IRON OXIDE-(COPPER-GOLD) SYSTEMS RELATED TO THE 1.86 GA GREAT BEAR CONTINENTAL ARC, NORTHWESTERN CANADIAN SHIELD

Fe oxide-(Cu-Au) systems in the Great Bear continental arc can be related to specific igneous events during its magmatic evolution. The calc-alkaline arc developed 1880-1840 Ma ago at the end of Wopmay orogeny, as a result of eastward subduction of an oceanic plate under the collision zone of the Paleoproterozoic Hottah terrane and the Archean Slave craton to the east. It evolved from (a) abundant volcanism 1870 to 1865 Ma ago, and intrusion of coeval porphyries and quartz monzonitic plutons, to (b) emplacement of calc-alkaline granitic batholiths ca. 1865 Ma, and then to (c) emplacement of late stage potassic granites ca. 1855 Ma.

Fe oxide-rich monometallic and polymetallic occurrences are distributed widely in the north-trending arc exposed over a 500x100 km area. They comprise 4 broad groups: (1) Stratiform massive magnetite beds with disseminated pyrite, chalcopyrite and uraninite in felsic tuffs. (2) Kiruna-type magnetite-apatite-actinolite veins, which are common at the margins and roof zones of the quartz monzonite intrusions, including the related laccoliths southeast of the arc along the Great Slave Lake. Some of the veins contain pyrite, copper sulphides, uraninite and nickel-cobalt arsenides. Most of them have strong sodic alteration. (3) Olympic Dam-type breccia-hosted polymetallic deposits with abundant magnetite and/or hematite matrix, which occur in the volcanic rocks. Main example is the Sue-Dianne deposit, which has drill indicated resources of 15 Mt averaging 0.78% Cu, 3.2 g/t Ag and 0.02 g/t Au. Brecciation is attributed to volalite-rich mineralizing fluids generated from differentiating felsic melts at depth. (4) Cloncurry-type polymetallic hydrothermal mineralization in the older iron oxide-rich meta-siltstone and argillite, at unconformity with the overlying felsic volcanic sequence of the arc. This is best developed in the Lou Lake (NICO) deposit, which contains drill indicated resources of 128 Mt averaging 0.54 g/t Au, 0.07% Co, 0.08% Bi, 0.05% Cu, and a notable amount of tungsten. The main ore minerals are arsenopyrite, chalcopyrite and pyrite. They form veinlets and disseminations, which extend across the unconformity. The mineralization was accompanied by strong and extensive potassium enrichment. It is interpreted as related to the late stage potassic granites.