Hydrothermal alterations of the chemical composition in grain size fractions of sediments in the Guaymas Basin,Gulf of California

The paper presents data on the contents of macro- and microelements (rare earth elements included) determined in grain size fractions of the Upper Pleistocene hydrothermally altered and unaltered sediments from the Guaymas Basin (Gulf of California). Sediments subjected to high-temperature hydrothermal alteration were recovered by DSDP Hole 477A. In the finely dispersed fractions, which are mainly composed of clay minerals, alteration of the chemical composition was provoked by the hydrothermal transformation of terrigenous clay minerals. The concentration of microelements in these fractions takes place primarily at the cost of the hydrothermal finely dispersed ore minerals. Alteration of the chemical composition of the coarse-grained fractions is related to the replacement of clastogenic minerals by the secondary varieties and the formation of new minerals (including ore minerals and native metals) from the solutions. Hydrothermal alterations of the chemical composition of bulk samples depend on the degree of chemical element concentration in fractions and their content in samples.     

超临界流体的快速泄出:来自霓辉石英脉的证据

河南外方山地区熊耳群火山岩中新发现了一种霓辉石英脉群,脉表现出独特的矿物组合及晶体特征,如霓辉石英脉具有复杂的矿物组成,表现出强烈的不平衡特点;霓辉石英脉具明显的对称分带特征,各分带中矿物组成及特征显著不同;存在具熔蚀边结构的钾长石-霓辉石集合体团块;霓辉石具在明显核幔结构,内核化学成分与幔边部存在较大差异;钾长石晶体元素分布存在似"文象"结构,表现出Fe元素的熔体分离特征;脉中矿物普遍发育环带结构;霓辉石英脉交代致密火山岩围岩形成蚀变带,新生大量榍石、霓辉石等矿物且含量随着与脉壁距离的增加急剧减少,显示出强烈的流体/气体扩散交代作用.霓辉石英脉独特的矿物组合及晶体特征反映了其成因的复杂性以及生长环境的变化.根据霓辉石英脉矿物的特点,推测其源于流体、熔体和晶体的混合物,并经历了快速侵位、快速冷却的过程,因而能够保存流体演化各阶段的结晶信息,保留不同成因的晶体群.外方山霓辉石英脉矿物可划分为捕虏晶亚群、熔体晶体群、超临界流体亚群、热液晶体亚群和凝聚晶体亚群等.外方山霓辉石英脉矿物晶体群的深入研究可能有助于揭示岩浆熔体向流体转换的机制及侵位机制,进而可揭示岩浆-流体成矿作用.     

Variation of the chemical composition of sediments under middle- and low-temperature hydrothermal conditions in the southern trough of Guaymas Basin,Gulf of California: Communication 1. results of the study of bulk sediment samples

The paper presents the results of the study of variations in the content of macro- and microelements (rare earth elements included) in the Upper Pleistocene sediments from DSDP Hole 477. The variations took place at the depth interval 110–191 m in the course of temperature drop to about 100°C in the upper part of the hydrothermal system. Variations in the content of most chemical elements in sediments from DSDP Hole 477 are manifested more weakly than in the underlying sediments of the interval 191–267 m of DSDP Hole 477A that were altered at a temperature of about 300°C. For many chemical elements, influence of the sedimentary cover on the compositional transformation of solutions during their interaction with sediments attenuates with the cooling of its upper part to the point of its complete termination.     

Peculiarities of Clay Mineral Formation in Sediments from the Hydrothermal System Center,Hole 858B,Juan de Fuca Ridge

Lithology and Mineral Resources - Clay minerals in Holocene–Pleistocene sediments from Hole 858B DSDP drilled at 20 m from the black smoker in the Dead Dog hydrothermal field, axial valley of...     

Mechanisms of Mg-phyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley,Juan de Fuca)

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24?Å stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14?Å with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ¹⁸O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H₂O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300°?C±30° for chlorite-quartz at 32?m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.     

