Geochemistry

Distribution by Scientific Domains
Distribution within Earth and Environmental Science

Kinds of Geochemistry

  • element geochemistry
  • isotope geochemistry
  • trace element geochemistry
  • whole-rock geochemistry


  • Selected Abstracts


    SOURCE ROCK EVALUATION AND GEOCHEMISTRY OF CONDENSATES AND NATURAL GASES, OFFSHORE NILE DELTA, EGYPT

    JOURNAL OF PETROLEUM GEOLOGY, Issue 2 2003
    L. M. Sharaf
    Geochemical analyses of mudstones from wells in the NE offshore Nile Delta suggest that the Early Miocene Qantara Formation has "good" potential to generate hydrocarbons at the studied locations. Its generating capability and oil-proneness increase northwards, towards areas where better organic-matter preservation and a greater contribution from marine source material can be expected. By contrast, the Middle Miocene Sidi Salem Formation has "poor to fair" potential to generate mixed gas and oil, while the overlying Wakar and Kafr El Sheikh Formations have "poor" capability to generate gas with minor oil. Based on pyrolysis Tmax and thermal alteration index assessments, the Wakar and Kafr El Sheikh Formations are immature in the study area. The Sidi Salem and Qantara Formations are immature in the southern part of the study area, but are within the oil window in the north, around well Temsah-4. Biomarker distributions based on GC-MS analyses of two condensate samples from the Wakar and Sidi Salem Formations indicate that hydrocarbons are derived from siliciclastic source rocks containing significant terrestrial material and limited marine organic matter. The condensates were generated during early maturation of Type III kerogen from deeper and more mature source rocks than those encountered in the drilled wells. Geochemical and isotopic data from natural gas produced from the Kafr El Sheikh Formation suggest mixed biogenic and thermogenic sources. [source]


    Measuring the plasma environment at Mercury: The fast imaging plasma spectrometer

    METEORITICS & PLANETARY SCIENCE, Issue 9 2002
    P. L. KOEHN
    Three primary populations of ions exist at Mercury: solar wind, magnetospheric, and pickup ions. These pickup ions are generated through the ionization of Mercury's exosphere or are sputtered particles from the Mercury surface. A comprehensive mission to Mercury, such as MESSENGER (MErcury: Surface, Space ENvironment, GEochemistry, Ranging), should include a sensor that is able to determine the dynamical properties and composition of all these plasma components. An instrument to measure the composition of these ion populations and their three-dimensional velocity distribution functions must be lightweight, fast, and have a very large field of view. The fast imaging plasma spectrometer (FIPS) is an imaging mass spectrometer, part of NASA's MESSENGER mission, the first Mercury orbiter. This versatile instrument has a very small footprint, and has a mass that is ,1 order of magnitude less than other comparable systems. It maintains a nearly full-hemisphere field of view, suitable for either spinning or three-axis-stabilized platforms. The major piece of innovation to enable this sensor is a new deflection system geometry that enables a large instantaneous (,1.5,) field of view. This novel electrostatic analyzer system is then combined with a position sensitive time-of-flight system. We discuss the design and prototype tests of the FIPS deflection system and show how this system is expected to address one key problem in Mercury science, that of the nature of the radar-bright regions at the Hermean poles. [source]


    Geochemistry and petrography of basalt grindstones from the Karak Plateau, central Jordan

    GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 1 2004
    Brandon G. Watts
    Seventeen basalt grindstone fragments from central Jordan's Karak Plateau were studied. Most of these artifacts are vesicular or amygdaloidal with calcite as the dominant mineral filling the voids. The major minerals are olivine (with iddingsite rims), plagioclase, clinopyroxene, magnetite, and apatite. Glass is present in some samples. One basalt fragment is quite different in appearance and composition and may have come from flows closer to the Dead Sea. Grindstone fragment compositions plot in the tephrite-basanite and basalt fields. A plot of the concentrations of niobium, zirconium, and yttrium reveal that the sample compositions plot in the "within-plate alkali basalt" and "within-plate tholeiite" fields. The acquisition of basalts for preparing such implements appears to have been random. Some may have been introduced through trade and migration. Archaeological and environmental studies on the Karak Plateau are urgently needed because Jordan's population growth and economic development are destroying many sites and their environmental contexts. 2004 Wiley Periodicals, Inc. [source]


    Geochemistry of the Bagnres-de-Bigorre thermal waters from the North Pyrenean Zone (France)

    GEOFLUIDS (ELECTRONIC), Issue 1 2002
    S. Levet
    Abstract Thermal springs are poorly known in the sedimentary sites of the Pyrenees. In this paper, we describe the ,Bagnres-de-Bigorre' springs which occur in a remarkably active seismotectonic context. A chemical and isotopic study of 15 spring waters (both cold and thermal, ranging in temperature from 7.0 to 49.9C), and continuous monitoring of a single spring allow us to characterise water,rock interactions, fluids paths and mixing processes. Three groups of waters are distinguished: (I) SO42+,Ca2+,Cl, thermal waters (II) SO42+,Cl,,Ca2+ thermal waters and (III) HCO3,,Ca2+ cold shallow waters. Their characteristics suggest interactions with Mesozoic evaporite and carbonate formations. O and D isotopes from thermal waters indicate a local meteoric origin of Atlantic signature and a recharge elevation of 800 to 1000 m, which corresponds to a single feeding area. Their ,13C values (,2.8 to ,9.6,) are consistent with carbonate dissolution, slight fractionation and a surficial organic input leading to ,13C depletion. Sr isotopes (0.70751 to 0.70777), Na+/Cl, and (Ca2+ + Mg2+)/SO42, ratios as well as thermodynamic calculations show that the dissolution of anhydrite and halite-bearing Triassic layers control the chemical composition of group-I and -II waters. The contrasting trends of cation/Cl, ratios and TDS of waters from groups I and II suggest the existence of two different circulation paths at depth as well as dilution with surficial waters similar to group III. Calculated mixing proportions show that three waters from group I are diluted from 17 to 66%, whereas all waters from group II are mixed. The aquifer temperature is estimated to be in the range 55,64C using the retrograde and prograde solubilities of anhydrite and chalcedony, respectively. Accordingly, the mean depth of the reservoir is around 1.7 km, which allows us to constrain the depth of the Triassic layer. [source]


