Fluid Inclusion Water Research Articles

Geology, mineralization, and geochronology of the Qianhe gold deposit, Xiong’ershan area, southern North China Craton

The Qianhe gold deposit in the Xiong’ershan area is located along the southern margin of the Archean-Paleoproterozoic North China Craton. The deposit consists of six orebodies that are hosted in Paleoproterozoic andesites to basaltic andesites and structurally controlled by roughly EW-trending faults. Individual orebodies comprise auriferous quartz veins and disseminated Au-bearing pyrite within hydrothermally altered rocks on both sides of, or close to, the veins. Ore-related hydrothermal alteration has produced various mixtures of K-feldspar, quartz, sericite, chlorite, epidote, carbonate, and sulfides. Pyrite is the most important ore mineral, associated with minor amounts of galena, sphalerite, and chalcopyrite. Other trace minerals include molybdenite, arsenopyrite, scheelite, rutile, xenotime, and parisite. Gold occurs mostly as native gold and electrum enclosed in pyrite or along microfractures of sulfides and quartz. Microthermometric measurements of primary inclusions in auriferous quartz suggest that gold and associated minerals were precipitated in the range of 160–305 °C from aqueous or carbonic-aqueous fluids with salinities of 6–22 wt% NaCl equiv. Samples of molybdenite coexisting with Au-bearing pyrite have Re–Os model ages of 134–135 Ma, whereas ore-related hydrothermal sericite separates yield 40Ar/39Ar plateau ages between 127 and 124 Ma. The Re–Os and 40Ar/39Ar ages are remarkably consistent with zircon U–Pb ages (134.5 ± 1.5 and 127.2 ± 1.4 Ma; 1σ) of the biotite monzogranite from the Heyu-intrusive complex and granitic dikes in and close to the Qianhe gold mine, indicating a close temporal and thus possibly genetic relationship between gold mineralization and granitic magmatism in the area. Fluid inclusion waters extracted from auriferous quartz have δD values of −80 to −72 ‰, whereas the calculated δ 18OH2O values range from 3.1 to 3.8 ‰. The hydrogen and oxygen isotopes from this study and previous work indicate that ore fluids were likely derived from degassing of magmas, with addition of minor amounts of meteoric water. Gold mineralization at Qianhe is temporarily coincident with pervasive bimodal magmatism, widespread fault-basin formation, and well development of metamorphic core complexes in the whole eastern North China Craton that have been interpreted as reflecting reactivation of the craton in the late Mesozoic after prolonged stabilization since its formation in the late Paleoproterozoic. It is therefore concluded that the Qianhe gold deposit formed as a result of this craton reactivation event.

Paleoclimate reconstruction during MIS5a based on a speleothem from Nerja Cave, Málaga, South Spain

Speleothems from Nerja Cave in southern Spain provide a record during interglacial period MIS5a. Period of speleothem deposition occurred from 70,000 90,000 yr ago. Oxygen (δ18O) and hydrogen (δD) isotope ratios of speleothem and fluid inclusions enable the reconstruction of climatic variability in this region of southern Spain. Fluid inclusions trapped in speleothems represent samples of drip water from which the speleothems grew. The isotopic compositions of cave dripwaters approximate average annual δ18O and δD of precipitation, therefore δ18O can be calculated from D/H of inclusion water using the MWL relationship δD = 8δ18O + 10. The measurements of the δD values of fluid-inclusion water and δ18O values from speleothems have been applied to paleoclimate reconstruction in Southern Spain indicating a colder condition than at present.

