Fluid Inclusion Trapping Temperatures Research Articles

High temperature wall-rock alteration zoning in the Sanjin deposit, Hishikari gold mine, Japan: Implication for exploration of mature mining districts

The Sanjin deposit is one of three major ore zones hosted by Pleistocene quartz-adularia veins being mined at the Hishikari low-sulfidation epithermal gold mine, which has produced more than 242 t of gold at an extraordinary average grade since 1985. Hydrothermal alteration zoning of the Sanjin deposit was examined with respect to mineralogy, geochemistry, fluid inclusion microthermometry, and oxygen and hydrogen isotopes. Clay minerals are dominated by interstratified chlorite-smectite (C/S) and interstratified illite-smectite (I/S) with <20% smectite. Epidote and prehnite are recorded for the first time at Hishikari in the southeastern part of the Sanjin deposit, typically coexisting with chlorite. Trapping temperatures of fluid inclusions from associated ore zone quartz veins typically range from 195 to 230 °C, with higher temperatures prevalent in the southeastern part of the Sanjin deposit. The calculated fluid δ18O and δ2H values from clay minerals and quartz cannot be explained by a simple water-rock interaction or a simple fluid mixing model, since variable isotopic exchange temperature and endmembers have to be considered. This suggests that both water-rock interaction and mixing of fluids occurred between dynamically variable end members during mineralization. The average estimated formation temperature of chlorites in the Sanjin deposit using chlorite geothermometry is 233 ± 19 °C, in agreement with the highest temperature zone in Hishikari, estimated by homogenization temperature of fluid inclusions of ore veins. In addition, the estimated formation temperatures of chlorite in epidote- and/or prehnite-rich altered rocks are higher (avg. 240 ± 17 °C) than those in epidote- and/or prehnite-poor altered rocks (avg. 216 ± 9 °C). Thus, the chlorite-epidote/prehnite assemblage can be an index of a high temperature alteration zone in the Sanjin deposit. Considering the position of the paleo-water table of the three ore zones, these factors are consistent with the formation of the Sanjin deposit proximal to the upflow zone responsible for gold mineralization at Hishikari. We suggest that our approach could be utilized to understand the thermal structure of epithermal gold system, which may be important to explore for blind veins at mature mining districts.

Natural fractures in carbonate-rich tight oil reservoirs from the Permian Lucaogou Formation, southern Junggar Basin, NW China: Insights from fluid inclusion microthermometry and isotopic geochemistry

Natural fractures within tight reservoirs form preferred pathways for hydrocarbon charge and provide space for hydrocarbon preservation. However, the mechanism that controls the formation of natural fractures in tight reservoirs remains controversial. This study investigates natural fractures within carbonate-rich tight oil reservoirs from the Permian Lucaogou Formation in the Jimsar Sag, southern Junggar Basin. Drill core and image log observations show that fractures in the Lucaogou Formation can be categorized into two types: bedding-parallel fractures and inclined fractures. The carbon and oxygen isotopic compositions of calcite cements in the bedding-parallel fractures range from 0.6‰ to 6.0‰ (δ13CPDB), and from −15.5‰ to −8.9‰ (δ18OPDB), respectively, which are similar to those of the organic-rich host rock. This suggests that the bedding-parallel fractures likely formed in a closed fluid system. Fluid inclusion analyses of the aqueous inclusions in calcite cements from the bedding-parallel fractures record homogenization temperatures of 91.5 °C–130.4 °C, indicating that they formed at the Cretaceous to Cenozoic, during when the Lucaogou Formation reached peak oil generation stage. Therefore, the bedding-parallel fractures were most likely generated by hydrocarbon generation overpressure. The presence of hydrocarbon inclusions in bedding-parallel fracture cements supports natural fracturing by high pore-fluid pressure. The δ13CPDB and δ18OPDB of calcite cements within the inclined fractures vary from 3.6‰ to 9.6‰, and from −11.0‰ to −2.9‰, respectively, which are close to those of the carbonate-rich parent rock. This also supports a closed fluid system scenario. The relatively lower fluid inclusion trapping temperatures (70.5 °C–109.7 °C) suggest that they were likely generated by decrease of confining stress during uplift processes. Therefore, both an increase in pore-fluid pressure and a decrease in confining stress contribute to the formation of natural fractures in the Lucaogou Formation tight oil reservoirs.

