WHERE ARE THE LARGE HYDROTHERMAL SULFIDE DEPOSITS IN THE OCEANS

Large sulphide deposits have been identified on slow and fast spreadin g ridges and back-arc basins. Their formation is controlled by a combi nation of several conditions, each of which alone is often only compat ible with tile formation of small and unstable deposits. The geologica l control of deposits has to be considered both at the regional and lo cal scales. The convective system is dependent on the morphology of th e heat source (magma chamber) and the magma supply. Major sites are co ntrolled by regional topographic highs that are the locus of the highe st magma and heat supply along the ridge. On slow spreading ridges the flow of hydrothermal fluids can also be controlled by major regional rift valley faults. The discharge within a field is controlled by the local near surface permeability related to faulting or permeability of rocks. Recent discoveries considerably enlarge the potential location s of hydrothermal activity. On slow spreading ridges we have now to co nsider the base and top of the rift valley walls and the non-transform offsets, in addition to the relatively well documented control by vol canic topographic highs. Known sites also demonstrate that slow spread ing ridges are more favourable for the formation of extensive minerali zation. On fast spreading ridges, deposits are numerous and very small because the upflow zone is relatively narrow and subject to perturbat ion by frequent tectonic and volcanic activity. However, near fast spr eading ridges, first order sulphide deposits can be formed on off-axia l seamounts. Geological and physical conditions are key parameters con trolling the morphology and potential size of deposits. Among these pa rameters, boiling, mixing within the crust, or precipitation under an impermeable cap rock, can enhance the formation of extensive subsurfac e mineralization within the oceanic crust. However, the knowledge of t hese deposits requires further investigation in the vertical dimension .