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1.
This paper contributes to the understanding of the genesis of epigenetic, hypogene BIF-hosted iron deposits situated in the eastern part of Ukrainian Shield. It presents new data from the Krivoy Rog iron mining district (Skelevatske–Magnetitove deposit, Frunze underground mine and Balka Severnaya Krasnaya outcrop) and focuses on the investigation of ore genesis through application of fluid inclusion petrography, microthermometry, Raman spectroscopy and baro-acoustic decrepitation of fluid inclusions. The study investigates inclusions preserved in quartz and magnetite associated with the low-grade iron ores (31–37% Fe) and iron-rich quartzites (38–45% Fe) of the Saksaganskaya Suite, as well as magnetite from the locally named high-grade iron ores (52–56% Fe). These high-grade ores resulted from alteration of iron quartzites in the Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and were a precursor to supergene martite, high-grade ores (60–70% Fe). Based on the new data two stages of iron ore formation (metamorphic and metasomatic) are proposed.The metamorphic stage, resulting in formation of quartz veins within the low-grade iron ore and iron-rich quartzites, involved fluids of four different compositions: CO2-rich, H2O, H2O–CO2(± N2–CH4)–NaCl(± NaHCO3) and H2O–CO2(± N2–CH4)–NaCl. The salinities of these fluids were relatively low (up to 7 mass% NaCl equiv.) as these fluids were derived from dehydration and decarbonation of the BIF rocks, however the origin of the nahcolite (NaHCO3) remains unresolved. The minimum P–T conditions for the formation of these veins, inferred from microthermometry are Tmin = 219–246 °C and Pmin = 130–158 MPa. The baro-acoustic decrepitation analyses of magnetite bands indicated that the low-grade iron ore from the Skelevatske–Magnetitove deposit was metamorphosed at T = ~ 530 °C.The metasomatic stage post-dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high-grade iron ores. The genesis of these ores, which are located in the Saksaganskiy tectonic block (Saksaganskiy ore field), and the factors controlling iron ore-forming processes are highly controversial. According to the study of quartz-hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow, simultaneous with thrusting and deformation. During the 1st episode three types of fluids were introduced: CO2–CH4–N2(± C), CO2(± N2–CH4) and low salinity H2O–N2–CH4–NaCl (6.38–7.1 mass% NaCl equiv.). The 2nd episode included expulsion of the aqueous fluids H2O–N2–CH4–NaCl(± CO2, ± C) of moderate salinities (15.22–16.76 mass% NaCl equiv.), whereas the 3rd event involved high salinity fluids H2O–NaCl(± C) (20–35 mass% NaCl equiv.). The fluids most probably interacted with country rocks (e.g. schists) supplying them with CH4 and N2. The high salinity fluids were most likely either magmatic–hydrothermal fluids derived from the Saksaganskiy igneous body or heated basinal brines, and they may have caused pervasive leaching of Fe from metavolcanic and/or the BIF rocks. The baro-acoustic decrepitation analyses of magnetite comprising the high-grade iron ore showed formation T = ~ 430–500 °C. The fluid inclusion data suggest that the upgrade to high-grade Fe ores might be a result of the Krivoy Rog BIF alteration by multiple flows of structurally controlled, metamorphic and magmatic–hydrothermal fluids or heated basinal brines. 相似文献
2.