Mineralogical and geochemical evidence for multi-stage origin of mineral veins hosted by teschenites at Tichá, Outer Western Carpathians,Czech Republic

Numerous mineral veins are hosted in a body of teschenite which is situated within the Lower Cretaceous flysch siliciclastics of the Silesian Unit at Tichá. Mineralogy, fluid inclusions, stable isotopes and trace elements have been studied in order to assess the origin of this mineralization. Three stages of vein cementation have been recognized, each of them being characterized by distinct mineral composition and genetic conditions. The first stage is composed of titanite, aegirine-augite to aegirine, annite, analcime and strontian apatite. These minerals originated from NaCl-rich, CaCl₂-poor magmatic brine (total fluid salinities range between 47 and 57 wt%), leaving after crystallization of host teschenite in low-pressure (<1 kbar) environment. Crystallization temperatures reached ～390–510 °C for early phases, titanite and aegirine-augite. The second stage is formed by calcite, chlorite, dolomite, siderite, strontianite, quartz, pyrite and sphalerite. The parent fluids were low-salinity (0.5–4.5 wt% NaCl eq.) aqueous solutions with low content of strong REE-complexing ligands, that were progressively cooled during mineral precipitation (up to ～190 °C at the beginning, ～90 °C at the end of crystallization). These fluids are interpreted to be predominantly of external origin, derived from surrounding sedimentary sequences during diagenetic dewatering of clay minerals. The highly positive δ¹⁸O and near-zero δ¹³C values indicate an interaction of fluids with sedimentary carbonates. The third stage is formed by a dense net of calcite veinlets, which probably originated during tectonic deformations connected with orogenetic movements during the Tertiary. The source of strontium for first stage mineralization was probably related to the special conditions of magmatic evolution of the host teschenite, whereas strontium for second stage minerals could have been remobilized during hydrothermal alteration from earlier teschenite-hosted mineral phases and/or limestone.     

Metabasalts from the Mid-Atlantic Ridge: new insights into hydrothermal systems in slow-spreading crust

An extensive suite of hydrothermally altered rocks were recovered byAlvin and dredging along the MARK [Mid-Atlantic Ridge, south of the Kane Fracture Zone (23–24°N)] where detachment faulting has provided a window into the crustal component of hydrothermal systems. Rocks of basaltic composition are altered to two assemblages with these characteristics: (i) type I: albitic plagioclase (An_02–10)+mixed-layer smectite/chlorite or chlorite±actinolite±quartz±sphene, <10% of the clinopyroxene is altered, and there is no trace metal mobility; (ii) type II: plagioclase (An_10–30)+amphibole (actinolite-magnesio-hornblende) +chlorite+sphene, >20% of the clinopyroxene is altered, and Cu and Zn are leached. The geochemical signature of these alteration types reflects the relative proportion and composition of secondary minerals, and the degree of alteration of primary phases, and does not show simple predictive relationships. Element mobilities indicate that both alteration types formed at low water/rock ratios. The MARK assemblages are typical of the greenschist and transition to the amphibolite facies, and represent two distinct, albeit overlapping, temperature regimes: type I-180 to 300°C and type II-250 to 450°C. By analogy with DSDP/ODP Hole 504B and many ophiolites, the MARK metabasalts were altered within the downwelling limb of a hydrothermal cell and type I and II samples formed in the upper and lower portions of the sheeted like complex, respectively. Episodic magmatic and hydrothermal events at slow-spreading ridges suggest that these observed mineral assemblages represent the cumulative effects of more than one hydrothermal event. Groundmass and vein assemblages in the MARK metabasalts indicate either that alteration conditions did not change during successive hydrothermal events or that these assemblages record only the highest temperature event. Lack of retrograde reactions or overprinting of lower temperature assemblages (e.g., zeolites) suggests that there is a continuum in alteration conditions while crustal segments remain in the ridge axis environment. The type II samples may be representative of thereaction zone where compositions of hydrothermal fluids actively venting at the seafloor today become fixed. This prediction necessitates interaction between hydrothermal fluids and intersertal glass and/or mafic phases, in addition to plagioclase, in order to produce the observed range in vented fluid pH.     

