Identifying low-temperature hydrothermal karst and palaeowaters using stable isotopes: a case study from an alpine cave,Entrische Kirche,Austria

The area south of the prominent east–west trending Salzach Valley at the northern rim of the Central Alps of Austria has long been known to host anomalously warm springs emerging from a highly deformed calcite marble (Klammkalk). This unit also hosts cavities whose shapes suggest a hydrothermal karst origin and which are lined by calcite spar. We report here petrographic and isotopic evidence suggesting that dissolution by ascending low-temperature thermal waters also played an important role in the origin of a large cave in this region, Entrische Kirche. A paleo cave wall, preserved behind a thick flowstone in the interior of this cave, revealed a brownish bleaching zone which contrasts to the medium grey colour of the unaltered marble beneath. Across this zone the C and O isotope values gradually decrease by 3 and 11‰, respectively. These compositions are very different from those of the speleothem above but are similar to phreatic calcite spar from hydrothermal karst cavities in other outcrops in the area, where the absence of two-phase fluid inclusions suggests a low-temperature (less than ca. 50°C) hydrothermal origin. U/Th dating of the flowstone capping the alteration zone yielded a minimum age of the thermal water invasion in Entrische Kirche of ca. 240 kyr. There is no evidence in Entrische Kirche that these palaeowaters reached the point of calcite precipitation, but it is physically conceivable that higher and as yet unexplored parts of this deep (ca. 900 m) cave contain cavities lined by phreatic cave spar.     

Assessing the past thermal and chemical history of fluids in crystalline rock by combining fluid inclusion and isotopic investigations of fracture calcite

Fluid inclusion studies combined with the isotope geochemistry of several generations of fracture calcite from the Olkiluoto research site, Finland, has been used to better understand the past thermal and fluid history in the crystalline rock environment. Typically, fracture mineral investigations use O and C isotopes from calcite and an estimate of the isotopic composition of the water that precipitated the calcite to perform δ¹⁸O geothermometry calculations to estimate past temperature conditions. By combining fluid inclusion information with calcite isotopes, one can directly measure the temperature at which the calcite formed and can better determine past fluid compositions. Isotopic, petrologic and fluid inclusion studies at the Olkiluoto research site in Finland were undertaken as part of an investigation within the Finnish nuclear waste disposal program. The study revealed that four fluids were recorded by fracture calcites. From petrologic evidence, the first fluid precipitated crystalline calcite at 151–225°C with a δ¹³C signature of −21 to −13.9‰ PDB and a δ¹⁸O signature of 12.3–13.0‰ SMOW. These closed fracture fillings were found at depths greater than 500 m and were formed from a high temperature, low salinity, Na–Cl fluid of possible meteoric water altered by exchange with wallrock or dilute basinal origin. The next fluid precipitated crystalline calcite with clay at 92–210°C with a δ¹³C signature of −2.6 to +3.8‰ PDB and a δ¹⁸O signature of 19.4–20.7‰ SMOW. These closed fracture fillings were found at depths less than 500 m and were formed from a moderate to high temperature, low to moderate salinity, Na–Cl fluid, likely of magmatic origin. The last group of calcites to form, record the presence of two distinct fluid types. The platy (a) calcite formed at 95–238°C with a δ¹³C signature of −12.2 to −3.8‰ PDB and a δ¹⁸O signature of 14.9–19.6‰ SMOW, from a high temperature, low salinity, Na–Cl fluid of possible magmatic origin. The platy (b) calcite formed at 67–98°C with a δ¹³C signature of −13.0 to −6.2‰ PDB and a δ¹⁸O signature of 15.1–20.1‰ SMOW, from a low temperature, high salinity, Ca–Na–Cl fluid of possible basinal brine origin. The two calcites are related through a mixing between the two end members. The source of the fluids for the platy grey (a) calcites could be the olivine diabase dykes and sills that cut through the site. The source of fluids for the platy (b) calcites could be the Jotnian arkosic sandstone formations in the northern part of the site. At the Olkiluoto site, δ¹⁸O geothermometry does not agree with fluid inclusion data. The original source of the water that forms the calcite has the largest effect on the isotopic signature of the calcites formed. Large isotopic shifts are seen in any water by mineral precipitation during cooling under rock–water equilibrium fractionation conditions. Different calcite isotopic signatures are produced depending on whether cooling occurred in an open or closed system. Water–rock interaction, at varying W/R ratios, between a water and a host rock can explain the isotopic shifts in many of the calcites observed. In some cases it is possible to shift the δ¹⁸O of the water by +11.5‰ (SMOW) using a realistic water–rock ratio. This process still does not explain some of the very positive δ¹⁸O values calculated using fluid inclusion data. Several other processes, such as low temperature recrystallization, boiling, kinetic effects and dissolution of calcite from fluid inclusion walls can affect isotopic signatures to varying degrees. The discrepancy between fluid inclusion data and δ¹⁸O geothermometry at the Olkiluoto site was most likely due to poor constraint on the original source of the water.     

