Conditions and processes leading to large-scale gold deposition in the Jiaodong province,eastern China

The gold deposits in the Jiaodong Peninsula constitute the largest gold mineralized province in China. The mineralization shows common characteristics in their tectonic setting, ore-forming fluid and metallogenic system. Sulfidation and fluid immiscibility are two important mechanisms controlling gold precipitation, both of which consume sulfur in the oreforming fluids. The escape of H_2S from the main ore-forming fluids and the decrease of total sulfur concentration not only lead to the efficient precipitation of gold, but also result in the crystallization of reducing minerals such as pyrrhotite and oxidizing minerals such as magnetite. Quartz solubility shows strong dependence on temperature, pressure, and CO_2 content. The dependence of quartz solubility on pressure is weak at low temperatures, and progressively stronger at higher temperatures.Similarly, the temperature dependence of quartz solubility is relatively low at low pressures, but becomes gradually stronger at high pressures. The results of solubility modeling can constrain the dissolution and reprecipitation behavior of quartz in the oreforming veins and the formation mechanism of different types of quartz veins. The multi-stage mineralization fluid activity resulted in the complex dissolution structure of quartz in the Jiaodong gold veins. Pyrite in the main metallogenic period in the Jiaodong gold deposits shows complex microstructure characteristics at single crystal scale. The trace elements(mainly the coupling of As-and Au-rich belt) and sulfur isotope composition also display a certain regularity. The As-rich fluids might have formed by the initial pulse of ore-forming fluids through As-rich metasedimentary strata, while the As-Au oscillation zone at the margin of pyrite grains is related to the pressure fluctuation caused by fault activity and the local phase separation of fluids. There is a temporal and spatial evolution of gold fineness in the Jiaodong gold deposits. Water/rock reaction(sulfidation) was the main ore-forming mechanism of early gold mineralization, forming relatively high fineness gold, while significant pressure drop in the shallow part accompanied by fluid phase separation promoted the late gold mineralization, forming low fineness gold. Under cratonic destruction setting, dehydration of the amphibolite and granulite facies metamorphic lower-crust resulted in the formation of Au-CO_2-rich ore-forming fluids, which rose along the deep fault and secondary structure, and formed the largescale fault-controlled gold deposits in Jiaodong.     

Oxygen isotopic composition of low-temperature authigenic clinoptilolite

Oxygen isotope ratios were obtained from authigenic clinoptilolites from Barbados Accretionary Complex, Yamato Basin, and Exmouth Plateau sediments (ODP Sites 672, 797, and 762) in order to investigate the isotopic fractionation between clinoptilolite and pore water at early diagenetic stages and low temperatures. Dehydrated clinoptilolites display isotopic ratios for the zeolite framework (δ¹⁸O_f) that extend from +18.7‰ to +32.8‰ (vs. SMOW). In combination with associated pore water isotope data, the oxygen isotopic fractionation between clinoptilolite and pore fluids could be assessed in the temperature range from 25°C to 40°C. The resulting fractionation factors of 1.032 at 25°C and 1.027 at 40°C are in good agreement with the theoretically determined oxygen isotope fractionation between clinoptilolite and water. Calculations of isotopic temperatures illustrate that clinoptilolite formation occurred at relatively low temperatures of 17°C to 29°C in Barbados Ridge sediments and at 33°C to 62°C in the Yamato Basin. These data support a low-temperature origin of clinoptilolite and contradict the assumption that elevated temperatures are the main controlling factor for authigenic clinoptilolite formation. Increasing clinoptilolite δ¹⁸O_f values with depth indicate that clinoptilolites which are now in the deeper parts of the zeolite-bearing intervals had either formed at lower temperatures (17–20°C) or under closed system conditions.     

