No abstract

Optical microscopy, Raman spectroscopy and electron probe microanalyses were conducted on a migmatitic metapelite to investigate the effects of alpha radiation and subsequent alteration at the interface between uraninite inclusions and the host minerals biotite, chlorite and albitic plagioclase. The study reveals (1) anomalous colouration under the polarizing microscope; (2) pertinent changes in the characteristic Raman spectra of host phases; (3) reaction aureoles, composed of secondary phases (viz. chlorite and K-feldspar) of various sizes (∼15–45 μm) within biotite and plagioclase, respectively; (iv) K-feldspatization and sericitization of plagioclase at the grain boundaries; (5) agreement between the observed α-penetration depths and the Monte-Carlo simulation results; and (6) dissolution textures within the matrix monazites. Analysis of the compositions of the reaction aureoles in albitic plagioclase reveals a systematic distribution of K-feldspar-, LREE- and clay-rich zones; while the same in biotite is composed of secondary chlorite. The growth sequence of the secondary phases indicates an influx of a K-rich fluid, following intense radiation damage, efficiently superimposed by LREE-metasomatism and later acidic alteration. These changes took place under low-temperature (≤150°C) conditions, wherein radial cracks (within plagioclase) and cleavages/fractures (within biotite) favoured fluid infiltration-circulation into the reaction aureoles. Depletion of the LREEs from the dissolved matrix monazites and their enrichment as a discrete LREE phase within the damaged aureoles in plagioclase demonstrate micrometre-scale LREE mobility.

Several radioactive anomalies due to uranium and thorium, associated with the mesedimentary enclaves (Archaean) within granite (Archaean to Early-Proterozoic) have been recorded in parts of Karimnagar Granulite Terrain, Karimnagar Dist. At Peddur and Kottur, Uraninite has been identified in the samples of metasediments. The metasediment from these two places have been subjected to granulite facies of metamorphism and host high values of uranium with negligible thorium. In Peddur, samples of metasediments have assayed as high as 1.96% U 3 O 8 with negligible thorium, and in Kottur up to 0.059% U 3 O 8 . Leaching studies on these samples have indicated that most of the U 3 O 8 present is leachable. This discovery has opened up the possibility of finding uranium mineralisation in Archaean metasediments and thus provides a thrust for uranium exploration in similar geological environs in India. Further, the basement granite along with the metasedimentary enclaves has the potential to act as a provenance for a possible unconformity type or sandstone type U-deposit in the rocks of overlying Pakhal and Gondwana Supergroup, in Pranhita-Godavari Basin, situated to the east of this area.

The NNW-SSE trending leucogranites in and around Mastipuram, Wanaparthi district, Telangana are intrusive into Peninsular Gneissic complex. The leucogranite is metaluminous to mildly peraluminous, alkali-feldspar granite with I-type signatures, probably formed from a fractionated melt involving crustal component. Magmatic and hydrothermal contributions are supported by wide range of U/Th ratios in mineralized samples, which also show Na-enrichment as compared to non-mineralized leucogranites. Average U and Th contents and U/Th ratio in mineralized leucogranite is 541ppm, 147 ppm and 2.85, respectively. Main ore mineral is uraninite and its altered form - gummite, while secondary uranium minerals such as schoepite, uranophane and haiweeite are also identified in leucogranite by XRD. Reconnoitory drilling of ten boreholes have confirmed sub-surface continuation of uranium mineralization with varying width of 1m to 31m at vertical depth of about 50m. Thus, the leucogranites bordering Gadwal schist belt around Mastipuram forms a very potential target.