Syn-metamorphic gold mineralisation,Invincible Vein,NW Otago Schist,New Zealand

The Invincible Vein fills a fault zone which strikes northeast and dips steeply southeast in the lower Rees Valley, NW Otago. The vein cuts north striking foliation in lower greenschist facies Otago Schist. Structures associated with the fault zone are both brittle and ductile, and the fault zone has had a complex history of post-mineralisation reactivation. Mineralised vein material filling parts of the fault zone consist of quartz, albite, muscovite, chlorite, calcite, pyrite, arsenopyrite and minor gold. These minerals have been strained and locally recrystallised during ductile deformation. Fluid inclusion homogenisation temperatures (140–175°C) and ice melting temperatures (0 to –1°C) indicate that the mineralising fluid was low salinity, low CO₂ water with a density between 0.88 and 0.93 g/cm₃. Arsenopyrite geothermometry implies a temperature of mineralisation of 370 ± 70°C. Mineralisation pressure lay between 2 and 5 kbar. Mineralisation pressure-temperature conditions and mineralogy are essentially the same as for metamorphism of the host schist. Vein calcite oxygen isotope ratios (+12 to +15 per mil) are similar to host schist values. Carbon isotope ratios of vein calcite (– 3 to –5 per mil) are distinctly different from ratios in host schist (–7 to –10 per mil). Elevated vein Cr contents, and isotopically depleted carbon data, are consistent with some degree of equilibration with metavolcanic rocks. It is inferred that metavolcanic rocks of the underlying Aspiring Terrane were a significant source for mineralising fluid and metals. Invincible mineralisation occurred in the latter stages of metamorphism, and is the earliest recognised gold-bearing vein system in the Otago Schist.     

湘西花垣李梅铅锌矿区古热液卡斯特特征及其成因研究

发育于湘西花垣县李梅铅锌矿区内下寒武统清虚洞组藻灰岩中的一套角砾岩,具古卡斯特特征。溶蚀洞穴、溶蚀崩解、溶蚀变形构造以及特殊的洞穴充填角砾和洞穴矿物可作为鉴别和区分不同于一般沉积角砾岩的主要特征。X衍射、包体测温、包体成分及氢氧同位素组合分析表明:本区古卡斯特为一种低温(150℃±)、高盐度(14.89～39.24％)、弱酸性(pH=5.75.t=150℃)至弱碱性(pH=6.88、t=150℃)、还原型(Eh:-0.7721～-0.8699)的CaCl型深层热卤水的溶蚀作用所造成。     