Pyroxenes of the Khibiny alkaline pluton, Kola Peninsula

Seven pyroxene varieties were identified in nepheline syenites and foidolites of the Khibiny pluton: enstatite, ferrosilite, diopside, hedenbergite, augite, aegirine-augite, and aegirine. Enstatite and augite are typical of alkaline and ultramafic rocks of dike series. Ferrosilite was found in country quartzitic hornfels. Diopside is a rock-forming mineral in alkaline and ultramafic rocks, alkali gabbroids, hornfels in xenoliths of volcanic and sedimentary rocks and foyaite, melteigite-urtite that assimilate them, and certain hydrothermal pegmatite veins. Hedenbergite was noted in hornfels from xenoliths of volcanic and sedimentary rocks and in a hydrothermal pegmatite vein at Mount Eveslogchorr. Aegirine-augite is the predominant pyroxene in all types of nepheline syenites, phonolites and tinguaites, foidolites, alkaline and ultramafic rocks of dike series, fenitized wall rocks surrounding the pluton, and xenoliths of Devonian volcanic and sedimentary rocks. Aegirine is an abundant primary or, more often, secondary mineral in nepheline syenites, foidolites, and hydrothermal pegmatite veins. It occurs as separate crystals, outer zones of diopside and aegirine-augite crystals, and homoaxial pseudomorphs after Na-Ca amphiboles. Microprobe analyses of 265 pyroxenes samples allowed us to distinguish ten principal trends of isomorphic replacement and corresponding typomorphic features of pyroxenes. Compositional variations in clinopyroxenes along the sampled 35-km profile from the margin of the Khibiny pluton to its center confirm the symmetric zoning of the foyaite pluton relative to semicircular faults of the Minor Arc and the Main (Central) Ring marked by Devonian volcanic and sedimentary rocks, foidolites, and related metasomatic rocks (rischorrite, albitite, and aegirinite). Changes in the composition of pyroxenes are explained mainly by the redistribution of elements between coexisting minerals of foyaites in the process of their intense differentiation under the effect of foidolite melts that have intruded into the circular fault zones.     

Fluid inclusion evidence for hydrothermal fluid evolution in the Darreh-Zar porphyry copper deposit,Iran

The Darreh-Zar porphyry copper deposit is associated with a quartz monzonitic–granodioritic–porphyritic stock hosted by an Eocene volcanic sedimentary complex in which magmatic hydrothermal fluids were introduced and formed veins and alteration. Within the deepest quartz-rich and chalcopyrite-poor group A veins, LVHS2 inclusions trapped high salinity, high temperature aqueous fluids exsolved directly from a relatively shallow magma (0.5 kbar). These late fluids were enriched in NaCl and reached halite saturation as a result of the low pressure of magma crystallization and fluid exsolution. These fluids extracted Cu from the crystallizing melt and transported it to the hydrothermal system. As a result of ascent, the temperature and pressure of these fluids decreased from 600 to 415 °C, and approximately 500–315 bars. At these conditions, K-feldspar and biotite were stabilized. Type A veins were formed at a depth of ∼1.2 km under conditions of lithostatic pressure and abrupt cooling. Upon cooling and decompressing, the fluid intersected with the liquid–vapor field resulting in separation of immiscible liquid and vapor. This stage was recorded by formation of LVHS1, LVHS3 and VL inclusions. These immiscible fluids formed chalcopyrite–pyrite–quartz veins with sericitic alteration envelopes (B veins) under the lithostatic–hydrostatic pressure regime at temperatures between 415 and 355 °C at 1.3 km below the paleowater table. As the fluids ascended, copper contents decreased and these fluids were diluted by mixing with the low salinity-external fluid. Therefore, pyrite-dominated quartz veins were formed in purely hydrostatic conditions in which pressure decreased from 125 bars to 54 bars and temperature decreased from 355 to 298 °C. During the magmatic-hydrothermal evolution, the composition and P–T regime changed drastically and caused various types of veins and alterations. The abundance of chalcopyrite precipitation in group B veins suggests that boiling and cooling were important factors in copper mineralization in Darreh-Zar.     

