Acta Geochimica (v.30, #3)

Comparison between the Xishimen gold deposit and the Shihu gold deposit in western Hebei and analysis of the potential for ore exploration by Chao Chen; Shuyin Niu; Zili Wang; Zhong Guo; Aiqun Sun; Baode Wang; Yincang Gao (281-289).
The Xishimen gold deposit is located in the tectonic zone southeast to the Shihu gold deposit, both of which belong to the same orefield. Then, how is the prospecting potential for the Xishimen gold deposit? A comparison was made between the Xishimen gold deposit and the Shihu gold deposit on the basis of their geological background of metallogenesis, geological characteristics of the ore deposits, the sources of ore-forming materials, etc. The results showed that both the deposits possess many similarities in the respects mentioned above. And their metallogeneses can be compared. Therefore, there is great potential for ore prospection in the periphery or at the deep levels of the Xishimen gold deposit. Ore exploration should be well programmed on the basis of a better dissection of the tectonic setting of the orefield. A breakthrough in ore prospection of the Xishimen gold deposit will be of great instructive significance in regionally geological prospecting.
Keywords: gold deposit; metallogenesis; orefield structure; comparative study; potential; West Hebei

Semial ophiolite veins of United Arab Emirates—A fluid inclusions study by Mohamed El Tokhi; Abdulla Musallam (290-294).
Calcite and quartz veins in the gabbroic and dunite rocks of the Semial ophiolite (UAE) were selected for fluid inclusion analysis. The inclusions contain both aqueous low-salinity and hydrocarbon-dominated fluids. Microthermometry data indicate that the aqueous fluids contain 0.22 to 1.45 equivalent wt% NaCl and occasionally contain traces of hydrocarbons. Homogenization to liquid occurred between 91 and 152°C. Modeling based on these fluid inclusion observations indicates that the trapping conditions of the studied rocks were subjected to temperatures of 162 and 172°C, occasional pressures of 610–710 Pa and the sequence percolation of aqueous and hydrocarbon fluids.
Keywords: mafic; ultramafic; fluid inclusion; hydrocarbon; vein; UAE

Variations in nitrogen isotopic values among various particle-sized fractions in modern soil in northwestern China by Weiguo Liu; Zhoufeng Wang; Zheng Wang; Xiahong Feng; Pu Zhang (295-303).
Ratios of stable nitrogen isotopes in organic matter derived from plants and preserved in soil are potential tracers for nitrogen cycles in natural ecosystems and valuable for evaluation of climate change. However, the relationship between nitrogen isotopic compositions in surface soil and in plant litter during the decomposition process from plant litter to soil organic matter is not well understood. By using nitrogen isotopic analysis of soil particle-sized fractions, nitrogen isotope discrimination between plant litter and surface soil organic matter in various modern ecosystems in northwestern China was conducted. The results of our study indicate that: (1) in general, the nitrogen isotopic compositions of particle-sized fractions from surface soil are different, and δ15N values increase from plant litter to fine soil organic matter; (2) the δ15N values in the soil particle-sized fractions become larger with increasing relative humidity and temperature, and the largest variation in the δ15N values is from −5.9‰ to −0.3‰; and (3) under a controlled climate, significant nitrogen isotope differences in δ15N values (Δδ15Nplant-soil) between plant litter and bulk soil organic matter were observed, with the values of 1.52 to 4.75 at various sites. Our results suggested that comparisons of Δδ15N values between bulk soil and the particle-sized fractions of soil could reveal the effect of humidity on transferring process of nitrogen from plant to soil in arid and semi-arid ecosystems.
Keywords: nitrogen isotope; soil; particle-sized fraction; plant decomposition

