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    2012, Volume 19 Issue 3
    25 May 2012
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    The IUGS/IAGC Task Group on Global Geochemical Baselines
    2012, 19(3): 1-6. 
    Abstract ( 2379 )   PDF (390KB) ( 2181 )  

    The Task Group on Global Geochemical Baselines, operating under the auspices of both the International Union of Geological Sciences (IUGS) and the International Association of Geochemistry (IAGC), has the longterm goal of establishing a global geochemical database to document the concentration and distribution of chemical elements in the Earth’s surface or nearsurface environment. The database and accompanying element distribution maps represent a geochemical baseline against which future humaninduced or natural changes to the chemistry of the land surface may be recognized and quantified. In order to accomplish this longterm goal, the activities of the Task Group include: (1) developing partnerships with countries conducting broadscale geochemical mapping studies; (2) providing consultation and training in the form of workshops and short courses; (3) organizing periodic international symposia to foster communication among the geochemical mapping community; (4) developing criteria for certifying those projects whose data are acceptable in a global geochemical database; (5) acting as a repository for data collected by those projects meeting the criteria for standardization; (6) preparing complete metadata for the certified projects; and (7) preparing, ultimately, a global geochemical database. This paper summarizes the history and accomplishments of the Task Group since its first predecessor project was established in 1988.

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    Global Geochemical Baselines: Understanding the past and predicting the future.
    WANG Xue-Qiu
    2012, 19(3): 7-18. 
    Abstract ( 4553 )   PDF (1493KB) ( 6795 )  

    In  order to recognize  the evolution of the Earth and global changes,  it is necessary to set up  consistent Globalscale Geochemical Baselines as a reference. “Geochemical Baseline” is known as the database and the accompanying  element distribution maps by systematically documenting the concentration and spatial distribution of chemical elements and compounds  on the Earths surface, which can be applied to quantified  the  future humaninduced or natural changes in the chemistry of the present Earth and the past chemical evolution induced by geological processes. China Geochemical Baselines (CGB), as a part of  the Global Geochemical Baselines, is to document both concentrations and spatial distribution of nearlyall chemical elements across  whole China. China Geochemical Baseline Networks  are  designed based on 1∶200000 map sheets of 1°(lng.)×40′ (lat.), approximately 1500 grids covering all over China. Approximately 10000 rock samples for primary lithosphere baselines and 6000 soil samples for secondary pedosphere had been collected across the China land surface. Eightyone geochemical indicators including 78 chemical elements are determined by ICPMS/AES following 4acid digestion and by XRF following fusion as backbone methods combined with other 10 analytical methods. Analytical quality is under strict control by using standard reference samples to obtain harmonious highquality data. In the present paper the author shows  some of the examples for the  use of preliminary results of the spatial distribution of elements to understand the past and to predict the future. Ir geochemical anomalies have been discovered in the suspected Cretaceous and Tertiary (KT) boundary ranging from 0.2×10-9 to 0.8×10-9 whereas Ir concentration baselines in Cretaceous and Tertiary rocks are less than 0.02×10-9. It is likely that the Iridium in the suspected boundary is of extraterrestrial origin, providing us with understanding  the extinction of dinosaurs due to the impact of an earthcrossing asteroid. It has been found that  the dramatic changes of CaO distribution occurred in the past 15 years from 1994 to 2009. The average concentration decreases from 0.53% to 0.41%, the  area of CaO<1% extends from 872000 km2 to 1073000 km2, increased 23%  as compared to that in 1994, due to the leaching of CaCO3 by acid rains caused by human activities.

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    History and Progress of the North American Soil Geochemical Landscapes Project, 2001-2010
    David B.Smith, William F.Cannon, Laurel G.Woodruff, et al.
    2012, 19(3): 19-32. 
    Abstract ( 2387 )   PDF (1123KB) ( 2662 )  

    In 2007, the U.S. Geological Survey, the Geological Survey of Canada, and the Mexican Geological Survey initiated a lowdensity (1 site per 1600 km2, 13323 sites) geochemical and mineralogical survey of North American soils (North American Soil Geochemical Landscapes Project). Sampling and analytical protocols were developed at a series of workshops in 20032004 and pilot studies were conducted from 20042007. The ideal sampling protocol at each site includes a sample from 05 cm depth, a composite of the soil A horizon, and a sample from the soil C horizon. The <2mm fraction of each sample is analyzed for Al, Ca, Fe, K, Mg, Na, S, Ti, Ag, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Ga, In, La, Li, Mn, Mo, Nb, Ni, P, Pb, Rb, Sb, Sc, Sn, Sr, Te, Th, Tl, U, V, W, Y, and Zn by inductively coupled plasmamass spectrometry and inductively coupled plasmaatomic emission spectrometry following a neartotal digestion in a mixture of HCl, HNO3, HClO4, and HF. Separate methods are used for As, Hg, Se, and total C on this same size fraction. The major mineralogical components are determined by a quantitative Xray diffraction method. Sampling in the conterminous U.S. was completed in 2010 (c. 4800 sites) with chemical and mineralogical analysis currently underway. In Mexico, approximately 66% of the sampling (871 sites) had been done by the end of 2010 with completion expected in 2012. After completing sampling in the Maritime provinces and portions of other provinces (472 sites, 7.6% of the total), Canada withdrew from the project in 2010.  Preliminary results for a swath from the central U.S. to Florida clearly show the effects of soil parent material and climate on the chemical and mineralogical composition of soils. A sample archive will be established and made available for future investigations.