The geochemical controls on vent fluids from the Lucky Strike vent field, Mid-Atlantic Ridge

Hydrothermal vent fluids were collected from the Lucky Strike site at 37°17′N on the Mid-Atlantic Ridge in both 1993 and 1996. Seven vents were sampled with the DSV Alvin in 1993 and six vents were sampled in 1996 using the ROV Jason during the LUSTRE '96 Cruise. As three of the vents were sampled in both 1993 and in 1996, a time series of vent fluid chemistry is also reported. Measured temperatures ranged from 202 to 333°C at the 1618–1726 m depth of the vent field, which is located on Lucky Strike Seamount. These fluids are either equal to or less than the local bottom seawater in chlorinity. While the range in fluid compositions at Lucky Strike is generally within that observed elsewhere, the unusual aspects of the fluid chemistries are the relatively high pH and low Fe, Mn, Li and Zn. We attribute this, as well as an usually low Sr/Ca ratio, to reaction with a highly altered substrate. The high Si and Cu contents suggest a deep, as well as hot, source for these fluids. The fluid compositions therefore suggest formation by super-critical phase separation at a depth not less than 1300 m below the seafloor, and reaction with a relatively oxic, and previously altered, substrate. There is temporal variability in some of the vent fluid compositions as Li, K, Ca and Fe concentrations have increased in some of the vents, as has the Fe/Mn (molar) ratio, although the chlorinities have remained essentially constant from 1993 to 1996. While there is not a simple relationship between vent fluid compositions (or temperatures) and distance from the lava lake at the summit of the seamount, the vent fluids from many of the vents can be shown to be related to others, often at distances >200 m. The most southeasterly vents (Eiffel Tower and the Marker/Mounds vents) are distinct in chlorinity and other chemical parameters from the rest of the vents, although closely related to each other within the southeastern area. Similarly all of the vents not in this one area, appear closely related to each other. This suggests one or two source fluids for many of the vents, as is also inferred to be the case at TAG, but which is in contrast to observations on faster spreading ridges. This may suggest inherently different plumbing for hydrothermal systems at slower versus faster spreading ridges.     

Experimental hydrogen isotope studies,II. Fractionations in the systems epidote-NaCl-H2O,epidote-CaCl2-H2O and epidote-seawater,and the hydrogen isotope composition of natural epidotes

The hydrogen isotope fractionation factors between epidote and aqueous 1 M and 4 M NaCl, 1 M CaCl₂ solutions, and between epidote and seawater, have been measured over the temperature range 250–550°C over which the degree of dissociation of dissolved species varies significantly. Measured fractionations at 350°C are decreased by up to 12‰, 9‰ and 7‰ relative to pure water in seawater, 1 M CaCl₂ and 1 M NaCl respectively, while above 500°C fractionations are not measurably dependent on fluid composition. Water—solution fractionation factors are derived which are generally applicable to the correction of mineral—water hydrogen isotope fractionations for the composition of the fluid phase.The hydrogen isotope compositions of natural epidotes are interpreted in the light of experimental fractionation data for situations where temperature, δD (fluid), and, in some cases, fluid chemistry, are independently known. Epidotes from active geothermal systems have hydrogen isotope quench temperatures consistent with or close to measured well temperatures unless the measured temperature has declined substantially since epidote formation or there is uncertainty in the D/H ratio of the water associated with the epidote because of isotopic heterogeneity in the well waters. Hydrothermal and metamorphic epidotes show closure temperatures of 175–225°C and 200–250°C. There is no evidence that retrograde metamorphic fluids, if present, are isotopically different from prograde fluids.The water-solution fractionations indicate strong solute-solvent interactions between 250 and 450°C and imply that both dissociated and associated species contribute to the fractionation effects through modification of the orientations and structure of the water molecules. Solute-solvent interactions become negligible at temperatures around 550°C.     

Regional metamorphism and hydrothermal alteration related to Miocene submarine volcanism ('green tuff') in the Yurihara oil and gas field, northwest Honshu Island, Japan

Abstract Alteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin-fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite-pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5-15.5 Ma, determined by planktonic foraminifera assemblages of inter-bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K-Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The maximum Th of the fluid inclusions is 222°C and Tm indicates trapped fluids of up to 3.3 wt% equivalent NaCl (i.e. almost the same as seawater). A Th versus Tm plot indicates mixing occurred between hydrothermal fluids and formation water that has low salinity. Corrensite and chlorite form veins, and the temperatures of their formation, estimated by the extent of aluminium substitution into the tetrahedral site of chlorite, ranges between 165 and 245°C in the centre of the hydrothermal alteration zone (zone IIIa). This is consistent with the result of Th analyses. The deposition temperature of chlorite associated with prehnite in veins ranges between 190 and 215°C in zones IIa and IIb.     

