Theoretical and numerical analyses of convective instability in porous media with upward throughflow

Exact analytical solutions have been obtained for a hydrothermal system consisting of a horizontal porous layer with upward throughflow. The boundary conditions considered are constant temperature, constant pressure at the top, and constant vertical temperature gradient, constant Darcy velocity at the bottom of the layer. After deriving the exact analytical solutions, we examine the stability of the solutions using linear stability theory and the Galerkin method. It has been found that the exact solutions for such a hydrothermal system become unstable when the Rayleigh number of the system is equal to or greater than the corresponding critical Rayleigh number. For small and moderate Peclet numbers (Pe ⩽ 6), an increase in upward throughflow destabilizes the convective flow in the horizontal layer. To confirm these findings, the finite element method with the progressive asymptotic approach procedure is used to compute the convective cells in such a hydrothermal system. Copyright © 1999 John Wiley & Sons, Ltd.     

A model of thermo-diffusive mass transport in geothermal systems using a stability theory formalism

The coexistence of chemical potential gradients of solute species in a hydrothermal solution, and a temperature gradient imposed by a heat source may lead to complex coupled transport of matter and heat. If the geological material is permeable to solutes, thermal diffusion (Soret effect) may constitute an important transport mechanism that could redistribute ionic species in the fluid phase. An attempt is made to explain, by stability theory, the non-steady-state behaviour of the system and the temperature oscillations within a thin convective layer adjacent to the walls of the heat source.     

含矿流体混合反应与成矿作用的动力平衡模拟研究

本文在约定热液体系中成矿元素成矿速率(成矿过程中单位时间内单位体积所合成矿元素重量的变化)的基础上。借助于物质-热-化学-成矿四重全耦合的研究思路,构建了均匀热液体系、层状热液体系、岩浆侵入热液体系下成矿元素的迁移、富集、溶解与沉淀作用数值模型。模拟结果表明;(1)硫化物(H2S)和硫酸盐(SO42-)流体的混合反应是成矿热液体系中铅、锌、铁成矿元素成矿的重要控制因素;(2)均匀介质、岩浆侵入或地质构造的存在,对成矿元素在成矿流体运移的速度、流线、温度分布和成矿元素的溶解与沉淀分布都有着各自的特征．不同的成矿环境或成矿背景制约了成矿元素的迁移与富集以及矿体的产出定位。暗示成矿环境及成矿速率对热液体系中成矿元素的沉淀与溶解具重要作用;成矿流体的混合反应是成矿作用发生的重要机制之一。在成矿理论研究中必须充分考虑不同地质构造因素的约束。     

HHP花岗岩和喷气沉积贱金属硫化物矿床间的成因关系

热源一直是沉积喷气型块状硫化物矿床成因模式难以解决的阿题．在该类矿床下盘岩石中HHP花岗岩的发现为其问题的解决提供了新途径。HHP花岗岩含有很高的放射性U、Th和K，能长期持续地提供热能．从而在上覆沉积岩中驱动热液对流循环系统。如果保持中等程度高的渗透率．一个宽大于10km的HHP花岗岩驱动并维持热液对流循环系统脉动性地活动10^5-10^4a．耶么温度为235℃．且最初由花岗岩缓冲的还原卤水能提供10Mt铅锌，所以确定HHP花岗岩及其形成时代有助于大型Sedex矿床的发现。     

