Saline brines and metallogenesis in a modern sediment-filled rift: the Salton Sea geothermal system, California, U.S.A.

The Salton Sea geothermal system (SSGS) is the site of active hydrothermal metamorphism and metallogenesis in the delta of the Colorado River, which partially fills the Salton Trough rift zone at the head of the Gulf of California. Growth of the delta across the rift has isolated the northern part of the Salton Trough since the Pleistocene, forming the evaporative Salton Sea basin whose sediments host the SSGS. More than 70 commercial geothermal wells, including a 3.2 km deep research borehole drilled as part of the Salton Sea Scientific Drilling Project (SSSDP), are yielding a wealth of new data from this system.Within the SSGS, active greenschist facies metamorphism is occuring at temperatures 365°C at only 2–3 km depth, by reaction of NaCaKFeMnCl brines with the deltaic and lacustrine Pilo-Pleistocene sediments. Two kinds of base metal ore mineralization occur at depth: stratabound diagenetic Fe sulfides, and epigenetic vertical veins containing FeZnCuPb sulfides and Fex oxides. The vein mineralization occurs as two types: an older, reduced carbonate-sulfide assemblage, and a modern, oxidized, silicate-hematite-sulfide assemblage. The earlier assemblage formed at temperatures up to 100°C higher than the ambient temperatures measured in the wells today, implying that cooler, oxidized fluids have displaced hot, reduced fluids.A sharp interface between shallow fluids containing <12 wt % TDS and deep hypersaline brines containing 15–27 wt % TDS exists in the SSGS. The deeper hypersaline brines are rich in base metals (Fe 1500 ppm, Mn 1000 ppm, Zn 500 ppm) whereas the overlying lower-salinity fluids contain less than 100 ppm each of Fe, Mn and Zn. The modern silicate-hematite-sulfide vein assemblage is precipitating where the two fluids appear to be mixing. The two fluid types also have distincty different δ¹⁸O and δD relations. The shallow lower-salinity fluids have only partially-exchanged oxygen with deltaic and lacustrine host rocls. The deep hypersaline brines have exchanged oxygen extensively at 250°C with the deltaic sediments. δD values of the hypersaline brines are typically lighter than either the lower-salinity fluids or the modern evaporating groundwaters occupying the Salton Trough, implying a different source for the original fluids.The high salinities of the geothermal brines are derived from a combination of evaporation of fossil lakewaters, groundwater dissolution of shallow lacustrine evaporites, and subsurface hydrothermal metamorphism of buried lacustrine evaporites. Episodic filling and desiccation of the closed Salton Sea basin has allowed cold saline brines to form and percolate down into the sedimentary section. In some wells anyhydrite meta-evaporites and interbedded solution collapse breccias occur at 1 km depth. The anyhydrite contains fluid inclusions that are saturated in halite at their homogenization temperatures of 300°C, recording the hydrothermal dissolution of bedded salt. Based on Sr and Pb isotopic data and whole-rock chemical data, the source of the metals in the hypersaline brines is from leaching of the host sediments. However, the origin of reduced S for ore mineralization remains an enigma. δ³⁴S values for vein sulfides cluster uniformly around zero per mil, implying that an isotopically-homogeneous source of reduced sulfide exists in the brines. It is possible that the vein sulfides receive a constribution from a magmatic S source.Movement and mixing of brines of different chemistry and oxidation states play a major role in ore genesis. Our model envisages an early stage in which a deep brine pool accumulated at depth in the sedimentary section by partial section by partial evaporation of basin and dissolution of bedded salts. Intrusion of rift-related basaltic magma into the base of the sedimentary caused heatingand fracturing of the sediments, resulting in precipitation of the carbonate-sulfide veins during pore fluid expulsion. Heating also caused a diapir of the hypersaline brine to rise and displace colder, less saline, shallower pore fluids. This brine intrusion was accompanied by pervasive and extensive mineralization. As this diapir cooled it began to move downward, drawing in shallow, more oxidized fluids and causing the formation of the modern silicate-hematite-sulfide vein ore zone.     

The association of organic molecules with clay minerals in aqueous solutions

The uptake of dissolved organic molecules by kaolinite and montomorillonite clay minerals was measured in distilled water and saltwater solutions. Glucose and valine exhibited low affinities for both clay minerals over a wide range of concentrations in distilled water and seawater solutions. Stearic acid was efficiently removed by both clay minerals from all solutions over the concentration range 10–1000 ppb. These experiments suggest that some dissolved organic molecules may be preferentially removed by clay minerals in natural waters. It is unlikely, however, that the partitioning of simple organic molecules between natural waters and suspended clay minerals could produce the high concentrations of organic matter that occur in most fine-grained sediments.     

