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1.
流体的沸腾和混合在热液成矿中的意义 总被引：67，自引：0，他引：67
讨论了流体沸腾和混合在热液成矿中的重要作用及研究进展。成矿汉体的降温可能不是许多金属矿物沉淀的最有效机制；流体的沸腾作用对浅成热液帮床，斑岩铜钼等矿床中矿物的沉淀作用很大，造成的矿化具有强度大，品位富及垂向分带较发育的特征，同时因其影响范围小，作用时间短，限制了它的作用的发挥。 相似文献
2.
辽宁是国内萤石重要产地之一,分布着阜新哈尔套-义县地藏寺、辽阳隆昌—金州杏树屯两个萤石成矿带,带内已发现大中型矿床7处、中小型矿床和矿(化)点60余处。通过对辽宁萤石成矿特征和地层、侵入岩、构造等控矿条件与成矿关系的分析,认为萤石成矿类型为中低温岩浆期后热液充填交代型,围岩类型主要为碳酸盐岩和硅酸盐岩;萤石成矿时期为燕山期、印支期和华力西期,且以燕山期为主;萤石矿体多赋存在花岗岩岩体内部断裂带或与围岩外接触带的断裂带位置。依据萤石矿床的地质特征和成矿规律,结合构造、岩石与围岩、地球物理与地球化学等找矿标志,圈定出6个成矿远景区。 相似文献
3.
萤石是我国战略性矿种之一,提高其资源储备对国民经济发展具有重要意义。本研究对河南省萤石矿床的地质特征、时空分布、成矿物质来源等进行分析,系统总结萤石矿床的成矿规律,并提出河南省萤石找矿方向。结果表明：（1）河南省萤石矿床类型以热液充填型为主,全省可划分为三个萤石成矿带;（2）河南省萤石矿床主要分布于中生代花岗质侵入岩体的内部或其周边地层的北西向、北东向断裂构造带中,方城地区萤石矿床主要赋存于岩体附近的地层中;（3）萤石矿床的成矿年龄在120 Ma左右,方城地区萤石矿床可能形成于新元古代;（4）中生代花岗质岩浆作用或其期后热液活动为萤石成矿提供重要的物质和驱动热;地层岩石为萤石成矿提供重要的氟,其丰富钙源很可能是巨量萤石富集的关键;（5）萤石主成矿阶段的成矿流体应属低温、低盐度、低密度的流体系统,大气降水对萤石成矿具有重要作用;（6）河南省萤石找矿重点地区应在太华群地层岩石和中生代花岗质侵入岩体广泛发育的北带和中带,特别是合峪、太山庙、天目山、铜山等岩体的内部或其周围。 相似文献
4.
Major minerals (sulfates, sulfides, quartz) are distributed in different parts of submarine hydrothermal ore deposits. For instance, the abundance of barite increases stratigraphically upwards in the massive orebodies of the Kuroko deposits (black and yellow ores), while quartz is abundant in the lower parts (siliceous ore). The different distribution of barite and quartz in the Kuroko deposits can not be accounted for by thermochemical equilibrium calculations based on the precipitation due to mixing of ascending hydrothermal solutions with ambient cold seawater. In the present study, a coupled fluid flow‐precipitation kinetics model was used to calculate the amounts of quartz, barite, and anhydrite precipitated from a hydrothermal solution mixed with seawater, assuming reasonable values for temperature, precipitation rate, fluid flow velocity, mineral surface area/fluid mass ratio (A/M), and initial concentrations of hydrothermal solution and seawater before mixing occurred. The results indicate that barite precipitates more efficiently than quartz from discharging fluids with relatively higher flow velocity, lower temperatures and under the condition of lower A/M ratios on the seafloor (black ore), whereas quartz precipitates more effectively from solutions with lower flow velocity, higher temperatures and higher A/M ratios beneath the seafloor (siliceous ore) and in the orebody (barite ore, ferruginous chert ore). Anhydrite precipitates in shallow sub‐seafloor environments with lower precipitation rates and higher A/M ratios than barite and higher precipitation rates and lower A/M ratios than quartz. These results explain the observed occurrences of barite, anhydrite, and quartz in the Kuroko deposits. Namely, barite is abundant in black ore and barite ore which formed above the seafloor, anhydrite formed in high‐permeability tuff breccias, and quartz formed in low permeability dacite intrusive bodies in the sub‐seafloor environment. 相似文献
5.
