naa a22 4500 510755003 CHVBK 20180411083119.0 cr unu---uuuuu 180411e20130101xx s 000 0 eng 10.1007/s12594-013-0009-7 doi (NATIONALLICENCE)springer-10.1007/s12594-013-0009-7 Texture, microstructure and geochemistry of magnetite from the Banduhurang uranium mine, Singhbhum Shear Zone, India — implications for physico-chemical evolution of magnetite mineralization [Elektronische Daten] [Dibakar Ghosh, Tusar Dutta, Susanta Samanta, Dipak Pal] The Singhbhum Shear Zone in eastern India is one of the largest repositories of uranium and copper in India. Besides uranium and copper, apatite-magnetite mineralization is widespread in this shear zone. This study aims at deciphering the physico-chemical evolution of magnetite mineralization in relation to progressive shearing integrating field relations, micro-textures, structures and compositions of magnetite in the Banduhurang uranium mine. Apatite-magnetite ores occur as discrete patches, tongues, and veins in the strongly deformed, fine grained quartzchlorite schist. Textures and microstructures of magnetite indicate at least three stages of magnetite formation. Coarsegrained magnetite (magnetite-1) with long, rotational, and complex strain fringes, defined by fibrous and elongate quartz, is assigned to a stage of pre-/early-shearing magnetite formation. Medium grained magnetite (magnetite-2), characterized by single non-rotational strain fringe equivalent to the youngest fringe of magnetite-1, grew likely at the mid-/late-stage of shearing. Fine grained magnetite (magnetite-3) is generally devoid of any pressure shadow. This indicates even a much later stage of formation of this magnetite, presumably towards the closing stage of shearing. Some of the magnetite-1 grains are optically heterogeneous with a dark, pitted Cr-Ti-bearing core overgrown by lighter, fresh rim locally containing pyrite, chalcopyrite, and chlorite inclusions. The cores are also locally characterized by high Al and Si content. Homogeneous magnetite-1 is optically and compositionally similar to the overgrowth of heterogeneous magnetite-1. This homogeneous magnetite-1 that grew as separate phase is contemporaneous with the overgrowth on pitted core of heterogeneous magnetite-1. Magnetite-2 is compositionally very similar to homogeneous magnetite-1, but is devoid of sulfide inclusion. Magnetite-3 is generally devoid of any silicate or sulfide inclusion and is most pure with least concentrations of trace/minor elements. The high Al and Si content in some magnetite can be explained by coupled substitution that involves substitution of Si4+ for Fe3+ in the tetrahedral sites and Fe2+ for Fe3+ in the octahedral sites, with a simple substitution of Al3+ for Fe3+ in the octahedral sites. The mode of occurrences of apatite-magnetite ores indicates a predominantly hydrothermal origin of most magnetite. However, the Cr-Ti-bearing magnetite-1 cores and inferred mafic nature of the original protolith indicates that some magnetite was inherited from the original igneous rock. We propose that the pre-/early-shearing hydrothermal event of magnetite formation was associated with sulfide mineralization and alteration of existing magmatic magnetite. The second stage of magnetite formation at the mid-/late-stage of shearing was not associated with sulfide formation. Finally, fine-grained compositionally pure magnetite formed at the closing stage of shearing likely due to metamorphism of Fe-rich protolith. Geological Society of India, 2013 Magnetite nationallicence Texture nationallicence Microstructure nationallicence Composition nationallicence Singhbhum nationallicence Ghosh Dibakar Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut Dutta Tusar Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut Samanta Susanta Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut Pal Dipak Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut Journal of the Geological Society of India Springer-Verlag 81/1(2013-01-01), 101-112 0016-7622 81:1<101 2013 81 12594 https://doi.org/10.1007/s12594-013-0009-7 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1007/s12594-013-0009-7 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Ghosh Dibakar Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut NATIONALLICENCE 700 1- Dutta Tusar Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut NATIONALLICENCE 700 1- Samanta Susanta Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut NATIONALLICENCE 700 1- Pal Dipak Department of Geological Sciences, Jadavpur University, 700 032, Kolkata, India aut NATIONALLICENCE 773 0- Journal of the Geological Society of India Springer-Verlag 81/1(2013-01-01), 101-112 0016-7622 81:1<101 2013 81 12594 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer