SMOW Values Research Articles

Naryn-Gol Creek Sapphire Placer Deposit, Buryatia, Russia

A new gem corundum occurrence has been discovered in the Naryn-Gol Creek placer of the Dzhida volcanic field (Russia). In this placer deposit, sapphire associates with large crystals of garnet, spinel, augite, olivine, enstatite, ilmenite, Ti-magnetite, and alkali feldspar. Such a combination of minerals is typical for the placer deposits associated with alkali basalts widely distributed in Southeastern Asia and Australia. We have also found sapphire crystals in phonotephrites of the nearby Cenozoic alkali-basalt paleovolcano Barun Khobol Pravyi, and in basalt sample and trachybasalt from the valley flood basalts. The chemical composition of sapphire is generally typical for ‘basalt’ corundum: it is rich in Fe, and depleted in Ti and Cr. The δ18O SMOW values of corundum and related megacrysts range from 4.6 to 6.8 ‰, thus corresponding to the isotopic signature of igneous rocks. Etched and corroded surfaces of sapphire and other megacrysts indicate that they are in non-equilibrium with their host alkali basalts. Volatile components, CO2 in particular, played a significant role during sapphire formation as gas inclusions reveal.

AGE OF THE KINTEREP FORMATION OF THE NORTHWESTERN SALAIR: CHEMOSTRATIGRAPHY AND U-Pb ZIRCON DATING

A comprehensive study of the stratotype section of the Kinterep formation of the North-Western Salair was carried out, including geochemical and isotope (Sr, C, O) studies of carbonate rocks, and U-Pb dating (LA-ICP-MS) of igneous zircon crystals from tuffites. The studied carbonate rocks are pure limestones with Mg/Ca less than 0.007 and a low proportion of insoluble residue (average 5 %). Kinterep limestones are characterized by δ 18 O SMOW values from 19.8 to 23.8 ‰ and δ 13 С PDB values from –0.7 to +0.9. The Sr isotope composition in limestones of the Kinterep formation varies in a narrow range of 87 Sr/ 86 Sr values between 0.70851 and 0.70859. Comparison of the obtained isotopic characteristics ( 87 Sr/ 86 Sr and δ 13 С PDB ) of Kinterep formation limestones with the generalized global 87 Sr/ 86 Sr variation curve and δ 13 С PDB values in the pale-ocean suggests two equally probable interpretations of the time of accumulation of Kinterep formation limestones: 550–540 and 525–510 Ma. U-Pb dating of magmatic zircon crystals from tuffite, which forms an interlayer among limestones of the Kinterep formation, showed the age at the boundary of ~515 Ma. Thus, using a combination of isotope (Sr, C) chemostratigraphy of carbonate rocks and U-Pb dating of igneous zircon crystals from tuffites syngenetic to limestones, the age limit of 525–510 Ma was established for the time of the formation of the Kinterep formation of the Salair basin. Carbonate rocks of similar age and similar features of the isotopic composition are known in neighbouring areas (for example, the Kuznetsk Alatau) and in regions of the Siberian Platform and microcontinents of Central Asia (Tuva-Mongolian and Dzabkhan) spatially distant from the Salair basin.

Chemical and isotope composition of the oilfield brines from Mishrif Formation (southern Iraq): Diagenesis and geothermometry

This paper focuses on the geochemical composition and isotope geochemistry of brines in the Cenomanian–Turonian carbonate Mishrif reservoir of southern Iraq. Main dissolved constituents, trace elements, δ2H and δ18O, 87Sr/86Sr, mineral saturation indices and thermodynamic calculations were investigated in formation waters from the Mishrif Formation to obtain a better understanding of brine evolution and diagenetic effects over geological time. Previous published δ11B data were also reinterpreted as a geothermometer tool. The results are compared with previous published data for local oilfields and coeval formations in the Arabian Gulf. The Mishrif brine has a marine origin and is diagenetically modified to Ca-excess and Na-deficit. Formation waters are quartz supersaturated and are in equilibrium with chalcedony and calcite-dolomite in the temperatures range of 50–75 °C, which is also confirmed by calcite-water oxygen isotope fractionation and δ11B geothermometer. The potential role of clays in conditioning brine chemistry during diagenetic processes was highlighted by activity diagrams; in particular, their adsorption/exchange effect on sodium could explain the lower temperature obtained by the Na/Li geothermometer (42 ± 6 °C). The δ2H and δ18O values show that oxygen isotope composition of the brines was isotopically more affected by interaction with limestone during diagenesis than seawater evaporation. The main effect is an 18O-enrichment on the brine starting from the SMOW value. Locally, dilution by present-day meteoric water was also detected (Rumaila South), which is shifted towards the local meteoric water line. The strontium isotope ratios range from 0.707713 to 0.707749 and correspond to a marine strontium of late Cenomanian–early Maastrichtian age, except for the Majnoon sample, which shows a more radiogenic value (0.708043). Radiogenic strontium and gypsum and anhydride saturation indices of the Majnoon sample could indicate the contribution of calcium and sulphate from the strontium-rich sulphate minerals of the Cambrian salt domes occurring in the oilfields of southern Iraq. The higher manganese concentration (4 mg/l) and the slightly higher temperature inferred by geothermometers (up to 74 °C) in comparison with present-day could indicate that the Majnoon brine is a hot fluid, probably related to a deeper structure such as the Zagros Foredeep Fault.

