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.