JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.81, January 2013, pp.101-112
Texture, Microstructure and Geochemistry of Magnetite from the Banduhurang Uranium Mine, Singhbhum Shear Zone, India – Implications for Physico-chemical Evolution of Magnetite Mineralization DIBAKAR GHOSH, TUSAR DUTTA, SUSANTA K. SAMANTA and DIPAK C. PAL Department of Geological Sciences, Jadavpur University, Kolkata - 700 032 Email:
[email protected];
[email protected] Abstract: 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. Keywords: Magnetite, Texture, Microstructure, Composition, Singhbhum. INTRODUCTION
The ~200 km long arcuate intensely deformed Singhbhum shear zone (SSZ), in eastern India is one of the most important poly-metallic mineral districts in India. The SSZ hosts several U, Cu and apatite-magnetite deposits hosted in hydrothermally altered, deformed and metamorphosed rocks. Previous studies propose multiple
stages of mineralization/ mobilization of U, Cu and rare earth elements starting early in or prior to the beginning of formation of the shear zone (Rao and Rao, 1983; Sarkar, 1984; Pal et al. 2009, 2010, 2011a, b). Similar studies on the physical and chemical evolution of apatite-magnetite ores, that constitute an important part of the mineralization in the Singhbhum shear zone, are not available. Recognizing
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the events of mineralization/mobilization at the different stages of deformations, utilizing mode of occurrences, textures and microstructures of the ore minerals in such metamorphosed terrain is a difficult task as progressive ductile shearing, metamorphism and repetitive metasomatism can potentially obliterate most of the earlier ore fabric and ore mineral compositions. Magnetite is one of the most physically robust minerals that can record and retain the physical and chemical imprints acquired during progressive deformation. In this study we examine the potential use of magnetite in deciphering the physico-chemical evolution of magnetite-rich ores in general and the apatite-magnetite ores of the Singhbhum shear zone in particular. We combine the mode of occurrences, microtextures, microstructures and compositions of magnetite from the apatite-magnetite (±Cu, U) ores of the Banduhurang uranium mine in the Singhbhum shear zone to trace the evolution of mineralization in relation to progressive deformation. GEOLOGICAL BACKGROUND
The three major tectono-metamorphic belts (from north to south) of the eastern Indian shield are (a) Chottanagpur Gneissic Complex (CGNC) to the north (b) the North Singhbhum Fold Belt at the centre and (c) the Singhbhum cratonic nucleus to the south (Fig.1a). The Singhbhum shear zone (SSZ) occurs close to the boundary between the Archean cratonic nucleus to its south, and the Proterozoic North Singhbhum Fold belt to its north. Two prominent basins, namely the Iron ore basin (Iron ore Group greenstone sequence) and the Dhanjori basin (Dhanjori Group of rocks) occupy the north-western and south-eastern parts of the cratonic nucleus, respectively. The fold belt, near the northern margin of the craton, is occupied by predominantly siliciclastic rocks of the Singhbhum Group. For a more detailed discussion on the regional geology see Pal et al. (2009) and references therein. The Singhbhum shear zone cuts the rocks of the Singhbhum Group, Dhanjori Group and the Iron Ore Group. It perhaps represents a deep seated tectonic boundary (Dunn and Dey 1942; Sarkar and Saha 1962; Naha 1965; Roy 1966; Banerji 1981) along which the rocks of the northern fold belt were thrust southward on the Archaen cratonic nucleus (Ghosh and Sengupta 1987, 1990; Mukhopadhyay and Deb 1995). This southward movement of the crustal block initiated progressive ductile shearing that generated southvergent isoclinal folds with SE-NW/E-W fold axes and pervasive mylonitic foliation steeply dipping towards NE/ N (Ghosh and Sengupta 1987, 1990). The subsequent
deformation developed sub-horizontal folds parallel to the SSZ followed by local folding transverse to the shear-zone trend. The prograde metamorphism that followed the ductile shearing culminated in the epidote-amphibolite facies and somewhat outlasted the ductile shearing (Sengupta et al. 2005). The Banduhurang Uranium deposit in the Singhbhun shear zone hosts the first open cast uranium mine in India. The deposit is located almost at the central segment of the SSZ and at the western fringe of the segment that hosts the presently working uranium and copper mines in the Singhbhum shear zone. The rock types in and around the Banduhurang mine consist of chlorite schist, biotite schist, quartzite and feldspathic schist (Fig. 1b). The quartzite is locally ferruginous and rich in oxidized magnetite. In the mine, chlorite schist and sericite schist are the most prominent rock units along with feldspathic schist. The chlorite schist and sericite schist comprises variable proportions of quartz, chlorite and sericite. The rocks are strongly sheared, fine grained, heterogeneous and display prominent shear fabric. Presence of prominent c-c’ fabric and very fine grained nature of the rocks indicate that the rocks under study are from the ultra-mylonite zone. Foliation is commonly defined by oriented flakes of chlorite and sericite. Stair like quartz veins, folded pinch and swell structure and asymmetric folding of foliation are commonly observed in the field. The feldspathic schist is characterized by alternate light colored quartz-feldspar-rich and dark colored chlorite rich bands. This quartz-chlorite schist is the main host rock of uranium as well as other oxide and sulfide mineralization. Previous studies (Banerji, 1962) have proposed a major antiformal fold within the shear zone in and around Banduhurang area (Fig. 1b). The fold occurs at the core of the shear zone and affects the group of chlorite schist of this area. This fold is isoclinal, trending generally ESE to WNW with the axial plane dipping 200 to 600 towards north. The culminations and depressions along the major antiformal fold axes, within, the shear zone are due to a set of cross fold axes that affect the major folds about axes slightly oblique to the direction of dip of their axial planes (Banerji, 1962). METHODOLOGY
For the present investigation, we studied 30 polished thin sections, cut perpendicular to the foliation and parallel to lineation, from 20 hand specimens collected from the quartz-chlorite schist. These sections were examined using standard transmitted and reflected light microscopic techniques. Chemical characters of different varieties of JOUR.GEOL.SOC.INDIA, VOL.81, JAN. 2013
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Fig.1. (a) Simplified geological map of Eastern Indian Shield (redrawn from Saha, 1994). (b) Lithology and overall structure in and around the Banduhurang uranium deposit (redrawn from Banerji, 1962).