Ore geology, fluid inclusions, and C–H–O–S–Pb isotopes of Nagengkangqieergou Ag‐polymetallic deposit, East Kunlun Orogen, NW China

Several large Ag‐polymetallic deposits have been newly discovered in the eastern segment of the East Kunlun Orogen, with the largest one being the high‐grade Nagengkangqieergou deposit (3,017 tons Ag at 310.04 g/t). The Ag ore veins are hosted in the Palaeoproterozoic Jinshuikou Group and Upper Triassic Elashan Formation, principally along NW‐trending faults and volcanogenic ring fractures. Alteration and mineralization processes occurred in two periods (J and E), which are hosted in two different ore‐bearing sequences (Jinshuikou Group and Elashan Formation respectively). These two periods can be subdivided into Stages J‐I, J‐II, J‐III, and J‐IV and Stages E‐I, E‐II, and E‐III. Fluid inclusions in the ore‐bearing quartz and calcite comprise predominantly of vapour‐rich two‐phase (vapour + liquid) type and minor pure‐liquid type. Fluid inclusions in Stage J quartz and calcite homogenized mainly at 270–330°C, 250–310°C, 250–290°C, and 170–250°C, with salinities of 5–6 wt.%, 4–6 wt.%, 4–5 wt.%, and 3–5 wt.% NaCl equiv., respectively. Fluid inclusions in Stage E quartz homogenized mainly at 210–250°C, 170–230°C, and 150–170°C, with salinities of 6–7 wt.%, 4–5 wt.%, and 1–2 wt.% NaCl equiv., respectively. δ13C and δ18OSMOW values of the calcite samples are of −6.0‰ to −2.8‰ and 5.2‰ to 20.5‰, respectively. The results indicate a mantle‐derived magmatic source and low temperature alteration. and δDV–SMOW values of the quartz and calcite from the Jinshuikou Group vary from −1.9‰ to 9.0‰ and −116.5‰ to −69.3‰, respectively, whilst those of quartz from the Elashan Formation vary from −11.6‰ to 7.8‰ and −99.7‰ to −86.7‰. The results indicate that the ore‐forming fluids are of medium‐low temperatures and low salinities and were originated from magmatic water to meteoric water. Sulfide δ34SV–CDT values of the Jinshuikou Group and the Elashan Formation are of −6.1‰ to 0.8‰ and −3.4‰ to 0.9‰, respectively, suggesting a mantle‐derived magmatic source. Sulfide 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of the J‐period ores (18.190–18.622, 15.598–15.725, and 38.383–39.103, respectively) and E‐period ores (18.408–18.420, 15.746–15.758, and 38.879–38.895, respectively) are similar to those of granodiorite porphyry and Elashan Formation (sub)‐volcanic rocks, respectively, indicative of genetic connections between them. Integrating new evidence from fluid inclusions and C–H–O–S–Pb isotopes, we suggest that (a) mantle‐derived materials and their mixture with crust‐derived ones were critical to the ore formation, (b) the Nagengkangqieergou deposit was formed in a magmatic system and a (sub)‐volcanic system under the syn‐ and post‐collisional tectonics of the East Kunlun Orogen, respectively.

C and O stable isotopes and rare earth elements in the Devonian carbonate host rock of the Pivehzhan iron deposit, NE Iran

The Pivehzhan iron deposit is located at about 80km southwest of Mashhad, NE Iran. They occur within the Devonian carbonates as lenticular and massive bodies, as well as veinlets of magnetite and iron sulphides, transformed to goethite and haematite by weathering process. The hydrothermal calcite is the most important gangue mineral, which is observed in the form of veins/veinlets and open-space filling. The iron ores are accompanied by some minor elements such as Mn, Ti, Cr, and V and negligible amounts of Co and Ni. The distribution pattern of Rare Earth Elements (REEs) normalized to Post Archean Australian Shale (PAAS), which is characterized by the upward convex, as well as the positive Eu anomalies indicate the activity of reduced and acidic hydrothermal fluids. The negative Ce anomalies of host carbonates, although slight, point to the dominance of anoxic conditions during interaction with hydrothermal fluids. The hydrothermal calcite and quartz coexisting with the iron minerals contain principally fluid, which were homogenized into liquid phase. The homogenization temperature (TH (L-V) ) and the salinity of the analysed fluid inclusions range from 129°C to 270°C and from 0.4wt.% to 9.41wt.% NaCl eq., respectively. The δ 13 C PDB and δ 18 O SMOW values ranges from -2.15‰ to -5.77‰ (PeeDee Belemnite standard, PDB) and from +19.87‰ to +21.64‰ (Standard Mean Ocean Water standard, SMOW) in hydrothermal calcite veinlets occurring with iron minerals and -0.66‰ to -4.37‰ (PDB) and +15.55‰ to +20.14‰ (SMOW) within the host carbonates, respectively. The field relations and petrographic examination along with geochemical and isotopic considerations indicate that the Pivehzhan iron deposit was formed through replacement processes by reducing and acid fluids containing light carbon and oxygen isotopes. Variations in the physico-chemical conditions of hydrothermal fluids and their interaction with carbonates were the most effective mechanisms in the formation of this iron deposit. The potential source of iron was probably the basement magmatic rocks from which iron was leached by hydrothermal solutions.

