Table of Content

20 December 2009, Volume 15 Issue 4

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Article

Discovery of Impact Ejecta from Taihu Lake Impact Crater

WANG He-nian, XIE Zhi-dong*, QIAN Han-dong

2009, 15(4):  437-444. 

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The hypothesis of impact origin of the Taihu Lake was proposed about twenty years ago, but never confirmed.Recently some weird-shaped concretions were found embedded in the mud layer in the vicinity of the Taihu Lake, which might be impact ejecta from the Taihu impact crater. These new samples comprise two categories. One is rich in iron with fine siderite as concretion matrix, including abundant small spheroid concretions, stick-shaped concretions, and irregular shaped concretions.The other one is poor in iron with calcite as concretion matrix, including twisted-shaped concretions. The fragments embedded in matrix mainly consist of angular-sharp-edged quartz grains, and minor fragments of clay and plagioclase minerals. The sizes of the ejected materials range from centimeters of massive rocks to millimeters even to micrometers of the spheroids. Most hand samples show abundant semi-plastic features, such as twisted shape and molten crust, which might be caused by their flying in the air in the molten or semi-molten status. All these features suggest that these new samples once experienced a series of the impact cratering processes, such as the shock-induced fragmentation, shock-induced melting, ejection into the air, flying in the air, and falling in and around the impact crater. The components of the ejected materials are consistent with the target rocks in the Taihu Lake area. The confirmation of impact ejecta is another important discovery after the discovery of the shock-induced microfeatures in deformed quartz in the sandstone of islands in the Taihu Lake. The discovery of the ejection materials, combining with other features, confirmed the impact origin of the Taihu Lake.

A Study on Formation Mechanism of Gullies on Mars Based on the Fractal Theory

XU Da-liang1, ZENG Zuo-xun1,2, YUE Zong-yu3, ZHANG Zhen-fei4, YAN Dan1

2009, 15(4):  445-452. 

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It is one of the most important challenges to research the controlling factors and formation mechanisms of Martian gullies at impact crater walls and sequentially to explore the geological processes near the surface of Mars. In this paper, the fractal theory was used to study the gully's alcoves in different orientations of nine craters at the mid-latitudes in the southern hemisphere of Mars. The results suggest significant positive correlation between the fractal characteristics of the gullies and the maturity of their development. The fractal values of the gully's alcoves have close relation to the orientations of gullies and the poleward facing gullies have higher fractal values than those of equatorward facing gullies. The local differences of climate characteristics in different orientations of the same crater walls mainly control the existence and amount of water, which causes the differences of the maturity and fractal values of gullies in different orientations of the crater walls. The results of our research provide one of the evidences for the accumulation/melting of surface snowpacks formation model.

Some Neoproterozoic Geological Events Involved in the Development of the Jiangnan Orogen

ZHOU Jin-cheng, WANG Xiao-lei, QIU Jian-sheng

2009, 15(4):  453-459. 

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The characteristics of the komatiitic basalts, quartz-keratophyre and rhyolitic tuff formed in the subducted stages of the oceanic crust as well as the two kinds of basic rocks formed in the post-orogenic extensional stage in the development of the (middle-)western Jiangnan orogen are reported in the paper. The kamatiitic basalts are considered to be the high-MgO basalts formed in island-arc settings. Their high Ni and Cr contents suggest that they were the primary basalts derived from the sub-arc mantle.Their high MgO contents indicate that the basaltic magmas were extracted from its mantle source under relatively dry conditions.The two subtypes of the mafic rocks formed in the post-orogenic extensional stage might be derived from the partial melting of the asthenospheric and lithospheric mantles， respectively， and show contrasting geochemical features. U-Pb zircon dating data for samples of the mafic and acidic rocks as well as sandstones hosted in the Neoproterozoic basement strata in the western Jiangnan orogen reveal that deposition of the terrigenous fragments in the island-arc area took place between 872 Ma and 835 Ma. The magmatism with arc signatures occurred from 878 Ma to 822 Ma. Therefore, we believe that there existed coeval arc magmatism and sedimentation along the southeastern margin of the Yangtze Block during the Neoproterozoic.

Application of Meta-Analysis to Deep Prospecting Predictions:An Example from the Tongshan Copper Deposit, Tongling, Anhui Province

PENG Sheng-lin1, WANG Ying1,2, SHAO Yong-jun1, XIAN Cheng1,LAI Jian-qing1, WANG Xiong-jun1, ZHANG Jian-dong1

2009, 15(4):  460-469. 

