The Science of Seafloor Massive Sulfides (SMS) in the Modern Ocean A New Global Resource for Base and Precious Metals

Abstract

High temperature hydrothermal activity was first observed 30 years ago in the modern oceans where hot springs are precipitating sulfide-sulfate-silica mounds and columnar edifices (“chimneys”) of calcium, barium, iron, copper, zinc, lead, silver and gold with other minor elements. Hydrothermal fields are now known in several major geodynamic settings (slow and fast spreading ridges, back-arc basins, arcs, and fore arcs) and associated with various types of basement rocks (basalt, andesite and dacite volcanic; sediment; and ultramafic intrusions from the mantle). According to their geodynamic setting and the composition of the basement rocks, hydrothermal sulfide deposits can be divided in five major types: 1) Mid ocean ridges + basalt = oceanic crust type; 2) Slow spreading ridges + ultramafic rocks = mantle type; 3) Arc or immature back-arc + felsic lava = Back-arc type); 4) Mid-ocean ridge + sediments + basalt = sedimented ridge type; and 5) Back-arc + continental sediments + felsic lava = sedimented back arc type. Active sites are known at water depths from a few hundreds of meters to 4100 m. The mineral and chemical compositions of sulfides are strongly dependent on the basement rock composition, the degree of maturation of the deposit, the geodynamic setting and, in some cases, on the input of magmatic fluids. At another scale, the composition of fluids and sulfide mineralization is controlled by various physical and chemical processes. One important process, related to pressure and water depth, is phase separation. Modern hydrothermal fields provide insight into geological controls, as well as the mode of formation of Submarine Massive Sulfide (SMS) deposits. On fast spreading ridges, the discharge is unstable and the style of activity varies according to the relative importance of tectonic and volcanic activities. Axial hydrothermal fields are small; however, large sulfide deposits can be formed on off-axial volcanoes. On slow spreading ridges, the hydrothermal activity is more stable and better focused. Hydrothermal fields are much larger than on fast spreading ridges but the spacing between fields is greater. Geological controls are variable: the top of the axial volcanoes where the control is volcanic is one control, but also the base and the top of the rift valley walls, as well as non-transform discontinuities where the control is tectonic and basement rocks often dominated by ultramafic rocks. Back-arc hydrothermal fields also vary depending on the importance of tectonic versus volcanic activity. The style of the discharge and the morphology of mineralization are influenced by the strong permeability of the felsic, vesicular and brecciated lava. Discharge occurs often as extensive (>1km) low temperature deposits at the top of the volcaniclastic ridges. The first observations of black smokers led to the understanding that the SMS deposits were formed primarily by accumulations of chimneys on the oceanic floor. The most recent investigations, and in particular operations of the Ocean Drilling Program showed that they are formed by three principal processes: 1) Accumulations of chimneys on the seafloor; 2) sulfate and sulfide precipitates within the mound; and 3) Replacement of basement rocks (volcanic, ultramafic rocks or sediments). The morphology of mineralization is controlled by the permeability of basement rocks. In the more mature mounds, zone refining processes produces a mineral and chemical zonation of the sulfide mounds.