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1.
Numerous stable isotope studies of whole rocks and mineral separates in epithermal systems indi-cate that even though meteoric waters are dominant components in epithermal systems ,fluids of other origins,such as sedimentary or meta-sedimentary fluids,magmatic waters and even evolved meteoric waters ,may also play a role in the formation of epithermal ore deposits.Usually the more depleted the wall rocks,the larger the size of ore deposits ,and the least depletion degrees in whole rocks for economic mineralization are by about 3.5‰.The depletion in δ^18O in wall rocks,however,may be complicated by the superimposition of low temperature-hydration over high-temperature altera-tion or vice versa,the existence of primary low-^18O and high-^18O magmas ,and alteration by vol-canic gases.The depletion in δ^18O in wall rocks is controlled by the composition and nature of flu-ids,the temperature of fluids,the elevation of rocks at the time of alteration ,lithology,boiling effects of fluids ,and alteration style,as well as by water/rock ratios.In addition ,the fluids re-sponsible for epithermal deposits have experienced positive δ^18O shifts .It seems that when the above complications and controlling factors are well defined,oxygen isotope studies would be a promising and powerful exploration tool.  相似文献   

2.
Mapping of the oxygen and carbon isotopic composition of hydrothermally altered wall rocks was conducted during blind ore prospecting for Pb---Zn skarn deposits in the Kamioka mining district, central Japan. The wall rocks consist of heterogeneous rock units. Oxygen and carbon isotope ratios were determined for 35 limestones and 33 silicate rocks from the area around the Mozumi deposit (3 km × 3 km) in the Kamioka mining district. The results (δ18Ospsmow of − 1.1 to + 17.3% and δ13Csppdb of − 5.0 to +4.8% for limestones, and δ18Ospsmow of −0.8 to + 12.5% for silicate rocks) show isotope zonations of the wall rocks, with lighter isotopic compositions toward the center of mineralization. The isotope zonations likely formed by interaction of thermal waters with the wall rocks during skarn mineralization. The isotopically light zone indicates a higher paleotemperature or higher water-to-rock ratios, and occurs in the footwall of the 7Gohi fault. This structure is spatially related to the distribution of orebodies, indicating that the fault was the main conduit of the ore-forming fluids. The oxygen and carbon isotopic compositions of limestones vary regularly across limestone bodies hosted by the silicate wall rocks, suggesting that the thermal waters were pervasive throughout the wall rocks at the time of mineralization.An isotopically light zone was also found in the southeastern corner of the study area, where significant mineralization had yet to be identified. This suggested an extension of the extinct hydrothermal system to this area, and the possibility of hidden orebodies underneath. Recent drilling in this area has intercepted a zone 45 m thick with a grade of 13.4% Zn, 0.03% Pb and 8 g per metric ton at about 380 m depth.  相似文献   

3.
Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boléo district of the Miocene Santa Rosalía basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boléo Formation is characterized by modified marine carbon (δ 13CPDB=−6.0 to +4.4‰) and oxygen (δ 18OSMOW=+19.5 to +26.2‰) isotope composition due to the influx of 13C- and 18O-depleted fluvial water. Sulfate sulfur isotope composition (δ 34SCDT=+17.21 to +22.3‰ and δ 18OSMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ 34S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ 34S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ 13C=−11.6 to −3.2‰ and δ 18O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ 13C=−7.9 to +4.3‰ and δ 18O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.  相似文献   

