Sulphur isotopic fractionation between sulphate and sulphide in hydrothermal ore deposits: disequilibrium vs equilibrium processes

Correlative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ³⁸S versus Δ³⁴S diagrams are deciphered theoretically. Taking into account dissolved H₂S and SO₄^2- in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4-species closed system is suggested for describing the conservation of mass among all sulphur-bearing species on the δ-Δ diagrams. The covariation in the δ³⁴S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ³⁴S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H₂S and/or SO₄^2-. Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.     

The geochemistry of volatile species in melt inclusions and sulfide minerals from Izu-Oshima volcano, Japan

Olivine-hosted melt inclusions in the O95 pyroclastic layer of Izu-Oshima volcano, Japan are basaltic to basaltic-andesitic in composition. The negative correlation between SiO₂ and H₂O in melt inclusions and reverse compositional zoning observed in olivine and other mineral phenocrysts is inferred to arise from mixing between a highly evolved and a less evolved magma. The latter is characterized by the highest S (0.15 wt.%) and H₂O (3.4 wt.%) concentrations among those described in reports of previous studies. The S⁶⁺/S_total ratios in melt inclusions were 0.64?–?0.73, suggesting a relatively high oxidation state (NNO + 0.87 at 1150°C). The presence of pyrrhotites, which are found only in titanomagnetite microlites, suggests that sulfide saturation occurred during microlite growth under at a sulfur fugacity (log fS₂) value of around + 0.5 for T = 1060°C. The groundmass glass compositions are more evolved (andesitic composition) than any melt inclusions containing high amounts of Cl (0.13 wt.%) but negligible H₂O (0.20 wt.%) and S (< 70 ppm), suggesting that Cl was retained in the magma, in contrast to S and H₂O, which degassed strongly during magma effusion.     

Chemical and isotopic evidence for secondary alteration of natural gases in the Hetianhe Field, Bachu Uplift of the Tarim Basin

H₂S and CO₂ are found in elevated concentrations in the reservoirs near the Carboniferous–Ordovician unconformity in the Hetianhe Field of the Tarim Basin, NW China. Chemical and isotopic analyses have been performed on produced gases, formation waters and reservoir rocks to determine the origin of CO₂ and H₂S and to explain the heterogeneous distribution of isotopic and geochemical characteristics of petroleum fluids. It is unlikely that H₂S and CO₂ had a mantle component since associated helium has an isotope ratio totally uncharacteristic of this source. Instead, H₂S and CO₂ are probably the result of sulphate reduction of the light hydrocarbon gases (LHG). Increasing H₂S concentrations and CO₂/(CO₂+ΣC_1–4) values to the west of the Hetianhe Field occur commensurately with increasingly heavy hydrocarbon gas δ¹³C values. However, thermochemical sulphate reduction is unlikely because the temperatures of the reservoirs are too low, no H₂S or rare pyrite was detected in deeper reservoirs (where more TSR should have occurred) and inferred δ³⁴S values of H₂S (from late-stage pyrite in the Carboniferous and Ordovician reservoirs) are as low as −24.9‰. Such low δ³⁴S values discount the decomposition of organic matter as a major source of H₂S and CO₂. Bacterial sulphate reduction of the light hydrocarbon gases in the reservoir, possibly coupled indirectly with the consumption of organic acids and anions is most likely. The result is the preferential oxidation of ¹²C-rich alkanes (due to the kinetic isotope effect) and decreasing concentration of organic acids and anions. Modern formation water stable isotope data reveal that it is possible that sulphate-reducing bacteria were introduced into the reservoir by an influx of meteoric water from the west by way of an inversion-related unconformity. This may account for the apparently stronger influence of bacterial sulphate reduction to the west of the Hetianhe Field, and the consequent greatest decrease of the δ¹³C-CO₂ values and the greatest increase in δ¹³C values of the alkane gases.     

Geochemistry of hydrothermal solutions from 9°50′ N at the East Pacific Rise (EPR) within twelve years after the eruption of a submarine volcano

