Craig W. Harvey

The acoustic properties of ores containing the common sulphide minerals (pyrite, chalcopyrite, sphalerite, pyrrhotite and/or pentlandite) and their felsic and mafic-hosts were determined. P and S-wave velocities were measured on dry oriented mini-cores to 600 MPa confining pressure using the pulse transmission technique of Birch (1960). To document the compositions of the samples, the modal mineralogy was estimated by a variety of techniques; visual estimation, density analysis, oriented thin section microprobe point counting and manual and microprobe grain-mounted point counting techniques. Of these, the grain-mounted manual point counting technique proved the most accurate.

Correlations between velocity and modal sulphide abundance indicate that sulphide mineral assemblages have velocities related to the end-member phases by linear mixing lines. Acoustic properties of the sulphide minerals obtained from extrapolation of these mixing lines are: pyrite ( = 4.91 g/cc, Vp = 7.99 km/s, Vs = 4.94 km/s), chalcopyrite ( = 4.28 g/cc, Vp = 5.12 km/s, Vs = 2.49 km/s), sphalerite ( = 4.07 g/cc, Vp = 5.44 km/s, Vs = 2.80 km/s), pyrrhotite ( = 4.71 g/cc, Vp = 4.60 km/s, Vs = 2.73 km/s) and pentlandite ( = 4.68 g/cc, Vp = 4.56 km/s, Vs = 2.95 km/s). The results indicate that all sulphide-assemblages have higher acoustic impedances than most common mafic and felsic host rocks and pyrite-rich rocks have much higher impedances. For example, a 100% pyrite body in contact with a felsic host is predicted to generate P and S-wave reflections which are 4.4 and 9x stronger than those from a felsic-mafic contact, respectively. For P and S-wave high resolution surveys (100 Hz and 50 Hz, respectively) sulphide bodies must be at least 15-25 m thick by 250-270 m wide at 500 meters depth to be resolved (~5 Mt deposit) and 8-13 m thick by 60-100 meters wide to be detected. Synthetic modelling of a hypothetical felsic-hosted VMS deposit and a mafic-hosted Ni-Cu deposit confirm that pyrite-rich bodies will generate `bright spots' whereas pyrrhotite-pentlandite-chalcopyrite bodies will generate detectable but weaker reflections.

The calculated acoustic properties of sulphide minerals can be used in seismic surveying and tomographic imaging. In seismic studies, velocities and densities could be used to constrain possible lithologies and to identify zones of possible sulphide concentration. The use of both P and S-wave velocities and Poisson's ratio can be used to help improve the quality of seismic and VSP interpretations.

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Supervisor: Matt Salisbury