Mixing Of Meteoric Water Research Articles

Geochemical Characterization of the Calacoa Geothermal Zone

The Calacoa geothermal zone (CGZ) is located in the western Cordillera of the Andes in southern Peru, approximately 10km on the western slope of Ticsani Volcano. In Calacoa there are mainly volcanic rocks that have been deposited over a sedimentary Cretaceous basement. The geochemical interpretation of the results shows that thermal waters are an alkaline-chloride-sulphate water type. The Cl-SO4-HCO3 diagram shows that most of the geothermal waters plot close to the mature water portion, which is typical of geothermal deep fluids. High B concentrations lead to a relatively high B/Cl ratios as shown on a B-Cl binary diagram. This can be used to elucidate the reactions of waters with sedimentary marine rocks at deep depths. The δ18O vs δD plot indicates that the geothermal water originates by a mixing of meteoric water with magmatic water. The results for chemical geothermometry allowed us to estimate temperatures between 170 and 230°C at the depth of the geothermal resources.

Comparison of stable isotopes, ratios of 36Cl/Cl and 129I/127I in brine and deep groundwater from the Pacific coastal region and the eastern margin of the Japan Sea

Abstract Fifty-three samples, including brines associated with oil and natural gas reservoirs and groundwater samples from deep boreholes, were collected from the Pacific and Japan Sea coastal regions in Japan. The 129 I/ 127 I and 36 Cl/Cl ratios, and stable isotopes (δD and δ 18 O) are compared to investigate differences related to the geotectonic settings of the two regions. The δD and δ 18 O data indicate that brine and groundwater from the Pacific coastal region reflect mixing of meteoric water with connate seawater in the pores of sedimentary rocks. On the other hand, brine and groundwater from the Japan Sea coastal region have been hydrothermally altered. In particular, brines associated with petroleum accumulations at Niigata and Akita showed the same isotopic characteristics as fluids found in the Kuroko deposits of the Green Tuff region in northeastern Japan. There is little difference in the 36 Cl/Cl ratios in brine and groundwater from the Pacific and Japan Sea coasts. Most brine and some deep groundwater, except those from the Pleistocene Kazusa Group, have already reached the average secular equilibrium ratio of 9.9 ± 2.7 × 10 −15 for their mudstone and sandstone reservoirs. There was no correlation between the 36 Cl/Cl ratios and differences in geotectonic setting between the Pacific and the Japan Sea coast. The molar I/Br ratio suggests that the I in all of water samples was of biogenic origin. The average 129 I/ 127 I ratio was 290 ± 130 × 10 −15 to 294 ± 105 × 10 −15 in both regions, showing no relationship to the different geotectonic settings. The uncontaminated brine and groundwater samples are likely to have retained the original 129 I/ 127 I ratios of marine I released from the old organic matter stored in sedimentary rock.

Subepithermal Au-Pd Mineralization Associated with an Alkalic Porphyry Cu-Au Deposit, Mount Milligan, Quesnel Terrane, British Columbia, Canada

At the Mount Milligan Cu-Au porphyry deposit, Quesnel terrane, British Columbia, Canada, barren and weakly mineralized, late-stage hydrothermal veins occur in volcanic rocks adjacent to zones of Cu-Au porphyry mineralization, and have overprinted the porphyry-stage veins. The earliest of the late-stage hydrothermal veins are barren and consist of quartz ± pyrite ± carbonate ± chlorite ± tourmaline. These veins are similar to “transitional” to late-stage hydrothermal veins in other alkaline porphyry Cu-Au deposits, and we consider these to be the equivalent of transitional (post-porphyry, pre-epithermal) quartz-sericite-pyrite veins in calc-alkaline porphyry environments. A later generation of volumetrically minor, mineralized veins are composed of pyrite (Hg- and As-bearing) ± quartz ± carbonate ± chlorite and contain early electrum, arsenopyrite, tetra-hedrite-tennantite, platinum-group element (PGE) tellurides, galena, sphalerite, barite, and chalcopyrite as inclusions in pyrite, and a later assemblage of electrum, PGE tellurides, arsenides and antimonides, galena, sphalerite, chalcopyrite, and various Au-Ag-Te-Bi minerals in annealed fractures and open-space infillings in quartz and pyrite. Metal precipitation in these veins was temporally and spatially associated with the deposition and later recrystallization of pyrite. Primary fluid inclusions in quartz in the barren and weakly mineralized veins are two-phase (L+V), homogenize to liquid over a narrow range in T (~170°–270°C; n = 96, 12 veins), and show a wide range in salinity (4.2 wt % NaCl equiv to 28.7 wt % CaCl2 equiv) when all samples are considered. However, individual veins show narrow ranges in salinity and homogenization temperature. LA-ICP-MS analyses indicate that the fluids were highly enriched in As (to 2,260 ppm), Sb (to 230 ppm), B (to 5,400 ppm), Au (~1–2 ppm) and Pd (~0.5–1 ppm) but depleted in Cu ( 80 ppm) compared to typical porphyry-stage fluids. Metal ratios in the fluids overlap with bulk rock metal ratios in the mineralized veins. The inclusions are interpreted to contain a contracted magmatic vapor (produced by boiling) that lost Cu during the formation of porphyry stage veins at depth. Fluids show decreasing B, As, Sb, and increasing Sr, Ca, and salinity with time. Stable C, O, and H isotope analyses of vein minerals indicate that mixing of this magmatic fluid with meteoric water was not responsible for metal deposition. Rather, metal precipitation was possibly the result of mixing of the magmatic-derived fluid with a heated saline groundwater. The precious and accessory metal mineralogy of the hydrothermal veins is similar to that found in low- to intermediate-sulfidation epithermal systems. Fluid inclusion microthermometry and chlorite thermometry constrain the approximate formation conditions of the veins between ~200 and 1,500 bars and ~240° and 280°C. After the formation of the mineralized veins, circulation of low salinity, metal-depleted fluids occurred. These latest stage fluids may have formed by mixing of the saline magmatic fluid-groundwater hybrid with meteoric water. The results of this study suggest a genetic link between porphyry-stage events and the deposition of Au and PGE in late-stage veins in an alkalic igneous environment. Recognition of hydrothermal processes involving the transport of Au-PGE-As-Sb-Bi-Te-B-rich fluids in the “subepithermal” regimes implies that low-sulfidation epithermal Au deposits may have been present in the shallower parts of the magmatic-hydrothermal complex and that there is potential for the discovery of PGE-rich epithermal veins in less deeply exhumed terranes. On the other hand, the formation of high-grade, low-sulfidation epithermal Au-PGE deposits may be prohibited if porphyry-epithermal transitional fluids precipitate ore metals through mixing with groundwater prior to reaching the level where meteoric water mixing and epithermal boiling normally occur.