Hydrothermal solutions were examined in a circulation system that started to develop after the 1991 volcanic eruption in the axial EPR segment between 9°45′ N and 9°52′ N. Within the twelve years elapsed after the eruption, the diffusion outflow of hot fluid from fractures in basaltic lavas gave way to focused seeps of hot solutions through channels of hydrothermal sulfide edifices. The example of field Q demonstrates that the concentrations of H₂S decreased from 86 to 1 mM/kg from 1991 to 2003, and the Fe/H₂S ratio simultaneously increased by a factor of 1.7, a fact that can explain the disappearance of the microbial mats, which were widespread at the fields before 1991. The S isotopic composition of H₂S is independent of the H₂S concentration, a fact testifying to the rapid evolution of the hydrothermal system in the early years of its evolution. Carbon in CH₄ from the hot fluid sampled in 2003 is richer in the light ¹²C isotope than carbon in the fluid from the hydrothermal field at 21° N in EPR, which suggests that methane comes to field Q from more than one source. The composition of particulate matter in the hydrothermal solutions indicates that it was contributed by biological material. Experimental solutions with labeled substrates (t < 70°C) show evidence of the active processes of methane oxidation and sulfate reduction. Our results indicate that, during the 12 years of the evolution of the hydrothermal system, the composition of its solutions evolved and approached the compositions of solutions in mature hydrothermal systems in EPR.     

Stable isotope study of base metal deposits from the Eastern High Atlas,Morocco

Tertiary epigenetic lead ± zinc and copper mineralizations occur in the Mesozoic carbonate cover and stibnite, barite and copper mineralization in the Precambrian to Palaezoic basement in the Eastern High Atlas, Morocco. The carbon isotope data from host carbonates range from +3 to -1‰ PDB. The data are typical for marine carbon. There is no difference between surrounding dolomites and younger vein dolomites. The oxygen isotopic composition of the samples is enriched in ¹⁶O; the range is from -3 to -11‰ PDB with respect to that of carbonates in equilibrium with marine water. It is postulated that the isotopic composition was changed during dolomitization being caused by transition of connate formation waters diluted to some degree by meteoritic water. The bacteriogenically reduced sulphur from syngenetic sulphides and Mesozoic marine sulphate mobilized during Tertiary orogenesis are thought to be the source of H₂S and SO₄. Cambro-Ordovicien sulphate is also supposed to be the source of sulphur in the Jebel Zelmou barite deposit in the basement. Isotopic composition of lead from galenas in the Tertiary deposits from the Mesozoic cover are strikingly homogenous. Model ages vary between 230–180 Ma. It is suggested that important mobilization processes are responsible for Tertiary metallogeny in Morocco.

     

The origin of sulphur in gypsum and dissolved sulphate in the Central Namib Desert,Namibia

This study investigates the sulphur source of gypsum sulphate and dissolved groundwater sulphate in the Central Namib Desert, home to one of Africa's most extensive gypsum (CaSO₄·2H₂O) accumulations. It investigates previously suggested sulphate precursors such as bedrock sulphides and decompositional marine biogenic H₂S and studies the importance of other potential sources in order to determine the origin of gypsum and dissolved sulphate in the region. An attempt has been made to sample all possible sulphur sources, pathways and types of gypsum accumulations in the Central Namib Desert. We have subjected those samples to sulphur isotopic analyses and have compiled existing results. In addition, ionic ratios of Cl/SO₄ are used to determine the presence of non-sea-salt (NSS) sulphur in groundwater and to investigate processes affecting groundwater sulphate. In contrast to previous work, this study proposes that the sulphur cycle, and the formation of gypsum, in the Namib Desert appears to be dominated by the deposition of atmospheric sulphates of phytoplanktonic origin, part of the primary marine production of the Benguela upwelling cells. The aerosol sulphates are subjected to terrestrial storage within the gypsum deposits on the hyper-arid gravel plain and are traceable in groundwater including coastal sabkhas. The hypothesis of decompositional marine biogenic H₂S or bedrock sulphide sources, as considered previously for the Namib Desert, cannot account for the widespread accumulation of gypsum in the region. The study area in the Central Namib Desert, between the Kuiseb and Omaruru rivers, features extensive gypsum accumulations in a ca. 50–70 km wide band, parallel to the shore. They consist of surficial or shallow pedogenic gypsum crusts in the desert pavement, hydromorphic playa or sabkha gypsum, as thin isolated pockets on bedrock ridges and as discrete masses of gypsum selenite along some faults. The sulphur isotopic values (δ³⁴S ‰CDT) of these occurrences are between δ³⁴S +13.0 and +18.8‰, with lower values in proximity to sulphuric ore bodies (δ³⁴S +3.1 and +3.4‰). Damaran bedrock sulphides have a wide range from δ³⁴S −4.1 to +13.8‰ but seem to be significant sources on a local scale at the most. Dissolved sulphate at playas, sabkhas, springs, boreholes and ephemeral rivers have an overall range between δ³⁴S +9.8 and +20.8‰. However, they do not show a systematic geographical trend. The Kalahari waters have lower values, between δ³⁴S +5.9 and +12.3‰. Authigenic gypsum from submarine sediments in the upwelling zone of the Benguela Current between Oranjemund and Walvis Bay ranges between δ³⁴S −34.6 to −4.6‰. A single dry atmospheric deposition sample produced a value of δ³⁴S +15.9‰. These sulphur isotopic results, complemented by meteorological, hydrological and geological information, suggest that sulphate in the Namib Desert is mainly derived from NSS sulphur, in particular oxidation products of marine dimethyl sulphide CH₃SCH₃ (DMS). The hyper-arid conditions prevailing along the Namibian coast since Miocene times favour the overall preservation of the sulphate minerals. However, sporadic and relatively wetter periods have promoted gypsum formation: the segregation of sulphates from the more soluble halite, and the gradual seaward redistribution of sulphate. This study suggests that the extreme productivity of the Benguela Current contributes towards the sulphur budget in the adjacent Namib Desert.     

