Meteoric Groundwater Research Articles

Distinction between epigenic and hypogenic maze caves

Certain caves formed by dissolution of bedrock have maze patterns composed of closed loops in which many intersecting fractures or pores have enlarged simultaneously. Their origin can be epigenic (by shallow circulation of meteoric groundwater) or hypogenic (by rising groundwater or production of deep-seated solutional aggressiveness). Epigenic mazes form by diffuse infiltration through a permeable insoluble caprock or by floodwater supplied by sinking streams. Most hypogenic caves involve deep sources of aggressiveness. Transverse hypogenic cave origin is a recently proposed concept in which groundwater of mainly meteoric origin rises across strata in the distal portions of large flow systems, to form mazes in soluble rock sandwiched between permeable but insoluble strata. The distinction between maze types is debated and is usually based on examination of diagnostic cave features and relation of caves to their regional setting. In this paper, the principles of mass transfer are applied to clarify the limits of each model, to show how cave origin is related to groundwater discharge, dissolution rate, and time. The results show that diffuse infiltration and floodwater can each form maze caves at geologically feasible rates (typically within 500ka). Transverse hypogenic mazes in limestone, to enlarge significantly within 1Ma, require an unusually high permeability of the non-carbonate beds (generally ≥10−4cm/s), large discharge, and calcite saturation no greater than 90%, which is rare in deep diffuse flow in sedimentary rocks. Deep sources of aggressiveness are usually required. The origin of caves by transverse hypogenic flow is much more favorable in evaporite rocks than in carbonate rocks.

Submarine groundwater discharge from oceanic islands standing in oligotrophic oceans: Implications for global biological production and organic carbon fluxes

We investigated submarine groundwater discharge (SGD)–associated nutrient fluxes and budgets in two coastal embayments, Hwasun Bay and Bangdu Bay, off the volcanic island of Jeju, Korea. SGD in Hwasun Bay is a composite of marine and meteoric groundwater, while that in Bangdu Bay mainly includes marine groundwater. The submarine inputs of groundwater into Hwasun and Bangdu Bays were approximately 0.12 and 0.27 m3 m−2 d−1, respectively, on the basis of the 222Rn mass balance models. The nitrogen:phosphorus ratios in coastal groundwater (85 ± 96) were considerably larger than those in the seawater (3.8 ± 1.6) of both bays. Fluxes of dissolved inorganic nitrogen (DIN) through SGD were more than 90% of the net DIN input into both bays; approximately 93% and 39% of SGD‐driven DIN was consumed inside Hwasun and Bangdu Bays, respectively. The discharge of DIN through SGD from the entire island was approximately 2.1 × 109 mol yr−1, which is equivalent to that of some large rivers, potentially supporting approximately 1.6 × 1011 g carbon yr−1 of new primary production. Because Jeju accounts for less than 1% of the total land mass of the volcanic islands, SGD‐driven nutrient fluxes from highly permeable islands standing in oligotrophic oceans could be very important for global nutrient budgets.

Comparison of fluid geochemistry and microbiology of multiple organic-rich reservoirs in the Illinois Basin, USA: Evidence for controls on methanogenesis and microbial transport

Microbial methane in sedimentary basins comprises approximately 20% of global natural gas resources, yet little is known about the environmental requirements and metabolic rates of these subsurface microbial communities. The Illinois Basin, located in the midcontinent of the United States, is an ideal location to investigate hydrogeochemical factors controlling methanogenesis as microbial methane accumulations occur: (1) in three organic-rich reservoirs of different geologic ages and organic matter types – Upper Devonian New Albany Shale (up to 900 m depth), Pennsylvanian coals (up to 600 m depth), and Quaternary glacial sediments (shallow aquifers); (2) across steep salinity gradients; and (3) with variable concentrations of SO 4 2 - . For all three organic-rich reservoirs aqueous geochemical conditions are favorable for microbial methanogenesis, with near neutral pH, SO 4 2 - concentrations <2 mM, and Cl − concentrations <3 M. Also, carbon isotopic fractionation of CH 4, CO 2, and DIC is consistent with microbial methanogenesis, and increased carbon isotopic fractionation with average reservoir depth corresponds to a decrease of groundwater flushing rates with average depth of reservoir. Plots of stable isotopes of water and Cl − show mixing between a brine endmember and freshwater, suggesting that meteoric groundwater recharge has affected all microbial methanogenic systems. Additionally, similar methanogenic communities are present in all three reservoirs with comparable cell counts (8.69E3–2.58E6 cells/mL). TRFLP results show low numbers of archaea species with only two dominant groups of base pairs in coals, shale, and limestone aquifers. These results compare favorably with other methanogen-containing deep subsurface environments. Individual hydrogeochemical parameters that have a Spearman correlation coefficient greater than 0.3 to variations in methanogenic species include stable isotopes of water (δ 18O and δD), type of substrate (i.e. coals versus shale), pH, and Cl − concentration. The matching of variations between methanogenic TRFLP data and conservative tracers suggests that deep circulation of meteoric waters influenced archaeal communities in the Illinois Basin. In addition, coalification and burial estimates suggest that in the study area, coals and shale reservoirs were previously sterilized (>80 °C in nutrient poor environments), necessitating the re-introduction of microbes into the subsurface via groundwater transport.

