Fluid δ18O Research Articles

The identification of the Guqiong orogenic Ag–Au polymetallic deposit in southern Tibet: Evidences from mineralogy, geochronology and fluid evolution

Guqiong is an Ag–Au polymetallic deposit located in the eastern part of the Au–Sb ore belt along the Indus-Yarlung Zangbo suture in the North Himalaya. In this study, we have identified three major alteration/mineralization stages for Guqiong based on the mineral assemblages and their paragenetic relationships, namely the (I) pre-ore alteration, forming mainly ore-barren thick quartz veins and vuggy quartz; (II) main ore stage that comprises quartz veins with sulfide mineralization including Ag (-Au)-bearing minerals. In this stage, sericite occurs closely associated with sulfides; (III) the post-ore stage characterized by the formation of calcite and quartz with trace sulfides. Fluid inclusion assemblage (FIA) microthermometry was conducted on barren and mineralized quartz, and sphalerite. Three types of inclusions are identified based on their petrographic features and laser Raman spectra, i.e., the pure carbonic (PC), aqueous-carbonic (three-phase C1 and two-phase C2) and aqueous (W) types. The FIAs in stage I ore-barren quartz are mainly PC-type and C1-type, which are characterized by being high temperatures (260 to 300 °C), low salinities (0.8 to 3.5 wt% NaCl eqv.) and with abundant CO2. Sphalerite of main ore stage contains mainly W-type (with limited C2-type) of low temperatures (158 to 180 °C) and medium salinities (5.6 to 6.7 wt% NaCl eqv.). Meanwhile, the FIA in mineralized quartz is featured by the general coexistence of C2-type and W-type (with local PC-type and C1-type) inclusions. The fluid system may have been a mixture of the two aforementioned fluids in barren quartz and sphalerite, as indicated by two significantly different FIAs (C-type and W-type) with contrasting homogenization temperatures (260 to 290 °C and 180 to 220 °C, respectively) but comparable salinities (0.2 to 8.2 and 0.2 to 8.1 wt% NaCl eqv., respectively). The fluid δD, δ18O and δ13C values of the main ore stage quartz are −172 to −114%, 8.8 to 13.1% and −13.4 to −7.7%, respectively. The negative δ13C values are interpreted as devolatilization products of biogenic carbon with a metamorphic origin, while the δD and δ18O values likely reflect meteoric water involvement into metamorphic fluids. We propose that fluid mixing had occurred and caused sulfide precipitation at the main ore stage. Additionally, the estimated fluid trapping depth of 5 to 6 km (obtained using 3-D phase diagram) is consistent with the formation depth of many orogenic-type deposits worldwide. 40Ar/39Ar dating on sericite yielded a well-defined plateau age of 42.03 ± 0.42 Ma, with normal and inverse isochron ages of 42.8 ± 1.6 Ma and 42.9 ± 1.9 Ma, respectively. These ages bracket the Ag–Au mineralization at Guqiong, place it in the main-stage India-Asia collision and further support its orogenic-type origin.

Clumped isotopic signatures in land-snail shells revisited: Possible palaeoenvironmental implications

It is crucial to understand the clumped isotope compositions (Δ47) of modern land-snail shells and their relationship with environmental parameters for palaeoenvironmental-studies. Previous studies have shown significant variations in relationships between snail-shell Δ47 and ambient air temperature, and the reliability of shell Δ47 in indicating air temperature remains uncertain. This study examined the shell Δ47 from China with mean annual temperatures ranging from 5 to 23 °C and the snail body fluid δ18O estimated from shell δ18O and Δ47-derived temperatures. For all snails studied in this study, site-averaged Δ47 values yield snail calcification temperatures ranging from 25 to 36 °C, which are higher-than-expected in either environmental temperatures or estimated snail activity temperatures. We suggest that snails up-regulate their body temperatures towards their preferred living conditions likely through behaviour or/and physiology adaptation, especially at low temperature environments, which is likely significant than observed previously. Δ47 disequilibrium associated with CO2 degassing during snail calcification is potential but it could possibly be corrected by using an empirical calibration based on natural land-snails. Varying degrees of δ18O enrichment (>8‰) relative to rainwater are observed in snail body fluids, possibly due to variable evaporation during shell calcification. Coupled with model calculations, the relationship between snail body fluid δ18O and rainwater δ18O could be improved by using Δ47-based temperature and δ18O thermometry, if snails prefer to be active at ~90% relative humidity. This study highlights the importance of land-snail shell Δ47 coupled with shell δ18O to indicate temperature and rainwater δ18O at micro-environments scales.

