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1.
Techniques of X-ray photoelectron and Auger electron spectroscopy, scanning probe microscopy were used to demonstrate that the natural surface of hydrothermally synthesized pyrite, as well as vacuum fractures, contain a number of sulfide-sulfur species: disulfide, monosulfide, and, more rarely, polysulfide. The natural surface of hydrothermal pyrite is chemically modified compared to the inner volume into a nonautonomous phase film up to ～500 nm thick, which has a variable composition resembling that of pyrrhotite but with broader variations toward FeS2. Its principal distinctive feature is the presence of a peak at ～710 eV in the XPS Fe 2p3/2 spectrum, which is often higher than the main peak of bivalent low-spin Fe(II) in the pyrite structure (707 eV). The “basic” structure of the nonautonomous phase is a layer of variable composition Fe2+[S, S2, S n ]2?, whose S/S2 ratio varies from ～0.5 to ～2.0, averaging at ～1.1. This layer may include admixtures of minor elements, as follows from the appearance of a nonautonomous phase in the presence of As, which does not, however, form an individual phase. The polymerization of S at the surface is thereby more significant. The major oxisulfide components of this phase may be the sulfite and thiosulfate ions at a subordinate concentration of sulfate because of the instability of coexisting sulfate and disulfide ions, which results, in the presence of oxygen, in sulfite (thiosulfate) and sulfide ions in the nonautonomous phase. In line with XPS, scanning probe microscopic (SPM) data show that, at a high S activity in the “pure” system, the surface of the crystals contains practically no nanometer-sized phases and is characterized by low roughness (14–17 nm). At a low S fugacity in equilibrium with pyrrhotite and sphalerite, the average roughness of the surface increases to 25–65 nm, with the maximum height of the surface features of ～100–500 nm. This is consistent with Auger spectroscopic data, obtained after the etching (ion milling) of the surface with Ar+, on the thickness of the nonstoichiometric surface layer. Comparison with analogous data on other sulfides shows that crystals growing in hydrothermal environments have surface layers up to ～500 nm thick, which are different from the main volume of the crystal in chemistry, stoichiometry, and, possibly, also structure. This is scale of the surface heterogeneity at which the typochemistry of mineral surfaces may be manifested. The typochemistry of pyrite stems from the ability of the nonautonomous phase to “record” the growth conditions of crystals in terms of two major factors: the purity of the system (the occurrence of other phases, including virtual ones, i.e., potentially possible phases of admixture elements) and S fugacity (which influences the S/S2 ratio at the surface). The geochemical role of the surface nonautonomous phase in pyrite may be very significant, particularly when minor elements are captured that are incompatible with the pyrite structure but can be easily accommodated in the less rigid structure of the nonautonomous phase.  相似文献   

2.
The use of trace elements (TE) as geochemical indicators is complicated by the dualism of their distribution coefficients D due to the additional (i.e., above the concentrations of an isomorphic component) incorporation of elements at structural defects of various nature (including the surface of the crystal). A pressing problem in this situation is to determine the true D values that pertain to the structural component of an admixture D str and evaluate effects of other modes of TE occurrence. Only upon distinguishing D str in the bulk coefficient D bulk it is possible to evaluate the ore potential of fluid in terms of certain TE from the composition of a mineral containing the TE. Pyrite synthesized in solutions of variable pH at 450°C and 1 kbar (100 MPa) at fluid portions sampled in a trap is utilized to demonstrate the role of a surface nonautonomous phase (NP) in the incorporation of gold in this mineral. The distribution coefficient of gold between pyrite and hydrothermal solution is 0.14 for “pure” pyrite and 0.05 for As-bearing pyrite (containing 0.02–0.05 wt % As), and these coefficients for NP are 310 and 170, respectively. This increases the D bulk for evenly distributed (“invisible”) gold by factors of four and nine. In contrast to the results of earlier studies conducted at room temperature and pressure or parameters close to them, our data demonstrate that the accumulation of “invisible” Au in pyrite is controlled not only by reducing adsorption with the development of Au(0) particles and films but also by Au incorporation in NP developing in the surface layer of the crystal approximately 500 nm thick as chemically bound Au [most likely as Au(I)]. The possible reason for the high absorption capacity of NP is the defect (pyrrhotite-like) structure, which is not saturated with bonds of excess S and sulfoxi onions.  相似文献   

