Geochemical characteristics and environmental implications of loess sequences in the Loess Plateau

The major,trace,and rare earth elements of the Duanjiapo(DJP)section in the south of the Loess Plateau,the Jiaxian(JX)section in the north,and the Jiuzhoutai(JZT)section in the west are studied.The results show that the main elemental characteristics of loess in three profiles are consistent with the upper continental crust(UCC).In terms of trace elements,Th,Nb,Zr,Hf,Y,Cs,W,Cr,V,Li,and Pb show relative enrichment compared with UCC;Rb,Ba,Sr and Be exhibit relative depletion.The average ofΣREE is 171.91ppm with a negative anomaly forδEu and essentially no anomaly forδCe.The results of K_(2)O/Na_(2)O,Rb/Sr ratios,as well as the leaching co-efficient from three profiles,indicate DJP>JX>JZT,suggesting that DJP experiences the strongest weathering leaching effect.The chemical index of alteration(CIA)reveals that all three profiles of loess are in the primary stage of Ca and Na depletion.DJP is generally in the early to moderate stage of chemical weathering,while JX and JZT are both in the early stage.

Accumulation Mechanisms and Evolution History of the Giant Puguang Gas Field,Sichuan Basin,China

Solid bitumens were found throughout the carbonate reservoirs in the Puguang gas field, the largest gas field so far found in marine carbonates in China, confirming that the Puguang gas field evolved from a paleo-oil reservoir. The fluid conduit system at the time of intensive oil accumulation in the field was reconstructed, and petroleum migration pathways were modeled using a 3-D model and traced by geochemical parameters. The forward modeling and inversion tracing coincided with each other and both indicated that oils accumulated in the Puguang-Dongyuezhai structure originated from a generative kitchen to the northwest of the Puguang gas field. The deposition of organic-rich Upper Permian source rocks dominated by sapropelic organic matter in the Northeast Sichuan Basin, the development of fluid conduit system that was vertically near-source rock and laterally near-generative kitchen, and the focusing of oils originated from a large area of the generative kitchen, were the three requirements for the formation of the giant paleo-oil reservoir from which the giant Puguang gas field evolved. The Puguang gas field had experienced a three-stage evolution. The post-accumulation processes, especially the organic-inorganic interaction in the hydrocarbon-water-rock system, had not only profoundly altered the composition and characteristics of the petroleum fluids, but also obviously changed the physicochemical conditions in the reservoir and resulted in complicated precipitation and solution of carbonate minerals.

Autunite-Group Minerals and Their Paragenesis from the Sheared Granite of Gabal El Sela, South Eastern Desert, Egypt

G. El Sela is located in the Southern Eastern Desert of Egypt cropping as two parts, occupied by monzogranites that were categorized as biotite granite, muscovite granite and two mica granites. The northern part is more significant according its high concentrations of uranium that controlled by complicated structure regime;faulting, infrastructures and shearing are the most common structural criteria of this part. The Egyptian Nuclear Materials Authority (NMA) mined this part to produce the uranium ore. The previous mineralogical studies indicated that this granite was dominated by primary uranium minerals (pitchblende and coffinite) and secondary minerals belong to the autunite group (autunite, metautunite, phurcalite) in addition to uranophane. In the present work, petrographic and mineralogical studies are applied for the granites using the polarized and stereo microscopes and followed by electron microscope and XRD. The result of the microscopic examinations revealed the tectonic regime controlling the radioactivity and recognized the sodic autunite (meta-natroautunite) beside the pre-mentioned autunite group minerals completing the paragenetic sequrnce of these minerals. In this study, it is concluded that the sheared biotite granite is monzogranite originated during the episode of the continental plate collision (syncollision). The study finished to presence of two main types of the alteration corresponding to the two high levels of radioactivity (moderate and anomalous). The first is the thermal alteration (saussiritization, sericitization, kaolinization, silicification and hematization) and the second is the chemical transformation (oxidation, dehydration, ion substitutions and confusion) responsible for formation of the secondary uranium minerals. The temperature needed for the thermal alteration is sourced by the hydrothermal solutions, while the temperature needed for the uranium minerals transformation may be generated during the episode of the continental plate collision (syncollision). Paragenesis of these minerals indicates that they represent a series of uranyl phosphate minerals (autunite group) with paragenetic sequence starting by autunite (calcic uranyl phosphate) and ends by meta-natroautunite (sodic uranyl phosphate). An advanced process of dehydration accompanies the process of mineral transformation from autunite to meta-natroautunite leading to formation of the anhydrous uranyl mineral (phurcalite) formed by oxidation and dehydration of autunite. Meta-autunite is recorded as a transitional mineral composed of sodic-calcic uranyl phosphate. Uranophane is created by thermal confusion of autunite with the silica.

Neoproterozoic sedimentary facies and glacial periods in the southwest of Tarim Block

Based on the analysis of sedimentary facies and chemical index of alteration(CIA) in Nanhua and Ediacaran Systems in the southwest of Tarim Block,some features of glacial records in Neoproterozoic become more clear.Six sedimentary facies have been divided in the study area,including alluvial fan facies,lacustrine facies,glacial facies,littoral facies,neritic facies,and lagoonal facies,showing that this area underwent a process from continent to marine,with mainly littoral and neritic sedimentation.Two cold events have been recognized by analysis of CIA values in the study area,called Bolong and Yutang glaciation,respectively.They present as thick-layer tillite deposition in the Bolong Formation and thin-layer tillite deposition in the Yutang Formation,respectively.The Bolong glacial period in the study area can be correlated to the Yulmeinak glacial period in Aksu area,Tereeken glacial period in Qurugtagh area,and the Nantuo glacial period in South China,which is equivalent to the universally acknowledged Marinoan glacial period.The Yutang glacial period can be correlated to the Hankalchough glacial period in Qurugtagh,which is equivalent to Gaskers glaciation in Newfoundland.