Theoretical phase relations involving cordierite and garnet in the system MgO-FeO-Al2O3-SiO2

Theoretical stability relations have been derived between the phases cordierite (Cd), garnet (Ga), hypersthene (Hy), olivine (Ol), sapphirine (Sa), spinel (Sp), sillimanite (Si) and quartz (Qz) in the system MgO-FeO-Al₂O₃-SiO₂. Natural rock data and experimental evidence suggest that the Mg/Mg+Fe²⁺ ratio (X) of coexisting ferromagnesian phases decreases as follows: X _Cd>X _Sa>X _Hy>X _Ol>X _Sp>X _Ga. By use of this information four stable invariant points are proposed involving the phases: Cd, Hy, Sa, Ga, Si, Qz; Cd, Sa, Ga, Sp, Si, Qz; Cd, Hy, Sa, Ga, Sp, Qz; Cd, Ga, Hy, Ol, Sp, Qz. All univariant curves in the system are nonterminal, representing the breakdown of a join rather than the stability limit of an individual phase. A detailed treatment of divariant equilibria involving two and three ferromagnesian solid solutions illustrates the potential of these equilibria as Pressure-Temperature indicators. Interactions between solid-solid reactions and dehydration reactions involving biotite in the system MgO-FeO-Al₂O₃-SiO₂-K₂O-H₂O have been graphically analysed. The addition of biotite to anhydrous divariant assemblages does not affect the composition of coexisting phases at constant P and T but can affect their relative proportions.     

Ore transport and deposition in the Red Sea geothermal system: a geochemical model

Thermodynamic calculation of distribution of dissolved aqueous species in the Red Sea geothermal brine provides a model of ore transport and deposition in good agreement with observed accumulations of base metal sulfides, anhydrite, and barite.The Red Sea brine is recirculated seawater that acquires high salinity by low-temperature interaction with Miocene evaporites and is subsequently heated to temperatures in excess of 200°C by interaction with recent rift zone intrusive rocks. At temperatures up to 250°C, NaSO^?₄ and MgSO⁰₄ are the dominant sulfur-bearing species. H₂S forms by inorganic sulfate reduction at the higher temperatures but is maintained at a uniform concentration of about 2 ppm by the strength of the sulfate complexes.Chloride complexes solubilize metals at the higher temperatures, and thus sulfide and metals are carried together into the Atlantis II Deep. Below 150°C, the brine becomes supersaturated with respect to chalcopyrite, sphalerite, galena, and iron monosulfide due to chloride-complex dissociation. Sulfide precipitation rates, based on the rate of brine influx, are in good agreement with measured sedimentation rates. Anhydrite precipitates as crystalline fissure infillings from high-temperature inflowing brine. Barite forms from partial oxidation of sulfides at the interface between the lower hot brine and the transitional brine layer.     

The thermodynamic properties and phase relations of some minerals in the system CaO-AI2O3-SiO2-H2O

The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K^?1 mol^?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K^?1 mol^?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10^?3T ? 15.27 × 10⁵T^?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10^?3T ? 30.24 × 10⁵T^?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10^?3T ? 25.02 × 10⁵T^?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10^?3T ? 24.08 × 10⁵T^?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al₂O₃-SiO₂-H₂O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H₂O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.     

Chromium speciation in oxide-type compounds: application to minerals,gems, aqueous solutions and silicate glasses

Cr K-edge XANES spectra were obtained for a variety of Cr-bearing model compounds containing Cr(II), Cr(III), Cr(IV), Cr(V) and Cr(VI), in which the Cr-site symmetry is D4h, Oh and Td. The centroid position of the pre-edge feature is a better indicator of the Cr valence than the edge position. In Cr-rich oxides, higher-energy transitions must be excluded in order to refine a robust valence for Cr. The pre-edge for chromates is not unique and varies as a function of the CrO₄ ^2? moiety distortion, which is often related to Cr-polymerization (monochromate vs. dichromate). Both the analogy with the Mn K-pre-edge information and ab initio FEFF calculations of the pre-edge feature for Cr(III) and Cr(VI) confirm the experimental trends. This methodology is applied to the Cr K-edge pre-edge feature collected in gems (emerald, spinel and ruby), the layered minerals fuchsite and kämmererite, two Cr-bearing aqueous solutions and a set of sodo-calcic silicate glasses used for bottling sparkling white wine. In emerald and fuchsite, the Cr-site is differently distorted than its ruby or spinel counterpart. In a Cr(III)-bearing aqueous solution and sodo-calcic glass, no evidence for Cr(III) with Td and C3v symmetry is detected. However, minor amounts of chromate moieties (most likely monomeric) are detected in a glass synthesized in air. Preliminary spectra for the wine bottle glass suggest that only trace amounts of chromates might possibly be present in these glasses.     

