首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The Dexing deposit is located in a NE‐trending magmatic belt along the southeastern margin of the Yangtze Craton. It is the largest porphyry copper deposit in China, consisting of three porphyry copper orebodies of Zhushahong, Tongchang and Fujiawu from northwest to southeast. It contains 1168 Mt of ores with 0.5% Cu and 0.01% Mo. The Dexing deposit is hosted by Middle Jurassic granodiorite porphyries and pelitic schist of Proterozoic age. The Tongchang granodiorite porphyry has a medium K cal‐alkaline series, with medium K2O content (1.94–2.07 wt%), and low K2O/(Na2O + K2O) (0.33–0.84) ratios. They have high large‐ion lithophile elements, high light rare‐earth elements, and low high‐field‐strength elements. The hydrothermal alteration at Tongchang is divided into four alteration mineral assemblages and related vein systems. They are early K‐feldspar alteration and A vein; transitional (chlorite + illite) alteration and B vein; late phyllic (quartz + muscovite) alteration and D vein; and latest carbonate, sulfate and oxide alteration and hematite veins. Primary fluid inclusions in quartz from phyllic alteration assemblage include liquid‐rich (type 1), vapor‐rich (type 2) and halite‐bearing ones (type 3). These provide trapping pressures of 20–400 ´ 105 Pa of fluids responsible for the formation of D veins. Igneous biotite from least altered granochiorite porphyry and hydrothermal muscovite in mineralized granodiorite porphyry possess δ18O and δD values of 4.6‰ and ?87‰ for biotite and 7.1–8.9‰, ?71 to ?73‰ for muscovite. Stable isotopic composition of the hydrothermal water suggests a magmatic origin. The carbon and oxygen isotope for hydrothermal calcite are ?4.8 to ?6.2‰ and 6.8–18.8‰, respectively. The δ34S of pyrite in quartz vein ranges from ?0.1 to 3‰, whereas δ34S for chalcopyrite in calcite veins ranges from 4 to 5‰. These are similar to the results of previous studies, and suggest a magmatic origin for sulfur. Results from alteration assemblages and vein system observation, as well as geochemical, fluid inclusion, stable isotope studies indicate that the involvement of hydrothermal fluids exsolved from a crystallizing melt are responsible for the formation of Tongchang porphyry Cu‐Mo orebodies in Dexing porphyry deposit.  相似文献   

2.
Recent (<50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean δ18O value of ?5–9‰ PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (?7–5‰SMOW) assuming a mean water temperature of 9°C. The mean δ18O of lake data, ?4–5‰ PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to ?12‰ PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light ‘soil zone’ carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO2 and aqueous HCO?3. We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary δ18O values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.  相似文献   

3.
The skarn‐type tungsten deposit of the Date‐Nagai mine is genetically related to the granodiorite batholith of the Iidateyama body. Skarn is developed along the contact between pelitic hornfels and marble that remains as a small roof pendant body directly above the granodiorite batholith. Zonal arrangement of minerals is observed in skarn. The zonation consists of wollastonite, garnet, garnet‐epidote, and vesuvianite‐garnet zones, from marble to hornfels. Sheelite is included in garnet, garnet‐epidote, and vesuvianite‐garnet zones. The oxygen isotope values of skarn minerals were obtained as δ18O = 4.2–7.7‰ for garnet, 5.9–6.9‰ for vesuvianite, ?0.3–3.4‰ for scheelite, 6.0–10.9‰ for quartz, and 8.2‰ for muscovite. The temperature of skarn‐formation was calculated from oxygen isotopic values of scheelite‐quartz pairs to be 288°C. Calculated oxygen isotope values of fluid responsible for skarn minerals were 6.1–9.5‰ for garnet, 1.2–4.8‰ for scheelite, ?1.3‐3.6‰ for quartz, and 4.5‰ for muscovite. Garnet precipitated from the fluids of different δ18O values from scheelite, quartz, and muscovite. These δ18O values suggest that the origin of fluid responsible for garnet was magmatic water, while evidence for the presence of a meteoric component in the fluids responsible for middle to later stages minerals was confirmed.  相似文献   

