Evidence of hydrothermal fluid flow in a hyperextended rifted margin: the case study of the Err nappe (SE Switzerland)

This paper investigates hydrothermal fluid circulation in pre- and syn-tectonic sediments associated with detachments faults. The study area, located in the Err Nappe (SE-Switzerland), preserves a portion of the Adriatic distal margin. Two sites were studied in combining fieldwork, petrography, geochemistry and fluid inclusion analysis: the Piz Val Lunga and Fuorcla Cotschna areas. Both preserve relationships between a spectacularly exposed rift-related extensional detachment fault and its footwall and hangingwall that consist of extensional allochthons and syn- to post-tectonic sediments. These areas register a complex fluid flow history characterized by dolomitization, de-dolomitization, calcite cementation, dolomite and quartz veining and diffuse silicification. Meso- and micro-scale observations allow defining two steps in fluid evolution, which are related to Jurassic rift activity. A first carbonate-rich event occurred before the exhumation of the granitic basement, and this was followed by a second event marked by a change in the fluid towards a silica-dominated chemistry. Homogenization temperatures of fluid inclusions (average T_h = 120?130 °C), negative δ¹⁸O values and a radiogenic ⁸⁷Sr/⁸⁶Sr signatures of carbonate minerals support the hypothesis that both the pre-tectonic rocks constituting the allochthons and the syn-tectonic sediments overlying the detachment fault were crossed by a flux of over-pressured hydrothermal fluids originating from seawater that penetrated into the basement through fault and fracture systems. Field relationships show that this fluid circulation started latest in middle Early Jurassic time, when fault activity migrated from the proximal to the future distal margin. We propose that it evolved chemically as a result of the involvement of the granitic basement forming the footwall of the extensional detachment system. Hydrothermal activity continued until the Middle/Late Jurassic, when tectonic activity shifted outwards leading to the exhumation of mantle rocks. This paper provides an original contribution to better understand the complex evolution of hyperextended continental rift domains and to constrain their thermal regimes.     

Insights to magmatic-hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE_N patterns measured in anhydrite from mid-ocean ridge deposits. The REE_N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. ⁸⁷Sr/⁸⁶Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ³⁴S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ³⁴S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO₂ degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.     

Trace-element and Sr–Nd isotopic evidence for the origin of the Sardinian fluorite mineralization (Italy)

The fluorite-bearing hydrothermal mineralization in Sardinia mainly occurs within Paleozoic volcanic and metasedimentary rocks. Only 3 occurrences are located in volcanic and siliciclastic Cenozoic rocks. Most Sardinian fluorites exhibit relatively high rare earth and Y (REY) contents, strong positive Y anomalies, slightly negative Ce and generally positive Eu anomalies. These features indicate that the REY were mobilized mainly from non-carbonate rocks. Neither Sr nor Nd isotopes can be used to date radiometrically the Sardinian fluorites. However, the measured Sr-isotope ratios of the fluorites hosted by Paleozoic rocks fit mixing lines in the 1000/Sr versus ⁸⁷Sr/⁸⁶Sr plot once recalculated at 280 Ma, suggesting that the age inferred for the correction probably represents that of the formation of the fluorite mineralization. Mixing likely occurred between diluted surficial waters and brines circulating mainly through the Lower Paleozoic metasedimentary basement. The Cenozoic fluorites exhibit chemical and isotopic features similar to those of the Paleozoic fluorites, except the Nuraghe Onigu fluorite displaying a possible contribution of Sr from Cenozoic magmatic rocks. The initial ε_Nd values of the Paleozoic fluorites fit the age proposed for the formation of the deposits. Moreover, the values suggest that radiogenic Nd was provided to the fluids from the Ordovician siliciclastic basement, except for 3 deposits where the potential source rocks of Nd were mainly Ordovician acidic magmatic rocks. The initial ε_Nd values of the Cenozoic fluorites suggest a provenance of Nd essentially from the leaching of Variscan granitoids.     