The variability of δ34S and sulfur speciation in sediments of the Sulejów dam reservoir (Central Poland)

The study was carried out on the Sulejów dam reservoir (Central Poland). Water and sediment samples were collected between February and October 2006. Sulfur compounds in the sediment were chemically extracted and subjected to isotopic analysis.Large variability of SO₄²⁻ concentration in the water column (from 10.3 to 36.2 mg/dm³) and the isotopic composition of sulfur (δ³⁴S from 2.1 to 5.4‰) was observed. The main identified sources of SO₄²⁻ were watercourses, surface runoff, and phosphorus fertilizers.Both oxidized sulfur species (SO₄²⁻) and its reduced forms were found in sediments. Particular sulfur forms were characterized by large variations in both, concentrations and the isotopic composition of sulfur. SO₄²⁻ in the sediment and in the water column had different genesis. Bacterial oxidation of organic sulfur and its binding in SO₄²⁻ were observed in the sediment. Under reducing conditions, oxidized and organic sulfur is converted to H₂S which reacted with Fe or other metallic ions leading to metal sulfide precipitation. Monosulfides were shown to have a very low concentration, ranging up to 0.07 mg/g of sediment. The transformation of elemental sulfur from sulfides through their chemical oxidation occurred in the sediment.     

Low-temperature isotopic exchange in obsidian: Implications for diffusive mechanisms

While a great deal is known about the interaction between water and rhyolitic glasses and melts at temperatures above the glass transition, the nature of this interaction at lower temperatures is much more poorly understood. This paper presents the results of a series of isotopic exchange experiments aimed at further elucidating this process and determining the extent to which a point-by-point analysis of the D/H or ¹⁸O/¹⁸O isotopic composition across the hydrated rim on a geological or archaeological obsidian sample can be used as a paleoclimatic monitor. Experiments were performed by first hydrating the glass for 5 days in water of one isotopic composition, followed by 5 days in water of a second composition. Because waters of near end-member compositions were used (nearly pure ¹H₂¹⁶O, ¹H₂¹⁸O, and D₂¹⁶O), the relative migration of each species could be ascertained easily by depth-profiling using secondary ion mass spectrometry (SIMS). Results suggest that, during hydration, both the isotopic composition of the waters of hydration, as well as that of intrinsic water remaining from the initial formation of the glass vary dramatically, and a point-by-point analysis leading to paleoclimatic reconstruction is not feasible.     

Gas genetic type and origin of hydrogen sulfide in the Zhongba gas field of the western Sichuan Basin, China

Natural gases and associated condensate oils from the Zhongba gas field in the western Sichuan Basin, China were investigated for gas genetic types and origin of H₂S by integrating gaseous and light hydrocarbon geochemistry, formation water compositions, S isotopes (δ³⁴S) and geological data. There are two types of natural gas accumulations in the studied area. Gases from the third member of the Middle Triassic Leikoupo Formation (T₂l³) are reservoired in a marine carbonate sequence and are characterized by high gas dryness, high H₂S and CO₂ contents, slightly heavy C isotopic values of CH₄ and widely variable C isotopic values of wet gases. They are highly mature thermogenic gases mainly derived from the Permian type II kerogens mixed with a small proportion of the Triassic coal-type gases. Gases from the second member of the Upper Triassic Xujiahe Formation (T₃x²) are reservoired in continental sandstones and characterized by low gas dryness, free of H₂S, slightly light C isotopic values of CH₄, and heavy and less variable C isotopic values of wet gases. They are coal-type gases derived from coal in the Triassic Xujiahe Formation.The H₂S from the Leikoupo Formation is most likely formed by thermochemical SO₄ reduction (TSR) even though other possibilities cannot be fully ruled out. The proposed TSR origin of H₂S is supported by geochemical compositions and geological interpretations. The reservoir in the Leikoupo Formation is dolomite dominated carbonate that contains gypsum and anhydrite. Petroleum compounds dissolved in water react with aqueous SO₄ species, which are derived from the dissolution of anhydrite. Burial history analysis reveals that from the temperature at which TSR occurred it was in the Late Jurassic to Early Cretaceous and TSR ceased due to uplift and cooling thereafter. TSR alteration is incomplete and mainly occurs in wet gas components as indicated by near constant CH₄ δ¹³C values, wide range variations of ethane, propane and butane δ¹³C values, and moderately high gas dryness. The δ³⁴S values in SO₄, elemental S and H₂S fall within the fractionation scope of TSR-derived H₂S. High organo-S compound concentrations together with the occurrence of 2-thiaadamantanes in the T₂l reservoir provide supplementary evidence for TSR related alteration.     