方解石充填物对于深层古岩溶洞穴保存的独特意义--以塔河地区奥陶系为例

塔河油田奥陶系古岩溶洞穴埋藏深度之大在世界范围内罕见,但对其为何没有垮塌殆尽的原因却知之甚少。为了探究其保存机制,对比了Fort Worth盆地奥陶系深埋洞穴,发现大量缝、洞充填方解石是塔河地区洞穴中常见且特有的充填物。在统计和综合分析缝洞方解石充填物地球化学数据的基础上,以洞穴充填方解石成因类型为切入点,详细分析了构造运动背景、热液活动、油气充注、方解石的产状以及洞穴受力情况,通过与Fort Worth盆地深埋洞穴埋藏史的对比和分析,提出塔河地区深埋岩溶洞穴的一种保存机制。研究区岩溶洞穴方解石主要是长期浅埋阶段,以大气淡水成因为主,海水成因为辅形成,深埋过程还受到了溶蚀性流体（岩浆热液和有机酸）后期改造。方解石充填物增加了洞穴的抗压性,岩溶洞穴内部充填的方解石在埋藏过程中支撑了洞穴;岩溶洞穴顶部裂纹中充填的方解石还“愈合”了洞穴周边的裂纹,强化了洞穴的顶板,增加了洞穴整体的稳定性。充填的方解石在垮塌前占据了部分洞穴空间,在埋藏后,高温有机酸、CO₂、H₂S和岩浆热液沿着深大断裂等通道对奥陶系碳酸盐岩改造溶蚀作用明显,也对处于优势通道和良好储集性能的洞穴充填方解石进行了改造,恢复了部分洞穴空间。因而认为地表、近地表方解石充填过程是碳酸盐岩洞穴型储层的“保持性成岩作用”。     

Structural control and hydrothermal alteration at the BIF-hosted Raposos lode-gold deposit, Quadrilátero Ferrífero, Brazil

In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D₁, D₂ and D₃). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D₃. The orebodies constitute sulfide-bearing D₁-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H₂O-rich containing CO₂ + Na⁺ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H₂O is indicated. The CO₂ is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H₂O to CO₂-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration.     

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.     

Two hundred million years of karst history, Dachstein Limestone, Hungary

Platform carbonates of the Upper Triassic Dachstein Limestone in Naszály Hill have been karstified extensively over the past 200 million years. They provide an excellent example of polyphase karstic diagenesis that is probably typical of many subaerially exposed carbonate sequences. Seven karstic phases are recognized in the area, each of which include polyphase karstic events. The first karst phase was associated with the Löfer cycles. Meteoric waters caused dissolution; enlarged fractures and cavities, were filled by marine and/or vadose silts and cement. The second karst phase was caused by local tectonic movements. Bedding-plane-controlled phreatic caves were formed, and filled by silts. The third karst phase lasted from the end of the Triassic to the Eocene. This was a regional, multiphase karstic event related to younger composite unconformities. Bauxitic fill is the most characteristic product of this phase. The karst terrain reached its mature or senile stage with very little porosity. Narrow veins and floating rafts of white calcite marks karst phase 4, which resulted from hydrothermal activity associated with Palaeogene magmatism. The early Rupelian phase of Alpine uplift caused large-scale rejuvenation of the former karst terrain (karst phase 5). Subsequently Naszály Hill was buried as an area of juvenile karst with significant porosity. A second period of hydrothermal activity in the area (karst phase 6) was induced by Miocene volcanism, which resulted in wide, pale green calcite veins. Finally karst phase 7 was of tectonic origin. Following the most recent, Miocene uplift of the Naszály Hill, the carbonates have again become the site of vadose karst development.     

