Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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Guide to the Proterozoic Rocks of the Northwestern
Lake Superior Area, Ontario
INTRODUCTtON:
The Froterozoic rocks of Northwestern Ontario, which form part
of the "Animikie" and Keweenawan unit; represent one of the most
complete geological records of middle and late Proterozoic sedimentation
and igneous activity in eastern North America. These rocks
are virtually uninetamorphosed and only slightly deformed.
Mineral deposits in these Proterozoic rocks include silver in
Keweenawan dykes and the Rove Formation, iron in the Gunf lint Formation,
nickel in mafic intrusive rocks, copper in various volcanic
and sedimentary strata, and lead-zinc-barite associated with the
Sibley Group. During the last century, the famous Silver Islet mine
produced over three million dollars in silver. Currently, a minor
amount of silver is recovered from the Creswel mine near Stanley
(Fig. 1).
GENERAL GEOLOGY
The Proterozoic rocks lie unconforinably on the peneplained
Archean surface. Archean meta volcanic and meta sedimentary rocks
form a "belt" extending from west of Shebandowan to Thunder Bay city.
Another similar belt crops out in the Schreiber—Big Duck Lake area
(Pye, 1964). To the north, the Geraldton-Beardmore belt may be
traced westward by aeromagnetic interpretation under Lake Nipigon,
and may possibly join with the Lac Des Mille Lacs—Atikokan belts.
The remainder of Archean outcrop is composed of intrusive and metamorphic
granitic rocks, and small ultrainafic bodies.
The Proterozoic rocks are subdivided as shown in Table 1.
- TABLE 1 —
Proterozoic Stratigraphy of Northwestern Ontario
Neohelikian
Osler Group: basalt, minor rhyolite and sedimentary rocks
Intrusive rocks: gabbro plugs
undersaturated plugs
layered bodies
northeast trending dykes
Logan diabase sills
Paleohelikian
Sibley Group: red beds, stromatolite zone
Apheb ian
Animikie Group
Rove Formation: shale
Gunf lint FormatioB: iron formation