Unusual Uranium‐rich Fluid Flow During Exhumation of Deeply Subducted Continental Crust

AbstractIn‐situ SIMS analyses of O and U‐Pb isotopes were carried out for zircons from a quartz vein hosted by ultrahigh‐pressure metagranite (UHP) in the Dabie orogen. The results are integrated to decipher the property of unusual U‐rich aqueous fluids and their effects on both metamorphic and magmatic zircons during exhumation of the UHP metagranite. In CL images, most zircon grains show distinct core‐rim structures. Relict cores are bright and exhibit oscillatory or patchy zonation, giving Neoproterozoic upper‐intercept ages of 795 ± 26 Ma. Newly grown rims are dark and exhibit no zoning, yielding Triassic concordant ages of 215 ± 5 Ma. The cores give Th contents of 59 to 463 ppm and U contents of 98 to 558 ppm, with Th/U ratios of 0.263 to 1.423. The rims yield reduced Th contents of 11 to 124 ppm but significantly elevated U contents of 1051 to 3531 ppm, with Th/U ratios of 0.010 to 0.035. Comparison with the cores of magmatic origin, the unusual enrichment in U but depletion in Th in the rims of metamorphic origin are interpreted as zircon growth from Cl‐rich oxidized vein‐forming aqueous fluids that were produced by dehydration reactions of the wallrock during continental exhumation. The cores have variably positive δ18O values with concordant or discordant Neoproterozoic U‐Pb ages, suggesting their solid‐state modification of both O and U‐Pb isotopes through interaction with the fluids. The rims yield negative δ18O values, indicating their growth from the negative δ18O fluids. Taken together, the proposed Cl‐rich oxidized negative‐δ18O vein‐forming aqueous fluids have such an ability to not only cause variable metamorphic recrystallization in the relict magmatic zircons but also produce dramatic fractionation of U over Th in the metamorphic zircons during quartz veining, and potentially impact on the overlain metasomatite in the mantle wedge.

Origin and Evolution of Ore-Forming Fluid and Gold-Deposition Processes at the Sanshandao Gold Deposit, Jiaodong Peninsula, Eastern China

The Early Cretaceous Sanshandao gold deposit, the largest deposit in the Sanshandao-Cangshang goldfield, is located in the northwestern part of the Jiaodong peninsula. It is host to Mesozoic granitoids and is controlled by the north by northeast (NNE) to northeast (NE)-trending Sanshandao-Cangshang fault. Two gold mineralizations were identified in the deposit’s disseminated and stockwork veinlets and quartz–sulfide veins, which are typically enveloped by broad alteration selvages. Based on the cross-cutting relationships and mineralogical and textural characteristics, four stages have been identified for both styles of mineralization: Pyrite–quartz (stage 1), quartz–pyrite (stage 2), quartz–pyrite–base metal–sulfide (stage 3), and quartz–carbonate (stage 4), with gold mainly occurring in stages 2 and 3. Three types of fluid inclusion have been distinguished on the basis of fluid-inclusion assemblages in quartz and calcite from the four stages: Pure CO2 gas (type I), CO2–H2O inclusions (type II), and aqueous inclusions (type III). Early-stage (stage 1) quartz primary inclusions are only type II inclusions, with trapping at 280–400 °C and salinity at 0.35 wt %–10.4 wt % NaCl equivalent. The main mineralizing stages (stages 2 and 3) typically contain primary fluid-inclusion assemblages of all three types, which show similar phase transition temperatures and are trapped between 210 and 320 °C. The late stage (stage 4) quartz and calcite contain only type III aqueous inclusions with trapping temperatures of 150–230 °C. The δ34S values of the hydrothermal sulfides from the main stage range from 7.7‰ to 12.6‰ with an average of 10.15‰. The δ18O values of hydrothermal quartz mainly occur between 9.7‰ and 15.1‰ (mainly 10.7‰–12.5‰, average 12.4‰); calculated fluid δ18O values are from 0.97‰ to 10.79‰ with a median value of 5.5‰. The δDwater values calculated from hydrothermal sericite range from −67‰ to −48‰. Considering the fluid-inclusion compositions, δ18O and δD compositions of ore-forming fluids, and regional geological events, the most likely ultimate potential fluid and metal would have originated from dehydration and desulfidation of the subducting paleo-Pacific slab and the subsequent devolatilization of the enriched mantle wedge. Fluid immiscibility occurred during the main ore-forming stage due to pressure decrease from the early stage (165–200 MPa) to the main stage (90–175 MPa). Followed by the changing physical and chemical conditions, the metallic elements (including Au) in the fluid could no longer exist in the form of complexes and precipitated from the fluid. Water–rock sulfidation and pressure fluctuations, with associated fluid unmixing and other chemical changes, were the two main mechanisms of gold deposition.

