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   

等高线图信息定量度量研究   总被引：2，自引：0，他引：2  

刘文锴  乔朝飞  陈云浩  赵仁亮 《武汉大学学报(信息科学版)》2008,33(2):157-159,176

在分析现有的地图信息定量度量方法的基础上,提出了一组测量等高线地形图的几何信息和拓扑信息的定量度量方法,并进行了实例分析.  相似文献   

Retailing local food in the Scottish-English borders: A supply chain perspective     

B. Ilbery  D. Maye 《Geoforum》2006,37(3):352-367

Local food is championed as one alternative response to industrial systems of food production and supply. While advocacy for local food is high, there is a lack of empirical evidence about the actual shape and scale of such food supply chains, especially from a retail perspective. Using supply chain diagrams, this paper presents a summary of ‘new’ agro-food geographies for five different retail types—farm shops, butchers, caterers, specialist shops, supermarkets/department stores—that all source local food from suppliers in the Scottish-English borders. Presented as five separate ‘shopping trips’, the paper examines where, how and why retailers source local food. Results reveal the complex nature of local food systems, especially in terms of intra-sector competitive dynamics (with a notable tension between direct forms of retail and established (independent) retailers), links and overlaps with ‘normal’ food retail systems and elastic notions of the ‘local’. The paper also draws a key distinction between locally produced and locally supplied food products.  相似文献   

Tetrahedral Plots of the Phase Relations for Basalts     

Sven Maal?e  Richard N. Abbott Jr. 《Mathematical Geology》2005,37(8):869-893

The phase relations of quaternary systems are generally represented by projections onto ternary compositional planes. Such projections often obscure relationships that would only be evident in a three-dimensional tetrahedral plot. The tetrahedral plot requires that compositions of the minerals and melts be transformed into Cartesian coordinates. It is shown here how this transformation is carried out. The application is demonstrated by tetrahedral plots of experimental melt compositions of partially molten lherzolite. Furthermore, the plot can be used to evaluate whether or not a particular basaltic composition represents a primary melt. The methods are applicable to any four-component system.  相似文献   

一种稀土参数图解新方法:以内蒙古拜仁达坝-维拉斯托闪长岩成因研究为例     

赵保具  颜开  肖荣阁 《现代地质》2021,35(3):608-624

稀土元素的示踪作用在地质学、地球化学研究中具有重要意义,稀土参数图解可以揭示岩石的形成机理、条件和成因分类,追踪各种复杂地质作用的演化历史。利用稀土元素参数REE与SiO₂、REE与LREE/HREE、δEu与LREE/HREE图解,在内蒙古拜仁达坝-维拉斯托铅锌银多金属矿区从偏基性端元向偏酸性端元演化的闪长岩类野外岩相学观察和室内测试的基础上,发现其全岩稀土参数之间的相关关系,进而结合岩石地球化学数据、稀土参数图解新方法与微量元素聚类分析,判别其闪长岩的成因和岩浆源区。结果显示,拜仁达坝赋矿的闪长岩具有高硅、富钠及较高MgO和Al₂O₃含量的特征,属钙碱性系列,显示地幔岩浆分异与下地壳熔融岩浆混合的特点;拜仁达坝-维拉斯托闪长岩LREE/HREE-REE和δEu-LREE/HREE曲线显示,稀土高值组端元分别呈负相关和正相关关系,负Eu异常明显,相对富集Rb、Th、U、K等大离子亲石元素,亏损Nb、Ta、Sr、P、Ti等高场强元素,而稀土低值组端元反之。上述特征表明, 拜仁达坝-维拉斯托闪长岩为一套活动大陆边缘幔源为主、壳幔混合成因的钙碱性岩浆岩系列。实例研究初步证实,提出的稀土参数图解新方法是有效的,具有较强的普适性,适合在常规的地质科研中完善并推广。  相似文献   

论构造地球化学图解的局限性          下载免费PDF全文

万渝生 《岩石矿物学杂志》1988,7(4):318-325

本文从实践和理论两方面讨论了应用构造地球化学图解分辨玄武质岩石产出的大地构造环境这一方法所存在的问题,认为J. A. Pearce等给出的构造地球化学图解,在很大程度上,是一种岩性分辨图解,图解分辨中存在不确定性的根本原因是控制玄武质岩石形成的条件参数与构造环境并不完全存在一一对应的映射关系,应用图解应持谨慎态度。  相似文献   

Development of algebra algorithms for automated generation of grain‐size distribution maps     

Alessandro Sarretta  Mauro Masiol  Emanuela Molinaroli 《地球表面变化过程与地形》2007,32(7):1116-1127

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Catastrophe theory, caustics and traveltime diagrams in seismology     

Michael G. Brown  Frederick D. Tappert 《Geophysical Journal International》1987,88(1):217-229

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低温平衡ZnS-PbS-FeS-H_2S四组分体系的热力学分析     

李小伟  莫宣学  赵志丹 《岩石学报》2010,26(10):3153-3157

本文介绍了热力学相图在浅成低温热液硫化物矿床中的应用,并以三江地区金顶铅锌矿床中的矿物含量为例进行定量分析。一方面在相同温度下,浓度与矿物(组合)之间存在对应关系,并且一些矿物(组合)对应特定的酸碱条件,例如在400K和105Pa条件下,黄铁矿与方铅矿的组合在碱性环境下稳定,而方铅矿与闪锌矿的组合在酸性环境中稳定,且酸性条件下当lg[Pb2+]≥-11.29时,才开始出现方铅矿;另一方面对比了298.15K和400K条件下矿物共生组合的及介质pH值的变化情况,并得到400K条件下形成矿物或矿物组合所需离子的浓度更高。  相似文献   

硫化铜矿生物浸出过程热力学研究     

周娥  段东平  陈思明  王晓冬 《盐湖研究》2014,(1):47-55

绘制了Cu-Fe-S-H2O体系在不同温度、反应物浓度条件下的E-pH图。通过分析比较发现,体系反应温度升高,有利于矿物溶解浸出反应的进行,反应物浓度应保持在一定的范围内,更有利于浸出反应的进行;热力学计算的反应物及产物在氧化浸出体系的变化与工业生产基本一致。  相似文献