Fluid inclusion and O–H–C isotopic constraints on the origin and evolution of ore-forming fluids of the Cenozoic volcanic-hosted Kuh-Pang copper deposit, Central Iran

The Kuh-Pang copper deposit (2.8 Mt @ 1.65% Cu, 0.52 g/t Au, 34 g/t Ag) is a volcanic-hosted epithermal deposit in the central part of the Late Eocene-Oligocene Urmia-Dokhtar Magmatic Arc of Iran. Three stages of fluid evolution are identified at the Kuh-Pang deposit: (i) early pre-ore stage: with homogenization temperature of 205–372 °C (298 ± 45 °C), salinity of 11.3 ± 2.9 wt% NaCl equiv., fluid trapped at ∼200 bars, (ii) main-ore stage: with homogenization temperature of 175–310 °C (253 ± 43 °C), salinity of 2.1–12.5 wt% NaCl equiv. at ∼150 bars, and (iii) late post-ore stage: with homogenization temperatures of 148 to 231 °C (199 ± 24 °C), and salinity of 1.1–9.8 wt% NaCl equiv. These data record an evolution of mineral precipitation from deeper (>2 km) to shallower environments (<500 m). Fluids trapped in the early (quartz I) and main-ore stages (quartz II) yielded δ18Ofluid values of +5.7 to +5.9‰ and +3.2 to +3.8‰, and δDfluid values of −77 to −41‰ and −84 to −60‰, respectively. These data indicate a major magmatic fluid source, with slightly increasing mixing of meteoric waters. Calcite in the late post-ore stage has δ13CV–PDB values of −5.8 to −5.6‰ and δ18OV–SMOW values of 8.2–8.6‰, with calculated δ18Ofluid and δ13Cfluid values of −1.0 to −1.4‰ and −5.4 to −5.2‰ respectively, that are also consistent with a predominantly magmatic carbon source and a significant fluid mixing by meteoric waters.

Mechanism of mineralization in the Changjiang uranium ore field, South China: Evidence from fluid inclusions, hydrothermal alteration, and H–O isotopes

The Changjiang uranium ore field, which contains >10,000tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O2, and those of the late syn-ore and post-ore stage contain H2 and CH4. The fluid inclusions in quartz of the syn-ore stage include H2O, H2O–CO2, and CO2 types, and they occur in clusters or along trails. Homogenization temperatures (Th) for the H2O–CO2 and two-phase H2O inclusions range from 106°C to >350°C and cluster in two distinct groups for each type; salinities are lower than 10wt% NaCl equiv. The ore-forming fluids underwent CO2 effervescence or phase separation at ∼250°C under a pressure of 1000–1100bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe2O3/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe2O3/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe2O3/FeO ratios in the alteration zones. The δ18OW–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δDW–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO2 effervescence.

Interstellar and Solar System organic matter preserved in interplanetary dust

AbstractInterplanetary dust particles (IDPs) collected in the Earths stratosphere derive from collisions among asteroids and by the disruption and outgassing of short-period comets. Chondritic porous (CP) IDPs are among the most primitive Solar System materials. CP-IDPs have been linked to cometary parent bodies by their mineralogy, textures, C-content, and dynamical histories. CP-IDPs are fragile, fine-grained (&lt; um) assemblages of anhydrous amorphous and crystalline silicates, oxides and sulfides bound together by abundant carbonaceous material. Ancient silicate, oxide, and SiC stardust grains exhibiting highly anomalous isotopic compositions are abundant in CP-IDPs, constituting 0.01-1% of the mass of the particles. The organic matter in CP-IDPs is isotopically anomalous, with enrichments in D/H reaching 50x the terrestrial SMOW value and 15N/14N ratios up to 3x terrestrial standard compositions. These anomalies are indicative of low T (10-100 K) mass fractionation in cold molecular cloud or the outermost reaches of the protosolar disk. The organic matter shows distinct morphologies, including sub-um globules, bubbly textures, featureless, and with mineral inclusions. Infrared spectroscopy and mass spectrometry studies of organic matter in IDPs reveals diverse species including aliphatic and aromatic compounds. The organic matter with the highest isotopic anomalies appears to be richer in aliphatic compounds. These materials also bear similarities and differences with primitive, isotopically anomalous organic matter in carbonaceous chondrite meteorites. The diversity of the organic chemistry, morphology, and isotopic properties in IDPs and meteorites reflects variable preservation of interstellar/primordial components and Solar System processing. One unifying feature is the presence of sub-um isotopically anomalous organic globules among all primitive materials, including IDPs, meteorites, and comet Wild-2 samples returned by the Stardust mission. We will present an overview of the current state of understanding of the properties and origins of organic matter in primitive IDPs.

