Geology of the Thunder bay south District
The Thunder Bay South District (outlined in the map below) is underlain by a variety of Archean and Proterozoic rocks of the Superior and Southern structural provinces, respectively.
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Archean Superior Province
The Thunder Bay South District straddles three subprovinces of the Archean Superior Province. The Wawa and Wabigoon are volcano-plutonic subprovinces, consisting of metamorphosed supracrustal rocks (i.e. greenstone belts) surrounded and cut by granitic plutons and batholiths. The intervening Quetico subprovince represents accumulations of metamorphosed clastic sedimentary rocks, derived migmatites and granitoids. The three subprovinces were thrust-imbricated along steeply dipping shear zones during a late Neoarchean, transpressional tectonic event. Continued compression resulted in the development of regional, dextral, strike-slip shear zones along the subprovince boundaries.
Wawa and Wabigoon Subprovinces
The supracrustal component of these subprovinces consist largely of submarine, komatiite to tholeiitic basalt, with local centres of submarine to subaerial, calc-alkalic, intermediate to felsic volcanic rocks. Associated sedimentary rocks consist mainly of turbiditic wacke, minor conglomerate and iron formation. Volcano-sedimentary sequences are commonly tectonically juxtaposed to form greenstone belts; some of their representative ages are shown in the tables below:
Wawa Subprovince Belts:
| Greenstone Belt |
Rock Type(s) |
Location |
Age (Ma) |
| Schreiber - Hemlo |
Felsic volcanics |
Winston Lake |
2723 |
| Felsic volcanics |
Heron Bay |
2695 |
| Felsic volcanics |
North Limb, Hemlo |
2692-2698 |
| Felsic batholiths |
Hemlo |
2719-2720 |
| Felsic plutons |
Hemlo |
2678-2690 |
| Manitouwadge |
Felsic volcanics |
Manitouwadge |
2720-2722 |
| Sediments |
Manitouwadge |
2693 |
| Felsic plutons |
Manitouwadge |
2677-2687 |
| Shebandowan |
Mafic - felsic volcanics |
Shebandowan |
2720 |
| Felsic volcanic/plutonic |
Shebandowan |
2695 |
| Timiskaming sediments |
Shebandowan |
2690 |
| Felsic plutons |
Shebandowan-Thunder Bay area |
2670-2684 |
Wabigoon Subprovince Belts:
| Greenstone Belt |
Rock Type(s) |
Location |
Age (Ma) |
| Lumby Lake |
Felsic volcanics |
Lumby Lake |
2992 -2999 |
| Lumby Lake |
Felsic batholiths |
Marmion Lake |
3003 |
| Lumby Lake |
Felsic plutons |
Atikokan-Ignace |
2665-2730 |
| Calm Lake – Lac des Mille Lacs |
Felsic volcanics |
Calm Lake |
2722 |
Quetico Subprovince:
| Quetico Subprovince |
Rock Type(s) |
Location |
Age (Ma) |
| Quetico Subprovince |
Sediments |
Thunder Bay - Atikokan |
2690-2693 |
| Quetico Subprovince |
Alkalic Plutons |
Atikokan |
2679-2682 |
| Quetico Subprovince |
Felsic plutons |
Thunder Bay - Atikokan |
2685-2688;
265 -2671 |
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Proterozoic Southern Province
The Southern Province comprises a variety of Proterozoic rocks that either rest unconformably on, or intrude into, the older (Archean) rocks of the Superior Province. The constituent groups or "supergroups" of Proterozoic rocks are situated around the Lake Superior basin. They consist of a variety of relatively undeformed, flat-lying sedimentary and volcanic rocks and associated intrusive rocks.
The Thunder Bay South District is somewhat unique in that it is underlain by rocks representing three distinct tectonic settings, ranging from the Paleoproterozoic to the Mesoproterozoic, spanning some 800 million years. These three divisions are represented by the Animikie and Sibley groups, and the Keweenawan Supergroup, as shown in the following table:
| AGE |
GROUP |
SUBDIVISIONS |
ROCK TYPES |
Mesoproterozoic
(ca. 1.1 Ga) |
Keweenawan Supergroup |
Osler Group |
Basalt, rhyolite, conglomerate, sandstone |
Mesoproterozoic
(ca. 1.1 Ga) |
Keweenawan Supergroup |
Intrusive Igneous Rocks |
Diabase, gabbro, carbonatite, syenite |
Mesoproterozoic
(ca. 1.55 Ga) |
Sibley Group |
Kama Hill Formation |
Red shale, mudstone |
| Rossport Formation |
Sandy dolostone |
| Pass Lake Formation |
Quartz sandstone, conglomerate |
| Paleoproterozoic (ca.1.8 Ga) |
Animikie Group |
Rove Formation |
Shale, wacke |
| Gunflint Formation |
Conglomerate, shale, chert-carbonate, tuff, iron formation |
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Economic Geology
A variety of precious and base metal ore deposits have been discovered in Archean Superior Province greenstone belts within the Thunder Bay South District.
Wawa Subprovince Belts:
| Greenstone Belt |
Deposit / Camp |
Type of Deposit |
Size and Grade of Deposit(s) |
| Schreiber – Hemlo |
Hemlo |
Lode Gold |
>80 mt @ 0.25 opt Au
(= 20 m oz Au) |
| Schreiber – Hemlo |
Winston Lake |
Volcanogenic Massive Sulphide |
4.2 mt @ 16.8% Zn, 1% Cu, 37 g/t Ag |
| Manitouwadge |
Geco – Willroy |
Volcanogenic Massive Sulphide |
64 mt @ ~2% Cu, 4% Zn, 50 g/t Ag. 0.3% Pb |
| Shebandowan |
North Coldstream |
Mafic Intrusion – Hosted(?) |
~ 2.75 mT @ 1.875% Cu ; Au, Ag |
| Shebandowan |
Shebandowan |
Ultramafic Intrusion – Hosted |
8.7 Mt @ 2.0% Ni, 1% Cu |
Wabigoon Subprovince Belts:
| Greenstone Belt |
Deposit(s) |
Type of Deposit |
Size and Grade of Deposit(s) |
| Finlayson Lake |
Steep Rock Lake |
Iron Formation |
>700 mt of high-grade iron ore |
| Heaven Lake |
Lac des Iles |
Mafic Intrusion - Hosted |
>42.8 mT @ 0.07 opT Pd, 0.006 opT Pt, 0.006 opT Au, 0.075% Cu, 0.092% Ni |
With respect to the Southern Province rocks, Animikie and Sibley group rocks in the Thunder Bay area host no economic mineral deposits, other than dimension stone, that are associated directly with their deposition.
However, a variety of economic and sub-economic, syn- and epigenetic, metallic mineral deposits are related to magmatic and associated hydrothermal activity during Midcontinent (Keweenawan) rifting. These include:
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Geological Map of the Thunder Bay South Districtx
The map below depicts the general geology of the Thunder Bay South District.