4.
We present the results of a study on gabbroic rocks, syenites, pegmatites, carbonatites, and hydrothermal products of the Oshurkovo apatite-bearing massif. The results include Nd and Sr isotope ratios; the isotope compositions of carbon and oxygen in calcite; oxygen in apatite, magnetite, and silicate minerals (phlogopite, titanite, diopside, amphibole, K-feldspar, and quartz); sulfur in barite; and hydrogen in mica. The isotopic data are close to the EM-1 enriched mantle values and confirm a comagmatic relationship between the gabbros and carbonatites. The binary plot ?Nd vs. 87Sr/86Sr demonstrates strong differentiation between silicate rocks and carbonatites, as is the case with the other Late Mesozoic carbonatite occurrences of southwestern Transbaikalia. The oxygen isotope composition of all comagmatic phases also falls within the range of mantle values. A clear trend toward heavier oxygen and lighter carbon isotope compositions is observed in all successively emplaced phases, which is consistent with a trend defined by hydrothermal products formed under the influence of the parent magma chamber. Carbonates formed during the greenstone alteration of gabbroic rocks are enriched in the light oxygen isotope (δ18O from ?2.8 to ?7.3‰), suggesting a contribution of vadose water.  相似文献   

5.
The Ohori ore deposit is one of the Cu–Pb–Zn deposits in the Green Tuff region, NE Japan, and consists of skarn‐type (Kaninomata) and vein‐type (Nakanomata) orebodies. The former has a unique origin because its original calcareous rocks were made by hydrothermal precipitation during Miocene submarine volcanism. Carbon and oxygen isotope ratios of skarn calcite and sulfur isotope ratios of sulfides were measured in and around the deposit. Carbon and oxygen isotope ratios of the skarn calcite are δ13C = ?15.51 to ?5.1‰, δ18O = +3.6 to +22.5‰. δ13C values are slightly lower than those of the Cretaceous skarn deposits in Japan. These isotope ratios of the Kaninomata skarn show that the original calcareous rocks resemble the present submarine hydrothermal carbonates at the CLAM Site, Okinawa Trough, than Cenozoic limestones, even though some isotopic shifts had occurred during later skarnization. δ34S ratios of the sulfide minerals from the Kaninomata and Nakanomata orebodies are mostly in a narrow range of +4.0 to +7.0‰ and they resemble each other, suggesting the same sulfur origin for the both deposits. The magnetite‐series Tertiary Kaninomatasawa granite is distributed just beneath the skarn layer and has δ34S ratios of +7.5 to 8.1‰. The heavy sulfur isotope ratio of the skarn sulfides may have been affected by the Kaninomatasawa granite.  相似文献   

6.
The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ13C values from–6.6 to 1.3‰ and δ18O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ13C from–5.2 to–2.9 and δ18O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ13C from–16.4 to–13.1 and δ18O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ13C from–13.0 to–10.4‰ and δ18O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ18O from 18.7 to 17.6‰ and δ13C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO2. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.  相似文献   

7.
Two quartz samples of igneous origin, UNIL‐Q1 (Torres del Paine Intrusion, Chile) and BGI‐Q1 (Shandong province, China), were calibrated for their oxygen isotope composition for SIMS measurements. UNIL‐Q1 and BGI‐Q1 were evaluated for homogeneity using SIMS. Their reference δ18O values were determined by CO2 laser fluorination. The average δ18O value found for UNIL‐Q1 is 9.8 ± 0.06‰ and that for BGI‐Q1 is 7.7 ± 0.11‰ (1s). The intermediate measurement precision of SIMS oxygen isotope measurements was 0.32–0.41‰ (2s; UNIL‐Q1) and 0.40–0.48‰ (2s; BGI‐Q1), respectively. While less homogeneous in its oxygen isotope composition, BGI‐Q1 is also suitable for SIMS trace element measurements.  相似文献   

