Geology of the Thunder bay North District

The Thunder Bay North District encompasses four main subprovinces within Archean terrain of the Eastern Canadian Shield--the Wabigoon, Uchi, Berens River and Sachigo subprovinces.

Wabigoon Subprovince

Rocks of the Wabigoon Subprovince occupy the southern most portion of the district and are the most significant in terms of both current exploration activity and past production. East of Lake Nipigon, the subprovince is divided into two belts: the main east-trending Beardmore-Geraldton metavolcanic-metasedimentary belt to the south and the older (2.7 to 2.9 billion years old) Onaman-Tashota metavolcanic belt to the north. These belts are separated by an east-trending regional transcurrent fault known as the Paint Lake fault. This fault is the current focus of major gold exploration (i.e. Ontex Resources Ltd.’s Brookbank deposit).

The main Beardmore-Geraldton belt is dominated by iron tholeiite flows and tuffs, wacke, conglomerate, siltstone and prominent magnetite-hematite-chert iron formation rocks. Since the mid-1930s within this belt, the Beardmore-Geraldton camp has produced 4.12 million ounces of gold from 19 past producing mines.

The Onaman-Tashota metavolcanic belt consists of a felsic to mafic (calc-alkaline and tholeiitic) sequence deformed by the emplacement of intervening granodioritic to gabbroic intrusions. Regional lineaments or faults trend north and northeasterly. The rocks range from mafic metavolcanic pillowed flows and tuffs to felsic tuffs, porphyries and pyroclastic breccia. Metamorphic grade is commonly greenschist but ranges to amphibolite grade. Base metal, volcanogenic massive sulphide (VMS) and gold exploration is ongoing within the belt.

East Uchi, Berens River and Sachigo Subprovinces

The eastern portion of the Uchi Subprovince, within the Fort Hope area, consists of a tabular, east-trending belt (080° ) of metavolcanic and metasedimentary rocks forming a semi-continuous supracrustal network. The Uchi Subprovince extends for over 625 km and includes such prolific gold producing camps as Red Lake and Pickle Lake. Volcanogenic massive sulphides have been mined and are the target of active exploration in the East Uchi Subprovince.

The Sachigo and, to a lesser degree, the Berens River subprovinces are the most northerly granite-greenstone subprovinces in Ontario. These subprovinces are made up of isolated, thin greenstone belts surrounded by extensive granite and gneiss units (Thurston, Osmani and Stone 1991).

A total of 42 mineral occurrences (prospects and deposits) are known to exist within these three subprovinces (primarily within the East Uchi Subprovince). Eight gold deposit types and three base metal types including magmatic, VMS and massive sulphide/banded iron formation were identified.

The map below depicts the general geology of the Thunder Bay North District.

References

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