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1.
Nitrogen loading from anthropogenic sources, including fertilizer, manure, and sewage effluents, has been linked with declining water quality in coastal lagoons worldwide. Freshwater inputs to mid-Atlantic coastal lagoons of the USA are from terrestrially influenced sources: groundwater and overland flow via streams and agricultural ditches, with occasional precipitation events. Stable nitrogen isotopes ratios (δ15N) in bioindicator species combined with conventional water quality monitoring were used to assess nitrogen sources and provide insights into their origins. Water quality data revealed that nutrients derived from terrestrial sources increased after precipitation events. Tissues from two bioindicator species, a macroalgae (Gracilaria sp.) and the eastern oyster (Crassostrea virginica) were analyzed for δ15N to determine spatial and temporal patterns of nitrogen sources. A broad-scale survey assessment of deployed macroalgae (June 2004) detected regions of elevated δ15N. Macroalgal δ15N (7.33 ± 1.15‰ in May 2006 and 6.76 ± 1.15‰ in July 2006) responded quickly to sustained June 2006 nutrient pulse, but did not detect spatial patterns at the fine scale. Oyster δ15N (8.51 ± 0.89‰) responded slowly over longer time periods and exhibited a slight gradient at the finer spatial scale. Overall, elevated δ15N values in macroalgae and oysters were used to infer that human and animal wastes were important nitrogen sources in some areas of Maryland’s coastal bays. Different nitrogen integration periods across multiple organisms may be used to indicate nitrogen sources at various spatial and temporal scales, which will help focus nutrient management.  相似文献   

2.
Isotopic composition of monthly composite precipitation samples from Kozhikode (n = 31), a wet tropic station and Hyderabad (n = 25), a semi-arid station across southern India were studied for a period of four years from 2005 to 2008. During the study period, the Kozhikode station recorded an average rainfall of 3500 mm while the Hyderabad station showed an average rainfall of 790 mm. The average stable isotope values in precipitation at the Kozhikode station were δ 18O = −3.52‰, d-excess = 13.72‰; δ 18O = −2.94‰, d-excess = 10.57‰; and δ 18O = −7.53‰, d-excess = 13.79‰, respectively during the pre-monsoon (March–May), monsoon (June–September) and post-monsoon (October–February) seasons. For the Hyderabad station, the average stable isotope values were δ 18O = −5.88‰, d-excess = 2.34‰; δ 18O = −4.39‰, d-excess = 9.21‰; and δ 18O = −8.69‰, d-excess = 14.29‰, respectively for the three seasons. The precipitation at the two stations showed distinctive isotopic signatures. The stable isotopic composition of precipitation at the Hyderabad station showed significant variations from the global trend while the Kozhikode station almost followed the global value. These differences are mainly attributed to the latitudinal differences of the two stations coupled with the differences in climatic conditions.  相似文献   

3.
Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ18O across the batholith (9.5 ± 0.4‰n = 12) suggests V1 precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ18O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ18O (−3 to + 1.2‰), δ13CCO2 (−19 to 0‰) and δ34Ssulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, δD and δ18O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ18O (1.2–2.5‰), high-δ13CCO2 (> −4‰), low-δ34Ssulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ18O (−5.4 to −3‰), low-δ13C (<−10‰), high-δ34Ssulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated. Received: 3 April 1996 / Accepted: 5 May 1997  相似文献   

