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《Geochimica et cosmochimica acta》1986,50(9):1837-1845
During supergene alteration of auriferous carbonate ore, the weathering fluids formed are likely to be alkaline and therefore unsuitable as a medium for gold transport as a chloride complex. Secondary gold remobilization in such deposits can often be attributed instead to gold complexing by sulphur-bearing ligands. Gold and silver solubility in the systems AuSO2H2O and AgSO2H2O respectively, from the thermodynamic data available, is due to complex formation with thiosulphate and bisulphide ligands. The most stable gold complexes, Au(S2O3)23− (at φO2 > 10−60) and Au(HS)−2 (atφO2 < 10−60), exist in neutral or alkaline solutions. Like gold, silver forms a stable thiosulphate complex, Ag(S2O3)3−2 in moderately oxidizing, and bisulphide complexes, AgHS0 and Ag(HS)−2 in reducing, alkaline media. Silver solubility in highly oxidized, neutral or acid solutions is increased by formation of AgS2O3−, Ag+ and AgSO4− complexes.Colloidal, crystalline and alloyed gold and silver reacted with 0.1 M Na2S2O3 do not, however, demonstrate independent solubility. The rate of gold solubility in 0.1 M Na2S2O3, for example, is increased both by the presence of silver-thiosulphate complexes and alloyed silver. It is possible that such behaviour is due to the formation a mixed metal complex of the type (Au, Ag)(S2O3)23−.The nature and mineral association of secondary gold in the oxidized zone of carbonate ore at Wau. in Papua New Guinea, is consistent with prior remobilization as a thiosulphate complex. Here the secondary gold is coarsely crystalline, alloyed with 50–75 at% Ag and enriched at the watertable and with manganese dioxide in the oxidized zone. 相似文献
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《Geochimica et cosmochimica acta》1987,51(4):869-878
K-Ar ages of illite alteration associated with Middle Proterozoic Athabasca unconformity-type U deposits in Saskatchewan range from 414 to 1493 Ma. The K-Ar ages correlate with water contents and δD values such that illites with young K-Ar ages have δD values as low as −169 and water contents as high as 7.7 wt.% whereas illites with older ages have δD values near −70 and water contents near 4 wt.%. Water extracted at 400°C from illites with low δD values and high water contents has low δD and δ18O values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration. The alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the K-Ar ages and introduction of excess structural water in the form of interlamellar water. The interlamellar water is enriched in 18O by about 7 per mil relative to the water that physically surrounded the clay particle. Further alteration decreases the δD value and increases the δ18O value of the illite by isotopic exchange between the mineral and the interlamellar water. Although the chemical compositions and XRD patterns of the altered illites indicate that no detectable smectite component is present in the samples, the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed-layer illite/smectite by retrograde alteration of pure illite. The wide variation of δD values of chlorite and kaolinite from these U deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial. The general association of altered clays with areas containing the highest concentrations of U is probably related to localized permeability within the ore zone. 相似文献
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《Geochimica et cosmochimica acta》1986,50(1):107-110
The Fe/Mn-Fe/Mg relations of published microprobe analyses of ureilite olivine cores suggest that FeO reduction in the magmatic stage was important in the genesis of some of these meteorites. Consideration of Mg/Mn and Fe/Mn partitioning between olivine and pigeonite cores shows that these two phases are not in equilibrium. The data are consistent with combined fractional crystallization and FeO reduction being the major processes of ureilite genesis. Few, if any, of the analyzed ureilites are consistent with a partial melting residue model. 相似文献
