An example of identifying karst groundwater flow

 Hydrogeological investigations for the purpose of regulating the karst aquifer were carried out in the mountain massif of Kucaj in the Carpatho-Balkan range of eastern Serbia. Different geophysical methods were applied in order to identify the position of karstified zones with active circulation of karst underground streams. Especially good results were obtained by using the spontaneous potential method for the exploration and construction of boreholes and wells. In the valleys of Crni Timok and Radovanska reka the measurements have been carried out upstream along the whole width of the alluvium to the limestone periphery. A number of positive and negative anomalies have been recorded. In the centres of positive anomalies several boreholes were located: HG-19 (centre of anomaly +30 mV, total length of the biggest cavern is 9 m); HG-1 (+20 mV, cavern of 2 m); HG-15 (max. +114 mV, effective cavernousness is 17%). Received: 20 February 1995 · Accepted: 12 September 1997     

Hydrogeology and geochemistry in the Curuksu (Denizli) hydrothermal field, western Turkey

 Curuksu is a low temperature hydrothermal system located within the upper sector of the B. Menderes Graben. The hydrologic structure of the Curuksu hydrothermal system is largely controlled by major graben faults where it is characterized by the presence of two thermal reservoirs. One is formed by Paleozoic quartzite, schist and marble units, and the second consists of Pliocene limestone-travertine units. The thermal conditions in the Curuksu region indicate that the regional tectonics and resulting local stress field control low temperatures activity. Temperatures of 30 springs emerging in the study area range between 15 and 55  °C. These springs are classified as cold fresh, warm mineral and thermal waters. Pamukkale, Karahayıt and Honaz springs are steam condensate waters, whereas Curuksu springs are commonly steam-heated waters with respect to the major anion concentrations. The reservoir temperatures have been estimated from chemical compositions by utilizing simultaneously, geothermometers and mixing models. According to these thermometric methods, the most probable subsurface temperature is in the range of 62–90  °C. However, the mixing models suggest a temperature level of 80  °C for the parent water. The system has low total dissolved solid (TDS) of ∼1000–1500 mg/l, which indicate that these waters undergo conductive cooling within the reservoir. Received: 9 September 1999 · Accepted: 14 February 2000     

Paleomagnetism of the Society Cliffs dolostone and the age of the Nanisivik zinc deposits, Baffin Island, Canada

Dolostones of the ∼1200 Ma Society Cliffs Formation within the hydrothermal zone surrounding the Nanisivik zinc deposits retain a stable characteristic remanent magnetization (ChRM) on alternating field and thermal step demagnetization. Based on the thermal data and saturation isothermal remanence analyses, the ChRM resides in pseudosingle domain magnetite and hematite. A paleomagnetic fold test favours a post-folding ChRM, and a paleomagnetic contact test, using a Franklin gabbro dike, indicates that the ChRM predates ∼720 Ma. The pole position calculated from the ChRM direction is at 168.2°E, 42.8°N (δ_p=4.9°, δ_m=6.8°), giving an age of 1095 ± 10 Ma on the well-defined “Logan Loop” portion of the North American apparent polar wander path. This age is considered to date recrystallization of the dolostone host rocks in the halo around the hydrothermal sulfide deposits. No evidence is found for a postulated Cretaceous remagnetization event in the region. Received: 9 January 1999 / Accepted: 3 March 2000     

CO2-rich thermomineral groundwater in the Betic Cordilleras, southeastern Spain: Genesis and tectonic implications

The CO₂-rich thermal groundwater in the Betic Cordilleras in Spain has been studied with regard to the geological and hydrogeological setting, physical and chemical characteristics, and ¹³C-isotope content. The study area is about 60 km northeast of Almería city, in southeastern Spain. The thermomineral waters are plentiful and are related to regional geothermal anomalies. Temperatures of 20 −41°C, high bicarbonate concentrations (183–1824 mg/L), and high amounts of PCO₂ (<1.1 bar) characterize the groundwater. CO₂ spatial variations are related to proximity to the Carboneras, Palomares, and Guadalentín fault systems, which may be the surface representation of the zone of crustal thinning and magmatism. δ ¹³C values probably indicate a deep source for the CO₂, either the mantle or perhaps carbonate rocks in the metamorphic substratum. The high amount of CO₂ in the groundwater causes problems in wells and severely restricts water usage. The hydrothermal features of this area are probably related to neotectonic activity. Received, September 1998/Revised, June 1999, September 1999/Accepted, December 1999     

