Mineralogy and Mineral Chemistry of the Manganese-Poor and Manganiferous Iron-Formations from the High-Grade Metamorphic Terrain of Southern Karnataka, India

The iron-formations of the high-grade metamorphic terrain of Southern Karnataka comprise both Mn-poor (av. 0.3% MnO) and Mn-rich (av, 7.4% MnO) types. The two types are distributed throughout the 1273km² area studied. Each type occurs constituting a large proportion of an individual body and within each body, complex variations from one type to the other, both along and across the strike, might exist. Despite their high-grade metamorphism the rocks have preserved primary sedimentary banding; there are, however, no relics of original textures or minerals. A very striking mineralogical feature of these iron-formations is the common occurrence of pyroxenes and garnets as major mineral phases in addition to magnetite and quartz. Mn-rich types are typically enriched in garnet whereas the Mn-poor types are generally almost devoid of it or contain' a small proportions of it. Pyroxenoids appear in the Mn-rich iron-formation analysing over 6% MnO. The other minerals of restricted occurrence include ferroactinolite, grunerite, Ksfeldspar (orthoclase-microcline), plagioclase (An 20-25), olivine, biotite and chlorite. Apatite is the most common accessory; zircon is scarce.

A detailed electronprobe microanalysis has revealed; (1) the low MnO content of less than 2%,1%,0.4% and 0.03% respectively, of garnet (70-81% almandine), orthopyroxene (ferrohypersthene-eulite ; 30-22 En), clinopyroxene (augite-ferroaugite to saliteferrosalite) and magnetite in the Mn-poor iron-formation, (2) the high MnO content of 10-20.5%, 2.5 to 15%, 2.25·7.5 and 0.2-0.3% respectively of garnet (almandine 30-54 - spessartite 39-49), orthopyroxene (manganoan ferrohypersthene-rnanganoan eulite), clinopyroxene (manganoan augite-ferroaugite to manganoan salite-ferrosalite) and magnetite in the Ma-rich iron-formation, (3) the occurrence of pyroxenoids varying in character from pyroxferroite to pyroxmangite with the Fe: Mn ratios either just below or above 1 : 1, (4) the common occurrence of (exsolution) intergrowths of one pyroxene in the other, (5) the occurrence of Mn-poor magnetite (≤0.03%MnO) carrying abundant exsolutions of hercynite and Mn-poor (around 1% MnO), ilmenite in the Mn-poor iron-formation and (6) magnetites (around 0.3% MnO) carrying Mn-rich ilmenite (MnO 15-30%) exsolutions in the Mn-rich iron-formation. An empirical study of the Mg-Fe-Mn distribution in the co-existing pyroxene and garnet has shown that these have crystallised in equilibrium metamorphic conditions corresponding to granulite facies. Estimations based on better suited pyroxene-garnet compositions occurring in the associated granulite facies rocks have indicated the formation of ironformations of the area at temperatures of 720-820°C and pressures of 7-11 Kbars.