This study is concerned with the radioactivity and mineralogy of the younger granites and pegmatites in the Wadi Haleifiya area, southeastern Sinai Peninsula, Egypt. The area is occupied by metasediments, migmatites, older and younger granites. Most of these rocks, especially granites, are dissected by mafic and felsic dykes as well as pegmatites.The younger granites are represented by three main varieties: monzogranites, syenogranites and alkali feldspar granites. The monzogranite consists essentially of quartz, plagioclase, potash feldspar and biotite with minor muscovite. Iron oxide, titanite, zircon and allanite are the main accessory minerals. Syenogranite is massive, medium- to coarse-grained and commonly exhibits equigranular and hypidiomorphic textures. It is made up essentially of potash feldspar, quartz, plagioclase and biotite. Iron oxides, allanite, epidote, titanite, and zircon are accessory minerals. The alkali feldspar granite consists mainly of perthite, quartz, alkali amphibole (arfvedsonite and riebekite), biotite, subordinate plagioclase and aegirine. Iron oxide, zircon and apatite are accessory minerals, whereas chlorite and sassurite are secondary minerals.The altered monzogranite and pegmatite recorded high radioelement contents. The eU reaches up to 120 (av.=82×10−6) in the altered monzogranite and up to 55 (av.=27×10−6) in the pegmatites. The high radioactivity in the altered monzogranite is due to the presence of thorite, uranothorite and metamict zircon. In the pegmatites, it is related to the presence of uranophane, uranothorite, thorite, zircon, samarskite, monazite, xenotime, magnetite, ilmenite, hematite and rutile.
Keywords: Wadi Haleifiya; younger granite; pegmatite; uranophane; uranothorite; thorite

Rainfall runoff is a critical hydrological process related to soil erosion and agricultural non-point pollution. In this study, 25 simulation experiments on rainfall were carried out in five runoff plots. Rape (Brassica campestris) was planted on the downslope of the plots. Experiments were conducted when the vegetation coverage reached 80%. Each plot was subjected to five rainfall events differing in intensity. The results showed: (1) the runoff coefficients of overland flow and subsurface flow were less than 0.6 and 0.005, respectively; (2) the discharge of overland flow was the quadratic function of time; (3) runoff coefficient was the function of slope gradient and rainfall intensity. When the slope gradient increased from 8.7% to 46.6%, the runoff coefficient of overland flow first increased and then decreased. The runoff coefficient reached the maximum when the slope gradient was within the range of 17.6%–36.4%; and (4) the process of subsurface flow generation included the increasing phase and recession phase. Discharge was a logarithm function of time in the increasing phase, and an exponential function in the recession phase. Runoff coefficient of subsurface flow decreased first and then increased when the slope gradient varied from 8.7% to 46.6% and was not correlated with rainfall intensity.
Keywords: purple soil; sloping cropland; rainfall simulation; overland flow; subsurface flow

The Wadi Sikait area lies at about 95 km southwest of Marsa Alam City along the Red Sea Coast, Eastern Desert, Egypt. It is occupied by Precambrian rocks of ophiolitic mélange, metamorphosed sandstones (MSS), gabbros and monzogranites which were later intruded by lamprophyre dykes and quartz veins.The lamprophyre dykes were extruded in NW-SE and NE-SW trends cutting monzogranites and metamorphosed sandstones. The lamprophyres are porphyritic and composed of clinopyroxene, olivine and amphibole phenocrysts enclosed in a fine-grained groundmass of clinopyroxene, amphibole, opaque and lithium mica. The alteration products are represented by amphibole (tremolite-actinolite and hornblende), carbonate, epidote, chlorite, iddingsite, clay minerals, limonite and serpentine.The Sikait lamprophyre dykes can be classified as alkaline lamprophyres characterized by silica contents ranging from 41.65 wt% to 50.88 wt% and Na2O>K2O. They are enriched in LILE, LREE and HFSE, but strongly depleted in compatible elements such as Cr and Ni relative to the primitive mantle. Sikait lamprophyres have moderate Zr/Hf (35.6–52.8) and Nb/Ta (20.5–22.5) ratios. Most of these features are attributed to the origin of these dykes from the metasomatized mantle affected by subduction-related fluid. These lamprophyres are compositionally similar to Salu lamprophyres in eastern China.The Sikait lamprophyre samples have high LREE (320×10−6–419×10−6) relative to HREE (20×10−6–33×10−6) with ratios (LREE/HREE=11.6–18.7) and no negative Eu anomaly (Eu/Eu*=0.9–1.04). The relative presence of positive Ce anomaly (Ce/Ce*=1.04) in lamprophyre samples suggests the oxidizing condition under which the REEs were precipitated due to the common occurrence of fluorite and apatite.
Keywords: Sikait; lamprophyre; metasomatised; REEs

