Carbon-oxygen isotopic geochemistry of the Yangla Cu skarn deposit,SW China: Implications for the source and evolution of hydrothermal fluids

The Yangla Cu deposit is the largest Cu skarn deposit in the Jinshajiang tectonic belt. Based on the detailed observation of crosscutting relationships, three mineralization stages (i.e., pre-ore, ore and supergene) have been identified in the Yangla deposit. The pre-ore stage is dominated by prograde skarn. The ore stage is characterized by the precipitation of hydrous silicate minerals, Fe-oxides, Fe-Cu-Mo-sulfides, quartz and calcite, whose mineral assemblages were formed in the early and late sub-ore stages. The early sub-ore stage is marked by retrograde alteration with the deposition of hydrous silicate minerals (e.g., actinolite, epidote and chlorite), Fe-oxides, abundant Fe-Cu-Mo-sulfides, quartz and minor calcite. Whilst, the late sub-ore stage, associated with silicic and carbonate alteration, is represented by widespread thick quartz or calcite veins with disseminated pyrite, chalcopyrite, galena and sphalerite. We present new carbon-oxygen (C-O) isotopic compositions of the ore-hosting marble and hydrothermal calcite of this deposit. The hydrothermal calcite in the Yangla deposit was precipitated from both the early and late sub-ore stages. Calcite I from the early sub-ore stage is anhedral, and occurs as spot in the skarn or locally replaces the skarn minerals. Calcite II from the late sub-ore stage is distinguished by being coarse-grained, subhedral to euhedral and its occurrence in thick veins. Calcite I contains lower δ¹³C_PDB (−7.0‰ to −5.0‰) and δ¹⁸O_SMOW (7.2‰ to 12.7‰) than Calcite II (δ¹³C_PDB = −4.5‰ to −2.3‰; δ¹⁸O_SMOW = 10.7‰ to 19.4‰). In the δ¹³C_PDB vs. δ¹⁸O_SMOW diagram, the Calcite I and Calcite II data fall close to the igneous carbonatite field and between the fields of igneous carbonatite and marine carbonates, respectively. This suggests a dominantly magmatic origin for the early sub-ore fluids, and there might have been increasing carbonate wall rock involvement towards the late sub-ore stage. The ore-hosting marble (δ¹³C_PDB = −4.8‰ to −0.3‰; δ¹⁸O_SMOW = 10.2‰ to 23.9‰) also shows a positive δ¹³C_PDB vs. δ¹⁸O_SMOW correlation, which is interpreted to reflect the decreasing alteration intensity during the interactions between the hydrothermal fluids and ore-hosting carbonates. Simulated calculation suggests that both the Calcite I and Calcite II precipitated at 350 °C to 250 °C and 250 °C to 150 °C, respectively. We proposed that CO₂ degassing and water/rock interactions were likely the two major processes that precipitated the calcite and led to the observed C-O isotopic features of the Yangla Cu deposit.     

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

The oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene-Pleistocene) dolomitization of these rocks. Diagenetic carbonates included early siderite (δ¹⁸O_SMOW=+24.4 to +26.5‰δ¹³C_PDB=?1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ¹⁸O_SMOW=+18.4 to +25.8‰; δ¹³C_PDB=?2.1 to +0.2‰), and calcite (δ¹⁸O_SMOW=+21.3 to +32.6‰; δ¹³C_PDB=?4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values. A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ¹⁸O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post-dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ¹⁸O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.     

Carbon and Oxygen Isotope Compositions of Calcite and Rhodochrosite from Geothermal Exploratory Drills TH‐4 and TH‐6 near the Toyoha Deposit,Hokkaido, Japan

We studied calcite and rhodochrosite from exploratory drill cores (TH‐4 and TH‐6) near the Toyoha deposit, southwestern Hokkaido, Japan, from the aspect of stable isotope geochemistry, together with measuring the homogenization temperatures of fluid inclusions. The alteration observed in the drill cores is classified into four zones: ore mineralized zone, mixed‐layer minerals zone, kaolin minerals zone, and propylitic zone. Calcite is widespread in all the zones except for the kaolin minerals zone. The occurrence of rhodochrosite is restricted in the ore mineralized zone associated with Fe, Mn‐rich chlorite and sulfides, the mineral assemblage of which is basically equivalent to that in the Toyoha veins. The measured δ¹⁸O_SMOW and δ¹³C_PDB values of calcite scatter in the relatively narrow ranges from ?2 to 5‰ and from ?9 to ?5‰, respectively; those of rhodochrosite from 3 to 9‰ and from ?9 to ?5‰, excluding some data with large deviations. The variation of the isotopic compositions with temperature and depth could be explained by a mixing process between a heated surface meteoric water (100°C δ¹⁸O =?12‰, δ¹³C =?10‰) and a deep high temperature water (300°C, δ¹⁸O =?5‰, δ¹³C =?4‰). Boiling was less effective in isotopic fractionation than that of mixing. The plots of δ¹⁸O and δ¹³C indicate that the carbonates precipitated from H₂CO₃‐dominated fluids under the conditions of pH = 6–7 and T = 200–300°C. The sequential precipitation from calcite to rhodochrosite in a vein brought about the disequilibrium isotopic fractionation between the two minerals. The hydrothermal fluids circulated during the precipitation of carbonates in TH‐4 and TH‐6 are similar in origin to the ore‐forming fluids pertaining to the formation of veins in the Toyoha deposit.     

