Studies of sulfide cores from Site 856, Middle Valley, northern Juan de Fuca Ridge, established the vertical zonation of a large inactive oceanic massive sulfide deposit. Zone 1 consists mainly of pyritic massive sulfide with minor sphalerite and magnetite (0 to 28 mbsf in Hole 856H and 0 to 40 mbsf in Hole 856G). Compared with the rest of the deposit, this zone is strongly enriched in Zn and a large number of trace metals (in particular Cd, Mn, Sn, and Sb), and in elements contained in gangue minerals (Si, Al, Mg, and Ba). Most of these elements reach their maximum concentrations in the upper part of the zone; the lower part is enriched in Cu, Se, and Ca. Zone 2 consists dominantly of pyrrhotite-rich massive sulfide with minor pyrite and magnetite (28 to 48 mbsf in Hole 856H) and is depleted in minor elements. Zone 3 is almost pure pyritic massive sulfide (48 to 75 mbsf in Hole 856H and 40 to 65 mbsf in Hole 856G) and is relatively enriched in Ge, As, Sb, Pb, and Tl. Zone 4 consists of dominant pyrrhotite with minor chalcopyrite (75 to 95 mbsf in Hole 856H) and shows a maximum enrichment in Cu, Co, and Bi. Correlation analysis shows that elements enriched in zone 1 are associated with different ore-forming and gangue minerals: sphalerite (Cd and Pb), talc and/or chlorite (Sc, Th, and REE), silica (Hg and Ta), dolomite (V), and barite (Sr, Zr, and Hf). Sb, As, Sn, Au, Ag, Mn, Mo, and Tl form a separate association, presumably related to sulfosalt minerals. Enrichment of Zn and minor elements in the upper part of the deposit, which is typical of ancient massive sulfide deposits, is attributed to the zone-refining process of progressive upward replacement of lower temperature minerals by higher temperature minerals during growth of the sulfide body, with the concomitant hydrothermal leaching of trace metals. Later, the deposit was hydrothermally altered, probably due to lateral flow of evolved seawater along turbidite layers. The alteration produced replacement of primary pyrrhotite by pyrite in zone 3 and by pyrite + magnetite assemblages in the upper zones. In the upper levels of the deposit, sulfate-rich water acted as a strong oxidant. At lower levels a hotter and more evolved sulfur-enriched pyritizing solution also introduced additional Pb, Sb, As, and other sediment-derived elements into zone 3. The absence of light rare earth element enrichment and the existence of a positive Eu anomaly in the massive sulfides of this zone is connected with the alteration of plagioclase in the associated turbidites. The secondary convection system that caused alteration of the massive sulfides could have been driven by one of the sills emplaced into Middle Valley sediments subsequent to sulfide formation.
