Mineralogical and oxygen isotopic constraints on the origin of the contact-metamorphosed bedded manganese deposit at Nagasawa, Japan

Abstract

The Nagasawa deposit, in Japan, is a Paleozoic to Mesozoic chert-hosted bedded manganese deposit that has undergone intense contact metamorphism due to intrusion of a granitic pluton ca. 59 to 62 Ma in age. The manganese ores occur as stratiform manganese silicates and quartz-dominant veins. A detailed petrographic and mineralogical study of ore samples reveals that Fe-,Co-,Ni-,Cu-,Zn-,As-,Te-,Pb-, and Bi-minerals (including native elements, sulfides, arsenides, and tellurides) are commonly present in both types of ore, and that molybdenite occurs in the veins. These minerals are closely associated with garnet in the pyroxmangite - garnet - amphibole ore that was metamorphosed from the rhodochrosite - quartz - chlorite assemblage commonly observed in unmetamorphosed manganese carbonate ores. The calculated oxygen isotopic temperature based on mineral pairs obtained from the same hand samples of bedded ore scatter widely and include the temperature range estimated from equilibria among metamorphic minerals. The inconsistency in temperature ranges is ascribed to oxygen isotope disequilibrium among all the analyzed minerals from the bedded ores and suggests that externally derived fluid were not important during their formation. In contrast, mineral pairs from the quartz veins give oxygen isotopic temperature of ~440-570°C that are in excellent agreement with those determined from equilibria among metamorphic minerals. Minor elements characteristic to the Nagasawa deposit, in addition to Mn and Fe, are those that are commonly concentrated in seafloor ferromanganese nodules. The above observations indicate that the minor elements were first scavenged by manganese oxide or hydroxide phases from seawater, pore water in deep-sea sediments, or seafloor hydrothermal fluids, were then released and incorporated in chlorite- and pyrite-bearing assemblages during the diagenetic reconstitution of manganese oxides and hydroxides to manganese carbonates, and were finally recrystallized as the minor sulfide and other minerals during contact metamorphism. The sulfur in the sulfides could have been derived from the replacement reaction of pyrite by pyrrhotite.