What lies beneath: The wealth beneath the Marianas waves

Seamount crusts/Photo courtesy of NOAA

According to the U.S. Geological Survey, the following minerals are likely present in the CNMI waters:

Cobalt-rich iron-manganese crusts occur on the sea­mounts located east of the Mariana Trench. Data for crusts from nearby seamounts indicate that crusts on sea­mounts within that sector of the CNMI EEZ have an excel­lent potential to be thick and have high contents of cobalt, nickel, and platinum. It is recommended that these sea­mounts be explored for their mineral potential. These crust deposits represent an intermediate-term resource potential for cobalt, nickel, platinum and perhaps other metals.

Iron-manganese nodules have seldom been recovered from the abyssal plain surface to the east of the Mariana Trench. Nodules have been recovered at depth in sediment cores, indicating that the current high rates of sedimentation do not favor nodule occurrence on the sea floor.

Phosphorite deposits occur on the Cretaceous sea­mounts with iron-manganese crusts, and their distribution and grade are also poorly known. These deposits are typi­cally either small, high-grade deposits or more widespread, low-grade deposits. This deposit type probably has a long-term resource potential.

Gold deposits: Epithermal gold has not been found in the CNMI EEZ, but geologic considerations indicate that it may occur along the Mariana Arc in submerged calderas (such as Maug and Esmeralda Bank) and in areas of back-arc spreading, such as the Mariana Trough. Recent work along the Mariana Arc supports these speculations. If high-grade gold deposits do occur offshore, they would offer a near- to intermediate-term resource.

Manganese oxides of hydrothermal origin are known to occur extensively along the Mariana, Yap and Palau volca­nic arcs. From analysis of 26 samples from the Mariana Arc, manganese averages 47 percent and zinc 0.26 percent. Man­ganese oxides cement sandstone and form layers within the sediments. In some places, the manganese deposits have high con­tents of molybdenum, nickel, zinc and/or chromium. How­ever, because large deposits of most of these metals occur on the continents, they offer only an intermediate- to long-term resource potential.

Polymetallic sulfides, sulfates and sulfur of hydro­thermal origin have been found within the CNMI EEZ at four submarine sites along the Mariana Arc and at three sites along the eastern margin of the Mariana Trough spreading center. There is a good potential to find large, high-grade polymetallic sulfide deposits along the active Mariana Arc and back-arc basin. If gold- and silver-rich polymetallic sulfides are found within the CNMI EEZ, they would have a short- to intermediate-term resource potential.

Insular phosphorites averaging 20 to 26 percent P2O5 occur on Saipan, Rota, Tinian and Aguijan Islands. During the Japanese occupation of the islands, phosphate mining began on Rota in 1937 and on Saipan in 1938 and continued until the United States took control of the islands in 1944. During those few years, 236,000 metric tons of ore were mined from Rota and 88,000 from Saipan. Rota may contain as much as 300,000 tons of reserves and Tinian hosts about 30,000 tons of reserves, both at about 23 percent P2O5.

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