Seismic reflection imaging of mineral systems: Three case histories

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 fau lt systems, suggesting that the faults acted as fluid migration pathways th rough the crust. Large faults often have broad internal zones of deformatio n fabric, which is anisotropic. This coupled with the metasomatic effects o f fluids moving along faults while they are active, can make the faults sei smically reflective. For example, major gold deposits in the Archaean Easte rn Gold-fields province of Western Australia lie in the hanging-wall block of regional-scale faults that differ from other nearby faults by being high ly reflective and penetrating to greater depths in the lower crust. Coupled thermal, mechanical, and fluid-flow modeling supports the theory that thes e faults were fluid migration pathways from the lower to the upper crust. S trong reflections are also recorded from two deeply penetrating faults in t he Proterozoic Mt. Isa province in northeastern Australia. Both are closely related spatially to copper and copper-gold deposits. One, the Adelheid fa ult, is also adjacent to the large Mt. Isa silver-lead-zinc deposit. In con trast, other deeply penetrating faults that are not intrinsically reflectiv e but are mapped in the seismic section on the basis of truncating reflecti ons have no known mineralization. Regional seismic profiles can therefore b e applied in the precompetitive area selection stage of exploration. Applyi ng seismic techniques at the orebody scale can be difficult. Orebodies ofte n have complex shapes and reflecting surfaces that are small compared to th e diameter of the Fresnel zone for practical seismic frequencies. However, if the structures and alteration haloes around the orebodies are targeted r ather than the orebodies themselves, seismic techniques may be more success ful. Strong bedding-parallel reflections were observed from the region of a lteration around the Mt. Isa silver-lead-zinc orebodies using high-resoluti on :profiling. In addition, a profile in Tasmania imaged an internally nonr eflective bulge within the Que Hellyer volcanics, suggesting a good locatio n to explore for a volcanic hosted massive sulfide deposit. These case stud ies provide a pointer to how seismic techniques could be applied during min eral exploration, especially at depths greater than those being explored wi th other techniques.