The Chadbourne Mine, N oranda, Quebec: A Gold-Bearing Breccia

Special Volume, Vol. SV 24, No. 1982, 1982

The Chadbourne mine is within the town of Noranda, Rouyn Township, Quebec. After attempts in 1923, 1932, 1951 and 1969, production began in January 1978, with reserves of 1.1 million tonnes of 4.52 g Au/ tonne plus trace amounts of silver, copper and zinc. This deposit is in the northern twothirds of a breccia which cuts felsic and mafic metavolcanic rocks of the Blake River Group, within the Archean Abitibi Greenstone Belt. The breccia is elliptical at surface, 300 m from north to south and 120 m from east to west; the plunge is 80 degrees southeast and it persists at least 750 m below surface. The breccia contains clasts of andesite and minor amounts of rhyolite which are mostly 0.10 to 1.5 m in diameter, but in some instances andesite clasts are up to 15m in diameter. Clasts are rectangular to lath-like, have quartz, sericite and carbonate rims, and locally have 30% pyrite. They are almost totally matrix supported, have little or no comminution and tend to lie en echelon and overlapping. The matrix is quartz, albite, calcite, ankerite and dolomite, with quartz dominant, and locally up to 30% as carbonate minerals. There is accessory magnetite, specular hematite and tourmaline. The ore, defined as greater than 2.2 g Au/ tonne, is funnel shaped with a surface diameter of 76 m tapering to a diameter of 16m, 240m below surface. Ore cuts off abruptly against andesite to the north and northwest and grades south and southeast into barren breccia. Native gold occurs as microscopic inclusions within 0.1- to 1. 0-cm pyrite cubes in the breccia. Minor visible gold is present in quartz veinlets cutting andesite clasts. The greatest gold content is associated with the more siliceous matrix. The shape, lack of comminution and en-echelon orientation of the clasts and their matrix-supported nature suggests that the breccia is neither explosive nor intrusive, but rather was produced by hydrofracturing of rocks by hydrous fluids localized by fractures along faults. Metals were leached from host volcanic rocks, with subsequent deposition in zones of greater dilatancy. This process is analogous to observed hot spring activity where hot, metal-bearing fluids are circulated by movement along regional faults.
Keywords: Gold, gold mining, mineralization,