Geochemical Characteristics of Natural Gases in the Sichuan Basin—A Further Discussion

This paper is the continuation of the paper published in 1985, with the emphasis on the irregular components of natural gases from the Sichuan Basin—the isotopic compositions of C,H and S, the n-and iso-alkane predominance (C₆-C₁₃) of methane homologues, the compositional characteristics of benzene and methylbenzene, and the distribution and geochemical characteristics of organic sulfur-bearing compounds including thiophen, thio-alcohol, etc. It is considered that natural gases from the Sichuan Basin have different distributional characteristics in different layers and locations, suggesting that they are controlled by a combination of factors such as kerogen type, maturity of organic matter and wall-rock assemblage.

1. The type of source material (or precursor) is a main factor affecting the geochemical characteristics of natural gases. T_3x-h and P₂l produce coal-series gases with type-III source material dominant. The C and H isotopic compositions of natural gases are heavier, the contents of C₆H₆ and C₇H₈ are high, C₇H₈/C₆H₆ < 1. the content of C₄H₄S is low, and the predominance in isoalkane (C₆C₁₃) is obvious. J₁t. P₁y. Z₂b. etc. produce oil-series gases with type-I and -II kerogens dominant, indicating that the geochemical characteristics of natural gases are different from those described above.
2. The maturity of organic matter is an important factor affecting the composition of natural gases.With increasing maturity of organic matter, the C and H isotopic compositions tend to become light. C₁ and C₂ isotopic values are nearly equal or even inverse, and C₄H₄S decreases in content.
3. Wall-rock assemblage has apparent influence on the S isotopic composition. The Middle-Lower Triassic series belongs to sulfate-carbonate formations enriched in gypsum. Therefore, the (δ³⁴S values of natural gases are relatively high (< 20 %.) against the lower values for other layers due to the absence of gypsum layers.

     

Stable sulfur isotope partitioning during simulated petroleum formation as determined by hydrous pyrolysis of Ghareb Limestone, Israel

Hydrous pyrolysis experiments at 200 to 365°C were carried out on a thermally immature organic-rich limestone containing Type-IIS kerogen from the Ghareb Limestone in North Negev, Israel. This work focuses on the thermal behavior of both organic and inorganic sulfur species and the partitioning of their stable sulfur isotopes among organic and inorganic phases generated during hydrous pyrolyses. Most of the sulfur in the rock (85%) is organic sulfur. The most dominant sulfur transformation is cleavage of organic-bound sulfur to form H₂S_(gas). Up to 70% of this organic sulfur is released as H₂S_(gas) that is isotopically lighter than the sulfur in the kerogen. Organic sulfur is enriched by up to 2‰ in ³⁴S during thermal maturation compared with the initial δ³⁴S values. The δ³⁴S values of the three main organic fractions (kerogen, bitumen and expelled oil) are within 1‰ of one another. No thermochemical sulfate reduction or sulfate formation was observed during the experiments. The early released sulfur reacted with available iron to form secondary pyrite and is the most ³⁴S depleted phase, which is 21‰ lighter than the bulk organic sulfur. The large isotopic fractionation for the early formed H₂S is a result of the system not being in equilibrium. As partial pressure of H₂S_(gas) increases, retro reactions with the organic sulfur in the closed system may cause isotope exchange and isotopic homogenization. Part of the δ³⁴S-enriched secondary pyrite decomposes above 300°C resulting in a corresponding decrease in the δ³⁴S of the remaining pyrite. These results are relevant to interpreting thermal maturation processes and their effect on kerogen-oil-H₂S-pyrite correlations. In particular, the use of pyrite-kerogen δ³⁴S relations in reconstructing diagenetic conditions of thermally mature rocks is questionable because formation of secondary pyrite during thermal maturation can mask the isotopic signature and quantity of the original diagenetic pyrite. The main transformations of kerogen to bitumen and bitumen to oil can be recorded by using both sulfur content and δ³⁴S of each phase including the H₂S_(gas). H₂S generated in association with oil should be isotopically lighter or similar to oil. It is concluded that small isotopic differentiation obtained between organic and inorganic sulfur species suggests closed-system conditions. Conversely, open-system conditions may cause significant isotopic discrimination between the oil and its source kerogen. The magnitude of this discrimination is suggested to be highly dependent on the availability of iron in a source rock resulting in secondary formation of pyrite.     