Significance of hypoburrow nodule formation associated with large biogenic sedimentary structures in open-marine bay siliciclastics of the Upper Eocene Birket Qarun Formation, Wadi El-Hitan, Fayum, Egypt

Unusually large biogenic sedimentary structures from the shallow quiescent-marine siliciclastics of the Upper Eocene Birket Qarun Formation in the Fayum area of Egypt display pronounced concretion formation around the trace fossils. The structures are massive, and vary morphologically, forming branched pillars (up to dm-scale), vertical (up to 180 cm height) amphora-like masses, and 3-D box-work “maze”. Bioturbation, mainly Thalassinoides attributable to the Glossifungites ichnofacies, mediated and modified the physical and chemical microenvironments influencing early diagenesis; i.e., burrows promote the precipitation of pervasive calcite-dominated cement. The inferred paragenesis, combined with the negative (light) carbon and oxygen stable-isotopic values of the bulk calcite (δ 13C PDB from −0.94 to −4.98‰ and δ 18O PDB from −4.63 to −7.22‰) and bulk dolomite (δ 13C PDB from −2.05 to −8.23‰ and δ 18O PDB from −1.41 to −11.20‰), imply that the pore-water carbon was derived directly from seawater and dissolution of metastable carbonate, which was mediated by bacterial decomposition of organic matter and mixing of meteoric ground water. Thereby, the carbonate cement precipitated mostly under eodiagenetic conditions near the sediment/water interface (<~3 m in depth). The distribution of these structures is confined to parasequence-bounding flooding surfaces (generally expressed as transgressive surfaces of erosion). Notably, sedimentological, ichnological and paragenetic data can be related to stratigraphic evolution such that geochemical and textural evidence is distinctly associated with (1) early cementation of the host sandstone during highstands of relative sea level, (2) the formation of firmgrounds during low relative sea level, (3) the development of a Glossifungites-demarcated discontinuity during initial relative sea-level rise, and (4) continued cementation with rising relative sea level. This was followed by burial diagenesis, evidence for which is derived from petrographic and isotopic data.

EL DISTRITO DE ORO EL BRONCE Y SU RELACION CON LA CALDERA MORRO HEDIONDO, REGION DE VALPARAISO, CHILE

RESUMEN:The El Bronce gold district is located in Central Chile (32°11'S-70°56'W), and contains epithermal polimetallic (Au, Ag, Cu, Pb, Zn) vein deposits. The district has been mined since the end of the 18th century and its estimated production up to the present is more than 4 x 105 oz Au and 106 oz Ag, besides an important volume of copper. The geological framework of the district is represented by Cretaceous volcanic and intrusive rocks. The oldest stratigraphic units are the Cerro Morado Formation (late Lower Cretaceous) and Las Chilcas Formation (late Lower Cretaceous-Upper Cretaceous (?)), formed by andesitie breccias, agglomerates and lava flows, with volcaniclastic interbeddings. An Upper Cretaceous eruptive centre unconformably overlies these units cutting the volcanic sequence. The centre is defined by the presence of a caldera structure (Morro Hediondo Caldera), of a 7 km radius semicircular section. Dacitic lapilli tuffs (K-Ar: 86±3 Ma), andesitic lavas (K-Ar: 82-80 Ma), and breccias, assigned to the Lo Valle Formation, are the rocks associated with this centre. Two groups of intrusive rocks are recognized in the area. The western group is the oldest one, and is composed of quartz monzodiorites belonging to a batholith dated (K-Ar) as 134 - 86 Ma, which extends between 31° and 32°S. The eastern group is younger and is composed by dioritie (andesitic) to granodioritic (dacitic) stocks, dikes and sills, among which the outstanding components are the Porfido Petorea (K-Ar: 86±3 Ma) and a Ring Dike (K-Ar: 80-79 Ma), which define the outer border of the above mentioned caldera. Extensive hydrothermal alteration zones considered as genetically associated with the intrusives, according to their spatial distribution and K-Ar data (109±4; 82±9; 81±14 Ma) also crop out in the district. The rocks of the area are cut by a number of mainly NW striking faults and dikes, which played an important role in the control of mineralization. Some of these structures show a radial or concentric pattern to me Hediondo Caldera. About 90 ore bodies, mainly polimetallie veins, some copper veins, and one copper breccia pipe (Dulcinea Mine) have been identified. The most important and productive ore deposit is the El Bronce vein, emplaced in a 7 km long, NS-N20°E striking fault. Different lenticular, structurally controlled ore shoots are observed along the fault, ranging from 100-600 m in length, 200-400 m in depth, and 1-20 m in width. The mineralogy consists of pyrite, sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bornite, arsenopyrite, pyrrhotite, hematite, and magnetite in that order of abundance. Gangue minerals are quartz (massive, microcrystalline, amethyst, and crystalline-transparent), ankerite, siderite, and barite. These minerals occur as massive concentrations, disseminated grains and in stockworks. A barren andesitic dike is also emplaced along the mineralized structure. A few meters thick, hydrothermally altered (sericite, clays, chlorite and carbonates) halo surrounds the vein. According to paragenetic studies, mineralization would have been produced in three stages, gold and silver being deposited during the two earlier ones, associated with quartz, pyrite, sphalerite and chalcopyrite. Fluid inclusion studies show homogenization temperatures ranging between 150° and 349°C, and salinities (weight %NaCI) from 1 to 10, while sulfide isotope analyses yielded values of 34S between +0,5 and +2,3. The above data support the hypothesis that the ore was formed at a depth of less than 1,000 m below paleosurface, due to a mixing process between hydrothemal magmatic ore fluids and meteoric ground waters. Keywords: Paragenesis, Hidrothermal alteration, El Bronce district, Epithermal ores, Gold, Morro Hediondo caldera, K-Ar Dating, Fluid inclusions, Upper Cretaceous, Valparaiso Region, Chile.