相山铀矿田西部地区深部多金属矿化成矿年代与成矿流体演化:Rb-Sr同位素体系的制约

江西相山铀矿田科学深钻3号孔在深部-700 m发现大量铅锌多金属矿化脉,垂向上呈"上铀下多金属"的分布特征。本文选取深部多金属矿脉主成矿阶段(S3)自形闪锌矿样品6件和不同阶段的毒砂、黄铁矿、方铅矿、方解石等样品12件,以及围岩全岩样品17件,进行了Rb、Sr同位素组成研究。结果表明:(1)由闪锌矿Rb-Sr等时线法确定的相山铀矿田深部多金属矿化形成于121. 0±3. 5Ma,与围岩火山岩存在较大时差,可能与晚于围岩的深部次火山有关。根据穿插关系,多金属矿化略晚于碱性交代铀矿化,但明显早于酸性交代铀矿化;(2)多金属矿化脉体中金属矿物的Rb和Sr含量分别介于0. 041×10~(-6)～1. 38×10-6和2. 35×10-6～23. 11×10-6之间,Sr同位素初始比值(87Sr/86Sr)i变化较大,介于0. 706114～0. 718814之间,平均值为0. 713579,暗示相山铀矿田深部多金属矿化的成矿物质主要来源于地壳。初始流体Sr同位素值(0. 718665)明显高于成矿时赋矿围岩(流纹英安岩为0. 714581,碎斑流纹岩为0. 714417)的Sr同位素组成,表明多金属成矿流体和物质并非来自围岩火山岩;(3)由早到晚阶段的(87Sr/86Sr)i呈明显降低的演化趋势,表明成矿流体演化过程中受到大气降水的不断稀释作用。相山矿田的铀矿和深部多金属矿化同形成于华南中生代板内伸展构造背景。     

火山成因块状硫化物矿床研究进展

火山成因块状硫化物（VMS）矿床可形成于太古宙至现代各个地质时期.现代海底热液成矿作用是赋存于海相火山岩系中的古代VMS矿床成矿作用的再现.VMS矿床可形成于多种构造环境,但均与拉张背景有关.按照构造环境和容矿岩系将VMS矿床分为黑矿型、塞浦路斯型、别子型和沙利文型.VMS矿床的热液蚀变由下盘蚀变带和上盘蚀变带两个结构单元组成.下盘蚀变一般有两种类型：（1）不整合的蚀变岩筒,直接产在块状硫化物带的下方;（2）半整合或层控的上盘蚀变带,并可以在区域范围内展布.与下盘蚀变类似,上盘蚀变也具有蚀变岩筒和半整合的蚀变带两种类型.成矿金属主要有两种可能的来源：（1）在岩浆侵入体和浅位岩浆房之上被加热的循环海水对含矿火山岩系及下伏基底物质的淋滤;（2）深部岩浆房挥发分通过释气作用直接释放.下渗的海水是成矿流体的主要来源,同时流体包裹体和稳定同位素资料显示岩浆流体对成矿流体系统亦有较大贡献.VMS矿床具有明显的金属分带现象,其金属序列组合从下至上依次为Fe→Fe-Cu→Cu-Pb-Zn→Pb-Zn-Ba,这种金属分带的演化反应了流体的演化和块状硫化物丘的生长机制.根据成矿流体温度变化而不断演化的4阶段成矿模式可以很好的解释经典丘堆式矿床的形成过程.     

An experimental study of alteration of oceanic crust and terrigenous sediments at moderate temperatures (51 to 350°C): insights as to chemical processes in near-shore ridge-flank hydrothermal systems

Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 15 × 10⁶ mol U yr⁻¹ from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 10⁶ mol yr⁻¹), which concords with arguments that the Δ_element/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement.     