    Geochemistry of Extremely Alkaline (pH > 12) Ground Water in Slag-Fill Aquifers

    GROUND WATER, Issue 6 2005
    George S. Roadcap
    Extremely alkaline ground water has been found underneath many shuttered steel mills and slag dumps and has been an impediment to the cleanup and economic redevelopment of these sites because little is known about the geochemistry. A large number of these sites occur in the Lake Calumet region of Chicago, Illinois, where large-scale infilling of the wetlands with steel slag has created an aquifer with pH values as high as 12.8. To understand the geochemistry of the alkaline ground water system, we analyzed samples of ground water and the associated slag and weathering products from four sites. We also considered several potential remediation schemes to lower the pH and toxicity of the water. The principal cause of the alkaline conditions is the weathering of calcium silicates within the slag. The resulting ground water at most of the sites is dominated by Ca2+ and OH, in equilibrium with Ca(OH)2. Where the alkaline ground water discharges in springs, atmospheric CO2 dissolves into the water and thick layers of calcite form. Iron, manganese, and other metals in the metallic portion of the slag have corroded to form more stable low-temperature oxides and sulfides and have not accumulated in large concentrations in the ground water. Calcite precipitated at the springs is rich in a number of heavy metals, suggesting that metals can move through the system as particulate matter. Air sparging appears to be an effective remediation strategy for reducing the toxicity of discharging alkaline water. [source]


    Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, Korea

    ISLAND ARC, Issue 4 2002
    SEONG HEE CHOI
    Abstract Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB,oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history. [source]


    Geochemistry, dispersal, volumes and chronology of Holocene silicic tephra layers from the Katla volcanic system, Iceland

    JOURNAL OF QUATERNARY SCIENCE, Issue 2 2001
    Gudrn Larsen
    Abstract At least 12 silicic tephra layers (SILK tephras) erupted between ca. 6600 and ca. 1675 yr BP from the Katla volcanic system, have been identified in southern Iceland. In addition to providing significant new knowledge on the Holocene volcanism of the Katla system which typically produces basaltic tephra, the SILK tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts. With one exception the SILK tephras have a narrow compositional range, with SiO2 between 63 and 67%. Geochemically they are indistinguishable from ocean transported pumice found on beaches in the North Atlantic region, although they differ significantly from the silicic component of the North Atlantic Ash Zone One (NAAZO). Volumes of airborne SILK tephra range from 0.05 to 0.3 km3. We present new isopach maps of the six largest layers and demonstrate that they originate within the Katla caldera. The apparently stable magma system conditions that produced the SILK tephras may have been established as a consequence of the eruption of the silicic component of NAAZO (ca. 10.3 ka) and disrupted by another large-scale event, the tenth century ad Eldgj eruption (ca. 1 ka). Despite the current long repose, silicic activity of this type may occur again in the future, presenting hitherto unknown hazards. Copyright 2001 John Wiley & Sons, Ltd. [source]


    Geochemistry of Persististrombus latus Gmelin from the Pleistocene Iberian Mediterranean realm

    LETHAIA, Issue 2 2010
    TRINIDAD De TORRES
    De Torres, T., Ortiz, J.E., Arribas, I., Delgado, A., Juli, R. & Martn-Rub, J.A. 2009: Geochemistry of Persististrombus latus Gmelin from the Pleistocene Iberian Mediterranean realm. Lethaia, Vol. 43, pp. 149,163. In this paper the organic and inorganic geochemistry of fossil and extant Persististrombus latus are compared, together with other strombid species (Lentigo lentiginosus, Lobatus gigas, Strombus alatus, Lobatus raninus, Laevistrombus canarium and Tricornis latissimus). Using a large sample of well-preserved fossil P. latus shells from the Mediterranean realm, we examined the warming period of sea water in the Middle Pleistocene. A mineralogical study of the shells demonstrates the continuous presence of calcite and a complex organic matter distribution, which was well preserved in many cases, thereby making the U/Th dating of strombid shells unreliable. U/Th analysis of coral samples and amino acid racemization dating of pelecypod shells confirmed that P. latus entered the Mediterranean realm in MIS 7 and 5. The oscillations of the ,18O values reflect annual growth periods and provide a mixing of the first signal record (primary growth) and successive overgrowths. ,Amino acid racemization, Mediterranean Sea, Persististrombus latus, shell mineralogy, U/Th, ,18O. [source]


    Geochemistry and origin of metal, olivine clasts, and matrix in the Dong Ujimqin Qi mesosiderite

    METEORITICS & PLANETARY SCIENCE, Issue 3 2008
    Ping Kong
    According to silicate textures and metal composition, this meteorite is classified as a member of subgroup IB. Instrumental neutron activation analyses (INAA) of metals show that the matrix metal has lower concentrations of Os, Ir, Re, and Pt, but higher concentrations of Ni and Au than the 7.5 cm metal nodule present in the meteorite. We attribute these compositional differences to fractional crystallization of molten metal. Studies of olivine clasts show that FeO contents are uniform in individual olivine crystals but are variable for different olivine clasts. Although concentrations of rare earth elements (REEs) change within olivine clasts, they all exhibit a vee-shaped pattern relative to CI chondrites. The relatively high concentrations of REEs in olivine and the shape of REE patterns require a liquid high in REEs and especially in light REEs. As such a liquid was absent from the region where basaltic and gabbroic clasts formed, mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed. [source]