Fluid inclusion geochemistry and ore genesis of the Longmendian Mo deposit in the East Qinling Orogen: Implication for migmatitic-hydrothermal Mo-mineralization

The 1.85Ga Longmendian Mo deposit in East Qinling is the oldest Mo system recognized in China. It is unique for three reasons: (1) quartzofeldspathic orebodies are away from any intrusion or fault, (2) it is closely associated with migmatitic rocks, and (3) it has remarkably high Re content in molybdenite (504 to 1660ppm). The origin of the deposit is poorly constrained.In the Longmendian deposit, strong Mo mineralization is always associated with hydrothermally altered migmatitic amphibolites. To probe into ore genesis, detailed fluid inclusion studies are carried out on both mineralized migmatitic amphibolites and ore-barren rocks. Four compositional types of fluid inclusions are observed, including CO2±CH4 (PC-type), CO2–H2O (C-type), daughter mineral-bearing (S-type) and H2O–NaCl (W-type). Quartz in mesosome and melanosome of mineralized migmatitic amphibolites contains all of the four types of inclusions. Measurements of immiscible inclusion assemblages in the ores constrain the Mo-mineralization temperatures and pressures to be 225–390°C and 114–265MPa, respectively. Primary fluid inclusions in barren migmatitic gneisses are dominated by S-type, and minor of C- and PC-types which are identical to those in the leucosome of migmatitic amphibolites. These inclusions yield lower homogenization temperatures than those in the mineralized mesosome and melanosome of migmatitic amphibolites, suggesting that the ore-causative, injected melts should have higher temperatures and originated from depths. Such features of the ore-forming fluids indicate that the Longmendian deposit was a migmatitic-hydrothermal system caused by high-temperature melt injection. This interpretation can also be supported by the observations below: (1) tourmaline is abundant in melanosome, but absent in mesosome; (2) the consistent quartzofeldspathic composition of leucosome is independent of mesosome; (3) leucosome in migmatitic amphibolite crosscuts each other; (4) halite-bearing fluid inclusions are prevalent in studied samples, which is the feature of granitic rocks, instead of the scenarios of in situ migmatites; and (5) the trapping temperatures of fluid inclusions in mineralized migmatitic amphibolite are much lower than those required for partial melting, but higher than those obtained from barren migmatitic rocks.

Fluid overpressure along an Oligocene out-of-sequence thrust in the Shimanto Belt, SW Japan

Out-of-sequence thrusts (OSTs) exposed in ancient accretionary prisms are considered as fossil analogs of present-day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity. In modern as well as in ancient accretionary prisms, first-order megasplay faults or OSTs truncate or merge with faults of lesser importance called second-order OSTs. Structural analysis of the Makinokuchi fault, a branch of an Oligocene to lower Miocene second-order OST in the Tertiary Shimanto Belt of central Kyushu, SW Japan, brings information about the conditions of deformation at the time of thrusting. The studied exposure shows that the fault footwall and, to a much lesser extent, the fault hanging-wall, consist of quartz-cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component. The footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias. The continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault. Fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions (245–285°C and 5–8km depth). These similarities suggest that the fluids responsible for syn-tectonic hydraulic brecciation were collected from the footwall through the conduits. The fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone. Our analysis shows that fluid overpressures can play a key role in the growth and activity of second-order OSTs in accretionary prisms and suggests that fluids collected along second-order OSTs or splay faults may flow upward along first-order OSTs or megasplay faults.

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

The Bohaun gold deposit in County Galway, western Ireland, is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks that have only reached the late diagenetic zone of low-grade metamorphism. The mineralization consists of undeformed veins, stockworks, and breccias. Quartz dominates all the veins and commonly shows multiple growth stages and a variety of classic epithermal textures, including comb, plumose, and banded, indicative of low confining pressures. The veins have a simple mineralogy made up of quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite. Vein quartz is associated with two fluids compositions: type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126° to 257°C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175° and 245°C. The geodynamic setting of Bohaun, which is located in a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition, and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine-type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic-grade gold mineralization, independent of other fluids.

