GEMOC ARC National Key CentreJohn Ridley, GEMOC Macquarie
The uniformity of many chemical characteristics of mesothermal gold deposits of Archean granite-greenstone terrains and geologically more recent low-grade metamorphic terrains implies a uniform genesis for these deposits. A specific source for the gold-carrying fluid remains, however, unresolved, despite data for a large number of geochemical and isotopic tracers from a large number of deposits. Individual tracers have been used to argue for specific fluid sources at specific deposits, however, wherever multiple tracers are available, a single source can rarely be implied. Data from multiple tracers has been used to rule out specific sources, to argue for derivation from mixed and homogenised or average crust, or for the fluid being a mixture from different source rocks.
Fluid flow at the deposits is channelised along fractures and structural discontinuities in the rock mass, with chemical interchange between the fluid and wallrock during flow is indicated by alteration haloes around channelways. Several lines of evidence imply that many deposits are kilometres distant from the fluid source, in particular the lack of clear lithological associations of deposits, the mass balance requirements of the volume of source rock or fluid needed to form large deposits, and the vertical continuity of deposits.
Order-of-magnitude mass balance calculations based on quartz precipitation in veins and lodes, and the average thickness of alteration haloes around lodes, suggest effective fluid : rock ratios after a few kilometres of fluid travel in these hydrological systems are of the order 10 : 1, and are thus such that the systems would be neither fluid dominated nor rock dominated. How individual tracer components will behave in such systems will be controlled by the fluid : rock partitioning of the component (Kd). We now have some multi-element composition data on the gold ore fluid from fluid inclusion analyses and phase equilibria calculations, and can hence estimate Kd's for many components. Many of the commonly used source tracers (Pb, Sr and O isotopes, K/Rb ratios) have intermediate Kd values such that they will be neither ėrobust' and reflect the source composition, nor be controlled by equilibration with immediate wallrock. Rather, the source and all rock types with which the fluid has interacted with along the channelway will have influenced the fluid composition with respect to the component at a deposit. Data from multiple geochemical tracers could be inverted to determine likely source rocks, but this would require a good knowledge of the geological environment of the deposit. At present our tracer data does not distinguish between the various specific source rocks proposed (e.g. granitoid, metamorphic devolatilisation of mafic rocks, mantle) for the mesothermal gold fluid.