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1.
 Foresighted and determined local authorities, purposeful exploration (i.e. by seismic reflection) and extensive testing led to the discovery of a substantial groundwater resource near the community of Seon (Switzerland) at a depth of 268–305 m. Production tests revealed a hydraulic conductivity of ∼5.10–5 m/s, transmissivity of ∼5.10–4 m2/s and a storage coefficient of ∼2% in the aquifer. Pumping up to 1500 l/min is sustainable; the water quality complies chemically and bacteriologically with drinking-water requirements. The residence time of several 103 years, determined by isotope techniques, guarantees protection from surface contamination. The elevated temperature of 19.5  °C of the produced water enables combined use for drinking water and space heating. The environmental benefits are substantial: the emission reduction amounts up to 780 tons/year CO2 and 1 ton/year SO2. Received: 21 September 1998 · Accepted: 10 February 1999  相似文献   

2.
 The Alto Guadalentín detrital aquifer is both overexploited and polluted. Water conductivity ranges between 1200 and 4900 μS cm–1, HCO3 – between 1000 and 1990 mg l–1, and PCO2 between 0.041 and 1.497 bars. The temperature and chemical composition of the water show a positive thermal anomaly directly attributable to the neotectonic activity in the area. The high CO2 content has caused the abandonment of numerous wells due to water corrosiveness which attacks pumping equipment. Received: 10 October 1995 · Accepted: 14 November 1995  相似文献   

3.
Synorogenic veins from the Proterozoic Eastern Mount Isa Fold Belt contain three different types of fluid inclusions: CO2-rich, aqueous two-phase and rare multiphase. Inclusions of CO2 without a visible H2O phase are particularly common. The close association of CO2-rich inclusions with aqueous two-phase, and possibly multiphase inclusions suggests that phase separation of low- to -moderate salinity CO2-rich hydrothermal fluids led to the selective entrapment of the CO2. Microthermometric results indicate that CO2-rich inclusions homogenize between –15.5 and +29.9 °C which corresponds to densities of 0.99 to 0.60 g.cm−3. The homogenization temperatures of the associated aqueous two-phase inclusions are 127–397 °C, with salinities of 0.5 to 18.1 wt.% NaCl equivalent. The rarely observed multiphase inclusions homogenize between 250 and 350 °C, and have salinities ranging from 34.6 to 41.5 wt.% NaCl equivalent. Evidence used to support the presence of fluid immiscibility in this study is mainly derived from observations of coexisting H2O-rich and CO2-rich inclusions in groups and along the same trail. In addition, these two presumably unmixed fluids are also found on adjacent fractures where monophase CO2-rich inclusions are closely related to H2O-rich inclusions. Similar CO2-rich inclusions are widespread in mineral deposits in this region, which are simply metal-enriched synorogenic veins. Therefore, we argue that fluid immiscibility caused volatile species such as CO2 and H2S to be lost from liquid, thus triggering ore deposition by increasing the fluid pH and decreasing the availability of complexing ligands. Received: 28 April 1997 / Accepted: 4 January 1999  相似文献   

4.
Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H2O; 5% CO2; 0.5% CH4; 0.2% N2; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO2, CH4 and N2). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO2, CH4 and N2), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO2)-rich aqueous inclusions, aqueous inclusions (H2O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999  相似文献   