The Ulu Sokor gold deposit is one of the most famous and largest gold deposits in Malaysia and is located in the Central Gold Belt. This deposit consists of three major orebodies that are related to NS- and NE-striking fractures within fault zones in Permian-Triassic meta-sedimentary and volcanic rocks of the East Malaya Block. The faulting events represent different episodes that are related to each orebody and are correlated well with the mineralogy and paragenesis. The gold mineralization consists of quartz-dominant vein systems with sulfides and carbonates. The hydrothermal alteration and mineralization occurred during three stages that were characterized by (I) silicification and brecciation; (II) carbonatization, sericitization, and chloritization; and (III) quartz–carbonate veins.Fluid inclusions in the hydrothermal quartz and calcite of the three stages were studied. The primary CO2–CH4–H2O–NaCl fluid inclusions in stage I are mostly related to gold mineralization and display homogenization temperatures of 269–389 °C, salinities of 2.77–11.89 wt.% NaCl equivalent, variable CO2 contents (typically 5–29 mol%), and up to 15 mol% CH4. In stage II, gold was deposited at 235–398 °C from a CO2 ± CH4–H2O–NaCl fluid with a salinity of 0.83–9.28 wt.% NaCl equivalent, variable CO2 contents (typically 5–63 mol%), and up to 4 mol% CH4. The δ18OH2O and δD values of the ore-forming fluids from the stage II quartz veins are 4.5 to 4.8‰ and − 44 to − 42‰, respectively, and indicate a metamorphic–hydrothermal origin. Oxygen fugacities calculated for the entire range of T-P-XCO2 conditions yielded log fO2 values between − 28.95 and − 36.73 for stage I and between − 28.32 and − 39.18 for stage II. These values indicate reduced conditions for these fluids, which are consistent with the mineral paragenesis, fluid inclusion compositions, and isotope values.The presence of daughter mineral-bearing aqueous inclusions is interpreted to be a magmatic signature of stage IIIa. Combined with the oxygen and hydrogen isotopic compositions (δ18OH2O = 6.8 to 11.9‰, δD = − 77 to − 62‰), these inclusions indicate that the initial fluid was likely derived from a magmatic source. In stage IIIb, the gold was deposited at 263° to 347 °C from a CO2–CH4–H2O–NaCl fluid with a salinity of 5.33 to 11.05 wt.% NaCl equivalent, variable CO2 contents (typically 9–15 mol%), and little CH4. The oxygen and hydrogen isotopic compositions of this fluid (δ18OH2O = 8.1 to 8.8‰, δD = − 44 to − 32‰) indicate that it was mainly derived from a metamorphic–hydrothermal source. The CO2–H2O ± CH4–NaCl fluids that were responsible for gold deposition in the stage IIIc veins had a wide range of temperatures (214–483 °C), salinities of 1.02 to 21.34 wt.% NaCl equivalent, variable CO2 contents (typically 4–53 mol%), and up to 7 mol% CH4. The oxygen and hydrogen isotopic compositions (δ18OH2O = 8.5 to 9.8‰, δD = − 70 to − 58‰) were probably acquired at the site of deposition by mixing of the metamorphic–hydrothermal fluid with deep-seated magmatic water and then evolved by degassing at the site of deposition during mineralization. The log fO2 values from − 28.26 to − 35.51 also indicate reduced conditions for this fluid in stage IIIc. Moreover, this fluid had a near-neutral pH and δ34S values of H2S of − 2.32 to 0.83‰, which may reflect the derivation of sulfur from the subducted oceanic lithospheric materials.The three orebodies represent different gold transportation and precipitation models, and the conditions of ore formation are related to distinct events of hydrothermal alteration and gold mineralization. The gold mineralization of the Ulu Sokor deposit occurred in response to complex and concurrent processes involving fluid immiscibility, fluid–rock reactions, and fluid mixing. However, fluid immiscibility was the most important mechanism for gold deposition and occurred in these orebodies, which have corresponding fluid properties, structural controls, geologic characteristics, tectonic settings, and origins of the ore-forming matter. These characteristics of the Ulu Sokor deposit are consistent with its classification as an orogenic gold deposit, while some of the veins are genetically related to intrusions. 相似文献
3.
The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO2 inclusions, CO2–H2O inclusions, hydrocarbon inclusions, and hydrocarbon–H2O inclusions. The ore-forming fluids are characterized by a H2O + CO2 + CH4 ± N2 system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H2O + NaCl → H2O + NaCl + CO2 + CH4 ± N2 → H2O + NaCl ± CH4 ± CO2 with a slight decrease in homogenization temperature and salinity. The δ18O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δDH2O and calculated δ18OH2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ13CPDB and δ18OSMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H2O–NaCl–CO2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold. 相似文献
4.