3.
The FeS2–Ag–Pt–As system was studied using hydrothermal thermogradient synthesis (with internal sampling) of pyrite crystals at a temperature of 500°C and pressure of 1 kbar in ammonium chloridebased solutions. The modes of occurrence of precious metals (PM) were determined using atomic absorption spectrometry (AAS) in its version of statistical selections of analytical data on single crystals (SSADSC), electron microprobe analysis (EMPA), scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The concentration of Pt in its structural mode in pyrite is as high as 10–11 ppm and is practically not correlated with the As concentration. The dualistic distribution coefficient of Pt between pyrite and hydrothermal solution is 21 ± 7 for the structural mode and 210 ± 80 for the surface-related mode of this element. No inclusions of either any Pt-bearing minerals or Pt itself was detected. Platinum is an element highly compatible with hydrothermal pyrite and is different in this sense from gold, and pyrite is underestimated as a potential concentrator of platinumgroup elements (PGE). The distribution of Ag in pyrite is highly heterogeneous. The likely reason for this is that the Ag solid solution cannot be quenched, and hence, the Ag concentrations broadly vary and are very unsystematically distributed in natural pyrite crystals. Assuming this hypothesis, the limit for Ag accommodation in FeS2 can be estimated using SSADSC at 0.09 ± 0.06 wt % under the experimental parameters, and the distribution coefficient of the structural Ag mode is thereby evaluated at 1400 ± 700. When crystallizing together with FeS2 proustite (Ag3AsS3) near its melting point, forms mixtures with dervillite (Ag2AsS2), in which Ag deficit is counterbalanced by excess divalent As. The limit of As incorporation into pyrite under these conditions is ≤0.1 wt %. SEM-EDS and XPS data indicate that the surface phases are of three types. In the course of crystal growth, practically two-dimensional nonautonomous phases (NP) are aggregated into submicroscopic and micrometer-sized crystalline bodies (mesocrystals) that largely inherit their unusual minor-element composition from NP and are enriched in Ag, Pt, As, and other minor elements. NP and mesocrystals are enriched in Al, which was transferred into them from the Al-bearing Ti alloy of the reaction containers. Silver occur in the volume of the crystals and on their surface as monovalent silver sulfide. Arsenic was detected mostly in the form of di- and trivalent arsenic sulfides. Pentavalent arsenic oxide was identified only on the surface of the crystals and can be easily eliminated by ion milling.  相似文献   

4.
《Ore Geology Reviews》2011,43(1):32-46
Hydrothermal pyrite contains significant amounts of minor and trace elements including As, Pb, Sb, Bi, Cu, Co, Ni, Zn, Au, Ag, Se and Te, which can be incorporated into nanoparticles (NPs). NP-bearing pyrite is most common in hydrothermal ore deposits that contain a wide range of trace elements, especially deposits that formed at low temperatures. In this study, we have characterized the chemical composition and structure of these NPs and their host pyrite with high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), analytical electron microscopy (AEM), and electron microprobe analysis (EMPA). Pyrite containing the NPs comes from two types of common low-temperature deposits, Carlin-type (Lone Tree, Screamer, Deep Star (Nevada, USA)), and epithermal (Pueblo Viejo (Dominican Republic) and Porgera (Papua New-Guinea)).EMPA analyses of the pyrite show maximum concentrations of As (11.2), Ni (3.04), Cu (2.99), Sb (2.24), Pb (0.99), Co (0.58), Se (0.2), Au (0.19), Hg (0.19), Ag (0.16), Zn (0.04), and Te (0.04) (in wt.%). Three types of pyrite have been investigated: “pure” or “barren” pyrite, Cu-rich pyrite and As-rich pyrite. Arsenic in pyrite from Carlin-type deposits and the Porgera epithermal deposit is negatively correlated with S, whereas some (colloform) pyrite from Pueblo Viejo shows a negative correlation between As + Cu and Fe. HRTEM observations and SAED patterns confirm that almost all NPs are crystalline and that their size varies from 5 to 100 nm (except for NPs of galena, which have diameters of up to 500 nm). NPs can be divided into three groups on the basis of their chemical composition: (i) native metals: Au, Ag, Ag–Au (electrum); (ii) sulfides and sulfosalts: PbS (galena), HgS (cinnabar), Pb–Sb–S, Ag–Pb–S, Pb–Ag–Sb–S, Pb–Sb–Bi–Ag–Te–S, Pb–Te–Sb–Au–Ag–Bi–S, Cu–Fe–S NPs, and Au–Ag–As–Ni–S; and (iii) Fe-bearing NPs: Fe–As–Ag–Ni–S, Fe–As–Sb–Pb–Ni–Au–S, all of which are in a matrix of distorted and polycrystalline pyrite. TEM-EDX spectra collected from the NPs and pyrite matrix document preferential partitioning of trace metals including Pb, Bi, Sb, Au, Ag, Ni, Te, and As into the NPs. The NPs formed due to exsolution from the pyrite matrix, most commonly for NPs less than 10 nm in size, and direct precipitation from the hydrothermal fluid and deposition into the growing pyrite, most commonly for those > 20 nm in size. NPs containing numerous heavy metals are likely to be found in pyrite and/or other sulfides in various hydrothermal, diagenetic and groundwater systems dominated by reducing conditions.  相似文献   

