Minati Roy

C-O Isotope Analysis of Dolostone of Vempalle Formation in Dhone-Gudipadu-Korivipalle Sector, Western Part of Papaghni Sub-basin, Andhra Pradesh and Its Impact on Uranium Mineralization

Cuddapah basin is an important uranium province, wherein unconformity related uranium deposit occur in Srisailam sub-basin in northern part and the dolostone hosted strata bound uranium deposit in Vempalle dolostone in southern part. The latter host the low-grade, large-tonnage uranium deposit. Vempalle Formation in Dhone-Gudipadu-Korivipalle (DGK) sector, along western part of the Papaghni sub-basin consists of three major units viz. (a) lower dolostone, (b) middle purple shale and (c) upper cherty dolostone. The lower dolostone has lot of detritus, represented mainly by quartz, feldspar, mica and clay. The upper cherty dolostone has chert as layers, nodules, rip-up clasts and at places do have conspicuous reddish brown shale bands with <2mm size siliceous oolite layers. They exhibit shallowing-upward carbonate depositional system with tidal flat environment. A number of uranium occurrences in Gudipadu-Korivipalle sector are hosted by the lower dolostone unit, especially at its lower stratigraphic level, i.e. 15-42m above the Gulcheru-Vempalle (GV) contact. Micro-stylolites are indicative of comparatively more burial pressure. Presence of fenestral voids filled with recrystallised fibrous calcite and thin mud drapes in the dolostone is indicative of precipitation in a shallow marine-tidal flat environment with late burial dolomitisation. Depletion of heavier C and O isotopes [13C: (-2.31 to 1.32%) and 18O: (-6.21 to -17.3%)] in the lower dolostone indicate mixing of fresh water with sea water in the depositional environment of tidal flat regime

Ore mineralogy and mineral-chemistry of uranium mineralisation, and geochemistry of metasomatised calc-silicate rocks at Raghunathpura, Mahendragarh district, Haryana

Uranium mineralization at Raghunathpura, Mahendragarh district, Haryana is confined to a major shear zone within the metasedimentary lithounits which is marked by prominent alkali metasomatism. The metasedimentary lithounits of the area belong to the Ajabgarh Group of Delhi Supergroup and form part of the Khetri basin. Alkali metasomatised calc-silicate rock is the main host for anomalous uranium mineralisation and is mainly composed of tremolite-actinolite, diopside, scapolite (mariolite type), wollastonite, calcite, quartz, graphite and sphene. Hydrothermal/metasomatic alterations are manifested as multiple episodes of albitisation with minor microclinisation, carbonitisation, silicification and U-Th-sulfide-clay formation. The radioactive mineral phases are uraninite (uraninite I and II), coffinite, brannerite, U-Ti complex, uranothorite, thorite, allanite, monazite and associated sulphides are pyrrhotite, pyrite, chalcopyrite, molybdenite and galena. Repeated hydrothermal alterations in this shear zone has remobilised the uranium as seen by the alteration of uraninite, metamictised uranothorite and formation of remobilized graphite. Mineral chemistry of uraninites and their chemical ages indicate two distinct phases of formation of which the rounded uraninite I, rich in Th and REE represents the earliest phase with a chemical age of 809 Ma. Uraninite II with a chemical age of 745Ma is Thpoor and the mineral chemistry indicates its origin from a relatively low temperature fluid. Th-bearing and Th-poor coffinite are recorded, of which Th-bearing coffinite is the product of alteration of uraninite I and Th-poor is of remobilized origin. The observed mineralogical and chemical features are suggestive of a hydrothermal mineralisation associated with one or more episodes of alkali metasomatism.