内蒙古东胜铀矿床成矿主岩中球状磁铁矿的成因  

Origin of Spherical Magnetite from Host Sandstone of the Dongsheng Uranium Deposit, Inner Mongolia

作　　者：李宏涛[1] 蔡春芳[1] 李开开[1] 罗晓容[1] 

机构地区：[1]中国科学院地质与地球物理研究所

出　　处：《地质论评》2007年第4期564-570,共7页Geological Review

基　　金：国家重点基础研究发展计划(973计划)项目"多种能源矿产共存成藏(矿)机理与富集分布规律"(编号2003CB214605)的成果

摘　　要：在东胜砂岩型铀矿床中,发现了球状磁铁矿、黄铁矿和方解石胶结物紧密共生的现象。为分析球状磁铁矿成因,进行了主岩岩石学观察、同位素测试及包裹体烃色谱—质谱分析。结果显示,包裹体石油烃具有"鼓包",并检测到25-降藿烷系列,说明油气已被生物降解了。黄铁矿硫同位素低达-19.8‰,形成于细菌硫酸盐还原作用;方解石胶结物碳同位素低至-19.7‰,表明部分CO2来自油气的氧化作用。于是,硫酸盐还原细菌有可能厌氧降解了油气,并用厌氧呼吸还原Fe3+为Fe2+,形成球状磁铁矿、黄铁矿。这一成因联系,对本区铀矿和油气的勘探可能具有启发意义。Spherical magnetite has been found in the Middle Jurassic Zhiluo Formation sandstone in Dongsheng area, Inner Mongolia, in the northern of the Ordos basin. In order to assess its origin, the host sandstone has been analyzed for fluid inclusion oil biomarkers, pyrite δ^14S, calcite δ^13C and observed under scanning electron microscope （SEM） and X-ray energy dispersive spectroscopy analyzer （EDS）. Fluid inclusion oils in the host sandstone are characterized by significant amounts of demethylated hopanes and tricyclic terpanes and existence of unresolved complex mixtures. The features indicate the result of biodegradation. Pyrite has δ^34ScDTvalues as low as --19. 8‰, indicating that the sulfur was originated from bacterial sulfate reduction. Calcite cement exhibits δ^13 CPDB values from --12. 3‰ to --19. 7‰, suggesting that partial CO2 was derived from petroleum oxidation. Biogenic pyrite was found to intimately intergrow with calcite cements with petroleum-derived carbon, suggesting that petroleum was likely oxidized by sulfate reducing bacteria （SRB） and thus supplied a source of the carbon in the calcite. Furthermore, SRB may reduce Fe^3＋ to Fe^2＋ by anaerobic respiration, resulting in magnetite precipitation. These lines of evidence lead to proposal that SRB may have degraded hydrocarbons coupled with Fe^3＋ reduction and thus spherical magnetite formation. The association has important significance likely to uranium ore and petroleum exploration.

关 键 词：东胜铀矿床 球状自生磁铁矿 碳同位素 硫同位素 烃类包裹体 硫酸盐还原菌(SRB) 

分 类 号：P619.14[天文地球—矿床学]