Giant Mesozoic gold provinces related to the destruction of the North China craton

Lode gold deposits in Precambrian cratons represent the world's major gold source and were mostly generated during formation and stabilization of the cratons. However, there is an extraordinary exception in the North China craton (NCC), where lode gold deposits formed after prolonged stabilization of the craton. Molybdenite Re–Os and hydrothermal sericite and biotite 40Ar/39Ar dating of major gold deposits from the Xiaoqinling district, southern NCC, bracket their emplacement in the range of 154.1±1.1 to 118.9±1.2Ma (n=23), postdating formation of the craton by more than 1.7 billion years. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (1.52–0.22Ra) and mantle-like Ne isotopes (20Ne/22Ne=10.02−9.22 and 21Ne/22Ne=0.033−0.027), indicating presence of mantle-derived fluids in the ore system. Measured δ34S of pyrite and δD and δ18O of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic/mantle source for sulfur and ore fluids. Gold deposits of similar ages also widely occur in the eastern and northern margins of the NCC, which, together with those in the Xiaoqinling district, have a total reserve of ∼2500t gold, forming the only known giant late Mesozoic gold province in the world's Precambrian cratons. These deposits formed coevally with extensive felsic to mafic magmatism, development of intracontinental rift basins, and exhumation of metamorphic core complexes across the eastern NCC, events interpreted as indicating thinning and destruction of the lithosphere beneath the craton. Rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids, sulfur and, by inference, other ore components to form the giant gold provinces.

Origin and evolution of hydrothermal fluids in the Taochong iron deposit, Middle–Lower Yangtze Valley, Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Middle–Lower Yangtze River Valley is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. The Taochong deposit is located in the northern part of the Fanchang iron ore district of the Middle–Lower Yangtze River metallogenic belt. The Fe-orebody is hosted by Middle Carboniferous to Lower Permian limestones. Skarns and Fe-orebodies occur as tabular bodies along interlayer-gliding faults, at some distance from the inferred granitic intrusions. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity—the skarn, iron oxide (main mineralization stage), and carbonate stages—all contributed to the formation of the Taochong iron deposit. The skarn stage is characterized by the formation of garnet and pyroxene, with high-temperature, hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids. These fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite, chlorite, quartz, calcite and hematite. The hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor-rich inclusions with anomalously low δD values (−66‰ to −94‰). The decrease in ore fluid δ18Owater with time and decreasing depth is consistent with the decreases in fluid salinity and temperature. The fluid δD values also show a decreasing trend with decreasing depth. Both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water. This process was probably induced by a drop in pressure from lithostatic to hydrostatic. The carbonate stage is represented by calcite veins that cut across the skarn and orebody, locally producing a dense stockwork. This observation indicates the veins formed during the waning stages of hydrothermal activity. The fluids from this stage are mainly represented by a variety of low-salinity fluid inclusions, as well as fewer high-salinity inclusions. These particular fluids have the lowest δ18Owater values (−2.2‰ to 0.4‰) and a wide of range of δD values (−40‰ to −81‰), which indicate that they were originated from a mixture of residual fluids from the oxide stage, various amounts of meteoric water, and possibly condensed vapor. Low-temperature boiling probably occurred during this stage.We also discuss the reasons behind the anomalously low δD values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures, and suggest that calcite data provide a possible benchmark for adjusting low δD values found in quartz intergrown with calcite.

The origin of NO3− and N2 in deep subsurface fracture water of South Africa

Deep (>0.8km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40μM of NO3−, up to 50mM N2 (90 times air saturation at surface) and 1 to ~400μM NH3/NH4+. To determine whether the oxidized N species were introduced by mining activity, by recharge of paleometeoric water, or by subsurface geochemical processes, we undertook N and O isotopic analyses of N species from fracture water, mining water, pore water, fluid inclusion leachate and whole rock cores.The NO2−, NO3− and NH3/NH4+ concentrations of the pore water and fluid inclusion leachate recovered from the low porosity quartzite, shale and metavolcanic units were ~104 times that of the fracture water. The δ15N–NO3− and δ18O–NO3− of the pore water and fluid inclusion leachate, however, overlapped that of the fracture water with the δ15N–NO3− ranging from 2 to 7‰ and the δ18O–NO3− ranging from 20 to 50‰. The δ15N–NO3− of the mining water ranged from 0 to 16‰ and its δ18O–NO3− from 0 to 14‰ making the mining water NO3− isotopically distinct from that of the fracture, pore and fluid inclusion water. The δ15N–N2 of the fracture water and the δ15N–N from the cores ranged from −5 to 10‰ and overlapped the δ15N–NO3−. The δ15N–NH4+ of the fracture water and pore water NH3/NH4+ ranged from −15 to 4‰. Although the NO3− concentrations in the pore water and fluid inclusions were high, mass balance calculations indicate that NO3− accounts for ≤10% of the total rock N, whereas NH3/NH4+ trapped in fluid inclusions or NH4+ present in phyllosilicates account for ≥90% of the total N.Based on these findings, the fluid inclusion NO3− appears to be the source of the pore water and fracture water NO3− rather than paleometeoric recharge or mining contamination. Irradiation experiments indicate that radiolytic oxidation of NH3 to NO3− can explain the fluid inclusion NO3− concentrations and, perhaps, its isotopic composition, but only if the NO3− did not attain isotopic equilibrium with the hydrothermal fluid 2billion years ago. The δ15N–N, δ15N–N2 and δ15N–NH4+ suggest that the reduction of N2 to NH4+ also must have occurred in the Witwatersrand Basin in order to explain the abundance of NH4+ throughout the strata. Although the depleted NO3− concentrations in the fracture water relative to the pore water are consistent with microbial NO3− reduction, further analyses will be required to determine the relative importance of biological processes in the subsurface N cycle and whether a complete subsurface N cycle exists.

Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain, Nevada, USA, revealed by the isotope composition of fluid-inclusion water

Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ 18O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ 18O values were calculated from the δ 18O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry. The δD values measured in all samples range between − 110 and − 90‰, similar to Holocene meteoric water. Coupled δ 18O–δD values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive δ 18O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 °C), which is only possible in the deep-seated hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin.

Fossil dripwater in stalagmites reveals Holocene temperature and rainfall variation in Amazonia

Most proxy records used for reconstruction of Holocene climate of Amazonia are unable to quantitatively distinguish between the effect of temperature and rainfall amounts. We present a new isotope technique applied to a ∼ 13,500 yr stalagmite archive from Peruvian Amazonia. By analysing the coupled isotope composition of fossil dripwater trapped in stalagmite fluid inclusions, and that of the calcite hosting the fluid inclusions, we were able to calculate independent paleotemperatures and rainfall amounts. This stalagmite record shows that Holocene climate variation was controlled by orbitally-forced Southward migration of the Inter Tropical Convergence Zone. While temperature remained constant, isotope variation of rainwater, reflected in fluid inclusion water δ 18O composition, suggests a ∼ 15–30% increase in convective rainfall through the Holocene. A comparison of the low-land Peruvian fluid inclusion record with the high Andean Huascaran ice core record shows a constant ∼ 12‰ offset of δ 18O curves for the Holocene, suggesting that Andean vertical temperature gradients (lapse rates) did not vary much over the last 9000 years. During the Younger Dryas interval, however, the offset of δ 18O values was much higher than in the Holocene. This may be attributed to a relative drop in air temperatures in the highlands (higher lapse rate), caused by long distance teleconnections to climate perturbations in the North Atlantic. In a wider perspective, fluid inclusion isotope analysis drastically improves paleotemperature reconstructions based on speleothem calcite δ 18O data, because it provides the δ 18O value of drip water through time, which is usually the most important unknown in paleotemperature equations.

Hydrogen isotope determination of fluid inclusion water from hydrothermal fluorite: Constraining the effect of the extraction technique

Natural waters of various origins exhibit systematic differences in their D/H and 18O/ 16O ratios. Several recent spectroscopic and isotopic studies indicate that in addition to molecular water from fluid inclusions isotopically shifted structurally bound water may contribute to the hydrogen isotope budget in hydrothermal minerals. To further clarify these effects, a combined FTIR and stable isotope study has been carried out on hydrothermal fluorite from fluorite–barite veins, Schwarzwald district, Germany. Water for δD analysis was extracted at two different temperatures from the same samples, at 400 °C and 650 °C. In one series of experiments, different sample aliquots were used for the runs at both temperatures, whereas in a second series a step heating technique was employed to extract water from the same sample aliquot. Compared to the results at 400 °C, the δD values obtained at 650 °C are systematically more negative for most of the samples. The isotopic shift ranges from a few permil up to as much as − 55‰. FTIR spectroscopic measurements of the fluorites show that a broad absorption band at 3400 cm − 1 is present, which results from O–H stretch vibrations. A band at 5200 cm − 1 , which is related to a combination of bend and stretch vibrations from molecular H 2O, was not found in the inclusion-free areas of the fluorites. The spectroscopic observations show that significant (but variable) amounts of structurally bound water are present in the hydrothermal fluorites. The results of our study indicate that most likely isotopically distinct fluid inclusion and structurally bound water contribute to the bulk δD signature. This calls for extreme caution in setting up appropriate analytical procedures for determination of δD values by thermal decrepitation and has considerable implications for the interpretation of paleofluid signatures.