Thermal water prospecting results at Jelenia Góra-Cieplice (Sudetes, Poland) versus geothermometric forecasts

 To provide additional volume of thermal water for balneotherapy and space heating the well Cieplice-1 has been deepened from 661 m to 2002.5 m. A spontaneous outflow of water with temperatures reaching 87.8  °C was obtained. Thermal logging revealed 97.7  °C at 1870 m. These results are consistent with earlier estimates based on various chemical geothermometers that indicated water temperatures in the lower parts of the geothermal system possibly exceeding 100  °C. This was also confirmed by the cationic composition geothermometer. The estimates based on the dependence of saturation indices on temperature did not prove correct, suggesting 65  °C as the most probable maximum temperature of the water. Received: 11 Sebtember 1998 · Accepted: 7 January 1999     

Thermal waters of the Hornonitrianska kotlina depression and their utilization

 Slovakia has many areas rich in thermal waters one of which is the Hornonitrianska kotlina depression. At four localities three types of waters are found. The first belongs to the Ca–Mg–HCO₃ type with T.D.S. 0.7 g/l, the second to the Ca–Mg–SO₄ type with T.D.S. 1.37–2.01 g/l and the third to the Ca–Mg–SO₄–HCO₃ type with T.D.S. 0.97 g/l. Discharge at individual localities varies up to 30 l/s and temperatures of water reach 32.5–66.6  °C. The waters are predominantly used for healing, rehabilitation purposes, recreation and heating. Received: 8 March 1999 · Accepted: 7 June 1999     

Groundwater problems resulting from CO2 pollution and overexploitation in Alto Guadalentín aquifer (Murcia,Spain)

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

Mazeno Pass Pluton and Jutial Pluton, Pakistan Himalaya: age and implications for entrapment mechanisms of two granites in the Himalaya

Zircon and monazite U-(Th)-Pb ion microprobe analysis were performed on the Mazeno Pass pluton and the Jutial pluton, two leucogranite bodies within the Nanga Parbat-Haramosh massif (NPHM), Pakistan Himalaya. Zircon rim ages and monazite ages indicate the Mazeno Pass pluton in southwest NPHM intruded at 1.40 ± 0.05 Ma; the Jutial pluton, to the north, similarly yields concordant zircon and monazite ages suggesting crystallization at 9.45 ± 0.06 Ma. The Jutial pluton was subsequently intruded by leucogranite dikes at 5.3 Ma, as revealed by monazite ages. Concordancy of U-Pb and Th-Pb accessory mineral ages demonstrates the robustness of the technique on young rocks. Both plutons, some of the youngest in the Himalaya, have a general association with nearby shear zones that we interpret to have played an integral role in granite evolution and emplacement setting (`deformation enhanced ascent'). Together with new field observations, these results provide an insight on the spatial and temporal relationship between plutonism and deformation relating to the development of the massif. Received: 31 August 1998 / Accepted: 10 March 1999     

Removal of heavy metals from mine water by carbonate precipitation in the Grootfontein-Omatako canal, Namibia

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

The thermal springs of Aachen/Germany – what Charlemagne didn't know

 The Aachen hot springs flow out from Upper Devonian limestones at the Variscan thrust front of the Rhenish Massif. Hydrogeochemical analyses were used to determine the origin of the water and show that sodium, chloride, and sulfate originate from dissolving evaporites that are influenced by meteoric groundwater. The circulation depths of the Aachen thermal water is estimated to be >3500 m using SiO₂–geothermometry and the regional geothermal gradient of about 30  °C/km. The deep circulation in Paleozoic sandstones, siltstones and shales and a considerable residence time allow rock water reactions which explain the concentrations of Ba, Sr, F and Si. While the thermal water rises into Upper Devonian limestones carbonates are dissolved. Cooler fresh water from the vicinity contributes to the thermal aquifer depending on the pressure head of the springs. Sulfate reduction coupled with methane oxidation can be observed as soon as the water temperature is below 65  °C. Received: 21 September 1998 · Accepted: 19 May 1999     

