MASSIVE SULFIDE CHIMNEYS FROM THE EAST PACIFIC RISE AT 7-DEGREES-24'SAND 16-DEGREES-43'S

Massive sulfide chimneys from two hydrothermal fields at 7 degrees 24' S and 16 degrees 43'S on the East Pacific Rise were investigated using microscopic and chemical methods. The studies reveal three stages of growth for chimneys derived from black smokers in both locations: I. E uhedral, partially porous aggregated Fe-sulfides crystallized at high temperatures (> 300 degrees C) building the central vent II. Clogging of mesh-structured (spongy) central conduit by idiomorphous base metal sulfides; percolation of solutions through porous chimney walls; part ial solution of initial precipitates (S isotope disequilibrium!), quen ching in outer portions of chimney walls, cementation with inward grow th of front; late stage (150-100 degrees C) epithermal impregnation of early (partially friable) precipitates by layered colloidal and lamin ated fine-grained sulfide specimensIII. Emanation of fluids at the chi mney base or from secondary vents branching off the chimney; direct se awater mixing and precipitation of quenched sulfides on the outer shel l of the initial central vent with partial impregnation of walls Stage s II and III likely proceed simultaneously, provided the entire smoker builds in a single fluid-expulsion event, with a slow temperature dec rease of emanating fluids. None of these cycles is marked by the conce ntration of specific chemical elements; pyrite, chalcopyrite, and spha lerite are most abundant, and ubiquitous in various forms in all porti ons of the smokers. Marcasite occurs laminated, interlayered with gel- pyrite. Multivariate factor analysis reveals a preferred accumulation of As, Cr, and Ni in the crystal lattice of iron sulfides, while chalc opyrite scavenges Se, Hg, and Pi Sphalerite tends to concentrate Cd, A u, Ag, Ge, Pb, Mn, Ga, and In.