CO2, CH4, and N2 adsorption and gas-induced swelling were quantified for block Blind Canyon, Pittsburgh #8 and Pocahontas Argonne Premium coals that were dried and structurally relaxed at 75 °C in vacuum. Strain measurements were made perpendicular and parallel to the bedding plane on ~ 7 × 7 × 7 mm3 coal blocks and gravimetric sorption measurements were obtained simultaneously on companion coal blocks exposed to the same gaseous environment. The adsorption amount and strain were determined after equilibration at P ≤ 1.8 MPa. There is a strong non-linear correlation between strain and the quantity of gas adsorbed and the results for all gases and coals studied follow a common pattern. The dependence of the coal matrix shrinkage/swelling coefficient (Cgc) on the type and quantity of gas adsorbed is seen by plotting the ratio between the strain and the adsorbate concentration against the adsorbate concentration. In general, Cgc increases with increasing adsorbate concentration over the range of ~ 0.1 to 1.4 mmol/g. Results from the dried block coals are compared to CO2 experiments using native coals with an inherent level of moisture as received. The amount of CO2 adsorbed using native coals (assuming no displacement of H2O by CO2) is significantly less than the dried coals. The gas-induced strain (S) and adsorption amount (M) were measured as a function of time following step changes in CO2, CH4, and N2 pressure from vacuum to 1.8 MPa. An empirical diffusion equation was applied to the kinetic data to obtain the exponent (n) for time dependence for each experiment. The data for all coals were pooled and the exponent (n) evaluated using an ANOVA statistical analysis method. Values for (n) near 0.5 were found to be independent on the coal, the gas or type of measurement (e.g., parallel strain, perpendicular strain, and gas uptake). These data support the use of a Fickian diffusion model framework for kinetic analysis. The kinetic constant k was determined using a unipore diffusion model for each experiment and the data were pooled for ANOVA analysis. For dry coal, statistically significant differences for k were found for the gases (CO2 > N2 > CH4) and coals (Pocahontas >Blind Canyon > Pittsburgh #8) but not for the method of the kinetic measurement (e.g., strain or gas uptake). For Blind Canyon and Pittsburgh #8 coal, the rate of CO2 adsorption and gas-induced strain for dry coal was significantly greater than that of the corresponding native coal. For Pocahontas coal the rates of CO2 adsorption and gas-induced strain for dry and native coal were indistinguishable and may be related to its low native moisture and minimal amount of created porosity upon drying. 相似文献
5.
《Chemie der Erde / Geochemistry》2014,74(4):681-690
This study presents isotope geochemical analyses conducted on water column samples and core sediments collected from the Swan Lake Basin. Water analyses include the dissolved methane (CH4) content and the ratio of carbon-13 to carbon-12 (δ13C) in dissolved inorganic carbon (DIC). The core sediments – sandy muds containing inorganic calcite, organic matter, and opal phases ± ostracods – were examined by X-ray diffraction, dated by radiocarbon (14C), analyzed for wt% organic carbon, wt% organic nitrogen, wt% organic matter, wt% calcite, δ13C of bulk-sediment insoluble organic matter (kerogen), 18O:16O ratio (δ18O) and δ13C of bulk and ostracod calcite. Of particular significance is the large enrichment in carbon-13 (δ13C = +4.5 to +20.4‰ V-PDB) in the calcite of these sediments. The 13C-enriched calcite is primarily formed from DIC in the water column of the lake as a result of the following combined processes: (i) the incorporation of 13C enriched residual carbon dioxide (CO2) after partial reduction to CH4 in the sediments and its migration into the water column-DIC pool; (ii) the preferential assimilation of 12C by phytoplankton during photosynthesis; (iii) the removal of 13C-depleted CH4 by ebullition and of organic matter by sedimentation and burial. The 13C enrichment was low between 3624 and 2470 yr BP; high between 2470 and 1299 yr BP; and moderate since 1299 yr BP. Low 13C enrichment was formed under low water-column carbon levels while higher ones were formed under elevated rates of biomass and calcite deposition. These associations seem to imply that biological productivity is the main reason for carbon-13 enrichments. 相似文献
6.