Cryogenic carbonates in cave environments: A review

Cryogenic cave carbonate (CCC) represents a specific type of speleothem. Its precipitation proceeds at the freezing point and is triggered by freezing-induced concentration of solutes. Compared to classical speleothems (stalagmites, flowstones), CCC occurs as accumulations of loose uncemented aggregates. The grain sizes range from less than 1 μm to over 1 cm in diameter. Karst groundwater chemistry and its freezing rate upon entering the cave are responsible for highly variable grain morphology. Rapid freezing of water results in the formation of CCC powders with grain size typically below 50 μm. Slow freezing of water in caves (usually in systems where the CO₂ escape is partly restricted; e.g., ice covered water pools) results in the formation of large mineral grains, with sizes from less than 1 mm to about 20 mm. The range of carbon and oxygen stable isotope compositions of CCC is larger than for a typical carbonate speleothem. Rapid freezing of water accompanied by a quick kinetic CO₂ degassing results in large ranges of δ¹³C of the CCC powders (between –10‰ and +18‰ PDB). Slow freezing of water, with a restricted CO₂ escape results in gradual increase of δ¹³C values (from −9‰ to +6‰ PDB; data ranges in individual caves are usually much more restricted), accompanied by a δ¹⁸O decrease of the precipitated carbonate (overall range from −10‰ to −24‰ PDB). These unusual trends of the carbonate δ¹⁸O evolution reflect incorporation of the heavier ¹⁸O isotope into the formed ice. New isotope data on CCC from three Romanian ice caves allow better understanding of the carbon and oxygen isotope fingerprint in carbonates precipitated from freezing of bulk water. CCCs are proposed as a new genetic group of speleothems.     

Thermal history of the unsaturated zone at Yucca Mountain,Nevada, USA

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T_h) up to ∼80 °C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ¹⁸O values, and depositional timing is constrained by the ²⁰⁷Pb/²³⁵U ages of chalcedony or opal in the deposits. Fluid inclusion T_h from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90 °C. Calcite δ¹⁸O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest T_h and the lowest δ¹⁸O values are found in the older calcite. Calcite T_h and δ¹⁸O values indicate that most calcite precipitated from water with δ¹⁸O values between −13 and −7‰, similar to modern meteoric waters.     

Carbon isotopic study on graphite and carbonate in the Kamioka mining district,Gifu Prefecture,central Japan,in relation to the role of graphite in the pyrometasomatic ore deposition

Pyrometasomatic lead-zinc ore deposits of the Kamioka mine occur in the Hida metamorphic complex of central Japan. The pyrometasomatic ore deposition was followed by small scale hydrothermal ore deposition. Flaky graphite characteristically occurs in skarn, ores and in the surrounding crystalline limestone. ¹³C values of graphite in the skarn and the ores are close to those of graphites in the crystalline limestone. Graphite in the skarn and ores is considered to be remains of graphite in the crystalline limestone which was replaced by the skarn and the ores. At the pyrometasomatic stage, the oxygen fugacity of fluid would control the carbon isotopic composition of calcite precipitated. On the assumption that graphite played a role of oxygen buffer, the oxygen fugacity of the fluid was estimated to be from 10^–31,4 to 10^–301 bars at 350 °C and total pressure of 1,000 bars. The predominant carbon species in the fluid would be carbon dioxide. The ¹³C value of total carbon in fluid was estimated to be –3.6±1.7 (PDB) for the Tochibora and Maruyama deposits of the Kamioka mine, and the oxygen fugacity in the fluid was probably constant during the pyrometasomatic stage. It is likely that an important source of carbon was the carbon remained after the decarbonation of crystalline limestone. Oxygen isotopic studies on calcite of pyrometasomatic and hydrothermal stages revealed that meteoric water was an important source for most of the oxygen in fluid of both stages.     