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    Analytical scheme and quality monitoring system for China Geochemical Baselines.
    ZHANG Qi, BAI Jin-Feng, WANG Ye
    2012, 19(3): 33-42. 
    Abstract ( 1877 )   PDF (565KB) ( 1248 )  

    According to the related specification and requirements, analytical scheme and quality monitoring system for the analysis of 76 elements and 81 analysis parameters of crust rock and soil samples from the program of China Geochemical Baselines was designed based on the analytical system developed for the 76 elements geochemical mapping. The sophisticated noncontamination sample preparation procedure for rock samples was established, analytical technique with good accuracy was employed for analyzing major elements. The accuracy and precision was improved for major elements. It has been successfully applied in the China Geochemical Probe Program.

     

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    Global geochemical data management: Development of Chemical Earth software.
    2012, 19(3): 43-48. 
    Abstract ( 2083 )   PDF (1017KB) ( 1217 )  

    “Digital Earth” concept was initiated in 1990s; the “Google Earth” as the representative of the “Digital Earth” has become popular. A “Chemical Earth” or “Element Earth” software based on Internet for managing global geochemical data and visualizing global distribution of chemical elements in the Earths crust was developed. The software is focused on (1) building a unified “Chemical Earth” software platform in twodimensions and threedimensions based on ArcGIS Server, similar to Google Earth, which is a Digital Earth software platform to obtain the piece of available geochemical data and to demonstrate graphics timely in twodimensions and threedimensions; and (2) providing database and GUIbased tool to implement very conveniently the statistical database queries in various ways, such as displaying different scale geochemical maps, interactive queries of graphics and data, query of sampling information, etc.

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    Regional geochemical indicators of largesized silver polymetallic deposits in Daxing anling Metallogenic Belt
    2012, 19(3): 49-58. 
    Abstract ( 1918 )   PDF (1044KB) ( 2148 )  

    Regional geochemical indicators for prediction of largesized ore deposits were established by using geochemical mapping data in the Daxinganling Metallogenic Belt, Inner Mongolia. Geochemical provinces delineated by 1∶1000000 geochemical mapping are closely correlated  with ore districts and regional geochemical anomalies delineated by 1∶200000 geochemical mapping have a good relationship with largesized ore deposits. Geochemical indicators based on the three large AgPbZn polymetallic deposits in Daxinganling show that (1) Ag, Pb and Zn anomalies are consistently distributed across the large deposits; that (2) 3level nested geochemical anomalies occur around the large deposits, ie a geochemical province (>500 km2) containing regional anomalies (>100 km2) and local concentration centers (n×10 km2); that (3) normalized threshold values are more than 4.5 and concentration coefficient more than 2.0.

     

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    Relationship of copper geochemical blocks and metallogenic provinces in South China Continent.
    2012, 19(3): 59-69. 
    Abstract ( 2372 )   PDF (2337KB) ( 5142 )  

    Based on the 1∶200000 stream sediments data from Regional GeochemistryNational Reconnaissance (RGNR) Project in South China Continent, this paper described the spatial distribution of copper geochemical blocks, and studied their relationship with geologic bodies and metallogenic provinces. It was found that copper geochemical blocks in South China Continent are mainly distributed in the southwestern margin of Yangtze Block, the middle and lower reaches of the Yangtze River, West Qinling, the JinshajiangLancangjiangNujiang Rivers and the border area of Hunan, Guangdong and Guangxi Provinces. The geochemical block in southwestern margin of Yangtze River is largely due to high background copper value in Emeishan basalt. The rest is consistent with the copper deposits related to Hercynian maficultramafic rocks and strata bound type copper deposits. The geochemical block in the middle and lower reaches of the Yangtze River coincides with its mineralization belt. The geochemical block in West Qinling area is concerned with magmatic rocks, copper and other metal deposits. Many copper ores and metal deposits are distributed in the geochemical blocks of JinshajiangLancangjiangNujiang Rivers, and the border area of Hunan, Guangdong and Guangxi Provinces. The conclusion was drawn that copper geochemical block is related to high background copper value in rocks, metallogenic provinces or other deposits, in which copper acts as an accompanying element. The supply of huge metallogenic materials is basic for forming large ore deposits, such as in the middle and lower reaches of the Yangtze River, where copper geochemical block provides metallogenic materials for forming copper deposits. However, not each geochemical block results in metallogenic province, like the geochemical block in Emeishan basalt.