4.
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   

5.
In order to reconstruct the formation and exhumation mechanisms of UHP metamorphic terrains, the Chinese Continental Scientific Drilling Program (CCSD) has been carried out in Donghai of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, East China. Eclogite, gneiss, amphibolite (retrograded from eclogite), ultramafic rocks, and minor schist and quartzite have been drilled. Aiming to reveal the fluid behaviour in a vertical sequence of an UHP slab, we investigated fluid inclusion and oxygen isotope characteristics of selected drillcores from the main hole and the pilot-holes PP2 and ZK 703 of the CCSD. More than 540 laser-ablation oxygen isotope analyses on garnet, omphacite, quartz, kyanite, amphibole, phengite, rutile, epidote, amphibole, plagioclase, and biotite from various rocks in the depth range of 0–3,000 m (mainly eclogite and gneiss) show that the investigated rocks can be divided into two groups: 18O-depleted rocks (as low as δ18O = −7.4‰ for garnet) indicate interaction with cold climate meteoric waters, whereas 18O-normal rocks (with bulk δ18O > +5.6‰) have preserved the O-isotopic compositions of their protoliths. Meteoric water/rock interaction has reached depths of at least 2,700 m. Oxygen isotope equilibrium has generally been achieved. Isotopic compositions of mineral phases are homogeneous on a mm to cm scale regardless of lithology, but heterogeneous on the scale of a few metres. Oxygen isotope distributions in the vertical sections favour an “in situ” origin of the UHP metamorphic rocks. The very negative δ18O eclogites usually have higher hydroxyl-mineral contents than the normal δ18O rocks, indicating higher water content during UHP metamorphism. Fluid inclusion data suggest that rocks with depleted 18O compositions have had different fluid histories compared to those with normal δ18O values. Rocks with depleted 18O mainly have primary medium-to-high salinity inclusions in omphacite, kyanite and quartz, and abundant secondary low-salinity or pure water inclusions in quartz, indicating a high-salinity-brine-dominated fluid system during peak UHP metamorphism; no carbonic inclusions have been identified in these rocks. By contrast, primary very high-density CO2 inclusions are commonly found in the rocks with normal δ18O values. These observations suggest that fluid and oxygen isotope composition of minerals are related and reflect variable degrees of alterations of the Dabie-Sulu UHP metamorphic rocks.  相似文献   

6.
The oxygen isotopic composition was studied in minerals and rocks from the critical zone of the Ioko-Dovyren layered pluton. The δ18O values vary from +5.4 to +6.1‰ in rocks, from +4.8 to +5.8‰ in olivine, from +5.5 to 6.5‰ in pyroxene, and from +5.8 to +6.9‰ in plagioclase and fall into the interval of mantle values for continental mafic and ultramafic rocks. A decrease in δ18O could have been caused by penetration of meteoric water. Postmagmatic (retrograde) oxygen isotopic redistribution in the slowly cooling rocks is responsible for disturbance of oxygen isotope equilibria in the coexisting minerals, which were crystallized from the same magma at a high temperature. The nonequilibrium oxygen isotopic composition in the associated minerals and calculated temperature of the final isotopic equilibration do not contradict the model of “fluid” formation of low-sulfide PGE mineralization in the Ioko-Dovyren layered pluton.  相似文献   

7.
Copper–nickel sulfide mineralization in the Partridge River Intrusion of the 1.1 Ga Duluth Complex is restricted primarily to a 100 m thick zone near the base of the intrusion, which is heterogeneous at meter scales in terms of both sulfide contents and rock types, which include dunite, melatroctolite, troctolite, leucotroctolite, gabbro, olivine gabbro, gabbronorite, and rare norite. Olivine-rich troctolites and melatroctolites appear to have required mineral accumulation on a substrate, whereas augite troctolite and gabbros are thought to have formed via in situ crystallization of magmas ranging in composition from high-Al olivine tholeiite to high-Ti tholeiite. δ18O values of orthopyroxene-poor rocks in the Partridge River Intrusion range from 5.2 to 6.7‰. δ18O values of 6.7‰ are consistent with less than 20% contamination by high-18O metasedimentary country rock, either via devolatilization or local partial melting. Rocks with greater than ∼15% orthopyroxene, gabbronorites, and norites, are characterized by δ18O values in excess of 6.9‰, and required the assimilation of larger amounts of siliceous country rocks. Sulfur isotopic values in leucotroctolitic rocks that contain less than ∼400 ppm S and that overlie the basal zone range between −1.5 and 2‰, values that are consistent with those of mantle-derived sulfur. In contrast, δ34S values in the basal zone range from −1.4 to 10.5‰, where the 34S-enriched samples require an input of sulfur from metasedimentary country rocks. δ34S values of the rocks in the basal zone correlate with variations in olivine Fo content but not with S abundance. The wide range in δ34S values of rocks in the basal zone strongly suggests that magmas interacted with layers in the sedimentary country rocks that were themselves characterized by variable sulfide contents and δ34S values. The S isotopic data suggest that the heterogeneity observed in the basal zone results from the emplacement of relatively thin sheets of compositionally distinct magma. All rock types present in the basal zone can be produced as a result of variable degrees of fractionation of a parental high-Al olivine tholeiite, followed by varying degrees of contamination of derivative liquids by country rocks. The S-contamination process was essential for the development of Cu–Ni mineralization, and was restricted to the earliest stages in the development of the Duluth Complex at a time when volatile species such as S and H2O, and low-T partial melts of country rocks, were available to magmas. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.  相似文献   