平衡热液体系中硫同位素演化的几个图解

根据含硫矿物的同位素组成推断热液矿床成因是很有意义的。 1968年首先由H.Sakai指出热液的温度和pH值可以影响硫化物的同位素组成。接着,1972年H.Ohmoto以及1979年他和R.O.Rye系统讨论了平衡条件下热液的物理化学条件对硫同位素分馏的影响,建立了高温热液系统和低温热液系统的热液流体以及含硫矿物与热液成分和物理化学条件(温度、压力、氧逸度和酸碱度等)之间的数学表达式。     

O and S isotopic composition of dissolved and attached oxidation products of pyrite by Acidithiobacillus ferrooxidans: Comparison with abiotic oxidations

The acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, plays a part in the pyrite oxidation process and has been widely studied in order to determine the kinetics of the reactions and the isotopic composition of dissolved product sulphates, but the details of the oxidation processes at the surface of pyrite are still poorly known. In this study, oxygen and sulphur isotopic compositions (δ¹⁸O and δ³⁴S) were analyzed for dissolved sulphates and water from experimental aerobic acidic (pH < 2) pyrite oxidation by A. ferrooxidans. The oxidation products attached to the pyrite surfaces were studied for their morphology (SEM), their chemistry (Raman spectroscopy) and for their δ¹⁸O (ion microprobe). They were compared to abiotically (Fe³⁺, H₂O₂, O₂) oxidized pyrite surface compounds in order to constrain the oxidation pathways and to look for the existence of potential biosignatures for this system.The pyrite dissolution evolved from non-stoichiometric (during the first days) to stoichiometric (with increasing time) resulting in dissolved sulphates having distinct δ¹⁸O (e.g. +11.0‰ and −2.0‰, respectively) and δ³⁴S (+4.5‰ and +2.8‰, respectively) values. The “oxidation layer” at the surface of pyrite is complex and made of iron oxides, sulphate, polysulphide, elemental sulphur and polythionates. Bio- and Fe³⁺-oxidation favour the development of monophased micrometric bumps made of hematite or sulphate while other abiotic oxidation processes result in more variable oxidation products. The δ¹⁸O of these oxidation products at the surface of oxidized pyrites are strongly variable (from ≈−40‰ to ≈+30‰) for all experiments.Isotopic fractionation between sulphates and pyrite, Δ³⁴S_SO4-pyrite, is equal to −1.3‰ and +0.4‰ for sulphates formed by stoichiometric and non-stoichiometric processes, respectively. These two values likely reflect either a S-S or a Fe-S bond breaking process. The Δ¹⁸O_SO4-H2O and Δ¹⁸O_SO4-O2 are estimated to be ≈+16‰ and ≈−25‰, respectively. These values are higher than previously published data and may reflect biological effects. The large δ¹⁸O heterogeneity measured at the surfaces of oxidized pyrites, whatever the oxidant, may be related (i) to the existence of local surface environments isolated from the solution in which the oxidation processes are different and (ii) to the stabilization at the pyrite surface of reaction intermediates that are not in isotopic equilibrium with the solution. Though the oxygen isotopic composition of surface oxidation products cannot be taken as a direct biosignature, the combined morphological, chemical and isotopic characterization of the surfaces of oxidized pyrites may furnish clues about a biological activity on a mineral surface.     

Proportional variations of geochemical δ 18O scales —an interlaboratory comparison

Nineteen laboratories, of nine countries, have reported independent determinations of the oxygen and carbon isotopic composition of the TKL-1 and K-2 calcite reference samples. Results for the individual samples demonstrate a need to replace exhausted primary reference standards and to adopt precise values for constants such as α_CO2-H2O^25°C. The reported differences in δ¹⁸O between the two samples vary between 23.0 and 23.6%., reflecting mass spectrometer errors that are approximately proportional to measured differences in isotopic composition. In order to eliminate such errors, isotopic reference scales should be based on at least two primary standards.     

