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

The Ansil Cu-Au volcanogenic massive sulfide deposit is located within an A rchean-age cauldron infill sequence that contains the well-known Noranda ba se metal mining district. The deposit is unusual in that 17% of the massive pyrrhotite-chalcopyrite orebody is replaced by semi-massive to massive mag netite. Temporally associated with the magnetite formation are several calc -silicate mineral assemblages within the massive sulfide lens and the under lying sulfide stock-work vein system. Coarse-grained andradite-hedenbergite and ferroactinolite-ilvaite alteration facies formed in the immediate foot wall to the massive magnetite-sulfide lens, whereas an epidote-albite-pyrit e-rich mineral assemblage overprints the margins of the chlorite-rich stock work zone. The epidote-rich facies is in turn overprinted by a retrograde c hlorite-magnetite-calcite mineral assemblage, and the andradite-hedenbergit e 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-t rondhjemite complex. Early phases of this intrusive complex are affected to varying degrees by calc-silicate-rich mineral assemblages that are commonl y confined to miarolitic cavities, pipe vesicles and veins. The vein trends parallel the orientation of synvolcanic faults that controlled volcanism a nd hydrothermal fluid migration in the overlying cauldron succession. The m agnetite-rich calc-silicate alteration facies are compositionally similar t o those of volcanic-hosted Ca-Fe-rich skarn systems typical of oceanic are 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 tr ondhjemites generated relatively oxidized, acidic, Ca-Fe-rich magmatic-hydr othermal fluids either through interaction with convecting seawater, or by assimilation of previously altered rocks. These fluids migrated upsection a long synvolcanic faults that controlled the formation of the original volca nogenic massive sulfide deposit. This is one of the few documented examples of intense metasomatism of a VMS orebody by magmatic-hydrothennal fluids e x-solved from a relatively primitive composite sub-seafloor intrusion.