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The KAr isochron method is an attempt to obtain the initial age or reset age and the extraneous argon isotopic ratio of a suite of cogenetic samples of different K-contents. Some samples contain excess argon-40 and others lose argon-40. The resultant ages on single samples are not significant geologically. We have discussed the principles of three types of isochrons commonly used, the 40Ar vs 40K isochron, the 40Ar/36Ar vs 40K/30Ar isochron, and the 40Ar/36Ar vs 39Ar/36Ar isochron, and evaluated the first two types for ten different hypothetical cases. If a straight line is obtained in the 40Ar vs 40K isochron, a positive intercept indicates an approximately constant amount of excess argon, whereas a negative intercept indicates argon loss. A curved line or scattering of points indicates that the basic assumptions are not valid for the set of samples under consideration. The 40Ar/36Ar vs 40K/36Ar isochron method is valid, rigorously, only when all samples of the system under consideration have the same non-radiogenic argon isotope composition. This requires that either no excess argon is present in the system, or else each analysis contains the same proportion of excess and atmospheric argon. If these conditions do not hold, approximately, invalid ages and invalid intercepts are obtained. Any KAr isochron needs to be used with caution. 相似文献
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Na 属于热扩散元素,具有不稳定性,准确测定有难度。前人对含Na 玻璃和具有不同晶体结构的含Na 矿物的电子探针定量分析发现,随着分析时间的延长,Na 的计数明显下降,并且含Na 玻璃计数的下降要比含Na 矿物相比更为明显,表明Na 计数与试样的结构有关,但对其内在控制因素还不太明确。文章在前人研究的基础上,选择具有代表性不同结构含Na 硅酸盐矿物和非晶质含Na 玻璃作为研究对象,通过对不同晶体结构中Na 元素所处的位置、配位数、键性类型的特征进行总结,针对不同晶体结构中Na 原子扩散条件和是否存在附加阴离子与之相伴等因素,对比分析不同晶体结构中Na 计数下降的程度,深入探讨了导致Na 计数下降的内在原因。结果显示,试样分析过程中Na 计数下降顺序为：非晶质> 架状>层状> 环状> 单链状> 双链,且有附加阴离子的含Na 矿物要大于无附加阴离子的矿物。通过对不同测试条件下Na 计数的对比分析发现,加速电压、束流、束斑直径等改变,都会对Na 计数产生一定的影响。最终总结得出了准确测定含Na 硅酸盐矿物的最佳分析条件,为准确测定含Na 矿物和玻璃中的Na 含量提供参考。 相似文献
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埃达克岩的Na亏损及其对地幔Na交代的指示意义 总被引：1，自引：3，他引：1
埃达克岩是玄武质洋壳部分熔融的产物。然而,与实验室玄武岩部分熔融产生的埃达克质熔体相比,天然埃达克岩明显地高Mg、Cr和Ni,这表明埃达克岩浆在上升过程中有地幔成分的加入。本文的观察结果表明,全球新生代埃达克岩的Na2O含量低于5.8%,大约95%的新生代埃达克岩样品Na2O含量小于5.0%。然而,在埃达克岩产生的压力范围(1.5～3.0GPa),实验的玄武岩部分熔体大多数Na2O含量超过5.0%,最高达到9.0%,显示埃达克岩具有明显的Na亏损现象。我们认为这是埃达克熔体在热的地幔楔中与地幔橄榄岩反应的结果。在俯冲带,大洋板片熔融产生的熔体(埃达克熔体)上升并与地幔橄榄岩发生反应,原始的埃达克熔体获得MgO、Cr及Ni等地幔组分,但其Na2O和SiO2等通过反应进入地幔,导致地幔交代作用。根据长英质熔体与橄榄岩反应体系的相关系,我们认为,地幔单斜辉石、橄榄石、尖晶石的混染作用以及钠质角闪石和斜方辉石的分离结晶作用,是改变埃达克熔体组成并导致其Na亏损的一个重要的过程。埃达克岩的Na亏损为地幔Na交代作用和一些富Na的弧岩浆成因提供了重要证据。 相似文献
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《Precambrian Research》1987,37(3):191-197
A low pressure metamorphic Pan African terrain, composed mainly of schists and hornfels, ranging from high greenschist to low amphibolite facies, is exposed west of Dahab, southeastern Sinai, located between the metamorphic Kid Complex and the Feirani Volcanics.The studied metamorphic unit was dated using RbSr and KAr methods. A RbSr whole-rock isochron (based on seven samples) yields an age of 602 ± 11 Ma with an initial (87Sr/86Sr) ratio of 0.7041 ± 4. The RbSr age is assumed to be the age of a metamorphic phase (600 ± 10 Ma) well known in the area. On the other hand, KAr whole-rock ages (based on nine samples) show different values ranging from 590 to 526 Ma. These low KAr ages are due to the resetting of the KAr system by a thermal event, before 530 Ma affecting the Ar behaviour without disturbing the RbSr system. The Ar escape took place mainly from K-bearing feldspars, which were not affected by later textural, crystallographic or mineralogic variations. 相似文献
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David Alan Spears 《Geochimica et cosmochimica acta》1974,38(2):235-244
