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.
