1976 Volume 26 Issue 139 Pages 311-325
In this paper, physicochemical environments during burial metamorphism of the Dales Gorge Member in the Witterioom area were estimated based on the geotectonic history in the Hamersley area (MIYANO, 1976a) and mineral assemblages in banded iron formation (MIYANO, 1976b) as follows;
Rock pressure (Ps): 700-1300 bars
Temperature
Maximum: 230±20°C (within 1000±300 bars)
Minimum: 100±20°C (within 1000±300 bars)
Dominant range: 80-160°C
Partial pressures of fluids
Po2 (=fo2): 10-58.5-10-42.8 bars
Dominant range: 10-55.8-10-42.8 bars
Ps2 (=fs2): 10-27.1-10-15.2 bars
Dominant range: 10-22.0-10-15.2 bars
PCO2: about 1-55 bars (within 120±40°C)
The phase equilibria of banded iron formation were discussed in terms of the systems of Fe-Si-O-H and Fe-Si-C-O-H. In the Fe-Si-O-H system, pure minnesotaite does not coexist with hematite. Considering, however, impurities in minnesotaite, the assemblage of two minerals can be associated. Pure greenalite seem to be unstable at higher temperatures by taking into account the effect of error in the Gibbs free energy, ΔG°f, 298.15, of minnesotaite. In the Fe-Si-C-O-H system, four phases of hematite, magnetite, siderite, and minnesotaite (impure phase) can coexist together in the absence of graphite. Considering common impurities in greenalite, it is predicted that five phases, hematite, magnetite, siderite, greenalite, and minnesotaite, can coexist together(Fig. 12(C)). But greenalite is extremely rare in the Hamersley area. Then it is likely that greenalite is unstable under the CO2 pressure estimated above (Fig. 12 (D)).
From analysis of T-XCO2 diagram, burial metamorphism of the Dales Gorge Member seem to have taken place within one phase region of H2O and CO2, where XCO2 gives 0.001 to 0.05 within the estimated temperature at PCO2+PO2H=1000 bars.