    Geochemistry of Darwin glass and target rocks from Darwin crater, Tasmania, Australia

    METEORITICS & PLANETARY SCIENCE, Issue 3 2008
    Kieren T. Howard
    Target rocks at Darwin crater are quartzites and slates (Siluro-Devonian, Eldon Group). Analyses show 2 groups of glass, Average group 1 is composed of: SiO2 (85%), Al2O3 (7.3%), TiO2 (0.05%), FeO (2.2%), MgO (0.9%), and K2O (1.8%). Group 2 has lower average SiO2 (81.1%) and higher average Al2O3 (8.2%). Group 2 is enriched in FeO (+1.5%), MgO (+1.3%) and Ni, Co, and Cr. Average Ni (416 ppm), Co (31 ppm), and Cr (162 ppm) in group 2 are beyond the range of sedimentary rocks. Glass and target rocks have concordant REE patterns (La/Lu = 5.9,10; Eu/Eu* = 0.55,0.65) and overlapping trace element abundances. 87Sr/86Sr ratios for the glasses (0.80778,0.81605) fall in the range (0.76481,1.1212) defined by the rock samples. ,-Nd results range from ,13.57 to ,15.86. Nd model ages range from 1.2,1.9 Ga (CHUR) and the glasses (1.2,1.5 Ga) fall within the range defined by the target samples. The 87Sr/86Sr versus 87Rb/86Sr regression age (411 42 Ma) and initial ratio (0.725 0.016), and the initial 43Nd/144Nd ratio (0.51153 000011) and regression age (451 140 Ma) indicate that the glasses have an inherited isotopic signal from the target rocks at Darwin crater. Mixing models using target rock compositions successfully model the glass for all elementsexcept FeO, MgO, Ni, Co, and Cr in group 2. Mixing models using terrestrial ultramafic rocks fail to match the glass compositions and these enrichments may be related to the projectile. [source]


    Drill core LB-08A, Bosumtwi impact structure, Ghana: Geochemistry of fallback breccia and basement samples from the central uplift

    METEORITICS & PLANETARY SCIENCE, Issue 4-5 2007
    Ludovic FERRIRE
    Two boreholes were drilled to acquire hard-rock samples of the deep crater moat and from the flank of the central uplift (LB-07A and LB-08A, respectively) during a recent ICDP-sponsored drilling project. Here we present results of major and trace element analysis of 112 samples from drill core LB-08A. This core, which was recovered between 235.6 and 451.33 m depth below lake level, contains polymict lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment. The basement is dominated by meta-graywacke (from fine-grained to gritty), but also includes some phyllite and slate, as well as suevite dikelets and a few units of a distinct light greenish gray, medium-grained meta-graywacke. Most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types, as well as from aqueous alteration processes, which have undeniably affected the different rocks. Suevite from core LB-08A (fallback suevite) and fallout suevite samples (from outside the northern crater rim) display some differences in major (mainly in MgO, CaO, and Na2O contents) and minor (mainly Cr and Ni) element abundances that could be related to the higher degree of alteration of fallback suevites, but also result from differences in the clast populations of the two suevite populations. For example, granite clasts are present in fallout suevite but not in fallback breccia, and calcite clasts are present in fallback breccia and not in fallout suevite. Chondrite-normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other. Siderophile element contents in the impact breccias are not significantly different from those of the metasediments, or compared to target rocks from outside the crater rim. So far, no evidence for a meteoritic component has been detected in polymict impact breccias during this study, in agreement with previous work. [source]


    Geochemistry and shock petrography of the Crow Creek Member, South Dakota, USA: Ejecta from the 74-Ma Manson impact structure

    METEORITICS & PLANETARY SCIENCE, Issue 1 2004
    Crispin KATONGO
    The shocked minerals represent impact ejecta from the 74-Ma Manson impact structure (MIS). This study was aimed at determining the bulk chemical compositions and analysis of planar deformation features (PDFs) of shocked quartz; for the basal and marly units of the Crow Creek Member. We studied samples from the Gregory 84-21 core, Iroquois core and Wakonda lime quarry. Contents of siderophile elements are generally high, but due to uncertainties in the determination of Ir and uncertainties in compositional sources for Cr, Co, and Ni, we could not confirm an extraterrestrial component in the Crow Creek Member. We recovered several shocked quartz grains from basal-unit samples, mainly from the Gregory 84-21 core, and results of PDF measurements indicate shock pressures of at least 15 GPa. All the samples are composed chiefly of SiO2 (29,58 wt%), Al2O3 (6,14 wt%), and CaO (7,30 wt%). When compared to the composition of North American Shale Composite, the samples are significantly enriched in CaO, P2O5, Mn, Sr, Y, U, Cr, and Ni. The contents of rare earth elements (REE), high field strength elements (HFSE), Cr, Co, Sc, and their ratios and chemical weathering trends, reflect both felsic and basic sources for the Crow Creek Member, an inference, which is consistent with the lithological compositions in the environs of the MIS. The high chemical indices of alteration and weathering (CIA' and CIW': 75,99), coupled with the Al2O3 -(CaO*+Na2O)-K2O (A-CN'-K) ratios, indicate that the Crow Creek Member and source rocks had undergone high degrees of chemical weathering. The expected ejecta thicknesses at the sampled locations (409 to 219 km from Manson) were calculated to range from about 1.9 to 12.2 cm (for the present-day crater radius of Manson), or 0.4 to 2.4 cm (for the estimated transient cavity radius). The trend agrees with the observed thicknesses of the basal unit of the Crow Creek Member, but the actually observed thicknesses are larger than the calculated ones, indicating that not all of the basal unit comprises impact ejecta. [source]