8.
付旭东 《沉积学报》2017,35(1):67-74
巴丹吉林沙漠是世界上沙丘最高大的沙漠,其沙源研究对认识沙漠形成、高大沙山发育和防沙治沙工程有重要意义。石英是沙漠沉积物中常见的矿物,其氧同位素值可示踪物源。采集沙漠西北部、东部、东南部高大沙山、丘间低地与湖泊以及雅布赖山前的表层沉积物,测定了样品不同粒级的石英δ18O值。结果表明:①石英δ18O值随粒级减小有增大趋势,同一样品不同粒级石英δ18O值存在较大差异,相同粒级石英δ18O值也有变化。②石英δ18O值介于9.4‰~19.3‰,均值为13.3‰（n=55）;其中沙丘沙的石英δ18O值介于9.5‰~16.6‰,均值为12.9 ‰（n=39）;湖相沉积物的石英δ18O值介于9.4‰~19.3‰,均值为14.2‰（n=16）。③区域内,<16 μm粒级的石英δ18O值与16~64 μm、125~154 μm、200~250 μm、> 300 μm粒级的石英δ18O值都存在显著差异,但200~250 μm与 > 300 μm粒级的石英δ18O值没有显著差异;经区域对比,巴丹吉林沙漠 < 16 μm粒级的石英δ18O值与柴达木盆地沙漠、蒙古戈壁风成沉积物 < 16 μm石英δ18O值无显著差异,但巴丹吉林沙漠16~64 μm粒级的石英δ18O值与蒙古戈壁风成沉积物16~64 μm石英δ18O值存在显著差异;这似乎暗示研究区的细颗粒物质可能是远源的。巴丹吉林沙漠沉积物的石英δ18O值位于火成岩石英、砂岩和变质岩石英δ18O值分布阈值内,受区域地质条件、物源混合、粒级效应等因素的影响,砂粒级的石英δ18O值所指示的母岩成份特征与祁连山区岩石的岩性有较好吻合。  相似文献   

9.
《Geochimica et cosmochimica acta》1999,63(11-12):1825-1836
Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF3. 18O/16O ratios are determined on oxygen converted to CO2 over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O2) enhanced CO production in the graphite reactor causes a systematic shift in both δ13C and δ18O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ13C (CO2) depletion relative to δ13C (graphite), is used to obtain corrected δ18O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ13C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ18O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ18O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ18O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.  相似文献   

10.
This study couples in situ 16O, 17O and 18O isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic-hydrothermal quartz crystals. Euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic-hydrothermal ore deposits: El Indio Au–Ag–Cu deposit; Summitville Au–Ag–Cu deposit; North Parkes Cu–Au deposit and Kingsgate quartz-Mo–Bi–W deposit. The internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps. Quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry (for 40 trace elements in quartz) and sensitive high-resolution ion microprobe (for 3 isotopes in quartz). Imaging revealed fine oscillatory zoning, sector zoning, complex “macromosaic” textures and hidden xenocrystic cores. In situ oxygen isotope analyses revealed a δ18O range of up to 12.4 ± 0.3 ‰ in a single crystal—the largest isotopic range ever ascribed to oscillatory zonation in quartz. Some of these crystals contain a heavier δ18O signature than expected by existing models. While sector-zoned crystals exhibited strong trace element variations between faces, no evidence for anisotropic isotope fractionation was found. We found: (1) isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis (e.g., δD–δ18O) using bulk analytical techniques, (2) the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and (3) in three of the four samples, both textural and isotopic data indicate non-equilibrium deposition of quartz.  相似文献   

11.
《Chemical Geology》1992,94(3):215-227
Tourmaline is a ubiquitous mineral in the Mid-Proterozoic, peraluminous, syn- to post-tectonic granites and aplites and the related hydrothermal rocks of the Karagwe-Ankolean belt in northwest Tanzania. Electron microprobe analysis indicates that tourmalines from all of the intrusive and hydrothermal lithologies: (1) belong to the schorl-dravite solid-solution series; and (2) plot within the field occupied by tourmaline from Li-poor granitoids on the Fe-Al-Mg classification diagram. Oxygen isotope compositions range from +12.2 to +11.6‰ (SMOW) for magmatic tourmalines and from +10.8 to +9.8‰ for those of hydrothermal origin. Hydrogen isotope compositions vary from −79 to − 65‰ (SMOW) for magmatic tourmalines and from −99 to −84‰ for hydrothermal tourmalines. Water contents measured by manometry are constant at 3.0–3.2 wt.%. Within the broad grouping there arc systematic variations in both chemical [particularly Fetot/(Fetot + Mg ratio)] and isotopic composition that relate to evolving magmatic and hydrothermal conditions. Igneous differentiation [increasing Fetot/(Fetot + Mg) in magmatic tourmaline] has produced trends with higher δ18O in quartz, lower δ18O in tourmaline, and larger ΔQTZ.−TOUR.-values, that reflect a combination of a reduction of crystallization temperature and an increase of Fetot/ (Fetot + Mg) in the residual melt. Subsequent cooling and interaction of an exsolved, B-rich magmatic fluid with the pelitic country rocks, resulted in the deposition of hydrothermal tourmaline with increasing Fetot/(Fetot + Mg) ratios, and progressively lower δ18O and δD -values.  相似文献   

