7. Seismic Exploration of the Manitouwadge Greenstone Belt, Ontario: A Case History

Abstract

The Manitouwadge greenstone belt in the Archean Superior Province of Northern Ontario, a highly deformed and metamorphosed remnant of supracrustal rocks, has been a focus of mining activity since the 1950s. However, known economic mineral deposits have been fully exhausted, providing impetus for the application of geophysical techniques that can guide further exploration for deep orebodies. A multidisciplinary approach was adopted in which the groundwork was laid for using seismic techniques to image key horizons of the major structure, the Manitouwadge synform, at depth. The use of seismic-reflection techniques as an exploration tool in crystalline rocks with a complex geological setting is unconventional. Detailed geological mapping was used to generate a working model of the subsurface geometry. This was followed by physical rock property and borehole studies, which provided the acoustic properties of the main lithological units. Three-D forward modeling using the Born approximation confirmed that, given the working geological model, reflections from the main contacts should provide a framework for the interpretation of the observed data. A seismic program consisting of three intersecting 2D, high-resolution, reflection profiles successfully imaged several important horizons. The top of the volcanic sequence in which the known mineral deposits are hosted can be broadly imaged throughout the subsurface extent of the Manitouwadge synform, providing some guidelines for exploration. There is also an indication that the Geco mine horizon, which is host to the known mineral deposits, can be imaged with the seismic data, thus defining tighter guidelines on where further exploration is warranted. Several deep holes were drilled subsequently in order to both confirm the interpretation and investigate a number of high-amplitude anomalies observed in the seismic data. The drilling confirmed the interpretation of the main cornacts imaged in the synform, but the amplitude anomalies were found to correspond to amplitude focusing artifacts along several high-impedance contacts, rather than massive sulfides as hoped. This case history demonstrates that the use of seismic-reflection technology, where appropriate and logistically feasible, can be a significant aid in mineral exploration, both for the delineation of key marker horizons and, potentially, for direct detection of massive sulfide deposits.