5.
 This paper presents a site-specific conceptual model of groundwater flow in fractured damage zones associated with faulting in a package of sedimentary rocks. The model is based on the results of field and laboratory investigations. Groundwater and methane gas inflows from fault-fracture systems in the West Elk coal mine, Colorado, USA, have occurred with increasing severity. Inflows of 6, 160 and 500 L s−1 discharged almost instantaneously from three separate faults encountered in mine workings about 460 m below ground level. The faults are about 600 m apart. The δ 2H and δ 18O compositions of the fault-related inflow waters and the hydrodynamic responses of each fault inflow indicate that the groundwaters discharge from hydraulically isolated systems. 14C data indicate that the groundwaters are as much as 10,500 years old. Discharge temperatures are geothermal (≈30°C), which could indicate upwelling from depth. However, calculations of geothermal gradients, analysis of solute compositions of groundwater in potential host reservoirs, geothermometer calculations, and results of packer testing indicate that the fractured groundwater reservoir is the Rollins Sandstone (120 m thick) directly beneath the coal seams. The packer test also demonstrates that the methane gas is contained in the coal seams. A geothermal gradient of 70–80°C km−1, related to an underlying intrusion, is probably responsible for the slightly elevated discharge temperatures. Large discharge volumes, as great as 8.2×105 m3 from the 14 South East Headgate fault (14 SEHG), rapid declines in discharge rates, and vertical and horizontal permeability (matrix permeability generally <0.006 Darcy) indicate fracture flow. An in-mine pumping test demonstrates that the 14 SEHG fault has excellent hydraulic communication with fractures 50 m from the fault. Aeromagnetic data indicate that the faults are tectonically related to an igneous body that is several thousand meters below the coal seams. Exploratory drilling has confirmed a fourth fault, and two additional faults are projected, based on the aeromagnetic data. The conceptual model describes a series of parallel, hydraulically separate groundwater systems associated with fault-specific damage zones. The faults are about 600 m apart. Groundwater stored in fractured sandstone is confined above and below by clayey layers. Received March 1999 / Revised, November 1999 / Accepted, December 1999  相似文献   

6.
 The aquifer system of the Upper Triassic Keuper Sandstone, an important source of drinking water in northern Bavaria, is affected by elevated arsenic concentrations. Within the study area of 8000 km2, no evidence exists for any artificial source of arsenic. Data from about 500 deep water wells show that in approximately 160 wells arsenic concentrations are 10–150 μg/L. The regional distribution of arsenic in the groundwater shows that elevated arsenic concentrations are probably related to specific lithofacies of the aquifers that contain more sediments of terrestrial origin. Geochemical measurements on samples from four selected well cores show that arsenic has accumulated in the rocks. This indigenous arsenic is the source of arsenic in the groundwater of certain facies of the middle unit of the Keuper Sandstone. Received, June 1998 / Revised, January 1999, May 1999 / Accepted, June 1999  相似文献   

7.
Fluid inclusions were studied in samples from the Ashanti, Konongo-Southern Cross, Prestea, Abosso/Damang and Ayanfuri gold deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralised veins of the Ashanti, Prestea, Konongo-Southern Cross, and Abosso/Damang deposits contain almost exclusively volatile species. The primary setting of the gaseous (i.e. the fluid components CO2, CH4 and N2) fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids. Microthermometric and Raman spectroscopic analyses of the inclusions revealed a CO2 dominated fluid with variable contents of N2 and traces of CH4. Water content of most inclusions is below the detection limits of the respective methods used. Aqueous inclusions are rare in all samples with the exception of those from the granite-hosted Ayanfuri mineralisation. Here inclusions associated with the gold mineralisation contain a low salinity (<6 eq.wt.% NaCl) aqueous solution with variable quantities of CO2. Microthermometric investigations revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm3 at Ashanti, 0.85 to 0.98 g/cm3 at Prestea, up to 1.02 g/cm3 at Konongo-Southern Cross, and 0.8 to 1.0 g/cm3 at Abosso/Damang. The fluid inclusion data are used to outline the PT ranges of gold mineralisation of the respective gold deposits. The high density gaseous inclusions found in the auriferous quartz at Ashanti and Prestea imply rather high pressure trapping conditions of up to 5.4 kbar. In contrast, mineralisation at Ayanfuri and Abosso/Damang is inferred to have occurred at lower pressures of only up to 2.2 kbar. Mesothermal gold mineralisation is generally regarded to have formed from fluids characterized by H2O > CO2 and low salinity ( ±  6 eq.wt.%NaCl). However, fluid inclusions in quartz from the gold mineralisations in the Ashanti belt point to distinctly different fluid compositions. Specifically, the predominance of CO2 and CO2 >> H2O have to be emphasized. Fluid systems with this unique bulk composition were apparently active over more than 200␣km along strike of the Ashanti belt. Fluids rich in CO2 may present a hitherto unrecognised new category of ore-forming fluids. Received: 30 May 1996 / Accepted: 8 October 1996  相似文献   