Origin and evolution of fluids from mud volcanoes in the Barbados accretionary complex

A large collection of fluids (54 interstitial fluids and four expelled fluids) were sampled at the Manon site, at the outer edge of the Barbados accretionary complex. These warm fluids (up to 20°C) are expelled by sub-marine (5000 mbsl) mud volcanoes consisting of diapirs (unchanneled flow) and diatremes (channeled).Chlorine stable isotope ratios of these fluids were measured by IRMS with a reproducibility of ± 0.05‰ (1σ) versus SMOC (Standard Mean Ocean Chloride).A large range of δ³⁷Cl between −5.3‰ and +0.1‰ is observed. Data from each volcanic structure describe a mixing between seawater and a low-δ³⁷Cl fluid. The whole set of data is interpreted as the result of a mixing between two deep components and seawater. The two deep fluids are chemically distinct (e.g., in Ca, Mg, K, Li, Sr and Br contents and Br/Cl ratio). They display low and significantly different ⁸⁷Sr/⁸⁶Sr ratios (0.707790 and 0.707892, respectively) and δ³⁷Cl values (−4.51 and −5.24‰, respectively).Physicochemical processes such as mineralogical transformation, diffusion, compaction or ion filtration are known to fractionate chlorine stable isotopes and can produce fluids with negative δ³⁷Cl values. Ion filtration due to sediment compaction appears to be the more likely process to explain the negative δ³⁷Cl values observed at the Manon site. A model for the generation of these signatures is proposed where a residual negative δ³⁷Cl fluid reservoir is created at the bottom of the prism or the sediment pile. Further compaction/fracturing and/or dewatering of the slab may flush out these fluids and focus them towards the décollement zone. Mixing between the fluids and ultimately with seawater and water released during gas hydrate destabilizations may explain the data set within the individual cores and between the different structures.     

Elemental and Sr–Nd–Pb isotopic geochemistry of Late Paleozoic volcanic rocks beneath the Junggar basin, NW China: Implications for the formation and evolution of the basin basement

The basement beneath the Junggar basin has been interpreted either as a micro-continent of Precambrian age or as a fragment of Paleozoic oceanic crust. Elemental and Sr–Nd–Pb isotopic compositions and zircon Pb–Pb ages of volcanic rocks from drill cores through the paleo-weathered crust show that the basement is composed mainly of late Paleozoic volcanic rock with minor shale and tuff. The volcanic rocks are mostly subalkaline with some minor low-K rocks in the western Kexia area. Some alkaline lavas occur in the central Luliang uplift and northeastern Wulungu depression. The lavas range in composition from basalts to rhyolites and fractional crystallization played an important role in magma evolution. Except for a few samples from Kexia, the basalts have low La/Nb (<1.4), typical for oceanic crust derived from asthenospheric melts. Zircon Pb–Pb ages indicate that the Kexia andesite, with a volcanic arc affinity, formed in the early Carboniferous (345 Ma), whereas the Luliang rhyolite and the Wucaiwan dacite, with syn-collisional to within-plate affinities, formed in the early Devonian (395 and 405 Ma, respectively). Positive ε_Nd(t) values (up to +7.4) and low initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of the intermediate-silicic rocks suggest that the entire Junggar terrain may be underlain by oceanic crust, an interpretation consistent with the juvenile isotopic signatures of many granitoid plutons in other parts of the Central Asia Orogenic Belt. Variation in zircon ages for the silicic rocks, different Ba, P, Ti, Nb or Th anomalies in the mafic rocks, and variable Nb/Y and La/Nb ratios across the basin, suggest that the basement is compositionally heterogeneous. The heterogeneity is believed to reflect amalgamation of different oceanic blocks representing either different evolution stages within a single terrane or possibly derivation from different terranes.     