Metasomatic hydrous fluids in amphibole peridotites from Zabargad Island (Red Sea)

Combining textural, petrological, chemical and isotopic (Sr, H and O) data for amphiboles and whole rocks from the Zabargad peridotite diapir allows three different events to be distinguished. During each event, which can be related to a specific tectonic process of the rifting of the Red Sea, hydrous fluids produced amphiboles.

The first and the second generations of amphiboles have characteristics consistent with the involvement of mantle-derived hydrous fluids. The first generation consists of scarce Ti-pargasites which crystallized from small amounts of fluid at temperatures of around 900–1000°C. Their growth was linked to magma percolation in the peridotites before their deformation during diapiric uplift. The second generation consists of Cr-pargasites which crystallized locally (and abundantly) during reaction between the peridotites and a sodium/potassium-bearing hydrous fluid at temperatures of around 700–800°C. These amphiboles grew synchronously with the diapiric uplift. The hydrous fluids probably originated in the sub-continental mantle and were released during the diapiric uplift of the peridotites.

The third generation consists of amphiboles (pargasitic hornblende, hornblende sensu lato and tremolite) which are localized in shear zones and veins. They crystallized at temperatures estimated between 700°C and 450°C, again from a sodium/potassium-bearing hydrous fluid. However, this fluid is extraneous to the peridotites, as shown by the Sr, H and O isotope compositions which suggest seawater penetration either during or after the final emplacement of the peridotite diapir.

Although the peridotite diapir was emplaced in granulitic gneisses of the pan-African deep continental crust, no evidence was found for a contribution of hydrous continental fluids in the production of the amphiboles present in the peridotite bodies of Zabargad Island.     

Stress And Seismicity In The Lower Continental Crust: A Challenge To Simple Ductility And Implications For Electrical Conductivity Mechanisms

The lower crust is generally considered to be an aseismic, weak zone where fluid distribution might be governed by textural equilibrium geometries. Saline fluids below the transition from brittle to ductile rheology have been advanced as a joint explanation for deep crustal conductivity and seismic reflectivity, the depth of onset of both phenomena being apparently bounded by isotherms in the 300–450 °C temperature range. Some petrologists, meanwhile, contest that the deep crust should be devoid of extensive fluid networks. This review exposes some geophysical exceptions to the statistical norm suggested by global geophysical data compilations and presents counter-arguments that the lower crust in places may be both dry and strong, that fluids if at all present at such depths may not necessarily be connected and that fluid mobility in the lower crust may be more limited and heterogeneous than commonly assumed.Laboratory data on crustal rocks implies that the transition from brittle to ductile rheology actually occurs over a much broader range of temperatures than 300–450 °C, and the apparent association of deep crustal conductive horizons with a temperature field of 300–450 °C may be interpretable in terms of formation temperatures of graphite, rather than fluids and brittle-ductile transition rheology.High v_P/v_S ratios from a 6 km thick, seismically layered zone below the Weardale granite, NE England can be explained by underplated mafic material. They are unlikely to be explained by fluids in an area where deep crustal conductance has been shown to be relatively low, unless conventional assumptions regarding deep crustal fluid distribution are inadequate or false.Perusal of the literature reveals that lower crustal seismicity is less seldom than generally appreciated. Interpretation of earthquakes nucleating at lower crustal depths is ambiguous, but in some tectonic regimes may indicate preservation of brittle rheology to the Moho and a lower crust that is predominantly mafic and dry.A better understanding of lower crustal deformation mechanisms and history may provide better insight into deep crustal conductivity mechanisms. Recent rock mechanical experiments suggest that permeability (and thus fluid connectivity) may be decreased by ductile shearing, whereas ductile shearing may aid graphitisation at lower crustal temperatures. If the lower crust in some regions is strong, this may explain the apparent preservation of both extant- and palaeostress orientations in interpretations involving electrical anisotropy.     