8.
Summary The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112–97 Ma), the largest of which occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25‰ (V-SMOW). The δ18O values decrease down to values of 12‰ towards the contact with dolostones. This 18O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which vary from 87Sr/86Sr = 0.7087–0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with 87Sr/86Sr = 0.7094 near dolomitization fronts. Dolostones have δ18O values (about 9‰) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (87Sr/86Sr = 0.7109 to 0.7130). Hydrothermal calcites have δ18O values close to dolostones but substantially lower δ13C values, down to −6.5‰, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with 87Sr/86Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of the O isotope composition of the infiltrating water at 300 °C, which is in the δ18O = 2–4.5‰ range. Hydrogen isotope compositions of talc and chlorite indicate a δD = 0 to −20‰. This water probably derived from seawater, with minor contribution of evolved continental water.  相似文献   

9.
Whole rock and mineral stable isotope and microprobe analyses are presented from granitoids of the North Chilean Precordillera. The Cretaceous to Tertiary plutonic rocks contain important ore deposits and frequently display compositional and textural evidence of hydrothermal alteration even in barren rocks. Deuteric alteration includes replacement of biotite and amphibole by chlorite and epidote, sericitization and saussuritization of feldspars, and uralitization of clinopyroxene and/or amphibole. While whole rock compositions are not significantly affected, compositional variations in amphiboles suggest two types of hydrothermal alteration. Hornblende with actinolitic patches and rims and tight compositional trends from hornblende to Mg-rich actinolite indicate increasing oxygen fugacity from magmatic to hydrothermal conditions. Uralitic amphiboles exhibiting irregular Mg-Fe distribution and variable Al content are interpreted as reflecting subsolidus hydration reactions at low temperatures. The δD values of hydrous silicates vary from −63 to −105‰. Most δ18O values of whole rocks are in the range of 5.7 to 7.7‰ and are considered normal for igneous rocks in the Andes. These δ18O values also coincide well with the oxygen isotope composition of geochemically similar recent volcanics from the Central Andean Volcanic Zone (δ18O = 7.0–7.4‰). Only one sample in this study (δ18O = 3.0‰) appears to be depleted by isotope exchange with light meteoric water at high temperatures. The formation of secondary minerals in all other intrusions is mainly the product of deuteric alteration. This also holds true for the sample from El Abra, the only pluton associated with mineralization. This indicates the dominant role of a magmatic rather than a meteoric fluid in the alteration of the Cretaceous and Tertiary granitoids in northern Chile. Received: 8 July 1998 / Accepted: 15 April 1999  相似文献   

10.
1 IntroductionThe compositional characteristics of natural gasesand their carbon isotopic composition are controlledmainly by the type of organic matter and the degree ofits thermal evolution in the source rocks (Song Yan,1995; Xu Yongchang et al., 1994; …  相似文献   