Fluid Inclusion and Isotopic Constraints on the Origin of the Paleoproterozoic Yinachang Fe-Cu-(REE) Deposit, Southwest China

The Yinachang deposit in the Kangdian Fe-Cu metallogenic province, Southwest China, contains approximately 20 Mt ore @ 41.9 to 44.5 wt % Fe and 15 Mt ore @ 0.85 to 0.97 wt % Cu, with potentially significant REEs. Orebodies are hosted in the late Paleoproterozoic Dongchuan Group, and consist mainly of massive and banded replacement ores. The paragenetic sequence of this deposit includes pre-ore Na-(Fe) alteration (stage I), Fe-(REE) mineralization dominated by magnetite and siderite with subsidiary apatite and fluorite (stage II), and Cu-(REE) mineralization with chalcopyrite, ankerite, biotite, and subordinate apatite, fluorite, allanite, and synchysite (stage III). This sequence is similar to those of many iron oxide copper-gold (IOCG) deposits elsewhere. There are two distinct types of fluid inclusions within apatite of both stages II and III. Type 1 fluid inclusions with liquid and vapor phases have relatively low homogenization temperatures (98°–345°C) and salinities (4.7–16.2 wt % NaCl equiv), whereas type 2 fluid inclusions with one or more solid phases, a liquid phase, and a vapor phase have higher homogenization temperatures (273°–>500°C) and salinities (36.2–>59.8 wt % NaCl equiv). Fluids in sulfur isotope equilibrium with chalcopyrite of stages III have δ 34 S CDT values ranging from −2.8 to +2.7‰, consistent with a magmatic origin for the sulfur. Thus, sulfur isotopie compositions imply that magmatic-hydrothermal fluids, probably represented by the hot and saline type 2 fluid inclusions, were involved in the ore-forming process, although δ 34 S close to 0 might also mean that sulfur was leached from igneous rocks by nonmagmatic fluids. Siderite of stage II and ankerite of stage III have δ 13 C PDB values ranging from −12.4 to −7.5‰ and from −8.5 to −4.4‰, respectively. The large variation of δ 13 C PDB values reveals a strong interaction between the magmatic-hydrothermal fluids and the dolostone in the ore-hosting sequence. Calculated δ 18 O SMOW values of the fluids in stages II and III are broadly similar, and vary from 11.1 to 17.1‰ (avg = 13.5‰). Such values are higher than that of magmatic-hydrothermal fluids and thus, may indicate extensive fluid–wall-rock interaction and/or mixing of magmatic-hydrothermal fluids with shallow-level nonmagmatic fluids. This interpretation is consistent with the radiogenic Sr isotope compositions of bulk ores (( 87 Sr/ 86 Sr) i = 0.710404–0.734034) and fluorite separated from ores (( 87 Sr/ 86 Sr) i = 0.709851–0.723658). These nonmagmatic fluids are probably representative of the type 1 fluid inclusions. Bulk ores and mineral separates have ɛ Nd(t) values ranging from −8.1 to +0.4 (mostly −0.9 to +0.4), similar to the coeval mafic intrusions in the region (+2.0 to +2.8). This similarity suggests that REEs in the Yinachang deposit might have been derived dominantly from mantle-derived magmas. On the other hand, relatively low ɛ Nd(t) values (−5.5 and −8.1) of some samples indicate insignificant REE contributions from wall rocks. The Yinachang deposit is temporally associated with regional mantle-derived mafic magmatism, and may have a genetic relationship with it. Fluids derived from deep-seated magmas may have supplied abundant sulfur and carbon as well as some of the ore metals in this deposit. The mafic magmatism could also have provided heat to induce the shallow level nonmagmatic fluids to interact with the sedimentary-volcanic sequence, thus leaching additional ore metals into the ore-forming system. Fluid mixing can effectively trigger saturation and deposition of ore minerals. Our work highlights that nonmagmatic fluids are not a prerequisite for Cu-(Au-REE) mineralization, but may facilitate metal deposition in magmatic-hydrothermal IOCG systems.

