Cook Islands,Deepsea Mineral Resource Survey Report & Atlas #5

The Cooperative Study on the Deepsea Mineral Resources in Selected Offshore Areas of the SOPAC Region, 2001 -Sea Area of Cook Islands. The survey which began in the year 2000 continues as 2 terms of 3 years projects.
The present survey constituting the work of the first year was carried out in an area of about 431,000km2 in the Southern Penrhyn Basin of the southern part of the Cook Islands sea area. The survey area consists of three sea areas (South, Center, and North Areas) and manganese nodule sampling was carried out in these areas at a total of 111 points of 37 stations (37 stations × 3 sampling points). Seafloor photographs were taken simultaneously with sampling and the modes of manganese nodule occurrences, grade, nature of deep-sea sediments, and relevant characteristics were clarified. In the Center Sea Area, FDC seafloor observation was conducted at one track line (about 29 nm). During cruise from one station to the next, acoustic exploration by SBP and MBES was made, and seafloor topographic maps were prepared, surface sediments were investigated and manganese nodule abundance was estimated by MFES. In addition to them, environmental survey was carried out.The cruise lasted for 24 days and the objective of the survey was manganese nodules.Latitude Longitudea) 6°30’N 160°30’Nb) 6°30’N 159°30’Nc) 8°30’N 159°30’Nd) 8°30’N 159°00’Ne) 10°00’N 159°00’Nf) 10°00’N 158°30’Ng) 17°30’N 158°30’Nh) 17°30’N 160°30’Ni) 6°30’N 160°30’NCENTER AREA:The seafloor of this area consists of a flat zone of 5,000 ~ 5,200m depth extending in the northwest and southeast direction from the central part, and zones of complex seafloor topography with very many undulations with sea knolls and seamount chains in the southwestern and northeastern parts shallower than 5,000m. SBP type indicating the existence of acoustically transparent layer in the upper horizon is distributed in the plain flat zones where pale color MBES acoustic intensity images occur indicating the existence of unconsolidated sediments. On the other hand, the uppermost layer of the sea knolls and seamounts are acoustically opaque. Most of the deep-sea sediments are brown clay and the deposition has continued from pre-late Miocene. The surface part is believed to have deposited after 500ka. The productivity of the sea surface of this area is believed to have been low from the above time to the present. Spheroidal manganese nodules are dominant and the size are mostly 2~6cm, and the average abundance of the area is high at 28.88kg/m2. High-abundance zones exceeding 30kg/m2 extend over relatively flat parts deeper than 5,000m constituting 42% (7,021km2) of the total sea area. The abundance is less than 10kg/m2 in zones shallower than 4,900m consisting of plateaus, knolls and seamounts. In these shallow zones bedrocks covered by manganese crusts are exposed. The average grade of the manganese nodules is; Ni 0.25%, Cu 0.14%, Co 0.50%, Mn 15.33%, Fe 18.48% and the Ni and Cu contents are lower and Co content higher than those of the Clarion-Clipperton area. Mn content is low and Fe high resulting in low Mn/Fe ratio. Thus these nodules are chemically similar to the hydrogenous manganese nodules (s type) of the highly abundant in northeastern part of the Central Pacific Basin and the Penrhyn Basin. The total metal contents of the calculated area of 16,600km2 are Ni 797,000t, Cu 454,000t, and Co 1,681,000t. The present survey clarified that high-abundance manganese nodule zones exceeding 30kg/m2 extend over an area of 7,000km2 in the Center Area.SOUTH AREA:This is is a relatively flat 4,600 ~ 5,200m deep zone in the Aitutaki Passage, which borders the South Penrhyn Basin and the Southwest Pacific Basin. SBP survey indicated the dominance of acoustically opaque layers, and the average MBES acoustic reflection intensity tends to be high. Thus the development of surface sediments is very limited. Almost all of the deep-sea sediments are brown clay, and fossil studies indicate that the deposition has continued from pre-Pliocene time. The surface is believed to have deposited under tropical to subtropical environment after 500ka.Spheroidal manganese nodules are dominant and most of them are 2 ~ 6cm in diameter, and those larger than 8cm also exist more than in the other two areas. The average nodule abundance of the whole area is 12.29kg/m2, and zones with the average exceeding 20kg/m2 occur in the central part occupying 30% (about 40,000km2) of the total sea. Within these high-abundance zones, those with average values of more than 30kg/m2 occupy 8,000km2 in the relatively flat parts deeper than 4,800m. The average grade of the nodules are; Ni 0.32%, Cu 0.19%, Co 0.46%, Mn, 13.76%, and Fe 18.79% which is similar to the grade of the Center Area. Namely it is chemically similar to the hydrogenous manganese nodules (s type) with low Ni and Cu content and high Co. The total metal contents of an area of about 114,000km2 are calculated to be Ni 3,310,000t, Cu 1,942,000t, and Co 5,825,000t.The present survey clarified the existence of high-abundance zones exceeding 30kg/m2 and also showed that the areal extent of the zones with abundance higher than 20kg/m2 is estimated to be 40,000km2.NORTH AREA:This area generally consists of flat low-relief topography of 5,200 ~ 5,600m depth. Some seamounts with relative height of 1,000m and lower knolls occur isolated in the area. The results of the SBP survey indicate that, to the north of 11°S, coarse to consolidated sediments are distributed and MBES acoustic reflection intensity images have pale color indicating smooth seafloor. On the other hand, to the south of 11°S, SBP transparent layer type and opaque layer type appear alternatively and the MBES images also show alternating pale and dark color tones. Deep-sea sediments consist of brown clay and insoluble brown clay, and fossil studies show that deposition of the sediments continued from pre-Miocene time, and the surface is considered to have deposited under tropical to subtropical environment after 500ka. Almost all manganese nodules are spheroidal and pebbles, and 2 ~ 6cm is their dominant size. The abundance at four stations is 1.47~20kg/m2, and the average grade is Ni 0.61%, Cu 0.38%, Co 0.35%, Mn 18.39%, and Fe 13.57%. Ni, Cu, Mn content is somewhat higher and Co and Fe is lower than the average grade of the nodules of the Center and South Areas. The present survey could not clarify the detailed distribution of the nodules of this area, but zones with abundance higher than 20kg/m2 exists and the Ni and Cu contents are higher than those of the other two areas.