8.
Water from the Kombat mine was delivered to the Omatako dam via the 263-km-long Grootfontein-Omatako canal during test runs in 1997. It is intended to supply water from Kombat and other underground mines in the Otavi Mountain Land to the capital Windhoek. The Cu-Pb-Zn orebodies are hosted by carbonate rocks and the mine waters are supersaturated with respect to calcite and CO2. Along the length of the canal, the CO2 partial pressure drops from 10−2.1 atm at the inlet of the Kombat mine to 10−3.5 atm at the end of the canal. This is accompanied by a drop in Ca concentration from about 60 to about 20 mg/l. The heavy metal concentrations (Cd, Cu, Mn, Pb and Zn) drop along the course of the canal to values far below the national drinking-water standard. Scavenging by calcium carbonate precipitation is the major depletion mechanism. Received: 21 June 1999 · Accepted: 29 August 1999  相似文献   

9.
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10.
The system KAlO2–MgO–SiO2–H2O–CO2 has long been used as a model for the processes of granulite-facies metamorphism and the development of orthopyroxene-bearing mineral assemblages through the breakdown of biotite-bearing assemblages. There has been considerable controversy regarding the role of carbon dioxide in metamorphism and partial melting. We performed new experiments in this system (at pressures of 342 to 1500 MPa with T between 710 and 1045 °C and X Fl H2O between 0.05 and 1.00), accurately locating most of the dehydration and melting equilibria in P-T-X Fl H2O space. The most important primary result is that the univariant reaction Phl + Qtz + Fl = En + Sa + melt must be almost coincident with the fluid-absent reaction (Phl + Qtz = En + Sa + melt) in the CO2-free subsystem. In conjunction with the results of previous measurements of CO2 solubility in silicate melts and phase equilibrium experiments, our theoretical analysis and experiments suggest that CO2 cannot act as a flux for partial melting. Crustal melting in the presence of H2O–CO2 mixed fluids will always occur at temperatures higher than with pure H2O fluid present. Magmas produced by such melting will be granitic (s.l.) in composition, with relatively high SiO2 and low MgO contents, irrespective of the H2O–CO2 ratio in any coexisting fluid phase. We find no evidence that lamprophyric magmas could be generated by partial fusion of quartz-saturated crustal rocks. The granitic melts formed will not contain appreciable dissolved CO2. The channelled passage of hot CO2-rich fluids can cause local dehydration of the rocks through which they pass. In rock-dominated (as opposed to fluid-dominated) systems, minor partial melting can also occur in veins initially filled with CO2-rich fluid, as dehydration and local disequilibrium drive the fluid towards H2O-rich compositions. However, CO2 is unlikely to be a significant agent in promoting regional granulite-grade metamorphism, melting, magma generation, metasomatism or long-range silicate mass transfer in Earth's crust. The most viable model for the development of granulite-facies rocks involves the processes of fluid-absent partial melting and withdrawal of the melt phase to higher crustal levels. Received: 28 November 1996 / Accepted: 25 June 1997  相似文献   

11.
 Marine contamination of groundwater may be caused by seawater intrusion and by salt spray. The role of both processes was studied in the Cyclades archipelago on four small islands (45–195 km2) whose aquifers consist essentially of fractured, weathered metamorphic rocks. Annual rainfall ranges from 400 to 650 mm and precipitation has high total dissolved solids contents of 45–223 mg l–1. The chemical characteristics of the groundwater, whose salinity is from 0.4 to 22 g l–1, are strongly influenced by seawater intrusion. However, the effect of atmospheric input is shown in certain water sampling locations on high ground elevation where the dissolved chloride contents may attain 200 mg l–1. Received: 14 November 1995 · Accepted: 9 September 1996  相似文献   

12.
The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T < 600 °C, XCO2 < 0.4 at P ≈ 6 kbar. The stabilities of margarite and paragonite in equilibrium with quartz are also consistent with T < 600 °C and XCO2 < 0.4 at 6 kbar. Additional assemblages consisting of calcite + clinochlore + quartz + talc + margarite indicate T < 500 °C with XCO2 > 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path. Received: 2 April 1996 / Accepted: 15 November 1996  相似文献   