南海西南次海盆扩张期热液循环与洋壳增生的数值模拟

洋壳厚度受多方面因素的影响，前人大多关注地幔温度、地幔源成分等岩石圈深部因素，很少关注岩石圈浅层的热液循环对洋壳厚度的影响。利用基于有限元的数值模拟手段，对扩张期不同背景(洋中脊、拆离断层)、不同扩张速率的热液循环与洋壳增生的关系进行研究。结果表明：洋壳增生达到稳定前，热液循环导致理论洋壳厚度发生阶段性减薄，减薄量随时间改变，并且推迟了上地幔中熔融体出现的时间；当洋壳增生达到稳定后，热液循环下产生的理论洋壳厚度反而比无热液循环的更厚。结合洋壳增生过程中对流热通量的变化分析，在洋壳增生前期的上地幔温度低，驱动热液循环的热源小，产生的对流热通量相对较小且不稳定，热液循环缓慢冷却上地幔顶部的温度，进而推迟上地幔初始熔融的时间，减弱上地幔的熔融，并造成一定时间阶段内的生成理论洋壳比正常理论洋壳厚度更薄；当洋壳增生达到稳定后，对流热通量达到最大并稳定，热液循环持续快速的冷却上地幔顶部温度，导致上地幔深部的热向上地幔顶部补给，反而增大了上地幔顶部的温度和熔融量，进而增大了理论洋壳厚度。随着扩张速率的增大，理论洋壳厚度增大，对流热通量增大，热液循环导致的洋壳阶段性减薄的最大减薄量也增大，阶段性减薄的时间缩短。结合南海西南次海盆的洋壳结构特征分析：两条横跨南海西南次海盆的地震剖面显示，海盆内存在异常薄的洋壳区域，并且两条地震剖面的最薄洋壳厚度相差0. 85 km,推测海盆内异常薄洋壳和不同扩张时期的最薄洋壳厚度差异受到扩张期热液循环阶段性减薄洋壳作用的影响。     

Double Diffusion-Driven Convective Instability of Three-Dimensional Fluid-Saturated Geological Fault Zones Heated from Below

We conduct a theoretical analysis to investigate the double diffusion-driven convective instability of three-dimensional fluid-saturated geological fault zones when they are heated uniformly from below. The fault zone is assumed to be more permeable than its surrounding rocks. In particular, we have derived exact analytical solutions to the total critical Rayleigh numbers of the double diffusion-driven convective flow. Using the corresponding total critical Rayleigh numbers, the double diffusion-driven convective instability of a fluid-saturated three-dimensional geological fault zone system has been investigated. The related theoretical analysis demonstrates that: (1) The relative higher concentration of the chemical species at the top of the three-dimensional geological fault zone system can destabilize the convective flow of the system, while the relative lower concentration of the chemical species at the top of the three-dimensional geological fault zone system can stabilize the convective flow of the system. (2) The double diffusion-driven convective flow modes of the three-dimensional geological fault zone system are very close each other and therefore, the system may have the similar chance to pick up different double diffusion-driven convective flow modes, especially in the case of the fault thickness to height ratio approaching 0. (3) The significant influence of the chemical species diffusion on the convective instability of the three-dimensional geological fault zone system implies that the seawater intrusion into the surface of the Earth is a potential mechanism to trigger the convective flow in the shallow three-dimensional geological fault zone system.     

现代海底热液活动与块状硫化物矿床成因研究进展

现代海底热液活动的发现及对其分布特征和成矿机理的研究是近年来海洋地质学和矿床地质学的一大进展。对现代海底金属硫化物成矿作用的研究大大推动和促进了对古代块状硫化物矿床成因的认识。有关研究成果综合分析表明：(1)深部热液对流循环系统是块状硫化物成矿的核心，对流循环模式有简单的热液对流模式和双扩散对流模式。(2)块状硫化物矿床集中分布在大洋中张裂性活动板块边界，与大地构造活动紧密联系。(3)成矿流体与成矿物质均有多源性，在强调海水循环淋滤的同时，通过应用新的方法技术，岩浆来源物质(流体及成矿金属等)对一些块状硫化物矿床成矿的直接贡献得到初步确认。(4)在高温热液活动区及金属硫化物沉积中发现大量生命活动和生物群体，意义重大。     

Characteristics of the Reservoir of the Rehai Geother-mal Field in Tengchong,Yunnan Province,China1

