The nature of dilution in narrow vein mining operations

The underground mining of metalliferous narrow veins is always associated with dilution. Dilution is the addition of waste to ore, which consequently increases tonnage and reduces grade. Excessive dilution is a commonly quoted reason for mine failure world-wide. These features are often not addressed adequately in reserve determination, though are an integral, if subjective part of the mining reserve. The unpredictability of the ore/waste contact is principally responsible for the shortfall between estimated and realized mill feed tonnages and grades. There are two types of dilution. Planned dilution arises from material below the cut-off that lies within the stope boundaries determined by the selectivity of the mining method and complexity of the orebody. Additional dilution is extra material generally waste and derived from rockmass failure and overbreak. It is routinely recorded by mine operators though is not determined in an identical fashion. Planned dilution included in the mine design and reserve has to be tolerated and cannot be avoided. Additional dilution is often controllable and related to careless drilling and blasting. Unexpected geomechanical problems may occur during stope drawdown or mucking. Preventative measures to control dilution include placing of rock/cable bolts and/or pillars. In many stopes support systems need only be short-medium term to permit the safe removal of ore. Mining quality control as part of the grade control programme is an important part of dilution limitation.