摘要
Apatite grains in a stratabound manganese ore body from Eastern Ghats in Leliguma, Koraput district south Orissa, India were studied under optical and electron microscope. Apatite shows bimodal occurrence: one associated with granite/pegmatitic phase and other with manganese phase, and exhibits different microstructure and composition. Apatite in pegmatitic association (occurring as inclusion in quartz and feldspar) is small in size and occurs as subhedral grains having irregular boundaries and poor in manganese content (MnO: 0.16%). Apatite associated with manganese mineral phases (cryptomelane / romanechite) is mostly euhedral, relatively larger in size, contains higher manganese value, and exhibits some peculiar features like twinning, zoning, overgrowth, and occasionally contains inclusion of quartz and feldspar grains. Apatite grains occurring as inclusions within quartz and feldspar are of hydrothermal origin and formed along with pegmatitic minerals. Such apatite is almost devoid of manganese but relatively enriched in Sr, F and LREE content. The apatite in manganese mineral association formed during supergene process. The apatite of latter generation appears as idiomorphic crystals;contains up to 6.85% of MnO in solid solution and shows relatively higher HREE values. The higher HREE values may be due to its derivation from stratiform Mn-ore bodies and associated granitised rock during remobilization, solution and precipitation of Mn –rich fluid along structurally weak planes resulting in development of a stratabound ore body.
Apatite grains in a stratabound manganese ore body from Eastern Ghats in Leliguma, Koraput district south Orissa, India were studied under optical and electron microscope. Apatite shows bimodal occurrence: one associated with granite/pegmatitic phase and other with manganese phase, and exhibits different microstructure and composition. Apatite in pegmatitic association (occurring as inclusion in quartz and feldspar) is small in size and occurs as subhedral grains having irregular boundaries and poor in manganese content (MnO: 0.16%). Apatite associated with manganese mineral phases (cryptomelane / romanechite) is mostly euhedral, relatively larger in size, contains higher manganese value, and exhibits some peculiar features like twinning, zoning, overgrowth, and occasionally contains inclusion of quartz and feldspar grains. Apatite grains occurring as inclusions within quartz and feldspar are of hydrothermal origin and formed along with pegmatitic minerals. Such apatite is almost devoid of manganese but relatively enriched in Sr, F and LREE content. The apatite in manganese mineral association formed during supergene process. The apatite of latter generation appears as idiomorphic crystals;contains up to 6.85% of MnO in solid solution and shows relatively higher HREE values. The higher HREE values may be due to its derivation from stratiform Mn-ore bodies and associated granitised rock during remobilization, solution and precipitation of Mn –rich fluid along structurally weak planes resulting in development of a stratabound ore body.