Magma-hydrothermal system interaction inferred from volcanic gas measurements obtained during 2003–2008 at Meakandake volcano, Hokkaido, Japan

Phreatic eruptions occurred at the Meakandake volcano in 1988, 1996, 1998, 2006, and 2008. We conducted geochemical surveillance that included measurements of temperature, SO2 emission rates, and volcanic gas composition from 2003 to 2008 at the Nakamachineshiri (NM), Northwest (NW), and Akanuma (AK) fumarolic areas, and the 96–1 vent, where historical eruptions had occurred. The elemental compositions of the gases discharged from the different areas are similar compared with the large variations observed in volcanic gases discharged from subduction zones. All the gases showed high apparent equilibrium temperatures, suggesting that all these gases originated from a common magmatic gas. The gases discharged from each area also exhibited different characteristics, which are probably the results of differences in the conditions of meteoric water mixing, quenching of chemical reactions, and vapor-liquid separation. The highest apparent equilibrium temperatures (about 500°C) were observed in the case of NW fumarolic gases, despite the low outlet temperature of about 100°C at these fumaroles. Since the NW fumaroles were formed as a result of the 2006 phreatic eruption, the high-temperature gas supply to the NW fumarole suggests that the phreatic eruption was caused by the ascent of high-temperature magmatic gases. The temperatures, compositions, and emission rates of the NM and 96–1 gases did not show any appreciable change after the 2006 eruption, indicating that each fumarolic system had a separate magmatic-hydrothermal system. The temperatures, compositions, and emission rates of the NM fumarolic gases were apparently constant, and these fumaroles are inferred to be formed by the evaporation of a hydrothermal system with a constant temperature of about 300°C. The 96–1 gas compositions showed large changes during continuous temperature decrease from 390° to 190°C occurred from 2003 to 2008, but the sulfur gas emission rates were almost constant at about four tons/day. At the 96–1 vent, the SO2/H2S ratio decreased, while the H2/H2O ratio remained almost constant; this was probably caused by the rock-buffer controlled chemical reaction during the temperature decrease.

Petrology and palaeoenvironmental significance of authigenic iron-rich clays, carbonates and apatite in the Claiborne Group, Middle Eocene, NE Texas

The Claiborne Group (Eocene) in North East Texas consists of clayey sandstones and mudrocks, both with variable proportions of dark green to brown clay pellets deposited in a marginal to open marine setting on the Gulf Coast margin. The composition of concretionary carbonate and clay pellets, from 5 localities, has been investigated using a wide variety of analytical techniques. Our palaeontological, sedimentological, C/S and stable isotope data support an interpretation of marginal marine facies with limited freshwater input, for the Claiborne Group. Dark green (mature) pellets are predominantly associated with transgressive, bioturbated, marine sediments. From the Weches Formation through to the youngest Crockett Formation there is a trend from pellets composed mostly of dark green serpentine-rich mixed layer clay to those of mostly brown (immature) nontronite-rich mixed layer clay. The serpentine is intermediate between odinite and berthierine and may have been originally odinite that has undergone partial transformation to berthierine. Pellet maturity apparently corresponds with the degree of winnowing and sedimentation rate. It does not appear to correspond with global temperature fluctuations, though the serpentine-rich rather than glauconite-rich composition may do. Siderite cement, mostly in the form of concretions, post-dates the iron-rich clay pellets and pre-dates calcite cement. Carbonate and apatite concretions show a wide range of mineral cement textures, reflecting variation in sedimentary environment and early diagenetic processes. Based on water temperatures derived from verneriacardia bivalves a precipitation temperature of 20 °C is assumed for the concretionary siderite. Using this temperature we surmise that the siderite precipitated from marine pore waters with intermittent meteoric water mixing.