Genesis of the Sanbaqi deposit: a paleokarst-hosted uranium deposit in China

The Sanbaqi uranium deposit in Hunan Province, south China, is the largest of a group of paleokarst-hosted uranium deposits in Lower Carboniferous limestone. Mineralization is localized in cavities and fault-breccias formed by dissolution of carbonates. Four episodes of karst formation are recognized: late Triassic-early Jurassic, late Jurassic-early Cretaceous, Cretaceous-Tertiary and Recent. Field relations indicate that the main uranium mineralization is related to the second karst episode. This is supported by isotopic ages of two pitchblende samples at 129 Ma and 134 Ma, as indicated by their nearly concordant data points on concordia plot. These ages are in the time range of the early Yanshanian tectonic movements that affected southern China, and the faulting related to the movements likely triggered the mineralization process at the Sanbaqi deposit. Associated minerals include pyrite, millerite, ullmannite, niccolite, molybdenite, chalcopyrite, sphalerite, galena, calcite and dolomite. Fluid inclusion studies on calcite reveal that temperature of ore deposition was from 181° to 150 °C. The δ¹⁸O and δD values of the ore fluids range from 1.5 to 7.9 per mil and from −30.4 and −54.8 per mil, respectively. The mineralogical, fluid inclusion and isotopic data indicate that the minerlization took place in episodic pulses of hydrothermal fluids that were introduced along a set of ring faults. Mobilization and redeposition of earlier formed ore minerals in an open system added to the complexity of the paragenetic sequence. Younger episodes of mineralization occurred during the later karst events as suggested by the geological and additional pitchblende U-Pb isotopic data, during the Cretaceous-Tertiary late Yanshanian tectonic movements and recently. Finally, a comparison of the Sanbaqi uranium deposit with the uranium deposits hosted by solution collapse breccia pipes of the Colorado Plateau, USA, shows that they have many similarities. Received: 9 July 1996 / Accepted: 17 January 1997     

Occurrence of hypogenic caves in a karst region: Examples from central Italy

The caves of the Umbria and Marche regions in central Italy are made up of three-dimensional maze systems that display different general morphologies due to the various geological and structural contexts. At the same time, the internal morphologies of the passages, galleries, and shafts present some similarity, with solutional galleries characterized by cupolas and blind pits, anastamotic passages, roof pendants, and phreatic passages situated at different levels. Some of these caves are still active, as is the case for Frassassi Gorge, Parrano Gorge, and Acquasanta Terme, with galleries that reach the phreatic zone, where there is a rising of highly mineralized water, rich in hydrosulfydric acid, and with erosion of limestone walls and the formation of gypsum. Elsewhere there are fossil caves, such as Monte Cucco and Pozzi della Piana, where large speleothems of gypsum are present 500 m or more above the regional water table. In all of these important karst systems it is possible to recognize basal input points through fracture and intergranular porosity networks at the base of the oxidizing zone in the core of the anticline, where mineralized water rises up from the Triassic evaporitic layers in small hydrogeological circuits. Different underground morphologies can derive from the presence of a water table related to an external stream or from the confined setting of the carbonate rocks, underlying low permeable sedimentary cover, where artesian conditions can occur.     

Siderite mineralization of the Gemericum superunit (Western Carpathians, Slovakia): review and a revised genetic model

The Gemericum is a segment of the Variscan orogen subsequently deformed by the Alpine–Carpathian orogeny. The unit contains abundant siderite–sulphide and quartz–antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano-sedimentary sequences. The siderite–sulphide veins and siderite replacement deposits of the Gemericum represent one of the largest accumulations of siderite in the world, with about 160 million tonnes of mineable FeCO₃. More than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern, reflecting the distribution of regional metamorphic zones. Siderite–sulphide veins are typically contained in low-grade (chlorite zone) sedimentary, volcano-sedimentary or volcanic Lower and Upper Paleozoic rocks. Quartz–antimony veins are hosted by higher-grade units (biotite zone). Siderite–sulphide veins are dominated by early siderite followed by a complex set of stages, including quartz–sulphide (chalcopyrite, tetrahedrite), barite, tourmaline–quartz, and sulphide-remobilization stages. The temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes, within-vein replacement and recrystallization. Siderite–sulphide veins show considerable vertical (up to 1200 m) and lateral (up to 15 km) extent, and a thickness typically reaching several metres. Carbonate-replacement siderite deposits of the Gemericum are hosted by a Silurian limestone belt and are similar to stratabound siderite deposits of the Eastern Alps (e.g., Erzberg, Austria).Based on a review of geological, petrological and geochronological data for the Gemericum, and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals, the siderite–sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense. The deposits were formed during several stages of regional crustal-scale fluid flow. Isotope (S, C, Sr, Pb) fingerprinting identifies the metamorphosed rock complexes of the Gemericum as a source of most components of hydrothermal fluids. Fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types, and the existence of contrasting P–T conditions during vein evolution. A high-δ¹⁸O, medium- to high-salinity, H₂O-type fluid is the most important component during siderite deposition, whereas H₂O–CO₂-type fluid inclusion containing dense liquid CO₂ and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline–quartz stage. Younger quartz–ankerite(±siderite)–sulphide stages are characterized by high-salinity (17 to 35 wt.% NaCl equivalent) and low-temperature (T_h=90 to 180 °C) H₂O-type fluids.The vein deposits are interpreted as a result of multistage hydrothermal circulation, with Variscan and Alpine mineralization phases. Based on available indirect data, the most important mineralization phase was related to regional fluid flow during the uplift of a Variscan metamorphic core complex, producing siderite–sulphide (±barite) mineralization, while tourmaline–quartz stage and sulphide remobilization stages are related to Alpine processes. Two phases of vein evolution are evident from two groups of ⁸⁷Sr/⁸⁶Sr isotope ratios of Sr-rich, Rb-poor hydrothermal minerals: 0.71042–0.71541 in older barite and 0.7190–0.7220 in late-stage celestine and strontianite.     