In order to better understand the paleoceanographic sedimentary environment of the Lower Cambrian black shales extensively distributed in South China, outcropped along the present southern margin of the Yangtze Platform with a width of ca. 200–400 km and a length of more than 1500 km, we present new paired δ13C data on carbonates (δ13Ccarb) and associated organic carbon (δ13Corg) and δ34Spy data on sedimentary pyrite in black shales from three sections (Ganziping, Shancha and Xiaohekou) located in NW Hunan, China. In these sections, a total of 82 Lower Cambrian black shale samples have δ13Ccarb values ranging from −4.0‰ to 1.7‰ with an average value of −2.1‰, and δ13Corg values between −34.9‰ and −28.8‰, averaging −31.9‰. The δ34Spy values of 16 separated sedimentary pyrite samples from the black shales vary between +10.2‰ and +28.7‰ with an average value of +19.5‰, presenting a small isotope fractionation between seawater sulfate and sedimentary sulfide. The model calculation based on credible data from the paired analyses for δ13Ccarb and δ13Corg of 11 black shale samples shows a high CO2 concentration in the Early Cambrian atmosphere, about 20 times higher than pre-industrial revolution values, consistent with previous global predictions. The small sulfur isotope fractionation between seawater sulfate and sedimentary sulfide in black shales, only 15.5‰ on average, implies a low sulfate level in the Early Cambrian seawater around 1 mmol. In combination with a high degree of pyritization (DOP) in the black shales, it is suggested that sulfidic deep-ocean water could have lingered up to the earliest Cambrian in this area. The black shale deposition is envisaged in a stratified marine basin, with a surface euphotic and oxygenated water layer and sulfidic deeper water, controlled by a continental margin rift.
Keywords: black shale; stable isotope; trace element; sedimentary environment; Lower Cambrian; Yangtze Platform

Based on the geochemical characteristics of oil-cracking gas and kerogen-cracking gas revealed by simulation experiments and the chemical composition of natural gases in actual gas reservoirs, two kinds of natural gases with different relationships between C2/C3 and C1/C2, C2/C3 and C1/C3, C2/C3 and 100×C1/(C1–C5) were identified in the Tarim Basin, and proposed further by the authors. The relationship charts of C2/C3 and C1/C2, C2/C3 and C1/C3, C2/C3 and 100×C1/(C1–C5) can be used to effectively distinguish oil-cracking gas from kerogen-cracking gas. Petroleum geological analysis of the oil-cracking gas reservoirs showed that the distribution of oil-cracking gas is mostly related with deep-seated faults or faults with a large fault throw, and the burial depth of paleo-oil reservoir is relatively high; crude oil-cracking gas resources have been evaluated by using both forward and inversion methods. The plots of C2/C3 vs. C1/C2, C2/C3 vs. C1/C3, and C2/C3 vs. 100×C1/(C1–C5) were used to distinguish between oil-cracking gas and kerogen-cracking gas, and estimate the mixed ratios of the two kinds of natural gases in the main gas reservoirs of the platform area.
Keywords: oil-cracking gas; identification mark; forming condition; resource evaluation; Tarim Basin

Geochemistry and origin of deep-seated cracked gas on the northern slope of the Dongying Sag, Shandong Province by Xiangchun Chang; Zuozhen Han; Youde Xu; Xiaofei Shang; Chengpeng Yan (353-358).
Deep-seated cracked gas exploration was achieved great breakthrough in Es4 L of the Minfeng area on the northern slope of the Dongying Sag. Carbon isotopic and molecular compositions studies revealed the characteristics of wet gas and a normal trend of carbon isotopic composition. Empirical cutoff points of δ13C2 and δ13C3 and light hydrocarbon compositions distinguished the cracking gas as sapropelic gas. Variations in i/nC5, i/nC4 and δ13C2 further confirmed that the gas was cracked from residual kerogen. Source characteristics indicated that the gas was derived from mature-highly mature source rocks of Es4 with kerogen type II being dominant mixed with some oil-cracking gas. Burial history modeling indicated that there were two hydrocarbon charging periods in Es4 L reservoirs. The first period refers to the Guantao-Minghuazhen stage dominated by oil charging, while the second period refers to the Minghuazhen stage and has been dominated by cracked gas charging till now.
Keywords: cracking gas; source rock; geochemistry; Dongying Sag

