Detection directe d'un gisement de sulfures massifs volcanogenes par sismique reflexion (French and English text).

摘要

The objective of this study is to evaluate the applicability of the 3D seismic reflection method, which is routinely applied in oil and gas exploration, to prospecting for deep volcanogenic massive sulphides (VMS). The Matagami mining camp of the Abitibi greenstone belt is well suited for this study because geological units have a moderate dip (∼45°) and most geological contacts can be traced for tens of kilometres. Petrophysical studies performed in situ (boreholes) and on rock samples have demonstrated that rhyolite-gabbro contacts were likely to generate strong seismic reflections. The good reflectivity of lithological contacts in the Matagami mining camp allow seismic methods to map them to depths exceeding 3 km. Sample analyses from the Isle-Dieu orebody and borehole logging of holes that intersect the Bell Allard deposit have also shown that massive and semi-massive sulphides could be reflective. In particular, sulphides rich in high-impedance minerals (e.g. pyrite and magnetite) should generate anomalously strong reflections if their size is sufficient. In 1994, a 2D seismic profile acquired directly above Bell Allard identified a geophysical anomaly corresponding to the ore deposit. Since 2D seismic reflection profiles are inherently ambiguous as to the source of a given reflection, a vertical seismic profile (VSP) was deemed necessary to demonstrate the reflectivity of Bell Allard. The VSP section displays a strong compressional wave reflection and converted shear wave produced by the Bell Allard sulphides. A modelling study was undertaken, using existing borehole geometries, to evaluate the performance of VSP technology as a mineral exploration tool. The modelled VSP section showed weak amplitude reflections for any shot location. Hence, the VSP method is not recommended for exploration in the Matagami mining camp.; In 1996, a 20 km2 3D seismic survey was collected above the 1 km deep Bell Allard ore deposit. The survey objectives were to identify the seismic response of a VMS ore deposit and to map the main lithological contacts. The Bell Allard VMS deposit appears as a 100 m wide zone of high seismic amplitudes. The deposit is underlain by a larger amplitude anomaly that we attribute to a gabbro sill, or possibly to the alteration zone of the deposit. (Abstract shortened by UMI.)