About the sources of ore material supply to the ocean, there were once three major presumptions:
(1) Main ore material supply to the ocean comes from terrigenous and volcanic, and also hydrothermal material.
(2) Volcanic material is the major source of the supply of ore matter to the ocean.
(3) Terrigenous is the major source of the supply of ore matter to the ocean.
Goldschmidt was the first to open the problem of balance for discussion. In 1954, he pointed out that absolute masses of some metals in the ocean exceed their supply from continental rock weathering over geological time. This problem has risen a great interest in geochemistry. It is sustained by the problem of manganese nodule origin and also by the problem of hydrothermal matter supply to the ocean. In 1966, Scientists computed the balance of some elements in the ocean. Their computations revealed an actual excess of manganese over its supply from continental sources.The computation result also revealed that elements such as chlorine, sulfur, bromine, iodine, molybdenum were "excessive." According to the calculating result of Elderfield (1976), dissolved manganese supply to the ocean from various sources is 2.2-4.0 million tons annually; 0.7-7.4 million tons of manganese sink to the ocean bottom annually; 0.5-10 million tons of hydrothermal manganese are supplied to the ocean annually. But according to different estimates, the supply of labile and dissolved manganese to the ocean is 0.2-0.4million tons yearly through river discharge; 0.05-0.3 million tons yearly by atmospheric material; 1-10 million tons by hydrothermal material; and 0.4-4 million tons yearly by diagenetic flow. So, the amplitude of minimum-maximum summary estimates is 1.2-14.7 million tons per year. These are the general profiles of the accumulation of manganese nodules. The accumulation rate of manganese should be studied in individual oceanic regions.
Classification of Manganese Nodules
Oceanic nodules may be formed from the ore material of polygenic origin, although some researchers would emphasize the predominant role of hydrothermal sources. Manganese nodules may grow due to a supply of material both from above (out of oceanic water) and below (out of host sediments). Depending on the major ore elements (manganese, iron and base metals) the nodules contain, three major types were distinguished:
(1) hydrogenous, i.e., formed due to slow deposition of metals out of sea water and characterized by a high concentration of base metals and varying Mn/Fe rations (from 0.5-5)
(2) hydrothermal, i.e., rich in iron and depleted of other metals, and turns out to be having an extremely wide range of Mn/Fe rations
(3) diagenetic, i.e., characterized by high Mn/Fe ratios and relatively low concentration of base metals.