浙江省萤石矿床区域成矿规律与找矿方向研究 总被引：2，自引：1，他引：1
萤石矿是浙江省的优势矿种之一,储量在中国名列前茅。根据矿床成因可分为火山-次火山热液充填型、岩浆期后热液充填型和岩浆热液交代型3种类型。火山-次火山热液充填型萤石矿床主要分布于浙东南火山岩分布区,成矿作用与中生代火山活动关系密切;岩浆期后热液充填型和岩浆热液交代型萤石矿床主要分布于浙西北燕山期花岗岩周围,成矿作用与燕山期岩浆侵入活动密切相关。文章根据浙江省萤石矿床的成因类型、成矿作用、构造环境、成矿时代等特征,可将其归纳为2个成矿系列:与中生代火山-次火山热液有关的萤石矿床成矿系列和与中生代岩浆侵入作用有关的萤石矿床成矿系列。其中,与中生代岩浆侵入作用有关的萤石矿床成矿系列又可分为与燕山期中酸性岩浆期后热液充填作用有关的萤石矿床成矿亚系列及与燕山期中酸性岩浆热液交代作用有关的萤石矿床成矿亚系列。在此基础上,提出了2大萤石矿床成矿系列的找矿方向分别为浙东南晚白垩世火山断陷盆地边缘断裂、火山口环形和放射状断裂及浙西北高氟岩体。 相似文献
6.
Nasser Mourad Mahdy Mahmoud Hany Shalaby Hassan Mohamed Helmy Ali Farage Osman El Sayed H. El Sawey Ehab Korany Abu Zeid 《Arabian Journal of Geosciences》2014,7(7):2573-2589
Uranium mineralizations occur and form in a broad range of geologic setting and age, including magmatic to surfacial conditions, and there are numerous controls on their transportation and deposition, such as redox, pH, ligand concentration, complexation, and temperature. These temporal and spatial variations have caused a range of ore deposit mineral assemblages. Consequently, understanding their conditions of formation is still in its infancy. This research reports rare earth elements (REE) and trace elements of fluorite associated with hexavalent uranium mineralizations and tests of genetic models for the deposits. These data contribute to a better understanding of the variables controlling fluorite formation and uranium ore composition through understanding the evolution of these ore-forming hydrothermal systems. Fluorite in Gabal Gattar granite occurs as disseminations and/or thin veinlets and encrustations filling some uranium mineralized fissures and fractures along the northern margin of host granite mass. In the U-poor samples, fluorite forms well-developed large crystals that are commonly zoned. The zones are represented by alternating colorless and violet zones, and the outer zones are frequently dark violet. In the U-rich samples, fluorite is usually anhedral, unzoned, and has a dark violet color. The results of analysis of REE and trace element contents of fluorites using laser ablation inductively coupled plasma mass spectrometry indicate that total REE in the anhedral unzoned fluorite are elevated compared to the well developed zoned fluorite, and also total REE in dark violet zones of zoned fluorite are elevated with respect to the colorless zones. The fluorites and host granite are generally characterized by strongly negative Eu anomalies and slightly negative or chondritic Ce anomalies. Accordingly, REE patterns of the fluorite and host granite are roughly alike, indicating that the source of REE and trace elements of hydrothermal fluids is the host granite leached by fluids. Y/Y*, Ce/Ce,* and Eu/Eu* patterns show that fluorite clearly records the compositional evolution of the hydrothermal solutions that have transferred trace and REE from host granite during the fluid–wall rocks interactions. The high uranium contents of fluorite in Gabal Gattar granite suggest that parent fluids bearing fluorine have interacted with host granite to leach uranium from the accessory minerals of granite and tetravalent uranium minerals in reduced or weakly oxidized zones. 相似文献
7.
