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.
Highlights
For the past three decades, the seismic reflection method has shown promising results for the exploration of volcanogenic massive sulfide (VMS) deposits [1,2,3]
The direct detection of sulfide ore will likely remain the primary objective of any 3D seismic exploration endeavor, seismic imaging of the more regional-scale hydrothermal alteration system associated with VMS deposits may provide important geological insight, widening the scope of seismic exploration from direct detection to indirect targeting in areas
This paper reports the integrated modeling of seismic rock properties, lithogeochemistry, and estimates of mineral abundance from scanning electron microscopy-X-ray energy dispersive spectroscopy (SEM-EDS) to gain more insight into the largely unknown potential of detecting
Summary
For the past three decades, the seismic reflection method has shown promising results for the exploration of volcanogenic massive sulfide (VMS) deposits [1,2,3] This success is largely attributable to their ore composition of high-density sulfide minerals, including low-seismic velocity minerals, such as chalcopyrite, pyrrhotite, sphalerite, and high-velocity pyrite. We model the seismic properties of mineral mixtures by estimating their abundances from SEM-EDS images of thin sections cut from 11 drill core samples When these estimates are combined with the appropriate mineral velocities and densities, the mixture-modeled P-wave velocity and density can be computed, which can be compared with the seismic rock property measurements of these samples. On the basis of these results, we revisit previously reported interpretations of the 3D seismic cube acquired over the Lalor VMS deposit [4,5] to investigate if the response of hydrothermal alteration can be recognized in the seismic data
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