Magmatic fluids of Tatun volcanic group, Taiwan

Two distinctive magmatic fluids were recognized in the Tatun volcanic group (TVG), Taiwan. One is a relatively reduced fluid represented by the fumarolic gases at Hsiao-you-ken (HYK) geothermal field. Another is an oxidized fluid containing high concentrations of HCl represented by the fumarolic gases at Da-you-ken (DYK). An intermediate gas was recognized at Gung-tze-ping (GTP) and She-hung-ping (SHP). The fumarolic gases at HYK and GTP possess the features of so-called primary steam generated on mixing of magmatic gas and meteoric groundwater. The fumarolic gases at DYK are a simple mixture between magmatic gas and water vapor of meteoric origin. The CO2/H2O molar ratio of the magmatic component in the fumarolic gases at DYK was estimated to be 0.018, meanwhile it was estimated to be 0.027 for the fumarolic gases at HYK and GTP, suggesting the magma beneath DYK is depleted in volatiles relative to the magma beneath HYK and GTP. The estimated CO2/H2O ratio for the magmatic component is comparable to that of some active volcanoes in Japan, suggesting the enrichment of volatiles in the magmas beneath TVG.

A hydrogeological type section for the Duna-Tisza Interfluve, Hungary

The Duna-Tisza Interfluve, Hungary has an agricultural economy but is plagued by severe problems of soil and wetland salinization despite 200 years of intensive research. The study’s objective was to determine the origin of salts and the mechanisms of salinity distribution. To this end, flow-patterns and chemistry of groundwater were evaluated in a 100 km × 65 km area, with emphasis on the Kolon- and Kelemenszek Lakes region. The lakes are located 13 km apart and have chemically contrasting water and soil types. Two groundwater flow-domains were identified: a gravity-drive meteoric fresh water and an over-pressured deeper domain of saline water. The waters are channeled by a highly permeable gravel aquifer to the surface and may merge near Kelemenszek Lake, causing it to be saline. Kolon Lake receives meteoric groundwater only, hence its fresh chemical character. The cross-formational ascent of the deep waters, combined with the gravitational systems’ geometry and the flow-channeling effect of the near-surface rocks, explains the contrasting chemistry between lakes, and the origin and pattern of soil salinization. The scheme is proposed as a generally valid hydrogeological profile for the interfluve, and has been named the Duna-Tisza Interfluve Hydrogeological Type Section.

新潟地域の大規模地すべりと異常高圧熱水系

High salinity Na-Cl-type geothermal waters have often been found in the anticlinal hilly terrains and landslide-prone areas of Niigata Prefecture by boring to depths of more than 1,000 meters. These fluid pressure gradients are much higher than the hydrostatic pressure gradient and approach the lithostatic pressure gradient with increasing depth. The geothermal waters in the Matsunoyama area, Tokamachi City, have the highest orifice temperature in Niigata Prefecture. Eight geothermal water wells in this area were drilled along the anticlinal axis of a nearby hilltop or higher breast of the Matsunoyama dome and ranged in depth from 170 to 1,170 meters and in temperature from 35 to 95°C. For example, the Takanoyu-1 geothermal water well is only 170 meters in depth but has an orifice temperature of 90°C. These waters show geyser action associated with methane gas, and typically have very high salinity with considerable amounts of chloride. Hydrogen and oxygen isotope values and chloride concentration of approximately 9,000mg/L suggest that the origin of the waters is altered fossil seawater trapped in organic-matter-bearing sedimentary rocks. The temperature and depth of the primary reservoir are estimated to be 139°C and 3,000 to 4,000 meters, respectively, using a Li-Mg geothermometer and the mean geothermal gradient of 30-40°C/km in the Niigata sedimentary basin. Na-Cl-type groundwaters emerged from several landslides located in the Higashi-kubiki and Naka-kubiki areas including the Matsunoyama area. Well loggings were carried out to profile the electric conductivity and hydrochemistry of groundwater in the Utsunomata landslide in the neighborhood of the Matsunoyama area. Profiles in the active landslide mass showed sharp increases of the electric conductivity and the NaCl components at around the depth of the sliding surface. In contrast, no significant variations of the profiles were recognized in the inactive landslide mass. Na-Cl-type groundwaters are formed by mixing deep Na-Cl-type geothermal waters with meteoric groundwaters. These phenomena suggest that the geothermal water injection into shallow aquifers in landslide mass generates a partially high pore water pressure around the sliding surface and causes landslides.

Biogeochemical profiles in deep sedimentary rocks in an inland fore-arc basin, Central Japan

Microorganisms thrive in the deep subsurface, but their extent and distribution, type and rate of metabolism are not well constrained, at least in part, owing to geochemical, lithological, hydrological and tectonic variations. We obtained deep sedimentary rocks from a tectonically stable inland basin in central Japan by drilling with minimized microbial and geochemical disturbance. Along with hydrogeological and microbial characterizations, geochemical properties of drilled cores from depths between ~ 200 and ~ 350 m were investigated. Except for a depth of 202 m, nitrate and/or nitrite were present in pore-water at all depths investigated in the present study. In most of the pore-water samples, the levels of dissolved Fe and H 2S were low. Additionally, the presence of microbially reducible Fe(III) was not evident in most of the core samples. Based on the acetylene blockage method, it was revealed the denitrification potential was high at depths of 302 and 351 m where pore-water was depleted in nitrate and enriched in nitrite, while the denitrification potential was relatively low at depths of 300 and 340 m where nitrate and nitrite were both detected in pore-water. Potential rates of methane production via CO 2 reduction were higher than those via acetate fermentation in the 302- and 350-m deep core samples. In the silty sandstone interval at depths of 340 and 351 m where permeability was relatively low, isotopically light methane of microbial origin was slightly enriched with sodium and chloride in pore-water. As a moderately mineralized fluid similarly enriched in sodium and chloride is entrained in the deeper subsurface with a depth over 1200 m around the drilling site, the increase in dissolved aqueous species is attributed to the remnant of the mineralized groundwater after the circulation of meteoric groundwater. The ubiquity of type of the sedimentary rocks suggests that the biogeochemical processes presented in this study might not be limited to the terrestrial sedimentary basin in central Japan.