Multi-episode fluid boiling in the Shizishan copper-gold deposit at Tongling, Anhui Province: its bearing on ore formation

The Shizishan copper-gold deposit at Tongling, Anhui Province consists of two magmato-hydrothermal mineralization types: the crypto-explosive breccia type and the skarn type. At least four episodes of boiling occurred to the ore-forming fluids in this deposit. The first episode took place in accompany with the formation of the crypto-explosive breccias. The melt-fluid inclusions giving temperatures above 600℃ and salinities higher than 42% NaCl equiv represent a residual magma related to this episode. The second episode occurred during skarnization, giving fluid temperatures of 422℃-472℃, averaging 458℃, and salinities of 10.2%-45.1% NaCl equiv. The third episode corresponds to the main mineralization stage, i.e., the quartz-sulphide stage. Fluid temperatures of this episode vary in a range of 337℃-439℃ with an average of 390℃, and salinities in a range of 3%-30% NaCl equiv. The forth episode happened at the waning stage of mineralization, giving fluid temperatures below 350℃ with an average of 265℃ and salinities of 2.1%-40.4% NaCl equiv.     

Multi-stage fluid circulation in a hydraulic fracture breccia of the Larderello geothermal field (Italy)

The deep well MV5A, drilled in the western part of the Larderello geothermal field, crossed a 20-cm-thick hydraulic fracture breccia unit at a depth of 1090 m below ground level (b.g.l.). This breccia occurs in a fine-grained Triassic metasandstone and consists of angular to subangular clasts of up to some centimeters in size. Pervasive alteration has affected the breccia clasts and wall rock around the breccia, with the formation of Mg–Fe chlorite. After such alteration, hydrothermal circulation caused the precipitation of two generations of calcite cement. Then, ankerite partially replaced these two calcite generations. Ankerite also precipitated in late veinlets with chlorite. Late hydrothermal activity led to the crystallization of albite, quartz and finally, anhydrite. The calcite contains vapor-rich inclusions and two populations of liquid-rich (L1 and L2) inclusions. L1 inclusions are characterized by homogenization temperatures between 304 and 361°C and salinities from 7.4 to 11.6 wt.% NaCl equivalent; L2 inclusions revealed homogenization temperatures in the range of 189–245°C and salinities from 2.6 to 6.3 wt.% NaCl equivalent. The fluids contained in L2 inclusions were probably trapped coevally with some vapor-rich inclusions under boiling conditions after the L1 inclusions formed. Some of the abundant vapor-rich inclusions in calcite may also represent early, low-temperature inclusions affected by decrepitation and/or stretching and/or leaking during L1 trapping. The liquid-rich (L) inclusions trapped at later stages in ankerite, albite and anhydrite display, respectively, homogenization temperature ranges of 189–198°C, 132–145°C, and 139–171°C, and salinities ranging from 1.6 to 1.7 wt.% NaCl equivalent, 1.4 to 2.1 wt.% NaCl equivalent and 3.7 to 6.2 wt.% NaCl equivalent. The inclusions studied record the evolution, over time, of the fluids flowing in the breccia level: L1 inclusions capture high-temperature fluid (about 300 to 350°C) of high salinity (around 10 wt.% NaCl equivalent) at above-hydrostatic pressures (up to about 150 bar). The L2 inclusions in calcite and liquid-rich inclusions in ankerite and albite represent subsequent hydrothermal fluid evolution toward lower temperatures (about 250 to 130°C), pressures (45 to a few bar) and salinities (6.3 to 1.4 wt.% NaCl equivalent). During this stage, boiling processes and infiltration of meteoric waters probably occurred. Finally, moderately saline fluids (around 5 wt.% NaCl equivalent) at a temperature (about 160°C) close to that of present-day in-hole measurements was trapped in the anhydrite inclusions. The liquids trapped in liquid-rich inclusions circulated at 41,000 years (maximum age of calcite) or later. This age represents an upper limit for the development of vapor-dominated condition, in this part of the geothermal system. The fluids circulating at the breccia level were probably meteoric and/or connate waters. These fluids may have interacted with the anhydrite and carbonate bearing formations present in the Larderello area. The occurrence of the hot and saline fluids, trapped in L1 inclusions at above-hydrostatic pressure, suggests that similar fluids but with higher pressure (≥167 bar) and temperature (≥360°C) may have been responsible for rock fracturing.     