The carbon isotopic composition of diamonds: relationship to diamond shape,color, occurrence and vapor composition

Three hundred and thirty new ¹³C analyses of diamonds are presented, indicating, in conjunction with earlier published work, a range of about 30%. in the carbon isotopic composition of diamonds. The frequency distribution of diamond δ¹³C analyses shows a very pronounced mode at ?5 to ?6%.vs PDB, a large negative skewness, and a sharp boundary at about ?1%.. Analyses of diamonds from the Premier and Dan Carl mines, South Africa, demonstrate that: (1) differences in ¹³C content that can be related to diamond color and shape are smaller than 1%.; (2) the mean ¹³C content of kimberlite carbonates is 1–2%. lower than that of associated diamonds; (3) significant differences in ¹³C content exist between the mean isotopic compositions of diamonds from these two pipes; (4) the variability in δ¹³C differs from one mine to the other.Computations were carried out evaluating the effect on the ¹³C content of diamonds of: (i) various precipitation processes; (ii) the abundance of the species H₂, H₂O, CH₄, CO, CO₂ and O₂ in the vapor; (iii) the initial isotopic composition variability of the source carbon; (iv) variations of the carbon isotope effects resulting from changes in pressure and temperature and (v) reservoir effects (Rayleigh fractionation). Fifty-eight genetic models were investigated for compatibility with the ¹³C distribution in diamonds and associated carbonate. The modeling does not permit an unambiguous answer to the question whether or not a vapor participated in diamond formation, although the presence of methane during diamond formation is compatible with the carbon isotopic composition data, possible oxygen fugacities in the mantle and with the composition of gases liberated from diamonds. In all probability carbon isotope effects in the diamond formation process were small, and the very large range in δ¹³C observed was inherited from the source carbon.     

Zonation of Sulfate and Sulfide Minerals and Isotopic Composition in the Far Southeast Porphyry and Lepanto Epithermal Cu–Au Deposits,Philippines

The world‐class Far Southeast (FSE) porphyry system, Philippines, includes the FSE Cu–Au porphyry deposit, the Lepanto Cu–Au high‐sulfidation deposit and the Victoria–Teresa Au–Ag intermediate‐sulfidation veins, centered on the intrusive complex of dioritic composition. The Lepanto and FSE deposits are genetically related and both share an evolution characterized by early stage 1 alteration (deep FSE potassic, shallow Lepanto advanced argillic‐silicic, both at ~1.4 Ma), followed by stage 2 phyllic alteration (at ~1.3 Ma); the dominant ore mineral deposition within the FSE porphyry and the Lepanto epithermal deposits occurred during stage 2. We determined the chemical and S isotopic composition of sulfate and sulfide minerals from Lepanto, including stage 1 alunite (12 to 28 permil), aluminum–phosphate–sulfate (APS) minerals (14 to 21 permil) and pyrite (?4 to 2 permil), stage 2 sulfides (mainly enargite–luzonite and some pyrite, ?10 to ?1 permil), and late stage 2 sulfates (barite and anhydrite, 21 to 27 permil). The minerals from FSE include stage 2 chalcopyrite (1.6 to 2.6 permil), pyrite (1.1 to 3.4 permil) and anhydrite (13 to 25 permil). The whole‐rock S isotopic composition of weakly altered syn‐mineral intrusions is 2.0 permil. Stage 1 quartz–alunite–pyrite of the Lepanto lithocap, above about 650 m elevation, formed from acidic condensates of magmatic vapor at the same time as hypersaline liquid formed potassic alteration (biotite) near sea level. The S isotopic composition of stage 1 alunite–pyrite record temperatures of approximately 300–400°C for the vapor condensate directly over the porphyry deposit; this cooled to <250°C as the acidic condensate flowed to the NW along the Lepanto fault where it cut the unconformity at the top of the basement. Stage 1 alunite at the base of the advanced argillic lithocap over FSE contains cores of APS minerals with Sr, Ba and Ca; based on back‐scattered electron images and ion microprobe data, these APS minerals show a large degree of chemical and S‐isotopic heterogeneity within and between samples. The variation in S isotopic values in these finely banded stage 1 alunite and APS minerals (16 permil range), as well as that of pyrite (6 permil range) was due largely to changes in temperature, and perhaps variation in redox conditions (average ~ 2:1 H₂S:SO₄). Such fluctuations could have been related to fluid pulses caused by injection of mafic melt into the diorite magma chamber, supported by mafic xenoliths hosted in diorite of an earlier intrusion. The S isotopic values of stage 2 minerals indicate temperatures as high as 400°C near sea level in the porphyry deposit, associated with a relatively reduced fluid (~10:1 H₂S:SO₄) responsible for deposition of chalcopyrite. Stage 2 fluids were relatively oxidized in the Lepanto lithocap, with an H₂S:SO₄ ratio of about 4. The oxidation resulted from cooling, which was caused by boiling during ascent and then dilution with steam‐heated meteoric water in the lithocap. This cooling also resulted in the sulfidation state of minerals increasing from chalcopyrite stability in the porphyry deposit to that of enargite in the lithocap‐hosted high‐sulfidation deposit. The temperature at the base of the lithocap during stage 2 was ≥300°C, cooling to <250°C within the main lithocap, and about 200°C towards the limit of the Lepanto orebody, approximately 2 km NW of the porphyry deposit. Approximate 300°C and 200°C isotherms, estimated from S isotopic and fluid inclusion temperatures during stage 1 and stage 2, shifted towards the core of the FSE porphyry deposit with time. This general retreat in isotherms was more than 500 m laterally within Lepanto and 500 m vertically within FSE as the magmatic–hydrothermal system evolved and collapsed over the magmatic center. During this evolution, there is also evidence recorded by large S isotopic variations in individual crystals for sharp pulses of higher temperature, relatively reduced fluid injected into the porphyry deposit.     