Formation process of recent fumarolic gases at the Mt. Mihara summit peak of the Izu-Oshima volcano, Japan

Abstract We have investigated the chemical and isotopic composition of recently formed gases emitted by three fumaroles on the Mt. Mihara summit peak of the Izu-Oshima volcano in order to assess the formation process of these gases. The CO2/H2O ratio of these gases varied greatly, with the σD and σ18O of the H2O being as low as a vapor phase equilibrated with the local meteoric water. These features are explained by a mixing of the vapor equilibrated with local meteoric groundwater and primary steam, which is a vapor phase generated by the mixing of magmatic gas and a cold meteoric groundwater. We concluded that there was no direct mixing between the magmatic vapor and a vapor phase of meteoric origin in the processes involved in fumarolic gas formation.

Cementation of kerogen-rich marls by alkaline fluids released during weathering of thermally metamorphosed marly sediments. Part II: Organic matter evolution, magnetic susceptibility and metals (Ti, Cr, Fe) at the Khushaym Matruk natural analogue (Central Jordan)

Spontaneous combustion, less than 1 Ma ago, affected a 60-m thick sediment pile of biomicrite at the Khushaym Matruck site (Jordan). The present study shows that three retrograde alteration stages occurred: weathering, thermal stress and oxidative alkaline perturbation. μ-FT-i.r. spectra of isolated kerogens and oxygen index of whole rocks indicate that oxidation of organic matter occurred down to ∼10 m beneath the metamorphosed zone at Khushaym Matruck. The occurrence of the oxidative weathering bacterially mediated, as suggested by the mass chromatograms of saturated hydrocarbons, can explain high Rock-Eval T max values and low petroliferous potential measured along the sedimentary pile. On the other hand, the thermal extent of combustion events was limited to the first 2 m from the contact. The mean reflectance of 0.20–0.24% and porosity of ca. 50% of the grey clayey biomicrites indicate that organic matter was very immature and sediments were unconsolidated at the time of the combustion event. Using mineralogy, microscopic analyses of vegetable debris and magnetic susceptibility, a suite of characteristic points corresponding to the thermal imprint can be assessed: (i) x = 0 m, T ∼ 1000 °C, (ii) x = 1 m, T ∼ 350 °C, (iii) x = 2 m, T ∼ 150 °C and (iv) x > ∼ 8 m, T ∼ 30 °C. Paleocirculation of meteoric groundwater in the ‘cement-marbles’ generated high-pH fluids that have circulated via fractures and through the matrix porosity of the underlying biomicrites but have also induced alkaline hydrolysis and oxidative attack of the organic matter. The polysaccharide/lignin ratio derived from μ-FT-i.r. analyses shows that the delignification of vegetable debris and degradation of polysaccharides progressively decline in the indurated zone, which indicates a decrease in the pH of migrating solutions. The latter also severely oxidized organic matter at 2.10 and 3.05 m as revealed by the oxygen index and induced the generation of bitumen. The spatial correlation between the oxidation levels of organic matter and the metal contents (Fe, Ti and Cr) suggests that redox reactions were responsible for the immobilization of metals in the indurated biomicrites. The intensity of these reactions is attributed to changes in the fluid flow regime within the sedimentary column.

Br/Cl ratios and O, H, C, and B isotopic constraints on the origin of saline waters from eastern Canada

Saline groundwaters were recovered from undisturbed (Restigouche deposit) and active (Brunswick #12 mine) Zn–Pb volcanogenic massive sulfide deposits in the Bathurst Mining Camp (BMC), northern New Brunswick, Canada. These groundwaters, along with fresh to brackish meteoric ground and surface waters from the BMC, have been analyzed to determine their major, trace element and stable isotopic (O, H, C, and B) compositions. Saline groundwaters (total dissolved solids = 22–45 g/L) are characterized by relatively high Na/Ca ratios compared to brines from the Canadian Shield and low Na/Cl molar and δ 11B isotopic compositions (−2.5‰ to 11.1‰) compared to seawater. Although saline waters from the Canadian Shield commonly have oxygen and hydrogen isotopic compositions that plot to the left of the global meteoric water line, those from the BMC fall close to the water line. Fracture and vein carbonate minerals at the Restigouche deposit have restricted carbon isotopic compositions of around −5‰ to −6‰. The carbon isotopic compositions of the saline waters at the Restigouche deposit (+12‰ δ 13C DIC) are the result of fractionation of dissolved inorganic carbon by methanogenesis. We suggest that, unlike previous models for shield brines, the composition of saline waters in the BMC is best explained by prolonged water–rock reaction, with no requirement of precursor seawater. We suggest that elevated Br/Cl ratios of saline waters compared to seawater may be explained by differential uptake of Br and Cl during groundwater evolution through water–rock reaction.