Fluid mixing and thermal regimes beneath the PACMANUS hydrothermal field, Papua New Guinea: Helium and oxygen isotope data

Fluid mixing processes and thermal regimes within the Snowcap and Roman Ruins vent sites of the PACMANUS hydrothermal system, Papua New Guinea, were investigated using 3He/ 4He ratios from fluid inclusions within pyrite and anhydrite and the δ 18O signature of anhydrite. Depressed 3He/ 4He ratios of 0.2–6.91R A appear to be caused by significant atmospheric diffusive exchange, whilst He–Ne diffusive fractionation precludes correction using 20Ne. 40Ar/ 36Ar ratios of 295–310 are elevated above seawater, indicating the majority of argon is seawater derived but with a magmatic component. δ 18O anhydrite ratios are 6.5‰ to 11‰ for Snowcap and 6.4‰ to 11.9‰ for Roman Ruins. Using oxygen isotope fractionation factors for the anhydrite–water system, the temperatures calculated assuming isotopic equilibrium at depth are up to 100 °C cooler than fluid inclusion trapping temperatures. It is likely that anhydrite is precipitated rapidly, preventing δ 18O equilibration. By comparing new δ 18O values for anhydrite with corresponding published 87Sr/ 86Sr ratios, seawater is inferred to penetrate deep into the Snowcap system with little conductive heating. A simple fluid mixing model has been constructed whereby the differing venting styles can be explained by a plumbing system at depth which favors delivery of end-member hydrothermal fluid to the high temperature sites.

Round Hill Shear Zone-Hosted Gold Deposit, Macraes Flat, Otago,New Zealand: Evidence of a Magmatic Ore Fluid

The 0.8-Moz Round Hill shear zone-hosted gold deposit is one of several located on, or near, the Hyde-Macraes shear zone, Macraes Flat, New Zealand. Field relationships show stockwork veins are the oldest part of the hydrothermal system, followed by flat and ramp veins. Hanging-wall shear veins formed early but have been modified by late deformation. Fluid inclusion microthermometric results for dominant type I inclusions show that flat and ramp veins have the same average wt percent NaCl equiv values (2.2± 0.6%) and were precipitated from the same fluid. Salinity data for these veins overlap with those from hanging-wall veins (average 1.3 ± 0.3%) and stockwork veins (1.6 ± 0.6% for vein margins; 0.9 ± 0.3% for vein centers), although both of the latter trend toward lower overall wt percent NaCl equiv values, suggesting mixing between relatively dilute and more saline end members. All of the above veins have similar temperatures of homogenization (T h ), ranging between 110° and 188°C. Fluid inclusion trapping temperatures are considered to be ~300°C, equating to a ~10-km depth of formation. Carbon dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities. Stockwork and hanging-wall veins thus have true salinities of <1 wt percent NaCl equiv (<0.17 molal) and flat and ramp veins have true salinities of Fluid inclusion volatiles from Round Hill and other Macraes deposits are dominated by H 2 O (99.02–99.69 mol %), with lesser CO 2 (0.14–0.76 mol %), N 2 (0.03–0.32 mol %), and CH 4 (0.06–0.17 mol %). Noticeable concentrations of hydrocarbons occur in many of the samples. Fluid inclusion cation and anion data show few components of relatively low concentration are dissolved in the hydrothermal fluid, consistent with CO 2 -corrected microthermometric measurements. A range in fluid inclusion δ D H 2 O values obtained for the Round Hill deposit is coincident with the different vein types and also indicates mixing between two fluids. One fluid is considered to be O isotope shifted meteoric water, as trapped by the early stockwork veins, and has the lowest δ D H 2 O values of –80 per mil. The other fluid is represented by flat and/or ramp and hanging-wall veins that have δ D H 2 O values up to –40 per mil. The Macraes δ D H 2 O data show a trend toward the magmatic water end member for geothermal systems associated with convergent plate boundaries and are consistent with a 25 to 50 percent magmatic component in the Macraes hydrothermal system.