Contexto geológico, estudos isotópicos (C, O e Pb) e associação metálica do depósito aurífero Tocantinzinho, domínio Tapajós, Província Tapajós-Parima

The Tocantinzinho ore deposit is located along a NW-SE-trending lineament, southwestern of Itaituba (Para, Brazil), and is the largest known gold deposit of the Tapajos Province. The host Tocantinzinho granite is essentially isotropic and dominated by syenogranites and monzogranites that have been weakly to moderately altered by hydrothermal fluids. Microclinization (earliest), chloritization, sericitization, silicification and carbonatization (latest) are the main types of alteration. Most mineralization was contemporaneous with the sericitization/silicification and is represented by sulfide- and gold-bearing veinlets which locally occur as stockwork. Pyrite, chalcopyrite, sphalerite and galena are the most common sulfides. Among the ore metals, Cu, Pb and Zn present the highest contents, but Mo, As and Bi locally show anomalous concentrations. The relationship of Au with Cu, Pb or Zn is at random and the Au/Ag ratios range from 0.05 to 0.5. The higher the sulfide contents, the higher the Au concentrations, though it occurs mainly included in pyrite. Zircon monocrystals from the Tocantinzinho granite yielded an average Pb-Pb age of 1982 ± 8 Ma and may represent an earlier event of the Creporizao magmatic arc. δ13C PDB values for calcite from the carbonatization stage fall dominantly between -3.45 and -2.29‰, being compatible with a deep crustal source that may include carbonatite reservoirs. In turn, δ18O SMOW values vary from +5.97 to +14.10‰, being indicative of magmatic derivation, although the less positive values suggest contribution from surficial waters. Unpublished fluid inclusion study reveals the presence of aquo-carbonic fluids, whose CO2 could have been dissolved in the granitic magma rather than being related to the shear zone. The available data allow the Tocantinzinho deposit to be classified as a granite-hosted, intrusion-related gold deposit.

Geochemistry and Metallogeny of Ag-Pb-Zn Veins in the Purcell Basin, British Columbia

The Mesoproterozoic Purcell Basin in Canada contains three different types of veins: (1) type 1 Pb-Zn-Ag veins are composed of Fe-rich sphalerite, pyrrhotite, galena, freibergite, and pyrite with minor quartz gangue; (2) type 2 Pb-Ag-Cu-Au veins are characterized by galena, pyrite, freibergite, and gold in a quartz gangue; and (3) type 3 Ag-Pb-Zn vein and replacement deposits contain Fe-poor sphalerite, galena, pyrite, and freibergite in a quartz-dolomite gangue that locally replaces host dolostone. The sulfur isotope composition of type 1, 2, and 3 vein and replacement deposits closely reflects their respective stratigraphic positions. Type 1 and 2 veins hosted in lower Purcell sedimentary rocks have light sulfur isotope compositions (−4.8 to +2.5‰) similar to diagenic pyrite disseminated in clastic sedimentary host rocks. Type 3 veins and the replacement deposits hosted in upper Purcell Supergroup rocks have heavy sulfur isotope compositions (8.3–17.2‰) indicative of thermo-chemical reduction of marine sulfate in carbonate host rocks. Quartz from type 1 Pb-Zn-Ag veins has δ 18 O SMOW values ranging from 7 to 17‰, whereas type 2 and 3 vein and replacement deposits have δ 18 O values ranging from 9 to 20‰. The range of δ 18 O values for type 1, 2, and 3 veins is the result of mixing of metamorphic fluids with a δ 18 O value higher than 11.5‰ and an upper crustal fluid with a δ 18 O lower than 1.2‰, in equilibrium with the host rocks during Proterozoic (type 1) and Mesozoic-Cenozoic (type 2 and 3) hydrothermal events. Galena and freibergite lead isotope compositions plot in two groups, consistent with previous studies. Type 1 Pb-Zn-Ag veins have a nonradiogenic lead signature ( 206 Pb/ 204 Pb: 16.322–16.435) and are considered Proterozoic in age. Type 2 Pb-Ag-Au-Cu veins and type 3 Ag-Pb-Zn vein and replacement deposits have a radiogenic lead signature ( 206 Pb/ 204 Pb: 17.842–19.347) and are interpreted to be Mesozoic-Cenozoic in age. Hydrothermal sericite from the type 3 Ptarmigan replacement deposit yields a 40 Ar/ 39 Ar age of 133.1 ± 0.7 Ma, indicating a Cretaceous age for type 3 deposits. Type 1 veins are interpreted to be the result of hydrothermal fluids generated during the Proterozoic East Kootenay Orogeny, whereas type 2 and 3 veins and replacement deposits are interpreted to be the result of Late Cretaceous-Paleocene tectonism.

Decoupling of O and Pb isotope systems of uraninite in the early Proterozoic Conglomerates in the Elliot Lake district