11.
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   

12.
Summary Telluride-bearing gold deposits of the Pingyi area, western Shandong, China, are located on the southeastern margin of the North China Craton. There are two main types of deposits: (i) mineralized cryptoexplosive breccia, e.g., Guilaizhuang; and (ii) stratified, finely-disseminated mineralization hosted in carbonate rocks, e.g., Lifanggou and Mofanggou deposits. In Guilaizhuang, the cryptoexplosive breccia is formed within rocks of the Tongshi complex and Ordovician dolomite. The mineralization is controlled by an E–W-trending listric fault. Stratified orebodies of the Lifanggou and Mofanggou deposits are placed along a NE-trending, secondary detachment zone. They are hosted within dolomitic limestone, micrite and dolomite of the Early-Middle Cambrian Changqing Group. The mineralization in the ore districts is considered to be related to the Early Jurassic Tongshi magmatic complex that formed in a continental arc setting on the margin of the North China Craton. The host rocks are porphyritic and consist predominantly of medium- to fine-grained diorite and pyroxene (hornblende)-bearing monzonite. SHRIMP U–Pb zircon dating of diorites give a 206Pb/238U weighted mean age of 175.7 ± 3.8 Ma. This is interpreted as representing the crystallization age of the Tongshi magmatic complex. Considering the contact relationships between the magmatic and host sedimentary rocks, as well as the genetic link with the deposits, we conclude that this age is relevant also for the formation of mineralization in the Pingyi area. We hence consider that the deposits formed in the Jurassic. The principal gold minerals are native gold, electrum and calaverite. Wall-rock alteration comprises pyritization, fluoritization, silicification, carbonatization and chloritization. Fluid inclusion studies indicate that all the analyzed inclusions are of two-phase vapor–liquid NaCl–H2O type. Homogenization temperatures of the fluid inclusions vary from 103 °C to 250 °C, and the ice melting temperatures range from −2.5 °C to −13.5 °C, corresponding to a salinity range of 4.65 to 17.26 wt.% NaCl equiv. The δ34S values of pyrite associated with gold mineralization exhibit a narrow range of −0.71 to + 2.99‰, implying that the sulfur was probably derived from the mantle and/or dioritic magma. The δ13CPDB values of the fluid inclusions in calcite range from −7.3 to 0.0‰. The δ18OSMOW values of vein quartz and calcite range from 11.5 to 21.5‰, corresponding to δ18Ofluid values of −1.1 to 10.9‰; δD values of the fluid inclusions vary between −70 and −48‰. The isotope data for all three deposits suggest mixing of ore-forming fluids derived from the mantle and/or magma with different types of fluids at shallow levels. Pressure release and boiling of the fluids, as well as fluid-rock interaction (Lifanggou and Mofanggou) and mixing of magmatically-derived fluids with meteoritic waters (Guilaizhuang) played an important role in the ore-forming processes.  相似文献   

13.
The relationships between the δ18O of quartz veins and veinlets pertaining to the main stage of gold mineralization at the Sukhoi Log deposit and metasomatically altered host slates are estimated. The oxygen isotopic composition of veined quartz and host slates is not uniform. The δ18O of quartz veins from the Western, Central, and Sukhoi Log areas of the deposit vary from +16 to + 18 ‰. The δ18O range of metasomatically altered slates in the Western and Sukhoi Log areas attains 6 ‰. The δ18O of quartz veins are always higher than those of host slates by 3–7‰. The regular difference in the δ18O between quartz veins and host slates indicates that the oxygen isotopic composition of the ore-bearing fluid forming the system of quartz veins and veinlets at the Sukhoi Log deposit could have formed as a result of interaction with silicate rocks, for instance, terrigenous slates enriched in δ18O. Such interaction, however, took place at deeper levels of the Sukhoi Log deposit. It is suggested that the fluid phase participating in the formation of the vein and veinlet system had initially high δ18O(>+10‰) due to interaction with the rocks enriched in δ18O at a low fluid/rock ratio. The oxygen isotope data indicate that the fluid participating in the formation of gold mineralization at the Sukhoi Log deposit was not in equilibrium with igneous rocks at high temperatures.  相似文献   