Synthetic fluid inclusion logging to measure temperatures and sample fluids in the Kakkonda geothermal field, Japan

Synthetic fluid inclusion logging is a new tool to measure temperatures and sample fluids in high-temperature geothermal wells. Fluid in the microcracks of a crystal can be trapped in inclusions through healing. Fluid inclusions in quartz, for example, can be synthesized easily in geothermal boreholes and can be used as long as the host crystal is stable (e.g. α-quartz is stable up to 573°C). This technique can be applied to high-temperature geothermal wells where conventional temperature measurement methods are not feasible. Cracked crystals of quartz, soaked in silica-saturated solutions in gold or platinum capsules mounted on containers, are placed in a geothermal borehole. Geothermal fluid enters the microcracks in the crystals at the selected sampling depths, and inclusions containing ambient fluid are formed through crack healing. Trapping temperatures of fluid inclusions in quartz are determined by microthermometry using a heating stage with pressure corrections. Other cracked crystals mounted in containers with rupture disks are used for fluid sampling. The first borehole experiment was conducted at WD-1, a deep research hole drilled in the Kakkonda geothermal field, northeast Japan, from September to October 1994 (24 days). Results from the experiment confirmed that temperatures measured from fluid inclusions are consistent with borehole temperatures measured by conventional logging tools.

Petrological, mineralogical, fluid inclusion, and stable isotope studies of the Gies gold-silver telluride deposit, Judith Mountains, Montana

The Gies gold-silver telluride deposit, Judith Mountains, Montana, occurs in steeply dipping quartz veins near the contact of Late Cretaceous to Paleocene alkaline porphyritic intrusions (quartz monzonite and syenite stocks, and tinguaite dikes), and sedimentary rocks (shale, sandstone, conglomerate, limestone, and dolomite) of Middle Cambrian to late Paleocene age. Trace element geochemistry suggests that these intrusions were possibly derived from the shallowly dipping subducting Farallon plate lithosphere or the overlying asthenospheric mantle.Vein-related alteration is variable and does not extend more than 100 m away from veins. Potassic alteration and silicification are most closely associated with telluride mineralization. The vein mineralogy is complex and consists of at least 36 minerals including two unknown possibly new minerals and six tellurides. Four stages of vein mineralization are recognized in the Gies deposit, the dominant mineral assemblages are (I) quartz-pyrite-roscoelite-carbonates, (II) quartz-pyrite-base metal sulfides-carbonates-minor tellurides-roscoelite, (III) quartz-tellurides-pyrite-roscoelite-base metal sulfides-sulfosalts, and (IV) sulfides-sulfosalts-native gold. Fluid inclusion studies of quartz from stages I, II, and III yield homogenization temperatures of 270 degrees to 319 degrees C (mean = 293 degrees + or - 10 degrees C), 222 degrees to 282 degrees C (mean = 256 degrees + or - 9 degrees C), and 195 degrees to 250 degrees C (mean = 220 degrees + or - 13 degrees C), respectively. Assuming hydrostatic pressure, a 5 degrees C correction due to pressure yields mean trapping temperatures of approximately 300 degrees , 260 degrees , and 225 degrees C, respectively, for stages I, II, and III. Salinities of primary fluid inclusions in all three quartz stages range from 5.7 to 8.0 wt percent NaCl equiv.Isotopic studies show that delta 34 S values between -1.0 to 3.1 per mil for vein sulfides are most consistent with a magmatic origin. Oxygen isotope values of quartz in stages I, II, and III range from 13.1 to 18.1 per mil, whereas values for feldspar in unaltered alkaline intrusions range from 7.1 to 9.8 per mil, feldspar in altered alkaline intrusions from 7.3 to 9.8 per mil, and a single sample of stage III kaolinite is 7.9 per mil. Values of delta 18 O water computed from quartz and fluid inclusion trapping temperatures for stage I, II, and III fluids are 5.9 to 9.4 per mil, 6.4 to 8.7 per mil, and 3.6 to 6.2 per mil, respectively. A delta 18 O value of 3.3 per mil for fluid in equilibrium with stage III kaolinite agrees well with values derived from fluids in equilibrium with stage III quartz. Hydrogen isotope values of stage II roscoelite, stage III kaolinite, and fluid inclusions in stage I, II, and III quartz range from -115 to -78 per mil, whereas fluids in equilibrium with these minerals range from -115 to -88 per mil.Oxygen and hydrogen isotope evidence suggests that gold telluride mineralization precipitated by conductive cooling in response to increased mixing of evolved meteoric water with magmatic fluids. Concomitant with a decrease in temperature from stage I through stage IV are decreases in pH, f (sub S 2 ) , and f (sub O 2 ) and increases in f (sub Te 2 ) and vanadium content of the ore-fluid. Geologic and mass balance calculations are consistent with a low water/rock ratio (<0.1) and indicate that magmatic fluids and many of the ore-forming components (e.g., Au, Ag, Te, V, S) have possibly been derived from a buried intrusion (e.g., tinguaite or carbonatite) at approximately 59 Ma.