Geological and geochemical characteristics of the Baogudi Carlintype gold district(Southwest Guizhou,China) and their geological implications

The newly discovered Baogudi gold district is located in the southwestern Guizhou Province,China,where there are numerous Carlin-type gold deposits.To better understand the geological and geochemical characteristics of the Baogudi gold district,we carried out petrographic observations,elemental analyses,and fluid inclusion and isotopic composition studies.We also compared the results with those of typical Carlin-type gold deposits in southwestern Guizhou.Three mineralization stages,namely,the sedimentation diagenesis,hydrothermal(main-ore and late-ore substages),and supergene stages,were identified based on field and petrographic observations.The main-ore and late-ore stages correspond to Au and Sb mineralization,respectively,which are similar to typical Carlin-type mineralization.The mass transfer associated with alteration and mineralization shows that a significant amount of Au,As,Sb,Hg,Tl,Mo,and S were added to mineralized rocks during the main-ore stage.Remarkably,arsenic,Sb,and S were added to the mineralized rocks during the late-ore stage.Element migration indicates that the sulfidation process was responsible for ore formation.Four types of fluid inclusions were identified in ore-related quartz and fluorite.The main-ore stage fluids are characterized by an H2O–NaCl–CO2–CH4±N2system,with medium to low temperatures(180–260℃)and low salinity(0–9.08%NaCl equivalent).The late-ore stage fluids featured H2O–NaCl±CO2±CH4,with low temperature(120–200℃)and low salinity(0–7.48%Na Cl equivalent).The temperature,salinity,and CO2and CH4concentrations of ore-forming fluids decreased from the main-ore stage to the late-ore stage.The calculated δ^13C,d D,and δ^18O values of the ore-forming fluids range from-14.3 to-7.0%,-76 to-55.7%,and 4.5–15.0%,respectively.Late-ore-stage stibnite had δ^34S values ranging from-0.6 to 1.9%.These stable isotopic compositions indicate that the ore-forming fluids originated mainly from deep magmatic hydrothermal fluids,with minor contributions from strata.Collectively,the Baogudi metallogenic district has geological and geochemical characteristics that are typical of Carlin-type gold deposits in southwest Guizhou.It is likely that the Baogudi gold district,together with other Carlin-type gold deposits in southwestern Guizhou,was formed in response to a single widespread metallogenic event.     

Chemical mass balance in a reaction zone between serpentinite and metapelites in the Nishisonogi metamorphic rocks, Kyushu, Japan: Implications for devolatilization

Abstract   Reaction zones of 0.5–10.0 m thick are commonly observed between serpentinite and pelitic schist in the Nishisonogi metamorphic rocks, Kyushu, Japan. Each reaction zone consists of almost monomineralic or bimineralic layers of talc + carbonates, actinolite (or carbonates + quartz), chlorite, muscovite and albite from serpentinite to pelitic schist. Magnesite + quartz veins extend into the serpentinite from the talc + carbonates layer, while dolomite veins extend into the pelitic schist from the muscovite layer. These veins are filled by subhedral minerals with oriented growth features. Primary fluid inclusions yield the same homogenization temperatures (145–150°C) both in the reaction zone and in the veins, suggesting their simultaneous formation. Mass-balance calculations using the isocon method indicate that SiO₂, MgO, H₂O and K₂O are depleted in the reaction zone relative to the protoliths. These components were probably extracted from the reaction zone as fluids during the formation of the reaction zone.     

Chemical composition of thermal springs, cold springs, streams, and gas vents in the Mt. Amiata geothermal region (Tuscany, Italy)

A geochemical study of thermal and cold springs, stream waters and gas emissions has been carried out in the Mt. Amiata geothermal region.The cold springs and stream waters do not seem to have received significant contribution from hot deep fluids. On the contrary, the thermal springs present complex and not clearly quantifiable interactions with the hot fluids of the main geothermal reservoir.The liquid-dominated systems in the Mt. Amiata area, like most of the high-enthalpy geothermal fields in the world, are characterized by saline, NaCl fluids. The nature of the reservoir rock (carbonatic and anhydritic), and its widespread occurrence in central Italy, favor a regional circulation of “Ca-sulfate” thermal waters, which discharge from its outcrop areas. Waters of this kind, which have been considered recharge waters of the known geothermal fields, dilute, disperse and react with the deep geothermal fluids in the Mt. Amiata area, preventing the use of the main chemical geothermometers for prospecting purposes. The temperatures obtained from the chemical geothermometers vary widely and are generally cooler than temperatures measured in producing wells.Other thermal anomalies in central Italy, apart from those already known, might be masked by the above-mentioned circulation. A better knowledge of deep-fluid chemistry could contribute to the calibration of specific geothermometers for waters from reservoirs in carbonatic rocks.     

