A NOTE ON FLUID DYNAMIC PROCESSES WHICH CAN INFLUENCE THE DEPOSITION OF MASSIVE SULFIDES

The dynamics of hydrothermal fluids venting into the sea are summarized, and are noted to be contrary to expectations and to pose problems for current ideas of massive sulphide formation. Thus, calculations show that the fine sulphides from a black smoker plume are widely dispersed and cannot form an ore deposit. Three fluid dynamic models deserve consideration: 1) jets of dense hydrothermal fluid; 2) plumes venting in a submarine depression; and 3) precipitation under an anhydrite cap. The last is favoured as best explaining deposition at the sea-floor from buoyant fluids, base metal zoning, replacement textures and the prevalent sedimentary textures and structures, probably originated in late-stage dissolution of anhydrite. On the other hand, no anhydrite caps have been described above any Cu-Zn massive sulphide deposits. -G.J.N.