西オーストラリア,Hamersley地域における原生代縞状鉄鉱の鉱物共生

抄録

Mineralogical studies of banded iron formation (BIF) of the Dales Gorge Member were undertaken in this paper. The sequence of mineral developments during burial metamorphism was determined under microscope from mineral relationships within BIF through the member. The sequence is shown in Fig. 14. Final stage (Stage IV, in Fig. 14) gives present mineral assemblages observed in BIF, but all minerals in the stage are not simultaneously associated together (see text). The assemblages are usually formed on the basis of an assemblage, quartz-hematite-magnetite-siderite, although siderite is in places distributed sparsely in amount. Most of minerals seem to have developed on the process of increasing in the reducing potential with increasing metamorphic grade. Pyrrhotite indicates that carbonaceous matters gave the reducing environment. Minnesotaite and stilpnomelane in the earlier stage seem to have been continuously stable, or seem to have responded to reactions of substitution and reduction as Mg²⁺ Fe₂₊and Fe³⁺→Fe₂₊during metamorphism. The occurrence of quartz veinlets (10μ to 3 mm in width)shows that other metallic elements than K and Na were little mobile during metamorphism. The chemical composition in each mesoband, except fluids (H₂O, CO₂, etc), K, and Na, appears to reflect the primary precipitation of BIF.