A sulfur isotope study of volcanogenic massive sulfide deposits of the Eastern Black Sea province,Turkey

Kuroko-type massive sulfide deposits of the Eastern Black Sea province of Turkey are related to the Upper Cretaceous felsic lavas and pyroclastic rocks, and associated with clay and carbonate alteration zones in the footwall and hangingwall lithologies. A complete upward-vertical section of a typical orebody consists of a stringer-disseminated sulfide zone composed mainly of pyrite and chalcopyrite; a massive pyrite zone; a massive yellow ore consisting mainly of chalcopyrite and pyrite; a black ore made up mainly of galena and sphalerite with minor amounts of chalcopyrite, bornite, pyrite and various sulfosalts; and a barite zone. Most of the deposits in the province are associated with gypsum in the footwall or hangingwall. The paragenetic sequence in the massive ore is pyrite, sphalerite, chalcopyrite, bornite, galena and various sulfosalts, with some overlap between the mineral phases. Massive, stringer and disseminated sulfides from eight kuroko-type VMS deposits of the Eastern Black Sea province have a ³⁴S range of 0–7 per mil, consistent with the ³⁴S range of felsic igneous rocks. Sulfides in the massive ore at Madenköy (4.3–6.1 per mil) differ isotopically from sulfides in the stringer zone (6.3–7.2 per mil) suggesting a slightly increased input of H₂S derived from marine sulfate with time. Barite and coarse-grained gypsum have a ³⁴S range of 17.7–21.5 per mil, a few per mil higher than the ³⁴S value of contemporaneous seawater sulfate. The deposits may, therefore, have formed in restricted basins in which bacterial reduction of sulfate was taking place. Fine-grained, disseminated gypsum at Kutlular and Tunca has ³⁴S values (2.6–6.1 per mil) overlapping those of ore sulfides, indicating sulfide oxidation during waning stages of hydrothermal activity.     

Origin of ultramafic-hosted magnesite on Margarita Island,Venezuela

Ultramafic-hosted deposits of magnesite (MgCO₃) have been studied on Margarita Island, Venezuela, to elucidate the source of carbon and conditions of formation for this type of ore. Petrographic, mineralogic, and δ¹⁸O data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. δ¹³C ranges from −9 to −16‰ PDB within the magnesite and −8 to −10‰ PDB within some calcite and dolomite elsewhere on the island. The isotopically light dolomite fills karst and the calcite occurs as stock-work veins which resemble the magnesite deposits. These carbon isotopic ratios are consistent with a deep-seated source rather than an overlying source from a zone of surficial weathering. However, there is not much enrichment of precious metals and no enrichment of heavy rare-earth elements, as would be expected if the carbon had migrated upward as aqueous carbonate ions. The carbon probably has risen as a gaseous mixture of CO₂ and CH₄ which partially dissolved in near-surface water before leaching cations and precipitating as magnesite and other carbonates. The process probably is ongoing, given regional exhalation of carbonaceous gases.     