Theoretical and experimental investigation of nickel isotopic fractionation in species relevant to modern and ancient oceans

Nickel plays a central role as an enzyme co-factor in the metabolism of methanogenic Archaea. Methanogens can fractionate Ni isotopes during assimilation, opening the possibility of using the stable isotopic composition of Ni as a biomarker. However, other sources of Ni isotopic variations need to be evaluated before one can establish Ni isotopes as an unambiguous tracer of methanogenesis in the rock record. Equilibrium exchange of Ni between the different species present in the ocean is a potential source of isotopic fractionation. Through controlled laboratory experiments and theoretical calculations, we quantify equilibrium Ni isotope fractionation between different species relevant to the modern and ancient ocean: Ni(H₂O)₆²⁺, Ni(H₂O)₁₈²⁺, NiOH(H₂O)₅⁺, Ni(OH)₂(H₂O)₄, NiCl(H₂O)₅⁺, cis-NiCl₂(H₂O)₄, trans-NiCl₂(H₂O)₄, NiHS(H₂O)₅⁺, Ni(HS)₂(H₂O)₄, NiSO₄(H₂O)₄, NiHCO₃(H₂O)₄⁺, NiCO₃(H₂O)₄, and organic ligands (crown ether and oxalic acid). The magnitude of ligand-controlled Ni isotopic fractionation, approximately 1.25‰/amu (2.5‰ for the ⁶⁰Ni/⁵⁸Ni ratio), is similar to that previously measured in cultures of methanogenic Archaea.     

Further stable isotope investigations of human urinary stones: comparison with other body components

Carbon and sulphur isotope investigations of human urinary stones have been expanded to relate such data to various body components and to diet. Techniques include isotopic determinations for various body components, for example, hair and urine, as well as trace sulphate and sulphide in apatite-struvite stones.Hair from individuals in Calgary was found to be, on average, about 3‰ depleted in¹³C in comparison to samples from Hawaii. Uric acid stones from both locations were found to be 1 to 3‰ enriched in¹³C, compared to hair. Oxalate stones from Calgary had δ¹³C values very close to those of hair. In contrast, oxalate stones from 31 patients from Honolulu fit the regression lineδ¹³C_oxalate= 0.8 δ¹³C_hair− 4.4‰, with a correlation coefficient of 0.77. It remains debatable as to whether the isotopic differences between the stones and hair reflects preferential incorporation of dietary components or kinetic isotope effects during biochemical conversions. There was no evidence in the data from Honolulu that ethnic background significantly influenced the carbon isotope composition of hair or kidney stones. There was a suggestion that recent arrivals had hair and stones slightly depleted in¹³C as compared to longer residents.The δ³⁴S values of cystine stones from Calgary were markedly consistent, near 0‰, and isotopic variations among different body components of individuals were of the order of 1‰.The trace sulphate content of a bladder stone from Papua New Guinea, was 300ppm S, whereas the sulphide content was negligible (determined by in vacuo Kiba extraction). The total S content of three samples from Calgary averaged 250 ppm, whereas 150 ppm was found for two stones each from Quito, Ecuador and Honolulu, Hawaii. For stones other than the speciment from Papua New Guinea, the sulphate-to-sulphide ratio varied from 1 to 4. The source of sulphide is uncertain but degradation of organic S could contribute to this fraction during Kiba extraction. The small range of δ³⁴S values (+3.5 to +7.4‰) for trace total S in the phosphate-containing urinary stones is believed to reflect only a fraction of the global variation of these materials. Trace sulphate was variably enriched in³⁴S (0 to 9‰) as compared to sulphide. Neither these enrichments, nor the sulphate-to-sulphide ratio, could be related to the struvite-to-apatite ratio.There were no significant differences in the carbon and sulphur isotope compositions of hair from patients and non-stone formers. Both the carbon and sulphur isotope variations can be attributed to the isotopic compositions of diets and the superposition of small kinetic isotope effects during biochemical conversions.     

