Fluid Chemistry of Orogenic Lode Gold Deposits and Implications for Genetic Models

Abstract The source of the ore fluid and hence of the gold in orogenic lode gold deposits remains unresolved. The consensus that the ore fluid of orogenic deposits of all ages and settings is a low-salinity mixed aqueous-carbonic fluid, and hence different from that for most other gold deposit types, stands up on examination of available fluid inclusion data. A minority of deposits have only low salinity aqueous fluid inclusions. The composition and isotope chemistry of the fluid is uniform in many respects. For many components it appears that the fluid is in approximate equilibrium with the host-rock sequence, although not with the immediate wall rock. There is evidence that fluid compositions varied in space and time in the deposits with respect to a number of critical components. Assessment of mass balance of fluid-rock interaction in these hydrothermal systems shows that fluid compositions should not be expected to reflect the source after the fluid travel distances implied but rather to reflect fluid-rock interactions along the fluid pathway or mixed signatures of the source and the wall rocks. With this principle, it is seen that either a granitoid magmatic or a metamorphic devolatilization model for the fluid source is allowable given our present knowledge.