Fluid inclusion and stable isotope (O, H, C) constraints on the genesis of the Pedra Branca gold deposit, Troia Massif, Borborema Province, NE Brazil: An example of hypozonal orogenic gold mineralization

At the Archean-Paleoproterozoic Troia Massif, NE Brazil, two major Paleoproterozoic greenstone belts are documented (Algodões and Serra das Pipocas). They share similar lithostratigraphic characteristics with other 2.2–2.1 Ga greenstone belts of the surrounding cratonic domains (e.g. Guiana shield and West Africa craton). Gold mineralization in the Serra das Pipocas greenstone belt is associated with a regional NE-trending shear zone. In the mineralized area (the Pedra Branca gold deposit) the main gold mineralization is found in association with quartz veins, calc–silicate alteration (e.g. diopside, K-feldspar, amphibole, titanite, biotite, pyrite, albite, magnetite ± carbonates) and albitization. Pyrrhotite is commonly found as inclusions in pyrite, and late magnetite replaces pyrite, suggesting progressive oxidation of the ore–forming fluid. Free-milling gold commonly precipitates at the late stages of alteration, in close association with magnetite and tellurides. The main tellurides found are hessite, altaite, sylvanite, calaverite and tellurobismuthite. Two fluid inclusion assemblages were identified from mineralized quartz veins. Assemblage 1 is characterized by pseudo-secondary trails that show the coexistence of CO2–rich and low salinity (0–8 wt% NaCl equiv.) CO2–H2O–NaCl and H2O–NaCl inclusions, suggesting formation during phase separation (fluid immiscibility). Assemblage 2 is represented by late secondary low–temperature (Th < 200 °C) H2O–NaCl inclusions, probably unrelated to gold mineralization. The δ18O, δD and δ13C values of hydrothermal minerals (quartz, calcite, biotite, hornblende and magnetite) and fluid inclusions are compatible with a magmatic–hydrothermal ore-forming fluid that underwent fluid–rock interaction with the greenstone sequence. The calculated fluid δ18O values range from +8.3 to +11.0‰ (n = 59), fluid δD from −98 to –32‰ (n = 24) and δ13C values of calcite from −6.35 to −9.40‰ (n = 3). Oxygen isotope thermometry for quartz–magnetite pairs yields temperatures from 467 to 526 °C (n = 7, average 503 °C). This probably represents the temperature of gold deposition, because gold is generally associated with magnetite in the ore mineral assemblage. The estimated PT conditions of the gold mineralization range from 467 °C and 1.93 kbar (7 km) to 526 °C and 3.48 kbar (13 km), repectively, and they were obtained from the intersection of isochores with the temperature range derived from the quartz–magnetite isotope geothermometer. This PT range is akin to hypozonal orogenic gold deposits. The association of gold with magnetite and tellurides implies an ore–forming fluid derived from oxidized magmas, similar to those interpreted as ‘orogenic oxidized intrusion-related gold deposits’ in other Precambrian greenstone belts (e.g. Abitibi and Eastern Goldfields).

Seafloor weathering and the oxygen isotope ratio in seawater: Insight from whole-rock δ18O and carbonate δ18O and Δ47 from the Troodos ophiolite

The controls on, and history of, the oxygen isotope ratio in seawater continue to be debated after many decades of research with the lack of consensus in large part reflecting uncertainty in the role of hydrothermal exchange between seawater and the oceanic crust. We have investigated this using new carbonate Δ47 and δ18O data, and whole-rock O-isotope data, for samples from the lava section of the Troodos ophiolite. Carbonate data confirm that fluid-to-rock ratios in the upper lavas during off-axis hydrothermal circulation are generally sufficiently large that both the fluid δ18O and temperature are similar to those of bottom water. However, some samples require more complicated interpretations that could reflect changes in the rate of calcite formation. Whole-rock data indicate that O-isotope exchange in the lavas is directly linked to the major element exchange that leads to alkalinity production (i.e., CO2 consumption) and both are dependent on bottom water temperature. This means that the O-isotopic composition of seawater is linked to the long-term C-cycle. The data are used to parameterise a simple model of the evolution of the O-isotopic composition of seawater driven by changes in solid earth CO2 degassing. Alkalinity balance links the total extent of weathering of the continents and seafloor, which are sinks for high δ18O material, to CO2 degassing rate and surface temperature. The modelling suggests that if solid earth CO2 degassing and the rate of formation of oceanic crust are linked, the O-isotopic composition of the ocean (including any ice sheets) is unlikely to have varied more than ±1‰ over the Phanerozoic.

Taking the Temperature of Hydrothermal Ore Deposits Using Clumped Isotope Thermometry

Abstract Better tools are needed to map the thermal structure of ore deposits. Here, carbonate clumped isotope thermometry is applied for the first time in epithermal, skarn, and carbonate-hosted deposits to identify the conditions involved in metal transport and deposition. Clumped isotope temperature calibrations were tested by measurement of carbonates from three geothermal fields in the Taupo volcanic zone, New Zealand, that record growth temperatures between 130° and 310°C. Results for modern Taupo volcanic zone calcites were paired with known fluid δ18O values and these indicate precipitation in equilibrium with produced geothermal waters. Measurements carried out at the Waihi low sulfidation deposit in New Zealand, the Antamina polymetallic skarn in Peru, and the Mount Isa sediment hosted Pb-Zn and Cu deposit in Queensland, Australia, demonstrate that clumped isotope values are sensitive to temperature gradients defined using other methods. At Waihi, an andesite-hosted deposit, temperature controls the majority of variation in carbonate mineral δ18O. At Mount Isa, ~300° to 400°C temperatures were recorded in a 1.5 Ga orebody, which are consistent with fluid inclusion values, highlighting the longevity of clumped isotope archives in dolomite minerals. Collectively, these results demonstrate the potential for clumped isotopes to delineate the heat footprint around deposits that contain carbonates, and to more effectively disentangle magmatic and meteoric fluid δ18O signals.

