Overpressure Evolution Recorded in Fluid Inclusions in the Dongpu Depression,Bohai Bay Basin,North China

The Dongpu depression,like other depressions in the Bohai Bay Basin,is characterized by overpressure,however,the published studies have not clearly given the overpressure formation mechanism and paleo-pressure evolution history in the Dongpu depression.In addition,the current researches are mainly focused on the northern Dongpu depression,while rare investigation has been conducted in the southern part.In this context,it is urgent to explore the comprehensive overpressure evolution and formation mechanism in the Dongpu depression in academic and industrial fields.This paper provided a systematic analysis of fluid inclusions,including fluid inclusion petrography,microthermometry,vapor phase filling degree measurement and PVT-x modeling.By using homogenization temperature-burial history projection,the trapping time of the oil inclusions are determined.Four pressure evolution stages are identified in both the northern and the southern Dongpu depression.The results indicate that the pressure accumulation stage is derived by disequilibrium compaction and hydrocarbon generation in the northern part because of the salt rock distribution,while the stage is only derived by hydrocarbon generation in the southern part because of no salt rock distribution.The pressure release stages are mainly caused by the tectonic uplifting movements at the end of Dongying and Minghuazhen Formation.The overpressure in most areas in the northern part has been preserved till now,which is considered as a result of the good plasticity and sealing ability of salt rock that prevent the pressure leakage by smearing on the fault.Areas without salt rock distribution show normal pressure,which means the salt rock is an important factor that contributes to the overpressure preservation.However,there are exceptions that in Baimiao and Qiaokou,the overpressure still exists even without salt rock distribution,which is due to fault closure.

Mass Transfer during Hematitization and Implications for Uranium Mineralization in the Zoujiashan Deposit,Xiangshan Volcanic Basin

The Zoujiashan uranium deposit in the Xiangshan ore field is the largest volcanic-related uranium deposit in China.Hematite-and fluorite-type ores are the predominant mineralization styles.Hematitization in the Xiangshan ore field is closely associated with uranium mineralization,mainly occurring as hematitized rocks enclosing fluorite-type vein ores developed in pre-ore illitized porphyritic lava.Detailed petrographic and mass balance calculation studies were conducted to evaluate the mechanisms for uranium precipitation and mass transfer during hematitization.Petrographic observations suggest that in the hematitized rocks,orthoclase is more altered than plagioclase,and quartz dissolution is common,whereas in the illitized rocks,pyrite commonly occurs within the altered biotite grains,and chlorite grains are locally found.Mass balance calculations indicate that Na2O and U were gained,K2O,Ca O and Si O2were lost,whereas Fe2O3-t remained more or less constant during hematitization.These observations suggest that the hydrothermal fluids were Na-and U-rich and Ca-K-poor,and the Fe2+used for hematitization was locally derived,most likely from biotite,pyrite and chlorite in the host rocks.The Fe2+is inferred to have played the role of reductant to precipitate uranium,and calculation indicates that oxidation of Fe2+provided by host rocks is sufficient to form ores of economic significance.Consequently,the hematite-type ore is interpreted to be generated by the reaction between oxidized ore fluids and reduced components in host rocks.The development of calcite and pyrite in the fluorite ores suggests that perhaps mixing between the U-rich fluid and another fluid carrying reduced sulfur and carbon may have also contributed to uranium mineralization,in addition to temperature and pressure drop associated with the veining.