magnetite were first studied by Scanning Electron Microscope and Energy Dispersive X-ray analysis (SEMEDX). Selected samples were then analyzed by Electron Probe Micro Analysis (EPMA). Both these studies were carried out in the Geological Survey of India, Kolkata. For SEM-EDX studies Zeiss-Evo-40, INCA penta FETx3 was used with a beam current 20kv. Electron Probe Micro Analysis (EPMA) was carried out on the polished section using CAMECA Sx100 EPMA with an accelerating voltage 20 kV and a beam current of 15 nA. Initially analyses of all possible elements were attempted in magnetite during SEMJOUR.GEOL.SOC.INDIA, VOL.81, JAN. 2013
EDX study. However, besides Fe the other elements those were detected during this preliminary study includes Cr, Ti, Si, Al, Ca, Mg, Ni, Co, P, Na, V, Mn. Accordingly these elements were analyzed during EPMA investigation. Following standards were used for EPMA; albite (Si, Na), magnetite (Fe), olivine (Mg), wollastonite (Ca), apatite (P), corrundum (Al), manganosite (Mn), nickel metal (Ni), eskolaite (Cr), cobalt metal (Co), rutile (Ti). Detection limit (in wt. %) for elements in EPMA are: Na-0.08, Si-0.05, Al-0.05, Co-0.12, Mg-0.05, Ca-0.06, Ti-0.18, Cr-0.15, V0.2, Mn-0.23, Fe-0.02, Ni-0.2.
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MAGNETITE TEXTURE AND COMPOSITION
Magnetite-apatite rich zones are present as discrete patches, pockets, veins and intruded tongue in the chlorite schist (Fig. 2). The host rock comprises variable proportions of quartz, chlorite, and sericite. The ore minerals include magnetite, ilmenite, apatite, uraninite, chalcopyrite, pyrite, allanite and some unidentified REE-oxides. The rock shows prominent mylonitic foliation and lineation defined by quartz ribbon, pressure fringe and shadow on magnetite, apatite and pyrite. The rock has distinct c-c' fabric and the c-fabric defines the pervasive foliation. Texture and Microstructure of Magnetite
Under the microscope the magnetite-rich rock comprises magnetite of different grain size, coarse (~500-1300 µm), medium (~300-500 µm) and fine ( not detected in EPMA; very high SiO2 and Al2O3 content (analysis #30, #50) may be due to sub-microscopic inclusions; see text for discussion
Si Al3+ Co2+ P5+ Mg2+ Ca2+ Ti2+ Cr3+ V3+ Fe3+ Fe2+
1.19 0.81 n.d. n.d 0.38 0.10 n.d. 0.86 n.d. 86.94 90.28
#29
0.309 0.375 2.887 0.864 0.037 0.217 0.301 0.363 0.632 0.029 0.032 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.116 0.178 0.176 0.276 0.000 0.063 0.034 0.000 0.043 0.000 0.045 0.000 0.000 0.000 0.000 0.000 0.214 0.122 0.046 0.000 0.000 0.000 0.000 0.000 0.000 15.077 14.735 9.742 13.594 15.896 8.141 8.164 10.711 8.545 8.037
0.99 0.59 0.13 n.d 0.25 0.19 0.19 n.d. n.d. 89.09 91.43
SiO2 Al2O3 CoO P2O5 MgO CaO TiO2 Cr2O3 V2O3 FeO Total
4+
#23
Oxide
Core
Table 1. Representative compositions of different textural types of magnetite
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fresh rim portion (Fig. 3c). In magnetite-1 grains those are optically and compositionally (see next section) homogeneous, sulfide inclusions do not have any preferred distribution (Fig. 3e). At places chalcopyrite is present at the pressure shadow of magnetite-1. The rim portion of heterogeneous grain and whole of the homogeneous grains of magnetite-1 contains innumerous silicate (quartz, chlorite) inclusion. Magnetite-1 locally contains ilmenite inclusions (Fig. 3d). Magnetite - 2
Medium-grained (~300-500 µm) magnetite-2 is characterized by small, single non rotational fringe (Fig. 3f). Fringes are mostly straight and simple in nature. Some of the grains also contain massive (non-fibrous) strain shadows. These zones are mainly located at the extensional regime of the porphyroblast in bulk simple shear movement (Samanta et al. 2002). The single set of non-rotational strain fringes of magnetite-2 is parallel to the youngest pressure fringes (attached to the core object) of magnetite-1. This type of magnetite is mostly free from sulfide inclusion. However, at some places chalcopyrite is present within these non-rotational pressure shadows. Magnetite-2 contains chlorite and quartz inclusions throughout the whole grain. Magnetite - 3
Fine-grained magnetite-3 (