Calculation of individual isotope equilibrium constants for geochemical reactions

Theory is derived from the work of Urey (Urey H. C. [1947] The thermodynamic properties of isotopic substances. J. Chem. Soc. 562-581) to calculate equilibrium constants commonly used in geochemical equilibrium and reaction-transport models for reactions of individual isotopic species. Urey showed that equilibrium constants of isotope exchange reactions for molecules that contain two or more atoms of the same element in equivalent positions are related to isotope fractionation factors by α = (K^ex)^1/n, where n is the number of atoms exchanged. This relation is extended to include species containing multiple isotopes, for example ¹³C¹⁶O¹⁸O and ¹H²H¹⁸O. The equilibrium constants of the isotope exchange reactions can be expressed as ratios of individual isotope equilibrium constants for geochemical reactions. Knowledge of the equilibrium constant for the dominant isotopic species can then be used to calculate the individual isotope equilibrium constants.Individual isotope equilibrium constants are calculated for the reaction CO_2g = CO_2aq for all species that can be formed from ¹²C, ¹³C, ¹⁶O, and ¹⁸O; for the reaction between ¹²C¹⁸O_2aq and ¹H₂¹⁸O_l; and among the various ¹H, ²H, ¹⁶O, and ¹⁸O species of H₂O. This is a subset of a larger number of equilibrium constants calculated elsewhere (Thorstenson D. C. and Parkhurst D. L. [2002] Calculation of individual isotope equilibrium constants for implementation in geochemical models. Water-Resources Investigation Report 02-4172. U.S. Geological Survey). Activity coefficients, activity-concentration conventions for the isotopic variants of H₂O in the solvent ¹H₂¹⁶O_l, and salt effects on isotope fractionation have been included in the derivations. The effects of nonideality are small because of the chemical similarity of different isotopic species of the same molecule or ion. The temperature dependence of the individual isotope equilibrium constants can be calculated from the temperature dependence of the fractionation factors.The derivations can be extended to calculation of individual isotope equilibrium constants for ion pairs and equilibrium constants for isotopic species of other chemical elements. The individual isotope approach calculates the same phase isotopic compositions as existing methods, but also provides concentrations of individual species, which are needed in calculations of mass-dependent effects in transport processes. The equilibrium constants derived in this paper are used to calculate the example of gas-water equilibrium for CO₂ in an acidic aqueous solution.     

Isotopic constraints on growth conditions of multiphase calcite–pyrite–barite concretions in Carboniferous mudstones

Carbonate concretions in the Lower Carboniferous Caton Shale Formation contain diagenetic pyrite, calcite and barite in the concretion matrix or in different generations of septarian fissures. Pyrite was formed by sulphate reduction throughout the sediment before concretionary growth, then continued to form mainly in the concretion centres. The septarian calcites show a continuous isotopic trend from δ¹³C=?28·7‰ PDB and δ¹⁸O=?1·6‰ PDB through to δ¹³C=?6·9‰ PDB and δ¹⁸O=?14·6‰ PDB. This trend arises from (1) a carbonate source initially from sulphate reduction, to which was added increasing contributions of methanogenic carbonate; and (2) burial/temperature effects or the addition of isotopically light oxygen from meteoric water. The concretionary matrix carbonates must have at least partially predated the earliest septarian cements, and thus used the same carbonate sources. Consequently, their isotopic composition (δ¹³C=?12·0 to ?10·1‰ PDB and δ¹⁸O=?5·7 to ?5·6‰ PDB) can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and, directly or indirectly, also from skeletal carbonate. Concretionary growth was therefore pervasive, with cements being added progressively throughout the concretion body during growth. The concretions contain barite in the concretion matrix and in septarian fissures. Barite in the earlier matrix phase has an isotopic composition (δ³⁴S=+24·8‰ CDT and δ¹⁸O=+16·4‰ SMOW), indicating formation from near‐surface, sulphate‐depleted porewaters. Barites in the later septarian phase have unusual isotopic compositions (δ³⁴S=+6 to +11‰ CDT and δ¹⁸O=+8 to +11‰ SMOW), which require the late addition of isotopically light sulphate to the porewaters, either from anoxic sulphide oxidation (using ferric iron) or from sulphate dissolved in meteoric water. Carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments.