试论稀土铁建造

Geological and geochemical characteristics of REE iron formation, a term proposed by Prof. Tu Kuang-chih to specify a special type of Precambrian iron formation rich in REE, are discussed in this paper with special reference to its REE contents, REE distribution patterns, model of formation, the relationship between its development and the multistage evolution of terrestrial crust in China, and its role as an indicator of oxidation status for ancient atmosphere. Major conclusions are as follows: (1) REE iron formation is characterized by high REE concentrations as opposed to very low REE level in normal Precambrian iron formations. (2) REE iron formation was resulted from marine sedimentary-diagenetic process in miogeosyncline or intermediate regions during Proterozoic Era. To some extent,volcanic activity may be responsible for the deposition of ore-forming materials. In a broader sense, it belongs to Fe-bearing dolomite formation. On most REE iron formations there may be later mineralizations such as hydrothermo-metasomatisn, migmatitization and metamorphism superimposed. Generally, REE iron formations share common fundamental features with stratabound ore deposits. (3) It is similar to Precambrian iron formation in the enrichment of light REE.But, the degree to LREE is enriched in REE iron formation is noticed to increase with the increase of total REE eontent. Most REE iron formations are characterized hy the high ratios of ∑Ce/∑Y, Mg Fe/Ca, Na/K, Nb/Ta, Zr/Hf, Th/U, Ba/Sr, and etc. (4) The widespread occurrence of REE iron formation indicates a higher REE abundance in terrestrial crust of China and adds further support to the multistage theory regarding the chemical element evolution and differentiation in the terrestrial crust of China. (5) Preliminary data seem to support the time dependence of REE pattern and relative Eu content,