13.
 Karstic aquifers are highly susceptible to rapid infiltration of river water, particularly during periods of high flow. Following a period of sustained rainfall in the Suwannee River basin, Florida, USA, the stage of the Suwannee River rose from 3.0 to 5.88 m above mean sea level in April 1996 and discharge peaked at 360 m3/s. During these high-flow conditions, water from the Suwannee River migrated directly into the karstic Upper Floridan aquifer, the main source of water supply for the area. Changes in the chemical composition of groundwater were quantified using naturally occurring geochemical tracers and mass-balance modeling techniques. Mixing of river water with groundwater was indicated by a decrease in the concentrations of calcium, silica, and 222Rn; and by an increase in dissolved organic carbon (DOC), tannic acid, and chloride, compared to low-flow conditions in water from a nearby monitoring well, Wingate Sink, and Little River Springs. The proportion (fraction) of river water in groundwater ranged from 0.13 to 0.65 at Wingate Sink and from 0.5 to 0.99 at well W-17258, based on binary mixing models using various tracers. The effectiveness of a natural tracer in quantifying mixing of river water and groundwater was related to differences in tracer concentration of the two end members and how conservatively the tracer reacted in the mixed water. Solutes with similar concentrations in the two end-member waters (Na, Mg, K, Cl, SO4, SiO2) were not as effective tracers for quantifying mixing of river water and groundwater as those with larger differences in end-member concentrations (Ca, tannic acid, DOC, 222Rn, HCO3). Received, March 1999 / Revised, July 1999 / Accepted, July 1999  相似文献   

14.
The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) was investigated through the study of melt inclusions (MI) in crystals of different origins. FTIR spectroscopy and EMPA were used to measure H2O, CO2, S and Cl of the different melts. This allowed us to define the volatile content of the most evolved, phonolitic portion of the magma chamber and of the mafic melts feeding the chamber. MI in sanidine from phonolitic and tephri-phonolitic pumices show systematic differences in composition and volatile content, which can be explained by resorption of the host mineral during syn-eruptive mixing. The pre-eruption content of phonolitic magma appears to have been dominated by H2O and Cl (respectively 6.0 to 6.5 wt% and 6700 ppm), while magma chamber refilling occurred through the repeated injection of H2O, CO2 and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO2 degassing probably occurred during the decompressional path of mafic batches towards the magma chamber, while sulphur was probably released by the magma following crystallization and mixing processes. Water and chlorine strongly accumulated in the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been strongly compositionally controlled, and Cl release was inhibited by groundmass crystallization of leucite, which shifted the composition of the residual liquid towards higher Cl solubilities. Received: 28 October 1999 / Accepted: 21 April 2000  相似文献   

15.
Sabkhas are ubiquitous geomorphic features in eastern Saudi Arabia. Seven brine samples were taken from Sabkha Jayb Uwayyid in eastern Saudi Arabia. Brine chemistry, saturation state with respect to carbonate and evaporate minerals, and evaporation-driven geochemical reaction paths were investigated to delineate the origin of brines and the evolution of both brine chemistry and sabkha mineralogy. The average total dissolved solids in the sabkha brines is 243 g/l. The order of cation dominance is Na+   >>  Mg2+ >>  Ca2+>K+, while anion dominance is Cl− >> SO4 2− >> HCO3 −. Based on the chemical divide principle and observed ion ratios, it was concluded that sabkha brines have evolved from deep groundwater rather than from direct rainfall, runoff from the surroundings, or inflow of shallow groundwater. Aqueous speciation simulations show that: (1) all seven brines are supersaturated with respect to calcite, dolomite, and magnesite and undersaturated with respect to halite; (2) three brines are undersaturated with respect to both gypsum and anhydrite, while three brines are supersaturated with respect to both minerals; (3) anhydrite is a more stable solid phase than gypsum in four brines. Evaporation factors required to bring the brines to the halite phase boundary ranged from 1.016 to 4.53. All reaction paths to the halite phase boundary follow the neutral path as CO2 is degassed and dolomite precipitates from the brines. On average, a sabkha brine containing 1 kg of H2O precipitates 7.6 g of minerals along the reaction path to the halite phase boundary, of which 52% is anhydrite, 35.3% is gypsum, and 12.7% is dolomite. Bicarbonate is the limiting factor of dolomite precipitation, and sulfate is the limiting factor of gypsum and anhydrite precipitation from sabkha brines.  相似文献   

