The Bousquet Pyritic Gold Deposit, Bousquet Region, Quebec: Descriptive Geology and Preliminary Interpretations on Genesis

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

The Bousquet deposit consists of a number of stratiform lenses of pyrite with gold as native metal and as telluride minerals in volcanic rocks of the Archean Abitibi Greenstone Belt, 6.5 km north of Cadillac, Quebec. The Number 3 lens, which went into production in 1979, contains 1 million tonnes grading 7.5 grams of gold per tonne. It is 500 m long, 6 m thick and consists of recrystallized quartz with 2-mm-thick anastomosing layers of graphite and muscovite averaging 8 per cent subhedral pyrite. One per cent of the Number 3 lens is massive pyrite in layers up to 1.5 m thick which are most abundant near the center of the orebody. Grains of native gold up to 5 p.m in diameter are within or at the edges of quartz or pyrite grains. Gold is also present as the telluride minerals petzite and calaverite. The gold content is greatest near the center of the zone in association with layers of massive pyrite. Chalcopyrite and altaite are common accessory minerals. The Number 3 lens is adjacent to massive rhyolite near the top of a mafic to felsic volcanic sequence. The immediate hanging wall to the orebody is a rhyolite breccia and the immediate footwall is a manganiferous garnet schist with minor chloritoid, sphalerite and argentiferous galena. This overlies a mafic tuff in which plagioclase has been altered to muscovite and quartz and in which there is an increase in absolute amount of K and Si and a decrease in Na and Ca relative to tuffs 400 m laterally from the orebody. The interpretation is that the Number 3 lens precipitated in a sea-floor depression as chert, pyrite, native gold and telluride minerals from hydrothermal fluids which ascended fractures and altered the footwall tuffs before mixing with seawater.

Mots Clés: Gold, gold deposits, geology, Bousquet deposit, pyrite, Number 3 lens