    Geochemistry and source waters of rock glacier outflow, Colorado Front Range

    PERMAFROST AND PERIGLACIAL PROCESSES, Issue 1 2006
    M. W. Williams
    Abstract We characterize the seasonal variation in the geochemical and isotopic content of the outflow of the Green Lake 5 rock glacier (RG5), located in the Green Lakes Valley of the Colorado Front Range, USA. Between June and August, the geochemical content of rock glacier outflow does not appear to differ substantially from that of other surface waters in the Green Lakes Valley. Thus, for this alpine ecosystem at this time of year there does not appear to be large differences in water quality among rock glacier outflow, glacier and blockslope discharge, and discharge from small alpine catchments. However, in September concentrations of Mg2+ in the outflow of the rock glacier increased to more than 900,eq,L,1 compared to values of less than 40,eq,L,1 at all the other sites, concentrations of Ca2+ were greater than 4,000,eq,L,1 compared to maximum values of less than 200,eq,L,1 at all other sites, and concentrations of SO reached 7,000,eq,L,1, compared to maximum concentrations below 120,eq,L,1 at the other sites. Inverse geochemical modelling suggests that dissolution of pyrite, epidote, chlorite and minor calcite as well as the precipitation of silica and goethite best explain these elevated concentrations of solutes in the outflow of the rock glacier. Three component hydrograph separation using end,member mixing analysis shows that melted snow comprised an average of 30% of RG5 outflow, soil water 32%, and base flow 38%. Snow was the dominant source water in June, soil water was the dominant water source in July, and base flow was the dominant source in September. Enrichment of ,18O from ,10, in the outflow of the rock glacier compared to ,20, in snow and enrichment of deuterium excess from +,17.5, in rock glacier outflow compared to +,11, in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow. Copyright 2005 John Wiley & Sons, Ltd. [source]


    REE, Mn, Fe, Mg and C, O Isotopic Geochemistry of Calcites from Furong Tin Deposit, South China: Evidence for the Genesis of the Hydrothermal Ore-forming Fluids

    RESOURCE GEOLOGY, Issue 1 2010
    Yan Shuang
    Abstract The Furong tin deposit in the central Nanling region, South China, consists of three main types of mineralization ores, i.e. skarn-, altered granite- and greisen-type ores, hosted in Carboniferous and Permian strata and Mesozoic granitic intrusions. Calcite is the dominant gangue mineral intergrown with ore bodies in the orefield. We have carried out REE, Mn, Fe, and Mg geochemical and C, and O isotopic studies on calcites to constrain the source and evolution of the ore-forming fluids. The calcites from the Furong deposit exhibit middle negative Eu anomaly (Eu/Eu*= 0.311,0.921), except for one which has an Eu/Eu* of 1.10, with the total REE content of 5.49,133 ppm. The results show that the calcites are characterized by two types of REE distribution patterns: a LREE-enriched pattern and a flat REE pattern. The LREE-enriched pattern of calcites accompanying greisen-type ore and skarn-type ore are similar to those of Qitianling granite. The REE, Mn, Fe, and Mg abundances of calcites exhibit a decreasing tendency from granite rock mass to wall rock, i.e. these abundances of calcites associated with altered granite-type and greisen-type ores are higher than those associated with skarn-type ores. The calcites from primary ores in the Furong deposit show large variation in carbon and oxygen isotopic compositions. The ,13C and ,18O of calcites are ,0.4 to ,12.7, and 2.8 to 16.4,, respectively, and mainly fall within the range between mantle or magmatic carbon and marine carbonate. The calcites from greisen and altered granite ores in the Furong deposit display a negative correlation in the diagram of ,13C versus ,18O, probably owing to the CO2 -degassing of the ore-forming fluids. From the intrusion to wall-rock, the calcites display an increasing tendency with respect to ,13C values. This implies that the carbon isotopic compositions of the ore-bearing fluids have progressively changed from domination by magmatic carbon to sedimentary carbonate carbon. In combination with other geological and geochemical data, we suggest that the ore-forming fluids represent magmatic origin. We believe that the fluids exsolved from fractionation of the granitic magma, accompanying magmatism of the Qitianling granite complex, were involved in the mineralization of the Furong tin polymetallic deposit. [source]


    Geochemistry of K-feldspar and Muscovite in Rare-element Pegmatites and Granites from the Totoral Pegmatite Field, San Luis, Argentina

    RESOURCE GEOLOGY, Issue 4 2009
    Julio Oyarzbal
    Abstract The geochemistry of K-feldspar for K, P, Sr, Ba, Rb, Cs, Ga, and of muscovite for the same elements plus Nb and Ta, was used for proving the parental relationships of S-type granites and LCT (Li, Cs, Ta) rare-element pegmatites in the southernmost pegmatitic field of the Pampean pegmatite province in Argentina. The variation of K/Rb-Cs, K/Cs-Rb, K/Rb-Rb/Sr, K/Rb-Ba in K-feldspar from the granites and pegmatites show that they form an association with the evolutional sequence: granites , barren- to transitional pegmatites , beryl type, beryl-columbite-phosphate pegmatites , complex type of spodumene subtype pegmatites , albite-spodumene type , albite type pegmatites. This sequence reflects the regional distribution of the different magmatic units. The Ta-Cs diagram for muscovite reveals that none of the studied pegmatites exceed the threshold established in previous studies for being considered with important tantalum oxide mineralization. The granites and pegmatites constitute a rare-element pegmatitic field in which different magmatic units form a continuous fractionation trend, extended from the less evolved granitic facies to the most geochemically specialized pegmatites [source]


    Geochemistry, Petrography and Spectroscopy of Organic Matter of Clay-Associated Kerogen of Ypresian Series: Gafsa-Metlaoui Phosphatic Basin, Tunisia