Implication of Apatite and Anhydrite for Formation of an Iron‐Oxide‐Apatite (IOA) Rare Earth Element Prospect,Benjamin River,Canada

The Benjamin River apatite prospect in northern New Brunswick, Canada, is hosted by the Late Silurian Dickie Brook plutonic complex, which is made up of intrusive units represented by monzogranite, diorite and gabbro. The IOA ores, composed mainly of apatite, augite, and magnetite at Benjamin River form pegmatitic pods and lenses in the host igneous rocks, the largest of which is 100 m long and 10–20 m wide in the diorite and gabbro units. In this study, 28 IOA ore and rock samples were collected from the diorite and gabbro units. Mineralogical observations show that the apatite–augite–magnetite ores are variable in the amounts of apatite, augite, and magnetite and are associated with minor amounts of epidote‐group minerals (allanite, REE‐rich epidote and epidte) and trace amounts of albite, titanite, ilmenite, titanomagnetite, pyrite, chlorite, calcite, and quartz. Apatite and augite grains contain small anhydrite inclusions. This suggests that the magma that crystallized apatite and augite had high oxygen fugacity. In back scattered electron (BSE) images, apatite grains in the ores have two zones of different appearance: (i) primary REE‐rich zone; and (ii) porous REE‐poor zone. The porous REE‐poor zones mainly appear in rims and/or inside of the apatite grains, in addition to the presence of apatite grains which totally consist of a porous REE‐poor apatite. This porous REE‐poor apatite is characterized by low REE (<0.84 wt%), Si (<0.28 wt%), and Cl (<0.17 wt%) contents. Epidote‐group minerals mainly occur in grain boundary between the porous REE‐poor apatite and augite. These indicate that REE leached from primary REE‐rich apatite crystallized as allanite and REE‐rich epidote. Magnetite in the ores often occurs as veinlets that cut apatite grains or as anhedral grains that replace a part of augite. These textures suggest that magnetite crystallized in the late stage. Pyrite veins occur in the ores, including a large amount of quartz and calcite veins. Pyrite veins mainly occur with quartz veins in augite. These textures indicate pyrite veins are the latest phase. Apatite–augite–magnetite ore, gabbro–quartz diorite and feldspar dike collected from the Benjamin River prospect contain dirty pure albite (Ab₉₈Or₂–Ab₁₀₀) under the microscope. The feldspar dikes mainly consist of dirty pure albite. Occurrences of the dirty pure albite suggest remarkable albitization (sodic alteration) of original plagioclase (An_25.3–An₆₀ in Pilote et al., 2012) associating with intrusion of monzogranite into gabbro and diorite. SO₄^2? bearing magma crystallized primary REE‐rich apatite, augite and anhydrite reacted with Fe in the sodic fluids, which result in oxidation of Fe²⁺ and release of S^2? into the sodic fluids. REE, Ca and Fe from primary REE‐rich apatite, augite and plagioclase altered by the sodic fluids were released into the fluids. Then Fe³⁺ in the sodic fluids precipitated as Fe oxides and epidote‐group minerals in apatite–augite–magnetite ores. Finally, residual S^2? in sodic fluids crystallized as latest pyrite veins. In conclusion, mineralization in Benjamin River IOA prospect are divided into four stages: (1) oxidized magmatic stage that crystallized apatite, augite and anhydrite; (2) sodic metasomatic stage accompanying alteration of magmatic minerals; (3) oxidized fluid stage (magnetite–epidote group minerals mineralization); and (4) reduced fluid stage (pyrite mineralization).     

Clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin (Gulf of California)

The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.     

Phyllosilicates in hydrothermally altered basalts from DSDP Hole 504B,Leg 83 — a TEM and AEM study

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or ohvine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from< 100 Å to a few hundred Å. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystal-linity of saponite. by contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of Å thick. The Si/(Si+Al) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe+Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+Al) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite±mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.Contribution No. 488 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan     

Fluid–Sediment Interaction in a Marine Shallow-Water Hydrothermal System in the Wakamiko Submarine Crater, South Kyushu, Japan

The Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow‐water hydrothermal activity associated with fumarolic gas emissions at the crater sea floor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05‐13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper‐Dolphine (Japan Agency for Marine‐Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fluid chemistry were studied. The pore fluids of these sites showed a drastic change in chemical profile from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end‐member composition. Occurrence of illite/smectite interstratified minerals in the North site sediment is attributed to in situ fluid–sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the significant low potassium concentration of the hydrothermal end‐member, the abundant occurrence of low‐temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fluid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fluid and alteration minerals in the North site sediment. This difference may reflect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.     

