Anomalous changes of CH4/Ar ratio in subsurface gas bubbles as seismogeochemical precursors at Matsuyama,Japan

The results of seismogeochemical monitoring of subsurface gases associated with ground-waters from deep artesian wells in Matsuyama, Japan, are presented. The weekly collections of bubble-gas samples from the two borehole wells and gas chromatographic determinations of their He/Ar, H₂/Ar, N₂/Ar, and CH₄/Ar ratios for the recent nineteen months have revealed that the bubble-gas CH₄/Ar ratios at the two stations are correlated with the seismic activity in this area. Positive anomalies of the CH₄/Ar ratio were accompanied by five earthquakes with relatively large magnitudes and/or relatively small epicentral distances among thirty earthquakes that occurred during the monitoring period. The anomalous increases in the CH₄/Ar ratio had evidently begun prior to the respective earthquakes.The occurrence of the geochemical anomaly appears to depend strongly upon the earthquake magnitude and distance, because the earthquakes that accompanied the anomalies and those that did not can be reasonably distinguished in the plot of magnitude vs. distance. Similar anomalies can be also seen in the variations of He/Ar and N₂/Ar ratios, but their magnitudes are much smaller than those of the CH₄/Ar ratio. These features have been discussed in relation to the possible mechanism of the seismogeochemical phenomena.     

Characterization of Aquatic Dissolved Organic Matter by Spectral Analysis： From Watershed to Deep Ocean

Since 1990s， spectral analysis has become an important technique to characterize the properties of chromophoric and fluorescent dissolved organic matter （CDOM and FDOM） from various aquatic systems and a series of spectral indices have been suggested to trace the sources of DOM and their biogeochemical regulation processes. DOM samples were collected from an aquatic continuum from watershed to deep ocean， i.e. Zhangjiang River and Estuary， Dongshan Bay， Taiwan Strait， Northeast basin of the South China Sea， Luzon Strait and the vertical profile of the Kuroshio region of the West Pacific Ocean. This continuum covered many critical interfaces （land-ocean， shelf-basin， marginal sea basin-open ocean and euphotic and aphotic layer）. The spatial distribution and variation of various qualitative and quantitative parameters along the continuum were clearly revealed. Combined with literature review， the sources and sinks of CDOM/FDOM and their inherent regulation processes under significant hydrological and biogeochemical gradient variation were systematically summarized. The geochemical differentiation of the quantitative DOM spectral index in various aquatic systems was discussed. The tracing ability of the qualitative DOM spectral index was commented. The coupling study of soil-river organic matter systems， mechanism of mineralization-related microbial production of CDOM/FDOM， quantified geochemical framework concept and perturbation of global change on CDOM/FDOM dynamics were suggested as future key topics.     

Groundwater geochemistry of Ain Azel area,Algeria

Hydrogeochemical data for 18 groundwater samples and 11 hydrochemical parameters were subjected to Q- and R-mode cluster analysis and inverse geochemical modeling. Q-mode cluster analysis resulted in three distinct water types (brackish water type, saline water type and highly saline water type). R-mode cluster analysis led to the conclusion that the water–rock interaction is the major source of contamination for the groundwater in the area. Geochemical modeling results show that carbonates, gypsum, halite, carbon dioxide (gas), and chlorite are dissolving, whereas Ca-montmorillonite, gibbsite, illite, K-mica, kaolinite, and quartz are mostly precipitating along different flow paths in the groundwater system of the area.     

Geochemical comparison of waters and stream sediments close to abandoned Sb-Au and As-Au mining areas,northern Portugal

Waters from abandoned Sb-Au mining areas have higher Sb (up to 2138 μg L⁻¹), As (up to 1252 μg L⁻¹) and lower Al, Zn, Li, Ni and Co concentrations than those of waters from the As-Au mining area of Banjas, which only contain up to 64 μg L⁻¹ As. In general, Sb occurs mainly as SbO₃⁻ and As H₂AsO₄⁻. In general, waters from old Sb-Au mining areas are contaminated in Sb, As, Al, Fe, Cd, Mn, Ni and NO₂⁻, whereas those from the abandoned As-Au mining area are contaminated in Al, Fe, Mn, Ni, Cd and rarely in NO₂⁻. Waters from the latter area, immediately downstream of mine dumps are also contaminated in As. In stream sediments from Sb-Au and As-Au mining areas, Sb (up to 5488 mg kg⁻¹) and As (up to 235 mg kg⁻¹) show a similar behaviour and are mainly associated with the residual fraction. In most stream sediments, the As and Sb are not associated with the oxidizable fraction, while Fe is associated with organic matter, indicating that sulphides (mainly arsenopyrite and pyrite) and sulphosalts containing those metalloids and metal are weathered. Arsenic and Sb are mainly associated with clay minerals (chlorite and mica; vermiculite in stream sediments from old Sb-Au mining areas) and probably also with insoluble Sb phases of stream sediments. In the most contaminated stream sediments, metalloids are also associated with Fe phases (hematite and goethite, and also lepidocrocite in stream sediments from Banjas). Moreover, the most contaminated stream sediments correspond to the most contaminated waters, reflecting the limited capacity of stream sediments to retain metals and metalloids.     

