Numerous gold bearing quartz vein systems are found in oxide facies banded iron formations of the Beardmore-Geraldton area. They exhibit a spatial relationship with zones of dilated cleavage and fracture networks that transect the limbs and hinges of asymmetric folds. Many of the gold deposits in the region are located in close proximity to east-west trending fault zones that may have acted as plumbing systems for the auriferous hydrothermal fluids.
In this thesis the geological setting of the gold mineralization is discussed with particular emphasis on mineralogy, alteration and structural control of the vein systems. Several gold bearing veins were studied from a select number of mine sites and gold occurrences to obtain information on the mineralization processes.
The mineralogy of the quartz veins and spatially associated sulphidation and carbonatization alteration zones were examined in detail. The results suggest the precipitation of gold from hydrothermal solutions was concomitant with the replacement of the iron formations by pyrite and arsenopyrite.
Gold may have been transported as reduced sulphur complexes in CO2 rich fluids of low salinity derived by the metamorphic devolitization of a greenstone terrain at the transition between greenschist and amphibolite facies. The emplacement of the veins is predominantly a late stage event although evidence suggests some veining may have occurred concomitant with a regional episode of deformation.
The iron formations in the study area were important as both chemical and structural traps for gold precipitation. The competency of the units allowed for ductile and brittle deformation providing low pressure zones for the hydrothermal fluids. They also supplied abundant Fe for sulphidation processes initiating the reduction of gold in solution to elemental gold following the reaction:
2Fe3O4 + 6Au(HS)2- + 6H + 6FeS2 + 8H2O + 6Au
(magnetite) (bisulphide gold complex) (pyrite) (gold)