Geochemistry and depositional environment of Mn oxide deposits in the Tokoro Belt, northeastern Hokkaido, Japan

Abstract

Numerous Early Cretaceous Mn oxide and Mn-Fe deposits are distributed in the Tokoro belt, northeastern Hokkaido, Japan. The Mn oxide deposits usually occur in the southwestern part of the belt and within bedded cherts whereas the Mn-Fe deposits are restricted to the northeastern part of the belt and occur between bedded cherts and underlying basaltic pillow lava.The Mn oxide ores are characterized by low Fe/Mn ratios, and the trace element concentrations and rare earth element (REE) patterns are similar to those of the submarine hydrothermal Mn deposits. Distribution patterns of trace elements (Co, Ni, Cu, and Zn) and REE in the Mn oxide ores determine two types of deposits: hydrogenous deposits which have relatively high concentrations, and submarine hydrothermal deposits which have low concentrations.These compositional trends and geologic evidences of the Tokoro Mn deposits suggest that the Fe phases precipitated first closer to their source whereas the Mn phases deposited later at a distance from their source. Some hydrogenous effects on the Mn oxide deposits are due to the adsorption of trace elements from seawater. Trace element concentrations of Tokoro Mn oxide deposits show that they formed within 40 km from their source.