Geochemical comparison between gas in fluid inclusions and gas produced from the Upper Triassic Xujiahe Formation,Sichuan Basin,SW China

Natural gas in the Xujiahe Formation of the Sichuan Basin is dominated by hydrocarbon (HC) gas, with 78–79% methane and 2–19% C₂₊ HC. Its dryness coefficient (C₁/C_1–5) is mostly < 0.95. The gas in fluid inclusions, which has low contents of CH₄ and heavy hydrocarbons (C₂₊) and higher contents of non-hydrocarbons (e.g. CO₂), is a typical wet gas produced by thermal degradation of kerogen. Gas produced from the Upper Triassic Xujiahe Formation (here denoted field gas) has light carbon isotope values for methane (δ¹³C₁: −45‰ to −36‰) and heavier values for ethane (δ¹³C₂: −30‰ to −25‰). The case is similar for gas in fluid inclusions, but δ¹³C₁ = −36‰ to −45‰ and δ¹³C₂ = −24.8‰ to −28.1‰, suggesting that the gas experienced weak isotopic fractionation due to migration and water washing. The field gas has δ¹³C_CO2 values of −15.6‰ to −5.6‰, while the gas in fluid inclusions has δ¹³C_CO2 values of −16.6‰ to −9‰, indicating its organic origin. Geochemical comparison shows that CO₂ captured in fluid inclusions mainly originated from source rock organic matter, with little contribution from abiogenic CO₂. Fluid inclusions originate in a relatively closed system without fluid exchange with the outside following the gas capture process, so that there is no isotopic fractionation. They thus present the original state of gas generated from the source rocks. These research results can provide a theoretical basis for gas generation, evolution, migration and accumulation in the basin.     

南天山哈拉奇地区地球化学异常特征及地质意义

此区开展1/5万水系沉积物测量的目的是为了解区内元素的地球化学特征和分布、分配规律,圈定异常,查明异常与地质、矿产的关系。本文分析了区内元素的相关性、因子组合及其地质意义,认为异常（高值区）严格受喀拉铁克断裂带、可牙克断裂带、塔什纳拉他乌断裂带、热液活动、脉岩等其中某几种因素联合控制。根据异常特征与成矿地质条件,圈定了三处成矿矿远景区,即喀拉铁克成矿远景区、布隆成矿远景区、萨喀尔得成矿远景区,其中喀拉铁克成矿远景区意义重大,为该区下一步的找矿工作指明了方向。     

冀西北银地球化学块体与遥感成矿信息集成研究

基于冀西北1∶20万地球化学数据、遥感线环构造解译成果及蚀变遥感异常提取成果,开展了冀西北银地球化学块体与遥感成矿信息集成研究。认为银地球化学块体剩余金属量是成矿物质富集的表现,环形构造是银金属活化的动力标志,北东向断裂构造是成矿物质运移存储的空间,北东向区域蚀变异常带是成矿作用发生的标志。将地球化学块体理论与遥感成矿信息集成,为提高成矿预测的准确性、缩小找矿靶区提供了理论依据。     

探地雷达在古城墙病害探测中的应用

古城墙由于长年雨、雪水的侵蚀,容易造成城墙内部砖层、土体疏松等病害区域的发生,然而如何确定这些病害的位置、深度、规模等,是非常棘手的问题。探地雷达作为一种无损探测手段,具有探测效率高、分辨率高、结果直观等特点,对探测目标体不会造成任何损伤。结合工程实例,采用探地雷达技术对古城墙顶面进行探测。根据对探地雷达图谱进行处理分析,并选取5处有代表性的雷达图谱进行轻型动力触探验证。结合轻型动力触探验证资料,综合解释分析,提高了查找病害区域的准确性,同时说明探地雷达在古城墙探测中的适用性。     

Distribution of base metals and the related elements in the stream-sediments around the Ahar area (NW Iran) and their implications