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A new concept of effectiveness was introduced in this paper to evaluate the prediction effect of a detection technology so as to acquire the optimization and integration of technologies for ore exploration. Meta-analysis, initially used in medical science, is an unconventional statistical method that makes comprehensive analysis and statistics on various results of researches on the same subject with specific constraints. In this paper, the principle and feasibility of the Meta-analysis was first studied and then, as a case study, applied in metallogenic predictions in Tongshan copper deposit by quantitative comparison on the effectiveness of prediction of 2-D inversion sections from three geophysical techniques, i.e. CSAMT, EH4 and TEM by Metaanalysis.It is shown that CSAMT and EH4 are much more effective than TEM, though there is no apparent difference in the prediction effectiveness between CSAMT and EH4.

Formation of Pyrite Framboids in the Chamber of Foraminiferas and Its Geological Significance: A Case Study of the Foraminiferas Fossils in the Qixia Formation in the Yanmenkou Area, Hubei Province

LI Hong-xing1??LU Xian-cai1??BIAN Li-zeng1??XU Wei-wei1??LI Juan1 ZHANG Zhuang-zhi2??ZHAO Hua-ping3??GONG Hong-liang

2009, 15(4):  470-476. 

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Abundant benthic foraminiferas, identified mainly as Nodosaria, Pachyphloia and Langella, were discovered in the muddy limestone of the Qixia Formation in Yanmenkou area, Jingshan County, Hubei Province. Most of the foraminiferas chamber were filled by pyrite framboids. Systematic microscopic, scanning electron microscopic observations and energy spectrum analysis of pyrite were carried out. The maximum diameter of pyrite framboids is 15 μm, and the average is about 10 μm. The sulfur to iron atom ratio (S/Fe) is 1.90 ± 0.10. All these features indicate that these chamber were formed under a weak to moderate dysoxic micro-environments. Based on the discussion of the formation mechanisms of pyrite framboids, it is thought that these pyrite framboids in foraminiferas chamber have restricted significance in the study of deposition and diagenesis environment.

A Preliminary Study on the Space Distribution Regularity of “Increasing Distance Pattern” of Mineralized Veins and Its Structural Relation:Taking Nanzhuhe tin deposit in Dupangling area, South China as an example

XIE Hui1,2, TIAN Jing-chun1, ZHANG Gui-lin2, ZHU Ying-tang1, LU Xiao-ping3,HUANG Shou-wei3, HU Fang4, YANG Chen-yu1

2009, 15(4):  477-484. 

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The Nanzhuhe tin deposit in Dupangling area is developed in the endo- and exo-contact zones of a granite body. The mineralized veins are mainly controlled by the fracture syetem. Through a combined study of both ground surface and deep structures, this paper found that the mineralized veins of this deposit have a“ increasing distance pattern” in space distribution:namely, all the intervals between mineralized veins within a vein group in the crosswise direction, the intervals between vein groups in the crosswise direction, and the intervals between the mineralized blocks within a mine district in the longitudinal direction, have a regularity of progressively increasing distance pattern. This regularity is also valid for the Furong and Jiumao-Liuxiu tin ore-fields etc. in the Nanling metallogenic belt and of significance for exploration work in the study area and the Nanling Mountains region as a whole. The relation of this regularity with fracture structure development is also discussed in this paper.

Lala Iron Oxide-Copper-Gold Deposit in Sichuan Province:Evidences from Mineralography

ZHU Zhi-min1, 2, ZENG Ling-xi2, ZHOU Jia-yun2, LUO Li-ping2, CHEN Jia-biao2, SHEN Bing2

2009, 15(4):  485-495. 

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The Lala copper deposit of Sichuan is one of the important large deposits in SW China not only for Cu, but also for considerable economic amount of Au-Mo-Co-REE-Fe. Systematic ore mineralogy research shows that ore minerals are mainly composed of hydrothermal magnetite, chalcopyrite and molybdenite. The wall-rock alterations include biotitization, silicification,carbonatization, albitization, potash feldspathization, apatitization, actinolitization, fluoritization, etc. The ores have euhedralsubhedral-allotriomorphic crystalline grained texture, metasomatic relict texture, poikilitic texture, and dendritic texture, and have disseminated, banded, vein-stockwork and breccia structures. Copper is mainly found in chalcopyrite, cobalt mainly in pyrite and marcasite, molybdenum mainly in molybdenite and gold mainly in chalcopyrite, pyrite and pilsenite as native or silverbering native gold. The contents of Ti in magnetite and S in ore are low, with a very small amount of Pb-Zn sulfide minerals.Consequently, the Lala copper deposit is a typical iron oxide-Cu-Au (IOCG) deposit.