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    Temporal and spatial distribution of tungsten and tin in South China Continent.
    2012, 19(3): 70-83. 
    Abstract ( 1953 )   PDF (1586KB) ( 4148 )  

     Based on tungsten and tin analytical data from stream sediments of 1∶200000 Regional GeochemistryNational Reconnaissance Project and the study of elemental abundance of crust and rocks in the eastern part of China, the temporal and spatial distribution characteristics of tungsten and tin in South China Continent (SCC) are discussed in this paper. On the temporal distribution, the high background contents of tungsten and tin in SCC are distributed in the South China Orogenic Belt and Coastal Volcanic Rocks Belt of Cathaysia Block, in the lower Yangtze Platformal Fold Belt and the Jiangnan Earth Axis of the Yangtze Block, in east of the YueyangHuaihuaGuilinWuzhouMaoming and south of Yangtze River. The concentrated distribution area of high contents of tungsten and tin in Nanling Region corresponds with important tungsten and tin orefield in China and the world as a whole, which occurred in the metallogenic domain of South China Mesozoic great granite province. On the lithology, alkalic feldspar granite and syenogranite, slate, phyllite and schist in basement in the southeast part of South China Block are characterized by high background contents of tungsten and tin. On the temporal distribution, the sandstones and mudstones in basement and cover from the Mesoproterozoic era are enriched in tungsten and tin; the extensive granitoid volcanicintrusive activities during the Indosinian and Yanshanian periods, in which the alkalic feldspar granite and syenogranite enriched in tungsten and tin played the main parts, reached a climax in the late stage of crustal evolution. These brought about the distribution of high background contents of tungsten and tin, and formed the geochemical domain. In addition, strong metallogenesis and a lot of middlelarge deposits of tungsten and tin widespread in the southeast part of SCC led to forming plentiful regional geochemical anomalies and local geochemical anomalies of tungsten and tin.

     

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    The significance of different scale copper geochemical anomalies and large ore deposit prediction in the middlelower Yangtze River.
    XU Shan-Fa, WANG Wei
    2012, 19(3): 84-92. 
    Abstract ( 1835 )   PDF (937KB) ( 1184 )  

    Based on the 1∶200000 regional geochemical data of copper element as the foundation in the middlelower Yangtze River, the different scales of geochemical anomaly map can be used for different purposes. (1)The 1∶200000 geochemical anomalies can be used for outlining anomalous target area of a deposit and forecasting large deposits. Twelve of 13 large deposits fell in threetier structure of geochemical anomalies in the study area; the correlation coefficient between the ore reserves of known deposits and areal productivity of geochemical anomalies is 0.94; the correlation coefficient of known deposit reserves and geochemical anomalous areas is 0.95, showing a good correlation between the size of ore reserve and the scale of regional geochemical anomalies. (2)The 1∶500000 geochemical anomalies can be used to delineate anomalous target areas of an orefield, to predict the favorable mining area in a metallogenic belt. (3)The 1∶1000000 geochemical anomalies can be used to delineate a large ore district or a metallogenic belt target for ore district prediction. If we consider an area greater than 1000 km2 including three deposits as an ore district, then, there are three large ore districts in the middlelower Yangtze River: Maanshan  Nanjing ore district, Jiujiang  Ruichang  Daye ore district and Dexing  Huangshan Anqing  Tongling ore district (actually containing two Dexing and Tongling ore districts). Large deposits often occurred in the areas with multilayered geochemical anomalies, and the geochemical anomaly in a large ore district has at least a 3 layered structure; there exists a corresponding correlation between the concentration centers and the large deposits. The discovery of these laws provides an important geochemical marker for predicting new large ore district in different mineralization areas.

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     Geochemical characteristics of the Mesozoic volcanic belt in southeast coast of China.
    2012, 19(3): 93-100. 
    Abstract ( 2026 )   PDF (671KB) ( 2300 )  

    Southeast coast of China lies in the plate junction between the Pacific and the Eurasian plates, where volcanic rocks are widely exposed and largescale mineralization were developed in the Mesozoic. There is a largescale distribution of acidic volcanic rocks while the mafic and  intermediate volcanic rocks are exposed sporadically.The acid and intermediate volcanic rocks, which are characterized by high alkali, high K, high Al and low Ti, belong to highK calcalkaline series while the mafic volcanic rocks which are characterized by high Si and low alkali belong to highmidK calcalkaline series. The REE fractionation and negative anomaly of europium are more and more obvious from mafic volcanic rocks to acidic volcanic rocks. The mafic volcanic rocks are depleted in Nb, Zr, Y and slightly enriched in Ba, Hf, Th, Ce. The acid and intermediate volcanic rocks are depleted in Ba, Ti, P, Nb, Zr, Ta, Sr and strongly enriched in Pb. The studies of major elements, trace elements and REE suggest that the source of mafic volcanic rocks is separated from the source of acidic volcanic rocks. The remelting of the lower crust caused by heat from basaltic underplating which was widespread in southeast coast of China in the Mesozoic generated a lot of acidic volcanic rocks. The intermediate volcanic rocks, geochemical characteristics of which are controlled by acid volcanic rocks, are generated by mixing of acid magma and mafic magma.