The study area is located in the Ahar region, NW Iran. Volcanic rocks of Eocene cover major parts of the area, within which granitic-granodioritic intrusive bodies of Oligocene intruded and produced hydrothermal alterations and Cu-Au mineralization. This paper aims to explore anomalies of base metals and related elements across the region based on systematic sampling of stream sediments and using the secondary geochemical halos. In this regard, by taking into account factors such as stratigraphy, lithology, tectonics and the topologic center of the drainage system, 620 samples were taken from stream sediments and analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES) method.All the distinguished anomalies correlate well with Oligocene granitic-granodioritic rocks and the related hydrothermal alterations occurred within the Eocene andesitic-basaltic volcanics, especially at the NE part of the quadrangle, as well as with alterations within trachy-andesitic and andesitic volcanics of Pliocene at the SE part of the quadrangle, where epithermal gold and Pb-Zn mineralization is found. Most of he studied elements also show moderate to strong anomalies over the Sonajil porphyry-type Cu mineralization. Copper, and to some extent Mo, as well as Pb, Zn, Sn, W, As and Sb are the best examples of this association. Bismuth has more limited anomalies across the region, showing correlation with the granitoid intrusion at the east of Ahar and the hydrothermal alterations within the Pliocene andesitic and basaltic rocks at SE of Ahar quadrangle which, considering the presence of epithermal gold and Pb-Zn veins in both areas, can be attributed to epithermal processes. However, anthropogenic pollutions are also found for As, Fe, V, Ti, Ni and Co downstream the urban and rural areas. In this regard, besides the Sonajil area, where porphyry-type Cu mineralization is discovered, the NE and SE parts of the quadrangle present promising areas for further investigations.     

Petrography,mineralogy and geochemistry of the Late Eocene oolitic ironstones of the Jebel Ank,Southern Tunisian Atlas

The oolitic ironstones ore deposit of Jebel Ank (central Tunisia), is a simply folded stratiform ore body of about 2.5–8 m thickness located in the upper part of the epicontinental Souar Formation (Late Eocene) and is covered by the continental Segui Formation (Mio-Pliocene). The deposit contains about 20 Mt of ore with an average grade of 50% Fe. Generally, oolitic iron deposition occurs in shallow water lagoonal environments. The Jebel Ank deposit lies between two regional disconformities (Late Eocene and Miocene), and is evidence of a transitional stage at the end of regional regression before renewed transgression. The footwall of the oolitic iron ore-bearing bed consists of a fine-grained sandstone bed (10–20 cm-thick) pinching out laterally westward into green clays. The hanging wall is composed of thin-bedded limestone and clay alternations (2–3.5 m-thick).Iron occurs in the form of cryptocrystalline goethite with limited Al-Fe substitution. The goethite contains around 48% Fe, 5% Al and up to 1.5% P. Jarosite, alunite and manganese minerals (cryptomelane, psilomelane and manjiorite) are supergene secondary minerals, probably related to descending surface fluids. These manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies (local cement, space filling and free centimeter sized nodules). Fe-oolites in the deposit are similar to those documented in many other oolitic ironstone deposits. The dominant Fe-oolite type (>90%) has a concentrically laminated cortex with no nucleus. The nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains.Major elements in high grade samples (Fe₂O₃ > 65%) vary within a limited range and show higher concentrations of SiO₂ (average 7.85%) and Al₂O₃ (average 5.1%), with minor TiO₂, MnO, MgO, Na₂O, K₂O, and SO₃ (less than 1%). PAAS-normalized trace elements of bulk samples and Fe-oolite generally show similar behavior, both are enriched in V, Co, Ni, Mo, As, Zn, and Y and are depleted in Cu, Rb, Zr, Nb, Ba, and Hf. Anomalous V, Cr, Ni, Zn, and REE-Y are correlated with goethite. PAAS-normalized REE-Y patterns of both bulk samples and Fe-oolite show slight HREE enrichment, positive Ce with negative Y anomalies.The mineralogy (goethite and cryptomelane) along with the geochemistry (Si vs. Al; As + Cu + Mo + Pb + V + Zn vs. Ni + Co binary plots; Zn–Ni–Co triangular diagram, REE-Y content and patterns and Ce/Ce¹ vs. Nd and Ce/Ce¹ vs. Y_N/Ho_N binary plots) of the studied oolitic ironstone are congruent with a hydrogenetic type. While two possible sources of iron for Jebel Ank ironstone can be proposed: (i) submarine weathering of glauconite-rich sandstone and (ii) detrital iron from adjacent continental hinterland, the later is the more plausible source of iron, based on paleogeographic setting, the occurrence of fine sandstone underlying the iron level, occurrence of Mn-ores in the lower part of the Fe-ores succession, high phosphorous, zinc, ∑REE-Y concentrations and Y/Ho ratios, and low La/Ce ratios.