Fluid inclusion study of radioactive granitoids and cherty cataclasite in the southeastern part of Nanded district, Maharashtra: implications for Uranium mineralisation

Radioactive granitoids and cherty cataclasites are delineated in Thadisaoli–Khatgaon and Shahpur–Sujayatpur sectors in southeastern part of Nanded district, Maharashtra, which have recorded anomalous radioelemental contents (Granites: upto 1% U₃O₈ and 0.20% ThO₂; Cherty cataclasites upto 0.11% U₃O₈ and <0.005% ThO₂) and enrichment in trace element and rare metal and rare earth element concentration (Nb: upto 146ppm, Y: upto 226ppm, Zr: upto 559ppm and total REE: upto 2010ppm). The mineralised granitoids are affected by profuse pegmatitic/quartzo-feldspathic, quartz and epidote venations and mainly confined along the NE–SW and NNE–SSW faults/shear zones. Radioactive phases are represented by discrete uranium/thorium ore minerals (uraninite, β-uranophane and thorite) and high content of resistates viz., apatite, zircon, allanite, sphene, cerianite, monazite and ilmenite.

Fluid inclusion studies of the mineralised and sheared granites, quartzo-feldspathic veins (QFV's) and quartz veins indicate the presence of 6 to 32μm size aqueous and carbonic inclusions of three principle types and represented by six phases viz., aqueous mono-, bi- and poly-phases, aqueous-carbonic, carbonic mono-, bi- and poly-phases. Aqueous biphase inclusions show wide range of salinity (0.71 to 19.99 wt.% eNaCl) and homogenisation temperatures (T_h: 127.5–280.8ºC), while aqueous-carbonic inclusions exhibit restricted salinity (5–8.4 wt.% eNaCl) and T_h (28.3–30.9ºC). The composition of these aqueous fluids varies from NaCl– KCl dominant in quartz veins and radioactive granite to MgCl₂ dominant in sheared radioactive granite. Similarly, co-eval aqueous biphase and aqueous-carbonic inclusions have yielded 678–958 bar pressure and 201–233ºC temperature in quartz vein samples while higher values of 1120–1550 bar and 304–360ºC are indicated by sheared granite. The presence of more than one population without much change in fluid composition signifies their origin at different stages of deformation. Besides, mixing of a moderate temperature and low salinity fluid (meteoric/basinal brine) with a comparatively high temperature and hypersaline fluid (magmatic/evolved brine) might have also played an important role in inclusion characteristics. Based on these fluid inclusion characteristics, it appears that multi-episodic hydrothermal activity under an extensional tectonic regime in this fertile granitic province has led to precipitation of uranium along suitable structural aureoles from uraniferous hydrotherms.