Genesis and Metallogenic Process of the Lujing Uranium Deposit,Southwest Jiangxi Province,China: Constraints of Micropetrography and S–C–O Isotopes

The Lujing uranium deposit, located in the southeastern part of the Nanling metallogenic province, is one of the representative granite‐related hydrothermal uranium deposits in South China. Basic geology, geochemistry, and geochronology of the deposit have been extensively studied. However, there is still a chronic lack of systematic research on the genesis and metallogenic process of the deposit. Thus, we recently carried out an electron microprobe and stable isotopic analysis. The main research results and progresses are as follows: Uranium minerals in this deposit include coffinite, pitchblende, and uranothorite, and small amounts of uranium exist in accessory minerals in the form of isomorphism. Coffinite, which occurs predominantly as the pseudomorphs after pitchblende, also occurs as a primary mineral and is locally formed from the remobilization of uranium from adjacent uranium‐bearing minerals. The mineralizing fluid was originally composed of a magmatic fluid generated by late Yanshanian magmatism. The high As content of pyrite in ores may reflect the addition of meteoric water, or the formation water (or both), to the magmatic hydrothermal system. The δ³⁴S values vary from −14.4‰ to 13.9‰ (mean δ³⁴S = −3.9‰), showing a range that is similar to nearby Cambrian metamorphic strata and Indosinian granites, indicating that these host rocks represent the source of sulfur; however, the possibility of a mantle source cannot be completely ruled out. According to our new isotopic data and recent Pb isotopic data, we conclude that the uranium in ores was derived by leaching dominantly from the uranium‐rich host rocks, especially the Cambrian metamorphic strata. The δ¹³C_PDB values (−8.75‰ to 1.40‰; mean δ¹³C_PDB = −5.41‰) and δ¹⁸O_SMOW values (5.45–18.62‰; mean δ¹⁸O = 13.02‰) of reddish calcite from the ore‐forming stage suggest that the CO₂ in the mineralizing fluids was derived predominantly from the mantle, with a small component contributed by marine carbonates. Based on these new data and previous research results, this paper proposes that uranium metallogenesis in the Lujing deposit is closely associated with mafic magmatism resulting from crustal extension during the Cretaceous to Paleogene in South China.     

Geology and Isotope Geochemistry of the Dongzhongla Pb–Zn Deposit in Tibet: Implications for the Origin of the Ore‐Forming Fluids and Storage Condition of Certain Metals

As a newly discovered medium‐sized deposit (proven Pb + Zn resources of 0.23 Mt, 9.43% Pb and 8.73% Zn), the Dongzhongla skarn Pb–Zn deposit is located in the northern margin of the eastern Gangdese, central Lhasa block. Based on the geological conditions in this deposit of ore‐forming fluids, H, O, C, S, Pb, Sr, and noble gas isotopic compositions were analyzed. Results show that δ¹⁸O_SMOW of quartz and calcite ranged from −9.85 to 4.17‰, and δD_SMOW ranged from −124.7 to −99.6‰ (where SMOW is the standard mean ocean water), indicating magma fluids mixed with meteoric water in ore‐forming fluids. The δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from −1.4 to −1.1‰ and from 5.3 to 15.90‰, respectively, show compositions consistent with the carbonate limestone in the surrounding rocks, implying that the carbon was primarily sourced from the dissolution of carbonate strata in the Luobadui Formation. The ore δ³⁴S composition varied in a narrow range of 2.8 to 5.7‰, mostly between 4‰ and 5‰. The total sulfur isotopic value δ³⁴S was 4.7‰ with characteristics of magmatic sulfur. The ³He/⁴He values of pyrite and galena ranged from 0.101 to 5.7 Ra, lower than those of mantle‐derived fluids (6 ± 1 Ra), but higher than those of the crust (0.01–0.05 Ra), and therefore classified as a crust–mantle mixed source. The Pb isotopic composition for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of the ores were in the ranges of 18.628–18.746, 15.698–15.802, and 39.077–39.430, respectively, consistent with the Pb isotopic composition of magmatic rocks in the deposit, classified as upper‐crust lead. The ore lead was likely sourced partially from the crustal basement of the Lhasa Terrane. The initial (⁸⁷Sr/⁸⁶Sr)_i value from five sulfide samples ranged from 0.71732 to 0.72767, and associated ore‐forming fluids were mainly sourced from the partial melting of the upper‐crust materials. Pb isotopic compositions of ore sulfides from the Dongzhongla deposit are similar to that of the Yuiguila and Mengya'a deposit, indicating that they have similar sources of metal‐rich ore‐forming solution. According to basic skarn mineralogy, the economic metals, and the origin of the ore‐forming fluids, the Dongzhongla deposit was classified as a skarn‐type Pb–Zn deposit.     

