Hydrothermal mineralization at oceanic ridges

A compilation of data relating to more than 100 mineral occurrences at oceanic ridges and rifts comprising the global sea- floor spreading system is presented in terms of volcanic- vs sediment-hosted and of stage (early or advanced) and rate (slow- or intermediate- to fast-spreading) of opening of an ocean basin. Although this represents systematic sampling of <1% of the approx 55 000 km global length of spreading centres, examples of almost all major varieties of volcanic- and sediment-hosted hydrothermal deposits associated with basaltic rocks in the geological record have been found. The data indicate that a range of hydrothermal mineral-deposit sizes from small to > or =1 x 10 (super 6) tonnes occurs at all sea-floor spreading rates. Also it suggests that larger deposits, but fewer per unit length of spreading axis, form at slow- than at intermediate- to fast-spreading centres. Larger deposits are more common in sediment- than in volcanic-hosted settings regardless of spreading rate. A spectrum of hydrothermal deposit varieties (stratiform, stockwork and disseminated sulphides; various forms of sulphate, carbonate, silicate, oxide and hydroxide deposits) occurs in all of the tectonic settings. High-intensity, ore-forming, subsea-floor, hydrothermal convection systems that conserve heat and mass, and concentrate hydrothermal precipitates, are extremely localized by anomalous physical and chemical conditions relative to nearly ubiquitous low-intensity hydrothermal activity at, and flanking, sea-floor spreading axes at all spreading rates. Two distinct shapes of volcanic-hosted hydrothermal deposits at sea-floor spreading centres may be explained by differences in fluid dynamic behaviour controlled by T-salinity properties of solutions. Massive sulphide deposits that are mound-shaped in profile (the TAG massive sulphide mound on the Mid-Atlantic Ridges and the Archaean Noranda-area deposits) are constructed by hydrothermal solutions that discharge as buoyant plumes. Massive sulphide deposits that are saucer- or bowl-shaped in profile (the Atlantis II Deep deposits of the Red Sea and the Cretaceous Troodos deposits) are formed from ponded solutions denser than the surrounding sea-water. A review of data relating to 508 massive sulphide deposits suggests more than twice as many volcanic- and sediment-hosted ones are associated with rhyolitic than with basaltic rocks. This suggests that sea-floor spreading centres have been significant as tectonic settings for massive sulphide deposit formation through geological time, although subsidiary to continental rifts and volcanic island arcs. [This paper and those of the following 25 abstracts represent the papers and proceedings of a conference convened by the Mineral Exploration Research Institute in Montreal, February 1987].