     

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    Nanogeochemistry: Deeppenetrating geochemical exploration through cover.
    2012, 19(3): 101-112. 
    Abstract ( 2053 )   PDF (1309KB) ( 2686 )  

    Deeppenetrating geochemistry have revealed that anomalies do exist at surface overlying mineral deposits covered by transported overburden. We do not yet fully understand the dispersion processes though many researchers recently have given different migration models. Nanoparticles of metals were observed in gases, soils and ore rocks at the coppernickel and gold deposits using a transmission electron microscope (TEM) equipped with an energy dispersive spectroscope. Particle diameters range from 10 to 200 nm. Most of them are generally tens of nanometers in diameters with ordered crystal structure observed by the TEM. The particles tend to assemblage shaped like a grape cluster. The particles are composed of native copper, compound metals of CuFe, CuFeMn, CuAg, CuCr, CuNi, and compound metals with Si, Al, Ca, O, P from the coppernickel deposit and Au, CuAu, CuFe, CuTi from the gold deposit. The facts show that nanoscale particles both in gases and soils are from the same source and the nanoscale particles with ordered crystal structures further indicate that they originated from the concealed CuNi deposit. The migration model can be interpreted as that nanoscale particles of metals from the ore body are adsorbed onto surface of gas bubbles and migrated with an ascending flow of gases upward to the surface, arriving at the surface, part of particles are persisting in the soil gases and the other part of particles are trapped by soil geochemical barriers. The findings of nanoscale particles of mineralization metals provide direct observation evidence for migration mechanism of deeppenetrating geochemistry and a new method for separation of nanoscale particles from soils to search for concealed deposits.

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    Characteristics, formation and migration of the particles carried by ascending gas flow from the concealed metal deposits.
    2012, 19(3): 113-119. 
    Abstract ( 2108 )   PDF (742KB) ( 3361 )  

     Characteristics, formation and migration of the particles carried by ascending gas flow in the concealed metal deposits were studied. In the research, the theory and method of geochemistry and of nano scale science and technology were  adopted and more than ten metal deposits had been studied. The results showed that particles carried by ascending gas flow in surface sediments above concealed metal orebodies occur in forms of individual particles and particle aggregations. The shapes of individual particles are spherical, nearspherical, irregular, regular polygon, and long columnar. The particles are crystalline, amorphous, or partly ordered semicrystalline. The shapes of aggregations are irregular, spherical, or nearspherical. Their mineral compositions are dominantly native gold, native copper, chloride, oxide, sulfate, and hydroxide. The oreforming elements such as Au, Cu may be in the form of native metals or native alloys. The oreforming elements such as Cu, Pb, Zn may be in the form of oxide, or sulfate. They, too, may be the impurity elements existing in particles such as chloride, oxide, sulfate, and hydroxide. The particles are the indicators of concealed metal orebodies. After mineralization, the particles related to the concealed metal orebodies were mainly formed by faulting, oxidation and biological effects. In the ascending gas flow, particles are in Brownian motion.

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    Occurrences and formation of copper nanoparticles over the concealed oredeposits.
    2012, 19(3): 120-129. 
    Abstract ( 2103 )   PDF (1118KB) ( 1883 )  

    Nanoscale particles of copper or copper alloy were widely discovered in soils and gases over concealed ore deposits, such as CuNi ore deposit, Au ore deposit and CuAu deposit. The particles in size diameters, shapes and compositions are similar to those in ore rocks. The facts show that copper minerals widely occur in native copper in various types of mineral deposits. The nanoscale particles of copper or alloy from the ore body could migrate upward to the surface, arriving at the surface, part of particles are persisting in the soil gases and the other part of particles may be trapped by soil geochemical barriers such as iron and manganese oxide coatings, clay minerals, colloids, soluble salts and secondary carbonates. The discovery of nanoscale particles of copper provides direct observation evidence for deeppenetrating geochemistry using soil and geogases as sampling media to search for deeply concealed ore deposits.

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    Threedimensional geochemical distribution patterns in regolith over a concealed gold deposit in arid desert terrains.
    2012, 19(3): 130-137. 
    Abstract ( 1680 )   PDF (1223KB) ( 1267 )  

    In recent years, deeppenetrating geochemistry has been proven to be effective in search for concealed deposits in arid desert terrains. However, we have a little understanding of the dispersion patterns. In this study, Air Reverse Circulation (ARC) drilling technology is used to revealed the 3D distribution patterns of elements in regolith sediments over Jinwozi gold deposit, which is covered by tens of meters of regolith sediments, in order to trace the migration track and to understand the migration mechanisms. 3D distribution maps of elements generated show that (1) anomalies continuatively occur in the different depths of regolith sediments over the ore body, that (2) gold tend to be enriched in top and bottom horizon, and depleted in middle horizon in vertical direction, that (3) the anomalous distribution in bottom is restricted in the lowest places at the interface of sediments and bedrocks, and that (4) laterally the anomalous extension in the bottom is narrow within a width of tens of meters, and that in top soils is much wider and extends to several kilometers.

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    Progress in geochemistry of sandstonetype uranium deposit in North Ordos Basin.
    2012, 19(3): 139-146. 
    Abstract ( 2186 )   PDF (19200KB) ( 860 )  

    This paper describes the progress of new research about sandstonetype uranium deposits from elemental geochemistry, isotopic geochemistry and organic geochemistry in the Northern Ordos Basin. Through elemental geochemistry, researchers understand the geochemical characteristics of ores and wall rocks, the geological environment of uranium mineralization and the relationship between ores and wallrock. The stage of uranium mineralization period was determined by isotopic geochronology. The researchers constructed the areas thermal history by fission tracks. Oreforming age similar to thermal history suggests the link between the oreforming and the tectonic evolution. The uranium source, the source of the oreforming fluids and the role played by organic matter in uranium have been discussed by the comprehensive consideration of the results of isotopic geochemistry, organic geochemistry and fission track studies.