14.
Sulfur isotope composition of carbonate-associated sulfate (δ34SCAS) and carbon isotope composition of carbonate (δ13Ccarb) were jointly investigated on the Late Permian rocks at Shangsi Section, Guanyuan, Northeast Sichuan, South China. Both δ34SCAS and δ13Ccarb show gradual decline trends in Late Permian strata, inferring the occurrence of the long-term variation of marine environmental conditions. Associated with the long-term variation are the two coincident negative shifts in δ34SCAS and δ13Ccarb, with one occurring at the boundary between Middle Permian Maokou Formation and Late Permian Wujiaping Formation and another at Middle Dalong Formation. Of significance is the second shift which clearly predates the regression and the biotic crisis at the end of Permian at Shangsi Section, providing evidence that a catastrophic event occurred prior to the biotic crisis. The frequent volcanisms indicated by the volcanic rocks or fragments, and the upwelling are proposed to cause the second negative excursion. An abrupt extreme negative δ34SCAS (ca. −20‰) associated with a low relative concentration of CAS and total organic carbon without large change in δ13Ccarb is found at the end of the second shift, which might arise from the short-term oxygenation of bottom waters and sediments that resulted from the abrupt sea level drop.  相似文献   

15.
The Mont-de-l’Aigle deposit is located in the northern part of Dome Lemieux, in the Connecticut Valley-Gaspé Synclinorium, Gaspé Peninsula, Québec. The Dome Lemieux is a subcircular antiform of Siluro–Devonian sedimentary rocks that is cut by numerous mafic and felsic sills and dikes of Silurian to Late Devonian age. Plutonism occurred in a continental within-plate extensional setting typical of orogenic collapse. The Cu−Fe (± Au) mineralization of Mont-de-l’Aigle occurs in veins, stockworks, and breccias. Mineralization is located near or within N−S and NW−SE faults cutting sedimentary rocks. IOCG mineralization postdates intrusions, skarns, hornfels, and epithermal mineralization typical of the southern part of the Dome Lemieux. The paragenetic sequence comprises: (1) pervasive sodic, potassic, chlorite, and silica alteration, (2) hematite, quartz, pyrite, magnetite, and chalcopyrite veins, stockworks and breccias and, (3) dolomite ± hematite veins and veinlets cutting the earlier mineralization. Intrusions display proximal sodic and potassic alteration, whereas sedimentary rocks have proximal decalcification, silicification, and potassic alteration. Both intrusive and sedimentary rocks are affected by a pervasive distal chlorite (± silica) alteration. The sulfur isotope composition of pyrite and chalcopyrite (δ34S=−1.5 to 4.8‰) suggests that sulfur was derived mainly from igneous rocks. Fluid δ18O (−0.4 to 2.65‰) indicates meteoric or seawater that reacted with the country rocks. Mixing of hot magmatic fluids with a cooler fluid, perhaps meteoric or seawater is suggested for mineral deposition and alteration of the Mont-de-l’Aigle deposit. The mineralogy, alteration, and sulfur isotope composition of the Mont-de-l’Aigle deposit compare well with IOCG deposits worldwide, making the Mont-de-l’Aigle deposit a rare example of Paleozoic IOCG mineralization, formed at shallow depth, within a low metamorphic grade sedimentary rock sequence.  相似文献   