Sulfide remobilization during low temperature alteration of seafloor basalt

The behavior of sulfide minerals during the physical and chemical changes accompanying seafloor alteration was studied in three basalt flows from the bottom of D.S.D.P. Hole 418A, Leg 53. The rocks are mildly altered, and contain primary, authigenic, and vein sulfide minerals. Sulfide habit, mineralogy, and trace element content are inter-related and are correlated with the extent and type of silicate and oxide alteration. Incipient alteration at > 90°–100°C was accompanied by low temperature reequilibration of pyrrhotite, and locally, by the oxidation of pyrrhotite to pyrite plus magnetite. The dominant stage of alteration, at ≤90°C, is characterized by dissolution and local redistribution of pyrite and chalcopyrite, whose precipitation appears to be controlled by the water/rock ratio and the extent to which the water has been modified by reaction with the basalt. Chalcopyrite was concentrated relative to pyrite by slight changes in fluid composition caused by reaction with other minerals. Concurrent precipitation of smectite causes a net increase in rock volume, tending to restrict seawater access. Calculations of rock cooling rate through time suggest that the most prolonged hydrothermal circulation occurs at low temperatures, giving rise to pervasive low temperature alteration assemblages.     

Highly Oxidized Magma and Fluid Evolution of Miocene Qulong Giant Porphyry Cu‐Mo Deposit,Southern Tibet,China

The Miocene Qulong porphyry Cu‐Mo deposit, which is located at the Gangdese orogenic belt of Southern Tibet, is the largest porphyry‐type deposit in China, with confirmed Cu ～10 Mt and Mo ～0.5 Mt. It is spatially and temporally associated with multiphase granitic intrusions, which is accompanied by large‐scale hydrothermal alteration and mineralization zones, including abundant hydrothermal anhydrite. In addition to hydrothermal anhydrite, magmatic anhydrite is present as inclusions in plagioclase, interstitial minerals between plagioclase and quartz, and phenocrysts in unaltered granodiorite porphyry, usually in association with clusters of sulfur‐rich apatite in the Qulong deposit. These observations indicate that the Qulong magma‐hydrothermal system was highly oxidized and sulfur‐rich. Three main types of fluid inclusions are observed in the quartz phenocrysts and veins in the porphyry: (i) liquid‐rich; (ii) polyphase high‐salinity; and (iii) vapor‐rich inclusions. Homogenization temperatures and salinities of all type inclusions decrease from the quartz phenocrysts in the porphyry to hydrothermal veins (A, B, D veins). Microthermometric study suggests copper‐bearing sulfides precipitated at about 320–400°C in A and B veins. Fluid boiling is assumed for the early stage of mineralization, and these fluids may have been trapped at about 35–60 Mpa at 460–510°C and 28–42 Mpa at 400–450°C, corresponding to trapping depths of 1.4–2.4 km and 1.1–1.7 km, respectively.     

Hydrothermal alterations in the Echassières granitic cupola (Massif central,france)