5.
The system magnetite-Au-hydrothermal solution was employed to continue studying the distribution coefficients of trace elements in system with real crystals. The role of surface nonautonomous phase (NP) is elucidated. The distribution coefficient of an Au structural admixture between magnetite and hydrothermal solution at the experimental conditions [450°C, 1 kbar (100 MPa), and fluid sampling by a trap] is, according to the most representative data, 1.0 ± 0.3, and Au is thus not an incompatible element in magnetite, in contrast to pyrite and arsenopyrite [1], minerals for which this coefficient is much lower than one. The NP is enriched in Au with respect to the rest of the crystal by a factor of more than 4000, and this results in an one order of magnitude increase in the bulk distribution coefficient. Similar to pyrite, the reason for the dualistic nature of the distribution coefficient is the presence of an NP, which contains ∼2000 ± 500 ppm Au. The NP occupies the approximately 330-nm surface layer of the crystal, and the chemically bound Au [Au(III), according to XPS data] admixture is evenly distributed with depth within the layer, which is the reason for the strongly determinate dependences of the concentrations of the evenly distributed Au admixture on the size and specific surface area of the crystal. The occurrence of an NP is controlled by the chemistry of the system. The partial substitution of Fe for Mn and the synthesis of a phase close to jacobsite MnFe2O4 results in the disappearance of both the NP itself and the size dependence of the Au concentration. The XPS spectra of O 1s and Fe 2p are used to analyze two models: (i) a single goethite-like (O2−/OH−∼ 1) phase of variable composition and Fe in more than one valence state and (ii) a heterogeneous structure of alternating domains of wuestite- and goethite-like NP. The reason for the “excess” admixture in the former instance can be vacancies at Fe sites, whereas that in the latter one is the interaction of the admixture with nanometer-in-size nanometer in-size strained domains on the surface of the crystal.  相似文献   

6.
The gold distribution in 32 pyrite samples and some samples of other ore minerals is studied using the method of statistical samplings of analytical data for single crystals. The samples were recovered from deposits of different genetic types within the largest gold provinces of Russia and Uzbekistan. The contents of uniformly distributed gold and the ratios of its structurally to superficially bound forms have been determined. According to the Au–As diagram for the chemical states of gold, uniformly distributed gold in pyrite is chemically bound in the overwhelming majority of cases. The previous experimental data suggest that it is partly incorporated into pyrite and partly into the structures of nanosized nonautonomous phases on the surface of the pyrite crystals. Micro- and nanoparticles of native gold might appear during postgrowth transformations of these phases. Data on the other ore minerals suggest that the dependence of the content of uniformly distributed gold on the size or specific surface area of the crystal and the superficial position of its considerable part are common to the ore minerals. It is shown for pyrite that the observed features are commonly found at deposits of different genetic types, only with differences in the slope and determination coefficients of the dependences. The size dependences of the contents of gold and other elements in pyrite are genetically significant, because they give an insight into the ore-forming processes. The data on structurally bound gold permit comparative evaluation of gold concentrations in ore fluids forming gold deposits of different genetic types.  相似文献   

7.
The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO2 content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, fO 2 between 10−38.8 and 10−39.6 bars, and mΣs between 10−3.2 and 10−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.  相似文献   