Influence of contaminated Vistula River water on the groundwater entering the Zakrzówek limestone quarry, Cracow region, Poland

 Chemical composition of water inflows in the Zakrzówek quarry, developed in fractured and karstified Upper Jurassic limestones, is controlled by infiltration of polluted water from the Vistula River and by infiltrating meteoric water. The river water TDS value is 2.5 g/dm³. The quarry waters have 0.6–2.0 g/dm³ TDS. Highly mineralised waters belong to Cl-Na type. With decreasing TDS the percentage of sulphates, calcium, magnesium and hydrocarbonates increases. This seems to result from various processes including dilution of polluted river water, leaching of aquifer rocks, and ion exchange. The transfer time of river water to the quarry is about 100–120 days. Concentration of contaminants contained in the river water declines during the migration through limestones to the quarry. Received: 7 March 1997 · Accepted: 7 December 1998     

Petrology, isotope geochemistry and chemical budgets of oceanic gabbros-seawater interactions in the Equatorial Atlantic

 Petrological and chemical variations, as well as oxygen and strontium isotopic data are presented for metagabbros from the Romanche and Vema fracture zones. These rocks were affected by several types and degrees of alterations ranging from slight hydrothermal alteration to complete amphibolitization. Five major kinds of alteration processes ranging from late-magmatic deuteric alteration (stage I) to low temperature (<150 °C) alteration (stage V) were identified. Water-rock interactions between 300 and 650 °C are the most dominant interactions resulting in the most prevailing secondary mineralogical assemblages which characterize the amphibolite and/or greenschist facies (amphibole ± plagioclase ± epidote ± titanite ± chlorite ± prehnite). Hydrothermal alteration of these gabbroic rocks results in isotopic exchanges between rocks and seawater-derived fluids. These exchanges lead to decrease of gabbroic δ¹⁸O toward values as low as +3.9‰, and larger Sr isotopic variations than other oceanic gabbroic rocks (⁸⁷Sr/⁸⁶Sr ratios shift to 0.7029–0.7051). Calculation of a chemical budget indicates that metagabbros are hydrated and enriched in Fe and probably in Mg and Cl, while Si, Ca and Ti are released to the hydrothermal fluids. In addition to metamorphic recrystallization and geochemical transformation, hydrothermal alteration of oceanic gabbros contributes to the control of the global ocean geochemistry. Received: 8 March 1999 / Accepted: 12 July 1999     

Third dimension of a presently forming VMS deposit: TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N