Geological and geochemical characteristics of the Sawaya'erdun gold deposit, southwestern Chinese Tianshan

The Sawaya'erdun gold deposit in the western segment of the South Tianshan, Xinjiang, China, is a large, low-grade orogenic gold deposit. An abundance of index fossils in the ore-hosting strata, including brachiopoda, corals and Schwagerina, show that the fossil age of the country rocks ranges from Late Carboniferous to Early Permian. Whole rock Rb–Sr and Sm–Nd isochron ages, ranging from 292.43 ± 0.38 to 304.7 ± 11.6 Ma, are concordant with a Late Carboniferous age. Gold mineralization is strictly controlled by a fault zone. Ore minerals are dominated by sulfides, with gold inhomogeneously distributed within them; gangue minerals are quartz, siderite or calcite. Data suggest that the ore-forming fluid was derived predominantly from an active meteoric groundwater system. Ore-forming temperatures are estimated to have been within the range 110 to 220 °C. New 40Ar/ 39Ar isotope age determinations of auriferous quartz are in excellent agreement with Rb–Sr ages obtained from fluid inclusions in quartz. They suggest gold mineralization at 206 to 213 Ma (Late Indosinian). Such an age is consistent with the age of formation given for other gold deposits in the region, such as Bulong and Dashankou. Combined with K–Ar geochronological data and observed geological features, it is concluded that the Indosinian metallogentic epoch represents a previously unrecognized, major period of gold metallogeny in the southwestern Chinese Tianshan. In some ways, the Sawaya'erdun gold deposit can be compared with the Muruntau gold deposit in Uzbekistan with respect to geological setting, host lithology, mineralization style, mineral assemblages, geochemical association and metallogenic processes involved. Despite this, the Sawaya'erdun gold deposit possesses a number of characteristics typical of only low-temperature metallogenesis.

Sequence stratigraphic controls on synsedimentary cementation and preservation of dinosaur tracks: Example from the lower Cretaceous, (Upper Albian) Dakota Formation, Southeastern Nebraska, U.S.A.

A thin cemented sandstone bed in the Upper Albian Dakota Formation of southeastern Nebraska contains the first dinosaur tracks to be described from the state. Of equal importance to the tracks are stable-isotope (C, O) analyses of cements in the track bed, especially in the context of data derived from generally correlative strata (sandstones and sphaerosiderite-bearing paleosols) in the region. These data provide the framework for interpretations of paleoenvironmental conditions, as well as a novel approach to understanding mechanisms of terrestrial vertebrate track preservation. High minus-cement-porosity (> 47%) and low grain-to-grain contacts (∼2.5) in the track bed indicate early (pre-compaction) lithification. Although phreatic cements dominate, the history of cementation within this stratigraphic interval is complex. Cathodoluminescence petrography reveals two distinct calcite zones in the track-bearing horizon and four cement zones in stratigraphically equivalent strata from a nearby section. The earliest calcite cements from both localities are likely coeval because they exhibit identical positive covariant trends ( δ 18O values of − 9.89 to − 6.32‰ and δ 13C values of − 28.01 to − 19.33‰ VPDB) and record mixing of brackish and meteoric groundwaters. All other calcite cements define meteoric calcite lines with δ 18O values clustering around − 9.42‰ and − 8.21‰ VPDB from the track-bearing horizon, and − 7.74‰, − 5.81‰, and − 3.95‰ VPDB from the neighboring section. Distinct meteoric sphaerosiderite lines from roughly correlative paleosols serve as a proxy for locally recharged groundwaters. Back-calculated paleogroundwater δ 18O estimates from paleosol sphaerosiderites range from − 7.4 to − 4.2‰ SMOW; whereas, meteoric calcite lines from the track horizon are generally more depleted. Differences in cement δ 18O values record changes in paleogroundwater recharge areas over time. Early calcite cements indicate mixing of fresh and brackish groundwaters during the syndepositional lithification of the track horizon. Later calcite cements, however, indicate recharge from a larger catchment basin that extended far inland. Therefore, the cements likely record a rise and subsequent fall in relative sea level. We conclude that scrutiny of the cement isotope geochemistry of genetically significant surfaces, especially track beds, can provide new data for interpreting sea level change.

Tectonic-hydrothermal brecciation associated with calcite precipitation and permeability destruction in Mississippian carbonate reservoirs, Montana and Wyoming