12.
The origin of monzonitic intrusions that are associated with Proterozoic massif-type anorthosite complexes is controversial. A detailed oxygen isotope study of the Sybille intrusion, a monzonitic intrusion of the Laramie Anorthosite Complex (Wyoming), indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component (∼20%) incorporated early in its magmatic history, or a partial melt of lower crustal rocks is consistent with the data. The oxygen isotope compositions of plagioclase, pyroxene and zircon from the Sybille monzosyenite, the dominant rock type in the Sybille intrusion, were analyzed in order to establish the isotopic composition of the source of the magma. Plagioclase δ18O values range from 6.77 to 9.17‰. We interpret the higher plagioclase δ18O values (average 8.69 ± 0.30‰, n = 19) to be magmatic in origin, lower plagioclase δ18O values (average 7.51 ± 0.44‰, n = 22) to be the result of variable subsolidus alteration, and pyroxene δ18O values (average 6.34 ± 0.38‰, n = 19) to be the result of closed-system diffusional exchange during cooling. Low magnetic zircons, which have been shown to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration, have δ18O values (7.40 ± 0.24‰, n = 11) which are consistent with our interpretation of the plagioclase and pyroxene results. Oxygen isotope data from all three minerals indicate that the magmatic oxygen isotope composition of the Sybille intrusion is enriched in 18O relative to the composition of average or “normal” mantle-derived magmas. This enrichment is approximately twice the oxygen isotope enrichment that could result from closed-system fractionation, rendering a closed-system, comag- matic petrogenetic model between the Sybille intrusion and the mantle-derived anorthositic lithologies of the Laramie Anorthosite Complex improbable. Received: 7 April 1998 / Accepted: 19 January 1999  相似文献   

13.
北淮阳新开岭地区花岗岩锆石U-Pb年龄和氧同位素组成   总被引：9，自引：2，他引：7  
对大别造山带北麓的北淮阳新开岭地区岩浆岩进行了锆石阴极发光显微结构观察和SHRIMP法锆石微区UPb定年.在锆石阴极发光图像中, 一个花岗岩样品中的大部分锆石颗粒具有明显的初始岩浆振荡环带, 为典型的岩浆锆石, 少有蚀变的颗粒和/或区域; 而另一个花岗岩样品中的锆石虽然同样具有振荡环带, 但是大部分颗粒中心的初始岩浆环带被扰动, 指示这些锆石为岩浆锆石, 受到了较强的后期热液蚀变的改造.对锆石具有初始岩浆环带和溶蚀结构的区域分别进行SHRIMP法UPb微区定年结果表明, 这些岩浆岩的形成年龄为(820±4) Ma, 热液蚀变作用发生的时间为(780±4) Ma.新开岭地区新元古代花岗质岩石的形成和后期超固相热液蚀变作用分别对应于超大陆裂解之前的约830~795Ma岩浆活动和裂解过程中约780~745Ma的岩浆作用.单矿物激光氟化氧同位素分析结果表明, 这些岩浆岩具有非常低的δ18O值, 其中锆石为1.90‰~5.78‰, 石英为-2.88‰~-7.67‰, 斜长石为-4.01‰~-11.40‰.锆石和其他矿物之间表现出强烈的氧同位素不平衡, 而其他矿物之间则达到了氧同位素的再平衡.结合不同δ18O值锆石的内部结构特征, 认为该地区的热液蚀变作用为超固相条件下的高温热液蚀变.这一过程不但改变了石英等矿物的氧同位素组成, 同时也不同程度地改变了锆石的氧同位素组成, 所以这些样品中低δ18O值锆石可能是超固相条件下热液蚀变的结果.石英中具有异常低的δ18O值表明蚀变流体来源应为寒冷气候大气降水.所以, 新开岭地区亏损18O蚀变岩石的形成与裂谷岩浆作用和雪球地球事件相耦合的高温大气降水热液蚀变有关.   相似文献   

14.
江西德兴铜厂铜矿水－岩体系氢氧同位素演化   总被引：4，自引：0，他引：4       下载免费PDF全文
 江西铜厂铜矿床露天采场岩石76个全岩氧同位素组成表明,该超大型铜矿的形成与具有5个水-岩交换成矿体系汇集在一起有关。水-岩体系计算表明,成矿流体储库形成是大气降水与千枚岩和花岗闪长斑岩在300℃以及W/R比值为0.5左右条件下形成,而后上升进入矿化沉积体系时温度降低,W/R比值超过10.0,计算的最少水量达1.9×1010t以上。  相似文献   

