Stable isotope studies of paleokarst-hosted uranium deposits in China

摘要

The Devonian-Carboniferous carbonate rocks in South China host several important uranium deposits, which are characterized by their occurrence in solution collapse breccias and by the mineral association of pitchblende, coffinite, carbonates and Fe, Pb, Cu, Zn, Ni sulfides. The deposits represent an economically new, important uranium deposit type in China. The results of sulfur, carbon, oxygen and hydrogen isotope studies on fluid inclusions, carbonate and pyrite from three typical deposits of this type, i.e., Sanqilinyi, Sanbaqi and Saqisan, provide information concerning the nature of the fluids and sources of sulfur and carbon responsible for uranium mineralization. The delta(14)S values of pyrite from the ores and host carbonate range from 1.0 to -39.8 parts per thousand and from 8.2 to -38.1 parts per thousand, respectively. Depletion in S-34 and a wide range of the delta(34)S values for sulfide indicate that the sulfur in both the ores and host carbonates is biogenic. The delta(13)C values of calcite from the ores range from 2.9 to -4.4 parts per thousand, which are similar to those of the host carbonate, ranging from 3.3 to -1.8. The majority of delta(13)C and delta(18)O values of carbonate from the deposits lie within the range for carbon and oxygen isotope compositions that exist in marine sedimentary carbonate. The similarity between the sulfur and carbon isotope compositions of the ores and host carbonate indicates that the sulfur and carbon as well as metallic elements in the ores may have derived from the host marine sedimentary carbonate. The delta(18)O and delta D values cf the mineralizing fluids range from 12.6 to -2.1 parts per thousand and from -26.8 to -54.8 parts per thousand, respectively. The oxygen and hydrogen isotope data demonstrate that the mineralizing fluids were derived from metamorphic water and were mixed with shallow groundwater. Such fluids may have been released by faulting and shearing, which also opened the channel ways for circulation of minerarizing fluids and facilitated their mingling with meteoric water. [References: 25]