16.
The sediment-hosted stratiform Cu–Co mineralization of the Luiswishi and Kamoto deposits in the Katangan Copperbelt is hosted by the Neoproterozoic Mines Subgroup. Two main hypogene Cu–Co sulfide mineralization stages and associated gangue minerals (dolomite and quartz) are distinguished. The first is an early diagenetic, typical stratiform mineralization with fine-grained minerals, whereas the second is a multistage syn-orogenic stratiform to stratabound mineralization with coarse-grained minerals. For both stages, the main hypogene Cu–Co sulfide minerals are chalcopyrite, bornite, carrollite, and chalcocite. These minerals are in many places replaced by supergene sulfides (e.g., digenite and covellite), especially near the surface, and are completely oxidized in the weathered superficial zone and in surface outcrops, with malachite, heterogenite, chrysocolla, and azurite as the main oxidation products. The hypogene sulfides of the first Cu–Co stage display δ34S values (−10.3‰ to +3.1‰ Vienna Canyon Diablo Troilite (V-CDT)), which partly overlap with the δ34S signature of framboidal pyrites (−28.7‰ to 4.2‰ V-CDT) and have ∆34SSO4-Sulfides in the range of 14.4‰ to 27.8‰. This fractionation is consistent with bacterial sulfate reduction (BSR). The hypogene sulfides of the second Cu–Co stage display δ34S signatures that are either similar (−13.1‰ to +5.2‰ V-CDT) to the δ34S values of the sulfides of the first Cu–Co stage or comparable (+18.6‰ to +21.0‰ V-CDT) to the δ34S of Neoproterozoic seawater. This indicates that the sulfides of the second stage obtained their sulfur by both remobilization from early diagenetic sulfides and from thermochemical sulfate reduction (TSR). The carbon (−9.9‰ to −1.4‰ Vienna Pee Dee Belemnite (V-PDB)) and oxygen (−14.3‰ to −7.7‰ V-PDB) isotope signatures of dolomites associated with the first Cu–Co stage are in agreement with the interpretation that these dolomites are by-products of BSR. The carbon (−8.6‰ to +0.3‰ V-PDB) and oxygen (−24.0‰ to −10.3‰ V-PDB) isotope signatures of dolomites associated with the second Cu–Co stage are mostly similar to the δ13C (−7.1‰ to +1.3‰ V-PDB) and δ18O (−14.5‰ to −7.2‰ V-PDB) of the host rock and of the dolomites of the first Cu–Co stage. This indicates that the dolomites of the second Cu–Co stage precipitated from a high-temperature, host rock-buffered fluid, possibly under the influence of TSR. The dolomites associated with the first Cu–Co stage are characterized by significantly radiogenic Sr isotope signatures (0.70987 to 0.73576) that show a good correspondence with the Sr isotope signatures of the granitic basement rocks at an age of ca. 816 Ma. This indicates that the mineralizing fluid of the first Cu–Co stage has most likely leached radiogenic Sr and Cu–Co metals by interaction with the underlying basement rocks and/or with arenitic sedimentary rocks derived from such a basement. In contrast, the Sr isotope signatures (0.70883 to 0.71215) of the dolomites associated with the second stage show a good correspondence with the 87Sr/86Sr ratios (0.70723 to 0.70927) of poorly mineralized/barren host rocks at ca. 590 Ma. This indicates that the fluid of the second Cu–Co stage was likely a remobilizing fluid that significantly interacted with the country rocks and possibly did not mobilize additional metals from the basement rocks.  相似文献   

17.
Stable carbon and oxygen isotopic analyses of the planktonic foraminifera (Globigerinoides ruber) from a deep sea sediment core (GC-1) in the Andaman Sea show high glacial-to-Holocene δ18O amplitude of 2.1%o which is consistent with previously published records from this marginal basin and suggest increased salinity and/or decreased temperature in the glacial surface waters of this region. A pulse of18O enrichment during the last deglaciation can be attributed to a Younger Dryas cooling event and/or to a sudden decrease of fresh water influx from the Irrawady and Salween rivers into the Andaman Sea. High δ13C values observed during the isotopic stages 2 and 4 are probably due to the enhanced productivity during glacial times in the Andaman Sea.  相似文献   

18.
Natural and anthropogenic impacts on karst ground water, Zunyi, Southwest China, are discussed using the stable isotope composition of dissolved inorganic carbon and particulate organic carbon, together with carbon species contents and water chemistry. The waters can be mainly characterized as HCO3–Ca type, HCO3 · SO4–Ca type, or HCO3 · SO4–Ca · Mg type, according to mass balance considerations. It is found that the average δ13CDIC values of ground waters are higher in winter (low-flow season) than in summer (high-flow season). Lower contents of dissolved inorganic carbon (DIC) and lower values of δ13CDIC in summer than in winter, indicate that local rain events in summer and a longer residence time of water in winter play an important role in the evolution of ground water carbon in karst flow systems; therefore, soil CO2 makes a larger contribution to the DIC in summer than in winter. The range of δ13CDIC values indicate that dissolved inorganic carbon is mainly controlled by the rate of carbonate dissolution. The concentrations of dissolved organic carbon and particulate organic carbon in most ground water samples are lower than 2.0 mg C L−1 and 0.5 mg C L−1, respectively, but some waters have slightly higher contents of organic carbon. The waters with high organic carbon contents are generally located in the urban area where lower δ13CDIC values suggest that urbanization has had an effect on the ground water biogeochemistry and might threaten the water quality.  相似文献   