15.
The oxygen isotope composition of nitrate is used increasingly for identifying the origin of nitrate in terrestrial and aquatic ecosystems. This novel isotope tracer technique is based on the fact that nitrate in atmospheric deposition, in fertilizers, and nitrate generated by nitrification in soils appear to have distinct oxygen isotope ratios. While the typical ranges of δ18O values of nitrate in atmospheric deposition and fertilizers are comparatively well known, few experimental data exist for the oxygen isotope composition of nitrate generated by nitrification in soils. The objective of this study was to determine δ18O values of nitrate formed by microbial nitrification in acid forest floors.Evidence from laboratory incubation experiments and field studies suggests that during microbial nitrification in acid forest floor horizons, up to two of the three oxygen atoms in newly formed nitrate are derived from water, particularly if ammonium is abundant and nitrification rates are high. It was, however, also observed that in ammonium-limited systems with low nitrification rates, significantly less than two thirds of the oxygen in newly formed nitrate can be derived from water oxygen, presumably as a result of heterotrophic nitrification. It can be concluded from the presented data that the δ18O values of nitrate formed by microbial nitrification in acid forest floors typically range between +2 and +14‰, assuming that soil water δ18O values vary between −15 and −5‰. Hence, oxygen isotope ratios of nitrate formed by nitrification in forest floors are usually distinct from those of other nitrate sources such as atmospheric deposition and synthetic fertilizers and, therefore, constitute a valuable qualitative tracer for distinguishing among these sources of nitrate. A quantitative source apportionment appears, however, difficult because of the wide range of δ18O values, particularly for atmospheric nitrate deposition and for nitrate from microbial nitrification.  相似文献   

16.
Oxygen isotopic compositions of quartz from silty sediments buried to 5400 m depth from two wells in the Gulf of Mexico each showed δ518O variations of less than 1.7%. Clay diagenesis has been reported within this depth. The observed variations in the quartz do not appear to be primarly diagenetic effects, but rather are mainly depositional features reflecting variations in the sources of the sediments. Sources may be influenced by the variation of distance from the shore at different depths in a given sampling location and by sediment production by continental glaciations. Stability of the oxygen isotopic composition of quartz in the 10–20 μm size range under long-time humid, temperate weathering conditions was studied by analysis of saprolites formed from Pennsylvanian to Precambrian crystalline rocks. In four of the five cases, the 10–20 μm fraction was found to have only 0.1–0.5%. greater δ18O than the corresponding 20–50 μm fraction. This increase may be attributable either to a slight oxygen isotopic exchange with ambient ground waters or original differences within the rock since the saprolites were sufficiently coherent to make an influx of extraneous detrital silt unlikely. The amount of oxygen isotopic exchange in silt size quartz over periods of many million years of shallow burial or weathering appears to be small enough to permit the use of the oxygen isotopic ratio of quartz in tracing the origin of eolian and fluvial additions of minerals to continental soils and pelagic sediments.  相似文献   

17.
We discuss the nature of the ore-forming hydrothermal fluid in the Noya gold-bearing calcite-quartz-adularia veins of central Kyushu, Japan on the basis of oxygen, carbon, and strontium isotope ratios, and aqueous speciation calculations for the present-day geothermal fluid. The isotopic values of the Noya ore-forming fluid were estimated to be −6.5‰ for δ13C and −7.5‰ for δ18O. The oxygen isotopic equilibrium temperatures for vein calcite are more than 180°C at the bottom of the Noya mineralization zone, and decrease with increasing elevation. As the temperature decreased, the dominant carbon species in the fluid changed from H2CO3 to HCO3- at about 120°C. The equilibrium temperatures for vein quartz are consistent with the calcite calculations. The carbon and oxygen isotope trends of the Noya vein calcite and the isotope ratios of strontium suggest that the fluids that precipitated the Noya veins were controlled by an andesite-dominated geology. Chondrite-normalized REE patterns for the white-colored veins from wells 51-WT-1 and 51-WT-2 displayed a light REE-rich pattern with positive Eu anomalies, suggesting the existence of a reducing environment for the fluid. The pyrite-rich gray-colored veins and a silicified rock from well 51-WT-2 showed higher REE concentrations than did the white veins. Altered host andesitic rocks have similar REE patterns to that of the silicified rock, and have higher REE contents than the others in the drill cores. Aqueous speciation calculations showed that the fluid in the hydrothermal reservoir is currently in muscovite stability. The fluid at the ore-mineralization stage may have contained more potassium or have had a higher pH, so that adularia precipitated with calcite and quartz, as well as gold. Fluid boiling at depth in the system produced the gold-bearing calcite-quartz-adularia veins.  相似文献   