Electron-Microprobe Compositions and Genesis of Beryls from the Nanping No. 31 Granitic Pegmatite (Fujian Province, Southeastern China)

RAO Can1,2, WANG Ru-cheng1, HU Huan1

2009, 15(4):  496-505. 

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The Nanping granitic pegmatite is one of the most important rare-metal deposits in China. In this pegmatite field the No.31 granitic pegmatite is the most highly evolved and well mineralized one, where beryl is the most important beryllium mineral. The beryls were studied in this paper by using the electron microprobe analyses and back scattered-electron imaging. The results show that the beryls in this pegmatite can be distinguished into primary beryl and late beryl. The primary beryl crystals were crystallized early from the pegmatitic magma in association with rock forming minerals such as quartz, muscovite, albite and spodumene.The late beryl is typically associated with the primary beryl, and was either crystallized from the Be-rich hydrothermal fluids, or the replacement product of Cs-rich primary beryl by hydrothermal fluids along its cleavages, fissures or porosities. The electronmicroprobe results reveal that the primary beryl is also enriched in Fe, Mg, Na, Cs, and contains up to 0.28% FeO, 0.89% MgO, 1.36% Na2O and 3.92% Cs2O. Overall, in the primary beryl crystals from zone I to zone IV, the contents of FeO and MgO decrease, butcontents of Na2O and Cs2O increase. Especially, the contents of Cs2O in the primary beryl show wide variations. Some primary beryl crystals are found in spodumene in zones III and IV, and are enriched in Fe and Mg. The late secondary beryls with low contents of Cs2O are attributed to crystallization of pullucite and nanpingite. There is possibly a re-equilibrium between the late hydrothermal fluids and the Cs-rich beryl.

Trace Elements Features of the Vein Quartzs, Arsenian Pyrites and Their Fluid Inclusions in Jinfeng (Lannigou) Gold Deposit, Guizhou Province, China

HU Ying1, CHEN Mao-hong1, 2, DONG Qing-ji1, HUANG Qing-wen1

2009, 15(4):  506-516. 

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Jinfeng (previously known as Lannigou) is the largest known Carlin-type gold deposit in the Yunnan-Guizhou-Guangxi triangle area of southwestern China. Arsenian pyrite is the most important Au-bearing mineral, and silicification with quartz veins is the well-marked hydrothermal alteration. The trace elements of arsenian pyrites, vein quartzs and fluid inclusions in these minerals were analyzed by ICP-MS in this paper. The results show that in contrast to wall rocks and upper crust, the arsenian pyrites are enriched in chalcophile elements, but depleted in V，Sr and HFSE. The syngenetic pyrites bear the similar features.The vein quartzs have high lithophile elements contents, as well as chalcophile elements, Mo and Bi. However, compared with the upper crust, they are poor in most elements. The trace elements in fluid inclusions are usually lower than in corresponding hosting minerals, but with consistent variation features. Combining with the REE data of corresponding minerals and inclusions, it is clear that the arsenian pyrites, vein quartzs and fluid inclusions in these minerals are depleted in HFSE, with the Th/La and Nb/La ratios less than 1, suggesting a Cl-rich character of the ore-forming fluids. The Co/Ni ratios of arsenian pyrites and fluid inclusions are also lower than 1, indicating a lower ore-forming temperature. Consequently, it is inferred that the ore-forming fluids were mainly derived from the sedimentary formation water of the basin.

Practical and Fast Velocity Analysis by Multiple Pre-Stack Time Migration

YE Jing-yan1, ZHENG Hui1, LU Jun1, LI Qing2

2009, 15(4):  517-521. 

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A practical and fast approach is presented to update the migration velocity model for improved imaging. It is achieved by analyzing multiple migration sections in the control lines by scanning the migration velocity from 80% to 120% of the best available velocity model. The different percentages of the velocity model produce reasonable or unreasonable imaging sections with varying degrees of clarity. The correct migration velocity can be estimated by integrating the right percentage of the velocity model, which produces the greatest improvement. Applying this approach to real data shows that the estimated migration velocity reliably converges to the correct velocity. The consumed CPU time of the multiple migration is not too much, because multiple travel-time calculations for the different percentages of the velocity model are unnecessary.

