No abstract

A new type of uranium mineralization, associated with late stage pegmatitic activity and quartzo-feldspathic injections has been identified in Rohil West that lies at about 500 m west of main Rohil uranium deposit. The injections have caused extensive deformation and silicifications of host quartz-muscovite-biotite schist. Uranium mineralization is represented by uranitite occurring in association with high temperature phases like allanite, monazite and zircon. Uranitite forms coarse to medium euhedral - subhedral grains showing intergrowth textures with minerals like quartz, feldspar and biotite of quartzo-feldspathic injections. Uraninites contained high ThO2 2.67-7.21wt% (av. 4.78%, n=30) and high Re2O3 (2.35-4.31 wt.%, n=30). Chemical age of uranitite is calculated as 675-834Ma. In view of intergrowth textures, high ThO2 content and association with quartzo-feldspathic injections uranium mineralization in Rohil West block is interpreted as Pegmatitic Type. The recognition of granite-pegmatite related uranium mineralization in Rohil West has opened up another dimension for the exploration, in addition to the largely Metasomatite Type uranium mineralization at Rohil. The possible extensions of newer type of uranium mineralization in NDFB need to be interpreted from heliborne geophysical data for the search of satellite deposit to Rohil.

Results of multi-method geochronological study (U-Pb, Pb-Pb and Sm-Nd), on uranium mineralized rocks along the albitite line in the North Delhi Fold Belt (NDFB, Rajasthan, India) are presented. The Pb-Pb ages obtained, on uranium mineralized core samples, from Rohil uranium deposit, Sikar district, Rajasthan (south western part of NDFB), are 836 ± 5 Ma (Pb Sequential Leaching-PbSL, weighted average of 11 isochron ages), 826 ± 9 Ma (Pb-Pb isochron age on uraninite), 842 ± 18 Ma (U-Pb concordia upper intercept age, on uraninite). Similar ages of uraninites were also obtained from areas close to Rohil uranium deposit i.e. Jahaz (823 ± 10 Ma, U-Pb and 826 ± 14 Ma, Pb-Pb) and Ghateshwar areas (834 ± 14 Ma, U-Pb concordia upper intercept age and 826 ± 25 Ma, Pb-Pb isochron age). The ages of uranium mineralisation (PbSL) further in northeastern part of NDFB i.e. in Gorir area, Jhunjhunu district, Rajasthan, are 827 ± 7 Ma, 818 ± 24 Ma (PbSL) and in Dhancholi area, Mahendragarh Dist., Haryana, are 824 ± 12 Ma (U-Pb) and 824 ± 6.5 Ma (Pb-Pb). The above age data in general indicates age of uranium mineralization as 830 Ma in the NDFB. These ages are consistent with age of uranium mineralization in albitites from Brazil.

Uranium mineralization in Jahaz prospect of North Delhi Fold Belt, Rajasthan is closely associated with deformed and metasomatized meta-sediments of Ajabgarh Group. Detailed petrographic and electron microprobe studies of bore-hole samples indicate that the host rocks consist of metamorphic mineral assemblage such as quartz-annite-muscovite-hornblende (magnesiohornblende - ferrohornblende) - anorthite–bytownite with almandine - staurolite-corundum- ilmenite. Brittle-ductile deformation is evident by brecciation, micro-folds and fracture filled micro-veinlets of quartz and calcite. Fluid induced alteration products are pennine-albite-phlogopite-orthoclase-sphene-rutile-actinolite-ferropargasite-sericite-calcite-apatite. Two types of albite are prominent, albite-1 formed by alteration of parent metamorphic plagioclase, whereas albite-2 is feathery and found to be in open spaces and formed by precipitation of sodium rich fluid. Sulfide minerals (pyrrhotite-chalcopyrite) are most common in albitized rocks and commonly associated with calcite. The above studies clearly indicate the geological processes responsible for the formation of metamorphic-metasomatic mineral assemblages in host rocks associated with uranium mineralization.

Geological and radiometric studies of outcrops aided by extensive subsurface exploration through drill holes in an otherwise soil-covered terrain revealed the existence of low grades, medium tonnage, and metasomatite types of polymetallic uranium deposits at Rohil in India. Microscopic studies, electronprobe micro analyses, and geochemical analyses of samples from lodes indicate the polymetallic nature of mineralisation involving copper and molybdenum, in addition to uranium. Wide variations in the composition of fluid (S-, F-, P-, and O-rich) led to the formation of sulphides, fluorite, U-phosphosilicate, quartz, and magnetite, respectively, and are associated with uraninite. Litho-geochemical analyses from the Rohil deposit indicate multifarious metasomatic alterations associated with polymetallic mineralisation occurring in veins. The major mineralogical and metasomatic controls on rock compositions and the extent of material transfer processes that influenced the host rocks and mineralisati on are quantified by molar element ratio studies and alteration plots. General element ratio (GER) diagrams on chemical analyses of rock samples reveal albitisation and chloritisation as major and microclinisation, sericitisation, carbonatisation, and silicification as minor wall rock alterations associated with ore mineralisation. The alteration box plot between the chlorite–carbonate–pyrite index (CCPI) and the Ishikawa alteration index (AI) indicates the influence of hydrothermal activity and dominance of both albitisation and chloritisation. The ore zone is controlled by meso- and microstructures and the geometry of the soda- and potash-metasomatised zone around hydrothermal veins. This zone contains several anastomosing mineralised veins defined by a prominent joint that is set in quartzite that strikes subparallel to the axial surface of the F2 isoclinal folds and the pervasive schistosity S1 in the quartz–feldspar–biotite schist. Aventurisation of albite and microcline, established through electron probe micro analyses, can be considered as a pathfinder for uranium mineralisation. The close association of uranium and metallic sulphide mineralisation with microstructural, mineralogical (albitisation, chloritisation, and microclinisation), and geochemical variations can be applied as suitable exploration guides in a similar geological set-up worldwide.