4.
The Marcona–Mina Justa deposit cluster, hosted by Lower Paleozoic metaclastic rocks and Middle Jurassic shallow marine andesites, incorporates the most important known magnetite mineralization in the Andes at Marcona (1.9 Gt at 55.4% Fe and 0.12% Cu) and one of the few major iron oxide–copper–gold (IOCG) deposits with economic Cu grades (346.6 Mt at 0.71% Cu, 3.8 g/t Ag and 0.03 g/t Au) at Mina Justa. The Middle Jurassic Marcona deposit is centred in Ica Department, Perú, and the Lower Cretaceous Mina Justa Cu (Ag, Au) prospect is located 3–4 km to the northeast. New fluid inclusion studies, including laser ablation time-of-flight inductively coupled plasma mass spectrometry (LA-TOF-ICPMS) analysis, integrated with sulphur, oxygen, hydrogen and carbon isotope analyses of minerals with well-defined paragenetic relationships, clarify the nature and origin of the hydrothermal fluid responsible for these contiguous but genetically contrasted deposits. At Marcona, early, sulphide-free stage M-III magnetite–biotite–calcic amphibole assemblages are inferred to have crystallized from a 700–800°C Fe oxide melt with a δ18O value from +5.2‰ to +7.7‰. Stage M-IV magnetite–phlogopite–calcic amphibole–sulphide assemblages were subsequently precipitated from 430–600°C aqueous fluids with dominantly magmatic isotopic compositions (δ34S = +0.8‰ to +5.9‰; δ18O = +9.6‰ to +12.2‰; δD = −73‰ to −43‰; and δ13C = −3.3‰). Stages M-III and M-IV account for over 95% of the magnetite mineralization at Marcona. Subsequent non-economic, lower temperature sulphide–calcite–amphibole assemblages (stage M-V) were deposited from fluids with similar δ34S (+1.8‰ to +5.0‰), δ18O (+10.1‰ to +12.5‰) and δ13C (−3.4‰), but higher δD values (average −8‰). Several groups of lower (<200°C, with a mode at 120°C) and higher temperature (>200°C) fluids can be recognized in the main polymetallic (Cu, Zn, Pb) sulphide stage M-V and may record the involvement of modified seawater. At Mina Justa, early magnetite–pyrite assemblages precipitated from a magmatic fluid (δ34S = +0.8‰ to +3.9‰; δ18O = +9.5‰ to +11.5‰) at 540–600°C, whereas ensuing chalcopyrite–bornite–digenite–chalcocite–hematite–calcite mineralization was the product of non-magmatic, probably evaporite-sourced, brines with δ34S ≥ +29‰, δ18O = 0.1‰ and δ13C = −8.3‰. Two groups of fluids were involved in the Cu mineralization stage: (1) Ca-rich, low-temperature (approx. 140°C) and high-salinity, plausibly a basinal brine and (2) Na (–K)-dominant with a low-temperature (approx. 140°C) and low-salinity probably meteoric water. LA-TOF-ICPMS analyses show that fluids at the magnetite–pyrite stage were Cu-barren, but that those associated with external fluids in later stages were enriched in Cu and Zn, suggesting such fluids could have been critical for the economic Cu mineralization in Andean IOCG deposits.  相似文献   

5.
Geochemical mixing models were used to decipher the dominant source of freshwater (rainfall, canal discharge, or groundwater discharge) to Biscayne Bay, an estuary in south Florida. Discrete samples of precipitation, canal water, groundwater, and bay surface water were collected monthly for 2 years and analyzed for salinity, stable isotopes of oxygen and hydrogen, and Sr2+/Ca2+ concentrations. These geochemical tracers were used in three separate mixing models and then combined to trace the magnitude and timing of the freshwater inputs to the estuary. Fresh groundwater had an isotopic signature (δ 18O = −2.66‰, δD −7.60‰) similar to rainfall (δ 18O = −2.86‰, δD = −4.78‰). Canal water had a heavy isotopic signature (δ 18O = −0.46‰, δD = −2.48‰) due to evaporation. This made it possible to use stable isotopes of oxygen and hydrogen to separate canal water from precipitation and groundwater as a source of freshwater into the bay. A second model using Sr2+/Ca2+ ratios was developed to discern fresh groundwater inputs from precipitation inputs. Groundwater had a Sr2+/Ca2+ ratio of 0.07, while precipitation had a dissimilar ratio of 0.89. When combined, these models showed a freshwater input ratio of canal/precipitation/groundwater of 37%:53%:10% in the wet season and 40%:55%:5% in the dry season with an error of ±25%. For a bay-wide water budget that includes saltwater and freshwater mixing, fresh groundwater accounts for 1–2% of the total fresh and saline water input.  相似文献   