The 2D Velocity and Density Structure of the Mesozoic Sediments in the Chaoshan Depression

RUAN Ai-guo1,2, NIU Xiong-wei1,2, WU Zhen-li1,2, WU Zhao-cai1,2, XUE Bin1,2

2009, 15(4):  522-528. 

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The wide angle seismic profile OBS2006-3 crossing the Dongsha Rise and the Chaoshan and Chaonan Depressions of South China Sea was completed by the Second Institute of Oceanography, SOA in the autumn of 2006. Based on the inversed velocity model of the whole profile, a more accurate 2D tomography imaging was carried out in this paper for a smaller model and only refracted waves recorded by seven OBS stations in the Chaoshan and Chaonan Depressions were used. The imaging results show that there are three sediment layers in the depression area. The thicknesses of the first two layers of the Cenozoic sediments are very thin, less than 2 km with velocities of 2.2 km/s and 3.6 km/s, respectively. In the Mesozoic sediment layer the velocity increases gradually from 4.4 km/s at the top to 5.4 km/s at the bottom with a maximum thickness of 8 km and can be considered as a horizontal sub-layer. Based on the gravity simulation of OBS2006-3, the average density of the Mesozoic layer in the Chaoshan and Chaonan Depressions is 2.45 g/cm3. The average densities of the profile OBS2006-3 in the crust, the high velocity layer and the upper mantle beneath Moho are 2.86 g/cm3, 3.05 g/cm3 and 3.32 g/cm3, respectively.

The Structural Styles of Different Structural Layers in the Western Part of North Tarim Uplift and Their Genetic Mechanisms

CHENG Hai-yan, LI Jiang-hai, ZHAO Xing

2009, 15(4):  529-536. 

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The Western part of North Tarim Uplift underwent several phases of tectonic evolution. Three structural layers were formed including Sinian-Paleozoic, Mesozoic-Paleogene and Neogene-Quaternary structural layers. The thrust faults, folds,strike-slip faults and magmatic diapir structures were formed in the lower layer; the negative inversion and normal faults were developed in the middle layer; while in the upper layer, the thrust faults were developed in the Yingmaili region, and the negative inversion structures were developed in the Yaha region continuously. The relations of the structures of different layers are diverse in different places. The structures of the middle and upper layers inherited those in the lower layer in the Yaha region. The deep thrusts extended upwards and controlled the strike and the motion of the upper faults; however, in the Yingmaili region, the gypsum-salt beds separated the structures of different layers.

Noble Gas Geochemistry of CO2 Gas Pool in Gaoqing-Pingnan Fault Zone,Jiyang Depression

SHEN Bao-jian1,2, QIN Jian-zhong2, HU Wen-xuan1, HUANG Zhi-long3, WANG Jie2

2009, 15(4):  537-546. 

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In the surroundings of the Gaoqing-Pingnan fault zone five CO2 gas reservoirs are developed. Based on the data on compositions, isotope geochemistry and CO2/3He of the reservoirs, it is shown that CO2 distributed in the Gaoqing area was mostly originated from the mantle-sourced inorganic material associated with magmatic rocks. The 3He/4He ratios range in 2.02×10-6~8.34×10-6 and 40Ar/36Ar ratios range in 628.9~3178. The higher 3He/4He and 40Ar/36Ar ratios in CO2 gas pools prove a major mantle-magmatic origin of the noble gases, with a part of crustal source gas mixed. The average 4He/40Ar* of 2.15 shows a slow degassing rate of magma. Its formation mechanism can be interpreted as follows: During the magma rising, the pressure and te perature would decrease, the magma composition would change, and the solubility of CO2 and noble gases in magma would also decrease. Consequently, the CO2 and noble gases could be separated from the magma, and large amount of CO2 and rare gases could be transported to the various stratigraphic traps along the deep faults and some second- and third-order faults.

Argillaceous Hydrocarbon Source Rock Density Separation and Dstribution of Oganic Carbon

YE Wen-qing1, CAI Jin-gong1*, FAN Fu1, BAO Yu-jin2, XU Jin-li2

2009, 15(4):  547-556. 

