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The mineralized veins of the Dolgellau Gold Belt, their host shales (the Clogau Shales and Maentwrog Formation) and the underlying Cambrian/Pre-Cambrian' sequence of the Harlech Dome with its associated igneous rocks have all been studied by a combination of geochemical, fluid inclusion and stable isotope techniques. This has enabled the characterization of the fluids associated with the metamorphic and mineralizing events in the Dome and estimation of the pressure-temperature conditions of these events. Fluid inclusion studies. show the metamorphic fluid in the "Lower Formations" (those below the Clogau Shale) to be an aqueous Na-K-Cl brine while in the Clogau Shales it is an immiscible pair of CH4-H2O fluids with Na-K-Cl dissolved in the aqueous-rich phase. This implies that conditions were far more reducing in the Clogau Shales during metamorphism due to the presence of graphite in the shale. Geothermometry and geobarometry using the immiscible fluid pair in the Clogau Shales gives metamorphic conditions of 300-3650C, 3.0-3.3 Kb. The mineral ized-vein inclusions also contain an immiscible CH4-H20 pair which yeild conditions for the mineralizing event of 245- 3200C, 1.6 Kb at the maximum temperature. Both metamorphic and mineralized vein samples show late-stage calcic and weakly saline aqueous inclusions. Oxygen isotopes show widespread resetting of carbonate and mica minerals, probably an effect of the late stage fluids noted above, while quartz retains a metamorphic value. Isotopic temperature estimates for metamorphic and mineralizing conditions are consistent with those from fluid inclusionson the basis of the fluid inclusion evidence a model has been proposed for the genesis of the ores of the Gold Belt. This model postulates that an oxidized metamorphic fluid from the Lower Formations was introduced into the Clogau Shale. Here it underwent reaction with its new host and became more reduced as some of the introduced water reacted with graphite in the shale to produce methane with consequent deposition of the ores. The stable isotope data and theoretical consideration of the fluid inclusion and geochemical data all support this model. They also suggest that a basic/ultrabasic igneous lithology was the source of the gold-mineralizing fluid |