利用流体包裹体分析技术研究地震和断层活动的进展

矿物中的流体包裹体记录了地球古流体的形成和演化、矿物的形成环境等各种地质信息。利用微区微量测量技术测定断裂带脉石矿物流体包裹体可以获得断层和地震活动的信息,延长认识地震复发周期的时间,对确定地震活动规律有重要意义。迄今为止,地震流体研究主要是关于宏观区域流体(水和气体)变化规律及其与地震的关系,对微区微量流体的研究很少。本文扼要介绍了地震和构造活动中流体作用与流体包裹体拉曼光谱测量技术,综述了流体包裹体(FI)分析在地震与断裂活动方面的研究进展,并提出了进一步研究的领域,以期促进微区微量地震流体研究和应用。     

Geochemistry of hydrothermal deposits from Loihi submarine volcano,Hawaii

Dredging across the northeast rim of the summit crater of Loihi Seamount recovered several morphologically similar but chemically and mineralogically distinct hydrothermal deposits encrusting the surface of fresh pillow lava talus. The multicolored deposits suggest a precipitation sequence that may be controlled by an oxidation-reduction gradient in which smectite ranging in composition from Fe-montmorillonite to nontronite has precipitated along with iron oxide under slightly reducing conditions. This deposition was apparently followed by amorphous iron oxide and silica precipitation, possibly under more oxic conditions. Oxygen isotope geothermometry indicates formation temperatures in the range of 31–57°C for the Loihi smectites. Trace element enrichments appear to be positively correlated with the isotopic formation temperature of the smectite, suggesting either increased trace element solubility within the higher-temperature vent fluids or increased smectite and iron oxide scavenging with increased precipitation rates. The trace element abundances further suggest the presence of polymetallic sulfides that either are directly associated with the smectites as amorphous phases or occur beneath these deposits in the volcanic pile of the seamount.     

Evolving patterns of the fluids within the TAG hydrothermal field

The mixing of seawater/hydrothermal fluid within the large seafloor hydrothermal sulfide deposits plays a key role in the formation processes of the sulfide deposits. Some issues attract considerable attentions in the study of seafloor hydrothermal system in recent years, such as the relationships among different types of vent fluids, the characteristics of chemical compositions and mineral assemblages of the hydrothermal deposits and their governing factors. Combined with the measured data of hydrothermal fluid in the TAG field, the thermodynamic model of mixing processes of the heated seawater at different temperatures and the hydrothermal fluid is calculated to understand the precipitation mechanism of anhydrite and the genetic relationships between the black and white smoker fluids within the TAG mound. The results indicate that the heating of seawater and the mixing of hydrothermal fluid/seawater are largely responsible for anhydrite precipitation and the temperature of the heated seawater is not higher than 150°C and the temperature of the end-member hydrothermal fluid is not lower than 400°C. Based on the simulated results, the evolving patterns of fluids within the TAG deposit are discussed. The mixed fluid of the end-member hydrothermal fluid and the seawater heated by wall rock undergoes conductive cooling during upflowing within the deposit and forms “White Smoker” eventually. In addition, the end-member hydrothermal fluid without mixed with seawater, but undergoing conductive cooling, vents out of the deposit and forms “Black Smoker”. Supported by China Ocean Mineral Resources Research and Development Association Program (Grant No. DY115-02-1-01) and National Basic Research Program of China (Grant No. G2000078503)     

Trace element compositions of jadeite (+omphacite) in jadeitites from the Itoigawa-Ohmi district, Japan: Implications for fluid processes in subduction zones

