Hydrothermal alterations and U-REE mineralisation in the Narwapahar uranium deposit, Singhbhum shear zone, India

The polymetallic mineral district in the Singhbhum shear zone (SSZ), eastern India, hosts some of the most important Palaeoproterozoic uranium, copper and apatite–magnetite deposits in India. Although the shear zone had been the locale of profuse hydrothermal alteration and there is general consensus about hydrothermal mineralisation, the alteration types vis-à-vis ore mineralisation are not amply clear. In this paper, we describe ore mineralogy and multiple alteration types vis-à-vis uranium-rare earth element (U-REE) mineralisation in the Narwapahar uranium deposit located in the SSZ. Uranium mineralisation is hosted by intensely deformed and metamorphosed tourmalinite, albite schist and chlorite schist, which are mineralogically heterogeneous showing extreme variations in constituent minerals and their modal abundances in the outcrop, hand specimen and microscopic scales often grading to nearly monomineralic rocks. Thorium-poor uraninite is an ubiquitous ore mineral of uranium. High uranium content is also noted in ilmenorutile and some unidentified U silicates. Allanite is the most abundant REE-bearing mineral whereas monazite and xenotime are subordinate. Low-Ti magnetite is nearly ubiquitous in the U ores. Based on locally preserved cross-cutting hydrothermal veins and frozen-in mineral replacement textures, the sequential metasomatic events deciphered from the Narwapahar deposit are B (boron) → Na–Fe (±Ca)→ K–Fe (±Ca)→ K→ H+. Uranium mineralisation is associated with B, Na–Fe and H+ metasomatism and perhaps with K–Fe metasomatism too whereas REE mineralisation is associated with K–Fe metasomatism and resultant hydrothermally altered rocks. The alteration types, especially the well-known pervasive regional-scale alkali metasomatism, the alteration paragenesis as described here and metal association (Fe–Cu–U–REE–Co–Ni–Mo–Au) are akin to Fe oxide (Cu–U–REE) (IOCG)-type mineralisation. The presence of a substantial amount of uranium in ilmenorutile and very fine-grained inclusion of uraninite in magnetite may potentially affect the recovery of uranium during beneficiation leading to low leachability of some uranium ores. The close association of REE-bearing minerals with the U ores requires focused study to examine the potential of U ores as a source of by-product REEs.