    RESOURCE GEOLOGY, Issue 4 2008
    Mongi Felhi
    Abstract This work presents geochemical characterization of isolated kerogen out of clay fraction using petrography studies, infrared absorption and solid state 13C nuclear magnetic resonance (NMR) spectroscopy, with N -alkane distributions of saturated hydrocarbon. Mineralogical study of clay mineral associations was carried out using X-ray diffraction (XRD), on Ypresian phosphatic series from Gafsa-Metlaoui basin, Tunisia. The XRD data indicate that smectite, palygorskite and sepiolite are the prevalent clay minerals in the selected samples. In this clay mineral association, the N -alkane (m/z = 57) distribution indicates that the marine organic matter is plankton and bacterial in origin. The kerogens observed on transmitted light microscopy, however, appear to be totally amorphous organic matter, without any appearance of biological form. The orange gel-like amorphous organic matter with distinct edges and homogenous texture is consistent with a high degree of aliphaticity. This material has relatively intense CH2 and CH3 infrared bands in 13C NMR peaks. This aliphatic character is related to bacterial origin. Brown amorphous organic matter with diffuse edges has a lower aliphatic character than the previous kerogen, deduced from relatively low CH2 and CH3 infrared and 13C NMR band intensities. [source]


    Sulfur Isotope Geochemistry of the Supergiant Xikuangshan Sb Deposit, Central Hunan, China: Constraints on Sources of Ore Constituents

    RESOURCE GEOLOGY, Issue 4 2006
    Dong-sheng Yang
    Abstract. The supergiant Xikuangshan Sb deposit is located in the Middle to Upper Devonian limestone of central Hunan, China. Primary ores are composed of early-stage stibnite and calcite with rare pyrite, early main-stage stibnite and quartz, and late main-stage stibnite and calcite. New sulfur isotope data reveal the clustering of ,34S values (+5 , +8 %) for both early and late main-stage stibnite; a single early-stage stibnite exhibits ,34S value (+7.5 %) identical to its main ore-stage counterparts and the coexisting calcite has almost unmodified carbon isotope composition (-4.4 %). The data suggest a probable common source of sulfur for stibnite that was deposited at different paragenetic stages. A much wider variation in ,34S values for early main-stage stibnite (+3.5 to +16.3 %, av. +7.5 %) compared to that for late main-stage stibnite (+5.3 to +8.1 %, av. +6.2 %) can be interpreted to be due to local interaction of earlier ore fluid with Devonian host rocks. The previous studies show that the Precambrian basement contains elevated Sb concentrations, and two distinctive sulfur reservoirs with ,34Spyrite values at ca. +11 , +24 % and -7.0 ,-11 %. The homogenizing effect for sulfur hydrothermally leached from the two reservoirs might have provided ore constituents for the Xikuangshan fluids. [source]


    Helium Isotope Geochemistry of Ore-forming Fluids from Furong Tin Orefield in Hunan Province, China

    RESOURCE GEOLOGY, Issue 1 2006
    Zhao-li Li
    Abstract. The Furong tin orefield, located in southern Hunan, China, is a newly-discovered super-large tin orefield. In contrast to most other tin deposits associated with S-type granites, the Furong tin deposit is closely associated with the Qitianling A-type granite. The 3He/4He ratios of fluid inclusions in pyrite and arsenopyrite from this orefield range from 0.13 to 2.95 Ra. The influence of various post-mineralization processes on the helium isotopic composition of ore-forming fluid inclusions are estimated negligible. Thus, the analytical values of helium isotopic composition basically represent the original values of ore-forming fluids at the time they were trapped. The 3He/4He ratios of ore-forming fluids from the Furong orefield indicate the existence of mantle-source components. It supports the idea that both the Furong tin orefield and Qitianling granite formed under the geodynamic background of mantle upwelling and crustal extension. [source]


    Mineralogy, Geochemistry, and Age Constraints on the Beni Bou Ifrour Skarn Type Magnetite Deposit, Northeastern Morocco

    RESOURCE GEOLOGY, Issue 1 2002
    Mohammed EL RHAZI
    Abstract: The Beni Bou Ifrour deposit of northeastern Morocco is a skarn type magnetite deposit. K-Ar age determination suggests that the mineralization occurred at 7.040.47 Ma. The spatial relationship between skarn and dikes of microgran-odiorite derived from the batholith of Wiksane Granodiorite, and the similarity of age (8.020.22 Ma), confirms that the Wiksane Granodiorite is the igneous rock most probably related to mineralization. The skarn is distributed asymmetrically in the limestone, and magnetite ore was developed just below the calc-silicate skarn as two parallel beds separated by 100 m of barren limestone and schist. The mineralization can be divided into three stages. The early stage is characterized by the formation of calc-silicate minerals, mainly clinopyroxene (80,70 % diopside) and garnet (early almost pure andradite to the late 60 % andradite). The main stage is characterized by the formation of a large amount of magnetite. Epidote and quartz formed simultaneously with magnetite. Fluid temperatures exceeded 500 C during the early to main stages. Fluid with very high salinity (50,75 wt% NaCl equiv.) was responsible for the formation of the magnetite ore. The oxygen isotope composition, together with the fluid inclusion data, suggests that magmatic fluid was significant for the formation of calc-silicate skarn minerals and magnetite. Low temperature (-230C) and low salinity (-10 % NaCl equiv.) hydrothermal fluids dominated by meteoric water were responsible for the late stage quartz and calcite formation. [source]


    A Special Orogenic-type Rare Earth Element Deposit in Maoniuping, Sichuan, China: Geology and Geochemistry

