Origin of a complex alteration assemblage, Four Corners Cu-Zn prospect, Quebec, Canada

Abstract

Three types of hydrothermal alteration spatially associated with Cu-Zn mineralization within the Archean Blake River Group of greenschist facies metavolcanic rocks, Noranda, Quebec, were studied. Alteration was mapped around a partly fragmental rhyolite dome, enclosed in mafic flows. Sodic regional alteration and calcic-silicic background alterations were overprinted by potassic and magnesian pipe alteration. Pipe alteration was mapped only in conical breccia pipes within the rhyolite dome, which terminate at a pyroclastic rhyolite-rhyolite breccia contact. Cu-Zn sulfide mineralization was found within the pipes and in stratiform exhalative tuff on overlying volcanic rock contacts. Mass balance studies suggest a general trend from base-fixing regional and background alteration to base-leaching pipe alteration.Textural and geochemical evidence and similarities with presently active geothermal systems indicate alteration and mineralization were products of an Archean convective system, closely related in time and space to volcanism. Albite-rich rocks (regional alteration) are mineralogically similar but chemically distinct from greenschist facies metavolcanic rocks in the rest of the Noranda area. They are chemically similar to spilitic rocks from Thingmuli volcano, Iceland, and are the earliest evidence of seawater-rock interaction. This interaction, in conjunction with further reactions during subsequent background alteration, converted neutral to alkaline seawater into a hot, metal-enriched acid brine. Final reactions between this brine and the rocks through which it passed immediately beneath the sea floor resulted in pipe alteration and in Cu-Zn mineralization where the brine was expelled onto the sea floor. Stratiform pillowed andesite flows showing calcic background alteration were altered during their deposition by continuing exhalative activity. These distinctive altered flows represent excellent field indicators of base metal mineralization, best identified and traced through detailed mapping.