Organic geochemistry of paleokarst-hosted uranium deposits, South China

The paleokarst-hosted uranium deposits in organic-matter, clay-rich Devonia n-Carboniferous carbonates are an economically important, new type of urani um deposit in China. The organic matter intimately associated with the uran ium mineralization in this type of deposit has been characterized by petrog raphic, isotopic, gas chromatographic, pyrolysis-gas chromatographic, infra red spectroscopic and elemental geochemical methods. Comparing genetic type s of the organic matter in unmineralized and mineralized samples indicates that no fundamental differences are found. The organic matter is chiefly of marine origin and contains a minor terrestrial component. The immature nat ure of the indigenous organic matter in the unmineralized samples shows gen erally a low-temperature history (less than or equal to max. 65 degrees C), and geologic data show a shallow maximal burial depth. By combining the organic geochemistry with the geological data, U-Pb dating and temperature determinations, an overall formation process for this type of uranium deposit is deduced. The formation of the paleokarst-hosted uran ium deposits in South China is the result of: (1) repeated paleokarstificat ions of the Devonian and Carboniferous organic, clay-rich carbonate along t he faults and unconformities between different strata because of the Hercyn ian and Yanshanian regional tectonism, and extensive formation of solution- collapse, solution-fault breccias; (2) accumulation of organic matter and c lays in the paleocaverns and matrix of the breccias, fixation and adsorptio n of uranium by the organic matter and clays from the paleokarst waterflows that leached metals from the uranium-bearing host carbonates during their passage towards the karst zones, (3) reduction of uranium by the organic ma tter and formation of protore and low-grade ore; (4) circulation of heated formational waters and deep circulating, uraniferous meteoric waters by tec tonic pumping, reworking the uranium-rich, paleocave-fillings, protore and low-grade ore, reduction and formation of primary uranium minerals (uranini te and coffinite) because of the reducing environment resulting from organi c matter and sulfide. (C) 2000 Elsevier Science B.V. Ail rights reserved.