of the Cobalt Fjnbayment has been outlined by Meyn (1973a) who indicates that the Huronian Supergroup in the area is over 6.5 km thick. The basal Huronian strata in this area (Fig. 2) are
predominantly arkosic and subarkosic sandstones with lesser amounts of conglo-merate and modstone. Sandstones dominate the upper part of the sequence, with conglomerates confined to the lower 100 m. The basal sequence is traditionally ascribed to the "Mississagi Formation" in the sense originally used by Collins
(1925) . They may be correlative with parts of the Hough and Elliot Lake Groups (Table 1) although clear cut stratigraphie subdivisions, apparent in the Blind River - Elliot Lake area, and in the area south of Wanapitei Lake, cannot be traced with certainty into the Cobalt Embayment (Meyn 1973a; Meyn and Matthews, in press; Long, in press) . Most of the monotonous sequences of planar and
trough cross-stratified sandstones in the upper part of the "Mississagi Formation"
of the Cobalt Embayment are probably correlative with the Mississagi Formation (Hough Lake Group) in the more restrictive useage (Table 1). Conglomerates near the base of the sequence may be in part equivalent to the Matinenda Formation, or may represent faciès equivalents of stratigraphically higher units.
STRATIGRAPHY
The "Mississagi Formation" in the southern part of the Cobalt Embayment unconformably to disconformably overlies Archean metasediments, metavolcanics and granites. In places the contact is fresh, while in others Archean rocks show signs of paleosoil development related to Proterozoic weathering phenomena.
Thickness of the basal sequence (0-55Qm) is highly irregular due to deposition of the sequence on an irregular topographic surface, and erosional truncation of the top of the sequence. The latter is related to glacial scouring which preceded deposition of both the Bruce and Gowganda Formations. Contacts with the Bruce Formation are cainmonly conformable, while those with the Gowganda Formation may be locally disconformable, indicating a period of subdued tectonic activity between these two major glacial advances.
On a regional basis the "Mississagi Formation" can be subdivided into two menbers: a lower member consisting of conglomerates, sandstones and mud-stones; and an upper mejnber dominated by planar and trough cross stratified
sandstones (Fig. 3) . Major uranium showings are confined to the lower member.
The stratigraphy is discussed below fron west to east; a more detailed discussion is presented in Long (in press) .
Roberts and Creelman Townships
On the west side of the area (Fig. 2) the "Mississagi Formation" outcrops in a discontinuous belt extending from Roberts Township to Lake Wanapitei.
In Roberts Township (Fig. 3A-C) the basal member is between 10 and 50 m thick
\A/ 3 Mixtite
"""i ""b"'— --r--^ K:
;--¥"¥••
Roberts Parkin G r i g g Stobie Seagram DeMorest Pardo Vogt
Figure 3. Representative stratigraphie sections of the Mississagi Formation.
and consists of polymict and oligcmict conglomerate, feldspathic sandstone and minor argillite, with marked lateral and vertical changes in both thickness and faciès over relatively short distances (cf Fig. 4). Typical conglomerates are of pebble and cobble grade; they include both massive and plane bedded varieties which are typically clast supported (Fig. 5) and have low lateral continuity. Clasts within the conglcnerates are predominantly subangular to subrounded, although angular clasts are present locally. Clasts within the