ODP drilling of the active TAG hydrothermal mound at 26°N on the Mid-Atlantic Ridge provided the first insights into the third dimension of a volcanic-hosted massive sulfide (VMS) deposit on a sediment-free mid-ocean ridge. Sulfide precipitation at this site started at least 20,000 years ago and resulted in the formation of a distinctly circular, 200-m diameter, 50-m-high pyritic mound and a silicified stockwork complex containing approximately 3.9 million tonnes of sulfide-bearing material with an average of 2.1 wt% Cu and 0.6 wt% Zn in 95 samples collected from 1–125 m below the seafloor. The periodic release of high-temperature hydrothermal fluids at the same location for several thousand years with intermittent periods of hydrothermal quiesence is the dominating process in the formation of the TAG hydrothermal mound. Distinct geochemical, mineralogical and isotopic zonation as well as a complex assemblage of sulfide-anhydrite-quartz bearing breccias can be related to this process. Geochemical depth profiles indicate extremely low base and trace element concentrations for the interior of the mound, which clearly contrasts with published analyses of samples collected from the surface of the TAG mound. This is explained by continued zone refining during which metals were mobilized from the interior of the mound by upwelling, hot (>350 °C) hydrothermal fluids. Mixing of these fluids with infiltrating ambient seawater subsequently caused redeposition of metals close to the mound-seawater interface. The sulfur isotopic composition of bulk sulfides (+4.4 to +8.2‰δ³⁴S; average +6.5‰) is unusually heavy when compared to other sediment-free mid-ocean ridge deposits and implies the introduction of heavy seawater sulfur to the hydrothermal fluid. The slight increase in sulfur isotope ratios with depth and distinct variations between early, disseminated sulfides related to wallrock alteration, and massive as well as late vein sulfides indicates widespread entrainment of seawater deep into the system. Fluid inclusion measurements in quartz and anhydrite reveal high formation temperatures throughout the TAG mound (up to 390 °C) at one time with an overall increase in trapping temperatures with depth. Lower formation temperatures close to the paleo-seafloor indicate local entrainment of seawater into the mound. Formation temperatures for a central anhydrite-bearing zone range from 340–360 °C and are slightly lower than the exit temperature of hydrothermal fluids presently venting at the Black Smoker Complex (360–369 °C). Fluid inclusions in quartz and anhydrite from the stockwork zone are characterized by formation temperatures higher than 375 °C, indicating conductive cooling of the hydrothermal fluids or mixing with ambient seawater prior to venting. Formation temperatures for quartz from an area of extremely low heat flow at the western side of the mound reach up to 390 °C, implying that this area was once part of a high-temperature hydrothermal upflow zone. The low heat flow and the absence of anhydrite within this part of the mound are strong indications that the recent pulse of high-temperature hydrothermal activity is not affecting this area and provides evidence for significant changes in the fluid flow regime underneath the deposit between hydrothermal cycles. Received: 16 November 1998 / Accepted: 19 August 1999     

The Hanshan gold deposit in the Caledonian North Qilian orogenic belt, NW China

The recently discovered Hanshan gold deposit in northern Gansu Province, northwestern China, is hosted by a WNW-striking shear zone in Ordovician andesite and basalt. Mineralization consists of surface to near-surface oxidized ore (the yellow sandy gossan type) and three types of primary ore, i.e. early-stage quartz-sericite-pyrite ores in stockworks, early-stage disseminated ore, and the most important late-stage quartz ± calcite-sulfide veins. The ore system is characterized by variable degrees of potassic and silicic alteration. Late-stage gold-related fluid inclusions have homogenization temperatures between 170 to 310 °C, with a peak around 260 °C and low salinities. The ore fluids had high contents of CO₂, CH₄, and N₂. Sulfur isotope measurements of −1.9 to +1.7 per mil for hydrothermal pyrites could be consistent with a hydrothermal fluid source from the mantle, but the oxygen and carbon isotope data from calcite and quartz suggest mixing between mantle and crustal fluid sources. K-Ar ages for hydrothermal sericite from ore zones are 213.9 ± 3.1 and 224.4 ± 3.2 Ma. Due to the arid Cenozoic climate, a yellow gold-bearing gossan developed, which consists of jarosite, gypsum, and relict quartz. It could be a widespread and useful prospecting guide for gold in northwestern China. Received: 1 February 1999 / Accepted: 1 August 1999     

Kinetics of dehydration of Ca-montmorillonite

Isothermal thermogravimetric experiments have been carried out to determine the reaction kinetics of the dehydration processes in fuller's earth, a natural Ca-montmorillonite. Dehydration in swelling clays is a complex reaction, and analysis of the thermogravimetric data using empirical rate equations and time-transformation analysis reveals that the nature of the rate controlling mechanism is dependent upon both the temperature regime of the sample as well as the extent of reaction. For fuller's earth, we find that the dehydration kinetics are dominated by a nucleation and growth mechanism at low temperatures and fractions transformed (stage I), but above 90 °C the last stages of the reaction are diffusion controlled (stage II). The activation energy for dehydration during stage I is around 35 kJ · mol⁻¹, whereas the removal of water during stage II requires an activation energy of around 50 kJ · mol⁻¹. These two stages of dehydration are associated with primary collapse of the interlayer (stage I) and movement of water that is hydrated to cations within the interlayer (stage II). Received: 28 August 1998 / Revised, accepted: 27 January 1999     

