THE ROLE OF SEDIMENTS IN ANCIENT SEAFLOOR MASSIVE SULFIDES FORMED IN SEDIMENTED SETTINGS

Modern seafloor hydrothermal deposits hosted in part or largely by sediments (e.g., Red Sea, Guaymas Basin, Middle Valley, Escanaba Trough) have ancient analogs that are variously classified as “Besshi-type” or “mafic-siliciclastic” (e.g., Besshi, Japan; Windy Craggy, B.C.), “bimodal-siliciclastic”, or hybrid sedimentary exhalative-volcanogenic massive sulfide (e.g., Bathurst, N.B.; Wolverine, Yukon; Neves Corvo, Portugal). These are generally much larger than those in non-sedimented settings and are therefore a more desirable target for explorationists. Several Canadian deposits will be used as examples to illustrate the role of sediments in the mineralizing process. Sediments focused hydrothermal discharge by insulating the mineralizing fluids from cold ambient seawater and suppressing conductive and convective heat loss. Sediments also served as a chemical trap during fluid interaction with the sedimentary rocks; they can be the source of metals and other constituents in the mineralization (e.g., Pb, S, Se, carbonate). The proximity of the site of mineralization to land, and the tectonic setting (e.g., continental margin versus backarc basin) may have controlled the composition of sediments deposited (carbonate-rich, mature versus immature), thus influencing metal tenors (e.g. Pb abundance) and gangue composition (e.g., carbonate content) of the deposit. At high sedimentation rates, turbidites may have predominated, controlling the style of volcanism, which occurred largely by sill and dike injection. At lower sedimentation rates, hemipelagic sediments or black shales may have been deposited under reduced ambient bottom water conditions, and this may have contributed to deposit preservation. Sediments also influenced the morphology and architecture of the deposits. As in non-sedimented settings, constructive mounds remain important; however, replacement-style mineralization can be significant, particularly in porous and permeable coarser-grained sediments.