6.
The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenite-hosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively (events II–IV). The δ18O-δD and δ34S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event I fluids are of mixed origin. They are metamorphic fluids (H2O-CO2-CH4-NaCl; δ18O=4.7 to 9.3‰; δD ab.−34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H2O-NaCl, 4.5–6.4 wt% NaCl eq.; δ18O=7.3–7.8‰; δD=−77 to −74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H2O-NaCl, <8 wt% NaCl eq.; δ18O=−0.4 to 3.4‰; δD=−75 to −58‰). They were probably formed by local hydrothermal cells that were spatially and temporally related to the youngest Variscan granites, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor influence of magmatic fluids confirms that the intrusion of these granites was essentially water-undersaturated, as most of the hydrothermal fluids were external to the igneous rocks. The fluids involved in the younger hydrothermal systems (events II–III) are very similar. The waters involved in the formation of episyenites, chlorite-rich greisens, retrograde skarns and phyllic and chlorite-rich alterations in the shear zones show no major chemical or isotopic differences. Interaction of the hydrothermal fluids with the host rocks was the main mechanism of ore formation. The composition (H2O-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and the δ18O (−4.6 to 6.3‰) and δD (−51 to −40‰) values are consistent with a meteoric origin, with a δ18O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective cells and were then channelled along major crustal discontinuities. In these shear zones the more easily altered minerals such as feldspars, actinolite and chlorite had their δ18O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower portions of the hydrothermal systems, basement-cover fluorite-barite-base metal veins formed by mixing of these deep fluids with downwards percolating brines. These brines are also interpreted as of meteoric origin (δ18O< ≈ −4‰; δD=−65 to −36‰) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hosted in the post-Variscan sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper Paleozoic meteoric waters in central Iberia had very negative δD values (≤−52‰) whereas those of Early Mesozoic age ranged between −65 and −36‰. Received: 9 June 1999 / Accepted: 19 January 2000  相似文献   

7.
The present study reports nitrogen and carbon stable isotope data (δ15N and δ13C) from four large (63–400 km2), shallow (∼1 m) coastal lagoons on Egypt’s Nile Delta. While the lagoons all receive sewage and agricultural drainage, the magnitude of loading varies. In this preliminary survey, we document wide variability in the δ15N and δ13C isotope values of major fish groups among these lagoons. There were no consistent or significant differences among the major groups of fish, including carp, catfish, mullet, and tilapia. There was a strong positive correlation (R 2 = 0.84) between the average δ15N values of fish muscle and estimated water residence time among the lagoons. This preliminary evidence suggests that nitrogen cycle transformations may be more important than primary N source differences in determining N isotopic ratios of organisms in the lagoons. The δ13C results point to the probable importance of autochthonous particulate organic matter rather than terrestrial detritus or marine plankton in the diets of resident fish populations in the lagoons.  相似文献   

8.
Vein-type tin mineralization in the Dadoushan deposit, Laochang ore field, Gejiu district, SW China, is predominantly hosted in Triassic carbonate rocks (Gejiu Formation) over cupolas of the unexposed Laochang equigranular granite intrusion. The most common vein mineral is tourmaline, accompanied by skarn minerals (garnet, diopside, epidote, phlogopite) and beryl. The main ore mineral is cassiterite, accompanied by minor chalcopyrite, pyrrhotite, and pyrite, as well as scheelite. The tin ore grade varies with depth, with the highest grades (~1.2 % Sn) prevalent in the lower part of the vein zone. Muscovite 40Ar–39Ar dating yielded a plateau age of 82.7 ± 0.7 Ma which defines the age of the vein-type mineralization. Measured sulfur isotope compositions (δ 34S = −4.1 to 3.9 ‰) of the sulfides (arsenopyrite, chalcopyrite, pyrite, and pyrrhotite) indicate that the sulfur in veins is mainly derived from a magmatic source. The sulfur isotope values of the ores are consistent with those from the underlying granite (Laochang equigranular granite, −3.7 to 0.1 ‰) but are different from the carbonate wall rocks of the Gejiu Formation (7.1 to 11.1 ‰). The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (δ 18OH2O = −2.4 to 5.5 ‰, δD = −86 to −77 ‰) suggest an initially magmatic fluid which gradually evolved towards meteoric water during tin mineralization.  相似文献   