Relationship of present saline fluid with paleomigration of basinal brines at the basement/sediment interface (Southeast basin – France)

This paper investigates the isotopic composition (O, D, Sr, O_SO4, S_SO4, Cl, He) of a present saline fluid sample collected at the sediment/basement interface in the Permian continental formation at 634 m depth in the SE margin of the Massif Central shield (Ardèche margin of the Southeast basin of France). The fluid sample shows clear water–rock interaction processes, such as feldspar dissolution and kaolinite precipitation, which have led to high Na concentrations and water stable isotopes above the local meteoric water line. The geological formations of the SE margin of the Massif Central shield show that intensive fluid circulation phases occurred across the margin from the late Triassic to the middle Jurassic. The fluids most probably originated from fluid expulsion during burial of the thick Permo-Carboniferous sedimentary succession. These circulation phases were responsible for cementation of the margin and for the solutes in the matrix microporosity which were extracted by leaching core samples.The chemical and isotopic composition of the saline fluid sample at 634 m in the Permian rock is very similar to that of fluids in the microporosity of the rock matrix. Their S_SO4, O_SO4 and Sr isotopic compositions are close to those of cements investigated in fracture fillings in the same geological formations. Simple diffusion computations and comparison of the chemical composition of the present free fluid sample with matrix porosity fluids indicate that the solutes in the present free fluid sample are related to solutes originating from fluid circulation events which occurred 160–200 Ma ago through their diffusion from the matrix microporosity.A two-stage fluid flow regime is proposed to interpret the chemistry of present and paleo-fluids. (1) During the extensional context (Permian to Cretaceous), basinal brines migrated along the basement/sediment interface after expulsion from the subsiding basin. This fluid migration would be responsible for the solutes in the rock matrix microporosity and the solutes in the present free fluid sample. (2) Following the Alpine and Pyrenean compressive phases, gravity-driven meteoritic fluids slowly migrated from the surface down to the basement along major faults. This fluid regime would be responsible for the meteoric water collected in the present free fluid sample. Several investigations in Europe have shown that the existence of other saline fluids sampled elsewhere could be explained by these phases of fluid circulation related to specific geodynamic events.     

Dolomitization of the Devonian Swan Hills Formation, Rosevear Field, Alberta, Canada

The Swan Hills Formation (Middle-Upper Devonian) of the Western Canada Basin is host to several NW-SE-trending gas fields developed in massive replacement dolostone. One of these, the Rosevear Field, contains two major dolostone trends along opposing margins of a marine channel that penetrates into a platform-reef complex. Dolostones consist predominantly of branching and bulbous strdmatoporoid floatstones and rudstones with well-developed moldic and vuggy porosity. Replacement dolomite is coarsely crystalline (100-600 μm), inclusion-rich, composed of euhedral through anhedral crystals and has a blotchy to homogeneous red cathodoluminescence. Geochemically, replacement dolomite is characterized by (i) nearly stoichiometric composition (50.1-51.1 mol% CaCO₃), (ii) negative δ¹⁸O values (mean=-7.5‰, PDB) and (iii) variable ⁸⁷Sr/⁸⁶Sr ratios ranging from values similar to Late Devonian-Early Mississippian seawater (～0.7082) to radiogenic compositions comparable to saddle dolomite cements (>0.7100). Dolomitization began after widespread precipitation of early, equant calcite spar and after the onset of pressure solution, implying that replacement dolomite formed in a burial environment. Oxygen isotope data suggest that dolomite formed at 35-75°C, temperatures reached during burial in Late Devonian through Jurassic time, at minimum depths of 450 m. The linear NW-SE orientation of most dolomite fields in the Swan Hills Formation is suggestive of fault control on fluid circulation. Two models are proposed for fault-controlled circulation of dolomitizing fluids at the Rosevear Field. In the first, compaction-driven, updip fluid migration occurred in response to basin tilting commencing in the Late Palaeozoic. Deep basinal fluids migrating updip were focused into channel-margin sediments along fault conduits. The second model calls upon fault-controlled convective circulation of (i) warm Devonian-Mississippian seawater or (ii) Middle Devonian residual evaporitic brines. The overlap in ⁸⁷Sr/⁸⁶Sr and δ¹⁸O compositions, and similar cathodoluminescence properties between replacement and saddle dolomites provide evidence for neomorphism of some replacement dolomite. Quantitative modelling of Sr and O isotopes and Sr abundances suggests partial equilibration of some replacement dolomite with hot radiogenic brines derived during deep burial of the Swan Hills Formation in the Late Cretaceous-Palaeocene. Interaction of replacement dolomite with deep brines led to enrichment in ⁸⁷Sr while leaving δ¹⁸O similar to pre-neomorphism values.     