Fluid Evolution and Metallogenic Dynamics during Tectonic Regime Transition: Example from the Jiapigou Gold Belt in Northeast China

AbstractThe Jiapigou gold belt, one of the most important gold‐producing districts in China, is located in the northern margin of the North China Craton (NCC). The tectonic evolution of the gold belt is closely related to the Siberian Plate (SP) in the north, Yangtze Craton (YC) in the south and Pacific Plate in the east. In order to investigate the nature of the tectono‐fluid‐metallogenic system, the authors investigated the relationships among the tectonic regimes, fluid evolution and metallogenesis. This paper examined the corresponding spatial–temporal relationship between the ore‐controlling tectonic regime and hydrothermal fluid evolution in the Jiapigou gold belt. There are two types of gold mineralization: disseminated ores that are distributed within the NW‐trending main ductile shear zone and gold‐bearing quartz veins and minor disseminated ores that are distant to the ductile shear zone. The fluid inclusions in quartz contain a large amount of CO2. Metamorphic fluids of middle to high temperatures and pressures and meteoric waters of low temperatures and pressures mixed together during mineralization. A proposed ore‐forming model is as follows: in the pre‐ore phase, the collision of SP and NCC resulted in the NS‐trending compression of the ore belt. This formed the NE‐trending and NW‐trending shear faults and EW‐trending folds. During the ore‐forming phase, the collision of YC and NCC resulted in dextral shearing of the NW‐trending Jiapigou fault and the NE‐trending Green faults. High‐pressure fluids caused by the compression flowed into the dilatant zone. This may have caused both phase separation of CO2‐bearing fluids and the mixing of meteoric waters, metamorphic waters and magmatic source fluids and finally resulted in the disequilibrium of the ore fluids and precipitation of ore minerals.

The groundwaters of Fontevivo (Parma Province, Italy): redox processes and mixing with brine waters

This paper describes the chemical and isotopic characterization of H 2 S-bearing groundwaters of the Fontevivo area, northern Italy. Groundwaters from Fontevivo (Parma Province) contain dissolved H 2 S and minor hydrocarbons, which are released from the truncated front of a buried geological structure (Calabrian– Miocene terrains) and through abandoned unsealed oil wells. H 2 S concentration is up to 5.54 mg/l in groundwaters from the topographical high of the village and its distribution in the investigated area is inversely related with those of NH 4 + and SO 4 2− . Groundwaters are dominantly Ca-HCO 3 type with lesser Ca-SO 4 and Na-Cl types and display two compositional trends: group A, waters from Ca-HCO 3 to Ca-SO 4 ; group B, waters from Ca-HCO 3 to Na-Cl. Group A water compositions are influenced by redox processes including the oxidation of H 2 S to SO 4 2− . Primary sulphate from dissolution of Messinian evaporite is rare. Group B waters represent mixing of meteoric water with small amounts of brine. The most Cl (Br, I)-rich groundwaters are located in the NE of the investigated area, where a Cl–Br–I-rich brine was encountered by an exploration well in the Calabrian stratum. Berner has provided a classification of redox environments starting from the observation of the sequence of reduction/oxidation processes as shown by groundwater composition. At Fontevivo the Berner redox zones show an areal distribution where the anoxic zones occur in the topographical high. The δ 2 H and δ 18 O values of groundwaters plot close to the Global Meteoric Water Line. They indicate that local precipitation recharges the shallow aquifer (<30 m deep), whereas rainwater from the higher elevated Apennine ridge recharges the deeper aquifer (≧30 m deep) via the Taro river and its alluvial fan. Dissolved H 2 S is depleted in 34 S consistent with biogenic sulphate reduction. Secondary origin from oxidation of H 2 S in shallow groundwater is invoked for depleted δ 34 S (SO 4 2− ). The studied saline sample (27.8 g/l TDS) show a δ 34 S (H 2 S) value of +24.9‰, suggesting a nearly complete reduction of Messinian sulphate within a system closed to H 2 S. A hydrogeological model is presented based on a chemical-thermodynamic, trace element statistical, and multi-isotope approach.

Hydrochemistry and isotope compositions of groundwater from the Shihongtan sandstone-hosted uranium deposit, Xinjiang, NW China

Groundwaters and surface water in the Shihongtan sandstone-hosted U ore district, Xinjiang, NW China, were sampled and analyzed for their major-, and trace element concentrations and oxygen, hydrogen, boron and strontium isotope compositions in order to assess the possible origins of the waters and water–rock interactions that occurred in the deep aquifer system. The waters in the study district have been grouped into three hydrochemical facies: Facies 1, potable spring-water, is a pH neutral (7.0), Na–Ca–HCO 3 type water with low total dissolved solids (TDS; 0.2 g/l, fresh) and has δ 18O of − 8.3‰, δD of − 48.2‰,δ 11B of 1.5‰, and 87Sr/ 86Sr of 0.70627. Facies 2 groundwaters are mildly acidic to mildly alkaline (pH of 6.5–8.0, mean 7.3), Na–Ca–Mg–Cl–SO 4 type waters with moderate TDS (8.2 g/l–17.2 g/l, mean 9.3 g/l, brackish) and haveδ 18O values in the − 5.8‰ to − 9.3‰ range (mean − 8.1‰), δD values in the − 20.8‰ to − 85.5‰ range (mean − 47.0‰),δ 11B values in the + 9.5‰ to + 39.1‰ range (mean + 17.1‰), and 87Sr/ 86Sr values in the 0.70595 to 0.70975 range (mean 0.70826). Facies 3, Aiting Lake water, is a mildly alkaline (pH = 7.4), Na–Ca–Mg–Cl–SO 4 type water with the highest TDS (249.1 g/l, brine) and has δ 18O of − 2.8‰, δD of − 45.8‰,δ 11B of 21.2‰, and 87Sr/ 86Sr of 0.70840. The waters from the study district show a systematic increase in major, trace element and TDS concentrations and δ 11B values along the pathway of groundwater migration which can only be interpreted in terms of water–rock interaction at depth and strong surface evaporation. The hydrochemical and isotopic data presented here confirm that the groundwaters in the Shihongtan ore district are the combined result of migration, water–rock interaction and mixing of meteoric water with connate waters contained in sediments.