Water and the density of silicate glasses

A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial molar volume of water is 12.0 ± 0.5 cm³/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H₂O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO₂–M₂O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H₂O is about 4 × 10⁻⁵ K⁻¹ in silicate glasses. Received: 30 June 1999 / Accepted: 5 November 1999     

Water contamination and remedial measures at the Troya abandoned Pb-Zn mine (The Basque Country, Northern Spain)

 This article describes a case of contamination of a karstic aquifer by abandoning an underground mine exploiting sulphide ore body. To exploit the ore, the aquifer was drained and the water level declined about 230 m, drying up the spring that had drained the aquifer up to that moment. When the mining activity ceased, the piezometric level recovered and contaminated water began to flow out from a mine adit. The water is high in sulphates and dissolved Fe, although the pH is neutral. When this water reached the nearby creek, the fish population was eliminated, principally due to the presence of toxic metals and the precipitation of Fe hydroxides. The contamination originated in an area of the partially flooded mine rooms where the ore is in contact with both air and water. The acidity generated by pyrite oxidation is neutralized by calcite dissolution. Presently, the mine water is diverted to the old tailings pond which functions as an aerobic wetland. This action has allowed the fish population in the creek to be restored. Received: 20 January 1999 · Accepted: 15 March 1999     

New thermochronologic constraints on the evolution of the Zaldívar porphyry copper deposit,Northern Chile

Life spans and thermal evolution of hydrothermal systems are of fundamental metallogenic importance. We were able to establish the chronology and cooling history of the Zaldívar porphyry copper deposit (Northern Chile) by applying a combination of different isotopic dating methods in minerals with different closure temperatures, including ⁴⁰Ar/³⁹Ar geochronology and zircon fission track thermochronology, together with fluid inclusion thermometry and previous published U–Pb zircon geochronology. The hydrothermal mineralization in the Zaldívar deposit is genetically related to the Llamo Porphyry unit. Samples of igneous biotites from this intrusion yielded ⁴⁰Ar/³⁹Ar plateau ages between 35.5 ± 0.7 and 37.7 ± 0.4 Ma defining a weighted average of 36.6 ± 0.5 Ma (2σ). In contrast, one sample from the Zaldívar porphyry, one from the andesites, and two from the Llamo porphyry yielded considerably younger fission track ages of approximately 29 Ma with a weighted mean for all ages of 29.1 ± 1.7 Ma (2σ). Thermal and compositional constraints for the hydrothermal system in the Zaldívar deposit from fluid inclusions thermometry show that at least three fluid types broadly characterize two main hydrothermal episodes during the evolution of the deposit. The main mineralization and alteration event is characterized by high temperature (above 320°C) hypersaline fluids (salinity between 30 and 56 wt.% NaCl equivalents) coexisting with low-density gas-rich inclusions (salinity less than 17 wt.% NaCl equivalents) that homogenizing into the gas phase at temperatures above 350°C. The second episode corresponds to a low-temperature event which is characterized by liquid-rich fluid inclusions that homogenize into the liquid phase at temperatures ranging from 200°C to 300°C with salinities lower than 10 wt.% NaCl equivalents. The ⁴⁰Ar/³⁹Ar data (36.6 ± 0.5 Ma, weighted average) obtained from igneous biotites represent the minimum age for the last high-temperature (above 300°C) hydrothermal pulse. When compared with previously published U–Pb ages (38.7 ± 1.3 Ma) in zircons from the Llamo porphyry, a close temporal relationship between crystallization of the parental intrusion and the thermal collapse of the last high-temperature hydrothermal event is evident. Cooling took place from approximately 800°C (crystallization of the intrusive complex defined by zircon U–Pb ages) to below 300 ± 50°C (biotite ⁴⁰Ar/³⁹Ar closure temperature) within approximately 1.5 m.y. Because the thermal annealing of fission tracks in zircons occurs at temperatures of 240 ± 30°, the zircon fission track (ZFT) ages of 29.1 ± 1.7 Ma (2σ) mark the end of the thermal activity in the Zaldívar area, specifically the time when the whole area cooled below this temperature, well after the collapse of the main hydrothermal event in the Zaldívar porphyry copper deposit. This cooling age roughly coincides with the age defined for the emplacement of dacitic dikes at 31 ± 2.8 Ma (2σ) (published K–Ar whole rock), 5 km south of the Zaldívar deposit, in the Escondida area. This late magmatic pulse probably is responsible for high heat flow in the Zaldívar deposit as late as 29 Ma. There is no evidence that the low temperature hydrothermal pulse recognized by fluid inclusion studies is related to this thermal event. The zircon fission track cooling ages are interpreted to be related to the time lag required for complete relaxation of the perturbation of the isotherms in the geothermal field imposed by the intrusion of magmatic bodies, with or without any association with low temperature hydrothermal activity.     