Metal sources of the Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr isotope evidence for deep hydrothermal convection

Nd and Sr isotope analyses are presented for gangue mineral samples from the giant carbonate-hosted Navan Zn–Pb deposit, Ireland, and for rocks from which Navan metals may have been derived. Analysis of gangue minerals spanning the Navan paragenetic sequence reveals systematic evolution in the composition of the mineralising fluid. Early fluid represented by replacive dolomite exhibits the lowest initial ⁸⁷Sr/⁸⁶Sr ratio (0.7083–0.7086), closest to that of the host limestone and to Lower Carboniferous seawater, and the highest ¹⁴³Nd/¹⁴⁴Nd ratio (0.51161–0.51176). Later generations of dolomite, barite and calcite, which encompass sulphide precipitation, have higher initial ⁸⁷Sr/⁸⁶Sr ratios (maximum 0.7105) and lower initial ¹⁴³Nd/¹⁴⁴Nd ratios (minimum 0.51157). All samples have initial Nd isotope ratios that are too low to have been acquired only from the host limestone. Drill core samples of presumed Ordovician volcanic and sedimentary rocks from beneath the Navan orebody have ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios at the time of mineralisation of 0.51184–0.51217 and 0.7086–0.7138, respectively. The data are interpreted to indicate mixing of sulphide-rich, limestone-buffered brine, with a metal-bearing hydrothermal fluid, which had passed through sub-Carboniferous rocks, consistent with published fluid inclusion and S isotope data. The ¹⁴³Nd/¹⁴⁴Nd ratio of this basement-derived fluid is too low to have been imparted by flow through the Devonian Old Red Sandstone, as required in models of regional fluid flow in response to Hercynian uplift. Irrespective of whether such regional fluid flow occurred, the hydrothermal Nd must have been derived from sub-Devonian rocks. These conclusions broadly support the hydrothermal convection cell model in which brines, ultimately of surface origin, penetrated to a depth of several kilometres, leaching metals from the rocks through which they passed. The data also support increasing depth of penetration of convection cells with time. Metals were subsequently precipitated in carbonate rocks at sites of mixing with cooler, sulphide-rich fluids. However, comparison of the Navan hydrothermal gangue Nd–Sr isotope data with data from Lower Palaeozoic rocks strongly suggests that the latter cannot alone account for the “basement” signature. As the Navan deposit lies immediately north of the Iapetus Suture, this suggests that the Laurentian margin includes Precambrian basement.     

Genesis of Pb–Zn Mineralization Beneath the Xiangshan Uranium Orefield,South China: Constraints from H–O–S–Pb Isotopes and Rb–Sr Dating

The volcanic‐hosted Xiangshan uranium orefield is the largest uranium deposit in South China. Recent exploration has discovered extensive Pb–Zn mineralization beneath the uranium orebodies. Detailed geological investigation reveals that the major metallic minerals include pyrite, sphalerite, galena, and chalcopyrite, whilst the major non‐metallic minerals include quartz, sericite, and calcite. New δ¹⁸O_fluid and δD_fluid data indicate that the ore‐forming fluids were mainly derived from magmatic, and the sulfide δ³⁴S values (2.2–6.9‰) suggest a dominantly magmatic sulfur source. The Pb isotope compositions are homogeneous (²⁰⁶Pb/²⁰⁴Pb = 18.120–18.233, ²⁰⁷Pb/²⁰⁴Pb = 15.575–15.698, and ²⁰⁸Pb/²⁰⁴Pb = 37.047–38.446). The ⁸⁷Sr/⁸⁶Sr ratios of sulfide minerals range from 0.7197 to 0.7204, which is much higher than volcanic rocks and fall into the range of metamorphic basement. Lead and strontium isotopic compositions indicate that the metallogenic materials probably were derived from metamorphic basement. Pyrite Rb–Sr dating of the ores yielded 131.3 ± 4.0 Ma, indicating that the Pb–Zn mineralization occurred in the Early Cretaceous.     