As–(Ag) sulphide veins in the Spanish Central System: further evidence for a hydrothermal event of Permian age

The Spanish Central System (SCS) has been subjected to repeated deformation and fluid flow events which have produced both sulphide-bearing and barren vein systems. Although several hydrothermal episodes took place between 300 and 100 Ma, fluid circulation during the Permian was especially important, giving rise to a range of different types of deposits. This study presents a multidisciplinary approach leading to the characterisation of the chemistry and age of the hydrothermal fluids that produced the As–(Ag) mineralised stockwork of Mónica mine (Bustaviejo, Madrid). Fluid inclusion data indicate the presence of two different fluids. An initial ore stage (I) formed from a low- to moderate salinity (3–8 wt.% eq. NaCl) H₂O–NaCl–CO₂–CH₄ fluid, at minimum trapping temperature of 350±15 °C and 0.3 kbar. A second H₂O–NaCl fluid is found in three types of fluid inclusions: a high temperature and low salinity type (340±20 °C; 0.8–3.1 wt.% eq. NaCl) also associated to ore stage I, a moderate temperature and very low salinity type (160–255 °C; 0–1.5 wt.% eq. NaCl) represented in ore stage III, and a very low temperature and hypersaline type (60–70 °C; 30–35 wt.% NaCl), unrelated to the mineralising stages and clearly postdating the previous types. ⁴⁰Ar–³⁹Ar dating on muscovite from the early As–Fe stage (I) has provided an age of 286±4 Ma, synchronous with the late emplacement phases of La Cabrera plutonic massif (288±5 Ma) and with other Permian hydrothermal events like Sn–W skarns and W–(Sn) sulphide veins. δ¹⁸O of water in equilibrium with stage I quartz (5.3–7.7‰), δD of water in equilibrium with coexisting muscovite (−16.0‰ to −2.0‰), and sulphide δ³⁴S (1.5–3.6‰) values are compatible with waters that leached metamorphic rocks. The dominant mechanism of the As–(Ag) deposition was mixing and dilution processes between aqueous–carbonic and aqueous fluids for stage I (As–Fe), and nearly isobaric cooling processes for stages II (Zn–Cu–Sn) and III (Pb–Ag). The origin and hydrothermal evolution of As–(Ag) veins is comparable to other hydrothermal Permian events in the Spanish Central System.     

Stable isotope variations in the Quaternary epithermal calcite-fluorite deposit at Monte delle Fate near Cerveteri (Latium,central Italy)

Carbon, oxygen and hydrogen isotope variations have been measured in samples from the epithermal fluorite vein deposit at Monte delle Fate, Latium. The ranges in ¹³C and ¹⁸O of calcite are –1.3 to 3.4 and 9.5 to 17.3, respectively. D values of water extracted from fluid inclusions are –49 to –39 for calcite and –41 to –34 for fluorite. Fluid inclusion filling temperatures (225°–240°C) and salinites (3.75) are nearly the same for both fluorite and sparry calcite. An elongated form of calcite, of minor abundance, precipitated at lower temperatures. The data indicate that (1) the CO₂ involved in the mineralization was provided by the local marine limestones, (2) the waters were meteoric in origin and underwent an ¹⁸O shift of 10 permil by exchange with marine country rocks, and (3) all geochemical features can be explained by the action of two hydrothermal fluids. Hot brines recently discovered in the Cesano geothermal area, 30 km to the east, have temperatures and some chemical characteristics similar to the hydrothermal fluids at Monte delle Fate.     