Evidence for Li-rich brines and early magmatic fluid-rock interactionin the Larderello geothermal system

The geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the Larderello geothermal system (Italy). Deep drilling has provided samples which preserve evidence of this early geothermal activity. Four wells (San Pompeo 2, Monteverdi 7, Sasso 22, and Serrazzano, VC 11) penetrated the deeper parts of the Larderello system, located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the main aquifer at Larderello. The drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity. Fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages (plagioclase-actinolite-biotite-tourmaline; clinopyroxene ± andradite-wollastonite) and also from magmatic quartz in a leucogranite dike. The inclusions are mainly secondary in origin, oriented in fluid inclusion planes (FIP) related to hydrothermal circulation in the Larderello system. Several generations of high temperature fluids were trapped and include: 1. (1) H 2OCO 2 dominated vapors displaying variable but significant contents of CH 4 and N 2; 2. (2) aqueous vapors containing LiCl, with variable salinity; 3. (3) aqueous LiCl brine, often oversaturated with respect to halite at room temperature; 4. (4) complex brine, always oversaturated at room temperature with respect to two (halite and sylvite) or more ( n ≤ 4) salts. The presence of LiCl was confirmed by identification of the salt hydrate (LiCl5H 2O) at very low temperature using Raman spectroscopy. Bulk salinities could be roughly estimated at around 30 wt% eq. LiCl for the LiCl brine. Geometric and chronologic relationships between FIP reveal close relationships and mutual contamination between the H 2OCO 2 vapors and LiCl brine, indicating synchronism in their trapping. These fluids were generated and trapped at pressures of 100–130 MPa, nearly 23 MPa above the estimated present-day lithostatic pressure. This implies a denudation rate between 0.2 and 0.5 mrn· a −1 since the onset of hydrothermal activity, compatible with the setting of Larderello in a young (Tortonian) collision belt. Fluid inclusion trapping temperatures (425–650°C) show a monotonous increase towards the inferred granite, and are around 100–200°C higher than the highest present-day temperatures. The results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early Larderello system. The H 2OCO 2 vapors resulted from the reheating of the basement metamorphic series (often C-rich) under relatively high temperatures during contact metamorphism. Lirich fluids expelled from an underlying Li-rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling. The fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field.

Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska

The Prince William Sound region is located approximately 100 km east-southeast of Anchorage, Alaska, and contains the geologically diverse Late Cretaceous to Eocene Valdez and Orca Groups, which comprise a portion of the accretionary prism complex of south-central Alaska. The rocks host more than 600 known massive sulfide deposits and prospects. Historically, the deposits have been described as epigenetic vein, replacement, and shear zone-hosted ores. We define the Prince William Sound massive sulfide deposits in terms of two genetically related, yet geochemically distinct, types of deposits, both related to submarine volcanism. The first type is hosted in submarine pillow basalt, massive basalt flows, and mafic tuffs and is analogous to Cyprus-type deposits. The second type is hosted in intercalated slate and graywacke and is analogous to Besshi-type deposits. Both types are similar morphologically and are characteristically stratiform to strata bound. The deposits vary in size from less than 1 million tons to greater than 5 million tons, with the majority of the largest deposits forming in sediment-dominated Besshi-type environments.The massive sulfide deposits are Fe-Cu-Zn systems dominated mineralogically by pyrite, pyrrhotite, chalcopyrite, and sphalerite. Minor phases include cubanite, arsenopyrite, tetrahedrite, and secondary marcasite. Silicate gangue is comprised predominantly of quartz and chlorite, with minor sericite, talc, and calcite. Primary sedimentary features preserved within massive sulfide are ubiquitous within the deposits and include planar bedding, crossbedding, and graded bedding.Fluid inclusion microthermometry of quartz gangue from the massive sulfide deposits is consistent with hydrothermal fluid temperatures found in genetically related modern seafloor hydrothermal environments. Minimum trapping temperatures of fluid inclusions in quartz from the stockwork feeder zone of the volcanic-hosted deposits generally range from 210 degrees to 260 degrees C. Fine-grained anhedral quartz gangue in massive sulfide layers also contains sparse primary fluid inclusions; typically the homogenization temperature is lower in the sediment-hosted deposits. Hydrothermal fluids in the volcanic-hosted settings exited the volcanic pile directly into seawater and were rapidly quenched, thus fluid inclusions only record high-temperature fluid conditions. Similar hydrothermal fluids in the sediment-hosted settings exited the volcanic pile into warm, seawater-saturated sediments and were cooled more slowly, in an environment where lower temperature fluid inclusions were formed and preserved.Sulfur and oxygen isotope data support a submarine origin for the deposits. Sulfur isotope values range from delta 34 S GDT = 0.8 to 5.9 per mil for volcanic-hosted deposits, and from 2.7 to 9.2 per mil for sediment-hosted deposits. The higher values for sediment-hosted ores reflect increased amounts of reduced seawater sulfate in the hydrothermal fluid, with sulfate reduction occurring in the seawater-hydrothermal fluid mixing zone within the sediment pile below the seawater-sediment interface. Oxygen isotope values for quartz gangue associated with massive sulfide also reflect the lower temperature deposition that is characteristic of the sediment-hosted deposits: delta 18 O quartz = 7.0 to 7.8 per mil (SMOW) for volcanic-hosted deposits and delta 18 O quartz = 9.6 to 16.9 per mil for sediment-hosted ores.Whole-rock geochemical analyses of the basalts and related igneous rocks from both the Valdez and Orca Groups show that these rocks are consistent with formation in midocean ridge and/or seamount environments. The geochemistry of the Valdez Group rocks suggests that sediment contamination of basaltic magma has occurred, imparting a primitive arclike signature. Discrimination diagrams show that the rocks from both groups have normal midocean ridge basalt, enriched midocean ridge basalt, and low K arc tholeiite affinities, and plot along the tholeiitic-calc-alkaline boundary.The fluid inclusion, stable isotope, and whole-rock data collected in this study are all consistent with the hypothesis that the massive sulfide deposits in the Prince William Sound district are sea-floor volcanogenic massive sulfide deposits that formed in both sediment-starved and sedimented midocean ridge settings.

Sulfur, carbon, and oxygen isotope variations in submarine hydrothermal deposits of Guaymas Basin, Gulf of California, USA