Post-Variscan hydrothermal vein mineralization,Taunus, Rhenish Massif (Germany): Constraints from stable and radiogenic isotope data

Post-Variscan hydrothermal base-metal mineralization of the Taunus ore district, SE Rhenish Massif (Germany), has been studied through combination of stable (S, C, O) and radiogenic (Pb) isotope geochemistry. Based on field and textural observations, five hydrothermal mineralization types can be distinguished. These are (1) tetrahedrite–tennantite bearing quartz–ankerite veins, (2) quartz veins with Pb–Zn–Cu ores, (3) giant quartz veins, (4) metasomatic dolomite in Devonian reef complexes, and (5) calcite–(quartz) mineralization in Devonian reefs. The δ¹⁸O_V-SMOW quartz values of base-metal veins are in the range of 18.0–21.5‰, whereas those of giant quartz veins have lower values of 15.9–18.6‰. This difference reflects the higher fluid fluxes and smaller extent of rock-buffering for the giant quartz veins. Hydrothermal carbonates from the tetrahedrite and Pb–Zn–Cu veins have variable but distinctly negative δ¹³C_V-PDB values. They can be explained by contributions from fluids that had picked up low δ¹³C_V-PDB carbon via oxidation of organic matter and from fluids that interacted with Devonian reef carbonate having positive δ¹³C_V-PDB. Metasomatic dolomite has positive δ¹³C_V-PDB values that closely reflect those of the precursor limestone. By contrast, carbonates of calcite–(quartz) mineralization have negative δ¹³C_V-PDB values which are negatively correlated with the δ¹⁸O values. This pattern is explained by fluid mixing processes where contributions from descending cooler fluids with rather low salinity were dominant. The isotope data suggest that tetrahedrite veins, Pb–Zn–Cu veins, and giant quartz veins formed from fluid mixing involving two end-members with contrasting chemical features. This is supported by fluid inclusion data (Adeyemi, 1982) that show repeated alternation between two different types of fluid inclusions, which are hotter intermediate- to high-salinity NaCl–CaCl₂ fluids and cooler low-salinity NaCl-dominated fluids. The metal-rich saline fluids were likely generated at the boundary between the pre-Devonian basement and the overlying Devonian–Carboniferous nappe pile. Fault activation resulted in strong fluid focusing and upward migration of large volumes of hot Na–Ca brines, which mixed with cooler and more dilute fluids at shallower crustal levels. Variable contributions from both fluid types, local fluid fluxes, temperature variations, and variations in pH and oxidation state have then controlled the vein mineralogy and metal inventory.     

Evolution of a hydrothermal fluid-rock interaction system as recorded by Sr isotopes: A case study from the Schwarzwald, SW Germany