Natural gas reservoirs in organic-rich shales in the Appalachian and Michigan basins in the United States are currently being produced via hydraulic fracturing. Stratigraphically-equivalent shales occur in the Canadian portion of the basins in southwestern Ontario with anecdotal evidence of gas shows, yet there has been no commercial shale gas production to date. To provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production, such as leakage of natural gas, saline water, and/or hydraulic fracturing fluids, and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general, this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern Ontario. An extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow, domestic wells in bedrock and overburden aquifers throughout southwestern Ontario was utilized. In addition, select wells (n = 36) were resampled for detailed dissolved gas composition, δ13C of CH4, C2, C3, and CO2, and δD of CH4. Dissolved gases in groundwater from bedrock and overburden wells were composed primarily of CH4 (29.7–98.6 mol% of total gas volume), N2 (0.8–66.2 mol%), Ar + O2 (0.2–3.4 mol%), and CO2 (0–1.2 mol%). Ethane was detected, but only in low concentrations (<0.041 mol%), and no other higher chain hydrocarbons were present, except for one well in overburden overlying the Dundee Formation, which contained 0.81 mol% ethane and 0.21 mol% propane. The highest methane concentrations (30 to >100 in situ % saturation) were found in bedrock wells completed in the Upper Devonian Kettle Point Formation, Middle Devonian Hamilton Group and Dundee Formation, and in surficial aquifers overlying these organic-rich shale-bearing formations, indicating that bedrock geology is the primary control on methane occurrences. A few (n = 40) samples showed Na–Cl–Br evidence of brine mixing with dilute groundwater, however only one of these samples contained high (>60 in situ % saturation) CH4. The relatively low δ13C values of CH4 (−89.9‰ to −57.3‰), covariance of δD values of CH4 and H2O, positive correlation between δ13C values of CH4 and CO2, and lack of higher chain hydrocarbons (C3+) in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin. The presence or absence of alternative electron acceptors (e.g. dissolved oxygen, Fe, NO3, SO4), in addition to organic substrates, controls the occurrence of microbial CH4 in shallow aquifers. Microbial methane has likely been accumulating in the study area, since at least the Late Pleistocene to the present, as indicated by the co-variance and range of δD values of CH4 (−314‰ to −263‰) and associated groundwater (−19‰ to −6‰ δD-H2O). 相似文献
7.
《Applied Geochemistry》2004,19(11):1655-1686
Water samples from short-screen monitoring wells installed along a 90-km transect in southwestern Kansas were analyzed for major ions, trace elements, isotopes (H, B, C, N, O, S, Sr), and dissolved gases (He, Ne, N2, Ar, O2, CH4) to evaluate the geochemistry, radiocarbon ages, and paleorecharge conditions in the unconfined central High Plains aquifer. The primary reactions controlling water chemistry were dedolomitization, cation exchange, feldspar weathering, and O2 reduction and denitrification. Radiocarbon ages adjusted for C mass transfers ranged from <2.6 ka (14C) B.P. near the water table to 12.8 ± 0.9 ka (14C) B.P. at the base of the aquifer, indicating the unconfined central High Plains aquifer contained a stratified sequence of ground water spanning Holocene time. A cross-sectional model of steady-state ground-water flow, calibrated using radiocarbon ages, is consistent with recharge rates ranging from 0.8 mm/a in areas overlain by loess to 8 mm/a in areas overlain by dune sand. Paleorecharge temperatures ranged from an average of 15.2 ± 0.7 °C for the most recently recharged waters to 11.6 ± 0.4 °C for the oldest waters. The temperature difference between Early and Late Holocene recharge was estimated to be 2.4 ± 0.7 °C, after taking into account variable recharge elevations. Nitrogen isotope data indicate NO3 in paleorecharge (average concentration=193 μM) was derived from a relatively uniform source such as soil N, whereas NO3 in recent recharge (average concentration=885 μM) contained N from varying proportions of fertilizer, manure, and soil N. Deep water samples contained components of N2 derived from atmospheric, denitrification, and deep natural gas sources. Denitrification rates in the aquifer were slow (5 ± 2× 10−3 μmol N L−1 a−1), indicating this process would require >10 ka to reduce the average NO3 concentration in recent recharge to the Holocene background concentration. 相似文献
8.
The main aim of this study was to assess the natural and anthropogenic contributions of CO2 in the urban atmosphere of Wrocław City (SW Poland) using combined quantitative (CO2 concentrations) and qualitative analysis (δ13C of CO2). Between 21 January 2011 and 22 December 2011, 17 sampling campaigns were performed at 3-week intervals and in total 255 samples were collected. The mean CO2 concentration was 469 ± 71 ppm and the mean δ13C(CO2) was −10.8 ± 1‰.The measured δ13C(CO2) values of major end-members for two winter heating seasons (−25.7‰ in January–March of 2011 and −27.6 ‰ in October–December of 2011) and for one vegetative season (−20.4‰ in April–September of 2011) suggest soil respiration as a main source of atmospheric CO2 during the vegetative season, and a very significant impact of fossil fuel combustion during the winter heating seasons. There were significant increases of CO2 concentrations at many sampling locations after the opening of a new motorway on 31 August 2011. The authors hypothesise that the new motorway contributes to the increase of CO2 across the city. 相似文献
9.