Detailed petrographic and mineralogic investigations of an albite-lepidolite granite at Echassières (Massif Central, France; scientific deep drill program) shows the existence of hydrothermal stages which are closely related to the magmatic and structural history. According to fluid inclusion data, K-Ar datations and ¹⁸O/¹⁶O-D/H compositions of secondary minerals, two successive hydrothermal periods have been recognized. The early one (273–268 million years) produced a series of aluminous phyllosilicates: muscovite, pyrophyllite, donbassite, tosudite, kaolinite which are observed as vein deposits (<10 mm wide) and alteration products of primary minerals in wall-rocks. The vein system was sealed by monomineralic assemblages during a cooling period (400–150°C). This early hydrothermal alteration stage was controlled by interactions of rock with low salinity (1–10 wt% NaCl equivalent) fluids expelled from the granitic body during the cooling processes. The chemical properties of these fluids were the following: low pH, very low Mg and Fe and high Li, Na and K contents. Thermodynamic calculations show that the sequence pyrophyllite, Li-bearing donbassite, tosudite is mostly temperature dependent. From the chemical composition of secondary minerals and isotopic data it can be deduced that these fluids, which have a meteoric origin, have been expelled from the granite body during its cooling period and after interaction with it at high temperature. The late hydrothermal stage corresponds to deposits of fluorite and Fe-Mg rich illite (151 million years) in subvertical fractures. Temperature conditions did not exceed 250° C and fluids came through the surrounding metamorphic rocks into the granitic body. IIlite/smectite mixed-layer minerals have been identified in subvertical fractures which were opened during Tertiary periods. In the host micaschists, successive hydrothermal alterations took place during the cooling of the Beauvoir granite. Early magmatic fluids interacted with these micaschists. Locally, the metamorphic assemblage is replaced by a metasomatic one. Secondary topaz and (F, Li)-rich mica crystals were formed over a range of 450 of 150°C. Later hydrothermal fluids reacted with the country rocks to form phengite-biotite, chlorite-illite and kaolinite over a range of 300 to 150°C. Illite/smectite mixed-layer minerals crystallized in the roof micaschists and within the Beauvoir granite during the Tertiary alteration period. Meteoric water invaded open fractures producing supergene alteration mineral assemblages.     

Occurrence and origin of marialitic scapolite in the Humboldt lopolith,N.W. Nevada

The occurrence and origin of marialitic scapolite in the Humboldt lopolith was investigated in the field and in the laboratory using petrographic and experimental techniques. Scapolite occurs in three modes: as a pervasive replacement of plagioclase and other minerals in gabbro, diorite and extrusive rocks; as a poikiloblastic mineral in scapolitite dikes; and as a fracture-filling mineral with analcime, albite and sphene in scapolite veins. Additional secondary minerals associated with scapolite include epidote, prehnite, hornblende and diopside-salite clinopyroxene. Relations with these minerals suggest that most marialitic scapolite grew at temperatures around 400° C. Scapolite composition varies from EqAn₁₂ to EqAn₃₇, containing from 72 to 96 atomic% Cl in the R position. Experiments on systems of similar compositions indicate that NaCl-H₂O fluid having more than 40 mol% NaCl is needed to stabilize the scapolite.Variation in scapolite compositions is due to thermal and fluid compositional gradients normal to conduits of hydrothermal fluids, and occurs on a scale up to 100 m. The likely source of Na and Cl is pre-existing evaporites or evaporitic brine derived from the wallrocks. Salinity could have been increased to a level sufficient to stabilize scapolite by hydration of an originally dry magma, possibly aided by hydrothermal boiling. Results may be applied to hydrothermal alteration in areas of rifting or back-arc spreading, and in mid-ocean ridge hydrothermal systems.     

The Asachinskoe epithermal Au-Ag deposit in southern Kamchatka: physicochemical conditions of formation

The Asachinskoe epithermal Au-Ag deposit (southern Kamchatka) is referred to as low-sulfidation in quartz-adularia-sericite in Corbett's classification. Research into fluid inclusions of its minerals gave an insight into the PT-conditions of formation and gas composition of ore-forming fluids as well as the vertical variations in these parameters to a depth of more than 200 m within a 2 km long horizontal site of the deposit ore zone. It is shown that mineral assemblages formed at 320 to <100 °C. Ore-forming hydrothermal solutions were poor in salts (3–9.2 wt.% NaCl equiv.), with NaCl being the main component. Mineral assemblages with high contents of Au crystallized at 250–175 °C. Ore-free quartz-carbonate veins formed at 80–120 °C. High-temperature (300–320 °C) veins also lack ores. Rich gold ores were deposited in the environments where ore-bearing fluids boiled, mixing with meteoric waters.     

