Abstract:

The recent upticks in market value of various commodities has generated great interests for low grade high volume deposits. Such deposits can be mined at relatively low unitary costs via open pit mining by leveraging capabilities for large production tonnage through heavily mechanized processes. Many operations of that nature have been developed in the past few years near abandoned underground mines. These historic sites present readily available information on regional and local geology from past exploration work while leaving much of the low grade zones unexploited. Surface excavations near or above abandoned underground workings present important geotechnical challenges regarding surface pillar stability separating the pit from the old openings. Planning of the mining sequence must take into account the eventual collapse of these surface pillars.

Of relevance to the aforementioned mining approach, the Detour Lake Mine, located in northeastern Ontario, currently mines a low grade disseminated porphyritic deposit for gold from a conventional open pit. The mine is situated above the historic Detour Lake underground mine operated from 1983 to 1999 by the company Placer Dome. During that period, the mine produced approximately 1.8 million ounces of gold. The mine recently performed a surface pillar blast above abandoned underground openings. Careful planning of the drill pattern, loading configurations, and blasting sequence lead to a successful blast result. The learnings from such special types of blast conditions are worth sharing to properly understand the underlying geotechnical nature of these situations, and help enhance safe and efficient performances.

This work will present the steps and procedures followed by the Detour lake mine engineering team to safely execute the blasting of the sill pillar without direct access to the surface of the pillar. The study will present the results and data collected during the drilling and mining of the sill pillar area. The work will have an emphasis on the safety procedures surrounding the sill pillar collapse blast as well as what happened at Detour lake mine so that future projects with similar conditions can know what to expect.

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Adele Malo