Geological and Geochemical Characteristics of the Zhulazhaga Gold Deposit in Inner Mongolia, China

Abstract Located in Alxa Zuoqi (Left Banner) of Inner Mongolia, China, the Zhulazhaga gold deposit is the first large‐scale gold deposit that was found in the middle‐upper Proterozoic strata along the north margin of the North China craton in recent years. It was discovered by the No. 1 Geophysical and Geochemical Exploration Party of Inner Mongolia as a result of prospecting a geochemical anomaly. By now, over 50 tonnes of gold has been defined, with an average Au grade of 4 g/t. The ore bodies occur in the first lithological unit of the Mesoproterozoic Zhulazhagamaodao Formation (MZF), which is composed mainly of epimetamorphic sandstone and siltstone and partly of volcanic rocks. With high concentration of gold, the first lithological unit of the MZF became the source bed for the late‐stage ore formation. Controlled by the interstratal fracture zones, the ore bodies mostly appear along the bedding with occurrence similar to that of the strata. The primitive ore types are predominantly the altered rock type with minor ore belonging to the quartz veins type. There are also some oxidized ore near the surface. The metallic minerals are composed mainly of pyrite, pyrrhotite and arsenopyrite with minor chalcopyrite, galena and limonite. Most gold minerals appear as native gold and electrum. Hydrothermal alterations associated with the ore formation are actinolitization, silicatization, sulfidation and carbonation.A total of 100 two‐phase H2O‐rich and 7 three‐phase daughter crystal‐bearing inclusions were measured in seven gold‐bearing quartz samples from the Zhulazhaga gold deposit. The homogenization temperatures of the two‐phase H2O‐rich inclusions range from 155 to 401°C, with an average temperature of 284°C and bimodal distributions from 240 to 260°C and 300 to 320°C respectively. The salinities of the two‐phase H2O‐rich inclusions vary from 9.22wt% to 24.30wt% NaCl equiv, with a mode between 23wt% and 24wt% NaCl equiv. Comparatively, the homogenization temperatures of the three‐phase daughter crystal‐bearing inclusions vary from 210 to 435°C and the salinities from 29.13wt% to 32.62wt% NaCl equiv. It indicates that the ore‐forming fluid is meso‐hypothermal and characterized by high salinity, which is apparently different from the metamorphic origin with low salinity. It suggests a magmatic origin of the gold‐bearing fluid.The δ18O values of quartz from auriferous veins range from 11.9 to 16.3 per mil, and the calculated δ18O values in equilibrium with quartz vary from 1.06 to 9.60 per mil, which fall between the values of meteoric water and magmatic water. It reflects that the ore‐forming fluid may be the product of mixing of meteoric water and magmatic water. Based on geological and geochemical studies of the Zhulazhaga gold deposit, it is supposed that the volcanism in the Mesoproterozoic might make gold pre‐concentrate in the strata. The extensive and intensive Hercynian tectono‐magmatic activity not only brought along a large number of ore‐forming materials, but also made the gold from the strata rework. It can be concluded that the ore bodies were mainly formed in late hydrothermal reworking stage. Compared with typical gold deposits associated with epimetamorphic clastic rocks, the Zhulazhaga deposit has similar features in occurrence of ore bodies, ore‐controlling structure, wall‐rock alterations and mineral assemblages. Therefore, the Zhulazhaga gold deposit belongs to the epimetamorphic clastic rock type.

THE OILFIELD WATER IN TAHE AREA, TARIM BASIN —CONSTRAINTS FROM ITS ELEMENT COMPOSITION

The wide range variation of composition of (oilfield) formation water in the different geological era are likely to represent the complex origin and evolution history of fluid. It is thus important to understand how to describing the evolution of water. In order to demonstrate the relative richness of elements in the evaporation and condensation of seawaters and effect on composition variation of oilfield water in water-rock reaction, we have studied the evolution of water in terms of oxygen, carbon isotopes and a great number of chemical components of oilfield water, combined with the comparison among the components of oilfield water or the average seawater composition or brine water undergone the evaporation and condensation of seawater or meteoric water in the dilution and weathering process. The results suggest the marine carbonate rocks -related origin of Ordovician formation water with good proxy indicator of ratio of Cl~-/Br~- (322～337),which approach to the minimum richness content in a preserved pHanerozoic marine seawater, as well as high richness of halogen elements of Br,Cl and I, volatility elements of F,Cl and B,and Iron cluster elements of V,Cr, and so on , the Carboniferous formation water is characteristic of dilution and filtration process with ratio of Cl~-/Br~- as high as 1 399.8,while the Triassic formation water contains relatively high contents of Pro-copper cluster elements and germanium(Ge),indicating a significant input of meteoric water during the late diagenetic process. the isotope records of δ ~(18)O、 δ D and~(87)Sr/~(86)Sr of formation water shows a significant feature of “metamorphic water ”with a much negative shift of δ ~(18)O of Triassic formation water. In conclusion, the composition of Ordovician formation water have been mainly controlled by evaporation and condensation of ancient seawater, undergone a series of dilution input and mixing of meteoric water in the early period of Hercynian, the Carboniferous formation water have been related to dilution and filtration as well as chemical interaction between the meteoric water and gypsum-bearing or halite rocks; while the Triassic formation water maybe derived from mixing and rock interaction with meteoric water.

