Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

A gold copper deposit in Archean mafic volcanic rocks, near the eastern limit of the Abitibi Greenstone belt about 2 km north of the Grenville Front, and 65 km southwest of Chibougamau, Quebec, is described with reference to major mineralogy, textures and alteration. Host rocks consist of an intrusive subvolcanic gabbro sill and mafic and felsic volcanic rocks formed in a volcano-sedimentary environment* This sequence was intruded by felsic and diabase dykes. Mineralization consists of native gold and chalcopyrite in brecciated quartz veins, formed as fracture-fillings, located in three subparallel shear zones cutting the gabbro, mafic and felsic volcanic rocks. Chalcopyrite, pyrite, pyrrhotite and minor sphalerite are found in breccia interstices and in disseminations near the veins but are not spatially related to the gold mineralization. The gold is contained in quartz and albite. Textures show it was the last mineral to be deposited in the veins.

A mineral identified as anhydrite is found in the 'main' shear and the mafic volcanic rock unit of the mine sequence. As anhydrite has not been found elsewhere in the region it may be possible to use its presence as a stratigraphie indicator.

A hydrothermal lateral secretion origin is postulated for the deposit. Basic, reducing, carbonate and H2S-bearing fluids have the capability to transport the mineralization of the deposit and to produce the alteration noted in the host rocks. The deposition of the gold mineralization took place during late stages of, or after, regional metamorphism.