Mapping closely to how ore deposit geology is now taught, this textbook systematically describes and illustrates the major ore deposit types, linking this to their settings in the crust and the geological factors behind their formation. Written for advanced undergraduate and graduate students with a basic background in the geosciences, it provides a balance of practical information and coverage of the relevant geological sciences, including petrological, geochemical, hydrological and tectonic processes. Important theory is summarized without unnecessary detail and integrated with students' learning in other topics, including magmatic processes and sedimentary geology, enabling students to make links across the geosciences. Students are supported by further reading, a comprehensive glossary, and problems and review questions that test the application of theoretical approaches and encourage students to use what they have learnt. A website includes visual resources and combines with the book to provide students and instructors with a complete learning package.
Textures in the blueschist metabasic rocks of the island of Syros, Greece, indicate a reaction forming omphacite and garnet from earlier glaucophane and epidote. A balanced 'whole rock' reaction can be written using the phase compositions and modal mineralogy of the products observed. This reaction is continuous in P-T space if Mg and Fe 2+ are independent components. The available thermodynamic data suggest that the 'eclogite' (garnet plus omphacite) assemblage is favoured by higher temperatures.It cannot, however, be conclusively shown that the rocks went through an up-temperature interval of metamorphism. The reaction seen could be promoted by reduced silica, or water activity. The details of the reaction textures and mineral zonation patterns show furthermore that chemical equilibrium was not maintained throughout the rock development. The reaction textures differ in detail in different samples. In those samples in which the textures are most consistent with maintained chemical equilibrium however, the mineral zonation patterns do imply up-temperature metamorphism.
Abstract.A low-salinity, mixed aqueous-carbonic fluid is common to all Arch~ean lode-gold deposits throughout the range of mineralising conditions from sub-greenschist to lower-granulite facies temperatures. Alteration assemblages and fluid-inclusion data give constraints on the fluid composition. Fluid Xco2 is 0.1-0.3 in typical greenschist-facies (mesothermal) deposits. At higher temperatures, the assemblages are consistent with formation from a fluid of similar composition, but slightly higher or lower Xco2 cannot be ruled out, and fluid-inclusion data indicate that CH4 may be an important component in ore fluids at these temperatures. Fluid pH is neutral or weakly alkaline at all conditions. A range of relative oxidation states of four orders of magnitude fo2 is indicated at any temperature, with deposits more oxidising relative to QFM at lower temperature. Sulphur contents of the fluids vary from ~ 10 to 10-3"Sm~s , with a trend towards lower sulphur contents at lower temperatures. The relative concentrations of major cations in solution are similar at all conditions with Na >> K _ Ca, although Ca may be less abundant at low temperatures. The broad similarities in ore-fluid composition at all temperatures give support to 'crustal-continuum' models, in which Arch~ean lode-gold mineralisation involved either a single fluid moving through the middle and upper crust, or derivation of ore fluids by similar processes at different crustal levels. Many of the compositional differences between high-and lowtemperature ore-fluids may be attributed to evolution of deep-sourced hydrothermal solutions as they rise along structurally-controlled conduits. The constancy of major ore-fluid component concentrations (e.g. CO2, C1, __ K) suggests fluid-buffering and high fluid-rock ratios along fluid pathways. Fluid-buffered conditions can also explain the ore-fluid fo=-temperature relations; witla equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state. In contrast, the concentrations of components present in lesser abundance in Arch~ean gold ore-fluids (e.g. S, Ca, H +) were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature, or were rock-buffered through fluid-rock reactions. Extrapolation to high temperatures of the K, Na and Ca contents of the gold-bearing fluids indicates that their composition is consistent with derivation from, or final equilibration with, rocks of intermediate-granitic composition, thus giving support to isotopic and geological arguments for orefluid source regions external to the greenstone belts. The fluid oxidation states are characteristic of a wide range of potential source rocks, including mantle-derived igneous rocks, calc-alkaline granitoids and magmas, and seaflooraltered metabasalts. Strongly oxidised magmatic sources or unusually oxidising source processes (e.g. COg-streaming during granulitisation of the lower crust) are therefore not required in the genesis of Archman lode-gold deposit...
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