Metasomatic and Sr-isotopic changes, associated with formation of zoned alteration halos along hydrothermal veins, are documented for a gneiss from the Artenberg quarry near Steinach (Kinzigtal, Schwarzwald, SW Germany). Veins are postorogenic, SW-NE-oriented, and cut straight through metaquartzdioritic Variscan gneiss, where flow of low-temperature fluids (～100–200°C) caused adularia-quartz-sericite-type alteration. Fluid-rock interaction occurred nearly 50 Ma after Variscan metamorphism, as constrained by a Rb–Sr multimineral isochron for unaltered gneiss of 327.1?±?3.1 Ma, and by two independent ages of 279.2?±?3.1 Ma and 274?±?13 Ma, based on Rb–Sr systematics of late-stage quartz from the veins. In a profile from unaltered gneiss towards a vein, alteration-induced mineralogical changes correlate with metasomatic net addition of K, Rb, and Cl to the alteration zone, combined with net loss of Na, Ca, and Sr. Strontium isotopes give a more detailed insight into the fluid-rock interaction process. ⁸⁷Sr/⁸⁶Sr ratios in a profile across the alteration zone are incompatible with simple Sr leaching but reflect partial replacement of the rocks’ Sr by fluid-derived Sr, the isotopic composition of which varied with time. Early fluids, with high ⁸⁷Sr/⁸⁶Sr ratios compared to unaltered gneiss, evolved into fluids with somewhat lower ratios, and finally reached a second maximum in ⁸⁷Sr/⁸⁶Sr ratios. This Sr-isotopic fluid evolution is equally revealed by the mineral sequence of the vein mineralization. It appears that the compositional evolution of the fluids correlates with the sequence of mineral breakdown reactions in the gneissic host rock, and that the Sr-isotopic evolution of the fluids can be fully explained as the result of internal, progressive reaction of fluid with the local rocks. Results also show that the spatial distributions of Sr isotopes in metasomatic alteration zones may reflect the complex evolution of fluid-rock interaction systems, and ultimately constrain the factors controlling both fluid compositions and alteration patterns.     

Dolomitization of Quaternary reef limestone, Aitutaki, Cook Islands

Six holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void-filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ¹⁸O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO₃ and broad and weak ordering X-ray reflections at 2·41 and 2·54 A. The fine-scale replacement of skeletal grains and freshwater void-filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ¹⁸O=+2·6‰₀ PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater-seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ¹³C=2·7‰) and magnesium were derived from seawater. Low-temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section. The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.     

Fluid-rock interactions in a geothermal Rotliegend/Permo-Carboniferous reservoir (North German Basin)

Comprehensive data on the chemical composition of reservoir rocks and geothermal brines from the geothermal well doublet Groβ Schönebeck (North German Basin) drilled into a Rotliegend sedimentary and Permo-Carboniferous volcanic rock reservoir were sampled over the past years. They were characterized with respect to their major and minor elemental composition including various isotope ratios. The study considered the impact of drilling and reservoir operations on fluid composition and aimed at determining fluid–rock interactions to gain information on fluid origin and hydraulic pathways.The highly saline fluids (up to 265 g/L TDS) show δ ¹⁸O and δD of water (2.7–5.6 and −3.1–15, respectively) as well as δ ³⁴S of sulfate (3.6–5), and ⁸⁷Sr/⁸⁶Sr ratios (0.715–0.716) that resemble Rotliegend brines from an area located around 200 km in the west (the Altmark). Halogen ratios indicated that brines developed predominantly by evaporation of meteoric water (primary brine) together with halite dissolution brine (secondary brine). Indication for mixing with Zechstein brine or with younger meteoric water was not found.No geochemical distinction was possible between fluids deriving from different rock formations (dacites or sedimentary rocks, respectively). This is due to the evolution of the sediments from the effusive rocks resulting in a similar mineralogical and chemical composition and due to a hydraulic connectivity between the two types of rock. This connection existed probably already before reservoir stimulation as indicated by a set of faults identified in the area that could connect the Rotliegend formation with both, the volcanic rocks and the lower units of the Zechstein. Additional geochemical indication for a hydraulic connectivity is given by (1) the very high heavy metal contents (mainly Cu and Pb) in fluids and scaling that derive from the volcanic rocks and were that were also found in increased amounts up at the Zechstein border (Kupferschiefer formation). (2) The ⁸⁷Sr/⁸⁶Sr isotope ratios of fluid samples correspond to the ratios determined for the sedimentary rocks indicating that initially the fluids developed in the sedimentary rocks and circulated later, when faults structures were created by tectonic events into the volcanic rocks.     