Groundwater and microbial processes of Alabama coastal plain aquifers

We integrate groundwater geochemistry, microbiology, and numerical modeling techniques to study the origin of elevated salinity and chemical evolution of groundwaters in the coastal plain aquifers of Alabama. Our field data indicate that chemical composition of groundwater evolves by various geochemical and microbial processes as it moves deeper into the subsurface. Sequential peaks of Ca2+, Mg2+, K+, and Na+ along flow paths indicate that separation of ions may be driven by cation exchange. Microbial‐mediated reactions are important for the formation of several discrete hydrochemical zones containing Fe2+, Mn2+, Sr2+, and SO42− rich groundwaters. Elevated Fe2+, Mn2+, and Sr2+ concentrations may be derived from bacterial iron and manganese reduction. High sulfate concentrations observed a short distance from the recharge may be partly explained by microbial sulfur oxidation and nitrate reduction (denitrification). The presence of denitrifying and sulfur‐oxidizing bacteria in water further supports these reactions. Major ion compositions and δD and δ18O values are used to determine the source of salinity and the nature of mixing of different groundwaters. Three water types were identified; these include carbonate groundwater, brines associated with evaporites, and groundwater of meteoric origin. Groundwater age differences and flow velocities were calculated using the 36Cl/Cl ratios. Calculated groundwater flow velocities within the Eutaw and Tuscaloosa aquifers are about 0.20 m/yr and 0.15 m/yr, respectively. We modeled basin‐scale hydrologic and solute transport processes in a cross section extending from the aquifer outcrops to the Gulf Coast. The modeling result shows that the buried Jurassic Louann Salt can significantly increase groundwater salinity in the overlying coastal plain aquifers by density‐driven advection and hydrodynamic dispersion. The modeling results are consistent with Cl/Br ratios and O/H isotope signatures, which indicate that salinity of the groundwater could be derived from seawater that has been evaporated beyond halite saturation. The predicted groundwater flow pattern reveals the mixing of meteoric water, carbonate groundwater (from the Ordovician Knox Group), and saline brines associated with the Louann Salt. The hydrologic model is consistent with the hydrochemical facies distribution in the Alabama coastal plain.

Source processes of the thermal waters from the Phlegraean Fields (Naples, Italy) by means of the study of selected minor and trace elements distribution

The geochemical characteristics of hydrothermal waters from the Phlegraean Fields (P.F.) (Naples, Italy) were analysed for minor and trace elements, selectively mobilised in hydrothermal systems such as B, F, Hg, As, Pb and Tl. The water samples, collected from a shallow aquifer likely to be fed by deeper fluids, showed various geochemical features, resulting from the mixing of three components: (1) surface waters of meteoric origin; (2) hot deep waters deriving from water–rock interaction and including deep waters of marine origin; (3) magmatic fluids rising from the local magma chamber, lying a few kilometres below the town of Pozzuoli. This setting, although very complex, provides a reliable means of studying the distribution of the investigated trace elements. In particular, within the Phlegraean area, high contents of B (0.1–48 mg/l), F (0.5–8 mg/l), As (16–6050 μg/l) and Hg (0.7–232 μg/l) were observed. The levels of thallium in the springs close to Solfatara (about 7 μg/l) were in line with those normally recorded in hydrothermal areas, whereas high levels of this element (up to 23.3 μg/l) were identified in other wells of the study area. Lead (1.3 to 29.1 μg/l) appears to be anomalous with respect to its normal content in groundwater (about 1 μg/l), owing to the presence of high-density brines at depth, which enhance the solubility of Pb in volcanic rocks under hydrothermal conditions. The distribution of the investigated trace elements in the Phlegraean Fields thermal area is probably related to the different ascent pathways of the fluids. Clearly, apart from the influence exerted by anomalous thermal conditions, each element shows a different behaviour, depending on its geochemical affinity with mineral phases and as a consequence of the different enthalpy values, which determine ion partitioning in gaseous phases. Based on geochemical evidences and on the distribution of minor and trace elements, the source processes of the investigated hydrothermal waters were defined. Five main groups were identified: (1) acid sulphate waters, resulting from mixing of meteoric water with magmatic gases (mainly H 2S); (2) high sulphate–chloride waters, from a deep reservoir located in the major upflow zone; (3) waters associated with significant degassing of magmatic CO 2; (4) waters from a deep geothermal neutral chloride reservoir, resulting from heating of marine water modified by water–rock interaction processes; (5) cold waters from the inner area, influenced by low-temperature, water–rock interaction processes.