9.
Gossan Hill is an Archean (∼3.0 Ga) Cu–Zn–magnetite-rich volcanic-hosted massive sulfide (VHMS) deposit in the Yilgarn Craton of Western Australia. Massive sulfide and magnetite occur within a layered succession of tuffaceous, felsic volcaniclastic rocks of the Golden Grove Formation. The Gossan Hill deposit consists of two stratigraphically separate ore zones that are stratabound and interconnected by sulfide veins. Thickly developed massive sulfide and stockwork zones in the north of the deposit are interpreted to represent a feeder zone. The deposit is broadly zoned from a Cu–Fe-rich lower ore zone, upwards through Cu–Zn to Zn–Ag–Au–Pb enrichment in the upper ore zone. New sulfur isotope studies at the Gossan Hill deposit indicate that the variation is wider than previously reported, with sulfide δ34S values varying between −1.6 and 7.8‰ with an average of 2.1 ± 1.4‰ (1σ error). Sulfur isotope values have a broad systematic stratigraphic increase of approximately 1.2‰ from the base to the top of the deposit. This variation in sulfur isotope values is significant in view of typical narrow ranges for Archean VHMS deposits. Copper-rich sulfides in the lower ore zone have a narrower range (δ34S values of −1.6 to 3.4‰, average ∼1.6 ± 0.9‰) than sulfides in the upper ore zone. The lower ore zone is interpreted to have formed from a relatively uniform reduced sulfur source dominated by leached igneous rock sulfur and minor magmatic sulfur. Towards the upper Zn-rich ore zone, an overall increase in δ34S values is accompanied by a wider range of δ34S values, with the greatest variation occurring in massive pyrite at the southern margin of the upper ore zone (−1.0 to 7.8‰). The higher average δ34S values (2.8 ± 2.1‰) and their wider range are explained by mixing of hydrothermal fluids containing leached igneous rock sulfur with Archean seawater (δ34S values of 2 to 3‰) near the paleoseafloor. The widest range of δ34S values at the southern margin of the deposit occurs away from the feeder zone and is attributed to greater seawater mixing away from the central upflow zone. Received: 10 June 1999 / Accepted: 28 December 1999  相似文献   

10.
Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22; mean δ18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ18O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ18O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.  相似文献   

11.
Bulk δ 34Srock values, sulfur contents, and magnetic susceptibility were determined for 12 gold-related granitoid intrusions in southwestern New Brunswick, the Canadian Appalachians. The sulfur isotope compositions of sulfide minerals in some of the granitoid samples were also analyzed. This new dataset was used to characterize two distinctive groups of granitoids: (1) a Late Devonian granitic series (GS) and (2) a Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS). The GS rocks have a large range in δ 34S values of −7.1‰ to +13‰ with an average of 2.2 ± 5.0‰ (1σ), low bulk-S contents (33 to 7,710 ppm) and low magnetic susceptibility values (<10−4 SI), consistent with reduced ilmenite-series granites. The GMS rocks have a relatively narrower variation in δ 34S values of −4.4‰ to +7.3‰ with an average 1.2 ± 2.9‰ but with larger ranges in bulk-S contents (45 to 11,100 ppm) and high magnetic susceptibility values (>10−3 SI), indicative of oxidized magnetite-series granites. The exceptions for the GMS rocks are the Lake George granodiorite and Tower Hill granite that display reduced characteristics, which may have resulted from interaction of the magmas forming these intrusions with graphite- or organic carbon-bearing sedimentary rocks. The bulk δ 34S values and S contents of the GMS rocks are interpreted in terms of selective assimilation–fractional crystallization (SAFC) processes. Degassing processes may account for the δ 34S values and S contents of some GS rocks. The characteristics of our sulfur isotope and abundance data suggest that mineralizing components S and Au in intrusion-related gold systems are dominantly derived from magmatic sources, although minor contaminants derived from country rocks are evident. In addition, the molar sulfate to sulfide ratio in a granitic rock sample can be calculated from the δ 34Srock value of the whole-rock sample and the δ 34Ssulfide (or δ 34Ssulfate) value of sulfide and/or sulfate mineral in the sample on the basis of S-isotope fractionation and mass balance under the condition of magmatic equilibrium. This may be used to predict the speciation of sulfur in granitic rocks, which can be a potential exploration tool for intrusion-related gold systems.  相似文献   

