Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization     

R. POWELL  T. M. WILL  G. N. PHILLIPS 《Journal of Metamorphic Geology》1991,9(2):141-150

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (X_CO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al₂O₃–SiO₂–H₂O–CO₂, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower X_CO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.  相似文献   

A wavelet method for detecting S‐waves in seismic data     

P.J. Oonincx 《Computational Geosciences》1999,3(2):111-134

Seismic signals consist of several typically short energy bursts, called phases, exhibiting several patterns in terms of dominant frequency, amplitude and polarisation. We present a fast algorithm to detect the so‐called S‐phase in a three‐component seismic signal. This new approach combines traditional S‐phase detection methods and the discrete wavelet transform. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

    

Z Dhurmeea  H Pethybridge  EV Romanov  C Appadoo  N Bodin 《African Journal of Marine Science》2020,42(2):131-140

The diet of albacore Thunnus alalunga from their spawning ground in the waters of Mauritius was investigated. The stomach contents of 249 albacore individuals, caught by industrial longliners and artisanal fishers, was analysed, and the dominant prey taxa were selected for lipid and fatty acid (FA) analysis. The FA profiles of prey were compared with those of liver tissue of spawning-capable and post-spawning female albacore, using multivariate analysis. Whereas stomach content analysis identified cephalopods and crustaceans as the dominant prey items in number, FA-profile analysis identified crustaceans and fishes as the most-frequently consumed prey of post-spawning female albacore. In contrast, the FA profiles of spawning-capable albacore and those of prey showed very low similarity. Analysis of the prey suggests that although cephalopods, crustaceans and fishes could all provide albacore with the required lipids and FAs, cephalopod prey seem to be less desirable owing to their lower energy content (i.e. low in total lipid, triacylglycerol, 16:0, 18:0 and 18:1ω9). Instead, the most beneficial food appeared to be locally available fishes, which are more energy-rich and have a higher docosahexaenoic acid/ eicosapentaenoic acid ratio. These results provide new information on the trophic ecology of albacore, promote our understanding of the importance of prey type for successful reproduction of albacore, and highlight the advantages of FA profiling to study diet.  相似文献   

    

Omran Frihy  Essam Deabes  Wahid Moufaddal  Adam El-Shahat 《Marine Georesources & Geotechnology》2013,31(5):408-418

Mineralogical and textural analyses of 45 undisturbed short cores and 80 grab sediment samples, collected from five frequently dredged navigational areas within harbors and water pathways of the Nile delta littoral system, were utilized for evaluation of these sediments as potential source of economic heavy minerals (EHMs). Results of mineralogical characterization indicate that the average total heavy mineral (HM) concentrations are as follows: Abu Qir Bay (1.7%), Rosetta estuary (3.1%), Burullus fishing port (4.5%), Damietta Harbor (2.9%), and El Gamil lagoon inlet (1.9%). Assessment of HM grades indicates predominance of magnetite, ilmenite, hematite, leucoxene, garnet, zircon, and rutile. Results of the feasibility analysis indicate that dredged sediments at these study areas are considered as a potential source of EHMs and economically promising to be mined for HMs. The present study suggests a practical operative plan of two successive phases for HMs recycling: (1) in-situ initial separation of HMs on the dredger deck using wet-gravity spirals, then (2) transportation of the recovered HM concentrates to an onshore processing plant to selectively separate individual HMs via wet and dry magnetic and electrostatic separators. Alternatively, dredged sediments can be directly pumped to a nearby onshore area as stockpile to be recycled afterward in inland processing plant.  相似文献   

白云岩体表生成岩裸露期古风化壳岩溶的规模   总被引：9，自引：5，他引：9  

侯方浩  方少仙  沈昭国  董兆雄  蒋裕强 《海相油气地质》2005,10(1):19-30

白云岩体表生成岩裸露期古风化壳岩溶对白云岩体古岩溶储渗体的形成至关重要。表生成岩裸露期古风化壳岩溶可形成地表残积层和表层之下大小不等的溶蚀空间。地表残积层与上覆盖层间可成为油气的储渗空间和通道,表层之下大小不等的溶蚀空间则可被塌积岩、冲积岩、填积岩和淀积岩充填,但其内常可保存部分孔隙,并为后来的埋藏溶解提供流体扩溶的初始空间和流体流动的通道。野外露头和井下岩心中的大量现象和事实说明,白云岩体表生成岩裸露期古风化壳的规模远小于石灰岩体。最主要原因是白云岩(石)可溶性远较石灰岩(方解石)小,同样组构的白云岩的物理力学强度亦较石灰岩小。白云岩体本身的岩性和组构、层理类型、泥质含量、构造裂隙的发育程度等都会影响裸露期古风化壳的规模。  相似文献   

