Large sulphide deposits have been identified on slow and fast spreading ridges and back–arc basins. Their formation is controlled by a combination of several conditions, each of which alone is often only compatible with the formation of small and unstable deposits. The geological control of deposits has to be considered both at the regional and local scales. The convective system is dependent on the morphology of the heat source (magma chamber) and the magma supply. Major sites are controlled by regional topographic highs that are the locus of the highest 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 locations of hydrothermal activity. On slow spreading ridges we have now to consider the base and top of the rift valley walls and the non–transform offsets, in addition to the relatively well documented control by volcanic topographic highs. Known sites also demonstrate that slow spreading ridges are more favourable for the formation of extensive mineralization. On fast spreading ridges, deposits are numerous and very small because the upflow zone is relatively narrow and subject to perturbation by frequent tectonic and volcanic activity. However, near fast spreading ridges, first order sulphide deposits can be formed on off–axial seamounts. Geological and physical conditions are key parameters controlling the morphology and potential size of deposits. Among these parameters, boiling, mixing within the crust, or precipitation under an impermeable cap rock, can enhance the formation of extensive subsurface mineralization within the oceanic crust. However, the knowledge of these deposits requires further investigation in the vertical dimension.