The giant Jianchaling gold deposit is located in the Shaanxi Province, China. The mineralization is hosted by WNW-trending faults in the Mianxian-Lueyang-Yangpingguan (MLY) area. The mineralization can be divided into three stages based on mineralogical assemblages and crosscutting relationships of mineralized quartz veins. These stages, from early to late, are characterized by the mineral assemblage of: (1) quartz – coarse-grained pyrite – pyrrhotite – pentlandite – dolomite; (2) quartz – pyrite – gold – sphalerite – galena – carbonate – arsenopyrite – fuchsite; and (3) dolomite – calcite – quartz – fine-grained pyrite – realgar – orpiment.Three types of fluid inclusions have been recognized in this study based on petrographic and microthermometric measurements, including pure CO2 and/or CH4 (PC-type), NaCl-H2O (W-type), and NaCl-CO2-H2O (C-type) fluid inclusions. These fluid inclusion types are present in quartz from the Stage 1 and 2 assemblages, whereas the Stage 3 quartz only contains W-type fluid inclusions. The Stage 2 assemblage is associated with the mineralization at the Jianchaling deposit. Fluid inclusions of Stage 1 quartz homogenize mainly between 250° and 360 °C, with salinities up to 15.6 wt.% NaCl equiv., whereas the Stage 3 dolomite with homogenization temperatures of 160° – 220 °C and salinities of 1.1–7.4 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO2-rich, probably metamorphic hydrothermal to CO2-poor, meteoric fluid. All three types of fluid inclusions can be observed in the Stage 2 quartz, suggesting that this heterogeneous association was trapped from a boiling fluid system. These inclusions homogenized at temperatures of 200°–250 °C and salinities of 1.2–12.4 wt.% NaCl equiv. The estimated trapping pressures of the fluid inclusions are between 117 and 354 MPa in Stage 1, suggesting an alternating lithostatic–hydrostatic fluid system, which was controlled by a fault-valve at the depth of ~ 12 km.Two fuchsite samples collected from the Stage 2 polymetallic-quartz veins yielded well-defined 40Ar/39Ar isotopic plateau ages of 197 ± 2 and 194 ± 2 Ma, and 39Ar/36Ar-40Ar/36Ar normal isochrones of 198 ± 2 and 199 ± 2 Ma. This indicates that the mineralization at Jianchaling is Early Jurassic (ca. 198 Ma) in age. We propose that Jianchaling is an orogenic gold deposit, and formed during continental collision related to the northward subduction of the Mian-Lue oceanic plate during the Early Jurassic. We also conclude that the beginning of the continental collision between the Yangtze and the North China Cratons took place around 200 Ma. 相似文献
10.
The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO2 concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO2 varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO3 and Fe(CO3)33−, with formation constants log*β°1,1,1 = 10.76 ± 0.38 and log β°1,0,3 = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO2 and at T = (25 ± 1) °C, as log*Ks,0 = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO2-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers. 相似文献
11.
《Chemie der Erde / Geochemistry》2016,76(3):327-352
Soils act as sources and sinks for greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Since both storage and emission capacities may be large, precise quantifications are needed to obtain reliable global budgets that are necessary for land-use management (agriculture, forestry), global change and for climate research. This paper discusses exclusively the soil emission-related processes and their influencing parameters. It reviews soil emission studies involving the most important land-cover types and climate zones and introduces important measuring systems for soil emissions. It addresses current shortcomings and the obvious bias towards northern hemispheric data.When using a conservative average of 300 mg CO2e m−2 h−1 (based on our literature review), this leads to global annual net soil emissions of ≥350 Pg CO2e (CO2e = CO2 equivalents = total effect of all GHG normalized to CO2). This corresponds to roughly 21% of the global soil C and N pools. For comparison, 33.4 Pg CO2 are being emitted annually by fossil fuel combustion and the cement industry. 相似文献
12.
