Elucidating the Effects of Hydrothermal Alteration on Seismic Reflectivity in the Footwall of the Lalor Volcanogenic Massive Sulfide Deposit, Snow Lake, Manitoba, Canada

Abstract

The integrated analysis of seismic rock properties, lithogeochemical data, and mineral compositional data, estimated via scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), provides insight into the effects of hydrothermal alteration on seismic reflectivity in the footwall of the Lalor volcanogenic massive sulfide (VMS) deposit, Manitoba, Canada. The effects of hydrothermal alteration on variations in acoustic impedance are secondary in magnitude and superimposed on the dominant acoustic impedance contrast between felsic and mafic volcanic protoliths. This secondary effect is due to an increase in P-wave velocity with increasing intensity of hydrothermal alteration, as measured by the Ishikawa and Carbonate-Chlorite-Pyrite alteration indices. Mixture modeling of the seismic rock properties and mineral percentages suggests that the increase in seismic velocity is due to an increase in abundance of cordierite, which is one of the diagnostic aluminum silicates for hydrothermally-altered volcanic rocks metamorphosed in the upper almandine amphibolite facies. The synthetic seismic data of a simple VMS model consisting of mafic-felsic host rock contacts, a sulfide ore lens, and a discordant hydrothermal conduit, consisting of the amphibolite-facies mineral assemblage (600 °C, 6 kbar) encountered at Lalor, show enhanced seismic reflections at conduit-host rock contacts, in comparison to its greenschist facies equivalent (350 °C, 2.5 Kbar). This zone of enhanced seismic reflectivity in the footwall of the massive sulfide ore zone is also recognized on the Lalor seismic data suggesting that high-grade terrains hosting VMS deposits possess enhanced potential for the seismic detection of their footwall hydrothermal alteration zones.