Hydrothermal brecciation due to fluid pressure fluctuations: examples from the Olary Domain, South Australia

Abstract

Two breccias, the Doughboy and Cathedral Rock breccias, hosted by the Willyama Supergroup in the Olary Domain, northeastern South Australia, were mapped and described in this study. Mapping of the breccias has demonstrated that they are controlled by the regional deformational history of the area and they are intimately related to episodes of hydrothermal fluid flow and alteration. The clast morphology, particle size distribution, dilation ratio, structural position and contact relationships with the host rocks allow the degree of maturity and the nature of the fluid rock interactions for each breccia to be determined. Breccia clast size distribution and morphology were quantified using fractal analysis techniques. Textural analysis revealed significant differences in particle size distribution and clast morphology for the different breccias. The results of the fractal analysis indicate that the Doughboy breccia is the result of multiphase tectonic and fluid-assisted failure on a fault plane, whereas the Cathedral Rock breccia is the result of fluid-assisted failure during fold-related faulting. Due to the high fluid pressure constraints on reverse fault initiation, large fluid pressure differentials are generated during the opening of dilatant sites. It is the opening of these dilatant spaces which provides the room to accommodate and the mechanism for brecciation. Reverse faulting created dilatant spaces that, due to the large pressure differential between the wall rock and the dilational space, initiated failure of the wall rock. Whether this was a single event such as at Cathedral Rock or occurred during multiple events such as at Doughboy can be deduced via the fractal characteristics. Collectively, these observations shed light on the processes that occur during regional deformational events associated with multiple generations of hydrothermal activity. The relationship between fluid pressure and brecciation may explain why they are the sites where the most intense effects of hydrothermal activity in the Olary Domain occur.