18.
The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix-supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM-CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM-CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a-axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ18O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ18O composition of +20·0‰. Estimates of the δ18O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ18O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.  相似文献   

19.
The Bairendaba vein-type Ag–Pb–Zn deposit, hosted in a Carboniferous quartz diorite, is one of the largest polymetallic deposits in the southern Great Xing'an Range. Reserves exceeding 8000 tonnes of Ag and 3 million tonnes of Pb?+?Zn with grades of 30 g/t and 4.5% have been estimated. We identify three distinct mineralization stages in this deposit: a barren pre-ore stage (stage 1), a main-ore stage with economic Ag–Pb–Zn mineralization (stage 2), and a post-ore stage with barren mineralization (stage 3). Stage 1 is characterized by abundant arsenopyrite?+?quartz and minor pyrite. Stage 2 is represented by abundant Fe–Zn–Pb–Ag sulphides and is further subdivided into three substages comprising the calcite–polymetallic sulphide stage (substage 1), the fluorite–polymetallic sulphide stage (substage 2), and the quartz–polymetallic sulphide stage (substage 3). Stage 3 involves an assemblage dominated by calcite with variable pyrite, galena, quartz, fluorite, illite, and chlorite. Fluid inclusion analysis and mineral thermometry indicate that the three stages of mineralization were formed at temperatures of 320–350°C, 200–340°C, and 180–240°C, respectively. Stage 1 early mineralization is characterized by low-salinity fluids (5.86–8.81 wt.% NaCl equiv.) with an isotopic signature of magmatic origin (δ18Ofluid = 10.45–10.65‰). The main ore minerals of stage 2 precipitated from aqueous–carbonic fluids (4.34–8.81 wt.% NaCl equiv.). The calculated and measured oxygen and hydrogen isotopic compositions of the ore-forming aqueous fluids (δ18Ofluid = 3.31–8.59‰, δDfluid?=??132.00‰ to??104.00‰) indicate that they were derived from a magmatic source and mixed with meteoric water. Measured and calculated sulphur isotope compositions of hydrothermal fluids (δ34S∑S?=??1.2–3.8‰) indicate that the ore sulphur was derived mainly from a magmatic source. The calculated carbon isotope compositions of hydrothermal fluids (δ13Cfluid?=??26.52‰ to??25.82‰) suggest a possible contribution of carbon sourced from the basement gneisses. The stage 3 late mineralization is dominated (1.40–8.81 wt.% NaCl equiv.) by aqueous fluids. The fluids show lower δ18Ofluid (?16.06‰ to??0.70‰) and higher δDfluid (?90.10‰ to??74.50‰) values, indicating a heated meteoric water signature. The calculated carbon isotope compositions (δ13Cfluid?=??12.82‰ to??6.62‰) of the hydrothermal fluids in stage 3 also suggest a possible contribution of gneiss-sourced carbon. The isotopic compositions and fluid chemistry indicate that the ore mineralization in the Bairendaba deposit was related to Early Cretaceous magmatism.  相似文献   

20.
The Pongkor gold–silver deposit is the largest low‐sulfidation epithermal precious metal deposit in Indonesia, and is of Pliocene age. The deposit consists of nine major subparallel quartz–adularia–carbonate veins with very low sulfide content. Vein infill records five paragenetic sequences, dominated by calcite in the early stage and quartz in the later stage of the hydrothermal evolution. Fluid inclusions in hydrothermal calcite and quartz of all stages indicate a temperature ranging from 180 to 220°C and a meteoric water origin (very low salinity close to 0 wt% NaCl equivalent). Carbon isotope data on calcite display a narrow range from ?6.5 to ?3.0‰δ13C. The oxygen isotope values have a wider range of +4.6 to +10.1‰δ18O. The broadly positive correlation of the δ13C versus δ18O plot suggests that the carbon species, which equilibrated during the formation of calcite, is dominated by H2CO3 not far from equilibrium with HCO3?. The abundance of rare earth and yttrium (REY) in carbonate samples is very low (>REY mostly <2 ppm). However, there is always a positive Eu anomaly, which indicates a deeper fluid reservoir at >250°C.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号