Stable isotope (O and C) geochemistry of non-sulfide Zn–Pb deposits; case study: Chah-Talkh non-sulfide Zn–Pb deposit (Sirjan,south of Iran)

The study of oxygen and carbon isotopic ratios has gained importance to determine the origin of ore-bearing fluids, carbon origin, and also to determine the formation temperature of non-sulfide Pb and Zn minerals. In order to determine the origin of fluids and carbon existing in Zn carbonate minerals in Chah-Talkh deposit, initially the amounts of δ¹⁸O_SMOW and δ¹³C_PDB changes in various zinc minerals in important deposits in Iran and the world were studied, and then by comparing these values in Chah-Talkh deposit with those of other deposits, the origin of fluids responsible for ore forming, carbon, and formation temperature of Chah-Talkh deposit was determined. The range of δ¹⁸O_SMOW changes in smithsonite mineral in non-sulfide lead and zinc deposits varies from 18.3 to 31.6 ‰, and δ¹⁸O_SMOW in hydrozincite mineral varies from 7.8 to 27 ‰. Due to the impossibility of smithsonite sampling from Chah-Talkh deposit (due to it being fine-grained and dispersed), hydrozincite minerals which have high isotopic similarities with smithsonite are used for the isotopic analysis of carbon and oxygen. The range of δ¹⁸O_SMOW changes in hydrozincite mineral of Chah-Talkh deposit varies from 7.8 to 15.15 ‰, which places in the domain of metamorphic water. The extensiveness of δ¹⁸O_SMOW changes in Chah-Talkh indicates the role of at least two fluids in the formation of non-sulfide minerals. The obtained formation temperature of non-sulfide minerals (hydrozincite) in Chah-Talkh deposit is 70 to 100 °C, which indicates the role of metamorphic fluids in the formation of deposit. Complete weathering of sulfide minerals to a depth of 134 m confirms the role of rising metamorphic fluids in the formation of non-sulfide minerals. The δ¹³C_PDB values of Chah-Talkh deposit are set in the range of atmospheric CO₂ and carbonate rocks, in which the existence of atmospheric CO₂ indicates the role of atmospheric fluids, and the existence of carbonate carbon rock indicates of the role of metamorphic fluids in the precipitation of non-sulfide Zn minerals.     

Geochemical and isotopic evidence for a magmatic-hydrothermal origin of the polymetallic vein-type Zn-Pb deposits in the northwest margin of Jiangnan Orogen,South China

Polymetallic vein-type Zn-Pb deposits are located in the Xiangxi–Qiandong zinc-lead metallogenic belt (XQMB) of the northwestern margin of the Jiangnan Orogen, South China. Ores are mainly found in fault-bounded quartz veins hosted in the upper part of the Banxi Group that consists of low-grade metamorphic sandstone, siltstone with minor tuff interbeds. The Zn-Pb deposits primarily contain sphalerite, galena, chalcopyrite and pyrite, accompanied by quartz and minor calcite. Zinc, lead, copper, indium and gallium are enriched in these ores. Investigation of the ore fluid reveals low temperature (87–262 °C) with scattered salinity (range from 2.73 to 26.64 wt% NaCl_eqv.). Hydrogen and oxygen isotopic compositions of fluid inclusions in quartz indicate mixing of magmatic hydrothermal fluid and meteoric water (δ¹⁸O_{H2O SMOW} = 0.2‰ to 4.2‰; δD_{H2O SMOW} = −126‰ to −80‰). Carbon and oxygen isotopic composition of carbonate samples indicate the magmatic hydrothermal origin of CO₃²⁻ or CO₂ in ore-forming fluid (δ¹³C_PDB = −6.9‰ to −5.7‰, δ¹⁸O_SMOW = 11.3‰ to 12.7‰). Sulfur and lead isotopic compositions (δ³⁴S_VCDT = 8.8–14.2‰ and ²⁰⁶Pb/²⁰⁴Pb = 17.156–17.209, ²⁰⁷Pb/²⁰⁴Pb = 15.532–15.508, ²⁰⁸Pb/²⁰⁴Pb = 37.282–37.546) demonstrate that sulfur sources were relatively uniform, and low radiogenic lead isotopic compositions indicate that ore metals were derived from a relatively unradiogenic source, probably by mixing of mantle with crust. Therefore, polymetallic vein-type Zn-Pb mineralization in this area probably arose from a magmatic-related hydrothermal system, and the deposition of sulfides occurred in response to cooling and boiling of magmatic hydrothermal fluids (high salinity, high δ¹⁸O_H2O and δD_H2O and metal-bearing), and is mainly the result of emplacement into open space and mixing with meteoric water (low salinity, low δ¹⁸O_H2O and δD_H2O). This study provides direct evidence that magmatism was involved in the ore-forming processes of the low temperature metallogenic district, South China, and it raises awareness about the presence of polymetallic vein-type Zn-Pb deposits in the northwest margin of Jiangnan Orogen and their potential as a source of zinc, copper, indium and gallium.     