Contrasting paleofluid systems in the continental basement: a fluid inclusion and stable isotope study of hydrothermal vein mineralization, Schwarzwald district, Germany

AbstractAn integrated fluid inclusion and stable isotope study was carried out on hydrothermal veins (Sb‐bearing quartz veins, metal‐bearing fluorite–barite–quartz veins) from the Schwarzwald district, Germany. A total number of 106 Variscan (quartz veins related to Variscan orogenic processes) and post‐Variscan deposits were studied by microthermometry, Raman spectroscopy, and stable isotope analysis. The fluid inclusions in Variscan quartz veins are of the H2O–NaCl–(KCl) type, have low salinities (0–10 wt.% eqv. NaCl) and high Th values (150–350°C). Oxygen isotope data for quartz range from +2.8‰ to +12.2‰ and calculated δ18OH2O values of the fluid are between −12.5‰ and +4.4‰. The δD values of water extracted from fluid inclusions vary between −49‰ and +4‰. The geological framework, fluid inclusion and stable isotope characteristics of the Variscan veins suggest an origin from regional metamorphic devolatilization processes.By contrast, the fluid inclusions in post‐Variscan fluorite, calcite, barite, quartz, and sphalerite belong to the H2O–NaCl–CaCl2 type, have high salinities (22–25 wt.% eqv. NaCl) and lower Th values of 90–200°C. A low‐salinity fluid (0–15 wt.% eqv. NaCl) was observed in late‐stage fluorite, calcite, and quartz, which was trapped at similar temperatures. The δ18O values of quartz range between +11.1‰ and +20.9‰, which translates into calculated δ18OH2O values between −11.0‰ and +4.4‰. This range is consistent with δ18OH2O values of fluid inclusion water extracted from fluorite (−11.6‰ to +1.1‰). The δD values of directly measured fluid inclusion water range between −29‰ and −1‰, −26‰ and −15‰, and −63‰ and +9‰ for fluorite, quartz, and calcite, respectively.Calculations using the fluid inclusion and isotope data point to formation of the fluorite–barite–quartz veins under near‐hydrostatic conditions. The δ18OH2O and δD data, particularly the observed wide range in δD, indicate that the mineralization formed through large‐scale mixing of a basement‐derived saline NaCl–CaCl2 brine with meteoric water. Our comprehensive study provides evidence for two fundamentally different fluid systems in the crystalline basement. The Variscan fluid regime is dominated by fluids generated through metamorphic devolatilization and fluid expulsion driven by compressional nappe tectonics. The onset of post‐Variscan extensional tectonics resulted in replacement of the orogenic fluid regime by fluids which have distinct compositional characteristics and are related to a change in the principal fluid sources and the general fluid flow patterns. This younger system shows remarkably persistent geochemical and isotopic features over a prolonged period of more than 100 Ma.