    RESOURCE GEOLOGY, Issue 3 2001
    Denghong WANG
    Abstract: The Maoniuping REE deposit is the second largest light rare earth elements deposit in China, explored recently in the northern Jinpingshan Mountains, a Cenozoic intracontinental orogenic belt in southwestern China. It is a vein-type deposit hosted within, and genetically related to, carbonatite-alkalic complex. Field investigation and new geochemical data of the carbonatites from the carbonatite-alkalic complex support an igneous origin for the Maoniuping carbonatites and related REE mineralization. Carbonatite itself carries rare earth elements which were enriched by hydrothermal solution. It is known that most of the REE deposits related to carbonatite-alkalic complexes were formed in relatively stable tectonic setting such as cratonic or rifting environment. The Maoniuping deposit, however, was formed during the processes of Cenozoic orogeny. Although the Maoniuping deposit is located in the north sector of the Panxi paleo-rift zone, the rift had been closed before early Cenozoic and evolved into an intracontinental orogenic belt, i.e., the Jinpingshan Orogen, which was formed since later Mesozoic to early Cenozoic. Geochronological and geochemical data also prove that the Maoniuping REE deposit was formed in an intracontinental orogenic belt instead of rift system or stationary block. The Maoniuping REE deposit is similar to the Mountain Pass REE deposit in many respects such as the high contents of bastnaesite and barite, the low content of niobium, and the common occurrence of sulfides. The discovery of the Maoniuping deposit and other REE deposits during the past two decades suggest a good potential for prospecting REE deposits along the alkalic complex belt located on the eastern side of the Qinghai,Xizang,West Sichuan Plateau. [source]


    Geochemistry and Radioactive Potentiality of Um Naggat Apogranite, Central Eastern Desert, Egypt

    RESOURCE GEOLOGY, Issue 1 2000
    AFANDY, Adel H. EL
    Abstract: The northern part of Um Naggat granite massif (UNGM) has suffered extensive post-magmatic metasomatic reworking which results into the development of (Zr, Hf, Nb, Ta, U, Th, F), and albite-enriched and greisenized apogranite body (UNAP) of 600 m thick, and more than 3 km in the strike length. Albitization produced an enrichment in Zr (av. 2384 ppm), Hf (61), Nb (419), and U (43). The Th/U ratio ranges between 1. 33 and 1. 90. Extreme albitization (i. e. the albitite rock) is characterized by sharp decrease in the rare metals contents. However, extreme greisenization (i. e. endogreisen bodies) is characterized by a considerable enrichment in Zr (av. 5464 ppm), Hf (143), Nb (2329), Ta (152), U (66) and Th (178). The Th/U ratio ranges between 1. 57 and 3. 60. In contrast to extreme greisenization, it seems that extreme albitization does not apparently change the fluid pH and therefore poor amounts of rare metals are localized in the albitites. It is suggested that the presence of Na+, H+ and F - in the ore fluids was essential to stablize complexes of Zr, Hf, Nb, Ta, U, Th, and HREE during extraction and transportation. In contrast, contemporaneous decrease of temperature and increasing pH due to decreasing pressure are considered the essential factors for localization of disseminated mineralization of Zr and Nb in the apical parts of the UNAP. The enhanced uranium content in the alteration facies of UNAP coupled with the absence of significant uranium mineralization may indicate the metalliferous rather than mineralized nature for the UNAP. The high uranium contents are stabilized in refractory accessory minerals. However, with repect to Zr and Nb, the UNAP especially the albitized and greisen facies, can be categorized as a mineralized productive granite. [source]


    Geochemistry as an aid in archaeological prospection and site interpretation: current issues and research directions

    ARCHAEOLOGICAL PROSPECTION, Issue 1 2009
    S. Oonk
    Abstract Human habitation may significantly affect the chemical soil composition leading to enrichments and depletions of specific chemical elements and the formation of archaeological soils. Prospection and interpretation of sites by means of element analysis of soils requires appropriate and well-tested geochemical methodologies. In this paper element analysis in archaeological prospection and site interpretation is briefly reviewed and three major unresolved issues are discussed: (i) how to distinguish archaeological chemical signals in soils from modern and geogenic signals; (ii) what role do geochemical processes play in the formation of archaeological soils; (iii) how to implement geochemical methods in archaeological research programmes. These issues are addressed with reference to the geochemical literature on retention and sequestration pathways of presumed anthropogenic elements in soils and analytical procedures. Based on the acquired knowledge, several directions for future research are proposed. Copyright 2009 John Wiley & Sons, Ltd. [source]


    Regional Spatial Modeling of Topsoil Geochemistry

    BIOMETRICS, Issue 1 2009
    C. A. Calder
    Summary Geographic information about the levels of toxics in environmental media is commonly used in regional environmental health studies when direct measurements of personal exposure is limited or unavailable. In this article, we propose a statistical framework for analyzing the spatial distribution of topsoil geochemical properties, including the concentrations of various toxicants. Due to the small-scale heterogeneity of most geochemical topsoil processes, direct measurements of the processes themselves only provide highly localized information; it is thus financially prohibitive to study the spatial patterns of these processes across a large region using traditional geostatistical analyses of point-referenced topsoil data. Instead, it is standard practice to assess geochemical patterns at a regional scale using point-referenced measurements collected in stream sediment because, unlike topsoil data, individual stream sediment geochemical measurements are representative of the surrounding area. We propose a novel multiscale soils (MSS) model that formally synthesizes data collected in topsoil and stream sediment and allows the richer stream sediment information to inform about the topsoil process, which in environmental health studies is typically more relevant. Our model accommodates the small-scale heterogeneity of topsoil geochemical processes by modeling spatial dependence at an aggregate resolution corresponding to hydrologically similar regions known as watersheds. We present an analysis of the levels of arsenic, a toxic heavy metal, in topsoil across the midwestern United States using the MSS model and show that this model has better predictive abilities than alternative approaches using more conventional statistical models for point-referenced spatial data. [source]