Nd,Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (_Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, ⁸⁷Sr/⁸⁶Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (²⁰⁶Pb/²⁰⁴Pb=18.35–18.84, ²⁰⁷Pb/²⁰⁴Pb=15.59–15.97 and ²⁰⁸Pb/²⁰⁴Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.     

Recurrent hydrothermal activity induced by successive extensional episodes: the case of the Berta F–(Pb–Zn) vein system (NE Spain)

A mineralogical and geochemical (fluid inclusion, stable and radiogenic isotopes) study of the Berta F–(Pb–Zn) vein system has identified the source and temperature of the fluid reservoirs involved and proved the existence of two separate hydrothermal events at the mine scale, which reflect distinct periods of regional fluid circulation. Main stage minerals (fluorite I, sulphides, calcite I and barite I) precipitated by mixing between a polysaline H₂S bearing (δ³⁴S=11‰) brine (up to 23% NaCl eq salinity) and a more dilute fluid (δ¹⁸O from −3.2‰ to 0‰), at temperatures between 80 and 150°C. The progressive increase in ⁸⁷Sr/⁸⁶Sr ratio from the early precipitated minerals (0.71242 in calcite I) to the late ones (0.71894 in fluorite II) is mainly (but not exclusively) due to a difference in age separating the two hydrothermal events. The assumed genetic model for the main stage fluorite (I) is based on a convective circulation of surficial waters leaching the crystalline basement rocks acquiring a high salinity, high ⁸⁷Sr/⁸⁶Sr ratios and a high temperature. These fluids then mixed with low salinity–low temperature waters, having a low ⁸⁷Sr/⁸⁶Sr ratio. An at least Jurassic age is suggested for the main period of vein filling, contemporaneous with the extensional regime during the Mesozoic, when fluid circulation was probably enhanced by crustal thinning. During the early Burdigalian (lower Miocene), a new period of important extension in this area took place. Hydrothermal activity related to this new and younger extensional regime is geochemically different and produced a distinctive mineralogical record, developing a set of veinlets filled with green octahedral fluorite (fluorite II), calcite (II) and barite (II). The Sr isotope compositions of these late stage vein minerals are compatible with leaching the granodiorite host-rocks during recent times. The existence of successive hydrothermal events in the same area is not surprising as geothermal systems, like La Garriga–Samalus, are still active and currently precipitating fluorite.     

Formation and age of sphalerite mineralization in carbonate rocks of Bajocian age in the Swiss Jura Mountains: evidence of Mesozoic hydrothermal activity

A combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated, Cd-rich (up to 0.6 wt%), sphalerite (ZnS) mineralization in the northeastern part of the Jura Mountains, Switzerland. The Rb–Sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late Middle and early Late Jurassic, at around 162 Ma. The negative δ³⁴S values (?22.3 to ?5.3 ‰) suggest that biogenic sulfide sulfur was involved in ZnS precipitation. The strontium isotope composition is more radiogenic than that of contemporaneous seawater, reflecting the interaction of mineralizing fluids with silicate rocks. Lead isotope signatures are very uniform (²⁰⁶Pb^/204Pb = 18.63–18.67, ²⁰⁷Pb^/204Pb = 15.63–15.64, ²⁰⁸Pb^/204Pb = 38.51–38.63), indicating an isotopically well-homogenized fluid system. The basement rocks underlying the Jurassic strata are considered to be the main source of metals for the sphalerite mineralization. The migration of deep-sourced hydrothermal saline metal-bearing fluids into the Bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides. The period of sphalerite formation near the Middle–Late Jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in Europe, related to the opening of the Central Atlantic and tectonic/thermal subsidence during spreading of the Alpine Tethys. Our study provides evidence that the Bajocian carbonate rocks in the Jura Mountains area were affected by the circulation of deep-sourced metal-bearing hydrothermal fluids in response to these continent-wide tectonothermal events. The presence of sphalerite mineralization and associated geochemical anomalies in Zn and Cd contents in carbonate rocks may also be used to trace basement features.     

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.