16.
 The Heretaunga Plains, Hawke's Bay, New Zealand, is underlain by Quaternary fluvial, estuarine-lagoonal, and marine deposits infilling a subsiding syncline. Within the depositional sequence, river-channel gravels form one of the most important aquifer systems in New Zealand. An interconnected unconfined–confined aquifer system contains groundwater recharged from the Ngaruroro River bed at the inland margin of the plain, 20 km from the coast. At the coast, gravel aquifers extend to a depth of 250 m. In 1994–95, 66 Mm3 of high quality groundwater was abstracted for city and rural water supply, agriculture, industry, and horticulture. Use of groundwater, particularly for irrigation, has increased in the last 5 years. Concern as to the sustainability of the groundwater resource led to a research programme (1991–96). This paper presents the results and recommends specific monitoring and research work to refine the groundwater balance, and define and maintain the sustainable yield of the aquifer system. Three critical management factors are identified. These are (1) to ensure maintenance of consistent, unimpeded groundwater recharge from the Ngaruroro River; (2) to specifically monitor groundwater levels and quality at the margins of the aquifer system, where transmissivity is <5000 m2/d and summer groundwater levels indicate that abstraction exceeds recharge; (3) to review groundwater-quality programs to ensure that areas where contamination vulnerability is identified as being highest are covered by regular monitoring. Received, January 1998 / Revised, August 1998, March 1999 / Accepted, April 1999  相似文献   

17.
Geochemical processes that take place in the aquifer have played a major role in spatial and temporal variations of groundwater quality. This study was carried out with an objective of identifying the hydrogeochemical processes that controls the groundwater quality in a weathered hard rock aquifer in a part of Nalgonda district, Andhra Pradesh, India. Groundwater samples were collected from 45 wells once every 2 months from March 2008 to September 2009. Chemical parameters of groundwater such as groundwater level, EC and pH were measured insitu. The major ion concentrations such as Ca2+, Mg2+, Na+, K+, Cl−, and SO4 2− were analyzed using ion chromatograph. CO3 − and HCO3 − concentration was determined by acid–base titration. The abundance of major cation concentration in groundwater is as Na+ > Ca2+ > Mg2+ > K+ while that of anions is HCO3 − > SO4 2− > Cl− > CO3 −. Ca–HCO3, Na–Cl, Ca–Na–HCO3 and Ca–Mg–Cl are the dominant groundwater types in this area. Relation between temporal variation in groundwater level and saturation index of minerals reveals the evaporation process. The ion-exchange process controls the concentration of ions such as calcium, magnesium and sodium. The ionic ratio of Ca/Mg explains the contribution of calcite and dolomite to groundwater. In general, the geochemical processes and temporal variation of groundwater in this area are influenced by evaporation processes, ion exchange and dissolution of minerals.  相似文献   

18.
The concentrations of water and carbon dissolved in an icelandite glass quenched from 1400 °C and 10 kbar were measured using Fourier transform infra-red spectroscopy and elemental analyses of carbon and hydrogen. Only carbon dioxide and water were observed in the fluid phase as analysed after quenching with a qudrupole mass analyser. The mole fraction of carbon dioxide in the fluid phase ranged from 0.36 to 0.95. Carbon is dissolved as carbonate except at the highest CO2 fluid fugacity, where a small amount of molecular CO2 is observed. Dissolved carbon in the glasses, calculated as CO2, remained constant at approximately 1 wt %, in spite of the different CO2 fluid fugacities. Water was dissolved as molecular water and as hydroxyl groups, the hydroxyl concentration in the quenched glasses remaining almost constant over the whole interval, whereas the molecular water dissolves in accordance with Henry's law. Molecular water peaks at 5200␣cm−1 and 1630 cm−1, the hydroxyl peak at 4500␣cm−1, and the carbonate peaks at 1400 cm−1–1550 cm−1 have been calibrated using elemental analyses of C and H in the quenched glasses. As molecular water decreases in the melt the higher wavenumber carbonate peak is observed to move towards the molecular water peak at 1630 cm−1 causing a split of the carbonate peaks, ranging from 45 cm−1 to 100 cm−1. Received: 15 November 1995 / Accepted: 21 September 1996  相似文献   

