Review of layered deposits of manganese oxides and carbonates throughout the world indicates that most of the large concentrations are in Precambrian or Tertiary rocks. Iron is generally 30 to 90 times as abundant as manganese in ordinary rocks, whereas iron is subordinate to manganese in most bedded manganese deposits. The lack of complementary iron deposits in basins containing manganese deposits therefore suggests that the rocks surrounding the basin are not an adequate source. Upward increase in the Mn-Fe ratio in hydrothermal deposits, together with common evidence for contemporaneous vulcanism, favor an exhalative sedimentary origin.

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