MINERALOGY AND CHEMISTRY OF METALLIFEROUS MUDS FORMING THE TOPSTRATUMOF A MASSIVE SULFIDE-METALLIFEROUS SEDIMENT SEQUENCE FROM EAST PACIFIC RISE 18-DEGREES-S - ITS ORIGIN AND IMPLICATIONS CONCERNING THE FORMATION OF OCHROUS SEDIMENTS IN CYPRUS-TYPE DEPOSITS

A stratified mound surrounding a massive sulfide chimney was sampled o n the East Pacific Rise (EPR) at 18 degrees S and studied with respect to its mineralogical and chemical composition. A tripartite successio n was observed, composed-from bottom to top-of massive sulfides, sulfi dic debris, and metalliferous mud. These types Of Sulfide deposits dif fer from each other significantly by the quantity of ore and gangue mi nerals, texture, and grain size. Different modifications of Fe disulfi des, pyrrhotite, sphalerite, Cu-Fe-sulfides, and Zwischenprodukt (inte rmediate compounds) make up the sulfidic portion of the mineral assemb lage. Ferrihydrite was formed during the incipient stages of oxide pre cipitation and was diagenetically altered to goethite. The reaction of ferrihydrite with silica of hydrothermal and to a lesser degree, biog enic origin, resulted in the formation of nontronite, which is the onl y 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 conglom erate with increased sheet silicate content The formation of this mine ral assemblage can be explained as follows. The mineralization commenc ed with the precipitation of Cu-Fe sulfides from solutions at temperat ures higher than 350 degrees C. With increasing P-O2, sulfides gradual ly passed into Fe-oxide/hydroxides, nontronites, and Fe-sulfates, whic h formed at pH < 3 and Eh > 0.6 V (estimated). The ancient analogues o f these present-day metalliferous sequences may be traced along the Te thyan spreading rone (Cyprus-type deposits). The ochre and massive sul fides 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 min eralization of Cyprus-type deposits to have formed during waning stage s of hydrothermal activity, whose brines were mixed up with the silice ous sediment lend of marine currents.