Mineralogy and chemistry of metalliferous muds forming the topstratum of a massive sulfide‐metalliferous sediment sequence from East Pacific rise 18°S: Its origin and implications concerning the formation of ochrous sediments in cyprus‐type deposits

Abstract

A stratified mound surrounding a massive sulfide chimney was sampled on the East Pacific Rise (EPR) at 18°S and studied with respect to its mineralogical and chemical composition. A tripartite succession was observed, composed—from bottom to top—of massive sulfides, sulfidic debris, and metalliferous mud. These types of sulfide deposits differ from each other significantly by the quantity of ore and gangue minerals, texture, and grain size. Different modifications of Fe disulfides, pyrrhotite, sphalerite, Cu‐Fe‐sulfides, and Zwischenprodukt (intermediate compounds) make up the sulfidic portion of the mineral assemblage. Ferrihydrite was formed during the incipient stages of oxide precipitation and was diagenetically altered to goethite. The reaction of ferrihydrite with silica of hydrothermal and, to a lesser degree, biogenic origin, resulted in the formation of nontronite, which is the only phyllosilicate recognized in this argillaceous rock. In the case of prevailing sulfidic debris, the material may be...