Temperature — composition relationships of authigenic micaceous minerals in the Los Azufres geothermal system

Smectite, illite, celadonite and chlorite are the major products of alteration of rhyolites and andesites, in the upper part of the Los Azufres geothermal system. Changes in mineral assemblages and composition of phases are observed as a function of depth and host rock lithology. Two different sequences characterize the rhyolites and the andesites from the surface to a depth of about 1500 m: kaolinitesmectite (±interlayered illite/smectite)illitemuscovite (rhyolites), and kaolinitesmectite (±interlayered illite/smectite)illite+celadoniteillite+chloritechlorite (andesites).Illite, and chlorite at depth, are largely dominant. Similar substitutions and correlations among chemical constituents characterize illites from rhyolites or andesites, but their compositions in the two host rock lithologies exhibit slight but significant differences, especially in their Fe and Mg contents which are the highest in illites from andesites. Illite exhibits progressive changes in composition with depth: a strong increase in the K content in the interlayer, together with an increase of the Fe content in the octahedral site. These changes correspond to a slight increase in the molar fraction of Fe-(Mg) celadonite end-members, and mostly to a dramatic decrease of pyrophyllite with increasing temperature.Temperature of the mineralogical and compositional changes was estimated from fluid inclusions studies, combined with other geothermometric approaches (chemical geothermometers and direct measurements). Variation of X-pyrophyllite with temperature is proposed as a geothermometer for different host rock lithologies. Transitions between the stability fields of illite±interstratified illite-smectite and illite+chlorite is around 200±30° C, and between illite+chlorite and chlorite around 290±20° C.     

不同有机酸对矿物溶解的动力学实验研究

用石英、微斜长石和方解石混合颗粒模拟碎屑组分分别与不同有机酸水溶液进行溶蚀实验,以比较储集层内不同矿物在含低分子量有机酸地层水中溶解的速率,并试图探讨矿物溶蚀的微观机理。结果表明：①所有矿物颗粒都发生了不同程度的溶解,表现为颗粒失重,溶液中SiO2和金属阳离子含量增加,pH值上升。②优先溶解的是方解石,其次是硅酸盐矿物。③温度增加,硅酸盐矿物在水中的溶解度明显增加,而方解石的溶解度基本不变。④由于不同有机酸与二氧化硅和金属阳离子生成的络合物稳定性不同,因此,在不同有机酸水溶液中,矿物的溶蚀速率有较大差别。多官能团有机酸的水溶液中,不同矿物有更大的溶解;但在含有丰富Ca2＋的草酸水溶液中,由于矿物颗粒表面难溶草酸钙的沉淀,方解石的溶解变得更加困难。⑤多官能团有机酸与SiO2形成的多环螯合物由于完全取代了硅氧四面体的氧原子,在水中具有高度稳定性,可能有助于(铝)硅酸盐的溶蚀和硅元素的迁移     

Paragenetic relations of diamond with silicate and carbonate minerals in the carbonatite-diamond system: Experiments at 8.5 GPa

Phase relations of diamond and syngenetic minerals were experimentally investigated in the multicomponent system natural carbonatite-diamond at a pressure of 8.5 GPa and temperatures of 1300–1800°C (within the thermodynamic stability field of diamond). Under such conditions, the natural carbonatite of the Chagatai complex (Uzbekistan) acquires the mineralogy of Ca-rich eclogites (grospydites). The melting phase diagram of this system (syngenesis diagram) was constructed; an important element of this diagram is the diamond solubility curve in completely miscible carbonate-silicate melts (solubility values are 15–18 wt % C). The diamond solubility curve divides the phase diagram into two fields corresponding to (1) phase relations involving diamond-undersaturated melts-solutions of carbon with garnet as a liquidus phase (region of diamond dissolution) and (2) phase relations with diamond-saturated melts-solutions with diamond as a liquidus phase (region of diamond crystallization). During a temperature decrease in the region of diamond crystallization from carbonate-silicate melts, the crystallization of diamond is accompanied by the sequential formation of the following phase assemblages: diamond + garnet + melt, diamond + garnet + clinopyroxene + melt, and diamond + garnet + clinopyroxene + carbonate + melt, and the subsolidus assemblage diamond + garnet + clinopyroxene + carbonate is eventually formed. This is indicative of the paragenetic nature of silicate and carbonate minerals co-crystallizing with diamond and corresponding primary inclusions trapped by the growing diamond. A physicochemical mechanism was proposed for the formation of diamond in carbonate-silicate melts. The obtained results were used to analyze the physicochemical behavior of a natural diamond-forming magma chamber.     

