総説
地球表層環境における鉄の濃集と鉄鉱床の形成
キーワード:
Iron deposits,
magmatic,
hydrothermal,
sedimentary,
banded iron formation,
magnetite,
iron isotope
2021 年 71 巻 2 号 p. 57-73
詳細
2021 年 71 巻 2 号 p. 57-73
Iron (Fe) is the fourth abundant element on the Earth’s surface, and it has been mined and utilized for various industrial activities. Whereas average continental crust contains ~5 wt% of Fe (as FeO), the minimum Fe grade for minable ore is 20–25 wt%. Formation of Fe ore deposits requires crystallization of iron as oxides, neither silicates nor sulfides, which occur more commonly in ordinary igneous and sedimentary rocks. There are various geological processes that concentrate Fe and form Fe ore deposits, including magmatic, hydrothermal, and sedimentary processes. Fe ore deposits can be divided into orthomagmatic type (or Fe-Ti-V type), iron oxide-apatite (IOA) type (or Kiruna type), skarn, submarine-hydrothermal, banded iron formations (BIF)-hosted, and phanerozoic ironstone. Production of iron ores, particularly high-grade ores, in the world is currently dominated by those associated with or derived from BIFs, Fe-rich chemical sedimentary rocks formed in Precambrian era. A complication of ore reserves and average grade on various Fe ore deposits over the world demonstrates that a number of large and high-grade Fe deposits belong to BIF-associated and derived deposits whereas some other types of Fe deposits, such as orthomagmatic, IOA-type, and skarn, contain gigantic deposits. Although other hydrothermal and sedimentary deposits contain fairly large deposits, the average ore grades are not as great as those associated with BIFs. In magmatic processes, fractional crystallization may concentrate Fe as magnetite. However, further enrichment of Fe to form Fe-Ti-V or IOA type deposits requires a decrease in SiO2 activity in magma, possibly due to immiscible segregation of oxide melts from silicate melts. Although hydrothermal processes typically precipitate Fe as sulfides, highly oxidized and Cl--rich fluids may cause enrichment of Fe as oxides in some magmatic-hydrothermal systems. Other important parameters to form hydrothermal Fe deposits include CO2 fugacity and temperature for skarn and submarine-hydrothermal Fe deposits, respectively. In sedimentary processes, redox state of seawater is the key parameter However, primary precipitates and geochemical processes that govern the formation of BIFs in Archean is still controversial. Recently, trace element chemistry of magnetite as well as Fe and O isotopes are developing geochemical indicators for Fe ore genesis. There will be need for more case studies to verify the indicators.