    Outlines of New Global Geochemical Mapping Program

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010
    Xuejing XIE
    Abstract: Since 1988, great efforts and enthusiasm had been paid by applied geochemists in the implementation of global geochemical mapping through the International Geological Correlation Program's Projects 259 and 360, and the Task Group on ,Global Geochemical Baselines' established by the International Union of Geological Sciences (IUGS), in collaboration with the International Association of Geochemistry (IAGC). But how to use extremely low-density sampling to obtain a global picture of the distribution of most elements in the periodic table in a reasonably short time is still a great challenge faced by the applied geochemistry community. It will depend on the continuous development of new mapping concept, and the advisable and courageous innovation of methodology for searching other suitable sample media and sampling layout. Based on the encouragement results obtained from the representativeness study of delta sediments conducted at the mouth of Yangtze River, and at the mouths of its four major tributary, it is expected to broadly apply the geochemical fractal self-similarity nature to main rivers and their estuaries with catchments up to hundreds of thousands or over a million square kilometers in the world. With this new mapping concept, a new outlines of a Global Geochemical Mapping Program was advanced and the establishment of an International Research Center of Global Geochemical Mapping was also suggested to facilitate the programs implementations. [source]


    Neoproterozoic Mafic Dykes and Basalts in the Southern Margin of Tarim, Northwest China: Age, Geochemistry and Geodynamic Implications

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010
    Chuanlin ZHANG
    Abstract: Neoproterozoic rifting-related mafic igneous rocks are widely distributed both in the northern and southern margins of the Tarim Block, NW China. Here we report the geochronology and systematic whole-rock geochemistry of the Neoproterozoic mafic dykes and basalts along the southern margin of Tarim. Our zircon U-Pb age, in combination with stratigraphic constraint on their emplacement ages, indicates that the mafic dykes were crystallized at ca. 802 Ma, and the basalt, possibly coeval with the ca. 740 Ma volcanic rocks in Quruqtagh in the northern margin of Tarim. Elemental and Nd isotope geochemistry of the mafic dykes and basalts suggest that their primitive magma was derived from asthenospheric mantle (OIB-like) and lithospheric mantle respectively, with variable assimilation of crustal materials. Integrating the data supplied in the present study and that reported previously in the northern margin of Tarim, we recognize two types of mantle sources of the Neoproterozoic mafic igneous rocks in Tarim, namely the matasomatized subcontinental lithospheric mantle (SCLM) in the northern margin and the long-term enriched lithospheric mantle and asthenospheric mantle in the southern margin. A comprehensive synthesis of the Neoproterozoic igneous rocks throughout the Tarim Block led to the recognition of two major episodes of Neoproterozoic igneous activities at ca. 820,800 Ma and ca. 780,740 Ma, respectively. These two episodes of igneous activities were concurrent with those in many other Rodinian continents and were most likely related to mantle plume activities during the break-up of the Rodinia. [source]


    Petrology and Geochemistry of the Huangshan Granitic Intrusion in Anhui Province, Southeast China: Implications for Petrogenesis and Geodynamics

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010
    Shu ZHANG
    Abstract: The Huangshan granitic intrusion in Anhui province, SE China, is tectonically located at the southeastern boundary of the Yangtze Block. Based on the contact relation and the petrography, the Huangshan granitic intrusion can be divided into four stages, from early to late, medium-grained monzogranite, coarse-grained porphyric granite, fine-medium grained porphyric granite, and fine-coarse grained granite. All rocks from the Huangshan granitic intrusion display similar petrological and geochemical characteristics, i.e. relatively high SiO2 (>75%) and alkali (7.85%,8.59%), low CaO(<1%), high Fe-number (FeOT/(MgO+FeOT) = 0.93,0.97) and A/CNK (atomic Al/(Ca+Na+K)) = 1.04,1.19. They are also enriched in rare earth elements (REE, except for Eu, with a total REE contents ranging from 116 ppm to 421 ppm), high strength field elements such as Zr, Hf, Nb, but depleted in Ba, Sr and Ni. The 10 000Ga/Al ratios are higher than 2.6, which are consistent with the A-type granitoids. Based on the classification diagrams proposed by Eby, the Huangshan granite can be classified into the A2 group, which is usually believed to be formed under an extensional tectonic setting. Their Nd isotopic compositions suggest that the primary magmas of the Huangshan granite are predominantly derived from the Proterozoic andesitic rocks in the region, and this conclusion is also supported by REE modeling. The systemic investigations on the geochemistry of the Huangshan granitic intrusion can provide significant implications for the understanding of the petrogenesis and the geodynamic regime of southeastern China during the Late-Mesozoic. [source]


    REE and C-O Isotopic Geochemistry of Calcites from the World-class Huize Pb-Zn Deposits, Yunnan, China: Implications for the Ore Genesis

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010
    Zhilong HUANG
    Abstract: The world-class Huize Pb-Zn deposits of Yunnan province, in southwestern China, located in the center of the Sichuan-Yunnan-Guizhou Pb-Zn polymetallic metallogenic province, has Pb+Zn reserves of more than 5 million tons at Pb+Zn grade of higher than 25% and contains abundant associated metals, such as Ag, Ge, Cd, and Ga. The deposits are hosted in the Lower Carboniferous carbonate strata and the Permian Emeishan basalts which distributed in the northern and southwestern parts of the orefield. Calcite is the only gangue mineral in the primary ores of the deposits and can be classified into three types, namely lumpy, patch and vein calcites in accordance with their occurrence. There is not intercalated contact between calcite and ore minerals and among the three types of calcite, indicating that they are the same ore-forming age with different stages and its forming sequence is from lumpy to patch to vein calcites. This paper presents the rare earth element (REE) and C-O isotopic compositions of calcites in the Huize Pb-Zn deposits. From lumpy to patch to vein calcites, REE contents decrease as LREE/HREE ratios increase. The chondrite-normalized REE patterns of the three types of calcites are characterized by LREE-rich shaped, in which the lumpy calcite shows (La)N < (Ce)N < (Pr)N, (Nd)N with Eu/Eu* < 1, the patch calcite has (La)N < (Ce)N < (Pr)N, (Nd)N with Eu/Eu* > 1, and the vein calcite displays (La)N > (Ce)N > (Pr)N > (Nd)N with Eu/Eu* > 1. The REE geochemistry of the three types of calcite is different from those of the strata of various age and Permian Emeishan basalt exposed in the orefield. The ,13CPDB and ,18OSMOW values of the three types of calcites vary from ,3.5, to ,2.1, and 16.7, to 18.6,, respectively, falling within a small field between primary mantle and marine carbonate in the ,13CPDB vs ,18OSMOW diagram. Various lines of evidence demonstrate that the three types of calcites in the deposits are produced from the same source with different stages. The ore-forming fluids of the deposits resulted from crustal-mantle mixing processes, in which the mantle-derived fluid components might be formed from degassing of mantle or/and magmatism of the Permian Emeishan basalts, and the crustal fluid was mainly provided by carbonate strata in the orefield. The ore-forming fluids in the deposits were homogenized before mineralization, and the ore-forming environment varied from relatively reducing to oxidizing. [source]