Based on the chromatograms of oils and saturated hydrocarbons, biomarkers and stable carbon isotope analyses, the geochemical characteristics and oil family Classification of crude oils from the Markit Slope in the southwest of the Tarim Basin were investigated. The results showed that crude oils from the Markit Slope are divided into two oil family Classification. Oils collected from the Bashituo oilfield in the western part of the Markit Slope are characterized by high contents of tricyclic terpanes, pregnane, and homopregnane, low contents of garmmacerane (G/H<0.20), dibenzofuran, and methyl cyclohexane, and light stable carbon isotopic values (the δ13C values of saturated hydrocarbons and aromatic components are less than −34‰ and −32‰, respectively), with the distribution type of steranes being C27≫C2829. Condensate oils collected from the Hetian River gasfield in the eastern part of the Markit Slope are characterized by low contents of tricyclic terpanes, pregnane, and homopregnane, high contents of garmmacerane (0.2013C of saturated hydrocarbons and aromatic components (> −32‰ and −30.6‰, respectively), with the distribution type of steranes being C27>C28≪C29. In addition, oils from Well Qu 1 have some characteristics similar to those of the above oils, with some special characteristics including high contents of β-carotenes and special distribution of steranes (C2728 29).
Keywords: Tarim Basin; Markit Slope; Bashituo oilfield; Hetian River gasfield; genetic type

To assess the effects of single and combined pollution of cadmium (Cd) and mercury (Hg) on soil microbial community structural and functional diversities, an incubation experiment was conducted, by employing two soils, namely, the marine sediment silty loam soil and the yellowish-red soil, in which five levels of Cd, Hg and Cd and Hg in combination were added. After being incubated for 56 days, the phospholipid fatty acids (PLFAs) profile and sole carbon source utilization pattern (BIOLOG) of the samples were tested. The results showed that the composition of the microbial communities changed significantly at different levels of metals application. The principal component analyses (PCA) of PLFAs indicated that the structure of the microbial community was also significantly altered with increasing levels of metals, with increasing PLFAs biomarkers for fungi and actinomycetes, and increasing ratio of Gram-positive to Gram-negative bacteria. Sole carbon source utilization pattern analysis revealed that single and combined application of Cd and Hg inhibited significantly the functional activity of soil microorganisms, the functional diversity indices [Richness (S), Shannon-Wiener indices (H) and Evenness (E H )] were significantly lower in polluted soils than those in non-polluted soils, which also significantly altered with increasing levels of metals. PCA for the sole carbon source utilization pattern also indicated that the metal contamination could result in a variable soil microbial community. The results revealed that the combination of Cd and Hg had higher toxicity to soil microbial community structural and functional diversities than the individual application of Cd or Hg.
Keywords: microbial community; structural diversity; functional diversity; cadmium; mercury; soil contamination; PLFA; Biolog

Adsorption experiments were made at room temperature and neutral pH value on different types of minerals associated with the Lower Cambrian black shale series polymetallic layers in Hunan and Guizhou provinces on nanometer-sized Pt colloids and PtCl4 2−-bearing ionic solutions with an attempt to constrain the relationship between the different types of minerals in the polymetallic layers and the enrichment of platinum group elements (PGEs). Experimental results showed that the different types of minerals show strong selectivity to the adsorption of nanometer-sized Pt colloids and PtCl4 2−-bearing ionic solutions. Metallic sulfides, organic matter and clay minerals are the strong adsorbents of PGEs, while quartz, albite, muscovite and other silicate minerals show a week adsorbility to both of them. This phenomenon is well consistent with the geological fact that metallic sulfides, organic matter and clay minerals in the polymetallic layers of the black shale series are the major carrier minerals of PGEs, giving a thorough explanation to the mechanism of enrichment of previous metal elements. Adsorption may be a principal mechanism of enrichment of precious metal elements under lower temperature conditions. The presence of the aforementioned strong adsorbents is the good geochemical barriers for the enrichment of PGEs.
Keywords: Hunan; Guizhou; Lower Cambrian; black shale; adsorption; precious metal; enrichment process