Origin, chemistry and flow of formation waters in the Mississippian–Jurassic sedimentary succession in the west-central part of the Alberta Basin, Canada

The flow and chemistry of formation waters was analyzed in the Mississippian–Jurassic succession in the west-central part of the Alberta Basin, using publicly available standard chemical analyses of formation waters and drillstem test data collected by the petroleum industry in Alberta. The interpretation of the thoroughly culled hydrogeological data show that a transition between a southern and a northern basin-scale flow system takes place in the study area. In the southeastern half of the study area, the Mississippian–Jurassic hydrostratigraphic group represents a single continuous carbonate–sandstone aquifer system, consisting of the Lower Jurassic Nordegg Member, the Triassic Montney Formation, the Permian Belloy Formation and the Mississippian Stoddart and Rundle groups. The confining units are the competent Fernie and Exshaw–Banff aquitards at the top and base, respectively. The salinity of formation waters increases northwestwards from ∼50 to ∼150g/l, while the bicarbonate content decreases in similar fashion from >3 to ∼0.5g/l. The flow pattern, based on hydraulic heads, and chemical composition of formation waters indicate decreasing mixing of meteoric water, carried northward in the southern basin-scale long-range system, with heavier connate water present in the northern part of the study area. In the center and in the northeast, the Permian and Mississippian carbonates, and the Triassic Montney and Charlie Lake–Halfway sandstones form individual aquifers separated by the intervening Doig–Montney aquitard. The flow in the Charlie Lake–Halfway and the Permo-Mississippian aquifers is directed east–northeastward from the deformation front into the overlying aquitards, or discharging into the southern basin-scale flow system, respectively. Only in the Montney aquifer, the flow is directed inward, south–southwestward, drawn into a local hydrocarbon-saturated sink, which is created by a higher rate of gas escape than of gas generation through a low-permeability and capillary-seal region. The high salinity, in the 130–150g/l range, and low bicarbonate content (∼0.5g/l) of formation waters in the northern part of the study area indicate a connate origin. The relation of Na–Ca–Cl–Br concentrations in the formation waters suggest that these waters initially originated as seawater that was altered to some degree by subaerial evaporation and dolomitization, but halite dissolution probably is the main source of salinity. The driving mechanism for the flow of these waters could be either tectonic expulsion from deep strata in the deformed part of the basin, as suggested by the high salinity of these waters, or topography-driven meteoric recharge in the Rocky Mountains through a tortuous path, unidentified as yet. The integrated interpretation of flow and chemistry of formation water shows that Mississippian–Jurassic formation waters in the deep, west-central parts of the Alberta Basin have been influenced less by mixing with meteoric water than was identified previously in the central, southern and northern parts of the basin. The main reasons for the isolation of ‘relict’ Mississippian–Jurassic formation waters in the deep Alberta Basin are effective confining aquitards and the fact that this area is influenced only marginally by mixing with meteoric water carried by a gravity-driven flow system.

Hydrothermal bismuth mineralization of the Yucheon mine, South Korea: Oxygen and hydrogen isotope study

Hydrothermal bismuth ore deposit of the Yucheon mine, Kyongsang Basin, consists of quartz veins that filled fault-related fractures within Cretaceous sedimentary and Tertiary igneous rocks. Geologic setting and K−Ar age of muscovite (45.4±0.6 Ma) indicate that the ore mineralization was associated with calc-alkaline granitism during the Early Tertiary. The three phases of mineralization are: early phase of pyrite+arsenopyrite+pyrrhotite+sphalerite+chalcopyrite; middle phase of base-metal sulfides +bismuthinite+electrum+Bi-, Sb- and Te-bearing minerals; and late phase of fluorite+calcite in vugs. Based on fluid inclusion data and oxygen isotope geothermometry, the early mineralization phase was formed at higher temperatures (from 350° to 480°C) than the middle phase (230°–350°C). Such cooling was a result of progressive meteoric water mixing, likely due to further fracturing of veins. The main deposition of bismuth occurred as a result of cooling at temperatures between 250° and 350°C. The early to middle phases of mineralization occurred from high temperature fluid (up to 480°C) with the oxygen isotope composition and salinity of 5.5 to −1.4‰ and 0.7 to 18.4 wt.% eq. NaCl, respectively. The fluid experienced progressive meteoric water inundation toward the late mineralization (248−167°C; δ18Owater=−3.9 to −13.9‰; down to 1.9 wt.% eq. NaCl). The oxygen and hydrogen isotope data indicate the extensive evolution of hydrothermal fluids at Yucheon, characterized by the progressive increasing influx of an isotopically less-evolved (approaching the meteoric water composition) meteoric water into a deeply circulating highly-evolved meteoric water or a magmatic fluid.

Groundwater flow patterns in a large peatland

Groundwater flow patterns and geochemistry were studied in the Mer Bleue bog, near Ottawa, Ontario, Canada. Groundwater flow patterns alternated between recharge, i.e. head gradients producing flow from the surface of the peatland to the deeper peat, and discharge, i.e. head gradients indicating flow from the deeper peat towards the surface of the peatland, during the summer of 1998. The patterns were controlled by changes in precipitation, evapotranspiration and the differential head response of catotelm peat (lower layer) to changes in water table elevation. Above-average rainfall in the spring created recharge patterns of groundwater flow in the peatland. Evapotranspiration exceeded precipitation for a three-week period in mid-summer decreasing head at the water table and reversing flow from recharge to discharge. A sustained moisture deficit maintained a flow reversal for 32 days, until a 46 mm rainfall raised the water-table, reversing the vertical hydraulic gradients and restoring recharge flow. The mixing of meteoric water and deeper groundwater controls geochemical profiles. Diffusion modeling shows that the peatland is a long-term recharge system. However, electrical conductivity and cation concentrations in peat pore-waters increase (up to 60%) during a flow reversal, and then decrease when recharge conditions are re-established. The redistribution of substrates from reversed flow is likely important to peatland biogeochemical function.

