科研之友
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摘要
The sandstone-hosted Qianjiadian uranium deposit, hosted in the Upper Cretaceous Yaojia Formation in the Qianjiadian area, Songliao Basin, NE China, has been studied for over twenty years. However, there exists debate on whether mineralization fluid is low-temperature groundwater in favor of biogenic mineralization or diabaserelated magmatic hydrothermal fluid for this deposit and other adjacent geologically linked sandstone-hosted uranium deposits (Baixingtu, Baolongshan and Huitianzhao deposits) in the North China. This study provided new data from petrographic and geochemical analyses, geochronology of uranium minerals (the EMPA chemical dating method) and petroleum hydrocarbon biomarkers in host sandstones. Two types of host sandstone were recognized. In calcareous sandstones, pitchblende coexists with colloform pyrite and poikilitic calcite cement while quartz and feldspars were extensively corroded. In non-calcareous sandstones, coffinite coexists with colloform pyrite and only feldspars were slightly corroded. This suggests that pitchblende formed in high pH (pH > similar to 9) fluid, while the pH of ore-forming fluid for coffinite was lower (pH = 7-9). Besides, EMPA chemical dating revealed two stages of uranium mineralization. Stage-A occurred in 43-28 Ma with pitchblende as the only uranium mineral species. Stage-B occurred in 19-3 Ma with the formation of both pitchblende and coffinite. The uranium mineralization was biogenic under low-temperature groundwater condition and thus not from a diabase-related mafic magmatic hydrothermal fluid based on the following lines of evidence: (i) some coffinite occurs as phosphorus-rich microorganism-like microspherules structure; (ii) pyrite aggregates were generated from bacterial sulfate reduction as indicated by the S34S values as low as -41.4%o; (iii) calcite cement in calcareous sandstone contains only single phase aqueous inclusions, and have delta C-13 values as low as- 11.2 parts per thousand, indicating that part of the carbon was derived from organic matter oxidation; (iv) the organic matter oxidation is further supported by hydrocarbons extracted from petroleum inclusions within calcite cement, showing occurrence of C-26-C-31 17 alpha, 21 beta 25-norhopanes, typically resulting from heavy biodegradation.
- 出版日期2018-10
- 单位中国科学院地质与地球物理研究所; 中国科学院地质与地球物理研究所兰州油气资源研究中心; 绵阳师范学院; 中国科学院大学; 长江大学
