The place of sulfide mineralization in the volcanic sequence at Noranda, Quebec

Abstract

Copper-zinc massive sulfide ore deposits in Noranda, Quebec, occur in a thick pile of Archean andesites and rhyolites that form the upper part of the Blake River Group. The flows are weakly metamorphosed and over a large part of the area have only shallow dips. Flows are extensive, often thick, sheets, and several features imply their extrusion in a deep-water setting. Subsidence during the buildup of the pile maintained relatively subhorizontal surfaces over large areas. Five zones of rhyolitic lavas mark five successive periods of rhyolitic activity.The ore deposits, which overlie chloritic pipes of alteration, occur at or near the top of rhyolitic formations and many are associated with primary volcanic features such as lava domes and explosive breccias. They show a zoning of copper-zinc ratios and evidence of fragmentation of some massive sulfides prior to their having been covered by later flows of andesite or rhyolite. They are attributed to submarine volcanogenic processes forming sulfide sinters over hot springs.Two main types of deposit, copper-zinc and zinc-rich, are recognized as occurring, respectively, in the third and fourth rhyolitic zones, while only a massive pyrite body is known in the fifth zone.Deposits in the third zone are divided into three groups which lie at different contacts due to the wedging out of several formations. The north group, often associated with rhyolite domes, formed on the Waite Rhyolite and may be placed precisely in a narrow time-span in the succession. They were deposited during the waning stage of the third period of rhyolitic activity. This waning was concurrent with the onset of the eruption of the overlying Amulet Andesite. Deposits of the center group occur at or above the upper contact of the Amulet Rhyolite, an older formation of the third zone. A wider time-span for their formation is possible but, as several deposits of this group occur in the Amulet Andesite and are in part associated with rhyolite domes, they are interpreted as having formed contemporaneously with those in the north group. Though deposits of the south group overlie rhyolites equated with the Amulet or Waite Rhyolites, they are overlain by rhyolites of the fourth zone and similarly were deposited in the same narrow time span as those of the other groups.Zinc-rich deposits formed with renewed hydrothermal activity higher in the pile, completing a cycle of copper to zinc with time, a feature also noted in most deposits.Ore deposits are restricted to a small part of the pile, that in the center of the area which is probably that over the magma chamber whose position is reflected by one of the granitic bodies and evidence of subsidence.