Diversity and origin of quartz cements in continental carbonates: Example from the Lower Cretaceous rift deposits of the South Atlantic margin

Silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis. The Lower Cretaceous rift deposits of the South Atlantic equatorial margin, which are intensively explored by petroleum companies, provide good examples of such silicifications in carbonates, exhibiting a wide diversity of petrographic habit of early to late quartz cements. In order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity, we integrated detailed petrography of diagenetic sequences and quartz habit with δ18Oquartz measurements (by SIMS) of individual cements observed in samples from the offshore and onshore basins of the West African margin. The petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length-fast or length-slow habit, in addition to laminated microquartz and micro- or mega-quartz forms. Amongst the isotopic analysis, the δ18Oquartz data show that length-slow cements are generally strongly enriched in 18O (δ18Oquartz ranging from 31 to 37‰ SMOW), whereas length-fast forms show less elevated values (<32‰ SMOW). The highest δ18Oquartz values for fibrous microquartz are interpreted to reflect precipitation from evaporated meteoric fluids at temperatures >25 °C and <100 °C. The alkalinity required to favor the precipitation of length-slow fibrous microquartz cements is probably related to fluid/rock interactions with underlying mantel-related or basic volcanic rocks. Such interactions would be in agreement with the recent geodynamic models of the South Atlantic passive margin. The length-fast fibrous microquartz associated with δ18Oquartz values ranging from 27 to 32‰ SMOW, probably reflect precipitation from moderately to non evaporated, fairly neutral to acid, fluids. The partial dissolution of carbonate cements prior to quartz cementations represents the signature of those acidic conditions. We therefore suggest that acidic pH was obtained through fluid/rock interactions with the intermediate to acid volcanic rocks encountered along the palaeohydrological pathway. Other quartz phases, such as the megaquartz cement, exhibit highly variable δ18Oquartz values ranging from 20 to 40‰. This variation may reflect significant variation in temperature conditions (between 100 and 200 °C) or changes in fluid δ18O at the small scale. For these populations of non-fibrous quartz cement, the very high δ18Oquartz values may reflect a contribution of fluids that have either suffered strong evaporation or strong water/rock interaction.

Isotopic constraints on fluid evolution and ore precipitation in a sediment-hosted Pb-Ag-Ba-Zn-Cu-Au deposit in the Capricorn Orogen, Western Australia

The Abra Pb-Ag-Ba-Zn-Cu-Au deposit in the Capricorn Orogen, Western Australia is primarily a lead and silver resource currently estimated at 47.8 Mt (indicated and inferred) of 7.3–10.1% Pb and 18–28 gt-1 Ag, although significant Cu-Au zones are also identified. The deposit is unique within sediment-hosted Pb-Zn deposits for its low Zn content, significant Cu-Au zone and high Fe content, providing a case study where the source of fluid and ore-forming processes are contentious. The combination of whole-rock hydrogen and oxygen isotope data, in situ oxygen isotope data in quartz, and in situ sulphur isotope data of pyrite and chalcopyrite, has been used to reconstruct a complex history of overprinting, involving stages of sedimentation, diagenesis and hydrothermal activity. The host sedimentary rocks consist of detrital quartz (δ18O ∼11–18‰) and whole rock δ18O values (∼9–16‰) reflecting the combined composition of detrital and authigenic minerals, diagenetic-metamorphic exchange, chlorite and iron content. Quartz in recrystallised chemical sedimentation, quartz cementation, and quartz-barite veins at low temperatures (∼100–250 °C) involved predominantly surface and formation fluids with a wide range of fluid δ18O values between ∼ -5‰ and 2.6‰. Quartz in chloritized host rock with disseminated pyrite and chalcopyrite-galena veins at 250–320 °C reflect exchange with fluids(s) having a narrow range of δ18O values (∼5–9‰), most likely formation fluids. The fluid responsible for iron oxide, pyrite and polymetallic carbonate veins appears to be a mixture of formation and lighter surface fluids, with a range of fluid δ18O values (∼0.8–5.5‰). In situ sulphur isotopes are consistent with reduced seawater sulphate source in all samples, therefore it is likely that metal-rich formation fluids have interacted with reduced sulphate in the host sediments to precipitate as sulphide. Mineralisation and associated alteration at Abra has caused whole rock δ18O values to decrease in the deposit which may be useful as a tool for exploration in similar sediment-hosted base-metal deposits. We have shown the combination of different isotopic systems, and utilisation of in situ techniques, can constrain the sources and evolution of fluid and sulphur involved in basin formation, hydrothermal alteration and base metal mineralisation. Isotopic values can be directly related to different mineral populations within a relative temporal framework and can be used to distinguish fluids between multiple events.