We investigate the helium, carbon and oxygen–hydrogen isotopic systematics and CO2/3He ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western Anatolia (Turkey). 3He/4He ratios of the samples (R) normalized to the atmospheric 3He/4He ratio (RA = 1.39 × 10? 6) range from 0.27 to 1.67 and are significantly higher than the crustal production value of 0.05. Fluids with relatively high R / RA values are generally found in areas of significant heat potential (K?z?ldere and Tuzla fields). CO2/3He ratios of the samples, ranging from 1.6 × 109 to 2.3 × 1014, display significant variation and are mostly higher than values typical of an upper mantle source (2 × 109). The δ13C (CO2) and δ13C (CH4) values of all fluids vary from ? 8.04 to + 0.35‰ and ? 25.80 to ? 23.92‰ (vs. PDB), respectively. Stable isotope values (δ18O–δD) of the geothermal waters are conformable with the Mediterranean Meteoric Water Line and indicate a meteoric origin. The temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers, implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface.Evaluation of He–CO2 abundances indicates that hydrothermal degassing and calcite precipitation (controlled probably by adiabatic cooling due to degassing) significantly fractionate the elemental ratio (CO2/3He) in geothermal waters. Such processes do not affect gas phase samples to anywhere near the same extent. For the gas samples, mixing between mantle and various crustal sources appears to be the main control on the observed He–C systematics: however, crustal inputs dominate the CO2 inventory. Considering that limestone is the main source of carbon (～ 70 to 97% of the total carbon inventory), the carbon flux from the crust is found to be at least 20 times that from the mantle. As to the He-inventory, the mantle-derived component is found to vary up to 21% of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension. The range of 3He/enthalpy ratios (0.000032 to 0.19 × 10? 12 cm3 STP/J) of fluids in western Anatolia is consistent with the release of both helium and heat from contemporary additions of mantle-derived magmas to the crust. The deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface. 相似文献
13.
Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO2 ± CH4 ± H2O) and aqueous-carbonic (H2O–NaCl–CO2 ± CH4) inclusions along intragranular trails, whereas aqueous inclusions (H2O–NaCl ± CO2) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ34S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ34SΣs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO2 via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites. 相似文献
14.
The Qiangma gold deposit is hosted in the > 1.9 Ga Taihua Supergroup metamorphic rocks in the Xiaoqinling terrane, Qinling Orogen, on the southern margin of the North China Craton. The mineralization can be divided as follows: quartz-pyrite veins early, quartz-polymetallic sulfide veinlets middle, and carbonate-quartz veinlets late stages, with gold being mainly introduced in the middle stage. Three types of fluid inclusions were identified based on petrography and laser Raman spectroscopy, i.e., pure carbonic, carbonic-aqueous (CO2–H2O) and aqueous inclusions.The early-stage quartz contains pure carbonic and CO2–H2O inclusions with salinities up to 12.7 wt.% NaCl equiv., bulk densities of 0.67 to 0.86 g/cm3, and homogenization temperatures of 280−365 °C. The early-stage is related to H2O–CO2 ± N2 ± CH4 fluids with isotopic signatures consistent with a metamorphic origin (δ18Owater = 3.1 to 5.2‰, δD = − 37 to − 73‰). The middle-stage quartz contains all three types of fluid inclusions, of which the CO2–H2O and aqueous inclusions yield homogenization temperatures of 249−346 °C and 230−345 °C, respectively. The CO2–H2O inclusions have salinities up to 10.9 wt.% NaCl equiv. and bulk densities of 0.70 to 0.98 g/cm3, with vapor bubbles composed of CO2 and N2. The isotopic ratios (δ18Owater = 2.2 to 3.6‰, δD = − 47 to − 79‰) suggest that the middle-stage fluids were mixed by metamorphic and meteoric fluids. In the late-stage quartz only the aqueous inclusions are observed, which have low salinities (0.9−9.9 wt.% NaCl equiv.) and low homogenization temperatures (145−223 °C). The isotopic composition (δ18Owater = − 1.9 to 0.5‰, δD = − 55 to − 66‰) indicates the late-stage fluids were mainly meteoric water.Trapping pressures estimated from CO2–H2O inclusions are 100−285 MPa for the middle stage, suggesting that gold mineralization mainly occurred at depths of 10 km. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ18Owater values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Qiangma gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic−Early Cretaceous continental collision between the North China and Yangtze cratons. 相似文献
15.