The RbSr age dating method has been applied to a sequence of Carboniferous shales for which some geochemical information was already available. In the marine shales, the Rb/Sr ratio is higher than in the non-marine and brackish water shales. Although the range of values can be attributed to depositional processes, the range of Rb/Sr values is too small to give a depositional isochron, and thus the age of deposition can not be determined. If samples with a variable mineralogy had been selected for the isotope study, the range of Rb/Sr could have been extended. Only by chance, however, would the depositional age have been obtained.A comparison of the RbSr isotope values in the Carboniferous shales with those in probable source rocks shows that a reduction in the Sr87/Sr86 ratio relative to the Rb87/Sr86 ratio may have taken place. Such a reduction could have occurred on the Carboniferous land-mass during the formation of the clay minerals. 相似文献
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《Lithos》1987,20(1):59-76
Plastically deformed ultramafic rocks in the Table Mountain and Blow-Me-Down Mountain ophiolites comprise a basal unit of slightly depleted Iherzolites, an intermediate sequence of strongly depleted harzburgites and an upper zone of dunites, also referred to as a transition zone intensively percolated by basaltic melts or magmatic fluids. Thirty-five samples including all of the above rock types have been investigated on 100 polished thin sections for CuFeNiS mineral assemblages. Most of them contain traces of CuFeNi sulfides, native metals and locally Ni arsenide. Compositional features of opaque assemblages as well as their textural sites in the rocks indicate that the present CuFeNiS minerals derive from an upper-mantle sulfide component through extensive subsolidus re-equilibration down to 100°C. The primitive component (predominant pentlandite, minor pyrrhotite and chalcopyrite) is preserved as sulfide inclusions in chromites of the transition zone, due to a subsolidus re-equilibration in a closed system. On the contrary, sulfide assemblages interstitial to silicates and spinel have extensively reacted with reducing serpentinizing fluids to produce sulfur-deficient sulfides such as heazlewoodite and mackinawite and native metals (native copper and awaruite). Microscale variations of redox conditions and the removal of Fe from the silicate during serpentinization may account for the peculiar “grain-by-grain” equilibrium state of intergranular assemblages. In spite of low-temperature alteration, a gradual depletion in sulfide component has been recognized from the basal lherzolites to the intermediate harzburgites while the sulfide content gradually increases in the transition zone (up to 0.2% by volume). The first pattern is consistent with the low-melting nature of the sulfide component in mantle melting processes. Microstructural criteria such as the absence of sulfide inclusions in olivine neoblasts demonstrate that the sulfide component postdates plastic deformation of the transition zone. The sulfide-enrichment pattern is thus ascribed to the percolation of a sulfur-saturated basaltic magma into residual dunites. 相似文献
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《Geochimica et cosmochimica acta》1987,51(6):1665-1673
An extraction-quench apparatus was used to measure calcite solubilities in supercritical CO2H2O mixtures. Experiments were conducted at 1 kbar and 2 kbar, between 240°C and 620°C and from XCO2 = .02 toXCO2 = .15 in order to determine the solubility behavior as a function of pressure, temperature and CO2 content. The results indicate that calcite solubilities under these conditions behave similarly to previously investigated calcite solubilities at lower pressures and temperatures (SHARP and Kennedy, 1965). At constant XCO2, the solubility increases with increasing pressure, but it decreases with increasing temperature. When the temperature and pressure are constant, the calcite solubility rises with increasing XCO2 to a maximum value