Seismic reflection imaging of mineral systems: Three case histories

Abstract

Mineral deposits can be described in terms of their mineral systems, i.e., fluid source, migration pathway, and trap. Source regions are difficult to recognize in seismic images. Many orebodies lie on or adjacent to major fault systems, suggesting that the faults acted as fluid migration pathways through the crust. Large faults often have broad internal zones of deformation fabric, which is anisotropic. This, coupled with the metasomatic effects of fluids moving along faults while they are active, can make the faults seismically reflective. For example, major gold deposits in the Archaean Eastern Goldfields province of Western Australia lie in the hanging‐wall block of regional‐scale faults that differ from other nearby faults by being highly reflective and penetrating to greater depths in the lower crust. Coupled thermal, mechanical, and fluid‐flow modeling supports the theory that these faults were fluid migration pathways from the lower to the upper crust. Strong reflections are also recorded from two deeply penetrating faults in the Proterozoic Mt. Isa province in northeastern Australia. Both are closely related spatially to copper and copper‐gold deposits. One, the Adelheid fault, is also adjacent to the large Mt. Isa silver‐lead‐zinc deposit. In contrast, other deeply penetrating faults that are not intrinsically reflective but are mapped in the seismic section on the basis of truncating reflections have no known mineralization. Regional seismic profiles can therefore be applied in the precompetitive area selection stage of exploration. Applying seismic techniques at the orebody scale can be difficult. Orebodies often have complex shapes and reflecting surfaces that are small compared to the diameter of the Fresnel zone for practical seismic frequencies. However, if the structures and alteration haloes around the orebodies themselves, seismic techniques may be more successful. Strong bedding‐parallel reflections were observed from the region of alteration around the Mt. Isa silver‐lead‐zinc orebodies using high‐resolution profiling. In addition, a profile in Tasmania imaged an internally nonreflective bulge within the Que Hellyer volcanics, suggesting a good location to explore for a volcanic hosted massive sulfide deposit. These case studies provide a pointer to how seismic techniques could be applied during mineral exploration, especially at depths greater than those being explored with other techniques.

Understanding regional structural controls on mineralization at the century deposit: A numerical modelling approach

“…The Widdallion unit is simulated as a low permeability unit (as suggested by Broadbent et al, 1998), although the higher permeability case is also tested. It is worth noting that the TRF and RL, the largest fault and shear zones in the region, form a "Y-front" architecture, similar to that proposed for major faults in the Yilgarn Craton (Drummond et al, 2000). Two main scenarios are simulated here:…”

Towards direct detection of gold bearing rock formations from seismic data, St. Ives gold camp, Western Australia

“…The St. Ives gold camp is located on the Yilgarn craton, an archaean mafic sequence made up of basalts, ultra-mafics and interflow sediments. These formations are found primarily in a southerly plunging anticline, overlain with unknown crusts of felsic composition intruded by dolerites and felsic porphyries (Drummond et al, 2000). As illustrated in Error!…”

What can seismic in hard rocks do for you?

“…After sporadic trials in Australia in mid-seventies, Geoscience Australia collected in 1990s several province scale seismic traverses throughout the Eastern Goldfields (Drummond et al, 2000). These sections provided exciting new information about major structures in the province, and stimulated new ideas about why the big gold deposits occur where they do.…”