Both isotopic (Pb and O) and chemical compositions were measured by two in-situ techniques, SIMS and EPMA, for ∼20µm-diameter areas of over forty eight individual grains of uraninite in the early Proterozoic quartz pebble conglomerate uranium deposits in the Elliot Lake district, in order to constrain the origin of uraninite and its post-mineralization history. Together with textural observation by SEM, Pb isotope analyses and chemical compositions of brannerite and uranothorite, our data revealed a protracted uranium remobilization history for the uranium deposits in the Elliot Lake district.All grains of uraninite examined show consistently high Th contents, supporting detrital origin of uraninite derived from pegmatitic rocks. The unimodal distribution of Th concentration of uraninite excludes the possibility that uraninite formed by multiple pathways, where some grains are detrital and some are later diagenetic/hydrothermal in origin. The measured uraninite grains yield U–Pb discordia age of 1.8Ga, which is much younger than the depositional age of the host Huronian Basin (2.45 to 2.2Ga). This age closely corresponds with the age of peak metamorphism in the Huronian Basin, suggesting that all uraninite grains completely lost its Pb during this time.The least texturally and chemically altered grains of uraninite yield δ18O–SMOW values of −10 to −22 ‰. This range of δ18O value is lower than that expected for uraninite from granitic/pegmatitic rocks by 10 ‰. It is concluded that the oxygen isotope ratios of uraninite is completely reset during the peak metamorphism and/or by interaction with recent meteoric water. The δ18O of uraninite, however, do not show systematic variation with Th/U, Pb/U and 207Pb/206Pb ratios, indicating uraninite exchanged its O isotopes without much disturbance in its chemical composition.Among the various geochemical signatures in uraninite, high Th content is the only original signature preserved since detrital deposition of uraninite. Both Pb and O isotope systems have been disturbed in various degrees by later events. The coupling/decoupling of the composition, Pb and O isotope systematics of uraninite reflect the protracted mineralization/remobilization history of the oldest uraninite.

Oxygen isotopes in detrital zircons: Insight into crustal recycling during the evolution of the Greenland Shield

Insight into the interactions between crust and hydrosphere, through the protracted evolution of the Greenland Shield, can be provided by oxygen isotopes in the mineral remnants of its denuded crust. Detrital zircons with ages of 3900 Ma to 900 Ma found within an arkosic sandstone dike of the Neoproterozoic (?Marinoan) Moraeneso Formation, North Greenland, provide a time-integrated record of the evolution of part of the Greenland Shield. These zircon grains are derived from a wide variety of sources in northeastern Laurentia, including Paleoproterozoic and older detritus from the Committee-Melville orogen, the Ellesmere-Inglefield mobile belt, and the subice continuation of the Victoria Fjord complex. Archean zircon crystals have a more restricted range of δ 18 O SMOW values (between 7.2‰ and 9.0‰ relative to standard mean ocean water [SMOW]) in comparison to Paleoproterozoic 1800–2100 Ma grains, which display significant variation in δ 18 O SMOW (6.8‰–10.4‰). These data reflect differences in crustal evolution between the Archean and Proterozoic Earth. Through time, remelting or reworking of high δ 18 O materials has become more important, consistent with the progressive emergence of buoyant, cratonized continental lithosphere. A secular increase in the rate of crustal recycling is implied across the Archean-Proterozoic boundary. This rate change may have been a response to differences in the composition of sediments and/or the stabilization of continental crust. One Eoarchean oscillatory-zoned zircon grain, free of cracks and with concordant U-Pb systematics, has an elevated δ 18 O SMOW value of 7.8‰. This is interpreted to reflect a primary magmatic signature, supporting the presence of heavy oxygen that may be compatible with a hydrosphere on early Earth, as previously determined only from Jack Hills zircons.

High precision, high accuracy measurement of oxygen isotopes in a large lunar zircon by SIMS

Ninety-seven high precision, high spatial-resolution ion microprobe analyses of oxygen isotopes in a single, large zircon crystal from a lunar soil sample yield a surprising and unexpectedly large range in δ 18O SMOW from 4.5 to 9.2‰. Detailed consideration of sample mounting, polishing, and sample-standard orientation suggests that this variation was caused by analytical artefact(s) degrading the accuracy of δ 18O values. Careful control of these parameters effectively eliminated the variation and the two most reliable sets of analyses, obtained when the sample and standard were mounted within 2 mm in the centre of the mount, yield a combined weighted average δ 18O SMOW value of 5.70 ± 0.07‰ (95% confidence). This value is indistinguishable from bulk rock analyses of low-Ti lunar basalts and suggests that, at least under anhydrous conditions, there is minimal fractionation accompanying zircon crystallisation.

THE ARSHAN REE CARBONATITES, SOUTHWESTERN TRANSBAIKALIA, RUSSIA: MINERALOGY, PARAGENESIS AND EVOLUTION

The Arshan carbonatites in the Transbaikalia region of Siberia consist of sovite with high concentrations of the REE, Sr, Ba, and SO 3 , and low concentrations of Nb and P 2 O 5 . There was a high fugacity of oxygen and fluid activity at the time of formation of these carbonatites, and they were later altered by hydrothermal solutions. Bastnasite formed as a magmatic mineral in the carbonatites, on the basis of petrographic evidence and fluid-inclusion data. The recrystallization of calcite, the replacement of bastnasite and phlogopite, and the formation of secondary sulfates, strontianite, calcite and fluorite resulted from a hydrothermal process. The 87 Sr/ 86 Sr values of minerals in the Arshan carbonatites are higher than in many other carbonatites, but are similar to those in the continental basalts of Transbaikalia. The δ 34 S CDT values resemble those of other Transbaikalia carbonatites and alkaline rocks. The δ 13 C V–PDB isotopic values in the carbonatite plot within the field defined for primary igneous carbonatites. On the other hand, δ 18 O V–SMOW values for the Arshan bastnasite is only partially within the usual range of mantle values. The altered carbonatites are characterized by lighter δ 18 O values, probably because of the influence of meteoric water. The carbonatites were formed from an enriched source in the mantle and crystallized rapidly at shallow depth.

