Geological and Geochemical Significance of Red Sea Evaporites

INTRODUCTION A body of data on the hot brine-metal deposits of the Red Sea has emerged since the well-known monograph edited by Degens and Ross appeared in 1969 (1). Many of the more accessible deeps of the Red Sea have been explored in the course of the German Valdivia and earlier expeditions, and the subsurface of the rift valley regions has been probed by Leg 23 of' the Deep Sea Drilling-Project. This may be an appropriate time to evaluate the accumulated data and reassess major questions on the origin and significance of the metalliferous systems. High brine content (>90%) and other properties of the uppermost metal-enriched sediments in the Atlantis II Deep show that they were deposited as precipitates from hot brines that discharged into the floor of the basin. The main valuable elements in the sediments of Atlantis II Deep as given by investigations up through 1969 averages follows :Cu 8.05 × 105, Pb .8 × 105, Ag 45× 102, Au 45 tons (2,3). The very detailed Valdivia investigations of 1971–72 have implied an increase in the assayed volume and tonnages of metals by a factor of 2–3. However, a careful survey of the rift regions reveals no deeps containing significant economic deposits of metalliferous sediments other than the Atlantis II Deep (4). STRUCTURAL SETTING - SEA FLOOR SPREADING The Deep Sea Drilling Project (DSDP) Leg 23 revealed evidence that spread confusion rather than sea floors in the Red Sea. The entire Red Sea from the central area to at least 150 S was shown to contain buried evaporites at relatively shallow (few hundred meters,) depth (5). Moreover, the DSDP data coupled with seismic reflection evidence (e.g., Ross and Schlee, 1973) (6) showed relatively conformable strata overlying the Late Miocene top of the evaporite sequence from the disrupted rift area to the flanks of the Red Sea. In the rift regions Pleistocene V and Mo-bearing black shales from the DSDP coreholes (7) correlate well with similar facies found in Mobil Oil Co. drillholes on the Ethiopian shelf (8). Thus, in the author's view, the earlier concept of a spreading history for the Red Sea during Miocene and part of post-Miocene time (9, 10) or its post Leg 23 attempts at modification (11) cannot be sustained, for there is no evidence of the time-bounded facies changes that would be expected in the case of a widening Red Sea. This in turn virtually excludes the possibiIity that there is a succession of older "Atlantis II Deep" deposits of heavy metals, corresponding to earlier hot brine sites, landward from the present rift area. What underlies the thick Miocene evaporates and presumed deeper clastics remains in a controversial state (12).