Oxidation of methane at the CH 4/H 2O–(CO 2) transition zone in the external part of the Central Alps, Switzerland: Evidence from stable isotope investigations

With the aim of understanding the mechanisms that control the metamorphic transition from the CH 4– to the H 2O–(CO 2)-dominated fluid zone in the Helvetic domain of the Central Alps of Switzerland, fluid inclusions in quartz, illite “crystallinity” index, vitrinite reflectance, and the stable isotope compositions of vein and whole rock minerals and fluids trapped in quartz were investigated along four cross-sections. Increasing temperature during prograde metamorphism led to the formation of dry gas by hydrocarbon cracking in the CH 4-zone. Fluid immiscibility in the H 2O–CH 4–(CO 2)–NaCl system resulted in cogenetic CH 4- and H 2O-dominated fluid inclusions. In the CH 4-zone, fluids were trapped at temperatures ≤ 270 ± 5 °C. The end of the CH 4-zone is marked by a sudden increase of CO 2 content in the gas phase of fluid inclusions. At temperatures > 270 ± 5 °C, in the H 2O-zone, the total amount of volatiles within the fluid decreased below 1 mol% with no immiscibility. This resulted in total homogenization temperatures of H 2O–(CO 2–CH 4)–NaCl inclusions below 180 °C. Hydrogen isotope compositions of methane in fluid inclusion have δD values of less than − 100‰ in the CH 4-zone, typical for an origin through cracking of higher hydrocarbons, but where the methane has not equilibrated with the pore water. δD values of fluid inclusion water are around − 40‰, in isotopic equilibrium with phyllosilicates of the whole rocks. Within the CH 4 to H 2O–(CO 2) transition zone, δD(H 2O) values in fluid inclusions decrease to − 130‰, interpreted to reflect the contribution of deuterium depleted water from methane oxidation. In the H 2O-zone, δD(H 2O) values increase again towards an average of − 30‰, which is again consistent with isotopic equilibrium with host-rock phyllosilicates. δ 13C values of methane in fluid inclusions from the CH 4-zone are around − 27‰, in isotopic equilibrium with calcite in veins and whole rocks. The δ 13C(CH 4) values decrease to less than − 35‰ at the transition to the H 2O-zone and are no longer in equilibrium with the carbonates in the whole rocks. δ 13C values of CO 2 are variable but too low to be in equilibrium with the wall rock fluids, compatible with a contribution of CO 2 from closed system oxidation of methane. Differences in isotopic composition between host-rock and Alpine fissure carbonate are generally small, suggesting that the amount of CO 2 produced by oxidation of methane was small compared to the C-budget in the rocks and local pore fluids were buffered by the wall rocks during precipitation of calcite within the fissures.

Zinc Deposits and Related Mineralization of the Burketown Mineral Field, Including the World-Class Century Deposit, Northern Australia: Fluid Inclusion and Stable Isotope Evidence for Basin Fluid Sources

