Breccia formation by particle fluidization in fault zones: Implications for transitory, rupture-controlled fluid flow regimes in hydrothermal systems

Abstract

Breccias in the Rusey Fault (Cornwall, UK) and the Roamane Fault (Porgera gold deposit, Papua New Guinea) provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid. These faults contain cockade-like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals. At Rusey, the overgrowths are quartz-dominant, whereas in the Roamane Fault overgrowths are composed of calcite, quartz, or gold-rich pyrite. Although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours, the spheroidal overgrowths do contact each other and are at least partially cemented together. The hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment, or finer-grained, inequigranular to mesh-like intergrowths. Concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles.

The breccias occur as fault-parallel layers and lenses, each up to several tens of centimeters thick. Adjacent layers are characterized by texturally-distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths. In the Rusey Fault, many texturally-distinct breccia layers are present within a 3m wide fault core. Some layers truncate others and many breccia layers exhibit grainsize grading or banding. Clasts in the breccias include fragments of wall-rock, veins and various fault damage products, including fragments of earlier generations of cemented breccia. Brecciation was episodic and separated by periods of cementation.

The distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime. Hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments. Layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow. Particle size distributions indicate that fluid velocities during fluidization were in the range 0.1 ms⁻¹ to 1 ms⁻¹. The maximum flow rates correspond to fluid fluxes of 10 to 100 L.s⁻¹ per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide. Such high flow rates characteristically induce intense swarm seismicity rather than mainshock-aftershock seismicity, and have implications for the dynamics and rates of formation of fault-related hydrothermal ore deposits.