19.
A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hydrocarbons contained in minerals. Application of this technique allows the identification of low-molecular-weight n-alkanes and some aromatic hydrocarbons in sulfides and gangue minerals from epigenetic Variscan and post-Variscan lead–zinc deposits in the Rhenish Massif, Germany. Based on the analysis of 200 samples, Variscan and post-Variscan mineralization can be distinguished by the composition of associated hydrocarbons. Variscan sulfides and gangue minerals contain high abundancies of methane. In contrast, n-alkanes in the C2–C9 range and aromatic hydrocarbons (benzene, toluene) are dominant in post-Variscan mineralization. The absence of high-molecular-weight hydrocarbons in ore minerals suggests highly mature gas associated with hydrothermal activity, during which hydrothermal fluids caused an increase in thermal maturation of organic matter and the generation of low-molecular-weight hydrocarbons in the adjacent organic-rich rocks. The hydrocarbon compositions contained in fluid inclusions of Variscan and post-Variscan minerals are probably governed by the maturation level of the potential source rocks. In Variscan time tectonic brines (T > 175 °C) generated predominantly methane, whereas basement brines (T < 175 °C) expelled higher-molecular-weight hydrocarbons (wet gases, condensates, aromatic hydrocarbons) from adjacent rocks during the Mesozoic event. The specific role of hydrocarbons in sulfide precipitation via thermochemical sulfate reduction is indicated by geochemical characteristics of organic matter associated with the Plombières Pb–Zn deposit, in eastern Belgium. Intense alteration phenomena were observed in near-ore kerogens, compared with unaltered kerogens far from the ore body, as well as by a very high maturity (5.40% Ro), a systematic depletion in 12C towards the vein-type mineralization, high atomic S/C ratios (0.49), and by low atomic H/C ratios (0.29). The data suggest that hydrothermal solutions caused a drastic increase in the thermal maturation of organic matter within the adjacent wall rock. Increased thermal maturation resulted in increased δ13C-values of organic carbon due to the preferential release of 12C. The change in the organic matter to a H-depleted and S-enriched bulk composition in association with sulfide ores strongly suggests that thermochemical sulfate reduction was responsible for organic degradation. Thus, thermochemical sulfate reduction probably triggered base metal sulfide precipitation in Variscan and post-Variscan ore deposits of the Rhenish Massif. Finally, based on data from this study and previous investigations, new genetic models are presented for both Variscan and post-Variscan mineralization in the Rhenish Massif. Received: 15 September 1999 / Accepted: 2 December 1999  相似文献   

20.
Iron-oxide–Cu–Au deposits, particularly those formed in deeper level (plutonic) environments, are commonly characterized by regional scale sodic(–calcic) alteration, which typically formed pre- or syn-Cu–Au mineralization. The sodic(–calcic) assemblages include albite, scapolite, pyroxene, actinolite, apatite, titanite, epidote and calcite. The consistent presence of coexisting hypersaline aqueous and CO2-rich fluids in minerals from sodic(–calcic) alteration and associated Fe-oxide–Cu–Au deposits is the result of unmixing of H2O–CO2–NaCl ± CaCl2–KCl magmatic fluids. Experimental evidence indicates that the Na/(Na + K) ratio of fluids in equilibrium with two alkali feldspars in CO3 2−-bearing parent fluids would be significantly higher than in unmixed chloride-bearing aqueous fluids. Therefore, fluid unmixing caused by decreases in temperature and/or pressure, will result in albitization of wall rocks, as is observed in most deeper level Fe-oxide–Cu–Au deposits. This alteration style may be succeeded by K-feldspathization with decreasing temperature because of the increase in equilibrium Na/(Na + K) in chloride-bearing fluids buffered by alkali feldspars. Received: 26 May 1999 / Accepted: 8 June 2000  相似文献   

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