The stratiform Century Zn-Pb deposit and the discordant Zn-Pb lode deposits of the Burketown mineral field, northern Australia, host ore and gangue minerals with primary fluid inclusions that have not been affected by the Isan orogeny, thus providing a unique opportunity to investigate the nature of the ore-forming brines. All of the deposits are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. According to Pb model ages, the main ore stage of mineralization at Century formed at 1575 Ma, some 20 m.y. after deposition of the host shale sequence. Microthermometry on undeformed, primary fluid inclusions hosted in porous sphalerite shows that the Zn at Century was transported to the deposit by a homogeneous, Ca 2+ - and Na + -bearing brine with a salinity of 21.6 wt percent NaCl equiv. δD fluid of the fluid inclusion water ranges from −89 to −83 per mil, consistent with a basinal brine that evolved from meteoric water. Fluid inclusion homogenization temperatures range between 74° and 125°C, which are lower than the 120° to 160°C range calculated from vitrinite reflectance and illite crystallinity data from the deposit. This discrepancy indicates that mineralization likely formed at 50 to 85 Mpa, corresponding to a depth of 1,900 to 3,100 m. Transgressive galena-sphalerite veins that cut stratiform mineralization at Century and breccia-filled quartz-dolomite-sphalerite-galena veins in the discordant Zn-Pb lodes have Pb model ages between 1575 and 1485 Ma. Raman spectroscopy and microthermometry reveal that the primary fluid inclusions in these veins contain Ca 2+ , Na + , but they have lower salinities between 23 and 10 wt percent NaCl equiv and higher δD fluid values ranging from −89 to −61 per mil than fluid inclusions in porous sphalerite from Century. Fluid inclusion water from sphalerite in one of the lode deposits has δ 18 O fluid values of 1.6 and 2.4 per mil, indistinguishable from δ 18 O fluid values between −0.3 to +7.4 per mil calculated from the isotopic composition of co-existing quartz, dolomite, and illite. The trend toward lower salinities and higher δD fluid values relative to the earlier mineralizing fluids is attributed to mixing between the fluid that formed Century and a seawater-derived fluid from a different source. Based on seismic data from the Lawn Hill platform and paragenetic and geochemical results from the Leichhardt River fault trough to the south, diagenetic aquifers in the underlying Calvert superbasin appear to have been the most likely sources for the fluids that formed Century and the discordant lode deposits. Paragenetically late sphalerite and calcite cut sphalerite, quartz, and dolomite in the lode deposits and contain Na + -dominated fluid inclusions with much lower salinities than their older counterparts. The isotopic composition of calcite also indicates δ 18 O fluid from 3.3 to 10.7 per mil, which is larger than the range obtained from synmineralization minerals, supporting the idea that a unique fluid source was involved. The absolute timing of this event is unclear, but a plethora of Pb model, K-Ar, and 40 Ar/ 39 Ar ages between 1440 and 1300 Ma indicate that a significant volume of fluid was mobilized at this time. The deposition of the Roper superbasin from ca. 1492 ± 4 Ma suggests that these late veins formed from fluids that may have been derived from aquifers in overlying sediments of the Roper superbasin. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralization record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district. Fluid inclusions in these veins indicate formation from a low-salinity, 300° ± 80°C fluid. Temperatures approaching 300°C recorded in organic matter adjacent to faults and at sequence boundaries correspond to K-Ar ages spanning 1300 to 1100 Ma, which coincides with regional hydrothermal activity in the northern Lawn Hill platform and the emplacement of the Lakeview Dolerite at the time of assemblage of the Rodinia supercontinent.

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain–Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J0, J1, J2, and J3, were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J0 formed during an early event. Petrographic observations, geochemistry, and δ18O values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J1, J2, and J3 jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in J0 jasperoids caused them to have higher δ18O values than J1,2,3 jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of δ18O values (24.5 and −3.7‰). Jasperoids hosted in limestones with the most anomalous Au, Ag, Hg, ±(As, Sb, Tl) concentrations and the highest δ18O values are associated with the largest deposits. The 28‰ range of jasperoid δ18O values is best explained by mixing between an 18O-enriched fluid and an 18O-depleted fluid. The positive correlation between the sizes of gold deposits and the δ18O composition of jasperoids indicates that gold was introduced by the 18O-enriched fluid. The lowest calculated δ18O value for water in equilibrium with late-ore-stage quartz at 200°C (−15‰) and the measured δD value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (−116‰) indicate that the 18O-depleted fluid was composed of relatively unexchanged meteoric water. The source of the 18O-enriched ore fluid is not constrained. The δ34S values of late-ore-stage realgar, orpiment, and stibnite (5.7–15.5‰) and barite (31.5–40.9‰) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the δ13C and δ18O values of late-stage calcite (−4.8 to 1.5‰ and 11.5 to 17.4‰, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.

A continuous‐flow crushing device for on‐line δ2H analysis of fluid inclusion water in speleothems

A method for the isotope analysis of fluid inclusion water in speleothem calcite is presented. The technique is based on a commercially available continuous-flow pyrolysis furnace (ThermoFinnigan TC-EA). The main adaptation made to the standard TC-EA configuration is the addition of a crusher and cold trap unit, which is connected to the carrier gas inlet at the top of the TC-EA reactor tube. A series of tests conducted with this device shows that: (1) standard waters, injected in the crusher, and passed through a cryogenic trapping routine, yield accurate delta(2)H values; (2) crushed cubes of speleothem calcite from two Peruvian caves with rather dissimilar seepage water delta(2)H values yield fluid inclusion delta(2)H values in good accordance with these drip waters. The clear advantage of this continuous-flow technique for fluid inclusion isotope analysis is that it is relatively quick compared with other techniques. Since the conditions of water sample introduction into the TC-EA are identical for delta(2)H and delta(18)O analysis, we expect that only limited adaptations to the extraction procedure are required to provide delta(18)O analysis of fluid inclusion samples with the same device.