Hydrothermal alteration and zeolitization of the Fohberg phonolite,Kaiserstuhl Volcanic Complex,Germany

The subvolcanic Fohberg phonolite (Kaiserstuhl Volcanic Complex, Germany) is an economic zeolite deposit, formed by hydrothermal alteration of primary magmatic minerals. It is mined due to the high (>40 wt%) zeolite content, which accounts for the remarkable zeolitic physicochemical properties of the ground rock. New mineralogical and geochemical studies are carried out (a) to evaluate the manifestation of hydrothermal alteration, and (b) to constrain the physical and chemical properties of the fluids, which promoted hydrothermal replacement. The alkaline intrusion is characterized by the primary mineralogy: feldspathoid minerals, K-feldspar, aegirine–augite, wollastonite, and andradite. The rare-earth elements-phase götzenite is formed during the late-stage magmatic crystallization. Fluid-induced re-equilibration of feldspathoid minerals and wollastonite caused breakdown to a set of secondary phases. Feldspathoid minerals are totally replaced by various zeolite species, calcite, and barite. Wollastonite breakdown results in the formation of various zeolites, calcite, pectolite, sepiolite, and quartz. Zeolites are formed during subsolidus hydrothermal alteration (<150 °C) under alkaline conditions. A sequence of Ca–Na-dominated zeolite species (gonnardite, thomsonite, mesolite) is followed by natrolite. The sequence reflects an increase in \(\log [(a_{{{\text{Na}}^{ + } }} )/(a_{{{\text{H}}^{ + } }} )]\) and decrease in \(\log [(a_{{{\text{Ca}}^{2 + } }} )/(a_{{{\text{H}}^{ + } }}^{2} )]\) of the precipitating fluid. Low radiogenic ⁸⁷Sr/⁸⁶Sr values indicate a local origin of the elements necessary for secondary mineral formation from primary igneous phases. In addition, fractures cut the intrusive body, which contain zeolites, followed by calcite and a variety of other silicates, carbonates, and sulfates as younger generations. Stable isotope analysis of late-fracture calcite indicates very late circulation of meteoric fluids and mobilization of organic matter from surrounding sedimentary units.     

Variation of the chemical composition of sediments under middle- and low-temperature hydrothermal conditions in the southern trough of Guaymas Basin,Gulf of California: Communication 2. Results of the study of grain size fractions of sediments

The chemical composition of grain size fractions of the Upper Pleistocene sediments in the southern trough of Guaymas Basin (Gulf of California) is studied based on materials from DSDP Hole 477 (191 m). The sediments are located in the upper part of the main long-lived hydrothermal system. Therefore, they were subjected to long-term reworking by the middle- and low-temperature solutions (100?300°C) and short-term hydrothermal impact during the intrusion of basalt sills. The upper part of the main hydrothermal system is divided into three middle- to low-temperature hydrothermal alteration zones: lower (III), middle (II), and upper (I). In zone III (250?300°C), hydrothermal alterations of sediments are most significant. The coarsegrained fractions are enriched in Fe, S, Ni, Cu, Zn, Mo, Co, Se, Cd, Bi, Pb, and Ag due to the formation of sulfides and precipitation of native metals. The silt fractions are enriched in Be, Sc, Nb, Ta, W, Th, U, Y, and REE. In contrast, the pelite fractions are depleted in most of these elements. The fine-dispersed (<0.001 mm) pelite fractions are appreciably enriched in Mg (due to the formation of the authigenic Mg-chlorite), as well as MnO, Cr, V, Ga, Pb, and Zr. Virtually all grain size fractions are depleted in K₂O, Li, Rb, Cs, and Tl due to the dissolution of the K-bearing terrigenous minerals. In zones II (146?170 m) and I (110?146 m), where the temperature of hydrothermal solutions dropped successively from 250 to 180?195 and 100°C, the sedimentary environment was unfavorable for the concentration of Fe, S, and a large group of ore elements that are typical for sediments in zone III. Intrusion of basalt sills and a short-term hydrothermal impact changed concentrations of several components in the sediments overlying the sill complex: CaO, K₂O, Li, Rb, Cs, and Tl acquired a trend typical of the high-temperature lower zone III, whereas Fe, MgO, P₂O₅, TiO₂, V, Sc, and Y distribution pattern became typical of the low-temperature upper zone I. Significant compositional variations in the grain size fractions of sediments are lacking beneath the sill base.