油气、微生物与砂岩型铀矿形成关系研究——以东胜铀矿床和钱家店铀矿床为例

Sandstone-type uranium deposits are frequently found close to oil fields or uraniferous sandstones contain bitumen or petroleum. However, few evidence has been presented to indicate the association of uranium mineralization with petroleum oxidation. Thus, Dongsheng uranium deposit in Ordos Basin and Qianjiadian deposit in Kailu Basin are taken for examples to solve the puzzle. Integration data from sedimentary petrology, mineralogy, race elements geochemistry, isotope geochemistry and organic geochemistry, the uranium and petroleum sources, and diagenetic paragenesis of the host sandstone are analyzed, and then the genetic relationship between microbes, petroleum and uranium deposits are discussed. The observation under microscope shows that the host sandstone samples from Middle Jurassic Zhiluo Formation in the Dongsheng deposit contained different kinds of metamorphic rock fragments, which should have been derived form outcrops north to this basin. The LREE/HREE ratios of gneiss and amphibolite sampled from outcrops were close to the highest and the lowest LREE/HREE ratios of the sandstones with well-compared chondrite-normalized REE patterns, respectively. So these results consistently indicated that parent rocks of sandstones were mainly contributed from these two kinds of metamorphic rocks. There was very high Th/U ratio for granite gneiss, which was a mainly potential U resource. Hydrocarbon inclusions and adsorbed hydrocarbons are observed under fluorescence microscope in the host sandstone of Dongsheng uranium deposit, suggesting that the sandstones may have been utilized as oil migration pathways. Based on biomarker parameters, it is indicated that the inclusion oils and adsorbed hydrocarbons were marginally mature to mature, and were derived from humic-sapropel type organic matter under poor reducing freshwater to semi-saline environment. The features are similar to those of organic matter extracted from Triassic sandstone and source rock, but are different from that of cretaceous sandstone. Thus, it can be concluded that the inclusion oils and adsorbed hydrocarbons were mainly derived from Triassic lacustrine facies source rock. Observation results under Scanning Electron Microscopy and Electron Microprobe with Energy Spectrum Analysis show that, in Dongsheng area, the main uranium ore mineral is coffinite. The coffinite is intimately intergrown or coexists with pyrite and calcite, thus, the solution during mineralization stage is inferred to be alkaline. The alkaline environment is not favored for uranium to be pre-concentrated by absorption, and then be reduced abiogenetically. δ34S of pyrite and δ13C of calcite indicate that pyrite was formed by bacterial sulfate reduction (BSR) and part of the carbon of calcite has been dirived from oxidation of petroleum, respectively. Additionally, petroleum is found biodegraded. All the lines of evidence consistently indicate that petroleum was involved in uranium mineralization. Coffinite with microbe-like structures is found in the high U sandstone samples and is composed of nanoparticles, indicating the coffinite is biogenic. The conclusion are also supportted by laboratory experiment studies, which have shown that SRB are capable of utilizing U(VI) as the preferred electron acceptor for respiration and reduce U(VI) to U(IV) directly, coupled the oxidaton of organic matter and sulfate reduction. Based on the research results mentioned above, in the Dongsheng area, coffinite is likely to have formed by mixing of brine containing petroleum derived from Triassic with uranium-bearing meteoric water from outcrops north to Ordos Basin. SRB utilize hydrocarbon as carbon source, and directly reduce U(VI) resulting in precipitation of coffinite. The product of metabolism, H2S and CO2, was precipitated as pyrite and calcite during mineralization stage. Petroleum in fluid inclusions and adsorbed type in host sandstone from Lower Cretaceous Yaojia Formation in Qianjiadian uranium deposit, Kailu Basin, are derived from Jurassic Jiufotang Formation in this basin and the uranium mineral consists mainly of pitchblende. The δ34S and δ13C values of pyrite and calcite during mineralization stage indicate SRB have likely degraded petroleum, which is similar to that of Dongsheng deposit. The alkaline environment as indicated by the diagenetic mineral assemblage calcite, Fe dolomite, pyrite and pitchblende deposit suggests that U ore in the Qiangjiajiadian has a similar origin, i.e., direct reduction by SRB. However, less part of pitchblende is intergrown with kaolinite, suggesting the solution during mineralization stage is acidic. The environment is favorable for U(VI) to be adsorded on quartz or other mineral, and then reduced by H2S produced by SRB. Thus, it can be concluded that U(VI) reduction with petroleum oxidation by SRB and other microbes is an important ore-forming mechanism in petroleum-related sandstone-type uranium deposits. The finding is significant in that it provides a theoretical basis for exploration of both uranium and petroleumr.

塔里木盆地柯坪-巴楚露头区白云岩成岩作用机理研究

Through field outcrop dolomite observation, laboratory petrography (macroscopy, microscopy, cathodeluminescence and scan electronic microscopy), geochemistry (carbon-oxygen-strontium isotopes and trace elements) and fluid inclusion microthermometry study in Keping-Bachu area of Tarim Basin, it can be inferred that there are existing eight dolomite texture types within four evolution phases in Keping-Bachu area of Tarim Basin. The paragenesis of different dolomite texture types and associated minerals in Keping-Bachu area has been established. The carbon and oxygen isotopes of saddle dolomites and matrix dolomites overlap greatly. The Strontium isotopes results of Keping-Bachu outcrop area show that the strontium isotopes differentiation of the matrix and saddle dolomites is not obvious, the reason of which is that there is thousands of Cambrian-Ordovician dolomite strata below the stratum bearing the saddle dolomite. In the process of the heat flow upward migration, the isotopes of the heat interacts with the host rock, which leads to the similarity betwwen the strontium of the saddle dolomite and matrix dolomite. The strontium isotope of the saddle dolomite is not very radiogenic. the six types samples within four phases in the study area show Eu negatively. Comparing to the other types of samples, the δEu of saddle dolomite is relatively high falling into the range of 0.510-0.874, which shows that the saddle dolomite forms in the hydrothermal setting and is affected by the hydrothermal activity to some extend.The Lan/Ybn of saddle dolomite is high up to 15.726, which means that the HREE is very rich. It belongs to the typical hydrothermal genesis model. The δCe of saddle dolomite is positive anomaly, which is the result of high effect from the land source debris. The homogeneous temperature of the saddle dolomite falls into two ranges 110-120℃ and 125-160℃, after pressure correction, they are 141-152℃，157.5-196℃, the salinity of the saddle dolomite can reach to 20-25%. With the comparing with the burial history, the Th of the saddle dolomite is high than the ambient strata temperature, these data show that the saddle dolomite is of hydrothermal origin. The evolution trend of different dolomite and associated minerals is from matrix dolomite, dolomite cementation, saddle dolomite, quartz to calcite. Alonging with this evolution trend, the temperature of the diagenetic flow initiated from 80-100℃, after rising to 135-160℃, then gradually declined. Finally, a structurally-controlled dolomitization model is established in Keping-Bachu area of Tarim Basin.