    Geochemistry and Genesis of the Late Jurassic Granitoids at Northern Great Hinggan Range: Implications for Exploration

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2010
    Guang WU
    Abstract: The Longgouhe and Ershiyizhan intrusions of the Late Jurassic, located in the Upper Heilongjiang Basin of the northern Great Hinggan Range, are closely related to porphyry Cu-Au mineralizations. In lithology the intrusions are quartz diorite, quartz monzodiorite and granodiorite of high-K calc-alkaline series, with minor aspects of shoshonite series. Their SiO2 and Al2O3 contents range from 61.37% to 66.59% and 15.35% to 17.06%, respectively. The MgO content ranges from 2.02% to 3.47%, with Mg# indices of 44,59. The (La/Yb)N and Eu/Eu* values range from 16.85 to 81.73 and 0.68 to 0.93, respectively, showing strong differentiation rare earth element (REE) patterns similar to those of adakites. The rocks are enriched in Ba, Sr and light REE (LREE), obviously depleted in Nb and Ta, slightly depleted in Rb and Ti, and poor in Yb and Y, with Yb and Y contents of 0.31,1.32 ppm and 4.32,12.07 ppm, respectively. As indicated by Sr/Y ratios of 67.74,220.60, the rocks are characterized by low-Y and high-Sr contents, which characterize the adakites in the world. Holistically, geochemical tracers suggest that the interested intrusions are adakitic rocks. Given that the Paleo-Asian Ocean and Mongol-Okhotsk Ocean were closed in the Late Paleozoic and Permian-Middle Jurassic, respectively, the interested intrusions should be formed by partial melting of delaminated crust, which had been thickened during collisional orogeny between the Siberian and Mongolian-Sinokorean continents. [source]


    Thermochemical Sulfate Reduction in the Tazhong District, Tarim Basin, Northeast China: Evidence from Formation Water and Natural Gas Geochemistry

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2010
    Caifu XIANG
    Abstract: Systematic analyses of the formation water and natural gas geochemistry in the Central Uplift of the Tarim Basin (CUTB) show that gas invasion at the late stage is accompanied by an increase of the contents of H2S and CO2 in natural gas, by the forming of the high total dissolved solids formation water, by an increase of the content of HCO,3, relative to Cl,, by an increase of the 2nd family ions (Ca2+, Mg2+, Sr2+ and Ba2+) and by a decrease of the content of SO2,4, relative to Cl,. The above phenomena can be explained only by way of thermochemical sulfate reduction (TSR). TSR often occurs in the transition zone of oil and water and is often described in the following reaction formula: ,CH+CaSO4+H- 2O,H2S+CO2+CaCO3. (1) Dissolved SO2,4 in the formation water is consumed in the above reaction, when H2S and CO2 are generated, resulting in a decrease of SO2,4 in the formation water and an increase of both H2S and CO2 in the natural gas. If formation water exists, the generated CO2 will go on reacting with the carbonate to form bicarbonate, which can be dissolved in the formation water, thus resulting in the enrichment of Ca2+ and HCO,3. The above reaction can be described by the following equation: CO2+H2O+CaCO3,Ca2++2HCO,3. The stratigraphic temperatures of the Cambrian and lower Ordovician in CUTB exceeded 120C, which is the minimum for TSR to occur. At the same time, dolomitization, which might be a direct result of TSR, has been found in both the Cambrian and the lower Ordovician. The above evidence indicates that TSR is in an active reaction, providing a novel way to reevaluate the exploration potentials of natural gas in this district. [source]


    Geochemistry of Heavy Oil in the T Block, Oriente Basin and its Origin Mechanism

    ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2010
    Ying HU
    Abstract: Exploration and development experience show that there is obvious oil gravity difference between the southern part and northern part of the "M1" reservoir in the Fanny oil field in the slope of the Oriente Basin, Ecuador. The American Petroleum Institute Gravity (API) values of oils in the northern part are higher than the one in the southern part of the Fanny oil field, with the values of 20 and 10.0,13.0, respectively. So the primary purpose of this study was to analyze the heavy oil characteristics of biodegradation and the oil,oil correlation according to the biomarker data and the ,13C signature of oil samples from T block. The results of the hydrocarbon gas chromatography fingermark and the inversion attribute characteristics indicated that there are fluid compartments between the "M1" reservoir of Fanny south. Finally, the models of oil,gas accumulation under the control of multiple-activities of complicated fault systems, as well as the origin of heavy oil, are contended. The early stage oils from the western part of the basin were biodegraded heavily in varying degrees in the whole basin, and the later stage oils were derived from the southern part in a large scale and were mature and lighter. Generally, oil mixing is the primary control of net oil properties, such as API gravity in Oriente Basin. We therefore predicted that the API gravity variation of oil pools radically depends on the injection amount of the later stage oil. Because of the shale barrier in the "M1" reservoir of Fanny south, the later stage hydrocarbon could not pass through and contribute to increase the oil API value. [source]