S, C, O, H Isotope Data and Noble Gas Studies of the Maoniuping LREE Deposit, Sichuan Province, China： A Mantle Connection for Mineralization

The Maoniuping REE deposit, located about 22 km to the southwest of Mianning, Sichuan Province, is the second largest light REE deposit in China, subsequent to the Bayan Obo Fe-Nb-REE deposit in the Inner Mongolia Autonomous Region. Tectonically, it is located in the transitional zone between the Panxi rift and the Longmenshan-Jinpingshan orogenic zone. It is a carbonatite vein-type deposit hosted in alkaline complex rocks. The bastnaesite-barite, bastnaesite-calcite, and bastnaesite-microcline lodes are the main three types of REE ore lodes. Among these, the first lode is distributed most extensively and its REE mineralization is the strongest. Theδ34Sv-CDT values of the barites in the ore of the deposit vary in a narrow range of +5.0 to +5.1‰in the bastnaesite-calcite lode and +3.3 to +5.9‰in the bastnaesite-barite lode, showing the isotopic characteristics of magma-derived sulfur. Theδ13Cv-PDB values and theδ518OV-SMOW values in the bastnaesite-calcite lode range from -3.9 to -6.9‰and from +7.3 to +9.7‰, respectively, which fall into the range of "primary carbonatites", showing that carbon and oxygen in the ores of the Maoniuping deposit were derived mainly from a deep source. Theδ13Cv-PDB values of fluid inclusions vary from -3.0 to -5.6‰, with -3.0 to -4.0‰in the bastnaesite-calcite lode and -3.0 to -5.6‰in the bastnaesite-barite lode, which show characteristics of mantle-derived carbon. TheδDv-SMOW values of fluid inclusions range from -57 to -88‰, with -63 to -86‰in the bastnaesite-calcite lode and -57 to -88‰in the bastnaesite-barite lode, which show characteristics of mantle-derived hydrogen. Theδ18OH2OV-SMOW values vary from +7.4 to +8.6‰in the bastnaesite calcite lode, and +6.7 to +7.8‰in the bastnaesite-barite lode, almost overlapping the range of +5.5 to +9.5‰for magmatic water. The 4He content, R/Ra ratios are (13.95 to 119.58×10-6 (cm3/g)STP and 0.02 to 0.11, respectively, and 40Ar/36Ar is 313±1 to 437±2. Considering the 4He increase caused by high contents of radioactive elements, a mantle-derived fluid probably exists in the inclusions in the fluorite, calcite and bastnaesite samples. The Maoniuping deposit and its associated carbonatite-alkaline complex were formed in 40.3 to 12.2 Ma according to K-Ar and U-Pb data. All these data suggest that large quantities of mantle fluids were involved in the metallogenic process of the Maoniuping REE deposit through a fault system.     

The origin of the Maozu carbonate-hosted Pb–Zn deposit,southwest China: Constrained by C–O–S–Pb isotopic compositions and Sm–Nd isotopic age

The Maozu Pb–Zn deposit, located on the western margin of the Yangtze Block, southwest China, is a typical carbonate-hosted deposit in the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province with Pb + Zn reserves of about 2.0 million tonnes grading 4.15 wt.% Pb and 7.25 wt.% Zn. Its ore bodies are hosted in Sinian (635–541 Ma) Dengying Formation dolostone and show stratiform, vein and irregular textures. Ores are composed of sphalerite, galena, pyrite, calcite, dolomite, quartz and fluorite with massive, banded, disseminated and veined structures. The C–O–Sm–Nd isotopic compositions of hydrothermal calcites and S–Pb isotopic compositions of sulfides were analyzed to constrain the origin of the Maozu deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcites range from −3.7‰ to −2.0‰ and +13.8‰ to +17.5‰, respectively, and plot near the marine carbonate rocks field in a plot of δ¹³C_PDB vs. δ¹⁸O_SMOW, with a negative correlation. It suggests that CO₂ in the hydrothermal fluids was mainly originated from marine carbonate rocks, with limited influence from sedimentary organic matter. δ³⁴S_CDT values of sulfides range from +9.9‰ to +19.2‰, similar to that of Cambrian to Triassic seawater sulfate (+15‰ to +35‰) and evaporate (+15‰ to +30‰) in the Cambrian to Triassic sedimentary strata. It suggests that reduced sulfur was derived from evaporate in sedimentary strata by thermo chemical sulfate reduction. Sulfides have low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.129–18.375, ²⁰⁷Pb/²⁰⁴Pb = 15.640–15.686 and ²⁰⁸Pb/²⁰⁴Pb = 38.220–38.577) that plot in the field between upper crust and the orogenic belt evolution curve in the plot of ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb, and similar to that of age corrected Proterozoic basement rocks (Dongchuan and Kunyang Groups). This indicates that ore-forming metals were mainly derived from basement rocks. Hydrothermal calcite yields a Sm–Nd isotopic age of 196 ± 13 Ma, possibly reflecting the timing of Pb–Zn mineralization in the SYG province, younger than the Permian Emeishan mantle plume (∼260 Ma). All data combined suggests that hydrothermal fluids circulated through basement rocks where they picked up metals and migrated to surface, mixed with reduced sulfur-bearing fluids and precipitated metals. Ore genesis of the Maozu deposit is different from known magmatic–hydrothermal, Sedimentary Exhalative or Mississippi Valley-types, which maybe represent a unique ore deposit type, named as the SYG-type.     

