Paragenesis of Uraninite and Associated Ore Minerals in the Precambrian Oligomictic Conglomerates from Walkunji, South Kanara District, Karnataka, India

Mineragraphic study of the Early Proterozoic uraniferous oligomictic quartzpebble conglomerate of Walkunji reveals 'three types of uraninites, designated as U-I, U-II and U-III respectively. Textural and mineralogical characteristics indicate . that (i) U-I with plenty of filled-in fractures is subrounded to rounded in shape, detrital, allogenic and oldest in the sequence; (ii) U-II and U-III are authigenic with fine-grained U-II, occurring as fillings in gangue minerals, and appear to be hydrothermal; and (iii) colloform-textured U-III (pitchblende) is the youngest formed at low temperature due to remobilization of earlier phases. In addition, a little detrital, partially altered brannerite occurs with anatase.

Among the sulphides, melnicovite-pyrite and pyrite-marcasite are the most dominant followed by galena pyrrhotite and marcasite. The oxide sulphide mineral assemblage suggests that mineralization was primarily strata-bound in the initial stages followed by hydrothermal phase and ended with remobilization that resulted in the formation of U-lIl. The textural relationships among different ore minerals indicate the following paragenetic sequence: U-I, brannerite, pyrite, melnicovite-pyrite and gel. pyrite (oldest); U-II, arsenopyrite, pyrrhotite-marcasite, chalcopyrite, galena, sphalerite and molybdenite; and U-lIl, gel-pyrite and marcasite (youngest).