13:45 〜 14:00
[HRE12-01] 地球表層環境における鉄の濃集と鉄鉱床の形成：レビュー
キーワード：鉄、濃縮度、マグマ、熱水、堆積、酸化還元条件
Iron 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%. Although its concentration factor (4–5) is not as high as that for some other elements, formation of iron ore deposits requires crystallization of iron as oxides (e.g., magnetite and hematite), not either silicates or sulfides, which occurs more commonly in ordinary igneous and sedimentary rocks. There are various geological processes, including magmatic, hydrothermal, and sedimentary processes, to concentrate Fe and form Fe ore deposits. However, ore genesis may be ambiguous for some deposits, which makes exploration difficult. The objective of this study is to review various types of Fe ore deposits and geochemical factors controlling concentration of Fe in the Earth’s surface environments. Some recent geochemical techniques to indicate Fe ore genesis are also reviewed.
Based on previous literatures, Fe ore deposits can be divided into orthomagmatic (or Fe-Ti-V type), iron oxide-apatite (IOA) type (or Kiruna type), skarn, submarine volcanogenic iron oxide (SVIO), banded iron formations (BIF), and phanerozoic iron stone. Production of iron ores, particularly high-grade ores, in the world is currently dominated by those associated with BIFs, Fe-rich chemical sedimentary rocks formed in Precambrian era. Complication of ore reserves and average grade on various (~200) Fe ore deposits over the world demonstrates that a number of large and high-grade Fe deposits belong to BIF-associated deposits although some other types of Fe deposits, such as orthomagmatic, Kiruna-type, skarn, contain gigantic deposits (>1 billion ton reserve). 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 since it is one of the minerals that crystallize at the early stage of magma cooling. However, further enrichment of Fe to form Fe-Ti-V or IOA type deposits requires to 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 (e.g., pyrite), 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 SVIO Fe deposits, respectively. In sedimentary processes, redox state of sweater is the key parameter since reduced seawater is needed to dissolve and transport Fe in a large scale in the surface environments. However, primary precipitates and its mechanism to form BIF 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 although there will be more case studies needed to verify the indicators.
Based on previous literatures, Fe ore deposits can be divided into orthomagmatic (or Fe-Ti-V type), iron oxide-apatite (IOA) type (or Kiruna type), skarn, submarine volcanogenic iron oxide (SVIO), banded iron formations (BIF), and phanerozoic iron stone. Production of iron ores, particularly high-grade ores, in the world is currently dominated by those associated with BIFs, Fe-rich chemical sedimentary rocks formed in Precambrian era. Complication of ore reserves and average grade on various (~200) Fe ore deposits over the world demonstrates that a number of large and high-grade Fe deposits belong to BIF-associated deposits although some other types of Fe deposits, such as orthomagmatic, Kiruna-type, skarn, contain gigantic deposits (>1 billion ton reserve). 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 since it is one of the minerals that crystallize at the early stage of magma cooling. However, further enrichment of Fe to form Fe-Ti-V or IOA type deposits requires to 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 (e.g., pyrite), 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 SVIO Fe deposits, respectively. In sedimentary processes, redox state of sweater is the key parameter since reduced seawater is needed to dissolve and transport Fe in a large scale in the surface environments. However, primary precipitates and its mechanism to form BIF 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 although there will be more case studies needed to verify the indicators.