The Mississippian Madison Formation contains abundant fracture zones and breccias that are hydrothermal in origin based on their morphology, distribution, and geochemical signature. The hydrothermal activity is related to crustal shortening during the Laramide orogeny. Brecciation is accompanied by dedolomitization, late-stage calcite precipitation, and porosity occlusion, especially in outcrop dolomites. The tectonic-hydrothermal late-stage calcite reduces permeability in outcrops and, potentially, high-quality subsurface reservoir rocks of the subsurface Madison Formation, Bighorn Basin. The reduction of permeability and porosity is increased along the margins of the Bighorn Basin but not predictable at outcrop scale. The destruction of porosity and permeability by hydrothermal activity in the Madison Formation is unique in comparison to studies that document enhanced porosity and permeability and invoke hydrothermal dolomitization models. Hydrothermal breccias from the Owl Creek thrust sheet are classified into four categories based on fracture density, calcite volume, and clast orientation. Shattered breccias dominate the leading edge of the tip of the Owl Creek thrust sheet in the eastern Owl Creek Mountains, where tectonic deformation is greatest, whereas fracture, mosaic, and chaotic breccias occur throughout the Bighorn Basin. The breccias are healed by calcite cements with 18O values ranging between 26.5 and 15.1 Peedee belemnite (PDB), indicating that the cements were derived from isotopically depleted fluids with elevated temperatures. In the chaotic and mosaic breccia types, large rotated and angular clasts of the host rock float in the matrix of coarse and nonzoned late-stage calcite. This appearance, combined with similar 18O values across even large calcite veins, indicates that the calcite precipitated rapidly after brecciation. Values for 13C (5–12 PDB) from the frontal part of the Owl Creek thrust sheet indicate equilibrium between methane and CO2-bearing fluids at about 180C. Fluid inclusions from the eastern basin margin show that these cements are in equilibrium with fluids having minimum temperatures between 120 and 140C and formed from relatively low-salinity fluids, less than 5 wt.% NaCl. Strontium isotope ratios of these hydrothermal fluids are more radiogenic than proposed values for Mississippian seawater, suggesting that the fluids mixed with felsic-rich basement before migrating vertically into the Madison Formation. We envisage that the tectonic-hydrothermal late-stage calcite-cemented breccias and fractures originated from undersaturated meteoric groundwaters that migrated into the burial environment while dissolving and incorporating Ca2+ and and radiogenic Sr from the dissolution of the surrounding carbonates and the felsic basement, respectively. In the burial environment, these fluids were heated and mixed with hypersaline brines from deeply buried parts of the basement. Expulsion of these fluids along basement-rooted thrust faults into the overlying strata, including the Madison Formation, occurred most likely during shortening episodes of the Laramide orogeny by earthquake-induced rupturing of the host rock. The fluids were injected forcefully and in an explosive manner into the Madison Formation, causing brecciation and fracturing of the host rock, whereas the subsequent and sudden decrease in the partial pressure of CO2 caused the rapid precipitation of calcite cements. The explosive nature of hydrothermal fluid migration ultimately produces heterogeneities in reservoir-quality carbonates. In general, flow units in the Madison Formation are related to sequence boundaries, which create vertical subdivisions in the porous dolomite. The late-stage calcite cement surrounds hydrothermal breccia clasts and invades the dolomite, reducing porosity and permeability of the reservoir-quality rock. As a consequence, horizontal flow barriers and compartments are established that are locally unpredictable in their location and extent and regionally predictable along the margins of the Bighorn Basin.

Stable Isotope Evidence for Large-Scale Infiltration of Metamorphic Fluids Generated during Shallow Subduction into the Eastern Peninsular Ranges Mylonite Zone (EPRMZ), Southern California

New δ18O and δD data confirm and amplify the conclusion by Holk et al. (1991) that, during the late Cretaceous or earliest Tertiary, there was large-scale infiltration of metamorphic water at depths of at least 15 km into the lower plate of the EPRMZ, a major crustal shear zone that transects a 15 x 15 km tonalite pluton west of Borrego Springs, California. Whole-rock δ18O values in undeformed tonalite are +11.2 ± 0.4 (n = 9) and in mylonitized tonalite are +12.3 ± 0.3 (n = 21), indicating a 1 per mil increase in 18O during open-system water-rock exchange. Quartz, feldspar, and biotite δ18O values in undeformed tonalite are +12.9 ± 0.4 (n = 8), +11.0 ± 0.6 (n = 8), and +7.7 ± 0.7 (n = 8), compared with +13.5 ± 0.2 (n = 8), +11.8 ± 0.3 (n = 9), and +9.2 ± 0.3 (n = 10) in mylonitized tonalite. The extremely uniform quartz-feldspar and quartz-biotite 18O fractionations in the mylonitized tonalite indicate equilibration at 500-600°C. Biotite and hornblende δD values from undeformed tonalite are -73 ± 3 (n = 12), compared with -52 to -68 (n = 21) in mylonitic tonalite, even though the biotite and hornblende have the same Fe/Mg (≈1.25; Anderson, 1983) in both kinds of tonalite. This implies that the EPRMZ was infiltrated by large amounts of high-D water (δD ≈ -10 to -30) at 500-600°C, probably derived at depth by dehydration of subducted oceanic crust or underplated volcanogenic sediments. Syndeformational tonalitic pegmatites have quartz and feldspar δ18O values of +13.5 ± 0.3 (n = 4) and +12.2 ± 0.3 (n = 6), and biotite and muscovite δD values of -64 ± 1 (n = 2) and -42 (n = 1), suggesting a genetic relationship between the pegmatites and these deep-seated fluids. Higher whole-rock δ18O values (+13.2 to +16.1, n = 3) and much lower δD values (-72 to -114, n = 3) occur where the EPRMZ cuts upper-plate amphibolite-facies metasediments. The shift to lower δD values in these finer grained rocks probably occurred during interactions with heated meteoric groundwaters during Pliocene detachment faulting associated with the opening of the Gulf of California.