Abstract The Rehai geothermal field in Tengchong County, Yunnan Province is a significant high- temperature hydrothermal convective system. The geothermal reservoir is composed of granite. Various geothermometers are used to evaluate the reservoir temperature. The most likely temperature of the reservoir as represented by T_Na-K-Ca is about 230 °C. The chemical and isotopic compositions of fluids before boiling within the reservoir are estimated. The mixing and dilution of cold and warm waters are discussed. The Rehai geothermal field is a high- temperature (hot) water system with the subsurface boiling zone close to the surface. The reservoir pressure at different depths is calculated. And finally the water-rock equilibration is inferred.     

Numerical investigation of salinity in controlling ore-forming fluid transport in sedimentary basins: example of the HYC deposit, Northern Australia

This paper presents the first hydrogeological model that fully couples transient fluid flow, heat and solute transport associated with the formation of the HYC SEDEX deposit in the McArthur Basin, northern Australia. Numerical results reveal that salinity plays an important role in controlling hydrothermal fluid migration. In particular, it appears that it is the distribution of evaporitic units within a given basin, rather than their absolute abundance, that controls the development of free convection. Relatively saline conditions at the seafloor strengthen the thermally-induced buoyancy force and hence promote free convection of basinal solutions; whereas high salinities at the bottom counteract the thermal function of natural geothermal gradient and suppress the development of convective hydrothermal fluid circulation. In the latter case, higher thermal gradients are required to initiate substantial free convective fluid flow.Numerical experiments also suggest the position of an ore body with respect to its vent system may be controlled by the spatial and temporal salinity distributions in the basin. Vent-distal ore formation, a result of exhalation of brines that are denser than seawater and hence can flow away from the vent region, is promoted by moderate salinity at the seafloor and higher salinity in the aquifer. Vent-proximal ore accumulation, a result of pluming upon exhalation of brines less dense than seawater, is favored by the highest salinity conditions occurring near the level of the seafloor.Editorial Handling: G. Beaudoin     

东秦岭中生代钼铅锌银多金属矿床模型及其找矿评价

最近几年不仅在东秦岭地区又发现和探明一批大型—超大型斑岩型钼矿床,而且在南泥湖矿田（包括南泥湖-三道庄-上房沟矿床）和东沟超大型斑岩钼矿床外围探明了一批脉状铅锌银矿。这些脉状铅锌银矿与斑岩钼矿具有明显的时空关系,而且互为找矿指示。以南泥湖和东沟为代表的斑岩钼矿分别形成于晚侏罗世—早白垩世（156~137Ma）和白垩纪中期（125~114Ma）,其成矿有关岩体指示成矿物质分别来源于壳幔同熔和壳源。尽管有物质来源的差别,但是在2个不同时代斑岩钼矿的外围都有一系列脉状铅锌矿的发育,而且通常在断裂膨大或拐弯处和2组断裂的交会部位形成大矿和富矿,也见出现在地层不整合界面的部位。这种钼与铅锌矿的密切时空分布关系构成一个很好的成矿系统,斑岩－矽卡岩钼矿与铅锌银矿互为找矿的指示标志。矿床模型的建立有利于进一步推动在中国东部地区开展此类矿床组合的找矿勘查。     

安徽铜陵凤凰山铜矿床地球化学特征及其意义

通过对凤凰山铜矿床化探样品测试数据的系统分析研究,初步确定了本矿床在成矿作用过程中至少经历了两次大的热液流体活动:第一次是花岗闪长岩的侵入,不仅是岩浆热液使成矿元素迁移富集,而且在岩体侵入过程中强大热能的驱动下地层中的大气水参与流体的对流循环,使地层中的成矿元素被迁移;第二次是石英二长闪长岩的侵入,它在带来大量成矿物质的同时,也使地层中、特别是前期花岗闪长岩中的成矿元素在大气水的对流循环作用下被迁移并在岩体边缘及附近富集成矿.     

