Stratigraphy and Nature of uranium mineralization from Precambrian Basement Granitoid -Srisailam Formation contact around Chitrial area, Cuddapah Basin, Telangana

Abstract

The biotite rich granitoid rocks exposed around Chitrial village varies in character from a porphyritic to massive granite to gneiss or mylonite with characteristic foliations defined by alternate quartzo-feldspathic and biotite rich layers in the latter units. It is intruded by ca. 1.9-1.8 Ga age mafic dyke sets and is overlain by Mesoproterozoic Srisailam Formation rocks of the Cuddapah Supergroup represented by an arenaceous gritty or pebbly sandstone interspersed with thin shale and siltstone horizons. The granitoid locally becomes uranium-rich near its contact with the overlying Srisailam Formation rocks. The present work encompasses stratigraphic, petrological, geochemical and geochronological analyses of the granitoids and accompanying supracrustals with special emphasis on nature and localization of uranium mineralization. Major element geochemical data characterize the granitoid rocks as monzogranites and alkali feldspar granite and the cover rocks as quartz arenites. The trace and rare earth element data were used to identify the protolith history of the granitoid rocks as well as about the nature of provenance of the cover sediments. The geochemical data further provide clues regarding probable tectonic and geodynamic setting of these rocks. A marked enrichment in U, Th and REE (particularly LREE) content of the granitoids has been noted. The overall REE pattern suggests a similar source for all the granitoid types. Several tectonic discrimination diagrams suggest a volcanic arc tectonic setting for these rocks. The recycled mature quartzose cover rocks show distinctly similar geochemical characteristics as the granitoids suggesting a granitoid/felsic source of mature continental provenance. REE patterns of the basement granite and the cover sandstone show similar variation which represents that the derivation of the sediments could be from the underlying basement granite. Recent exploration programme by Atomic Minerals Directorate for Exploration and Research (AMDER) has led to the discovery of a number of potential radioactive mineralized zones in the northwestern part of the Cuddapah basin such as around the Chitrial area. Uranium bearing minerals are intimately associated with sulphide rich minerals within the basement granitoids of the area. There is ample evidence of hydrothermal activity straddling across the unconformity surface which includes- (1) development of fracture filling veins of various dimensions comprising quartz, quartz-epidote, quartz-chlorite or pyrite, (2) hydrothermal alteration of granitoids adjacent to these veins resulting in chloritization and sericitization and (3) epigenetic uranium mineralization in micro-fractures and inter-granular spaces within granitoids. Evidence of uranium mineralization within the cover rocks is comparatively less. In the uraniferous zones in granitoid and overlying quartzite, pitchblende and coffinite are the main uranium phases occurring in micro-fractures and inter-granular spaces of host rock, often in association with pyrite. Depending upon micro-textural data, the paragenetic history of the mineralization has been divided into seven stages in the present study. U-Pb zircon radiometric dating of the basement granitoids reveals that the main tectonothermal event took place in Chitrial area between ca. 2535 Ma and 2519 Ma. Granitoid samples including the grey massive variety, pink granite, granite gneiss, foliated granite and alkali feldspar granite show emplacement ages of 2525±20 Ma, 2519±12 Ma, 2524±18 Ma, 2514±22 Ma and 2524±20 Ma respectively. Hence, it can be concluded that major tectonothermal event affected these rocks of the study area around 2535 to 2514 Ma. Probability density plot of weighted mean ages for the sample CT206 (granite gneiss) shows a strong peak at ca. 2465 Ma while the sample CT207 (foliated granite) shows another strong peak at ca. 2455 Ma which may be related to a second phase of tectonothermal event. U-Pb zircon detrital age of the cover rocks of the Chitrial area gives major cluster ages at ca. 2468 and 2488 Ma that may be correlated with this second tectonothermal event. From these rocks, diagnostic detrital zircons show age peaks at ca. 2520 Ma, 3000 Ma and 3200 Ma, which correspond to the established emplacement ages of the basement granitoid plutons in the Eastern Dharwar Craton. Younger dates are discordant with a lower intercept ages of near 200 Ma in the Wetherill concordia. Older zircon cores with spot data ranging from 2636±28 Ma to 3200±7 Ma are interpreted as grains inherited from the crustal source region or from the wall-rock of the granite intrusion. From the detrital zircon data, it can be inferred that source of the sediments is proximal. EPMA chemical dates of uraninites from the drill core sample 226B shows that the area underwent several episodes of hydrothermal activity, which have left their imprints on the isotope systematics of uraninite. Thus the younger ages furnished by U-Pb zircon radiometric dating of uranium rich in-situ zircon grains of the granite drill core sample (289Av) shows a group age of 172 Ma, possibly related to the much younger tectonothermal event. From this study, it is concluded that the Chitrial granitoids are ‘S’ type in character and formed by intracrustal melting of the deeply buried clastic sediments and subsequent incubational heating. It also implies crustal recycling could be the likely mechanism for granite magmatism during ca. 2535-2514 Ma. Uranium mineralization in the granite was influenced by increased fracture volume in the rocks and was controlled by oxygen fugacity in the ore-bearing hydrothermal fluid. This mineralization is related with the later stage fracture reactivation of the Eastern Dharwar Craton during a major younger tectonic activity.