为探究不同类型的萤石矿所反映的自然重砂矿物组合所携带的对各类型萤石矿的指示特征,统计了浙江、湖南、湖北、青海、广西、福建、安徽、新疆、甘肃、河北共计10个地区23个典型萤石矿床的自然重砂矿物报出情况,计算各重砂矿物报出率,分析得出热液型矿、后期热液型及热液充填型3种类型萤石矿床各自对应的自然重砂矿物组合和标型矿物组合。结果显示,各类型矿床自然重砂矿物组合既有相同之处也有显示各自的特点。因此,按照矿床类型建立的自然重砂矿物组合及其含量变化,对于新一轮的矿产资源勘查具有重要的意义。 相似文献
8.
南岭萤石矿床成矿规律及成因 总被引：2，自引：0，他引：2
南岭成矿带是我国最重要的萤石矿聚集区之一。本文从矿床类型、空间分布格局、成岩成矿时代、稀土元素组成、成矿流体特征、成矿物质来源、矿床成因等方面对区内萤石矿床进行了研究与总结。南岭萤石矿床的数量东多西少,储量则是"中间大,两头小",矿床类型中亚带以伴生型为主,东亚带和西亚带主要为单一型。成矿时限介于160~100 Ma之间,集中在160~135 Ma,由中亚带向西亚带和向东亚带有变年轻的趋势。单一萤石矿床以大气降水为主的成矿热液沿破碎带循环、淋滤,使花岗岩中的F以及地层中的Ca再次活化、富集、重结晶成矿,成矿相对于成岩具有较为明显的滞后性,时差为10~20 Ma。伴生型萤石矿床首先经历岩浆-热液-成矿作用,由花岗岩浆的F与地层的Ca结合形成,成岩与成矿作用近于同步,后期又经历了大气降水改造和重结晶。 相似文献
9.
The results are given of textural and compositional investigations carried out on carbonate materials outcropping in various localities in northwestern Sicily where fluorite, barite and calcite mineralizations of hydrothermal origin occur. Observation of the textural features indicate variations in the degree of calcite recrystallization and silicification that appear to be more marked in rocks that show more evident effects of mineralization. The geochemical behaviour of the minor elements indicate variations in chemical composition (increase of Mn, Fe, and Sr and removal of Mg) as a result of interactions between mineralizing fluids and host-rocks. Comparison between the isotopic compositions of oxygen and carbon in mineralized and unmineralized limestones shows that isotopic exchange took place between CaCO3 and hydrothermal fluids. The values obtained support relatively short limestone/water contact times and moderate temperature values. It is suggested that the ore-forming solutions were essentially heated meteoric waters. Precipitation of the fluorite was caused by temperature decrease, reaction with Ca-rich wall rock, and increase in pH of initially rather acid hydrothermal solutions.This work was supported by a grant from the Ente Minerario Siciliano. 相似文献
10.
近年来浙西北地区在岩浆期后热液型萤石矿床勘查中取得重大突破,先后探明常山、淳安、开化、安吉等地多个大中型萤石矿床。这些矿床主要分布在燕山期(早白垩世)花岗岩类内外接触带中,成矿作用与岩浆侵入活动密切相关。通过对浙西北地区7个典型矿床进行矿床地质特征、成矿岩体及控矿构造研究,探讨岩体与萤石成矿之间的关系,将萤石矿床成因初步划分成2个亚类:与A型花岗岩有关的岩浆期后热液型萤石矿床和与I型花岗岩有关的岩浆期后热液型萤石矿床。通过对该区萤石矿床成矿物质来源、成矿温度、萤石稀土地球化学特征、成矿时代与成矿构造环境分析,较系统总结了浙西北主要萤石矿床的成因。在此基础上,结合区域F元素地球化学特征及剩余重力异常特征,提出本区萤石矿的找矿方向为高氟岩体分布区、F元素异常区及剩余重力异常负值区。 相似文献
11.
The factors determining the REE distribution in natural hydrothermal systems are studied by a numerical experiment. The behavior of REE is examined based on the composition of ore-bearing hydrothermal solutions and the parameters of ore formation processes at different fluorite deposits. These data were obtained in studies of fluid inclusions. Some regularities of the REE behavior during the formation of fluorite deposits have been revealed. It is shown that the REE distribution in fluorites is related mainly to changes in the composition of mineral-producing fluid solution. 相似文献
12.
13.
