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  Friday, July 25, 2008

RESIDENT GEOLOGIST PROGRAM

Geology of the Red Lake District

The Red Lake District is entirely underlain by Archean (i.e. greater than 2.5 billion year old) rocks of the Superior Province of the Canadian Shield. These fall into four categories which are generally distinguished by the subprovince in which they occur:

• Uchi Subprovince rocks, which contain the northeast-trending Red Lake and Birch-Confederation Lake greenstone belts, where the bulk of the exploration and mining activity has taken place;
• English River Subprovince rocks, south of the Uchi Subprovince, which are predominantly sedimentary and minor intrusive rocks similar to those in the Quetico Subprovince; and
• The Berens River Subprovince, north of the Uchi Subprovince, and the Sachigo Subprovince, north of the Berens River Subprovince. Both of these subprovinces consist of relatively small, isolated greenstone belts surrounded by extensive granitic and gneissic units.

The Uchi Subprovince metavolcanic rocks have been subdivided into assemblages with ages ranging from the youngest the Confederation Assemblage 2730 to 2800 mY through the Bruce Channel and Woman Assemblages 2800 to 2900 MY to the Balmer and Ball Assemblage 2900 to 3000 MY. The Balmer Assemblage rocks form the core of the Red Lake Greenstone belt and host several of the largest and most prolific gold mines. Small mafic to ultramafic intrusives cut all the asemblages. The Balmer consists of basaltic tholeiite and komatiite flows with intercalated magnetite-quartz iron formation. Felsic pyroclastic rocks occur as comparitively thin units. Balmer Assemblage rocks are the oldest with ages 2958 to 2992 Ma. The Ball Assemblage, located in the northwestern corner of the Red Lake Belt, is slightly younger with ages 2940 to 2925 Ma in the upper part of the assemblage. The main mass of the Ball Assemblage is composed of calc-alkalic mafic flows and intermediate to felsic calc-alkalic flows and tuffs. Well preserved stromatolitic dolomitic carbonate rocks are found in the upper part of the Ball Assemblage in Ball and Todd Townships. The Bruce Channel Assemblage is poorly exposed in the eastern part of the belt. It is composed of basaltic flows capped by minor felsic pyroclastic rocks with a common age of 2894 Ma and clastic and iron formation metasediments. Woman Assemblage rocks are restricted to 2830 Ma felsic rocks in the central part of McKenzie Island. The Confederaion Assemblge is found on the northern and southern flanks of the Red Lake Belt. The rocks are predominantly calk-alkalic with substantial amounts of felsic pyroclastic deposits that can be correlated stratigraphically with simillar rocks in the Birch-Uchi Greenstone belt. The relationships between these assemblages is still under study.

Gold has been produced from greenstone belts of the Uchi, Berens River and Sachigo Subprovinces while iron and base metals have been produced from the Uchi Subprovince. Due to the isolated nature of the northern greenstone belts only high value commodities such as gold were explored for in the past. Gold mining has taken place at two isolated deposits in the northern greenstone belts. Gold, iron and base metal occurrences have been reported from many parts of the District. Rare metal pegmatites are found along the southern boundary of the Uchi Subprovince and along the boundary between the Berens River and Sachigo Subprovinces.

The map below depicts the general geology of the Red Lake District.