Cyprus type: associated with tholeiitic basalts in ophiolite sequences (back arc spreading ridge), copper rich with
gold. e.g. Troodos Massif (Cyprus).
Besshi-type: associated with plate volcanics and continental turbidites, copper/zinc with gold and silver.
e.g. Sanbagwa (Japan).
Kuroko-type: associated with felsic volcanics particularly rhyolite domes (back arc rifting), copper/zinc/lead with gold and silver, e.g. Kuroko deposits (Japan).
Primitve-type: associated with differentiated magmas of uncertain origin, copper/zinc and gold.
e.g. Canadian Archean rocks.
The stockwork zone beneath these deposits is the conduit through which the hydrothermal fluids rise and consists of vein sulphide mineralisation. Hydrothermal alteration forms a pipe around the stockwork zone and grades from an inner chloritised zone to an outer sericitic zone. The thin bedded exhalite which exists around the edges of the sulphide mound generally consists of pyritic, hematitic or siliceous material.
Formation of the massive sulphide lens depends on the conditions at eruption. If the density of the hydrothermal fluid is greater than that of the seawater it will settle in ponds and form brine pools, however if the density is less than that of seawater the fluid will rise forming plumes from which the sulphide and oxide particles will rain down.
A mound structure as shown above is formed as the hydrothermal fluid erupts and is slowed by the presence of eruption breccia at the vent. This allows the formation of chimney structures which collapse and add to the amount of sulphide breccia, further slowing the eruption of later fluids. This gives these fluids more time to cool and mix with seawater, increasing sulphide precipiation into the mound. With a crust developing on the outer surface the mound grows from within, with earlier sulphides being replaced by later eruptions.
Most of the sulphide formations would be oxidised in seawater as soon as hydrothermal activity ceased, therefore the mounds must be well preserved for deposits to exist. This is likely to occur with anoxic seafloor conditions, immediate burial under other flows or resulphidation due to burial metamorphism.
The origin of the fluids is dominantly seawater but magmatic sources have been advocated while the metals originate from the footwall lithologies up to 3km below the deposit.