     

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    History of formation and evolution on the crust around the Ordos Basin: Evidences from UPb dating and Hf isotopic composition of zircons.
    2012, 19(3): 147-166. 
    Abstract ( 2622 )   PDF (1435KB) ( 1632 )  

    U-Pb dating and Hf isotopic composition of zircons from the northern margin of North China Platform, the Great HinganMongolian Orogen, and the Yangtze PlatformQinlingDabieSulu Orogen around the Ordos Basin show that its surrounding tectonic units have different histories of formation and evolution. Average age is 1837 Ma based on U-Pb dating of 885 zircon grains from the northern margin of North China Platform. The most intensive magmatism peak occurred between 2200 and 1800 Ma; the zircon grains within this range amount to 40% of the total. The secondary intensive period is between 2800 and 2200 Ma; percentage of zircon grains in this range is ~30%. The zircons crystallized in 15001200 Ma and 500100 Ma occupy ~15% respectively. The northern margin of North China Platform is characterized by a lack of 1000700 Ma zircons. The zircons older than 3000 Ma are very rare. The averaged Hf isotopic depletion mantle model age of zircons is 2.55 Ga, which is older than that of U-Pb ages, suggesting that zircons crystallized between 2200 and 1800 Ma contain older recycled precursor crustal components. The most intensive peak of Hf isotopic depletion model age is ~2.8 Ga, consistent with that of Nd isotopic depletion mantle model ages. The zircon grains with Hf model ages between 3.0 and 2.25 Ga account for ~95%, which implies that the crustal accretion from the depleted mantle took place between the Archean and the Paleoproterozoic. Other ~5% zircon grains were formed by Mesoproterozoic crustal accretion event. The Late PaleozoicMesozoic magmatism was predominately controlled by anatexis of precursor crustal components, especially the Mesoproterozoic juvenile crust. The percentage of zircon grains with the Hf model ages older than 3.0 Ga is less than 0.1%. The average U-Pb age of zircons in the Great HinganMongolian Orogen is 497 Ma; the most intensive peak is around the Carboniferous, i.e. ~320 Ma. CarboniferousPermian zircons account for over 30% in the orogen. Another population of zircon U-Pb ages is around the NeoproteozoicEarly Paleozoic, i.e. 600440 Ma, whose percentage is >55% of the total. The percentage of Late MesoproterozoicNeoproterozoic (1200600 Ma) zircons is ~4%. The corresponding average Hf isotopic depletion model age of the zircons in the orogen is 1.13 Ga, and the most intensive peak is around ~0.6 Ga. The Hf model ages of zircons dominantly distribute between 0.7 and 0.28 Ga; the zircons within this range account for 57% of the total, which implies the most intensive accretion period of the orogenic crust from the depleted mantle. Other intensive accretion episode is between the Mesoproterozoic and the Neoproterozoic (1.50.7 Ga); the percentage of zircons in this range is up to ~38%. The zircons with Hf model age between 3.3 and 1.5 Ga is less than 5%. The orogenic crust almost has no juvenile component from the depleted mantle since the Paleozoic. The average U-Pb age of zircons in the Yangtze PlatformQinlingDabieSulu Orogen is 799 Ma. The zircon population between 1300 and 750 Ma is ~70% of the total. The other intensive magmatism period is between the Late Paleozoic and the Yanshanian; the zircons formed in this period account for ~20%. PaleoarcheanPaleoproterozoic (35002650 Ma) and PaleoproterozoicMesoproterozoic (21181680 Ma) zircons account for ~5%, respectively. On the basis of averaged Nd isotopic depletion mantle model age of 1.56 Ga, it is suggested that much old precursor crustal components were involved in the magmatism event between 1300 and 750 Ma. The tectonic units around the Ordos Basin have different histories of formation and evolution. Their crustal growth has been episodic. But the age and intensity of each episode show significant differences among the tectonic units. Thus, the strata of the basin may decipher corresponding provenances through U-Pb dating and Hf isotopic composition of detrital zircons in them.

     

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    Pilot study of deeppenetrating geochemical exploration for sandstonetype uranium deposit, Ordos Basin.
    2012, 19(3): 167-176. 
    Abstract ( 2360 )   PDF (124438KB) ( 789 )  

    Abstract: Ordos Basin contains concealed sandstone type uranium deposits in the lower Jurassic Zhiluo Formation. The study of the deeppenetrating geochemical methods will contribute to uranium exploration in the basin. The study shows that the collection and analysis of the clayrich microsurface soil provides a special application to regional geochemical exploration of sandstone type uranium deposit in the semiarid terrains. Pilot geochemical mapping project covering 2000 km2 around Dongsheng uranium deposit area with the sampling density of 1 sample per 4 km2,successfully delineated the clear geochemical anomalies at the redox frontier zone and the concealed deposit belts. The study of the distribution patterns of R=lnCe+lnY+lnZr+lnNb shows that the mobile forms of uranium should be the key factor causing the variation of the total content in surface soil samples. The soil sieving operation can be deemed to be the procedure of physical extraction of the deeppenetrating information for the concealed deposit. The content of uranium in the surface soil can be used as the key indexes for geochemical exploration of sandstone type uranium deposits in the basin. Due to the terrain features, the clayrich microsurface soil samples can be applied as the media for delineating the regional geochemical anomalies, but it cannot be always obtained in the exploration stage of locating the concealed orebodies. The application of the extractant of MMLU to selectively leaching the mobile forms of elements from soil along traverses over the Sunjialiang section of Dongsheng uranium deposit well delineated the U, Mo and V anomalies over the concealed orebodies.