Hydrothermal source rocks of the Meng'entaolegai Ag-Pb-Zn deposit in the granite batholith, Inner Mongolia, China: Constrained by isotopic geochemistry

The Meng'entaolegai Ag-Pb-Zn-In polymetallic deposit is located in the eastern part of Inner Mongolia. It consists of a hydrothermal quartz-sulfide vein deposit hosted within a Hercynian granite massif which is about 400 km2 in size. More than 40 orebodies are found in the orefield which is 6 km in length from east to west and 200 to 1,000 m in width from south to north. All orebodies are controlled by E-W trending faults. Economic resources are dominated by Pb and Zn (reserves of Pb and Zn are 0.17 Mt and 0.37 Mt, and their grades are 1% and 2.3%, respectively), with Ag, Sn, In and Cd (1,800 t Ag, >2,000 t Sn, >500 t In and 1800 t Cd) as by-products. The δ34SCDT values of sulfides range from -1.7‰ to +4.6‰, averaging 1.4‰, indicating that sulfur is magmatic in origin. The H-O isotopic compositions (δD values from -52.8‰ to -66.9‰ and δ18OH2O values from 4.8‰ to 7.9‰) of the fluid inclusion water in quartz show that the ore-forming fluid has a mainly magmatic source. The lead isotopic compositions of ore are relatively homogeneous and are significantly lower than those of feldspars and whole rock of the Hercynian Meng'entaolegai granite and the Yanshanian Duerji granite. Neither these granites nor the regional basement Precambrian metamorphic rocks provided ore with lead. By comparison, ore lead isotopic composition is the same as that of E-W trend diorite dykes which intruded the granites and this indicates that the ore lead is probably derived from magamatism which is related to these dykes. We considered therefore that although orebodies occurred within the Hercynian Meng'entaolegai granite, the deposit origin is not related to this stage of magmatism.

Reconnaissance Stable Isotope (C, O, H, S) Study of Paleoproterozoic Gold Deposits of the São Luis Craton and Country Rocks, Northern Brazil: Implications for Gold Metallogeny

Caxias, Areal, and Pedra de Fogo are structurally controlled syn- to late tectonic and post-metamorphic gold deposits of Paleoproterozoic age located in the São Luis craton, northern Brazil. Previous fluid inclusion studies indicated reduced, low-salinity CO2-H2O (±CH4-N2) fluids trapped at 260°-300°C (Caxias, Areal) to 330°-400°C (Pedra de Fogo), and at 2 kbar as responsible for the mineralization. Oxygen isotope ratios of quartz (+10.4 to +16.2‰), chlorite (+7.6‰), and sericite (+5.1‰), and hydrogen isotope ratios of chlorite (-46‰), sericite (-58‰), and fluid inclusion water (-32 to -70‰) indicate fluid Δ18O values of +2.6 to +5.6 per mil (Caxias), +0.6 to +3.5 per mil (Areal), and +10.3 to +12.1 per mil (Pedra de Fogo) and fluid ΔD composition of -24 to -53 per mil (Caxias), -49 to -62 per mil (Areal), and -70 per mil (Pedra de Fogo) at the assumed temperatures. These estimated fluid compositions are consistent with metamorphic sources. Calcite and fluid inclusion CO2 Δ13C values between -3.1 and -10.9 per mil are not diagnostic of a particular origin, but a more negative value (-20.2‰) found at Caxias indicates a greater organic component, at least locally. Sulfide Δ34S values of -2.8‰ to -11.0‰ reflect possible achievement of more oxidized conditions. Hydrogen and oxygen isotope analyses of the country rocks suggest minor isotope disequilibrium and resetting of oxygen isotope geothermometers. This might indicate subsolidus post-crystallization isotopic exchange, linked with metamorphism and/or hydrothermal alteration.

Changing moisture sources over the last 330,000 years in Northern Oman from fluid-inclusion evidence in speleothems

Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δ D values of water extracted from speleothem fluid inclusions (δ D FI) are between −60 and −20‰ (VSMOW) and δ 18O values of speleothem calcite (δ 18O C) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δ D) or modern stalagmites (for δ 18O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ 18O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.

