VOLCANOGENIC MASSIVE SULFIDES THROUGH TIME

Volcanogenic massive sulfide deposits (VMS) occur in submarine volcanic strata that span ~3.5GA in five lithotectonic environments: 1: Bimodal volcanic, mafic-dominated in ocean-ocean arcs (Zn-Cu); 2: Bimodal volcanic, felsic-dominated in ocean-continent arcs (Zn-Pb-Cu); 3: Ophiolite -associated, in mafic volcanic dominated oceanic rift and oceanic backarc-rifts (Zn-Cu); 4: Basalt - pelagic sediment, mafic dominated oceanic backarc-rifts (Cu-Zn); 5: Felsic volcanic-siliciclastic dominated in ocean-continent backarc (Zn-Pb-Cu). Most require a shallow crustal heat source to drive a convective hydrothermal system. All form in extensional basins, with discharge along caldera- and rift-boundary faults. Significant variations occur in contents of Pb, Au, Ag and Sn within and between tectonic classes. Archean deposits are all Type 1. Most have low gold (~1 ppm); a few are anomalously gold-rich (~3+ ppm). Proterozoic deposits are all Types 1, 2 and 4, with elevated Au (~2ppm). Conventional back-arc systems seem absent in the Archean; both ocean-ocean (Trans Hudson) and ocean-continent (Skellefte) systems and rift-related backarcs (Outokumpu) appear in the early Proterozoic. Siliciclastic-dominant (Type 5) backarc settings are uncommon before the Phanerozoic (Bathurst). Hydrothermal fluids formed in basalt-dominated environments probably all had similar compositions, with narrow pH-pO2, temperature and salinity ranges. Fluids in felsic-dominated systems had a wider compositional range with lower pH, higher pO2, but similar salinities as basaltic fluids. Felsic systems may have had direct input of magmatic fluids (Sn). A control on Au (& Pb) contents in both may have been water depth and pO2. Au-rich deposits are almost exclusively formed in pyroclastic-dominated areas, constrained to < 2000m water depth. Shallow conditions induce boiling, where Au was preferentially bisulphide complexed, and Cu was fractionally precipitated, sub seafloor. Here, Au would precipitate (with Zn) very efficiently if the ocean water was oxidized. Cu-Au rich deposits are more possibly magmatic. All Au-rich Archean examples are in shallow-water, pyroclastic-dominated environments. Perhaps the Archean ocean was layered, with sufficient oxygen in its upper parts to permit efficient gold precipitation.