Mineralogical and fluid inclusion studies of low-sulfidation epithermal veins at Osilo (Sardinia), Italy

Several precious metal-bearing, low sulfidation epithermal veins occur in the rolling topography of the Osilo area, northern Sardinia. The Sa Pala de Sa Fae and the Sa Pedra Bianca veins were subject to intense diamond drilling exploration in the mid 1990 s. The veins extend for 1–3 km, dip steeply, and range from 1 to 10 m in width. High K-calc-alkaline volcanic deposits containing plagioclase phenocrysts (along with lesser pyroxene, amphibole, magnetite, olivine and sanidine) form the main host rocks. Gold grades in drill intersections range from <0.1 to <20 ppm, with silver-gold ratios of around 4 to 7. Mineralogical studies show a systematic distribution of three hydrothermal mineral assemblages. At distances >50 m from the vein, the assemblage albite + Fe-chlorite + illite + pyrite (± montmorillonite ± calcite ± K-feldspar) prevails regionally, and its formation is attributed to minor metasomatism of the country rock involving the addition of water, carbon dioxide and hydrogen sulfide. At distances <10 m from the vein, the assemblage quartz + K-feldspar + pyrite ± illite dominates, forming an alteration envelope that cross cuts regional alteration. Quartz and K-feldspar increase in abundance towards the vein. Quartz is the main vein mineral, and it displays a range of morphologies and textures including crustiform colloform banding, quartz pseudomorphs of platy calcite, breccias and coarse euhedral crystals. Electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb-shaped crystals of K-feldspar (adularia). Pyrite, plus lesser marcasite, arsenopyrite, stibnite and sphalerite, are the other sulfide phases in veins. Kaolinite ± halloysite ± jarosite form a late assemblage overprinting earlier hydrothermal alteration. It is mostly restricted to shallow depths of a few meters, except near veins. Most of this assemblage likely formed from weathering and oxidation of sulfides. Microthermometric measurements were made on quartz-hosted, two-phase (liquid + vapor) inclusions, containing ∼75% liquid; mean homogenization temperatures (∼750 measurements) range from 220 to 250 °C, and ice-melting temperatures (∼550 measurements) range from 0.0 to −2.3 °C. The presence of co-existing vapor-rich and liquid rich inclusions, with quartz pseudomorphs of platy calcite, indicate that boiling conditions existed. Slight vapor-bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas. This study shows that gold-silver mineralization formed in subvertical channels from ascending solutions at 250 °C at around 300 to 450 m below the paleo-water table in a typical low-sulfidation epithermal environment. Hydrothermal solutions that produced vein mineralization and related alteration were dilute (<4.1 equivalent wt.% NaCl and <4 wt.% CO₂), near neutral pH, reduced and, at times, boiling. Received: 19 May 1998 / Accepted: 8 March 1999     

Fracture flow and groundwater compartmentalization in the Rollins Sandstone, Lower Mesaverde Group, Colorado, USA

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