12.
In the Mazowe area some 40 km NW of Harare in Zimbabwe, gold mineralization is hosted in a variety of lithologies of the Archean Harare-Bindura-Shamva greenstone belt, in structures related to the late Archean regional D2/3 event. Conspicuous mineralzogical differences exist between the mines; the mainly granodiorite-hosted workings at Mazowe mine are on pyrite-rich reefs, mines of the Bernheim group have metabasalt host rocks and are characterized by arsenopyrite-rich ores, and Stori's Golden Shaft and Alice mine, both in metabasalts, work sulfide-poor quartz veins. In contrast to the mineralogical diversity, near-identical fluid inventories were found at the different mines. Both H2O-CO2-CH4 fluids of low salinity, and highly saline fluids are present and are regarded to indicate fluid mixing during the formation of the deposits. Notably, these fluid compositions in the Mazowe gold field markedly contrast to ore fluids “typical” of Archean mesothermal gold deposits on other cratons. Stable isotope compositions of quartz from the various deposits (δ18O=10.8 to 13.2‰ SMOW), calcite (δ18O=9.5 to 11.9‰ SMOW and δ13C=−3.2 to −8.0‰ PDB), inclusion water (δD=−28 to −40‰ SMOW) and sulfides (δ34S=1.3 to 3.2‰ CDT) are uniform within the range typical for Archean lode gold deposits worldwide. The fluid and stable isotope compositions support the statement that the mineralization in the Mazowe gold field formed from relatively reduced fluids with a “metamorphic” signature during a single event of gold mineralization. Microthermometric data further indicate that the deposits formed in the PT range of 1.65–2.3 kbar and 250–380 °C. Ages obtained by using the Sm/Nd and Rb/Sr isotope systems on scheelites are 2604 ± 84 Ma for the mineralization at Stori's Golden Shaft mine, and 2.40 ± 0.20 Ga for Mazowe mine. The Archean age at Stori's is regarded as close to the true age of gold mineralization in the area, whereas the Proterozoic age at Mazowe mine probably reflects later resetting. Received: 30 September 1998 / Accepted: 17 August 1999  相似文献   

13.
The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ34S and δ13C data which deviate from a negative correlation line toward higher δ13C values suggest methanogenic CO2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH4 and CO2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U6+ to U4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH4 and HCO3 − existed and reduced sulfur species (H2S, HS−) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO4 2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U4+ is stable has been maintained since precipitation of uranium minerals. Received: 9 February 1996 / Accepted: 11 April 1997  相似文献   

14.
Whole-rock chemical composition and 11B/10B isotope ratios in tourmaline was investigated to study the geochemical recycling of boron during the evolution of the Andean basement from the Palaeozoic to Mesozoic. In the basement (Cambrian to Ordovician high-grade paragneisses, migmatites and orthogneisses, the Eocambrian Puncoviscana Formation, and Paleozoic-Mesozoic granitoid igneous rocks) whole-rock B contents are generally below 100 ppm, but B contents of ˜1 wt% are found in cogenetic aplite and pegmatite dikes and in tourmaline–quartz rocks. In the metasedimentary rocks, no systematic variation in B content because of metamorphic grade and no correlation of B with other incompatible elements are apparent. Tourmalines from the high-grade metamorphic basement yield δ11B values ranging from −11.2 to −6.8‰ and isotope fractionation during migmatisation was small. Metamorphic tourmalines from the Puncoviscana Formation have δ11B values between −6.3 and −5.8‰. The calculated (corrected for fractionation) δ11B values of −6 to −2‰ for the sedimentary protolith of the metamorphic basement indicate a continental B source with subordinate marine input. Tourmalines from Palaeozoic and Mesozoic granitoids display an identical range of δ11B values from −12 to −5.3‰ and indicate a similarly homogeneous B source throughout time. Tourmalines from pegmatites and tourmaline–quartz rocks record the average δ11B values of the parental granitic magma. We assume that B in the Palaeozoic and Mesozoic granitoids is derived from the local metamorphic basement supporting the hypothesis that recycling of the lower Palaeozoic crust is the dominant process in granitic magma formation from Palaeozoic to Mesozoic. Received: 15 December 1999 / Accepted: 11 July 2000  相似文献   

