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Large-Scale Fluid Flow |
National Key Centre in Economic Geology, James Cook University, 4811, Townsville, Australia
Fluids are active at all levels in the crust during metamorphism. Devolatilization of many rock packages leads to the evolution of H2O, CO2 and/or S as a fluid phase that is commonly of low salinity. The composition of the fluid phase released during devolatilization is controlled by the dominant mineral assemblage in the source region, and the evolution of such fluid is most dramatic where major changes in modal mineralogy occur; e.g. across metamorphic facies boundaries. Evidence for this loss of volatiles during metamorphism is provided by metamorphic assemblages that typically contain less volatile components as grade increases.
The metamorphism of mafic rock successions can yield a metamorphic fluid rich in H2O, CO2 and S, and close in composition to fluids inferred in the formation of many gold-only deposits. The close affinity between gold and these fluids arises from the stability of the Au-S complexing.
Where evaporites are present in the succession, the fluids released during devolatilization can be quite saline. A further potential source of saline fluids in these environments is from crystallization of certain magmas. Saline fluids have a composition favourable for transporting gold and base metals, which is likely to explain the occurrence of gold as a co-product in many deposits where saline fluids are inferred.
The contrast between saline and low salinity fluids during metamorphism is not only reflected in ore metals, but also in the associated alteration and element mobility. Only a few elements are mobile in the low salinity fluids, and the alteration mineralogy is strongly influenced by the nature of the host rocks. Saline fluids readily transport many elements, and a similar alteration assemblage can be superimposed on many different precursors.
