Metacolloid pyrite-pitchblende veinlets of high-grade hydrothermal ores at the Dalmatovskoe uranium deposit,Transural Region,Russia: New data on the mineralogy,geochemistry, age,and uranium sources

Micro to nano-scale structures of pockets of solidified uranium oxide gel were found for the first time and deciphered ontogenetically during electron-microscopic mineralogical study of four samples of high-grade uranium ores from the Dalmatovskoe deposit. Mineralogical-geochemical features of paragenesis and structural features are reported for unique hydrothermal veinlets consisting of variably crystallized gelpyrite and subveinlets of gel-pitchblende of several generations. Mineralogical features indicate repeated opening of veinlets during hydrothermal ore formation and provide insight into physical and chemical features of non-solidified gel, its mobility, and aggressive behavior during metasomatism. It was proposed that the veinlets are genetically related to the main hydrothermal sources and possibly to the endogenous uranium occurrence, which was formed owing to opening of main fault marked by paleoriver bed. The find of such veinlets is considered as evidence for repeated influx of hydrothermal uranium in the uranium ores of the sedimentary-epigenetic deposit simultaneously with accumulation and transformation of sediments, which made it possible to ascribe this deposit to the hydrothermal-sedimentary type. The depth of core sampling of high-grade ores corresponds to the “ore zone” of the Dalmatovskoe Deposit, near the Paleozoic basement. For this reason, the entire “zone” was ascribed to the specific endogenous metasomatic epihydrothermal occurrences, possibly, of elision nature, with approximate temperature of ore formation of 150–250°C.