Origin and migration of brines from Paleozoic strata in Central Tarim, China: constraints from 87Sr/ 86Sr, δD, δ 18O and water chemistry

Chemistry of major and minor elements, 87Sr/ 86Sr, δD, and δ 18O of oilfield waters, and 87Sr/ 86Sr of whole rock were measured from Paleozoic strata in the Central Tarim basin, NW China. The aim is to elucidate the origin and migration of formation water and its relation to petroleum migration. High salinity oilfield waters in Carboniferous, Silurian and Ordovician reservoirs have maintained the same Na/Cl ratio as seawater, indicative of subaerially evaporated seawater. Two possible sources of evaporitic water are Carboniferous (C II) and Cambrian, both of which contain evaporitic sediments. Geographic and stratigraphic trends in water chemistry suggest that most of the high salinity water is from the Cambrian. Strontium, H and O isotopes as well as ion chemistry indicate at least 3 end member waters in the basin. High-salinity Cambrian evaporitic water was expelled upward into Ordovician, Silurian and Carboniferous reservoirs along faults and fractures during compaction and burial. Meteoric water has likely invaded the section throughout its history as uplift created subaerial unconformities. Meteoric water certainly infiltrated Silurian and older strata during development of the C III unconformity and again in recent times. Modern meteoric water enters Carboniferous strata from the west and flows eastward, mixing with the high salinity Cambrian water and to a lesser degree with paleometeoric water. The third end member is highly radiogenic, shale-derived water which has migrated eastward from the Awati Depression to the west. Enrichment of Ca and Sr and depletion of K, Mg, and SO 4 relative to the seawater evaporation trajectory suggest waters were affected by albitization of feldspars, dolomitization, illitization of smectite, and SO 4 reduction. The mixing of meteoric water occurred subsequently to seawater evaporation, main water-rock interactions, and brine migration. The direction of brine migration is consistent with that of petroleum migration, suggesting water and petroleum have followed the same migration pathways.

Evolution and paragenetic context of low δD hydrothermal fluids from the Panasqueira W-Sn deposit, Portugal: new evidence from microthermometric, stable isotope, noble gas and halogen analyses of primary fluid inclusions

The evolution, paragenetic context and origin of remarkably low δD hydrothermal fluids from the Hercynian Panasqueira W-Sn-Cu(Ag) deposit have been investigated through a combined microthermometric, stable isotope, halogen and noble gas fluid inclusion study. Large variations in δD between −60 ‰ and −134‰ have been observed in primary fluid inclusions from growth zones in a quartz crystal (Pa66) paragenetically constrained to the main sulfide stage (MSS) of ore formation. The same fluids exhibited relatively constant fluid inclusion homogenisation temperatures (254 to 260°C), salinities (7.4 to 8.7 wt.% NaCl equivalent) and calculated fluid δ 18O (3.8 to 4.4‰). All the fluids exhibited 40Ar excesses. Mean molar Br/Cl and I/Cl ratios varied from 2.3 × 10 −3–4.2 × 10 −3 and 8 × 10 −4–10 × 10 −4 respectively, with the more Br-rich fluids being associated with the more deuterium-depleted fluids. The low palaeolatitude of Panasqueira throughout the main stages of ore formation and the overwhelming predominance of meteoric fluids in the main, late and post-ore mineralising fluids with δD of −40 to −65‰ lends little support for any models involving low δD palaeometeoric water. The limited range of 40Ar∗/Cl ratios (1.1 × 10 −5–1.6 × 10 −5) also precludes boiling during the MSS or mixing of meteoric water with highly fractionated deuterium-depleted magmatic waters as viable mechanisms for producing the low δD fluids. The high Br/Cl and I/Cl of the MSS fluids indicate significant fluid interactions with organic rich sediments or metasediments, in particular regionally abundant Carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85‰ compared to palaeometeoric water. The large range of δD observed in the MSS fluids may be explained by isotopic exchange of palaeometeoric water with these coals at varying water/rock (wt./wt.) ratios between 0.02 and 0.002 with fluid oxygen isotopic compositions controlled by exchange with the Beira Schists. Such a model is consistent with the requirements that any satisfactory model must take into account viz. (i) the relative constancy of temperature, salinity, 40Ar∗ and fluid δ 18O during excursions to low δD; (ii) the high I/Cl and Br/Cl ratios observed; and (iii) the coincidence of these low δD fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions. The model indicates that (meta)sedimentary sources of ore forming components became increasingly important during the evolution of the Panasqueira Sn-W deposit and accounts in part for the observed progression from oxide-dominated to base-metal sulfide-dominated parageneses.

