PROBING THE DEPTHS

In 1987-88, Lithoprobe researchers focused their efforts on the Canadian Shield and tested the feasibility of studying greenstone belts with seismic reflection techniques. Significant additions to the scientific program were made possible by supplemental funding from the Ontario Geological Survey, the Ministere de l’energie et des ressources du Quebec and Minnova Inc. Even at an early stage of processing, the quality of the seismic images obtained from the pilot survey has exceeded most expectations. Seismic profiles reveal complex geometries of volcanic rocks, intrusions and major faults known for their association with economic mineralization. Results from this work should have an impact on our understanding of a mining district in Ontario and Quebec which is intensively explored for precious and base metals. Last year’s survey has led to high expectations for the continuation of scientific activities in this region scheduled from 1990 to 1992. Major Subhorizontal Features

Some of the available profiles have unequivocally imaged major subhorizontal features as shallow as a few kilometres; previous models of the Abitibi belt that envisage only steeply dipping strata or structures to great depths will thus require revision. Flat- lying structural elements provide excellent indirect evidence of steep discontinuities along which mineralization is often found. The diagram on page 45 schematically illustrates the geometry of lithological blocks inferred from seismic profiles in the vicinity of the Porcupine-Destor break, north of Kirkland Lake, Ont., and of the Larder Lake-Cadillac break, south of Rouyn-Noranda, Que. Major offsets in structures imaged between two to four seconds listening time (~6 to 12 km depth) project to the surface near mapped branches of the Porcupine- Destor break. Similarly, relatively flat- lying reflectors south of Rouyn abut against a steeply north-dipping discontinuity related to the Cadillac fault. Evidence gathered from decades of exploration and mining activity suggests that mineralization associated with regional breaks may have resulted from deep-seated fractures channelling mineralized fluids that originate from lower, crustal levels. Lithoprobe reflection profiling now provides evidence that such fractures can be traced to depth. Not only can the geometries of structures be better understood, but new ideas regarding the source and migration of mineralizing fluids can be formulated as a result of reliable sub-surface control. The recognition of similar structures in poorly exposed terranes now seems possible using the seismic reflection profiling technique.

In addition to developing new geological concepts, research that stems from Lithoprobe is concerned with testing new acquisition methods and designing new processing schemes. Unlike some other geophysical techniques, seismic reflection surveying requires site-specific selection of a large number of acquisition and data- processing variables, and it is only through experimentation that expertise relevant to Precambrian shields will evolve. Results from the 1987-88 campaign have demonstrated that special consideration must be given to the imaging of steeply dipping structures. Although Lithoprobe seismic reflection work is designed to target crustal-scale features, the acquisition technology developed through the program may be scaled down to map features of more immediate interest to the mining community. Technological spinoffs from Lithoprobe will thus benefit those in industry who cannot otherwise afford to test the feasibility of this expensive, but powerful, new exploration tool.

As with other government-sponsored mapping programs, Lithoprobe is chiefly committed to developing a better understanding of the crust, not to finding specific petroleum or metal deposits. In so doing, Lithoprobe builds a knowledge base useful for developing strategies for resource exploration and for assessing earth- related hazards. Work initiated in 1984 has targeted the Appalachian and Cordilleran mountain belts and associated oil-producing basins, earthquake-prone areas of western Canada, ancient continental break-ups in the Great Lakes region, major faults in the Kapuskasing area of Ontario, and the continental margin along Canada’s east coast. L. Mayrand, A. G. Green, B. Milkereit are with the Geological Survey of Canada; P. Verpaelst is with Ministere de l’energie et des ressources du Quebec; R. H. Sutcliffe and S. L. Jackson are with the Ontario Geological Survey; and C. Hubert and J. N. Ludden are with the Universite de Montreal.

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