8.
水热法合成黄铁矿微观形貌和结构的观测与表征   总被引：1，自引：0，他引：1       下载免费PDF全文
模拟热液型金矿床中黄铁矿生成的地质条件,采用硫酸亚铁(FeSO4)和硫代乙酰胺(CH3CSNH2)为铁源和硫源,在Fe/S比为1∶3、温度180~200 ℃、加热时间24 h条件下考察黄铁矿的结晶情况。用SEM扫描电镜观察在不同水热条件下合成黄铁矿的形态及成分;用多晶X射线衍射仪(XRD)确定了产物物相组成;用透射电子显微镜(TEM)观测表征了黄铁矿晶体的形态和结构。结果表明：在200 ℃时黄铁矿为粒状,粒度较均匀,粒径1 μm左右。在180 ℃时黄铁矿除有尺寸在1 μm左右粒状黄铁矿外,还有不规则带状黄铁矿晶体,带宽为200 nm左右。认为水热条件中,随着结晶温度的逐步升高,黄铁矿的形貌逐步由不规则形状向规则形状的转变。所以在实验温度较高阶段,形成规则的粒状形貌;在低温阶段,则会出现不规则带状形貌。  相似文献   

9.
山西堡子湾金矿床黄铁矿标型特征   总被引：6，自引：1，他引：6  
从成因矿物学及找矿矿物学观点出发，系统研究了堡子湾金矿床黄铁矿的产状，形态，化学成分，热电性质和热爆特征，该矿床黄铁矿富含Co,Ni,As,Ag,Au与W，Cu,Hg,Ag，As，Bi,Ni,Co,Pb相关性较好，构成特征元素组合，与通常认为的与火山－次火山热液有关的明矾石－高岭土型浅成中低温热液型金矿床特征元素及其组合基本一致。黄铁矿空穴型导电性与明矾，石－高岭土型浅成中低温热液型金矿床特征元素圾其组合基本一致。黄铁矿空穴型导电性与As,Au正相关，与Co，Ni呈明显的负相关，P型和N型导电性是由As/(Co Ni)值决定的。利用黄铁矿热电性及热爆特征空间分带与赋矿空间的对应关系进行成矿预测，效果明显。  相似文献   

10.
猫场式黄铁矿矿床地质特征及成因探讨   总被引：2，自引：0，他引：2  
甘朝勋 《矿床地质》1985,4(2):51-57
在川滇黔交界地区二叠系峨眉山玄武岩被边缘地带,广泛分布着猫场式黄铁矿矿床(图1)。这一类型矿床具有典型特征,其成矿与峨眉山期火山岩密切相关,展布稳定,厚度大,品位较高,为我国西南硫矿带中佼佼者,颇具经济意义和地质找矿意义。  相似文献   

11.
The low‐grade metasediments of the Cameros Basin, northern Spain, host a number of deposits of spectacular quality pyrite mineralization. These formed during regional metamorphism and the pyrite crystals exhibit a wide range of morphologies. On the basis of pyrite crystal habit, the deposits can be classified into two groups: Group I comprises deposits with cubic, elongated or platy crystals; Group II comprises deposits characterized by pyritohedra and cubo‐pyritohedra with striated faces, along with blocky crystals and fine‐grained aggregates. Group I deposits are formed in sequences dominated by meandriform fluviatile sediments, while Group II is hosted by deltaic plain and lacustrine metasediments. Temperature differences between deposits and As content are possible causes of the different pyrite morphologies in the deposits, but no significant variation exists between the two groups for either factor. Comparison with experimentally grown pyrite crystals suggests that Group I deposits have morphologies indicative of lower degrees of pyrite supersaturation than pyrite crystals in Group II deposits. The sedimentary facies hosting Group II deposits provides a greater availability of sedimentary sulphur (pyrite and sulphates). Moreover, reactions involving sulphate during metamorphism may have modified fluid chemistry, which would also act to produce higher degrees of pyrite saturation in fluids derived from the sulphate‐rich deltaic plain and lacustrine metasediments hosting the Group II deposits. This hypothesis is confirmed by sulphur isotope data on the pyrites, which show a larger component of34S‐enriched sulphate‐derived sulphur in these deposits. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

12.
This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1−-pyrite, in which As1− substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3++yAu++1-y(□)⇔2Fe2+; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1−-pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems.  相似文献   

13.
One of the key-principles of the iron-sulphur world theory is to bring organic molecules close enough to interact with each other, using the surface of pyrite as a substrate in a hydrothermal setting. The present paper explores the relationship of pyrite and organic matter in a hydrothermal setting from the geological record; in hydrothermal calcite veins from Carboniferous limestones in central Ireland. Here, the organic matter is accumulated as coatings around, and through, pyrite grains. Most of the pyrite grains are euhedral-subhedral crystals, ranging in size from ca 0.1-0.5 mm in diameter, and they are scattered throughout the matrix of the vein calcite. The organic matter was deposited from a hydrothermal fluid at a temperature of at least 200°C, and gives a Raman signature of disordered carbon. This study points to an example from a hydrothermal setting in the geological record, demonstrating that pyrite can have a high potential for the concentration and accumulation of organic materials.  相似文献   