Stable isotope and fluid inclusion study of sediment-hosted stratiform copper deposits from the Neuquén Basin, Argentina

The Los Chihuidos and El Porvenir deposits of the Neuquen Basin are examples of sediment-hosted stratiform Cu deposits generated by interaction of hydrocarbons and formation water with host red beds and metal-charged basinal brines. During early diagenesis of red beds precipitated hematite, kaolinite at 60 °C from meteoric water followed by calcite 1 (δ13C − 8 to − 3.3‰) and barite (δ34S + 4.1‰) with increasing temperature (75–85 °C) from an evolved surface fluid (δ18Ofluids + 0.9 to + 2.7‰). During the Tertiary Andean orogeny, hydrocarbons and formation water migrated and reacted with the red beds resulting in bleaching of the sandstone. Smectite (fluid δ18O − 2.2‰ and δD − 73.7‰), chlorite-smectite mixed-layer minerals (fluid δ18O − 6.9‰ and δD − 84‰), pyrite (δ34S + 10.2‰), and calcite 2 (δ13C − 12.9 to − 6.8‰) formed as by-product of the redox reactions. Calcite 2 formed from low-salinity fluids (0.4 to 5.9 wt% NaCl equiv.) at slightly higher temperatures (125–145 °C) at El Porvenir and at Los Chihuidos deposit (80–105 °C). During subsequent uplift, metal-charged basinal brines flowed into the bleached sandstone and precipitated chalcopyrite-bornite (δ34S + 12.3‰) followed by chalcocite-spionkopite (δ34S − 64 to + 4.1‰). Calcites 3a (δ13C − 19 to − 10.1‰) and 3b (δ13C − 31.4 to − 9.5‰) that accompanied Fe-Cu and Cu sulfides, respectively, formed from saline fluids (up to 21.3 wt% NaCl equiv.) at temperatures of 159–70 °C. The δ18O values (+ 5.6 to + 11.4‰) of fluids in equilibrium with calcite 3a are similar to oilfield and basinal brines. Fluids in equilibrium with calcite 3 in Los Chihuidos and calcite 3b in El Porvenir show much lighter δ18O values (− 7.3 to − 4.6‰ and − 0.6 to + 2.3‰, respectively), which suggests the involvement of methane as the main reductant and possibly meteoric water interaction.

Assessment of Factors Controlling Clumped Isotopes and δ18O Values of Hydrothermal Vent Calcites

AbstractThe clumped isotope composition of CaCO3 (Δ47) is a geochemical proxy that can provide mineral formation temperatures and, together with measured carbonate δ18O, inferred fluid δ18O values. Under natural conditions, carbonates form within a relatively wide pH range and varying growth rates which are typically not reflected in laboratory‐based calibrations (mostly ∼pH 8, moderate growth rates). A pH and growth‐rate dependence is known for oxygen isotopes and was also postulated for clumped isotopes. Theoretical predictions suggest that Δ47 values could lie between the carbonate equilibrium value and the value inherited from the dissolved inorganic carbon (predicted offset: +0.04‰ pH &lt; 4 and −0.025‰ at high pH &gt; 12). Here we test whether pH (in addition to temperature) is recorded in the carbonate clumped isotope composition using modern calcites from natural travertine‐forming streams and scales precipitated in pipes of deep geothermal wells from Italy, Hungary, and Turkey (pH: 6.1–7.5, T: 33–100°C). Although a comparison of all samples with expected equilibrium values in this pH range and known formation temperatures reveals only an insignificant Δ47 offset (0.006 ± 0.004‰, 1SE, n = 9), the clumped isotope values of samples with the highest growth rates (0.014 ± 0.007‰, 1SE, n = 5) are consistent with the theoretical prediction attributable to pH of 0.01‰. Similarly, deviations in δ18O of up to −2‰ follow a growth‐rate dependence. This field‐based study shows that pH‐related effects are mostly small for Δ47 in the subsurface environment at lower pH and that high mineral growth rates control the magnitude of this disequilibrium.

Hydrothermal evolution and isotope studies of the Baghu intrusion-related gold deposit, Semnan province, north-central Iran