Abstract   Trace-element compositions of jadeite (±omphacite) in jadeitites from the Itoigawa-Ohmi district of Japan, analyzed by a laser-ablation inductively coupled plasma mass spectrometry technique showed chemical zoning within individual grains and variations within each sample and between different samples. Primitive mantle-normalized patterns of jadeite in the samples generally showed high large-ion lithophile element contents, high light rare earth element/heavy rare earth element ratios and positive anomalies of high field strength elements. The studied jadeitites have no signatures of the protolith texture or mineralogy. Shapes and distributions of minerals coupled with chemical zoning within grains suggest that the jadeitites were formed by direct precipitation of minerals from aqueous fluids or complete metasomatic modification of the precursor rocks by fluids. In either case, the geochemical characteristics of jadeite are highly affected by fluids enriched in both large-ion lithophile elements and high field strength elements. The specific fluids responsible for the formation of jadeitites are related to serpentinization by slab-derived fluids in subduction zones. This process is followed by dissolving high field strength elements in the subducting crust as the fluids continue to circulate into the subducting crusts and serpentinized peridotites. The fluids have variations in chemical compositions corresponding to various degrees of water–rock interactions.     

Ore-forming mechanism for the Xiaoxinancha Au-rich Cu deposit in Yanbian,Jilin Province,China: Evidence from noble gas isotope geochemistry of fluid inclusions in minerals

The Xiaoxinancha Au-rich copper deposit is one of important Au-Cu deposits along the continental margin in Eastern China. The deposit consists of two sections: the Beishan mine (North), composed of altered rocks with veinlet-dissemination sulfides and melnicovite-dominated sulfide-quartz veins, and the Nanshan mine (South), composed of pyrrhotite-dominated sulfide-quartz veins and pure sulfide veins. The isotope compositions of noble gases extracted from fluid inclusions in ore minerals, i.e. ratios of 3He/4He, 20Ne/22Ne and40Ar/36Ar are in the ranges of 4.45―0.08 Ra, 10.2―8.8 and 306―430, respectively. Fluid inclusions in minerals from the Nanshan mine have higher 3He/4He and 20Ne/22Ne ratios whereas those from the Beishan mine have lower 3He/4He ratios. The analysis of origin, and evolution of the ore fluids and its relations with the ore-forming stages and the ages of mineralization suggests that the initial hydrothermal fluids probably come from the melts generated by partial melting of oceanic crust with the participation of fluids from the mantle (mantle-plume type)/aesthenosphere. This also corresponds to the continental margin settings during the subduction of Izanagi ocaneic plate towards the palaeo-Asian continent (123―102 Ma). The veinlet-dissemination ore bodies of the Beishan mine were formed through replacement and crystallization of the mixed fluids generated by mixing of the ascending high-temperature boiling fluid with young crustal fluid whereas the melnicovite-dominated sulfide-quartz veins were formed subsequently by filling of the high-temperature ore fluid in fissures. Pyrrhotite-dominated sulfide-quartz veins in the Nanshan mine were formed by filling-deposition-crystallization of the moderate-temperature ore fluids and the pure sulfide veins were formed later by filling-deposition-crystallization of ore substance-rich fluids after boiling of the moderate-temperature ore fluids. The metallogenic dynamic processes can be summarized as: (1) formation of fluidand ore substance-bearing Adakitic magma by degassing, dewatering and partial melting during subduction of the Izanagi plate; (2) separation and formation of ore fluids from the Adakitic magma; and (3) success-sive ascending of the ore fluids and final formation of the Au-rich Cu deposit of veinlet-dissemination and vein types by secondary boiling.     

An oxygen isotopic profile through the upper kilometer of the oceanic crust, DSDP Hole 504B

DSDP Hole 504B is the deepest basement hole in the oceanic crust, penetrating through a 571.5 m pillow section, a 209 m lithologic transition zone, and 295 m into a sheeted dike complex. An oxygen isotopic profile through the upper crust at Site 504 is similar to that in many ophiolite complexes, where the extrusive section is enriched in¹⁸O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 haveδ¹⁸O values generally ranging from +6.1 to +8.5‰ SMOW(mean= +7.0‰), although minor zeolite-rich samples range up to 12.7‰. Rocks depleted in¹⁸O appear abruptly at 624 m sub-basement in the lithologic transition from 100% pillows to 100% dikes, coinciding with the appearance of greenschist facies minerals in the rocks. Whole-rock values range to as low as +3.6‰, but the mean values for the lithologic transition zone and dike section are +5.8 and +5.4‰, respectively.

Oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone. The pillow section reacted with seawater at low temperatures (near 0°C up to a maximum of around 150°C) and relatively high water/rock mass ratios (10–100); water/rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section. The transition zone and dikes were altered at much higher temperatures (up to about 350°C) and generally low water/rock mass ratios ( 1), and hydrothermal fluids probably contained mantle-derived CO₂. Mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient ( 2.5°C/m) across a 70 m interval at the top of the lithologic transition zone. Progressive reaction during axial hydrothermal metamorphism and later off-axis alteration led to the formation of albite- and Ca-zeolite-rich alteration halos around fractures. This enhanced the effects of cooling and¹⁸O enrichment of fluids, resulting in local increases inδ¹⁸O of rocks which had been previously depleted in¹⁸O during prior axial metamorphism.     

Trace-element compositions of single fluid inclusions in the Kofu granite, Japan: Implications for compositions of granite-derived fluids

Fluid inclusions in quartz from miarolitic cavities, pegmatites, and quartz veins in Miocene biotite-granite plutons, Kofu, Japan, were analyzed by particle-induced X-ray emission to examine chemistries and behaviors of granite-derived fluids in island-arc granite. Most inclusions are aqueous two-phase inclusions, and halite-bearing polyphase inclusions are also observed in quartz veins in the upper part of the plutons. From element contents of fluid inclusions in the miarolitic cavities, the original fluid released from the granite plutons during solidification is inferred to have concentrations of Mn, Fe, Cu, Zn, Ge, Br, Rb, Pb, and Ba of several tens to hundreds of parts per million by weight (ppm) and a salinity of about 10 wt% NaCl equivalent. We estimated the formation conditions of the fluid to have been at about 1.3–1.9 kb and 530–600°C on the basis of the homogenization temperatures of the inclusions and the solidification conditions of the plutons. The polyphase inclusions probably originated from hypersaline fluid by boiling of part of the released fluid during its ascent in the plutons. The polyphase inclusions contain several hundreds to tens of thousands of ppm of Fe and Mn, and tens to several hundreds of ppm of Cu, Zn, Br, Rb, and Pb. The salinities are about 35 wt% NaCl equivalent. Compositional variations in two-phase inclusions from the miarolitic cavities and quartz veins are primarily explained by mineral precipitation with dilution by surface water exerting a secondary influence. Thus, chemistries and behaviors of the granite-derived fluids in the plutons can be explained by mineral precipitation, boiling, and dilution of the originally released fluid.     

Geochemistry of volcanic and hydrothermal gases of Mutnovsky volcano,Kamchatka: evidence for mantle,slab and atmosphere contributions to fluids of a typical arc volcano

We report chemical compositions (major and trace components including light hydrocarbons), hydrogen, oxygen, helium and nitrogen isotope ratios of volcanic and geothermal fluids of Mutnovsky volcano, Kamchatka. Several aspects of the geochemistry of fluids are discussed: chemical equilibria, mixing of fluids from different sources, evaluation of the parent magmatic gas composition and contributions to magmatic vapors of fluids from different reservoirs of the Kamchatkan subduction zone. Among reactive species, hydrogen and carbon monoxide in volcanic vapors are chemically equilibrated at temperatures >300°C with the SO₂-H₂S redox-pair. A metastable equilibrium between saturated and unsaturated light hydrocarbons is attained at close to discharge temperatures. Methane is disequilibrated. Three different sources of fluids from three fumarolic fields in the Mutnovsky craters can be distinguished: (1) magmatic gas from a large convecting magma body discharging through Active Funnel, a young crater with the hottest fumaroles (up to 620°C) contributing ~80% to the total volcanic gas output; (2) volcanic fluid from a separate shallow magma body beneath the Bottom Field of the main crater (96–280°C fumaroles); and (3) hydrothermal fluid with a high relative and absolute concentrations of CH₄ from the Upper Field in the main crater (96–285°C fumaroles). The composition of the parent magmatic gas is estimated using water isotopes and correlations between He and other components in the Active Funnel gases. The He-Ar-N₂ systematics of volcanic and hydrothermal fluids of Mutnovsky are consistent with a large slab-derived sedimentary nitrogen input for the nitrogen inventory, and we calculate that only ~1% of the magmatic N₂ has a mantle origin and <<1% is derived from the arc crust.