Compositional characteristics and phase equilibria in manganiferous iron formations from a high‐grade terrain near Satnuru, Karnataka, India

Abstract

Manganese‐rich and manganese‐poor iron formations which occur as thin layers in the Halaguru‐Satnuru area, south of Kabbaldurga, Karnataka, India are chemically intermediate between the ‘Algoma’and ‘Lake Superior’types, but higher in their MnO and TiO 2 contents. The rocks are of four petrographic varieties: (a) quartz‐magnetite‐orthopyroxene‐clinopyroxene, (b) quartz‐magnetite‐orthopyroxene‐clinopyroxene‐garnet, (c) quartz‐magnetite‐clinopyroxene‐garnet, and (d) quartz‐magnetite‐clinopyroxene‐garnet‐plagioclase. In the orthopyroxene‐clinopyroxene pairs, Mn‐Mg and Mn‐Fe exchange is ideal irrespective of the MnSiO 3 contents of orthopyroxenes (0.6–1.8 mol. % in Mn‐poor and 15–25 mol. % in Mn‐rich compositions). Mg‐Fe exchange in the same pair is however non‐ideal. Mn‐Fe exchange in orthopyroxene‐garnet pairs is ideal. The distribution patterns in the other binaries are inconclusive regarding ideality of exchange. Orthopyroxene‐garnet and clinopyroxene‐garnet geothermometers, modified for high spessartine contents, give temperatures of 800 ± 30° C. A modified version of the Harley (1984) geothermometer registers 740 ± 60° C, in agreement with the consensus temperature value. The equilibrium log ffo 2 values in the iron formations, as calculated from the reaction 6FeSiO 3 + O 2 = 2Fe 3 O 4 + 6SiO 2 are in the range of −14.2 to −15.5. Algebraic analysis of variations of f o2 with composition of phases indicates buffering of O 2 in the rocks. The absence of grunerite in these assemblages is compatible with X H2O being less than 0.3 in the ambient fluid. Computations from volatile equilibria in the C‐O‐H system, however, predict high X H2O values (>0.7) at a c = 1.0, implying that the activity of graphite must have been greatly reduced—this is in accordance with the absence of graphite in these rocks.