The phase behavior of CO2–CH4–H2S–brine systems is of importance for geological storage of greenhouse gases, sour gas disposal and enhanced oil recovery (EOR). In such projects, reservoir simulations play a major role in assisting decision makings, while modeling the phase behavior of the relevant CO2–CH4–H2S–brine system is a key part of the simulation. There is a need for an equation of state (EOS) for such system which is accurate, with wide application range (pressure, temperature and aqueous salinity), computationally efficient and easy for implementation in a reservoir simulator.In this study, an improved cubic EOS model of the system CO2–CH4–H2S–brine is developed based on the modifications of the binary interaction parameters in Peng–Robinson EOS, which is widely implemented in reservoir simulators. Thus the new model is suited for numerical implementation in reservoir simulators.The available experimental data of pure gas brine equilibrium and gas mixture solubility in water/brine are carefully reviewed and compared with the new model. From the comparison, the new model can accurately reproduce (1) the CO2–brine mutual solubility data at temperature from 0 °C to 250 °C, pressure from 1 bar to 1000 bar and NaCl molality (mole number in 1 kg water, molal is used for short) from 0 to 6 molal, (2) CH4–brine mutual solubility data at temperature from 0 °C to 250 °C, pressure from 1 bar to 2000 bar and NaCl molality from 0 to 6 molal, (3) H2S–brine mutual solubility data at temperature from 0 °C to 250 °C, pressure from 1 bar to 200 bar and NaCl molality from 0 to 6 molal, and (4) has good accuracy for gas mixture solubility in brine. 相似文献
16.
New isotopic and chemical data on the sodium bicarbonate water and associated gases from the Razdolnoe Spa located in the coastal zone of Primorsky Kray of the Russian Far East, together with previous stable isotope data (δ18O, δD, δ13C), allow elucidation of the origin and evolution of the groundwater and gases from the spa. The water is characterized by low temperature (12 °C), TDS – 2.5–6.0 g/L, high contents of B (∼5 mg/L) and F (4.5 mg/L) and low contents of Cl and SO4. Water isotopic composition indicates its essentially meteoric origin which may comply with an older groundwater that was recharged under different (colder) climatic conditions. Major components of bubbling gases are CH4 (68 vol%), N2 (28%) and CO2 (4%). The obtained values δ13C and δD for CO2 and CH4 definitely indicate the marine microbial origin of methane. Thus the high methane content in the waters relates to the biochemical processes and presence of a dispersed organic matter in the host rocks. Based on the regional hydrogeology and the geological structure of the Razdolnoe Spa, Mesozoic fractured rocks containing Na–HCO3 mineral water and gases are reservoir rocks, a chemical composition of water and gases originates in different environmental conditions. 相似文献
17.
Thomas Pape André Bahr Janet Rethemeyer John D. Kessler Heiko Sahling Kai-Uwe Hinrichs Stephan A. Klapp William S. Reeburgh Gerhard Bohrmann 《Chemical Geology》2010,269(3-4):350-363
Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C1–C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (δ13C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C1–C4, CO2]) relative to the other gas types and the virtual lack of 14C–CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1–C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C–CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area. 相似文献
18.
An experimental study on the origin of ferric and ferrous carbonate-silicate melts, which can be considered as the potential metasomatic oxidizing agents and diamond forming media, was performed in the (Ca,Mg)CO3-SiO2-Al2O3-(Mg,Fe)(Cr,Fe,Ti)O3 system, at 6.3 GPa and 1350–1650 °C. At 1350–1450 °C and ?O2 of FMQ + 2 log units, carbonate–silicate melt, coexisting with Fe3 +-bearing ilmenite, pyrope-almandine and rutile, contained up to 13 wt.% of Fe2O3. An increase in the degree of partial melting was accompanied by decarbonation and melt enrichment with CO2, up to 21 wt.%. At 1550–1650 °C excess CO2 segregated as a separate fluid phase. The restricted solubility of CO2 in the melt indicated that investigated system did not achieve the second critical point at 6.3 GPa. At 1350–1450 °C and ?O2 close to CCO buffer, Fe2 +-bearing carbonate–silicate melt was formed in association with pyrope-almandine and Fe3 +-bearing rutile. It was experimentally shown that CO2-rich ferrous carbonate-silicate melt can be an effective waterless medium for the diamond crystallization. It provides relatively high diamond growth rates (3–5 μm/h) at P,T-conditions, corresponding to the formation of most natural diamonds. 相似文献
19.