广东三坑地热田形成条件及开发潜力分析

广东三坑地热田位于吴川－四会深大断裂带的东北侧,三坑向斜岩溶盆地内。地下热水主要赋存于ＮＥ与ＮＷ向断裂构造及其交汇部位的岩溶裂隙发育带中。根据地热地质条件及水热对流系统分析,地下热水是在水热对流系统中由深循环高温构造裂隙水与浅循环８常温岩溶水混合而成。三坑地热田热水资源具有水温适中、水量丰富、水质良好、动态稳定等特点。开发潜力很大。     

热液源研究的重要进展和“三源”交代热液成矿学说

热液来源在很大程度上决定了热液矿床的成因。过去由于缺乏研究热液来源的有效手段使矿床成因长期争论不休。近３０年来，热液水和成矿物质来源都有较好的测定方法，如热液的δ￣（１８）Ｏ、δＤ的测定，根据围岩的δ￣（１８）Ｏ变化、热液矿床的地理位置与热液水的δ￣（１８）Ｏ和δＤ的关系、流体包裹体的成分、矿床分布与围岩含水性的关系等确定热液水的类型和来源，根据围岩中成矿金属含量的变化而确定矿质来源等。这样在热液来源方面取得了一系列重要成果，在此基础上提出了热驱动对流循环成矿模式，进而发展为“三源”交代热液成矿学说。并根据新学说总结出新的成矿规律，建立了新的在实践中有明显效果的预测方法。但这方面的研究还刚刚开始，许多重要的成矿问题尚待研究。     

Thermodynamic model of ore-forming processes in a submarine island-arc hydrothermal system

A thermodynamic model suggested for ore-forming processes in a hydrothermal system (HS) in an island arc is based on the technique suggested earlier in [1] for simulating ore-forming hydrothermal systems in mid-oceanic ridges. This technique make use of the principle of flow-through multistep reactor and encompasses (a) the region where hydrothermal solutions are generated when seawater interacts with rocks (descending convection branch); (b) the region where material is transported with the solution at decreasing pressure (feeder channel); and (c) the region where the ore material is deposited (orebody). Hydrothermal systems in island arcs exhibit the following distinctive features taken into account in the model: (1) the composition of the host crustal rocks (rocks of mafic-acid composition instead of basalt and serpentinite) and (2) possible significant involvement of magmatic gases in the feeding of the hydrothermal system. The naturally occurring prototype of the simulated system is the hydrothermal system in the caldera of a submarine volcano in an island arc. The model is simulated in a number of variants in which the hydrothermal fluid is exogenic (heated seawater convecting through hot volcanic rocks), magmatic, or mixed (magmatic plus exogenic) is involved. The simulations were carried out using the HCh version 4.3 [2] program package for the multisystem H-O-K-Na-Ca-Mg-Fe-Al-Si-C-S-Cl-Cu-Zn-Pb-As-Sb-Ag-Au at temperatures of 25?C370°C and pressures of 10?C500 bar. The multisystem included 88 possible solid phases and aqueous solution with 95 species. The thermodynamic properties of compounds were calculated using the UNITHERM databank. The model is underlain by the principle of multiwave flow-through multistep reactor (MFTMR) with a starting rock/water (R/W) ratio of 1: 1. As progressively more solution portions passed through the rocks, the participation of fresh rock in the interaction accordingly diminished because the rock material was gradually exhausted in the system. The magmatic fluid had a composition selected based on data on fumaroles at Kudryavyi volcano [3] with a correction for the degassing pressure. The evolution of ore deposition was simulated in compliance with the scheme described in [4], which was implemented using the technology of ??openness from above?? [3]. The model was simulated with various compositions of the host rocks (basalts, andesites, dacites, and rhyolites) and the origin of the fluid (magmatic fluid alone, seawater alone, and variable proportions of both). Our simulation results indicate that the metallogeny (relative enrichment in Pb, As, Sb, or Ag) of island-arc ore deposits is controlled by the abundances of metals in the host rocks predominant in the hydrothermal system. The mineralogy and geochemistry of ores generated in arc hydrothermal systems are predetermined by the effective transport of metalloids (S, As, and Sb) that have a high migration capacity in these systems. Magmatic gases introduced in the hydrothermal systems play dualistic roles in the ore-forming processes. If the hydrothermal fluid in a hydrothermal system is dominated by magmatic components, deposits of native sulfur are formed, and the precipitation of base metal is thereby suppressed because of the high acidity of the generated hydrothermal solutions. The involvement of magmatic gases in an amount of a few percent in a hydrothermal system enhances the overall oregenerating potential of the system in terms of sulfide ores.     