Sulfur, carbon, and oxygen isotope values were measured in sulfide, sulfate, and carbonate from hydrothermal chimney, spire, and mound samples in the southern trough of Guaymas Basin, Gulf of California, USA. δ 34 S values of sulfides range from −3.7 to 4.5%. and indicate that sulfur originated from several sources: 1. (1) dissolution of 0‰ sulfide contained within basaltic rocks, 2. (2) thermal reduction of seawater sulfate during sediment alteration reactions in feeder zones to give sulfide with positive δ 34 S, and 3. (3) entrainment or leaching of isotopically light (negative- δ 34 S) bacteriogenic sulfide from sediments underlying the deposits. δ 34 S of barite and anhydrite indicate sulfur derivation mainly from unfractionated seawater sulfate, although some samples show evidence of sulfate reduction and sulfide oxidation reactions during mixing within chimneys. Oxygen isotope temperatures calculated for chimney calcites are in reasonable agreement with measured vent fluid temperatures and fluid inclusion trapping temperatures. Hydrothermal fluids that formed calcite-rich chimneys in the southern trough of Guaymas Basin were enriched in 18O with respect to seawater by about 2.4‰ due to isotopic exchange with sedimentary and/or basaltic rocks. Carbon isotope values of calcite range from −9.6 to −14.0‰ δ 34 C pDB , indicating that carbon was derived in approximately equal quantities from the dissolution of marine carbonate minerals and the oxidation of organic matter during migration of hydrothermal fluid through the underlying sediment column. Statistically significant positive, linear correlations of δ 34 S, δ 34 C, and δ 18 O of sulfides and calcites with geographic location within the southern trough of Guaymas Basin are best explained by variations in water/rock ( w r ) ratios or sediment reactivity within subsurface alteration zones. Low w r ratios and the leaching of detrital carbonates and bacteriogenic sulfides at the southern vent sites result in relatively high δ 13 C and low δ 34 S in chimney carbonates and sulfides, respectively. In the north, where the depletion of alkalis in vent fluids indicates higher w r ratios, positive δ 34 S and more negative δ 13 c are due to increased contributions from organic matter oxidation and sulfate reduction reactions.

A comparative thermobarometric (chemical and isotopic) study of a tourmalinized pelite and its Sn-Be vein, Walmès, Morocco.

The P-T-t path has been determined for a vein and its tourmalinized metapelitic wall rocks from Walmes, central Morocco, by applying fluid-inclusion, chemical and isotopic (H-O) thermometers and mineral paragenesis analysis. Concordant temperatures have been derived from the tourmaline-biotite (Fe/Mg), tourmaline-muscovite (H-isotope) and quartz-muscovite-biotite (O-isotope isotherm) thermometers on the wall rocks. Biotite has undergone later H-isotope exchange in contrast to the more resistant muscovite and tourmaline. For the vein, concordant temperatures are given by the fluid-inclusion trapping temperatures, muscovite [Na/(Na+K)] and muscovite-H 2 O (H-isotope) systems

Laser microprobe analyses of Cl, Br, I, and K in fluid inclusions: Implications for sources of salinity in some ancient hydrothermal fluids

The relative concentrations of Cl, Br, I, and K in fluid inclusions in hydrothermal minerals were measured by laser microprobe noble gas mass spectrometry on irradiated samples containing 10 −10 to 10 −8 L of fluid. Distinctive halogen signatures indicate contrasting sources of fluid salinity in fluid inclusions from representative “magmatic” (St. Austell), “metamorphic” (Alleghany), and “geothermal” (Creede, Salton Sea) aqueous systems. Br/Cl mol ratios are lowest at Salton Sea (0.27–0.33 × 10 −3), where high salinities are largely due to halite dissolution; intermediate at St. Austell (0.85 × 10 −3), possibly representative of magmatic volatiles; and highest (near that of seawater) at Creede (1.5–2.1 × 10 −3) and Alleghany (1.2–2.4 × 10 −3), where dissolved halogens probably were leached from volcanic and (or) nonevaporitic sedimentary rocks. I C1 mol ratios are lowest (near that of seawater) at Creede (1–14 × 10 −6), possibly because organisms scavenged I during low temperature recharge; intermediate at Salton Sea (24–26 × 10 −6) and St. Austell (81× 10 −6); and highest at Alleghany (320–940 × 10 −6), probably because the fluids interacted with organic-rich sediments at high temperatures before being trapped. K Cl mol ratios indicate disequilibrium with respect to hypothetical feldspathic alkali-Al-silicate mineral buffers at fluid inclusion trapping temperatures at Creede, and large contributions of (Na, K)-bicarbonate to total fluid ionic strength at Alleghany. Significant variations in Cl/Br/I/K ratios among different fluid inclusion types are correlated with previously documented mineralization stages at Creede, and with the apparent oxidation state of dissolved carbon at Alleghany. The new data indicate that Cl/ Br/I ratios in hydrothermal fluid inclusions vary by several orders of magnitude, as they do in modern surface and ground waters. This study demonstrates that halogen signatures of fluid inclusions determined by microanalysis yield important information about sources of fluid salinity and provide excellent definition of fluid reservoirs and tracers of flow and interaction in ancient hydrothermal systems.