14.
牟云 《吉林地质》2010,29(3):6-8
吉林东部地区与火山岩-斑岩有关的金(铜银)矿床可分为浅成低温热液型金(银)矿和浅成中温斑岩。热液型金(铜)矿两个亚类。浅成低温热液型矿床的特征矿物组合为黄铁矿、石英、方解石、冰长石和重晶石,成因上与高钾的钙碱性火山岩。斑岩(137～177Ma)有关,成矿流体属浅成低温氧化流体,在成矿过程中大气降水混入的程度大。浅成中温斑岩,热液型矿床的特征矿物组合为黄铁矿、黄铜矿、磁黄铁矿和石英,成因上与钙碱性火山岩。斑岩(130～140Ma)有关,成矿流体属浅成中温还原流体,在成矿过程中大气降水混入的程度相对较小。由于火山岩,斑岩的来源深度不同,斑岩体在矿床的形成中所起的作用不同,成矿流体的性质不同,造成金矿床的类型不尽相同。  相似文献   

15.
The typomorphic features of pyrite of the Sukhoi Log deposit were studied by a set of volumetric and surface methods: electron probe microanalysis, scanning electron and probe microscopy, powder X-ray diffraction, X-ray photoelectron and Auger electron spectroscopy, atomic-absorption spectrometry in the SSADSC (method of statistical sample of analytical data for single crystals) version, and atomic-emission spectrometry. Pyrite from the Sukhoi Log deposit has the following distinctive features: permanent presence of sulfite ion, which often dominates over other surface sulfur anions; weakly determined size dependence of the content of uniformly distributed Au owing to the presence of an internal concentrator of gold—dispersed carbonaceous material—in pyrite from ore zones; cell sculptures of the crystal faces, which appeared owing to the nanofragmentation of the growth surface; micro- and nanoinclusions of carbonaceous phases within crystals, associated with defects in their structure; and thin films enriched in O and C on the surface of and within the crystals. It has been shown that gold-sulfide mineralization at the Sukhoi Log deposit formed in a single ore-generating hydrothermal system, in which gold, sulfur, and carbon belonged to a microparagenesis. Some features (composition of surface, characteristics of submicroscopic structure, and elemental composition) evidence that the conditions of crystallization of pyrite in inter-ore space were different from the conditions of its genesis in the ore zones, which suggests the presence of at least two genetic types of pyrite. Carbonaceous micro- and nanoparticles and O- and C-containing films can favor an increase in the adsorption of gold from cyanide solutions on pyrite. To reduce this effect during gold recovery, a technique for surface modification should be elaborated. The ways for solving the most complicated problems dealt with the source of noble metals (NM) and the ore specialization of the deposit have been outlined. For this purpose, a detailed analysis of the main ore minerals for trace-element speciation is required. In the case of the magmatic source of NM, correlation between the contents of Au and PGE structural forms should exist. On the other hand, there is no correlation between the structural forms of Au or Pt and elements whose contents in fluid are determined by the host rock rather than the magmatic source.  相似文献   

16.
与火山—次火山—侵入—热液作用有关的金矿床分类探讨   总被引：7，自引：0，他引：7  
讨论近年来在陆相火山－次火山－侵入－热液作用有关的金矿床分类中的一些争论问题。从具体资料出发，认为扩大浅成代温热注作用的范围，摒弃中温热液和高温热液矿床，以及经它们与斑岩矿床的关系是不妥当的；认为斑岩和系列是建立成矿模式，划分有产矿床的基础。  相似文献   

17.
Wulaga epithermal gold deposit is located in northeast China. Gold mineralization mainly occurs within the crypto‐explosive breccia belt of subvolcanic intrusion. Constraints on the precise timing of mineralization are of fundamental importance for understanding the ore genesis of the Wulaga gold deposit and its mineralization potential. Three hydrothermal stages have been identified: the early veiny quartz–euhedral pyrite stage; the fine pyrite–marcasite–gray or black chalcedony stage; and the late carbonate–pyrite stage. The Rb–Sr dating of gold‐bearing pyrites from the fine pyrite–marcasite–gray or black chalcedony stage is 113.8 ± 4.4 Ma with an initial 87Sr/86Sr ratio of 0.706346 ± 0.000019. The age of the gold deposit is consistent with the age of ore‐bearing volcanic (109–113 Ma) and subvolcanic intrusion (103–112 Ma) within the error limits, and the pyrite initial ratio has an identical value of 87Sr/86Sr to subvolcanic intrusion (0.705547 ± 0.000012). These indicate that crystallization of the wall rock and epithermal gold mineralization was coeval and likely cogenetic. Moreover, a lot of epithermal gold deposits that formed in Early Cretaceous volcanic and subvolcanic intrusions have been discovered in recent years in Heilongjiang province. Combined with the studies of tectonic and magmatic activities, we propose that the formation of the Wulaga gold deposit might be caused by the heated circum‐flow water related to the volcanic–subvolcanic intrusive hydrothermal event triggered by the ancient subduction of the Izanagi plate in the Early Cretaceous.  相似文献   