Geology and Isotope Geochemistry of the Yinchanggou-Qiluogou Pb-Zn Deposit, Sichuan Province, Southwest China

The Yinchanggou-Qiluogou Pb-Zn deposit,located in the western Yangtze Block,southwest China,is hosted by the Upper Sinian Dengying Formation dolostone.Ore bodies occur in the Qiluogou anticline and the NS-and NNW-trending faults.Sulfide ores mainly consist of sphalerite,pyrite,galena and calcite,with subordinate dolomite and quartz.Seventeen ore bodies have been discovered to date and they have a combined 1.0 million tons of sulfide ores with average grades of 2.27wt%Zn and 6.89wt%Pb.The δD_(H2O-SMOW) and δ~(18)O_(H2O-SMOW) values of fluid inclusions in quartz and calcite samples range from-68.9‰ to-48.7‰ and 7.3‰ to 15.9‰,respectively,suggesting that H_2O in the hydrothermal fluids sourced from metamorphic water.Calcite samples have δ~(13)C_(PDB) values ranging from-6.2‰ to-4.1‰ and δ~(18)O_(SMOW) values ranging from 15.1‰ to 17.4‰,indicating C and O in the hydrothermal fluids likely derived from a mixed source of metamorphic fluids and the host carbonates.The δ~(34)S_(CDT) values of sulfide minerals range from 5.5‰ to 20.3‰,suggesting that thermal chemical reduction of sulfate minerals in evaporates were the most probable source of S in the hydrothermal fluids.The ~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb ratios of sulfide minerals fall in the range of 18.11 to 18.40,15.66 to 15.76 and 38.25 to 38.88,respectively.The Pb isotopic data of the studied deposit plot near the upper crust Pb evolution curve and overlap with the age-corrected Proterozoic basement rocks and the Upper Sinian Dengying Formation hosting dolostone.This indicates that the Pb originated from a mixed source of the basement metamorphic rocks and the ore-hosting carbonate rocks.The ore geology and C-H-O-S-Pb isotopic data suggest that the YinchanggouQiluogou deposit is an unusual carbonate-hosted,strata-bound and epigenetic deposit that derived ore-forming materials from a mixed source of the underlying Porterozoic basements and the Sinian hosting carbonates.     

Stable isotopic composition of soil calcite (O,C) and gypsum (S) overlying Cu deposits in the Atacama Desert,Chile: Implications for mineral exploration,salt sources,and paleoenvironmental reconstruction

Soils overlying two porphyry Cu deposits (Spence, Gaby Sur) and the Pampa del Tamarugal, Atacama Desert, Northern Chile were collected in order to investigate the extent to which saline groundwaters influence “soil” chemistry in regions with thick Miocene and younger sediment cover. Soil carbonate (calcite) was analyzed for C and O isotopes and pedogenic gypsum for S isotopes. Soil calcite is present in all soils at the Spence deposit, but increases volumetrically above two fracture zones that cut the Miocene gravels, including gravels that overlie the deposit. The C isotope composition of carbonate from the soils overlying fracture zones is indistinguishable from pedogenic carbonate elsewhere at the Spence deposit; all δ¹³C_VPDB values fall within a narrow range (1.40–4.23‰), consistent with the carbonate having formed in equilibrium with atmospheric CO₂. However, δ¹⁸O_VPDB for carbonate over both fracture zones is statistically different from carbonate elsewhere (average δ¹⁸O_VPDB = 0.82‰ vs. −2.23‰, respectively), suggesting involvement of groundwater in their formation. The composition of soils at the Tamarugal anomaly has been most strongly affected by earthquake-related surface flooding and evaporation of groundwater; δ¹³C_VPDB values (−4.28‰ to −2.04‰) are interpreted to be a mixture of dissolved inorganic C (DIC) from groundwater and atmospheric CO₂. At the Spence deposit, soils only rarely contain sufficient SO₄ for S isotope analysis; the SO₄-bearing soils occur only above the fracture zones in the gravel. Results are uniform (3.7–4.9‰ δ³⁴S_CDT), which is near the middle of the range for SO₄ in groundwater (0.9–7.3‰). Sulfur in soils at the Gaby Sur deposit (3.8–6.1‰ δ³⁴S_CDT) is dominated by gypsum, which primarily occurs on the flanks and tops of hills, suggesting deposition from SO₄-rich fogs. Sulfate in Gaby Sur deposit gypsum is possibly derived by condensation of airborne SO₄ from volcanic SO₂ from the nearby Andes. At the Gaby Sur deposit and Tamarugal anomaly, pedogenic stable isotopes cannot distinguish between S from porphyry or redeposited SO₄ from interior salars.The three sites studied have had different histories of salt accumulation and display variable influence of groundwater, which is interpreted to have been forced to the surface during earthquakes. The clear accumulation of salts associated with fractures at the Spence deposit, and shifts in the isotopic composition of carbonate and sulfate in the fractures despite clear evidence of relatively recent removal of salts indicates that transfer from groundwater is an ongoing process. The interpretation that groundwaters can influence the isotopic composition of pedogenic calcrete and gypsum has important implications for previous studies that have not considered this mechanism.     