The stable isotope geochemistry of volcanic lakes, with examples from Indonesia

Stable isotope compositions (δD, δ18O and δ34S) of volcanic lake waters, gas condensates and spring waters from Indonesia, Italy, Japan, and Russia were measured. The spring fluids and gas samples plot in a broad array between meteoric waters and local high-temperature volcanic gas compositions. The δD and δ18O data from volcanic lakes in East Indonesia plot in a concave band ranging from local meteoric waters to evaporated fluids to waters heavier than local high-temperature volcanic gases. We investigated isotopic fractionation processes in volcanic lakes at elevated temperatures with simultaneous mixing of meteoric waters and volcanic gases. An elevated lake water temperature gives enhanced kinetic isotope fractionation and changes in equilibrium fractionation factors, providing relatively flat isotope evolution curves in δ18O–δD diagrams. A numerical simulation model is used to derive the timescales of isotopic evolution of crater lakes as a function of atmospheric parameters, lake water temperature and fluxes of meteoric water, volcanic gas input, evaporation, and seepage losses. The same model is used to derive the flux magnitude of the Keli Mutu lakes in Indonesia. The calculated volcanic gas fluxes are of the same order as those derived from energy budget models or direct gas flux measurements in open craters (several 100m3 volcanic water/day) and indicate a water residence time of 1–2decades. The δ34S data from the Keli Mutu lakes show a much wider range than those from gases and springs, which is probably related to the precipitation of sulfur in these acid brine lakes. The isotopic mass balance and S/Cl values suggest that about half of the sulfur input in the hottest Keli Mutu lake is converted into native sulfur.

Ion microprobe analysis of 18O/16O in authigenic and detrital quartz in the St. Peter Sandstone, Michigan Basin and Wisconsin Arch, USA: Contrasting diagenetic histories

The oxygen isotopic compositions of authigenic quartz cements in sandstones provide a monitor of the temperatures, compositions, and origins of pore-occluding fluids during diagenesis, but quartz overgrowths are too fine-grained to be amenable to conventional isotopic analysis. We have used a Cameca ims-4f ion microprobe to determine oxygen isotopic variations in authigenic and detrital quartz in four samples of the Ordovician St. Peter Sandstone from the Michigan Basin and Wisconsin Arch, midwestern USA. Ion microprobe isotopic analyses have been successfully accomplished with an internal precision of ±1‰ (1 σ) and a spatial resolution of 20–30 μm at low mass resolution using a high voltage offset technique. Repeated analyses of the quartz standard demonstrate a reproducibility of close to ±1‰ (1 sd) in good agreement with that expected from counting statistics. The four ion probe samples were chosen as representative of thirteen samples that were mechanically and chemically disaggregated, sieved, and analysed as a function of sieve size by conventional bulk-mineral techniques. Conventional and ion microprobe analyses are mutually consistent, supporting the accuracy of the ion microprobe analyses. Within-sample isotopic variations of up to 13%. and micro-scale isotopic variations of at least 4‰ over a distance of 100 μm have been measured within quartz overgrowths in a sandstone from the Wisconsin Arch. Overgrowths are uniformly higher in δ18O than detrital grains, and gradients of up to 25% exist across a few microns.18O-enriched quartz overgrowths in sandstones from the Wisconsin Arch show complex CL zonation and reflect one of two possible processes: (1) low-temperature quartz precipitation during mixing of meteoric waters with upwelling basinal fluids; (2) higher temperature quartz precipitation during episodic gravity-driven upwelling of warm basinal fluids (of comparable isotopic composition to Michigan Basin fluids) from the Illinois Basin, related to evolution of Mississippi Valley type PbZn ore-forming fluids. Quartz overgrowths in Michigan Basin sandstones, which derived their silica locally by pressure solution, are thought to have precipitated continuously over a range of temperatures from hot basinal fluids of restricted isotopic composition and circulation. Detrital quartz shows significant intea-grain and intergrain isotopic variation (5–13‰) but no significant inter-sample variation, consistent with a uniform source of granitoid and metamorphic quartz in space and time.

Pore-Water Chemistry and Diagenesis of the Modern Fraser River Delta

ABSTRACT Mixing of meteoric and marine water and diagenetic reactions within sediments of the modern (< 10,000 yr old) Fraser River Delta have been studied using sediment cores and pore-water compositions from 15 cored holes. Pore waters in the Fraser River delta sediments appear to be connate waters, and the concentration of major ions (Cl-, Mg2+, Na+ reflects the degree of mixing of meteoric water and marine water that occurred at the sediment-water interface as the sediments were deposited. Deviations from simple mixing of meteoric and marine water are observed for Ca2+, Mn2+, Fe2+, SO42-, and HCO3-, suggesting diagenetic modification of the pore water since sediment deposition. Although Na+ concentrations appear to be controlled by simple mixing rather than by diagenetic reactions, examination of reactions between clay minerals and water suggests that pore waters are in cation exchan e equilibrium with smectite. Some chemical modification of the pore waters has occurred subsequent to sediment deposition, but there is no chemical evidence either for extensive meteoric-water flushing of the sediments or for tidal influxes of marine water into the sediments. Framboidal pyrite and calcite are the only diagenetic minerals observed in the sediments. Calcite concretions formed at, or near, the sediment-water interface in distributary channels. Calcite in carbonate concretions is depleted in 13C, relative to PDB, because of oxidation near the surface of 13C-depleted methane formed at greater depths by methanogenesis. Oxidation of methane caused elevated HCO-3 concentrations that reacted with Ca2+ supplied by tidal influxes of marine water to promote calcite precipitation in this localized environment.