15.
A bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry Cu deposits in Butte, Montana, USA and Bingham Canyon, Utah, USA. The leachates mostly represent low-salinity magmatic–hydrothermal fluid inclusions. Their halogen ratios (Br/Cl) of fluid inclusion leachates were determined by ion chromatography, and δ37Cl values of the leachates were measured by continuous-flow isotope ratio mass spectrometry. Br/Cl ratios from early pre-Main stage and later Main stage veins at Butte range from 0.60 to 1.88 × 10−3 M. Ratios are similar in pre-Main stage veins with sericite bearing selvages and Main stage samples ranging from 0.81 to 1.08 × 10−3 and from 0.92 to 1.88 × 10−3 M, respectively, clustering below seawater (1.54 × 10−3 M) and overlapping mantle values (~1–2 × 10−3 M). Two samples associated with early pre-Main stage potassic alteration yield distinctly lower Br/Cl ratios of 0.60 and 0.64 × 10−3 M. Butte δ37Cl values range from −0.8‰ to −2.3‰ with no significant difference between pre-Main stage and Main stage samples. Br/Cl ratios for quartz veins from Bingham Canyon range from 0.18 to 3.68 × 10−3 M. Br/Cl ratios from Bingham range above and below previously reported for porphyry copper deposits. In contrast to Butte, δ37Cl values for Bingham are lower, ranging from −0.9‰ to −4.1‰. In the absence of any processes which can significantly fractionate chlorine isotopes at high temperatures, we suggest that the porphyry system at Bingham, and to a lesser extent at Butte, have inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures.  相似文献   

16.
The Kaoping (Taiwan) and Kapuas (Indonesia) Rivers differ in hydrological cycle, topography and landscape. These differences strengthen the use of 14C dating, lignin-derived phenols, δ13C values and C/N ratios to determine the sources and diagenesis of surface sedimentary organic carbon (OC) in both rivers. The Kapuas River is surrounded by forest, resulting in sedimentary OC with a 14C age between 600 and 740 years, Λ (total lignin expressed as mg/100 mg OC) values from 0.94 to 3.70, δ13C values from −27.87 to −30.00‰, C/N ratios from 6.7 to 30.8, %OC from 0.63 to 9.24% and vanillic acid to vanillin ratio, (Ad/Al)v, values from 0.73 to 2.09, all of which indicate the presence of recent plant-derived organic matter. The tributaries and three locations upstream of the Kaoping River are also surrounded by forests, resulting in Λ values (0.51–4.80), δ13C values (−23.85 to −27.08‰), C/N ratios (14.1–28.7), %OC (1.01–7.86%) and (Ad/Al)v values (0.86–1.88), which are indicative of a terrestrial signal. No lignin oxidation products were detected in the mainstream of the Kaoping River or its coastal zone, hence the surface sediments OC with a 14C age between 4,915 and 15,870 years, enriched δ13C values (−23.30 and −26.54‰), lower C/N ratios (6.0–17.5) and lower %OC (0.15–2.24%) likely represent old rock and soil material. Massive floods during typhoons most probably cause plant materials from the Kaoping River and its coastal zone to be transported into the deep sea.  相似文献   

17.
The Mississippi Valley-type (MVT) Pb–Zn ore district at Mežica is hosted by Middle to Upper Triassic platform carbonate rocks in the Northern Karavanke/Drau Range geotectonic units of the Eastern Alps, northeastern Slovenia. The mineralization at Mežica covers an area of 64 km2 with more than 350 orebodies and numerous galena and sphalerite occurrences, which formed epigenetically, both conformable and discordant to bedding. While knowledge on the style of mineralization has grown considerably, the origin of discordant mineralization is still debated. Sulfur stable isotope analyses of 149 sulfide samples from the different types of orebodies provide new insights on the genesis of these mineralizations and their relationship. Over the whole mining district, sphalerite and galena have δ 34 S values in the range of –24.7 to –1.5‰ VCDT (–13.5 ± 5.0‰) and –24.7 to –1.4‰ (–10.7 ± 5.9‰), respectively. These values are in the range of the main MVT deposits of the Drau Range. All sulfide δ 34 S values are negative within a broad range, with δ 34 S pyrite <δ 34 S sphalerite <δ 34 S galena for both conformable and discordant orebodies, indicating isotopically heterogeneous H2S in the ore-forming fluids and precipitation of the sulfides at thermodynamic disequilibrium. This clearly supports that the main sulfide sulfur originates from bacterially mediated reduction (BSR) of Middle to Upper Triassic seawater sulfate or evaporite sulfate. Thermochemical sulfate reduction (TSR) by organic compounds contributed a minor amount of 34S-enriched H2S to the ore fluid. The variations of δ 34 S values of galena and coarse-grained sphalerite at orefield scale are generally larger than the differences observed in single hand specimens. The progressively more negative δ 34 S values with time along the different sphalerite generations are consistent with mixing of different H2S sources, with a decreasing contribution of H2S from regional TSR, and an increase from a local H2S reservoir produced by BSR (i.e., sedimentary biogenic pyrite, organo-sulfur compounds). Galena in discordant ore (–11.9 to –1.7‰; –7.0 ± 2.7‰, n = 12) tends to be depleted in 34 S compared with conformable ore (–24.7 to –2.8‰, –11.7 ± 6.2‰, n = 39). A similar trend is observed from fine-crystalline sphalerite I to coarse open-space filling sphalerite II. Some variation of the sulfide δ 34 S values is attributed to the inherent variability of bacterial sulfate reduction, including metabolic recycling in a locally partially closed system and contribution of H2S from hydrolysis of biogenic pyrite and thermal cracking of organo-sulfur compounds. The results suggest that the conformable orebodies originated by mixing of hydrothermal saline metal-rich fluid with H2S-rich pore waters during late burial diagenesis, while the discordant orebodies formed by mobilization of the earlier conformable mineralization.  相似文献   

