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

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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 denominated as a grain‐supported conglomerate, which changes into a matrix‐supported conglomerate with increased sheet silicate content. The formation of this mineral assemblage can be explained as follows. The mineralization commenced with the precipitation ofCu‐Fe sulfides from solutions at temperatures higher than 350°C. With increasing PO2, sulfides gradually passed into Fe‐oxide/hydroxides, nontronites, and Fe‐sulfates, which formed at pH < 3 and Eh > 0.6 V (estimated). The ancient analogues of these present‐day metalliferous sequences may be traced along the Tethyan spreading zone (Cyprus‐type deposits). The ochre and massive sulfides in pillow lavas of Cyprus‐type deposits were subjected to strong early diagenetic alteration and halmyrolysis. By analogy with recent sequences described here from the EPR, we interpret this uppermost mineralization of Cyprus‐type deposits to have formed during waning stages of hydrothermal activity, whose brines were mixed up with the siliceous sediment load of marine currents.