The middle Eocene intrusion-related Baghu gold deposit is located in the Torud-Chahshirin segment of the Neotethyan magmatic arc of north-central Iran, Semnan province, Iran. The wallrocks at the deposit include a volcano-sedimentary sequence of calc-alkaline andesitic to dacitic lavas, marls, tuffs, and sandstones, which are intruded by epizonal granodioritic stocks and mafic dikes. Zircon U-Pb secondary ion mass spectrometry dating of a number of the granodiorite and micro-granite bodies yields an age of ca. 43.4 ± 1.3 Ma. Mineralization is in the form of subparallel sheeted quartz veins that are up to ∼1 km long within extensional faults. The ore-forming process included early potassic alteration with quartz + adularia + biotite + pyrite + chalcopyrite + native gold veins (average 0.25 g/t), and late phyllic alteration with quartz + sericite + pyrite + chalcopyrite + bornite + sphalerite + native gold veins (average 1.5 g/t). Supergene kaolinitization is also extensive in the Baghu area and has locally replaced up to 50 percent of the hypogene sulfide minerals. Turquoise is a diagnostic supergene phase, and locally, the iron oxy-hydroxides replacing the sulfides contain up to 57 g/t Au. The current hypogene and supergene resource estimate for the Baghu Au-Cu deposit is approximately 3.5 million tonnes averaging 1.17 g/t Au.Fluid inclusions in quartz from phyllic stage vein samples include monophase aqueous (I), monophase gas-rich (II), aqueous liquid + vapor (III), mixed aqueous-carbonic (IV), and multi-solid (V) inclusion types. The average homogenization temperatures and salinities of fluid inclusion assemblages from type III and IV inclusions range from 180 to 382 °C, with a mode at about 330 °C, and 4.2–5.4 wt% NaCl equiv., respectively. The type IV inclusions showed clathrate melting temperatures between +3.1 and +8.5 °C, indicating salinities of 2.8–11.5 wt% NaCl equiv. The type V inclusions also show a Th range from 310 to 478 °C (average 440 °C) corresponding to high salinities of 33–45 wt% NaCl equiv. The trapping pressure of the fluid inclusions was estimated to be 94–132 MPa corresponding to depths of 2.5–3.9 km during late stage of mineralization. Sericite from the stage II veins yields an 40Ar/39Ar age of ca. 44 Ma, suggesting that the mineralization at Baghu overlap granite emplacement. The δ18O of the ore-forming fluid also indicates no significant meteoric water input, despite gold deposition in a relatively shallow epizonal environment (δ18Ofluid = +8.0 to +10.3 per mil). In addition, δ34S values in stage II pyrite and chalcopyrite sulfide minerals range from +1.5 to +3.1 per mil, which is consistent with fluid saturation from a reduced magmatic source of sulfur. Historic gold production from occurrences hosted by the Torud-Chahshirin volcano-plutonic complex has been minor compared to that from orogenic gold deposits in Iran. However, identification of intrusion-related gold deposits in the complex indicates that more thorough exploration in the Eocene arc of north-central Iran could be promising.

Testing clumped isotopes as a reservoir characterization tool: a comparison with fluid inclusions in a dolomitized sedimentary carbonate reservoir buried to 2–4 km

Abstract Constraining basin thermal history is a key part of reservoir characterization in carbonate rocks. Conventional palaeothermometric approaches cannot always be used: fluid inclusions may be reset or not present, while δ 18 O palaeothermometry requires an assumption on the parent fluid composition. The clumped isotope palaeothermometer, however, is a promising technique for constraining the thermal history of basins. In this study, we test if clumped isotopes record temperatures of recrystallization in deeply-buried dolomitic reservoirs, through comparison with fluid-inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitized sedimentary carbonate hydrocarbon reservoir. It provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of c. 2000–4000 m below seafloor (mbsf) and the constituent dolomites are relatively homogeneous. Across this depth range, fluid-inclusion homogenization temperatures for the Pinda Formation record a range of temperatures from c. 110 to 170°C, increasing with depth. These closely match present-day ambient well temperatures, indicating recent resetting of the fluid inclusions. Clumped isotopes, however, record temperatures significantly ( c. 20–60°C) below fluid-inclusion and well temperatures for the seven samples analysed. The deepest five samples ( c. 2800–3700 mbsf) record clumped isotope temperatures of around 100–120°C, interpreted to represent a deep burial recrystallization event responsible for a massive (re)dolomitization of the reservoir. The lower clumped isotope temperatures (65 and 82°C) of the shallower (2055 and 2740 mbsf) samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallization of an early shallow dolomite. Determination of temperature through clumped isotopes allows calculation of the parent fluid δ 18 O values. In the five deepest samples, the fluid δ 18 O values of 3.7–6.5‰ cluster around the modern-day porewater composition (5‰), suggesting that burial dolomitization occurred in the presence of evolved brine. Mineral δ 18 O values of c . −7 to −4.5‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallization. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallization events. This study shows that clumped isotopes are a valuable tool in characterizing the thermal history of deeply-buried (&gt;2000 m) carbonate hydrocarbon reservoirs.

Fluid origin and evolution of Cu-Pb-Zn mineralization in rhyolite breccias in the Lón area, southeastern Iceland

