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between the Clipperton and Clarion Fracture Zones (figure. 1) is generally considered most favorable for future mining of manganese nodules and has therefore become the focus of recent academic and industry exploration. This large region of the eastern Pacific Ocean provides the requisite depths of more than 4,500 meters, has slow sedimentation rates, yields samples that consistently have high nickel and copper assays, and offers generally favorable weather and other operating conditions.

Designation of the eastern equatorial Pacific region as the 'only' or the 'most favorable' region for future mining is premature. Samples of nodules from other basins of the world's oceans have comparable high nickel and copper values and may represent individual deposits that are as rich or possibly richer than those of the favored band. Assayed samples with more than two percent total nickel and copper, generally considered the minimum to be of commercial interest, have come from basins northeast of the Hawaiian Islands in the north Pacific, west of Peru and northeast of the Tuamotu Islands in the equatorial South Pacific, south of Australia, and at several places in the Indian Ocean. Information concerning the nature of deposits in these basins is scant; many smaller basins of the world's oceans remain unexplored.

Many new instruments and techniques are being used to explore the manganese nodule deposits of the eastern Pacific; others are being developed. Among the most useful have been closed-circuit deep-sea television systems and deep-towed instrument packages that maintain constant distances above the bottom. Because of the great depths, precise horizontal positioning of equipment on the bottom relative to the surface ships has been a continuing problem in many of the exploration efforts.

Detailed investigations of the region between the Clarion and Clipperton Fracture Zones have shown that local relief of the sea floor amounts to about 200 meters consisting of north-south ridges and valleys bounded by gentle slopes, with a few elongated depressions bounded by steep slopes (Mudie and Grow, 1972). In a photo traverse near the center of the region, nodules were identifiable in about 75 percent of the pictures, and manganese pavements on lava outcrops were distinguishable in pictures of slopes (J.E. Andrews, oral communication; Andrews and Meylan, 1973, p. 339). In many bottom photographs, nodules cover 95. percent and more of the surfaces. The nodules, with sampled densities of from 2 to 8 kilograms per square meter (Andrews and Meylan, 1973), rest on surfaces of siliceous sediments that are geologically ancient (middle Tertiary) and indicative of slow sedimentation rates (Horn and Horn, 1973). In most cases, sediment cores have few or no nodules beneath the 2 to 6 centimeter layer of the surficial deposits.

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The Clipperton Fracture Zone forming the south boundary of the favorable band separates a shallower region with high sedimentation rates and sparse nodules to the south from the deeper region of slow sedimentation and abundant nodules to the north. The Fracture Zone and both regions lie beneath the northern half of equatorial belt of high biologic productivity which supplies a constant rain of dead skeletal material to deeper waters. To the south of the fracture zone, much of the skeletal material reaches the bottom, but in deeper waters to the north, it passed through a 'zone of carbonate compensation' where carbonate skeletal material is dissolved and only small amounts of siliceous material reach the bottom.

The Clarion Fracture Zone, which forms the north boundary of the favorable region, lies to the north of the equatorial belt of high productivity, but is nearly parallel to it and marks the approximate boundary between slow deposition of siliceous sediments to the south and red clays to the north. Nodules dredged from the red clay surfaces tend to have lower nickel and copper contents than those of siliceous surfaces to the south, although many exceptions exist (Horn, Horn and Delach, 1972). In general, bottom topography north of the Clarion Fracture Zone is more rugged than to the south.

In addition to relationships between nodules and sedimentation, considerable attention is now being devoted to properties of the nodules themselves and to factors that contribute to their formation. For example, recent determination of differences in the mineralogy of specimens from shallow and deep parts of the oceans helps explain the kinds and amounts of associated metals which they contain (Burns and Brown, 1972). Variations of composition within individual nodules has led to the suggestion that metals which compose them have come principally from igneous activity beneath the sea floor (Raab, 1972). Another study (Finkelman, 1972) has shown that deep sea nodules contain unusual concentrations of cosmic particles; the presence of sediment layers enriched in cosmic particles beneath some deposits of nodules supports another suggestion that the nodules consist chiefly of metals derived from older deposits that were buried and subsequently dissolved. Other suggested sources of the metals include submarine weathering of volcanic rocks and sea floor volcanic emanations (Bonatti, and others, 1972). Although seemingly esoteric or irrelevant, the accumulation of information and testing of suggestions such as these has already contributed significantly to the search for and evaluation of potentially mineable deposits; it is expected to be of even greater value when future recovery and processing of the nodules is undertaken.

Testing of prototype systems developed by industry to recover deep-sea manganese nodules has provided much information that is being used to evaluate potential environmental effects of future mining operations.

In addition to nodules, each of the systems delivers varying amounts of nutrient-rich deep ocean water to the surface. Currents provide the chief control in dispersal of this water and associated sediment spoils, but in general environmental effects are limited to the vicinity of the mining platform (Amos and others, 1972, 1973). An interesting observation entailed the proliferation (bloom) of diatoms that apparently form a dormant component of life among the nodules and in the sediments that were brought to the surface. (Roels, oral communication). These microscopic life forms constitute

a basic element of the biologic food chain; their rapid proliferation at the surface, together with the associated nutrients, might

attract high life forms, including fish, and enhance biologic activity in the vicinity of the mining vessel.

Because they are accessible for detailed study, deposits of manganese crusts and nodules discovered during 1970 on submarine slopes bounding Kauai and Oahu of the Hawaiian Islands have attracted considerable interest. East of Kauai, the crusts and nodules are found on the outer margins of submerged terraces at average depths of 800, 1400 and 2200 meters and on the generally smooth floor of the Kauai Channel below 2400 meters. Crusts within the Channel range from one to ten centimeters in thickness both at and buried beneath the surface. Studies of the crusts and comparison of their compositions with those of associated sediments and island soils has led to the conclusion that the crusts have formed chiefly through manganese enrichment resulting from seawater- sediment interaction aided by iron as a catalyst (Morgenstein and Fowler, 1971; Fein and Morgenstein, 1973). Although they also are enriched in the crusts, total contents of associated nickel, copper and cobalt average about one percent, and therefore the deposits are of little commercial interest.

OTHER METALLIFEROUS DEPOSITS

Results of early investigations of the metalliferous muds of the Red Sea have been assembled and published as a single volume (Degens and Ross 1969). Industry exploration of the deposits continues. During April 1972, three sites in the vicinity of the depresseions that contain the muds were occupied by the Deep Sea Drilling Project. At two of these sites and at another to the south, metal-rich shales and sandstones were encountered above layers of anhydrite and salt. These enriched sediments may be the source of the metals in the muds, and also constitute a new potential subsea source of copper, zinc and vanadium (Geotimes, v. 17, no. 7, July 1972 page 26).

A mineralized sample containing a vein of copper-iron sulphides was dredged from a depth of 3,500 meters in the rift valley of the Arabian-India Ridge (Rozanova and Baturin, 1971). It provides the first direct evidence of sulfide mineralization at and beneath the sea floor. Such mineralization was previously predicted on the basis of studies at Iceland, Cyprus and other places where rocks, which are thought to be oceanic crust, are exposed and contain ore deposits (McKelvey and Wang, 1969, p. 31)

The significance of metal-rich sediment layers contained in cores obtained at a number of deep ocean drilling sites occupied during the Deep Sea Drilling Project has not yet been fully evaluated. Of special interest are enriched layers that are found at the contacts of the sediments and sills of igneous rocks. These have been cited as evidence supporting derivation of metals forming manganese nodules from subsea floor igneous activity (Raab, 1972), however, as noted in the section on the nodules, other sources for the metals also exist.

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