ASPECTOS GEOLÓGICOS, PETROGRÁFICOS E GEOQUÍMICOS DOS MÁRMORES DOLOMÍTICOS COM NÓDULOS DE QUARTZO DA SEQUÊNCIA METAVULCANO-SEDIMENTAR DE ACARAPE-CE

The Acarape metavolcano-sedimentary sequence, Ceará State, Northeastern Brazil, consists of Neoproterozoic terrigenous metasedimentary rocks containing intercalations of marbles and calcsilicate rocks, as well as of metabasic and intermediate alcaline rocks. Deformation and metamorphism under amphibolite facies conditions affected the whole set. The marbles ocurr as lens-shaped, descontinuous layers that can reach more than 50 km and show locally (in some quarries) quartz concentration along the bedding planes. Quartz occurs as i) nodulate, subspheric structures; ii) centimetric-thick, metric-lenght beds; and iii) folded structures. Lozenge- and/or monoclinic-shaped molds and inclusions of sulfates, sometimes pseudomorphosed by dolomite in quartz crystals are common, suggesting a diagenetic origin for the silica as a result of precipitation in empty spaces generated by dissolution of sulfate in an evaporation-related, shallow-water marine environment. Deposition of the sulfate occurred probably on siliceous carbonatic mud, but later geochemical changes in the sedimentation setting, characterised by variations in the pH values, have caused its dissolution and quartz precipitation. The marbles present dolomitic- to calcitic-dolomitic composition, showing discreet variations inside the calcsilicate-rock field, which are reflected in the major-element diagrams CaO-MgO-SiO2. Some of the samples that plot in the calcsilicate-rock field correspond to siliceous marbles depleted in Al2O3 (&lt;1%), Na2O (&lt;0.1%), and K2O (&lt;0.04%), incompatible for calcsilicate rocks. The rare earth elements show an irregular pattern with a slight enrichment of LREE in relation to the HREE, strong positive Ce and moderate positive Eu anomalies. δ13C values are relatively homogeneous, ranging from +1.7 and -0.1, whilst the δ 18 O SMOW values vary between +18.3 and + 25‰, but one of the samples, collected next to a recent carstification zone and affected by intemperic changes, shows strong isotopic fluctuation (δ 18 O SMOW + 27.2‰ and δ 13 C –7.6‰). The δ 13 C values from the best preserved samples are compatible with the carbonates isotopic curve for the beginning of the Neoproterozoic.

Aspectos geológicos, petrográficos e geoquímicos dos mármores dolomíticos com nódulos de quartzo da sequência metavulcano-sedimentar de Acarape-CE

The Acarape metavolcano- sedimentary sequence, Ceara State, Northeastern Brazil, consists of Neoproterozoic terrigenous metasedimentary rocks containing intercalations of marbles and calcsilicate rocks, as well as of metabasic and intermediate alcaline rocks. Deformation and metamorphism under amphibolite facies conditions affected the whole set. The marbles ocurr as lens-shaped, descontinuous layers that can reach more than 50 km and show locally (in some quarries) quartz concentration along the bedding planes. Quartz occurs as i) nodulate, subspheric structures; ii) centimetric-thick, metric-lenght beds; and iii) folded structures. Lozenge- and/or monoclinic-shaped molds and inclusions of sulfates, sometimes seudomorphosed by dolomite in quartz crystals are common, suggesting a diagenetic origin for the silica as a result of precipitation in empty spaces generated by dissolution of sulfate in na evaporation-related, shallow-water marine environment. Deposition of the sulfate occurred probably on siliceous carbonatic mud, but later geochemical changes in the sedimentation setting, characterised by variations in the pH values, have caused its dissolution and quartz precipitation. The marbles present dolomitic- to calcitic-dolomitic composition, showing discreet variations inside the calcsilicate-rock field, which are reflected in the major-element diagrams CaO-MgO-SiO2. Some of the samples that plot in the calcsilicate-rock field correspond to siliceous marbles depleted in Al2O3 (<1%), Na2O (<0.1%), and K2O (<0.04%), incompatible for calcsilicate rocks. The rare earth elements show an irregular pattern with a slight enrichment of LREE in relation to the HREE, strong positive Ce and moderate positive Eu anomalies. a 13 C values are relatively homogeneous, ranging from +1.7 and -0.1, whilst the a 18 O SMOW values vary between +18.3 and + 25‰, but one of the samples, collected next to a recent carstification zone and affected by intemperic changes, shows strong isotopic fluctuation ( a 18 O SMOW + 27.2‰ and a 13 C –7.6‰). The a 13 C values from the best preserved samples are compatible with the carbonates isotopic curve for the beginning of the Neoproterozoic.

Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb–Zn deposit, Central Zambia

The carbonate-hosted Kabwe Pb–Zn deposit, Central Zambia, has produced at least 2.6 Mt of Zn and Pb metal as well as minor amounts of V, Cd, Ag and Cu. The deposit consists of four main epigenetic, pipe-like orebodies, structurally controlled along NE–SW faults. Sphalerite, galena, pyrite, minor chalcopyrite, and accessory Ge-sulphides of briartite and renierite constitute the primary ore mineral assemblage. Cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore (willemite) and mineralized jasperoid that consists largely of quartz, willemite, cerussite, smithsonite, goethite and hematite, as well as numerous other secondary minerals, including vanadates, phosphates and carbonates of Zn, Pb, V and Cu. Galena, sphalerite and pyrite from the Pb–Zn rich massive orebodies have homogeneous, negative sulphur isotope ratios with mean δ 34S CDT permil (‰) values of − 17.75 ± 0.28 (1 σ), − 16.54 ± 0.0.27 and − 15.82 ± 0.25, respectively. The Zn-rich and Pb-poor No. 2 orebody shows slightly heavier ratios of − 11.70 ± 0.5‰ δ 34S for sphalerite and of − 11.91 ± 0.71‰ δ 34S for pyrite. The negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur. Carbon and oxygen isotope ratios of the host dolomite have mean δ 13C PDB and δ 18O SMOW values of 2.89‰ and 27.68‰, respectively, which are typical of marine carbonates. The oxygen isotope ratios of dolomite correlate negatively to the SiO 2 content introduced during silicification of the host dolomite. The depletion in 18O in dolomite indicates high temperature fluid/rock interaction, involving a silica- and 18O-rich hydrothermal solution. Two types of secondary fluid inclusions in dolomite, both of which are thought to be related to ore deposition, indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 °C, respectively. The high temperature fluid inclusions contain liquid + vapour + solid phases and have salinities of 15 to 31 eq. wt.% NaCl, whereas the low temperature inclusions consist of liquid + vapour with a salinity of 11.5 eq. wt.% NaCl. Fluid transport may have been caused by tectonic movements associated with the early stages of the Pan-African Lufilian orogeny, whereas ore deposition within favourable structures occurred due to changes in pressure, temperature and pH in the ore solution during metasomatic replacement of the host dolomite. The termination of the Kabwe orebodies at the Mine Club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite, indicate that ore emplacement occurred prior to the latest deformation phase of the Neoproterozoic Lufilian orogeny.

Emeralds from the Delbegetey deposit (Kazakhstan): mineralogical characteristics and fluid-inclusion study

AbstractThe aim of this study is to provide the first detailed mineralogical and fluid-inclusion description of emeralds from the Delbegetey deposit (Kazakhstan). The characteristic features of Delbegetey emeralds are established: they have dissolution figures on crystal faces, bluish colour and distinct colour zoning; the refractive indices are ω = 1.566–1.570, ε = 1.558–1.562, and the specific gravity is 2.65±0.005, relatively low for natural emeralds; they have very small concentrations of the impurities (Fe, Mg, Na and others) typical of other emeralds, and contain Cr and V; there is a significant preponderance of vapour in fluid inclusions of all types and there is liquid-to-vapour homogenization of primary fluid inclusions (at 395–420°C). The lattice oxygen isotope composition data obtained (δ18O SMOW value of 11.3%o) situate the deposit within the range characteristic of other granite-related emerald deposits. Emerald crystallization took place in low-density (0.40–0.55 g/cm3) aqueous fluid, with the following chemical composition (mol.%): 75.6-97.4 H2O, 0.0-18.4 CO2, 0.0-0.9 CH4, and 4.06-9.65 wt.% NaCl equiv. salinity. According to the calculated isochores, the pressure of formation of the Delbegetey emeralds can be estimated at 570–1240 bar.

Influence of localised igneous activity on cleat dawsonite formation in Late Permian coal measures, Upper Hunter Valley, Australia

Abstract Stable ( δ 13 C and δ 18 O) and radiogenic 87 Sr/ 86 Sr isotopic data have been used to investigate the origin of cleat dawsonite (NaAlCO 3 (OH) 2 ) in the Late Permian Wittingham Coal Measures of the Upper Hunter region in the Sydney Basin, New South Wales. The δ 13 C PDB values have a narrow range (− 1.7‰ to + 2.4‰), with an average of + 0.3‰, suggesting a magmatic source for the carbon. In contrast, δ 18 O SMOW values have a wide range (+ 13.6‰ to + 19.8‰), and decrease systematically with decreasing distance from a major intrusion. This systematic variation reflects establishment of localised hydrothermal cells. Water–rock interaction between fluids associated with these hydrothermal cells, and Rb-poor volcaniclastic detritus in the coal measures, produced mantle-like 87 Sr/ 86 Sr (0.705032 to 0.706464) in the dawsonite.