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11 samples of Tertiary argillaceous hydrocarbon source rocks in the Dongying Depression were selected and separated into three fractions according to density difference (＜1.6 g/cm3, 1.6-2.2g/cm3 and＞2.2 g/cm3) after crushing them in order to extract the free organic matter, inorganic minerals and organo-clay complex from the argillaceous hydrocarbon source rocks (labeled as 1, 2 and 3, respectively). The quantities and TOC of different fractions were tested and observed by scanning electron microscopy to explore the differences of the occurrence forms of organic matter and the modes of organic carbon distribution in different density fractions. After the density separation, the recovery rate of the quantity and the organic carbon of different fractions approximately reached 100%. These features show credibility of the density separation method and less destruction to hydrocarbon source rocks. Through the observation of each fraction by scanning electron microscopy, we found existence of a large number of biological debris in the fraction l. After hydrochloric acid treatment, the surface of the biological debris became clearer in the fraction l, while in the fractians 2 and 3, a large number of mineral grains can be observed. The energy spectrum analysis shows existence of element carbon. These features indicate that the fraction 1 is mainly the organic matter of biological debris, while the fractions 2 and 3 are the organic matter mixed with the inorganic minerals. Through analysis of the TOC and the amount of organic carbon of different fractions, we found that the average TOC content of the fraction 1 is about 14.24 %, but it occupies only about 1.11% of the total amount of organic carbon in the wholerock samples. The TOC values of the fractions 2 and 3 are 3.26% and 2.77 %, respectively, totally occupying more than 90% of the tatal amount of the organic carbon in the whole rocks. From this point of view, the mode of combination with the minerals is the most important occurrence form of organic matter in the hydrocarbon source rocks. This part of organic matter for the hydrocarbon-generating capacity of whole rocks can not be ignored.

Application of the Relative Free Swelling Ratio Testing of Clay Minerals in Sandstone Reservoirs Evaluation in Shengli Oil Field, China

WANG Bei-zhan1,2, LU An-huai3, LI Hong-nan4, WANG Lin5, XU Shou-yu4

2009, 15(4):  557-562. 

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The water sensitivity of clay minerals is one of the primary causes of reservoir damage in oil fields. As a new testing method of expansive soil, the relative free swelling ratio is defined as the ratio of the equilibrium sediment volume of 10 g ovendried soil in distilled water to that in coal soil (or CCl4). This paper applied this new testing method in swelling property analysis of clay minerals in sandstone reservoir. The relative free swelling ratios of 27 sandstone core samples with argillaceous contents of 10~50% in Shengli Oil Field were measured, the clay minerals contents were also identified by X-Ray, and then the positive correlation model between montmorillonite contents and free swelling ratios of sandstone reservoirs was established. The results showed that the free swelling ratio measurement can more directly and accurately describe the free swelling properties of clay minerals in sandstone reservoirs, and quantitatively reflect the potential swelling damage of reservoirs caused by water sensitivity of clay minerals, and consequently can be applied in the development of clay antiswelling agents for reservoirs, which is important and significant for adjusting the development plans of oil fields.

Grain-Size Characteristics and Climatic Changes of a Paleosol Sequence at Fenghuang Mountain in Chaoyang, Liaoning Province

CHEN Hui??WANG Qiu-bing??HAN Chun-lan

2009, 15(4):  563-568. 

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Based on the grain-size analysis of the paleosol section at Fenghuang Mountain in Chaoyang, Liaoning Province, the percentages of the coarse grains, fine grains and supper-fine grains were used as indicators of winter monsoon, westerly wind and summer monsoon strengths, respectively. The results show that the winter monsoon, westerly wind and summer monsoon change synchronously. The westerly wind and summer monsoon have the same variation trend, and there are 4 enhanced stages and 4 decreased stages plotted. The winter monsoon has the opposite variation trend. The climate was warm and wet at the following stages: 71-148 ka BP, 176-208 ka BP, 225-243 ka BP, and 311-403 ka BP；and the climate was cold and dry at the following stages: 148-176 ka BP, 208-225 ka BP, 243-311 ka BP and 403-423 ka BP.

Numerical Solutions of Two-Dimension Fractional Advection-Dispersion Equations

ZHOU Lu-ying1, WU Ji-chun1,2, XIA Yuan1

2009, 15(4):  569-575. 

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In this paper, two numerical schemes were developed for both two-dimensional temporally and two-dimensional spatially fractional advection-dispersion equations and their numerical solutions were achieved. We analyzed the variation of diffusion with the fractional order by applying the numerical scheme in a test case and verified the temporal and spatial correlation. Then we compared the calculation results of our new schemes with the solution of traditional two-dimensional advection-dispersion equation. When the fractional orders are integer, the calculation results of both two-dimensional temporally and two-dimensional spatially fractional advection-dispersion equations are the same as that of the traditional integer order advection-dispersion equation. These indicate that the numerical schemes for the two-dimensional fractional advection-dispersion equations developed in this paper are feasible.