• Hough, Rob
• Pearce, Mark

Abstract

Gold deposits are often associated with quartz and carbonate veining indicating mineralisation during deformation and fluid flow. Closer inspection of a number of greenstone hosted deposits from Western Australia has shown that gold mineralisation in rocks that contain veins is often located in the wall rock and not the veins themselves. This suggests that key processes are occurring in the wall rock causing the gold to precipitate from the fluid that carried it into the rocks. Continued reactions and deposition of gold demand that permeability can be created and maintained in the host rocks. Microstructures are used to decipher the reactions that occurred in response to the gold-bearing fluid and to investigate the mechanisms of permeability generation in the host rocks.We present quantitative electron back-scatter diffraction data combined with spatially referenced major and trace-element chemistry to infer the reaction mechanisms active during fluid-rock interaction which potentially lead to the gold mineralisation. Data are presented showing the chemically driven recrystallisation of large (several mm) calcite grains to produce iron rich carbonates. Recystallisation occurs by nucleation of new grains which are in low energy orientations with respect to the host. This iron is sourced from large biotite and ilmenite grains which are breaking down as a result of the carbonate metasomatism to form fine grained polycrystalline aggregates. Biotite is reacting to muscovite and gold grains are found associated with this reaction (inter-mingled with the two micas and at the reaction fronts between them). In these samples, the importance of brittle deformation seems to be limited to vein creation allowing large volumes of fluid into the rock in the first place. There is evidence for limited crystal plastic deformation in the older carbonate and biotite grains. Given that the reactions fronts in the biotite are perpendicular to the grain boundaries (as opposed to along the cleavage plains) it is inferred that grain boundary transport of fluid is important on the scale of the grains rather than smaller-scale fracturing. Therefore, the grain-size reduction caused by the chemically induced recrystallisation is potentially of important in speeding up fluid transport through the rock. Gold depositions could be caused by a number of mechanisms including creation of space due to local volume loss during reactions or redox reactions that occur when iron is removed from ilmenite and biotite and incorporated into the carbonates.

Topics

• Deposition
• impedance spectroscopy
• polymer
• grain
• grain boundary
• gold
• precipitate
• permeability
• iron