Exploration Potential, page-56

Impact of Earthquakes on Faulting and Gold Deposits at Hill End and Fosterville

Earthquakes played a critical role in the structural evolution of both the Hill End and Fosterville goldfields, influencing fault activity, fluid flow, and gold deposition. The episodic seismic activity during orogenic events created fracture networks and hydraulic pumping systems that controlled high-grade gold mineralization.

1. Fault Reactivation and Seismic Activity

• Hill End:

The region experienced multiple phases of fault reactivation, especially during the Tabberabberan and Kanimblan Orogenies.

• Earthquake-induced reverse faulting reactivated older, bedding-parallel structures, causing them to act as conduits for hydrothermal fluids.

• W-dipping reverse faults intersected E-dipping bedding-parallel quartz veins, creating zones of high dilation where gold-bearing fluids precipitated.

• Fosterville:

The reverse faults and fault splays associated with seismic activity led to the formation of dilational jogs, where high-grade gold was concentrated.

• Earthquakes caused brittle faulting, which re-opened sealed structures, allowing repeated fluid injections rich in gold.

• The deepest bonanza-grade gold zones are directly related to fault jogs formed by seismic slip events, with visible gold often found along micro-fractures.

2. Seismic Pumping and Fluid Migration

• Seismic pumping is a key mechanism where earthquake cycles (fault slip and relaxation) drive the movement of gold-bearing fluids through fault networks.

• During an earthquake:

• Rapid fault slip causes fracturing and creates temporary high-permeability pathways.

• Pressure drops in the fractures cause hydraulic fracturing, facilitating fluid flow.

• After the earthquake:

• The system reseals due to mineral precipitation, particularly quartz and sulfides.

• Repeated seismic cycles concentrate gold over time, especially in fault intersections and dilational zones .

At Hill End, seismic pumping helped explain the repeated stacking of gold-rich quartz veins, while at Fosterville, it accounts for the bonanza-grade gold zones associated with arsenopyrite-rich faults.

3. High-Grade Gold Shoots Formation

• Hill End:

• High-grade gold shoots formed where reverse faults intersected pre-existing quartz veins, creating localized zones of stress release during seismic events.

• These intersections acted as “pressure shadow zones”, ideal for gold precipitation due to sudden drops in fluid pressure during earthquakes .

• Fosterville:

• The link between seismic events and gold deposition is evident in the deep fault zones, where micro-fractures filled with visible gold are common.

• Bonanza-grade gold is often found in seismically induced breccia zones, where rapid fluid flow and pressure changes caused gold to precipitate almost instantly .

Key Differences

• Hill End shows evidence of episodic reactivation, with gold concentrated in stacked vein systems due to repeated seismic cycles.

• Fosterville experienced deep, high-pressure faulting, with seismic activity controlling the formation of bonanza-grade gold at depth.

Conclusion

Earthquakes significantly influenced both Hill End and Fosterville through:

1. Fault reactivation and creation of fluid pathways.

2. Seismic pumping, leading to repeated cycles of fluid flow and gold deposition.

3. Formation of high-grade gold shoots at fault intersections and seismically induced dilational zones.

The dynamic nature of these orogenic systems—with constant deformation, fault movement, and seismic activity—was essential for concentrating economic gold deposits in both regions.