Seismic reflection imaging over a massive sulfide deposit at the Matagami mining camp, Québec

A 2-D seismic reflection profile was shot across the southern flank of the Matagami mining camp, almost directly above the recently discovered Bell Allard massive sulfide deposit, now estimated at more than 6 million metric tons. All orebodies found in the southern part of the mining camp, including Bell Allard, are located at the contact between the primarily basaltic Wabassee Group and the underlying rhyolitic Watson Lake Group. Seismic reflections were recorded from the basalt-rhyolite contacts of the lower Wabassee Group, as well as from gabbro sills that intrude much of the volcanic stratigraphy. A strong reflection from the top of the Bell Allard orebody was also detected, but the reflection does not extend over the full width of the deposit as defined by drilling, appearing to correlate with the lower pyrite-rich zone. Faulting, which can be interpreted from discontinuities in the observed reflections, probably controlled the formation of the Bell Allard deposit. If the interpreted gabbro sills are accepted as isotime markers, then faulting of the deeper sill complex defines a series of half grabens within the rhyolitic Watson Lake Group. The Bell Allard deposit is found at the intersection of one of these apparently low-angle normal faults with the top of the Watson Lake Group, indicating that sulfide mineralization may have been associated with fluid flow along the fault, which likely penetrates to the underlying mafic intrusion. Although the precise geometry of subsurface faulting cannot be estimated from a single 2-D seismic profile, these results indicate that a full 3-D seismic survey should allow the mapping of many of the subsurface fault systems and the verification of hypotheses of fault-controlled deposit formation.