Massive sulfide chimneys from the east pacific rise at 7°24's and 16°43's

Abstract

Massive sulfide chimneys from two hydrothermal fields at 7°24'S and 16°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. Euhedral, partially porous aggregated Fe‐sulfides crystallized at high temperatures (> 300°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; partial solution of initial precipitates (S isotope disequilibrium!), quenching in outer portions of chimney walls, cementation with inward growth of front; late stage (150–100°C) epithermal impregnation of early (partially friable) precipitates by layered colloidal and laminated fine‐grained sulfide specimens III. Emanation of fluids at the chimney base or from secondary vents branching off the chimney; direct seawater mixing and precipitation of quenched sulfides on the outer shell of the initial central vent with partial impregnation of walls Stages II and III likely proceed simultaneously, provided the entire smoker builds in a single fluid‐expulsion event, with a slow temperature decrease of emanating fluids. None of these cycles is marked by the concentration of specific chemical elements; pyrite, chalcopyrite, and sphalerite are most abundant, and ubiquitous in various forms in all portions 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 chalcopyrite scavenges Se, Hg, and Bi. Sphalerite tends to concentrate Cd, Au, Ag, Ge, Pb, Mn, Ga, and In. Chemical disequilibrium is common among sulfides due to remobilization of early precipitates, “contamination”; by later‐stage hydrothermal solutions, and quenching.