     

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    The feature of lower crust of North China Craton(NCC) constrained by xenoliths.
    2012, 19(3): 177-187. 
    Abstract ( 1749 )   PDF (770KB) ( 1409 )  

    The data on lower crustal granulite xenoliths of the Cenozoic, the Mesozoic and the Paleozoic and their zircon ages were collected in this study. The results show that these xenoliths represent the compositions and formation ages of the bottom of lower crust in different domains and different times in NCC. The most useful information is from the Cenozoic basalts and their xenoliths such as from Hannuoba, Nushan, Junan and Qingdao. They may reflect almost the whole bottom of lower crust situations in NCC. Although the data from kimberlites provided the information only before 500Ma, they also can examine and confirm the accuracy of the results from the Cenozoic data. Granulites entrained by Mesozoic diorite in Inner Mongolia are cognate xenoliths, so they merely represent the bottom of lower crust of local area where these kinds of magmas distributed. The multistage geological events have occurred in NCC which were dominated by adding the juvenile mantle materials. The major events were happened in 2.83.0 Ga, 2.5 Ga, 1.8 Ga, 14090 Ma and 4547 Ma and the very important two lower crust growths appeared in ca. 2.5 Ga and 14090 Ma. Basic magma underplating at ca. 2.5 Ga was recorded in granulites of the Cenozoic and the Paleozoic from different places and it may relate to the “mantle plum” activity. Magma underplating in 14090 Ma is widespread in NCC including northern, eastern and southeastern parts. It is possible that the zircon ages of 2.3 Ga reported in granulite xenoliths from Nushan and Junan are the products of Mesozoic underplating magma mixed or/and interacted with the Archean lower crust. Data on the zircon ages of 1.92.0 Ga and 1.8 Ga attest it correlated to the tectonic movement at crust and the disturbance of instantaneous thermal event led to recrystallization of zircon. Tectonic background, a very important problem, must be pay attention to when you study the deepseat xenoliths, especially at the boundary of the firstorder tectonic units because of the complicated interaction between adjacent blocks, so it is necessary to distinguish which block be each xenolith derived from. This is helpful to the understanding of the complicated evolution history in different domain of NCC.

     

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    The discovery of Phanerozoic granulite in Chifeng area of North Craton and its implication.
    2012, 19(3): 188-198. 
    Abstract ( 1997 )   PDF (1247KB) ( 1869 )  

    It is found that the zircon SHRIMP U-Pb ages of the Permian mafic granulite and its host rockdioritein Chaihulanzi of Chifeng area are 262.3±3.1 Ma and 256.6±6.5 Ma respectively, and that the zircon SHRIMP U-Pb ages of the Triassic mafic granulite and its host rockdioritein Henanyingzi of Ningcheng area are 229.7±2.6 Ma and 221±3 Ma respectively. Other forms of magmation in this period were also discovered in these areas, including Permian accretion edges around the Archaeozoic zircon and Permian dioriteveins. Comparative study suggests that the Permian magmation possessed the characteristics of the initial stage of a continental rift with relatively low content of highfieldstrength elements, and that the increased Triassic highfieldstrength elements indicate a higher degree of mantle partial melting. Those granulites in combination recorded important information about the transformation of North China Craton from stable to active in its early stage. Isotopegeological study indicates that such transformation began with the crustmantle interaction. The Permian magmation here was related to the global tectonic evolution and bimodal volcanic activity in Hinggan Mongolia Orogen at that period. The Triassic magmation here was related to intracontinental extension under the setting of underplating.

     

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    Petrogenesis and tectonic implications of the Early Indosinian Meiwu Pluton in West Qinling, central China.
    2012, 19(3): 199-213. 
    Abstract ( 1888 )   PDF (1184KB) ( 1732 )  

     A combined study of UPb zircon dating, wholerock chemical and Sr-Nd isotopic composition was carried out for the Meiwu pluton in the middle part of the west Qinling. U-Pb zircon dating shows that the Meiwu pluton intruded at 245242 Ma. The Meiwu pluton, a complex batholith, is composed of quartzdiorite, granodiorite and biotite granite, with abundant mafic magmatic enclaves (MMEs) in the granodirite. The different rock types in the Meiwu pluton are characterized by distinct chemical evolution trend. Geochemical and SrNd isotopic composition indicate the magma of the Meiwu granodiorite and biotite granite was dominantly derived from crustal source. However, the granodiorite show relatively high Mg# (5058), Cr (217×10-6, (4693)×10-6) and Ni ((1250)×10-6), suggesting a minor contribution from the mantle. The quartzdiorite and mafic magmatic enclaves display high Mg# (63 and 62), Cr (217×10-6 and 318×10-6) and Ni (61×10-6 and 110×10-6), respectively, indicating that their magma were dominant mantlederived. Geochemical and U-Pb zircon ages, combined with regional setting, suggest the Meiwu Pluton in the middle part of the west Qinling resulted from breakoff of the subducted Animaque oeanic slab at early stage of postcollisional setting. The slab breakoff model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the west Qinling.

