Abstract:
Mineralizing processes at seafloor hydrothermal sites are directly observed and measured, reducing uncertainties inherent in interpretation of ancient, deformed and metamorphosed ore-bearing districts. The best example is volcanogenic massive sulfide (VMS) deposits. For these, seawater is down-drawn regionally to cool shallow crustal-level subvolcanic intrusions; these provide heat and drive convective circulation beneath caps which inhibit cross-stratal fluid migration. Trapped fluids reach approximately 400C and react with their enclosing strata, imparting a unique mineralogical imprint on the latter and releasing metals to the fluid. Fluids discharge in convective upwelling zones or along caldera- or rift-margin extensional faults. Near-seafloor reactions form distinct alteration pipes. Fluids discharge on or near the seafloor, where rapid cooling induces massive precipitation. Seafloor research contributes as follows: Magma-chamber petrochemical processes are unique to these environments, as shown by the specific occurrence of andesite (Galapagos VMS site). Identification of such petrochemically-unique units helps refine the land-based search in frontier areas. The ODP drill program (Legs 139 & 169; hole 504B) contributed to our knowledge of the reaction zone attributes; Site 857 at Middle Valley provided unequivocal mineralogical and compositional measures that now are applied in ancient sequences. Subseafloor alteration pipes establish sectoral and temporal variations in discharge-zone mineral and chemical attributes(Galapagos, TAG); these provide useable guides to ore in ancient pipes. The volcanological characteristics of caldera (Axial) and rift-related (Endeavour, EPR) synvolcanic faults provide a template for understanding similar preserved sequences. Finally, studies of vent fluids provide knowledge of metal speciation and predict characteristics such as gold contents (Axial, Lau, Havre Trough), sub-seafloor depositional and zone-refining attributes (ODP:TAG & Middle Valley), and far-field vectors from plume fallout. As research extends to new environments (arcs and backarcs) and to greater sub-seafloor depths (proposed TAG and other ODP sites) we will develop new, quantitative exploration guides that will be applied in searching for increasingly scarce mineral resources.