Iceland, the landward extension of the Mid-Atlantic Ridge, hosts dilute, predominantly meteoric hydrothermal systems that rarely form base metal (Cu-Pb-Zn) mineralization. One occurrence of Cu-Pb-Zn mineralization in intrusive rhyolitic breccias is in the Lón area of southeastern Iceland. Petrographic, electron-probe, fluid inclusion, stable isotope, and U-Pb zircon dating analyses on samples from Lón constrain the conditions and timing of sulfide mineral formation.Observations of outcrops and hand samples suggest that hydrothermal fluids precipitated chalcopyrite, sphalerite, galena, quartz, epidote, chlorite and calcite in rhyolitic breccia pipes and adjacent basalt flows. The mean salinity of liquid-dominated, multi-phase fluid inclusions in quartz coeval with chalcopyrite inclusions in quartz is 4.2wt% NaCl, and the mean trapping temperature is 332°C, which is consistent with the results of δ34S geothermometry of coexisting galena and chalcopyrite. Calculated δ18O and δD values of fluids in equilibrium with epidote coeval with chalcopyrite range from −5.2 to −2.7±0.1‰ and from −35.9 to −27.7±0.1‰, respectively. The δ18O values of fluids in equilibrium with quartz coeval with chalcopyrite are up to 5‰ larger than those of fluids in equilibrium with late stage quartz precipitated after chalcopyrite. The U-Pb crystallization age of magmatic zircons in the rhyolite breccia is 2.6±0.1Ma, significantly younger than the proximal 3.7 to 7.3Ma silicic intrusions of southeastern Iceland.Our results indicate that early-stage, mineralizing fluids derived from a mixture of meteoric water, seawater, and a minor magmatic water component exsolved from an evolved anatexis-produced melt. Late-stage fluids were derived exclusively from meteoric waters. Although anatectic dehydration melting of altered basalt produced millions of years of felsic magmatism in southeastern Iceland, only hydrothermal fluids that derived from a mixture of meteoric water, seawater, and brine exsolved from a highly evolved melt concentrated base metals in significant quantity to produce base metal sulfide mineralization.

Genetic nature of apatite–magnetite ore in North Gurvunur deposit, western Transbaikal region

The paper gives a mineralogical and geochemical characterization of the North Gurvunur deposit, which was discovered in the Eravna ore district. The ore is composed of apatite–magnetite paragenesis. Apatite is distinguished by elevated LREE concentrations; some of them are contained in emulsion-type impregnation of monazite. Hematitization, carbonate, quartz, and pyrite veinlets formed at the postore stage, and gypsum–anhydrite mineralization is widespread in the supraore sequence. Two groups of endogenic minerals are distinguished by oxygen isotopic composition. One of them comprises magnetite and apatite, which are characterized by a homogeneous composition throughout the section of the ore lode and are close to the mantle source. The oxygen–isotope temperature calculated for the apatite–magnetite couple (620–800°C) provides evidence for magmatic origin of ore. The δ18O of fluid in equilibrium with hematite is 8.0–8.5‰ and shows a certain enrichment in crustal component; carbonates of postore veinlets reveal participation of meteoric water. The study has made it possible to refer the North Gurvunur deposit to the Kiruna type.

Last glacial and Holocene stable isotope record of fossil dripwater from subtropical Brazil based on analysis of fluid inclusions in stalagmites

The stable isotope composition of fossil dripwater preserved in stalagmites fluid inclusions is a promising tool to reconstruct paleopluviosity in the tropics and subtropics. Here we present δD and δ18O records of fossil dripwater from two stalagmites collected in Botuverá Cave (subtropical Brazil), covering the last glacial period and the Holocene. The observed millennial-scale variability of fossil dripwater δD and δ18O values is of the order of 10 to 20‰ and 1 to 3.5‰ (V-SMOW), respectively. Fluid inclusion δD and δ18O values are higher than the weighted mean δD and δ18O values of modern precipitation, suggesting a possible intensification of extratropical cold surges during both the last glacial period and the Early Holocene, followed by a progressive Late Holocene transition towards modern pluviosity conditions. Fluid inclusions δ18O values vary in parallel with stalagmite δ18O values during the Holocene but not during the last glacial period. This suggests that the δ18O value of precipitation may not be the only dominant factor controlling the δ18O values of the analyzed stalagmites. Isotope disequilibrium effects arising from Prior Calcite Precipitation (PCP) possibly affected stalagmites δ18O values to a greater extent than previously considered, though this effect does not bias non-quantitative paleopluviosity reconstructions based on stalagmite δ18O values. Isotope-based paleotemperature calculated for the Late Holocene matches modern cave temperature (19°C) within ±2.4°C. This encourages further application of isotope analyses of fluid inclusion for climatic reconstructions.

Genesis of the Pb−Zn deposits of the Qingchengzi ore field, eastern Liaoning, China: Constraints from carbonate LA−ICPMS trace element analysis and C–O–S–Pb isotopes

