Magnetite-rich calc-silicate alteration in relation to synvolcanic intrusion at the Ansil volcanogenic massive sulfide deposit, Rouyn-Noranda, Quebec, Canada

Abstract

The Ansil Cu–Au volcanogenic massive sulfide deposit is located within an Archean-age cauldron infill sequence that contains the well-known Noranda base metal mining district. The deposit is unusual in that 17% of the massive pyrrhotite–chalcopyrite orebody is replaced by semi-massive to massive magnetite. Temporally associated with the magnetite formation are several calc-silicate mineral assemblages within the massive sulfide lens and the underlying sulfide stockwork vein system. Coarse-grained andradite–hedenbergite and ferroactinolite–ilvaite alteration facies formed in the immediate footwall to the massive magnetite–sulfide lens, whereas an epidote–albite–pyrite-rich mineral assemblage overprints the margins of the chlorite-rich stockwork zone. The epidote-rich facies is in turn overprinted by a retrograde chlorite–magnetite–calcite mineral assemblage, and the andradite–hedenbergite is overprinted first by ferroactinolite–ilvaite, followed by semi-massive to massive magnetite. The footwall sulfide- and magnetite-rich alteration facies are truncated by a late phase of the Flavrian synvolcanic tonalite–trondhjemite complex. Early phases of this intrusive complex are affected to varying degrees by calc-silicate-rich mineral assemblages that are commonly confined to miarolitic cavities, pipe vesicles and veins. The vein trends parallel the orientation of synvolcanic faults that controlled volcanism and hydrothermal fluid migration in the overlying cauldron succession. The magnetite-rich calc-silicate alteration facies are compositionally similar to those of volcanic-hosted Ca–Fe-rich skarn systems typical of oceanic arc terranes. Tonalite–trondhjemite phases of the Flavrian complex intruded to within 400 m of the base of the earlier-formed Ansil deposit. The low-Al trondhjemites generated relatively oxidized, acidic, Ca–Fe-rich magmatic–hydrothermal fluids either through interaction with convecting seawater, or by assimilation of previously altered rocks. These fluids migrated upsection along synvolcanic faults that controlled the formation of the original volcanogenic massive sulfide deposit. This is one of the few documented examples of intense metasomatism of a VMS orebody by magmatic–hydrothermal fluids exsolved from a relatively primitive composite sub-seafloor intrusion.