Hydrothermal vent deposits precipitates

Modem hydro- High-temperature hydrothermal vents currently active at mid-ocean ridges offer a thermal mineral- unique opportunity to study a hydrothermal mineral deposit in the process of ization at formation. The current working model assumes that cold seawater sulphate is mid-ocean ridges drawn down into sea-floor basalts, where it is heated in the vicinity of a magma chamber. Some sulphate is precipitated as anhydrite whilst the remainder is reduced to sulphide by reaction with the basalt. The fluid is vented back onto the seafloor at about 350 C laden with sulphides. On mixing with seawater these are precipitated onto the sea floor as a fine sulphide sediment whilst at the vent site itself the sulphides are built into a chimney a metre or so in height. 

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

Farrell and Holland (1983) cited ba,sed on Sr isotope study on anhydrite and barite in Kuroko deposits that the most appealing model for the formation of Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately below above the vent exit in the overlying seawater (Eldridge et al., 1983). The study on the chimney ores from Kuroko deposits support this model which is discussed below. 

HAs04 as well as the rare earth elements (REE). Precipitation of particulate Mn oxides takes place much more slowly, mainly in the neutrally buoyant plume where the oxidation is bacterially mediated. Because of the slow precipitation rate, particulate Mn concentrations increase in the plume to a maximum 80-150 km from the vent. 80% of the hydrothermal Mn is deposited on the sea floor within several hundred km of the vent field but the remaining Mn still raises the background concentration of Mn in seawater several-fold (Lavelle et al. 1992). The residence time of hydrothermal Mn in seawater is several years. German and Angel (1995) have estimated that the total hydro-thermal Mn flux to the oceans is 6.85-10 kgyr. This... 

At typical black smoker vents, a very large proportion (at least 90%) of the metals and sulfur carried in solution are lost to a hydrothermal plume in the overlying water column rather than deposited as chimneys. The metals are precipitated as sulfide particles in the plume above the black smokers and are rapidly oxidized and dispersed over distances of several kilometers from the vent (Feely et al. 1987, 1994a,b Mottl and McConachy 1990). Due to oxidation and dissolu-... 

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