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    Petrogenesis and tectonic implications of the Maozangsi and Huangyanghe granitic intrusions in Lenglongling area, the eastern part of North Qilian Mountains, NW China.
    2012, 19(3): 214-227. 
    Abstract ( 2561 )   PDF (1138KB) ( 1424 )  

    This paper reports a detailed study of UPb zircon dating, geochemical and SrNd isotopic compositions for the Maozangshi granodiorite and Huangyanghe Kfeldspar granite in Lenglongling area, North Qilian Mountains. UPb zircon dating by LAICPMS shows magma crystallization ages of 424±4 Ma for the Maozangshi granodiorite and 402±4 Ma for the Huangyanghe Kfeldspar granite. The Maozangshi granodiorite has high Mg# (~55), K2O/Na2O=0.77-0.91, A/CNK=0.92-0.94. It is rich in LILE and depleted in HFSE. REE data for the granodiorite show strongly fractionated REE patterns, with (La/Yb)N = 16.9-19.5 and Eu/Eu*=0.75-0.83. The granodiorite has ISr=0.7063-0.7065 and εNd(t) =1.5-1.1, with TDM=1.10-1.16 Ga. The geochemical characteristics and SrNd isotopic composition indicate that the magma of the granodiorite was derived from partial melting of basaltic rocks in lower crust, but with minor contribution from mantle material. The Huangyanghe Kfeldspar granite has K2O+Na2O=6.91%-7.66%, K2O/Na2O >1 and A/CNK=0.97-1.05. The Kfeldspar granite is rich in LILE and HFSE. It has moderately fractionated REE patterns with (La/Yb)N =10.6-17.8 and Eu/Eu*=0.43-0.68. These features show that the Kfeldspar granite has an affinity with aluminous Atype granite. The Kfeldspar granite shows ISr=0.7103-0.7113 and εNd(t)=6.7-6.0, with TDM =1.46-1.55 Ga, suggesting that the magma resulted from partial melting of felsic materials in crust. The diverse petrogenesis and evolutionary history of the granitoids in Lenglongling area reveal a tectonic transformation from compression to extension for North Qilian orogen. Both the Maozangshi granodiorite and Huangyanghe Kfeldspar granite formed in postcollisional tectonic setting. And the petrogenesis of the postcollisional granitoids in North Qilian Mountains may have close relation with the previously subducted North Qilian oceanic slab.

     

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    Late Cretaceous hightemperature metamorphism in the southeastern Lhasa terrane: Evidence from rutile exsolutions in quartz and zircon LAICPMS dating.
    2012, 19(3): 228-239. 
    Abstract ( 2700 )   PDF (1095KB) ( 1310 )  

    In this study, we present detailed petrographic study and LA-ICP-MS zircon U-Pb dating for a garnetbearing amphibolite from the Nyingchi Complex in the southeastern Lhasa terrane. Petrographic study indicates that the garnetbearing amphibolite had undergone two stages of peak granulitefacies and retrograde amphibolefacies metamorphism. The peak mineral assemblage is characterized by Grt+highTi Amp + Hy+Pl+Qtz+Rt. There are a large number of rutile exsolutions in the garnet, quartz and amphiboles, suggesting that the primary composition of these minerals have high Ti contents. The retrograde amphibolefacies assemblage is characterized by lowTi Amp+Pl+Czo+Qtz+Rt. Based on the Tiinquartz (TitaniQ) thermometers, the peak and retrograde metamorphic temperatures are 803924 ℃ and 555732 ℃, respectively. The zircons in the garnetbearing amphibolite show corerim structure in CL images. The cores are characterized by high Th/U ratios, enrichment in HREE, a positive Ce anomaly and a marked negative Eu anomaly, suggesting that they are typical of magmatic zircon. They yielded a weighted 206Pb/238U mean age of 89.3±0.6 Ma, which is interpreted as the crystallization age of the protolith of garnetbearing amphibolite. By contrast, the rims exhibit no zoning. They have low Th/U ratios and MHREE contents, and show weak or no Eu anomaly, indicating that they formed in the peak metamorphic stage. The 206Pb/238U age of 81.1±0.8 Ma obtained on these rims is interpreted as the peak metamorphic age. Combined with regional studies, we suggest that the about 81 Ma hightemperature metamorphism resulted from the upwelling of asthenosphere through the slab window opened as a result of ridge subduction.

     

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    Geochronology, geochemistry and geological significance of Yinchangping granite porphyry, Yunnan Province.
    2012, 19(3): 240-251. 
    Abstract ( 2177 )   PDF (175463KB) ( 882 )  

    Yinchangping granite porphyry intrusive outcrops around the southwestern segment of the Yangtze block, which is between the Xianshuihe-Xiaojiang alkaline rock zone and Jinshajiang-Ailaoshan alkali-rich porphyry rock zone. The major elements of this area show high alkaline, high ratio of Na2O/K2O and lower TiO2, presenting typical geochemical characteristic of high-K calc-alkaline rocks. Trace elements characteristics include LILE, LREE (with enrichments) and transitional elements (Cu, Ni and Co), as well as significant “Ta-Nb-Ti” negative anomaly and indistinctive Eu anomaly. LA-ICP-S zircon U-b dating shows that the emplacement age of Yinchangping granite porphyry is (37.39±0.33) Ma: εHf(t) value is in the range of  -1.8+1.8 with twostage Hf model age of 9961226 Ma; εNd(t) value is a little low within the range of -7.4-3.6; (87Sr/86Sr)i (0.706077 0.706231) is high; 207Pb/204Pb(15.56715.574) and 208Pb/204Pb(38.32138.335) present a typical characteristic of orogenic zone. All these analyses suggest that Yinchangping granite porphyry intrusion belongs to the Anninghe alkaline rocks zone  which is formed in the environment of near continental arc, where there is not only the geochemical characteristic of EM I-M II type source enrichment in mantle but also the porphyritic magma formed in “crust-antle mixed layer”. Yinchangping granite porphyry is the acid component at the end of fractional crystallization in Anninghe alkaline rock zone.

