Causes and effects of backfill dilution in open stoping method

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The use of mine backfill in open stoping method to mine steeply dipping ore deposits is very common in underground hard rock mines in Canada and around the world. In a sublevel open stoping environment, the use of backfill allows for safe, pillar-less, and economic extraction of an orebody with longitudinal and transverse stoping methods. This is particularly true when dealing with ore deposits that are situated at great depths below ground surface where the in situ stress fields are invariably high, and where complex geological features like faults, dykes and shear zones are frequently encountered. Once primary stopes are mined out and backfilled, mining of secondary stopes can be scheduled in accordance with a planned sequence that aims for full, pillar-less recovery while pushing high stress fields away from the area being mined. While widely accepted, the inherent weakness in this process is that, often enough, the backfill column side facing the stope will fail partially or not into the secondary stope during the ore mucking process, thus resulting in ore dilution. Such backfill dilution results in delayed productivity due to increased stope cycle time, and in lower mine profitability caused by the reduced mill grade. Depending on the significance of backfill column failure, this may also result in delays in the mining of upper stopes due to time lost in floor repair and preparation. The short term objectives of this project thus are: 1) to determine the causes of backfill dilution in open stoping method with respect to stope type, design, sequencing and cycle time, backfill material type, strength and fill practice, and stope blast design, and blasting sequence, and 2) quantify the effects of such dilution on stope recovery, mill grade and productivity. The impact of backfill dilution on mine scheduling and economics is not properly accounted for in the current practice of underground mine design. The proposed research is a first attempt to understand and document the causes and effects of backfill dilution. It is believed that the proposed research will set the stage for significant research partnership with the mining industry to develop together what would be the first Canadian best practices guidelines for mine design against ore dilution.