The relationship between the McLaughlin gold–mercury deposit and active hydrothermal systems in the Geysers–Clear Lake area, northern Coast Ranges, California

The Geysers–Clear Lake area has a long history of research on its active hydrothermal systems. It is a unique area containing a number of hydrothermal systems which include: the Geysers steam field, one of the largest vapor-dominated geothermal systems yet recognized; the McLaughlin gold deposit, an extremely well preserved hot-spring style gold deposit; and the Sulphur Bank mercury deposit, one of the first locations where geothermal systems were recognized as modern analogues to epithermal deposits. There is also a variety of active hot- and mineral-springs, including Wilbur Springs, or the Sulphur Creek district, which has been considered one of the type localities for connate fluids. The McLaughlin gold–mercury deposit is a fossil hot-spring system dominated by meteoric waters that exchanged with sedimentary rocks of the Great Valley sequence. Mineralization was syntectonic, occurring contemporaneously with fault movement. The fluids circulated in syntectonic dilation zones that resulted in, and maintained, high permeability of the fluid conduits permitting large volumes of fluid flow. The fluids precipitated metals in response to physical and chemical changes associated with boiling. The hydrothermal fluids that formed the McLaughlin deposit have the highest reservoir temperature, salinity and are isotopically the most enriched, of the Coast Range hydrothermal systems. The McLaughlin deposit is considered an end-member “fluid-dominated” hydrothermal system. The Geysers steam field, in its earliest phase was likely similar to the McLaughlin deposit being fluid-dominated and forming, at least on a small scale, a vein system enriched in silver and anomalous in gold, base metals, antimony and mercury. The hydrothermal system evolved into a vapor-dominated system as a result of decreased permeability of the reservoir, decreased recharge and/or increased heat flow. The modern day reservoir is encapsulated in impermeable rocks and is a “vapor-dominated” end-member hydrothermal system. Active hot- and mineral-springs in the Coast Ranges of northern California are intermediate between the fluid- and vapor-dominated end-member systems. The chemical and isotopic compositions of these fluids are the result of thermal processes and are not explained by simple mixing models between connate fluids and meteoric groundwater. Their isotopic and chemical composition is best explained by meteoric-dominated systems with repeated non-equilibrium subsurface vapor loss (evaporation) in a near closed system, with the relative deuterium and 18O enrichment proportional to the reservoir temperature.

Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence

Research Article| January 01, 2005 Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence T. White; T. White 1Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar B. Witzke; B. Witzke 2Iowa Geological Survey, Iowa City, Iowa 52242, USA Search for other works by this author on: GSW Google Scholar G. Ludvigson; G. Ludvigson 2Iowa Geological Survey, Iowa City, Iowa 52242, USA Search for other works by this author on: GSW Google Scholar R. Brenner R. Brenner 3Geoscience Department, University of Iowa, Iowa City, Iowa 52240, USA Search for other works by this author on: GSW Google Scholar Author and Article Information T. White 1Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA B. Witzke 2Iowa Geological Survey, Iowa City, Iowa 52242, USA G. Ludvigson 2Iowa Geological Survey, Iowa City, Iowa 52242, USA R. Brenner 3Geoscience Department, University of Iowa, Iowa City, Iowa 52240, USA Publisher: Geological Society of America Received: 14 Jul 2004 Revision Received: 27 Sep 2004 Accepted: 01 Oct 2004 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (1): 13–16. https://doi.org/10.1130/G20995.1 Article history Received: 14 Jul 2004 Revision Received: 27 Sep 2004 Accepted: 01 Oct 2004 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. White, B. Witzke, G. Ludvigson, R. Brenner; Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence. Geology 2005;; 33 (1): 13–16. doi: https://doi.org/10.1130/G20995.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present the results of oxygen isotope and electron-microprobe analyses of sphaerosiderites obtained from Cretaceous paleosols in Iowa. The sphaerosiderite δ18O values record Cretaceous meteoric groundwater chemistry and an overall waning of brackish groundwater inundation during alluvial-plain aggradation and soil genesis. We focus on horizons that precipitated from freshwater, in which δ18O values ranging from −3.3‰ to −6.8‰ relative to the Peedee belemnite standard are interpreted to record variations in the Cretaceous atmospheric hydrologic cycle. During relative sea-level highstands, moisture was derived from the Cretaceous Western Interior Seaway, whereas during lowstands, when the seaway narrowed and occasionally withdrew from the Midcontinent, the dominance of hemispheric-scale atmospheric moisture transport initiated in the tropical Tethys Ocean led to decreased precipitation rates. These processes did not operate like a switch, but rather as a continuum of competing moisture sources and mechanisms of transport between the nearby epicontinental sea and the distant tropics. The sphaerosiderite data demonstrate (1) temporal variation in the intensity of hemispheric-scale atmospheric moisture transport and (2) long-term amplification of the global hydrologic cycle marked by extreme 18O depletion at the Albian-Cenomanian boundary. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Stable-isotope geochemistry of the Pierina high-sulfidation Au–Ag deposit, Peru: influence of hydrodynamics on SO 42−−H 2S sulfur isotopic exchange in magmatic-steam and steam-heated environments