18.
Oxygen and hydrogen stable isotope ratios of eclogite-facies metagabbros and metabasalts from the Cycladic archipelago (Greece) document the scale and timing of fluid–rock interaction in subducted oceanic crust. Close similarities are found between the isotopic compositions of the high-pressure rocks and their ocean-floor equivalents. High-pressure minerals in metagabbros have low δ18O values: garnet 2.6 to 5.9‰, glaucophane 4.3 to 7.1‰; omphacite 3.5 to 6.2‰. Precursor actinolite that was formed during the hydrothermal alteration of the oceanic crust by seawater analyses at 3.7 to 6.3‰. These compositions are in the range of the δ18O values of unaltered igneous oceanic crust and high-temperature hydrothermally altered oceanic crust. In contrast, high-pressure metabasalts are characterised by 18O-enriched isotopic compositions (garnet 9.2 to 11.5‰, glaucophane 10.6 to 12.5‰, omphacite 10.2 to 12.8‰), which are consistent with the precursor basalts having undergone low-temperature alteration by seawater. D/H ratios of glaucophane and actinolite are also consistent with alteration by seawater. Remarkably constant oxygen isotope fractionations, compatible with isotopic equilibrium, are observed among high-pressure minerals, with Δglaucophane−garnet = 1.37 ± 0.24‰ and Δomphacite−garnet = 0.72 ± 0.24‰. For the estimated metamorphic temperature of 500 °C, these fractionations yield coefficients in the equation Δ = A * 106/T 2 (in Kelvin) of Aglaucophane−garnet = 0.87 ± 0.15 and Aomphacite−garnet = 0.72 ± 0.24. A fractionation of Δglaucophane–actinolite = 0.94 ± 0.21‰ is measured in metagabbros, and indicates that isotopic equilibrium was established during the metamorphic reaction in which glaucophane formed at the expense of actinolite. The preservation of the isotopic compositions of gabbroic and basaltic oceanic crust and the equilibrium fractionations among minerals shows that high-pressure metamorphism occurred at low water/rock ratios. The isotopic equilibrium is only observed at hand-specimen scale, at an outcrop scale isotopic compositional differences occur among adjacent rocks. This heterogeneity reflects metre-scale compositional variations that developed during hydrothermal alteration by seawater and were subsequently inherited by the high-pressure metamorphic rocks. Received: 4 January 1999 / Accepted: 7 July 1999  相似文献   

19.
The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene, grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO2 = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc) δ18O values of 9–18‰ and garnet (Gnt) δ18O values of 10–14‰. The δ18O(Gnt) values are only poorly correlated with δ18O(Cc), and the δ18O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the δ18O values of the rock were reset. The marbles contain  ≤15% garnet and, for water-rich fluids, garnet-forming reactions are predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively rapid. Texturally late epidote has δ18O values of 1.5–6.2‰ implying δ18O(H2O) values of 2–7‰. Waters with such low-δ18O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water into a cooling hydrothermal system. Received: 29 April 1996 / Accepted: 12 March 1997  相似文献   

20.
Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope （their δ^18O values range from -0.3‰- -0.1‰） and lighter oxygen isotope （their δ^18O values range from -22.1‰- -19.5‰）. Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later disso- lution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ^13C values ranging from -8.02‰ to -3.23‰, and lighter oxygen isotope with δ^18O values ranging from -22.9‰ to -19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous cal- cites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ^13C values ranging from -1.92‰ to -0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ^18O values ranging from -20.5‰ to -12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process.  相似文献   

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