《Chemie der Erde / Geochemistry》2016,76(2):275-297
Epithermal deposits mined for fluorite in Patagonia, Argentina, are closely related to late Triassic through Jurassic magmatic activity which brought about felsic to intermediate magmatic rocks. The fluorite mineralization in the Patagonian epithermal system resulted from gaseous F-and CO2-enriched magmas which lead to an explosive phreatomagmatic volcanism, when getting in contact with groundwater near the surface. As a result of these hydrothermal processes, rapid cooling took place in the epithermal mineralization. Changes in the viscosity along with the cooling down of mineralizing fluids caused mottled mineral colors blurring the boundaries between the stages and ore textures.The fluids accountable for the main constituents fluorite, quartz, barite and silica were operative over a vertical extension of roughly 600 m. Their temperature of formation dropped from 379 °C through 64 °C, while the pH decreased from the heat center towards the paleosurface under oxidizing conditions. This steep temperature gradient conducive to the telescoping of mineral associations into each other was accompanied by a rapid loss in CO2, and a mixing of meteoric and magmatic fluids. Even the boundary between the hypogene and supergene alteration cannot be drawn precisely within the assemblage of Mn oxides, which bridge the gap between hypogene and supergen mineralization. The physical-chemical parameters of the fluids, particularly, the redox conditions did not allow sulfides to be preserved. A classification of the epithermal system as to its degree of sulfidation is based on K-feldspar and kaolinite which are present in significant amounts, whereas APS (aluminum-phosphate-sulfate) minerals are absent. Therefore a categorization as an epithermal fluorite deposit of low- to intermediate sulfidation is justified, because the only mineral of economic interest in the system is fluorite.The data obtained during this joint study render the Patagonian fluorite district a reference type of fluorite in an epithermal system of low- to intermediate sulfidation which are widespread in Argentina, e.g., Sierras Pampeanas, and evolved on part of the stable craton, called Gondwana and which grade into epithermal Au, Ag, In, Pb and Zn deposits. 相似文献
14.
Abstract: Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K2 O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na2 O) content increases, and SiO2 content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow–mixing model, and the computed results are compared with the distribution of SiO2 minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10-4.2 m/sec, and ca. 0.10 m2 /kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain δ18 O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold. 相似文献
15.
额尔古纳超大型火山热液型铀成矿带地质特征及找矿前景 总被引：13，自引：0，他引：13
本文介绍了在额尔古纳超大型火山热液型铀成矿带中发现的超大型火山热液脉型铀矿末有５个，大型火山热液脉型铀矿床有６个，总共有铀金属储量３３万ｔ，在世界热液脉型铀矿资源量中占有重要地位。经研究认为，该成矿带内铀矿就成矿作用在时间上具有突发性特征，成矿时代集中在早白垩世，受统一中生代大陆裂陷－火山岩浆活动控制；成矿介质中深源的火山岩水，大气水和变质水三者相混合的具中低温的火山热液，成矿物质主要来自中生代深 相似文献
16.
《Applied Geochemistry》1993,8(5):495-506
The quantification of geochemical reactions in hydrothermal aquifers requires an exhaustive approach because of their interdependence. All chemical elements likely to have a quantitative influence on dissolution or precipitation reactions have to be taken into account. Geochemical constraints the number of which equals the number of the chemical elements, were determined with the help of chemical analyses of the solution. In the case study of the Dogger aquifer (Paris Basin, France), 12 elements are taken into account (Al, C, Cl, Ca, F, H, K, Mg, Na, O, S, Si). Three constraints apply to water-activity, neutrality and conservation of chloride in aqueous solutions. The determination of the remaining constraints was based on saturation indices, stability diagrams of minerals and partial pressure of carbon dioxide (pCO2). The present-day solutions are at equilibrium with respect to nine minerals (albite, anhydrite, chalcedony, calcite, dolomite, fluorite, kaolinite, K-feldspar and illite or chlorite). Thus, it was demonstrated that the composition of these solutions (including computed pCO2) is only a function of temperature and chloride content. Moreover, it was possible to test the validity of the geochemical system by computing its speciation and comparing these theoretical results with actual chemical analyses (pH and concentrations). Finally, geochemical simulations were used in predicting what quantities would be dissolved or precipitated, as either the temperature or the chlorinity varied. Although the rock is predominantly calcareous, these quantities could not be determined if the influence of the aluminosilicates were neglected. This chemical component approach with which one can pose and solve rigorously the chemical equilibrium problem constitutes a prerequisite for the quantitative study of geochemical processes related to fluid flow. 相似文献
17.