Theoretical phase relations involving cordierite and garnet revisited: the influence of oxygen fugacity on the stability of sapphirine and spinel in the system Mg-Fe-Al-Si-O

The theoreticalP-T grid for stability relations of the phases cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si), and quartz (Qz) of Hensen (1971), has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity. Both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel, which causes a topological inversion and the stabilisation of the invariant points [Sa], [Ga], and [Cd]. This implies the stable existence of the univariant equilibria (for buffered conditions): Sp+Qz=Ga+Hy+Si+O₂ (Sa, Cd), Cd+Sp+Qz=Hy+Si+O₂ (Sa, Ga) and Sa+Sp+Qz=Hy+Si+O₂ (Ga, Cd) and the divariant reaction: Sp+Qz=Hy+Si+O₂ (Sa, Ga, Cd). These redox equilibria are restricted to conditions of high oxygen fugacity. The proposed theoreticalP-T grids, for both low and high oxygen fugacity, satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites.     

Reaction of montmorillonite with acid aqueous solutions: solute activity control by a secondary phase

Al, Ca, Mg and Na montmorillonites were reacted with acid aqueous solutions in order to test the assumption of equilibrium underlying solubility determinations of thermodynamic properties of montmorillonite. The value calculated for the equilibrium constant varied with pH over the pH range 1.5–4.2. Hence montmorillonite was not in a unique state of equilibrium with all solutions over this range. Therefore the derivation of thermodynamic properties of montmorillonite by the application of a single step dissolution equation to results of solubility studies in mildly acidic solutions is invalid. Indications of solute activity control by a secondary solid phase were given by: (1) the dependence of the solution concentrations of some, but not all, montmorillonite constituent elements upon each other; and (2) a lack of congruency in the dissolution of montmorillonite. These indications were confirmed by a principal components statistical analysis, which showed that the solution activities of Si, Al and H after long-term treatments of the mineral from undersaturation were always related by the same expression. This particular expression is equivalent to an equilibrium constant for the reversible dissolution of a secondary solid phase that is enriched in silicon by comparison with montmorillonite. An aluminous solid phase was also shown to be formed.     

Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants,effective surface area,and the hydrolysis of feldspar

Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ? 10.6 ? (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10^?9 to 10^?8 cm² and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10^?13 to ~10^?7 moles cm^?2 sec^?1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole^?1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10^?16 to ~10^?11 moles cm^?2 sec^?1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole^?1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction.     

Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals,gases and an aqueous phase

The compositional characteristics of many geochemical systems which involve the interaction of natural aqueous solutions with minerals and gases are conveniently described using the following thermodynamic components: Cl^?, SO₄⁼, HS^?, CO₃⁼, H⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Fe⁺⁺, Zn⁺⁺, Cu⁺, Al⁺⁺⁺, SiO₂ and H₂O. A set of mass balance and mass action equations equal in number to the number of components plus the number of saturated minerals (and gases) is defined for a specified temperature, pressure and bulk composition. The mass balance equations include terms for minerals, gases and the molalities of aqueous complexes and dissociated species. This set of non-linear equations can be solved with the aid of a computer using'a Newton-Raphson technique. The calculation takes account of aqueous ion complexing, oxidation-reduction equilibria, activity coefficients, non-unit water activity and solid solutions. The use of H⁺, SO₄⁼, HS^? and H₂O as components allows straightforward treatment of processes involving oxidation-reduction, evaporation, boiling and changes of total aqueous H⁺ due to hydrolysis, mineral reaction or temperature change. One product of this approach is a technique for calculating pH at high temperature from measurement of pH at room temperature.By linking a series of discrete overall heterogeneous equilibrium calculations in which incremental changes of bulk composition, temperature or pressure are made, dynamic geochemical processes can be modeled. Example calculations for two such processes are given. These are the heating of seawater from 25° to 300°C and the isothermal irreversible reaction of rhyolite with an aqueous solution at 250°C.     

Mechanistic roles of soil humus and minerals in the sorption of nonionic organic compounds from aqueous and organic solutions

Mechanistic roles of soil humus and soil minerals and their contributions to soil sorption of nonionic organic compounds from aqueous and organic solutions are illustrated. Parathion and lindane are used as model solutes on two soils that differ greatly in their humic and mineral contents. In aqueous systems, observed sorptive characteristics suggest that solute partitioning into the soil-humic phase is the primary mechanism of soil uptake. By contrast, data obtained from organic solutions on dehydrated soil partitioning into humic phase and adsorption by soil minerals is influenced by the soil-moisture content and by the solvent medium from which the solute is sorbed.