at XCO2 between 0.02 and 0.05. For higher CO2 contents, up to XCO2 = .15, the calcite solubility decreases, probably due to the decrease of H2O activities to values significantly below unity.The solubility behavior can be successfully modeled by making the assumption that Ca++ is the dominant calcium species and that the carbon-bearing species are CO2(aq) and HCO−3. Since for these dilute H2OCO2 fluids, all activity coefficients can be assumed to not differ significantly from unity, ionization constants for the reaction H2O + CO2(aq) H+ + HCO−3 can be calculated at 1 and 2 kbar between 250°C and 550°C. These calculated values are in good agreement with the low temperature determinations of the ionization constants for this reaction determined by Read (1975). Values of the molal Gibbs free energy of CO2(aq) obtained in our study exhibit a much greater positive departure from ideality than those calculated with the modified Redlich-Kwong equations of either Flowers (1979) or Kerrick and Jacobs (1981) for dilute CO2 aqueous solutions. 相似文献
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《Geochimica et cosmochimica acta》1987,51(6):1651-1664
Microthermometry (MT) and laser Raman microprobe (LRM) spectroscopy (at room temperature and at about 0°C) were done on 33 synthetically produced CO2CH4H2O fluid inclusions in quartz (from R. Bodnar and M. Sterner). At room temperature, the inclusions consist of an aqueous liquid, a CO2CH4 supercritical carbonic fluid and (in most cases) graphite. In all these inclusions, the melting temperature for solid CO2 is less than that for the homogenization of the vapor bubble in the carbonic fluid.A method is described whereby MT data for CO2CH4H2O inclusions can be projected within the CO2CH4 binary phase diagram to infer CO2:CH4 ratios in the carbonic fluid (<2 to > 35 mole% CH4 in the inclusions under study). This method takes into account the formation of CO2CH4 clathrate hydrate during MT analysis. Unless clathrate formation is properly considered, errors arise in the determination of the bulk CO2CH4 ratio. For the inclusions in our study, these errors are on the order of 5 to 8 mole% CH4. Our interpretation of the MT data indicates that CH4 is preferentially partitioned into the clathrate over the coexisting carbonic fluid, in contradiction to the prediction from Parrish and Prausnitz's (1972) model for clathrate equilibria. Comparison of LRM analyses on the bulk carbonic fluid (in the absence of clathrate) and the residual carbonic fluid (in the presence of clathrate) confirm the preferential partitioning of CH4 into the clathrate. LRM analyses of the clathrate itself indicate that CH4 occupies both types of cage sites in the clathrate structure, whereas CO2 may only occupy one site. Two by-products of the combined LRM and MT analyses of the same inclusions are derivation of empirical ratios of Raman quantification factors for high-density CO2CH4 fluids and the ability to determine CO2:CH4 ratios of inclusions whose MT data lie near the critical region for CO2CH4. Thus, the joint use of LRM and MT techniques provides information that could not be obtained by either technique alone. 相似文献
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用四种不同类型高压釜研究Na2SnF6在200--647℃,、1kbar条件下的水解特征。实验表明,淬火速率影响实验结果的精度。快速淬火高压釜及金袋摇摆釜是研究这类化学平衡反应的理想设备，采用水解法可获得水解常数及热焓。 相似文献
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放射性钾元素(40 K)是地球演化过程中重要的热源之一,钠(Na)则是俯冲带岩石中的重要组成元素之一。俯冲板块是地壳物质运输到深部地幔的载体,而石榴石和单斜辉石是其中常见的矿物,因此K和Na在单斜辉石和石榴石中行为的研究具有重要意义。本文分以下3个方面总结了近50年来自然界以及高温高压实验中K和Na在石榴石和单斜辉石中行为的研究进展:1含Na、K石榴石的发现及其形成机制,以及Na、K进入石榴石对其晶格所产生的影响;2含K单斜辉石的发现及其形成机制,以及K进入单斜辉石对其晶格所产生的影响;3含K基性岩体系的实验岩石学研究现状。K作为大离子亲石元素,在高温高压下能够分别以KX+Si Y=(Ca,Mg)X+Al Y和MgM1+CaM2=Al M1+KM2的机制进入石榴石和单斜辉石晶格中,对其从地表到深部地幔中的循环具有重要贡献;而在持续俯冲板块中,Na元素在单斜辉石消失后以NaX+Si Y=(Ca,Mg)X+Al Y的机制进入石榴石晶格中,继续被运输到更深的地幔中去。前人在模拟地幔温压条件下对含K玄武岩和单斜辉石体系的高温高压实验研究结果表明,单斜辉石中K2O含量及石榴石中的Na2O含量与压力呈正相关。然而,到目前为止,K和Na在俯冲带温压条件下的富K基性岩(如榴辉岩)体系中的行为仍有待进一步深入研究。 相似文献