19.
The Spessart district (SW Germany), located at the southwestern margin of the Permian Kupferschiefer basin in Central Europe, hosts abundant stratabound and structurally controlled base metal mineralization. The mineralization styles identified are (1) stratabound Cu-Pb-Zn-(Ag) ores in Zechstein sedimentary rocks, (2) structurally controlled Cu-As-(Ag) ores in Zechstein sedimentary rocks, (3) crosscutting Co-Ni-(Bi)-As and Cu-Fe-As veins, (4) stratabound metasomatic Fe-Mn carbonate ores in Zechstein dolomite, (5) barren barite veins, and (6) Fe-Mn-As veins in Permian rhyolites. Building on previous work that involved mineralogical, textural, and chemical characterization of the major mineralization types, we have performed a comprehensive sulfur isotope study that applied both conventional and novel laser-ablation multi-collector inductively coupled plasma mass spectrometry techniques. The δ34S values of sulfide minerals from the different ore types are consistently negative and highly variable, in the range between −44.5‰ and −3.9‰, whereas the δ34S values of barite are all positive in the range between 4.7‰ and 18.9‰. Remarkably, stratabound and structurally controlled mineralization in Zechstein sedimentary rocks has the least negative δ34S values, whereas vein-type deposits have consistently more negative δ34S values. The observed pattern of sulfide δ34S values can be best interpreted in terms of fluid mixing at the basement-cover interface. Hydrothermal fluids originating from the crystalline basement migrated upward along subvertical fault zones and were periodically injected into groundwaters that were flowing in the post-Variscan sedimentary cover. These groundwaters had interacted with the Zechstein sedimentary rocks, resulting in fluids characterized by elevated concentrations of reduced sulfur (with negative δ34S values) and alkaline pH. Repeated mixing between both chemically contrasting fluids caused rapid and efficient precipitation of sulfide ore minerals in hydrothermal veins with highly variable but distinctly negative δ34S values.  相似文献   

20.
The Brandberg West region of NW Namibia is dominated by poly-deformed turbidites and carbonate rocks of the Neoproterozoic Damara Supergoup, which have been regionally metamorphosed to greenschist facies and thermally metamorphosed up to mid-amphibolite facies by Neoproterozoic granite plutons. The meta-sedimentary rocks host Damaran-age hydrothermal quartz vein-hosted Sn–W mineralization at Brandberg West and numerous nearby smaller deposits. Fluid inclusion microthermometric studies of the vein quartz suggests that the ore-forming fluids at the Brandberg West mine were CO2-bearing aqueous fluids represented by the NaCl–CaCl2–H2O–CO2 system with moderate salinity (mean=8.6 wt% NaClequivalent).Temperatures determined using oxygen isotope thermometry are 415–521°C (quartz–muscovite), 392–447°C (quartz–cassiterite), and 444–490°C (quartz–hematite). At Brandberg West, the oxygen isotope ratios of quartz veins and siliciclastic host rocks in the mineralized area are lower than those in the rocks and veins of the surrounding areas suggesting that pervasive fluid–rock interaction occurred during mineralization. The O- and H-isotope data of quartz–muscovite veins and fluid inclusions indicate that the ore fluids were dominantly of magmatic origin, implying that mineralization occurred above a shallow granite pluton. Simple mass balance calculations suggest water/rock ratios of 1.88 (closed system) and 1.01 (open system). The CO2 component of the fluid inclusions had similar δ 13C to the carbonate rocks intercalated with the turbidites. It is most likely that mineralization at Brandberg West was caused by a combination of an impermeable marble barrier and interaction of the fluids with the marble. The minor deposits in the area have quartz veins with higher δ 18O values, which is consistent with these deposits being similar geological environments exposed at higher erosion levels.  相似文献   

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