Stable isotope geochemistry of magnesite from Holocene salt lake deposits,Taoudenni, Mali

Stable isotope data (δ¹⁸O, δ¹³C) were obtained for magnesite (MgCO₃) from Holocene salt lake deposits of the Taoudenni–Agorgott basin, Mali, in which it is the only carbonate mineral present. The deposits have a high glauberite content (Na₂Ca(SO₄)₂), representing a type of continental evaporite formation that is commonly magnesite‐bearing. Samples from seven different levels in a 5 m long sequence were analysed. δ¹⁸O_V‐PDB values are between +3.17 and +5.91‰, which is lower than might be expected for evaporitic environments, recording reduced ¹⁸O enrichment at high salinity. δ¹³C_V‐PDB values are between −1.32‰ and −4.79‰, showing an influence of carbon derived from the decomposition of organic matter rather than from exchange with atmospheric CO₂. Covariance between δ¹⁸O and δ¹³C is strong for the lower part of the sequence, recording coupled ¹⁸O and ¹³C enrichment as commonly observed for closed basins. Covariance is lacking for the upper part of the sequence, with only periodic massive magnesite deposition in a more shallow lake. Copyright © 2012 John Wiley & Sons, Ltd.     

Carbon and oxygen isotopes suggesting deep-water basin deposition associated with hydrothermal events （Shangsi Section,Northwest Sichuan Basin-South China）

Carbon and oxygen isotope records for Shangsi Section in Northwest Sichuan Basin, South China can help investigating depositional environments and processes, including the burial rate and possible contribution of hydrothermal events. Samples from the lower Chihsian Formation show δ13CPDB and δ18OPDB values close to those of typical marine limestone. However, the overlying Permian middle-upper Chihsian, Wujiaping, and Maokou Formation samples reveal negative δ18OSMOW values and strong positive δ13CPDB values. These indicate high biological productivity and rapid burial of organic carbon. Samples from the Dalong Formation present both negative δ13CPDB and negative δ18OPDB values, which are quite different from the underlying Permian strata. These abnormal carbon and oxygen isotope characteristics in the Dalong Formation may suggest that hydrothermal processes contributed to deposition.     

Mixed Oceanic and Freshwater Depositional Conditions for Beachrocks of Northeast Brazil: Evidence from Carbon and Oxygen Isotopes

Holocene beachrocks of Northeast Brazil are composed predominantly of quartz (90%) with minor carbonate fragments (6% algal detritus) and feldspars (4%). The cement shows three textural varieties: (1) calciferous, surrounding siliciclastic grains; (2) micritic, with an acicular fringe; and (3) cryptocrystalline calcite in pores. Sandstone structures and composition show evidence of submerged and low-energy beaches. Cement is formed by ～20 mol% MgCO₃; the δ¹³C in cement ranges from ?1.3‰ to +3.5‰ PDB and δ¹⁸O varies from ?2.1 to +1.2‰_PDB. The cement was precipitated under high CO₂ pressure, as a result of the interaction of CaCO_3^? saturated seawater and nonsaturated groundwater, in a beach environment.     

Magmatic hydrothermal origin of the Hadamengou-Liubagou Au-Mo deposit,Inner Mongolia,China: Constrains on geology,stable and Re-Os isotopes