Early cambrian carbon isotope stratigraphy in the tarim basin and a correlation with the yangtze platform by Chunyan Yao; Dongsheng Ma; Haifeng Ding; Xiaoyong Zhang (382-390).
Carbon isotope chemostratigraphy has been used worldwide for stratigraphic correlation. In this study, δ13Ccarb values are estimated for the Early Cambrian Sugaitebulake section in the Tarim Basin, Xinjiang Autonomous Region, China. As a result, one positive and two negative carbon isotope excursions in the studied section were identified. The δ13Ccarb values reached the maximum negative excursion (N1: −12.39‰) at the basal of the Yuertusi Formation, and then increased to P1. After P1, δ13Ccarb values sharply decreased to about −7.06‰ (N2) in the studied section. The pattern of δ13Ccarb in the Early Cambrian is comparable to the synchronous records of other sections, such as the Laolin section, the Xiaotan section and the Anjiahe section of the Yangtze Platform. It is concluded that the Early Cambrian Yuertusi Formation from the Tarim Basin is within the Nemakit-Daldynian stage, and the lower strata of the Yuertusi Formation may belong to the Zhujiaqing Formation (Meishucun Formation) of the Yangtze Platform. The Ediacaran/Cambrian boundary of the Tarim Block should be located in cherts and phosphorites successions at the basal of the Yuertusi Formation. The δ13Ccarb negative excursion N1 is just across the PC/C boundary, and may be related to certain biomass extinction due to anoxic sedimentary environment, transgression and/or the oceanic overturn. The second δ13Ccarb negative excursion N2 may account for the sea-level falling in the Early Cambrian.
Keywords: carbon isotope; Early Cambrian; stratigraphic correlation; Tarim Basin

Geological and geochemical characteristics and metallogenic model of the Wenquan molybdenum deposit by Zicheng Wu; Jishun Liu; Haitao Han; Xin Dong; Yufei Ouyang (391-397).
The Wenquan molybdenum deposit is a kind of large-sized porphyry molybdenum deposit found in recent years. In this paper, on the basis of deposit geology, geochemistry and isotope geochronology data, the metallogenic model of this deposit was established. The Wenquan granitic batholith belongs to the K-rich (alkali-rich) calc-alkaline rock series, which is the mineralization parent rock. The rock massif shows the characteristics of both crust-remelting granite and mantle-source granite. At the same time, the data of REE contents, hydrogen and oxygen isotopes and inclusion temperatures showed that the metallogenic hydrothermal solution is a mixed mesothermal solution of magmato-hydrothermal fluid and meteoric water. Mineralization was dated at 214±7.1 Ma, basically identical with the parent rock’s age (207–226 Ma). This reflects that molybdenum mineralization has a close relation to tectonic magmatism evoked by orogenic processes, and molybdenum mineralization occurred mainly at the petrogenesis stage at the late stage of magma emplacement. Mixing with meteoric water led to a decrease in the salinity of magmato-hydrothermal solution and changes in other physical and chemical properties. During the tectonic process, ore-bearing hydrothermal solution ascended along favorable fault structure channels. With physicochemical changes, it filled in the surrounding rock joints on both sides of faults, forming ore deposits.
Keywords: geological and geochemical characteristics; granitic batholith; porphyry; molybdenum deposit; metallogenic model; Wenquan

The source of marine crude oils from Tarim Basin is still disputed. However, research on the reservoir bitumens may provide a piece of new evidence to elucidate oil source. Geochemical characteristics of reservoir bitumens from different strata from Well Zhong 1 in the Katake Uplift were discussed here in detail. Pr/Ph ratios of reservoir bitumen from Well Zhong 1 range from 1. 14 to 1. 39, CPI ratios from 1.01 to 1.08, which indicates no odd-even predominance of normal alkanes, with OEP ratios ranging from 0.98 to 1.05. The contents of gammacerane and C28 sterane of reservoir bitumen from Well Zhong 1 are low, while dibenzothiophene series are abundant. Sterane isomerization maturity parameter and Ts/(Ts+Tm) ratio indicate that the reservoir bitumen from Well Zhong1 is mature to highly mature. Based on the differences in biomarker distribution, it is concluded that reservoir bitumen from Well Zhong 1, of which molecular parameters are contrary to those in Cambrian oil from Well Tadong 2, originated from the Upper Ordovician source rocks.
Keywords: Katake Uplift; reservoir bitumen; biomarker assemblage; maturity; oil-source rock correlation