Dipak C. Pal

Texture, microstructure and geochemistry of magnetite from the Banduhurang uranium mine, Singhbhum Shear Zone, India — implications for physico-chemical evolution of magnetite mineralization

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.

Petrography and microthermometry of fluid inclusions in apatite in the Turamdih uranium deposit, Singhbhum shear zone, eastern India - An insight into ore forming Fluid

The Singbhum shear zone is one of most important polymetallic mineralized zone, characterized by uranium, Cu and apatite-magnetite mineralization. Although there is unanimity regarding the hydrothermal nature of different ores, the fluid characters, particularly related to apatite-magnetite mineralization, are not very well-constrained. This study aims at deciphering the fluid character involved in apatite-magnetite and associated mineralization through fluid inclusion studies in apatite from apatite-magnetite-bearing uranium ores in the Turamdih uranium deposit.The studied host rock is quartz-chlorite schist comprising predominantly of quartz, and chlorite with magnetite, apatite, uraninite, monazite, allanite chalcopyrite, and pyrite. Textural and micro-structural relations of apatite indicate that the studied apatite along with uraninite and monazite crystallized prior to or at the early stage of shearing. Based on the content of fluid inclusions in ambient room temperature, the primary inclusions are classified in to two groups, namely type-I and type-II. Aqueous bi-phase inclusions, defined as type-1, are most common and abundant. The type-II polyphase inclusions are characterized by the presence of aqueous liquid, vapor and one or more solid phases. Fluid inclusion microthermometric experiments suggest that apatite crystallized from highly saline fluid and the fluid composition can be best expressed as H2O-NaCl-CaCl2 (± MgCl2) brine. The salinity varies between ∼ 22 to 43 wt % NaCl equivalents. Although, the final melting temperature of hydrohalite could not be determined due to very small size of the inclusions, the minimum concentrations of CaCl2 was calculated considering the final ice melting temperature to be the hydrohalite melting temperature. The NaCl and CaCl2 content ranges between ∼ 2 to 21 and ∼ 4 to 28 wt. % respectively and the CaCl2:NaCl ratio are mostly above 1:1 indicating a calcic brine. The temperature of final homogenization (Th) of type-I inclusion (L+V→L) ranges mostly between 240° to 450°C. This study suggests that apatite started crystallizing at a higher temperature (Th ∼ 450°C) from a high salinity brine (∼35 wt% NaCl equivalent). Subsequently, this fluid mixed with a fluid of lower temperature (Th ∼ 300°C) and somewhat lower salinity (∼ 25 wt% NaCl equivalent). The associated uraninite and monazite likely precipitated along with apatite from the same fluid. However, the temperature of fluid entrapment and hence apatite crystallization must be higher as Th provides the minimum temperature of entrapment. Based on this study and several other lines of evidence, we propose that the mineralizing fluid was derived from basinal brine or from evaporite dissolution.