The Pierina high-sulfidation Au–Ag deposit formed 14.5 my ago in rhyolite ash flow tuffs that overlie porphyritic andesite and dacite lavas and are adjacent to a crosscutting and interfingering dacite flow dome complex. The distribution of alteration zones indicates that fluid flow in the lavas was largely confined to structures but was dispersed laterally in the tuffs because of a high primary and alteration-induced permeability. The lithologically controlled hydrodynamics created unusual fluid, temperature, and pH conditions that led to complete SO 4 2−–H 2S isotopic equilibration during the formation of some magmatic-steam and steam-heated alunite, a phenomenon not previously recognized in similar deposits. Isotopic data for early magmatic hydrothermal and main-stage alunite (δ 34S=8.5‰ to 31.7‰; δ 18O SO 4 =4.9‰ to 16.5‰; δ 18O OH=2.2‰ to 14.4‰; δD=−97‰ to −39‰), sulfides (δ 34S=−3.0‰ to 4.3‰), sulfur (δ 34S=−1.0‰ to 1.1‰), and clay minerals (δ 18O=4.3‰ to 12.5‰; δD=−126‰ to −81‰) are typical of high-sulfidation epithermal deposits. The data imply the following genetic elements for Pierina alteration–mineralization: (1) fluid and vapor exsolution from an I-type magma, (2) wallrock buffering and cooling of slowing rising vapors to generate a reduced (H 2S/SO 4≈6) highly acidic condensate that mixed with meteoric water but retained a magmatic δ 34S ΣS signature of ∼1‰, (3) SO 2 disproportionation to HSO 4 − and H 2S between 320 and 180 °C, and (4) progressive neutralization of laterally migrating acid fluids to form a vuggy quartz→alunite–quartz±clay→intermediate argillic→propylitic alteration zoning. Magmatic-steam alunite has higher δ 34S (8.5‰ to 23.2‰) and generally lower δ 18O SO 4 (1.0 to 11.5‰), δ 18O OH (−3.4 to 5.9‰), and δD (−93 to −77‰) values than predicted on the basis of data from similar occurrences. These data and supporting fluid-inclusion gas chemistry imply that the rate of vapor ascent for this environment was unusually slow, which provided sufficient time for the uptake of groundwater and partial to complete SO 4 2−–H 2S isotopic exchange. The slow steam velocities were likely related to the dispersal of the steam column as it entered the tuffs and possibly to intermediate exsolution rates from magmatic brine. The low δD values may also partly reflect continuous degassing of the mineralizing magma. Similarly, data for steam-heated alunite (δ 34S=12.3‰ to 27.2‰; δ 18O SO 4 =11.7‰ to 13.0‰; δ 18O OH=6.6‰ to 9.4‰; δD=−59‰ to −42‰) are unusual and indicate a strong magmatic influence, relatively high temperatures (140 to 180 °C, based on Δ 18 O SO 4–OH fractionations), and partial to complete sulfur isotopic exchange between steam-heated sulfate and H 2S. Restricted lithologically controlled fluid flow in the host tuffs allowed magmatic condensate to supplant meteoric groundwater at the water table and create the high-temperature low-pH conditions that permitted unusually rapid SO 4 2−–H 2S isotopic equilibration (50–300 days) and (or) long sulfate residence times for this environment. Late void-filling barite (δ 34S=7.4‰ to 29.7‰; δ 18O SO 4 =−0.4‰ to 15.1‰) and later void-filling goethite (δ 18O=−11.8‰ to 0.2‰) document a transition from magmatic condensate to dominantly meteoric water in steam-heated fluids during cooling and collapse of the hydrothermal system. These steam-heated fluids oxidized the top ∼300 m of the deposit by leaching sulfides, redistributing metals, and precipitating barite±acanthite±gold and goethite–hematite±gold. Steam-heated oxidation, rather than weathering, was critical to forming the orebody in that it not only released encapsulated gold but likely enriched the deposit to ore-grade Au concentrations.

Sulfur Isotope Evidence for Magmatic Contributions to Submarine and Subaerial Gold Mineralization: Conical Seamount and the Ladolam Gold Deposit, Papua New Guinea

Conical seamount is a submarine alkali basalt volcano situated 10 km south of Lihir Island in the Tabar-Feni island chain, Papua New Guinea. Polymetallic (Zn-Pb-Ag-Au-As-Sb) veins, pyritic stockwork, and hydrothermally altered rocks discovered on the summit of Conical seamount represent a unique example of submarine gold mineralization with mineralogical, chemical, and textural characteristics common to some subaerial epithermal systems. The giant Ladolam epithermal gold deposit (42 Moz Au) is situated in the crater of an alkali stratovolcano on Lihir Island. Portions of the epithermal mineralization and alteration assemblages at Ladolam are similar to those observed on Conical seamount. Conical seamount has the lightest, hydrothermal (i.e., nondiagenetic) sulfide d34S values (-17.5 to +6.1%) measured to date from modern sea-floor hydrothermal systems. A previous model for the Conical seamount hydrothermal system suggested a contribution of magmatic volatiles in the earliest stages of mineralization (stage 1), followed by a combination of sulfur leached from the underlying volcanic rocks and reduced seawater sulfate during the main base and precious metal precipitating event (stage 2). New sulfur isotope data allow a reinterpretation of this model that suggests a greater input of magmatic volatiles, plus boiling, throughout both stage 1 and 2 mineralization. Sulfides from the Ladolam gold deposit have a range of sulfur isotope values (-12.9 to +3.6%) similar to that of Conical seamount. These sulfur isotope data have previously been interpreted as the result of magmatic volatiles escaping from a crystallizing magma and mixing with dilute meteoric ground water. Anhydrite associated with the gold mineralization has two populations of d34S values (8-14 and 20-22%), which indicate the interaction of magmatic volatiles with seawater that has previously been discounted. The range of sulfide and sulfate d34S values at Ladolam is reinterpreted to be due to a complex interaction of magmatic hydrothermal fluids, meteoric ground water, and seawater. Native sulfur in thermal areas in Luise caldera and pyrite in offshore hydrothermal vents in Luise harbor have similar d34S values to the Ladolam epithermal mineralization, indicating that fluids similar to the young Ladolam ore-forming event are still reaching the floor of Luise caldera. Sulfur isotope data from Conical seamount and the Ladolam deposit suggest that magmatic volatiles have contributed a significant amount of sulfur to both these gold-rich hydrothermal systems.