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    Particularity of shale gas reservoir and its evaluation.
    2012, 19(3): 252-258. 
    Abstract ( 2006 )   PDF (698KB) ( 2482 )  

    In order to evaluate shale gas reservoir effectively, the particularity of shale gas reservoir relative to conventional one has been summed up in this paper, which includes richness in organic matter, abundance in clay minerals, finegrained texture, extremely low porosity and permeability, nanometer order of pore and throat structure, giant mineral surface area, complicated diagenesis, and great percentage of adsorbed natural gas. Based on the particularity of shale gas reservoir, the idea and method of evaluation for it have been discussed. It has been emphasized that the combination of direct measurement of evaluation parameters with indirect calculation, regular methods of measurement with special ones, evaluation of pore characteristics with gas adsorption capacity should be accepted. At last, the evaluation contents have been suggested, which should include mineral and organic components, porosity and permeability, types of pore space, gas adsorption capacity and rock mechanics.

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    Favorable depth zone selection for shale gas prospecting.
    2012, 19(3): 259-263. 
    Abstract ( 1976 )   PDF (524KB) ( 1886 )  

     In view of existing difficulties in deciding the shale gas favourable region, three important steps are proposed which should be considered especially in vertical,and the characteristic value of  corresponding parameters are put forward. (1)Make the exploration threshold depth clearly and definitely. According to basic model, gas generation threshold of typeⅠ,typeⅡ1,typeⅡ2 and type Ⅲ  organic matters Ro are ordinally 1.2%、0.9%、0.7% and 0.5%.(2)Determine the threshold gas content in different depth. According to the statistical analysis of gas producing shale formation parameters in America, the gas content threshold in 1000 m, 2000 m and 3000 m depth are ordinalty 0.5 m3/t, 1.0 m3/t and 1.5 m3/t. (3)Determine the approved shale. According to the research on Longmaxi shale of Lower Silurian in southeast Chongqing, if we want 1.0 m3/t gas content, the TOC of shale in the depth of 500 m, 1000 m and 2000 m must be ordinally more than 1.4%, 1.1% and 0.9%.

     

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    Characteristics of Mesozoic marine black shales and prediction of shale gas prospective area in Qiangtang Basin, Tibet.
    2012, 19(3): 264-279. 
    Abstract ( 2386 )   PDF (1870KB) ( 1447 )  

    Mesozoic marine black shales are mainly distributed in the Qiangtang Basin, QinghaiTibet plateau, in the Upper Triassic Xiaochaka Formation(T3x), the Jurassic Buqu Formation (J2b) and the Suowa Formation (J3x).  They are formed in the reducing sedimentary environment which is abundant in organics that are easily preserved. The sedimentary facies are mainly platform facies, followed by basinal facies and delta facies. The thickness is 41474 m; TOC is 0.25%8.34%; organic matter is typeⅡ; the average value of Ro is 1.21%3.0%, showing in the gas stage of highover mature. The brittle minerals are mainly quartz and feldspar, total contents are 65%70%, and the shale porosity is about 2%. Compared with each index of gas shales in the main basins of American, we can consider that the Mesozoic marine black shales in Qiangtang Basin have the features of large deposition thickness, low organic abundance, high maturity of thermal evolution, numerous species of brittle minerals, low content of quartz, high content of feldspar and clay minerals, and low porosity. It means that the shales have the condition of producing shale gas and the basin is one of the sedimentary basins, in which the marine shale gas could be explored. From the comprehensive evaluation of the shale gas prospective areas, it can be predicted that in the Upper Triassic Xiaochaka Formation shale gas is widely distributed, followed by the Buqu Formation and the Suowa Formation.

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    A study of shale gas preservation conditions for the Lower Paleozoic in Sichuan Basin and its periphery.
    2012, 19(3): 280-294. 
    Abstract ( 7138 )   PDF (86448KB) ( 20844 )  

    The exploration of shale gas indicated an indisputable fact that there is gas in shale, but the size of the shale gas reservoir and its industrial value need further study. Compared to the high production of U.S. shale gas, shale gas has its peculiar geological characteristics in China. The shale gas preservation conditions may be the focus of the shale gas exploration in China, which is less studied in U.S. shale gas exploration. This paper studied the shale gas preservation conditions by material basis, tectonism, evolutionary history, hydrogeology, seal and its microscopic properties, gas composition, pressure coefficient and other parameters of shale in the Lower Cambrian and the Upper OrdovicianLower Silurian of the Lower Paleozoic in Sichuan Basin and its periphery. The evaluation system of shale gas preservation of south China was put forward by comprehensive analysis of various parameters.

     

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