Origin of pedogenic needle-fiber calcite revealed by micromorphology and stable isotope composition—a case study of a Quaternary paleosol from Hungary

Pedogenic needle-fiber calcite was studied regarding its morphology, texture and stable isotope composition from the paleosol of the Quaternary Várhegy travertine (Budapest, Hungary). The needle-fiber calcite is composed of 40–200 μm long monocrystals. Smooth rods as well as serrated-edged crystals with calcite overgrowths were identified by SEM. Needles have several textural varieties: randomly distributed crystals in vugs and pores with calcite hypocoatings, bundles of subparallel crystals forming coatings around grains and alveolar structure with bridging needles in vugs.The morphological study of needle-fiber calcite suggests that needles are calcified fungal sheaths and produced by fungal biomineralization, a common process in recent and fossil soils and calcretes. The stable isotope composition of needle-fiber calcite (average: δ¹⁸O=-7.1‰ and δ¹³C=-7.3‰ vs. V-PDB) indicates significant incorporation of organically derived CO₂ and probably biological influence on needle genesis. Dissolved host rock travertine and/or atmospheric CO₂ could also contribute some carbon to the acicular calcite.     

Lewis Ponds, a hybrid carbonate and volcanic-hosted polymetallic massive sulphide deposit, New South Wales, Australia

The Lewis Ponds Zn–Pb–Cu–Ag–Au deposit, located in the eastern Lachlan Fold Belt, central western New South Wales, exhibits the characteristics of both volcanic-hosted massive sulphide and carbonate-hosted replacement deposits. Two stratabound massive to disseminated sulphide zones, Main and Toms, occur in a tightly folded Upper Silurian sequence of marine felsic volcanic and sedimentary rocks. They have a combined indicated resource of 5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main Zone is hosted by a thick unit of poorly sorted mixed provenance breccia, limestone-clast breccia and quartz crystal-rich sandstone, whereas Toms Zone occurs in the overlying siltstone. Pretectonic carbonate–chalcopyrite–pyrite and quartz–pyrite stringer veins occur in the footwall porphyritic dacite, south of Toms Zone. Strongly sheared dolomite–chalcopyrite–pyrrhotite veins directly underlie the Toms massive sulphide lens. The mineralized zones consist predominantly of pyrite, sphalerite and galena. Paragenetically early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone. Main and Toms zones are associated with a semi-conformable hydrothermal alteration envelope, characterized by texturally destructive chlorite-, dolomite- and quartz-rich assemblages. Dolomite, chlorite, quartz, calcite and sulphides have selectively replaced breccia and sandstone beds in the Main Zone host sequence, whereas the underlying porphyritic dacite is weakly sericite altered. Vuggy and botryoidal textures resulted from partial dissolution of the dolomite-altered sedimentary rocks and unimpeded growth of base metal sulphides, carbonate and quartz into open cavities. The intense chlorite-rich alteration assemblage, underlying Toms Zone, grades outward into a weak pervasive sericite–quartz assemblage with distance from the massive sulphide lens. Limestone clasts and hydrothermal dolomite at Lewis Ponds are enriched in light carbon and oxygen isotopes. The dolomite yielded ¹³C_VPDB values of –11 to +1 and ¹⁸O_VSMOW values of 6 to 16. Liquid–vapour fluid inclusions in the dolomite have low salinities (1.4–7.7 equiv. wt% NaCl) and homogenization temperatures (166–232°C for 1,000 m water depth). Dolomitization probably involved fluid mixing or fluid–rock interactions between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization. ³⁴S_VCDT values range from 2.0 to 5.0 in the massive sulphide and 3.9 to 7.4 in the footwall carbonate–chalcopyrite–pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at Lewis Ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization. Low temperature dolomitization of limestone-bearing facies in the Main Zone host sequence created secondary porosity and provided a reactive host for fluid-rock interactions. Main Zone formed by lateral fluid flow and sub-seafloor replacement of the poorly sorted breccia and sandstone beds. Base metal sulphide deposition probably resulted from dissolution of dolomite, fluid mixing and increased fluid pH. Pyrite, sphalerite and galena precipitated from a relatively low temperature, 150–250°C hydrothermal fluid. In contrast, Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During the D₁ deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins. Lewis Ponds is a rare example of a synvolcanic sub-seafloor hydrothermal system developed within fossiliferous limestone-bearing facies. The close spatial association between limestone, hydrothermal dolomite, massive sulphide and dacite provides a basis for new exploration targets elsewhere in New South Wales.Editorial handling: D. Lentz