Recent research on volcanic-hosted massive sulfide (VMS) deposits indicates that syngenetic subsea-floor replacement ores form an important component of many deposits. In the context of VMS deposits, subsea-floor replacement can be defined as the syn-volcanic formation of sulfide minerals within pre-existing volcanic or sedimentary deposits by infiltration and precipitation in open spaces (fractures, inter- and intra-granular porosity) as well as replacement of solid materials.There are five criteria for distinguishing subsea-floor replacement in massive sulfide deposits: (1) mineralized intervals are enclosed within rapidly emplaced volcanic or sedimentary facies (lavas, intrusions, subaqueous mass-flow deposits, pyroclastic fallout); (2) relics of the host facies occur within the mineral deposit; (3) replacement fronts occur between the mineral deposit and the host lithofacies; (4) the mineral deposit is discordant to bedding; and (5) strong hydrothermal alteration continues into the hanging wall without an abrupt break in intensity. Criteria 1–3 are diagnostic of replacement, whereas criteria 4 and 5 may suggest replacement but are not alone diagnostic. Because clastic sulfide ores contain accessory rock fragments collected by the parent sediment gravity flow(s) during transport, criteria 2 can only be applied to massive, semi-massive, disseminated or vein style deposits, and not clastic ores.The spectrum of VMS deposit types includes deposits that have accumulated largely subsea-floor, and others in which sedimentation and volcanism were synchronous with hydrothermal activity, and precipitation of sulfides occurred at and below the sea floor over the life of the hydrothermal system. Deposits that formed largely subsea-floor are mainly hosted by syn-eruptive or post-eruptive volcaniclastic facies (gravity flow deposits, water-settled fall, autoclastic breccia). However, some subsea-floor replacement VMS deposits are hosted by lavas and syn-volcanic intrusions (sills, domes, cryptodomes). Burial of sea-floor massive sulfide by lavas or sediment gravity flow deposits can interrupt sea-floor mineralization and promote subsea-floor replacement and zone-refining.The distance below the sea floor at which infiltration and replacement took place is rarely well constrained, with published estimates ranging from less than 1 to more than 500 m, but mainly in the range 10–200 m. The upper few tens to hundreds of metres in the volcano-sedimentary pile are the favoured position for replacement, as clastic facies are wet, porous and poorly consolidated in this zone, and at greater depths become progressively more compacted, dewatered, altered, and less amenable to large scale infiltration and replacement by hydrothermal fluids. Furthermore, sustained mixing between the upwelling hydrothermal fluid and cold seawater is regarded as a major cause of sulfide precipitation in VMS systems, and this mixing process generally becomes less effective with increasing depth in the volcanic pile.The relative importance of subsea-floor replacement in VMS systems is related principally to four factors: the permeability and porosity patterns of host lithofacies, sedimentation rate, the relative ease of replacement of host lithofacies (especially glassy materials) and early formed alteration minerals during hydrothermal attack, and physiochemical characteristics of the hydrothermal fluid. 相似文献
18.
Francisco González-Sánchez Antoni Camprubí Eduardo González-Partida Rafael Puente-Solís Carles Canet Elena Centeno-García Viorel Atudorei 《Mineralium Deposita》2009,44(3):343-361
Northeastern Mexico hosts numerous epigenetic stratabound carbonate-hosted low-temperature hydrothermal deposits of celestine,
fluorite, barite and zinc-lead, which formed by replacement of Mesozoic evaporites or carbonate rocks. Such deposits can be
permissively catalogued as Mississippi Valley-type (MVT) deposits. The deposits studied in the state of Coahuila are associated
with granitic and metasedimentary basement highs (horsts) marginal or central to the Mesozoic Sabinas Basin. These horsts
controlled the stratigraphy of the Mesozoic basins and subsequently influenced the Laramide structural pattern. The Sabinas
Basin consists of ~6,000-m-thick Jurassic to Cretaceous siliciclastic, carbonate and evaporitic series. The MVT deposits are
mostly in Barremian and in Aptian-Albian to Cenomanian formations and likely formed from basinal brines that were mobilized
during the Laramide orogeny, although earlier diagenetic replacement of evaporite layers (barite and celestine deposits) and
lining of paleokarstic cavities in reef carbonates (Zn–Pb deposits) is observed. Fluid inclusion microthermometry and isotopic
studies suggest ore formation due to mixing of basinal brines and meteoric water. Homogenization temperatures of fluid inclusions
range from 45°C to 210°C; salinities range from 0 to 26 wt.% NaCl equiv., and some inclusions contain hydrocarbons or bitumen.
Sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from Barremian to Cenomanian evaporites.
Regional geology and a compilation of metallogenic features define the new MVT province of northeastern Mexico, which comprises
most of the state of Coahuila and portions of the neighboring states of Nuevo León, Durango and, perhaps extends into Zacatecas
and southern Texas. This province exhibits a regional metal zonation, with celestine deposits to the south, fluorite deposits
to the north and barite and Zn–Pb deposits mostly in the central part. 相似文献
19.
浙江八面山萤石矿床流体包裹体地球化学研究 总被引：2，自引：1，他引：1
八面山萤石矿床流体包裹体可分为三大类型:Ⅰ气液包裹体,Ⅱ气体包裹体,Ⅲ含子矿物的多相包裹体;矿床成矿温度变化不大,主要集中在120～240°C之间。细粒条带状萤石矿石包裹体温度变化在115～250℃之间;巨晶块状萤石矿石和石英脉型萤石矿石包裹体温度集中在135～170℃之间。萤石矿床流体包裹体以低盐度成矿流体为主。成矿过程中起作用的成矿流体为KCl-H2O体系和CO2-CaF2-H2O体系,成矿溶液的离子类型属K+-Ca2+-HCO--F-型,KCl-H2O体系反映岩浆期后热液作用的结果,而CO2-CaF2-H2O体系可能反映了寒武纪矿源层成矿体系。通过包裹体研究,认为八面山萤石矿床是受地层-岩体-层间断裂共同控制"三位一体"的热液成因矿床。 相似文献
20.
Equilibrium path calculations have been used to model mixing between hot (350°C) hydrothermal solutions and ambient seawater, in an attempt to simulate mineral precipitation at seafloor vents. These calculations predict temperatures of precipitation, paragenetic sequence of minerals, and chemical composition of chimney deposits associated with vents on the seafloor at 21°N, EPR. Assuming sulfate-sulfide disequilibrium during mixing, the paragenetic sequence revealed is: chalcopyrite, anhydrite, pyrrhotite, pyrite, sphalerite, graphite, and barite. When sulfate-sulfide equilibria is permitted during mixing, however, reduction of small amounts of sulfate results in early precipitation of pyrite and a sequence of Cu-rich sulfide minerals (chalcopyrite-bornite-chalcocite-covellite). This sequence is analogous to that observed in thin chimney walls. The calculations indicate that sulfide mineral precipitation occurs in response to both cooling and change in composition of the hydrothermal solutions as a result of mixing. Varying the amount of mixing with respect to temperature, simulating conductive heating of seawater prior to mixing, results in only minor variations in the sequence and abundance of precipitated phases.Anhydrite precipitation during mixing occurs early, which is consistent with formation of an anhydrite leading edge of chimney structures. Similarly, extrapolation of warm spring data from Galapagos to zero SO4 concentration suggests anhydrite formation due to mixing with seawater beneath the seafloor, most likely below the level of reactive calcareous sediments. Subsequent interaction of the mixed hydrothermal solution with those sediments results in elevated and variable Ca concentrations estimated for end-member solutions from the Galapagos.Precipitation of Mg hydroxide sulfate hydrate in the walls of the vent chimneys at 21°N, EPR, occurs as a result of conductive heating of ambient seawater with only very minor amounts of mixing. In contrast, precipitation of amorphous silica in the vents must be due to conductive cooling of the hydrothermal solutions.Thus, incremental reaction calculations demonstrate that reactions occurring in and associated with venting ridge crest hydrothermal solutions can be effectively modeled using the thermodynamic data and reaction modeling codes available today. Departures from equilibrium required to accurately model the mixing process are easily accommodated and consistent with data from the vents and vent forming materials. 相似文献