The Qingchengzi ore field is an important Pb–Zn polymetallic ore district located in northeastern China, where more than ten Pb–Zn deposits and several Au-Ag deposits occur. The Pb–Zn mineralization is mainly hosted in the marble of the Dashiqiao Formation. Three distinct mineralization styles (strata-bound, open-space-filling and transitional type) are recognized. Two genetic models have been proposed to explain the Pb–Zn mineralization in this district: the sedimentary-exhalative model and the superimposed multi-stage model, in which syn-sedimentary deposition is overprinted by metamorphic and magmatic hydrothermal modification. Based on these three mineralization styles, this study provides new C–O–S–Pb isotopic data and LA–ICPMS trace element analyses of carbonates in order to place more constraints on the genesis of the Pb–Zn deposits.The δ13CPDB values of the syn-ore carbonates range from −5.11‰ to −1.06‰, and their δ18OSMOW values range from 3.30‰ to 13.2‰; these values are distinctively lower than those of barren meta-sedimentary rocks (δ13C=−1.98‰–−0.55‰, δ18O=19.69‰−22.76‰). The depleted C–O isotopic values of syn-ore carbonates are inferred to result from fluid-rock interactions combined with the impact of graphite. In situ LA–ICPMS trace element analyses show that syn-ore carbonates record enriched REE contents, pronounced positive Eu anomalies and lower Y/Ho ratios compared to barren carbonates. The depleted C–O isotopic signatures and distinctive REE characteristics of syn-ore carbonates indicate that the hydrothermal fluid is likely magmatic in origin and has mixed with low δ18O fluids, such as meteoric or formation water. Sulfide minerals have δ34S values ranging from 3.16‰ to 9.14‰ and display a gradually increasing trend from open-space-filling to transitional type to strata-bound mineralization styles, which reflect different mixing proportions of the two end-members of magmatic sulfur and sedimentary sulfur. The Pb isotopic compositions of sulfide minerals are relatively uniform (206Pb/204Pb=17.511–17.883, 207Pb/204Pb=15.549–15.64 and 208Pb/204Pb=37.670–38.178) for different mineralization styles; however, the open-space-filling mineralization records relatively lower lead isotope values. The linear correlation of ore lead isotopes between schist and Triassic intrusions indicates that lead may have originated from a mixture of magmatic and sedimentary end-members. The relatively lower Pb isotopes of the open-space-filling mineralization suggest the presence of higher proportions from a magmatic source. Mineralization styles respond to diverse ore-controlling structures and their proximity to the mineralizing source: open-space-filling mineralization shows the least stratigraphic control, with more pronounced “magmatic marks” (i.e., lower S–Pb isotopic values and more depleted C–O values), compared to strata-bound mineralization.These mineralization styles, combined with C–O–S–Pb isotopic geochemistry, consistently preclude a SEDEX origin for the Pb–Zn deposits in the Qingchengzi district, but suggest that they represent a distal hydrothermal mineralization type related to Triassic magmatic activity.

Local distribution of oxygen isotopes and fluid exchange during genesis of the corundum-bearing rocks of Khitostrov Island

New data are presented on the distribution of oxygen isotopes and conditions of the local isotope equilibrium in high-Al rocks rocks of Khitostrov Island showing abnormally low δ18O values (below–25‰). The temperatures of isotope equilibrium are within 400–475°C. The minimum δ18O values have been registered in the in plagioclase, whereas the same phases in kyanite-bearing rocks lacking corundum demonstrate δ18O values usually 3–5‰ higher. The fluid δ18O value varies from–22 to–16‰ at 475 ± 15°C, from–18 to–23‰ at 425 ± 25°C, and from–17 to–22‰ at 380 ± 15°C. The results obtained do not require abnormal depletion of δ18O values owing to the infiltration of an external fluid under the Svecofennian transformations. The association of corundum-bearing rocks with the basic intrusions, the presence of zircon cores of older ages compared to these rocks, and the peculiarities of rock chemistry may be ascribed to the fact that lower crustal layers of ancient rocks depleted in δ18O before metamorphism were captured by basite melts.

Evaluating formation fluid models and calibrations using clumped isotope paleothermometry on Bahamian dolomites

The use of stable oxygen isotopes to understand the mechanisms of dolomite formation has been hampered by the inability to precipitate well-ordered dolomite under normal Earth surface conditions. Several studies have attempted to address this problem, either by precipitating high-temperature ordered dolomites and extrapolating the data to low temperatures or by using more disordered very-high Mg-calcites as a proxy for low temperature dolomites. The result is eight equations that disagree significantly from each other (by as much as ∼3.6‰ in the δ18O value of the precipitating fluid at 25°C), and produce differences which can greatly affect the interpretation of the formation mechanisms for low temperature dolomites. However, by combining the recently developed clumped-isotope paleothermometer, an independent isotopic measurement (∆47) that directly relates to the temperature of formation, to Late Miocene to Pleistocene aged dolomites from the Bahamas with a well-constrained thermal and fluid history, we have attempted to narrow down the viable equations used to interpret the δ18O values of dolomites.The clumped-isotope temperatures measured on the Bahamian dolomites (16–37°C) agrees with the range of temperatures expected in the Bahamas. Pairing these temperatures with geological and mineralogical arguments, we favor the equation suggested by Matthews and Katz (1977), as it is the only one that produces realistic δ18O fluid values across the range of clumped-isotope temperatures. Both the clumped-isotope temperatures and δ18O values of the precipitating fluid show a strong positive covariance that we have interpreted as reflecting the mixing of surface brines that have undergone varying amounts of evaporation and normal seawater. The different mechanisms driving these fluids included formation by normal marine seawater driven by the compensatory flow of the mixing zone, bank wide Kohout convection, and evaporative brine reflux.

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation, southeastern California

ABSTRACTStable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.