The genesis of sediment-hosted,exhalative zinc + lead deposits

Large sediment-hosted lead+zinc deposits like Mount Isa, McArthur River, Navan, Rammelsberg and Sullivan form a distinctive group characterised by stratiform, syngenetic sulphide ores that formed in local basins on the sea floor as a result of protracted hydrothermal activity accompanying continental rifting. Generally there is a development of a sedimentary pre-ore phase mineralization often featuring manganese followed by zinc±lead, iron and chert. Lower main phase zinc+lead lenses are usually almost devoid of copper but Cu tenors increase toward the middle or top of the ore sequences. Hanging wall trace element haloes are common. These characteristics are accounted for by deriving the ore solutions from subsurface convective circulation of modified highly saline seawater. The circulation is initiated during rifting and driven by a high geothermal gradient. As a result of continued extensional strain and cooling of the rock column the brittle-to-ductile transition zone is depressed and the circulation penetrates to greater depth with time. Of the ore metals the downward-penetrating convection fluids first leach and transport zinc and lead, but with increasing temperature are later able to leach and transport some copper. Unless convective circulation ceases the metal sequence generally reverses as the cooling phase sets in. The minimum distance separating major coeval orebodies of this type is 18 km which is a function of the size of the convective systems.     

一个浅海裂谷盆地的古老热水沉积锰矿——以武陵山震旦纪锰矿为例

在我国贵州省东北部、湖南省西北部和四川省东南隅交界地带的武陵山区,分布着许多锰矿床(点),这就是产于上元古宇震旦系著名的大塘坡式锰矿。60年代后期至今的20年间进行了大量普查勘探工作,获得了丰富的矿床地质资料。80年代初期,笔者曾对黔东及邻区大塘坡式锰矿进行了较系统的研究,于1985年发表了相应的专著。近年又对其作了深入研究,在本文中笔者将简要论述这类古老锰矿床的成因及其热水成矿作用模式。     

辽宁兴城元台子银铅锌多金属矿成矿条件及找矿前景

元台子银铅锌多金属矿位于兴城首山—大英昌—杨家杖子NW向断裂带与黑风山—梁家沟—季家沟NE向断裂带的交汇部位。银铅锌多金属矿体主要赋存于长城系常州沟组石英岩与燕山早期花岗岩的接触带中,矿体多呈似层状、脉状产出,为热液充填型矿床,成矿与断裂构造与岩浆热液的活动关系密切。该区成矿地质条件优越,是银、铜、钼、铅锌等金属元素的富集区域,找矿潜力较大。     

Borosilicate alteration associated with U-Mo-Zn and Ag-Au-Zn deposits in volcanic rocks

