Causes and Methods of Reducing Dilution at Seabee Gold Mine

The failure of waste rock from the hangingwall and footwall of open panels dilutes ore grades, creates significant costs for underground mines and can reduce production rates. This research investigates the mechanisms causing dilution from the hangingwall and footwall of panels at SSR Mining’s Seabee Gold Mine, located in northern Saskatchewan. To conduct this research, estimations were made regarding the rock mass conditions and stress in panels so conventional analysis techniques for dilution could be applied. Underground field mapping and lab testing were conducted, and a new methodology was developed to assess large scale joint irregularities. Three joint sets were identified with Q’ ranging from 10.6 to 14.1 for the panel hangingwall. An instrumentation program was conducted with both closure stations and an extensometer. New data interpretation techniques were applied which suggested stresses normal to the ore body reached a stress of approximately 92 MPa, prior to panel mining. Conventional empirical dilution estimation analyses indicated that less than 0.5 metres of dilution should be expected from both the hanging wall and the footwall of all panels assessed at the Seabee Mine. Cavity Monitoring Survey (CMS) data was collected and interpreted and many dilution values were measured to be well in excess of 0.5 metres. It is known that open panel dilution can be caused by many mechanisms. These mechanisms, or causes of dilution, include: • rock mass instability due to panel dimensions exceeding stable limits which would be represented by an arch shaped failure; • panel top sill and bottom sill drifts extending past the ore – waste contact; and, • blast hole deviation causing the hangingwall and footwall rocks to be blasted. Each of these mechanisms have characteristic failure or dilution profiles. The approach to this research has been to link the detailed geometry of the hangingwall and footwall surfaces from which dilution has occurred, to the expected failure geometry from the common causes of panel dilution. This research shows that of the three failure mechanisms listed above, the dominant failure mechanism is based on geometrical instability represented by an arch shaped failure. The failure mechanisms of panel undercutting due to sill drifts extending past the ore – waste contact and blast hole deviation were not found to be significant. Efforts to minimize panel dilution are recommended based on their cause, and these efforts cannot be effective without an understanding of the mechanisms influencing dilution. The methodology developed in this thesis is applicable to all underground hard rock mining operations