Thermo-Chronological Approach to Reservoir Diagenesis in the Offshore Angola Basin: a Fluid Inclusion, {40}Ar-{39}Ar and K-Ar Investigation (1)

Microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in Neocomian sandstones and Albian dolomitic reservoirs from the offshore Angola basin have allowed determination of the age and the physical-chemical conditions for fluid migration in a late diagenetic stage. The {40}Ar-{39}Ar laser probe dating of adularia overgrowths from the Albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early Miocene (25.6 + or - 3.2 Ma). The K-Ar age of authigenic illites from the Neocomian sandstones is slightly younger (19 + or - 1 Ma), and the crystallization of regular illite-smectite mixed layers is related to a previous Eocene to Paleocene diagenetic stage (51-58 Ma). There appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the Miocene. The fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids, assuming that pressures during the Miocene were close to the present reservoir pressures, and by intersecting the isochores of aqueous and oil inclusions, assuming they were trapped at strictly similar pressure and temperature conditions. The fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content (AlIV) of authigenic chlorites: 168-206 degrees C in the Albian reservoirs at depths between 2350 and 3580 m, and 175-237 degrees C in the Neocomian sandstones at depths between 2770 and 4470 m. In both reservoir formations, Miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient.

Precambrian paleostress from microcracks and fluid inclusions in the Wolf River batholith of central Wisconsin

A strongly developed set of healed micro-cracks striking approximately northwest-southeast is present at 5 of 6 sample localities within the 1450 ± 20 Ma Wolf River batholith of central Wisconsin. Other minor sets are present but are less well defined. Fluid inclusions in healed microcracks show melting temperatures in 2 groups: 0 °C, indicating nonsaline water, and -3 to -30 °C, suggesting salinities above 4.5%. The in situ trapping temperatures of fluid inclusions, calculated from homogenization temperatures, show a consistent 250-400 °C for all the sets of healed microcracks. Rb-Sr dates of about 1440 Ma on biotites from the Wolf River batholith suggest that the 300 °C blocking temperature of biotite was reached very soon after intrusion and that no reheating event after primary cooling took place in the batholith. The fluid-inclusion data and Rb-Sr date interpreted together suggest that cracks were probably generated and healed during the initial cooling of the pluton, starting at near 400 °C. The preferred orientation of these microcracks implies that the maximum horizontal stress was northwest-southeast for this region about 1400-1450 Ma. A similar orientation of a major joint set in the area also might be correlated with the same event. Strongly developed N20°W fluid-inclusion planes in the Illinois deep borehole UPH-3, about 250 km to the southwest, with similar-age bedrock, may reflect a more distal effect of this same stress filed. The northwest-southeast orientation for the maximum horizontal stress direction during the cooling of the Wolf River batholith is subparallel to the proposed orientation of the paleostress field prior to the emplacement of the Wolf River batholith during an unnamed event that deformed upper Proterozoic quartzites, and it is consistent with a long period of continental collision and accretion, which formed northeast-trending belts along the southern and southeastern margin of the continent. The orientation of the regional stresses may have remained fairly constant during much of the period from Penokean to the start of the Keweenawan, when the maximum regional horizontal stress changed from a northwesterly orientation to a northeasterly one. The data show that even in periods of relative tectonic quiescence, such as during intrusion of the Wolf River batholith, micro-cracks may provide a record of paleostress orientations.