Borosilicate alteration is developed in many uranium-molybdenum-zinc deposits in volcanic piles such as at Novazza (Italy), Novoveska Huta (Slovakia), Ben Lomond (Queensland, Australia), the Saar-Nahe basin (Germany), the Shengyuan district (China), and at many deposits in Russia. These uranium-molybdenum-zinc deposits share similar styles of alteration with silver-gold-zinc deposits in volcanic rocks, like those of the Humboldt Range (Nevada, USA), the Basin district (Montana, USA) and Equity Mine (British Columbia, Canada). In thick volcanic piles, fluid-rock interaction commonly develops under the influence of convective hydrothermal systems. In borosilicate-bearing systems the alteration is characterized by changes in borosilicate mineralogy and chemistry under mesothermal conditions. Zonal distribution of different compositions of tourmaline can be observed including regular increases of alkali deficiency in the X(3a) site from peripheral (chlorite-epidote-prehnite) to internal (sericite and/or pyrophyllite) alteration zones. In the central part of these systems tourmaline is absent and the alteration assemblage is characterized by an andalusitedumortierite-quartz association. Such changes in mineral assemblage are related to an increase in temperature from 250 to 350–400 °C and to decreases in pH and Eh, from the outer toward the inner parts of the hydrothermal system. The zonation of the borosilicate-bearing alteration systems is similar to that of aluminosilicate-bearing hydrothermal systems (muscovite, pyrophyllite, andalusite) developed in volcanic rocks. A kaolinite-alunitejasperoid assemblage is generally associated with the borosilicate alteration, and seems related to a late hydrothermal phase mainly located in graben structures.     

广西贵港—平南地区铅锌矿带地质特征及找矿方向

广西贵港－平南地区分布有华南大型－超大型铅锌矿成矿的有利层位，北东向两条深断裂具有多期活动性，泥盆系沉积盖层中下部第一层碳酸盐岩与成矿关系密切。具有较广阔的找矿前景。主要铅锌矿类型有热水沉积＋构造热液迭加改造型、复合叠加型。     

Oxygen isotope mapping of the Archean Sturgeon Lake caldera complex and VMS-related hydrothermal system, Northwestern Ontario, Canada

The hydrothermal and magmatic evolution of the Sturgeon Lake caldera complex is graphically documented by a regional-scale (525 km²) analysis of oxygen isotopes. Spatial variations in whole-rock oxygen isotope compositions provide a thermal map of the cumulative effects of multiple stages of hydrothermal metasomatism before, during, and after volcanogenic massive sulfide (VMS) mineralization. There is a progressive, upward increase in δ¹⁸O from less than 2‰ to greater than 15‰ through a 5-km-thick section above the Biedelman Bay subvolcanic intrusive complex. This isotopic trend makes it clear that at least the earlier phases of this intrusive complex were coeval with the overlying VMS-hosting cauldron succession and provided thermal energy to drive a convective hydrothermal circulation system. The sharp contrast in δ¹⁸O values between late stage phases of the Biedelman Bay intrusion and immediate hanging wall strata indicates that the main phase of VMS-related hydrothermal activity took place before late-stage resurgence in the cauldron-related magmatic activity. Mineralogical and isotopic evidence indicates the presence of both syn- and postmineralization hydrothermal activity defined by the presence of widespread semiconformable and more restricted discordant alteration zones that affect the pre- and syncauldron strata. The semiconformable alteration zones formed during early stages of hydrothermal circulation and are defined by widespread silicification and carbonatization in association with relatively high δ¹⁸O values. The discordant alteration assemblages, containing Al-silicate minerals with chloritoid and/or Fe-rich carbonate or chlorite, centered on synvolcanic faults represent restricted zones of both seawater inflow and hydrothermal fluid upflow. A rapid increase in δ¹⁸O values (∼7–9‰) over a short distance (<200 m) suggests marked cooling of hydrothermal fluid from ∼350°C to less than 130°C either just before or during discharge onto the seafloor. Late emplacement of diorite sills and a dacite dome disrupted the isotopic imprint of cauldron-stage hydrothermal activity. The abrupt lowering of δ¹⁸O values at the transition from explosive to passive volcanism (andesite flows and dacite domes) indicates postcauldron emergence. Subsequently, thrust faulting disrupted the older synvolcanic hydrothermal isotopic patterns.