The Hadamengou-Liubagou Au-Mo deposit is the largest gold deposit in Inner Mongolia of North China. It is hosted by amphibolite to granulite facies metamorphic rocks of the Archean Wulashan Group. To the west and north of the deposit, there occur three alkaline intrusions, including the Devonian-Carboniferous Dahuabei granitoid batholith, the Triassic Shadegai granite and the Xishadegai porphyritic granite with molybdenum mineralization. Over one hundred subparallel, sheet-like ore veins are confined to the nearly EW-trending faults in the deposit. They typically dip 40° to 80° to the south, with strike lengths from hundreds to thousands of meters. Wall rock alterations include potassic, phyllic, and propylitic alteration. Four distinct mineralization stages were identified at the deposit, including K-feldspar-quartz-molybdenite stage (I), quartz-pyrite-epidote/chlorite stage (II), quartz-polymetallic sulfide-gold stage (III), and carbonate-sulfate-quartz stage (IV). Gold precipitated mainly during stage III, while Mo mineralization occurred predominantly in stage I. The δD_H2O and δ¹⁸O_H2O values of the ore-forming fluids range from −125‰ to −62‰ and from 1.4‰ to 7.5‰, respectively, indicating that the fluids were dominated by magmatic water with a minor contribution of meteoric water. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal carbonate minerals vary from −10.3‰ to −3.2‰ and from 3.7‰ to 15.3‰, respectively, suggesting a magmatic carbon origin. The δ³⁴S_CDT values of sulfides from the ores vary from −21.7‰ to 5.4‰ and are typically negative (mostly −20‰ to 0‰). The wide variation of the δ³⁴S_CDT values, the relatively uniform δ¹³C values of carbonates (typically −5.5‰ to −3.2‰), as well as the common association of barite with sulfides suggest that the minerals were precipitated under relatively high fo₂ conditions, probably in a magmatic fluid with δ³⁴S_ƩS ≈ 0‰. The Re-Os isotopic dating on molybdenite from Hadamengou yielded a weighted average age of 381.6 ± 4.3 Ma, indicating that the Mo mineralization occurred in Late Devonian. Collectively, previous ⁴⁰Ar-³⁹Ar and Re-Os isotopic dates roughly outlined two ranges of mineralizing events of 382–323 Ma and 240–218 Ma that correspond to the Variscan and the Indosinian epochs, respectively. The Variscan event is approximately consistent with the Mo mineralization at Hadamengou-Liubagou and the emplacement of the Dahuabei Batholith, whereas the Indosinian event roughly corresponds to the possible peak Au mineralization of the Hadamengou-Liubagou deposit, as well as the magmatic activity and associated Mo mineralization at Xishadegai and Shadegai. Geologic, petrographic and isotopic evidence presented in this study suggest that both gold and molybdenum mineralization at Hadamengou-Liubagou is of magmatic hydrothermal origin. The molybdenum mineralization is suggested to be associated with the magmatic activity during the southward subduction of the Paleo-Asian Ocean beneath the North China Craton (NCC) in Late Devonian. The gold mineralization is most probably related to the magma-derived hydrothermal fluids during the post-collisional extension in Triassic, after the final suturing between the Siberian and NCC in Late Permian.     

A study of oxygen isotopic fractionation during bio-induced calcite precipitation in eutrophic Baldeggersee,Switzerland

In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ¹⁸O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in ¹⁸O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (∼8.6) and increased [CO₃²⁻] (∼50 μmol/l) in the surface waters. The resulting weighted average δ¹⁸O value for the studied period is −9.6‰, compared with a calculated equilibrium δ¹⁸O value of −9.4‰. These data convincingly demonstrate that δ¹⁸O of calcite are, for the most part, a very reliable proxy for temperature and δ¹⁸O of the water.     

Environmental setting, (micro)morphologies and stable C–O isotope composition of cold climate carbonate precipitates—a review and evaluation of their potential as paleoclimatic proxies

Given the growing interest in carbonate deposits from polar regions as paleoclimatic proxies, this review paper first provides a classification of the various types of cold-climate carbonate precipitates followed by a summary of the ¹³C and ¹⁸O composition of the carbonate deposits and parent water from which the carbonates precipitated. The cold-climate carbonate precipitates were classified into three broad categories: powders, crusts and speleothem. The carbonate powders include those that precipitated in relation to aufeis aggradation (cryogenic aufeis calcite) and in relation to the growth of various annual/perennial ice formations in freezing caves (cryptocrystalline calcite and calcite pearls). The carbonate crusts can be further subdivided based on their lithic environment; those that precipitated on the upper surface of bedrock/clasts (i.e. subglacially precipitated calcite and evaporative calcite crusts); those that are located on the underside of clasts (i.e. pedogenic carbonates); and those that precipitated in rock outcrop fissures (i.e. endostromatolites). The cold-climate carbonate precipitates have a highly variable isotopic composition with δ¹⁸O values ranging between −6.5‰ and 28‰ VSMOW and δ¹³C values in the −10–20‰ VPDB range. However, each type of carbonate precipitates has a specific δ¹³C and δ¹⁸O range, suggesting that their environmental setting and the mechanism by which they formed controls their ¹³C and ¹⁸O signature. It was found that carbonate deposits that precipitated under equilibrium physico-chemical conditions had a δ¹³C value that is in equilibrium with that of the parent water, while its δ¹⁸O composition was more variable, as it is in part controlled by the temperature of reaction and by the δ¹⁸O and calcite saturation state of the parent water. By contrast, the δ¹⁸O composition of biologically precipitated carbonate deposits (endostromatolites) reflect that of the parent water, while its δ¹³C composition was enriched over that of the parent water due to bacterial methanogenesis. In the case of kinetically precipitated carbonate deposits, the δ¹⁸O and δ¹³C values are out-of-equilibrium relative to that of the parent water due to the faster rate of reaction.     