δD values of fluid inclusion water in quartz and calcite ejecta from active geothermal systems: do values reflect those of original hydrothermal water?

One of the most common methods for estimating paleohydrothermal δ D values in epithermal quartz veins is hydrogen isotope analysis of H 2 O extracted from fluid inclusions in quartz by thermal decrepitation. The validity of the method is questioned. The δ D values of water extracted from fluid inclusions in clear euhedral quartz from active geothermal systems are up to 30 per mil more negative than the δ D value (–42‰) of the geothermal water. Measured δ D values of the fluid inclusion water are dependent on the temperature at which the water in the quartz was extracted. Water extracted at 800°C has δ D values about 10 to15 per mil lower than fluid inclusion water extracted at 500°C (–53 to –61‰). Fluid inclusion water in calcite from an active geothermal well has δ D values that match those of the geothermal water. Calcite is thus potentially a more suitable mineral for estimating paleohydrothermal δ D values.

D values of fluid inclusion water in quartz and calcite ejecta from active geothermal systems: do values reflect those of original hydrothermal water?

D values of fluid inclusion water in quartz and calcite ejecta from active geothermal systems: do values reflect those of original hydrothermal water?

Do fluid inclusions preserve δ18O values of hydrothermal fluids in epithermal systems over geological time? Evidence from paleo- and modern geothermal systems, Milos island, Aegean Sea

Stable isotope compositions of quartz ( δ 18O quartz) and fluid inclusion waters ( δ 18O FI and δD FI) were analysed from Profitis Ilias, a low-sulphidation epithermal gold mineralisation deposit on Milos island, Greece, to establish if δ 18O FI preserve a record of paleogeothermal processes. Previous studies show that mineralisation at Profitis Ilias resulted from extreme boiling and vaporisation with a zone located at approximately 430 m above sea level (asl) representing the transition between liquid- and vapor-dominated systems [Miner. Depos. 36 (2001) 32]. The deposit is also closely associated with an active geothermal system, whose waters have a well-characterised stable isotope geochemistry [Pflumio, C., Boulegue, J., Liakopoulos, A., Briqueu, L., 1991. Source, Transport and Deposition of Metals. Balkema, Rotterdam]. The samples were collected over an elevation interval of 440 m (210–650 m asl) to give information on the liquid and vapor dominated sections of the paleosystem. The data show systematic variations with sample elevation. Samples from the highest elevations (ca. 650 m asl) have the lightest δ 18O FI (−7.3‰) and δD FI (−68.0‰) whilst the deepest (ca. 210 m asl) are isotopically heavier ( δ 18O FI, −0.3‰; δD FI, −19.0‰). Relative changes in δ 18O FI closely parallel those in δD FI. δ 18O quartz shows an opposite trend, from the lightest values (+13.9‰) at the lowest elevations to the heaviest (+15.1‰) at the highest elevations. δ 18O FI shows correlations with other parameters. For example, variable fluid inclusion homogenisation temperatures in the vapor-dominated part of the system correlate with a rapid shift in δD FI (−33.3‰ to −50.5‰) and δ 18O FI (−4.1‰ to −6.2‰). Gold contents also increase in the same zone (up to 50 ppm Au). Comparable correlations in δ 18O quartz or δ 18O calculated (estimated geothermal fluid from fluid inclusion homogenisation data) are absent. δ 18O calculated are always 5–10‰ heavier than δ 18O FI. Comparison with the present day geothermal field shows that δD FI and δ 18O FI are similar. Isotope data for the modern system and fluid inclusion waters fall on linear trends subparalleling the meteoric water line and project towards seawater values. Numerical modelling favours kinetically controlled fractionation to explain differences in δ 18O calculated and δ 18O fluid rather than diffusive posttrapping reequilibration. The evidence suggests that in low-temperature epithermal systems, δ 18O FI may represent a better record of fluid process and the isotopic composition of the paleogeothermal fluid than temperature-corrected quartz data.