地震活动的月相效应   总被引：9，自引：4，他引：9  

丁鉴海  黄雪香 《地震》1994,(4):7-13

本文利用中国大陆１９１０年以来６．０级以上和首都圈地区１９６６以来１．０级以上大量地震资料,对地震活动的月相效应进行了研究。结果表明,发生在各个月相上的地震频次明显受月相的调制和制约,并且有一定的规律性,月相效应的最大幅度可影响地震活动平均频次的２５％以上,证明了地震活动的月相效应明显存在,其影响不可忽略。结果还表明,月相效应的基本特征是具有半月和全月周期性,此外,对首都圈地区地震活动以及唐山,邢  相似文献   

成庄矿区构造特征及应力场分析     

王彦军  赵学晶 《安徽地质》2012,(2):93-97,102

本文对成庄矿区褶皱、断层及3号煤层的构造特征进行了总结论述.通过对矿区节理的统计、配套和分期,对该区燕山期以来的主应力方位、期次进行了划分,将主应力划分为3期,同时将成庄矿区含煤地层形成以后的构造演化划分为三个阶段:早期应力阶段奠定成庄矿区煤田构造基本格架;中期应力阶段对成庄矿区造成的影响很小;晚期应力阶段完成了矿区内煤田构造基本格局.  相似文献   

    

Hitoshi?Morikawa  "mailto:morika@enveng.titech.ac.jp  " title=  "morika@enveng.titech.ac.jp  " itemprop=  "email  " data-track=  "click  " data-track-action=  "Email author  " data-track-label=  "  ">Email author  Sumio?Sawada  Yusuke?Ono 《Journal of Seismology》2005,9(2):151-170

  相似文献   

GPS载波相位测量中的信号多路径效应影响研究   总被引：15，自引：1，他引：15  

袁林果  黄丁发  丁晓利  熊永良  钟萍  李成钢 《测绘学报》2004,33(3):210-215

当利用GPS进行高精度变形监测时,GPS信号的多路径效应成为影响GPS变形监测精度和可靠性的一种不可忽略的误差源.本文针对GPS载波相位测量中的多路径效应影响进行理论和试验分析.论述载波相位测量中GPS多路径信号的特征,分析它对载波相位测量的影响,提出在坐标域用移动平均进行短基线静态GPS测量多路径误差探测的方法.通过实测数据进行分析,取得满意的效果.  相似文献   

    

M. Matsumoto  S. Wallis  M. Aoya  M. Enami  J. Kawano  Y. Seto  N. Shimobayashi 《Journal of Metamorphic Geology》2003,21(4):363-376

A largely undocumented region of eclogite associated with a thick blueschist unit occurs in the Kotsu area of the Sanbagawa belt. The composition of coexisting garnet and omphacite suggests that the Kotsu eclogite formed at peak temperatures of around 600 °C synchronous with a penetrative deformation (D1). There are local significant differences in oxygen fugacity of the eclogite reflected in mineral chemistries. The peak pressure is constrained to lie between 14 and 25 kbar by microstructural evidence for the stability of paragonite throughout the history recorded by the eclogite, and the composition of omphacite in associated eclogite facies pelitic schist. Application of garnet‐phengite‐omphacite geobarometry gives metamorphic pressures around 20 kbar. Retrograde metamorphism associated with penetrative deformation (D2) is in the greenschist facies. The composition of syn‐D2 amphibole in hematite‐bearing basic schist and the nature of the calcium carbonate phase suggest that the retrograde P–T path was not associated with a significant increase or decrease in the ratio of P–T conditions following the peak of metamorphism. This P–T path contrasts with the open clockwise path derived from eclogite of the Besshi area. The development of distinct P–T paths in different parts of the Sanbagawa belt shows the shape of the P–T path is not primarily controlled by tectonic setting, but by internal factors such as geometry of metamorphic units and exhumation rates.  相似文献