The Loulo–Gounkoto complex in the Kédougou–Kéniéba Inlier hosts three multi-million ounce orogenic gold deposits, situated along the Senegal–Mali Shear Zone. This west Malian gold belt represents the largest West African orogenic gold district outside Ghana. The Gounkoto deposit is hosted to the south of the Gara and Yalea gold mines in the Kofi Series metasedimentary rocks. The ore body is structurally controlled and is characterised by sodic and phyllic alteration, As- and Fe-rich ore assemblages, with abundant magnetite, and overall enrichment in Fe–As–Cu–Au–Ag–W–Ni–Co–REE + minor Te–Pb–Se–Cd. Fluid inclusion analysis indicates that the deposit formed at P–T conditions of approximately 1.4 kbar and 340 °C and that two end member fluids were involved in mineralisation: (1) a moderate temperature (315–340 °C), low salinity (< 10 wt.% NaCl equiv.), low density (≤ 1 g·cm− 3), H2O–CO2–NaCl–H2S ± N2–CH4 fluid; (2) a high temperature (up to 445 °C), hypersaline (~ 40 wt.% NaCl equiv.), high density (~ 1.3 g·cm− 3), H2O–CO2–NaCl ± FeCl2 fluid. Partial mixing of these fluids within the Jog Zone at Gounkoto enhanced phase separation in the aqueo-carbonic fluid and acted as a precipitation mechanism for Au. These findings demonstrate the widespread, if heterogeneously distributed, nature of fluid mixing as an ore forming process in the Loulo–Gounkoto complex, operating over at least a 30 km strike length of the shear zone. Stable isotope analyses of ore components at Gounkoto indicate a dominant metamorphic source for H2O, H2S and CO2, and by extension Au. It thus can be reasoned that both the aqueo-carbonic and the hypersaline fluid at Gounkoto are of metamorphic origin and that the high levels of salinity in the brine are likely derived from evaporite dissolution. 相似文献
20.
The Shapinggou porphyry Mo deposit, one of the largest Mo deposits in Asia, is located in the Dabie Orogen, Central China. Hydrothermal alteration and mineralization at Shapinggou can be divided into four stages, i.e., stage 1 ore-barren quartz veins with intense silicification, followed by stage 2 quartz-molybdenite veins associated with potassic alteration, stage 3 quartz-polymetallic sulfide veins related to phyllic alteration, and stage 4 ore-barren quartz ± calcite ± pyrite veins with weak propylitization. Hydrothermal quartz mainly contains three types of fluid inclusions, namely, two-phase liquid-rich (type I), two- or three-phase gas-rich CO2-bearing (type II) and halite-bearing (type III) inclusions. The last two types of fluid inclusions are absent in stages 1 and 4. Type I inclusions in the silicic zone (stage 1) display homogenization temperatures of 340 to 550 °C, with salinities of 7.9–16.9 wt.% NaCl equivalent. Type II and coexisting type III inclusions in the potassic zone (stage 2), which hosts the main Mo orebodies, have homogenization temperatures of 240–440 °C and 240–450 °C, with salinities of 34.1–50.9 and 0.1–7.4 wt.% NaCl equivalent, respectively. Type II and coexisting type III inclusions in the phyllic zone (stage 3) display homogenization temperatures of 250–345 °C and 220–315 °C, with salinities of 0.2–6.5 and 32.9–39.3 wt.% NaCl equivalent, respectively. Type I inclusions in the propylitization zone (stage 4) display homogenization temperatures of 170 to 330 °C, with salinities lower than 6.5 wt.% NaCl equivalent. The abundant CO2-rich and coexisting halite-bearing fluid inclusion assemblages in the potassic and phyllic zones highlight the significance of intensive fluid boiling of a NaCl–CO2–H2O system in deep environments (up to 2.3 kbar) for giant porphyry Mo mineralization. Hydrogen and oxygen isotopic compositions indicate that ore-fluids were gradually evolved from magmatic to meteoric in origin. Sulfur and lead isotopes suggest that the ore-forming materials at Shapinggou are magmatic in origin. Re–Os dating of molybdenite gives a well-defined 187Re/187Os isochron with an age of 112.7 ± 1.8 Ma, suggesting a post-collisional setting. 相似文献