Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

B. S. Bisht ¹, I. H. Israili ², M. K. Khare ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Hyderabad-500016, IN
2 Regional Centre for Exploration and Research, AMD, Nagpur-440016, IN

Abstract

Metamorphic rocks of the Siang Group in West Siang and Upper Subansiri districts, Arunachal Pradesh, have anomalous concentration of intrinsic radioelements (viz., U, Th and K). The study reveals that psammopelites comprising paragneiss, carbonaceous phyllite and quartzite/phyllite/mica schist (av. 8.2-27.3 ppm U, 19.7-42.8 ppm Th, 2.2-4.0 % K); volcano-sedimentary garnet amphibolite (av. 14.3 ppm U, 23.1 ppm Th, 1.8 %K), and intrusive feldspar porphyry (av. 12.7 ppm U, 26.3 ppm Th, 5.9 %K) have higher radioelement contents, particularly U, in comparison to pelitic graphite schist, iron formation and metabasics (av. <4.2-4.7ppm U, 18.5-60.2 ppm Th, 3.3-4.9 %K). All rock types, except feldspar porphyry, exhibit noticeable difference in mean U and Ra (eU) values. Variation in U content has been conspicuous in carbonaceous rocks, with phyllitic units more enriched than graphitic schists. U and Th contents appear to be independent of K concentration in all rock types. Such features of variable radioelement concentration indicate differences in the composition of precursor sedimentary rocks and also differential mobility during metamorphism.

Keywords

Siang Group, Radioelements, Metamorphism, Arunachal Pradesh.

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B. S. Bisht ¹, M. A. Ali ¹, A. K. Pande ², R. Pavanagaru ³

Affiliations
1 Atomic Mineral Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad- 500 016, IN
2 Atomic Mineral Directorate for Exploration and Research, Department of Atomic Energy, Jaipur - 303 906, IN
3 Department of Geology, Osmania University, Hyderabad - 500 007, IN

Abstract

In the lesser Himalayan region of Arunachal Pradesh, northeastern India, iron-Uranium (Fe-U) mineralisation characterises the volcano-Sedimentary sequence belonging to the Middle-Late Proterozoic Siang Group. The host rocks metamorphosed in the greenschist to lower amphibolite facies are intensely deformed.

The lithological, structural, petrological and geochemical parameters indicate involvement of three distinct processes resulting in the Fe-U mineralisation. These comprise early sedimentary deposition, mainly of Fe-Oxides, Fe-Sulphides and minor U, followed by local metamorphic redistribution of U, and lastly hydrothermal mineralisation leading to deposition of mainly uranium and REE bearing minerals and minor Fe-Cu sulphides.

Syngenetic iron oxide mineralisation occurs in the form of ironstone containing 44 to 63 wt percent Fe. The lithological association of ironstone and low contents of Ti, V, Cr, Co and Ni in magnetite support this contention. Sulphides, mainly pyrite with subordinate chalcopyrite and molybdenite form bands, stringers and veins. The sulphur isotopic ratios (δ³⁴S) of pyrites in different formations exhibit a small range of +11.1 to +15.8 %. suggesting derivation of sulphur mainly from the seawater sulphate and hydrothermal sources.

Variation in the U-content of the rocks in the mineralisation zone is attributed to the differences in the composition of precursor sediments and also to differential mobility of uranium during progressive metamorphism.

Epigenetic U-Mineralisation related to quartzo-Feldspathic veins occurs along fractures and foliations and has caused wall rock alterations such as chloritisation, epidotisation, silicification, and sericitisation. Association of uraninite, brannerite, davidite, fluorite, tourmaline, albite, biotite and quartz characterise the uraniferous rocks. These rocks also contain significant REEs. The calc-Alkaline granitic magma of Cambro-Ordovician Period is considered to be the ultimate source of U.

Oxidation of host rocks resulted in extensive goethitisation and kaolinisation in the weathering zone.

Keywords

Fe-U Mineralisation, Himalayas, Arunachal Pradesh, India.

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S. K. Acharyya ¹, B. S. Bisht ²

Affiliations
1 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN
2 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad- 500 016, IN

Abstract

No Abstract.

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B. S. Bisht ¹, P. Rajasekaran ¹, R. M. Sinha ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research Department of Atomic Energy, Begumpet, Hyderabad- 500016, IN

Abstract

Aqueous bi-phase fluid inclusions in quartz from mineralised vein, granite and pebbly quartzite have been studied to decipher physico-chemical conditions of the unconformity-related uranium mineralisation at Lambapur- Peddagattu area in the northern fringe of Cuddapah basin. Fluid inclusions in vein- and granitic- quartz show significant similarity in phase ratios and homogenisation temperatures, though they differ in the mode of origin. main population of inclusions in vein quartz is of primary origin, while in granitic quartz it is of secondary origin. The size of fluid inclusions generally varies between lOpm to 20pm, with low gas to liquid ratios corresponding to high degree of fill (F=0.8 to 0.9). Inclusions in vein- and granitic- quartz homogenise to liquid phase between 140°C to 240°C (mode 150°C to 170°C) [T_H]. Similar temperature of homogenisation has been obtained in quartz of pebbly quartzite. The ice melting temperatures in vein quartz are in the range of -2°C to - 10°C [T_M] equivalent to salinity of 1-14 wt% eNaCl and in granitic quartz these are in the range of -6°C to -14"C[T_M] corresponding to salinity values of 10-18 wt% eNaC1. The low temperature fluid inclusions homogenising at similar range of temperatures but showing variable salinity suggest mixing of isothermal fluids of contrasting salinity. It is likely that uranium precipitated from the interaction of cooling low saline hydrothermal solution at the temperatures of 150-170°C with high saline mineralising fluid transported by recirculating meteoric water.

Keywords

Fluid Inclusions, Microthermometry, Uranium Mineralisation, Cuddapah Basin, Andhra Pradesh.

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B. S. Bisht ¹

Affiliations
1 Atomic Minerals Directorate of Exploration and Research, Department of Atomic Energy Begumpet, Hyderabad - 500 016, IN

Abstract

Compositional Characterisation of biotite from two distinctly different geological domains of the Gamkak and Jalyor areas in the West Siang district, Arunachal Pradesh has been carried out. The environments of formation of biotite in the areas have been inferred from the compositional data Conspicuous differences in chemistry have been observed in biotite from metamorphic mica schist and hydrothermal uraniferous vein Compositionally vein biotite associated with uranium minerals, uraninite, branncrite and occasionally davidite contain significantly higher (mean) MgO (17 38%), F (7 33%) and Si0₂, (38 95%) and lower (mean) Al₂0₃ (12 78%) when compared to the biotite in Non-Uranifeious mica schist (mean) MgO (5 67%), F (1 53%) SiO₂, (34 54%) and Al₂O₃ (17 58%). Minor differences do exist in the concentration of TIO₂, FeO and Na₂O in these biotite. The MgO-Fe0-Al₂o₃ , plot reveals that the biotite from uranifcrous vein is of calc-alkaline variety and that in mica schist is peialuminous.

Keywords

Blotite, Mica Schist, Uraniterous Vein, Gamkak Jaiyor, Arunachal Pradesh.

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