18.
李洪梁  李光明 《地学前缘》2019,26(3):202-210
黄铁矿作为成矿热液活动的产物,是热液金矿床中最为重要的载金矿物,其成分标型特征记录了大量矿床成因信息。在分类总结了各类型热液金矿床关键地质特征的基础上,采用比较矿床学的研究思路,较为系统地分析了不同类型热液金矿床中主成矿期或与金矿化同期的黄铁矿的主微量元素及组合特征、(Fe+S)As特征、δFeδS特征、AsCoNi特征以及热电性特征。结果表明,不同类型热液金矿床中的黄铁矿成分特征具有显著差异,彼此显示出特征性的成因标型特征,可作为判别热液金矿床成因的依据。结合各类热液金矿床形成的构造背景,提出热液金矿床是研究大陆动力学的理想对象。通过这些研究,初步建立了不同类型热液金矿床地质与黄铁矿成分标型特征之间的耦合关系,为今后热液金矿床成因的判别和黄铁矿标型特征的进一步研究提供一定的参考。  相似文献   

19.
Petrographic and sulphur isotope studies support the long‐held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz‐pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar‐type, clast‐supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source‐rocks during erosion and sediment transport. A large range in textures reflects source‐rock lithologies, with known varieties linked to sedimentary‐hosted diagenetic pyrite, volcanic‐hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ34S values (? 5·3 to + 6·7‰) than that of previously reported conventional bulk‐grain analyses (? 1 to + 4‰). Rounded pyrite from the Steyn Reef has significant variation in δ34S values (? 4·7 to + 6·7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ34S values (? 0·5 to + 2·5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand‐type Au‐U ore deposits, implies deposition of redox‐sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen‐poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au‐U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization.  相似文献   

20.
A comparative in situ LA-ICP MS trace-element study on pyrite from three different, variably auriferous, Archaean to Palaeoproterozoic palaeoplacer deposits in the Ouro Fino Syncline (Quadrilátero Ferrífero; Brazil), the Elliot Lake area north of Lake Huron (SE Canada) and several deposits within the Witwatersrand Basin (South Africa) revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits. Especially the Ni and Au concentrations as well as Co/Ni and Mo/Ni ratios were found to be systematically different in detrital compact, detrital porous and post-sedimentary/hydrothermal pyrite grains from different source areas. High Co/Ni ratios and low Au concentrations are typical of post-sedimentary pyrite, which is hydrothermal in origin. In contrast, relatively low Co/Ni ratios and high Au contents characterise detrital porous banded and concentric pyrite grains (Au > 1 ppm), which are syn-sedimentary in origin. In the Elliot Lake area and the Witwatersrand Basin, detrital compact rounded pyrite is characterised by high Co/Ni ratios, which is in agreement with derivation from a hydrothermal source. Low Au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks. In contrast, derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the Ouro Fino Syncline because of low to intermediate Co/Ni ratios. High Au contents may indicate a genetic relationship between pyrite and gold there. Systematic differences exist between the three areas with respect to Au, Ni, Co, Mo and Cu distributions in detrital pyrite, which reflects differences in the provenance. A predominantly mafic/ultramafic source is indicated for the Ouro Fino, a felsic source for the Elliot Lake, and a mixed felsic–mafic provenance for the Witwatersrand pyrite populations. Independently of pyrite type, the higher Au endowment of the studied Witwatersrand and Ouro Fino conglomerates are also reflected by an overall higher Au concentration in the respective pyrite grains compared to the relatively Au-poor samples from Elliot Lake. In general, a strong positive correlation between Au and Pb levels in the various pyrite grains is noted. Analogous to Pb, which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete PbS phases, Au is considered to be present mainly in the form of discrete Au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice.  相似文献   

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