Massive deep-sea sulphide ore deposits discovered on the East Pacific Rise

Abstract

Massive ore-grade zinc, copper and iron sulphide deposits have been found at the axis of the East Pacific Rise. Although their presence on the deep ocean-floor had been predicted rhere was no supporting observational euidence. The East Pacific Rise deposits represent a modern analogue of Cyprus-type sulphide ores associated with ophiolitic rocks on land. They contain at least 29% zinc meral and 6 % merallic copper. Their discovery will prouide a new focus for deep-sea exploration, leading to new assessmenrs of the concentration of metals in the upper layers of the oceanic crust.-THE area of the deposits of ore-grade zinc, copper and iron sulphide was explored and sampled in February-March 1978 by the manned diving saucer CYANA during the expedition CYAMEXi. The expedition, the only submersible diving programme that has so far been conducted on the East Pacific Rise (EPR), is part of the French-American-Mexican project RITA (Rivera-Tamayo), a 3-yr study devoted to detailed geological and geophysical investigations of the EPR crest. The ore deposits were sampled in water depths of close to 2,620 m a t two neighbouring sites near 20" 54' N 109" 03'W. (refs 2-5) about 9 0 km north of the Rivera transform fault and 240 km south of the Tamayo transform fault (Fig. 1). Three dives of Cyana (CY 78-06, 08 and 12) crossed the two sampling sites, and we collected samples during two of these dives (CY 78-08 and 12). However, during al1 dives in the EPR axial zone, signs of hydrothermal activity were seen, including colonies of dead giant clams, fields of pillow lavas with pronounced colourstaining at the base of pillows, and coloured deposits on exposed scarp surfaces of normal faults and open fissures'. Coral-like growths, possibly of native sulphur, occur in other locations, including a sedimented fault-scarp about 1.0 km to the West of where the sulphide ores were sampled. Sampling sites The two sites where the sulphides were sampled lie on the lightly sedimented flanks of steep-sided structural depressions, about 20-30 m deep, 20-30 m wide. and about 600-700 m west of the axis of the 'extrusion zone' where the youngest lavas occur. Whereas the extrusion zone is marked by a 50 m-high sedimentfree discontinuous ridge with n o fissures or faults, the structural *The authors are al1 members of the CYAMEX Scientific Team.

Geochemistry and Origin of Eocene-Oligocene Metalliferous Sediments from the Central Equatorial Pacific: Deep Sea Drilling Project Sites 573 and 574

“…Basal sediments in Bauer Basin directly overlie basalt (Dymond et al, 1976) and have lower Al and Si contents ( Table 7), suggesting that they are more akin to the East Pacific Rise deposits. In addition to the three main categories, local seafloor deposits of metal sulfides occur at 21 °N on the East Pacific Rise (Edmond, Measures, McDuff, et al, 1979;Edmund, Measures, Mangum, et al, 1979;Francheteau et al, 1979;Hekinian et al, 1980), and hydrothermal vents also precipitate sulfides in the Galapagos Mounds Hydrothermal Field Edmond, Measures, Mangum, et al, 1979). These are, however, very localized deposits that are mineralogically distinct from the others considered here.…”

Hydrothermal Activity in the Guaymas Basin, Gulf of California: A Synthesis

“…Subsequently, hydrothermal vents have been discovered in the Galapagos Rift (Corliss et al, 1979), on the East Pacific Rise at 21 °N (Francheteau et al, 1979;Spiess et al, 1980), and at other oceanic spreading centers. Chemical analyses of hydrothermal waters emanating from vents at the Galapagos Rift, as well as of those obtained at 21 °N, indicate profound changes in chemical composition of these waters when compared to the original sea water (Corliss et al, 1979;Edmond et al, 1979a, b, c), similar to those observed in hydrothermal sea water-igneous rock interaction experiments (Bischoff and Dickson, 1975;Hajash, 1975;Menzies and Seyfried, 1979;Mottl and Holland, 1978;Seyfried and Bischoff, 1977).…”

Axial summit trough of the East Pacific Rice 9°–10°N: Geological characteristics and evolution of the axial zone on fast spreading mid‐ocean ridge

“…The ASCI' floor contains a range of lava morphologies but is dominated by extensively collapsed, ropy, and jumbled lava surfaces and platy collapse crust (Plates 2f-2h and 3d-3f). Lava pillars [Ballard et al, 1979;Francheteau et al, 1979;Gregg and Chadwick, 1996] are often present on the trough floor (Plates 2g, 2h, 3e), indicating that liquid lava ponded temporarily and then drained away.…”