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

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. Zakaulla ¹, K. Umamaheswar ², P. V. Tirupathi ¹, U. P. Sharma ¹, H. Basu ¹, M. A. Ali ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Nagarabhavi, Bangalore, IN
2 Atomic Minerals Directorate for Exploration and Research, Nagarabhavi, Shillong, IN

Abstract

Gulcheru Formation constitutes 200 m-250 m thick siliciclastic sediments Based on grain size texture and colour of the sediments, four distinct facies are demarcated They are pink massive quartzite, cross bedded grey quartzite, purple siltstone and pitted quartzite The ratios of Si/Al is higher (62 9 to 292 18) in pitted quartzite and lowest (2 122 - 18 247) in siltstone, where as grey quartzite has wide range of 3 78 - 8637 67 Ths difference is essentially due to higher quartz content in pitted quartzite and dominance of clays/shaly component in siltstone The wide range of Si/Al ratio in grey quartzlie is due to interlayering of siltstone/shale laminations/bands Distribution of uranium indicates grey quartzite (av U₃0₈= 9 9 ppm) and siltstone (av 15 42 ppm) have higher intrinsic uranium than that of pitted quartzite (average 5 6 ppm) and pink massive quartzite (average 7ppm) which is considered very high in comparison to normal sandstone (av = 0 5ppm, Taylor, 1965) Trace elements like Mo, V, Ni, Cu and Pb are enriched in mineralised grey quartzite Pitchblende is the man uranium mineral in grey and pitted quartzite unlts It occurs as fracture fillings and also as interstital material, associated intimately with chlorite Close association between pitchblende and sulphides like pyrite, chalcopyrite and galena is seen when pitchblende occurs as fracture fills Uranium in siltstone, is generally in adsorbed form with ferruginous material, where as associated trace elements like Mo (r = 0 515), V (r = 0 806), Nt (r = 0 4) and even Ti0₂, (r = 0 87) are well correlated with Al₂0₃, suggesting their clays/phyllosilicates U/Pb dating of uraniferous grey quartzite indicated ages of 1336k±4 Ma and 446±29 Ma suggesting remobilisation and concentration of intrinsic uranium.

Keywords

Gulcheru Formation, Cuddapah Supergroup, Siliciclastic Sediments, Uranium incidence, Gandi Area, Andhra Pradesh.

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