Did an extensive forest ever develop on the Chinese Loess Plateau during the past 130 ka?: a test using soil carbon isotopic signatures

Pleistocene vegetation history on the Chinese Loess Plateau has been traditionally investigated using palynological methods, and questions remain regarding whether an extensive broadleaf deciduous forest ever developed on the loess table under favorable climatic conditions. The authors have employed a C isotope approach to address this question by comparing δ¹³C values in soil organic matter from different loess ecological domains with known source vegetation to the C isotope values obtained from a paleosol section that can be dated back to 130 ka. The C isotopic compositions of modern soils from the loess table and the loess–desert transition gave δ¹³C values of −24.5‰ to −18.2‰ and −25.7‰ to −20.7‰, respectively. These C isotopic ratios are consistent with the standing modern vegetation that is dominated by a mixture of C₃ and C₄ plants and can be distinguished from that in the patchy forest areas where exclusive C₃ trees yield a narrow δ¹³C value range from −26.9‰ to −25‰ (average −26.1‰). Obtained δ¹³C compositions from paleosols and loess sediments in the Lantian and the Luochuan profiles vary from −24‰ to −16.9‰, indicating a grass-dominated steppe with shifting C₃ and C₄ contributions controlled mainly by paleoclimatic changes during the late Pleistocene. The present results suggest no extensive forest coverage on the loess table during the past 130 ka even under the most suitable conditions for forest development. This conclusion supports the explanation of natural causes for the development of only patchy forests on the modern loess table and provides critical historical information toward the vegetation restoration project that is currently underway on the Chinese Loess Plateau.     

REE and C-O Isotopic Geochemistry of Calcites from the World-class Huize Pb-Zn Deposits, Yunnan, China: Implications for the Ore Genesis

<正>The world-class Huize Pb-Zn deposits of Yunnan province,in southwestern China,located in the center of the Sichuan-Yunnan-Guizhou Pb-Zn polymetallic metallogenic province,has Pb+Zn reserves of more than 5 million tons at Pb+Zn grade of higher than 25%and contains abundant associated metals,such as Ag,Ge,Cd,and Ga.The deposits are hosted in the Lower Carboniferous carbonate strata and the Permian Emeishan basalts which distributed in the northern and southwestern parts of the orefield.Calcite is the only gangue mineral in the primary ores of the deposits and can be classified into three types,namely lumpy,patch and vein calcites in accordance with their occurrence.There is not intercalated contact between calcite and ore minerals and among the three types of calcite,indicating that they are the same ore-forming age with different stages and its forming sequence is from lumpy to patch to vein calcites. This paper presents the rare earth element(REE) and C-O isotopic compositions of calcites in the Huize Pb-Zn deposits.From lumpy to patch to vein calcites,REE contents decrease as LREE/ HREE ratios increase.The chondrite-normalized REE patterns of the three types of calcites are characterized by LREE-rich shaped,in which the lumpy calcite shows(La)_N(Ce)_N(Pr)_N≈(Nd)_N with Eu/Eu~*1,the patch calcite has(La)_N(Ce)_N(Pr)_N≈(Nd)_N with Eu/Eu~*1,and the vein calcite displays(La)_N(Ce)_N(Pr)_N(Nd)_N with Eu/Eu~*1.The REE geochemistry of the three types of calcite is different from those of the strata of various age and Permian Emeishan basalt exposed in the orefield.Theδ~(13) C_(PDb) andδ~(18)O_(Smow) values of the three types of calcites vary from-3.5‰to-2.1‰and 16.7‰to 18.6‰,respectively,falling within a small field between primary mantle and marine carbonate in theδ~(13)C_(PDb) vsδ~(18)O_(Smow) diagram. Various lines of evidence demonstrate that the three types of calcites in the deposits are produced from the same source with different stages.The ore-forming fluids of the deposits resulted from crustal -mantle mixing processes,in which the mantle-derived fluid components might be formed from degassing of mantle or/and magmatism of the Permian Emeishan basalts,and the crustal fluid was mainly provided by carbonate strata in the orefield.The ore-forming fluids in the deposits were homogenized before mineralization,and the ore-forming environment varied from relatively reducing to oxidizing.     

The origin of the Dapingzhang volcanogenic Cu–Pb–Zn ore deposit,Yunnan province,SW China: Constraints from host rock geochemistry and ore Os–Pb–S–C–O–H isotopes

The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100 km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417 ± 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive ε_Nd(t) values (+ 4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ³⁴S values of the sulfides from the deposit vary from − 1.24 to + 4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.310–18.656, ²⁰⁷Pb/²⁰⁴Pb = 15.489–15.643 and ²⁰⁸Pb/²⁰⁴Pb = 37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ¹³C_PDB and δ¹⁸O_SMOW values of the associated hydrothermal calcite crystals vary from − 2.3‰ to + 0.27‰ and from + 14.6 to + 24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ¹³C_PDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ¹⁸O values for the hydrothermal fluids, calculated from the measured values for quartz, are between − 2.1‰ and + 3.5‰. The corresponding δD values for the fluids range from − 59‰ to − 84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.